shell array photoanode enhanced by a dual suppression strategy.

DOI: 10.1002/cssc.201403294

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Photoelectrochemical Water Oxidation Efficiency of a Core/Shell Array Photoanode Enhanced by a Dual Suppression Strategy Wanhong He,[a] Ye Yang,[c] Liren Wang,[a] Junjiao Yang,[a] Xu Xiang,*[a] Dongpeng Yan,*[a, b] and Feng Li[a] achieved over a wide range of potential. Mechanism studies show that the modification of phosphate leads to significantly improved charge separation. The amorphous hydroxide sheath could efficiently inhibit oxygen reduction reactions. Therefore, this strategy enables the simultaneous suppression of surface carrier recombination and back reactions, which is promising to improve the water oxidation efficiency of currently prevailing photoanodes.

The development of earth-abundant semiconductor photoelectrodes is of great importance to high-efficiency and sustainable photoelectrochemical water splitting. Herein, a one-dimensional TiO2 array photoanode was sheathed with an ultrathin overlayer of phosphated nickel–chromium double-metal hydroxide by a photoassisted modification and deposition strategy. The core/shell array photoanode resulted in a large cathodic shift of photocurrent onset potential ( 200 mV). Nearly 100 % oxidative efficiency for PEC water oxidation was

Introduction The development of materials capable of achieving solar fuel production in a clean, sustainable, and economic way is of great desire because this conversion leads to the reduction of our long-term demand for fossil fuels.[1] In this sense, lightdriven water splitting in a photoelectrochemical (PEC) cell is a promising method to convert and store solar energy in the form of chemical bonds, namely, as H2.[2] To date, the design and fabrication of stable, high-efficiency photoanodes are still challenging because water oxidation itself is kinetically hindered, and it also involves complex multiple proton-coupled electron transfer (PCET) processes.[3] Photogenerated holes in oxide semiconductors, for example, TiO2, can oxidize water, although its wide band gap leads to low light-absorbing efficiency.[4] Narrow band-gap materials, such as hematite (a-Fe2O3), BiVO4, and TaON, can improve light absorption, especially in the visible region.[5] However, the photocurrent onset potential

of these semiconductors is much more positive than that of TiO2 because of slower water oxidation kinetics that originate from severe photogenerated carrier recombination.[6] An effective way to lower the barrier is to attach a metal oxide or molecular electrocatalyst (EC) onto the surface of photoanodes. For instance, Co4O4 cubane,[7] IrO2,[8] Mn2O3,[9] Co(OH)2,[10] FeOOH,[11] Ni(OH)2, or NiOOH,[12] and cobalt–phosphate (Co-Pi), have been deposited onto semiconductors to lower the threshold overpotential, and thus, achieved high water oxidation reactivity.[13, 14] Another way to modify photoanodes is the introduction of an electron–hole-separating overlayer, which can also result in surface-state passivation.[15] An interesting finding, reported by Nocera et al., is that the phosphate electrolyte can facilitate the in situ formation of the Co-Pi catalyst and allow high activity for water oxidation.[16] A recent study also showed that the activity of photoanodes in a buffered phosphate electrolyte, as a proton-accepting electrolyte (PAE), was higher than that in an unbuffered sulfate electrolyte.[17] Another study reported that the water oxidation onset potential had a negative shift of about 0.2 V by in situ treatment or immersion of ZnO photoanode in borate buffer.[18] These results highlight the important role of the PAE during PEC water oxidation. The development of cobalt-free, earth-abundant catalysts, for example, the hydroxide or oxide of nickel, has also received much attention.[19] Also, recent studies suggested that double hydroxides containing chromium were efficient catalysts for photocatalytic water splitting.[20] However, double-metal hydroxide decorated photoanodes have rarely been studied. Therefore, it is worth investigating the role of the double hydroxide consisting of earth-abundant nickel and chromium for improving the PEC performance of photoanodes.

[a] W. He, L. Wang, J. Yang, Prof. X. Xiang, Prof. D. Yan, Prof. F. Li State Key Laboratory of Chemical Resource Engineering Beijing University of Chemical Technology Beijing 100029 (P.R. China) E-mail: [email protected] [email protected] [b] Prof. D. Yan Key Laboratory of Theoretical and Computational Photochemistry Ministry of Education College of Chemistry, Beijing Normal University Beijing 100875 (P.R. China) [c] Dr. Y. Yang Chemical and Materials Science Center National Renewable Energy Laboratory Golden, CO 80401 (USA) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201403294.

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Full Papers Herein, we report a modification method for a TiO2 photoanode through a simple photochemical treatment in a phosphate buffer solution (PBS). Furthermore, a photoassisted modification and deposition (PMD) strategy was established to obtain TiO2/phosphated NiCr double hydroxide core/shell array photoanode (NiCr-TiO2-P) for the first time. This strategy involves two sequential processes. First, the NiCr double hydroxide was predeposited onto TiO2 array electrodes by an electrochemical process. These electrodes were subsequently modified by a PEC treatment in PBS, resulting in a core/shell photoanode with an ultrathin and amorphous shell of phosphated NiCr hydroxide. The core/shell photoanode yields a large cathodic shift of photocurrent onset potential and remarkable enhancement in oxidative efficiency close to 100 % over a wide range of potential. Moreover, it was evidenced that the enhanced PEC performance stemmed from improved charge separation and suppression of back reactions (oxygen reduction). Therefore, our findings suggest that the high performance of photoanodes can be benefited by simultaneously suppressing carrier recombination and enhancing oxidative efficiency.

Figure 1. SEM images of (A) TiO2, (B) TiO2-P, and (C) NiCr-TiO2-P. (D) TEM and energy-dispersive X-ray spectroscopy (EDX) mapping of a single NiCr-TiO2-P nanorod (scale 100 nm). The elemental mappings show the distribution of O, Ti, Ni, Cr, and P.

tanium and oxygen. The appearance of the samples after modification (TiO2-P and NiCr-TiO2-P) is almost the same as that of TiO2 observed by the naked eye. The UV/Vis absorbance spectra show slight changes to the absorption properties within the UV region for these samples (Figure S3 in the Supporting Information). A high-resolution (HR) TEM image further reveals the ultrathin overlayer outside the TiO2 nanorod for the NiCr-TiO2-P sample (Figure S4 A in the Supporting Information). The interplanar spacing of the lattice fringe is 0.32 nm, which can be assigned to d110 of rutile TiO2.[21] The nanorod is uniformly covered by a thin, amorphous overlayer with a thickness of less than 10 nm (Figure S4 B in the Supporting Information). Morphological observations suggest that the deposition layer achieved by the PMD method is much thinner and more uniform than electrodeposited Co-Pi layers, which are usually sub-micrometer-sized particles.[14a] Elemental valence and composition analyses of the overlayer on the photoanodes were obtained by means of the X-ray photoelectron spectroscopy (XPS) technique. The O 1s spectra of TiO2 and TiO2-P were deconvoluted into two peaks at binding energies (BEs) of about 530.0 and 531.7 eV (Table S1 in the Supporting Information), which correspond to oxygen (O2) in the lattice of TiO2 and absorbed H2O on the surface, respectively (Figure 2 A and B).[19a, 22] The small difference in the BE of lattice oxygen (O2) verified that modification in phosphate solution did not cause any structural changes in TiO2. It may only lead to the modification of phosphate on the surface, which is consistent with a previous report.[22] The O 1s spectra of NiTiO2-P and NiCr-TiO2-P exhibit an additional peak at 531.1 eV, which is assigned to the hydroxyl group (OH ; Figure 2 C and D). The peak intensity of O2 is largely decreased compared with that of the dominant one from OH , which suggests the hydroxide nature of the overlayer.[19a] In particular, the O 1s spectra of NiCr-TiO2-P show a minor contribution from the oxide (O2). Therefore, it can be concluded that the TiO2 nanorods are homogeneously covered by the hydroxide layer (thickness < 10 nm).

Results and Discussion Characterization of photoanodes The possible phase structures of both the modified and unmodified TiO2 electrodes were investigated by XRD. For all samples (modified and unmodified), two intensive diffraction peaks at 2q = 36.1 and 62.78 attributed to the (101) and (002) planes, respectively, of rutile TiO2 (JCPDS no. 88-1175) were observed (Figure S1 in the Supporting information). The absence of other diffractions indicates that the TiO2 nanoarray (NA) exhibits oriented growth with respect to the fluorine-doped tin oxide (FTO) substrate.[21] The XRD patterns of the sample deposited with metal hydroxides (NiCr-TiO2 and NiCr-TiO2-P) are the same as that of pristine TiO2 ; this indicates that the deposition layers on TiO2 are amorphous and/or too thin to be detected by XRD. The morphologies of the samples are shown in Figure 1. TiO2-P (TiO2 modified in PBS) displays almost the same surface appearance as that of TiO2 (Figure 1 A and B). This is a clear indication that PEC modification in PBS does not change the structure of TiO2 nanorods. After being deposited with NiCr hydroxides and modified in the PBS (denoted NiCr-TiO2-P), the TiO2 nanorods were encapsulated with a highly uniform overlayer, which showed a rougher surface (Figure 1 C). In particular, the tips of the nanorods were covered by a cap-like aggregate, which is clearly different from the structures of TiO2 and TiO2-P. It is worth noting that modification with phosphate did not affect the morphology of the NiCr-TiO2 photoanode (Figure S2 in the Supporting Information). EDX mapping analyses show the elemental distribution on the nanorod (Figure 1 D). Titanium, oxygen, nickel, chromium, and phosphorus have a homogeneous distribution throughout the nanorod. The core part shows more intensive signals from titanium, which suggests that the dominant elements of the core might be ti-

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Full Papers PEC performances of photoanodes The photocurrent response of photoanodes was studied by current–voltage scans illuminated from the front side (Figure 3 A). The onset potential was defined as the potential at the intersection point of the tangent at the maximum slope of the photocurrent and dark current curve (the tangent lines are marked in the inset of Figure 3 A).[14g, 26] The pristine TiO2 photo-

Figure 2. O 1s core level spectra of (A) TiO2, (B) TiO2-P, (C) Ni-TiO2-P, and (D) NiCr-TiO2-P.

The presence of the element phosphorus was further confirmed by P 2p spectra (Figure S5 C in the Supporting Information). The spectra of all modified samples show a clear peak at about 133.2 eV, which is characteristic of phosphorus in phosphate.[14a] An appreciable amount of phosphorus was detected with atomic contents of 2.03, 2.88, and 4.39 % in TiO2-P, NiTiO2-P, and NiCr-TiO2-P, respectively (Table S1 in the Supporting Information). The ratio of P/(Ni + Cr) in NiCr-TiO2-P is around 0.28:1, which is much lower than that of Ni3(PO4)2 (0.67: 1) or CrPO4 (1: 1), and electro- and photochemically deposited Co-Pi systems (P/Co = 0.5:1 and P/Co 1.8:1, respectively).[14a,f] This result suggests that the overlayer may not be NiCr phosphate, but phosphated NiCr double hydroxide due to the low content of phosphorus. The Ni 2p spectra show a spin-orbital splitting with Ni 2p3/2 and Ni 2p1/2 regions (Figure S5 A in the Supporting Information). The Ni 2p spectra could be assigned to hydroxides of Ni2 + and Ni3 + .[23] The peaks assigned to Ni2 + and Ni3 + species in NiCr-TiO2-P appear at 856.4 and 858.1 eV, respectively; this shows a shift of around 0.5 eV towards higher BEs, relative to those of Ni-TiO2-P. The BE values could be affected by the incorporation of chromium species to form double hydroxides.[24] The BE value of Cr 2p3/2 (577.4 eV) in NiCr-TiO2-P (Figure S5 B in the Supporting Information) is also consistent with the that reported for the hydroxide of Cr3 + .[25]


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Figure 3. (A) Current–voltage curves of TiO2 (orange), TiO2-P (black), NiCrTiO2 (blue), and NiCr-TiO2-P (red) under illumination (solid lines) and in the dark (dashed lines). Solution: 0.1 m PBS (pH 7). Scan rate: 10 mV s1. Front illumination (intensity: 100 mW cm2); inset: enlargement of the image in (A) at low potentials. (B) Dark current–voltage curves of TiO2, TiO2-P, NiCr-TiO2, and NiCr-TiO2-P. Solution: 0.1 m PBS (pH 7). Scan rate: 10 mV s1.

anode has an onset potential of about 0.46 V, which is similar to those reported in the literature.[27] After modification in PBS, the photocurrent onset potential of TiO2-P shifts about 80 mV in the cathodic direction, relative to that of TiO2. Moreover, the NiCr-TiO2-P photoanode presents a cathodic shift of the onset potential by about 208 mV (Table 1). This is an indication that NiCr-TiO2-P has significantly decreased the overpotential for PEC water oxidation. However, NiCr-TiO2 shows a much lower photocurrent density than that of NiCr-TiO2-P above 0.8 V. On 3


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Full Papers performance limit to these photoanodes in the absence of surface electron–hole recombination, and thus, enable the study of PEC properties independently.[31] H2O2 has been reported as a suitable surrogate to assess the performance limits of aFe2O3 and BiVO4 photoanodes due to its much faster oxidation kinetics than water oxidation and low overpotential.[13, 32] Herein, we examined PEC oxidation of H2O2 on modified and unmodified photoanodes, and compared the results with water oxidation (Figure 4). The photocurrent of water oxidation is much lower than that of H2O2 oxidation on a pristine TiO2 photoanode at potentials below 1.0 V (Figure 4 A). This difference gradually diminishes at higher potential because a larger

Table 1. Cathodic shifts of the onset potential for water oxidation. Photoanode TiO2 TiO2-P NiCr-TiO2 NiCr-TiO2-P

Eonset shift[a] [mV] under illumination – 80 185 208

in the dark – 430 320 410

[a] The Eonset shift towards the cathodic direction relative to TiO2 under illumination and in the dark.

the other hand, modification with monometallic nickel hydroxide (Ni-TiO2-P) leads to an inferior photocurrent response (Figure S6 A in the Supporting Information). Its photocurrent density is around 0.53 mA cm2, which is much lower than that of NiCr-TiO2-P (0.94 mA cm2) at 1.23 V. These findings verify the superiority of the photoanode modified with both double hydroxide and phosphate. The dark current for water oxidation was measured and the onset potential was compared (Figure 3 B). The TiO2-P photoanode exhibits a shift of 430 mV towards the cathodic direction, which suggests that the phosphate may play a critical role in decreasing the overpotential of water oxidation, similar to that of borate reported.[18] Moreover, phosphate, as the PAE, facilitates the PCET process in the water oxidation reactions.[3a, 28] Recent Figure 4. Current–voltage curves of (A) TiO2, (B) TiO2-P, (C) NiCr-TiO2, and (D) NiCr-TiO2-P. The photocurrent for H2O2 studies reported that doping of oxidation (dashed line) and water oxidation (solid line) was measured in the presence or absence of 0.1 m H2O2 as phosphate into semiconductors a hole scavenger in 0.1 m PBS. Scan rate: 10 mV s1. (e.g., BiVO4, TiO2, and Fe2O3) improved their PEC photocurrent bias facilitates electron–hole separation. After TiO2 was modiresponse.[22, 29] However, none of these works observed a decrease in the overpotential. The NiCr-TiO2-P and NiCr-TiO2 phofied in PBS, the extent of water oxidation on TiO2-P was closer toanodes show cathodic shifts of 410 and 320 mV, respectively. to that of H2O2 oxidation (Figure 4 B). Furthermore, NiCr-TiO2-P This observation confirms that the modification of NiCr hydroxhas almost the same photocurrent response towards the oxiide does not cause an additional catalytic effect on electrodation of both H2O2 and water (Figure 4 D). It reveals almost chemical water oxidation. In other words, NiCr hydroxide on complete suppression of carrier recombination on the NiCrTiO2 does not act as a catalytically active site for oxygen-evolvTiO2-P photoanode/electrolyte interface and highly efficient ing reactions.[30] In contrast, Ni-TiO2-P has a smaller cathodic utilization of photogenerated holes involved in water oxidation.[12] As a control, the photocurrent of water oxidation on shift of about 140 mV in the dark (Figure S6 B in the Supporting Information). This could result from the unfavorable effect the NiCr-TiO2 photoanode is much lower than that of H2O2 oxiof nickel hydroxide on water oxidation, as discussed below. dation (Figure 4 C), which indicates that the phosphate species The addition of a hole scavenger to a water oxidation have a pivotal effect on the suppression of surface electron– system is a useful method to evaluate photogenerated elechole recombination. tron–hole separation efficiency.[13] It could reveal the ultimate

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Full Papers The sample modified with a nickel hydroxide overlayer also showed a large difference between the oxidation current of H2O2 and water at higher potentials (Figure S6 C in the Supporting Information). Hence, the involvement of chromium in the hydroxide shell is critical for improving electron–hole separation. Double-metal hydroxides could lead to high dispersion of metallic ions within the structural framework, which favors the efficient exposure of the active sites. Moreover, the incorporation of chromium may lead to the formation of trap states; thus slowing down the recombination rate of electrons and holes.[33] As to the kinetics of PEC water oxidation on a photoanode, the photocurrent density resulting from absorbed light is lost to bulk recombination (Jbr), leaving a charge-separation photocurrent density (Jsep). Jsep can be further lost by surface recombination (Jsr) and the remaining photocurrent participates in substrate oxidation reaction (Jox). As a result, the efficiency of substrate oxidation (fox) by surface-reaching holes equals 1Jsr/Jsep. The photocurrent density arising from substrate oxidation (JPEC) can be formulated as JPEC = Jsepfox.[14d, 34] In the case of rapid and easy substrate oxidation with H2O2, surface recombination can be considered to be completely suppressed, and thus, fox = 1. Consequently, the photocurrent density in the oxidation of H2O2 is expressed as JPEC = JH2O2 = Jsep. The oxidation efficiency, fox, can be calculated from Equation (1): ox ¼ JH2 O =JH2 O2

reaching holes into the solution species. This finding confirms the assumption that modification facilitates water oxidation by suppressing surface-reaching hole recombination. The efficiency was also compared with the state-of-the-art Co-Pi-modified photoanodes. For example, the fox value of Co-Pi/BiVO4 is around 0.9 from 0.9 to 1.4 V.[14d] In another study, the fox value of Co-Pi/W:BiVO4 reached 0.9 at 0.8 V (versus a reversible hydrogen electrode (RHE)) and remained nearly 1.0 from 0.9 to 1.4 V.[13] Therefore, the PEC oxidative efficiency of the photoanode developed herein is comparable to or even higher than that of the excellent Co-Pi modified photoanodes. As a control, NiCr-TiO2 shows a fox value of 0.9 at 0.4 V. However, the fox value dramatically decreases to 0.65 at 0.6 V. At higher potentials, the fox value of NiCr-TiO2 is much lower than that of NiCrTiO2-P. Additionally, Ni-TiO2-P has a lower fox value at a potential above 0.6 V (Figure S6 D in the Supporting Information). The findings reveal that severe carrier recombination occurs on both samples (NiCr-TiO2 and Ni-TiO2-P). Mechanism considerations Comparing the transient current of photoanodes is a way of exploring the effect associated with surface electron–hole recombination.[13] Pristine TiO2 showed a clear anodic photocurrent spike compared with the other samples when exposed to light (Figure S7 in the Supporting Information). These current spikes represent the recombination of accumulated surface holes with electrons from the conduction band due to slow water oxidation kinetics.[32] The anodic photocurrent spikes of NiCr-TiO2-P are nearly the same as that of TiO2-P, which suggests that there could be other reasons for the cathodic shifts of Eonset in NiCr-TiO2-P and the enhancement of fox. To characterize the electron transport properties at the photoanode/electrolyte interface during PEC water oxidation, electrochemical impedance spectroscopy (EIS) was performed at 1.23 V under illumination (Figure S8 in the Supporting Information). The charge-transfer resistance across the electrode/electrolyte interface (Rct) of NiCr-TiO2-P was 903 W, which was much smaller than the values of TiO2 (5319 W) and TiO2-P (4287 W). As a result, the electron conductivity is largely enhanced after modification with a phosphated NiCr hydroxide overlayer. In comparison, the Rct values of both NiCr-TiO2 (8900 W) and Ni-TiO2-P (9645 W) are much larger than that of NiCr-TiO2-P, which indicates that the inferiority in PEC performance of the first two samples can be partly caused by much slower charge transfer on the electrode/electrolyte interface. The Mott–Schottky plots of pristine TiO2 and NiCr-TiO2-P are displayed in Figure 6. Both samples show positive slopes in the corresponding plots, which reveal that they are n-type semiconductors.[35] No negative slope was present in the Mott– Schottky plots, which indicated that the NiCr-TiO2-P photoanode did not exhibit a p–n junction feature associated with nickel oxide/hydroxide, which was reported as a p-type semiconductor.[19b, 36] The flat band potential (EFB) and carrier density (Nd) were calculated based on Equations (2) and (3), respectively.[27]

ð1Þ

in which JH2O is the photocurrent density measured towards H2O oxidation on photoanodes and JH2O2 is the photocurrent density towards H2O2 oxidation on the TiO2 photoanode.[13] TiO2 has a fox value of less than 0.6 at potentials below 0.8 V (Figure 5). In comparison, the fox values of TiO2-P and NiCrTiO2-P reach 0.95 and 0.9, respectively, at 0.8 V. The fox value of NiCr-TiO2-P remains almost unity in the potential range of 1.0– 1.8 V, which is indicative of the complete injection of surface-

Figure 5. The oxidation efficiency of the surface-reaching holes injected into the solution species (fox) for TiO2 (orange circles), TiO2-P (black triangles), NiCr-TiO2 (blue triangles), and NiCr-TiO2-P (red squares) under PEC water oxidation conditions.


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Figure 7. Reduction of the dark current on TiO2-P and NiCr-TiO2-P with bubbling of different gases (N2 or O2). Scan rate: 10 mV s1. Solution: 0.1 m PBS (pH 7).

Figure 6. Mott–Schottky plots of TiO2 and NiCr-TiO2-P. Solution: 0.1 m PBS (pH 7).

1 2 kT ¼ ðE EFB Þ C 2 Nd ee0 e e "

Nd ¼

ð2Þ (Figure 7). Currents of both TiO2-P and NiCr-TiO2-P with N2 bubbling were lower than those with O2 bubbling. The slight cathodic current response under N2 bubbling may be caused by gas leakage due to an imperfect air-tight cell. However, TiO2-P showed a markedly reduced current with O2 bubbling into the electrolyte; this indicates that TiO2-P is sensitive to the O2 reduction reaction. It is worth noting that NiCr-TiO2-P shows a negligible O2 reduction current, which confirms that the O2 reduction reaction is largely suppressed on the surface of NiCrTiO2-P. It is known that O2 reduction is an undesired back reaction that can deteriorate the water oxidation performance. Hence, the larger cathodic shift of the onset potential on the NiCr-TiO2-P photoanode compared with that of TiO2-P is attributed to the remarkable suppression of the O2 reduction reaction. Based on the experimental results and analyses outlined above, we propose a possible scheme to demonstrate the cathodic shift of the onset potential and enhanced oxidation efficiency (Scheme 1). For n-type semiconductors, when the semiconductor is in contact with the electrolyte containing a redox couple (O2/H2O), electrons flow across the semiconductor/electrolyte interface until the equilibrium is established.[43] The electric field formed in the equilibrium leads to energy band bending, which directs photogenerated holes to move towards the surface through the semiconductor/electrolyte junction.[44] During PEC water oxidation on the photoanode, bulk recombination (Jbr), surface recombination (Jsr), and the O2 reduction reaction (Jor) are major loss pathways and result in decreased water oxidation performance (Jox), especially at lower potentials. After modification with the phosphated NiCr hydroxide (NiCr-Pi) ultrathin layer, the photoanode presents a significant cathodic shift of the photocurrent onset potential and a much higher fox value at a low potential. This is ascribed to the simultaneous suppression of surface carrier recombination and oxygen reduction reactions, which facilitates the reactivity of surface-reaching holes. The phosphate, as the PAE, subtracts protons at the solid/electrolyte junction, and pro-

#

2 dE ee0 e d C12

ð3Þ

in which C is the space charge capacitance in the semiconductor, Nd is the electron carrier density, e is the elemental charge value, e0 (8.86 1012 F m1) is the permittivity of a vacuum, e is the relative permittivity of the semiconductor (for rutile e = 170),[37] E is the applied potential, T is temperature, and k is the Boltzmann constant.[38] The EFB value is estimated by extrapolating the Mott–Schottky plots in the dark to 1/C2 = 0 to obtain the intercept.[35b, 39] TiO2 photoanodes doped with metallic ions have been reported to cause a positive shift of the EFB with significantly increased carrier density.[27, 40] However, in this work, the EFB was almost unchanged after surface modification. Furthermore, only a slight increase in the value of Nd was observed in NiCr-TiO2-P (18.3 1016 cm3) relative to TiO2 (8.2 1016 cm3). This result confirmed that the hydroxide overlayer barely affected the doping level or carrier density of the TiO2 photoanode.[12] Recently, it was reported that the back reactions at the surface of the photoanode could decrease the oxidation efficiency by reducing the photo-oxidation products.[41] Therefore, the suppression of back reactions on the surface of the photoanode could lead to a cathodic shift of the onset potential.[26, 39] Meanwhile, Bard et al. proposed that a thin overlayer of amorphous TiO2 could block the undesired back reduction on the surface of BiVO4, and thus, lead to an enhancement of the PEC performance.[41] Domen et al. reported that the decoration of CuCrOx nanoparticles could suppress O2 reduction on GaN:ZnO, and thus, increase the photocatalytic activity towards water splitting.[42] To explore the mechanism of a cathodic shift of the onset potential in NiCr-TiO2-P, we carried out gas bubbling experiments to measure the O2 reduction current. The reduction currents of TiO2-P and NiCr-TiO2-P were measured with bubbling of different gases in the dark

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Full Papers Experimental Section Materials Tetrabutyl titanate, Cr(NO3)3·9 H2O, Ni(NO3)2·6 H2O, KNO3, KH2PO4, and K2HPO4·3 H2O were purchased from Sinopharm Chemical Reagent (Beijing Co. Ltd.) All reagents were of analytical grade and were used without further purification. Deionized water was used throughout the experiments. FTO substrates (F: SnO2, 15 mW cm2) were received from Nippon Sheet Glass Co., Ltd.

Preparation of TiO2 NAs Scheme 1. Schematic illustration of carrier transport and water oxidation processes on a core/shell photoanode. NiCr-Pi (blue semicircles): the phosphated NiCr hydroxide overlayer, Jbr : current loss of bulk recombination, Jsr : current loss of surface recombination, Jor : current loss of O2 reduction reaction, Jox : current density of water oxidation, VB: valence band, CD: conduction band.

The 1D TiO2 NAs were grown on FTO substrates through a hydrothermal process, as described by Aydil et al.[21] In a typical synthesis, deionized water (3 mL) was mixed with concentrated hydrochloric acid (3 mL; 36.5 % by weight) in a Teflon-lined stainlesssteel autoclave (volume of 25 mL). The mixture was stirred for 5 min before the addition of tetrabutyl titanate (1 mL). One piece of FTO substrate was placed at an angle against the wall of the Teflon liner with the conducting side facing down. Hydrothermal growth was conducted at 150 8C for 5 h in an electric oven. After being rinsed with deionized water and dried in air, the prepared samples were annealed at 450 8C for 30 min in air. The TiO2 NAs sample was denoted as TiO2.

motes water oxidation associated with a PCET process.[16, 18, 29b, 45] Moreover, photocurrent stability is a critical factor for practical applications of the photoanodes. The stability of NiCr-TiO2P was evaluated at 1.23 V under illumination (Figure S9 in the Supporting Information). Only a slight decay ( 3 %) in the photocurrent was observed after 6 h of operation on the core/ shell photoanode, which was indicative of its high stability for PEC water oxidation and good resistance to photocorrosion. Photocurrent fluctuations in the course of monitoring were caused by gas bubble release from the surface of the photoanode.

Electrodepostion of the nickel–chromium hydroxide layer A nickel–chromium hydroxide overlayer was deposited onto TiO2 NAs through a potentiostatic deposition method by using an electrochemical workstation (CHI 660C, CH Instrument Co. USA). The TiO2 NAs were used as a working electrode. An Ag/AgCl electrode and Pt wire were used as the reference and counter electrodes, respectively. The electrolyte was prepared by dissolving 7.5 mm Cr(NO3)3·9 H2O, 22.5 mm Ni(NO3)2·6 H2O, and 0.3 m KNO3 in distilled water (150 mL). Potentiostatic deposition was carried out at a potential of 1.0 V versus Ag/AgCl for 50 s. The resulting sample was rinsed with deionized water and dried in air. The electrode after deposition was named NiCr-TiO2. TiO2 deposited with only a nickel hydroxide overlayer was prepared by following the same procedure without adding Cr(NO3)3·9 H2O.

Conclusions We presented a simple photoassisted modification to the TiO2 photoanode in PBS, which led to an enhancement in PEC water oxidation efficiency. The findings revealed that the phosphate species on the surface of the photoanode could lower overpotential, and thus, suppress surface electron–hole recombination. Furthermore, the photocurrent onset potential was shifted by about 200 mV in the cathodic direction on a core/ shell array photoanode with an ultrathin and amorphous phosphated double-hydroxide shell. The large cathodic shift could be due to the diminished accumulation of surface-reaching holes and the suppression of O2 reduction back reactions. The water oxidation efficiency (fox) was maintained at nearly 100 % throughout the potential range of 1.0–1.8 V, as a result of simultaneous depression of surface carrier recombination and electron back reactions. Therefore, this work not only supplies a facile and efficient strategy to obtain ultrathin-layer-covered core/shell photoanodes, but also provides a deeper understanding of the key role of the double-metal hydroxide overlayer on the reduction of carrier recombination and back reactions. Further work is still under way to develop other earth-abundant metal hydroxides for high-efficiency PEC water splitting. ChemSusChem 0000, 00, 0 – 0

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Preparation of core/shell array photoanodes by photoassisted electrochemical modification The modification was carried out in a three-electrode cell. The electrolyte was 0.1 m PBS (pH 7) at room temperature. TiO2, TiO2 deposited with nickel–chromium hydroxide, or nickel hydroxide were used as the working electrode. The electrode was irradiated from the front side by using a 300 W xenon lamp (light intensity: 100 mW cm2). During the process, the working electrodes were kept at a constant potential of 0 V versus Ag/AgCl for 30 min. After modification, the samples were thoroughly washed with deionized water. The electrodes were named TiO2-P, NiCr-TiO2-P, and Ni-TiO2-P.

Sample characterization XRD measurements: The XRD patterns of the samples were collected on a Shimadzu XRD-6000 diffractometer (40 kV, 30 mA, graphite-filtered CuKa radiation, l = 0.15418 nm), with a scan range between 3 and 708.

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Full Papers Acknowledgements

UV/Vis absorption spectra measurements: Solid-state UV/Vis absorption spectra were recorded at room temperature by using a Shimadzu UV-3000 spectrometer equipped with an integrating sphere attachment by using BaSO4 as a background sample. The signal from the FTO conductive glass was subtracted.

This work was supported by the 973 Program (grant no. 2011CBA00506), the National Natural Science Foundation of China (grant no. 21376020), the Beijing Natural Science Foundation (grant no. 2152022), the Program for Changjiang Scholars and Innovative Research Team in University (grant no. IRT1205), and the Fundamental Research Funds for the Central Universities (YS1406). X.X. thanks support from the Beijing Engineering Center for Hierarchical Catalysts.

SEM analyses: The morphologies of the samples were investigated by using a scanning electron microscope (SEM, Zeiss SUPRA 55) with an accelerating voltage of 20 kV. HRTEM analyses: HRTEM images were recorded by using a JEOL JEM-3010 microscope. For TEM observations, the samples were scraped from the FTO substrate and ultrasonically dispersed in ethanol, and then a drop of the suspension was deposited onto a carbon-coated copper grid followed by evaporation of the solvent in air.

Keywords: electrochemistry · nanostructures · oxidation · photochemistry · sustainable chemistry

Energy dispersive X-ray spectroscopy (EDS) mapping analyses: EDS mapping was performed by using a JEOL JEM-2010F microscope combined with an EDX (OxFord X-MaxN 80-TLE) spectrometer.

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XPS measurements: The XPS spectra were recorded on a Thermo VG ESCALAB MK II X-ray photoelectron spectrometer at a pressure of about 2 109 Pa by using AlKa X-ray as the excitation source (1486.6 eV). The positions of all BEs were calibrated by using the C 1s line at 284.8 eV.

Electrochemical and PEC experiments All electrochemical and PEC measurements were operated in a typical three-electrode configuration by using the prepared samples as working electrodes, Pt wire as the counter electrode, and Ag/AgCl as a reference electrode. All PEC and electrochemical measurements were conducted in 0.1 m PBS (pH 7), unless otherwise stated. The measured potential was converted into that versus RHE (in V) according to Equation (4): E RHE ¼ E Ag=AgCl þ E Ag=AgCl vs: NHE þ 0:059pH

ð4Þ

in which EAg/AgCl vs NHE is 0.197 V (versus a normal hydrogen electrode (NHE)) at 20 8C. All potentials refer to RHE, unless otherwise noted. The area of the samples was 2 cm2 and they were clamped with a copper tape and used as photoanodes for water oxidation in a PEC cell. They were illuminated from the front side by using a 300 W xenon lamp (illumination intensity: 100 mW cm2). EIS was performed by using the electrochemical workstation mentioned above at 1.23 V versus RHE under illumination with a 0.1 V amplitude perturbation between 100 000 and 0.1 Hz. The impedance data measured were fitted to an appropriate equivalent circuit by using ZView software (version 3.2c) to derive the resistance values. Mott–Schottky plots were measured in 0.1 m PBS (pH 7) at a frequency of 5000 Hz in the dark. The dark current of the electrodes with gas bubbling was measured in an air-tight electrochemical cell. Before measurements were taken, N2 or O2 gas was bubbled into the electrolyte for 30 min and maintained until the end of the measurement. PEC measurements with H2O2 as a hole scavenger were recorded in 0.1 m PBS electrolyte (pH 7) with the addition of 0.1 m H2O2.

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Received: November 19, 2014 Revised: January 21, 2015 Published online on && &&, 0000

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FULL PAPERS W. He, Y. Yang, L. Wang, J. Yang, X. Xiang,* D. Yan,* F. Li

You need to wear a coat! A core/shell array photoanode with an ultrathin overlayer is obtained by a photoassisted modification and deposition (PMD) strategy, which results in the simultaneous suppression of surface carrier recombination and back reactions during photoelectrochemical water oxidation.

&& – && Photoelectrochemical Water Oxidation Efficiency of a Core/Shell Array Photoanode Enhanced by a Dual Suppression Strategy

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