Enhanced Photoelectrochemical Water Splitting of Photoelectrode

Feb 5, 2018 - ZCF has a decreased arc radius compared to ZC, revealing that the presence of FeOOH could foster the interface charge transfer and inhib...
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Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 3565−3574

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Enhanced Photoelectrochemical Water Splitting of Photoelectrode Simultaneous Decorated with Cocatalysts Based on Spatial Charge Separation and Transfer Zhifeng Liu,*,†,‡ Jing Zhang,‡ and Weiguo Yan‡ †

Hubei Collaborative Innovation Center for High-efficiency Utilization of Solar Energy, Hubei University of Technology, Wuhan, 430068, China ‡ School of Materials Science and Engineering, Tianjin Chengjian University, 300384, Tianjin, China

ACS Sustainable Chem. Eng. 2018.6:3565-3574. Downloaded from pubs.acs.org by TULANE UNIV on 01/20/19. For personal use only.

S Supporting Information *

ABSTRACT: A ZnO/CdS heterojunction photoanode simultaneously decorated with separated Au and FeOOH cocatalysts is first established and applied in efficient photoelectrochemical water splitting. Enhanced optical performance is observed as well as the photocurrent density enhancement of 116% (5.38 mA·cm−2 at 1.23 V vs RHE) compared to that of a pure ZnO photoanode which can be ascribed to improved light absorption ability, enhanced charge transfer, and efficient charge separation. Furthermore, it is obviously seen that the onset potential is shifted negatively while the stability of the photoanode is promoted. This excellent performance is achieved because of the long light irradiation pathway of one-dimensional (1D) ZnO nanorods; a broadened light absorbance spectrum sensitized by CdS with narrow bad gap; a reduced charge recombination rate thanks to the appropriate gradient energy gap structure of ZnO/ CdS; and the bidirectional kinetics supplied by the dual Au and FeOOH cocatalysts to facilitate the photogenerated electrons and holes to flow in opposite directions and further promote charge separation. The concept of separating cocatalysts in a photoelectrochemical system is first established, highlighting the sight of an efficient photoelectrode for water splitting and hydrogen generation. KEYWORDS: Heterojunction, Photoelectrode, Cocatalysts, Charge separation, Photoelctrochemical water splitting

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INTRODUCTION With the development of industry especially architecture industry, a great amount of energy has been over consumed. Since the energy crisis and environment issues are the severe problems the world is facing today, renewable and green energy sources have received widespread attention.1−3 Among several alternative renewable energy sources, hydrogen generated by solar energy has been regarded extensively and intensively as one of the most attractive options. The discovery of TiO2 electrode applied in a photoelectrochemical (PEC) water splitting system by Fujishima and Honda in 1972,4 heralded a new era in the domain of solar-to-hydrogen (STH) energy conversion science. A significant amount of effort has been dedicated to develop semiconductor photocatalyst for efficient water splitting.5 Currently, the research on p-type semiconductors NiO, Co3O4, and MoO2 as photocathode as well as n-type semiconductors included TiO2, ZnO, and WO3 as photoanode received more and more attention.6−11 However, the main drawback influencing STH efficiency of semiconductors is the limiting light absorbance in the visible region. There is therefore an urgent need of wide light-absorption range © 2018 American Chemical Society

materials to address these issues. To this end, doping with metal or nonmetallic elements and coupling with smaller band gap semiconductors have been adopted to broaden the light response to the visible region.12−16 Zhang et al. doped Fe on WO 3 nanoflakes to achieve broadband light trapping modulation.12 Mukhopadhyay et al. adopted narrow-band gap CdS nanoparticles of 2.7 eV as sensitizer to establish ZnO/CdS heterostructured nanocomposites to capture more visible light.14 Liu and his group organized WO3 and Cu2O to construct an all-metal oxides heterojunction photoelectrode to realize a wide light-absorption range.15 Another key factor restricting the photoelectrocatalytic application of the above semiconductors is the high recombination rate of photogenerated electron and hole. As mentioned above, coupling with the narrow-band gap semiconductor could reduce charge recombination by their appropriate gradient energy gap structure.14,15 In photocatalytic systems, attempts have been made to reduce the recombination Received: October 25, 2017 Revised: January 3, 2018 Published: February 5, 2018 3565

DOI: 10.1021/acssuschemeng.7b03894 ACS Sustainable Chem. Eng. 2018, 6, 3565−3574

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ACS Sustainable Chemistry & Engineering

and collect photogenerated electrons from ZnO, while passing the electrons to the counter electrode. FeOOH cocatalyst is used for supplying the oxygen evolution reaction kinetics and then further facilitating the charge separation. Au and FeOOH dual cocatalysts are separately constructed to drive photogenerated electrons and holes to flow in opposite directions, further promoting charge separation. The charge transfer process has also been discussed in detail. In addition, the thickness of the FeOOH layer has been optimized to obtain efficient performance and the mechanism has been investigated. Hence, these findings will open up new opportunities in constructing a multielement system by assembling various materials to enhance photoelectrochemical activity further.

of charge carriers such as loaded with cocatalysts catalyze the hydrogen evolution reaction (HER) or oxygen evolution reaction (OER). As we all known, noble metals such as Au,16 Ag,7 Pt,17 and Pd18 have been investigated extensively for capturing photoinduced electrons, improving carrier mobility, and accelerating the separation of a photoinduced electron− hole. For instance, Lin et al. devised strategies to enhance charge separation by loading Pt, Au, and Cu cocatalyst on the mesoporous Nb2O5 photocatalysts. The photocatalytic activity is obviously improved compared to that of simple Nb2O5.16 Xu and his group developed Pd as a cocatalyst applied in the ternary In2S3-(RGO-Pd) composites to facilitate charge carrier transfers and promote visible-light photocatalysis.18 Alternatively, the design of coupling with oxygen evolution catalysts (OECs) such as oxides of Ir,19 Ru,20 and Co21,22 to supply oxygen evolution reaction kinetics has been employed as a promising strategy to further promote charge separation. On the basis of comprehensive theoretical investigations, Wang and his co-workers fabricated a versatile cocatalyst CoSe2 which can facilitate photoinduced hole transfer and thus enhance the photocatalytic water oxidation activity.21 Moreover, spatial separation cocatalysts which can facilitate electron and hole transfer has been proposed gradually to further reducethe charge recombination rate.23−25 Gong et al. described the design and synthesis of a Pt@TiO2@In2O3@MnOx mesoporous hollow sphere which can simultaneously reduce bulk and surface charge recombination.24 Wang and his co-workers fabricated Pt and Co3O4 simultaneously on the interior and exterior surface of hollow carbon nitride spheres to decrease the charge recombination and enhance photocatalytic water splitting activity.25 Recently, efficient OER cocatalysts have been introduced into PEC water splitting including noble metals (Ir and Ru),26,27 perovskite oxides,28 metal-based materials (Fe, Co, Ni, Mn, and Cu)29−33 as well as carbon materials.34 Petrykin et al. described the synthesis of active catalysts in the Ru−Zn−O system featuring high selectivity to the OER.35 Zhang et al. brought cobalt-based species cocatalysts onto the surface of graphitic carbon nitride which efficiently promoted the interface charge mobility.36 Kwong and his co-workers have reported a spray pyrolysis deposition route to fabricate WO3 thin films furnished by the electrodeposited solid state FeOOH as oxygen evolution catalyst (OEC) to reduce the recombination of charge carriers.32 Kim’s group have serially adopted two different OEC layers, FeOOH and NiOOH modified nanoporous bismuth vanadate (BiVO4) to achieve a photocurrent density of 2.73 mA·cm−2 (0.6 V vs RHE).33 Nevertheless, there are few reports involving modified semiconductor photoanodes by coupling spatial separation cocatalysts simultaneously applied in PEC water splitting so far. Herein, we are inspired by the concept of separated cocatalysts in the photocatalytic field to integrate the merits of diverse materials and first establish a quaternary system including Au, ZnO nanorods (NRs), CdS nanoparticles, and FeOOH simultaneously based on an indium tin oxide (ITO) substrate applied in PEC water splitting. One-dimensional (1D) ZnO nanorod possesses a long light irradiation pathway and is further sensitized by CdS with a narrow bad gap (2.4 eV) to broaden the light absorbance spectrum. Meanwhile the appropriate gradient energy gap structure contributes to the reduced charge recombination rate. Au cocatalyst and FeOOH cocatalyst are separately loaded on inner and outer surfaces. Au cocatalyst works as an electron absorber layer (EAL) to extract

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EXPERIMENTAL SECTION

Deposition of Au Layer. The Au layer was deposited directly onto the surface of ITO transparent conductive glass substrate with an ion beam sputter (IBS) evaporator as Qi et al. reported.37 The ITO substrates were cleaned ultrasonically with acetone, isopropyl alcohol and ethanol for each 15 min, respectively. The deposited thickness of the Au layer was controlled to be 20 nm. The deposition process was kept with a target substrate at room temperature with a depositing rate of 0.03 nm·s−1. Preparation of ZnO Nanorods. ZnO nanorods were obtained by an aqueous solution method.38 The substrates were immersed in the mixed aqueous solution containing zinc nitrate (Zn(NO3)2·6H2O, 99%) and hexamethylenetetramine (C6H12N4, 99.5%). The concentration of zinc nitrate was 0.1 M and the molar ratio of C6H12N4 to Zn(NO3)2 was 1:1. ZnO nanorods could be achieved after the system was heated at 90 °C and maintained for 4 h. All the samples were thoroughly washed with distilled water finally, and then naturally air dried. Fabrication of CdS Nanoparticles. CdS nanoparticles were fabricated through a modified successive ionic layer adsorption and reaction (SILAR) method based on the experiment of Liu et al.39 The substrates were successively immersed in the Cd2+ precursor solution containing 0.01 M of cadmium nitrate (Cd(NO3)2, 99%) and S2− precursor solution containing 0.01 M of sodium sulfide (Na2S, 99%) for 20 s, respectively. After each immersion process, the substrates were rinsed with deionized water and absolute ethanol to the remove the excess impurity ions remaining in it. Optimized CdS nanoparticles were achieved by repeating the immersion process for 15 cycles. Modification with FeOOH Layer. The FeOOH layer was synthesized via an electrodeposition route as reported in the reference.33 The process was conducted use a three-electrode configuration at 70 °C with 0.1 M ferrous sulfate (FeSO4·7H2O, 99%) solution as an electrolyte. The applied potential was set as 1.2 V vs Ag/AgCl electrode. The deposited FeOOH thicknesses were controlled by the time of electrodeposition which ranged from 20 to 80 s, and these samples were designated as F20, F40, and F80. Characterization. The surface morphology of the obtained samples was observed using a JEOL JSM-7800F scanning electron microscope (SEM) operated at an accelerating voltage of 10 kV. JEOL JEM-2100 transmission electron microscopy (TEM and HRTEM) was used to investigate the microstructures of the obtained samples. X-ray diffraction (XRD, Rigaku-D/max-2500; Cu Kα radiation; 40 kV; 150 mA) and energy dispersive X-ray spectroscopy (EDS, AZtec from Oxford) were used to identify the crystalline structures and element of the obtained samples. A DU-8B UV−vis double-beam spectrophotometer was used to examine optical absorption capabilities. The PEC performance was examined in 0.1 M sodium sulfate (Na2SO4, 99%) solution via an electrochemical workstation (LK2005A, Tianjin, China), with a three-electrode configuration, with the as-obtained samples as the working electrode, saturated Ag/AgCl as reference electrode, and a platinum (Pt) electrode as counter electrode, respectively. The potential of Ag/AgCl reported in the results has been converted to the reversible hydrogen electrode (RHE) in the electrochemical measurements results (ERHE = EAg/AgCl + 0.059pH + 3566

DOI: 10.1021/acssuschemeng.7b03894 ACS Sustainable Chem. Eng. 2018, 6, 3565−3574

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ACS Sustainable Chemistry & Engineering 0.1976 V). The working electrode was under AM 1.5 standard illumination (CHF-XM500, 100 mW·cm−2).

and the Au/ZnO/CdS (AZC) system was derived. The following chemical reaction may take place during this reaction:

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RESULTS AND DISCUSSION To get a full appreciation of how the structure influences the performance of the obtained samples, we must first turn to the design and synthesis of the Au/ZnO/CdS/FeOOH (AZCF) photoanode. Figure 1 depicts the formation processes and

Cd2 + + S2 − → CdS

(1)

At last, the Au/ZnO/CdS/FeOOH (AZCF) system was obtained by electrodepositing the amorphous FeOOH on the AZC system. With different deposition times, amorphous FeOOH with different thicknesses can be obtained. The typical SEM image of the Au layer is displayed as shown in Supporting Information, Figure S1a. What can be seen is that the surface of Au layer is very smooth which is coincident with its photo (the inset of Figure S1b). It is reasonable to form such morphologies under the ion-beam sputtering process. Material ions deposited at high energy contribute to form densely packed layers that are chemically and mechanically inert. The thickness of a layer can be precisely controlled throughout the deposition process via deposition time, in addition to the merits mentioned above. The presence of Au can be confirmed by the EDS elemental analysis spectrum as shown in Figure S1b. Additionally, the peaks of In, O, and Si elements also can be seen from the spectrum. Why? These peaks originated from the ITO transparent conductive glass substrate. Figure S2a shows the low magnification SEM image of AZ, which reveals that ZnO NRs have been synthesized on the ITO/Au substrate. Well-aligned vertical ZnO NRs are derived from the aqueous solution. From the enlarged view presented as Figure S2b,it can be seen that the surfaces of the ZnO NRs are smooth and the individual ZnO nanorod presents a diameter of approximately 300 nm. In contrast, a denser surface is displayed in Figure S2c than Figure S2a which is caused by CdS nanoparticles fabricated on the ITO/Au/ZnO NRs substrate after the SILAR method. What we can see from the enlarged view is that ZnO NRs are decorated with numerous nanoparticles. It is clear that the entire surface of the ZnO NRs is not smooth any more. Figure 2 displays that the ZnO nanorod which is decorated with CdS nanoparticles exhibits a diameter of approximately 400 nm. The element distribution mapping as shown in Figure 2 indicates the corresponding element distribution mapping of Zn, O, S, Cd, and Au of the AZC electrode as shown in Figure S2d, which could further confirm the presence of CdS decorated on the surface of the ZnO NRs.

Figure 1. Schematic representation of preparation process of the Au/ ZnO/CdS/FeOOH (AZCF) photoanode.

synthetic route of the AZCF photoanode, which includes the ion beam sputter deposition to load the Au layer, aqueous solution synthesis of ZnO nanorods, the sequential introduction of CdS nanoparticles by the SILAR method as well as the electrodeposition to fabricate the FeOOH layer. The whole reaction conditions were mild and facile so as to avoid the destruction of the previous substance and keep its integrity. To be specific, the Au cocatalyst with the thickness of 20 nm was deposited directly onto the surface of the ITO transparent conductive glass substrate to absorb and collect the photogenerated electrons from the ZnO NRs, while passing the electrons to the counter Pt electrode. ZnO NRs were synthesized on the top of the Au layer in the aqueous solution reaction as shown in Figure 1. After the mild reaction, the Au/ ZnO (AZ) system can be achieved. Sequentially CdS nanoparticles were fabricated via a modified SILAR method

Figure 2. Typical top view SEM image and the corresponding elemental distribution mapping of Zn, O, S, Cd, and Au of the AZC photoanode. 3567

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To illustrate the mechanism further, the band gap (Eg) of bare ZnO and ZnO/CdS composite can be estimated by the following equation.41

Specific microstructure information on ZnO NRs/CdS is investigated via TEM and HRTEM ulteriorly. After CdS was fabricated via a SILAR process, the surfaces of ZnO were well decorated with dense and uniform particles appearing as dark regions that can be observed in Figure 3a, which corresponds to

(αhν)n = A(hν − Eg )

(2)

Equation 2 is used for calculating the optical band gap energy (Eg) of samples, α represents the absorbance coefficient, hν represents the incident light intensity, and A is a constant. The direct band gap of the samples can be identified assigning 2 to n. According to eq 2, the band gaps of bare ZnO and ZnO/CdS composite were caculated to be 3.37 and 3.05 eV as shown in Figure 4b; the result is consistent with the differences in absorption spectra measurement. It indicates that CdS possesses a narrow band gap as well as a high optical absorption coefficient compared to ZnO so as to own an efficient light absorption capability. At the same time, the appearance of ZnO, Au/ZnO, ZnO/ CdS, and Au/ZnO/CdS are shown as photographs that are inserted in Figure 5. It can also be seen that the color of Au/

Figure 3. (a) TEM image and (b) HRTEM images of ZnO NRs/CdS.

the typical top view scan electron microscopy analysis as above (Figure 2). The CdS particles distributed on the ZnO NR assembles with sizes ranging from 10 to 30 nm to form intimate junctions with ZnO. The high-resolution transmission electron microscopy (HRTEM) image is presented as Figure 3b; the scale bar is 5 nm. Moreover, the lattice fringe of each object is easy to find in Figure 3b. The lattice distance is measured to be 0.260 nm, which can be indexed to the (002) plane of hexagonal ZnO. Moreover, the lattice spacing of the (200) face of the cubic CdS is observed as 0.286 nm which will be discussed in the XRD analysis as follows (Figure 6). To understand the truth of how the Au cocatalyst and CdS nanoparticles effect, it is necessary to analyze the UV−vis absorption spectra of the as-obtained samples as shown in Figure 4a. The absorption edge of samples is calculated by the intersection value of the decreasing boundary and the baseline of the spectrum based on the research by Li et al.40 According to the above investigation and study, pure ZnO NRs show an absorbed edge at around 380 nm. There is no obvious absorption shift after the Au layer was deposited. When the pure ZnO was coupled with CdS, the absorption edge presents an improvement with an extended absorption of 455 nm as well as the enhanced absorption intensity compared with pure ZnO. In another word, the ZnO/CdS system exhibits a remarkable red-shift and an increase in absorption intensity. What makes the enhanced optical activity of ZnO/CdS?

Figure 5. Photocurrent density−voltage curves and the photos (inset) of bare ZnO, AZ, ZC, and AZC photoanode.

ZnO/CdS becomes dark compared with that of ZnO/CdS which is yellow, Au/ZnO appears metallic and pristine, and ZnO appears white accompanied by the wider optical absorption. Photocurrent measurements are used to determine the PEC performance further. Figure 5 offers the photocurrent density−voltage (C−V) curves vs RHE of bare ZnO, AZ, ZC, and AZC photoanodes. As we can see, the photocurrent density of the AZC photoanode is 2.93 mA·cm−2 at 1.23 V vs RHE, whereas those of the bare ZnO, AZ, and ZC photoanode are about 0.51, 1.24, and 2.49 mA·cm−2, respectively. To be specific, AZ presents an enhanced photocurrent with a value of

Figure 4. (a) UV−vis absorbance spectra collected from bare ZnO, AZ, ZC, and AZC photoanode; (b) plot of (αhυ)2 versus hυ of bare ZnO and ZC. 3568

DOI: 10.1021/acssuschemeng.7b03894 ACS Sustainable Chem. Eng. 2018, 6, 3565−3574

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ACS Sustainable Chemistry & Engineering 1.24 mA·cm−2 compared to bare ZnO. In contrast, the photocurrent density of the ZC photoanode is about 4.88 times greater than that of the bare ZnO. The AZC photoanode generated the highest photocurrent density of 2.93 mA·cm−2 which is about 5.75, 1.36, and 1.18 times greater than the values of bare ZnO of 0.51 mA·cm−2, Au/ZnO of 1.24 mA·cm−2, and ZnO/CdS of 2.49 mA·cm−2, respectively. What makes this enhancement happen? It is necessary to carry out the electrochemical impedance spectroscopy (EIS) measurement. Figure S3 provided the Nyquist plots of the ZC and AZC photoanodes. Commonly, the arc radius of semicircles in the high frequency range reflects the charge transfer at the interface of the electrode/electrolyte. AZC possesses a smaller arc radius in the high frequence area contrasting with the ZC, which is consistent with the photocurrent measurements as shown in Figure 5. This could be attributed to the effect of deposition of Au film. The film works as an electron absorbant layer to supply the kinetics for photogenerated electron transfer. It also facilitates the transfer of interface charge carriers and restrains the recombination of photogenerated charge carriers.43 On the other hand, based on the construction of the ZnO/CdS heterojunction, the photogenerated electrons transfer from the conduction band (−0.52 eV) of CdS to the conduction band (−0.31 eV) of ZnO while the holes transfer from the valence band (2.89 eV) of ZnO to the valence band (1.88 eV) of CdS which restrains the recombination of photogenerated electron−hole.44 FeOOH cocatalyst layer has been electrodeposited onto the AZC system for supplying the oxygen evolution reaction kinetics to spatially separate from the inner Au cocatalyst. The XRD patterns of all the synthesized samples are presented in Figure 6. Four peaks marked with a black box “■” are

coincident with the ITO transparent conductive glass substrate as shown in the EDS elemental analysis spectrum (Figure S1b). In addition, a characteristic peak of Au observed at around 38.2° represents the (111) facet in the Au pattern, corresponding to the EDS analysis above. From the AZ pattern, the strongest (002) diffraction peak and some distinct peaks can be indexed as (100), (101), (102), (110), (103), and (112) diffraction directions of hexagonal ZnO (JCPDS Card No.36-1451), which indicates that the ZnO can be fabricated successfully via a mild one-step aqueous solution method. In the AZC pattern, the characteristic peak (200) is coincident with a space group of Fm/3m (225) (JCPDS Card No. 658873) of CdS which is not apparent yet. To keep the former substance such as Au and ZnO from being damaged, CdS is synthesized by a simple process without high temperatures or pressures, which causes the poor crystallinity of CdS. What is more, the AZCF XRD pattern presents no obvious difference compared to the XRD pattern of the AZC. This occurs because the FeOOH itself has no distinct Bragg reflections in the XRD pattern and appears to be amorphous, which is in agreement with the previous literature reported by Lhermitte.29 Further evidence to support the existence of FeOOH has been presented as Figure 7. The amorphous substance is apparently dominant on the surface of the AZC system as displayed in the typical top view SEM image after the FeOOH is introduced into the AZC system. The corresponding selected element distribution mapping images have been exhibited which consist of Fe, O, Cd, S, Zn, and the Au element from which the amorphous substance is identified as the FeOOH film. Moreover, the morphology and the existence of FeOOH can be identified by the TEM characterization as well as the EDS measurement as exhibited in Figure S4. The TEM image is presented as Figure S4a, from which it is observed that the FeOOH emerged as amorphous property and adhered to the surface of the CdS nanoparticles and the interstice of unsheathed ZnO by CdS. Figure S4b shows the spectra of the AZCF photoanode, implying that the Fe and O elements coexist in the pattern, which further proved the existence of FeOOH. Additionally, the lateral view of the AZCF photoanode is exhibited as Figure S5. It can be seen that the length of ZnO NRs can be measured close to approximately 2 μm, and the top surface of ZnO NRs/CdS is covered with a snowcap of amorphous FeOOH. The performance of the resulting photoanodes is first examined by optical analysis, and the light-absorption curves in the wavelength range 200−800 nm are shown in Figure 8a. What can be seen in the absorption spectrum curves is that the absorption edge of AZCF and ZCF is at around 400−420 nm which is better than that of AZ, while being slightly narrower

Figure 6. XRD patterns of Au, Au/ZnO, Au/ZnO/CdS, and Au/ ZnO/CdS/FeOOH.

Figure 7. Typical top view SEM image and the corresponding element distribution mapping of Fe, Zn, O, S, Cd, and Au of the AZCF photoanode. 3569

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Figure 8. (a) UV−vis absorbance spectra and (b) photocurrent density−voltage curves of AZ, ZC, ZCF, AZC, and AZCF photoanode.

Figure 9. (a) Nyquist plots and (b) the IPCE plots in the range of 300−900 nm measured at 1.23 V vs RHE of ZC, AZC, ZCF, and AZCF photoanode.

than that of ZC and AZC. It is necessary to conduct an investigation of the influence on PEC performance when FeOOH is loaded. The photocurrent density characterization versus RHE is further carried out to evaluate the photoelectrocatalytic activity. As displayed in Figure 8b, the photocurrent density of AZCF and ZCF is measured by 5.38 mA·cm−2 and 4.52 mA·cm−2 at 1.23 V vs RHE, respectively. Combined with those aforementioned measurements as shown in Figure 6, it can be observed that when the FeOOH is deposited on the ZC, the photocurrent density is enhanced 82% compared to that of ZC. Similarly, the photocurrent density of AZCF is improved 84% compared to that of AZC. Additionally, among these five photoanodes, the curve of AZCF exhibits a cathodic shift of the onset potential. To find out the mechanism, the electrochemical impedance spectroscopy (EIS) measurement is investigated for the evaluation of electrochemical characteristics and the EIS Nyquist plots of ZC, AZC, ZCF, and AZCF are exhibited in Figure 9a. Apparently, AZC possesses a smaller arc radius in the high frequence area contrasting with the ZC indicating the Au layer can act as the electron absorber cocatalyst. ZCF has a decreased arc radius compared to ZC, revealing that the presence of FeOOH could foster the interface charge transfer and inhibit the electron−hole recombination efficiently. It is reasonable to prove that the photocurrent of ZCF in Figure 8b is enhanced with the modification of FeOOH, which is due to the oxygen evolution reaction kinetics offered by FeOOH, and facilitates the transfer of photogenerated holes. The AZCF photoanode shows the smallest arc radius compared to other studied photoelectrodes, indicating Au and FeOOH dual cocatalysts are separated to further supply bidirectional kinetics which can foster the photogenerated electrons and holes transfer on the electrode surface in the opposite directions so as to further reduce the recombination rate.

Furthermore, to investigate the relevance of light absorbance and the photocurrent density, the incident phototo-current conversion efficiency (IPCE) at 1.23 V (vsRHE) bias is calculated using the following equation42 and presented in Figure 9b. IPCE % = 1240J /(λIlight)100

(3)

where Ilight stands for the incident light irradiance (mW·cm−2), J represents the photocurrent density (mA·cm−2), and λ is the incident light wavelength (nm) and measured ranged from 300 to 900 nm. Specifically, the ZCF owns the IPCE value of 15.1% while ZC is 8.3%, the incensement rate is 82%, which is ascribed to the oxygen evolution reaction kinetics supplied by FeOOH and prompted charge transfer. Meanwhile, the IPCE value of AZC is higher than that of ZC, which reveals that electrons could flow and be transferred to the counter electrode readily, thus enhancing the photogenerated electrons and holes separation rate with the assistance of Au. It is obviously seen that the AZCF occupies the supreme IPCE value of 17.9% which is 1.8 times higher than that of AZC at 9.8%. As a result of enhanced light harvesting and trapping, effective charge transfer as well as enhanced electron−hole separation have been obtained. Furthermore, to illustrate those points raised evocatively, it is necessary to develop a study of the internal charge transfer process and the energy level diagram of the AZCF photoanode (Figure 10). Combined with the above characterization analysis, a simplified schematic illustration is illustrated. Upon irradiation under visible light illumination for which the energy is equal to or beyond the band gap of the composite structure, electron hole pairs are induced. Thanks to the modification of CdS with a narrow band gap (2.4 eV) as certified by UV−vis spectra, a broadened light absorption has been achieved. The 3570

DOI: 10.1021/acssuschemeng.7b03894 ACS Sustainable Chem. Eng. 2018, 6, 3565−3574

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heterojunction which is confirmed by PEC measurements (Figure 5). Furthermore, the recombination of the electron− hole will be further reduced thanks to the dual cocatalysts which promote the transfer of photogenerated holes in the opposite directions as exhibited in the EIS results (Figure 9a). Moreover, the photoelectrocatalytic stability was then evaluated by linear sweep voltammetry (LSV). The measurement results of AZCF and AZC show a slight decrease after 100 cycles, indicating that the obtained samples possess photocorrosion resistance ability (Figure S6). To be specific, the photocurrent of AZC decreased obviously after 100 cycles, while the AZCF photoanode presents a more stable photocurrent density after 100 cycles, revealing a superior stability after loading the FeOOH cocatalyst layer. Therefore the modification of FeOOH results in a resistance to photocorrosion and protection of the semiconductor electrode. In addition, the varied thickness of the FeOOH layer is fabricated to investigate the effect of different thicknesses of the FeOOH layer. Figure 11 shows the typical top view SEM images of Au/ZnO/CdS (a) and Au/ZnO/CdS/FeOOH obtained with a variable deposition time of 20, 40, and 80 s, denoted as F-20 (b), F-40 (c), and F-80 (d), respectively. Along with the increase in deposition time, the thick amorphous FeOOH is aggregated. It is found that F-20 retains the morphology of AZC which is only covered with a thin layer of FeOOH. F-40 with further deposited material and distributed unevenly has more aggregated amorphous substances than F20. While F-80 presents a cotton-shaped structure and the morphology of ZnO/CdS can hardly be seen. The role of the FeOOH thickness with respect to the performance will be discussed in detail. To examine the optical property of the aforementioned samples with different thicknesses of FeOOH, the room temperature UV−vis absorption spectra are studied compared to that of AZC (Figure 12a). It can be seen that the absorbance peaks of AZCF shift to blue light gradually and the intensity is decreased compared with AZC along with the increased deposition time. Namely, the increased thickness of

Figure 10. Schematic of the AZCF photoanode applied in overall water splitting and simplified schematic illustration of the band gap energy diagram, showing the enhanced light-harvesting and chargetransfer processes.

photogenerated electrons are transported from the conduction band (CB) of the CdS to that of ZnO. In addition, a considerable number of photogenerated electrons employed the special structure of ZnO nanorods with high electron mobility moving toward the ITO substrate. It is believed that one-dimensional ZnO nanorods occupy a direct electrical pathway without crystal boundary resistance, which means an extremely high photogenerated electrons transport rate and low possibility of recombination for photoinduced electron−hole pairs. Additionally, the Au layer as deposited worked as the EAL to absorb the electrons from ZnO and facilitate the interfacial transfer to the counter Pt electrode as previously reported.43 Contrarily, the high energetic holes transfer from the valence band (VB) of the ZnO nanorod to that of CdS. Meanwhile, the photogenerated holes that accumulated on the surface of CdS can be transported to FeOOH and exhausted with the FeOOH deposit which acts as a trap for photogenerated holes and supplies the driving force for the transfer. Hence, the efficient separation of photogenerated electron− holes on the surface has been achieved through the ZnO/CdS

Figure 11. Typical top view SEM images of Au/ZnO/CdS (a) and Au/ZnO/CdS/FeOOH obtained with variable deposition time: (b) 20 s; (c) 40 s; (d) 80 s. 3571

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Figure 12. (a) UV−vis absorbance spectra and (b) photocurrent density−voltage curves of AZC and AZCF photoanode obtained with variable deposition time.

above indicate that with the assistance of Au, CdS, and FeOOH, the efficient AZCF photoanode is established and high photoelectrocatalytic activity can be achieved. Future prospects for an efficient photoelectrode accompanied by dual cocatalysts are becoming brighter.

FeOOH would impair the light absorbance inherent in the AZC components, which is similar finding to the previous research.29 It is thus clear that the thicker FeOOH might shorten the active surface area of the electrode and then reduce the photoabsorbance ability. It is vitally important to optimize the thickness of FeOOH. The photoelectrochemical responses of the samples depend on the structure with the changing thickness of the FeOOH cocatalyst in a certain range, which indicates that there is an optimum value of FeOOH cocatalyst that responds to the maximum PEC performance of sample under visible light. The photocurrent density−voltage curves of as-obtained AZCF with different deposition amounts of FeOOH are further researched as exhibited in Figure 12b. It is indicated that the densities of photocurrent of F-20, F-40, and F-80 are 5.38 mA·cm−2, 3.77 mA·cm−2, and 2.61 mA·cm−2 at 1.23 V versus RHE, respectively. That is, along with the increased thickness of FeOOH, the densities of photocurrent are decreased. From the comparison, the F-20 photoelectrode which is obtained when the deposition time is 20 s has a correspondingly high photocurrent. As the results show, the deposition time of FeOOH has great effect on the thickness and the differences of PEC performance. The FeOOH layer primarily supplies the driving force to facilitate the charge transfer and further promote the electron−hole separation.45 However, the decrease of optical absorbance along with the thicker FeOOH enhances even more the charge transfer, resulting in a net decline of PEC performance. As a result, the thickness of FeOOH is optimized to obtain efficient performance as discussed previously, and when the deposition time is 20 s, the resulting sample presents the best PEC performance.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b03894. Figure S1, (a) typical top view SEM image, (b) the EDS elemental analysis spectrum and the photo (inset) of Au layer deposited on the ITO transparent conductive glass substrate. Figure S2, typical top view SEM images of AZ (a) and AZC (c); the enlarged view of AZ (b) and AZC (d); with insets showing the cross sectional partial view of the corresponding AZ (a) and AZC (c), respectively. Figure S3, Nyquist plots measured at 1.23 V vs RHE of ZC and AZC photoanode. Figure S4, (a) TEM image and (b) the EDS element spectrum of AZCF photoanode. Figure S5, cross sectional SEM image of AZCF photoadode: ITO (blue), Au layer (orange), ZnO nanorods and CdS nanoparticles (green), amorphous FeOOH (purple). Figure S6, linear sweep voltammograms of AZC and AZCF photoanode (PDF)

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CONCLUSIONS In summary, a novel and flexible method is proposed to fabricate the ZnO/CdS heterojunction photoanode with dual Au and FeOOH cocatalysts. It is noteworthy that Au and FeOOH are for the first time adopted in photoanode modification simultaneously and applied in efficient PEC water splitting. The resulting optimized AZCF photoanode presents outstanding PEC water splitting performance with a high photocurrent density of 5.38 mA/cm2 and a negatively shifted onset potential. This enhancement may be attributed to the following: (i) a long light irradiation pathway owing to a one-dimensional (1D) ZnO nanorod; (ii) broaden light absorbance spectrum sensitized by CdS with narrow bad gap; (iii) reduced charge recombination rate thanks to the appropriate gradient energy gap structure of ZnO/CdS; (iv) the enhanced charge separation driven by bidirectional kinetics supplied by the dual Au and FeOOH cocatalysts. The results

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 22 23085236. Fax: +86 22 23085110. E-mail: [email protected]. ORCID

Zhifeng Liu: 0000-0002-1009-6267 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

The authors gratefully acknowledge financial support from Science Funds of Tianjin for Distinguished Young Scholar (No. 17JCJQJC44800), Natural Science Foundation of Tianjin (No. 16JCYBJC17900), and Open Foundation of Hubei Collaborative Innovation Center for High-efficient Utilization of Solar Energy (No. HBSKFZD2017001). 3572

DOI: 10.1021/acssuschemeng.7b03894 ACS Sustainable Chem. Eng. 2018, 6, 3565−3574

Research Article

ACS Sustainable Chemistry & Engineering

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