Boosting photocatalytic hydrogen production of CdS/BiVO₄ nanoplates by transferring in-plane plasmon resonant energy of gold nanoparticles

Abstract

A free-standing plate-type plasmon-enhanced Z-scheme photocatalytic system has been established via CdS deposition on Au-decorated BiVO₄ nanoplates (CdS/Au on BiVO₄). Through the enhancement from in-plane plasmon resonant energy transfer of gold, the hydrogen evolution rate of CdS/Au on BiVO₄ was enhanced about 7.7 times higher than that of the gold-free photocatalyst.

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Levels of carbon dioxide emissions from the combustion of fossil fuels have steadily increased over time year by year. The development of sustainable energy sources such as hydrogen energy has become a highly attractive topic in the literature due to environmental concerns.¹ Hydrogen is a promising energy carrier due to its high energy density and zero carbon emissions in combustion.² Hydrogen can be produced from multiple resources such as natural gas reforming, sugar fermentation and water splitting. Solar energy-driven water splitting is currently economically viable and environmentally benign compared with other energy sources. Since Honda and Fujishima discovered the decomposition of water on TiO₂ electrodes in 1972,³ semiconducting photocatalysts thus have drawn extensive and increasing attention due to their capabilities to convert solar energy to chemical energy (i.e., forming hydrogen via photocatalytic reduction of water). With the exception of TiO₂, the literature has reported numerous photocatalysts for water splitting.⁴ As is well known, rapid recombination of photo-generated charge carriers occurring in a single phase photocatalyst leads to a poor quantum efficiency and a low photocatalytic activity. Thus, the improvement of electron–hole separation efficiency of photocatalysts has become an emerging and attractive topic in photocatalysis. There are several strategies to improve the photo-generated charge separation efficiency of photocatalysts, such as impurity doping,⁵ construction of heterojunctions,^6–8 and conjugation of plasmonic metals (such as Au, Ag, and Cu).^9,10 Z-Scheme photocatalysts with a heterojunction in between can effectively reduce the recombination of photo-generated charge carriers via the formation of an interfacial electric field. Conjugated plasmonic metals on metal oxide surfaces usually give rise to the effects of direct electron transfer (denoted as DET) and plasmon resonant energy transfer (denoted as PRET). The former participates in the plasmonic enhancement effect via the injection of hot electrons. The latter is beneficial to charge separation by plasmonic energy transfer. There are numerous studies reporting the enhanced hydrogen production from plasmonic metal-conjugated semiconducting photocatalysts that possess negative conduction band edges enabling reduction of protons, thus producing hydrogen.⁹ Among these cases, particulate hybrid photocatalysts exhibited plasmon-enhanced photocatalytic activity boosted by DET and/or PRET. To develop highly efficient photocatalysts for hydrogen evolution, generating plenty of H₂-evolution centers around a plasmonic metal on semiconducting oxide nanoplates is the most feasible strategy.

To the best of our knowledge, very limited attention has been paid to the study of plasmonic enhancement in hydrogen evolution over free-standing Z-scheme nanoplate-type photocatalysts, especially when using a plasmonic metal solely for its enhancing role rather than serving as a shuttle mediator.¹¹ We choose bismuth vanadate nanoplates (BiVO₄ NPs) as a platform to construct a Z-scheme photocatalyst through cadmium sulfide deposition (denoted as CdS/BiVO₄). For demonstrating the plasmonic effect, gold nanoparticles were deposited on BiVO₄ NPs by photoirradiation prior to the deposition of CdS (denoted as CdS/Au on BiVO₄, Scheme 1). For comparison, a thin silica coated gold decorated Z-scheme photocatalyst (denoted as CdS/SiO₂-capped Au on BiVO₄) was prepared in order to exclude the enhancing contribution from the DET mechanism. These Z-scheme photocatalysts were evaluated in hydrogen evolution in the presence of sacrificial agents. The hydrogen evolution rate of CdS/Au on BiVO₄ was approximately 7.7 times higher than that of the gold-free photocatalyst, i.e., CdS/BiVO₄. Through the analyses of time-resolved photoluminescence spectroscopy, the results indicated that gold decoration effectively improved the separation efficiency of charge carriers, resulting in prolonged carrier lifetimes. Compared with CdS/SiO₂-capped Au on the BiVO₄ photocatalyst, it is believed that the dominant enhancing factor in hydrogen evolution is attributed to the PRET mechanism instead of DET.

Scheme 1 Schematic demonstrating the synthesis of gold-decorated BiVO₄ nanoplate supported CdS photocatalysts through deposition methods, including, photodeposition for gold nanoparticles and chemical deposition for CdS nanoparticles.

BiVO₄ is a typical semiconductor with a narrow bandgap of ca. 2.5 eV and a suitable valence band edge at 2.4 eV vs. NHE, providing adequate potential for holes to oxidize water.¹² The BiVO₄ nanoplates were prepared according to the literature report¹³ with slight modification (see ESI† for the details). The BiVO₄ nanoplates exhibit a thickness of ca. 40 nm (Fig. 1a) and size ranging from ca. 700 nm to 1–2 μm (Fig. 1b). The {010} crystal facets of BiVO₄ NPs possess a lower conduction band energy level as compared with other facets, thus accumulating photo-generated electrons for the reduction of AuCl₄⁻ ions. Fig. 1c shows that the majority of gold nanoparticles have a sphere-like shape and the size is about 12.0 ± 2.8 nm. The deposition of CdS nanoparticles was carried out via the assistance of electrostatic interactions between Cd ions and the negatively charged BiVO₄ surface. The particle size of CdS is about 11.7 ± 2.3 nm (Fig. 1d). The gold-decorated CdS/BiVO₄ photocatalyst was thus constructed through sequential deposition of gold and CdS nanoparticles (Fig. 1e and f). The scanning transmission electron microscope (denoted as STEM) image (Fig. 1e) clearly shows that some CdS nanoparticles are independently located on the surface. Some CdS nanoparticles surrounded the gold due to the thiol group of the precursor, thiourea. STEM with energy-dispersive X-ray spectroscopy (i.e., STEM-EDS) analysis showed that the CdS nanoparticles were deposited only next to the gold rather than on the top surface of the gold (data not shown). After sequential deposition, the {010} facets of BiVO₄ NPs existed as usual with only slight unevenness. However, due to the quite uniform deposition of Au and CdS, the surface unevenness is thus not a significant influencing factor in photocatalysis. Furthermore, to exclude the influence from DET, a thin silica layer was introduced to cap the naked gold surface prior to CdS deposition, resulting in the formation of CdS/SiO₂-capped Au on BiVO₄ (Fig. S1†). X-ray diffraction (XRD) patterns (Fig. 2a) confirm the deposition and crystal phases of CdS (ICDD no. 00-041-1049) and gold (ICDD no. 00-004-0784). The loading percentage by weight for CdS and Au was estimated as ca. 22% and 10%, respectively (Fig. S2†) by using an SEM equipped with an energy dispersive X-ray spectrometer. Through the characterization by diffuse reflectance spectroscopy (Fig. 2b), the absorption spectrum of CdS/BiVO₄ is nearly the same as that of pristine BiVO₄. Moreover, gold decoration gives rise to an obvious absorption band from 500 to 900 nm due to the distorted spherical shapes of gold. The spectrum without clear peaks is due to the fact that AuNPs with various sizes, shapes, and refractive indices of the surrounding medium, as evidenced by the TEM images (Fig. 1), have arisen. The photocatalytic activity in hydrogen evolution over the as-prepared samples was evaluated (Fig. 3a). In this system, SO₃²⁻ and S²⁻ are oxidized on BiVO₄ and water is reduced on CdS to produce hydrogen. The photocatalyst, CdS/Au on BiVO₄, shows that the evolution rate of hydrogen, i.e., 2366 μmol g⁻¹ h⁻¹, is about 7.7 times higher than that of CdS/BiVO₄ (305 μmol g⁻¹ h⁻¹). Furthermore, CdS/Au on the BiVO₄ photocatalyst was evaluated at different irradiation wavelengths (Fig. 3b). The evolution rate of hydrogen just remained 58% in the case of unfiltered irradiation, when using irradiation wavelengths ranging from 250 to 480 nm (away from the surface plasmon resonance band of gold nanoparticles) as excitation. The 42% decrease in activity can be ascribed to the removal of any possible influence from the surface plasmon resonance effect (due to its absorption at 500–900 nm). There is no activity under irradiation at a wavelength longer than 520 nm. That is because the irradiation wavelength was completely outside the adsorption window of CdS and BiVO₄. Therefore, the photocatalyst was inactive under irradiation at a wavelength longer than 520 nm. Moreover, it was difficult to obtain hydrogen from the CdS/BiVO₄ photocatalyst only through the excitation of gold using irradiation light longer than 520 nm. The effective PRET enhancement in the hydrogen evolution of CdS/Au on the BiVO₄ photocatalyst only occurred in the presence of efficient dipole–dipole interactions (i.e., spectral overlap) between Au and CdS/BiVO₄.¹⁴ According to the literature,¹⁴ the transfer of hot electrons from SPR resonant gold to BiVO₄ is not dominant in the system of Au on BiVO₄. However, the electron transfer from SPR resonant gold to CdS may occur to produce hydrogen, but the trace amount of hydrogen will be under the detection level based on our experimental conditions. Since BiVO₄ exhibits slow charge transportation and high recombination of photo-generated electrons and holes,¹⁵ it is believed that the efficiency of charge carrier separation could be improved obviously through the introduction of gold as a solid redox mediator in Z-scheme photocatalysts.¹⁶ However, in comparison with the evolution rate of CdS/SiO₂-capped Au on BiVO₄ (Fig. 3a), it is believed that the plasmonic effect of gold, not the solid redox mediator, is the main factor responsible for improving the hydrogen evolution rate of CdS/Au on the BiVO₄ photocatalyst. The DET and PRET plasmonic enhancements are usually entangled together. Therefore, in order to elaborate the enhancement mechanism from gold over the CdS/BiVO₄ Z-scheme photocatalyst, we measured the steady-state and time-resolved photoluminescence¹⁷ to investigate the separation efficiency of photo-generated charge carriers over our photocatalysts (Fig. 4a and b). In Fig. 4a, pristine BiVO₄ exhibits strong photoluminescence (denoted as PL) peaks at 473, 483, and 494 nm. When gold nanoparticles were deposited on BiVO₄, the PL intensities were decreased significantly (pink, olive, and orange lines in Fig. 4a). That is because the lifetimes of photo-generated charge carriers were prolonged. For CdS deposition, the PL intensity of CdS/BiVO₄ shows a slight increase accompanied by a slight shape change ranging from 500 to 530 nm due to extra luminescence from CdS. This indicates that CdS/BiVO₄ possesses a shortened carrier lifetime. The prolonged carrier lifetime of CdS/Au on BiVO₄ is potentially beneficial for photocatalysis, in agreement with its photocatalytic activity. Time-resolved photoluminescence spectroscopy was utilized to analyse the photo-generated charge carriers in-depth (Fig. 4b). Two time constants are extracted from the fitting: fast decay component, τ₁, originating from the electron trapping in the near CB edge of BiVO₄ and slow component, τ₂, originating from electron–hole recombination (Table 1). Due to the PL overlap of BiVO₄ with CdS in our system, the analysed results of the carrier lifetime are interpreted carefully. All samples exhibit prolonged τ₁ and τ₂ in comparison with that of pristine BiVO₄ nanoplates. The longer carrier lifetimes of τ₁ and τ₂ for Au on the BiVO₄ sample exhibit that gold decoration can effectively suppress the recombination of electron–hole carriers and prolong the trapping time of electrons in the CB edge of BiVO₄. The construction of Z-scheme CdS/BiVO₄ also prolongs the carrier lifetime as Au-decorated BiVO₄ NPs. The thin silica coating on the gold further improves the separation efficiency of photo-generated charge carriers. According to the analysed data (Table 1), it is clear that the reduction of carrier recombination indicates that PRET mainly dominates the photocatalysis instead of DET in our photocatalysts because if DET is dominant, the recombination rate would increase due to the injection of hot electrons (Fig. 5). This knowledge of the mechanism obtained in this study has made it possible to design an optimal system.

Fig. 1 Scanning electron microscope (SEM) (a) and transmission electron microscope (TEM) (b) images of BiVO₄ nanoplates (NPs). TEM images of Au-deposited (c) and CdS-deposited (d) BiVO₄ NPs. Scanning-TEM (STEM) (e) and SEM (f) images of Au-deposited BiVO₄ NPs, followed by CdS deposition (the inset in (b) shows the lattice fringes of the BiVO₄ (200) planes).

Fig. 2 XRD patterns (a) and diffuse reflectance absorption spectra (b) of the as-prepared photocatalysts (black, brown and pink labels in (a) indicate the crystal facets of BiVO₄, CdS, and Au, respectively).

Fig. 3 Photocatalytic hydrogen evolution of BiVO₄-based photocatalysts under the full spectrum of the Xe lamp (a) and CdS/Au on BiVO₄ at different irradiation wavelengths (b) (the evolution rates of hydrogen were calculated after 6 h of photocatalytic reactions).

Fig. 4 Steady-state photoluminescence (a) and time-resolved photoluminescence (i.e., TRPL) (b) spectra of the as-prepared samples under 280 nm excitation light (TRPL was monitored at 483 nm).

Table 1 Lifetimes of the as-prepared samples obtained via fitting of decay PL curves in Fig. 4b using a bi-exponential function

Sample	τ 1 (ns)	τ 2 (ns)
CdS/SiO2-capped Au on BiVO4	3.0	15.9
CdS/Au on BiVO4	3.5	12.6
CdS/BiVO4	4.5	21.3
Au on BiVO4	3.4	16.3
BiVO4 nanoplates	2.8	10.0

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Fig. 5 Schematic illustration of plasmon-enhanced photocatalytic hydrogen evolution over CdS/Au on the BiVO₄ photocatalyst (DET: direct electron transfer; PRET: plasmon resonant energy transfer).

Conclusions

We have successfully utilized free-standing Au-decorated BiVO₄ nanoplates as substrates to construct plasmon-enhanced Z-scheme photocatalysts by depositing CdS nanoparticles (CdS/Au on BiVO₄). The deposition of CdS nanoparticles occurred on the surface of BiVO₄ due to the electrostatic interaction between cadmium ions and the BiVO₄ surface. The photocatalytic hydrogen evolution of CdS/Au on the BiVO₄ photocatalyst has been effectively increased by about 7.7 times as compared with the gold-free CdS/BiVO₄ photocatalyst. The time-resolved photoluminescence spectroscopy analysis indicated that gold decoration improved the separation efficiency of carriers, resulting in the enhancement of hydrogen evolution. This enhancement is attributed to the in-plane plasmon resonant energy transfer not the direct electron transfer. We believe that this in-plane plasmon-enhanced Z-scheme photocatalytic system could provide ideas for designing various highly efficient two-dimensional plasmonic photocatalysts in the near future.

We gratefully acknowledge the funding for this work from Mitsui Chemicals, Inc. M.-H. Liu thanks Dr. Masaki Kudo (The Ultramicroscopy Research Center, Kyushu University) and Mr. Koki Nishimura (Center for Molecular Systems (CMS), Kyushu University) for the assistance in STEM-EDS (M. K.) and TRPL (K. N.) analyses.

Conflicts of interest

There are no conflicts to declare.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3cy01785k

‡ Present address: Department of Applied Chemistry, National Chiayi University, Chiayi City, 600355, Taiwan. E-mail: mhliu@mail.ncyu.edu.tw

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