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. 2022 Feb 16;2(3):216–224. doi: 10.1021/acsphyschemau.1c00053

Nanoscale Assembly of CdS/BiVO4 Hybrids for Coupling Selective Fine Chemical Synthesis and Hydrogen Production under Visible Light

Feng-Kang Shang 1, Ming-Yu Qi 1, Chang-Long Tan 1, Zi-Rong Tang 1,*, Yi-Jun Xu 1,*
PMCID: PMC9718317  PMID: 36855572

Abstract

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Simultaneously utilizing photogenerated electrons and holes in one photocatalytic system to synthesize value-added chemicals and clean hydrogen (H2) energy meets the development requirements of green chemistry. Herein, we report a binary material of CdS/BiVO4 combining one-dimensional (1D) CdS nanorods (NRs) with two-dimensional (2D) BiVO4 nanosheets (NSs) constructed through a facile electrostatic self-assembly procedure for the selectively photocatalytic oxidation of aromatic alcohols integrated with H2 production, which exhibits significantly enhanced photocatalytic performance. Within 2 h, the conversion of aromatic alcohols over CdS/BiVO4-25 was approximately 9-fold and 40-fold higher than that over pure CdS and BiVO4, respectively. The remarkably improved photoactivity of CdS/BiVO4 hybrids is mainly ascribed to the Z-scheme charge separation mechanism in the 1D/2D heterostructure derived from the interface contact between CdS and BiVO4, which not only facilitates the separation and transfer of charge carriers, but also maintains the strong reducibility of photogenerated electrons and strong oxidizability of photogenerated holes. It is anticipated that this work will further stimulate interest in the rational design of 1D/2D Z-scheme heterostructure photocatalysts for the selective fine chemical synthesis integrated with H2 evolution.

Keywords: CdS, BiVO4, hydrogen evolution, aromatic alcohols oxidation, photoredox dual reaction

1. Introduction

As ubiquitous intermediates for the production of many fine chemicals and pharmaceuticals, aromatic carbonyl compounds are of great significance in the industries of perfumes, dyes, and medicines.1 Aromatic carbonyl compounds can be directly obtained by selective transformation of aromatic alcohols, which is considered to be one of the most vital reactions in organic synthesis.2 As the simplest aromatic aldehyde with an active carbonyl group, benzaldehyde (BAD) occupies an essential position in organic reaction intermediates.3,4 The traditional industrial way to synthesize BAD is liquid toluene oxidation or benzoyl chloride hydrolysis, which not only generates various byproducts (bromobenzene, etc.), but also brings about severe environmental pollution and energy waste.5,6 In this context, solar-driven oxidation of benzyl alcohol (BA) has attracted more attention due to its advantages of benign environmental impact and low energy consumption.7−10 Most of the various researches on photocatalytic alcohols oxidation choose oxygen (O2) as the electron acceptor.7,11,12 In comparison, direct splitting of alcohols into hydrogen (H2) and corresponding carbonyl products not only makes full use of the energy of photoinduced electron–hole pairs, but also produces clean H2 fuels, thereby offering a cooperative photoredox reaction system.13,14 Accordingly, photocatalytic selective conversion of BA at ambient conditions coupling with protons reduction to produce H2 is a promising alternative.15−24

The monoclinic scheelite phase BiVO4 is a typical semiconductor with many advantages, including visible light response, suitable valence band (VB) position, good stability, extensive sources of its constituent elements, and mild environmental impact.25 Therefore, it is widely reported that the BiVO4 nanomaterials with various morphologies have been prepared and utilized in the research of photocatalytic alcohols conversion.26−28 Among them, benefiting from the large specific surface area and short carrier transport distance, ultrathin two-dimensional (2D) BiVO4 nanosheets (NSs) with several atomic layer thicknesses show excellent performance in heterogeneous photocatalysis.21,29,30 However, pure BiVO4 still has limitations: on the one hand, the rapid recombination of photoexcited electron–hole pairs severely limits the photoactivity of BiVO4; on the other hand, the conduction band (CB) is so positive that the electrons in it are difficult to use or transfer.25 Some strategies have been developed to overcome the aforementioned shortcomings, such as morphological control, loading cocatalysts, doping impurity, and designing type-II heterostructure composites. These methods can improve the shortcomings of low photogenerated carriers transport and separation efficiency, but cannot change the fact that the energy of the photogenerated electrons is not sufficient to reduce hydrogen protons.20,31−34 In comparison, designing composite materials with a Z-scheme charge transfer pathway is a feasible solution to achieve this goal for the photoexcited electrons and holes gathering in a more negative CB and a more positive VB, respectively, thereby retaining strong photoredox properties.29,30,35−38

Among various semiconductors that can form a Z-scheme system with BiVO4, CdS is one of the most promising candidates due to its suitable band gap and ideal position of CB for H2 production.39,40 Thereinto, one-dimensional (1D) CdS nanorods (NRs) exhibit excellent photocatalytic performance for H2 evolution, which is attributed to the high aspect ratio and single crystal with high crystallinity favoring the migration and separation of charge carriers.41,42 However, the applications of pure CdS NRs are severely limited due to the shortcomings of CdS, such as photocorrosion and rapid recombination of photogenerated carriers, which can be effectively solved by constructing a Z-scheme system.6,43−45 The CdS/BiVO4 binary system has been reported for dye degradation,30 CO2 reduction,29 H2 evolution,37 and conversion of biomass derivatives,38 but the selective transformation of photocatalytic biomass derivatives (e.g., alcohols) integrated with H2 production in one photoredox cycle over CdS/BiVO4 hybrids is still reported scarcely.

In this work, we have successfully constructed a CdS/BiVO4 1D/2D binary nanocomposite through a facile one-step electrostatic self-assembly method for coproduction of H2 and BAD under visible light illumination (λ > 420 nm), utilizing photoinduced electrons and holes simultaneously. A direct Z-scheme heterostructure can be formed between CdS NRs and BiVO4 NSs motivated by the substantial electrostatic attraction between CdS with a positive charge and BiVO4 with a negative charge, which can effectively promote the migration and separation of photoexcited electron–hole pairs.46 As expected, compared with bare CdS and BiVO4, the as-prepared CdS/BiVO4 hybrids display significantly enhanced photoactivity toward visible light-driven splitting of BA into BAD and H2. Particularly, the ratios of reduction product (H2) and oxidation products are calculated to be 1:1, approximately, indicating that a visible light-driven alcohol dehydrogenation reaction in a stoichiometric manner is realized in our system. It is hoped that this study could open up a new horizon for developing collaborative photoredox systems of 1D/2D composite materials with a Z-scheme heterostructure, thereby realizing low-cost and high-efficiency conversion of biomass derivatives and production of green energy.

2. Experimental Section

2.1. Materials

Ammonium metavanadate (NH4VO3), sodium hydroxide (NaOH), bismuth(III) nitrate pentahydrate (Bi(NO3)3·5H2O), thiourea (H2CNSNH2), nitric acid (HNO3), ethylenediamine (C2H8N2), dimethyl sulfoxide (C2H6OS, DMSO), tert-butyl alcohol (C4H10O, tBuOH), ethanol (EtOH), methyl cyanide (MeCN), carbon tetrachloride (CCl4), and triethanolamine (TEOA) were supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). 5,5-Dimethyl-1-pyrroline-N-oxide (DMPO), sodium dodecylbenzenesulfonate (C18H29NaO3S, SDBS), and cadmium nitrate tetrahydrate (CdN2O6·4H2O) were purchased from Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). All reagents are utilized without further purification. Deionized (DI) water utilized in all the experiments comes from local sources.

2.2. Synthesis Method

2.2.1. Synthesis of BiVO4 Nanosheets (NSs)

The BiVO4 NSs were synthesized via a hydrothermal method according to a literature procedure, which went through a few modifications.29,30 The typical experimental procedure is provided in the Supporting Information.

2.2.2. Synthesis of CdS Nanorods (NRs)

The CdS NRs were prepared via a solvothermal process according to a literature procedure.47,48 The typical experimental procedure is provided in the Supporting Information.

2.2.3. Preparation of CdS/BiVO4 Hybrids

Typically, the positively charged CdS NRs (100 mg) and the negatively charged BiVO4 NSs (a certain amount) were first dispersed into DI water (50 and 10 mL, respectively) by sonication for 0.5 h. Then, the CdS NRs suspension and BiVO4 NSs suspension were mixed and stirred vigorously for 1 h. Afterward, the resultant samples were collected via suction filtration, rinsed with DI water for several times, and dried in an oven for 6 h. The added weight amounts of BiVO4 NSs in the CdS/BiVO4 hybrids (CdS/BiVO4-X, X = 15, 20, 25, 30 and 35) were controlled to be 15, 20, 25, 30, and 35 wt % of CdS NRs, respectively.

2.3. Photocatalytic Conversion of BA

The photocatalytic splitting of BA into BAD and H2 was carried out in a single-walled quartz Schlenk tube. In a typical process, 0.1 mmol BA and 5 mg of photocatalyst were mixed into the quartz reactor prefilled with 5 mL of MeCN. Before irradiation, the reaction suspension was sonicated for 2 min to form a uniform suspension and degassed with nitrogen for 30 min to purge air. Subsequently, a 300 W Xe lamp (PLS-SXE 300D, Beijing Perfectlight Co., Ltd.) emitting visible light (λ > 420 nm) was used as the irradiation source to trigger the photocatalytic reaction. The energy output of the Xe lamp was measured to be 0.8 W cm–2 by a photoradiometer (PL-MW2000, Beijing Perfectlight Co., Ltd.). The operating conditions of the control experiments or extended experiments were the same as the above-mentioned photocatalytic process except for the controlled reaction conditions. The operation of the cycling tests is as follows. After the photocatalytic reaction is completed, the used photocatalyst is separated by centrifugation, washed with DI water, dried in a vacuum oven, and then utilized for the next activity test, which is repeated five times. After the reaction, the gas chromatograph (Shimadzu GC-2014C, MS-5 A column, TCD, Ar carrier) was utilized to quantitatively detect the gases products. After that, the suspension was filtered through a Nylon syringe filter (0.22 μm) to obtain a clear solution, which was qualitatively analyzed by gas chromatograph–mass spectrometer (Shimadzu GC-MS QP 2020, Q-Exactive) and quantitatively detected by a high-performance liquid chromatograph (Shimadzu HPLC-LC20AT, C18 column, SPD-M20A). The conversion of BA and selectivity of each oxidation product were calculated with the following equations:13,16

2.3.
2.3.
2.3.

where n0 represents the amount of BA that was added into the system initially; nBA represents the amounts of the residual BA; nBAD and nC–C, respectively, stand for the amounts of BAD and C–C products after reacting for a certain time.

3. Result and Discussion

The CdS/BiVO4 1D/2D hybrids are fabricated via a nanoscale self-assembly methodology, as depicted in Figure 1a. Initially, bare CdS NRs and BiVO4 NSs are synthesized through a solvothermal process, and their surface charge properties are ascertained by zeta potential measurements.30,48 As illustrated in Figure 1b, the surface of the BiVO4 NSs is obvious negatively charged (the zeta potential value is −16.3 mV), while CdS NRs exhibit positively charged (the zeta potential value is 15.4 mV) by reason of the residual amino ions.49,50 Consequently, the electrostatic interaction between CdS NRs and BiVO4 NSs allows the spontaneous formation of CdS/BiVO4 1D/2D heterostructure through electrostatic self-assembly strategy.38 The morphological characteristics of different samples are observed by field emission scanning electron microscope (FESEM). As illustrated in Figure 1c,d, the bare CdS has significant 1D nanorod morphology with an average diameter distribution of about 40–80 nm, while the bare BiVO4 manifests legible ultrathin 2D NSs, the thickness of which is about 30–50 nm. Figure 1e clearly shows that CdS NRs and BiVO4 NSs form a 1D/2D morphology with good interface contact through electrostatic interaction. After that, high resolution transmission electron microscope (HRTEM) characterizations have been further conducted to gain insight into the microstructure information of CdS/BiVO4 hybrids. As observed in Figure 1f, the 0.316 and 0.182 nm lattice fringes respectively correspond to the (101) facet of CdS and the (−202) facet of monoclinic BiVO4. Energy-dispersive X-ray spectroscopy (EDX) and element mapping have been carried out to determine the elements composition and distribution. The EDX spectrum plotted in Figure S1 ascertains the existence of Cd, S, Bi, V, and O in the composite material. Besides, the element mapping results illustrated in Figure 1g further demonstrate that CdS NRs are stacked well upon BiVO4 NSs.

Figure 1.

Figure 1

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(a) Schematic illustration of the fabrication of CdS/BiVO4 hybrids. (b) Zeta potentials of the as-synthesized CdS NRs and BiVO4 NSs. FESEM images of (c) CdS NRs, (d) BiVO4 NSs, and (e) CdS/BiVO4 hybrids. (f) HRTEM image of CdS/BiVO4 hybrids, in which the inset is the magnification of the image displayed in the panel surrounded by white dotted lines. (g) Corresponding elemental mapping images of CdS/BiVO4 hybrids.

The crystal structure characteristics of a series of CdS/BiVO4 hybrids have been probed via X-ray diffraction (XRD), as observed in Figure 2a. With regard to the composite samples, the peaks marked with red rounds are in agreement with the characteristic diffraction peaks of bare CdS, which can be indexed to hexagonal phase CdS (JCPDS No. 41-1049).49,51 The peaks marked with black quadrilaterals are consistent with the characteristic diffraction peaks of bare BiVO4 that can be indexed to monoclinic phase BiVO4 (JCPDS No. 14-0688).52 The diffraction peak intensities of BiVO4 become stronger as the content of BiVO4 NSs increases. Ultraviolet–visible diffuse reflectance spectroscopy (UV–vis DRS) was conducted to observe the optical absorption properties of the as-prepared samples. As mirrored in Figure 2b, all of the samples possess absorption bands in the visible light region from 450 to 550 nm. The absorption strength of the composite samples does not exhibit significant enhancement, suggesting that the formation of the 1D/2D heterostructure has no significant effect on the optical absorption of the composites. The band gap values of CdS and BiVO4 calculated according to the Kubelka–Munk function are respectively 2.42 and 2.53 eV (Figure S2), which means that they can be excited by visible light.30 In addition, on the grounds of the Mott–Schottky curves (Figure S3), both the CdS and BiVO4 exhibit the trends of n-type semiconductors, of which the flat band potentials are respectively situated in −0.70 and 0.00 V (vs Ag/AgCl). Since the conduction band (CB) positions of n-type semiconductors are close to their flat band potentials, the CB positions of CdS and BiVO4 are respectively estimated at −0.50 and 0.20 V (vs NHE). Considering the band gap values obtained before, the valence band (VB) positions of CdS and BiVO4 are respectively calculated to be 1.92 and 2.73 V on the grounds of the formula EVB = ECB + Eg (EVB, ECB, and Eg respectively are the energy values of VB, CB, and band gap).

Figure 2.

Figure 2

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(a) XRD patterns of bare CdS NRs, BiVO4 NSs, and a series of CdS/BiVO4 hybrids with different weight ratios of BiVO4 NSs. (b) UV–vis DRS spectra of blank CdS NRs, BiVO4 NSs, and a series of CdS/BiVO4 hybrids. High-resolution XPS spectra for (c) Cd 3d, (d) S 2p, and Bi 4f, (e) V 2p and (f) O 1s of the CdS/BiVO4 hybrid.

X-ray photoelectron spectroscopy (XPS) has been utilized to deeply analyze the surface valence states of the chemical elements in the CdS/BiVO4 hybrids. As displayed in the survey XPS spectrum of the CdS/BiVO4 hybrid (Figure S4), all elements (Cd, S, Bi, V, and O) related to CdS and BiVO4 are detected, which is consistent with the aforementioned EDX spectrum results. The two peaks of 405.14 and 411.88 eV depicted in the Cd 3d spectrum (Figure 2c) are respectively ascribed to Cd 3d5/2 and Cd 3d3/2, corresponding to Cd2+.53 In Figure 2d, the two peaks with binding energies of 161.23 and 162.41 eV deconvoluted from the S 2p spectrum belong to S 2p3/2 and S 2p1/2, which can be ascribed to S2–.54 And the two peaks with binding energies of 158.40 and 163.71 eV are respectively attributed to Bi 4f7/2 and Bi 4f5/2, which proves the existence of Bi3+.55 The V 2p spectrum (Figure 2e) displays that the binding energies of V 2p3/2 and V 2p1/2 are respectively fixed at 516.34 and 523.81 eV, indicating the valence state of V is +5.56 In Figure 2f, the O 1s spectrum can be deconvoluted into three peaks with binding energies of 532.91, 531.58, and 529.45 eV, which are in line with C–O/C=O, surface-adsorbed H2O, and the lattice oxygen of layer-structured Bi2O22+, respectively.57 All the above results further confirm that CdS and BiVO4 have been successfully combined.

Subsequently, the selective oxidation of BA and the simultaneous H2 production over the obtained samples were examined in the N2 saturated acetonitrile solution under visible light irradiation (λ > 420 nm) (Figure 3a and Figure S5). As plotted in Figure 3b and Figure S6, both bare CdS and bare BiVO4 show extremely low activity for the generation of BAD and H2 because of the rapid recombination of photoexcited electrons and holes, even the H2 evolution activity of BiVO4 is negligible owing to its overpositive CB position. In contrast, the photoactivity of the CdS/BiVO4 composite is significantly increased. Among them, CdS/BiVO4-25 exhibits the strongest activity with the conversion of BA reaching 50.5% within 2 h (43.96 μmol for BAD and 45.58 μmol for H2), which achieves about 9-fold and 40-fold improvement as compared to that for bare CdS and bare BiVO4, respectively. Given that the oxidation ability of BiVO4 is relatively strong with the products concentrated in BAD while the oxidation products of CdS tilt toward C–C coupled products due to the relatively mild oxidation ability of CdS, the selectivity of BAD increases as the content of BiVO4 increases owing to the BA oxidation competing between CdS and BiVO4. The selectivity of BAD has reached more than 90% when the BiVO4 content is greater than 25%. To explore the relationship between light absorption and photoactivity, the calculated apparent quantum yield (AQY) of the CdS/BiVO4-25 under different wavelengths of monochromatic light is measured and found to match substantially with its DRS spectrum (Figure S7). A long-term reaction test and recycling experiment are conducted to assess the stability and reusability of CdS/BiVO4 hybrids. As sketched in Figure 3c, after 8 h of exposure to visible light, the conversion of BA over CdS/BiVO4-25 reaches 100%, with the selectivity of BAD as high as 91.2%. Additionally, CdS/BiVO4-25 is not significantly inactivated after five cycles of testing (Figure S8), and its crystal structure also remains stable (Figure S9). The above results reveal the good stability of the CdS/BiVO4 hybrids.

Figure 3.

Figure 3

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(a) Formula for photoredox reaction coupling BA oxidation and H2 evolution. (b) Results of photocatalytic measurements for selective conversion of BA over bare CdS, bare BiVO4, and a series of CdS/BiVO4 hybrids. (c) Long-time experiments over CdS/BiVO4-25.

Aiming to further manifest the universality of CdS/BiVO4 in the photocatalytic conversion of aromatic alcohols, we have carried out photocatalytic activity tests of diverse aromatic alcohols (1-phenethyl alcohol, p-methoxybenzyl alcohol, p-methylbenzyl alcohol, p-fluorobenzyl alcohol, and p-chlorobenzyl alcohol). As illustrated in Table S2, it is seen that the CdS/BiVO4 hybrid has varying degrees of photoactivity for the transformation of all aromatic alcohols, and the main products are the corresponding aromatic aldehydes or aromatic ketones. Notably, for benzylic alcohols with different substituents, the introduction of electron-donating groups on the benzene ring significantly enhances the conversion rate of aromatic alcohols. For instance, the conversion of benzyl alcohol with a methoxy group (−OCH3) at the para position is significantly higher than that of benzyl alcohol with Cl at the para position, which implies the corresponding reaction mechanism, as discussed in detail below.

A set of control experiments with different reaction conditions have been performed to explore the potential mechanism of the photocatalytic reaction. As illustrated in Figure S10, with the hole scavenger (triethanolamine, TEOA) added to the photocatalytic reaction system, the generation of BAD is significantly restrained, indicating that the oxidation half reaction of BA is driven by holes. When adding carbon tetrachloride (CCl4) as the electron scavenger, the output of BAD has increased but the output of H2 is almost negligible, indicating that the reduction of H+ is driven by electrons, and the elimination of electrons promotes the oxidation reaction. Moreover, the control experiments show almost no activity on the generation of BAD and H2 in the absence of visible light irradiation or photocatalyst, strongly proving that the reaction is a photocatalytic process driven by visible light. In addition, adding 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) causes the conversion rate of BA to drop sharply, implying that the active species in the reaction is the carbon-center free radical.16

To gain insight into the information of the radical intermediates mechanism in the photocatalytic redox process, the electron paramagnetic resonance (EPR) technique was conducted to detect the free radicals in the reaction using DMPO as the trapping agent. As sketched in Figure 4a, no free radical signal appears in the dark, but six characteristic signal peaks with similar intensity at g = 2.0063 appear in all three samples under exposure to visible light. The result of a quantitative analysis for the above-mentioned EPR spectra suggests that the value of αH = 22.7 and αN = 16.0 (Figure S11) belong to •CH(OH)Ph intermediates.58,59 It is noteworthy that the characteristic peaks intensity follows the CdS/BiVO4 hybrid > bare CdS > bare BiVO4, which is in line with the law of reaction activity.

Figure 4.

Figure 4

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(a) EPR spectra of bare CdS, bare BiVO4 and CdS/BiVO4-25 with irradiation for 10 min. (b) Transient photocurrent–time (I–t) curves. (c) Schematic illustration of the migration pathway of photogenerated electrons and holes in accordance with type-II and direct Z-scheme system of the CdS/BiVO4 hybrid. DMPO spin-trapping EPR images of bare CdS, BiVO4, and CdS/BiVO4 for DMPO-•O2– (d) and DMPO-•OH (e) under visible light irradiation. (f) Illustration of the band structure for CdS/BiVO4 hybrids and the plausible reaction mechanism for the coupled reaction system of photocatalytic conversion of BA.

A set of photoelectrochemical measurements have been carried out to unveil the origin of the significantly enhanced photoactivity of the CdS/BiVO4 composites. The transient photocurrent responses analysis in Figure 4b display that the CdS/BiVO4 hybrid has a significantly improved photocurrent response compared to bare CdS and BiVO4, indicating that the recombination of photoinduced charge carriers has been significantly suppressed.60 The linear sweep voltammetry (LSV) curves (Figure S12) suggest that the CdS/BiVO4 hybrid has a higher current density at a similar potential range, proving that the formation of heterostructures effectively enhances charge transfer.61 Besides, the electrochemical impedance spectroscopy (EIS) has been utilized to study the inhibition of charge migration between the electrolyte and the sample. As depicted in Figure S13, the CdS/BiVO4 composite has the smallest arc radius at high frequency, which proves that it can more effectively promote the migration of photoexcited charges relative to the bare CdS and BiVO4.60 Photoluminescence (PL) measurement was performed to in-depth study the behavior of photogenerated carriers. As exemplified in Figure S14, the PL intensity of the CdS/BiVO4 hybrid is significantly quenched under 380 nm excitation, confirming that the formation of the heterostructure effectively hinders the recombination of photoexcited electron–hole pairs. As mentioned above, the construction of the CdS/BiVO4 heterostructure effectively promotes the separation and transfer of photoexcited electron–hole pairs, thereby improving the photocatalytic performance of the composite.

Taking into account the energy band structure of CdS and BiVO4, there are two possibilities for the charge carrier migration path existing in our photocatalytic system: traditional type-II heterojunction and direct Z-scheme system (Figure 4c). Given the respective CB positions of CdS and BiVO4, it is obvious that the CB position of BiVO4 is too low to drive protons reduction. Furthermore, the dominant oxidation product of BA over the optimal catalyst is BAD, indicating that the oxidation reaction is concentrated on the surface of BiVO4 NSs. EPR spin-trapping has been conducted to further verify the Z-scheme charge transfer mechanism.62 Under the condition of adding different catalysts, DMPO is deployed to trap •O2– and •OH in methanol solution and aqueous solution, respectively. Since the energy level of the O2/•O2– couple (−0.33 V) is more positive than the CB of CdS but more negative than the CB of BiVO4, only CdS can reduce O2 into •O2– (O2 + e– → •O2–). Analogously, the energy level of the OH–/•OH couple (2.40 V) is more positive than the VB of CdS but more negative than BiVO4, suggesting that only BiVO4 can oxidize H2O into •OH (OH– + h+ → •OH). As depicted in Figure 4d,e, after visible irradiation for a while, DMPO-•O2– is detected only in the samples of bare CdS or CdS/BiVO4 hybrid, and DMPO-•OH is only detected when present in the samples of bare BiVO4 or CdS/BiVO4 hybrid. Obviously, the signal peaks of CdS/BiVO4 hybrid are significantly higher than that of CdS or BiVO4. The above results indicate that the photoexcited electrons and holes are effectively separated and respectively concentrated in CdS and BiVO4, thereby evidently confirming a direct Z-scheme charge transfer mechanism, instead of a conventional type-II charge transfer mechanism, in our photocatalytic system.

Combining the aforementioned analyses, an underlying photocatalytic mechanism for the selective conversion of BA to BAD and H2 over the CdS/BiVO4 hybrid is proposed as follows (Figure 4f). Initially, both CdS and BiVO4 generate photoexcited electron–hole pairs under visible light illumination (λ > 420 nm). Owing to the ultrathin thickness of BiVO4 NSs and the close interfacial contact between CdS NRs and BiVO4 NSs, the photogenerated electrons on the CB of BiVO4 quickly transfer to the surface, and further migrate to the VB of CdS to recombine with the photogenerated holes, thereby realizing the effective separation of photoexcited electron–hole pairs. In this case, the electrons and holes of the composite accumulate in the CB of CdS and the VB of BiVO4, respectively, thereby maintaining a strong reduction and oxidation ability. Afterward, the hydrogen atom in α-C–H of BA can be abstracted and separated by photoexcited holes to form H+ and •CH(OH)Ph intermediates. These dropped H+ are quickly reduced by the electrons accumulated in the CB of the nearby CdS NRs, thereby generating H2. Meanwhile, the •CH(OH)Ph intermediates can be further oxidized to BAD, or C–C coupling products due to its instability. Notably, benzylic alcohols with electron-donating groups at the para position are apt to be attracted by holes due to the greater electron density, so that the hydrogen atoms in α-C–H are more likely to shed off. Consequently, benzylic alcohols with electron-donating groups exhibit significantly enhanced conversion, and conversely, benzylic alcohols with electron-withdrawing groups exhibit significantly decreased conversion.

4. Conclusion

In summary, we have successfully synthesized 1D/2D CdS NRs/BiVO4 NSs binary composites through a facile electrostatic self-assembly approach for the selective oxidation of BA accompanied by H2 evolution under visible light irradiation. Compared with bare CdS and bare BiVO4, the construction of the Z-scheme heterostructure remarkably hinders the recombination of photogenerated charge carriers and retains strong redox ability in the meantime. The photocatalytic activity over the CdS/BiVO4 hybrid is greatly enhanced toward merging BAD production and H2 evolution in comparison with that over bare CdS and BiVO4. It is anticipated that the semiconductor-based composite photocatalysts with a Z-scheme heterostructure exhibit promising potential in such dual-function photoredox systems, which can effectively utilize photogenerated holes and electrons to produce clean fuels and fine chemicals with high added value.

Acknowledgments

This work was supported by the Natural Science Foundation of China (22072023, 22172030, 21872029 and U1463204), the Program for National Science and Technology Innovation Leading Talents (00387072), the first Program of Fujian Province for Top Creative Young Talents, the Program for Leading Talents of Fujian Universities, the Award Program for Minjiang Scholar Professorship, and the Natural Science Foundation of Fujian Province (2017J07002, 2019J0106).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsphyschemau.1c00053.

  • Methods for the synthesis and characterization of catalysts, specific process, and results of the recycling tests, EDX spectrum of CdS/BiVO4, estimated band gap energy of CdS and BiVO4, Mott–Schottky plots, survey XPS spectrum, mass spectrogram, XRD patterns before and after cyclic tests, results of control experiments, quantitative analysis result of EPR spectrum, polarization curves, EIS Nyquist plots, PL spectra and results of extended experiments (PDF)

The authors declare no competing financial interest.

Supplementary Material

pg1c00053_si_001.pdf (1MB, pdf)

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