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ZnO heterostructured photoanode for efficient photoelectrochemical water splitting
Accepted Manuscript Title: Construction of CdS quantum dots modified g-C3 N4 /ZnO heterostructured photoanode for efficient photoelectrochemical water splitting Authors: Changhai Liu, Yangyang Qiu, Jin Zhang, Qian Liang, Naotoshi Mitsuzaki, Zhidong Chen PII: DOI: Reference:
S1010-6030(18)30769-X https://doi.org/10.1016/j.jphotochem.2018.11.008 JPC 11580
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date: Revised date: Accepted date:
4 June 2018 9 October 2018 6 November 2018
Please cite this article as: Liu C, Qiu Y, Zhang J, Liang Q, Mitsuzaki N, Chen Z, Construction of CdS quantum dots modified g-C3 N4 /ZnO heterostructured photoanode for efficient photoelectrochemical water splitting, Journal of Photochemistry and amp; Photobiology, A: Chemistry (2018), https://doi.org/10.1016/j.jphotochem.2018.11.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Construction of CdS quantum dots modified g-C3N4/ZnO heterostructured photoanode for efficient photoelectrochemical water splitting
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Changhai Liua, Yangyang Qiua, Jin Zhanga, Qian Liangb, Naotoshi Mitsuzakic, Zhidong Chenb, *
a
School of Materials Science & Engineering, Jiangsu Collaborative Innovation Center of
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Photovoltaic Science and Engineering, Changzhou University, Changzhou, Jiangsu 213164, P. R. China b
School of Petrochemical Engineering, Changzhou University, Changzhou, Jiangsu 213164, PR
c
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China
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Qualtec Co., Ltd, Osaka 590-0906, Japan
*
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Corresponding author. Tel.: +86 0519-86330232; Fax.: +86 0519-86330232. E-mail addresses:
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Graphical Abstract
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[email protected] (Zhidong Chen)
Highlights
CdS/g-C3N4/ZnO heterostructured photoanode was fabricated CdS/g-C3N4/ZnO photoanode showed 9.3 times more than that of pristine ZnO
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The ternary structure can efficiently improve the PEC performance and stability
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The band alignment can increased light absorption and efficient charge separation
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ABSTRACT
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CdS quantum dots modified g-C3N4/ZnO nanorods core/shell structured photoanode has been
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designed and hydrothermally grown on electrically conductive fluorine-doped tin oxide substrate
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(FTO). The CdS/g-C3N4/ZnO heterojunction is sequentially fabricated by one-step hydrothermal method in the g-C3N4 aqueous solution and successive ionic layer adsorption-reaction method. It
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has been shown that the interface of ternary semiconductors plays a key role in enhancement of photoelectrochemical activity for water splitting under simulated sunlight illumination. The increase
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of photocurrent is attributed to the redshift of absorption edge of CdS/g-C3N4/ZnO towards visible light in comparison with CdS/ZnO, g-C3N4/ZnO and pristine ZnO. Moreover, the heterojunction
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between CdS, g-C3N4 and ZnO can significantly enhance the separation efficiency of photogenerated charge carriers. In addition, g-C3N4 can serve as holes receptor of CdS and act as a protective layer which will reduce the photocorrosion of CdS and improve the stability of
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photoanode. The optimized of photoanode exhibits an improved PEC performance under visible light irradiation. This work provides a promising strategy for developing ternary photoanode with high stability and efficiency.
Keywords: Photoelectrochemical; CdS/g-C3N4/ZnO; photoanode; water splitting; band alignment
1. Introduction Photoelectrochemical (PEC) water splitting to store solar energy into chemical energy has been attracted intense research interests due to accelerated depletion of fossil fuels.1-3 Developing an efficient and stable photoanode for PEC cell is one of vital issues for sustainable PEC solar energy. Zincite (ZnO) with one-dimensional (1D) nanorods arrays (NAs), as an active n-type metal oxide semiconductor, is a promising photocatalyst for PEC applications, due to its large specific surface
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area, high stability, environment-friendly properties and inexpensive cost.4-7 Meanwhile, ZnO NAs possesses superior electron mobility, nearly two orders of magnitude higher than TiO2, which can
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lead to lower recombination of photogenerated electrons and holes in ZnO thus effectively improves
the quantum efficiencies of ZnO as photoanodes in PEC cells.8,9 However, due to the direct wide band gap (~ 3.3 eV) of ZnO, its photo-conversion efficiency is significantly hindered by the limited
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most crucial obstacles limiting its further applications.
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light absorption and charge transfer properties caused by the defects in the sample, which are the
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Attempts to enhance the photo-conversion efficiency of ZnO in a cost-effective manner, a large efforts have been made to extend the wavelength response of ZnO to visible region, such as
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decorating with noble metal nanoparticles (NMNPs, such as Au10, Ag11), forming heterostructure
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with oxygen evolution catalysts (OEC, such as Co3O412), doping non-noble metal (NNM, such as Mn13, Fe14), or depositing photocatalysts with narrow bandgap (such as CdS15, CdSe16, etc.). Among these strategies, the untilization of narrow band gap materials (CdS, Eg = 2.4 eV) as photoactive
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sensitizers of photoanodes based on ZnO NAs plays a significant role, attributing to its attractive light-harvesting properties, tunable band gap, easy fabrication and low cost.17-19 Therefore, the
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separation and transportation of photogenerated electron-hole pairs can be efficiently enhanced through the formation of a feasible band alignment with ZnO, thus the PEC performance can be improved theoretically. However, the inferior photochemical stability is severely limited its further
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application, which is caused by the photocorrosion under strong illumination. To solve the high recombination rate, enormous efforts have been made to couple CdS quantum dots (QDs) with other semiconductors or noble metals to build a strong interface electric field via band alignment, significantly accelerating the separation of photogenerated electron-hole pairs. Furthermore, with the purpose of alleviating corrosion of CdS by photogenerated holes, a robust O2 production materials can be combined with CdS as hole trappers.20-22 Among many candidates, graphitic carbon
nitride (g-C3N4), as a metal-free polymeric photocatalyst, has aroused much attention due to its easy fabrication, nontoxic substance, high thermal and chemical stability, and acceptable visible-light response (Eg = 2.7 eV).23-26 Therefore, the hybridization of π-conjugated g-C3N4 with various nanomaterials can effectively improve their visible-light responses and photocatalytic activities. Moreover, the photogenerated holes of CdS can effectively transfer to g-C3N4 with the higher HOMO level compared with that of CdS, thus the combination of CdS with g-C3N4 can provide a
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feasible approach to solve the two crucial drawbacks of low efficiency and inferior photochemical
stability simultaneously.27-29 Liu et al. reported a ternary g-C3N4/ZnO/ZnO photoanode toward
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highly-efficient photoelectrochemical water splitting, where the band alignment among the three
compoenhanced the separation efficiency of photogenerated carriers and lowered the recombination depletion of holes.30
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In this study, we fabricated a heterostructure photoelectrode, combined 1D ZnO as electron
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acceptor to avail charge transport and CdS/g-C3N4 as the light absorber, which show a dramatically
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enhanced PEC performance. The combination g-C3N4 with ZnO NAs was synthesized by in-situ hydrothermal process and CdS QDs were prepared from successive ionic layer adsorption-reaction
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(SILAR) process, as shown in Scheme 1. The ternary heterostructured PEC anode were
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characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), and UV-vis diffuse-reflectance spectra (DRS). Benefiting from the unique hierarchical structure of hetero-
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nanoarrays, the as-prepared CdS/g-C3N4/ZnO NAs could achieve a high photocurrent density of 3.34 mA cm-2 at 1.23 V vs. RHE under visible-light illumination, nearly 4.9 times more than that of
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g-C3N4/ZnO and 9.3 times more than that of pristine ZnO photoanode. The stability of the asprepared photoanodes during the PEC process was also investigated. More importantly, the electrochemical impedance spectroscopy (EIS) results also provide further insight for improving the
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efficiency of semiconductors by tuning suitable electron transfer pathways, which may be promising for rational construction of PEC devices.
2. Experimental 2.1 Preparation of g-C3N4
All chemical are analytical-grade reagents and used without further treatment. Graphitic C3N4 (gC3N4) was synthesized by the method reported previously.22,23,30 In a typical procedure, melamine as starting materials was loaded into a ceramic boat that was inserted into a quartz tube. After purging with nitrogen gas, the furnace was heated to 500 °C at a ramp rate of 10 °C/min and was kept for 4 h under N2 flow. Finally, the faint yellow product was got after the furnace was naturally cooled down to room temperature. In order to get fewer layered g-C3N4 with much smaller size and
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thickness, an aqueous solution with concentration of 2 mg/mL of g-C3N4 via ultrasound for 4 h to get a fewer layered g-C3N4 solution. Then, the solution was separated via centrifugalization with a
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slow rotate speed of 3000 rpm for 20 min. Finally, the supernatant was separated and kept for the further use.
2.2 Preparation of ZnO nanorods and g-C3N4/ZnO photoanode
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In a typical synthesis, the ZnO prepared on F-doped SnO2 coated glass (FTO; 14Ω per square)
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by a hydrothermal process. The FTO substrate was successively washed in acetone, alcohol and
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deionized water for 15 min each. Comparing with the traditional sol-gel method, the ZnO seed film
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was deposited on FTO substrate by electrodeposition at -1.2v vs. Ag/AgCl reference electrode, with a Pt wire as the counter electrode, in aqueous solution of 0.1 M Zn(CH3COO)2. Then, the plates
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were annealed in air at 300 °C for 1 h to transform the superficial layer of Zn into ZnO. A hydrothermal synthesis of ZnO nanostructures on the seeded FTO substrates was performed from
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0.04 M Zn(NO3)2 aqueous solution and 0.04 M Zn(NO3)2 with g-C3N4 supernatant at fixed pH 10.6, in tightly closed Teflon reactor at 80 °C. After 2.5 h, the reactor was quickly cooled down to ambient
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temperature. The samples were washed thoroughly with deionized water and calcined at 300 °C for 1 h.
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2.3 Preparation of CdS modified g-C3N4/ZnO photoanode A simple successive ionic layer adsorption-reaction (SILAR) method was used to deposit CdS
QDs on ZnO and g-C3N4/ZnO. To complete a SILAR cycle, the films were successively soaked in 0.2 M Cd(NO3)2 ethanol solution for 30 s, and then in 0.2 M Na2S solution which was made up of methanol and deionized water (V:V = 1: 1) for 30 s. A two-step low-temperature thermal treatment was then conducted on a hot plate to anneal the photoelectrodes at 150 °C for 10 min and then at 250 °C for another 10 min.
2.4 Characterization The morphology of the samples was characterized by field emission scanning electron microscopy (FESEM, Quanta 200F) and transmission electron microscopy (TEM, JEOL JEM2100F microscope at 200 kV). X-ray diffraction (XRD) patterns were carried out on a Shimadzu XRD 6000 diffractometer using Cu Kα X-ray radiation source (λ = 1.54056 Å) at 40 kV and 40 mA in a 2θ range of 30-80° to determine the crystal phase of the as-prepared samples. X-ray
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photoelectron spectra (XPS) obtained on an ESCALAB 250Xi with a monochromatic Al Kα source
(1486.6 eV) were employed to characterize the chemical composition and electronic structure of as-
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prepared samples. All XPS data were calibrated with the binding energy of C 1s (284.6 eV). The
UV-vis diffuse-reflectance spectra (DRS) were recorded on a UV–vis-NIR spectrophotometer (Hitachi U-4100).
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2.5 PEC measurements
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The photoelectrochemical (PEC) performance were investigated in a three-electrode
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electrochemical system (Potentiostat Model CHI 760E), using the as-electrodeposited ZnO-based nanostructural films on FTO glass as the working electrode, the Ag/AgCl in saturated KCl as the
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reference electrode, and a platinum plate as counter electrode. The electrolyte was 0.1 M Na2S and 0.2 M Na2SO3 (V:V = 1:1) aqueous solution buffered at pH = 7.25 (neutral solution) with phosphate
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buffered saline (DI water: 17.6~17.9 MΩ•cm). A simulated sunlight source, a 300 W xenon lamp with an intensity of 70 mW/cm2 coupled with an AM 1.5 G filter, was employed to evaluate the
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applied bias photon-to-current efficiencies (ABPE) of the photoanodes. The electrolyte was purged with N2 gas for at least 15 min before start of each PEC test with a mild agitation during the PEC
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tests. The linear-sweep voltammetry (LSV) was recorded with a scan rate of 3 mV s-1 and the scan range was from -1.0 to 0.7 VAg/AgCl. Capacitance was derived from the electrochemical impedance obtained at each potential with 1000 Hz frequency in the dark. Mott–Schottky plots were calculated
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from the capacitance values. Electrochemical impedance spectroscopy (EIS) measurement was carried out in the same workstation with a frequency range of 0.1–100 kHz with the oscillation potential amplitude of 10 mV under illumination condition. All the potential were reported with respect to reversible hydrogen electrode (RHE) by: ERHE= EAg/AgCl+0.0591*pH+0.197. The applied bias photon-to-current efficiencies (ABPE, η) for PEC water splitting of the photoanodes were estimated from the following equation31-33
j (1.23-Vapp ) 100% Pincident
=
(1)
where Vapp is the applied external potential vs. RHE, j is the measured current density (mA/cm-2) and Pincident is the power density of the incident light (mW/cm2).
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3. Results and discussion The crystal structures of the as-prepared CdS/g-C3N4/ZnO nanorods arrays were determined by X-ray diffraction (XRD) analysis. As shown in Figure 1, the XRD patterns presents typical XRD
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spectra of ZnO nanorod arrays with characteristic diffraction peaks, well indexed to the hexagonal phase ZnO (PDF # 36-1451) with the peaks at 31.9°, 34.6°,36.4°, 47.7°, 63.1° and 68.5° corresponding to the (100), (002), (101), (102), (103) and (112) planes, respectively.37,38 The
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strongest diffraction peaks at 2θ = 34.6° corresponding to (002) lattice plane demonstrates that the
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ZnO crystal is grown along c-axis, in accordance with the observation of vertically-aligned ZnO
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nanorods in the FESEM images. The weak diffraction peaks at 26.4°, 44.1° and 51.7° are the featured peaks of cubic CdS (PDF # 89-0440), which are corresponding to the (002), (103) and
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(112) lattice planes.39-41 There are no obvious diffraction peaks of g-C3N4, which may be due to its
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low content.
Raman scattering spectroscopy was also used to further clarify the crystal structure of CdS. Figure
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2 displays the Raman spectra of ZnO, g-C3N4/ZnO, CdS/ZnO and CdS/g-C3N4/ZnO nanorods. The dominating peak at about 440 cm-1 correspond to the E2H vibrated mode of hexagonal wurtzite ZnO
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NRs for these four photoanodes, indicating that all samples have perfect crystal quality. And then, the peaks located at 336 cm-1 and the broad peak at about 1152 cm-1 correspond to optical phonon overtone with A1 symmetry and the 2LO mode.42,43 Moreover, the inferior peaks at about 308 and
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619 cm-1 belong to the first-order longitudinal-optical (1LO) phonons and second-order longitudinal-optical (2LO) phonons of cubic CdS, respectively.44 However, we could not find the peaks of g-C3N4, which may be due to the low content of g-C3N4 and the CdS layer on the surface of g-C3N4. The dominating peak of 440 cm-1 of ZnO displays very clear, indicating that the incomplete coverage of CdS on the nanorods. In addition, the band at 440 cm-1 shifts towards lower wave number as shown in Figure 2b, which is presumably due to an increase in phonon interaction
between ZnO and CdS or g-C3N4. For CdS/g-C3N4/ZnO, the A1 peak diminishes, suggesting the reduction of disorder in the ZnO substrate.32 To further understand the catalytic performance of semiconductor materials, the band gap energy (Eg) is an important physical quantity, which can be calculated from Tauc plot via extrapolation intercept method, mathermatic relation of (F(R∞)hν)m versus incident photon energy (hν), where
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F(R∞) is absorption coefficient, h is Plank constant and ν is light frequency, respectively. In addition, F(R∞) can be converted from the UV-vis diffuse reflection absorption spectrum according to the Kubelka-Munk function.42 As shown in Figure 2c, the UV-vis diffuse reflection absorption spectra
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of different photoanodes are illustrated, where the absorption for the heterostructured photoanodes in visible region show an increased trend to different extends. There is no doubt that the red shifts
and absorption enhancements for heterostructure are significantly due to the addition of g-C3N4 and
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CdS with narrower band gap energy. Meanwhile, the Eg is extracted for all samples by Tauc plots
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as described in Figure 2d. For pristine ZnO and g-C3N4/ZnO, the Eg is about 3.0 eV, while the Eg
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of CdS/ZnO and CdS/g-C3N4/ZnO are 2.75 eV and 2.62 eV, respectively. Therefore, it can be
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confirmed that the introduction of g-C3N4 and CdS can be conducive to reduce the Eg and enhance the intrinsic optical absorption capability of photocatalysts.
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The morphology of the as-prepared samples was carried out with FESEM as shown in Figure 3ad. Intuitively, large-scale ZnO nanorods, the average diameter of which is about 200 nm, were
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vertically aligned on FTO glass substrate (Figure 3a). The ZnO nanorods possess smooth surface and a stacked or pagoda-like hexagonal cross section. With the addition of g-C3N4 nanosheets during
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the hydrothermal process of ZnO, the nanorods of g-C3N4/ZnO retained the similar morphology with pure ZnO, but the numbers of stacked layers for nanorods showed an increased trend and the apex of nanorods also get more sharpen compared with pure ZnO nanorods. Furthermore, with the
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addition of CdS QDs on the surface of ZnO nanorods, the average diameter of nanorods showed an obviously decrease, and it is obvious there are more ZnO nanorods in unit area (Figure 3c). Meanwhile, once the CdS QDs were decorated on the surface of g-C3N4/ZnO nanorods, the roughness of nanorods greatly increase, indicating there were more CdS QDs loaded on the surface of nanorods, which was due to the addition of g-C3N4 nanosheets on the surface of ZnO nanorods.
Such well-defined nanostructures of core-shell heterostructures could be further revealed by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) characterization. As shown in Figure 4a and b, the diameter of ZnO nanorods is measured about 174 nm and the HRTEM image reveals that the ZnO nanorods possessed a single-crystalline structure, which is clearly confirmed by the selected area electron diffraction (SEAD) of ZnO nanorod shown in the inset of Figure 4b. The lattice fringes of 0.26 nm corresponds to the reflection from the (002) plane of ZnO.
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HRTEM images of the CdS/g-C3N4/ZnO nanorods are showed in Figure 4c and d.34 The surface of nanorods become much rougher with the addition of CdS QDs during the SILAR process and a
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core-shell structure would be formed with CdS QDs as the shell, which could be determined from
the Figure 4d with a thickness of 10.7 nm. The lattice fringe space of 0.34 nm corresponds to the reflection from the (111) plane of CdS QDs.40 In addition, g-C3N4 nanosheets about 10 nm can be
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detected in the HRTEM image, the red dotted area in Figure 4d.35,36 The EDX results of Figure 4e
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and f, for pristine ZnO and CdS/g-C3N4/ZnO, respectively, show that the elemental signals of S and
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Cd in CdS/g-C3N4/ZnO, indicating the successful loading of CdS QDs on the surface of ZnO nanorods. In addition, the element mapping images in Figure 4g are also in good agreement with
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the HRTEM results of the uniform distribution of CdS QDs on the surface of ZnO nanorods.
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XPS characterization was carried out to determine the chemical composition of CdS/g-C3N4/ZnO nanorods arrays and identify the chemical state of the elements in the as-prepared sample. The two
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characteristic peaks centered at 404.2 and 411 eV in the Cd core level XPS spectrum (Figure 5a) are attributed to the Cd3d5/2 and Cd3d3/2, respectively. The two intense peaks at 160.9 and 162.1 eV
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in Figure 5b are ascribed to the S2p3/2 and S2p1/2, respectively, which are in good agreement with the values of CdS in the previous reports. In addition, the double peaks at 1020.9 eV (Zn2p3/2) and 1043.9 eV (Zn2p1/2) further confirm that the existence of Zn2+, as shown in Figure 5c. The high-
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resolution XPS spectrum of the C1s peak (Figure 5d) can be deconvoluted into three components at binding energies of 284.6, 285.9 and 288.2 eV. The peak at 288.2 eV corresponds to the sp2hybridized N-C=N in the heterocycle rings, the peak located at 285.9 eV is ascribed to C-O or C-N, and the peak at 284.6 eV is due to the C=C in adventitious carbon.43,44 Besides, Figure 5e shows the XPS spectrum of N 1s, which is subdivided into three peaks located at 398.7, 399.3 and 400.1 eV, corresponding to pyridinic nitrogen (C-N=C), tertiary nitrogen (NC3), and pyrrolic nitrogen. The
sp2-hybridized aromatic N and tertiary N as well as the sp2-hybridized carbon (N-C=N) can confirm the existence of tristriazine ring structure in the g-C3N4. The photoelectrochemical activity of ZnO-based photoanodes were investigated by measuring the photocurrent response. The increased photocurrent for these four different photoanodes are illustrated in Figure 6a, which are due to the photogenerated electrons transported from photoanode
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to Pt counter electrode under light illumination condition. Because the magnitude of the photocurrent represents the charge collection efficiency of the electrode surface, therefore, the
higher value of photocurrent demonstrates the heterostructure between ZnO and CdS or g-C3N4 can
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significantly improve the PEC properties. With the addition of g-C3N4 during the hydrothermal
process, the photocurrent density for g-C3N4/ZnO is twice than that of pristine ZnO nanorods, which is due to the slightly increase of light absorption in the range of 420~560 nm. Furthermore, with the
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CdS QDs decorated on the surface of pristine ZnO and g-C3N4/ZnO nanorods, the photocurrent
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densities of CdS/ZnO and CdS/g-C3N4/ZnO photoanodes show an obvious promotion, 2.56 and 3.34
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mA cm-2, respectively, which are almost 3.8 and 4.9 times more than that of g-C3N4/ZnO and 7.1
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and 9.3 times more than that of pristine ZnO photoanode. The significantly enhanced photocurrent density of CdS/g-C3N4/ZnO photoanode is because the further enhanced absorption of sunlight and
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the heterojunction formed between CdS and g-C3N4 conduce to inhibit the recombination of photogenerated electron-hole pairs. The transient amperometric I–t curves were performed at a
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constant potential of 1.23 V (vs. RHE) with light on/off cycles to examine the photo-response vs. time. As shown in Figure 6b, a spike in the photoresponse can be observed upon light irradiation,
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which is due to the transient effect in power excitation and quick return to steady state during the light on or off cycles. The CdS/g-C3N4/ZnO photoanode shows the highest photoresponse and an almost identical photocurrent during the chronopotentiometry test, implying the significant synergy
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effects between the hetero-structure for the improved separation of photogenerated electron-hole pairs. The ABPE conversion efficiency (η) for a PEC cell was calculated by the equation (2) in the
experimental section, as shown in Figure 6c.32 The pristine ZnO NAs show the maximum efficiency of only 0.17% at 0.78 V (vs. RHE), while an increase to 0.27% at 0.76 V (vs. RHE) for g-C3N4/ZnO was calculated. After decortated with CdS, the efficiency of CdS/ZnO photoanodes reaches 1.39%
at 0.67 V (vs. RHE). With the formation of advanced ternary heterostructure, the efficiency of CdS/g-C3N4/ZnO achieves 1.70% at 0.66 V (vs. RHE). Moreover, all of the as-prepared samples had been evaluated at 1.23 V vs. RHE under light irradiation for 3000 s to investigate the stability of different photoanodes. Figure 6d represents the I-t plots of as-prepared photoanodes, showing that the heterostructures exactly enhance the stability of photoanodes for a certain extent, where CdS/g-C3N4/ZnO and CdS/ZnO photoanodes can remain higher 50% efficiency compared to the
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~36% efficiency for pristine ZnO NAs. A possible reason for the improved stability is due to the
heterostrucuture of CdS QDs and g-C3N4 nanopsheets on the surface of ZnO can prevent the CdS
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and ZnO from dissolving thus keep a relative stability. Compared with the ternary g-C3N4/ZnO/ZnO photoanode, the as-prepared CdS/g-C3N4/ZnO also show higher photocurrent density and ABPE
value, about 4.4 times and 1.8 times more than those of g-C3N4/ZnO/ZnO photoanode in the
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reported work, respectively.30
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The open circuit potential (OCP) transient measurements of ZnO and CdS/g-C3N4/ZnO
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electrodes were also employed to investigate the injection direction of photogenerated electrons
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upon the light irradiation.45 As shown in Figure 7a, all these two samples show a negative increase in voltage upon light irradiation, indicating that the photogenerated electrons are injected for the
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semiconductor films into the FTO substrate, which result in the formation of anodic photocurrent in I-V and I-t measurements as shown in Figure 6. The negative increase for the potential and positive
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current value in the PEC tests confirm that the as-prepared ZnO and CdS/g-C3N4/ZnO NAs act as an n-type semiconductor materials. In addition, the generated photovoltage, that is the difference
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between the voltage in dark and under irradiation, for CdS/g-C3N4/ZnO sample (837 mV) is much larger than that of pristine ZnO sample (300 mV), suggesting that the heterostructured CdS/gC3N4/ZnO possesses remarkable photoelectric conversion ability. Meanwhile, it is noted that the a
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longer time will be needed for CdS/g-C3N4/ZnO during the light-off relaxation (after 150 s) from the illuminated state to the dark equilibrium, only about 25 s for pristine ZnO, indicating that CdS/gC3N4/ZnO has the longer decay time and the heterojunction structure can significantly prolong the electron-hole pair recombination process. Therefore, the photogenerated charge carriers can be more efficiently utilized and the enhanced PEC activity can be found for the hetrostructured photoanodes. Congruously, the PL spectra of different photoanodes (Figure 7b) display significant quenching of
the peak at about 437 nm, indicating a gradually decreased recombination in these heterostructured photoanodes compared to that in the bare ZnO. Note that the PL intensity of CdS/ZnO is lower than that of g-C3N4/ZnO, suggesting that the higher separation efficiency of photogenerated carriers and thus the higher photocurrent density in the CdS/ZnO. The emission peak for CdS/g-C3N4/ZnO almost disappear, which is in agreement with the most remarkable PEC performance where the CdS/g-C3N4/ZnO generates the highest photocurrent density.
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Electrochemical impedance spectroscopy (EIS) measurements were further conducted to evaluate the charge transfer characteristics of the different photoanodes. Figure 8a presents the Nyquist plots
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of four photoanodes, which were measured from the frequency range of 0.1 Hz to 100 kHz in a
mixed aqueous solution of 0.1 M Na2S and 0.2 M Na2SO3 under light irradiation. The equivalent circuit (EC) of Randles-Ershler model was used to fit the EIS data, obtained under light irradiation.
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The EC consists of three parts, electrolyte resistance (Rs), capacitance phase element for the
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semiconductor/electrolyte interface (Q), and the charge transfer resistance of across the
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semiconductor/electrolyte interface (Rct). The obtained fitting results are shown in Table 1, where exhibits the Rct of CdS/g-C3N4/ZnO (857.8 Ω) is the smallest compared to that of CdS/ZnO
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(1547Ω), g-C3N4/ZnO (2859 Ω) and pristine ZnO (5379 Ω). The smaller Rct can efficiently restrain
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the recombination of photogenerated carriers and contribute to the faster transfer of holes across the interface to the electrolyte.46 Meanwhile, the Nyquist and Bode plots (Figure 8b) display two relaxation times, where the high-frequency relaxation time is constant at the potential for different
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photoanodes. The similar impedance properties observed on different photoanodes reveal similar electrochemical process for these four photoanodes. Note that the peak position of Bode phase plots
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shifted toward a lower frequency region compared to that of pristine ZnO, indicating the prolonged electron lifetime in the heterostructured photoanodes, facilitated electron transfer and suppressed charge recombination.50 Therefore, the ternary heterostructure has been shown to optimize the
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interface of different semiconductors, which can reduce the charge transfer resistance and improve the PEC performance. The schematic diagram of PEC water splitting using CdS/g-C3N4/ZnO photoanode under artificial light irradiation is illustrated in Scheme 2. The position of valence band for the three
materials are confirmed according to a great deal of literature research. Clearly, CdS and g-C3N4
can be easily excited to form electron-hole pairs under visible light irradiation, while ZnO cannot be excited by visible light irradiation with energy less than 2.95 eV (420 nm) due to the wide band gap. The plenty of electrons photogenerated in CdS and g-C3N4 are thermodynamically transferred to conduction band (CB) of ZnO nanorods, because of the more negative CB of CdS QDs and gC3N4.8,47 Subsequently, the electrons are further transferred through the FTO to the Pt counter electrode and react with water to produce hydrogen molecules. Meanwhile, the electrical resistance
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is lower and electron-transfer efficiency higher in ZnO, because of 10-100 times of electron mobility in ZnO than that in TiO2.48 In addition, the plenty of photogenerated holes will be accumulated on
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valence band of g-C3N4, applying to the direct oxidation of S2-. During the reaction process, g-C3N4 acts as an efficient hole-acceptor because of the more negative valance band (VB) compared to ZnO
and CdS QDs. In this way, photoelectron-hole pairs can be separated efficiently. Therefore, these
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well-separated photoelectrons and holes would explain the higher PEC activity of CdS/g-C3N4/ZnO
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photoanode.
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4. Conclusions
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In summary, we have demonstrated a quantum dot sensitized nanorods PEC device based on the
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combination of ZnO nanorods arrays with CdS QDs and g-C3N4. The advanced photoanodes were successfully synthesized via combining the hydrothermal process and a simple successive ionic layer adsorption-reaction (SILAR) process. The as-prepared photoanode materials consist of core-
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shell nanorods with ZnO core and CdS-g-C3N4 two-component shell. The significantly enhanced
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photocurrent here confirm that this ternary structure can improve the PEC performance and facilitate the stability for PEC water splitting reaction. The highly enhanced PEC activity of the as-prepared photoanodes can be attributed to the increased light absorption and efficient charge separation. We believe that the facile and effective strategy can also be extended to other ternary photoanode
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systems and improve PEC performance through interface engineering.
Conflicts of interest There are no conflicts to declare
Acknowledgements
The authors greatly acknowledge financial support from the National Natural Science Foundation of China (51702025, 51574047, 21703019), Natural Science Foundation of
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Jiangsu Province (BK20160277, BK20150259).
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Scheme 1 The synthetic protocol for CdS/g-C3N4/ZnO nanorod arrays on FTO substrate, combing
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hydrothermal method and SILAR process.
Scheme 2 Possible mechanism of photoelectrochemical water splitting over CdS/g-C3N4/ZnO under
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visible light irradiation.
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Figure 1 XRD patterns of ZnO, g-C3N4/ZnO, CdS/ZnO and CdS/g-C3N4/ZnO nanorods.
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Figure 2 (a) Raman spectra and (b) selected area in the range of 400~470 nm of ZnO, g-C3N4/ZnO, CdS/ZnO and CdS/g-C3N4/ZnO nanorods; UV-vis diffuse reflection absorption spectra (c) and corresponding Tauc plots (d) of ZnO, g-C3N4/ZnO, CdS/ZnO and CdS/g-C3N4/ZnO nanorods.
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Figure 3. FESEM images of (a) ZnO, (b) g-C3N4/ZnO, (c) CdS/ZnO and (d) CdS/g-C3N4/ZnO
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nanorods.
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Figure 4 TEM, HRTEM images and EDX results of as-prepared (a,b,e) ZnO nanorods and (c,d,f) CdS/g-C3N4/ZnO nanorods. (g) Elemental mapping results of C, N, O, S, Zn and Cd elements on CdS/g-C3N4/ZnO nanorods.
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Figure 5 High resolution XPS spectra of (a) Cd3d, (b)S2p, (c) Zn2p, (d) C1s and (e) N1s of CdS/g-
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C3N4/ZnO nanorods.
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Figure 6 The comparisons of PEC performance of four different photoanodes: ZnO, g-C3N4/ZnO, CdS/ZnO and CdS/g-C3N4/ZnO nanorods. (a) Photocurrent densities-potential curves in dark and
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under simulated solar light irradiation (70 mW cm-2) with a 20 mV s-1 scan rate. (b) Chronoamperometric I-t curves recorded at 1.23 V vs. RHE with 30 s light on/off cycles under light
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irradiation. (c) Efficiency as a function of applied potential. (d) The stability test of photoanodes
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recorded at 1.23 V vs. RHE under light irradiation.
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Figure 7 (a) Transient OCP of ZnO and CdS/g-C3N4/ZnO electrodes in the dark and under light
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irradiation, (b) Room-temperature photoluminescence spectra of ZnO, g-C3N4/ZnO, CdS/ZnO and
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CdS/g-C3N4/ZnO nanorods, excited at 350 nm.
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Figure 8 (a) EIS of pristine ZnO and ZnO-based photoanodes under light irradiation at 0 V vs.
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Ag/AgCl, (b) Nyquist and bode plots showing EIS responses of different photoanodes in the
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electrolyte of mixed aqueous solution of 0.1 M Na2S and 0.2 M Na2SO3.
Table 1 The EIS results of ZnO, g-C3N4/ZnO, CdS/ZnO and CdS/g-C3N4/ZnO nanorods. Rs (Ω)
Q（10-4F）
Rct (Ω) R value
Error (%)
58.9
5379
7.092
2.889
g-C3N4/ZnO
56.6
2859
5.651
3.696
CdS/ZnO
63.5
1547
7.344
12.44
CdS/g-C3N4/ZnO
64.2
857.8
6.209
13.21
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ZnO
S1010-6030(18)30769-X https://doi.org/10.1016/j.jphotochem.2018.11.008 JPC 11580
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date: Revised date: Accepted date:
4 June 2018 9 October 2018 6 November 2018
Please cite this article as: Liu C, Qiu Y, Zhang J, Liang Q, Mitsuzaki N, Chen Z, Construction of CdS quantum dots modified g-C3 N4 /ZnO heterostructured photoanode for efficient photoelectrochemical water splitting, Journal of Photochemistry and amp; Photobiology, A: Chemistry (2018), https://doi.org/10.1016/j.jphotochem.2018.11.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Construction of CdS quantum dots modified g-C3N4/ZnO heterostructured photoanode for efficient photoelectrochemical water splitting
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Changhai Liua, Yangyang Qiua, Jin Zhanga, Qian Liangb, Naotoshi Mitsuzakic, Zhidong Chenb, *
a
School of Materials Science & Engineering, Jiangsu Collaborative Innovation Center of
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Photovoltaic Science and Engineering, Changzhou University, Changzhou, Jiangsu 213164, P. R. China b
School of Petrochemical Engineering, Changzhou University, Changzhou, Jiangsu 213164, PR
c
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China
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Qualtec Co., Ltd, Osaka 590-0906, Japan
*
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Corresponding author. Tel.: +86 0519-86330232; Fax.: +86 0519-86330232. E-mail addresses:
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Graphical Abstract
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[email protected] (Zhidong Chen)
Highlights
CdS/g-C3N4/ZnO heterostructured photoanode was fabricated CdS/g-C3N4/ZnO photoanode showed 9.3 times more than that of pristine ZnO
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The ternary structure can efficiently improve the PEC performance and stability
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The band alignment can increased light absorption and efficient charge separation
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ABSTRACT
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CdS quantum dots modified g-C3N4/ZnO nanorods core/shell structured photoanode has been
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designed and hydrothermally grown on electrically conductive fluorine-doped tin oxide substrate
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(FTO). The CdS/g-C3N4/ZnO heterojunction is sequentially fabricated by one-step hydrothermal method in the g-C3N4 aqueous solution and successive ionic layer adsorption-reaction method. It
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has been shown that the interface of ternary semiconductors plays a key role in enhancement of photoelectrochemical activity for water splitting under simulated sunlight illumination. The increase
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of photocurrent is attributed to the redshift of absorption edge of CdS/g-C3N4/ZnO towards visible light in comparison with CdS/ZnO, g-C3N4/ZnO and pristine ZnO. Moreover, the heterojunction
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between CdS, g-C3N4 and ZnO can significantly enhance the separation efficiency of photogenerated charge carriers. In addition, g-C3N4 can serve as holes receptor of CdS and act as a protective layer which will reduce the photocorrosion of CdS and improve the stability of
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photoanode. The optimized of photoanode exhibits an improved PEC performance under visible light irradiation. This work provides a promising strategy for developing ternary photoanode with high stability and efficiency.
Keywords: Photoelectrochemical; CdS/g-C3N4/ZnO; photoanode; water splitting; band alignment
1. Introduction Photoelectrochemical (PEC) water splitting to store solar energy into chemical energy has been attracted intense research interests due to accelerated depletion of fossil fuels.1-3 Developing an efficient and stable photoanode for PEC cell is one of vital issues for sustainable PEC solar energy. Zincite (ZnO) with one-dimensional (1D) nanorods arrays (NAs), as an active n-type metal oxide semiconductor, is a promising photocatalyst for PEC applications, due to its large specific surface
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area, high stability, environment-friendly properties and inexpensive cost.4-7 Meanwhile, ZnO NAs possesses superior electron mobility, nearly two orders of magnitude higher than TiO2, which can
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lead to lower recombination of photogenerated electrons and holes in ZnO thus effectively improves
the quantum efficiencies of ZnO as photoanodes in PEC cells.8,9 However, due to the direct wide band gap (~ 3.3 eV) of ZnO, its photo-conversion efficiency is significantly hindered by the limited
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most crucial obstacles limiting its further applications.
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light absorption and charge transfer properties caused by the defects in the sample, which are the
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Attempts to enhance the photo-conversion efficiency of ZnO in a cost-effective manner, a large efforts have been made to extend the wavelength response of ZnO to visible region, such as
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decorating with noble metal nanoparticles (NMNPs, such as Au10, Ag11), forming heterostructure
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with oxygen evolution catalysts (OEC, such as Co3O412), doping non-noble metal (NNM, such as Mn13, Fe14), or depositing photocatalysts with narrow bandgap (such as CdS15, CdSe16, etc.). Among these strategies, the untilization of narrow band gap materials (CdS, Eg = 2.4 eV) as photoactive
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sensitizers of photoanodes based on ZnO NAs plays a significant role, attributing to its attractive light-harvesting properties, tunable band gap, easy fabrication and low cost.17-19 Therefore, the
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separation and transportation of photogenerated electron-hole pairs can be efficiently enhanced through the formation of a feasible band alignment with ZnO, thus the PEC performance can be improved theoretically. However, the inferior photochemical stability is severely limited its further
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application, which is caused by the photocorrosion under strong illumination. To solve the high recombination rate, enormous efforts have been made to couple CdS quantum dots (QDs) with other semiconductors or noble metals to build a strong interface electric field via band alignment, significantly accelerating the separation of photogenerated electron-hole pairs. Furthermore, with the purpose of alleviating corrosion of CdS by photogenerated holes, a robust O2 production materials can be combined with CdS as hole trappers.20-22 Among many candidates, graphitic carbon
nitride (g-C3N4), as a metal-free polymeric photocatalyst, has aroused much attention due to its easy fabrication, nontoxic substance, high thermal and chemical stability, and acceptable visible-light response (Eg = 2.7 eV).23-26 Therefore, the hybridization of π-conjugated g-C3N4 with various nanomaterials can effectively improve their visible-light responses and photocatalytic activities. Moreover, the photogenerated holes of CdS can effectively transfer to g-C3N4 with the higher HOMO level compared with that of CdS, thus the combination of CdS with g-C3N4 can provide a
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feasible approach to solve the two crucial drawbacks of low efficiency and inferior photochemical
stability simultaneously.27-29 Liu et al. reported a ternary g-C3N4/ZnO/ZnO photoanode toward
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highly-efficient photoelectrochemical water splitting, where the band alignment among the three
compoenhanced the separation efficiency of photogenerated carriers and lowered the recombination depletion of holes.30
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In this study, we fabricated a heterostructure photoelectrode, combined 1D ZnO as electron
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acceptor to avail charge transport and CdS/g-C3N4 as the light absorber, which show a dramatically
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enhanced PEC performance. The combination g-C3N4 with ZnO NAs was synthesized by in-situ hydrothermal process and CdS QDs were prepared from successive ionic layer adsorption-reaction
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(SILAR) process, as shown in Scheme 1. The ternary heterostructured PEC anode were
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characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), and UV-vis diffuse-reflectance spectra (DRS). Benefiting from the unique hierarchical structure of hetero-
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nanoarrays, the as-prepared CdS/g-C3N4/ZnO NAs could achieve a high photocurrent density of 3.34 mA cm-2 at 1.23 V vs. RHE under visible-light illumination, nearly 4.9 times more than that of
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g-C3N4/ZnO and 9.3 times more than that of pristine ZnO photoanode. The stability of the asprepared photoanodes during the PEC process was also investigated. More importantly, the electrochemical impedance spectroscopy (EIS) results also provide further insight for improving the
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efficiency of semiconductors by tuning suitable electron transfer pathways, which may be promising for rational construction of PEC devices.
2. Experimental 2.1 Preparation of g-C3N4
All chemical are analytical-grade reagents and used without further treatment. Graphitic C3N4 (gC3N4) was synthesized by the method reported previously.22,23,30 In a typical procedure, melamine as starting materials was loaded into a ceramic boat that was inserted into a quartz tube. After purging with nitrogen gas, the furnace was heated to 500 °C at a ramp rate of 10 °C/min and was kept for 4 h under N2 flow. Finally, the faint yellow product was got after the furnace was naturally cooled down to room temperature. In order to get fewer layered g-C3N4 with much smaller size and
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thickness, an aqueous solution with concentration of 2 mg/mL of g-C3N4 via ultrasound for 4 h to get a fewer layered g-C3N4 solution. Then, the solution was separated via centrifugalization with a
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slow rotate speed of 3000 rpm for 20 min. Finally, the supernatant was separated and kept for the further use.
2.2 Preparation of ZnO nanorods and g-C3N4/ZnO photoanode
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In a typical synthesis, the ZnO prepared on F-doped SnO2 coated glass (FTO; 14Ω per square)
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by a hydrothermal process. The FTO substrate was successively washed in acetone, alcohol and
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deionized water for 15 min each. Comparing with the traditional sol-gel method, the ZnO seed film
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was deposited on FTO substrate by electrodeposition at -1.2v vs. Ag/AgCl reference electrode, with a Pt wire as the counter electrode, in aqueous solution of 0.1 M Zn(CH3COO)2. Then, the plates
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were annealed in air at 300 °C for 1 h to transform the superficial layer of Zn into ZnO. A hydrothermal synthesis of ZnO nanostructures on the seeded FTO substrates was performed from
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0.04 M Zn(NO3)2 aqueous solution and 0.04 M Zn(NO3)2 with g-C3N4 supernatant at fixed pH 10.6, in tightly closed Teflon reactor at 80 °C. After 2.5 h, the reactor was quickly cooled down to ambient
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temperature. The samples were washed thoroughly with deionized water and calcined at 300 °C for 1 h.
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2.3 Preparation of CdS modified g-C3N4/ZnO photoanode A simple successive ionic layer adsorption-reaction (SILAR) method was used to deposit CdS
QDs on ZnO and g-C3N4/ZnO. To complete a SILAR cycle, the films were successively soaked in 0.2 M Cd(NO3)2 ethanol solution for 30 s, and then in 0.2 M Na2S solution which was made up of methanol and deionized water (V:V = 1: 1) for 30 s. A two-step low-temperature thermal treatment was then conducted on a hot plate to anneal the photoelectrodes at 150 °C for 10 min and then at 250 °C for another 10 min.
2.4 Characterization The morphology of the samples was characterized by field emission scanning electron microscopy (FESEM, Quanta 200F) and transmission electron microscopy (TEM, JEOL JEM2100F microscope at 200 kV). X-ray diffraction (XRD) patterns were carried out on a Shimadzu XRD 6000 diffractometer using Cu Kα X-ray radiation source (λ = 1.54056 Å) at 40 kV and 40 mA in a 2θ range of 30-80° to determine the crystal phase of the as-prepared samples. X-ray
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photoelectron spectra (XPS) obtained on an ESCALAB 250Xi with a monochromatic Al Kα source
(1486.6 eV) were employed to characterize the chemical composition and electronic structure of as-
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prepared samples. All XPS data were calibrated with the binding energy of C 1s (284.6 eV). The
UV-vis diffuse-reflectance spectra (DRS) were recorded on a UV–vis-NIR spectrophotometer (Hitachi U-4100).
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2.5 PEC measurements
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The photoelectrochemical (PEC) performance were investigated in a three-electrode
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electrochemical system (Potentiostat Model CHI 760E), using the as-electrodeposited ZnO-based nanostructural films on FTO glass as the working electrode, the Ag/AgCl in saturated KCl as the
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reference electrode, and a platinum plate as counter electrode. The electrolyte was 0.1 M Na2S and 0.2 M Na2SO3 (V:V = 1:1) aqueous solution buffered at pH = 7.25 (neutral solution) with phosphate
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buffered saline (DI water: 17.6~17.9 MΩ•cm). A simulated sunlight source, a 300 W xenon lamp with an intensity of 70 mW/cm2 coupled with an AM 1.5 G filter, was employed to evaluate the
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applied bias photon-to-current efficiencies (ABPE) of the photoanodes. The electrolyte was purged with N2 gas for at least 15 min before start of each PEC test with a mild agitation during the PEC
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tests. The linear-sweep voltammetry (LSV) was recorded with a scan rate of 3 mV s-1 and the scan range was from -1.0 to 0.7 VAg/AgCl. Capacitance was derived from the electrochemical impedance obtained at each potential with 1000 Hz frequency in the dark. Mott–Schottky plots were calculated
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from the capacitance values. Electrochemical impedance spectroscopy (EIS) measurement was carried out in the same workstation with a frequency range of 0.1–100 kHz with the oscillation potential amplitude of 10 mV under illumination condition. All the potential were reported with respect to reversible hydrogen electrode (RHE) by: ERHE= EAg/AgCl+0.0591*pH+0.197. The applied bias photon-to-current efficiencies (ABPE, η) for PEC water splitting of the photoanodes were estimated from the following equation31-33
j (1.23-Vapp ) 100% Pincident
=
(1)
where Vapp is the applied external potential vs. RHE, j is the measured current density (mA/cm-2) and Pincident is the power density of the incident light (mW/cm2).
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3. Results and discussion The crystal structures of the as-prepared CdS/g-C3N4/ZnO nanorods arrays were determined by X-ray diffraction (XRD) analysis. As shown in Figure 1, the XRD patterns presents typical XRD
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spectra of ZnO nanorod arrays with characteristic diffraction peaks, well indexed to the hexagonal phase ZnO (PDF # 36-1451) with the peaks at 31.9°, 34.6°,36.4°, 47.7°, 63.1° and 68.5° corresponding to the (100), (002), (101), (102), (103) and (112) planes, respectively.37,38 The
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strongest diffraction peaks at 2θ = 34.6° corresponding to (002) lattice plane demonstrates that the
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ZnO crystal is grown along c-axis, in accordance with the observation of vertically-aligned ZnO
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nanorods in the FESEM images. The weak diffraction peaks at 26.4°, 44.1° and 51.7° are the featured peaks of cubic CdS (PDF # 89-0440), which are corresponding to the (002), (103) and
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(112) lattice planes.39-41 There are no obvious diffraction peaks of g-C3N4, which may be due to its
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low content.
Raman scattering spectroscopy was also used to further clarify the crystal structure of CdS. Figure
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2 displays the Raman spectra of ZnO, g-C3N4/ZnO, CdS/ZnO and CdS/g-C3N4/ZnO nanorods. The dominating peak at about 440 cm-1 correspond to the E2H vibrated mode of hexagonal wurtzite ZnO
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NRs for these four photoanodes, indicating that all samples have perfect crystal quality. And then, the peaks located at 336 cm-1 and the broad peak at about 1152 cm-1 correspond to optical phonon overtone with A1 symmetry and the 2LO mode.42,43 Moreover, the inferior peaks at about 308 and
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619 cm-1 belong to the first-order longitudinal-optical (1LO) phonons and second-order longitudinal-optical (2LO) phonons of cubic CdS, respectively.44 However, we could not find the peaks of g-C3N4, which may be due to the low content of g-C3N4 and the CdS layer on the surface of g-C3N4. The dominating peak of 440 cm-1 of ZnO displays very clear, indicating that the incomplete coverage of CdS on the nanorods. In addition, the band at 440 cm-1 shifts towards lower wave number as shown in Figure 2b, which is presumably due to an increase in phonon interaction
between ZnO and CdS or g-C3N4. For CdS/g-C3N4/ZnO, the A1 peak diminishes, suggesting the reduction of disorder in the ZnO substrate.32 To further understand the catalytic performance of semiconductor materials, the band gap energy (Eg) is an important physical quantity, which can be calculated from Tauc plot via extrapolation intercept method, mathermatic relation of (F(R∞)hν)m versus incident photon energy (hν), where
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F(R∞) is absorption coefficient, h is Plank constant and ν is light frequency, respectively. In addition, F(R∞) can be converted from the UV-vis diffuse reflection absorption spectrum according to the Kubelka-Munk function.42 As shown in Figure 2c, the UV-vis diffuse reflection absorption spectra
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of different photoanodes are illustrated, where the absorption for the heterostructured photoanodes in visible region show an increased trend to different extends. There is no doubt that the red shifts
and absorption enhancements for heterostructure are significantly due to the addition of g-C3N4 and
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CdS with narrower band gap energy. Meanwhile, the Eg is extracted for all samples by Tauc plots
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as described in Figure 2d. For pristine ZnO and g-C3N4/ZnO, the Eg is about 3.0 eV, while the Eg
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of CdS/ZnO and CdS/g-C3N4/ZnO are 2.75 eV and 2.62 eV, respectively. Therefore, it can be
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confirmed that the introduction of g-C3N4 and CdS can be conducive to reduce the Eg and enhance the intrinsic optical absorption capability of photocatalysts.
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The morphology of the as-prepared samples was carried out with FESEM as shown in Figure 3ad. Intuitively, large-scale ZnO nanorods, the average diameter of which is about 200 nm, were
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vertically aligned on FTO glass substrate (Figure 3a). The ZnO nanorods possess smooth surface and a stacked or pagoda-like hexagonal cross section. With the addition of g-C3N4 nanosheets during
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the hydrothermal process of ZnO, the nanorods of g-C3N4/ZnO retained the similar morphology with pure ZnO, but the numbers of stacked layers for nanorods showed an increased trend and the apex of nanorods also get more sharpen compared with pure ZnO nanorods. Furthermore, with the
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addition of CdS QDs on the surface of ZnO nanorods, the average diameter of nanorods showed an obviously decrease, and it is obvious there are more ZnO nanorods in unit area (Figure 3c). Meanwhile, once the CdS QDs were decorated on the surface of g-C3N4/ZnO nanorods, the roughness of nanorods greatly increase, indicating there were more CdS QDs loaded on the surface of nanorods, which was due to the addition of g-C3N4 nanosheets on the surface of ZnO nanorods.
Such well-defined nanostructures of core-shell heterostructures could be further revealed by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) characterization. As shown in Figure 4a and b, the diameter of ZnO nanorods is measured about 174 nm and the HRTEM image reveals that the ZnO nanorods possessed a single-crystalline structure, which is clearly confirmed by the selected area electron diffraction (SEAD) of ZnO nanorod shown in the inset of Figure 4b. The lattice fringes of 0.26 nm corresponds to the reflection from the (002) plane of ZnO.
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HRTEM images of the CdS/g-C3N4/ZnO nanorods are showed in Figure 4c and d.34 The surface of nanorods become much rougher with the addition of CdS QDs during the SILAR process and a
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core-shell structure would be formed with CdS QDs as the shell, which could be determined from
the Figure 4d with a thickness of 10.7 nm. The lattice fringe space of 0.34 nm corresponds to the reflection from the (111) plane of CdS QDs.40 In addition, g-C3N4 nanosheets about 10 nm can be
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detected in the HRTEM image, the red dotted area in Figure 4d.35,36 The EDX results of Figure 4e
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and f, for pristine ZnO and CdS/g-C3N4/ZnO, respectively, show that the elemental signals of S and
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Cd in CdS/g-C3N4/ZnO, indicating the successful loading of CdS QDs on the surface of ZnO nanorods. In addition, the element mapping images in Figure 4g are also in good agreement with
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the HRTEM results of the uniform distribution of CdS QDs on the surface of ZnO nanorods.
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XPS characterization was carried out to determine the chemical composition of CdS/g-C3N4/ZnO nanorods arrays and identify the chemical state of the elements in the as-prepared sample. The two
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characteristic peaks centered at 404.2 and 411 eV in the Cd core level XPS spectrum (Figure 5a) are attributed to the Cd3d5/2 and Cd3d3/2, respectively. The two intense peaks at 160.9 and 162.1 eV
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in Figure 5b are ascribed to the S2p3/2 and S2p1/2, respectively, which are in good agreement with the values of CdS in the previous reports. In addition, the double peaks at 1020.9 eV (Zn2p3/2) and 1043.9 eV (Zn2p1/2) further confirm that the existence of Zn2+, as shown in Figure 5c. The high-
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resolution XPS spectrum of the C1s peak (Figure 5d) can be deconvoluted into three components at binding energies of 284.6, 285.9 and 288.2 eV. The peak at 288.2 eV corresponds to the sp2hybridized N-C=N in the heterocycle rings, the peak located at 285.9 eV is ascribed to C-O or C-N, and the peak at 284.6 eV is due to the C=C in adventitious carbon.43,44 Besides, Figure 5e shows the XPS spectrum of N 1s, which is subdivided into three peaks located at 398.7, 399.3 and 400.1 eV, corresponding to pyridinic nitrogen (C-N=C), tertiary nitrogen (NC3), and pyrrolic nitrogen. The
sp2-hybridized aromatic N and tertiary N as well as the sp2-hybridized carbon (N-C=N) can confirm the existence of tristriazine ring structure in the g-C3N4. The photoelectrochemical activity of ZnO-based photoanodes were investigated by measuring the photocurrent response. The increased photocurrent for these four different photoanodes are illustrated in Figure 6a, which are due to the photogenerated electrons transported from photoanode
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to Pt counter electrode under light illumination condition. Because the magnitude of the photocurrent represents the charge collection efficiency of the electrode surface, therefore, the
higher value of photocurrent demonstrates the heterostructure between ZnO and CdS or g-C3N4 can
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significantly improve the PEC properties. With the addition of g-C3N4 during the hydrothermal
process, the photocurrent density for g-C3N4/ZnO is twice than that of pristine ZnO nanorods, which is due to the slightly increase of light absorption in the range of 420~560 nm. Furthermore, with the
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CdS QDs decorated on the surface of pristine ZnO and g-C3N4/ZnO nanorods, the photocurrent
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densities of CdS/ZnO and CdS/g-C3N4/ZnO photoanodes show an obvious promotion, 2.56 and 3.34
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mA cm-2, respectively, which are almost 3.8 and 4.9 times more than that of g-C3N4/ZnO and 7.1
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and 9.3 times more than that of pristine ZnO photoanode. The significantly enhanced photocurrent density of CdS/g-C3N4/ZnO photoanode is because the further enhanced absorption of sunlight and
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the heterojunction formed between CdS and g-C3N4 conduce to inhibit the recombination of photogenerated electron-hole pairs. The transient amperometric I–t curves were performed at a
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constant potential of 1.23 V (vs. RHE) with light on/off cycles to examine the photo-response vs. time. As shown in Figure 6b, a spike in the photoresponse can be observed upon light irradiation,
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which is due to the transient effect in power excitation and quick return to steady state during the light on or off cycles. The CdS/g-C3N4/ZnO photoanode shows the highest photoresponse and an almost identical photocurrent during the chronopotentiometry test, implying the significant synergy
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effects between the hetero-structure for the improved separation of photogenerated electron-hole pairs. The ABPE conversion efficiency (η) for a PEC cell was calculated by the equation (2) in the
experimental section, as shown in Figure 6c.32 The pristine ZnO NAs show the maximum efficiency of only 0.17% at 0.78 V (vs. RHE), while an increase to 0.27% at 0.76 V (vs. RHE) for g-C3N4/ZnO was calculated. After decortated with CdS, the efficiency of CdS/ZnO photoanodes reaches 1.39%
at 0.67 V (vs. RHE). With the formation of advanced ternary heterostructure, the efficiency of CdS/g-C3N4/ZnO achieves 1.70% at 0.66 V (vs. RHE). Moreover, all of the as-prepared samples had been evaluated at 1.23 V vs. RHE under light irradiation for 3000 s to investigate the stability of different photoanodes. Figure 6d represents the I-t plots of as-prepared photoanodes, showing that the heterostructures exactly enhance the stability of photoanodes for a certain extent, where CdS/g-C3N4/ZnO and CdS/ZnO photoanodes can remain higher 50% efficiency compared to the
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~36% efficiency for pristine ZnO NAs. A possible reason for the improved stability is due to the
heterostrucuture of CdS QDs and g-C3N4 nanopsheets on the surface of ZnO can prevent the CdS
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and ZnO from dissolving thus keep a relative stability. Compared with the ternary g-C3N4/ZnO/ZnO photoanode, the as-prepared CdS/g-C3N4/ZnO also show higher photocurrent density and ABPE
value, about 4.4 times and 1.8 times more than those of g-C3N4/ZnO/ZnO photoanode in the
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reported work, respectively.30
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The open circuit potential (OCP) transient measurements of ZnO and CdS/g-C3N4/ZnO
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electrodes were also employed to investigate the injection direction of photogenerated electrons
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upon the light irradiation.45 As shown in Figure 7a, all these two samples show a negative increase in voltage upon light irradiation, indicating that the photogenerated electrons are injected for the
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semiconductor films into the FTO substrate, which result in the formation of anodic photocurrent in I-V and I-t measurements as shown in Figure 6. The negative increase for the potential and positive
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current value in the PEC tests confirm that the as-prepared ZnO and CdS/g-C3N4/ZnO NAs act as an n-type semiconductor materials. In addition, the generated photovoltage, that is the difference
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between the voltage in dark and under irradiation, for CdS/g-C3N4/ZnO sample (837 mV) is much larger than that of pristine ZnO sample (300 mV), suggesting that the heterostructured CdS/gC3N4/ZnO possesses remarkable photoelectric conversion ability. Meanwhile, it is noted that the a
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longer time will be needed for CdS/g-C3N4/ZnO during the light-off relaxation (after 150 s) from the illuminated state to the dark equilibrium, only about 25 s for pristine ZnO, indicating that CdS/gC3N4/ZnO has the longer decay time and the heterojunction structure can significantly prolong the electron-hole pair recombination process. Therefore, the photogenerated charge carriers can be more efficiently utilized and the enhanced PEC activity can be found for the hetrostructured photoanodes. Congruously, the PL spectra of different photoanodes (Figure 7b) display significant quenching of
the peak at about 437 nm, indicating a gradually decreased recombination in these heterostructured photoanodes compared to that in the bare ZnO. Note that the PL intensity of CdS/ZnO is lower than that of g-C3N4/ZnO, suggesting that the higher separation efficiency of photogenerated carriers and thus the higher photocurrent density in the CdS/ZnO. The emission peak for CdS/g-C3N4/ZnO almost disappear, which is in agreement with the most remarkable PEC performance where the CdS/g-C3N4/ZnO generates the highest photocurrent density.
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Electrochemical impedance spectroscopy (EIS) measurements were further conducted to evaluate the charge transfer characteristics of the different photoanodes. Figure 8a presents the Nyquist plots
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of four photoanodes, which were measured from the frequency range of 0.1 Hz to 100 kHz in a
mixed aqueous solution of 0.1 M Na2S and 0.2 M Na2SO3 under light irradiation. The equivalent circuit (EC) of Randles-Ershler model was used to fit the EIS data, obtained under light irradiation.
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The EC consists of three parts, electrolyte resistance (Rs), capacitance phase element for the
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semiconductor/electrolyte interface (Q), and the charge transfer resistance of across the
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semiconductor/electrolyte interface (Rct). The obtained fitting results are shown in Table 1, where exhibits the Rct of CdS/g-C3N4/ZnO (857.8 Ω) is the smallest compared to that of CdS/ZnO
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(1547Ω), g-C3N4/ZnO (2859 Ω) and pristine ZnO (5379 Ω). The smaller Rct can efficiently restrain
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the recombination of photogenerated carriers and contribute to the faster transfer of holes across the interface to the electrolyte.46 Meanwhile, the Nyquist and Bode plots (Figure 8b) display two relaxation times, where the high-frequency relaxation time is constant at the potential for different
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photoanodes. The similar impedance properties observed on different photoanodes reveal similar electrochemical process for these four photoanodes. Note that the peak position of Bode phase plots
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shifted toward a lower frequency region compared to that of pristine ZnO, indicating the prolonged electron lifetime in the heterostructured photoanodes, facilitated electron transfer and suppressed charge recombination.50 Therefore, the ternary heterostructure has been shown to optimize the
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interface of different semiconductors, which can reduce the charge transfer resistance and improve the PEC performance. The schematic diagram of PEC water splitting using CdS/g-C3N4/ZnO photoanode under artificial light irradiation is illustrated in Scheme 2. The position of valence band for the three
materials are confirmed according to a great deal of literature research. Clearly, CdS and g-C3N4
can be easily excited to form electron-hole pairs under visible light irradiation, while ZnO cannot be excited by visible light irradiation with energy less than 2.95 eV (420 nm) due to the wide band gap. The plenty of electrons photogenerated in CdS and g-C3N4 are thermodynamically transferred to conduction band (CB) of ZnO nanorods, because of the more negative CB of CdS QDs and gC3N4.8,47 Subsequently, the electrons are further transferred through the FTO to the Pt counter electrode and react with water to produce hydrogen molecules. Meanwhile, the electrical resistance
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is lower and electron-transfer efficiency higher in ZnO, because of 10-100 times of electron mobility in ZnO than that in TiO2.48 In addition, the plenty of photogenerated holes will be accumulated on
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valence band of g-C3N4, applying to the direct oxidation of S2-. During the reaction process, g-C3N4 acts as an efficient hole-acceptor because of the more negative valance band (VB) compared to ZnO
and CdS QDs. In this way, photoelectron-hole pairs can be separated efficiently. Therefore, these
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well-separated photoelectrons and holes would explain the higher PEC activity of CdS/g-C3N4/ZnO
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photoanode.
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4. Conclusions
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In summary, we have demonstrated a quantum dot sensitized nanorods PEC device based on the
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combination of ZnO nanorods arrays with CdS QDs and g-C3N4. The advanced photoanodes were successfully synthesized via combining the hydrothermal process and a simple successive ionic layer adsorption-reaction (SILAR) process. The as-prepared photoanode materials consist of core-
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shell nanorods with ZnO core and CdS-g-C3N4 two-component shell. The significantly enhanced
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photocurrent here confirm that this ternary structure can improve the PEC performance and facilitate the stability for PEC water splitting reaction. The highly enhanced PEC activity of the as-prepared photoanodes can be attributed to the increased light absorption and efficient charge separation. We believe that the facile and effective strategy can also be extended to other ternary photoanode
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systems and improve PEC performance through interface engineering.
Conflicts of interest There are no conflicts to declare
Acknowledgements
The authors greatly acknowledge financial support from the National Natural Science Foundation of China (51702025, 51574047, 21703019), Natural Science Foundation of
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Jiangsu Province (BK20160277, BK20150259).
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Scheme 1 The synthetic protocol for CdS/g-C3N4/ZnO nanorod arrays on FTO substrate, combing
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hydrothermal method and SILAR process.
Scheme 2 Possible mechanism of photoelectrochemical water splitting over CdS/g-C3N4/ZnO under
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visible light irradiation.
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Figure 1 XRD patterns of ZnO, g-C3N4/ZnO, CdS/ZnO and CdS/g-C3N4/ZnO nanorods.
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Figure 2 (a) Raman spectra and (b) selected area in the range of 400~470 nm of ZnO, g-C3N4/ZnO, CdS/ZnO and CdS/g-C3N4/ZnO nanorods; UV-vis diffuse reflection absorption spectra (c) and corresponding Tauc plots (d) of ZnO, g-C3N4/ZnO, CdS/ZnO and CdS/g-C3N4/ZnO nanorods.
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Figure 3. FESEM images of (a) ZnO, (b) g-C3N4/ZnO, (c) CdS/ZnO and (d) CdS/g-C3N4/ZnO
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nanorods.
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Figure 4 TEM, HRTEM images and EDX results of as-prepared (a,b,e) ZnO nanorods and (c,d,f) CdS/g-C3N4/ZnO nanorods. (g) Elemental mapping results of C, N, O, S, Zn and Cd elements on CdS/g-C3N4/ZnO nanorods.
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Figure 5 High resolution XPS spectra of (a) Cd3d, (b)S2p, (c) Zn2p, (d) C1s and (e) N1s of CdS/g-
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C3N4/ZnO nanorods.
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Figure 6 The comparisons of PEC performance of four different photoanodes: ZnO, g-C3N4/ZnO, CdS/ZnO and CdS/g-C3N4/ZnO nanorods. (a) Photocurrent densities-potential curves in dark and
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under simulated solar light irradiation (70 mW cm-2) with a 20 mV s-1 scan rate. (b) Chronoamperometric I-t curves recorded at 1.23 V vs. RHE with 30 s light on/off cycles under light
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irradiation. (c) Efficiency as a function of applied potential. (d) The stability test of photoanodes
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recorded at 1.23 V vs. RHE under light irradiation.
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Figure 7 (a) Transient OCP of ZnO and CdS/g-C3N4/ZnO electrodes in the dark and under light
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irradiation, (b) Room-temperature photoluminescence spectra of ZnO, g-C3N4/ZnO, CdS/ZnO and
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CdS/g-C3N4/ZnO nanorods, excited at 350 nm.
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Figure 8 (a) EIS of pristine ZnO and ZnO-based photoanodes under light irradiation at 0 V vs.
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Ag/AgCl, (b) Nyquist and bode plots showing EIS responses of different photoanodes in the
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electrolyte of mixed aqueous solution of 0.1 M Na2S and 0.2 M Na2SO3.
Table 1 The EIS results of ZnO, g-C3N4/ZnO, CdS/ZnO and CdS/g-C3N4/ZnO nanorods. Rs (Ω)
Q（10-4F）
Rct (Ω) R value
Error (%)
58.9
5379
7.092
2.889
g-C3N4/ZnO
56.6
2859
5.651
3.696
CdS/ZnO
63.5
1547
7.344
12.44
CdS/g-C3N4/ZnO
64.2
857.8
6.209
13.21
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ZnO