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Article

One-Dimensional CdS/SrTiO3/Carbon Fiber Core–Shell Photocatalysts for Enhanced Photocatalytic Hydrogen Evolution

by
Qi Hu
1,2,
Jiantao Niu
3,
Ke-Qin Zhang
1,* and
Mu Yao
4
1
National Engineering Laboratory for Modern Silk, College of Textile and Clothing Engineering, Soochow University, Suzhou 215123, China
2
School of Social Development and Public Administration, Suzhou University of Science and Technology, Suzhou 215009, China
3
School of Textile and Clothing and Arts and Media, Suzhou Institute of Trade and Commerce, Suzhou 215009, China
4
School of Textile Science and Engineering, Xi’an Polytechnic University, Xi’an 710048, China
*
Author to whom correspondence should be addressed.
Coatings 2022, 12(9), 1235; https://doi.org/10.3390/coatings12091235
Submission received: 2 August 2022 / Revised: 22 August 2022 / Accepted: 22 August 2022 / Published: 24 August 2022
(This article belongs to the Section Surface Engineering for Energy Harvesting, Conversion, and Storage)
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Abstract

:
The photocatalytic hydrogen production efficiency of a single SrTiO3 photocatalytic catalyst is often low, which is mainly due to the serious combination of electrons and holes produced by photocatalysis as well as the mismatch of the redox capacity and light absorption range. Construction of semiconductor heterojunctions can solve these problems. CdS has a narrow band gap, which can effectively utilize visible light, and it has a band structure matched with that of SrTiO3. Therefore, CdS is considered as an ideal candidate for constructing heterojunctions with SrTiO3. In this paper, bamboo pulp fibers were used as the substrate, and SrTiO3 was coated on the substrate through the solvothermal process. CF/SrTiO3 rich in oxygen vacancies was formed by high temperature carbonization, and heterojunctions were formed by loading CdS on the surface of the CF/SrTiO3 composite material through the hydrothermal method, thus obtaining one-dimensional CF/SrTiO3/CdS core–shell photocatalysts. The structure and photocatalytic hydrogen production performance of the CF/SrTiO3/CdS core–shell photocatalysts were mainly studied. The photocatalytic hydrogen production experiment showed that the hydrogen production rate of the CF/SrTiO3/CdS-2 sample under the optimized process was as high as 577.39 μmol/g·h, which was about 11 times that of the CF/SrTiO3 sample. In this composite photocatalytic material system, the loading of the CdS nanospheres could enhance the visible light absorption capacity of the composite catalyst, promote the rapid separation and high-speed migration of photocarriers, and significantly improve the photocatalytic activity.

1. Introduction

Photocatalytic hydrogen production can not only produce highly efficient and clean hydrogen energy using clean and renewable solar energy, but it also requires simple equipment and mild reaction conditions and has low cost, which is the most attractive method for hydrogen production. The key to improving the efficiency of photocatalytic hydrogen production is to develop efficient and stable photocatalysts. Because of the excellent redox catalytic activity and environmental friendliness, SrTiO3 exhibits many intriguing properties that make it a good candidate for potential applications in a wide array of fields, which include water splitting, sewage treatment, air purification, and virus inactivation [1]. To produce efficient photocatalytic performance, the materials should possess high mobility carriers, suitable band gap width, and higher absorption capacity [2]. However, the wide band gap makes SrTiO3 photocatalysts only respond to ultraviolet light, so pure SrTiO3 normally exhibits mediocre photocatalytic activity because of its limited surface-active sites and fast recombination of the photoexcited electron–hole pairs. Various methods have therefore been proposed to address these issues, such as dopant introduction [3], heterojunction construction [4,5], loading precious metal [6], surface defect modification [7], surface decorating with 2D materials [8], and building sundry nanoarchitectures. Specifically, heterojunction construction has been proven to be one of the best methods to improve photocatalytic performance, and it is the most effective method with the most potential for practical application because of its feasibility and efficiency for the spatial separation of electron–hole pairs [9]. This paper focuses on the construction of heterojunctions to improve the photocatalytic activity of SrTiO3.
By virtue of its narrow bandgap (~2.4 eV) [10,11] and favorable band position, CdS has been widely explored as a photocatalyst for solar hydrogen generation. Previous studies on CdS-based photocatalysts mainly focused on the design of Janus-type nanostructures [12], core–shell nanostructures [13], hollow nanostructures [14], and surface-decorated heterostructure [15]. There are many reports about the heterojunction construction of CdS [16,17,18], but CdS composite photocatalyst loading on carbon fiber (CF) is scarcely studied. CdS has a band structure matched with that of SrTiO3. Therefore, CdS is considered as an ideal candidate for constructing heterojunctions with SrTiO3. When CdS and SrTiO3 are compounded, they have a synergistic effect in photocatalytic reaction. In addition, both semiconductor materials have a strong photocatalytic capacity, which is beneficial to accelerate the photocatalytic reaction rate and improve the efficiency of hydrogen production.
Bamboo pulp fiber is a relatively fine cellulose fiber, which contains a large number of hydroxyl groups and grooves on the surface. It is easy to load the photocatalytic semiconductor material onto the surface of the bamboo pulp fiber during the solvothermal process. Moreover, bamboo pulp fiber can be carbonized into carbon fiber at high temperature. In this work, bamboo pulp fiber was used as a substrate, SrTiO3 was coated on the surface of the substrate through the solvothermal process, and carbon fiber/SrTiO3 (CF/SrTiO3) rich in oxygen vacancies was formed by high temperature carbonization. Heterojunctions were formed by loading CdS on the surface of the CF/SrTiO3 through the hydrothermal method to thus prepare one-dimensional CF/SrTiO3/CdS core–shell photocatalysts, and the structure and photocatalytic hydrogen production performance of the CF/SrTiO3/CdS core–shell photocatalysts were mainly studied.

2. Experimental

2.1. Materials

Strontium acetate (C4H6O4Sr, 99%), cadmium acetate (C4H6CdO4·2H2O, 99%), and thiourea (CH4N2S, 99%) were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). Titanium (IV) isopropoxide (C12H28O4Ti, 97%) was from Macklin Co., Shanghai, China. Ethanol and ethylene glycol were from Sinopharm Chemical Reagent Co., Ltd., China. All the chemical reagents were of analytically pure grade and used as received without any purification.

2.2. Preparation of CF/SrTiO3/CdS Composite Material

In this work, firstly, bamboo pulp fiber (Hebei Jigao Chenical Fiber Co., Ltd., Shijiazhuang, China) was used as the precursor of carbon fiber, and the SrTiO3 nanolayer was loaded onto the surface of the bamboo pulp fiber, and then carbon fiber was carbonized to prepare CF/SrTiO3 core–shell composite fiber material. Finally, the CF/SrTiO3/CdS double-layer core–shell photocatalytic composite fiber material was prepared by loading CdS on CF/SrTiO3 composite fiber [19,20]. The specific preparation process is shown in Scheme 1.

2.2.1. Preparation of Bamboo Fiber/SrTiO3 Composite Fiber

The SrTiO3 nanosemiconductor material was coated on the bamboo pulp fiber through the solvothermal process. First, 19 mL of absolute ethyl alcohol and 15 mL of ethylene glycol were added separately to the beaker and mixed with adequate stirring. Then 0.85 g of titanium (IV) isopropoxide and 0.61 g of strontium acetate were added to the mixed solution and stirred adequately to obtain a homogeneous mixed solution. The above mixed solution was transferred to a PTFE reactor, 0.8 g of bamboo pulp fibers was added for impregnation, and the solvothermal reaction was carried out. The reaction temperature was kept at 180 °C, and the reaction time was 18 h. Following finish of the reaction and cooling to room temperature, the bamboo pulp fibers samples were draw out and rinsed with ethyl alcohol. The fiber samples were dried in a drying oven at 60 °C, thus obtaining the bamboo fiber/SrTiO3 composite material. In this experiment, the solvent was a mixture of anhydrous ethanol and ethylene glycol without water, which could help to prevent the bamboo pulp fibers from dissolving.

2.2.2. Preparation of CF/SrTiO3 Composite Fiber

CF/SrTiO3 photocatalyst with a core–shell structure was prepared by carbonization of cellulose fibers. The bamboo fiber/SrTiO3 composite material was prepared in a high-temperature tubular furnace by carbonizing cellulose fibers under the protection of an 800 °C argon atmosphere system for 2 h with a heating rate of 5 °C min. As the bamboo pulp fiber was converted into carbon fiber, the core–shell structured CF/SrTiO3 photocatalyst was achieved, and the surface of the carbon fiber was coated with SrTiO3 rich in oxygen vacancies.

2.2.3. Preparation of CF/SrTiO3/CdS Composite Fiber

CF/SrTiO3/CdS double-layer core–shell structure heterojunctions were prepared by the hydrothermal method. First, 50 mg, 100 mg, and 150 mg of cadmium acetate were respectively weighed and added to 35 mL of deionized water, and then 35 mg, 70 mg, and 100 mg of thiourea were respectively added [21]. The solution was transferred to the PTFE reactor after being stirred for 30 min and immersed in 0.07 g of CF/SrTiO3 photocatalytic material. Subsequently, the PTFE reactor was transferred to the oven, and the hydrothermal reaction was carried out at 120 °C for 2 h. After the hydrothermal reaction was over, the carbon fiber composite material was drawn out and rinsed with ethyl alcohol. The fiber samples were dried in the drying oven at 60 °C for 24 h, thus yielding the CF/SrTiO3/CdS double-layer core–shell structure heterojunctions [22]. In this process, the above samples with different proportions were respectively labeled as CF/SrTiO3/CdS-1, CF/SrTiO3/CdS-2, and CF/SrTiO3/CdS-3.
According to the test needs, pure SrTiO3 and CdS samples as well as carbon fiber samples were prepared by referring to the above methods.

2.3. Characterization Method

The crystalline structures of the samples were characterized by an X-ray diffractometer (XRD, Bruker D8 Discover, Billerica, MA, USA). The morphologies of the as-prepared products were characterized by an electron microscope (FESEM Hitachi S-4800, Tokyo, Japan) and a transmission electron microscope (TEM FEI talos F200x G2, Philips-FEI Corpocyclen, Eindhoven, The Netherlands). X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo ESCALAB 250XI (Thermo Fisher Scientific, Fairport, NY, USA), and the defects of the samples were tested and characterized by an electronic paramagnetic spectrometer (EPR, Bruker A300, Billerica, MA, USA). The electrochemical impedance and Mott–Schottky curve of the samples were tested by an electrochemical workstation (CHI660E, YIMA, Shanghai, China) with the standard three-electrode testing method. The UV–vis diffuse reflectance spectra (DRS) were performed on a Hitachi F-7000 spectrophotometer (Hitachi, Fukuoka, Japan). The specific surface area of the catalysts was calculated using the Brunauer–Emmett–Teller (BET, Bruker A300, Billerica, MA, USA) equation.

2.4. Test of Photocatalytic Hydrogen Production Performance

The photocatalytic hydrogen production performance of the samples was tested by the Labsolar III system (Beijing Perfectlight Technology Co. Ltd., Beijing, China) [23], including a gas chromatography (GC-7900, Tianmei, Shanghai, China). An external irradiation type photocatalytic reactor with 150 mL volume was used, which was connected to a closed gas circulation and evacuation system. High purity AR was used as the carrier gas, and the gas chromatography (GC-7900) equipped with a TCD detector and 5 Å molecular sieve column was used to analyze the evolved H2 gas. Before the experiment, 100 mg of catalyst sample was added into the mixture of 50 mL of Na2S (0.35 M) solution and 50 mL of Na2SO3 (0.25 M) solution, wherein Na2S and Na2SO3 were used as sacrificial agents in the photocatalytic hydrogen production experiment [24,25]. Prior to the irradiation test, the reactant solution was evacuated several times to remove air. The solution was added to the photocatalytic reactor and stirred by magnetic force. During the experiment, a plS-SE300C 300 W arc lamp (Beijing perfectlight Co., Ltd., Beijing, China) was used to simulate the solar light source for testing, and the light source intensity was 100 mW/cm2. The water in the photocatalytic reactor with an external cooling coil was cooled to keep the mixture temperature at about 25 °C.

3. Experimental Result and Analysis

The phase structure of the catalyst can be analyzed through X-ray crystal diffraction. The XRD energy spectra of the samples such as CF/SrTiO3/CdS, CF/SrTiO3, CF, SrTiO3, and CdS are each exhibited in Figure 1. The characteristic diffraction peaks of the pure SrTiO3 sample were mainly distributed at 32.4°, 39.9°, 46.5°, 57.8°, 67.8°, and 77.1°, which can be respectively identified as the (110), (111), (200), (211), (220), and (310) crystal faces (PDF#35-0734) [26,27]. Compared with the characteristic diffraction peak energy spectrum of the pure SrTiO3 sample, the shapes and positions of distinctive characteristic diffraction peaks in the XRD energy spectra of the CF/SrTiO3 and CF/SrTiO3/CdS samples were similar to those of the pure SrTiO3, indicating that SrTiO3 was coated smoothly on these carbon fibers. The XRD energy spectrum of CF formed by carbonization of bamboo pulp fibers showed only one broad carbon diffraction peak, indicating that the carbon fibers have an amorphous structure [28,29], and therefore the corresponding carbon diffraction peaks in the XRD energy spectra of CF/SrTiO3 and CF/SrTiO3 materials were not obvious. The pure CdS sample had obvious diffraction peaks at 24.8°, 26.4°, 28.1°, 43.9°, 47.9°, and 52.0°, respectively belonging to (100), (002), (101), (110), (103), and (112) crystal faces in accordance with PDF#41-1049. It could be observed from the XRD energy spectrum of the CF/SrTiO3/CdS sample that there were not only characteristic diffraction peaks attributed to SrTiO3, but also obvious diffraction peaks at 26.4°, 43.9°, and 52.1°, which could be attributed to CdS characteristic diffraction peaks (PDF#41-1049) [30]. This indicated that the SrTiO3 and CdS semiconductor materials were successfully loaded onto the surface of CF.
The microscopic morphologies of CF/SrTiO3/CdS, CF/SrTiO3, and pure carbon fiber were characterized by the SEM test, and the results are exhibited in Figure 2. These carbon fibers prepared from bamboo pulp fibers were elongated soft fibers with smooth surfaces and grooves [31], and the fiber diameter was about 5 µm (Figure 2a) [32]. It can be seen from Figure 2b that SrTiO3 nanoparticles were grown on the surface of each carbon fiber with relatively uniform distribution. As shown in Figure 2c, the coating layer was formed by SrTiO3 nanoparticles on the surface of each carbon fiber with core–shell structures, and the SrTiO3 material was integrated with the carbon fibers with compact structures. The SrTiO3 material was irregular nanoparticles with a size of 100–200 nm. Figure 2d–f show an SEM diagram of the CF/SrTiO3/CdS composite catalyst with different magnification factors, from which it can be observed that CF/SrTiO3/CdS had a complete morphology and a uniform thickness. CdS nanospheres were coated on the outer layer of CF/SrTiO3 to form a double-layer core–shell structured photocatalyst. As seen in Figure 2f, the CdS nanospheres were assembled and sedimented on the outer layer of CF/SrTiO3, and the size of the CdS nanospheres was about 80 nm.
The element distribution of CF/SrTiO3/CdS was studied by SEM mapping, as shown in Figure 3. SEM mapping was mainly used to study the element distribution over the surface of the CF/SrTiO3/CdS photocatalytic composite fibers, but carbon fibers were located in the center of the CF/SrTiO3/CdS core–shell structure. Therefore, as shown in Figure 3, relatively little carbon element was detected in the CF/SrTiO3/CdS fiber [31]. It can also be seen in Figure 3 that Ti, Sr, O, Cd, and S elements were uniformly distributed on the surface of the carbon fibers. Combined with Figure 1 and Figure 2, it can be determined that the preparation of CF/SrTiO3/CdS was successful, and SrTiO3 and CdS were evenly distributed on the carbon fibers.
Qualitative analysis and element valence analysis of surface elements of the CF/SrTiO3/CdS can be conducted by light ray photoelectron spectroscopy. A survey spectrum of the CF/SrTiO3/CdS and high resolution XPS spectra of C 1s, Sr 3d, Ti 2p, Cd 3d, S 2p, and O 1s are shown in Figure 4. Figure 4a shows an XPS full spectrum of the samples, confirming that there were mainly six elements (i.e., Sr, Ti, Cd, S, O, and C) on the surface of the CF/SrTiO3/CdS. Although the content of carbon element was high, the signal of the carbon element shown in the figure was moderate, which was mainly because the carbon fibers were located in the center of the CF/SrTiO3/CdS core–shell structure, while the signal of the Cd element was the strongest because the CdS nanospheres were coated on the CF/SrTiO3 photocatalytic fibers. As seen in Figure 4b, it could be presumed from the spectrum of C 1s that the three fitting peaks at the binding energies of 284.7 eV, 285.6 eV, and 288.1 eV were, respectively, corresponding to C–C bonds, C–O bonds, and C=O bonds [33]. Combined with the XRD energy spectra (Figure 1), it was analyzed and presumed that the carbon fibers mainly have an amorphous structure that was similar to the structure of graphite. As seen in Figure 4c, Ti 2P had two peaks, namely, the Ti 2p3/2 peak at the binding energy of 459.2 eV and the Ti 2p1/2 peak at the binding energy of 464.9 eV (the difference between the Ti 2p3/2 peak and the Ti 2p1/2 peak was 5.7 eV), both corresponding to Ti–O bonds, and the Ti ion valence was +4 [34]. As shown in Figure 4d, Sr 3d also had two peaks, namely, the Sr 3d5/2 peak at the binding energy of 133.8 eV and the Sr 3d3/2 peak at the binding energy of 135.5 eV, both corresponding to Sr–O bonds, and the Sr ion valence was +2 [35]. It can be seen in Figure 4e that the spectrum of O 1s was fitted into three peaks, and it could be known after analysis that the peak at 530.4 eV corresponded to lattice oxygen in the photocatalyst, while the peak at 531.8 eV could be ascribed to the surface adsorption oxygen, and the peak at 532.6 eV corresponded to adsorption oxygen in oxygen vacancies [36]. The results indicated that there were oxygen vacancies in the CF/SrTiO3/CdS, which was speculated to be due to the occurrence of oxygen defects in CF/SrTiO3 during the carbonization process of bamboo pulp fiber coated with SrTiO3 at high temperature [37]. In Figure 4f, S 2p had two peaks, namely, the S 2p3/2 peak at the binding energy of 162.1 eV and the S 2p1/2 peak at the binding energy of 163.3 eV, and the difference between the two peaks was 1.2 eV [38]. Figure 4g shows a high resolution XPS spectrum of Cd 3d. The two fitting peaks at the binding energies of 405.7 eV and 412.4 eV corresponded to Cd 3d5/2 and Cd 3d3/2 of Cd2+, respectively, the difference value between these two peaks was 6.7 eV, and both peaks were corresponding to Cd–S bonds [39].
In order to deeply confirm the existence of oxygen vacancies of SrTiO3 material, EPR testing was performed for CF/SrTiO3 (Figure 5). There was a strong single peak at g ≈ 2.002, explaining that oxygen vacancies existed in the CF/SrTiO3 catalyst material, which may be caused by the SrTiO3 material being loaded and oxygen vacancies being formed in the high temperature carbonization process of the bamboo pulp fibers [40,41]. The existence of the oxygen vacancies was beneficial to the absorption of visible light by the photocatalyst and the improvement of the charge transfer ability of the CF/SrTiO3/CdS material.
The specific surface area and internal pore size of CF/SrTiO3/CdS composite material were measured by the BET nitrogen adsorption volume method [42]. As shown in Figure 6a, it could be obtained by a BET calculation model that the specific surface area of the CF/SrTiO3/CdS photocatalytic fiber was 75.34 cm2/g, and it was calculated from a Barrett–Joyner–Halenda (BJH) equation that the average pore size of CF/SrTiO3/CdS was about 2.148 nm, as shown in Figure 6b. The loading of the CdS nanospheres on the surface of the CF/SrTiO3/CdS composite fiber was beneficial to improving the photocatalytic performance.
CF/SrTiO3/CdS is a fibrous catalyst with double-layer core–shell structure, and the central part of CF/SrTiO3/CdS is carbon fiber, which is not semiconductor material and cannot be split into water to produce hydrogen under the action of light. As shown in Figure 7, the approximate content of SrTiO3 and CdS in the CF/SrTiO3/CdS composite material could be determined by the TG–DSC test [43]. It could be observed that a slow weight loss (about 5%) occurred between 40 °C and 360 °C, and this phenomenon may have been caused by the evaporation of water and the oxidation of CdS. Rapid decomposition occurred, and an obvious endothermic peak appeared between 360 °C and 510 °C, and the weight loss rate was as high as about 31%, which was presumed to be mainly caused by the oxidation and combustion of carbon fibers in air conditions. According to the above analysis, we can presume that the content of SrTiO3 and CdS in CF/SrTiO3/CdS was about 64%.
To study the effect of CdS loading on the photocatalytic performance of CF/SrTiO3, the photocatalytic hydrogen production performances of CF, CF/SrTiO3, CF/SrTiO3/CdS-1, CF/SrTiO3/CdS-2, and CF/SrTiO3/CdS-3 samples were tested, respectively (Figure 8). In the photocatalytic hydrogen production experiment, a 300 W xenon lamp was used for simulating solar light, and Na2S and Na2SO3 were taken as corresponding sacrificial agents. The related data from Figure 8 indicated that carbon fibers alone had no hydrogen production evolution performance, while the photocatalytic hydrogen evolution of the CF/SrTiO3 sample loaded with SrTiO3 was about 53.27 μmol/g·h, which was close to that of pure SrTiO3 (51.84 μmol/g·h) [44]. This was attributed to the fact that carbon fibers acted as a co-catalyst, and the synergistic effect of the oxygen vacancies and the carbon fibers promoted the separation and migration of the photoelectron–hole pairs, thus significantly improving the photocatalytic activity of the SrTiO3 material. With the loading of CdS on the surface of CF/SrTiO3, the photocatalytic hydrogen production was further improved. The hydrogen production rates of the CF/SrTiO3/CdS-1 and CF/SrTiO3/CdS-3 samples were, respectively, 211.96 μmol/g·h and 330.34 μmol/g·h, which were, respectively, about 4 times and 6.3 times that of the CF/SrTiO3 sample. However, the hydrogen production rate of the CF/SrTiO3/CdS-2 sample was as high as 577.39 μmol/g·h, which was about 11 times that of the CF/SrTiO3 sample. This was mainly due to the much more efficient carrier utilization of compound photocatalyst caused by the increasing amount of CdS. Herein, the depressed performance of photocatalytic hydrogen generation was observed as the content of CdS was further increased from CF/SrTiO3/CdS-3. This phenomenon was consistent with those found in the literature, where the photocatalytic efficiency of type-II nanoheterostructures declined as the constituent content exceeded the optimal value [45,46]. CdS is an excellent photocatalytic material for visible light, but under the condition of photoexcitation, the photogenerated carriers are easy to recombine, resulting in low quantum efficiency [47]. It was considered that with the increasing of CdS content, electron–hole recombination suppressed the cross-interface transfer of photogenerated electrons, which was the main cause of the depressed photocatalytic hydrogen generation. The reference [32] showed that the photocatalytic hydrogen production performance of pure CdS was 55.84 μmol/g∙h, which was close to that of SrTiO3 (51.84 μmol/g·h), while the photocatalytic activity was significantly improved after the carbon fibers, SrTiO3, and CdS were compounded. The above test results showed that the carbon fibers and the oxygen vacancies improved the photocatalytic activity of the CF/SrTiO3 sample, and the coating of CdS on the surface in the later stage significantly enhanced photocatalytic hydrogen evolution of the sample.
The stability of the photocatalyst was examined to find out the cost effectiveness of the method by reusing the photocatalyst to its maximum extent [48]. To study the photocatalytic stability of CF/SrTiO3/CdS-2 composite fiber, the cyclic experiment of photocatalytic hydrogen production was carried out, and the test results are shown in Figure 9. After four consecutive cyclic tests of hydrogen production evolution performance, the average photocatalytic hydrogen evolution of the CF/SrTiO3/CdS-2 sample was about 540.39 μmol/g·h. There appeared to be a slight decline over the cyclic experiments, and the test results showed that CF/SrTiO3/CdS-2 maintained relatively stable photocatalytic performance.
The absorption response of catalyst to light was a key factor affecting the photocatalytic performance. The light absorption performances about CF/SrTiO3, CF/SrTiO3/CdS, CF, SrTiO3, and CdS were tested by using a Hitachi F-7000 spectrophotometer. As seen in Figure 10, it could be known from the analysis that the characteristic absorption edge of SrTiO3 was around 375 nm [49], which belongs to ultraviolet light absorption, while the characteristic absorption edge of CdS was around 565 nm, which had a good absorption capacity in the range of visible light. With the introduction of CdS, the light absorption capacity of the composite photocatalytic fiber was improved, which was beneficial to improving the photocatalytic hydrogen production performance.
Based on Kubelka–Munk theory [50], as shown in Figure 11, it can be obtained that the band gap of CdS alone was about 2.23 eV, and the band gap of pure SrTiO3 was about 3.32 eV.
Based on the Mott–Schottky equation [51], the tangent slopes of the Mott–Schottky spectral lines of SrTiO3 and CdS were positive (Figure 12), indicating that SrTiO3 and CdS were n-type. The flatband potentials about SrTiO3 and CdS were, respectively, −0.69 eV and −0.73 eV (calomel electrode vs. SCE). The potential of aqueous SCE relative to SHE was about 0.24 eV at 25 °C [52], and thus the Fermi levels of SrTiO3 and CdS were, respectively, about –0.45 eV and −0.49 eV. It was generally believed that the conduction band position of the n-type semiconductor was 0.1 eV to the Fermi level [53], so the conduction band positions of SrTiO3 and CdS were, respectively, −0.55 eV and −0.59 eV, and the conduction band position of CdS was higher than that of SrTiO3. The band gaps of SrTiO3 and CdS were, respectively, 3.32 eV and 2.23 eV, as shown in Figure 11. We can deduce that the valence band positions of SrTiO3 and CdS were, respectively, 2.77 eV and 1.64 eV.
Carrier separation and migration rates were the main factors affecting the performance of the photocatalyst, and the separation and migration rates of photoelectron–hole pairs could be evaluated by testing the transient photocurrent response and the electrochemical impedance [54,55]. Figure 13 shows the transient photocurrent response spectra and impedance spectra of the photocatalytic materials such as CF/SrTiO3, CF/SrTiO3/CdS-1, CF/SrTiO3/CdS-2, and CF/SrTiO3/CdS-3. As shown in Figure 13a, in the same illumination conditions, the photocurrent of CF/SrTiO3 was about 0.04 µA, while the photocurrent of CF/SrTiO3/CdS-2 was up to 0.23 µA, which was about six times that of CF/SrTiO3. The carbon fiber had no photocurrent response, but it could transfer charges. As for CF/SrTiO3/CdS-2, the substances which had transient photocurrent responses were mainly SrTiO3 and CdS, wherein SrTiO3 mainly absorbed ultraviolet light, and CdS absorbed visible light. The loading of CdS significantly increased the transient photocurrent of the catalyst as well as the generation and separation rates of photoelectron–hole pairs, which was beneficial to improving the photocatalytic hydrogen production performance. Figure 13b shows each of the impedance spectra of CF, CF/SrTiO3, CF/SrTiO3/CdS-1, CF/SrTiO3/CdS-2, and CF/SrTiO3/CdS-3. The high frequency region was composed of an arc, and the radius of the arc reflected the intensity of the charge transfer resistance, as the smaller the radius of the arc was, the higher the migration rate of the carriers would be [56]. Therefore, the photocarrier migration rate of CF/SrTiO3/CdS was higher than that of CF/SrTiO3 and carbon fiber. The above results showed that the separation and migration rates of carriers in the CF/SrTiO3/CdS-2 material was the highest, and that is why the photocatalytic hydrogen production performance was excellent, which was consistent with the results of the photocatalytic hydrogen production experiment shown in Figure 8.
To sum up, a photocatalytic hydrogen production mechanism of CF/SrTiO3/CdS was proposed. As shown in Figure 14, the loading of CdS significantly enhanced the visible light absorption capacity of the catalyst and promoted the rapid separation and high-speed migration of photocarriers. Carbon fibers themselves had no hydrogen production performance, and SrTiO3 and CdS alone showed relatively weak HER performances, but the combination of the three showed a relatively strong HER performance. This was because the carbon fibers had a strong electron conduction performance and acted as co-catalysts, CdS had a good absorption performance for visible light, SrTiO3 in the prepared composite catalyst contained abundant oxygen vacancies, and these three factors played a synergistic role in the formation of heterojunctions. As the conduction band position of CdS was higher than that of SrTiO3, photoelectrons migrated from the conduction band position of CdS to the surface of SrTiO3 and then to the carbon fibers. Furthermore, the holes at the valence band position of SrTiO3 migrated to the surface of CdS, thus promoting the fast migration of photocarriers and preventing the recombination of electron–hole pairs. Furthermore, the holes on the SrTiO3 and CdS were easily captured by the sacrificial agents (Na2S and Na2SO3), which promoted the generation and separation of photoelectron–hole pairs and facilitated the hydrogen production of the catalyst.

4. Conclusions

In this paper, bamboo pulp fibers were used as precursors of carbon fibers. CF/SrTiO3 composite photocatalytic materials were prepared first by loading and then carbonization, and heterojunctions were formed by loading CdS nanospheres on the surface of the CF/SrTiO3 composite material through the hydrothermal method in order to prepare CF/SrTiO3/CdS with a double-layer core–shell structure. By adjusting the concentration of CdS precursor reagent, the content of CdS in CF/SrTiO3/CdS composite fiber materials was regulated. The photocatalytic hydrogen production experiment showed that the hydrogen production rate of the CF/SrTiO3/CdS-2 sample under the optimized process was as high as 577.39 μmol/g·h, which was about 11 times that of CF/SrTiO3. Carbon fiber itself has no hydrogen production performance, and single SrTiO3 or CdS show relatively weak hydrogen production performance, but this composite photocatalytic material system showed relatively strong photocatalytic hydrogen production performance. The improvement of high photocatalytic hydrogen production performance was mainly attributed to the good light absorption capacity of CdS with a narrow band gap and the formation of excellent heterojunction structures after the composite with SrTiO3, as well as the oxygen vacancy in SrTiO3 and the excellent electrical conductivity of carbon fiber. Carbon fiber taken as the carrier for CF/SrTiO3/CdS could solve the difficult problem of recycling of traditional SrTiO3- and CdS-dispersed photocatalytic nanomaterials, which is conducive to recycling and improving economic benefits. The preparation methods of CF/SrTiO3 and CF/SrTiO3/CdS materials can also be used in environmental purification, CO2 reduction, electrocatalysis, and other fields and can also be extended to sensors and other directions.

Author Contributions

Conceptualization, K.-Q.Z. and M.Y.; data curation, Q.H. and J.N.; funding acquisition, J.N. and K.-Q.Z.; Investigation, Q.H. and J.N.; methodology, Q.H. and J.N.; supervision, K.-Q.Z. and M.Y.; writing—original draft, Q.H.; writing—review & editing, Q.H. and K.-Q.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by National Key Research and Development Program of China, grant number [2017YFA0204600], and the Qing Lan Project of Jiangsu Province in China.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data is contained within the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Scheme 1. Flow chart for preparing CF/SrTiO3/CdS double-layer core–shell structure heterojunctions.
Scheme 1. Flow chart for preparing CF/SrTiO3/CdS double-layer core–shell structure heterojunctions.
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Figure 1. XRD energy spectra of samples.
Figure 1. XRD energy spectra of samples.
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Figure 2. SEM diagram of samples: (a) carbon fiber (CF), (b,c) CF/SrTiO3, (d–f) CF/SrTiO3/CdS.
Figure 2. SEM diagram of samples: (a) carbon fiber (CF), (b,c) CF/SrTiO3, (d–f) CF/SrTiO3/CdS.
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Figure 3. Element distribution of CF/SrTiO3/CdS photocatalytic fiber: (a) C element, (b) Ti element, (c) Sr element, (d) O element, (e) S element, (f) Cd element, and (g) element content diagram of CF/SrTiO3/CdS.
Figure 3. Element distribution of CF/SrTiO3/CdS photocatalytic fiber: (a) C element, (b) Ti element, (c) Sr element, (d) O element, (e) S element, (f) Cd element, and (g) element content diagram of CF/SrTiO3/CdS.
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Figure 4. XPS spectra of CF/SrTiO3/CdS: (a) full spectrum, (b) C 1s spectrum, (c) Ti 2p spectrum, (d) Sr 3d spectrum, (e) O 1s spectrum, (f) S 2p spectrum, and (g) Cd 3d spectrum.
Figure 4. XPS spectra of CF/SrTiO3/CdS: (a) full spectrum, (b) C 1s spectrum, (c) Ti 2p spectrum, (d) Sr 3d spectrum, (e) O 1s spectrum, (f) S 2p spectrum, and (g) Cd 3d spectrum.
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Figure 5. EPR oxygen vacancies of CF/SrTiO3.
Figure 5. EPR oxygen vacancies of CF/SrTiO3.
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Figure 6. Nitrogen adsorption–desorption isothermal curve (a) and mesopore distribution (b) of CF/SrTiO3/CdS.
Figure 6. Nitrogen adsorption–desorption isothermal curve (a) and mesopore distribution (b) of CF/SrTiO3/CdS.
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Figure 7. TG–DSC curve of CF/SrTiO3/CdS.
Figure 7. TG–DSC curve of CF/SrTiO3/CdS.
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Figure 8. Photocatalytic hydrogen production rates (a) and time-dependent hydrogen production amounts (b) of different samples.
Figure 8. Photocatalytic hydrogen production rates (a) and time-dependent hydrogen production amounts (b) of different samples.
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Figure 9. Photocatalytic stability of CF/SrTiO3/CdS-2 composite fiber, (a) cycle curve of hydrogen production amount, and (b) the corresponding histogram.
Figure 9. Photocatalytic stability of CF/SrTiO3/CdS-2 composite fiber, (a) cycle curve of hydrogen production amount, and (b) the corresponding histogram.
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Figure 10. UV–vis diffuse reflection spectra of samples.
Figure 10. UV–vis diffuse reflection spectra of samples.
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Figure 11. Band gaps of SrTiO3 and CdS.
Figure 11. Band gaps of SrTiO3 and CdS.
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Figure 12. Mott–Schottky plots of samples: (a) SrTiO3 and (b) CdS.
Figure 12. Mott–Schottky plots of samples: (a) SrTiO3 and (b) CdS.
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Figure 13. Transient photocurrent response spectra (a) and impedance spectra (b) of different samples.
Figure 13. Transient photocurrent response spectra (a) and impedance spectra (b) of different samples.
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Figure 14. Photocatalytic hydrogen production mechanism of CF/SrTiO3/CdS.
Figure 14. Photocatalytic hydrogen production mechanism of CF/SrTiO3/CdS.
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Hu, Q.; Niu, J.; Zhang, K.-Q.; Yao, M. One-Dimensional CdS/SrTiO3/Carbon Fiber Core–Shell Photocatalysts for Enhanced Photocatalytic Hydrogen Evolution. Coatings 2022, 12, 1235. https://doi.org/10.3390/coatings12091235

AMA Style

Hu Q, Niu J, Zhang K-Q, Yao M. One-Dimensional CdS/SrTiO3/Carbon Fiber Core–Shell Photocatalysts for Enhanced Photocatalytic Hydrogen Evolution. Coatings. 2022; 12(9):1235. https://doi.org/10.3390/coatings12091235

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Hu, Qi, Jiantao Niu, Ke-Qin Zhang, and Mu Yao. 2022. "One-Dimensional CdS/SrTiO3/Carbon Fiber Core–Shell Photocatalysts for Enhanced Photocatalytic Hydrogen Evolution" Coatings 12, no. 9: 1235. https://doi.org/10.3390/coatings12091235

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