Anisotropic Photoelectrochemical (PEC) Performances of ZnO Single

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Anisotropic Photoelectrochemical (PEC) Performances of ZnO SingleCrystalline Photoanode: Effect of Internal Electrostatic Fields on the Separation of Photogenerated Charge Carriers during PEC Water Splitting Bo Zhang, Zeyan Wang, Baibiao Huang, Xiaoyang Zhang, Xiaoyan Qin, Huiliang Li, Ying Dai, and Yingjie Li Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02639 • Publication Date (Web): 30 Aug 2016 Downloaded from http://pubs.acs.org on September 3, 2016

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Chemistry of Materials

Anisotropic Photoelectrochemical (PEC) Performances of ZnO Single-Crystalline Photoanode: Effect of Internal Electrostatic Fields on the Separation of Photogenerated Charge Carriers during PEC Water Splitting Bo Zhang,† Zeyan Wang,*,† Baibiao Huang,*,† Xiaoyang Zhang,† Xiaoyan Qin,† Huiliang Li,† Ying Dai‡ and Yingjie Li‖ † ‡ ‖

State Key Laboratory of Crystal Materials, Shandong University Jinan 250100, P. R. China School of Physics, Shandong University, 250100, P. R. China School of Energy and Power Engineering, Shandong University, Jinan, 250061, P. R. China

ABSTRACT: This work investigates the anisotropic PEC performances of ZnO single-crystalline (SC) photoanodes and effect of internal electrostatic fields on the separation of photogenerated charge carriers during PEC water splitting. It was found the internal electrostatic field can greatly influence the bulk charge separation efficiencies during PEC water splitting depending on its orientations, which can only be promoted as the internal electrostatic field is accordance with the direction of holes’ transportation. Due to the surface stabilization of ZnO polar surfaces, the internal electrostatic field would be gradually decreased to zero near the surface of ZnO SC photoanodes. Therefore, the interfacial charge separation would be mainly determined by the interfacial electric fields in space charge region formed by the equilibration of the Fermi levels between ZnO and the electrolyte solution. However, the differences on the bulk charge separation efficiencies of ZnO SC photoanodes are much larger than that at the interface, which indicated the bulk charge separation could play a more important role on determining the overall charge separation during PEC water splitting. Therefore, the anisotropic PEC performances of ZnO SC photoanodes during PEC water splitting could be mainly attributed to the internal electrostatic fields. With the assistance of the internal electrostatic field, O-SC yield a record high solar to hydrogen conversion efficiency of 0.78% at 0.7 V vs RHE and a maximum photocurrent density of 1.84 mA cm-2 at 1.23 V vs RHE with ηb and ηi of 91.6 and 99.5 %, respectively. The results demonstrate the effectiveness of internal electrostatic fields in polar single crystals on promoting the bulk charge separation during PEC water splitting, and indicate polar single crystals could be good candidates to fabricate high efficient PEC photoanodes with high conversion efficiencies.

INTRODUCTION Photoelectrochemical (PEC) water splitting has attracted increasing interests owing to its potential applications on solving energy crisis by producing hydrogen from water and sunlight with high theoretical conversion efficiencies (>30%).[1-2] Since the first demonstration of PEC water splitting by Honda and Fujishima, great efforts have been made in the past few decades.[3] Various semiconductor PEC photoelectrodes have been fabricated and many strategies have been developed in order to improve the PEC solar-to-hydrogen (STH) conversion efficiencies.[4-7] However, at present, the STH conversion efficiency is still not satisfactory and some basic principles of PEC processes are still not fully understood. Therefore, it is still a challenging work to further improve the conversion efficiencies of PEC photoelectrodes. As one of the most important determining factors during PEC water splitting, the separation and transportation of photogenerated charge carriers inside PEC photoelectrodes can greatly influence the STH conversion efficiencies. Although many methods have been developed to improve the charge separation and transportation efficiencies, such as decreasing the recombination probability by lowering the defect densities of the semiconductor nanoparticles, doping to improve the carrier mobilities, and designing of new structures and morphologies to shorten the minority carriers’ diffusion lengths, etc. [8-13] These strategies are mainly focused on the optimization and modification of PEC photoelectrodes made by nanoparticles. However, people seem to be failed to take into account of the fact that the separation and transporta-

tion of photogenerated charge carriers are closely dependent on the anisotropy of materials, which is difficult to be effectively optimized in the photoelectrodes made by nanomaterials. For example, as a basic material parameter, the internal electrostatic fields originated from the asymmetric crystal structure of polar materials can greatly influence the separation and recombination of charge carriers in the materials. [14-20] Waltereit, et al. have reported that the radiative lifetime of charge carriers in GaN for C-plane wells (≥6 ns) is much longer than that for M-plane wells (450 ps) due to the existence of electrostatic field along c-axis.[21] The effect of internal electrostatic fields on improving the photocatalytic activities have already been recently demonstrated by our group and the others.[22-25] However, as the electrostatic fields can only be existed in the interior space of polar materials, it is almost impossible to construct a unique electrostatic field in a photoelectrode made by nanoparticles. In this regard, the investigations on the effect of the internal electrostatic fields on the charge separation and anisotropic performances of single crystalline (SC) photoelectrodes are important to understand the basic principles of photogenerated charge separation and further improve the STH conversion efficiencies during PEC water splitting, which have been rarely reported yet. ZnO is a semiconductor with asymmetric polar crystal structures (P63mc), where high quality bulk materials can be easily obtained at reasonable cost. ZnO has a strong spontaneous polarization along the [0001] direction (0.047 C/m2) and a high electron mobility (ca. 400 cm2 V-1 s-1 at 300 K).[26,27] All these attributes make it a good candidate to fabricate ZnO SC polar photoanodes.

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In this work, we fabricated ZnO SC PEC photoanodes and systematically investigated their anisotropic performances and the influence of the internal electrostatic fields on the separation of photogenerated charge carriers during PEC water splitting. The experimental results indicated that the internal electrostatic field in ZnO SC photoanodes can greatly influence the bulk charge separation efficiency depending its orientations. As the internal electrostatic field is accordance with the direction of photogenerated holes’ transportation, the bulk charge separation efficiencies can be greatly improved. And the interfacial charge separation efficiency are mainly determined by the interfacial electric fields. However, the differences on the bulk charge separation efficiencies of ZnO SC photoanodes determined by the internal electrostatic fields are much larger than that at the interface, which indicated the bulk charge separation could be dominant on determining the overall charge separation during PEC water splitting. Therefore, the anisotropic PEC performances of ZnO SC photoanodes during PEC water splitting could be mainly ascribed to the internal electrostatic fields. With the assistance of the internal electrostatic field, the charge separation efficiency can be greatly enhanced, which lead to an improved PEC performance during PEC water splitting. This provided a new way to fabricate high efficient PEC photoanodes with higher photoconversion efficiencies, which is important for the development and practical applications of PEC water splitting.

RESULTS and DISCUSSIONS

Figure 1, (a) Schematic diagram of as-prepared ZnO singlecrystalline photoanodes and ZnO nanorods photoanode; (b) the transmittance spectra and absorption spectra of c-plane, m-plane ZnO single crystal chips and ZnO nanorod arrays on FTO substrate. As illustrated in Figure 1a and S1, three ZnO single crystalline (SC) photoanodes were fabricated by c-cut and m-cut ZnO single crystal chips with different exposed crystal facets, namely, Zn-

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(0001), O-(000-1) and M-(10-10), and were denoted directly by the exposed crystal facets as Zn-SC, O-SC, and M-SC, respectively. The XRD patterns of the ZnO single crystal chips were shown in Figure S2. As the internal electrostatic field is along in ZnO single crystals (see Figure S1), the directions of the electrostatic field in Zn-SC, O-SC and M-SC are vertical inward, vertical outward and perpendicular to the surface exposed to electrolyte solutions as illustrated by the black arrows in Figure 1a, respectively. For comparison, ZnO nanorods photoanode (ZnO NRs) on FTO substrate was also fabricated. And the XRD pattern and SEM images shown in Figure S2 and S3 indicate the ZnO nanorods were grown on FTO substrates along direction, and the sizes of the ZnO nanorods are ~2 µm in lengths and 60-100 nm in diameters, respectively. The transmittance and absorption spectra of both ZnO SC chips and ZnO NRs are shown in Figure 1b. Due to the high absorption coefficient (~105 cm-1) and wide band gap (3.37 eV) of ZnO, both ZnO single crystal chips and ZnO NRs exhibit strong absorption in the UV region and high transparency in the visible region as expected. [28,29] And the band gap of c-plane, m-plane ZnO single crystal chips and ZnO nanorods is estimated to be 3.09, 3.12 and 3.24 eV, respectively. As the ZnO single crystals used in this work were grown by hydrothermal method, during which intrinsic defects, such as oxygen vacancies or interstitial Zn atoms, can be easily formed. The existence of these intrinsic defects make ZnO single crystals yellow in color, which could probably be responsible for the wider light absorption range than ZnO NRs.[30-31] And the small difference on band gaps between c-plane and mplane ZnO single crystal chips could be attributed to the strong internal electrostatic fields along direction, which result in a quantum confined Stark effect and poor electron-hole overlap similar as in GaN films. [32]

Figure 2, Mott-Schottky plots collected at a frequency of 1 kHz in the dark for (a) Zn-SC and M-SC, (b) O-SC, (c) ZnO NRs. In order to probe the electronic properties of ZnO SC and ZnO NRs photoanodes, Mott-Schottky measurements were conducted at a frequency of 1 kHz in dark. The donor densities, flatband potentials and the width of the space charge region at the electrode/electrolyte interface can be calculated by Mott-Schottky equation: 1⁄ = 2⁄ [ − − ⁄ Where e0 is the electron charge, ε the relative permittivity of ZnO (ε=10)[33], ε0 the permittivity of vacuum, Nd the donor density, and V the applied bias at the electrode, Vf the flatband potential, and kT/e0 is a temperature dependent correction term. As shown in


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Figure 2, all the ZnO photoanodes show a positive Mott-Schottky plots, as expected for n-type semiconductor. Vf of the photoanodes were determined from the extrapolation of X intercepts in MottSchottky plots, which were found to be -0.40, -0.38, -0.367 and 0.424 V vs Ag/AgCl for O-SC, M-SC, Zn-SC and ZnO NRs, respectively. The ZnO NRs shows the smallest slope of MottSchottky plot compared to ZnO SC photoanodes, indicating a higher donor densities. The surface donor densities and the width of the space charge region of the ZnO photoanodes were estimated using the following equations:

= 2⁄ [1⁄ /] W = [2 − / ]/ And the surface donor densities of O-SC, M-SC, Zn-SC and ZnO NRs were calculated to be 2.51×1018, 5.84×1017, 4.79×1017, 3.21×1019 cm-3, respectively. The donor density of ZnO NRs is about 1~2 orders higher than that of ZnO SC photoanodes. And the width of the space charge region for O-SC, M-SC, Zn-SC and ZnO NRs at a bias of 0.7 V vs NHE were calculated to be 14.8, 30.15, 32.8 and 4.25 nm, respectively.

Figure 3, (a) J-V curves recorded for Zn-SC, O-SC, M-SC, and ZnO NRs with a scan rate of 20 mV/s in dark and irradiated with the AM 1.5G simulated solar light at 100 mW/cm2. (b) J-t curves of Zn-SC, O-SC, M-SC, and ZnO NRs measured at 0.7 V vs RHE for 4800 s. (c) ABPEs of Zn-SC, O-SC, M-SC and ZnO NRs as a function of applied bias. (d) IPCEs for Zn-SC, O-SC, M-SC and ZnO NRs, measured in the wavelength range from 340 to 440 nm at applied voltage of 0.7 V vs RHE Photoelectrochemical measurements were performed by using a three-electrode electrochemical cell configuration with ZnO electrodes as the working electrode, a Pt sheet as the counter electrode, and Ag/AgCl as the reference electrode. Figure 3a shows the photocurrent density versus applied potential curves (J-V) recorded in the dark and under irradiation (AM 1.5G simulated solar light, 100 mW cm-2). Dark scan linear sweep voltamagrams from 0.05 to 1.23 V vs RHE for all ZnO photoanodes exhibit almost negligible current densities in the range of 10-7 A cm-2. And the onset potential for both ZnO SC photoanodes and ZnO NRs lie at about 0.2 V vs RHE, which is consistent with the flatband potentials observed in the Mott-Schottky plots in Figure 2. Under simulated sunlight irradiation, the ZnO NRs yield a photocurrent density of 0.95 mA cm-2 at 1.00 V vs RHE and the highest photocurrent density of 1.15 mA cm-2 at 1.23 V vs RHE, which is comparative with the reported value for ZnO nanorods photoanode.[26] However, the current densities for the three ZnO SC photoanodes varied a lot. O-SC exhibited the highest photocurrent density among the three SC photoanodes, where the maximum current density

reached 1.84 mA cm-2 at 1.23 V vs RHE, which is 1.6 times as high as that of ZnO NRs. At the same bias, the photocurrent density of Zn-SC and M-SC is only 0.86 and 1.35 mA cm-2, respectively. More interestingly, the photocurrent of O-SC increases rapidly with the increase of applied bias and begins to saturate at a bias of only 0.8 V vs RHE, which is much lower than that for the other photoanodes. The lower saturation potential can reduce the applied bias required to achieve the maximum photocurrent, which lead to more efficient charge separation and increase the overall efficiency of PEC water splitting. [34,35] As an important consideration for PEC water cleavage applications, the stabilities of O-SC, Zn-SC, M-SC and ZnO NRs were also investigated under prolonged illumination in 0.5 M Na2SO4 aqueous solution at potential of 0.7 V vs RHE. As shown in Figure 3b, the stability of ZnO NRs was very poor, where the photocurrent was time varying and rapidly decayed after 1 h. Only 12% residual photocurrent remained at the end of this experiment. This is similar as the recent reported results by Tang et al., which could be ascribed to the serious photocorrosions of the ZnO nanorods under irradiation during PEC water splitting experiment.[34]However, comparing to ZnO NRs, the stabilities of the three ZnO SC photoanodes are much higher with a steady photocurrent during the measurements (4800 s) as shown in Figure 3b. Although ZnO SC photoanodes can be also pohtocorroded due to the poor chemical stability of ZnO, the influences on photocurrents are much lower than that of ZnO NRs. The long-term current stability of O-SC was investigated by prolonging the irradiation time to 12 h as shown in Figure S4. The photocurrent was kept steady in the first 5 h, and then slightly changed with irradiation times. After 12 h, the initial current density of 1.47 mA cm-2 was slightly decreased to 1.40 mA cm-2. The enhanced current stability of ZnO SC photoanodes could be attributed to the excellent crystallinity and fewer defects of ZnO bulk single crystals, which could greatly reduce the photocorrosion rates during PEC water splitting. Additionally, owing to the large size of bulk ZnO, the photocurrent would not be greatly influenced even if the surface of ZnO SC photoanodes are slightly etched, and was kept stable even after irradiated for 12 h. Gas bubbles were observed to be evolved from both ZnO photoanodes and the counter electrode (Pt) continuously, which indicated the production of both H2 and O2. The gas products during the J-t measurements were collected every 15 min and analyzed with gas chromatography. Considering the poor stability of ZnO NRs, only the gas products in 1 h were analyzed. As shown in Figure S5, both H2 and O2 can be identified for all ZnO photoanodes as expected with the ratio H2:O2 2:1 within our experimental error. And the measured gas production rates were also perfectly overlapped with the theoretical values calculated from the photocurrents, which indicated the faradic efficiency was almost 100 % during the measurements. That means almost all the photogenerated charges in the current system were consumed for H2 and O2 production.[36] In order to evaluate the photoconversion efficiencies of these photoanodes, applied-bias photo-to-current efficiency (ABPE) curves as a function of applied potentials were calculated and plotted as shown in Figure 3c. Similar as the J-V curves in Figure 3a, O-SC showed the highest ABPE values of 0.78 % at a bias of 0.7 V vs RHE. And the optimal ABPE for M-SC, Zn-SC and ZnO NRs were 0.398, 0.274, and 0.271% at applied bias of 0.77, 0.78 and 0.85 V vs RHE, respectively. The conversion efficiency of 0.78 % for O-SC is nearly 2 times higher than the optimal ABPE of ZnO NRs with lower applied bias, which is also the highest value for ZnO materials ever reported to the best of our knowledge. [34,37,38] In order to evaluate the wavelength dependent photoconversion efficiencies, incident-photon-to-current-conversion efficiency



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(IPCE) measurements were performed with Zn-SC, O-SC, M-SC and ZnO NRs in the wavelength range from 350 to 410 nm at applied voltage of 0.7 V vs RHE as shown in Figure 3d. In comparison to photocurrent density obtained under white light illumination, IPCE can exclude the effects of light sources, which could be a better parameter to characterize the photoconversion efficiencies of photoanodes. Similar as the absorption spectra shown in Figure1b, the ZnO photoanodes showed photoresponses only in UV region due to their wide band gaps. The IPCE of ZnO NRs lies between 33.97~50.26 % with the optimal value of 50.26 % at 370 nm (350~375 nm), which is accordance with the recent reported values.[39,40] In comparison to ZnO NRs, all ZnO SC photoanodes exhibit enhanced conversion efficiencies over the entire UV region following the order O-SC > M-SC > Zn-SC. Particularly, the optimal IPCE value of O-SC is as high as 94.1 % at 367 nm, which indicates the separation and utilization of the photogenerated charge carriers in O-SC is very efficient. Meanwhile, the IPCEs of M-SC and Zn-SC at the wavelength of 367 nm is only 74.8 and 62.1 %, respectively. As the three ZnO SC photoanodes are fabricated with the same ZnO single crystal with the only differences on the crystal orientations and exposed crystal facets, the big differences on PEC performances could be attributed to the orientations of the internal electrostatic field and the Schottky barrier formed between Au and different exposed crystal facets of ZnO single crystals. In order to estimate the Schottky barriers formed between Au and different crystal facet of ZnO single crystals, valence band XPS were carried out (see Supporting Information). The Schottky barrier calculated by considering the conduction band bending for OSC, M-SC and Zn-SC is 0.558, 0.428 and 0.165 eV, respectively. According to the results reported by Wang et al., the photocurrent of O-SC with larger Schottky barrier should be weaker than that of Zn-SC and M-SC, which is opposite with the photocurrent observed above.[41] This indicate the effect of the Schottky barrier formed between Au and different crystal facet of ZnO single crystals could be less prominent in work. Therefore, the anisotropic PEC performances of ZnO SC photoanodes could be mainly ascribed to the internal electrostatic fields in this work. As the photogenerated electrons and holes are transported to the counter electrodes and the surface of the photoanodes to enable redox reactions during PEC water splitting, respectively. Once the internal electrostatic field is accordance with the direction of holes’ transportation, i.e., O-SC, an additional driven force would be provided by the electrostatic field to promote the charge separation. Oppositely, the internal electrostatic field will slow down the transportation of photogenerated charge carriers and increase the recombination of electrons and holes, which lead to less efficient charge separation just as the case in Zn-SC. In the case of M-SC, as the electrostatic field is perpendicular to the direction of charge transportation, the electrons and holes are separated by free diffusions, which yield a medium charge separation efficiency. As the separation of photogenerated charge carriers is comprised of two processes, namely, the charge separation in the bulk and the separation of charge carriers at the electrode/electrolyte interface. In order to investigate the effect of internal electrostatic field on the separation of photogenerated charge carriers in the bulk and interface during PEC water splitting, the bulk charge separation efficiency (ηb) and interfacial charge separation efficiency (ηi) as a function of applied potentials were also plotted following the method reported by Park et al.[42] as shown in Figure 4. The bulk and interfacial charge separation efficiency was estimated using the following equations: "#$#/%&' !

)"#$ /"#$# !

!

- .

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Where *+,# and *+,# # are the photocurrent density measured in aqueous solution without and with H2O2 as a hole scavenger, which were shown in Figure S6a and b respectively. Iabs is the photon absorption rate expressed as the photocurrent density, respectively. Iabs was estimated by integrating the overlap region of the AM1.5G spectrum with the absorption spectra over the wavelength (see Figure S7). In this study, the Iabs values of O-SC, M-SC, Zn-SC and ZnO NRs are calculated to be 2.04, 1.937, 2.04 and 2.09 mA/cm2, respectively

Figure 4. (a) the bulk charge separation efficiencies (ηb) and (b) interfacial charge separation efficiencies (ηi) of O-SC, M-SC, ZnSC, and ZnO NRs as a function of applied bias, respectively. As shown in Figure 4a, the ηb of all ZnO photoanodes increased constantly with the increase of applied bias, which is quite similar as the J-V curves shown in Figure 3a. This indicates higher applied bias can effectively enhance the bulk charge separation during PEC water splitting. The maximum ηb for O-SC, M-SC, ZnSC and ZnO NRs can be observed at the bias of 1.23 V vs RHE with the ηb value of 91.6, 83.2, 49.3 and 61.4%, respectively. As the internal electrostatic field can also be regarded as an additional bias. When the orientation of the internal electrostatic field is accordance with the direction of holes’ transportation, an additional bias was added to the applied bias, which lead to higher ηb as in O-SC. That means the same ηb can be obtained at a lower applied bias, which can also explain the rapid increase of ηb for OSC in Figure 4a. Oppositely, a higher applied bias is needed to balance the internal electrostatic field, which decreases the effect of applied bias on bulk charge separation and lead to a slower increased ηb curve as in Zn-SC. In M-SC, the internal electrostatic field is perpendicular with the charge transportation, therefore, the bias along charge transportation from the electrostatic field is zero, which yields a medium ηb. The bulk charge separation in ZnO NRs is similar as the case in M-SC. However, due to the higher defect densities in ZnO NRs, the ηb is much lower than that of MSC. The ηi of the ZnO photoanodes as a function of applied potentials were shown in Figure 4b. Similar as the bulk charge separation efficiencies as shown in Figure 4a, the ηi of those photoanodes also increased with the increase of applied bias. And the ηi of three ZnO SC photoanodes were quite close, which increased from 50 % to almost 100% as the bias increased from 0.3 to 1.23 V vs RHE. And the ηi of Zn-SC is only a slightly lower than that of O-SC and M-SC at low applied bias. That indicates the internal electrostatic field has less effects on the interfacial charge separation comparing to that in the bulk as shown in Figure 4a. This could be due to the cancelation of the electrostatic field by the self-stabilization of ZnO polar surfaces, where the electrostatic field near the surface of ZnO SC photoanodes gradually decreased. [43-44] Moreover, a space charge region (SCR) can be formed near the surface of the ZnO photoanodes, in which the energy bands bend upward and generate an interfacial potential due to the equilibration of the Fermi levels between ZnO and the electrolyte solution. As the interfacial potential is much larger than the electrostatic field in SCR, it will dominate the interfacial charge separation instead of the electrostatic field near the ZnO surfaces. However, as the differences on ηi between these ZnO SC photoanodes is not as large as that in the bulk (ηb). That means ηb


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determined by the internal electrostatic fields could play a more important role on determining the overall charge separation efficiency during PEC water splitting over ηi. Therefore, the anisotropic PEC performances of ZnO SC photoanodes can be mainly attributed to the internal electrostatic fields. The ηi of ZnO NRs was much lower than that of ZnO SC photoanodes as the applied bias was below 1.0 V vs RHE. The relative lower interfacial charge separation efficiency of ZnO NRs could be originated from the higher defect density and larger surface areas of ZnO nanorod arrays comparing to the planar ZnO SC photoanodes. A higher surface area leads to more active sites during PEC water splitting. However, on the other hand, a higher surface area also increases the number of surface trapping states, thus lower the interfacial charge separation efficiency as demonstrated by recent studies.[45,46]

Figure 5. Energy band schematic of O-SC, M-SC, Zn-SC and ZnO NRs to illustrate the bulk and interfacial charge separation. The electrostatic field induced bias (Ve) and the interfacial potential (Vi) in the space charge region (SCR) are also illustrated. Based on the results and analysis discussed above, the bulk and interfacial charge separation in O-SC, M-SC, Zn-SC and ZnO NRs can be summarized as illustrated in the energy band diagrams in Figure 5. As shown in this figure, the internal electrostatic field can greatly influence the bulk charge separation as an additional bias (Ve) during PEC water splitting, which is closely dependent on its orientations. As the internal electrostatic field is accordance with the direction of holes’ transportation, i.e., in OSC, the bulk charge separation can be greatly enhanced. On the contrary, the bulk charge separation would be greatly lowered as the internal electrostatic field is opposite along with the holes’ transportation as in Zn-SC. In the case of M-SC and ZnO NRs, the electrostatic field is perpendicular with the direction of charge transportation, the photogenerated charge carriers will be separated by free diffusions, which yield a medium ηb. However, due to the higher defect densities of ZnO nanorods, the ηb of ZnO NRs is much lower than that of M-SC. When the ZnO photoanodes are brought into contact with electrolyte solution, SCR would be formed near the surface of the photoanodes. And an upward band bending would be introduced to generate an interfacial potential (Vi) due to the equilibration of the Fermi levels between ZnO and the electrolyte solution. Therefore, the interfacial charge separation in ZnO SC photoanodes would be influenced by both the internal electrostatic field and the interfacial potential at the SCR synergistically. That indicates additional bias originated from both the electrostatic field and the interfacial potential would contribute to the interfacial charge separation, where the bias for O-SC, M-SC and Zn-SC is (Ve+Vi), Vi, and (Vi-Ve), respectively. However, due to the stabilization of the ZnO polar surfaces, the electrostatic field would gradually decreased to zero in the surface region, which would be negligible

comparing to the interfacial potentials in SCR. Therefore, the interfacial charge separation would be mainly determined by the interfacial electric fields, which is closely related to the surface donor densities of ZnO SC photoanodes. As the donor densities of ZnO SC photoanodes follow the order of O-SC > M-SC > Zn-SC. Therefore, the interfacial charge separation efficiency should also follow the same order as shown in Figure 4b. Same as the bulk charge separation discussed above, the interfacial charge separation in ZnO NRs is similar as that in M-SC. However, due to the higher defect densities and larger surface areas of ZnO nanorod arrays, more trapping sites would be provided as recombination centers of photogenerated charge carriers, which yields a lower ηi comparing to ZnO SC photoanodes with planar structures. In conclusion, we fabricated three ZnO SC photoanodes with c-cut and m-cut ZnO single crystal chips with different crystal facets exposed and investigated their PEC performances. Anisotropic PEC performances were observed in these ZnO SC photoanodes during PEC water splitting. By analyzing the charge separation in the bulk and at the interface separately, it was found that the charge separation in the bulk and at the interface was mainly determined by the internal electrostatic field and the interfacial electric field, respectively. However, the differences on the bulk charge separation efficiencies of ZnO SC photoanodes depending on the orientations of the internal electrostatic fields are much larger than that at the interface, which indicated the bulk charge separation could play a more important role on determining the overall charge separation during PEC water splitting. Therefore, the anisotropic PEC performances of ZnO SC photoanodes during PEC water splitting could be mainly attributed to the internal electrostatic fields. With the assistance of the internal electrostatic field, O-SC with planar structures yield a record high STH conversion efficiency of 0.78% at 0.7 V vs RHE and a maximum photocurrent density of 1.84 mA cm-2 at 1.23 V vs RHE with ηb and ηi of 91.6 and 99.5 %, respectively. The results in this work demonstrated the anisotropic PEC performances in ZnO SC photoanodes and the effectiveness of internal electrostatic fields in polar single crystals on promoting the bulk charge separation during PEC water splitting, which indicates polar single crystals could be good candidates to fabricate high efficient PEC photoanodes with high conversion efficiencies.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Experimental details; Crystal structure; XRD patterns; SEM images of the ZnO nanorods arrays; J-t curves of O-SC; Measured hydrogen/oxygen evolution; J-V curves recorded for Zn-SC, O-SC, M-SC, and ZnO NRs; AM 1.5 light spectrum (left) and UV-Vis absorption spectra (right) of ZnO nanorod arrays and m-plane and c-plane ZnO single crystal chips; Estimation of the Schottky barriers between Au and different crystal facets of ZnO single crystals.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected]

Author Contributions These authors contributed equally.

Notes The authors declare no competing financial interest

ACKNOWLEDGMENT



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This work is financially supported by the National Basic Research Program of China (the 973 Program, 2013CB632401), the National Natural Science Foundation of China (21333006, 11374190 and 51321091), and the Shandong Provincial Natural Science Foundation (ZR2014BM024). B. B. Huang acknowledged the support from Taishan Scholars Program of Shandong Province and Z. Y. Wang acknowledged the support from Young Scholars Program (2015WLJH35) and Fundamental Research Funds of Shandong University (2014JC049).

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Table of Contents Graphic

Anisotropic Photoelectrochemical (PEC) Performances of ZnO Single-Crystalline Photoanode: Effect of Internal Electrostatic Fields on the Separation of Photogenerated Charge Carriers during PEC Water Splitting Bo Zhang,† Zeyan Wang,*,† Baibiao Huang,*,† Xiaoyang Zhang,† Xiaoyan Qin,† Huiliang Li,† Ying Dai‡ and Yingjie Li‖ † ‡ ‖

State Key Laboratory of Crystal Materials, Shandong University Jinan 250100, P. R. China School of Physics, Shandong University, 250100, P. R. China School of Energy and Power Engineering, Shandong University, Jinan, 250061, P. R. China

Corresponding Author * E-mail: [email protected] * E-mail: [email protected]


Anisotropic Photoelectrochemical (PEC) Performances of ZnO Single

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