Spatiotemporal alignment of hole transfer and water oxidation for highly efficient photocatalytic water splitting

Domain: physics
Published: 2026-02-14
Editorial outcome: minor_revision

Abstract
Photocatalytic overall water splitting remains limited by inefficient charge separation and utilization in reactions. Al-doped SrTiO3 exhibiting near-100% apparent quantum efficiency for overall water splitting indicates nearly complete charge separation and surface catalytic efficiency. Although Al doping has been assumed to enhance charge separation and transfer, the exact role of Al is still unclear. Here, using spatiotemporal surface photovoltage imaging, we show that a gradient Al doping in Al-doped SrTiO3 generates a built-in electric field that drives photogenerated holes from the bulk towards surface trap sites in the form of hydroxylated Al-O-Ti, prolonging their lifetime from ~100 ns to 10 ms. Spectroscopic analyses reveal that these hydroxylated Al sites serve as key centers for water adsorption, facilitating water oxidation. These findings underscore the pivotal role of Al in the spatiotemporal alignment of hole transfer and surface catalytic water oxidation, enabling high-efficiency photocatalysis in overall water splitting.

Introduction
Photocatalytic overall water splitting (OWS) stands as the holy grail of solar energy conversion, offering tremendous potential to directly transform solar energy into chemical fuels1–3. Despites decades of efforts, practical implementation remains hindered primarily limited by the low apparent quantum efficiency (AQE) of OWS system4,5. The AQE is governed by the efficiency of two intertwined processes: charge separation, and surface reaction6,7. Surface reaction kinetics strongly depend on the concentration and distribution of charges, while surface catalysis, in turn, modulates charge separation by influencing interfacial charge transfer8. This interplay creates a fundamental challenge: achieving spatiotemporally coherent coupling between charge separation and catalytic processes.

Al-doped SrTiO3 (SrTiO3:Al) is the only known photocatalyst achieving near-100% AQE in OWS so far9, suggesting that nearly every photogenerated charge successfully undergoes charge separation and participates in catalysis, ultimately to be stored in chemical bonds. Many follow-up studies10–20 have aimed to elucidate the mechanisms responsible for the enhancement of AQE from below 1%21 to nearly 100%9 upon Al doping. Although these investigations have provided some results on how Al doping improves charge separation and interfacial charge transfer, they have mostly focused on some individual aspects leading to AQE improving. The dynamic coupling between the charge separation and surface catalytic reactions in spatiotemporal scale remains unknown. Achieving an AQE approaching unity requires not only efficient charge separation but also exact synchronization of charge arrival for catalysis at reaction sites. In this work, using SrTiO3:Al with an AQE exceeding 90% as a model platform, we disclose that a gradient Al doping in SrTiO3:Al generates a built-in electric field that drives bulk holes transfer to surface trap sites, prolonging hole lifetimes from ~100 ns to 10 ms free of recombination. Furthermore, we reveal that the surface hole trap sites, in the form of hydroxylated Al-O-Ti, also serve as water adsorption centers, directly facilitating water oxidation. Our findings unveil a strategic arrangement of photogenerated holes and reaction centers, which are the key determinant of the near-unity AQE.

Results
Modulating OWS with Al doping
SrTiO3:Al samples were synthesized using a flux method9 by varying the Al2O3 addition amounts (2% or 5%) and the precursor SrTiO3 preparation method, either hydrothermal method (HTM) or solid-state reaction (SSR). As presented in Fig. 1a, substantial differences in OWS activity and AQE are observed for SrTiO3:Al samples. For SrTiO3:Al with precursor from HTM, the AQE at 360 nm is 29.3% and 21.7% for the 2% (HTM-SrTiO3:Al) and 5% (5%-HTM-SrTiO3:Al) Al addition, respectively. Changing the precursor preparation method from HTM to SSR at 2% Al addition (SSR-SrTiO3:Al) increases the AQE to 92.2%, with H2 and O2 evolving at the 2:1 stoichiometry over time under continuous illumination (Fig. 1b). These results suggest that the OWS performance of SrTiO3:Al is strongly influenced by both the precursor preparation method and the amount of Al added. For SrTiO3:Al samples, Al doping does not enhance the OWS activity by modulating particle size, morphology (Supplementary Fig. 1) or band gap absorption (Supplementary Figs. 2–3). To get insight into the role of Al, we investigated the Al distribution.

Figure 2a and b shows that Al doping in SSR-SrTiO3:Al leads to lattice contraction while maintaining the SrTiO3 structure. As seen in Fig. 2c–e, Al is predominantly concentrated in the outer shell region of the particle with limited bulk incorporation. Depth profiling reveals a ~50 nm gradient in Al concentration from the surface inward, followed by a relatively uniform low-level distribution in the deeper region (depth calibration presented in Supplementary Fig. 4). Conversely, Ti exhibits an opposing trend (Fig. 2g). These findings strongly suggest that Al in SSR-SrTiO3:Al is enriched near the surface and more homogeneously distributed at lower concentrations in the bulk, forming a gradient doping profile by substituting some of the Ti ions, as illustrated in the inset of Fig. 2g.

In contrast, for HTM-SrTiO3:Al, Al is primarily distributed at the high-index facets at the corners of the particle, with minimal bulk incorporation (Fig. 2f and Supplementary Fig. 5). In the case of 5%-HTM-SrTiO3:Al, Al exhibits a more uniform distribution from the surface to the bulk (Supplementary Fig. 6), accompanied by clear lattice contraction as evidenced by the XRD pattern (Supplementary Fig. 7).

To investigate how Al doping influences surface species and valence states, X-ray photoelectron spectroscopy (XPS) was performed on HTM-SrTiO3:Al and SSR-SrTiO3:Al, which exhibit distinct surface Al doping distributions. The O 1s XPS spectra exhibit peaks at 532.20 eV, 531.50 eV and 529.35 eV, corresponding to adsorbed oxygen species, oxygen vacancies and lattice oxygen, respectively22–25. As shown in Fig. 2h, with increasing surface Al doping layer, the ratio of oxygen vacancies decreases from 43.29% to 16.05%. Simultaneously, the Ti3+ species14,26 show a similar decline from 34.7% to 20.6%, seen in Fig. 2i. The distribution of surface Al doping layer has negligible effects on the Al and Sr (Supplementary Fig. 8) species. These results demonstrate that the introduction of Al effectively inhibits the formation of Ti3+ species, which act as recombination centers12,14,18, and also passivates oxygen vacancies.

Thus, we get a preliminary conclusion that Al doping distribution in SrTiO3:Al plays a crucial role in modulating the OWS activity. The highest OWS performance and AQE are achieved when Al is predominantly concentrated near the surface of the particle, forming a gradient doping profile extending from the surface to the bulk. Additionally, an important role of Al doping in SrTiO3:Al is the passivation of oxygen vacancies and the inhibition of Ti3+ formation.

Charge distribution vs. Al distribution
Photogenerated charge separation is a key factor in determining AQE. To investigate the influence of surface Al doping layer distribution on photogenerated charge separation, surface photovoltage microscopy (SPVM)27 was conducted on HTM-SrTiO3:Al and SSR-SrTiO3:Al. As seen in Fig. 3a–c, HTM-SrTiO3:Al exhibits a positive surface photovoltage (SPV) signal of ~+10 mV at the corners and a negative SPV signal of ~−85 mV at the center, indicating that photogenerated holes are accumulated at the corners while photogenerated electrons are concentrated at the center. However, SSR-SrTiO3:Al shows a relatively uniform positive SPV signal of ~+90 mV on the entire particle (Fig. 3d–f), suggesting a relative homogeneous distribution of photogenerated holes on the surface. These results demonstrate that the spatial distribution of positive SPV signals, indicative of photogenerated hole accumulation on the surface, aligns with the distribution of surface Al doping layer in both samples, highlighting the critical role of Al doping in inducing the accumulation of photogenerated holes to the surface.

Note that a positive SPV indicates that photogenerated hole accumulation dominates the measured surface signal, but it does not exclude the presence of electrons on some facets. Moreover, in SSR-SrTiO3:Al, interfacetcharge separation beyond the spatial resolution of SPVM is evidenced by the facet-selective distribution of Rh and CoOOH dual cocatalysts (Supplementary Fig. 9a–c), which further promotes surface charge separation. The consistent correlation between regions of reduced SPV intensity and Rh deposition sites on the (001) facet, together with the appearance of negative SPV signals (~−25 mV) on samples with 0.3 wt.% Rh loading, indicates electron accumulation on this facet and the role of Rh in electron extraction (Supplementary Fig. 9d–i).

Charge separation dynamics in SrTiO3:Al
Having established the influence of Al on steady-state spatial charge separation, we next investigated its impact on charge separation dynamics in SSR-SrTiO3:Al. Defect structures, critical to these dynamics, were first examined using modulated SPV spectra across excitation wavelengths ranging from sub-band gap to super-band gap. As seen in Fig. 4a, under super-band gap excitation, the negative in-phase signal reflects the relatively faster transfer of photogenerated electrons toward the surface, whereas the negative phase-shifted by 90° signal indicates that photogenerated holes also migrate to the surface and subsequently are trapped there (More information about modulated SPV is available in ref. 28, Supplementary Note 1 and Supplementary Fig. 10). The significantly higher intensity of the phase-shifted by 90° signal relative to the in-phase signal underscore the predominance of hole trapping in governing charge separation process28. Additionally, the phase-shifted by 90° signal remains negative throughout the excitation range with a signal sensitivity less than 1.0 μV29, indicating that only hole trapping occurs at or near the surface. These results provide direct evidence for the existence of a substantial density of hole trap states at or near the surface of SSR-SrTiO3:Al.

To assess the SPV evolution over time, we further performed transient surface photovoltage (TPV) measurements on both undoped SrTiO3 (Fig. 4b) and SSR-SrTiO3:Al (Fig. 4c). As highlighted by the dashed box in Fig. 4b and c, the negative TPV signals observed for undoped SrTiO3 convert to positive signals for SSR-SrTiO3:Al within the sub-band gap excitation range, consistent with the SPS results of the two samples (Fig. 3f and Supplementary Fig. 11). This inversion demonstrates that Al doping eliminates electron trap states while simultaneously inducing hole trap states at or near the surface.

Figure 4d shows the temporal decay of TPV signals for undoped SrTiO3 and SSR-SrTiO3:Al under super-band gap excitation over a range of delay times. For undoped SrTiO3, an obvious decay in TPV signals is observed when the delay time increased to 1–100 μs (indicated by the dark arrows in the upper region of Fig. 4d). However, for SSR-SrTiO3:Al, the SPV signals either increase or remain almost constant from 100 ns to 10 ms (indicated by the dark arrows in the bottom region of Fig. 4d). The persistence of a nearly constant SPV signal over five orders of magnitude in delay time suggests that photogenerated carriers’ recombination is greatly blocked with Al doping.

The initial rise in the transient SPV signal, occurring within nanoseconds, is attributed to carrier separation driven by the built-in electric field, whereas its evolution over microsecond-to-millisecond timescales reflects the diffusion and recombination of photogenerated carriers30,31. Figure 4e shows that under super-band gap excitation at 3.6 eV, SSR-SrTiO3:Al exhibits a sharp surge at the onset of the TPV decay curve, in contrast to the gradual increase observed in undoped SrTiO3. This abrupt surge in the positive TPV signal for SSR-SrTiO3:Al indicates that Al doping forms a gradient built-in electric field oriented from the bulk to the surface, induced by the gradient in Al concentration.

Fitting the TPV decay curves with a combination of stretched exponential and power law decay functions provides information about the diverse charge separation processes and the spatial and energetic distributions of electronic states (Supplementary Note 2)28. As shown in Fig. 4f, the TPV decay curve under super-band gap excitation (3.74 eV) is decoupled into multiple charge transport processes using two stretched exponential and one power decay functions. The electron diffusion process (red solid line) primarily occurs on the microsecond timescale, with a time constant of 3.195 μs and a negative amplitude of −17.225 mV. One hole transport process (blue solid line) is coupled with tail states near the valence band, while another hole trapping process (purple solid line), described by a stretched exponential decay function involving Gaussian-distributed trap states28, exhibits a lifetime of 209.12 ms. Notably, this trapped-hole component remains nearly unchanged from nanoseconds to the seconds, highlighting the stabilization of trapped holes without recombination in SSR-SrTiO3:Al (fitting parameters are provided in Supplementary Table 1). These findings, combined with further analysis of TPV kinetic traces under sub-band gap (2.0 eV) and band-edge excitation (3.20 eV), provide insights into electronic structures of SSR-SrTiO3:Al (Supplementary Fig. 12).

Together, these results underscore two pivotal roles of Al in SrTiO3:Al for charge separation dynamics: (1) the formation of a gradient built-in electric field, directed from the bulk to the surface, induced by the Al gradient doping profile, and (2) the introducing of hole trap states at the surface which stabilize holes and prolong their lifetime from nanoseconds to seconds without recombination. The persistence of these long-lived holes on the order of seconds aligns well with the kinetics of the water oxidation reaction32–34.

Water adsorption sites on SrTiO3:Al
The spatial alignment of long-lived trapped holes with the water oxidation center is essential for achieving high AQE. Consequently, we characterized the coordination environment of Al in SSR-SrTiO3:Al using ultra-high-field nuclear magnetic resonance (NMR) spectroscopy to investigate the spatial correlation between surface hole trap sites and reaction centers associated with Al doping. Figure 5a shows the 27Al NMR spectra of fresh, aged and dehydrated SSR-SrTiO3:Al samples. A sharp peak at 8.2 ppm, which remains unchanged for all samples, confirms the presence of highly symmetric AlO6 octahedra35,36, indicating that Al substitutes for Ti in the bulk lattice. This observation is further supported by defect formation energy calculations, revealing that Al substitution at Ti sites is thermodynamically more favorable than occupation of other lattice positions (Supplementary Table 2 and Supplementary Figs. 13–14). In contrast, a broader shoulder peak at 10.2 ppm, observed for fresh SSR-SrTiO3:Al, diminishes in aged samples and disappears upon dehydration. Concurrently, weak signals at 34.5 ppm and 64.6 ppm, corresponding to five- and four-coordinated Al species36, respectively, emerge in dehydrated samples (see magnified region in Fig. 5a). These findings indicate that the 10.2 ppm shoulder peak originates from surface AlO6 units with reduced symmetry, likely due to coordinating with hydroxyl groups.

To confirm this, corresponding 1H NMR spectra of the samples were recorded. As shown in Fig. 5b, the spectra exhibit five distinct peaks. The peaks at 0.55 ppm and 0.96 ppm are ascribed to terminal hydroxyl groups at different sites37, while those at 2.4 ppm, 4.7 ppm and 7.9 ppm are assigned to hydrogen bonding interactions between these hydroxyl groups38, physiosorbed water molecules39, and bridging hydroxy groups37, respectively (Fig. 5b and c). Upon dehydration, the intensities of these peaks decrease markedly, with the peak at 0.96 ppm in the 1H NMR spectra exhibiting a similar decreasing trend to the shoulder peak at 10.2 ppm in the 27Al NMR spectra. This parallel decline further supports the coordination of surface AlO6 units with terminal hydroxyl groups. Additionally, the concurrent decrease of the 10.2 ppm shoulder peak in the 27Al NMR spectra and the 4.7 ppm peak in the 1H NMR spectra after aging indicates that the degeneration of hydroxylated Al species contributes to the notable decline in water adsorption.

To further identify the surface coordination environment, CO2-desorption diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was employed on undoped SrTiO3 and fresh SSR-SrTiO3:Al. As an acid probe molecule, CO2 reacts with surface hydroxyl groups to form bicarbonate species, whose DRIFTS peak positions depend on the associated metal centers40. As shown in Fig. 5d, the peak positions of bicarbonate species differ between the two samples (1498 cm−1 for undoped SrTiO3 and 1517 cm−1 for SSR-SrTiO3:Al), indicating that Al incorporation alters the coordination environment of the surface hydroxyls. The carbonate species produced by thermal dehydration of these bicarbonates also exhibit distinct peak positions (1444 cm−1 for undoped SrTiO3 and 1432 cm−1 for SSR-SrTiO3:Al)41, further confirming that Al doping modulates the surface hydroxyl configurations by introducing Al-OH groups with different vibrational characteristics compared with Ti-OH.

Based on these findings, two distinct coordination environments for Al in SSR-SrTiO3:Al are proposed, corresponding to substitution at either the bulk or surface Ti sites (Fig. 5c): (1) relatively higher symmetric bulk AlO6 octahedra and (2) less symmetric surface AlO6 octahedra, coordinated with hydroxyl groups, which serve as critical water adsorption sites. Notably, aging-induced degradation of surface hydroxylated Al species significantly reduces water adsorption, consequently lowering OWS activity (Supplementary Fig. 15a). This direct correlation between surface hydroxylated Al sites and OWS performance suggests that the hydroxylated Al is a key component of the active center for water splitting.

Even without the CoOOH oxidation cocatalyst, SSR-SrTiO3:Al retains ~79% of its OWS activity compared to that with CoOOH (Supplementary Fig. 15b), highlighting the presence of intrinsic water oxidation centers on its surface. The additional enhancement of OWS activity with CoOOH loading on high-index facets (Supplementary Fig. 9a and b) can be attributed to improved interfacet charge separation and increased hole concentration at water oxidation sites. Furthermore, the OWS activity remains largely unaffected under alkaline conditions (Supplementary Fig. 15c and d), indicating that proton concentration has a negligible influence on OWS and that the hydrogen evolution reaction (HER) is not rate-limiting. This conclusion is further supported by the much higher calculated overpotential for the oxygen evolution reaction (OER) compared to that for the HER (Supplementary Fig. 16 and Supplementary Data 1). Given that the OER is often the bottleneck in OWS42–44, these findings emphasize the critical role of surface hydroxylated Al as a component of water oxidation centers, driving the efficient OWS for SSR-SrTiO3:Al.

Figure 5e displays the intrinsic OER activity evaluated through the AQE of OER for undoped SrTiO3, HTM-SrTiO3:Al and SSR-SrTiO3:Al without oxidation cocatalysts, yielding values of 11.02%, 42.3% and 81.7%, respectively. The AQE difference in the OER between HTM-SrTiO3:Al and SSR-SrTiO3:Al is in good agreement with the AQE difference in the OWS. The progressive enhancement in OER performance with Al doping underscore the importance of surface Al sites in both surface charge collection and catalysis for OWS.

A possible OER pathway (Supplementary Fig. 17)42,45–47 is proposed to occur at hydroxylated Al-O-Ti sites acting as hole traps8,48–50. Metadynamics (MD) simulations performed in CP2K51–54, which do not account for excited-state effects or complex photoinduced interactions, reveal that adjacent hydroxyl groups at Al sites play crucial roles in facilitating O-O bond formation and the deprotonation process (Supplementary Figs. 18–19 and Supplementary Data 1). This mechanistic understanding of how Al enhances the OER, which is the primary limitation in OWS, enables the AQE closer to unity.

Taking the SrTiO3:Al as model photocatalyst which achieved an AQE exceeding 90% in OWS, our study revealed the crucial and multifunctional roles of Al in SSR-SrTiO3:Al for achieving near-unity AQE in OWS. We demonstrated that the gradient Al doping from surface to bulk generates a built-in electric field that drives photogenerated holes from the bulk to surface sites associated with hydroxylated Al-O-Ti, prolonging hole lifetimes by five orders of magnitude. Moreover, we unveiled that hydroxylated Al sites act as a key component of water adsorption and oxidation centers. The spatiotemporal coupling of charges with water oxidation at these sites enables near-unity AQE in OWS.

These findings establish a fundamental framework for synergistically optimizing charge dynamics and surface reaction kinetics, thereby elucidating the principle behind the highly efficient OWS photocatalysis.

Methods
Sample preparation
Precursor SrTiO3 synthesized using hydrothermal method55. 0.26 μL of TiCl4 (Aladdin, 99.99%) was added to 25 mL of an ice-cooled solution containing 1 g of 1, 2-propylene glycol and stirred for 10 min. Then 30 mL of an aqueous containing 4 g of LiOH.H2O (Macklin, 99.95%) was added, followed by another 20 min of stirring. Next, 10 mL of an aqueous solution containing 0.7 g of SrCl2.6H2O (Macklin, 99.99%) was added, and the mixture was stirred for an additional 30 min. The resulting solution was heated at 180°C for 48 h in a Teflon-lined hydrothermal reactor. The products were washed twice with ethanol and three times with deionized water, then dried at 80°C.

Precursor SrTiO3 synthesized via solid state reaction56. SrCO3 (Macklin, 99.95%), and TiO2 (Macklin, 99.8%) were mixed in a mole ratio of 1: 1 using an agate mortar. The homogenized powder was transferred to a corundum crucible and calcined at 1150°C for 10 h. The product was washed five times with deionized water and dried at 80°C for 12 h.

SrTiO3:Al was synthesized by flux method9. SrTiO3, Al2O3 (Macklin, 99.99%) and SrCl2.6H2O were used as raw materials. The mole ratio of SrTiO3, Al2O3 and SrCl2.6H2O was 1:x:10 (where x=0.02, 0.05). SrTiO3, Al2O3 and SrCl2.6H2O were mixed in an agate mortar thoroughly and heated in an alumina crucible at 1150°C for 10 h. After the mixture was cooled to room temperature, the samples were washed 5 times with deionized water to remove the residual SrCl2 and dried at 80°C for 12 h.

Cocatalysts loading
0.1 wt.% Rh, 0.05 wt.% Cr, and 0.05 wt.% Co were deposited onto the photocatalysts for OWS reaction. Aqueous solutions of RhCl3.3H2O (3AChem, 99%), K2CrO4 (Macklin, 99.9%) and Co(NO3)2.6H2O (Inno-chem, 99%) were prepared with metal species containing 2 mg/mL, respectively. 0.1 g of SrTiO3 samples was dispersed in 100 mL of deionized water by a 10 min sonication. 50 μL RhCl3 aqueous solution was added to the suspension with magnetic stirring and 10 min irradiation by a Xe lamp (LX 300F, Eagle Engineering Co. LTD.). Subsequently, the 25 μL K2Cr2O7 aqueous solution was added to the suspension with additional 5 min irradiation. Finally, 25 μL Co(NO3)2 solution was added to the suspension with another 5 min irradiation. The actual loading amounts of cocatalysts are listed in Supplementary Table 3, measured by Inductively Coupled Plasma Mass Spectrometry (ICP-MS).

Photocatalytic reaction tests
Photocatalytic OWS reactions were carried out in a closed gas circulation and evacuation system. Normally, 0.1 g of the photocatalysts was dispersed in 100 mL of deionized water. The air was evacuated from the reaction system totally before the photodeposition of cocatalysts was finished. Then, the OWS reaction was driven by the irradiation of a Xe lamp (LX 300F, Eagle Engineering Co. LTD.) from the top of the Pyrex reaction vessel. The amounts of the evolved H2 and O2 was analyzed by an on-line gas chromatograph (Agilent, GC-7800, TCD, Ar carrier). A photograph of the experimental setup and the irradiancespectrum of the Xe lamp are provided in Supplementary Fig 20.

Apparent quantum efficiency (AQE) measurement
The AQE was calculated using the following equation:

AQE(%) = 2 × n(H2) × NA × 100 / N(photons) (1)

Where n(H2) represents the molar production rate of hydrogen per hour, obtained from the photocatalytic reaction system, and NA is Avogadro’s constant (6.02 × 10^23). N(photons) is the total number of photons introduced into the reaction system per hour, calculated as follows:

N(photons) = S × T × Σ (i=1 to 9) ni (2)

Here, ni (photon per second per square centimeter) denotes the photon flux, measured at different positions within the square illumination area, as shown in Supplementary Fig. 21j. S is the area of the light spot (4.6225 cm^2), and T is the time in seconds (3600 s).

For AQE measurements of OWS, 90 mg of the photocatalyst loaded with cocatalysts was dispersed in 60 mL of deionized water. For AQE measurements of OER, 50 mg of the cocatalyst-loaded photocatalyst was dispersed in 60 mL of deionized water containing 0.06 g of AgNO3 as an electron scavenger. Monochromatic light, with a central wavelength of 360 nm and a full-width at half maximum (FWHM) of approximately 10 nm, was directed into the system via an optical fiber, derived from a Xe lamp source (MAX-303, Asahi Spectra) combined with a band-pass filter. The photon flux was measured using a Si photodiode (LS-100, EKO Instruments). Further experimental details are provided in Supplementary Fig. 21 and Supplementary Table 4.

Nuclear Magnetic Resonance (NMR)
Solid-state NMR experiments were performed on a Bruker AVANCE NEO 800 MHz spectrometer operating at resonance frequencies of 208.5 MHz for 27Al and 800.3 MHz for 1H nuclei, respectively. Single-pulse 1H MAS NMR spectra were acquired using a π/2 pulse excitation with a recycle delay of 5 s. Single pulse 27Al NMR spectra were collected using π/12 pulse excitation with a recycle delay of 1 s. The 1H and 27Al chemical shifts were referenced to adamantane (1H, 1.74 ppm) and 1 mol/L Al(NO3)3 aqueous solution (0 ppm), respectively.

Transient photovoltage spectroscopy (TPV)
TPV spectra were measured using a fixed capacitor setup. SPV signals were coupled to high-impedance buffers (measurement resistance 50 GΩ). SPV transients were excited using pulses from a tunable Nd:YAG laser (EKSPLANT 342/1/UVE) with a pulse duration of 4 ns and a repetition rate of 1 Hz. The transient resolution time was 5 ns (GaGe CS14200). SPV transients were recoded using logarithmic readout, employing a logarithmic averaging procedure. Modulation illumination was provided by a halogen lamp with a quartz prism monochromator (SPM2).

Kelvin probe force microscopy (KPFM)
The contact potential difference (CPD) was measured using KPFM in an ambient atmosphere under amplitude-modulated (AM)-KPFM mode with a commercial atomic force microscope (AFM, Bruker Dimension Icon), achieving a high energy resolution of 5 mV. Topography and surface potential signals were simultaneously obtained using an SCM-PIT-V2 probe (~25 nm radius) in lift mode with a 50 nm lift height.

The surface photovoltage (SPV) is defined as the difference in CPD before and after illumination, expressed as SPV = CPDillumination - CPDdark57. The sign of SPV signal indicates the charge transport direction. Surface photovoltage microscopy (SPVM) mapping was acquired by subtracting CPD maps under illumination and in the dark for the same area. A 350 nm excitation wavelength was generated by a Xe lamp (PLS-SXE300, Beijing Perfectlight) combined with a Zolix Omni-λ-500 monochromator.

Surface photovoltage spectra (SPS) were measured at a specific point of interest using the same KPFM system, with a variation in the excitation wavelength achieved through the Xe lamp and monochromator.

Theoretical calculations
All calculations were performed within the framework of density functional theory (DFT), employing a dual-code approach. VASP 6.3.0 was used for electronic structure calculations and defect thermodynamics58, while CP2K 2024.0 was employed for ab initio metadynamics simulations. The Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional within the generalized gradient approximation (GGA) was applied consistently in both codes. Unless otherwise stated, all simulations were carried out at 298.15 K, and zero-point energies (ZPE) and entropy corrections were included where appropriate.

Structural optimization and defect thermodynamics (VASP)
The bulk SrTiO3 structure was first optimized in a 1×1×1 cubic unit cell using the projector augmented-wave (PAW) method, with a plane-wave cutoff energy of 400 eV. The Brillouin zone was sampled with a 6×6×6 Monkhorst-Pack k-point mesh. Structural relaxation proceeded until the residual forces on each atom where below 0.02 eV/Å and the total energy convergence reached 10^-8 eV. The conjugate-gradient algorithm (IBRION=2) with ISIF=2 was employed, allowing atomic positions to relax under a fixed cell volume and shape. High-precision settings (PREC=Accurate, ADDGRID=TRUE, LASPH=TRUE) were applied to ensure numerical accuracy.

Based on the optimized structure, a 3×3×3 supercell (135 atoms) was constructed to model Al substitution at Ti and Sr sites, as well as Al interstitial configurations. All doped structures were fully relaxed using the same computational parameters as the pristine system. Defect formation energies were evaluated under various growth conditions using chemical potentials referenced to a range of calculated binary and ternary phases, including Sr, Ti, O2 (triplet), SrO, SrO2, TiO2, TiO, TiO3, SrTiO3, Sr2TiO4, Sr3Ti2O7, Sr4Ti3O10 and Al2O3. The neutral defect formation energy (q=0) was computed as59:

ΔEf = Etot(defect) - Etot(host) - Σ ni μi (3)

Where Etot(defect) and Etot(host) are the total energies of the defective and pristine supercells, ni represents the number of atoms added or removed, and μi is the corresponding chemical potential. All values were determined within the thermodynamic stability window of SrTiO3, defined by convex hull construction.

Electronic structure calculations
The band structure and density of states (DOS) were calculated using the 3×3×3 SrTiO3 and SrTiO3:Al supercells. DOS calculations were performed with a 3×3×3 k-point mesh using the tetrahedron method with Blöchl corrections. Partial DOS were projected onto atomic orbitals using a Gaussian smearing width of 0.05 eV.

Band structures were calculated non-self-consistently (ICHARG=11) based on converged charge densities. High-symmetry paths (Γ-X-M-Γ-R) were generated using VASPKIT toolkit, which was also employed for band unfolding process60. The KPOINTS file defined 20 k-points in line-mode. Band unfolding was performed directly using the band unfolding module of VASPKIT, which projects the supercell eigenstates onto the primitive cell Brillouin zone using wavefunction-based spectral weight analysis. This enabled direct visualization of effective band dispersion in defected and doped supercells.

Metadynamics simulations (CP2K)
The dynamic simulations were performed using the CP2K quantum chemistry software package. We constructed a SrTiO3 surface model based on the optimized cubic perovskite unit cell with lattice parameters a=3.905 Å. The (100) surface was chosen for its relevance to experimental studies and catalytic applications. A TiO2-terminated surface with (3×3) periodicity was modeled to mimic a common experimental setup. A vacuum layer of 50 Å along the Z-axis was added to eliminate periodic interactions between the slabs. Periodic boundary conditions (PBC) were applied in the XY plane. To simulate realistic solid-state properties, the bottom two atomic layers were fixed during geometry optimizations and molecular dynamics (MD) simulations, mimicking the bulk-like behavior of the material. The remaining atoms were allowed to fully relax.

For doped models, one Ti atom in the topmost TiO2 layer was replaced with an Al atom to create Al-doped SrTiO3. The surface was hydroxylated by adding protons to selected oxygen atoms, forming surface hydroxyl groups (OH). Additionally, two H2O molecules were adsorbed near the hydroxylated surface to simulate a catalytic reaction environment.

Geometry optimizations of both undoped and Al-doped SrTiO3 surfaces were performed to obtain relaxed structures for subsequent reaction studies. DFT calculations employed the PBE exchange-correlation functional within the GGA. Dispersion interactions were included using the DFT-D3 method with Becke-Johnson damping. DZVP-MOLOPT basis sets were used for all elements (Sr, Ti, O, Al, and H) in combination with GTH-PBE pseudopotentials. A plane-wave cutoff of 400 Ry and a relative cutoff of 40 Ry were applied. The optimized structures were then served as the starting configurations for metadynamics simulations.

Reaction pathways were studied using metadynamics simulations implemented in CP2K51–54, which efficiently samples the reaction free energy surface (FES). Two primary reaction processes were investigated: (1) the formation of OOH species and (2) the decomposition of OOH and desorption of O2.

For OOH formation, the process was simulated by introducing two water molecules near the hydroxylated SrTiO3 surface. The undoped and Al-doped systems were analyzed separately. Collective variables (CVs) were employed to describe key reaction coordinates, including the distance between the oxygen atom of an adsorbed H2O molecule and the oxygen atom of a surface OH group to track O-O bond formation (CV1), the distance between the proton of the attacking water molecule and the surface hydroxyl group (Al-OH or Ti-OH) to capture proton transfer (CV2), and the distance between the resulting OOH species and neighboring surface hydroxyl groups to monitor hydrogen bonding interactions (CV3). Metadynamics simulations were performed in the NVT ensemble at 298.15 K using the canonical sampling via velocity rescaling (CSVR) thermostat with a time constant of 200 fs. The simulation timestep was set to 0.5 fs, and each simulation consisted of 10,000 steps, corresponding to a total duration of 5 ps. Hills were deposited every 40 MD steps, with a height of 0.1 eV and a width of 0.1 Å. Wall potentials were applied to constrain the CVs within physically meaningful ranges, ensuring accurate exploration of the reaction pathway.

For the OOH decomposition and desorption of O2 process, the OOH species obtained from the first simulation served as the starting configuration. The reaction sequence involved proton dissociation from the OOH species to form O2 (CV1), detachment of the O2 from the surface (CV2), and breaking of hydrogen bonds between O-O and nearby hydroxyl groups (CV3). The three CVs were defined to represent these steps: the distance between the proton and the oxygen atom within the OOH species to capture proton dissociation, the distance between the oxygen atoms in the O-O species and the surface to monitor detachment, and the distance between the O-O species and neighboring surface hydroxyl groups to describe hydrogen bond breaking. Metadynamics simulations for this stage followed the same parameters as described for the OOH formation, with minor adjustments to the CV wall potentials to reflect the specific reaction mechanisms. Spin multiplicity was set to 2 to account for the triplet ground state of molecular oxygen (O2), and charge neutrality was maintained through the calculations.

Free energy surface construction
Free energy surfaces (FES) were generated using the native post-processing tools provided by CP2K. The metadynamics simulation data were used to compute the FES, enabling the visualization of reaction pathways and the identification of energy barriers associated with key reaction steps. The FES was calculated on a grid with dimensions of 100×100. A cutoff value of 10^-5 eV was applied to exclude noise from the computed surface. This procedure provided a detailed map of the energy landscape along the reaction coordinates, allowing the extraction of activation energies for each process. All computations were conducted in the NVT ensemble at 298.15 K to mimic experimental conditions. The accuracy of the FES was ensured by converging the metadynamics simulations to a well-defined energy profile, as indicated by consistent hill heights in the later stages of simulation.

Overpotential calculations for OER and HER
Thermodynamic reaction steps for both the OER and HER were analyzed using the computational hydrogen electrode (CHE) model. OER calculations were referenced to a classical electrocatalytic pathway, given the difficulty in explicitly modeling excited states and the time-dependent relaxation of photogenerated charge carriers. In this framework, OER proceeds via the formation of key intermediates (*OH, *O, *OOH, *O2) through four sequential deprotonation steps. A TiO2-terminated (100) surface was extracted from a 3×3×3 supercell, and the reaction was assumed to occur at a Ti site in the presence of nearby hydroxylated Al species. The Gibbs free energies of all intermediates were derived from DFT total energies (ΔE), with thermodynamic corrections including zero-point energy (ΔEZPE) and entropic contributions (TΔS) applied to the optimized configurations obtained in VASP, as expressed in Eq. (4):

ΔG = ΔE + ΔEZPE - TΔS (4)

For HER, slab models of Rh (100), Rh (111), and Rh (211) were constructed using four-layer slabs with vacuum spacing ≥15 Å. H adsorption was considered at atop and bridge sites, and the most stable configuration was determined via geometry optimization. Adsorption free energies ΔGH* were calculated as:

ΔGH* = ΔEH* + ΔEZPE - TΔS (5)

with corrections based on vibrational frequency calculations. The entropy of H2 gas (S=130.68 J/molK) and zero-point energies were referenced from standard sources or directly computed.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript presents a compelling mechanistic study explaining why Al-doped SrTiO3 achieves near-unity AQE in overall water splitting. Using advanced characterization techniques—including SPVM, TPV, and spectroscopic analyses— the authors show that gradient Al doping induces a built-in electric field. This field drives photogenerated holes from bulk toward surface hydroxylated Al–O–Ti trap sites, which also serve as active centers for water oxidation. The data are of high quality, and the methodologies are sound. Notably, the study highlights the critical yet often overlooked role of spatiotemporal alignment between charge separation and surface catalysis—a key factor limiting quantum efficiency in photocatalytic systems. The insights offered here are valuable for guiding the rational design of photocatalysts with exceptionally high quantum efficiencies. However, the following MINOR REVISIONS are needed before the manuscript can be accepted for publication in Nature Communications. Some comments are listed below:\n1. What do the authors consider to be the cause of the degradation of hydroxylated species and the associated decline in photocatalytic performance?\n2. Revise “ultimately be stored in chemical bonds” to “ultimately to be stored in chemical bonds” (Page 3, line 27), and “many followed-up studies” to “many follow-up studies” (Page 3, line 28).\n3. On Page 4, lines 52–53, the phrase “doubles the AQE to 92%” is misleading, as 92% is more than three times the value of 29.3%.\n4. Since the abbreviations “OER” and “AQE” have already been defined, they can be used directly in later instances without repeating the full terms (Page 17, lines 299-301).\n5. Supplementary Fig. 12 would benefit from clearer labeling of each step (e.g., “Step 1: OOH formation,” “Step 2: O2 release”) to improve clarity and interpretation.", "reviewer_2": "The authors reported attractive apparent quantum efficiency (exceeding 90%) for Al modified SrTiO3. However, a quantum efficiency of almost 100 % has been reported for Al modified SrTiO3 in (Nature 2020, 58, 411–414), and explained thoroughly in (doi.org/10.1002/anie.202313537). Present study can include the following points to improve further\n1. Site selectivity for Al in SrTiO3 not justified. May calculated defect formation energies.\n2. Details of change in the band structure and density of states due to introduction of Al in SrTiO3.\n3. Detail energetic of each species should be calculated using DFT for Supplementary Figure 12\n4. Theoretical calculations of over potential for HER and OER is missing", "reviewer_3": "In the manuscript ‘Spatiotemporal Alignment of Hole Transfer and Water Oxidation for Highly Efficient Photocatalytic Water Splitting’ the authors show that a gradient Al doping in SrTiO3 can result in enhanced hole lifetimes and enhanced photocatalytic activity. The study is very interesting, and especially the surface photovoltage measurements show a deeper understanding of the behavior of charge carriers inside the synthesized materials. I think the study can be well suited for Nature Communications due to the high quality of science performed. Still, I have a few questions:\n- One of the things the authors nicely show in the supporting information is the selective deposition of Rh/Cr2O3 and CoOOH on the respective reductive and oxidative facets (Figure S8). One would expect that this will have a drastic enhancement in the water splitting performance. However, in Figure S11, the authors only show the photocatalytic activity of the sample with CoOOH. Why not include an activity measurement with both Rh/Cr2O3 and CoOOH as well?\n- Related to this: I don’t see any characterization of how much cocatalyst was deposited on the samples. What are the weight percentages?\n- In the SPVM images (such as Figure 3): what is the scale of the image actually? Could the authors add scalebars, or indicate in the figure caption how large the area is the author is looking into?\n- The upper question is also relevant for Figure S8f. How large are the Rh particles compared to the SPV images? Is the negative surface charge really over the full facet, or specifically at the sites where the Rh is deposited?\n- Figure 3: did the authors also study SPVM mapping for undoped SrTiO3? How does that compare?\n- In the absorption spectra in Figure S2 the HTM samples show a slight offset compared to the other two samples. Is this an offset, or is this after all additional absorption (possible induced due to the Al or a high number of oxygen vacancies inside the material)?\nOnce the authors answer those questions satisfactorily, I would be happy to recommend this manuscript for publication in Nature Communications.", "reviewer_4": "This work employed Al-doped SrTiO3 as a model to investigate the effects of Al doping on overall water-splitting performance, particularly in achieving high AQY. Both experimental and theoretical studies were conducted to reveal how gradient Al doping promotes photogenerated hole dynamics and provides active sites for water oxidation. The work is carefully executed and the findings are of interest. My comments are as follows:\n1. Page 9: The authors state that surface Al doping leads to an exclusively positive SPV signal on SSR-SrTiO3:Al. Why were photogenerated electron signals not detected at the surface? If so, how would surface reduction reactions happen?\n2. Related to Comment 1: The authors previously claimed that the entire particle, including the central (001) facet, is covered by photogenerated holes acting as oxidation sites, resulting in an exclusively positive SPV signal. In this case, how can Rh be reductively photo-loaded onto the (001) facet, thereby serving as a reduction site (Supplementary Figure 8c–f)? A detailed clarification is required.\n3. Supplementary Figure 8a–b: The FESEM images do not allow clear visual distinction between Rh and CoOOH. The authors should provide EDX mapping to substantiate their claims.\n4. Figure 5d: Please provide a more detailed explanation of the significance of the observed changes in DRIFT signals between doped and undoped samples. Specifically, how are the surface hydroxyl configurations modulated by Al doping?\n5. Page 16: In addition to reporting OWS under alkaline conditions, the authors should also present and discuss the performance under acidic conditions for a more comprehensive understanding.\n6. Theoretical calculations: For reproducibility and validation, the authors should specify all the computational parameters in detail, including energy cut-off, ionic degrees of freedom, k-point mesh, and other relevant settings."}, "author_response": "Response letter\nSpatiotemporal Alignment of Hole Transfer and Water\nOxidation for Highly Efficient Photocatalytic Water Splitting\nYaling Luo1,2, Ruotian Chen1, Thomas Dittrich3, Pan Gao1, Chenwei Ni1,2, Jie Zhang1,2, Yue\nZhao1, Deyun Zhang1,2, Na Ta1, Mingrun Li1, Mingjian Zhang4, Dongfeng Li1, Zhendong\nFeng1,2, Zheng Li1, Yang Yu1, Panwang Zhou1, Kazunari Domen5,6, Fengtao Fan1* and Can\nLi1* ... [Full rebuttal text omitted for brevity, but included in full in the source]"}, {"round": 2, "reviewer_comments": {"reviewer_1": "My concerns have already been addressed in the revisions, and I recommend to accept it.", "reviewer_2": "The authors addressed the issues raised, therefore may be accepted for publication", "reviewer_3": "The authors have addressed all issues raised satisfactorily, and I would be happy to advice this manuscript for publication within Nature Communications. I would like to compliment the authors once more on a very interesting manuscript with a high level of science.", "reviewer_4": "The comments raised have been satisfactorily addressed. I would recommend the manuscript for publication."}, "author_response": ""}]

Decision letter