# Modulating the Optical Properties of Carbon Quantum Dots Through Cerium Doping to Boost the Photocatalytic Efficiency of Titanium Dioxide Composites

Bo-Rong Huang, Pei-Kai Hsu, Alexandre Gloter,* Chi-Liang Chen, Jenn-Ming Song, and Shih-Yun Chen*

In this study, cerium-doped carbon quantum dots (Ce-CDs) are first synthesized by hydrothermal decomposition of organic compounds mixed with different contents of cerium nitrate hexahydrate. The analysis results indicate that cerium doping effectively reduces the CD core's oxygen- and nitrogen-related structural defects. As the doping concentration increases, the incorporation of cerium further induces different defects in carbon quantum dots. Ultraviolet photoemission spectroscopy and Low Energy Inverse Photoemission Spectroscopy show Ce-CDs have similar band gap energy but different Fermi levels. Second, Ce-CDs are combined with TiO₂ to prepare TiO₂/Ce-CDs composites, which demonstrate strong optical properties and enhanced photocatalytic performance due to the structural modification of Ce-CDs. These composites improve visible light photocatalytic performance by improving the charge separation of electron-hole pairs. The main active species include hydroxyl radicals on the TiO₂ surface and superoxide anions on Ce-CDs. The best performance is observed in the TiO₂/0.5Ce-CDs composite, with a reaction rate constant as high as 0.087 min⁻¹ under visible light. This study demonstrates the potential of Ce doping on carbon quantum dots and their advanced photocatalytic applications.

## 1. Introduction

Environmental pollution, driven by the rapid growth of the global population, poses significant challenges. Sources of pollution include dyes from textile factories, pharmaceuticals and waste from the medical industry, and organic solvents from semiconductor manufacturing. This situation compels us to consider ways to repair and protect the environment. Photocatalysts provide a promising solution for environmental challenges, with solar photocatalysis being both eco-friendly and sustainable for degrading pollutants. This method efficiently oxidizes water contaminants without chemicals, using light energy to drive beneficial chemical reactions. These reactions can lead to antimicrobial properties, hydrogen generation, and the breakdown of organic pollutants. Additionally, solar photocatalysis is more energy-efficient than traditional methods, requiring minimal energy and operating effectively under moderate temperature and pressure conditions with

heterogeneous photocatalysts.[1–3] Titanium dioxide (TiO₂) is one of the most well-known photocatalytic materials, with research and development beginning ≈1972. However, titanium dioxide has a wide energy gap of 3.2 eV, meaning it can only catalyze reactions when exposed to ultraviolet light. This limitation restricts its practical applications.[4,5] Various strategies have been developed to address this issue. One notable approach is the production of composite structures, which has seen significant advances in recent years.[6,7] The resulting heterogeneous interface has been shown to effectively enhance the performance of individual materials and has found widespread application across different fields.[8–10]

Carbon quantum dots, also known as carbon dots (CDs), were first discovered in 2004 by Xu et al., who identified these nanometer-sized particles with fluorescent properties during the purification process of single-walled carbon nanotubes.[11] In 2006, Sun et al. demonstrated that carbon quantum dots exhibit a red-shift in emission wavelength as the excitation light's wavelength increases, which drew considerable interest from

<figure><img src="image_1.png" /></figure>

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adsu.202500283

DOI: 10.1002/adsu.202500283

<footnote>B.-R. Huang, P.-K. Hsu, S.-Y. Chen
Department of Materials Science and Engineering
National Taiwan University of Science and Technology
Taipei 106335, Taiwan
E-mail: sychen@mail.ntust.edu.tw</footnote>

<footnote>A. Gloter
Laboratoire de Physique des Solides
Université Paris-Saclay
CNRS UMR 8502, Orsay F-91405, France
E-mail: alexandre.gloter@universite-paris-saclay.fr</footnote>

<footnote>C.-L. Chen
National Synchrotron Radiation Research Center
Hsinchu 30076, Taiwan</footnote>

<footnote>J.-M. Song
Department of Materials Science and Engineering
National Chung Hsing University
Taichung 402202, Taiwan</footnote>
various research groups studying the fluorescence properties of these dots.[12] Wang et al. synthesized nitrogen-doped carbon quantum dots (N-CD) that exhibited significantly improved fluorescence intensity by using glycerin and melamine in a high-temperature reaction environment, with the assistance of an acid catalyst.[13] The charge transfer properties of carbon quantum dots enhance the photocatalytic degradation efficiency of various catalysts when combined with different semiconductor photocatalysts.[14-18] For example, Li et al. mixed carbon quantum dots with different metal oxides, producing composites that could completely degrade methylene blue after 15 min of visible light irradiation for SiO₂/CDs or 25 min for TiO₂/CDs.[19] The enhanced photodegradation efficiency was attributed to the up-conversion characteristics of carbon quantum dots. The same effect was observed by Zhang et al. in a silver phosphate (Ag₃PO₄) photocatalyst containing carbon quantum dots.[17] The authors suggested that carbon quantum dots act as electron acceptors, helping to inhibit the recombination of electron-hole pairs in the photocatalyst, thereby enhancing the degradation efficiency of silver phosphate for methyl orange (MO) under visible light irradiation.

Doping various elements into carbon quantum dots can significantly enhance photocatalytic efficiency. For instance, Ma et al.[20] developed a composite consisting of nitrogen-doped carbon quantum dots and cuprous oxide (Cu₂O/N-CDs). Their research demonstrated that nitrogen-doped carbon quantum dots act as electron acceptors during the photocatalytic process. They effectively capture electrons from the conduction band of cuprous oxide, facilitating the separation of photogenerated electrons and holes, which ultimately improves the degradation efficiency of methyl orange under visible light irradiation. Similarly, Ambade et al.[21] synthesized nitrogen and sulfur-doped carbon quantum dots and incorporated them into a reduced graphene oxide-titanium dioxide composite (rGO-TiO₂). Their findings showed that both nitrogen-doped and nitrogen-sulfur co-doped carbon quantum dots could extend the light absorption range of the rGO-TiO₂ composite into the visible light spectrum. The incorporation of sulfur effectively lowered the energy gap value of the carbon quantum dots, which helped suppress the recombination of photogenerated electrons and holes, thus enhancing the photodegradation efficiency.

Metallic dopants have been shown to significantly influence the properties of carbon quantum dots. For instance, metals such as zinc, silicon, and copper can enhance the fluorescence quantum yield of these dots.[22] Additionally, iron, cobalt, and manganese can impart magnetic properties to carbon quantum dots.[23] Certain rare-earth elements like europium, ytterbium, and iridium can enable carbon quantum dots to exhibit color-tunable up-conversion luminescence properties.[24] Mu et al. synthesized carbon quantum dots co-doped with copper and nitrogen, creating a composite material consisting of these co-doped quantum dots (Cu-N-CDs).[25] The nitrogen doping in the carbon quantum dots provides a higher electron affinity compared to carbon, which enhances their ability to capture electrons during photocatalytic reactions. Furthermore, the presence of copper in the carbon quantum dots improves electron transport and accelerates the reduction of oxygen molecules, leading to the formation of superoxide anion radicals.

Rare earth-doped carbon dots (RE-CDs) have recently gained attention for their unique properties. These materials take advantage of the larger atomic radii, additional electrons, and unoccupied orbitals present in rare-earth metal ions.[26,27] John et al.[28] utilized a microwave method to dope lanthanum into carbon quantum dots and subsequently prepare a composite photocatalyst (ZrO₂/La-CDs). According to the test results, the ZrO₂/La-CDs composites demonstrated excellent degradation capabilities for various dyes and antibiotics, particularly showing impressive results for rhodamine B, sunset yellow, and methyl orange. The analytical results suggest that the conversion characteristics of La-CDs can lower the energy gap of ZrO₂ and enhance light absorption in the visible spectrum. Additionally, the La ions create holes in the valence band, which helps to inhibit the recombination of photo-generated electron-hole pairs and extends the lifespan of these carriers. This process is the source of the excellent photocatalytic degradation efficiency of the ZrO₂/La-CDs composites.

However, research on the doping of rare earth elements in carbon quantum dots indicates that the literature concerning Ce doping is significantly scarcer than that for La, especially regarding its application in photocatalysis. This study begins by synthesizing carbon quantum dots through a hydrothermal process, incorporating different amounts of cerium doping. High-resolution spectroscopy and microscopy techniques were employed to investigate how Ce doping influences the structure of the carbon quantum dots and their optical properties. Following this, the Ce-doped carbon quantum dots (Ce-CDs) were combined with titanium dioxide (TiO₂) to create a composite structure (TiO₂/Ce-CDs). The catalytic performance of this composite under light irradiation was optimized by leveraging the effects of Ce-doped carbon quantum dots on the energy band structure.

## 2. Results and Discussion

### 2.1. Synthesis and Characterization of Ce-CDs

In this study, the amount of cerium doping in the carbon quantum dots was varied by adjusting the proportion of cerium nitrate hexahydrate in the precursor. The six resulting samples with different doping levels were named 0Ce-CDs, 0.25Ce-CDs, 0.5Ce-CDs, 1Ce-CDs, 2Ce-CDs, and 4Ce-CDs. According to the quantitative analysis from XPS, the Ce content in these samples accounts for 0, 0.03%, 0.06%, 0.1%, 3.14%, and 6.29% of the total elemental composition, respectively.

**Figure 1** presents the X-ray diffraction patterns for each sample. All samples exhibited broad diffraction peaks at 28° and 42°, corresponding to the (002) and (100) planes of graphitic carbon, respectively.[29] Even in the 4Ce-CDs sample, no other crystalline phases were detected. **Figure 2a–c** presents high-resolution TEM (HRTEM) images of the 0Ce-CDs, 0.5Ce-CDs, and 4Ce-CDs samples, all displaying distinct lattice fringes. HRTEM reveals that the carbon dots are ≈5 nm in size. The size distributions of the carbon dots from the three samples are displayed below the figures. The inset Fourier transform (FFT) diagrams show broad diffraction rings corresponding to the (002) and (100) planes of graphite, consistent with XRD results. Figure 2d includes STEM-HAADF images of the sample 0Ce-CDs, revealing crystalline
<figure><img src="image_2.png" /><figcaption>**Figure 1.** XRD analysis results of carbon dots with doping different amounts of cerium doping.</figcaption></figure>

domains of ≈3–5 nm (marked with red circles). Small clusters (marked with yellow circles), measuring 1–2 nm, are also visible in these HAADF images but are less detectable in the HRTEM images. EELS mapping of carbon on unstained carbon quantum dots on TiO₂ suggests actual sizes closer to 1–2 nm (not shown here). The larger sizes observed in HRTEM may result from aggregation or contrast artifacts due to the staining. The distribution of Ce in CDs was investigated by using EELS Ce M-edge mapping, and the result is shown in Figure S1 (Supporting Information). It can be observed that in both 0.5Ce-CDs and 1Ce-CDs, Ce is randomly distributed throughout the dot, and no obvious agglomeration or precipitation is observed.

The specific surface area of TiO₂ and TiO₂/0.5Ce-CDs was analyzed using the nitrogen adsorption and desorption method. As shown in Figure S2 (Supporting Information), the nitrogen adsorption-desorption curve displays type IV isotherms with a type H3 hysteresis loop, indicating a mesoporous structure. Compared to TiO₂, the incorporation of 0.5Ce-CDs enhanced the nitrogen adsorption capacity. According to the calculations based on the Brunauer-Emmett-Teller equation, the specific surface area of TiO₂/0.5Ce-CDs was slightly greater as compared with TiO₂, measuring 93 and 88 m²/g, respectively.

The bonding structure of carbon dots was analyzed using FTIR spectroscopy. Carbon quantum dots derived from organic compounds, such as citric acid, generally contain oxygen-bearing functional groups, including carboxyl, carbonyl, hydroxyl, and epoxy groups. The formation of these functional groups often disrupts the graphitic structure. As the formation energy of these groups becomes higher than that of carbon-oxygen single bonds, they are more likely to be located at the edges of the carbon dot nanosheets. When nitrogen is doped into these carbon dots, various bonding types may occur, including pyridinic N, pyrrolic N, and graphitic N. Some of these bonds are also prone to form either in the core, potentially disrupting the graphitic structures, or at the surface of the dots. **Figure 3** displays the FTIR spectra for

each sample. In the carbon quantum dot samples, C–H bending vibration peaks are observed at 786 cm⁻¹, while stretching vibration peaks are found at 2874 cm⁻¹. The C–O–C stretching vibration peaks appear at 1050 and 1222 cm⁻¹. The stretching vibration peaks for C–N and C=N are located at 1389 and 1527 cm⁻¹, respectively. Additionally, the stretching vibration peaks for C=O are recorded at 1558 and 1657 cm⁻¹. The N–H bending and stretching vibration peaks are at 2361 and 3239 cm⁻¹, respectively, while the stretching vibration peak for O–H is found at 3418 cm⁻¹. As anticipated, the 0Ce-CDs exhibit a variety of clearly visible peaks due to the different bonding interactions involving oxygen, nitrogen, and hydrogen with carbon. In the samples 0.25Ce-CDs, 0.5Ce-CDs, and 1Ce-CDs, the signal intensity for the C=O and C=N bonds is significantly reduced. Notably, the spectra for the 2Ce-CDs and 4Ce-CDs samples differ from the others, showing a clear suppression of double bonds and an increased presence of single C–O bonds.

XPS measurements were conducted at the C and N 1s edges for the carbon quantum dots. As shown in **Figure 4a**, the C 1s spectrum of the non-doped carbon quantum dots (0Ce-CDs) exhibits three primary peaks, which can be roughly assigned to different types of carbon bonding: the graphitic carbon bond at 284.5 eV, C–O/C–N single bonds ≈286 eV, and C=O/C=N double bonds at 288 eV. Before cerium doping, the signature of the graphitic carbon was weak, indicating a significant presence of nitrogen and oxygen-related defects within the carbon network, including in the core of the dots, which disrupted the C–C bonding. After doping with cerium, a stronger graphitic carbon signature is observed, while the signals associated with oxygen and carbon defects decrease. At higher concentrations of cerium, the peaks corresponding to the C=O and C=N double bonds are notably reduced (see Figure 4b).

In the case of the N 1s spectrum, as shown in Figure 4c, an intense pyridinic signature is measured for the 0Ce-CDs. As the carbon dots are doped with cerium, this pyridinic signature diminishes and is replaced by a more pyrrolic-type spectral signature. These data demonstrate the role of cerium and the structural evolution of the carbon dots. In the absence of cerium dopant, nitrogen and oxygen-related defects are prevalent, disrupting the carbon networks and indicating a strong presence of double bonds at the core of the dots. However, when the dots are synthesized in the presence of cerium, the carbon networks recover quickly, highlighting the catalytic role of cerium. At higher concentrations of cerium, there is a preferential reduction of C=O and C=N double bonds, as evidenced by XPS and FTIR measurements. A similar catalytic effect has been reported for iron atoms during the synthesis of carbon quantum dots, which resulted in the disappearance of pyridinic nitrogens.[30]

The evolution of cerium valence also indicates its interaction with the CDs, notably with a higher concentration of Ce⁴⁺ for lightly doped carbon dots, as demonstrated by the XAS Ce-M spectra (Figure S3, Supporting Information). At low doping levels, the cerium atoms likely interact directly with the pyridinic nitrogen in the carbon dots, which contributes to the decreased amounts of this nitrogen. Similar preferential bonding of metals to pyridinic nitrogen in nitrogen-doped graphene[31] and in carbon dots[32] has been observed. This interaction is thought to involve highly coordinated cerium single atoms, which facilitate strong charge transfer from ceria to the multiple
<figure><img src="image_3.png" /><figcaption>**Figure 2.** HRTEM images of sample a) 0 Ce-CDs, b) 0.5Ce-CDs, and c) 4Ce-CDs. FFT diagrams obtained from the dots are demonstrated in the insert. (d) STEM-HAADF image of sample 0 Ce-CDs.</figcaption></figure>

ligands and correlate with the high cerium valence observed at low concentrations. Conversely, at higher doping levels, Ce³⁺ may form due to different bonding interactions with the carbon quantum dots, potentially leading to the formation of CeOₓ clusters.

UPS and LEIPS were used to investigate the energy band structure of samples 0Ce-CDs, 0.5Ce-CDs, and 1Ce-CDs, specifically near the energy band gap.[33] The results are illustrated in Figure 5. All samples exhibit a band gap of ≈3 eV, with the conduction band at ≈−3.2 eV and the valence band at ≈−6.2 eV. This indicates that the carbon quantum dots are less than 2 nm in size.[34] The primary difference among the samples lies in the position of the Fermi level. Sample 0.5Ce-CDs exhibit the lowest electron doping, likely due to reduced defects in the carbon networks of their cores and the absence of a substantial amount of immobilized cerium atoms.

## 2.2. Optical Properties of Ce-CDs

**Figure 6a** displays the UV–Vis absorption spectra of each sample within the wavelength range of 200–550 nm. A characteristic ab-

sorption peak at 251 nm results from the π-π* transition within the graphite network, while an absorption peak at 350 nm is attributed to the n-π* transition of carbonyl functional groups.[35] The C≡N signature often appears at lower energy, particularly when associated with surface defects. However, in the case of substantial nitrogen doping, as it is the case there, it is possible for pyridine bonds to form clusters at the core of the dots. These clusters typically consist of trimer or tetramer pyridine N accompanied by neighboring carbon vacancies.[31] For such structures, the absorption bands occur at higher energy and may overlap with the carbonyl signatures.[36] As the amount of Ce doping increases, the intensity of both the π-π* and n-π* absorption peaks gradually declines, particularly the n-π* absorption peak associated with O and N defects in the core of the carbon quantum dots. This observation aligns with the decrease in nitrogen- and oxygen-containing functional groups noted in previous FTIR and XPS analyses.

**Figure 6b** displays 2D plots of PL emission intensities for various excitation and emission wavelengths of the 0Ce-CDs sample. The plots for the other samples can be found in Figure S4 (Supporting Information). The PL of the carbon quantum dots is
<figure><img src="image_4.png" /><figcaption>**Figure 3.** FTIR spectra of carbon dots doped with different concentrations of cerium.</figcaption></figure>

notably significant for excitation wavelengths ranging from 230 to 260 nm and from 280 to 400 nm, while the emission spectra are observed in the wavelength range of 370–700 nm. Each PL spectrum collected at a specific excitation wavelength is characterized by a broad, structureless band. The maximum intensity of the PL occurs at an excitation wavelength of 360 nm, with the peak emission observed ≈441 nm. Notably, the location of this peak is independent of the excitation wavelength.

Figure 6c compares the PL spectra of carbon quantum dots with varying concentrations of cerium, excited by radiation at a wavelength of 360 nm. First, it is observed that the peak shifts slightly toward the red region in the presence of cerium. Second, the PL intensity varies with different concentrations of cerium. The degree of PL enhancement or quenching is evaluated using the ratio F/F₀, where F represents the integral PL intensity of the carbon quantum dots with cerium doping, and F₀ represents the integral intensity of the carbon quantum dots without cerium. The inset figure shows that the emission intensity gradually increases for the 0.25Ce-CDs and 0.5Ce-CDs. However, for the 2Ce-CDs, PL quenching occurs. Furthermore, it is noted that in the case of the 0Ce-CDs, the spectra of the cerium-doped carbon quantum dots are broad and asymmetric, which may be attributed to multiple emission modes. The PL spectra of carbon dots can be fitted using two Gaussian functions, as illustrated in the lower part of Figure 6c. Peaks A and B correspond to PL emissions from the core, and similar dual emission modes have been linked to the presence of different carbon dot sizes.[37] A comparison of the peak positions for A and B in carbon quantum dots with varying cerium concentrations is shown in Figure 6d. Almost no shift was observed, confirming that there is no significant change in the energy band gap or size of the carbon quantum dots due to cerium doping. Importantly, in all cases, no emission associated with surface states was observed.

The effect of adding cerium during the synthesis of carbon dots on their optical properties can be understood through their structural evolution. At low concentrations, significantly below one cerium atom per carbon dot, the observed enhancement in PL is attributed to the structures of the carbon quantum dots.

XPS and FTIR results indicate a substantial reduction in defects, particularly a decrease in double bonding within the core of the carbon quantum dots, which is linked to the catalytic effect of cerium. Previous research has shown that different chemical forms of nitrogen can influence PL intensity. Specifically, pyrrolic bonding enhances photoluminescence, while pyridinic nitrogen tends to inhibit it.[30] This aligns with the structural changes observed in the carbon quantum dots as the amount of cerium varies. In contrast, a quenching effect may occur at higher concentrations, where multiple cerium atoms are bonded to the carbon dots. Various metal cations, such as Co²⁺, Cu²⁺, Mg²⁺, Ni²⁺, Pb²⁺, Zn²⁺, Al³⁺, Cr³⁺, and Fe³⁺, are known to quench the PL of carbon quantum dots through different mechanisms, including alterations to surface groups or dynamic quenching.[38] Notably, iron has been observed to significantly reduce the PL of nitrogen-doped carbon quantum dots, primarily by coordinating with carboxyl groups on the carbon quantum dot surface. This quenching is believed to originate from excited-state electron transfer between Fe³⁺ ions and carbon quantum dots when carboxylic acid forms complexes with Fe³⁺.[39] A similar electron transfer mechanism may also be present with the redox-active cerium atoms. Moreover, previous measurements of the optical properties and energy band structure have indicated that cerium doping increases the electron concentration within the dots. The increase in electron concentration could lead to reduced light absorption and, consequently, a decrease in photoluminescence.[29,40]

## 2.3. Structure and Optical Properties of TiO₂/Ce-CDs Composites

The results showed that the structure and optical properties of carbon quantum dots can be altered by doping with a small amount of cerium. To investigate the effect of different types of carbon quantum dots on the photocatalytic performance, various samples, namely, 0Ce-CDs, 0.25Ce-CDs, 0.5Ce-CDs, 1Ce-CDs, and 2Ce-CDs, were mixed with TiO₂ to form composites. Figure S5 (Supporting Information) displays the XRD results of these composites, which were validated by comparison with the anatase titanium dioxide standard (JCPDS 21-1272). The diffraction peaks observed in each sample corresponded to the anatase phase, and no additional crystalline phases were detected. Furthermore, the half-height width of the TiO₂ diffraction peak remained unchanged after mixing with the carbon dots, indicating that the formation of the composite structure does not affect the crystallinity of TiO₂. Additionally, Ti 2p XPS measurements were conducted (as shown in Figure S6, Supporting Information), and no significant differences were found between pristine TiO₂ and the composites.

The dispersion of carbon quantum dots among TiO₂ particles was investigated by utilizing STEM with HAADF imaging and EELS at angstrom resolution. The composites TiO₂/1Ce-CDs and TiO₂/2Ce-CDs were selected. As illustrated in Figure 7a for the TiO₂/2Ce-CDs sample, the crystallized TiO₂ particles are distinctly visible, showcasing several bright spots per particle, some of which are located on their surfaces. EELS maps are presented in Figure 7b. The HAADF images obtained alongside the EELS maps also reveal areas with bright spots. The spectra compare the C-K, N-K, Ti-L, O-K, and Ce-M edges,
<figure><img src="image_5.png" /><figcaption>**Figure 4.** XPS spectra of carbon dots doped with different concentrations of cerium. a) C 1s, b) intensity ratio of C=C, C-O/C-N, and C=O/C=N bonds of each dot, c) N 1s, and d) intensity ratio of pyridinic, pyrolic, and oxygen-related defects of each dot.</figcaption></figure>

integrating over the entire spectrum image or by selectively ex-
amining areas with and without bright spots in the HAADF
image (indicated by red and blue boxes, respectively). It is ev-
ident that the areas with bright spots exhibit an enrichment
in C, N, and Ce signals, indicating that these regions con-
tain cerium-bearing carbon dots attached to the anatase surface.
Figure 7c compares the Ti and C maps obtained from EELS
with the HAADF image, showing that the carbon dot size typi-
cally ranged from 1 to 2 nm, with considerable coverage on the

TiO₂ particles. Additionally, a few carbon quantum dots are vis-
ible attached to a TiO₂ crystal in the upper-right region (indi-
cated by the green box). The HAADF image reveals four bright
spots, and when EELS spectra are summed from these spots, an
intense Ce-M edge appears, confirming the presence of cerium
atoms in most aggregates of carbon quantum dots for this
sample.

In the TiO₂/1Ce-CDs sample, STEM-EELS analysis reveals the
presence of carbon quantum dots with spectral signatures of

<figure><img src="image_6.png" /><figcaption>**Figure 5.** UPS and LEIPS spectra of samples a) 0Ce-CDs, b) 0.5Ce-CDs, and c) 1Ce-CDs.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Figure 6.** a) UV–Vis absorption spectra of carbon dots doped with different concentrations of cerium within the wavelength range of 200 nm to 550 nm. b) 2D plots of PL emission intensities for various excitation and emission wavelengths of the 0Ce-CDs sample. c) PL spectra of carbon dots with varying concentrations of cerium, excited by radiation at a wavelength of 360 nm. The inset figure shows the variation of F/F₀ with Ce concentration. d) Peak positions for A and B in carbon dots with varying cerium concentrations.</figcaption></figure>

carbon and nitrogen. These dots are typically smaller than 2 nm and exhibit a high coverage (see Figure S7, Supporting Information), confirming their attachment to the TiO₂ surfaces. In this sample, cerium atoms are distributed and diluted (refer to Figure S8, Supporting Information), and never have two cerium atoms been seen simultaneously within a field of view of ≈10 by 10 nm. This observation supports the distinction between the two doping regimes: the 1Ce-CDs contain significantly less than one cerium atom per carbon quantum dot, while the 2Ce-CDs are likely to contain several.

The UV–visible light absorption spectra of the composites are illustrated in Figure S9a (Supporting Information). Pristine TiO₂ does not absorb light with wavelengths longer than 400 nm. However, upon mixing with carbon quantum dots, the absorption in the wavelength range of 400–600 nm increases. The luminescence characteristics of the composites were analyzed using 2D plots of PL intensities for various emission and excitation wavelengths. Composite TiO₂/0.5Ce-CDs was selected as an example, and this data is presented in Figure S9b (Supporting Information). While PL at 360 nm is observable, the intensity for excitation at 260 nm is suppressed compared to that of the pure carbon quantum dots, which is expected since it exceeds the energy gap of anatase.

Figure S9c (Supporting Information) compares the PL spectrum among the composites at an excitation wavelength of 360 nm. Notably, the intensity varies with the concentration of Ce. The relationship between intensity and Ce content is shown in Figure S9d (Supporting Information), which indicates that the PL intensity was enhanced by 1.5 times in composite TiO₂/0.25Ce-CDs, then the PL intensity gradually decreases as Ce concentration increases. Additionally, the PL intensity of the composite is lower than that of the raw carbon quantum dots, despite maintaining the same amount of carbon quantum dots in each composite, indicating a quenching effect in the composites. Compared to the spectra obtained from raw carbon quantum dots, the emission spectra are broad and asymmetric at a fixed excitation wavelength and can be decomposed into several modes. Along with the bulk contributions (peaks A and B), which are XPS nearly identical to those of the pure carbon quantum dots, a small contribution from surface states (peak SS) is discernible. The positions of the three peaks remain consistent across the composites, as depicted in Figure S9e (Supporting Information). Finally, except for the TiO₂/0.25Ce-CDs composite, the intensity of all three peaks decreases as Ce content increases, as illustrated in Figure S9f (Supporting Information), indicating that there is no preferential quenching of any mode.
<figure><img src="image_8.png" /><figcaption>**Figure 7.** a) Several HAADF-STEM images of the TiO₂/2Ce-CDs composite, showing visible TiO₂ nanoparticles along with some brighter individual spots corresponding to cerium atoms. b) HAADF-STEM images of the TiO₂/2Ce-CDs and the corresponding EELS spectra, extracted respectively from the entire spectrum images, the red box, and the blue box (top to bottom). The EELS spectra reveal strong C, N, and Ce signatures at the positions of the red box, indicating the presence of Ce doped CDs there. c) EELS Ti-L and C-K maps of TiO₂/2Ce-CDs, alongside the HAADF-STEM image. The carbon maps highlight features smaller than 2 nm, revealing the size of the CDs. The EELS spectra compare the Ti-L and Ce-M edges for regions corresponding to the three red boxes in the inset image and areas outside these boxes. By comparing the distributions of C, Ti, and Ce in the spectral data and the HAADF image, the area in the green box (or the inset) is estimated to contain approximately three CDs and four cerium atoms.</figcaption></figure>

The separation properties of photogenerated electrons and holes in different materials were studied by analyzing the transient photocurrent response, as illustrated in Figure S10 (Supporting Information). During on-off cycles of visible light irradiation ($\lambda \ge 400$ nm), TiO₂ exhibited a weak photocurrent response, with a density value of 0.001 µA cm⁻². In contrast, the TiO₂/0.5Ce-CDs composite catalyst demonstrated significant improvements over the single TiO₂ material, with the photocurrent density increasing to 0.0025 µA cm⁻². This indicates that the presence of 0.5Ce-CDs enhances the separation of photogenerated charge carriers. The enhanced separation of charge carriers in heterojunctions has been demonstrated in various systems.[41,42]

## 2.4. Photocatalytic Performance of TiO₂/Ce-CDs Composites

The photocatalytic performance was evaluated using methyl orange (MO) as the dye. Figure 8a illustrates the relationship between

the concentration of MO (C/C₀) and the irradiation time under UV light. It can be observed that within the first 30 min in the dark, an adsorption-desorption equilibrium is established between the photocatalyst and MO. No significant adsorption occurs with carbon dots alone. Additionally, in the composites formed by carbon dots doped with varying concentrations of cerium, the adsorption capacity remains similar, at ≈10%. Blank tests indicate that the MO molecules demonstrate good stability both in the dark and under UV light when no photocatalyst is present. When carbon dots (0Ce-CDs) form a complex with TiO₂, their degradation ability under UV light decreases significantly compared to pure TiO₂. As the concentration of cerium doping in the carbon quantum dots increases, the degradation efficiency declines even further.

In contrast, under simulated sunlight irradiation, neither the raw TiO₂ nor the carbon quantum dots exhibit any photocatalytic activity. However, the degradation ability of the TiO₂/CDs composites shows significant improvement, as indicated in
<figure><img src="image_9.png" /><figcaption>**Figure 8.** Photocatalytic performances of TiO₂, CDs, and composites under a) UV light and b) visible light. c) Rate constant K. d) The cycling test, effect of e) pH value, and f) ions (0.1 M) on the degradation efficiency of sample TiO₂/0.5Ce-CDs.</figcaption></figure>

Figure 8b. The reaction rate constant was calculated using the equation -ln(C/C₀) = K × t, and the results are presented in Figure 8c. For the sample TiO₂/0.5Ce-CDs, the K value reaches 0.087 min⁻¹, indicating an enhancement by almost a factor of ten compared to pure TiO₂. Such degradation efficiency is higher than that of most composites formed by TiO₂ and carbon compounds.[43-47] To assess the degree of mineralization during the photocatalytic degradation process, total organic carbon (TOC) analysis was performed using the TiO₂/0.5Ce-CDs photocatalyst. As shown in the Figure S11 (Supporting Information), the TOC removal profile of the MO solution treated with TiO₂/0.5Ce-CDs photocatalyst exhibited a gradual decrease. After 50 min of photocatalytic reaction, a TOC removal efficiency of ≈35% was achieved, indicating a moderate extent of mineralization.

Cycling experiments were conducted to evaluate the photo-
stability of the TiO₂/0.5Ce-CDs (Figure 8d). After seven runs,
the catalytic efficiency of the TiO₂/0.5Ce-CDs remained high at
82.2%, with only a slight decrease in activity of 17.8%. This con-
firms the composites' reusability. The changes in the functional
groups on the material surface after the reaction were detected
by XPS. The results have been added as Figure S12 (Support-
ing Information). Compared with the samples without photocat-
alytic reaction, it can be found that the spectra have no obvious
changes, indicating that the functional groups have not changed
significantly. The effect of pH on the photocatalytic properties of
TiO₂/0.5Ce-CDs was also evaluated (see Figure 8e). At pH values
of 3, 5, 7, 9, and 11, the TiO₂/0.5Ce-CDs were able to reduce the
concentration of MO by 99.1%, 100%, 100%, 99.6%, and 85.5%,
respectively. This indicates that TiO₂/0.5Ce-CDs operate effec-
tively across a broad pH range. Furthermore, several common

anions, including NO₃⁻, SO₄²⁻, PO₄³⁻, and Cl⁻, were introduced
into the TiO₂/Ce-CDs system to evaluate their impacts on the cat-
alytic outcome (Figure 8f). Cl⁻, NO₃⁻, and SO₄²⁻ showed no no-
ticeable effect on the overall elimination rate, while PO₄³⁻ anions
exhibited a slight inhibitory effect. This may be due to the trap-
ping of reactive oxygen species (ROS) or the generation of less
active species.[48]

To investigate the mechanism of dye degradation, we selected
sample TiO₂/0.5Ce-CDs for radical capturing trials. This was
done to identify the active species involved in the photocatalytic
reaction under simulated sunlight irradiation. As illustrated in
**Figure 9a,b**, the degradation of MO occurred within 40 min with-
out the addition of any scavengers. The degradation performance
remained similar when silver nitrate, which quenches electrons,
and EDTA, which quenches holes, were added. However, the
addition of isopropanol (IPA), a quencher of hydroxyl radicals
(●OH), and para-benzoquinone (p-BQ), a quencher of superoxide
(O₂●−), significantly reduced the degradation rates to 19% and
26%, respectively. Therefore, it can be concluded that ●OH and
O₂●− are the primary active species responsible for dye degrada-
tion under simulated sunlight irradiation.

Figure S13a,b (Supporting Information) show the EPR spec-
tra of DMPO•OH measured in aqueous and DMPO- O₂•-
measured in methanolic suspension, respectively. It is seen that
all samples did not show an EPR signal under dark condi-
tions. Under solar irradiation, six characteristic peaks belonging
to the DMPO-superoxide radical (O₂•−) were detected for the
pure TiO₂ sample (Figure S13a, Supporting Information). With
the presence of TiO₂/0.5Ce-CDs, the intensity of the DMPO-
O₂•− signals increased, which implies that a larger amount
of O₂•− is generated in TiO₂/0.5Ce-CDs than in pure TiO₂.
<figure><img src="image_10.png" /><figcaption>**Figure 9.** a) The scavenger experiment of sample TiO₂/0.5Ce-CDs. AgNO₃, benzoquinone (p-BQ), Ethylenediaminetetraacetic acid (EDTA) and iso-propanol (IPA) were applied as the scavengers of e⁻, O₂·⁻, h⁺ and ●OH. b) A comparison of the degradation with the presence of different scavengers. c) Schematic illustration of the charge transfer mechanism in the composite under visible light irradiation.</figcaption></figure>

From Figure S13b (Supporting Information), the intensity of the characteristic signals for the DMPO-hydroxyl radical (•OH) in TiO₂/0.5Ce-CDs was found to be stronger than that in pure TiO₂. The results prove that both the O₂•⁻ and •OH radicals are the main reactive species. It is consistent with the results of active species trapping experiments.

By comparing the energetic diagrams of TiO₂ (see Figure S14, Supporting Information) and carbon quantum dots (see Figure 5), the degradation mechanism of the composite under simulated sunlight irradiation can be interpreted. As illustrated in Figure 9c, the carbon quantum dots have a smaller bandgap than TiO₂, which extends their optical absorption range. However, the bottom of the conduction band of the carbon quantum dots is still at a higher energy level than that of TiO₂. When carbon quantum dots are adsorbed onto the surface of TiO₂ particles, their smaller energy gap allows electrons to be excited primarily within the carbon quantum dots by longer wavelengths of light. These excited electrons are then transferred to the conduction band of TiO₂, where they move to the surface and react with adsorbed oxygen molecules. Simultaneously, the holes in the carbon quantum dots migrate to their surface to react with water molecules. During this process, the main active species generated include hydroxyl radicals (•OH) and superoxide radicals (O₂•⁻), which are consistent with experimental findings involving scavenger addition.

The differences in the photocatalytic performance of the composites can be attributed to the structural evolution of carbon quantum dots caused by Ce doping. At low doping concentrations, defects are drastically reduced. At higher concentrations, multiple Ce atoms were bound to the carbon quantum dots. In addition, Ce-doping increases the electron concentration within the carbon quantum dots. The changes in the defect content and electron concentration change the Fermi level of the carbon dots. Therefore, when carbon dots form a complex with TiO₂, the Fermi levels of carbon dots doped with different concentrations of Ce are different, which affects the electric field formed at the interface and the charge separation effect, resulting in differences in photodegradation performance. Accordingly, the enhanced performance of the TiO₂/0.5Ce-CDs composite may be attributed to the lower Fermi energy of the 0.5Ce-CDs, resulting in a stronger interfacial electric field that

effectively separates charges when the interface with anatase is formed.

### 3. Conclusion

This study synthesized carbon quantum dots (CDs) through the hydrothermal decomposition of organic compounds using varying amounts of cerium nitrate hexahydrate. Structural analyses showed that lower cerium concentrations resulted in undoped carbon quantum dots, while higher concentrations successfully integrated cerium into the dots and aggregates. Cerium acted as a catalyst during synthesis, improving the carbon dots' structure by reducing C=O and C=N double bonds and strengthening graphitic networks, as confirmed by XPS and FTIR measurements. The PL of the Ce-CDs peaked ≈440 nm, indicating sizes below 2 nm. UPS and LEIPS suggested similar band gap energies but different Fermi levels among the Ce-CDs. Subsequently, TiO₂/Ce-CDs composites were developed, demonstrating strong optical properties and improved photocatalytic performance linked to the structural modifications in the Ce-CDs. The composites showed enhanced photocatalytic activity for MO degradation under visible light, primarily due to better charge separation of electron-hole pairs at the TiO₂/Ce-CDs interface. The optimal performance was observed in the TiO₂/0.5Ce-CDs composite, which had minimal defects and effective cerium doping, leading to efficient charge separation.

### 4. Experimental Section

*Preparation of Carbon Quantum Dots (CDs) and Cerium-Doped CDs (Ce-CDs):* First, 1 g of citric acid (C₆H₈O₇, 99%, Sigma-Aldrich) was dissolved in 20 mL of deionized water. Then, 0.5 mL of ethylenediamine (C₂H₈N₂, > = 99%, Aldrich) was added, and the solution was stirred at 600 rpm for 10 min. Next, the mixed solution was heated in an autoclave at 200 °C for 2 h, and carbon dots were obtained after the solution had cooled to room temperature. In another approach to obtain cerium-doped carbon quantum dots, 1 g of citric acid was dissolved in 20 mL of deionized water. Cerium (III) nitrate hexahydrate (Ce(NO₃)₃·6H₂O, 99.5%, Acros Organics BVBA) was then added in varying
amounts: 0 g, 0.25 g, 0.5 g, 1 g, 2 g, and 4 g. After this, 0.5 mL of ethylenediamine was incorporated into the solution. The mixture was stirred for 30 min before being heated in an autoclave reactor at 200 °C for 2 h. Once the solution cooled to room temperature, it was centrifuged to obtain the cerium-doped carbon dots.

**Preparation of TiO₂ Nanoparticles:** At first, an amorphous SiO₂ sphere was synthesized via the sol-gel method. Anhydrous ethanol (C₂H₅OH, > = 99.8%, Honeywell), ammonium hydroxide solution (NH₄OH, 28.0–30.0%, J. T. Baker), and deionized water were thoroughly mixed to prepare Solution A. Separately, anhydrous ethanol and tetraethoxysilane (Si(OC₂H₅)₄, 98%, Acros Organics BVBA) were mixed to form Solution B. After both solutions were individually and homogeneously mixed, the reaction was carried out under a controlled temperature of 30 °C. The addition of Solution B into Solution A was performed in two distinct stages: first, 9 mL of Solution B was added dropwise at a rate of 0.3 mL min⁻¹ to initiate nucleation. This was followed by the addition of an additional 18 mL of Solution B at a rate of 1 mL min⁻¹ to promote particle growth. Upon completion of the addition process, the resulting mixture was continuously stirred for 24 h, yielding monodisperse silica spheres. Then, 0.1 g of the SiO₂ sphere was mixed with 100 mL of anhydrous ethanol, 0.5 mL of ammonium hydroxide solution, and 0.53 mL of titanium (IV) isopropoxide (Ti[OCH(CH₃)₂]₄, 98%, Acros Organics BVBA) and stirred for 24 h at 45 °C. Next, 0.03 g of the obtained product was dispersed in a solution of 10 mL of sodium hydroxide (NaOH, 90–100%, J. T. Baker) and 1 mL of deionized water to remove the SiO₂ core, followed by treatment with 10 mL of hydrochloric acid (HCl, 36.5–38.0%, J. T. Baker). Finally, the material was annealed at 800 °C to yield TiO₂ nanoparticles.

**Synthesis of TiO₂/Ce-CDs Composites:** To synthesize TiO₂/CDs and TiO₂/Ce-CDs composites, 20 µL of either CDs or Ce-CDs were mixed with 10 mL of deionized water and 15 mg of TiO₂ particles. The mixture was then ultrasonically dispersed for 30 min. After dispersion, centrifugal separation was performed, and the resulting material was washed with ethanol and deionized water. Finally, the material was dried in an oven at 80 °C to obtain the TiO₂/CDs or TiO₂/Ce-CDs composites.

**Characterization:** The crystal structure of all samples was characterized using an X-ray powder diffractometer (XRD) with Cu Kα radiation (Bruker D2 PHASER XE-T XRD). Raman analysis was performed with a micro-Raman spectrometer (HORIBA iHR550) to investigate the structural features. Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) utilizing monochromatic Al Kα radiation (ULVAC-PHI, Inc./PHI 5000 Versa Probe III) were applied to analyze the chemical states of the elements. X-ray absorption spectroscopy (XAS) measurements were performed at the National Synchrotron Radiation Research Center (NSRRC) in Hsinchu, Taiwan. The spectra were collected with a monochromatic X-ray beam. Data acquisition and normalization were conducted to analyze the local electronic and coordination structure of the elements. The morphologies and elemental mapping were examined using transmission electron microscopy (TEM) with an accelerating voltage of 200 keV (FEI Tecnai™ G2 F-20 S-TWIN, Talos F200X G2). Scanning transmission electron microscopy (STEM) was conducted with a Cs-corrected Nion STEM microscope operating at 100 keV, achieving a typical probe size of 0.1

nm. High-angle annular dark field (HAADF) imaging was performed by selecting electrons scattered from 80 to 200 mrad. STEM-electron energy loss spectroscopy (EELS) was executed using a MerlinEM-Medipix3 detector, which provided high sensitivity, allowing the detection of down to a single cerium (Ce) atom via the measurement of the Ce-M edge EELS signature. For the STEM and STEM experiments, CDs and Ce-CDs were stained with uranyl acetate and deposited onto a continuous carbon film. The TiO₂/CDs and TiO₂/Ce-CDs composites were left unstained and deposited onto a lacey carbon grid. In this case, the self-supported samples were observed by STEM to isolate the EELS carbon signatures solely from the CDs and Ce-CDs. The UV-vis was measured at room temperature with JASCO UV-Vis/NIR spectrophotometer, covering a wavelength range from 190 to 2700 nm and equipped with an integrating sphere. The photoluminescence (PL) spectra were obtained by Jasco FP-6300 spectrofluorometer.

Photocurrent measurements were performed under visible light illumination using a 350 W xenon lamp. The TiO₂/CDs and TiO₂/Ce-CDs thin flake were fabricated from TiO₂/CDs and TiO₂/Ce-CDs composites using a conventional manual pressing method with a digital hydraulic press. For the construction of a photoconductor-type photodetector, the compressed thin flake was transferred onto a SiO₂ (300 nm)/n⁺-Si substrate. The substrate was subsequently mounted on a mica sheet using an adhesive. Silver (Ag) paste was employed as the electrode material, and enameled copper wires were connected to the silver contacts, serving as the electrical bridge between the thin flake and the external measurement system. The measurements were conducted at room temperature using a four-point probe electrical characterization system (Keithley 4200-SCS), with periodic light ON-OFF switching. The light intensity at the surface of the sample was measured with a handheld optical power meter, and the corresponding optical power density was determined to be 0.4 W cm⁻².

Ultraviolet photoemission spectroscopy (UPS) measurements were conducted using the VersaProbe III from ULVAC-PHI, Inc. This technique measures the kinetic energy of photoelectrons emitted from solid surfaces when they are irradiated with ultraviolet light (He I at 21.2 eV) in an ultrahigh vacuum environment, which is connected to a hemispherical energy analyzer. To prepare the sample, the powder was dispersed on a thin indium foil and fixed in place by pressing. A nitrogen gun was used to ensure there was no excess powder. The prepared sample was then secured on the stage with copper tape and covered with a mask to enhance conductivity for the UPS measurement. The work function (WF) was calculated by analyzing the high binding energy cut-off from the secondary electron contribution of the He I energy, using a -5 V bias applied during the spectrum acquisition. The valence band maximum (VBM) was identified from the onset of low binding energy. By combining the VBM obtained through UPS with the band gap (E_g) values derived from UV-vis absorption spectroscopy, the conduction band minimum (CBM) levels were deduced.

Low Energy Inverse Photoemission Spectroscopy (LEIPS) was used to measure the conduction band. The sample bias was ramped while a 5 eV incident electron beam was maintained, and the emitted ultraviolet or visible light was collected by selecting a suitable band-pass filter. By measuring this emitted light, the Bremsstrahlung Isochromat (BIC) mode provides
information on the unoccupied electronic states of the sample.
The low energy electron transmission (LEET) was measured to
the start point of the incident electron.

**Photocatalytic Activity:** Methylene Orange (MO) at a concentration of $10^{-5}$ M was used to evaluate the performance of the photocatalyst. Samples of 50 mg were mixed with 50 mL of the MO solution and stirred for 30 min in the dark. The samples were then illuminated using either UV or visible light. For UV light irradiation, a 400 W mercury lamp with a maximum wavelength of 365 nm was employed as the light source. For visible light irradiation, a 300 W sunlight-simulated lamp was used, producing radiation wavelengths ranging from 380 to 780 nm. The optical power density at the surface of the reactor was measured using a handheld optical power meter, yielding an optical power density of 0.93 W cm$^{-2}$. Every 10 min during UV light exposure and every 20 min during visible light exposure, ≈3 mL of the mixed solution was extracted and filtered for absorbance analysis. The absorbance of the MO was measured using a UV-Vis/NIR spectrophotometer. After each use, the photocatalyst was separated from the reaction mixture by centrifugation using a HERMLE centrifuge (Model Z206A) at 6000 rpm for 10 min. The solid photocatalyst was carefully collected and subsequently reintroduced into the photoreactor for subsequent photocatalytic reactions.

An Electronic Paramagnetic Resonance (EPR) spectrum was recorded using a Bruker EPR-plus spectrometer, with ≈0.003 g of sample utilized for each measurement. The settings for the EPR spectrometer were as follows: center field at 3000.00 G, modulation frequency at 100.00 kHz, microwave frequency at 9.84 GHz, and power set to 17.36 mW. A scavenger experiment was conducted to identify the main reactive species involved in the photodegradation process. Silver nitrate (AgNO$_3$, 100 mM), benzoquinone (p-BQ, 1 mM), ethylenediaminetetraacetic acid (EDTA, 10 mM), and isopropanol (IPA, in a 1:20 volume ratio) were employed as scavengers for electrons (e'), superoxide anions ($O_2^-$), holes ($h^+$), and hydroxyl radicals (●OH). To capture the different species, 1 mL of the scavenger was added to the dye solution.

## Acknowledgements

This work was supported by the National Science and Technology Council of Taiwan under Grants 113-2112-M-011-002, the "Sustainable Electrochemical Energy Development Center (SEED)" from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan, and the STEM experiments were supported by the ANR-23-CE42-0027 grant. We would thanks for the supporting of photocurrent measurement from Prof. Ruei-San Chen of Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology.

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