Electron Filtering by an Intervening ZnS Thin Film in the Au

ized plasmonic resonance excitation-induced electron injection from Au NP to CdS has been proved by photoelectrochemical measurements. Formation of Zn...
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Electron Filtering by an Intervening ZnS Thin Film in the Gold Nanoparticle-Loaded CdS Plasmonic Photocatalyst Kouichi Takayama,† Keigo Fujiwara,† Takahiro Kume,† Shin-ichi Naya,‡ and Hiroaki Tada*,†,‡ †

Department of Applied Chemistry, School of Science and Engineering and ‡Environmental Research Laboratory, Kindai University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan S Supporting Information *

ABSTRACT: In the gold nanoparticle (Au NP)-loaded CdS film on fluorinedoped tin oxide electrode (Au/CdS/FTO), the localized plasmonic resonance excitation-induced electron injection from Au NP to CdS has been proven by photoelectrochemical measurements. Formation of ZnS thin films between the Au NP and CdS film leads to a drastic increase of the photocurrent under visible-light irradiation (λ > 610 nm) in a 0.1 M NaClO4 aqueous electrolyte solution due to the electron filtering effect. The photocurrent strongly depends on the thickness of the ZnS film, and the maximum value is obtained at a thickness as thin as 2.1 nm. Furthermore, the ZnS overlayer significantly stabilizes the photocurrent of the CdS/FTO electrode in a polysulfide/sulfide electrolyte solution even under the excitation of CdS (λ > 430 nm). This work presents important information about the design for new plasmonic photocatalysts consisting of plasmonic metal NPs and chalcogenide semiconductors with high conduction band edge.

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in the Au/CdS system is very short.31 This originates from the large energy gap between the Fermi energy of the Au NP and the CB of CdS or the large driving force for the back IFET. While the lifetime of the CdS excitation-induced charge separation state in the Au/CdS system is much longer, CdS undergoes photodissolution.33 If the back IFET can be effectively suppressed without the forward IFET being retarded, the Au/CdS plasmonic photocatalyst would be highly promising for the solar-to-chemical transformations. Here we have shown in the Au/CdS film-coated fluorinedoped tin oxide (Au/CdS/FTO) photoanode of a photoelectrochemical (PEC) cell that the LSPR excitation of Au NPs induces the IFET from Au NP to CdS, and the charge separation is drastically enhanced by interposing an insulating ZnS thin film between the Au NP and CdS (Au/ZnS/CdS/ FTO). To our knowledge, this is the first report on the Au/ ZnS/CdS/FTO photoelectrode, presenting important information about the design for the Au/CdS plasmonic photocatalyst. CdS was deposited by the chemical bath deposition (CBD) method according to the procedure previously reported.34 The CdS thickness was controlled by the CBD time (td). To identify the deposits on FTO, X-ray diffraction (XRD) measurements were carried out for the samples without and with Au NPs loaded (Figure S1). In the XRD patterns, two peaks are observed except for those of FTO. A peak at 2θ = 44.1° can be indexed as the diffraction from the (220) and (110) crystal

oinage metal nanoparticle (NP)-loaded metal oxides have recently attracted much interest as a new class of visiblelight photocatalysts for solar-to-chemical transformations.1 The so-called “plasmonic photocatalysts” can be divided into the interfacial electron transfer (IFET) type2 and the electromagnetic field-enhanced (EM) type.3,4 Au NP-loaded TiO2 (Au/TiO2) is representative of the IFET type plasmonic photocatalyst with strong absorption due to the localized surface plasmon resonance (LSPR) in the visible region. The LSPR excitation of Au/TiO2 causes the IFET from Au NP to the conduction band (CB) of TiO2. As a result of the lowering in the Fermi energy in the Au NPs, an oxidizing ability is induced on their surface, while reduction can occur on the TiO2 surface. While various oxidative chemical transformations5−13 and water oxidation14−26 using the Au/TiO2 plasmonic photocatalyst have been reported to date, application to the reductive chemical transformation is only limited.27,28 This is partly because of the fairly weak reducing ability of the electrons, which is determined by the CB minimum level of TiO2. CdS with a higher-lying CB minimum has a much stronger reducing ability. Recently, the incorporation of Au NP into CdS particles (Au@CdS)29 and Au@CdS-incorporated metal−organic framework30 have been reported to boost the visible-light activity of CdS for hydrogen generation from an aqueous solution containing sulfide ions by working as an EM type plasmonic photocatalyst. On the other hand, it has been shown in the Au NP-loaded CdS (Au/CdS) particulate systems by transient absorption spectroscopy31 and high-resolution superlocalization imaging32 that the selective LSPR excitation of Au NP causes the IFET from Au NP to CdS. However, the lifetime of the LSPR excitation-induced charge separation state © XXXX American Chemical Society

Received: November 10, 2016 Accepted: December 12, 2016 Published: December 12, 2016 86

DOI: 10.1021/acs.jpclett.6b02642 J. Phys. Chem. Lett. 2017, 8, 86−90

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The Journal of Physical Chemistry Letters

selectively excited by irradiating Au/CdS/FTO with light (λ > 600 nm). To study the electron injection process from Au NP to CdS, the direct contact between Au NP and FTO must be strictly inhibited. The coverage of the FTO surface by the CdS film was checked by measuring the dark current−potential curves for a three-electrode EC cell with the structure of Au/CdS/FTO (working electrode) | a 0.1 M NaClO4 aqueous solution containing 10 mM K3Fe(CN)6 and 1 mM K4Fe(CN)6 | glassy carbon (counter electrode). 38 Figure 2A shows cyclic

planes of cubic and hexagonal CdS, respectively. Another peak at 2θ = 30.6° and a weak shoulder around 2θ = 26.6° are assignable to the diffraction from the (100) and (111) crystal planes of cubic and hexagonal CdS, respectively. Evidently, the as-grown CdS film consists of cubic and hexagonal crystals. Figure 1A shows the cross-sectional scanning electron

Figure 2. (A) CV curves for the FTO, CdS (td = 20 min)/FTO, and ZnS (N = 5)/CdS (td = 20 min)/FTO electrodes in a 0.1 M NaClO4 aqueous solution containing 10 mM K3Fe(CN)6 and 1 mM K4Fe(CN)6. (B) IPCE action spectrum for the CdS (td = 20 min)/ FTO, Au/CdS (td = 20 min)/FTO, and Au/ZnS (N = 2)/CdS (td = 20 min)/FTO electrodes in a 0.1 M NaClO4 aqueous solution. The absorption spectrum for each electrode is shown for comparison.

Figure 1. (A) Cross-sectional SEM images of CdS (td = 60 min)/ FTO. (B) TEM image of Au/CdS. (C) Au particle size distribution of Au/CdS (td = 20 min)/FTO. (D) UV−visible absorption spectra for CdS/FTO with varying td and Au/CdS (td = 20 min)/FTO.

voltammetry (CV) curves for the FTO and CdS (td = 20 min)/FTO electrodes. In the unmodified FTO electrode system, a pair of the Fe(CN)63−/K4Fe(CN)62− redox currents is observed at a half-wave potential of +0.28 V versus standard hydrogen electrode (vs SHE). In the CdS/FTO electrode system, the redox currents disappear. Evidently, in the CdS/ FTO sample, the FTO electrode is completely covered with CdS at td ≥ 20 min.38 Furthermore, the photocurrent was measured for a three-electrode PEC cell with the structure of Au/CdS/FTO (photoanode) | a 0.1 M NaClO4 aqueous solution | glassy carbon (cathode). Figure 2B shows the action spectrum of incident photon-to-current conversion efficiency (IPCE) for the CdS (td = 20 min)/FTO and Au/CdS (td = 20 min)/FTO electrodes in a 0.1 M NaClO4 aqueous solution and each absorption spectrum for comparison. In the CdS (td = 20 min)/FTO system, the photocurrent rises at λ ≈ 550 nm, which is close to the absorption edge of the CdS film. On the other hand, in the Au/CdS (td = 20 min)/FTO system, two photocurrent peaks are observed around 780 and 620 nm corresponding to the LSPR peaks of Au/CdS/FTO. The complete coverage of the FTO surface by the CdS film excludes the possibility of the electron injection from Au NPs to FTO. Clearly, the selective LSPR excitation of Au NPs in the Au/ CdS/FTO system gives rise to the IFET from Au NPs to CdS in a manner similar to the Au/CdS particulate system.31,32 Also, the Au NP loading significantly enhances the photocurrent at 520 < λ < 550 nm although the CdS absorption is very weak in the wavelength region. This finding may result from the enhancement of the CdS exciton generation by the LSPR of Au NPs.39 The rapid back electron transfer from the conduction band (CB) of CdS to Au NP could be responsible for the small photocurrent in the Au/CdS/FTO system.31,32 The effect by the ZnS overlayer with a much higher-lying CB minimum

microscopy (SEM) image for CdS (td = 60 min)/FTO. A fairly uniform polycrystalline CdS film is formed by the reaction of Cd2+ ions and thiourea on the surface of FTO with an apparently intimate contact. The thickness (l(CdS)) determined from the SEM observation increases with an increase in td according to an equation of l(CdS)/nm ≈ 481·{1 − exp(−0.0272td/min)} (Figure S2). In the presence of thiourea in excess (experimental details in the Supporting Information), the rate of film growth would obey the pseudo-first-order rate equation with respect to the Cd2+ concentration. Figure 1B shows a transmission electron microscopy (TEM) image for a part of the Au/CdS film detached from the FTO surface. A number of Au NPs were dispersed on the CdS surface, and some of them take a rodlike shape (Au NR). Figure 1C shows Au particle size distribution for Au/CdS/FTO. The Au particle has a wide size distribution with two peaks around 7 and 16 nm. Figure 1D shows ultraviolet (UV)−visible absorption spectra for CdS/FTO prepared by varying td and for Au/CdS (td = 20 min)/FTO. The absorption edge of FTO is ∼330 nm, and the deposition of the CdS film induces the visible-light absorption. The increase in td intensifies the absorption of the interband transition, and the absorption edge of λ = 520 nm is in agreement with the value for the bulk-state CdS.35 Au NPs (∼20 μg cm−2) were loaded on CdS/FTO by the seed-growth method.36 Loading Au NPs extends the absorption in the whole visible region due to the LSPR. A close inspection indicates two broad peaks around 620 nm (P1) and 780 nm (P2), which stem from not the absorption of CdS but the LSPR of Au NPs. The P1 and P2 peaks probably correspond to the transverse mode (T-mode) and longitudinal mode (L-mode) of the short Au NRs, respectively.37 Thus, the LSPR of Au NPs can be 87

DOI: 10.1021/acs.jpclett.6b02642 J. Phys. Chem. Lett. 2017, 8, 86−90

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The Journal of Physical Chemistry Letters (−1.85 V vs SHE) than CdS (−0.9 V vs SHE)40 on the photocurrent was studied. ZnS thin films were formed on the surface of CdS/FTO by the successive ionic layer adsorption and reaction (SILAR) method (ZnS/CdS/FTO).41 We confirmed that Au NPs are deposited on the ZnS/CdS/FTO surface in a fasion similar to the system without the ZnS layer (Figure S3), and the amount of CdS hardly changes during the SILAR process by inductively coupled plasma spectroscopy. Figure 3 shows X-ray photoelectron spectroscopy (XPS)

Figure 4. (A) Plots of the LSPR-induced photocurrent of Au/ZnS/ CdS (td = 20 min)/FTO vs l(ZnS). (B) log (τn/s)−Voc curves for the Au/ZnS/CdS (td = 20 min)/FTO electrodes with varying l(ZnS).

character of the ZnS film. Thus, the LSPR-excited hot electrons in the Au NPs effectively tunnel the insulating ZnS film to move to the CB of CdS at l(ZnS) ≤ 12.3 nm. The hot electrons are chemically active, and the hot electron-induced H2 dissociation on the Au NP surface has recently been reported.42 However, the data in Figure 4A rules out the mechanism in this reaction system, where the generation of a small amount of H2 was also detected by gas chromatography. The LSPR irradiation of the three-electrode PEC cell generates the photovoltage under the open-circuit condition (Voc). When the irradiation is interrupted, the Voc decays because of the back electron transfer from CdS to Au. The mean lifetime of the hot electrons in the CB of CdS (τn) can be estimated by analyzing the Voc-decay curve by the following equation:43 τn = −(kBT/q) (dVoc/dt)−1, where kB is the Boltzmann constant and q is the elementary charge of the electron. Figure 4B shows log(τn/s)−Voc curves for the ZnS/CdS (td = 20 min)/FTO electrodes with varying l(ZnS). The τn at the same Voc increases with an increase in l(ZnS) in the rage of l(ZnS) ≤ 12.3 nm. Evidently, the insulating ZnS overlayer on the CdS film effectively suppresses the back electron transfer from CdS to Au NP. On the basis of the results presented above, we propose the action mechanism in the present PEC cells using the Au/CdS/ FTO and Au/ZnS/CdS/FTO photoanodes (Scheme 1).

Figure 3. Zn 2p- and Cd 3d-XPS spectra of ZnS/CdS (td = 20 min)/ FTO with various SILAR cycle number N.

spectra for ZnS/CdS/FTO prepared by changing the SILAR cycle number (N). In the Zn 2p spectra, two signals due to the emission from the Zn 2p1/2 and Zn 2p3/2 orbitals of ZnS appear at the binding energies (EB) of 1044.2 and 1021.3 eV, respectively, with the SILAR process. The signals intensify as the N value increases. Conversely, in the Cd 3d-XPS spectra, the two signals at 411.8 and 405.0 eV due to the emission from the Cd 3d3/2 and Cd 3d5/2 orbitals of CdS, respectively, weaken with an increase in N. The intensities of the S 2p signals are almost independent of N (Figure S4). Furthermore, we carried out scanning electron microscopy (SEM) observation for the surface and cross section of ZnS (N = 20)/CdS/FTO (Figure S5). The elemental mapping of Cd and Zn by energy dispersive X-ray (EDX) spectroscopy indicates the formation of a ZnS overlayer on the surface of the CdS/FTO. These results reinforce the conclusion drawn from the XPS measurements that ZnS thin films are formed on the surface of the CdS film by the SILAR technique. The thickness of the ZnS layer (l(ZnS)) estimated from the loading amount of Zn and the density of ZnS increases in proportion to N according to the equation l(ZnS)/nm ≈ 1.33N − 0.80 (Figure S6). In Figure 2B, the IPCE action spectrum for the Au/ZnS (N = 2)/CdS (td = 20 min)/FTO electrode is also shown. Interestingly, the ZnS film formation on the CdS film drastically increases the IPCEs, while the profile is similar to that for the system without the ZnS film. Also, the IPCE action spectra in Figure 2B indicate that Au NPs are selectively excited at λ > 610 nm. The weak peak at λ ∼ 800 nm suggests that the LSPR excitation of not only the T-mode but also the L-mode causes the IFET from the Au NR to CdS. A similar result has recently been reported in the Au NR-loaded TiO2 system.21 Figure 4A shows the LSPR-induced photocurrent (λ > 610 nm) as a function of l(ZnS). Surprisingly, the ZnS overlayer with a thickness as small as ∼0.7 nm increases the photocurrent by a factor of approximately 1 order of magnitude. The photocurrent goes through a maximum at l(ZnS) = 2.1 nm to be attenuated to zero at l(ZnS) ≈ 40 nm. In the CV curve for the ZnS/CdS/FTO electrode in Figure 2, a small current flows only at E < −0.8 V. This fact is indicative of the good insulating

Scheme 1. Basic Reaction Mechanism on the Water Oxidation by the Au/ZnS/CdS/FTO Photoanode

Visible-light irradiation (λ > 610 nm) selectively excites the LSPR of Au NPs on the CdS surface. The hot electrons are injected from Au NPs to the CB of CdS. The rapid back electron transfer occurs in the Au/CdS/FTO system. When the ZnS thin film with thickness l < ∼10 nm is overlaid on CdS, the hot electrons can tunnel the ZnS overlayer to move to the CB of CdS. Because the hot electrons relax to the CB minimum of CdS by electron−electron scattering within 10−13 s,31 the back electron transfer is suppressed by a large energy barrier (∼1 88

DOI: 10.1021/acs.jpclett.6b02642 J. Phys. Chem. Lett. 2017, 8, 86−90

The Journal of Physical Chemistry Letters eV), and consequently, the effective charge separation is achieved. However, the thick ZnS overlayer with l > ∼20 nm decreases the tunneling probability in the forward electron transfer. Thus, an optimum thickness of the ZnS film is present around l ≈ 2.1 nm. As a result of the lowering in the Fermi energy of Au NPs (EF → EF′), water is oxidized on the surface.14−26 On the other hand, the electrons in the CB move to FTO to be further transferred to the cathode through the external circuit, and water is reduced to H2 there. The long-term stability of the PEC cell is also crucial. Importantly, the stability of CdS is guaranteed under the present conditions (λ > 610 nm) because the LSPR of Au NPs is selectively excited, and the CdS film only works as an electron transporter. Quantum dots (QDs) of metal chalcogenides such as CdS as well as Au NPs can be used as a promising sensitizer in the PEC cells for the solar-to-chemical transformations; however, they are known to undergo dissolution under photoexcitation. The use of the polysulfide/ sulfide electrolyte solution improves the cell stability by effectively suppressing the dissolution of the QDs.44 The effect of the ZnS overlayer on the photostability of the CdS/FTO electrode was examined for a three-electrode PEC cell with the structure of CdS/FTO (photoanode) | a 0.25 M Na2S + 0.35 M Na2SO3 aqueous solution | glassy carbon (cathode) under visible-light irradiation (λ > 430 nm). Clearly, the ZnS overlayer further stabilizes the photocurrent of the CdS/FTO electrode even under the excitation of CdS (Figure S7). In summary, this study has shown that the selective LSPR excitation of the Au/CdS/FTO photoanode in a PEC cell involving a 0.1 M NaClO4 aqueous electrolyte solution gives rise to the interfacial electron transfer from Au NP to CdS, and the ZnS overlayer on the CdS film drastically enhances the photocurrent by effectively decreasing the back electron transfer from CdS to Au NPs. In addition, the ZnS overlayer remarkably stabilizes the photocurrent of the CdS/FTO electrode in a polysulfide/sulfide electrolyte solution under the excitation of CdS. This is a basic proof-of-concept study, and its application to large-surface-area nanostructured systems would generate new plasmonic photocatalysts combining high reducing and oxidizing ability.

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ACKNOWLEDGMENTS

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REFERENCES

This work was partially supported by a Grant-in-Aid for Scientific Research (C) No. 15K05654 and MEXT-Supported Program for the Strategic Research Foundation at Private Universities.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b02642. Experimental details; XRD patterns of CdS/FTO (Figure S1); CdS film thickness (Figure S2); TEM image (Figure S3); S 2p-XPS spectra (Figure S4); EDX analysis (Figure S5); ZnS film thickness (Figure S6); photochronoamperometry profile (Figure S7) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +81-6-6721-2332. Fax: +81-6-6727-2024. E-mail: [email protected]. ORCID

Hiroaki Tada: 0000-0001-8638-0697 Notes

The authors declare no competing financial interest. 89

DOI: 10.1021/acs.jpclett.6b02642 J. Phys. Chem. Lett. 2017, 8, 86−90

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DOI: 10.1021/acs.jpclett.6b02642 J. Phys. Chem. Lett. 2017, 8, 86−90