Research Article pubs.acs.org/journal/ascecg
Plasmon-Enhanced Photoelectrochemical Water Splitting on Gold Nanoparticle Decorated ZnO/CdS Nanotube Arrays Ren-Bin Wei,† Pan-Yong Kuang,† Hui Cheng,† Yi-Bo Chen,† Jian-You Long,‡ Ming-Yi Zhang,*,§ and Zhao-Qing Liu*,† †
School of Chemistry and Chemical Engineering, and ‡School of Environmental Science and Engineering/Guangdong Provincial Key Laboratory of Radionuclides Pollution Control and Resources, Guangzhou University, Guangzhou Higher Education Mega Center, Waihuan Xi Road No. 230, Guangzhou, 510006, China § Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education, School of Physics and Electronic Engineering, Harbin Normal University, Harbin, 150025, China S Supporting Information *
ABSTRACT: The design and decoration of plasmonic metal hybrid photoanodes provide an effective strategy for highly efficient photoelectrochemical (PEC) water splitting. In this work, an Au nanoparticle (NP) decorated highly ordered ZnO/CdS nanotube arrays (ZnO/CdS/Au NTAs) photoanode has been rationally designed and successfully synthesized. By virtue of the favorable band alignment and specific nanotube structure of ZnO/CdS as well as the surface plasmonic effect of Au NPs, the ZnO/CdS/Au NTAs photoanode shows significantly enhanced PEC performance as compared to the ZnO/CdS/Au and ZnO/ CdS nanorod arrays (NRAs). Impressively, the optimized ZnO/CdS/Au NTAs photoanode exhibits the highest photocurrent density of 21.53 mA/cm2 at 1.2 V vs Ag/AgCl and 3.45% photoconversion efficiency (PCE) among the parallel photoanodes under visible light illumination (λ > 420 nm). KEYWORDS: Photoelectrochemical performance, Plasmon effect, Nanotube arrays, ZnO/CdS, Au nanoparticles
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INTRODUCTION Efficient photoelectrochemical (PEC) water splitting has attracted huge attention because it provides a promising route for solar-to-chemical energy conversion to deal with the increasingly serious global energy problem. Numerous metal semiconductors, such as ZnO, Cu2O, CdS, and Fe2O3, have been broadly researched and designed as photoanode materials for PEC water splitting since TiO2 was first used as a photoelectrode to decompose water into oxygen and hydrogen by Fujishima and Honda in 1972.1−5 As a universally used semiconductor, zinc oxide, ZnO, has drawn extensive attention due to its low cost and nontoxicity.6,7 Particularly, ZnO with one-dimensionally ordered structure can be widely applied in photocatalysis, solar cells, photovoltaic materials, and sensors owing to its easy fabrication, high charge carrier mobility, and large surface area.8−14 Despite these shining merits, the most crucial obstacles of ZnO for actively photo-to-hydrogen application, however, are the restricted light response range with the wide bandgap (∼3.2 eV) and frequent recombination © 2017 American Chemical Society
of photoinduced electron−hole pairs. Hence, how to address these disadvantages also becomes the focus of current research. Recently, many strategies have been designed to effectively improve the low photocatalytic activity of single ZnO through the doping, quantum dot sensitization, loading noble metal, and structuring heterojunction.15−19 Among them, combinations with narrow band gap semiconductor are commonly used to remarkedly enhance the PEC performance of ZnO. For example, Choi fabricated CdS and CuInS2 co-sensitized ZnO nanowires (NWs) that obtained a greatly improved photocurrent density of 13.8 mA cm2 at 0.3 V vs SCE under sunlight illumination.20 Zou synthesized vertically aligned Cu2O/ZnO p−n junction nanorod arrays (NRAs), exhibiting a significant improvement in photocatalytic applications under visible light irradiation when compared to the pure Cu2O and ZnO.21 As a Received: January 22, 2017 Revised: March 7, 2017 Published: March 31, 2017 4249
DOI: 10.1021/acssuschemeng.7b00242 ACS Sustainable Chem. Eng. 2017, 5, 4249−4257
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commonly used chalcogenide, CdS is an ideal modified choice because of its suitable energy band position and narrow Eg of 2.4 eV, which fully enables it to combine with ZnO and increase visible light absorption for photoelectrochemical water splitting.22,23 When coupling CdS with ZnO, the typical type II mode achieved due to their staggered band gap structures, which could not only effectively accelerate the photogenerated carrier transfer from CdS to ZnO but also greatly suppress the internal charge recombination.24 Therefore, CdS sensitized ZnO nanowires and nanorods may be favorable for electron transfer and result in enhanced photoelectrochemical performance.25,26 However, the PEC efficiency of nanowires and nanorods are still low and restrained on account of small specific surface and limited charge carrier collection ability.27 In contrast, the structure of the nanotube outperforms nanowire and nanorod because of the large surface-to-volume ratio (the surface of the internal and external) and high-efficiency charge carrier extraction, which is conducive to the trapping and utilization of photoinduced electron.28 In addition, the porous nanotube structure of the catalyst not only endows the catalyst with more reactive sites but also allows the fast charge transfer in reactions, resulting in the highly activity and carrier availability of catalyst. Accordingly, ZnO based nanotube arrays (NTAs) combined with CdS might propose a potential strategy to promote PEC efficiency for water splitting. Notably, using a noble metal (such as Au, Pt, and Ag) as cocatalyst coupled with semiconductor can largely enhance solar energy conversion efficiency and charge separation, which further boosts PEC performance.29,30 Generally, the Au plasmonic metals with nano-sized particles could act as photosensitizers and show broad absorption wavelengths (tunable from UV to near-IR) and high light harvesting efficiency owing to the unique LSPR effect.31 Furthermore, the plasmon-excited hot electrons in Au nanoparticles could be injected to the conduction band of the adjacent semiconductor and then participate in the following chemical reactions, which can synchronously suppress the recombination of photoinduced electron−hole pairs.32,33 Impressively, in the plasmonic metal-based semiconductor hybrid system, gold nanoparticles display inherently superior photostability during the photoreaction process. Therefore, Au nanoparticles have been widely used to construct plasmonic hybrid photocatalysts with optimizing the utilization of solar power and accelerating charge transfer, leading to greatly enhanced photoelectrochemical activities.34,35 In this work, an Au NP modified ZnO/CdS NTAs photoanode has been successfully fabricated through a threestep process of electrochemical deposition and chemical bath method followed by Au photoreduction reaction. In contrast to ZnO/CdS NRAs and ZnO/CdS/Au NRAs photoanodes, the optimized ZnO/CdS/Au NTAs photoanode exhibits greatly enhanced visible-light PEC performance, achieving the highest photocurrent density of 21.53 mA/cm2 at 1.2 V vs Ag/AgCl and 3.45% photoconversion efficiency (PCE) under visible light illumination (λ > 420 nm). Such improved photoelectrochemical performance could be mainly attributed to the structure feature of the ZnO/CdS nanotube and plasmon-induced hot electrons by Au NPs, benefiting from high-efficiency utilization of photoinduced electron and extended carrier density, respectively. Undoubtedly, this work provides a promising path for develop new water splitting photoelectrode materials.
Research Article
EXPERIMENTAL SECTION
Chemicals and Materials. Zn(NO3)2·6H2O, NH4Ac, C6H12N4 (HMT), Cd(NO3)2·2H2O, thiourea (CN2H4S), Auric chloride (HAuCl4), trisodium citrate dihydrate (C6H5Na3O7·2H2O), and NaBH4 were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All materials are analytical grade and used as received. Fabrication of ZnO/CdS/Au NTAs. The fabrication process for the ZnO/CdS/Au NTAs photoanode is proposed in Scheme 1. First,
Scheme 1. Formation of ZnO/CdS/Au NTAs Photoanode
the ZnO/CdS/Au NRAs were prepared via the chemical bath deposition (CBD) and Au reduction processes based on the asprepared ZnO NRAs according to the previous report.36 After the process of sulfuric acid corrosion, the ZnO/CdS/Au NTAs grown in FTO substrate were successfully obtained. Typically, a CdS shell layer was coated onto ZnO NRAs by the CBD method from the equimolar (0.01 M) aqueous solution of Cd(NO3)2 and thiourea at 90 °C for 20 min. Subsequently, the above prepared ZnO/CdS NRAs were immersed into 100 mL deionized water, and then a proper amount of 2.538 mM HAuCl4, 0.01 M C6H5Na3O7·2H2O, and 0.01 M NaBH4 were added in an orderly fashion. Then, this mixture was magnetically stirred for 1 h to achieve Au decoration. Finally, the fabricated ZnO/ CdS/Au NRAs were washed with deionized water and dipped in 20 mL 4 mM H2SO4 solution to acquire the ZnO/CdS/Au NTAs. Physicochemical Characterizations. The surface morphology and detailed microscopic structure of samples were analyzed using field emission scanning electron microscopy (FESEM, JSM-6701F) and transmission electron microscopy equipped with EDS (TEM, JEM2010-HR). The crystal phase of the samples were characterized by powder X-ray diffraction (XRD, Bruker, D8 Advance) with Kα irradiation (λ = 0.15418 nm). The chemical-state analysis of the products was measured with X-ray photoelectron spectroscopy (XPS, Thermo K-Alpha). The optical properties of samples were observed over Hitachi UV-3010 spectrophotometer using BaSO4 as a reference and the Raman spectrum was recorded on a Renishaw inVia spectrometer. Electrochemical Measurements. The PEC performances of the obtained photoanodes were measured by electrochemical workstation (CHI 760D, China Chenhua) under adjustable 350 W Xe lamp irradiation with a UV-light cutoff filter (λ > 420 nm) . In a complete test system, the as-prepared photoanode (effective area around 1.5 × 1.5 cm2) were served as the working electrode, a platinum sheet was used as the counter electrode, and Ag/AgCl electrode (saturated KCl) was employed as reference electrode. The 0.25 M Na2S + 0.35 M Na2SO3 aqueous solution (pH = 13.56) was added to the quartz cell (located the light source about 5 cm) as the electrolyte. A 350 W Xe lamp coupled with monochromator (AnHe, Inc., China) acted as the monochromatic light source for incident photon to current conversion efficiency (IPCE) measurement, and the intensity of monochromatic light was obtained by a radiometer (PM120VA, Thorlabs). 4250
DOI: 10.1021/acssuschemeng.7b00242 ACS Sustainable Chem. Eng. 2017, 5, 4249−4257
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Figure 1. (a, b) SEM, (c) TEM, (d) HRTEM, and (e−j) element mapping images of the ZnO/CdS/Au NTAs photoanode.
Figure 2. XPS spectra of the ZnO/CdS/Au NTAs and ZnO/CdS/Au NRAs photoanodes: (a) Zn 2p, (b) O 1s.
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RESULTS AND DISCUSSION Figure 1a shows the typical scanning electron microscopy (SEM) images of the ZnO/CdS/Au NTAs, which have uniform size and ordered shape with diameters of about 300−350 nm. Moreover, the homologous surface morphology of ZnO/CdS/ Au NRAs and ZnO/CdS NRAs are also obtained (Figures S1a, b and S2). The hexagonal nanotube hollow structure is clearly observed in Figure 1b. Accordingly, the transmission electron microscopy (TEM) images (Figures 1c and S1c) provide a more distinct comparison and confirmation for the nanotube hollow structure. The high-resolution transmission electron
microscopy (HRTEM) image (Figure 1d) shows Au nanoparticles (NPs) distributed on the CdS shell with diameters about 5−8 nm, the lattice spacing of 0.204 nm corresponds to the (200) plane of cubic Au.37 In addition, the lattice fringe spacing of 0.358 nm is consistent with the (100) plane of hexagonal CdS. Besides, elemental mapping images (Figures 1e−j and S1e−j), the X-ray diffraction (XRD) spectrum (Figure S3), and energy dispersive spectroscopy (EDS) (Figure S4) also present the elemental analysis and chemical component. Obviously, the photoanode is only comprised of Zn, O, Cd, S, and Au, indicating that the successful synthesis of 4251
DOI: 10.1021/acssuschemeng.7b00242 ACS Sustainable Chem. Eng. 2017, 5, 4249−4257
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Figure 3. (a) UV−vis diffuse reflectance spectra. (inset) Photographs of the samples of the ZnO/CdS NRAs, ZnO/CdS/Au NRAs, and ZnO/CdS/ Au NTAs (from left to right). (b) Plots (αhν)2 vs photo energy (hν) of the ZnO/CdS NRAs, ZnO/CdS/Au NRAs, and ZnO/CdS/Au NTAs.
Figure 4. (a) LSV versus the applied potential characteristics (10 mV/s). (b) Photoconversion efficiency. (c) Photocurrent response under visible light illumination (>420 nm). (d) Incident photon-to-current efficiency (IPCE) plot measured at 0.4 V of all samples.
ZnO and hydroxyl-type oxygen species adsorbed on the surface of as-prepared photoanode, respectively.39 In the sample of ZnO/CdS/Au NTAs, the intensity rate of Zn−O weakened due to the sulfuric acid dissolution when comparing with the ZnO/CdS/Au NRAs and the same tendency occurs in Zn 2p. On the contrary, there is almost no difference between ZnO/ CdS/Au NTAs and ZnO/CdS/Au NRAs of other elements. Meanwhile, the peaks at 405.0 and 411.8 eV are assigned to Cd 3d5/2 and Cd 3d3/2, while two peaks at 161.2 and 162.3 eV are attributed to S 2p3/2 and S 2p1/2, verifying the formation of CdS.40 The Au 4f featured peaks located at 84.0 and 87.7 eV match well with the binding energy of Au 4f7/2 and Au 4f5/2, which means that Au nanoparticles were successfully reduced.41 Based on the above discussion, the existence of ZnO, CdS, and Au components in the as-prepared photoanode is confirmed. UV−vis diffuse reflectance spectra can be performed to characterize optical properties of the as-prepared samples at different wavelengths. As presented in Figure 3a, all samples show a wide optical absorption range at about 550 nm and without distinct red shift occurred. However, the spectrum shows increased absorption intensity especially in the region of 550 to 700 nm due to the LSPR effect of Au NPs. As shown in
purified ZnO/CdS/Au NTAs. As shown in the XRD patterns of the ZnO/CdS/Au NTAs, strong diffraction peaks at 31.8, 34.4, 36.2, 47.5, and 62.8° match well with (100), (002), (101), (102), and (103) planes of hexagonal wurtzite ZnO (JCPDS: 36-1451). Moreover, the comparatively weak diffraction peaks at 24.8, 26.5, and 28.2° are in agreement with the hexagonal CdS phase (JCPDS: 41-1049) in the (100), (002), and (101) planes. Without explicit Au, diffraction peaks are detected, which may be ascribed to a very small amount of Au nanoparticles and the high intensity of the spike diffraction peaks of ZnO at 34.4°. The X-ray photoelectron spectroscopy (XPS) further confirms and provides the surface composition and elemental analysis of the as-prepared photoanodes (Figures 2 and S5). The full XPS spectrum also demonstrates that the photoanode is composed of Zn, O, Cd, S, and Au. Two weak peaks at binding energies of 1021.4 eV (Zn 2p3/2) and 1044.5 eV (Zn 2p1/2) are shown in Figure 2a, which confirmed the existence of Zn2+ in the low content ZnO of the ZnO/CdS/Au NTAs.38 The high-resolution spectrum of O 1s (Figure 2b) could be deconvoluted into two individual peaks at around 530.9 and 531.8 eV, corresponding to O2−-type oxygen species in the 4252
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Figure 5. LSV (a, c) and I−t (b, d) versus under the illumination of monochromatic light with 600 and 650 nm of the ZnO/CdS NRAs, ZnO/CdS/ Au NRAs, and ZnO/CdS/Au NTAs.
splitting,47 and the following equation is conducted to calculate PCE:
Figure 3b, the approximate optical bandgap energies of samples were obtained from the Kubelka−Munk equation. According to the equation of (αhν) = A(hν − Eg)n and plots of (αhν)2 versus photo energy (hν), the Eg values of ZnO/CdS NRAs, ZnO/ CdS/Au NRAs, and ZnO/CdS/Au NTAs are determined to be 2.20, 2.19, and 2.17 eV, respectively.42,43 These similar results indicate that introduction of Au nanoparticles hardly have no effect on the crystalline feature of ZnO/CdS.44,45 Meanwhile, the Raman spectra also reveals the optical effects of Raman scattering and directly confirms the existence of Au NPs (Figure S6). Two defined peaks are located near 300 and 600 cm−1 in all the samples, which are conformed to CdS structure.46 Apparently, the scattering intensity of ZnO/CdS/ Au NRAs and ZnO/CdS/Au NTAs are greatly enhanced, demonstrating the plasmonic effect is beneficial to promote the capability of visble-light absorption. The photoelectrochemical performances of the as-prepared photoanodes were evaluated in 0.25 M Na2S and 0.35 M Na2SO3 aqueous solution. Figure 4a displays the linear sweep voltammogram (LSV) curves of the samples versus Ag/AgCl applied potential from −1.0 to +1.2 V under visible-light illumination. The ZnO/CdS/Au NTAs exhibit a maximum current density of 21.53 mA/cm2 at 1.2 V vs Ag/AgCl, which is about 1.94 and 1.28 times compared with the value of ZnO/ CdS NRAs and ZnO/CdS/Au NRAs. The current density of different Au concentrations of ZnO/CdS/Au NTAs samples exhibits the appropriate Au NPs mass loading and the distinct color difference (Figure S7). Notably, the sample of 5 mL ZnO/CdS/Au NTAs shows lower current density about 12.63 mA/cm2 than that of 3 mL ZnO/CdS/Au NTAs, indicating that overmuch Au NPs agglomerate in the surface of the hybrid and hinder the transmission rate of charge carriers. The photoconversion efficiency (PCE) quantitatively evaluates the photo-to-hydrogen conversion efficiency in the process of water
PCE = J(1.23 − Vapp)/P
(1)
where J is the photocurrent density at the measured potential, Vapp is the experimental applied potential vs RHE (reversible hydrogen electrode), and P is the incident light intensity (100 mW/cm2). The measured electrode potential vs Ag/AgCl is converted to RHE via the Nernst equation: ERHE = EAg/AgCl + 0.059pH + EAg/AgClθ, wherein EAg/AgClθ is the standard electrode potantial of Ag/AgCl at 25 °C (0.197 V), and pH of the electrolyte is 13.56.48 As expected, the ZnO/CdS/Au NTAs possess the highest PCE of almost 3.45% (Figure 4b). These results may be mainly attributed to the decoration of Au NPs, which greatly increases the light absorption, generates more hot electrons, and further transfers to the conduction band of CdS, thus significantly enhances the density of charge carriers.49 Moreover, the unique nanotube hollow structure increases more contact reaction area between photoanode and electrolyte, and provides directly favorable migration pathway for charge carriers to fulfill preferable PEC performance. As presented in Figure 4c, the photocurrent density response (I−t) curves of different photoanodes are studied at an open circuit potential under visible light on−off cycles. The corresponding photocurrent densities of the ZnO/CdS NRAs, ZnO/CdS/Au NRAs, and ZnO/CdS/Au NTAs photoanodes are about 1.82, 3.89, and 4.77 mA/cm2, respectively. In addition, the ZnO/CdS/Au NTAs also displays the higher photocurrent response and larger span of photocurrent density, which further confirms the efficient separation of photoinduced electron−hole pairs and the accelerated transport of charge carriers in ZnO/CdS/Au NTAs photoanode. The incident photon to current conversion efficiency (IPCE) measurement is further performed to quantify the photoelectric conversion 4253
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Figure 6. Mott−Schottky plots (a) at a frequency of 1 kHz with an AC imposed bias of 5 mV and EIS spectra and (b) at 0.4 V with a frequency range between 100 kHz and 1 Hz and AC amplitude of 5 mV of the ZnO/CdS NRAs, ZnO/CdS/Au NRAs, and ZnO/CdS/Au NTAs in the dark.
CdS NRAs presents weak growth extent when contrast with ZnO/CdS/Au NTAs and ZnO/CdS/Au NRAs in the dark (Figure S9). This result identifies the considerable role of Au NPs in heightening the photoactivity of plasmonic heterojunction. By introducing Au NPs to decorate ZnO/CdS NRAs, the plasmonic-improved sunlight absorption and plasmoninduced hot electrons injection were achieved, which accounts for the pronounced promotion of photocurrent density. The intensified photocurrent response in the I−t curves tested at 0 V (Figure 5b, d) further verifies that Au NPs can increase the light harvesting and accelerate charge transportation because of the localized surface plasmon resonance effect. Mott−Schottky (MS) measurement determines the carrier density upon the formation of Schottky barrier between the photoanode materials and electrolytes, which further confirms the superior photoelectrochemical performance of the asprepared photoanodes (Figure 6a).51 All the samples display positive slopes and decrease step-by-step from ZnO/CdS NRAs, ZnO/CdS/Au NRAs, and ZnO/CdS/Au NTAs. Moreover, the slopes of ZnO/CdS/Au NTAs and ZnO/ CdS/Au NRAs are much smaller than that of the ZnO/CdS NRAs, demonstrating the great enhancement of carrier density after the decoration of Au NPs. Generally, the carrier density of the photoanode materials can be figured out through the following equation:52
efficiency of these photoanodes under various incident light wavelengths according to the following formula:50 IPCE = 1240J /λJlight
(2)
where J is the measured photocurrent density at the specific wavelength (mA/cm2), λ is the incident light wavelength, and Jlight is measured irradiance at the specific wavelength (mW/ cm2). All of the samples display the strong photoresponse peak at around 390 nm due to the light absorption of CdS and corresponding photoresponse edge at 550 nm when comparing with the absorption edge as shown in the UV−vis diffuse reflectance spectra (Figure 4d). Due to the typical type-II band gap structure between ZnO and CdS, which would effectively decrease the photoinduced electron−hole pairs recombination and greatly boost the photocatalytic activity, the pure ZnO/ CdS NRAs electrode has a high IPCE value approximately 25.13%. After the decoration of Au, the hybrids exhibit the distinct enhancement in the wavelength range of 380−550 nm. Furthermore, the maximum value of IPCE has increased to 64.15% and 88.76%, respectively. The distinct improvement of performance demonstrates that the introduction of Au can produce more hot electrons and contribute to provide more efficient conversion rate of photos-to-available electrons. Meanwhile, the ZnO/CdS/Au NTAs achieve a greater photoresponse than ZnO/CdS/Au NRAs, which may provide more reactive sites and efficient charge carrier transmission channels to capture light and excite electrons. Significantly, the stability of the photoanode is fundamental for water splitting and energy conversion. The persistent photocurrent density curve was obtained at 0.2 V under simulated sunlight (λ > 420 nm) for 1 h. The optimal ZnO/CdS/Au NTAs photoanode only exhibits a slight photocurrent density attenuation of 4.4% (Figure S8), implying the excellent stability of ZnO/CdS/Au NTAs photoanode for PEC water splitting. To further investigate the LSPR effect of Au NPs to plasmonic photoanodes, additional photoelectrochemical measurements were carried out under controllable irradiation wavelengths. Figure 5 demonstrates the LSV curves (a, c) and I−t curves (b, d) of the as-prepared photoanodes under the illumination of monochromatic light with 600 and 650 nm. For the ZnO/CdS/Au NTAs and ZnO/CdS/Au NRAs, the photocurrent density are 7.32 and 6.73 mA/cm2 at 1.2 V versus Ag/AgCl under 600 nm monochromatic light, which are much higher than the pure ZnO/CdS NRAs (1.97 mA/cm2). Simultaneously, with the illumination of 650 nm monochromatic light, the ZnO/CdS/Au NTAs and ZnO/CdS/Au NRAs shows comparative enhancement (4.68 and 3.76 mA/cm2) than pure ZnO/CdS NRAs (1.38 mA/cm2), respectively. The ZnO/
N = (2/εε0e0)[d(1/C 2)/dV ]−1
(3)
where ε denotes the dielectric constant of the material, ε0 denotes the permittivity of the vacuum (8.854 × 10−12 F m−1), e0 denotes the electronic charge unit (1.602 × 10−19 C), and V denotes the potential applied at the electrode. With the ε value of 10 for ZnO and CdS, and an ε0 value of 8.85 × 10−12 F m−1 for the permittivity of the vacuum, the carrier densities of these photoanodes are calculated to be 1.58 × 1019, 8.32 × 1019, and 1.47 × 1020, respectively. The carrier density of ZnO/CdS/Au NTAs is higher than that of ZnO/CdS/Au NRAs due to the larger light response area and higher efficiency charge carrier motion of the nanotube structure. The results of MS plots are in good agreement with the previous photoelectrochemical measurement and also demonstrate less recombination and more effective separation of photoinduced electron−hole pairs. Electrochemical impedance spectroscopy (EIS) can interpret the charge carrier migration activities and the interfacial charge transfer resistance (Rct). Typically, the smaller semicircle in a Nyquist plot suggests the lower electron transport resistance thus resulting in faster interfacial charge transfer and higher separation rate of electron−hole pairs.53 Figure 6b shows the EIS Nyquist plots of all the samples recorded at 0.4 V in the 4254
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dark with a frequency range between 100 kHz and 1 Hz and AC amplitude of 5 mV. The EIS plots reveal that the lowest Rct of the ZnO/CdS/Au NTAs photoanode, illustrating that Au NPs and the unique nanotube structure play crucial roles in the significantly enhanced PEC performance. On the basis of the above results, the internal nanotube structure and the tentative mechanism of ZnO/CdS/Au NTAs for photoelectrochemical water splitting can be proposed in Figure 7. Clearly, under
Research Article
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00242. TEM and corresponding EDS mapping images of ZnO/ CdS/Au NRAs, XRD and Raman spectra pattern of samples, XPS spectra of the ZnO/CdS/Au NTAs and ZnO/CdS/Au NRAs, current density curve, photocurrent density stability test plot, and LSV curve of samples in the dark (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. Fax: 86-20-39366908. Tel.: 8620-39366908 (Z.-Q. Liu). *E-mail:
[email protected] (M.-Y. Zhang). ORCID
Zhao-Qing Liu: 0000-0002-0727-7809 Notes
The authors declare no competing financial interest.
Figure 7. Schematic diagram of the proposed charge carriers transfer mechanism in ZnO/CdS/Au NTAs photoanode.
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ACKNOWLEDGMENTS R.-B.W. and P.-Y.K. equally contributed to this work. This work was financially supported by the Natural Science Foundation of China (Grant No. 21576056 and 21576057), the Natural Science Foundations of Guangdong Province (Grant No. 2014A030313520), Science and Technology Research Project of Guangdong Province (Grant No. 2016A010103043), Science and Technology Research Project of Guangzhou (Grant No. 201607010232, 201607010198), the Research Fund Program of Guangdong Provincial Key Laboratory of Radionuclides Pollution Control and Resources (Grant No. GZDX2016K002), and High Level University Construction Project (Regional Water Environment Safety and Water Ecological Protection).
visible light irradiation, as a narrow semiconductor with wonderfully high absorption, CdS is easily excited to generate photoinduced charge carriers. The photogenerated electrons can be quickly extracted and efficiently transferred through the type II band alignment and preponderant architecture of the nanotube ordered arrays, thus further driving water reduction for H2 production. Here, the photoinduced holes assemble in the valence band of CdS to perform superior redox solution under visible light irradiation. Meanwhile, Au NPs can generate more hot electrons and then inject them to the conduction band of adjacent CdS, which enables extended carrier density and accelarates the transfer of photoinduced electrons from CdS to ZnO. The synergistic effect occurred in the ZnO/CdS/ Au NTAs photoanode ensure that the efficient transport and utilization of photoinduced electrons, leading to the greatly improved PEC performance.
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REFERENCES
(1) Zhong, M.; Ma, Y. H.; Oleynikov, P.; Domen, K.; Delaunay, J. J. A Conductive ZnO−ZnGaON Nanowire-Array-On-A-Film Photoanode for Stable and Efficient Sunlight Water Splitting. Energy Environ. Sci. 2014, 7, 1693−1699. (2) Luo, J. S.; Tilley, S. D.; Steier, L.; Schreier, M.; Mayer, M. T.; Fan, H. J.; Grätzel, M. Solution Transformation of Cu2O into CuInS2 for Solar Water Splitting. Nano Lett. 2015, 15, 1395−1402. (3) Ai, G. J.; Li, H. X.; Liu, S. P.; Mo, R.; Zhong, J. X. Solar Water Splitting by TiO2/CdS/Co−Pi Nanowire Array Photoanode Enhanced with Co−Pi as Hole Transfer Relay and CdS as Light Absorber. Adv. Funct. Mater. 2015, 25, 5706−5713. (4) Lin, Y. J.; Zhou, S.; Sheehan, S. W.; Wang, D. W. Nanonet-Based Hematite Heteronanostructures for Efficient Solar Water Splitting. J. Am. Chem. Soc. 2011, 133, 2398−2401. (5) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37−38. (6) Yang, X.; Wolcott, A.; Wang, G.; Sobo, A.; Fitzmorris, R. C.; Qian, F.; Zhang, J. Z.; Li, Y. Nitrogen-Doped ZnO Nanowire Arrays for Photoelectrochemical Water Splitting. Nano Lett. 2009, 9, 2331− 2336. (7) Wang, T.; Jin, B. J.; Jiao, Z. B.; Lu, G. X.; Ye, J. H.; Bi, Y. P. Electric Field-Directed Growth and Photoelectrochemical Properties of Cross-Linked Au−ZnO Hetero-Nanowire Arrays. Chem. Commun. 2015, 51, 2103−2106. (8) Xitao, W.; Rong, L.; Kang, W. Synthesis of ZnO@ZnS−Bi2S3 Core−Shell Nanorod Grown on Reduced Graphene Oxide Sheets and
CONCLUSIONS
In summary, the highly ordered ZnO/CdS/Au NTAs photoanode has been rationally designed and successfully fabricated by a simple and effective strategy. The obtained ZnO/CdS/Au NTAs photoanode shows an optimal photocurrent density of 21.53 mA/cm2 at 1.2 V bias vs Ag/AgCl and a high 3.45% photoconversion efficiency under visible light illumination. The excellent performance of the ZnO/CdS/Au NTAs photoanode is responsible for the following aspects: the unique nanotube structure improves charge carrier extraction efficiency and enlarges the contact area with electrolyte, along with the favorable band alignment together promoting effective charge separation. Moreover, the transportation of photogenerated electrons can be greatly enhanced due to the hot electrons generated by the Au NPs, thus allowing more photoinduced electrons to participate in the process of photoelectrochemical water splitting. 4255
DOI: 10.1021/acssuschemeng.7b00242 ACS Sustainable Chem. Eng. 2017, 5, 4249−4257
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DOI: 10.1021/acssuschemeng.7b00242 ACS Sustainable Chem. Eng. 2017, 5, 4249−4257
