Heat-assisted hot-hole transfer increases the surface-enhanced Raman activity of Au-TiO_2 nanoarrays

Domain: biology
Published: 2026-03-17
Editorial outcome: major_revision

Abstract
Monitoring the evolution of molecules during photo and thermal synergistically induced physical and chemical processes is of paramount interest in fields including chemical, material, and energy research. Surface-enhanced Raman spectroscopy (SERS) is a highly promising technology in this regard, offering advantages of sensitivity, real-time, and label-free detection. However, the application of conventional SERS in high-temperature environments has faced challenges due to the inevitable loss of activity and decline in sensitivity. Herein, we synthesize Au-TiO2 nanoarrays as SERS substrates, and an anomalous enhancement of Raman signal with increasing temperature is observed. The signal intensity increases by 11.4 times at 180 ℃ compared to that at 22 ℃. This high-temperature enhancement in Raman activity is attributed to an underlying mechanism: heat-assisted hot-hole transfer, which enables 785 nm photon-induced hot-holes transfer from Au to TiO2. Our work expands the application of the SERS technique for high-temperature chemical analysis and molecular diagnostics.

Introduction
High temperature plays a key role in photocatalysis, as it can accelerate chemical reaction rates, improve energy conversion efficiency, and enable additional chemical reaction pathways1-4. Advanced plasmonic photocatalysis under high-temperature conditions combines photon and thermal effects, making it an effective method for optimizing chemical reactions1-3,5. Real-time observation of photocatalytic reactions at elevated temperatures is crucial for gaining a deeper understanding of the photo and thermal physical mechanisms in the catalytic reaction process. However, most existing in-situ techniques, such as scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES), require stringent vacuum environments, rendering them unsuitable for analyzing real-world thermal processes. In contrast, surface-enhanced Raman spectroscopy (SERS) can operate under ambient conditions, making it an appealing tool for the in-situ characterization of analyte transformations in complex environments6-9. Despite its potential, conventional SERS faces limitations at high temperatures, where the signal intensity typically diminishes or even disappears9-13. This poses a critical challenge in achieving reliable S Raman spectroscopy performance under such conditions.

Pioneers have made considerable efforts to improve the stability and sensitivity of high-temperature SERS9-13. One approach involves surface coating with inorganic materials to protect nanosubstrates from thermal degradation, thereby ensuring the stability of SERS analysis9-11. However, this method often sacrifices the SERS enhancement effect, as the coating inevitably impedes charge transfer between probe molecules and substrates, leading to relatively weak probe signals9-11. Alternatively, polymers have been tailored to optimize the localized surface plasmon resonance (LSPR) characteristics of certain plasmonic hybrid substrates through temperature-triggered volume-phase transitions, resulting in improved SERS signals at elevated temperatures12-14. However, this method is restricted by the thermal stability of the polymers and currently applicable only below 55 ℃, limiting its applications under higher temperature conditions12,13. To date, developing an active and robust in-situ SERS sensing technique for high-temperature applications remains challenging. Plasmonic metal–semiconductor hybrid materials have garnered attention owing to their enhanced stability by interfacial interactions15,16. These materials offer a promising opportunity for designing stable and sensitive SERS substrates that can withstand high-temperature conditions.

Here, using synthesized Au-TiO2 nanoarrays (NAs) as substrates and under 785 nm light excitation, we observe that SERS signals from molecules are enhanced by 11.4 times at 180 ℃ compared to those at room temperature (22 ℃). The application temperature of this high temperature-induced SERS (TI-SERS) technique is over 100 ℃ higher than that in previous temperature-dependent SERS studies12,13. Experimental observations from time-resolved spectroscopy and electron paramagnetic resonance spectroscopy (EPR) revealed the near-infrared driven hot-hole transfer from Au to TiO2, beyond the only occurrences of electron transfer. The transfer of hot holes with environmental heating changes the electron distribution near the Fermi level, which benefits the transfer of hot electrons from Au to the molecule, thereby enhancing Raman scattering. By contrast, at room temperature, the transfer of hot holes to TiO2 is impeded, leading to reduced hot-electron transfer. Moreover, our TI-SERS technique exhibits sensitivity, tunability, reproducibility, stability, and versatility, enabling real-time monitoring of the plasmon-activated photothermal catalytic reactions. Our method offers not only stable and sensitive molecular detection techniques for high-temperature applications but also provides insights into thermal plasmonic dynamics in heterojunction materials, which hold potential in the fields of photocatalysis, photovoltaics, and phototransduction.

Results
Synthesis and characterization of Au-TiO2 NAs
The original TiO2 NAs with a large specific surface area were synthesized via a hydrothermal method, followed by a 25 min photochemical growth of Au nanoparticles (NPs) with SERS activity on the TiO2 surface (Fig. 1a). Scanning electron microscopy (SEM) revealed the typical microstructures of the as-prepared Au-TiO2 NAs, where the TiO2 NAs were fully covered by a multitude of interconnected, even Au NPs with an average diameter at ≈19 nm (Fig. 1b and Supplementary Fig. 1). The densely packed and interconnected Au NPs on the TiO2 surface effectively promote LSPR effects because of the strong coupling between Au NPs6,17. High-resolution transmission electron microscopy (TEM) provided detailed structural insights into the interface between TiO2 and Au (Fig. 1c). The extended and interconnected Au lattice fringes at the boundary region indicate that the Au NPs are already overgrown on TiO2, rather than being physically mixed. Lattice-spacing measurements revealed distances of 0.236 nm, corresponding to the face-centered cubic structure of Au18, and 0.316 nm, corresponding to the (110) plane of rutile TiO2 19 (Fig. 1c and Supplementary Fig. 2). The ring-like concentric diffraction pattern observed in the selective area electron diffraction (SAED) analysis (Fig. 1d) confirms the polycrystalline nature of the sample, further supported by the X-ray diffraction (XRD) patterns shown in Fig. 1j18-20. High-angle annular dark-field scanning transmission electron microscopy (HAADF–STEM) and energy-dispersive X-ray spectroscopy (EDS) mapping (Fig. 1e–i) reveal a uniform distribution of Au, Ti, and O elements throughout the sample. The calculated relative atomic ratios of Au, Ti, and O are ≈8.3:30.7:61.0 (Supplementary Fig. 7d). The XRD pattern indicates that the diffraction peaks of TiO2 NAs are following the rutile phase (JCPDS No. 21-1276, Fig. 1j), and the Raman signal of TiO2 NAs also exhibits distinct characteristics of the rutile phase (Supplementary Fig. 3), collectively confirming the formation of TiO2 NAs in rutile crystal structure. The (110) plane of the TiO2 NAs is almost perpendicular to the substrate, thus is invisible in the XRD pattern (Fig. 1j, Supplementary Fig. 4). XPS analysis reveals changes in the chemical state and binding energy of the Au-TiO2 NAs (Fig. 1k). The Au 4f peaks at 84.1 and 87.8 eV corresponding to Au 4f7/2 and Au 4f5/2 exhibit distinct shifts in binding energy compared with pristine Au NPs due to the interaction and charge transfer between Au and TiO2, which simultaneously modulate the electronic structures of each component20,21, also resulting in the shift of Ti 2p peak (Supplementary Fig. 5). It reveals the first signs of Schottky barrier formation at the Au-TiO2 interface, which is beneficial for the transfer of photoexcited hot carriers at the interface and the chemical enhancement for SERS activity22.

We further investigated the optical properties of the Au-TiO2 NAs using absorption spectroscopy (Fig. 1l). TiO2 NAs exhibit sharp absorption peak with a cutoff at ≈380 nm. Following the overgrowth of plasmonic Au NPs on TiO2 NAs, the absorption can be significantly extended to the range of 500–1,000 nm, which substantially improves the light utilization efficiency of Au-TiO2 NAs. To better understand the state of Au in the samples, additional experiments were conducted by varying the duration of the light irradiation growth process. Characterization results, including SEM, EDS, absorption spectra, and SERS spectra, are presented in Supplementary Figs. 6–8 and Supplementary Note 1. The analysis reveals that Au-TiO2 NAs grown under light irradiation for 25 min exhibited the highest light-harvesting efficiency and LSPR properties. These samples were subsequently selected for high temperature SERS tests, and detailed findings are provided in Supplementary Figs. 6–8.

High-temperature-induced SERS activity
Exciting the nanoprobe in the near-infrared region (785 nm) effectively eliminates substrate fluorescence interference compared to excitation with visible light (532 and 633 nm)23. Fig. 2a shows the in-situ temperature-dependent SERS experiments conducted at 22, 90, 120, 150, and 180 ℃ using a common aromatic dye analyte24,25: crystal violet (CV) molecules (10−7 M) with an excitation wavelength of 785 nm (1.58 eV). The Raman signal of CV molecules increased as the temperature rose from 90 to 180 ℃. For instance, the Raman peak at 1172 cm−1 increased from ≈3236 counts s−1 at 22 ℃ to 14716 counts s−1 at 120 ℃, and then to a maximum of 36916 counts s−1 at 180 ℃, indicating a dramatic enhancement in SERS activity by a factor of 11.41. However, excessive heating (210 ℃ in this study) caused structural damage to the nanostructures, resulting in diminished SERS activity (Supplementary Fig. 9). The magnification factors for key Raman peaks of CV molecules at different temperatures are summarized in Fig. 2b. The data indicate obvious variability in the enhancement among different characteristic peaks in the TI-SERS measurements, with some peaks amplified by as large as 11.41 times, while some peaks had a lower enhancement factor of only 2.84 times. This selective enhancement of different Raman peaks and enhancement magnitude of TI-SERS (Fig. 2b and Supplementary Table 1) indicate an improved chemical enhancement in TI-SERS6,7,24. Control experiments were conducted to determine the contribution of individual components in Au-TiO2 NAs to the TI-SERS effect (Fig. 2c and Supplementary Fig. 10). No appreciable SERS enhancement was observed at 180 ℃ when Au NPs or TiO2 NAs were tested independently, ruling out their individual contributions to the TI-SERS effect. Also, room-temperature SERS testing of the 180 ℃-annealed Au-TiO2 NAs showed no detectable enhancement (Supplementary Fig. 10). In contrast, TI-SERS activity was extensively observed using other synthesized Au-TiO2 substrates with varying Au components (Supplementary Fig. 11 and Supplementary Note 2), suggesting that the TI-SERS effect arose from the synergistic interaction of Au and TiO2 complexes under high-temperature conditions. Finally, CV molecules at concentrations ranging from 10−7 to 10−12 M were accurately measured (Fig. 2d and Supplementary Fig. 12), demonstrating the femtomolar-level detection capability of the TI-SERS technology.

Tunability, reversibility, and stability
In addition to sensitivity, tunability, reversibility, and stability are also critical criteria for evaluating the practical performance of SERS substrates9-11,26,27. First, accurate control of the enhancement factor (EF), as a fundamental property of SERS, is necessary for quantitative detection26. As shown in Fig. 2e, the EF value clearly increased as the temperature rose from 90 to 180 ℃, and revealed a quantitative relationship (EF∝e−(T−180 °C)２, with R2 of only 0.995) between the enhancement factor and temperature. The EF decay during cooling still followed a similar quantitative relationship (Supplementary Fig. 13), demonstrating accurate and controllable regulation of EF through temperature, which contributes to more precise SERS applications. Second, the reversibility of the substrates was confirmed through Raman intensity measurements during five consecutive thermal field switching cycles (Fig. 2f). The enhanced Raman signals were well reproduced between 180 ℃ and room temperature with fluctuations of <4% (Fig. 2f and Supplementary Fig. 14), indicating the high reversibility of the substrates under the thermal field. Third, stability is among the most desirable features for high-performance SERS substrates in practical applications9. The Raman spectra can be clearly repeated even after 40 min of high-temperature treatment (Fig. 2g and Supplementary Figs. 15, 16). Moreover, the relative standard deviation of the Raman peak intensities at 1172 cm−1 was calculated to be ≈1.71%, highlighting the thermal stability of the TI-SERS system. In summary, the Au-TiO2 NAs used as TI-SERS substrates exhibited sensitivity, tunability, reversibility, and stability, making them applicable for the analysis of analytes in high-temperature environments.

Mechanisms underlying TI-SERS
The mechanism of action for TI-SERS remains unclear. On the one hand, some previous studies have shown that oxygen-related defects caused by high temperatures may enhance the Raman activity24,28,29. To verify whether TI-SERS is related to oxygen defects generated under high temperature conditions, we conducted a series of experiments, such as XPS, XRD, EPR, Raman and absorption spectra (Supplementary Figs. 17−21, 30, and Supplementary Notes 3−4). The results indicate that, compared with room temperature, heating the Au-TiO2 NAs substrates to 180 ℃ does not induce measurable changes in their structure or electronic configuration. This observation suggests that no oxygen defects are generated in the Au-TiO2 NAs upon heating to 180 °C, indicating that the TI-SERS mechanism is not attributable to oxygen defects.

On the other hand, photo-induced charge transfer (PICT) plays a crucial role in SERS, as described by chemical enhancement theory6,7,24,30. Transient absorption spectroscopy (TAS) is widely used to investigate the charge transfer of materials31,32. Therefore, we employed TAS to investigate the carrier dynamics of Au-TiO2 NAs. A 785-nm photon (1.58 eV) is used to excite the Au plasmon mode, but has no sufficient energy to excite TiO2 (bandgap: 3.10 eV; Supplementary Figs. 21, 22). As displayed in Fig. 3a,b, photoexcitation of the Au NPs LSPR results in a winglet signal and a bleach signal (centered at 491 and 678 nm, respectively)31,32. The maximum amplitude of the winglet signal and bleach signal of Au considerably increased with increasing temperature (Fig. 3b), indicating a larger electron population on Au at high temperatures. As the charge transfer process of the Au nanoparticles reduces the electron–phonon coupling time32-35 (τe–ph), we analyze the carrier dynamics of Au NPs and Au-TiO2 NAs at different temperatures. Signal fitting was performed using a double exponential decay convolved with the instrument response function. As illustrated in Fig. 3c, the dynamic process of Au NPs exhibited a τe–ph (8.6 ± 0.5 ps) at room temperature (bleach signal). At room temperature, Au-TiO2 NAs (bleach signal with blue circles) exhibited faster τe–ph (6.7 ± 0.4 ps) than Au NPs, which has been previously attributed to the electron transfer process from Au to TiO2 32. While the τe–ph (5.2 ± 0.3 ps) of Au-TiO2 NAs further decreases with increasing temperature to 180 ℃. The mechanism underlying the shortened relaxation at higher temperatures remains unclear. Previous standard “solid-state” integral model31,36,37 describes that the relaxation process slows down in metal with increasing temperature (laser heating), which is consistent with our experimental results (Supplementary Fig. 23). Moreover, the classic temperature-dependent permittivity models38-41 agrees with the increased absorption we measured for Au NPs at 785 nm with rising temperature (Supplementary Fig. 24). Our data confirm that the relaxation time of Au NPs lengthened as the temperature increases (Supplementary Fig. 24, 25 and Supplementary Note 5). However, the shortened relaxation time observed in Au-TiO2 NAs with increasing temperature (environmental heating) cannot be explained by the slowed relaxation predicted by the above models (Supplementary Figs. 24−26). The shorten τe–ph observed for Au-TiO2 NAs at high environment temperature could be explained by the higher efficiency in hot carrier transfer, wherein accelerated relaxation via charge transfer aligns with previous experimental observations31,32. Similar fast relaxation dynamics trend was also obtained for the winglet signal decay (Supplementary Fig. 27).

Furthermore, the selective chemical enhancement observed in our TI-SERS (11.4-fold at 1172 cm−1, 2.8-fold at 907 cm−1, Supplementary Table 1) also serves as evidence for efficient charge transfer, as opposed to a non-selective enhancement6,7,24,42,43 expected from purely electromagnetic mechanism. Next, direct evidence for hot-hole transfer was provided by our EPR observations.

In-situ EPR experiments were subsequently performed to further elucidate the roles of hot electrons or holes in the PICT process of Au-TiO2 NAs at various temperatures (Fig. 3d and Supplementary Fig. 28). From the original EPR spectra (Supplementary Fig. 28), we have obtained the in-situ EPR light-minus-dark difference spectra (i.e., the EPR spectra under light irradiation at the same temperature minus those under dark conditions) for Au-TiO2 at different temperatures, as shown in Fig. 3d. At relatively low temperatures (22–60 ℃) under 785 nm light excitation, the EPR light-minus-dark difference spectra were dominated by signals of Ti3+ (g=1.975, and 1.953 blue line) species, indicating that the transfer of photo-induced hot electrons from Au to TiO2 resulted in the reduction of Ti4+ to Ti3+44-46. These results are consistent with previous findings at room temperature44,47. Moreover, under high temperatures (90–180 ℃) combined with light irradiation, the new EPR-active O⁻ species signals emerged at g1=2.003, g2=2.013, and g3=2.024 (denoted by h+(O−), red line), along with the increasing of Ti3⁺ signals. This finding suggests the transfer of hot holes from Au to TiO2 44-46. Previous studies have confirmed that EPR-active O⁻ species are indicative of hot-hole transfer from Au to interface state of TiO2 under visible light excitation44-46. However, our results demonstrate that near-infrared light (785 nm) can induce hot-hole transfer from Au with the assistance of high temperature. These findings reveal a regime of interfacial energy transport. Furthermore, the EPR intensity of h+(O−) considerably increased with rising temperatures, reaching its highest value at 180 ℃ (red line). This indicates that high temperature not only facilitates but also enhances the hot-hole transfer process on Au under 785 nm excitation, highlighting the critical roles of thermal effects in expanding the scope of interfacial charge dynamics. Meanwhile, the computational investigations of Au-TiO2 NAs indicate an increased probability of hot-hole transfer from Au at high temperatures, with detailed analysis provided in Supplementary Figs. 31, 32 and Supplementary Note 6. These computational results are consistent with the experimental findings. In addition, Ti3+ signal also tended to increase as the temperature increased from 90 to 180 ℃ (Fig. 3d). We further confirmed that the decrease in Ti3+ does not affect the performance of TI-SERS through the EPR and Raman results in an oxygen atmosphere (Supplementary Fig. 29), thereby ruling out the contribution of Ti3+ to TI-SERS behavior. Moreover, we discussed the temperature-related quantum-tunneling effect48-53. The quantum-tunneling probability in plasmonic metal heterostructures is commonly considered to be weak dependence on temperature50-53. In our TI-SERS and EPR experiments, the signals are strongly dependent on temperature, indicating that the quantum-tunneling effect could be negligible.

Furthermore, density functional theory (DFT) calculations reveal that hot electrons of Au-TiO2 NAs can be effectively transferred to CV molecules within the TI-SERS system (2.13 |e⁻|, Fig. 3e and Supplementary Fig. 33). In general, extracting hot holes from Au NPs to semiconductors enables more charges to be available for chemical enhancement in SERS24,34. We proposed a chemical enhancement model dominated by PICT (Fig. 3f, Supplementary Figs. 34, 35, and Supplementary Notes 7, 8). Under high temperatures and 785 nm light excitation, hot holes on Au NPs can be efficiently transferred to TiO2 (Fig. 3f, PHT). This process facilitates charge separation and suppresses the recombination of hot electron–hole pairs in Au, thereby improving the transition efficiency of hot electrons from Au to the lowest unoccupied molecular orbital of CV molecules (Fig. 3f, μPICT1). As a result, high-temperature conditions considerably enhance the PICT process between the Au-TiO2 NAs and CV molecules under 785 nm excitation, which dramatically amplifies the molecular polarizability and the Raman scattering cross-section6,7,54, thus enhancing the Raman signal of CV. However, as the temperature increased, no obvious TI-SERS enhancement was observed when Raman lasers with excitation wavelengths of 532 and 633 nm (Supplementary Fig. 36) were used. Combining the above results, we conclude that high temperatures can trigger the photoexcitation of hot holes at higher energy levels, enabling their transfer.

Specifically, the photon energy of 785 nm (1.58 eV) is slightly below the threshold energy (1.6 eV) required for hot-hole transfer at Au-TiO2 interface44,47. However, works of pioneer have established that high temperatures could alter the electron distribution near the Fermi level5,55. This results in a consequent depletion of conduction band electrons near the Fermi level, thereby providing unoccupied states for the photoexcitation of hot holes at high energy levels (Fig. 3g). Therefore, Au-TiO2 NAs can achieve efficient near-infrared-driven hot-hole transfer under high temperature. We term this phenomenon as heat-assisted near-infrared driven hot-hole transfer, which is not permissible at room temperature (Fig. 3h).

High-temperature SERS sensing and photocatalytic activity
The TI-SERS effect originates from the modulation of plasmon-generated hot carriers by temperature on Au-TiO2 NRs substrates. This phenomenon is not limited to specific probe molecules, highlighting its potential for broad molecular detection applications. To demonstrate the versatility of the TI-SERS effect, various molecules were tested, including a typical aromatic dye24,25 methylene blue with large Raman cross-section, and a range of molecules with low Raman cross-section and weak affinity24,56-58, such as glibenclamide, metformin hydrochloride, thiamphenicol, streptomycin, folic acid, thiram, and penicillin G sodium (Fig. 4a−h, and Supplementary Table 1). The results revealed considerable enhancement in their TI-SERS spectra. Furthermore, Supplementary Fig. 37 and Supplementary Table 2 illustrate the SERS sensing of bisphenol S down to the nanomolar level, a highly carcinogenic health hazard that is released in substantial quantities during plastic decomposition at high temperatures59,60, demonstrating the practical applications and effectiveness of our TI-SERS technology (Supplementary Note 9).

Our method efficiently integrates thermal energy into conventional plasmonic photocatalytic reactions, leveraging the hot-carrier properties of Au-TiO2 NAs at high temperatures to enhance catalytic activity. On the basic of this principle, the synthesized Au-TiO2 NAs were designed for plasmon-mediated interfacial catalysis and in-situ SERS monitoring simultaneously at 180 ℃ with 785 nm laser excitation (Fig. 4i). The time-dependent Raman spectra revealed the emergence and growth of DMAB characteristic peaks at 1143 and 1440 cm⁻¹, which was accompanied by a reduction in the PNTP characteristic peak at 1340 cm⁻¹, confirming the dimerization of PNTP into DMAB61. Interestingly, the dimerization reaction was initiated almost instantaneously (within 2 s) upon laser irradiation on the Au-TiO2 NAs, which demonstrated that efficient hot-carrier transport between Au and TiO2 under the combination of high-temperature and optical fields was conducive to improving catalytic performance. Control experiments (Supplementary Fig. 38) confirmed that neither plasmon excitation nor high temperature (180 ℃) alone could trigger the catalytic reaction. Therefore, the synthesized Au-TiO2 NAs at high temperatures can efficiently drive chemical reactions that cannot occur at room temperature, with a reaction rate calculated to be 0.096 s−1 (Supplementary Fig. 39), which is superior to previously reported high-power plasmon-driven catalytic experiments61,62. Overall, our substrates with hot-carrier properties at high temperatures offer an alternative approach for plasmonic photocatalytic reactions in high temperatures.

Methods
Materials
Titanium butoxide (99.85%) and chloroauric acid (HAuCl4·4H2O, ≥47.8% Au basis) were purchased from China Sinopharm International (Shanghai) Co., Ltd. Hydrochloric acid (HCl, 99%) and ethanol (≥99.9%) were obtained from Macklin. CV (99%), bisphenol S (≥99%), and 4-nitrothiophenol (4-NTP, ≥95%) were purchased from Aladdin Chemistry Co., Ltd (Shanghai, China). In addition, polyvinylpyrrolidone (PVP, M =40000 g mol⁻1) was obtained from TCI (Shanghai). F-doped SnO₂ (FTO) conductive glass was customized in Guluo. All the reagents were of analytical grade and used as received without further purification. Deionized (DI) water, used throughout the fabrication and measurement processes, was prepared using a Millipore purification system (18.2 MΩ cm).

Synthesis of TiO2 NAs
Self-supported TiO2 NAs were synthesized on an FTO substrate using a hydrothermal method19,20. Initially, a 30.5 mL aqueous solution was prepared by mixing 15 mL of hydrochloric acid, 0.5 mL of titanium butoxide, and 15 mL of DI water in a glass dish, followed by stirring for 10 min to ensure homogeneity. The resulting mixture was then transferred to a 50 mL Teflon-lined autoclave. A precleaned FTO substrate was immersed in the prepared solution, and the hydrothermal reaction was conducted at 200 °C for 6 h. After the hydrothermal reaction, the sample was rinsed thoroughly with DI water. Finally, the self-supported TiO2 NAs were obtained by subjecting the sample to heat treatment in air at 400 °C for 1 h.

Synthesis of Au-TiO2 NAs
The growth of dense Au particles on the TiO2 NAs was achieved through ultraviolet (UV, 375 nm) laser irradiation64, as shown in Fig. 1a. Briefly, the as-prepared TiO2 substrate was immersed in a solution containing 20 µL of 0.05 M HAuCl₄, 0.5 mL ethanol, and 4 mL distilled water, then subjected to UV laser irradiation (wavelength of 375 nm, spot diameter of 2 cm, and power of ≈400 mW). The UV laser excited the TiO2 semiconductor, generating a large number of electron–hole pairs on its surface. The active electrons effectively reduced Au3+ to Au0 to realize the growth of Au NPs on the TiO2 surface. Simultaneously, the holes were rapidly consumed by the sacrificial reagent ethanol, which inhibited the recombination of photogenerated electron–hole pairs. This hole consumption also extends the electron–hole lifetime, promoting multielectron reduction of Au ions. Finally, Au-TiO2 composite structures with varying Au contents were obtained by irradiation with a UV laser for different durations (0–45 min) and washing with DI water for further measurements.

Synthesis of Au NPs
The Au NPs were synthesized through laser ablation using a Q-switched Nd-YAG (yttrium aluminum garnet) laser (Quanta Ray, Spectra-Physics). A 1064 nm pulsed laser beam (with a pulse width of 10 ns, energy of ≈250 mJ, and a repetition rate of 10 Hz) was employed to ablate the Au target in solution for 20 min, resulting in the formation of Au NPs. The as-synthesized Au NPs were then centrifuged at 6950×g relative centrifugal force for 15 min, followed by resuspension in DI water for further use.

SERS spectroscopy
SERS measurements were conducted using a confocal microprobe Raman spectrometer (Renishaw Raman spectroscopy) with a 50× objective. For a typical Raman spectroscopic analysis, the prepared substrates were individually immersed in 1 mL of analyte solutions (with ethanol as the solvent) and allowed to dry spontaneously for SERS measurements. Moreover, variable-temperature SERS tests for all the analytes were performed using accessories from the Linkam Scientific Instrument, and temperature increased at a constant rate of 10 ℃ min⁻1. Upon reaching the desired temperature, the temperature was stabilized for 5 min to ensure experimental accuracy. All SERS spectra were excited using a 785 nm laser with an output power of 0.45 mW. The acquisition time for each spectrum was set to 1 s. Furthermore, measurements at different temperatures were performed at the same sample location to ensure comparability of the experiments. The SERS enhancement factors of analytes were calculated by utilizing the Eq. (1):
EF = (NSERS × ISERS)/(NNOR × INOR) (1)
Where ISERS and INOR are the Raman intensities of SERS and Raman bands respectively (ISERS represents the intensities of TI-SERS bands). The NSERS, NNOR, and NSERS represent the corresponding number of analytes located in the laser spot area in the Raman, SERS, and TI-SERS measurements respectively, which can be approximately determined by the concentration of probe molecules24,65. It should be noted that all Raman measurements were carried out under identical experimental conditions. The analytes concentration of Raman were 0.2 M for CV, methylene blue, metformin hydrochloride, penicillin G sodium, streptomycin, folic acid, thiram, and 0.05 M for thiamphenicol, glibenclamide, respectively. The analytes concentration of SERS and TI-SERS were 10−7 M for CV, methylene blue, 10−6 M for glibenclamide, metformin hydrochloride, thiamphenicol, streptomycin, folic acid, thiram, and 10−5 M for penicillin G sodium, respectively.

Transient absorption spectroscopy
Femtosecond pulses at 800 nm with a pulse duration of 40 fs and a repetition rate of 1 kHz were generated by a Ti:sapphire oscillator-amplifier (Spectra-Physics). The femtosecond pulses were split into two beams: one directed into an optical parametric amplifier (Topas C, Light Conversion), and the other focused onto a CaF2 or sapphire crystal to produce probe pulses. The generated probe pulses (a broad spectrum) were focused onto the sample and overlapped with the pump beam. In this study, 50 μL of test solution spun onto a glass substrate was activated by the optical parametric amplifier. Transient spectra were detected using a UV-NIR detector from a fiber spectrometer (AvaSpec-ULS2048CLEVO, Avantes). The sample was heated in-situ using a Cindbest temperature-controlled stage equipped with a platinum resistance thermometer, which established a stable high-temperature environment through ambient thermalization. The instrument response function (IRF) is 200 fs. The extracted kinetic traces at the probe wavelength were normalized for comparison, and electronic relaxation signals were deconvoluted by fitting with the Eq. (2)32,66:
Se(t)=IRF⊗(A⋅e−t/τe–ph+B⋅e−t/τph–ph) (2)
where Se(t) is the electron signal intensity with time delay, and the symbol ⊗ denotes the convolution operation. A and B, τe–ph and τph–ph (phonon–phonon coupling time), are the amplitudes and time constants of the two exponential decay components.

Two-temperature model
The two-temperature model67-70 was used to simulate the relaxation phenomenon of Au NPs under different conditions, in which the energy exchange rate between electron and phonon are defined as Eqs. (3), and (4):
Ce(dTe/dt)=−G(Te−Tl) (3)
Cl(dTl/dt)=G(Te−Tl) (4)
Where Cl (2.5×106 J m−3 K−1)69 and Tl are the heat capacity and temperature of lattice. And Ce and Te are the heat capacity and temperature of electrons. For Te <3000 K, their relationship is characterized by a linear dependence71,72, Ce = γTe, where γ (68 J m−3 K−2)69 is the electronic heat capacity constant. And G (1.1×1016 W m−3 K−1)73 is the electron–phonon coupling constant of Au. Based on the absorbance (optical density, OD) of the Au nanoparticles (Supplementary Fig. 24) and 785 nm pump laser energy density (Ipump) at different ambient temperatures, the energy density absorbed by the Au NPs (Iabsorb) was calculated by Iabsorb =Ipump×(1-10-OD), yielding values of 160 μJ cm⁻2 at 22 °C and 222 μJ cm⁻2 at 180 °C with constant pump fluence. The corresponding peak electron temperatures were estimated to be 1563 K at 22 ℃ and 1864 K at 180 ℃, which were then used in the two-temperature model to simulate the temporal evolution of electron temperature in Au NPs under different temperatures.

First-principles calculations
The DFT calculations in this work were performed with a periodic slab model using the Vienna Ab Initio Simulation Package74-76. The Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional77 was applied with the generalized gradient approximation. The projector-augmented wave method78,79 was used to describe the electron–ion interactions, and the cut-off energy for the plane-wave basis set was 400 eV. The total energy convergence criterion for electronic interactions were converged within 10⁻5 eV. All the adsorption geometries were optimized using a force-based conjugate gradient algorithm. A 15 Å vacuum layer was added to all the slab models to prevent interactions between periodic images. The Bader charge was calculated using the Bader Charge Analysis module developed by Henkelman et al.80,81. The 1 × 1 × 1 k-point meshes were employed for the TiO2, Au-TiO2, and Au-TiO2 and CV molecule surface systems. The adsorption energy (Ead) is defined as Eq. (5):
Ead = Etotal − Eslab − Eadsorbate (5)
where Etotal denotes the total energy of the slab model with adsorbate atoms/molecules, Eslab denotes the total energy of the slab system without adsorbate, and Eadsorbate denotes the total energy of adsorbate atoms/molecules.

During the band calculation, the on-site Coulomb interactions of the Au 5d and Ti 3d electrons were treated within the Dudarev-type DFT+U formalism, with effective U values of 1.5 eV for Au and 4.2 eV for Ti. Through this calculation, we obtained the fatband structures and density of states.

Ultraviolet photoelectron spectroscopy
The energy band diagram of Au-TiO2 NAs was calculated from ultraviolet photoelectron spectroscopy (UPS) results. The UPS results were obtained on PHI5000 VersaProbe III instrument equipped with He-discharge lamp as the ultraviolet emission source (hν = 21.22 eV). The work functions (Φ) of TiO2 NAs and Au-TiO2 NAs can be determined by the Eq. (6):
Φ = 21.22−Ecut-off (6)
where 21.22 eV is the energy of the incident photon energy, and Ecut-off is the cut-off energy of secondary electrons. Moreover, the extrapolated set-off energy (Eset-off) represents the difference between the Fermi level (Ef) and the valence band (EVB). Based on these UPS results, we can evaluate the Φ or Ef, along with the EVB and conduction band (ECB) positions within this system.

Characterization
The morphologies and chemical compositions of the products were characterized using a focused ion beam electron microscope (Helios G4 UC) equipped with EDS. TEM images and SAED patterns were obtained with a JEOL JEM-2100F microscope operating at an accelerating voltage of 200 kV. Crystalline information was collected on the basis of XRD patterns using a Rigaku Smart Lab 9 kW system with Cu Kα radiation (λ= 0.15406 nm). The surface chemical states and compositions of the samples were analyzed through XPS using a PHI Quantera SXM. UPS measurements were conducted using a Thermo ESCALAB XI+ instrument with the He I line serving as the ultraviolet emission source (hν=21.22 eV). Absorption spectra were obtained using a UV-Vis-NIR spectrometer (20/30 PVTM micro-spectrophotometer), which was equipped with a Linkam Scientific Instrument Heating Cell Kit. The in-situ EPR investigation was performed by a Bruker A300 spectrometer operating at the X-band frequency, which was equipped with an in-situ digital temperature control systems ER4131VT and ER4141VT working in the temperature range of 100-600 K. The nanoarray sample was loaded into quartz glass tubes connected both to a high-vacuum pumping system and to a controlled gas feed (air or O2). Irradiation was performed by a 785 nm laser (Leoptics) and the output radiation focused on the samples in the cavity by an optical fiber. The spectra were recorded before and after 10 min of 785 nm laser irradiation inside the EPR cavity at different temperatures.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_2": "Zhang et al. present a new SERS substrate, composed of photodeposited gold nanoparticles on TiO2 nanorods. This\nsubstrate shows anomalous, and yet unobserved, high temperature SERS performance, with an optimum at 180°C. The\nsubstrate is used for the detection of various analytes, and the researchers attempt to explain the mode of action.\nThe observation of high temperature SERS is, without doubt, an important finding that warrants rapid and prestigious\npublication. Based on the data presented by the authors, I have no doubt that this effect occurs in a stable and reproducible\nmanner, and that it is highly useful for the detction of very small amounts of analytes. Regarding the mechanism, though, I\nhave doubts on the validity of the authors hypotheses, which are outlined in detail below. I think that a major revision is\nneeded to confirm and sharpen the hapotheses.\n(1) It is not clear which crystal phase dominates in the TiO2 NRs. The authors compare the XRD pattern of their sample to\nthe reference pattern of rutile, but the peaks do not match well. The peaks also do not match well with anatase. In addition,\nthe small peaks near 26, 34, 38 and 52° are likely to result from the underlying FTO substate, which should be clearly\nmarked. The authors instead conclude the presence of rutile from the TEM lattice fringes, but is this the only option? Or is it\npossible that the tubes are a mix of rutile and anatase, or that they are not well crystalline at all?\n(2) I have doubts about the accuracy of fit in Figure S17. All fits are located significantly above most measurement points. It\nis expected that by statistical error, in case of a good fit, about half of all measurement points are located above the fitting\ncurve and the other half below it. This is not the case here. Can you please comment how the fit was obtained, and why it\nwas done in this way? Furthermore, is a decrease by ~half a picosecond actually a \"significant decrease\", as the authors\nmention? Or would it be possible that, for example, different portions of the same sample show a similar deviation? In a\nsimilar manner, though the fit in Figure 4c looks much more reliable, is the difference in relaxation times extracted from the fit\nstatistically significant?\n(3) Figure S18 shows that a Ti3+ signal is already visible in the dark, which is easily possible, given that laser irradiation for\ngold deposition may have slightly (photo)reduced the material. Since all EPR spectra are plotted in arbitrary units, it is not\npossible for the reader to decide whether more Ti3+ is present under irradiation. This is additionally made more complicated\nby the differences in temperature between the two measurements. This makes the \"proof\" for hot electron transfer appear\nweak. In addition, can the authors cite an article why the peak just below g=2 is assumed to belong to the Ti3+ signal? This\nis unexpected, given that the signal in Figure S18 displays exactly he same shape, yet without the additional small signal\nbelow g=2.\n(4) UV-Vis spectra must be re-recorded and displayed (raw data) in the Supplementary Information. The Tauc plot cannot be\nperformed, the data is not clear. There is neither a clear baseline nor a clear absorption edge. Strictly speaking, the Tauc\nplot method is not appropriate anyway for composites of crystalline materials (see Klein et al., Adv. Funct. Mater. 33 (2023)\n2304523). The reviewer understands, though, that the method is applied in comparison with previous works. Still, the band\ngap energy is estimated from the intercept of the tangent to the absorption edge with the baseline of the spectrum (not a\nrandom zero line), and neither of the two features (absorption edge, baseline) is clearly visible, as outline above.\n(5) Considering points 1 and 3 above, it seems possible that the unexpected properties do not arise predominantly from an\nabsorption by gold, but by titania. It is well known that reduced titania is blue, and that is shows broad absorption across the\nred and infrared range. The color of titania would be obscured, however, by the purple color due to the deposited gold. The\nalternative hypothesis of an action of reduced titania might be ruled out, for example, by an oxidative treatment of the final\n(gold deposited) subtrate, for example at 180°C (where it is stable) with high amounts of oxygen in the surrounding\natmosphere. If the samples change color, and the SERS performance at 180° is lost or strongly attenuated, then the\nmechanism should potentially take into account a contribution of the reduced (and possibly amorphous) TiO2.", "reviewer_3": "The paper outlines work on SERS in high-temperature environments. The authors reported that\nAu-TiO₂ nanoarrays showed Raman signal enhancement with an increase in temperature. With a signal intensity increase at\n180°C compared to to that at 22°C. The paper ascribes this effect to phonon-assisted hot-hole energy upconversion. The\npaper showing that the increases in SERS signal,is probe molecule sensitive (Table S1 supplementary information).\nThe substrate consists of quasi-oriented nanotubes+silver nano particles (see Fig 4 supplementary information). The\nauthors reporting that the sample is polycrystalline. Ag nanoparticles are grown on the TiO2 through UV (375 nm) laser\nirradiation.\nTI-SERS (needs to be define, but presumably meaning temperature induced SERS)studies from room temp to 180 degree C\nwhere undertaken. Before the distribution of the SERS studies the paper outlines characterization of the sample.\nCan the Authors address the following\nCan information using methods be applied to determine in the impact of heating the substrate (upto 200 degrees), with\nregard to structure and electronic configuration (as seen via methods like XPS).Supplementary Fig. 7 shows that heating at\nc.a. 210 degrees is reported to course melting of the sample. A very important point of information is to clearly assess the\nimpact of heating to 180 degrees on the sample. Does this cause any persevered changes noting its near the melting point.\nThe use of UV light on TiO2 has been reported to cause the PIERS effect, similarly the heating of semiconductors like TiO2\nalso causes such effects as shown via heating semiconductors including TiO2 to c.a. 200 degrees (see Nature\nCommunications volume 7, Article number: 12189 (2016), Journal of Physical Chemistry C 124 (46), 25351-25360 2020 or\nACS Applied Nano Materials 3 (2), 1666-1673, 2020 ). Significantly this extends to enhancing plasmon assisted catalysis.\nThe author in Fig 5 showing that heating the substrate supports catalysis reactivity. Reports have shown that heating\nAg/TiO2 and other semiconductors (Journal of Physical Chemistry C 124 (46)) supports not only an increase in SERS but\nalso in catalysis.\nOverall. The noteworthy results are a demonstration that SERS at high temperatures in Ag/TiO2 systems can be improved,\nby an amount depending upon the probe molecule. More work in regard to assessing the impact of of thermal treatment of\nthe substrate is requested. Such as taking the substrate and examining the improvements in SERS at room temperature\nusing the oven treated substrate (as outlined in Journal of Physical Chemistry C 124 (46), 25351-25360 2020 or ACS\nApplied Nano Materials 3 (2), 1666-1673, 2020 ). A careful analysis of the Ag/TiO2 substrate following heat treatment is\nrequired. Eg after heating at room temperature what is the ESR data and what are the changes in structure, if any. Does the\npresence of thermally induced defect states explain the observed ESR data for example."}, "author_response": "Response to reviewer’s comments\nReviewer 1 (Remarks to the Author):\nThe authors suggest Au-TiO particles as a platform for high temperature SERS. They claim an\n2\nenhancement of about ao order of magnitude at about 100 degrees higher than before in a stable\nmanner. The results may be technically useful, but I am not convinced by the mechanism proposed,\nsince simpler explanations were not ruled out.\nAuthor Reply: Thank you to the reviewer for the valuable comments and for emphasizing the\nimportance of considering alternative mechanisms. We agree that simpler explanations should be\nexamined and will include additional discussion in the revised manuscript (See Q2). We appreciate\nyour insights, which help improve the clarity and robustness of our interpretation.\nQ1. In particular, one of the main findings (based on TAS) is a reduction of the e-ph decay time\nbetween 4.3 ps to 3.7 ps upon heating to 180℃. This effect of 14% can be easily explained via a\nstandard “solid-state” integral model of the effect, see, e.g., del Fatti et al., PRB 2000 (and countless\nre-uses of the approach for pulsed and CW illumination). There is also various simpler approaches\n(e.g., Grua et al., PRB 2003, Saavedra et al, ACS Photonics 2016). The effect may also be reflected\nvia temperature-dependent permittivity models, e.g., by the Shalaev group (various ellipsometry\nstudies for Au, Ag, ...), the Chu group (Scientific Reports 2016, Nanophotonics 2017, Opt. Exp.\n2017), Gurwich et al. (PRE 2017), Ferrara et al. (Phys Rev Mat 2019), etc. Such a simple\nelectromagnetism-based explanations should be accompanied by a quantification of the SERS\nenhancement, a procedure which is wel established (e.g., Greg Sun and Khurgin, or more\nsophisticated models that properly account for spatial variations of the field enhancement around\nthe plasmonic structure.)\nAuthor Reply: We appreciate the reviewer’s insightful comments. Based on the previous well-\nestablished standard “solid-state” integral model and experiments1-3, earlier studies have shown that\nupon photon absorption, the electron temperature in noble metals rises rapidly and subsequently\ntransfers energy to the lattice through electron–phonon coupling (Laser heating process). As\nreported in Laser heating studies, the measured electron-phonon relaxation process slows down with\nincreasing pump fluence1-3(Fatti et al., PRB, 2000, 61(24), 16956; Saavedra et al., ACS Photonics,\n2016, 3(9), 1637; Tagliabue et al., Nat. Mater., 2020, 19(12), 1312), which can also replicate in our\nmaterial, as shown in Supplementary Fig. 22. In contrast, in our experiment, the sample was heated\nin situ using the Cindbest temperature-controlled stage equipped with a platinum resistance\nthermometer (main lines 381−383). This configuration established a stable high-temperature\nenvironment, directly affecting the lattice temperature through ambient thermalization\n(Environmental heating process). For environmental heating process, the results show that for Au\nnanoparticles, when the temperature is raised from 22°C to 180°C, the relaxation dynamics\n(Supplementary Fig. 24) also slow down with increasing temperature, consistent with previous\nobservations under laser heating1-3 (Supplementary Fig. 22), suggesting that Au nanoparticles may\nconform to the standard “solid-state” integral model. However, in the Au-TiO₂ system, an\nanomalous behavior occurs: as the temperature rises from 22°C to 180°C, the relaxation dynamics\n(Fig. 4c) become faster, a phenomenon that cannot be explained by existing models. Next, in order\nto investigate the temperature-dependent permittivity of Au-TiO , we performed in situ steady-state\n2\ntemperature-dependent absorption measurements (Supplementary Fig. 23). Compared to 22°C, the\nabsorption of Au nanoparticles at 785 nm increases by ~40% at 180°C (Supplementary Fig. 23c),\nindicating that the rise in temperature significantly alters the dielectric constant of Au nanoparticles.\nThe observation of predecessors1-3 and our experimental measurements all indicate when a noble\nmetal absorbs more photons, electron relaxation should actually slow down (Supplementary Fig.\n24). However, the absorption spectra of Au-TiO₂ NRs increases by ~1% at 180°C compared to 22°C\n(Supplementary Fig. 23d), which cannot explain the faster decay of the dynamics of Au-TiO₂ NRs\n(Fig. 4c and Supplementary Fig. 25, main lines 193−205).\nOn the other hand, according to the temperature-dependent permittivity model4-7, the increase\nin the imaginary part of the dielectric constant at elevated temperatures leads to a reduction of the\nelectromagnetic field4-6 (Response Fig. 1, Shalaev et al., ACS Photonics, 2017, 4(5), 1083; Chu et\nal. Opt. Exp., 2017, 24(17), 19254; Gurwich et al., PRE, 2017, 96(1), 012212), thereby reducing the\nSERS enhancement. The model is consistent with the experimentally observed decrease in SERS\nperformance of Au NPs at 180℃ compared to 22℃ (Supplementary Fig. 9a), but contradicts with\nthe enhanced SERS activity on Au-TiO₂ NRs under the same thermal conditions (Fig. 3a). Therefore,\nthe electromagnetic explanations based on the temperature-dependent permittivity model fails to\naccount for the TI-SERS enhancement of Au-TiO₂ NRs. It is well known that the electromagnetic\nenhancement mechanism in SERS provides uniform enhancement across all Raman peaks8-10 (Greg\nSun and Khurgin, PRB, 2011, 84(4), 045415); Campion et al., Chem. Soc. Rev., 1998, 27(4), 241;\nZhao et al., J. Raman Spectrosc., 2018, 49(8), 1257), whereas our TI-SERS system exhibits a mode-\nselective enhancement compared to SERS at room temperature, consistent with the signal\namplification of specific vibrational modes dominated by charge transfer11-13 (Chemical\nenhancement mechanism, Supplementary Table 1). The corresponding discussion is added in the\nmain lines 193−209.\nResponse Fig. 1: The variations in a dielectric constant and b numerical simulation of field enhancement of single\ncrystalline silver films (solid lines), along with electron beam evaporated polycrystalline silver films (dashed lines)\nat higher temperatures4; c Temperature-dependent permittivity at selected wavelengths with unannealed gold films:\nthe imaginary part ε”5; d The Lorentz-Drude fitting (symbols) compared to experimental result (lines): the\nimaginary part ε” with the unannealed gold film5; e The temperature rise inside a Au nanosphere in an oil host\nilluminated at λ=550 nm as a function of the incident intensity (black solid line), the solutions accounting for up\nto third, second, and first-order terms are shown by magenta dashed, red dash-dotted, and blue dotted lines,\nrespectively6; f Imaginary parts of the Au permittivity as a function of the incoming intensity6, based on the\nvarious solutions for △T shown in e6; g Same as in f for the normalized electric field within the Au nanosphere6.\nRevision: 1) We added this part in the main lines 193−209: “The mechanism underlying the\nshortened relaxation at higher temperatures remains unclear. The previous standard “solid-state”\nintegral model1-3 describes that the relaxation process slows down in metal with increasing\ntemperature (Laser heating), which is consistent with our experimental results (Supplementary Fig.\n22). Moreover, the classic temperature-dependent permittivity models4-7 agrees with the increased\nabsorption we measured for Au NPs at 785 nm with rising temperature (Supplementary Fig. 23).\nAnd our data confirm that the relaxation of Au NPs slow down as the temperature increases\n(Supplementary Fig. 24). However, the shortened relaxation observed in Au-TiO NRs with\n2\nincreasing temperature (Environmental heating) cannot be explained by the slowed relaxation\npredicted by the above models (Supplementary Figs. 22-24). The shorten τ observed for Au-TiO\ne–ph 2\nNRs at high environment temperature could be explained by the higher efficiency in hot carrier\ntransfer, wherein accelerated relaxation via charge transfer aligns with previous experimental\nobservations3,14. And similar fast relaxation dynamics trend is also obtained for the winglet signal\ndecay (Supplementary Fig. 25). Furthermore, the selective chemical enhancement observed in our\nTI-SERS (11.4-fold at 1,172 cm−1, 2.8-fold at 907 cm−1, Supplementary Table. 1) also serves as\nevidence for efficient charge transfer, as opposed to a non-selective enhancement8,10-13 expected\nfrom purely electromagnetic mechanism. Next, direct evidence for hot-hole transfer was provided\nby our EPR observations.”\n2) We added this part in the main lines 381−383: “The sample was heated in situ using a Cindbest\ntemperature-controlled stage equipped with a platinum resistance thermometer, which established a\nstable high-temperature environment through ambient thermalization.”\n3) We added Supplementary Figs. 22−24.\nSupplementary Fig. 22: The dynamics of laser heating. Dynamics of a Au NPs and b Au-TiO2 NRs following laser\nenergy densities of 160 μJ cm−2 and 90 μJ cm−2, extracted at the maximum winglet peak and pumped at 785 nm.\nThe relaxation dynamics slow down with increasing laser energy densities, consistent with previous standard\n“solid-state” integral model and experiments1-3.\nSupplementary Fig. 23: The in situ steady-state absorption spectra of a Au NPs and b Au-TiO2 NRs, tested at room\ntemperature, 120℃ and 180℃; Differential-absorption spectra of c Au NPs and d Au-TiO2 NRs relative to room-\ntemperature absorptance, calculated from the date of a and b. The absorption of Au nanoparticles at 785 nm\nincreases by ~40% at 180°C compared to 22°C, consistent with previous temperature-dependent permittivity\nmodels4-7.\nSupplementary Fig. 24: Dynamics of Au NPs at 22℃ and 180℃, extracted at the maximum winglet peak of Au,\npumped at 785 nm. And Au NPs at 180℃ exhibited slower τe–ph (9.7 ± 0.6 ps) than Au NPs at room temperature\n(8.6 ± 0.5 ps). Under 785 nm pumping, the lengthened relaxation of Au NPs at 180°C is likely a consequence of\nthe increased photon absorption of Au at higher temperatures (Supplementary Fig. 23).\nQ2. Furthermore, the mechanism itself is unclear - the authors emphasize that their system is\ndescribed by a temperature, hence, the electrons should have a Fermi-Dirac distribution.\nIn that context, what is meant by” hot-hole energy upconversion via absorbed phonons”? hot holes usually\nrefer to a deviation of the e system from Fermi-Dirac distribution (i.e., a non-thermal effect); the\nterminology implies the se charges are coming from the valence band, where under thermal\nconditions, their occupation should be completely negligible;” absorbed phonons” - where? by\nwhat? Overall, I do not think that the paper can be accepted before the above issues are seriously\naddressed. This may involve authors of a different expertise, but this will ensure that unsubstantiated\nmechanisms will not be proposed, and the results will be far more convincing.\nAuthor Reply: Thank you for the reviewer’s comment. As the reviewer mentioned, the electrons\nshould have a Fermi-Dirac distribution at high temperatures15,16. In our text, the “hot holes” includes\n“hot nonthermal holes” (Those has not undergone carrier-carrier scattering) and “hot thermalized\nholes” (Experienced carrier-carrier scattering, but not experienced carrier-phonon scattering). As\npointed out by the reviewer, the simultaneous absorption of phonons by hot holes is likely\nchallenging due to the rapid relaxation of hot carriers (a timescale of hundreds of femtoseconds), in\ncontrast to the long-lived excited states in semiconductors which allow phonon absorption leading\nto energy upconversion17,18 (Response Fig. 2). We have revised our interpretation and now propose\na simpler mechanistic model. As shown in Fig. 4g, at high temperatures, thermal excitation induces\nthe depletion of conduction band electrons near the Fermi level, thereby creating unoccupied states\nthat facilitate the excitation of hot holes from higher energy levels. Thus, these hot holes at higher\nenergy levels of Au NPs could transfer to TiO (Fig. 4g). At room temperature, the photon energy\n2\nof 785 nm (1.58 eV) is slightly below the threshold energy (1.6 eV) required for hot-hole transfer at\nAu-TiO interface19,20, rendering this process inhibited (Fig. 4h). The relevant discussions have been\n2\nsupplied into the main text lines 247−257.\nResponse Fig. 2: a Schematic of luminescence upconversion for laser cooling of semiconductors17. Ultrafast\ncarrier longitudinal optical phonon collision aids efficient thermalization ② of photogenerated cold carriers ①\nand the sequential luminescence upconversion ③17; b Superposition of photoluminescence spectra for 1.96 eV\nexcitation (black) and 1.715 eV excitation (red), indicated by the red arrow. Inset: upconversion scheme of WSe2 18;\nc Schematic of the negatively charged and neutral exciton energy level diagram, showing the phonon-assisted\nupconversion process from X⁻ to X⁰18. The red curve is the charged exciton dispersion with its centre-of-mass\nmomentum k_18.\nRevision: 1) We have refined the title and related discussions of the manuscript. “Anomalous high-\ntemperature induced surface-enhanced Raman properties via heat-assisted hot-hole transfer”\nlines 65−70: “Experimental observations from time-resolved spectroscopy and electron\nparamagnetic resonance spectroscopy (EPR) firstly revealed the near-infrared driven hot-hole\ntransfer from Au to TiO₂, beyond the only occurrences of electron transfer. The transfer of hot holes\nhappened through environmental heating changes the electron distribution near the Fermi level,\nwhich benefit the transfer of hot electrons from Au to the molecule (Fig. 1a), thereby enhancing\nRaman scattering.”\nlines 294−297: “Environmental heating alters the electron distribution, leading to the formation of\nunoccupied states near the Fermi level, which realize the near-infrared photoexcitation of hot holes\nat high energy and facilitate their subsequent transfer from Au to TiO₂ (Fig. 4g).”\nlines 316−319: “Our results suggest that, before photoexcitation, environmental heating changes the\nelectron distribution near the Fermi level. Following photoexcitation, this distribution facilitates the\nexcitation of hot holes at high energy levels.”\n2) We added this part in the main lines 247−257: “Combined the above results, we conclude that\nhigh temperatures can trigger the photoexcitation of hot holes at higher energy levels, enabling their\ntransfer. Specifically, the photon energy of 785 nm (1.58 eV) is slightly below the threshold energy\n(1.6 eV) required for hot-hole transfer at Au-TiO interface19,20. However, the work of pioneers has\n2\nestablished that the high temperatures could alter the electron distribution near the Fermi level15,16.\nThis results in a consequent depletion of conduction band electrons near the Fermi level, thereby\nproviding unoccupied states for the photoexcitation of hot holes at high energy levels (Fig. 4g). It\nindicates that the Au-TiO₂ NRs can achieve efficient near-infrared-driven hot hole transfer under\nhigh temperature. We term this phenomenon as heat-assisted near-infrared driven hot-hole transfer.\nThis hot-hole transfer process is well known not permissible at room temperature (Fig. 4h).”\n3) We replaced Fig. 4g and Fig. 4h.\nFig. 4 Schematic diagram of hot holes transfer from Au to TiO2 at g high temperature and h room temperature.\nQ3: The amplitude of the absorption signals for the excited state of Au”- what does that mean?\nAuthor Reply: Thank you for the reviewer’s comment. We apologize for our negligence of the\nunclear expressions in the text. “The amplitude of the absorption signals for the excited state of Au”\nin previous versions described the transient absorption signal of Au increased with an increase in\ntemperature. And this explanation has been included in the revised version lines 182−185 to ensure\nclear presentation.\nRevision: lines 182−185: “As displayed in Fig. 4a−b, photoexcitation of the Au NPs LSPR results\nin a winglet signal and a bleach signal (centered at 491 and 678 nm, respectively)3,14. The maximum\namplitude of the winglet signal and bleach signal of Au considerably increase with increasing\ntemperature (Fig. 4b), indicating a larger electron population on Au at high temperatures.”\nQ4: fig. 2l- I expected to see numbers on the y-axis...\nAuthor Reply: Thank you for the reviewer’s comment. We have added numbers on the y-axis of\nFig. 2l in the revised manuscript.\nRevision:\nFig.2 l The absorption spectra of TiO2 NRs and Au-TiO2 NRs.\nQ5. refs 2-4 have comments written on them. They are most likely all wrong, so should not be\nquoted.\nAuthor Reply: Thanks for the reviewers’ valuable suggestions. We have deleted refs 2-4 in the text.\nQ6: Writing level and English grammar are on an insufficient level. MUST be improved.\nAuthor Reply: Thanks to the reviewer for the useful suggestion, and we understand that clear and\nconcise communication is crucial in scientific writing. In response, we have refined the language,\npaying close attention to sentence structure, word choice, and tone. Furthermore, we have polished\nthe language by the editing service of Springer Nature to improve readability of the manuscript, the\ncertificate is shown in Response Fig. 3. We appreciate for reviewers’ warm work earnestly and hope\nthat the correction will meet with approval.\nResponse Fig. 3: Certificate of English Language Editing.\nResponse to the comments of Reviewer 2:\nReviewer #2: Zhang et al. present a new SERS substrate, composed of photodeposited gold\nnanoparticles on TiO nanorods. This substrate shows anomalous, and yet unobserved, high\n2\ntemperature SERS performance, with an optimum at 180°C. The substrate is used for the detection\nof various analytes, and the researchers attempt to explain the mode of action. The observation of\nhigh temperature SERS is, without doubt, an important finding that warrants rapid and prestigious\npublication. Based on the data presented by the authors, I have no doubt that this effect occurs in a\nstable and reproducible manner, and that it is highly useful for the detction of very small amounts\nof analytes.\nAuthor Reply: We gratefully thank the reviewer for the positive comments and kind\nrecommendation for the publication of our manuscript.\nQ1. It is not clear which crystal phase dominates in the TiO NRs. The authors compare the XRD\n2\npattern of their sample to the reference pattern of rutile, but the peaks do not match well. The peaks\ndo not match well with anatase. In addition, the small peaks near 26, 34, 38 and 52° are likely\nto result from the underlying FTO substate, which should be clearly marked. The authors instead\nconclude the presence of rutile from the TEM lattice fringes, but is this the only option? Or is it\npossible that the tubes are a mix of rutile and anatase, or that they are not well crystalline at all?\nAuthor Reply: Thanks for the reviewer’s suggestions. In the revised manuscript, to further clarify\nthe crystal phase of the TiO₂ NRs, we measured their Raman spectrum (Supplementary Fig. 3). The\nresults indicate that the synthesized TiO₂ NRs are of the rutile phase. Furthermore, we performed a\nmore detailed measurement of the XRD data by reducing the scan rate from 10°/min to 2°/min. The\nresults (Fig. 2j) show that the (101), (111), (211), (002), (301), and (112) crystal planes of the TiO₂\nNRs match well with the reference pattern of rutile TiO₂. Combining the results of Raman, XRD\nand TEM results, the crystal phase dominates in the TiO NRs is rutile (main lines 97–101). In\n2\naddition, as suggested by the reviewer, we have marked the diffraction peaks originating from the\nFTO substrate (Fig. 2j and Supplementary Fig. 16b).\nRevision: 1) We updated the XRD pattern in Fig. 2j and Supplementary Fig. 16b.\nFig. 2j XRD patterns of TiO2 NRs and Au-TiO2 NRs substrates.\nSupplementary Fig. 16 b XRD patterns of the prepared Au-TiO2 NRs after heating at 180℃ for 70 min,\nmeasured at room temperature.\n2) We added this part in the main text lines 97–101: “The XRD pattern indicates that the diffraction\npeaks of TiO NRs are following the rutile phase (JCPDS No. 21-1276, Fig. 2j), and the Raman\n2\nsignal of TiO NRs also exhibits distinct characteristics of the rutile phase (Supplementary Fig. 3),\n2\ncollectively confirming the formation of TiO NRs in the rutile crystal structure.”\n2\n3) We added Supplementary Fig. 3.\nSupplementary Fig. 3: Raman spectrum of our TiO2 NR substrates as well as anatase TiO2 powder\n(aladdin 99.9%) and rutile TiO2 powder (aladdin 99.9%). Four major peaks that represent A1g, Eg, the second-\norder effect (SOE), and B1g Raman-active vibrational modes are located at 609, 446, 235, and 141 cm−1\nrespectively, indicating that the predominant phase of the TiO2 NRs is rutile.\nQ2. I have doubts about the accuracy of fit in Figure S17. All fits are located significantly above\nmost measurement points. It is expected that by statistical error, in case of a good fit, about half of\nall measurement points are located above the fitting curve and the other half below it. This is not\nthe case here. Can you please comment how the fit was obtained, and why it was done in this way?\nFurthermore, is a decrease by ~half a picosecond actually a \"significant decrease\", as the authors\nmention? Or would it be possible that, for example, different portions of the same sample show a\nsimilar deviation? In a similar manner, though the fit in Figure 4c looks much more reliable, is the\ndifference in relaxation times extracted from the fit statistically significant?\nAuthor Reply: Thank you for the reviewer’s comment. We are sorry that the signal-to-noise ratio\nof the TAS plots in previous submission was poor, which led to suboptimal accuracy in the lifetime\nfitting process. We have re-conducted the TAS experiments and replaced the original TAS spectra\nwith higher-quality ones in the revised version (Figs. 4a−c and Supplementary Figs. 21 and 25). We\nre-fit the dynamics referred to the previous works14,21(Nat. Commun., 2024, 15(1); Nat. Commun.,\n2022, 13(1)), and provided an explanation of the fitting method in the main texts (lines 187−192,\nand lines 383−388). The electronic dynamics of gold nanoparticles in the metallic state can be\neffectively described using the widely accepted two-temperature model22. This model accounts for\nthe nonequilibrium energy transfer processes that occur following laser excitation. Upon ultrafast\nphotoexcitation, conduction electrons in gold absorb photons and quickly thermalize via electron–\nelectron scattering, reaching an electron temperature much higher than that of the lattice. These hot\nelectrons then transfer energy to the lattice through electron–phonon coupling, causing lattice\nheating. Finally, the heated phonons undergo thermal diffusion through phonon–phonon coupling.\nAccording to the pervious works14,21, the electron–phonon and phonon–phonon coupling can be\nwell fitted with formula (1).\nt 𝑡\n− −\nS (t)=𝐼𝑅𝐹(𝐴∗𝑒 𝜏𝑒−𝑝ℎ+𝐵∗𝑒 𝜏𝑝ℎ−𝑝ℎ) (1)\ne\nAs shown in Fig. 4c, the dynamics probed at bleach peak of Au-TiO NRs at 180℃ exhibited\n2\nfaster relaxation time constant (5.2 ± 0.3 ps) than at room temperature (6.7 ± 0.4 ps), and similar\nfast relaxation dynamics trend was also obtained on the winglet peak (Supplementary Fig. 25).\nBesides, the temperature-dependent experiments of TAS were totally in situ experiments by using a\ncustom-made heating source to fix the samples (main texts lines 381−383), thereby avoiding the\noccurrence of similar deviations in different portions of the same sample. And we collected\ndynamics data from three randomly selected locations on Au-TiO NRs (Response Fig. 4),\n2\ndemonstrating good consistency and thus also excluding the aforementioned deviations. Moreover,\nthe original dynamic data show that even without fitting, Au-TiO NRs at 180℃ exhibits faster\n2\nrelaxation compared to at room temperature (Fig. 4c and Supplementary Fig. 25). And the fitting\nresults are well matched with the original data and have a consistent trend, the obtained lifetime\ndifference of 1.5 ps is also not within the error range. Therefore, the difference in relaxation times\nextracted from the fit is reliable.\nResponse Fig. 4 Dynamics from three different points (1, 2, 3) on the same Au-TiO2 NRs substrates at 22℃,\nextracted at the maximum winglet peak of Au. The τe-ph of points 1, 2, and 3 are 6.6 ± 0.4 ps, 6.6 ± 0.4 ps, and 6.5\n± 0.4 ps respectively, demonstrating good consistency across the sample.\nRevision: 1) We updated this part in the main text:\nFig. 4 TAS map of Au-TiO2 NRs obtained at a 22℃ and b 180℃ upon 785 nm pump, pulse energy density of 160\nμJ cm−2; c Dynamics of Au NPs at 22℃, Au-TiO2 NRs at 22℃ and 180℃, extracted at the maximum bleach peak\nof Au;\nlines 187−192: “Signal fitting is performed using a double exponential decay convolved with the\ninstrument response function. As illustrated in Fig. 4c, the dynamics of Au NPs exhibite a τ (8.6\ne–ph\n± 0.5 ps) at room temperature (bleach signal). At room temperature, Au-TiO NRs (bleach signal\n2\nwith blue circles) exhibit faster τ (6.7 ± 0.4 ps) than Au NPs, which has been previously attributed\ne–ph\nto the electron transfer process from Au to TiO 14. While the τ (5.2 ± 0.3 ps) of Au-TiO NRs\n2 e–ph 2\nfurther reduced with increasing temperature to 180°C.”\nlines 381−383: “The sample was heated in situ using a Cindbest temperature-controlled stage\nequipped with a platinum resistance thermometer, which established a stable high-temperature\nenvironment through ambient thermalization.”\nlines 383−388: “The instrument response function (IRF) is 200 fs. Electronic relaxation signals were\ndeconvoluted by fitting with the following formula14,21:\nt 𝑡\n− −\nS (t)=𝐼𝑅𝐹(𝐴∗𝑒 𝜏𝑒−𝑝ℎ +𝐵∗𝑒 𝜏𝑝ℎ−𝑝ℎ)\ne\nwhere S (t) is the electron signal intensity with time delay. A and B, τ and τ (phonon–\ne e-ph ph-ph\nphonon coupling time), are the amplitudes and time constants of the two exponential decay\ncomponents.”\n2) We updated this part in the supplementary information:\nSupplementary Fig. 21: TAS data after excitation at 785 nm. Representative TAS map for a Au NPs and b TiO2\nNRs.\nSupplementary Fig. 25: Dynamics of Au NPs at 22℃, Au-TiO2 NRs at 22℃ and 180℃, extracted at the maximum\nwinglet peak of Au, pumped at 785 nm. Au NPs exhibit τe-ph of 8.6 ± 0.5 ps. Au-TiO2 NRs at 180℃ exhibited\nfaster τe–ph (5.2 ± 0.3 ps) than that at room temperature (6.6 ± 0.4 ps).\nQ3. Figure S18 shows that a Ti3+ signal is already visible in the dark, which is easily possible,\ngiven that laser irradiation for gold deposition may have slightly (photo)reduced the material. Since all\nEPR spectra are plotted in arbitrary units, it is not possible for the reader to decide whether more\nTi3+ is present under irradiation. This is additionally made more complicated by the differences in\ntemperature between the two measurements. This makes the \"proof\" for hot electron transfer appear\nweak. In addition, can the authors cite an article why the peak just below g=2 is assumed to belong\nto the Ti3+ signal? This is unexpected, given that the signal in Figure S18 displays exactly the same\nshape, yet without the additional small signal below g=2.\nAuthor Reply: Thank you for the reviewer’s comment. We have changed the vertical axis of the\nEPR spectra from arbitrary units to signal intensity and added the corresponding signal intensity\nvalues to the Fig. 4d and Supplementary Figs. 26–28. Based on the EPR spectra in Supplementary\nFig. 26, we obtained the in situ EPR light-minus-dark difference spectra (i.e., the EPR spectra under\nlight irradiation at the same temperature minus those under dark conditions) for Au-TiO₂ at different\ntemperatures, as shown in Fig. 4d. The results indicate an increase in the h⁺(O⁻) and Ti³⁺ signals\ngenerated by light under high temperatures (90–180 °C). Additionally, the attribution of g=2 to Ti³⁺\nmay be questionable. We re-performed the EPR measurements on Au-TiO₂ NRs, including in situ\nEPR spectra under air and oxygen atmospheres, as well as spectra of heat-treated Au-TiO₂\n(Supplementary Figs. 26–28). None of the spectra measured in the three experiments detected this\nsmall signal, and the relevant results have been replaced in the revised manuscript.\nRevision: 1) We added this part in the main text lines 212−215: “On the basis of the EPR spectra in\nSupplementary Fig. 26, we obtained in situ EPR light-minus-dark difference spectra (i.e., the EPR\nspectra under light irradiation at the same temperature minus those under dark conditions) for Au-\nTiO₂ at different temperatures, as shown in Fig. 4d.”\nFig. 4: d In situ EPR light-minus-dark difference spectra of Au-TiO2 NRs at different temperatures under 785\nnm laser irradiation (the spectra were obtained in an air atmosphere).\n2) We added this part in supplementary information.\nSupplementary Fig. 26: The in situ EPR spectra of Au-TiO2 NRs at different temperatures under a 785 nm laser\nirradiation and b dark conditions in the air atmospheres, and R22℃ represents the in-situ cooling to 22℃.\nSupplementary Fig. 27 a The decrease of Ti3+ on Au-TiO2 NRs under 785 nm laser irradiation after adding\noxygen at 180℃.\nSupplementary Fig. 28: The comparison of EPR spectra of 180°C annealed (red) and unannealed (black) Au-TiO2\nNRs under 785 nm laser irradiation and dark (The spectra were obtained in an air atmosphere). The EPR spectra\nshow the peak positions and signal amplitude of the Au-TiO₂ nanoarrays after heat treatment at 180 °C exhibit no\nsignificant changes compared with the untreated sample.\nQ4. UV-Vis spectra must be re-recorded and displayed (raw data) in the Supplementary Information.\nThe Tauc plot cannot be performed, the data is not clear. There is neither a clear baseline nor a\nclear absorption edge. Strictly speaking, the Tauc plot method is not appropriate anyway for\ncomposites of crystalline materials (see Klein et al., Adv. Funct. Mater. 33 (2023) 2304523). The\nreviewer understands, though, that the method is applied in comparison with previous works. Still,\nthe band gap energy is estimated from the intercept of the tangent to the absorption edge with the\nbaseline of the spectrum (not a random zero line), and neither of the two features (absorption edge,\nbaseline) is clearly visible, as outline above.\nAuthor Reply: Thank you for the reviewer’s comment. We fully agree with your concern that the\nUV-Vis spectra and Tauc plot are not clear. To enable comparison with previous works, we have re-\nrecorded the UV-Vis spectra in Fig 2l and Supplementary Fig. 20, and the corrected Tauc plot result\nwith distinct absorption edge and baseline of the spectra is shown in Response Fig. 5. According to\nthe reviewer’s suggestion, we have revised the baseline of the spectra through linear fitting of the\nlow-energy data points (Supplementary Note 5), referred to the previous works23,24(Nat. Energy,\n2022, 7(12); J. Phys. Chem. Lett., 2018, 9(23)). As displayed in Response Fig. 5, the band gap values\nof TiO NRs and Au-TiO NRs obtained using the Tauc plots are 3.16 eV and 3.10 eV, respectively.\n2 2\nThe obtained band gap values for the TiO NRs from Tauc plots show good consistency with the\n2\nreported results obtained through the first-principles calculations25 (J. Semicond. 2019, 40, (9)).\nResponse Fig. 5 The Tauc plots for the TiO2 NRs and Au-TiO2 NRs, respectively.\nRevision: 1) we have adjusted the Fig. 2l and Supplementary Fig. 20.\nFig. 2 l The absorption spectra of TiO2 NRs and Au-TiO2 NRs.\nSupplementary Fig. 20: a The absorption spectra of the unannealed Au-TiO2 NRs tested at room temperature\n(unannealed, RT) and at 180℃ (unannealed, 180℃), as well as the 180℃-annealed Au-TiO2 NRs tested at room\ntemperature (annealed 180℃, RT); and b The corresponding Tauc plot analysis for band gap width.\n2) We corrected the Tauc plot and corresponding discussion in Supplementary Note 5.\nSupplementary Note 5. Data analysis of in situ absorption spectra from 22°C to 180°C on\nAu-TiO NRs.\n2\ncould be determined using the plots of (αhν)2 versus photon energy (hν) due to the direct gap of\nTiO 26. The baseline of the spectra was obtained through linear fitting of the low-energy data\n2\npoints23,24, and the band gap values are estimated from the intercept of the tangent to the absorption\nedge with the baseline of the spectra. The absorption spectra and bandgap during and after heating\nremained consistent with the pristine Au-TiO NRs, indicating that the heat treatment has no\n2\ninfluence on the energy band structure of our substrates.\nQ5. Considering points 1 and 3 above, it seems possible that the unexpected properties do not arise\npredominantly from an absorption by gold, but by titania. It is well known that reduced titania is\nblue, and that is shows broad absorption across the red and infrared range. The color of titania would\nbe obscured, however, by the purple color due to the deposited gold. The alternative hypothesis of\nan action of reduced titania might be ruled out, for example, by an oxidative treatment of the final\n(gold deposited) subtrate, for example at 180°C (where it is stable) with high amounts of oxygen in\nthe surrounding atmosphere. If the samples change color, and the SERS performance at 180° is lost\nor strongly attenuated, then the mechanism should potentially take into account a contribution of\nthe reduced (and possibly amorphous) TiO .\nthe reduced TiO in the origin of TI-SERS as a way to strengthen the rationality of this manuscript.\n2\natmosphere, the Raman signal of CV molecules notably increased as the temperature rose from 90°\nC to 180°C, and the Raman peak position and signal intensity were consistent with the results\nobtained in air (Supplementary Fig. 27). Besides, we further investigated the contribution of Ti3+\nusing in-situ temperature-dependent EPR spectra in the O atmosphere at 180°C. Under the oxygen\n2\natmosphere, the signal of Ti3+ decreased compared to that in air, while the TI-SERS behavior is not\naffected by oxygen (Supplementary Fig. 27), indicating TI-SERS is not caused by the reduced TiO .\n2\ncolor change of the sample (Response Fig. 6), while no obvious color changes were observed on the\nsurfaces of Au-TiO NRs (maintain purple-black color) compared to room temperature.\n180°C in the air and oxygen atmosphere respectively; b A schematic illustration with photographs for the\nsynthesis of Pt-DSA/TiO2 through the photochemical defect tuning process, and the reduced titania shows as blue\ncolor27.\nRevision: 1) We added this part in the main text lines 230−233: “Moreover, the Ti3+ signal also tend\nto increase as the temperature increases from 90°C to 180°C (Fig. 4d). We further confirmed that\nthe decrease in Ti3+ does not affect the performance of TI-SERS through the EPR and Raman results\nin an oxygen atmosphere (Supplementary Fig. 27), thereby ruling out the contribution of Ti3+ to TI-\nSERS behavior.”\n2) We added the new Supplementary Figs. 27 in the supplementary information.\nSupplementary Fig. 27 a The decrease of Ti3+ on Au-TiO2 NRs under 785 nm laser irradiation after adding\noxygen at 180℃; b The TI-SERS spectra recorded from CV (10−7 M) on Au-TiO2 NRs with temperature increased\nfrom 22 to 180°C in the O2 atmosphere. An external mechanical pump and mass flow controllers were used to\nregulate the flow rates of O2 at 40 ml/min to ensure the oxygen atmosphere; c The intensity comparison of the\ncorresponding different Raman signal peaks at 180°C in the air and oxygen atmosphere.\nResponse to the comments of Reviewer 3:\nThe paper outlines work on SERS in high-temperature environments. The authors reported that Au-\nTiO₂ nanoarrays showed Raman signal enhancement with an increase in temperature. With a signal\nintensity increase at 180°C compared to that at 22°C. The paper ascribes this effect to phonon-\nassisted hot-hole energy upconversion. The paper showing that the increases in SERS signal, is\nprobe molecule sensitive (Table S1 supplementary information). The substrate consists of quasi-\noriented nanotubes+silver nano particles (see Fig 4 supplementary information). The authors\nreporting that the sample is polycrystalline. Ag nanoparticles are grown on the TiO through UV\n2\n(375 nm) laser irradiation. TI-SERS (needs to be define, but presumably meaning temperature\ninduced SERS) studies from room temp to 180 degree C where undertaken. Before the distribution\nof the SERS studies the paper outlines characterization of the sample.\nAuthor Reply: We gratefully thank the reviewer for the positive comments and kind\nrecommendation for our manuscript. According to the reviewer’s suggestion, we have clearly\ndefined TI-SERS where it first appears in the revised manuscript (“high-temperature induced SERS”\n(TI-SERS), line 64).\nQ1: Can information using methods be applied to determine in the impact of heating the substrate\n(upto 200 degrees), with regard to structure and electronic configuration (as seen via methods like\nXPS). Supplementary Fig. 7 shows that heating at c.a. 210 degrees is reported to course melting of\nthe sample. A very important point of information is to clearly assess the impact of heating to 180\ndegrees on the sample. Does this cause any persevered changes noting its near the melting point.\nAuthor Reply: Thank you for the reviewer’s comment. According to the reviewer’s suggestion,\nwe have conducted in situ XPS analyses from 22 to 210°C to determine the impact of heating to\n210°C on our substrates. Compared with 22°C, the proportion of oxygen species (O, V , and O)\nO i\nin the structure remain unchanged at 180°C but undergo changes at 200°C and 210°C (Response\nFig. 7), especially the significant increase in oxygen incorporation (O). To assess the impact of\ni\nheating to 180°C on the sample, we have performed XRD, in situ XPS and absorption spectra of\nsample from 22°C to 180°C (Supplementary Figs.16−20). The results show no notable change in\nthe structure and electronic configuration when comparing samples at 180°C with room-\ntemperature controls. The detailed analysis and description have been added in Supplementary Note\n4.\nResponse Fig. 7 a In situ O 1s XPS spectra of the Au-TiO2 NRs tested at room temperature (22℃), 180℃, 200℃,\nand 210℃; b The corresponding proportion of O, V O , and O i of the Au-TiO2 NRs.\nRevision: 1) We added Supplementary Figs.17−20 and Supplementary Note 4 in the supplementary\ninformation.\nSupplementary Fig. 17: a−b XPS spectra of the unannealed Au-TiO2 NRs tested at room temperature (unannealed,\nRT) and at 180℃ (unannealed, 180℃), as well as the 180℃-annealed Au-TiO2 NRs tested at room temperature\n(annealed 180℃, RT).\nSupplementary Fig. 18: O 1s XPS spectra of the unannealed Au-TiO2 NRs tested at room temperature (unannealed,\nRT) and at 180℃ (unannealed, 180℃), as well as the 180℃-annealed Au-TiO2 NRs tested at room temperature\n(annealed 180℃, RT).\nSupplementary Fig. 19: Based on Supplementary Fig. 18, we obtained the proportion of O, VO, and Oi in the\nunannealed Au-TiO2 NRs tested at room temperature (unannealed, RT) and at 180℃ (unannealed, 180℃), as well\nas the 180℃-annealed Au-TiO2 NRs tested at room temperature (annealed 180℃, RT).\nSupplementary Fig. 20: a The absorption spectra of the unannealed Au-TiO2 NRs tested at room temperature\n(unannealed, RT) and at 180℃ (unannealed, 180℃), as well as the 180℃-annealed Au-TiO2 NRs tested at room\ntemperature (annealed 180℃, RT); and b The corresponding Tauc plot analysis for band gap width.\nSupplementary Note 4. Data analysis of in situ XPS from 22°C to 180°C on Au-TiO NRs.\n2\ninvestigate the changes of the sample during heating, and the XPS test at room temperature of the\nAu-TiO NRs annealed at 180°C (annealed 180°C, RT) was applied to reflect the changes of the\n2\nsamples after heating. As shown in Supplementary Fig. 17, the binding energies of Au 4f and Ti 2p\nshow no shift during and after heating compared to the pristine Au-TiO NRs (unannealed, 180°C).\ntransfer and band structure of TiO heterojunctions13,28,29. Therefore, the influences of heat treatment\n2\non chemical states of O 1s were characterized by XPS. As shown in Supplementary Figs. 18 and 19,\nthe XPS pattern of O1s can be deconvoluted into three peaks: 529.8 eV assigned to oxygen atoms\nin TiO (O), 531.2 assigned to the oxygen vacancies (V ), and 532.4 eV assigned to the additional\nO species adsorbed on the surface of the sample (O). It is obviously that the contents of O, V , and\ni O\nO remained unchanged during and after heating (Supplementary Figs. 18−19) compared to the\ni\npristine Au-TiO NRs, which demonstrates that 180°C heat treatment cannot induce oxygen-related\n2\ndefects in our substrate.\nQ2: The use of UV light on TiO has been reported to cause the PIERS effect, similarly the heating\n2\nof semiconductors like TiO also causes such effects as shown via heating semiconductors including\n(2), 1666-1673, 2020). Significantly this extends to enhancing plasmon assisted catalysis. The\nauthor in Fig 5 showing that heating the substrate supports catalysis reactivity. Reports have shown\nthat heating Ag/TiO and other semiconductors (Journal of Physical Chemistry C 124 (46)) supports\n2\nnot only an increase in SERS but also in catalysis.\nAuthor Reply: Thanks for the reviewer’s suggestions. The three works mentioned by the reviewers\nall focused on the improved room temperature SERS and photocatalytic performance, caused by\ngenerating oxygen defects (oxygen vacancies or oxygen incorporation) in the semiconductor after\nheat treatment or UV light13,28,29. Our work concentrated on the enhanced high temperature SERS\nand photocatalytic performance by in situ heating Au-TiO NRs. In detail, when heated at 180℃,\n2\nthe XPS spectra (Response Fig. 8) revealed the relative concentration of O exhibits a variation of\ni\nless than 1%, and absorption spectra (Response Fig. 9) showed the band gap changes by less than\n0.1 eV. The results are differed from the work of oxygen incorporation (Journal of Physical\nChemistry C 124 (46), 25351-25360 2020) that exhibits a significant increase in O content (more\ni\nthan 10%) and decrease in bandgap (more than 0.9 eV) after heated. It rules out oxygen\nincorporation as the underlying mechanism for TI-SERS.\nResponse Fig. 8 a O 1s XPS spectra of the unannealed Au-TiO2 NRs tested at room temperature (RT) and 180℃\n(in situ), and the 180℃ annealed Au-TiO2 NRs tested at room temperature, b O 1s XPS spectra of unannealed and\n200°C annealed WO3 nanowires, both of them tested at room temperature29.\nResponse Fig. 9 a The absorption spectra of the unannealed Au-TiO2 NRs tested at room temperature (Unannealed\n(RT)) and 180℃ (Unannealed (180℃)); b The corresponding Tauc plot analysis for band gap width; c The\nnormalized absorption spectra of unannealed and annealed WO3/AgNPs substrates29, and d The corresponding\nTauc plots used to determine the indirect bandgap value29.\nMeanwhile, at room temperature, Raman spectra (Response Fig. 10) analysis revealed no\nenhancement in signal intensity for the 180°C-annealed samples when compared to the untreated\ncontrol samples, differing significantly from the 2.5 fold enhancement achieved in the oxygen\nincorporation works (ACS Applied Nano Materials 3 (2), 1666-1673, 2020).\nResponse Fig. 10 a At room temperature, Raman spectra of 10−7 M CV on unannealed and 180°C annealed Au-\nTiO2 NRs; b At room temperature, Raman spectra of 10-4 M MB on Ag NPs on non-treated and annealed at 200°\nC TiO2 28.\nWhat’s more, the absorption spectra (Response Fig. 11) in our work show no LSPR peak shifts\nfor Au-TiO NRs during and after heating, which is different from the blue shift of LSPR peak\n2\ncaused by oxygen vacancies in UV light works (see Nature Communications volume 7, Article\nnumber: 12189 (2016)). These results demonstrate that the mechanism of TI-SERS is fundamentally\ndistinct from those oxygen-defect-mediated processes reported in previous studies.\nResponse Fig. 11 a Absorption spectra show no LSPR shifts for Au-TiO2 NRs at 180℃ compared with at room\ntemperature; b Absorption spectra of a thin film showing LSPR shifts for AuNPs on TiO2 (R) after irradiation with\n254nm light (3 h)13.\nRevision: We added this part in the main text lines 168−176: “The mechanism of action for TI-\nSERS remains unclear. On the one hand, some previous studies have shown that oxygen-related\ndefects caused by high temperatures may enhance the Raman signal13,28,29. To verify whether TI-\nSERS is related to oxygen defects generated under high temperature conditions, we conduct a series\nof experiments, such as XPS, XRD, EPR, Raman and absorption spectra (Supplementary Figs.\n16−20 and 28, and Supplementary Notes 4−5). The results indicate that, compared with room\ntemperature, heating the Au-TiO NRs substrates to 180°C does not induce measurable changes in\n2\ntheir structure or electronic configuration. This observation suggests that no oxygen defects are\ngenerated in the Au-TiO NRs upon heating to 180 °C, indicating that the TI-SERS mechanism is\n2\nnot attributable to oxygen defects.”\nQ3: Overall. The noteworthy results are a demonstration that SERS at high temperatures in Ag/TiO\n2\nsystems can be improved, by an amount depending upon the probe molecule. More work in regard\nto assessing the impact of thermal treatment of the substrate is requested. Such as taking the\nsubstrate and examining the improvements in SERS at room temperature using the oven treated\nsubstrate (as outlined in Journal of Physical Chemistry C 124 (46), 25351-25360 2020 or ACS\nApplied Nano Materials 3 (2), 1666-1673, 2020).\nAuthor Reply: Thank you for the reviewer’s comment. According to the reviewer’s suggestion,\nwe have added this experiment in Supplementary Fig. 9c, which involved taking the substrate and\nexamining the improvements in SERS at room temperature using the oven treated substrate (180°\nC annealed for 70 min), following the previous studies (Journal of Physical Chemistry C 124 (46),\n25351-25360 2020 or ACS Applied Nano Materials 3 (2), 1666-1673, 2020). While, the results\nindicated no SERS enhancement was observed at the 180°C annealed Au-TiO NRs compared to\n2\nthe unannealed samples (see Q2, Response Fig. 10).\nRevision: 1) We added this part in the main text lines 139−140: “Also, room-temperature SERS\ntesting of the 180°C-annealed Au-TiO NRs showed no detectable enhancement (Supplementary\n2) In SI, we add Supplementary Fig. 9c.\nSupplementary Fig. 9 c The Au-TiO2 NRs were annealed at 180℃ in an oven for 70 min. At room temperature,\nRaman spectra (10−7 M CV) revealed no SERS enhancement was observed for the 180°C-annealed Au-TiO2 NRs\n(annealed 180℃, RT) compared with the pristine samples (unannealed, RT).\nQ4: A careful analysis of the Ag/TiO substrate following heat treatment is required. Eg after heating\n2\nat room temperature what is the ESR data and what are the changes in structure, if any. Does the\npresence of thermally induced defect states explain the observed ESR data for example.\nAuthor Reply: Thank you for the reviewer’s comment. We have added the room-temperature EPR\ndata of 180 °C treated Au-TiO NRs substrates in Supplementary Fig. 28, which show that the EPR\n2\nsignal of the Au-TiO₂ NRs after heat treatment at 180 °C exhibit no changes compared with original\nsample. The EPR data do not support the contribution of thermally induced defects to TI-SERS.\nIn summary, the experimental findings demonstrate that oxygen defects are insufficient to explain TI-\nSERS, whereas our phonon-assisted hot hole energy upconversion mechanism provides a consistent\nexplanation for the observed TI-SERS enhancement.\nRevision: 1) In SI, we added Supplementary Fig. 28.\nSupplementary Fig. 28: The comparison of EPR spectra of 180°C annealed (red) and unannealed (black) Au-TiO2\nNRs under 785 nm laser irradiation and dark (The spectra were obtained in an air atmosphere). The EPR spectra\nshow the peak positions and signal amplitude of the Au-TiO₂ nanoarrays after heat treatment at 180 °C exhibit no\nsignificant changes compared with the untreated sample.\nReferences\n(2000).\nthermalization in small metallic nanoparticles. ACS Photonics, 3, 1637-1646 (2016).\nheterostructures. Nat. Mater., 19, 1312-1318 (2020).\npolycrystalline silver thin films. ACS Photonics, 4, 1083-1091 (2017).\nfilms. Opt. Express, 24, 19254-19263 (2016).\nunique case of nonperturbative nonlinear nanophotonics. Phys. Rev. E, 96, 012212 (2017).\nand implications for thermoplasmonics. Phys. Rev. Mater., 3, 105201 (2019).\ncomplex metal nanostructures. Phys. Rev. B, 84, 045415 (2011).\n250 (1998).\n2\ndifferences between chemical enhancement and electromagnetic enhancement in SERS. J.\nRaman Spectrosc., 49, 1257-1264 (2018).\nAngew. Chem. Int. Ed., 53, 4756-4795 (2014).\nspectroscopy. Nat. Rev. Methods Primers, 1, 87 (2021).\ndetection of explosives, pollutants and biomolecules. Nat. Commun., 7, 12189 (2016).\nsystem. Nat. Commun., 15, 445-456 (2024).\nheating? Light:Sci. Appl., 8, 89 (2019).\n12, 4283 (2021).\ndeep optical refrigeration by luminescence upconversion. Phys. Rev. B, 75, 155315 (2007).\nNature Physics, 12, 323-327 (2016).\nNano, 17, 7813-7820 (2023).\nin plasmonic devices. Nat. Commun., 9, 3394 (2018).\nCommun., 13, 7900-7909 (2022).\nprecise gold nanoparticles. Nat. Commun., 7, 13240 (2016).\nsemiconductor photocatalysts based on UV–Vis spectra. J. Phys. Chem. Lett., 9, 6814-6817\n(2018).\n2 2\nprinciples study. Journal of Semiconductors, 40, 092101 (2019).\nphotocatalytic degradation of phenol and antibiotics by Er-Fe-TiO . Chem. Eng. J., 410, 128319\n2\n(2021).\ndispersed catalysts. Nat. Mater., 23, 552-559 (2024).\nin silver nanoparticle-decorated ZnO nanowires. ACS Appl. Nano Mater., 3, 1666-1673 (2020).\nreactions on WO nanowire/AgNPs substrates via defect engineering. J. Phys. Chem. C, 124,\n3\n25351-25360 (2020)."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have answered the comments of the reviewers professionally, and improved their manuscript. I nevertheless\nstill think that their proposed mechanism (now changed!) is not substantiated with existing theoretical knowledge.\nSpecifically,\n1. The authors indicate that in Au samples, the e-ph energy transfer rate slows down upon absorption/increase of\ntemperature in their measurements and in previous work.\na. They mention changes to absorption upon, but it is not clear why is this relevant.\nb. They do not explain the physical reason for the slowing down. I believe it is due to the increase of the heat capacity of\nthe electrons with temperature. This is something that should be easy to quantify by using known temperature dependence of\nthe electron heat capacity and the temperature change.\n2. Then, the authors contrast their finding of a faster e-ph energy transfer rate in Au-TiO2 samples. Assuming the heat\ncapacity of electrons in that sample is the same, they need to suggest a stronger effect. They suggested that \"thermal\nexcitation induces the depletion of conduction band electrons near the Fermi level, thereby creating unoccupied states that\nfacilitate the excitation of hot holes from higher energy levels. \" – From Fig. 4g, I suppose they mean instead something like -\nexcitation of hot holes at energy levels deep below the Fermi energy of Au -. Either way, such a mechanism was suggested\nin the context of plasmonic catalysis, and is controversial; indeed famous studies of the Halas group using this combination\nof materials has already been shown to raise incorrect claims due to lack or erroneous control experiments. Therefore, the\nauthors should strengthen their (now fairly speculative) claim by quantifying it. Specifically, one should be able to predict the\nchange of occupation at the hole state and near the Fermi energy by using the formulation suggested already in the 1970's\nby Rosei et al.. Since the temperatures and temperature change are both much smaller than the Fermi energy of Au, I expect\nfrom earlier studies of transient permittivity studies (where heating can be more than 10 times stronger) that the impact of the\nhole population change to be much weaker than the e-ph changes observed for Au only samples by orders of magnitude,\nespecially since the transitions the authors seem to be discussing are near the X point. These are known to contribute much\nless than transitions near the L point. A possible exception may have been reported in the context of photoluminescence, but\nif so, should come with a strong justification. Moreover, the authors should be able to quantify not only the change in the\nnumber of thermal/non-thermal holes, but also their tunnelling probability to the TiO2, and then, relate the process to the\nexperimental observations of the SERS changes. All these steps are not trivial, and I am pessimistic they may yield\nconvincing conclusions… but the authors deserve a chance to demonstrate. some related work was done by the group of\nAndrea Baldi.\nI therefore suggest to make a decision about the suitability of the manuscript to Nature comm. Only after another revision.", "reviewer_2": "The article is a revised version that I had reviewed previously. The authors have submitted a rebuttal.\nIndeed, the new work performed by the authors significantly increases the quality of the article. It seems now almost ready\nfor publication, but a minor revision to clarify some additional questions is strongly advised:\n(1) Why is the intense peak of the reference pattern of rutile (~27°, (100)peak) not observed in the diffraction pattern of the\nauthors' sample? Does it have to do with the growth direction?\n(2) Has a similar amount of sample, or even better, the exact same portion of the sample, been used in the EPR\nmeasurements? Otherwise, they are not quantitatively comparable.\nEverything else is now clear, and I very much like to thank the authors for their effort.", "reviewer_3": "The paper reports a novel high-temperature enhanced SERS (TI-SERS) effect using Au–TiO₂ nanorod arrays (NRs) under\n785 nm excitation. Unlike traditional SERS, which usually weakens at elevated temperatures, this study shows a >11×\nenhancement at 180 °C. The authors attribute this to a heat-assisted hot-hole transfer mechanism from Au to TiO₂,\nwhich promotes hot-electron transfer to analyte molecules and thus enhances Raman scattering.\nThe authors have responded in detail to the list of questions/issues from the reviewers.\nOverall I am happy that the paper be published. Noting the comments below.\nI would like to point out that the manuscript is technically sound but occasionally verbose and could benefit from editing for\nclarity. For example:\n“Anomalous observed Raman signal enhancement” → “An anomalous enhancement of Raman signal was observed.”\nConsistency in units (°C spacing, scientific notation, etc.) should be improved."}, "author_response": "Response to the comments of Reviewer 1 (Remarks to the Author):\nThe authors have answered the comments of the reviewers professionally, and improved their\nmanuscript. I nevertheless still think that their proposed mechanism (now changed!) is not\nsubstantiated with existing theoretical knowledge. Specifically,\nAuthor Reply: We gratefully thank the reviewer for the insightful comments and kind\nrecommendation for the publication of our manuscript.\nQ1. The authors indicate that in Au samples, the e-ph energy transfer rate slows down upon\nabsorption/increase of temperature in their measurements and in previous work.\na. They mention changes to absorption upon, but it is not clear why is this relevant.\nb. They do not explain the physical reason for the slowing down. I believe it is due to the increase\nof the heat capacity of the electrons with temperature. This is something that should be easy to\nquantify by using known temperature dependence of the electron heat capacity and the temperature\nchange.\nAuthor Reply: Thank you for your comment. As the reviewer mentioned, the reduction in the e-ph\nenergy transfer rate is due to the increase in the electron heat capacity with the increase of\ntemperature. According to the absorption spectra (Supplementary Fig. 24), the absorption of Au\nnanoparticles at 785 nm is enhanced at 180 °C compared with that at 22 °C. The stronger optical\nabsorption leads to a higher electron temperature. Therefore, the electron–phonon energy relaxation\nat 180 °C become slower than those at 22 °C. We have further elaborated on the physical mechanism\nunderlying the extended electron–phonon relaxation time for Au NPs at high temperatures in\nSupplementary Fig. 26 and Supplementary Note 6.\nRevision: 1) Main text lines 200−202: “Our data confirm that the relaxation time of Au NPs\nlengthened as the temperature increases (Supplementary Fig. 24−25 and Supplementary Note 6).”.\n2) We added Supplementary Fig. 26 and Supplementary Note 6.\nSupplementary Fig. 26: a The heat capacity of the electrons as a function of electron temperature; b Two-\ntemperature model simulations showing the temporal evolution of electron temperatures in Au NPs at different\ntemperatures; c The corresponding normalized curves of the data in b.\nSupplementary Note 6. Analysis of the lengthened τ in Au NPs at higher temperatures.\ne–ph\nThe well-established two-temperature model1-4 was used to rationalize the slow down\nrelaxation of Au NPs at 180 °C (Supplementary Fig. 25), in which the energy exchange rate between\nelectron and phonon follows:\n𝐶\n𝑑𝑇𝑒\n=−𝐺(𝑇 −𝑇) (Supplementary Eqn. 2)\n𝑒 𝑒 𝑙\n𝑑𝑡\n𝐶\n𝑑𝑇𝑙\n=𝐺(𝑇 −𝑇) (Supplementary Eqn. 3)\n𝑙 𝑒 𝑙\n𝑑𝑡\nWhere C (2.5×106 Jm−3K−1)3 and T are the heat capacity and temperature of lattice. And C\nl l e\nand T are the heat capacity and temperature of electrons. For T ＜3000 K, their relationship is\ne e\ncharacterized by a linear dependence5,6, C = γT , where γ is the electronic heat capacity constant.\ne e\nUsing a reported value for (68 Jm−3K−2)3, we simulated the Cₑ–Tₑ relationship, as shown in\nSupplementary Fig. 26a. And G (1.1×1016 Wm−3K−1)7 is the electron–phonon coupling constant of\nAu. Based on the absorption spectra of the Au nanoparticles (A , Supplementary Fig. 24) and a\nabsorb\nconstant 785 nm pump laser energy density (W ) at different ambient temperatures, the energy\npump\ndensity absorbed by the Au nanoparticles (W ) was calculated by W =W *A , yielding\nabsorb absorb pump absorb\nvalues of 160 μJ·cm⁻² at 22 °C and 222 μJ·cm⁻² at 180 °C. Applying the two-temperature model,\nthe corresponding peak electron temperatures were estimated to be 1563 K at 22 °C and 1864 K at\n180 °C. We also used the two-temperature model to simulate the temporal evolution of electron\ntemperature in Au NPs under different peak electron temperatures (Supplementary Fig. 26b and c).\nThe simulations confirm that the electron-phonon relaxation prolonged by 1.5 ps at 180 ℃\ncompared to 22 ℃, which is in agreement with our experimental observations (Supplementary Fig.\n25).\nQ2. Then, the authors contrast their finding of a faster e-ph energy transfer rate in Au-TiO2 samples.\nAssuming the heat capacity of electrons in that sample is the same, they need to suggest a stronger\neffect. They suggested that \"thermal excitation induces the depletion of conduction band electrons\nnear the Fermi level, thereby creating unoccupied states that facilitate the excitation of hot holes\nfrom higher energy levels. \" – From Fig. 4g, I suppose they mean instead something like - excitation\nof hot holes at energy levels deep below the Fermi energy of Au -. Either way, such a mechanism\nwas suggested in the context of plasmonic catalysis, and is controversial; indeed famous studies of\nthe Halas group using this combination of materials has already been shown to raise incorrect claims\ndue to lack or erroneous control experiments. Therefore, the authors should strengthen their (now\nfairly speculative) claim by quantifying it. Specifically, one should be able to predict the change of\noccupation at the hole state and near the Fermi energy by using the formulation suggested already\nin the 1970's by Rosei et al.. Since the temperatures and temperature change are both much smaller\nthan the Fermi energy of Au, I expect from earlier studies of transient permittivity studies (where\nheating can be more than 10 times stronger) that the impact of the hole population change to be\nmuch weaker than the e-ph changes observed for Au only samples by orders of magnitude,\nespecially since the transitions the authors seem to be discussing are near the X point. These are\nknown to contribute much less than transitions near the L point. A possible exception may have\nbeen reported in the context of photoluminescence, but if so, should come with a strong justification.\nMoreover, the authors should be able to quantify not only the change in the number of thermal/non-\nthermal holes, but also their tunnelling probability to the TiO , and then, relate the process to the\n2\nexperimental observations of the SERS changes. All these steps are not trivial, and I am pessimistic\nthey may yield convincing conclusions… but the authors deserve a chance to demonstrate. some\nrelated work was done by the group of Andrea Baldi.\nI therefore suggest to make a decision about the suitability of the manuscript to Nature comm.\nOnly after another revision.\nAuthor Reply: Thank you for your comment. According to reviewer’s suggestions, we quantified\nthe change of the Fermi surface at 180 °C compared to 22 °C as 0.06 eV (Supplementary Note 7).\nIn Au-TiO NRs heterostructure, the interband transition thresholds of Au are calculated to be 1.55\n2\neV at the X point and 1.60 eV at the Γ point (Supplementary Figs. 31). The energy of our laser is\n1.58 eV. Therefore, 785 nm laser (1.58 eV) with the aid of 0.06 eV thermal energy could enhance\nthe hot hole transfer from Au (Main text lines 233−236 and Supplementary Note 7).\nWe also discussed the quantum tunneling effect in the main text lines 240−243. The quantum\ntunneling effect could be negligible.\nAccording to Monte Carlo modeling8,9, we calculate that laser heating raises the lattice\ntemperature by no more than 30 ℃. In experiments, at an ambient temperature of 90 ℃, the total\nlattice temperature under illumination does not exceed 120 ℃, and EPR reflects a pronounced hole\ntransfer. But, at an ambient temperature of 180 ℃ under dark conditions, no driven hole transfer is\nobserved by EPR. These results indicate that photoinduced hole transfer rather than merely\nphotothermal effects, aligning with Andrea Baldi et al’s. model of hot carriers dominated chemical\nreaction8,9.\nRevision: 1) Main text lines 233−236: “Meanwhile, the computational investigations of Au-TiO\ndetailed analysis provided in Supplementary Figs. 31–32 and Supplementary Note 7. These\ncomputational results are consistent with the experimental findings.”\n2) Main text lines 240−243: “Moreover, we discussed the temperature-related quantum tunneling\neffect10-15. The quantum tunneling probability in plasmonic metal heterostructures is commonly\nconsidered to be weak dependence on temperature12-15. In our TI-SERS and EPR experiments, the\nsignals are strongly dependent on temperature, indicating that the quantum tunneling effect could\nbe negligible.”\n3) We added this part in the Supplementary information:\nSupplementary Fig. 31: The calculated fatband structures for a Au component in the Au-TiO2 NRs, and b Au bulk;\nThe diameters of the circles represent the magnitude of density of states (DOS), and the\nFermi level is set at 0 eV.\nSupplementary Fig. 32: The calculated a band structures and b density of states for Au-TiO2 NRs; The Fermi level\nis set at 0 eV.\n4) We added this part in the Supplementary Note 7:\n“As proposed by Rosei et al.16, on increasing the temperature, the region between the Fermi\nsurface and the constant energy surface E = -4K T experiences a decrease of electron population,\nB\nthereby providing unoccupied states for the transition of low-energy d-band electrons. An increase\nin temperature from 22 °C to 180 °C (ΔT = 158 °C) leads to an energy shift of ΔE = 0.06 eV.\nSubsequently, to elucidate the mechanism of high-temperature-induced interfacial hot holes, we\nemployed first-principles calculations of the Au-TiO₂ heterostructure interface and computed its\nfatband structures. The band structure of the Au component in the Au-TiO heterostructure shows\n2\nan interband transition threshold of approximately 1.55 eV near the X point (Supplementary Fig.\n31a). Therefore, under 785 nm (1.58 eV) laser excitation, the energy is sufficient to trigger the\ninterband transition at the X point. However, the fatband structure exhibits a low density of states\nat the X point, indicating a low transition probability. In contrast, the interband transition near the Γ\npoint has a threshold of 1.60 eV coupled with a high electronic density of states. Thus, with the\nassistance of 0.06 eV thermal energy, the 785 nm laser excitation energy of 1.58 eV is sufficient\n(Supplementary Fig. 31a) to open the interband transition channel at the Γ point (1.60 eV),\nsignificantly enhancing the hot hole yield. Besides, we evaluated the change in the number of high-\nenergy hot holes in d-band based on the fatband structures. We extracted the density of states values\nfrom the calculated results for transitions at -1.55 eV near the X point and in the range of -1.60 to -\n1.64 eV near the Γ point. The results indicate that the generation of high-energy hot holes could be\nenhanced by ~20 times at 180 °C compared to 22 °C.\nWe also calculated the band structure of pure Au, which revealed an interband transition\nthreshold of approximately 1.80 eV near the X point (Supplementary Fig. 31b), in agreement with\nestablished literature17-19. The calculated interband transition energies of Au and the Au-TiO\n2\nheterostructure are 1.80 and 1.55 eV, indicating a 0.25 eV reduction for Au in the heterostructure.\nThis reduction also benefits the generation of hot holes under 785 nm laser excitation.”\nResponse to the comments of Reviewer 2:\nReviewer #2: Zhang et al. present a new SERS substrate, composed of photodeposited gold\nnanoparticles on TiO nanorods. This substrate shows anomalous, and yet unobserved, high\n2\ntemperature SERS performance, with an optimum at 180°C. The substrate is used for the detection\nof various analytes, and the researchers attempt to explain the mode of action. The observation of\nhigh temperature SERS is, without doubt, an important finding that warrants rapid and prestigious\npublication. Based on the data presented by the authors, I have no doubt that this effect occurs in a\nstable and reproducible manner, and that it is highly useful for the detction of very small amounts\nof analytes.\nAuthor Reply: We gratefully thank the reviewer for the positive comments and kind\nrecommendation for the publication of our manuscript.\nQ1. Why is the intense peak of the reference pattern of rutile (~27°, (100) peak) not observed in the\ndiffraction pattern of the authors' sample? Does it have to do with the growth direction?\nAuthor Reply: Thank you for your comment. The intense peak of the reference pattern of rutile\nTiO at ~27°corresponds to the (110) peak20-24. As the reviewer mentioned, we did not observe the\n2\n(110) peak in XRD pattern because the TiO nanoarrays were almost vertically grown on the FTO\n2\nsurface along the (110) plane. In a standard XRD scan, the detectable peaks arise from lattice planes\nparallel to the surface of FTO. Consequently, the (110) peak, which is almost perpendicular to the\nsurface of FTO, are usually absent in the XRD pattern22-24 (Response Fig. 1). To disrupt this vertical\nalignment, we removed TiO NRs powder from the FTO substrate via intensive ultrasound for XRD\n2\ntesting, which showed the intense (110) peak of the reference pattern of rutile at ~27°\n(Supplementary Fig. 4).\nResponse Fig. 1: a SEM images of the TiO2 nanorod arrays22; b XRD patterns of TiO2, defective TiO2 nanorod\narrays (def-TiO2), and Pt/def-TiO2 nanorod arrays22. c SEM images of our TiO2 NRs substrate; d XRD patterns of\nTiO2 NRs and Au-TiO2 NRs substrates.\nRevision: 1) We added this part in the main lines 102−104: “The (110) plane of the TiO NRs is\n2\nalmost perpendicular to the substrate, thus is invisible in the XRD pattern (Fig. 2j, Supplementary\nFig. 4).”\n2) We added this part in the supplementary information:\nSupplementary Fig. 4: XRD patterns of TiO2 NRs powder. To confirm that the absence of the (110) peak was due\nto the vertical alignment, we removed TiO2 NRs powder from the FTO substrate via intensive ultrasound for XRD\ntesting, which showed the intense (110) peak of the reference pattern of rutile at ~27°.\nQ2. Has a similar amount of sample, or even better, the exact same portion of the sample, been used\nin the EPR measurements? Otherwise, they are not quantitatively comparable.\nAuthor Reply: Thank you for your comment. We used the exact same portion of the sample in the\nin situ EPR measurements, thus these obtained data are quantitatively comparable. And the\nexperimental details were included in the main lines 408−415.\nRevision: We added this part in the main lines 408−415: “The in situ EPR investigation was\nperformed by a Bruker A300 spectrometer operating at the X-band frequency, which was equipped\nwith an in situ digital temperature control systems ER4131VT and ER4141VT working in the\ntemperature range of 100-600 K. The nanoarray sample was loaded into quartz glass tubes\nconnected both to a high-vacuum pumping system and to a controlled gas feed (air or O ). Irradiation\n2\nwas performed by a 785 nm laser (Leoptics) and the output radiation focused on the samples in the\ncavity by an optical fiber. The spectra were recorded before and after 10 min of 785 nm laser\nirradiation inside the EPR cavity at different temperatures.”\nResponse to the comments of Reviewer 3:\nThe paper reports a novel high-temperature enhanced SERS (TI-SERS) effect using Au–TiO\n2\nnanorod arrays (NRs) under 785 nm excitation. Unlike traditional SERS, which usually weakens at\nelevated temperatures, this study shows a >11× enhancement at 180 °C. The authors attribute this\nto a heat-assisted hot-hole transfer mechanism from Au to TiO , which promotes hot-electron\n2\ntransfer to analyte molecules and thus enhances Raman scattering. The authors have responded in\ndetail to the list of questions/issues from the reviewers. The authors have responded in detail to the list of questions/issues from the reviewers.\nOverall I am happy that the paper be published. Noting the comments below.\nAuthor Reply: We gratefully thank the reviewer for the positive comments and kind\nrecommendation for the publication of our manuscript.\nQ1: I would like to point out that the manuscript is technically sound but occasionally verbose and\ncould benefit from editing for clarity. For example:\n“Anomalous observed Raman signal enhancement” → “An anomalous enhancement of Raman\nsignal was observed.”\nConsistency in units (°C spacing, scientific notation, etc.) should be improved.\nAuthor Reply: Thank you for your comment to improve the clarity and conciseness of our writing.\nWe have carefully edited the entire manuscript, paying close attention to paragraph structure,\nsentence logic and tone. The original phrasing “firstly anomalous observed Raman signal\nenhancement with an increase in temperature” has been replaced with the more precise statement:\n“an anomalous enhancement of Raman signal with increasing temperature was observed for the first\ntime.” (Main lines 27−28). All units have also been checked to ensure consistency throughout the\nmanuscript, including the °C spacing, the scientific notation in Fig. 3e and Supplementary Fig. 13b,\nand the axis consistency in the TI-SERS plots (Fig. 5)."}, {"round": 3, "reviewer_comments": {"reviewer_1": "I read the response to previous round of comments. I find the answer of the authors confusing, and reflecting a lack of understanding of relevant basic concepts. I am either not able to follow their logic, or do not find it convincing qualitatively/quantitatively. Due to that, at this stage, I recommend to reject the paper.\nSpecifically,\n1. It is very unlikely that the absorption spectrum changed from 160 to 220 \\mu J / cm^2 (upon heating from 300K to 450K)\nwhich is ~40%. Indeed, the thermo-derivatives of the permittivity (and hence the absorption) were measured by several\ngroups to be on the order of 1e-3 - 1e-2 / K (see works by Shalaev group, Chu group and others).\n2. I do not understand what is the meaning of the statement – “the electron-phonon relaxation prolonged by 1.5 ps at 180 ℃\ncompared to 22 ℃”. what is the initial e-ph coupling time the authors have in mind? I would expect a change of decay time to\nbe reported in percent. The value reported reflects an extremely large, unrealistic change, and raises an alarm about the\nwhole argument. Does it even correlate to the experimental finding?\n3. Either way, quantifying the decay (cooling) rate should not be related to the absorption stage; thus, referring to it is quite\nsurprising, and raises doubt about the whole line of thought of the authors, especially since the equations 2-3 in the SI do not\ninvolve absorption...\n4. Instead, I would have expected the authors to perform at least a crude estimate of the e-ph energy transfer time. A simple\nanalysis reveals that what matters is not the heating level they refer to (which is similar for both cases, assuming the\nabsorption does not change drastically), but rather the temperature itself. Such estimate should then be compared to the\nexperimental measurement. No such analysis was done.\n5. The authors write – “we quantified the change of the Fermi surface at 180 °C compared to 22 °C as 0.06 eV\n(Supplementary Note 7).” This is unlikely - this is 3 times the temperature...\n6. The authors refer to a transition of “1.60 eV at the Γ point (Supplementary Figs. 31)”. This transition is irrelevant for\ninterband transitions in Au (because the Gamma point has many occupied states in the conduction band), and implies the\nauthors did not understand or did not read the earlier work by Rosei, which is a well accepted benchmark.\n7. The authors write – “Therefore, 785 nm laser (1.58 eV) with the aid of 0.06 eV thermal energy could ...” – this reflects a\nlack of understanding of how photon absorption works and generates non-thermal holes.", "reviewer_2": "In their second revision, the authors have answered my queries and clarified all open questions in full. Publication of the\narticle is now recommended."}, "author_response": ""}, {"round": 4, "reviewer_comments": {"reviewer_1": "I find the answers to the last review satisfactory. I am not convoionced that all the provided answers are included in the\nrevision. I encourage the authors to do that, as this strengthens their arguments."}, "author_response": ""}]

Decision letter