Superior performance in visible-light-driven hydrogen evolution reaction of three-dimensionally ordered macroporous SrTiO₃ decorated with Zn_xCd_1−xS

Abstract

It is of broad interest to develop emerging photocatalysts with excellent light-harvesting capacity and high charge carrier separation efficiency for visible light photocatalytic hydrogen evolution reaction. However, achieving satisfying hydrogen evolution efficiency under noble metal-free conditions remains challenging. In this study, we demonstrate the fabrication of three-dimensionally ordered macroporous SrTiO₃ decorated with Zn_xCd_1−xS nanoparticles for hydrogen production under visible light irradiation (λ>420 nm). Synergetic enhancement of photocatalytic activity is achieved by the slow photon effect and improved separation efficiency of photogenerated charge carriers. The obtained composites could afford very high hydrogen production efficiencies up to 19.67 mmol·g⁻¹·h⁻¹, with an apparent quantum efficiency of 35.9% at 420 nm, which is 4.2 and 23.9 times higher than those of pure Zn_0.5Cd_0.5S (4.67 mmol·g⁻¹·h⁻¹) and CdS (0.82 mmol·g⁻¹·h⁻¹), respectively. In particular, under Pt-free conditions, an attractive hydrogen production rate (3.23 mmol·g⁻¹·h⁻¹) was achieved, providing a low-cost and high-efficiency strategy to produce hydrogen from water splitting. Moreover, the composites showed excellent stability, and no obvious loss in activity was observed after five cycling tests.

Key words： three-dimensionally ordered macroporous SrTiO₃; Zn_xCd_1–xS; visible light; hydrogen production; promotion mechanism

1 Introduction

The utility of solar energy to produce hydrogen by semiconductor photocatalysts has become a promising alternative to fossil fuels to reduce global warming [1]. In the past decades, considerable effort has been devoted to adjust the energy band structure [2,3], enhance the charge separation efficiency [4,5], and increase the light-harvesting capacity [6,7] of semiconductors. However, developing stable and efficient photocatalysts with wide-spectrum response remains challenging [8]. Recently, three-dimensionally ordered macroporous (3DOM) semiconductor materials with inverse opal structure have received enormous attention in the photocatalysis field due to their unique slow photon effect and extremely improved light-harvesting efficiency [9–11]. Various 3DOM semiconductors, such as WO₃ [12,13], TiO₂ [14,15], g-C₃N₄ [16], MVO₄ (M= In, Bi) [17,18], and MoP [19], were synthesized and used as photocatalysts in water splitting and photodegradation of organic contaminants.

As a typical simple cubic perovskite material, strontium titanate (SrTiO₃) has extensive application prospects in the photocatalysis field due to its advantages of tailorable chemical and electronic structures, high redox property, good stability, low cost, and environmental friendliness [20,21]. Recently, the 3DOM SrTiO₃ materials (3D-STO) with varying macropore diameters were first reported by our group [22]. This kind of photocatalyst exhibited significantly improved photocatalytic efficiency in hydrogen production [23], CO₂ photocatalytic reduction [24], and contaminant photodegradation [25]. However, pristine 3D-STO often suffer from limited spectral response ability and serious photogenerated charge carriers’ recombination due to their large bandgap (ca. 3.2 eV) and excessive structural defects, thus impeding their real application in photocatalysis field.

S-based semiconductors have been widely investigated as photocatalysts for hydrogen evolution reaction from water splitting because their suitable valence band potential and narrow bandgap endow them with wide-spectrum responsibility [26–29]. In particular, the solid solution materials Zn_xCd_1−xS, which possess adjustable electronic band structure by tuning the molar ratio of Zn/Cd [30,31], improved stability, and less toxicity compared with CdS [32], exhibited superior photocatalytic hydrogen production performance under visible light irradiation [33–35]. However, a relatively high recombination rate of photogenerated electrons and holes were observed over pristine Zn_xCd_1−xS solid solution materials [36]. Therefore, decorating the Zn_xCd_1−xS nanoparticles into the macropores of 3D-STO should be an efficient strategy to combine the slow photon effect of 3DOM materials and inhibit recombination of charge carriers of heterojunction structure.

In this study, Zn_xCd_1−xS solid solution materials with different Zn/Cd ratios were synthesized through a chemical co-precipitation method in the presence of α-monothioglycerol. The obtained Zn_xCd_1−xS nanoparticles were dispersed into the macropores of 3D-STO with a variable mass ratio to fabricate composite photocatalysts. These photocatalysts exhibited excellent hydrogen evolution rates (up to 19.67 mmol·g⁻¹·h⁻¹) and good reusability under visible light irradiation (λ>420 nm). Notably, an attractive hydrogen evolution rate (3.23 mmol·g⁻¹·h⁻¹) can be achieved in the absence of Pt as co-catalyst. The promoted efficiency of the photocatalysts could be attributed to the synergistic effect of the slow photon effect of 3D-STO and the improved charge transfer efficiency. Thus, this study provides a promising strategy to fabricate highly efficient visible-light-driven photocatalysts for hydrogen evolution even under noble metal-free conditions.

2 Experimental

2.1 Materials

Methyl methacrylate (stabilized with 30 mg·L⁻¹p-methoxyphenol), tetrabutyl titanate [Ti(OBu)₄], and chloroplatinic acid hexahydrate (H₂PtCl₆·6H₂O) were obtained from Aladdin. Strontium nitrate [Sr(NO₃)₂] and lanthanum nitrate [La(NO₃)₂] was purchased from Tianjin Bodi Chemical Co., Ltd. Potassium persulfate (K₂S₂O₈) and citric acid monohydrate (C₆H₈O₇) were supplied by Sinopharm Chemical Reagent Co., Ltd. Sodium sulfide hydrate (Na₂S·9H₂O), cadmium acetate dihydrate [Cd(CH₃CO₂)₂·2H₂O], zinc acetate [Zn(CH₃CO₂)₂], Na₂SO₃, and α-monothioglycerol (C₃H₈O₂S) were obtained from Shanghai Macklin Biochemical Co., Ltd. Nafion® 117 solution (ca. 5% in a mixture of lower aliphatic alcohols and water) was purchased from Sigma-Aldrich. All chemicals were reagent grade and used as received without further purification.

2.2 Characterization

Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku SmartLab X-ray diffractometer (Cu Kα radiation) with a scanning rate of 6°·min⁻¹ at 40 kV and 40 mA. The morphology, elemental mapping, and lattice fringes of the samples were observed using a JEOL JSM-7500F field-emission scanning electron microscope (SEM) and a Philips Tecnai G2 F20 field-emission transmission electron microscope (TEM) equipped with energy dispersive X-ray spectrometers (EDS). The diffuse reflection ultraviolet-visible spectra (DR UV-Vis) of samples were tested by Shimazu UV-2550 spectrometer equipped with an integrating sphere using BaSO₄ as a reference in the range of 200–800 nm. Brauner-Emmett-Teller (BET) specific surface areas of the samples were detected by Micromeritics Tristar 3000 instrument. Photoluminescence (PL) spectra were collected using the Horiba Fluorolog-3 instrument, and the wavelength of the exciting light was 400 nm.

2.3 Synthesis of the samples

2.3.1 Synthesis of 3D-STO

The 3D-STO material was synthesized via a colloidal crystal template (CCT) method according to our previous report [22]. Polymethyl methacrylate (PMMA) microspheres with an average particle size of 300 nm were used. In the experiment, tetrabutyl titanate (0.01 mol) and acetic acid (10 mL) were added to a beaker. Then, deionized water (10 mL) was added dropwise with magnetic stirring. Afterward, a 10 mL aqueous solution containing 0.01 mol lanthanum nitrate and 10 mL citric acid solution (2 mol·L⁻¹) was sequentially added dropwise to the beaker. The resulting solution was stirred at room temperature for another 30 min, and then a clear solution was obtained. Then, the PMMA CCT was soaked in the abovementioned precursor solution for about 3 h and collected by filtration to remove excess precursor solution. After vacuum drying at 50 °C overnight, the samples were heated in a muffle furnace to 650 °C with a heating rate of 1 °C·min⁻¹ and calcined for 4 h. The obtained material was labeled 3D-STO. For comparison, a SrTiO₃ material with a disordered porous structure was prepared through a process similar to that of 3D-STO, except that the CCT was replaced by a disordered polymer microsphere template. The obtained sample was labeled Disorder-STO.

2.3.2 Synthesis of Zn_xCd_1−xS solid solution materials

The Zn_xCd_1−xS solid solution nanoparticles were prepared through a chemical co-precipitation method in the presence of α-monothioglycerol according to the literature [37]. For the synthesis of Zn_0.5Cd_0.5S, the solution of cadmium acetate (10 mL, 0.5 mol·L⁻¹), zinc acetate (10 mL, 0.5 mol·L⁻¹), sodium sulfide (20 mL, 0.5 mol·L⁻¹) and 20 mL α-monothioglycerol were added into a round bottom flask. The reaction was performed at 80 °C for 3 h under magnetic stirring. After cooling to room temperature, the orange precipitate was centrifuged and then washed with deionized water and ethanol several times. The samples were dried in a vacuum at 50 °C for 9 h. By tuning the molar ratio of Zn/Cd in the synthesis process, a series of Zn_xCd_1−xS (x = 0, 0.15, 0.25, 0.5, 0.75, 0.85, 1) solid solution materials were obtained. For simplicity, the Zn_0.5Cd_0.5S was labeled ZCS.

2.3.3 Synthesis of ZCS-modified 3D-STO

3D-STO (100 mg) and different amounts of ZCS (10, 25, 50, 75, and 100 mg) were dispersed in 35 mL deionized water under magnetic stirring for 4 h. Then, the solid powders were collected by centrifugation and dried under vacuum at 50 °C for 9 h. The obtained ZCS-modified 3D-STO photocatalyst was labeled SZy, where y represents the mass ratio of ZCS in the composites.

2.4 Photocatalytic hydrogen evolution reactions

The hydrogen evolution reactions were performed in a 150 mL quartz reactor with a continuous state of vacuum under visible light irradiation (λ>420 nm). Typically, the photocatalyst (25 mg) and 50 mL deionized water containing Na₂SO₃ (0.1 mol·L⁻¹) and Na₂S (0.1 mol·L⁻¹) were added into the quartz reactor. A certain amount of chloroplatinic acid aqueous solution was used as the precursor of Pt co-catalyst through an in situ photo-deposition method. Then, the reactor was irradiated by a 300 W Xe lamp with an ultraviolet cut-off filter (λ>420 nm) under magnetic stirring and degassing conditions. Online gas chromatograph was used to real-time monitor the hydrogen evolution rate in real time.

2.5 Photoelectrochemical measurements

The obtained photocatalyst (2 mg) was dispersed into a 1 mL mixed solution made up of ethanol and Nafion^® 117 with a volume ratio of 100:1. After ultrasonic treatment for 10 min, the supernatant of the mixture was dropped onto the fluorine-doped tin oxide (FTO) glass with an area of 1.0 cm × 1.0 cm and the obtained electrode was dried overnight at room temperature. The FTO glass containing the sample acted as the working electrode, Pt plate acted as the counter electrode, and Ag/AgCl (saturated KCl) was the reference electrode. A 0.1 mol·L⁻¹ Na₂SO₄ aqueous solution was used as the electrolyte. The electrochemical impedance spectroscopy (EIS) and Mott-Schottky (M-S) measurements were performed on an electrochemical workstation (CHI 760D) using a conventional three-electrode cell. Besides, the open-circuit voltage of the photocurrent-time was fixed at 0.36 V and the light source was supplied by a 300W Xe lamp with a 420 nm ultraviolet cut-off filter. The alternating current signal was set to a frequency of 1 kHz in the M-S tests.

3 Results and discussion

3.1 Characterization of the samples

The XRD patterns of as-synthesized samples are shown in Fig. 1. From Fig. 1(a), the characteristic diffraction peaks of pure CdS and ZnS were identified as cubic sphalerite phase (JCPDF No. 05-0566) and cubic hawleyite structure (JCPDF No. 10-0454), respectively. Meanwhile, with the increase in Zn molar ratio in Zn_xCd_1−xS, the diffraction peaks shifted to a larger diffraction angle direction due to the smaller lattice parameter of ZnS compared with CdS. No obvious peak broadening or splitting was observed, indicating the successful synthesis of Zn_xCd_1−xS solid solution materials. Figure 1(b) shows the powder XRD patterns of the obtained SZy photocatalysts. The characteristic diffraction peaks of 3D-STO could be indexed to a typical cubic perovskite phase (JCPDS No.35-0734). With the increase in the loading amount of ZCS, the (111) peak intensity of Zn_xCd_1−xS gradually increased. No other peaks related to impurities were observed from these XRD patterns, suggesting the successful synthesis of SZy photocatalysts.

Fig.1 Powder XRD patterns of the as-synthesized (a) Zn_xCd_1−xS solid solutions and (b) SZy composites.

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Figure 2 shows the SEM images of the obtained SZy photocatalysts. As shown in Fig. 2(a), a well-ordered inverse opal structure of pure 3D-STO was observed. The diameter of the macropores was 185 nm. From Figs. 2(b–f), the ZCS nanoparticles were decorated into the macropores of 3D-STO. With the increase in ZCS loading amount, the macropores of 3D-STO were gradually filled. However, the well-ordered inverse opal structure of 3D-STO was maintained on these SZy composites. As for SZ50, the surfaces of 3D-STO were nearly completely covered by ZCS (Fig. 2(f)). The SEM-EDS elemental mapping images of SZ33 are shown in Fig. S1 (cf. Electronic Supplementary Material, ESM). Six elements, including Sr, Ti, O, Zn, Cd, and S, were evenly dispersed in SZ33, suggesting the uniform dispersion of ZCS on the surface of 3D-STO. The SEM images of Disorder-SZ33 are shown in Fig. S2 (cf. ESM). Some ZCS nanoparticles were decorated on the surface of Disorder-STO with a disordered macroporous structure.

Fig.2 SEM images of (a) 3D-STO, (b) SZ9, (c) SZ20, (d) SZ33, (e) SZ42, and (f) SZ50.

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The TEM and high-resolution TEM (HRTEM) images of SZ33 are shown in Fig. 3. The ZCS nanoparticles with a particle size of 10–50 nm (as shown in Fig. S3 (cf. ESM)) dispersed onto the macropores of 3D-STO. The marked lattice spacings of 0.27 and 0.32 nm corresponded to the (110) plane of 3D-STO and (111) plane of ZCS, respectively. As shown in Fig. S4 (cf. ESM), the Ti, Sr, and O elements were evenly dispersed in SZ33 and some ZCS nanoparticles decorated on the surface of 3D-STO. The BET specific surface areas and Barrett-Joyner-Halenda (BJH) pore volumes of the samples were detected. The results are listed in Table 1. The BET surface areas of 3D-STO and ZCS were 44.4 and 154.4 m²·g⁻¹, respectively. The larger BET surface area of ZCS could be attributed to its smaller particle size. For SZy composites, with the increase in ZCS loading amount, their surface areas gradually increased from 45.8 to 104.1 m²·g⁻¹. However, the composite photocatalysts exhibited increased BJH pore volumes compared with pure 3D-STO and ZCS, which could be attributed to the aggregation of ZCS nanoparticles in the macropores of 3D-STO, resulting in the formation of accumulated mesoporous structure.

Fig.3 (a) TEM and (b) HRTEM images of SZ33.

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Tab.1 Specific surface areas and pore volumes of the samples

Sample	BET surface area/(m2·g–1)	BJH pore volume/(cm3·g–1)
3D-STO	44.4	0.164
SZ9	45.8	0.189
SZ20	57.1	0.190
SZ33	86.6	0.193
SZ42	135.9	0.305
SZ50	104.1	0.226
ZCS	154.4	0.091

Figure 4 presents the DR UV-Vis spectra and the (αhν)²-hν plots of the samples. As shown in Fig. 4(a), pure 3D-STO only absorbed ultraviolet light with a wavelength below 380 nm, whereas ZCS exhibited an obvious absorbance in the visible light region starting from 700 nm. The optical absorption edges of SZy composites were redshifted to approximately 560 nm compared with 3D-STO. Moreover, with the increase in ZCS loading amount, the light absorption ability of SZy in the visible light region gradually enhanced. The DR UV-Vis spectrum of Disorder-SZ33 is shown in Fig. S5 (cf. ESM). A very similar light absorption curve with SZ33 appeared. Through a transform of DR UV-Vis spectra using the Kubelka–Munk function, the plots of (αhν)²-hν of 3D-STO and ZCS were obtained (Fig. 4(b)) [38,39]. The bandgaps of 3D-STO and ZCS were determined as approximately 3.27 and 2.20 eV, respectively.

Fig.4 (a) DR UV-Vis spectra of 3D-STO, ZCS and SZy; (b) (αhν)²-hν plots of 3D-STO and ZCS.

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The surface composition and chemical states of SZ33 and its single counterparts were determined by XPS characterization. The results are shown in Figs. 5, S6 (cf. ESM), and S7 (cf. ESM). The Ti 2p XPS spectrum exhibited two signals of Ti 2p_3/2 and Ti 2p_1/2 with the binding energies of 458.2 and 464.0 eV, respectively (Fig. 5(a)), which could be ascribed to Ti⁴⁺. The Sr 3d XPS spectrum (Fig. 5(b)) showed two peaks at 132.6 and 134.4 eV, corresponding to the characteristic binding energies of Sr 3d_5/2 and 3d_3/2 of Sr²⁺, respectively. The Cd 3d XPS spectrum (Fig. 5(c)) exhibited two peaks at the binding energies of 404.6 and 411.3 eV, corresponding to Cd 3d_5/2 and 3d_3/2 of Cd²⁺, respectively [40]. Figure 5(d) showed that two peaks of Zn 2p_3/2 and Zn 2p_1/2 were discerned at 1021.9 and 1044.5 eV, which were the typical values of Zn²⁺. As depicted in Fig. 5(e), the O 1s XPS spectrum revealed several components. The peak at 529.6 eV corresponded to the lattice stoichiometric oxygen in STO, whereas the peak at 531.6 eV could be assigned to the chemisorbed oxygen species, which could be related to the surface defective oxygen [41]. The S 2p XPS spectrum of SZ33 (Fig. 5(f)) can be deconvoluted into three peaks with binding energies at 161.2, 162.7, and 164.1 eV. The peaks located at 161.2 and 162.7 eV could be, respectively, ascribed to the S 2p_3/2 and 2p_1/2 peaks of S^2– [42], which can also be observed in the S 2p XPS spectrum of pure ZCS (Fig. S5(c)). Notably, a new peak at 164.1 eV appeared in the S 2p XPS spectrum of SZ33 but not pure ZCS. It could be assigned to the formation of an interfacial metal−sulfur bond (e.g., Ti‒S bond) [43], which should facilitate charge transfer through the interface.

Fig.5 High-resolution XPS spectra of (a) Ti 2p, (b) Sr 3d, (c) Cd 3d, (d) Zn 2p, (e) O 1s, and (f) S 2p of SZ33.

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3.2 Photocatalytic hydrogen evolution reactions

The obtained photocatalysts were evaluated in photocatalytic hydrogen evolution reactions under visible light irradiation (λ>420 nm), and the results are shown in Fig. 6. As shown in Fig. 6(a), Zn_xCd_1−xS solid solutions exhibited obviously enhanced photocatalytic activity compared with pure ZnS and CdS. Among the Zn_xCd_1−xS samples, ZCS showed the highest hydrogen evolution rate (4.67 mmol·g⁻¹·h⁻¹), which was 8.1 times as high as that of pure CdS (0.58 mmol·g⁻¹·h⁻¹). Therefore, ZCS was selected for further investigation.

Fig.6 Photocatalytic hydrogen evolution rates under visible light irradiation (λ>420 nm) catalyzed by (a) Zn_xCd_1–xS and (b) SZy loading 1.5 wt-% Pt as co-catalyst, and (c) SZ33 with different Pt loading amount; (d) photocatalytic hydrogen evolution rates under visible light irradiation (λ>420 nm) on SZ33 and Disorder-SZ33.

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From Fig. 6(b), SZ33, SZ42, and SZ50 exhibited improved hydrogen evolution rates compared with pure ZCS. 3D-STO exhibited a negligible hydrogen evolution rate under visible light irradiation due to its large bandgap. In particular, among the SZy samples, SZ33 showed the highest hydrogen evolution rate (19.7  mmol·g⁻¹·h⁻¹), which was 4.2 times higher than that of pure ZCS. Moreover, compared with similar S-based semiconductor photocatalysts reported in the literature (as listed in Table S1, cf. ESM), SZ33 still exhibited significantly enhanced performance in hydrogen evolution reactions. SZ9 and SZ20 exhibited lower activity than pure ZCS, which could be attributed to the lower contents of ZCS with visible light responsibility. However, SZ42 and SZ50 with higher loading amount of ZCS exhibited lower activity than SZ33 because more ZCS nanoparticles filled up the macropores of 3D-STO, which not only decreased the efficiency of interfacial charge transfer but also affected the slow photon effect of 3D-STO.

Figure 6(c) shows the photocatalytic hydrogen evolution rates of SZ33 with different Pt loading amounts. The highest hydrogen evolution rate can be achieved when 1.5 wt-% Pt was in situ photo-deposited as co-catalyst. The hydrogen evolution performance of SZ33 would be suppressed under a low or high loading amount of Pt co-catalyst, which could be explained by the reduced proton reduction active sites or the increased particle size of co-catalysts. Remarkably, even under Pt-free conditions, SZ33 still exhibited a promising hydrogen evolution rate (3.23 mmol·g⁻¹·h⁻¹), which was close to the performance of pure ZCS using 1.5 wt-% Pt as co-catalysts, and much higher than that of pure CdS with 1.5 wt-% Pt. Therefore, SZ33 should be a promising candidate for noble metal-free photocatalysts in hydrogen evolution reaction under visible light irradiation. To properly evaluate the efficiency of the obtained photocatalysts, the apparent quantum efficiency value of SZ33 for hydrogen evolution was measured according to the reported method [44] under 420 nm wavelength light irradiation, and the calculated value was 35.9%.

To further confirm the slow photon effect of the SZy composites, the catalytic activity of SZ33 was compared with that of Disorder-SZ33, which was synthesized using Disorder-STO and ZCS. As shown in Fig. 6(d), SZ33 exhibited higher performance than Disorder-SZ33 (9.45 mmol·g⁻¹·h⁻¹). The hydrogen evolution rate of SZ33 was two times than that of Disorder-SZ33, confirming the contribution of the slow photon effect of 3D-STO.

The stability of SZy composite photocatalysts was evaluated by a cycling experiment as shown in Fig. 7. From Fig. 7(a), after five photocatalytic experiment cycles, the hydrogen evolution rate of SZ33 attained 16.3 mmol·g⁻¹·h⁻¹. Only a slight decrease of activity was observed, which might be attributed to the consumption of sacrifice reagents and the partial photocorrosion of some exposed ZCS [45]. In addition, no obvious changes can be observed from the XRD patterns of fresh and used SZ33 samples (Fig. 7(b)), indicating that the obtained composite photocatalysts were quite stable.

Fig.7 (a) Cycling experiments of SZ33 in photocatalytic hydrogen evolution under visible light irradiation (λ>420 nm); (b) powder XRD patterns of fresh and used SZ33.

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3.3 Promoting mechanism of photocatalysts

According to the discussion in the above section, the slow photon effect of 3D-STO was crucial in promoting the photocatalytic activity of SZy composites. Thus, the photonic stop-band of 3D-STO used in the experiments was calculated through modified Bragg’s law (Eq. (1)) [46,47]:

(1){\lambda }_{\max }=\mathrm{2}\sqrt{{\displaystyle \frac{\mathrm{2}}{\mathrm{3}}}}D\sqrt{n_{{}_{{\mathrm{SrTiO}}_{\mathrm{3}}}}^{\mathrm{2}}f+n_{\mathrm{water}}^{\mathrm{2}}\mathrm{(1}-f)-{\sin }^{\mathrm{2}}\theta },

where λ_max is the wavelength of the photonic bandgap, D is the diameter of macropores (D = 185 nm in this study), n is the refractive index of the substrate, f is the volume fraction of 3D-STO (usually given as 0.26), and θ is the incident angle of light. For normal incidence (θ = 0°), the calculated photonic stop-band was 507 nm, meaning that the incident light with a wavelength of 507 nm should be completely reflected. For the suspended photocatalysts, the multiple reflections of incident light should increase the photon absorption probability. As for the incident lights with the wavelength at the blue [48] and red edges [10] of the stop-band, their group velocity should be significantly suppressed and the photon-matter interaction should be enhanced. For SZ33, from Fig. 4(a), the effective light absorption range of SZ33 was located in the range of 420–570 nm, which exactly overlapped with the blue and red edges of the stop-band of 3D-STO. Therefore, it can be concluded that the slow photon effect of 3D-STO played a crucial role in promoting the light-harvesting efficiency of photocatalysts.

To investigate the charge transfer and separation efficiency of SZy photocatalysts, PL spectra and EIS Nyquist plots of the samples were recorded, and the results are shown in Fig. 8. Figure 8(a) shows the PL spectra of 3D-STO, ZCS, and SZ33. The emission intensity of SZ33 was lower than that of 3D-STO and ZCS, indicating the improved separation efficiency of electrons and holes in the SZ33 composite. Figure 8(b) shows the EIS Nyquist plots of the samples. SZ33 exhibited a smaller semicircle diameter than 3D-STO and ZCS, suggesting the smaller equivalent resistance of the charge transfer at the electrode interface. The results of PL spectra and EIS Nyquist plots indicated that the composite photocatalyst exhibited enhanced charge migration and separation efficiency compared with their single counterparts.

Fig.8 (a) PL spectra and (b) EIS Nyquist plots of 3D-STO, ZCS and SZ33.

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To investigate the charge transfer mechanism of obtained composite photocatalysts, the electronic band structure of 3D-STO and ZCS was determined using M-S tests and measured bandgap energies (Fig. 9). From Fig. 9(a), the slopes of the M-S curves of the 3D-STO and ZCS were positive, indicating that they are n-type semiconductors. The point that the tangent line of an M-S curve intersects the X-axis is the flat band potential of the sample. For n-type semiconductors, the flat band potential can be approximated as its conduction band (CB) potential. Based on the M-S tests, the CB potentials of 3D-STO and ZCS were determined as –0.65 and –0.75 V (vs. Ag/AgCl), respectively. According to E(NHE) = E(Ag/AgCl) + 0.197, their CB potentials can be calculated to be –0.45 and –0.55 V (vs. NHE), respectively. From Fig. 4(b), the bandgaps of 3D-STO and ZCS were 3.27 and 2.20 eV, respectively. Thus, their valence band potentials can be determined as 2.78 V and 1.63 V (vs. NHE) for 3D-STO and ZCS, respectively. Based on these results, the electronic band structure of the composite photocatalysts can be described in Fig. 9(b). The electrons in the ZCS will be separated from the holes under visible light irradiation. As the CB potential of 3D-STO was higher than that of ZCS, the electrons in the ZCS will migrate to 3D-STO, resulting in the separation of photogenerated electrons and holes. Therefore, the hydrogen evolution reaction occurred on the CB of 3D-STO, and the oxidation reaction occurred on the valence band of ZCS.

Fig.9 (a) M-S measurements in 0.1 mol·L⁻¹ Na₂SO₄ electrolyte solution of 3D-STO and ZCS samples; (b) schematic diagrams of band structures and proposed charge transfer mechanisms of the composite photocatalysts.

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4 Conclusions

In summary, a series of Zn_xCd_1–xS nanoparticle modified 3D-STO photocatalysts SZy with excellent performance in hydrogen evolution reaction under visible light irradiation were exploited in this study. These composite photocatalysts exhibited multiplied increased hydrogen evolution rates (up to 19.67 mmol·g⁻¹·h⁻¹) compared with pure Zn_xCd_1−xS, 3D-STO, CdS, and reported S-based semiconductor materials. Importantly, SZ33 showed an attractive hydrogen production rate of 3.23 mmol·g⁻¹·h⁻¹ in the absence of Pt co-catalysts. The promoted performance of SZy could be attributed to the synergetic effect of the slow photon effect of 3D-STO and the improved migration and separation efficiencies of photogenerated charge carriers. This study provides a convenient method to fabricate highly efficient visible-light-driven photocatalysts for hydrogen evolution reaction, even under noble metal co-catalyst-free conditions.

Acknowledgments

This work was supported by the Natural Science Foundation of Tianjin (Grant No. 17JCYBJC22600), Tianjin Development Program for Innovation and Entrepreneurship, and the Fundamental Research Funds for the Central Universities.

Electronic Supplementary Material

ƒSupplementary material is available in the online version of this article at https://dx.doi.org/10.1007/s11705-021-2089-z and is accessible for authorized users.

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