Enhanced Photoelectrochemical Water Splitting of Photoelectrode

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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 ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03894 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 6, 2018

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ACS Sustainable Chemistry & Engineering

Enhanced photoelectrochemical water splitting of photoelectrode simultaneous decorated with cocatalysts based on spatial charge separation and transfer Zhifeng Liu1,2∗, Jing Zhang2, Weiguo Yan2 (1 Hubei Collaborative Innovation Center for High-efficiency Utilization of Solar Energy, Hubei University of Technology, Wuhan, 430068, China. 2 School of Materials Science and Engineering, Tianjin Chengjian University, 300384, Tianjin, China.) Abstract: ZnO/CdS heterojunction photoanode simultaneously decorated with separated Au and FeOOH cocatalysts is firstly established and applied in efficient photoelectrochemical water splitting. The 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 pure ZnO photoanode which can be ascribed to the 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 owing to the long light irradiation pathway of one-dimensional (1D) ZnO nanorod; broaden light absorbance spectrum sensitized by CdS with narrow bad gap; reduced charge recombination rate thanks to the appropriate gradient energy gap structure of ZnO/CdS as well as 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 photoelectrochemical system is firstly established and it highlights the sight of efficient photoelectrode for water splitting and hydrogen generation. Keywords:

heterojunction;

photoelectrode;

cocatalysts;

charge

separation;

photoelctrochemical water splitting Introduction With the development of industry especially architecture industry, a great amount of energy has been over consumed. Since the energy crisis and environment issue are the severe problems the world facing today, renewable and green energy sources have ∗

corresponding author Tel: +86 22 23085236 Fax: +86 22 23085110 E-mail: address: [email protected] 1

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received widespread attention

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[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 option. The discovery of TiO2 electrode applied in photoelectrochemical (PEC) water splitting system by Fujishima and [4]

Honda in 1972

, 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 visible region. It is therefore in urgent need of wide light-absorption range materials to address these issues. To this end, doping with metal or non-metallic element and coupling with smaller band-gap semiconductors have been adopted to broaden the light response to visible region

[12-16]

. Zhang et al. doped Fe on WO3 nanoflakes to

achieve broadband light trapping modulation

[12]

. Mukhopadhyay et al. adopted

narrow-band-gap CdS nanoparticles of 2.7eV as sensitizer to establish ZnO/CdS heterostructured nanocomposites for capture more visible light

[14]

. Liu and his group

organized WO3 and Cu2O to construct an all metal oxides heterojunction photoelectrode to realize wide light-absorption range [15]. Another key factor restricting the photoelectrocatalytic application of the above semiconductors is the high recombination rate of photo-generated electron and hole. As mentioned above, coupling with 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 of charge carriers such as loaded with cocatalysts catalyze the hydrogen evolution reaction (HER) or oxygen evolution reaction (OER). As we all known, noble metal such as Au

[16]

, Ag

[7]

, Pt

[17]

and Pd

[18]

have been investigated extensively for

capturing photo-induced electrons, improving carrier mobility and accelerating the separation of photo-induced 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 carriers transfer and promote

2


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visible-light photocatalysis

[18]

. Alternatively, the design of coupling with oxygen

evolution catalysts (OECs) such as oxides of Ir

[19]

, Ru

[20]

and Co

[21,22]

to supply

oxygen evolution reaction kinetics has been employed as a promising strategy to further promote charge separation. Based on comprehensive theoretical investigations, Wang and his co-workers fabricated a versatile cocatalyst CoSe2 which can facilitates photo-induced hole transfer and thus enhances the photocatalytic water oxidation activity [21]. Moreover, spatial separation cocatalysts which can facilitate electron and hole transfer has been proposed gradually to further reduce charge recombination rate [23-25]

. Gong et al. described the design and synthesis of 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, the efficient OER cocatalysts have been introduced into PEC water splitting including noble metals (Ir and Ru) [26,27], perovskite oxides materials (Fe, Co, Ni, Mn and Cu)

[29-33]

as well as carbon materials

[28]

, metal-based

[34]

. Petrykin et al.

described the synthesis of active catalysts in 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 photoanode 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 firstly establish a quaternary system including Au, ZnO nanorods (NRs), CdS nanoparticles and FeOOH simultaneously based on ITO substrate applied in PEC water splitting. One-dimensional (1D) ZnO nanorod possesses long light irradiation pathway and further sensitized by CdS with narrow bad gap (2.4 eV) to broaden the light absorbance spectrum. Meanwhile the appropriate gradient energy gap structure

3



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contributes to the reduced charge recombination rate. Au cocatalyst and FeOOH cocatalyst are separately loaded on inner and outer surfaces. Au cocatalyst works as electron absorber layer (EAL) to extract and collect photogenerated electrons from ZnO, whilst 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 photo-generated 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 FeOOH layer has been optimized to obtain the efficient performance and the mechanism has been investigated. Hence, these findings will open up new opportunities in constructing multielement system by assembling various materials to enhance photoelectrochemical activity further. Experimental section Deposition of Au layer Au layer was deposited directly onto the surface of indium tin oxide (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,

isopropanol and ethanol for each 15 min, respectively. The deposited thickness of Au layer was controlled to be 20 nm. The deposition process was kept with 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 contained 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 should be thoroughly washed with distilled water finally, and then to be dried in air naturally. Fabrication of CdS nanoparticles CdS nanoparticles were fabricated through a modified successive ionic layer adsorption and reaction (SILAR) method based on Liu et al. experiment

[39]

. The

substrates were successively immersed in the Cd2+ precursor solution contained 0.01M of cadmium nitrate (Cd(NO3)2, 99%) and S2− precursor solution contained 0.01M of sodium sulfide (Na2S, 99%) for 20 s, respectively. After each immersion

4


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process, the substrates were rinsed with de-ionized water and absolute ethanol for the removal of excess impurity ions remaining in it. The optimized CdS nanoparticles were achieved by repeated the immersion process of 15 cycles. Modification with FeOOH layer The FeOOH layer was synthesized via electrodeposition route as the reported 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 ranged from 20 to 80 seconds, and these samples were signed with 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. DU-8B UV-Vis double-beam spectrophotometer was used for optical absorption capabilities examination. The PEC performance was examined in 0.1 M sodium sulfate (Na2SO4, 99%) solution, which was performed via an electrochemical workstation (LK2005A, Tianjin, China), 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 + 0.1976 V). The working electrode was under AM 1.5 standard illumination (CHF-XM500, 100 mW·cm-2). Results and discussion To get a full appreciation of how the structure influences the performance of the obtained samples, we must firstly turn to the design and synthesis of the Au/ZnO/CdS/FeOOH (AZCF) photoanode. Fig.1 depicts the formation processes and synthetic route of AZCF photoanode, which includes the ion beam sputter deposition to load Au layer, aqueous solution synthesis of ZnO nanorods, the sequential introduction of CdS nanoparticles by SILAR method as well as the electrodeposition

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to fabricate the FeOOH layer. The whole reaction conditions were mild and facile so as to avoid the destruction of previous substance and keep its integrality. To be specific, Au cocatalyst with the thickness of 20 nm was deposited directly onto the surface of ITO transparent conductive glass substrate and applied for absorbing and collecting the photo generated electrons from ZnO NRs, whilst passing the electrons to the counter Pt electrode. ZnO NRs were synthesized on the top of 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 and the Au/ZnO/CdS (AZC) system can be derived. The following chemical reactions may take place during this reaction: Cd2+ + S2- →CdS

(1)

At last, Au/ZnO/CdS/FeOOH (AZCF) system was obtained by electrodepositing the amorphous FeOOH on the AZC system. With different deposition time, amorphous FeOOH with different thicknesses can be obtained. The typical SEM image of Au layer is displayed as Fig.S1 (a). What can be seen is that the surface of Au layer is very smooth which is coincident with its photo (the inset of Fig.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 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 Fig.S1 (b). Additionally, the peaks of In, O, and Si elements also can be seen from the spectrum. Why? These peaks are originated from the ITO transparent conductive glass substrate. Fig.S2 (a) 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 Fig.S2 (b), the surfaces of the ZnO NRs are smooth and the individual ZnO nanorod presents a diameter of approximately 300 nm. In contrast, denser surface is displayed in Fig.S2 (c) than Fig.S2 (a) which is aroused 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. Fig.2 displays that the ZnO nanorod which decorated by CdS nanoparticles exhibits a diameter of

6


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approximately 400 nm. The element distribution mapping as shown in Fig.2 indicates the corresponding element distribution mapping of Zn, O, S, Cd and Au of AZC electrode shown in Fig.S2 (d), which could further confirms the presence of CdS decorated on the surface of ZnO NRs. Specific microstructure information of 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 appeared as dark regions that can be observed in Fig.3 (a), which is corresponding to the typical top view scan electron microscopy analysis as above (Fig.2). The CdS particles distributed on the ZnO NR resembles with the size ranging from 10 nm to 30 nm to form intimate junctions with ZnO. The high-resolution transmission electron microscopy (HRTEM) image is presented as Fig. 3 (b), the scale bar is 5 nm. Moreover, the lattice fringe of each object is easy to find in Fig. 3 (b). 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 (200) face of cubic CdS is observed as 0.286 nm which will be discussed in the XRD analysis as follows (Fig.6). To understand the truth of how Au cocatalyst and CdS nanoparticles effect, it is necessary to analyze the UV-Vis absorption spectra of the as-obtained samples as shown in the Fig.4 (a). The absorption edge of samples is calculated by the intersection value of the decreasing boundary and the baseline of the spectrum the 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? In order to illustrate the mechanism further, the band gap (Eg) of bare ZnO and ZnO/CdS composite can be estimated by the following equation [41]. (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 equation (2), the band gaps of bare ZnO and ZnO/CdS composite were

7



Page 8 of 27

caculated to be 3.37 and 3.05 eV and shown in Fig. 4 (b), the result is consistent with the differences in absorption spectra measurement. It can be attributed to CdS that possesses narrow band gap as well as the high optical absorption coefficient compared to ZnO so as to own 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 Fig.5. It can also be seen that the color of Au/ZnO/CdS become dark compared with ZnO/CdS showed yellow, Au/ZnO shows metal and pristine ZnO shows white accompanied with the wider optical absorption. Photocurrent measurements are used to determine the PEC performance further. Fig.5 offers the photocurrent density-voltage (C-V) curves vs. RHE of bare ZnO, AZ, ZC and AZC photoanode. As we can see, the photocurrent density of 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 1.24 mA·cm-2 compared to bare ZnO. In contrast, the photocurrent density of ZC photoanode is about 4.88 times greater than the bare ZnO. AZC photoanode generated the highest photocurrent density of 2.93 mA·cm-2 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. Fig.S3 provided the Nyquist plots of ZC and AZC photoanode. Commonly, the arc radius of semicircles in the high frequency range reflects the charge transfer at the interface of 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 Fig.5. It could be attributed to the effect of deposition of Au film worked as electron absorber layer to supply the kinetics for photogenerated electron transferring and facilitate the transfer of interface charge carriers and restrains the recombination of photogenerated charge carriers

[43]

. On the

other hand, based on the construction of 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

8


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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 Fig.6. Four peaks marked with “” are coincident with the ITO transparent conductive glass substrate as shown in the EDS elemental analysis spectrum (Fig.S1b). In addition, a characteristic peak of Au observed at around 38.2° represents (111) facet in the Au pattern, corresponding to the EDS analysis above. From the AZ pattern, strongest (002) diffraction peak and some distinct peaks can be indexed as (100), (101), (102), (110), (103) and (112) diffraction direction of hexagonal ZnO (JCPDS Card No.36-1451), which indicates that the ZnO can be fabricated successfully via 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. 65-8873) of CdS which is not apparent yet. In order to keep the former substance such as Au and ZnO not being damaged, CdS is synthesized by simple process without high temperatures or pressures, which causes the poor crystallinity of CdS. What’s more, the AZCF XRD pattern presents no obvious difference compared to the XRD pattern of the AZC owing to the FeOOH itself has no distinct Bragg reflections in XRD pattern and appears to be amorphous, which is in agreement with the previous literature reported by Lhermitte [29]. Further evidences to support the existence of FeOOH have been presented as Fig.7. The amorphous substance is apparently dominant on the surface of AZC system as displayed in the typical top view SEM image after FeOOH being introduced into the AZC system. The corresponding selected element distribution mapping images have been exhibited which consist of Fe, O, Cd, S, Zn and Au element from which the amorphous substance is identified as FeOOH film. Moreover, the morphology and the existence of FeOOH can be identified by the TEM characterization as well as EDS measurement as exhibited in Fig.S4. The TEM image is presented as Fig.S4 (a), 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. Fig.S4 (b) shows the spectra of the AZCF photoanode, implying Fe and O element coexist in the pattern, which further proved the existence of FeOOH. Additionally, the lateral view of AZCF photoanode is exhibited as Fig.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 like snowcapped by amorphous FeOOH. The performance of the resulting photoanodes is firstly examined by optical

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analysis and the light-absorption curves in the wavelength range 200-800 nm are shown in Fig.8 (a). 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 slight narrower than 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 Fig.8 (b), 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 Fig.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. In order 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 Fig.9 (a). 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 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 Fig.8 (b) is enhanced with the modification of FeOOH, which is owing to the oxygen evolution reaction kinetics offered by FeOOH, and facilitates the transfer of photogenerated holes. 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 photo-to-current conversion efficiency (IPCE) at 1.23 V (vs.RHE) bias is calculated using the following equation [42] and presented in Fig.9 (b). IPCE = 1240·J/ (λ·Ilight) × 100%

10


(3)

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where Ilight stands for the incident light irradiance (mW·cm-2), J represents the photocurrent density (mA·cm-2), λ is the incident light wavelength (nm) and measured ranged from 300 nm 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 the electrons could flow and transferred to the counter electrode readily thus enhance 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 AZC of 9.8%. As a result of enhanced light harvesting and trapping, effectively 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 AZCF photoanode (Fig.10). Combined with the above characterization analysis, a simplified schematic illustration is illustrated. Upon irradiated under visible light illumination that 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 narrow band gap (2.4 eV) as certified by UV-Vis spectra, broaden light absorption has been achieved. The 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 photo-induced electron-hole pairs. Additionally, Au layer is 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 accumulated on the surface of CdS can be transported to FeOOH and exhausted with the FeOOH deposited which acts as a trap for photogenerated holes and supply the driven force for the transfer. Hence, the efficient separation of photogenerated electron-hole on the surface has been achieved through the ZnO/CdS heterojunction

11



which confirmed by PEC measurements (Fig.5). Furthermore, the recombination of electron-hole will be further reduced thanks to the dual cocatalyst which promotes the transfer of photo-generated holes in the opposite directions as exhibited in EIS results (Fig.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 the obtained samples possess photocorrosion resistance ability (Fig.S6). To be specific, the photocurrent decreased obviously of AZC after 100 cycles while the AZCF photoanode presents more stable photocurrent density after 100 cycles, revealing a superior stability after loading FeOOH cocatalyst layer. It is attributed to the modification with FeOOH could resist photo corrosion and protect the semiconductor electrode. In addition, the varied thickness of the FeOOH layer is fabricated to investigate the effect of different thicknesses of FeOOH layer. Fig.11 shows the typical top view SEM images of Au/ZnO/CdS (a) and Au/ZnO/CdS/FeOOH obtained with variable deposition time of 20 sec, 40 sec and 80 sec named as F-20 (b), F-40 (c) and F-80 (d), respectively. Along with the deposition time increased, the thick amorphous FeOOH is aggregated. It is found that F-20 remains the morphology of AZC which is only covered with a thin layer of FeOOH. F-40 with further deposited and distributed unevenly has more amorphous substance as aggregation than F-20. While F-80 presents cotton-shaped structure the morphology of ZnO/CdS can hardly be seen. The role of FeOOH thickness will be discussed in detail regarding of the performance. 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 AZC (Fig.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 FeOOH would impair the light absorbance inherent in the AZC components, which is similar 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 photo absorbance ability. It is vital 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 indicating that there is an optimum value of FeOOH cocatalyst respond to the maximum PEC performance of sample under visible light. The

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photocurrent density-voltage curves of as-obtained AZCF with different deposition amount of FeOOH are further researched as exhibited in Fig.12 (b). 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 obtained when the deposition time is 20 seconds has a correspondingly high photocurrent. As the results shown, the deposition time of FeOOH has great effect on the thickness and the differences of PEC performance. The FeOOH layer primarily supplies the driven force to facilitate the charge transfer and further promote the electron hole separation45. However, the decrease of optical absorbance is even more than enhance of charge transfer along with the thicker FeOOH, resulting in a net decline of PEC performance. As a result, the thickness of FeOOH is optimized to obtain the efficient performance as above discussion and when the deposition time is 20 sec, the resulting sample presents the best PEC performance. 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 is the first time to be 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 of onset potential. This enhancement may be attributed to the following: (i) long light irradiation pathway owing to 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 cocatalyst. The results 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 efficient photoelectrode accompanied with dual cocatalyst are becoming brighter.

Acknowledgements The authors gratefully acknowledge financial support from Science Funds of 13



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Tianjin for Distinguished Young Scholar (No. 17JCJQJC44800), Natural Science Foundation of Tianjin (16JCYBJC17900) and Open Foundation of Hubei Collaborative Innovation Center for High-efficient Utilization of Solar Energy (No. HBSKFZD2017001). References [1] Ager, J. W.; Shaner, M. R.; Walczak, K. A.; Sharp, I. D.; Ardo, S. Experimental demonstrations of spontaneous, solar-driven photoelectrochemical water splitting. Energy Environ. Sci. 2015, 8: 2811-2824. [2] Li, H. J.; Tu, W. G.; Zhou, Y.; Zou, Z. G. Z-Scheme photocatalytic systems for promoting photocatalytic performance: recent progress and future challenges. Adv. Sci. 2016, 3: 1500389-1500404. [3] Deng, D. H.; Novoselov, K. S.; Fu, Q. Zheng, N. F.; Tian, Z. Q.; Bao, X. H. Catalysis with two-dimensional materials and their heterostructures. Nat. Nanotechnol. 2016, 11: 218-230. [4] Fujishima, A.; Honda, K. Electrochemical photolysis of water at a semiconductor electrode. Nature, 1972, 238: 37-38. [5] Hisatomi, T.; Kubota, J.; Domen, K. Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting. Chem. Soc. Rev. 2014, 43: 7520-7535. [6]

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Lists of the Figures: Fig.1 Schematic representation of preparation process of the Au/ZnO/CdS/FeOOH (AZCF) photoanode Fig.2 Typical top view SEM image and the corresponding elemental distribution mapping of Zn, O, S, Cd and Au of AZC photoanode Fig.3 (a) TEM image and (b) HRTEM image of ZnO NRs/CdS Fig.4 UV-Vis absorbance spectra collected from bare ZnO, AZ, ZC and AZC photoanode; (b) the plot of (αhυ)2 versus hυ of bare ZnO and ZC Fig.5 Photocurrent density-voltage curves and the photos (inset) of bare ZnO, AZ, ZC and AZC photoanode Fig.6 XRD patterns of Au, Au/ZnO, Au/ZnO/CdS and Au/ZnO/CdS/FeOOH Fig.7 Typical top view SEM image and the corresponding element distribution mapping of Fe, Zn, O, S, Cd and Au of AZCF photoanode Fig.8 (a) UV-Vis absorbance spectra and (b) photocurrent density-voltage curves of AZ, ZC, ZCF, AZC and AZCF photoanode Fig.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 Fig.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 charge-transfer processes Fig.11 Typical top view SEM images of Au/ZnO/CdS (a) and Au/ZnO/CdS/FeOOH obtained with variable deposition time: (b) 20 sec; (c) 40 sec; (d) 80 sec Fig.12 (a) UV-Vis absorbance spectra and (b) photocurrent density-voltage curves of AZC and AZCF photoanode obtained with variable deposition time

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

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

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

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Fig.4 (a) UV-Vis absorbance spectra collected from bare ZnO, AZ, ZC and AZC photoanode; (b) the plot of (αhυ)2 versus hυ of bare ZnO and ZC

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

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

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

(a)

(b)

Fig.8 (a) UV-Vis absorbance spectra and (b) photocurrent density-voltage curves of AZ, ZC, ZCF, AZC and AZCF photoanode

Fig.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

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Fig.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 charge-transfer processes

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

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(a)

(b)

Fig.12 (a) UV-Vis absorbance spectra and (b) photocurrent density-voltage curves of AZC and AZCF photoanode obtained with variable deposition time

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Supplemental information: Fig.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 Fig.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 Fig.S3 Nyquist plots measured at 1.23 V vs. RHE of ZC and AZC photoanode Fig.S4 (a) TEM image and (b) the EDS element spectrum of AZCF photoanode Fig.S5 Cross sectional SEM image of AZCF photoadode: ITO (blue), Au layer (orange), ZnO nanorods and CdS nanoparticles (green), amorphous FeOOH (purple) Fig.S6 Linear sweep voltammograms of AZC and AZCF photoanode

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ZnO/CdS heterojunction photoelectrode decorated with the separated cocatalysts Au and FeOOH promote charge separation in PEC water splitting.

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352x182mm (72 x 72 DPI)


Enhanced Photoelectrochemical Water Splitting of Photoelectrode

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