Facile Fabrication of SrTiO 3 /In 2 O 3 on Carbon Fibers via a Self-Assembly Strategy for Enhanced Photocatalytic Hydrogen Production

: Photocatalytic water splitting by semiconductors is considered a promising and cost-effective method for achieving sustainable hydrogen production. In this study, a CF/SrTiO 3 /In 2 O 3 photocatalytic material with a double-layer core–shell structure was developed. The experimental results indicated that the produced CF/SrTiO 3 /In 2 O 3 composite fiber displayed superior photo-catalytic hydrogen production performance, achieving a hydrogen evolution rate of approximately 320.71 µ mol/g · h, which is roughly seven times higher than that of the CF/SrTiO 3 fiber alone. The enhanced photocatalytic activity of the CF/SrTiO 3 /In 2 O 3 fiber can be attributed to the heterojunction structure enriched with oxygen vacancies. It was found that these oxygen vacancies created defective states that served as traps for photogenerated electrons, facilitating their migration to the surface defect states and enabling the reduction of H + in water to produce hydrogen. Furthermore, the synergy between the heterojunction structure and the conductivity of the carbon fiber promoted the generation and migration of photogenerated electrons, reduced the recombination of photogener-ated electron–hole pairs, and ultimately improved photocatalytic hydrogen production. This study presents a new approach for designing efficient photocatalysts with surface oxygen vacancies on carbon fibers, providing new insights into the sustainable application of photocatalysts.


Introduction
It is widely acknowledged that the ongoing consumption of non-renewable resources, such as coal, petroleum, and natural gas, is leading us toward a significant global energy shortage.In response to this foreseeable crisis, scientists around the world are actively exploring renewable energy sources as alternatives.Out of the many options available, hydrogen energy has emerged as a highly promising option.It offers significant advantages, including a high heating value, extensive availability, and the absence of secondary pollutants from its combustion.Additionally, hydrogen energy has the potential for efficient transportation and storage.
Since Fujishima and Honda first reported on photocatalytic water splitting in 1972, semiconductor photocatalytic water splitting has been identified as a viable and costeffective method for achieving sustainable hydrogen supply [1][2][3].SrTiO 3 , with its perovskite structure, is a material of interest for photocatalytic hydrogen evolution because of its excellent energy band position and great chemical stability.Nevertheless, the photocatalytic performance of pure SrTiO 3 is easily limited by the rapid recombination of photoinduced carriers, resulting in low photocatalytic efficiency and consequently impeding its practical application.In 2 O 3 , a significant N-type semiconductor, exhibits a band gap of approximately 2.8 eV and typically exists in a stable cubic crystal form with low resistivity and exceptional surface properties.It is an ideal material for photocatalytic applications and has seen widespread use in both photocatalytic sewage treatment and hydrogen evolution through photocatalytic water splitting [4].A multitude of studies have been conducted on the development of photocatalytic composite materials that incorporate In 2 O 3 .For example, Satyabadi et al. [5] modified ZnO with In 2 O 3 , and their experiments confirmed that this modification significantly enhanced the absorption ability of ZnO in the visible region, greatly reduced the particle size of ZnO, and delayed the recombination of carriers.In their study, all In 2 O 3 -modified ZnO samples exhibited high photocatalytic hydrogen evolution activity under visible light.Similarly, Liu et al. [6] successfully synthesized a hierarchical porous hydrangea-like In 2 S 3 /In 2 O 3 heterojunction structure using a simple in situ oxidation technique.Comparative studies demonstrated that this heterojunction structure substantially promoted photocatalytic hydrogen evolution compared to pure In 2 S 3 and In 2 O 3 .However, research on enhancing the photocatalytic hydrogen evolution performance of SrTiO 3 by modifying it with In 2 O 3 remains limited [7][8][9].
Carbon fiber (CF) is highly valued for its flexibility, corrosion resistance, and tensile strength, making it an ideal carrier material for photocatalysts.In addition, its excellent conductivity enhances the rapid transfer of photogenerated electrons, improving photocatalytic performance.In previous studies, we successfully synthesized CF/SrTiO 3 composite fiber materials and demonstrated that doping with Mn or incorporating CdS enhances light absorption and photogenerated electron transport, thereby improving the photocatalytic performance and facilitating the recyclability of the composite [10,11].In this work, CF/SrTiO 3 composite fiber material was employed as a substrate for the initial deposition of In seeds.Subsequently, a double-layer core-shell-structured CF/SrTiO 3 /In 2 O 3 photocatalytic composite fiber material was synthesized via a solvothermal method that enabled in situ growth.The enhancement mechanism of the CF/SrTiO 3 /In 2 O 3 composite fiber material in photocatalytic water-splitting hydrogen evolution was further discussed.

Preparation of CF/SrTiO 3 /In 2 O 3 Composite Material
The CF/SrTiO 3 composite fiber, featuring a core-shell heterojunction structure, was prepared using the method outlined in previous studies [10,11].Generally, this involves coating a SrTiO 3 nanolayer onto Tencel fibers via the solvothermal method at 180 • C for 7 h, followed by carbonizing the fibers at 800 • C for 2 h under nitrogen to form CF/SrTiO 3 .
CF/SrTiO 3 -In seed composite fibers were produced based on the seed growth method previously used for coating Bi 2 O 3 nanomaterial onto carbon fibers [12].Specifically, an InCl 3 /ethanol solution was prepared by dissolving 0.4 g of InCl 3 in 40 mL of ethanol, followed by 5 min of sonication and 30 min of stirring.The CF/SrTiO 3 composite fibers were immersed in this solution for 3 h, followed by a 3 h drying period, and subsequently subjected to heating in a muffle furnace at 300 • C for 6 min to obtain CF/SrTiO 3 -In seed composite fibers.CF/SrTiO 3 /In 2 O 3 composite fibers were then fabricated via the solvothermal method via in situ growth on the CF/SrTiO 3 -In seed composite fibers.In this process, 0.33 g of InNO 3 was added into a mixed solution of 15 mL of ethanol and 20 mL of glycol, and the mixture was stirred for 30 min to obtain a homogenous solution.The abovementioned CF/SrTiO 3 -In seed composite fibers were added to the mixture and transferred into a polytetrafluoroethylene reactor for solvothermal reaction at 180 • C for 20 h [13,14].Afterward, the composite was cleaned with absolute ethyl alcohol and dried in an oven at 60 • C for 24 h, resulting in the formation of CF/SrTiO 3 /In 2 O 3 composite fibers.The detailed preparation process is demonstrated in Figure 1.
cally, an InCl3/ethanol solution was prepared by dissolving 0.4 g of InCl3 in 40 mL o anol, followed by 5 min of sonication and 30 min of stirring.The CF/SrTiO3 com fibers were immersed in this solution for 3 h, followed by a 3 h drying period, and s quently subjected to heating in a muffle furnace at 300 °C for 6 min to obtain CF/Sr In seed composite fibers.
CF/SrTiO3/In2O3 composite fibers were then fabricated via the solvothermal m via in situ growth on the CF/SrTiO3-In seed composite fibers.In this process, 0.3 InNO3 was added into a mixed solution of 15 mL of ethanol and 20 mL of glycol, an mixture was stirred for 30 min to obtain a homogenous solution.The above-ment CF/SrTiO3-In seed composite fibers were added to the mixture and transferred into ytetrafluoroethylene reactor for solvothermal reaction at 180 °C for 20 h [13,14].ward, the composite was cleaned with absolute ethyl alcohol and dried in an oven °C for 24 h, resulting in the formation of CF/SrTiO3/In2O3 composite fibers.The de preparation process is demonstrated in Figure 1.

Characterization Method
An X-ray diffractometer (XRD, Bruker D8 Disvover, Bruker, Karlsruhe, Germ with a scanning speed of 0.2°/min was used to characterize the crystalline structu the as-prepared samples.The morphologies of the samples were examined using a ning electron microscope (FESEM Hitachi S-4800, Tokyo, Japan) and a transmission tron microscope (TEM FEI Talos F200x G2, FEI Company, Hillsboro, OR, USA).X-ray toelectron spectroscopy (XPS) measurement was carried out using a Thermo Scient Alpha spectrometer.An electronic paramagnetic spectrometer (EPR, Bruker EMX P Bruker, Karlsruhe, Germany) with a sweep width of 100 G was used to assess the d in the samples.Electrochemical properties were also tested using an electroche workstation (CHI660E, Shanghai Chenhua Instrumental Co. Ltd., Shanghai, China) b plying the standard three-electrode testing method.A Hitachi F-7000 spectrophoto was used to measure the light absorption response range of the samples.

Photocatalytic Evolution
A gas chromatograph (GC-7900, Techcomp (China) Co., Ltd., Shanghai, China used to identify the photocatalytic hydrogen evolution performance.For each test, 1 of photocatalyst was used.Na2S and Na2SO3 served as sacrificial agents, and the re

Characterization Method
An X-ray diffractometer (XRD, Bruker D8 Disvover, Bruker, Karlsruhe, Germany) with a scanning speed of 0.2 • /min was used to characterize the crystalline structures of the as-prepared samples.The morphologies of the samples were examined using a scanning electron microscope (FESEM Hitachi S-4800, Tokyo, Japan) and a transmission electron microscope (TEM FEI Talos F200x G2, FEI Company, Hillsboro, OR, USA).X-ray photoelectron spectroscopy (XPS) measurement was carried out using a Thermo Scientific K-Alpha spectrometer.An electronic paramagnetic spectrometer (EPR, Bruker EMX PLUS, Bruker, Karlsruhe, Germany) with a sweep width of 100 G was used to assess the defects in the samples.Electrochemical properties were also tested using an electrochemical workstation (CHI660E, Shanghai Chenhua Instrumental Co., Ltd., Shanghai, China) by applying the standard three-electrode testing method.A Hitachi F-7000 spectrophotometer was used to measure the light absorption response range of the samples.

Photocatalytic Evolution
A gas chromatograph (GC-7900, Techcomp (China) Co., Ltd., Shanghai, China) was used to identify the photocatalytic hydrogen evolution performance.For each test, 100 mg of photocatalyst was used.Na 2 S and Na 2 SO 3 served as sacrificial agents, and the reaction took place in 100 mL of aqueous solution.A 300 W xenon lamp (PLS-SE300C, Beijing perfectlight Co., Ltd., Beijing, China) was utilized to simulate sunlight [10,11].

Results and Discussion
The CF/SrTiO 3 /In 2 O 3 composite fibers with a double-layer core-shell structure were produced via an in situ seed growth method.The XRD patterns of CF/SrTiO 3 /In 2 O 3 , CF/SrTiO 3 , SrTiO 3 , and CF are shown in Figure 2. The characteristic diffraction peaks observed in both the CF/SrTiO 3 composite and CF/SrTiO 3 /In 2 O 3 composite correspond to JCPDS# 35-0734 on the XRD standard card, confirming the presence of a cubic phase perovskite structure of SrTiO 3 on the surface of the carbon fibers [15].However, the XRD spectra of the CF/SrTiO took place in 100 mL of aqueous solution.A 300 W xenon lamp (PLS-SE300C, Beijing perfectlight Co., Ltd., Beijing, China) was utilized to simulate sunlight [10,11].

Results and Discussion
The CF/SrTiO3/In2O3 composite fibers with a double-layer core-shell structure were produced via an in situ seed growth method.The XRD patterns of CF/SrTiO3/In2O3, CF/SrTiO3, SrTiO3, and CF are shown in Figure 2. The characteristic diffraction peaks observed in both the CF/SrTiO3 composite and CF/SrTiO3/In2O3 composite correspond to JCPDS# 35-0734 on the XRD standard card, confirming the presence of a cubic phase perovskite structure of SrTiO3 on the surface of the carbon fibers [15].However, the XRD spectra of the CF/SrTiO3/In2O3 composite fibers show no significant diffraction peaks attributable to In2O3, such as a signal near 30.6°[16].This absence may be due to the use of glycol as a solvent.Moreover, the mass of both the CF/SrTiO3/In2O3 composite and the CF/SrTiO3 was measured.The CF/SrTiO3/In2O3 composite exhibited only a 10% increase in mass compared to the CF/SrTiO3 material, suggesting a relatively low content of In2O3.These findings indicate a need for further characterization to confirm the presence of In2O3 in the composites.The SEM images of Tencel fibers, CF/SrTiO3 composite fibers, and CF/SrTiO3/In2O3 composite fibers are shown in Figure 3. Figure 3a presents Tencel fibers with a smooth surface, and each fiber shows a diameter of about 10 µm. Figure 3b,c reveal that the SrTiO3 material layer is coated on the carbon fibers, with the diameter reduced to 6-7 µm.The reduction is probably caused by the high-temperature carbonization [17].Figure 3b,c also show that the SrTiO3 material layer is composed of nanoparticles with a cubic phase structure, which are closely bonded to form a continuous coating layer.Figure 3d,e illustrate that the In2O3 material layer, prepared using the solvothermal method and applied like a paste, covers the CF/SrTiO3 composite fibers.This layer features a nanosheet structure that resembles a ceramic tile covering [18].The cross-section of the CF/SrTiO3/In2O3 composite fiber is displayed in Figure 3f, where the SrTiO3/In2O3 material layer is applied to the carbon fibers' surface, with In2O3 nanosheets forming the outermost layer.The SEM images of Tencel fibers, CF/SrTiO 3 composite fibers, and CF/SrTiO 3 /In 2 O 3 composite fibers are shown in Figure 3. Figure 3a presents Tencel fibers with a smooth surface, and each fiber shows a diameter of about 10 µm. Figure 3b,c reveal that the SrTiO 3 material layer is coated on the carbon fibers, with the diameter reduced to 6-7 µm.The reduction is probably caused by the high-temperature carbonization [17].Figure 3b,c also show that the SrTiO 3 material layer is composed of nanoparticles with a cubic phase structure, which are closely bonded to form a continuous coating layer.Figure 3d,e illustrate that the In 2 O 3 material layer, prepared using the solvothermal method and applied like a paste, covers the CF/SrTiO 3 composite fibers.This layer features a nanosheet structure that resembles a ceramic tile covering [18].The cross-section of the CF/SrTiO 3 /In 2 O 3 composite fiber is displayed in Figure 3f, where the SrTiO 3 /In 2 O 3 material layer is applied to the carbon fibers' surface, with In 2 O 3 nanosheets forming the outermost layer.
The elemental distribution of the CF/SrTiO 3 /In 2 O 3 composite fibers was analyzed using SEM mapping, as depicted in Figure 4.It demonstrated a homogeneous distribution of O, Sr, Ti, and In on the surface of the composite fibers.Additionally, it can be noticed that in Figure 4b, the C signal in the CF/SrTiO 3 /In 2 O 3 composite fibers seems relatively weak, but the signals on the periphery are relatively strong.This is because SEM mapping is mainly used to detect the elemental distribution on the surface of the material, but CF located in the core of the composite fiber is covered by SrTiO 3 /In 2 O 3 , and the C element around the fiber comes from the carbon conductive adhesive on the SEM stub.The elemental distribution of the CF/SrTiO3/In2O3 composite fibers was analyzed using SEM mapping, as depicted in Figure 4.It demonstrated a homogeneous distribution of O, Sr, Ti, and In on the surface of the composite fibers.Additionally, it can be noticed that in Figure 4b, the C signal in the CF/SrTiO3/In2O3 composite fibers seems relatively weak, but the signals on the periphery are relatively strong.This is because SEM mapping is mainly used to detect the elemental distribution on the surface of the material, but CF located in the core of the composite fiber is covered by SrTiO3/In2O3, and the C element around the fiber comes from the carbon conductive adhesive on the SEM stub.The elemental distribution of the CF/SrTiO3/In2O3 composite fibers was analyzed using SEM mapping, as depicted in Figure 4.It demonstrated a homogeneous distribution of O, Sr, Ti, and In on the surface of the composite fibers.Additionally, it can be noticed that in Figure 4b, the C signal in the CF/SrTiO3/In2O3 composite fibers seems relatively weak, but the signals on the periphery are relatively strong.This is because SEM mapping is mainly used to detect the elemental distribution on the surface of the material, but CF located in the core of the composite fiber is covered by SrTiO3/In2O3, and the C element around the fiber comes from the carbon conductive adhesive on the SEM stub.[20,21].Additionally, a double-layer structure can be seen in Figure 5a, where the upper layer mainly comprises SrTiO 3 nanoparticles, and the lower layer consists of In 2 O 3 nanosheets approximately 35 nm wide.These observations, in conjunction with Figure 3, all prove that the CF/SrTiO 3 /In 2 O 3 composite fibers possess a double-layer core-shell structure.
The microstructure of the composite material was further observed by the transmission electron microscopy.Figure 5 presents TEM and HRTEM images of the SrTiO3/In2O3, which was ultrasonically stripped from the surface of the CF/SrTiO3/In2O3 composite fibers.The interplanar spacing observed in Figure 5c is about 0.297 nm, corresponding to the (222) crystal face of In2O3 (JCPDS# 06-0416) [19].This confirms the successful loading of In2O3 nanosheets onto the surface of the CF/SrTiO3 composite fibers and the formation of a heterojunction structure between SrTiO3 and In2O3.Figure 5d shows an interplanar spacing of approximately 0.278 nm, matching the (110) crystal face of SrTiO3 (JCPDS# 35-0734) [20,21].Additionally, a double-layer structure can be seen in Figure 5a, where the upper layer mainly comprises SrTiO3 nanoparticles, and the lower layer consists of In2O3 nanosheets approximately 35 nm wide.These observations, in conjunction with Figure 3, all prove that the CF/SrTiO3/In2O3 composite fibers possess a double-layer core-shell structure.Qualitative and valence analysis of the elements on the surface of the CF/SrTiO3/In2O3 composite fibers was performed using X-ray photoelectron spectroscopy (XPS).Qualitative and valence analysis of the elements on the surface of the CF/SrTiO 3 /In 2 O 3 composite fibers was performed using X-ray photoelectron spectroscopy (XPS).Figure 7a shows a typical wide-scan spectrum of the CF/SrTiO Figure 7d shows the peaks in the Ti 2p spectrum that have been fitted.The peaks at binding energies of 458.7 eV and 464.6 eV correspond to the characteristic peaks of Ti 2p 3/2 and Ti 2p 1/2 , respectively, with a difference of 5.9 eV between them, representing the presence of Ti 4+ ions [26,27].Additionally, the peaks at binding energies of 458.2 eV and 464.1 eV are probably related to Ti 3+ ions.To further verify the existence of oxygen vacancies in the CF/SrTiO3/In2O3 composite fibers, an EPR test was conducted, as shown in Figure 8.This test reveals a strong signal at g ≈ 2.003, which is indicative of the existence of oxygen vacancies in the material [32].The use of 20 mL of glycol as a solvent and the deposition of In2O3 on the CF/SrTiO3 fibers likely contributed to a high concentration of oxygen vacancies within the In2O3 [33].And previous studies have indicated that the SrTiO3 in the CF/SrTiO3 composite fibers also contains oxygen vacancies [10].Hence, it can be deduced that the oxygen vacancies in the CF/SrTiO3/In2O3 composite fibers originate from both the SrTiO3 and In2O3 components, Figure 7e illustrates four characteristic peaks in the In 3d spectrum, located at binding energies of 448.0 eV and 455.6 eV, corresponding to the In 3d 5/2 and In 3d 3/2 peaks of In 3+ ions, with a difference of 7.6 eV between the two peaks [28,29].Moreover, these peaks suggest the presence of oxygen vacancies, which may cause a reduction in the valence of In 3+ ions, potentially to states between 0 and 2 + [30].Figure 7f shows the characteristic peaks in the Sr 3d spectrum at binding energies of 133.2 eV and 135.0 eV, attributed to Sr 3d 5/2 and Sr 3d 3/2 , respectively, with a 1.8 eV difference between them, indicating a Sr ion valence of +2 [31].
To further verify the existence of oxygen vacancies in the CF/SrTiO 3 /In 2 O 3 composite fibers, an EPR test was conducted, as shown in Figure 8.This test reveals a strong signal at g ≈ 2.003, which is indicative of the existence of oxygen vacancies in the material [32].The use of 20 mL of glycol as a solvent and the deposition of In 2 O 3 on the CF/SrTiO 3 fibers likely contributed to a high concentration of oxygen vacancies within the In 2 O 3 [33].And previous studies have indicated that the SrTiO 3 in the CF/SrTiO 3 composite fibers also contains oxygen vacancies [10].Hence, it can be deduced that the oxygen vacancies in the CF/SrTiO 3 /In 2 O 3 composite fibers originate from both the SrTiO 3 and In 2 O 3 components, which are believed to significantly improve the photocatalytic performance of the composite material.The mass composition of SrTiO3/In2O3 in the CF/SrTiO3/In2O3 composite fiber material was determined using a TG test conducted in an air atmosphere, as seen in Figure 9.The result indicates a gradual weight loss occurring between 100 °C and 420 °C, followed by a rapid weight loss between 420 °C and 550 °C, likely due to the combustion decomposition of the carbon fiber component.It is estimated that SrTiO3/In2O3 constitutes approximately 48% of the composite fiber material, given that the final weight loss in air conditions is approximately 52%.The mass composition of SrTiO 3 /In 2 O 3 in the CF/SrTiO 3 /In 2 O 3 composite fiber material was determined using a TG test conducted in an air atmosphere, as seen in Figure 9.The result indicates a gradual weight loss occurring between 100 • C and 420 • C, followed by a rapid weight loss between 420 • C and 550 • C, likely due to the combustion decomposition of the carbon fiber component.It is estimated that SrTiO 3 /In 2 O 3 constitutes approximately 48% of the composite fiber material, given that the final weight loss in air conditions is approximately 52%.
The photocatalytic water-splitting hydrogen evolution performance was evaluated using a 300 W xenon lamp to simulate sunlight, with the findings presented in Figure 10.The CF/SrTiO 3 sample exhibited a hydrogen evolution rate of about 45.18 µmol/g•h, whereas the CF/SrTiO 3 /In 2 O 3 sample showed a rate of approximately 320.71 µmol/g•h, nearly 7 times higher than that of the CF/SrTiO 3 sample.The significant enhancement can be attributed to the wider band gap of In 2 O 3 , which, even as a minor coating of nanosheets on the CF/SrTiO 3 fibers, markedly improves photocatalytic performance [34].It is apparent that CF alone does not contribute to photocatalytic hydrogen evolution under light, suggesting that the performances of both CF/SrTiO 3 and CF/SrTiO 3 /In 2 O 3 composites are primarily influenced by the SrTiO 3 and In 2 O 3 components.However, the excellent electrical conductivity of the carbon fibers promotes the separation, generation, and migration of photogenerated electron-hole pairs within these materials, thus enhancing their photocatalytic activity [35].In addition, the presence of abundant oxygen vacancies in the CF/SrTiO 3 /In 2 O 3 composite fiber material may also contribute to its improved performance.The mass composition of SrTiO3/In2O3 in the CF/SrTiO3/In2O3 composite fiber material was determined using a TG test conducted in an air atmosphere, as seen in Figure 9.The result indicates a gradual weight loss occurring between 100 °C and 420 °C, followed by a rapid weight loss between 420 °C and 550 °C, likely due to the combustion decomposition of the carbon fiber component.It is estimated that SrTiO3/In2O3 constitutes approximately 48% of the composite fiber material, given that the final weight loss in air conditions is approximately 52%.The photocatalytic water-splitting hydrogen evolution performance was evaluated using a 300 W xenon lamp to simulate sunlight, with the findings presented in Figure 10.The CF/SrTiO3 sample exhibited a hydrogen evolution rate of about 45.18 µmol/g•h, whereas the CF/SrTiO3/In2O3 sample showed a rate of approximately 320.71 µmol/g•h, nearly 7 times higher than that of the CF/SrTiO3 sample.The significant enhancement can be attributed to the wider band gap of In2O3, which, even as a minor coating of nanosheets on the CF/SrTiO3 fibers, markedly improves photocatalytic performance [34].It is apparent that CF alone does not contribute to photocatalytic hydrogen evolution under light, suggesting that the performances of both CF/SrTiO3 and CF/SrTiO3/In2O3 composites are primarily influenced by the SrTiO3 and In2O3 components.However, the excellent electrical conductivity of the carbon fibers promotes the separation, generation, and migration of photogenerated electron-hole pairs within these materials, thus enhancing their photocatalytic activity [35].In addition, the presence of abundant oxygen vacancies in the CF/SrTiO3/In2O3 composite fiber material may also contribute to its improved performance.The photocatalytic hydrogen evolution cyclic stability of the CF/SrTiO3/In2O3 composite fiber material was also measured, as shown in Figure 11.After four consecutive cyclic tests for photocatalytic water-splitting hydrogen evolution, the CF/SrTiO3/In2O3 composite fibers maintained an average hydrogen evolution performance of approximately 268 µmol/g•h, with no significant loss of activity observed.This indicates that the CF/SrTiO3/In2O3 composite catalyst possesses robust photocatalytic stability during the water-splitting hydrogen evolution process.The photocatalytic hydrogen evolution cyclic stability of the CF/SrTiO 3 /In 2 O 3 composite fiber material was also measured, as shown in Figure 11.After four consecutive cyclic tests for photocatalytic water-splitting hydrogen evolution, the CF/SrTiO 3 /In 2 O 3 composite fibers maintained an average hydrogen evolution performance of approximately 268 µmol/g•h, with no significant loss of activity observed.This indicates that the CF/SrTiO 3 /In 2 O 3 composite catalyst possesses robust photocatalytic stability during the water-splitting hydrogen evolution process.
site fiber material was also measured, as shown in Figure 11.After four consecutive cyclic tests for photocatalytic water-splitting hydrogen evolution, the CF/SrTiO3/In2O3 composite fibers maintained an average hydrogen evolution performance of approximately 268 µmol/g•h, with no significant loss of activity observed.This indicates that the CF/SrTiO3/In2O3 composite catalyst possesses robust photocatalytic stability during the water-splitting hydrogen evolution process.Figure 12 presents the electrochemical impedance diagram of CF, CF/SrTiO3, and CF/SrTiO3/In2O3 photocatalytic composite fiber materials.Among these three materials, the CF/SrTiO3/In2O3 composite displays the smallest first arc radius in the EIS, indicating higher charge mobility and faster separation of photoelectrons and holes, which implies lower resistance and higher photocatalytic activity [36,37].These observations are in agreement with prior experimental results from photocatalytic hydrogen evolution studies.Figure 12 presents the electrochemical impedance diagram of CF, CF/SrTiO 3 , and CF/SrTiO 3 /In 2 O 3 photocatalytic composite fiber materials.Among these three materials, the CF/SrTiO 3 /In 2 O 3 composite displays the smallest first arc radius in the EIS, indicating higher charge mobility and faster separation of photoelectrons and holes, which implies lower resistance and higher photocatalytic activity [36,37].These observations are in agreement with prior experimental results from photocatalytic hydrogen evolution studies.The absorption response range of a photocatalyst to light is crucial for its photocatalytic activity.Figure 13 shows the UV-Vis diffuse reflection spectra of CF/SrTiO3, CF/SrTiO3/In2O3, CF, and SrTiO3.It can be seen that the pure SrTiO3 sample exhibits a characteristic light absorption band edge near 375 nm, while the CF/SrTiO3 photocatalytic fiber sample shows a broader light absorption range.This may be attributed to the strong light absorption properties of carbon fibers.The band gap of the pure In2O3 sample is approximately 2.8 eV [38].Consequently, after coating the CF/SrTiO3 composite fibers with a thin layer of In2O3 nanosheets, the light absorption band edge of the CF/SrTiO3/In2O3 sample demonstrates a red shift toward visible light compared to the CF/SrTiO3 sample.The absorption response range of a photocatalyst to light is crucial for its photocatalytic activity.Figure 13  fiber sample shows a broader light absorption range.This may be attributed to the strong light absorption properties of carbon fibers.The band gap of the pure In2O3 sample is approximately 2.8 eV [38].Consequently, after coating the CF/SrTiO3 composite fibers with a thin layer of In2O3 nanosheets, the light absorption band edge of the CF/SrTiO3/In2O3 sample demonstrates a red shift toward visible light compared to the CF/SrTiO3 sample.According to our previous study [10], the band gap of the pure SrTiO3 sample is 3.32 eV, with a conduction band position at −0.55 eV and a valence band position at 2.77 eV.And it was reported that the band gap of the In2O3 material is about 2.77 eV, with a conduction band position of around −0.62 eV and a valence band position of 2.15 eV [39].Both SrTiO3 and In2O3 materials in the CF/SrTiO3/In2O3 composite fiber exhibit numerous oxygen vacancies, as confirmed by this work and previously published XPS and EPR tests According to our previous study [10], the band gap of the pure SrTiO 3 sample is 3.32 eV, with a conduction band position at −0.55 eV and a valence band position at 2.77 eV.And it was reported that the band gap of the In 2 O 3 material is about 2.77 eV, with a conduction band position of around −0.62 eV and a valence band position of 2.15 eV [39].Both SrTiO 3 and In 2 O 3 materials in the CF/SrTiO 3 /In 2 O 3 composite fiber exhibit numerous oxygen vacancies, as confirmed by this work and previously published XPS and EPR tests [10].The presence of these oxygen vacancies introduces a new donor level below the conduction band, forming an oxygen vacancy state (VOs).This state not only reduces the band gap and extends the light absorption boundary of the material but also serves as a trap for photogenerated electrons, enhancing the separation of electron-hole pairs and inhibiting their recombination [40][41][42].
Based on these insights, we propose a mechanism for the photocatalytic water-splitting hydrogen evolution in the CF/SrTiO 3 /In 2 O 3 composite fiber material, as illustrated in Figure 14

Conclusions
In this study, CF/SrTiO3/In2O3 composite fibers were fabricated as a photocatalytic material.Initially, Tencel fibers were coated with a SrTiO3 layer, and after carbonization, CF/SrTiO3 was produced.Subsequently, In seeds were deposited onto this substrate using an immersion heating method, and the final composite fiber was synthesized via a solvothermal method employing in situ growth on the CF/SrTiO3-In seed composite fibers.This process resulted in the formation of a SrTiO3/In2O3 heterojunction on the carbon fibers.XPS and EPR measurements suggested the presence of oxygen vacancies in both the SrTiO3 and In2O3 materials within the composite fibers.Under simulated sunlight using a xenon lamp, the photocatalytic hydrogen evolution rate of the CF/SrTiO3/In2O3 fibers reached approximately 320.71 µmol/g•h, about seven times higher than that of CF/SrTiO3.Subsequent cyclic tests confirmed the robust photocatalytic stability of the

Conclusions
In this study, CF/SrTiO 3 /In 2 O 3 composite fibers were fabricated as a photocatalytic material.Initially, Tencel fibers were coated with a SrTiO 3 layer, and after carbonization, CF/SrTiO 3 was produced.Subsequently, In seeds were deposited onto this substrate using an immersion heating method, and the final composite fiber was synthesized via a solvothermal method employing in situ growth on the CF/SrTiO 3 -In seed composite fibers.This process resulted in the formation of a SrTiO 3 /In 2 O 3 heterojunction on the carbon fibers.XPS and EPR measurements suggested the presence of oxygen vacancies in both the SrTiO 3 and In 2 O 3 materials within the composite fibers.Under simulated sunlight using a xenon lamp, the photocatalytic hydrogen evolution rate of the CF/SrTiO 3 /In 2 O 3 fibers reached approximately 320.71 µmol/g•h, about seven times higher than that of CF/SrTiO 3 .Subsequent cyclic tests confirmed the robust photocatalytic stability of the CF/SrTiO 3 /In 2 O 3 fibers.The mechanism can be explained by the presence of oxygen vacancies, which, along with the material's heterojunction structure and the carbon fibers' superior electrical conductivity, significantly enhance the separation, generation, and migration of photogenerated electrons.This improvement not only promotes photocatalytic activity but also reduces the recombination of electron-hole pairs, ultimately boosting photocatalytic hydrogen evolution performance in these fibers.
This study established a reference method for preparing photocatalytic composite materials using an in situ seed growth approach on carbon fibers.Furthermore, it opens up possibilities for exploring the use of carbon fiber-based photocatalytic composites as raw materials for constructing network materials via techniques such as knitting or bonding, potentially enabling the recycling of nanometer-scale photocatalysts and thereby boosting environmental sustainability and material efficiency.

3 /
In 2 O 3 composite fibers show no significant diffraction peaks attributable to In 2 O 3 , such as a signal near 30.6 • [16].This absence may be due to the use of glycol as a solvent.Moreover, the mass of both the CF/SrTiO 3 /In 2 O 3 composite and the CF/SrTiO 3 was measured.The CF/SrTiO 3 /In 2 O 3 composite exhibited only a 10% increase in mass compared to the CF/SrTiO 3 material, suggesting a relatively low content of In 2 O 3 .These findings indicate a need for further characterization to confirm the presence of In 2 O 3 in the composites.

Figure 4 .
Figure 4. SEM mapping diagram of CF/SrTiO 3 /In 2 O 3 sample, (a) SEM diagram of composite fiber, (b) C element, (c) O element, (d) Sr element, (e) Ti element, (f) In element, and (g) element content diagram.The microstructure of the composite material was further observed by the transmission electron microscopy.Figure 5 presents TEM and HRTEM images of the SrTiO 3 /In 2 O 3 , which was ultrasonically stripped from the surface of the CF/SrTiO 3 /In 2 O 3 composite fibers.The interplanar spacing observed in Figure 5c is about 0.297 nm, corresponding to the (222) crystal face of In 2 O 3 (JCPDS# 06-0416) [19].This confirms the successful loading of In 2 O 3 nanosheets onto the surface of the CF/SrTiO 3 composite fibers and the formation of a heterojunction structure between SrTiO 3 and In 2 O 3 .Figure 5d shows an interplanar spacing of approximately 0.278 nm, matching the (110) crystal face of SrTiO 3 (JCPDS# Figure 5 presents TEM and HRTEM images of the SrTiO 3 /In 2 O 3 , which was ultrasonically stripped from the surface of the CF/SrTiO 3 /In 2 O 3 composite fibers.The interplanar spacing observed in Figure 5c is about 0.297 nm, corresponding to the (222) crystal face of In 2 O 3 (JCPDS# 06-0416) [19].This confirms the successful loading of In 2 O 3 nanosheets onto the surface of the CF/SrTiO 3 composite fibers and the formation of a heterojunction structure between SrTiO 3 and In 2 O 3 .Figure 5d shows an interplanar spacing of approximately 0.278 nm, matching the (110) crystal face of SrTiO 3 (JCPDS# 35-0734)

Figure 6
Figure 6 shows the TEM EDS diagram of the nanosheet material obtained from the surface of the carbon fibers after ultrasonic treatment of the CF/SrTiO3/In2O3 composite fibers.The diagram also reveals a uniform distribution of In, Sr, Ti, and O elements throughout the material, suggesting that SrTiO3 and In2O3 are evenly dispersed and form a homogeneous heterojunction structure.

Figure 6 15 Figure 6 .
Figure 6 shows the TEM EDS diagram of the nanosheet material obtained from the surface of the carbon fibers after ultrasonic treatment of the CF/SrTiO 3 /In 2 O 3 composite fibers.The diagram also reveals a uniform distribution of In, Sr, Ti, and O elements throughout the material, suggesting that SrTiO 3 and In 2 O 3 are evenly dispersed and form a homogeneous heterojunction structure.Sustainability 2024, 16, x FOR PEER REVIEW 7 of 15 Figure 7a shows a typical wide-scan spectrum of the CF/SrTiO3/In2O3 composite fibers, which includes five elements: C, Sr, Ti In, and O.The signals of the In and O exhibit a comparatively high intensity, while the signals of the C, Sr, and Ti have a relatively low intensity.This is mainly due to the double-layer core-shell structure of the CF/SrTiO3/In2O3 composite, in which the In2O3 is situated in the outermost layer, and CF/SrTiO3 forms the inner layer.As illustrated in Figure 7b, two characteristic peaks at the binding energies of 284.3

3 /
In 2 O 3 composite fibers, which includes five elements: C, Sr, Ti In, and O.The signals of the In and O exhibit a comparatively high intensity, while the signals of the C, Sr, and Ti have a relatively low intensity.This is mainly due to the double-layer core-shell structure of the CF/SrTiO 3 /In 2 O 3 composite, in which the In 2 O 3 is situated in the outermost layer, and CF/SrTiO 3 forms the inner layer.As illustrated in Figure 7b, two characteristic peaks at the binding energies of 284.3 eV and 285.8 eV are identified in the C 1s spectrum, which corresponds to the C-C bond and C-O bond, respectively [22]. Figure 7c reveals the presence of three peaks in the O 1s high-resolution XPS spectrum.The peak observed at a binding energy of 529.8 eV corresponds to lattice oxygen in SrTiO 3 .On the other hand, the peak at 531.6 eV may indicate the presence of adsorption oxygen in oxygen vacancies, whereas the peak at 532.4 eV is associated with surface adsorption oxygen.And the highest peak at 531.6 eV indicates a significant abundance of oxygen vacancies in the CF/SrTiO 3 /In 2 O 3 material [23-25].

15 Figure 7 .
Figure 7. XPS diagram of CF/SrTiO3/In2O3 sample, (a) full spectrum, (b) C 1s spectrum, (c) O 1s spectrum, (d) Ti 2p spectrum, (e) In 3d spectrum, and (f) Sr 3d spectrum.In subfigures (b-f), the black curve are experimental data, the red curve is the fitting of of the fitting peak data, and the blue curve is the background from the simulation.

Figure 7 .
Figure 7. XPS diagram of CF/SrTiO 3 /In 2 O 3 sample, (a) full spectrum, (b) C 1s spectrum, (c) O 1s spectrum, (d) Ti 2p spectrum, (e) In 3d spectrum, and (f) Sr 3d spectrum.In subfigures (b-f), the black curve are experimental data, the red curve is the fitting of the fitting peak data, and the blue curve is the background from the simulation.

Figure 11 .
Figure 11.Photocatalytic stability diagram of CF/SrTiO3/In2O3 composite fiber material, (a) corresponding histogram of hydrogen evolution rate, and (b) cycle curve of hydrogen evolution amount.

Figure 11 .
Figure 11.Photocatalytic stability diagram of CF/SrTiO 3 /In 2 O 3 composite fiber material, (a) corresponding histogram of hydrogen evolution rate, and (b) cycle curve of hydrogen evolution amount.
shows the UV-Vis diffuse reflection spectra of CF/SrTiO 3 , CF/SrTiO 3 /In 2 O 3 , CF, and SrTiO 3 .It can be seen that the pure SrTiO 3 sample exhibits a characteristic light absorption band edge near 375 nm, while the CF/SrTiO 3 photocatalytic fiber sample shows a broader light absorption range.This may be attributed to the strong light absorption properties of carbon fibers.The band gap of the pure In 2 O 3 sample is approximately 2.8 eV [38].Consequently, after coating the CF/SrTiO 3 composite fibers with a thin layer of In 2 O 3 nanosheets, the light absorption band edge of the CF/SrTiO 3 /In 2 O 3 sample demonstrates a red shift toward visible light compared to the CF/SrTiO 3 sample.
. Since the conduction and valence band positions of pure In 2 O 3 are higher than those of SrTiO 3 , photogenerated electrons migrate from the conduction band of the In 2 O 3 nanosheets to that of the SrTiO 3 nanomaterial, while photogenerated holes move rapidly from the conduction band of SrTiO 3 to the valence band of In 2 O 3 nanosheets.The excellent conductivity of carbon fibers allows photogenerated electrons on SrTiO 3 to migrate to the carbon fiber surface, thereby inhibiting the recombination of the photogenerated electron-hole pairs.The presence of the VOs creates a trap for photogenerated electrons, facilitating their movement from the In 2 O 3 nanosheets to the surface oxygen vacancy state and promoting the reduction of H + in water to evolve hydrogen [43].In addition, holes on the surface of SrTiO 3 are readily captured by sacrificial agents (Na 2 S and Na 2 SO 3 ) in the aqueous solution, promoting the generation and separation of photogenerated electron-hole pairs and facilitating the water-splitting hydrogen evolution process of the catalyst [44].holepairs.The presence of the VOs creates a trap for photogenerated electrons, facilitating their movement from the In2O3 nanosheets to the surface oxygen vacancy state and promoting the reduction of H + in water to evolve hydrogen[43].In addition, holes on the surface of SrTiO3 are readily captured by sacrificial agents (Na2S and Na2SO3) in the aqueous solution, promoting the generation and separation of photogenerated electron-hole pairs and facilitating the water-splitting hydrogen evolution process of the catalyst [44].