Ni-Doped Overlayer Hematite Nanotube: A Highly Photoactive

Oct 29, 2012 - ... Overlayer Hematite Nanotube: A Highly Photoactive Architecture for ..... by a facile “top down” method for application in photo...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/JPCC

Ni-Doped Overlayer Hematite Nanotube: A Highly Photoactive Architecture for Utilization of Visible Light Weiren Cheng,† Jingfu He,† Zhihu Sun, Yanhua Peng, Tao Yao, Qinghua Liu,* Yong Jiang, Fengchun Hu, Zhi Xie, Bo He, and Shiqiang Wei* National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, P. R. China ABSTRACT: To efficiently transform absorbed photons to chemical energy is highly desired for the full utilization of visible light in solar hydrogen generation process. Here, a highly active photoanode consisting of a thin NixFe2−xO3 overlayer on the surface of hematite nanotube has been constructed to raise the utilization of the photoexcited carriers by Fe2O3 in the visible spectrum. We find that the obtained overlayer photoanodes promote the charge migration of photogenerated carriers to the surface, accelerating surface oxygen evolution and avoiding low-energy photoexcited holes recombination at the semiconductor−liquid junction. Relative to the pristine Fe2O3 photoanodes, a sustainably high incident photon to electron conversion efficiency from 40% at 400 nm until 10% at 500 nm is observed for Ni-doped overlayer hematite, yielding ∼280% enhancement of the photoconversion efficiency. Our results provide some guidance for the future design and optimization for the structure of photoanode.

■

INTRODUCTION Effectively transforming solar energy into chemical energy in the form of hydrogen is proposed to satisfy the increasing global energy demand.1,2 To achieve high transfer efficiency, great interests have been paid to look for semiconductor materials with the ability of fully utilizing visible light,3,4 which is the highest power density region of solar energy. Hematite (α-Fe2O3) with optimal optical band gap of 2.0−2.2 eV has attracted much attention due to the capability of absorbing more than half of the visible light in the solar spectrum.5 However, the oxygen evolution in the process of water oxidation by Fe2O3 suffers from a very low quantum yield, which greatly restricts the energy conversion efficiency.6 It is well recognized that this standing obstacle mainly originates from the short hole diffusion length (2−3 nm) in the bulk7 and the quick hole recombination on the surface of Fe2O3 photoanodes.8 In the past several decades, many efforts have been devoted to improving the conductivity of hematite via element doping such as Ti,9 Si,10 and Sn,11 and these impurity dopants are certainly beneficial for the enhancement of photocurrent. However, the charge carrier diffusion length is still on the order of 10 nm even for the classic work of Si-doped hematite;12 thus, only the charge carriers close to the surface could be successfully transferred to the semiconductor−liquid junction (SCLJ). Besides impurity doping, synthesis of the nanotube morphology of hematite photoanodes is another strategy to efficiently utilize photoexcited carriers. It has been revealed that the nanotube structure can markedly shorten the path of photogenerated carriers from bulk to surface of Fe2O3, significantly increasing the incident photon to electron © 2012 American Chemical Society

conversion efficiency (IPCE) with maximum value around 10% in its absorption range.13,14 For now, the common wall thickness of nanotube is around 7−15 nm, larger than the hole diffusion length, and the further decrease of wall thickness conflicts with the necessity of enough absorption length (400 nm). It is not easy to combine element doping with nanotube morphology to improve carrier mobility of hematite, and even if it were realized, there would still be rapid holes recombination at surface that acts as another factor limiting the energy conversion quantum yield in the visible light region. Recently, new methods of surface deposition, such as Co-based catalyst15 and Al2O3,16,17 have been reported to avoid holes accumulation or prolong the lifetime of photocarriers to inhibit surface recombination. But these treatments can only effect several nanometers (∼10 nm) from the surface and encounter new kinetic bottleneck symptoms because of the poor interfacial transition. Thus, to solve the holes diffusion problem and to decrease the recombination of photocarriers, it might be a good scheme to match the transfer path with the holes diffusion length via growth of an epitaxial layer on the nanotube surface. It is well-known that Ni and Fe can easily form a solid solution in common oxide, and Ni hydr(oxy)oxide surface structure exhibits the highest oxygen evolution reactivity among 3d transition metals.18 According to these facts, we propound the design of forming an NixFe2−xO3 overlayer on surface of hematite nanotube to enhance the quantum yield. Received: July 7, 2012 Revised: September 26, 2012 Published: October 29, 2012 24060

dx.doi.org/10.1021/jp306738e | J. Phys. Chem. C 2012, 116, 24060−24067

The Journal of Physical Chemistry C

Article

Figure 1. (a) IPCE (inset plot displays relative ratio of IPCE of pristine and Ni-doped samples), (b) UV−vis absorption spectra, (c) current density (inset plot is photochemical stability curves), and (d) PCE curves of pristine, Ni-deposited, and Ni-doped samples. The pink dashed line shown in (c) corresponds to the standard potential of O2/H2O couple.

5 kHz in 1 M KOH (pH 13.6). Electrochemical impedance spectroscopy (EIS) measurement was carried out in the same workstation with frequency range of 0.1−100 kHz under illumination condition at the potential of 0.45 V vs Ag/AgCl. A field-emission scanning electron microscope (FESEM: JEM2100F, USTC, China) was used to analyze the morphology of the hematite nanotube at an accelerating voltage of 5 keV. The crystalline structures of the samples were characterized by X-ray diffraction (XRD). UV−vis absorbance spectra of the samples were measured using a DUV-3700 spectrophotometer equipped with an integrating sphere. The Ni and Fe K-edge X-ray absorption fine structure (XAFS) spectroscopy were recorded in the fluorescence mode at U7C beamline in the National Synchrotron Radiation Laboratory (NSRL) and at BL14W1 beamline in the Shanghai Synchrotron Radiation Facility (SSRF), China. The band structure and total energy calculations were performed using a plane-wave basis set with the projector augmented plane-wave (PAW) method19 as implemented in the VASP package.20,21 The exchange-correlation interaction was described within the generalized gradient approximation (GGA) in the form of PW91. The cutoff energy for the planewave basis was 400 eV. It is well-known that the traditional DFT method in the frame of GGA usually underestimates the band gap for semiconductors.22,23 To overcome this deficiency, the GGA plus the onsite repulsion (GGA + U) method was employed. Here, we adopted the GGA + U approach with U = 5.5 eV and U = 8.0 eV for the Fe and Ni, respectively. We changed several U-values, and the results demonstrated that U = 5.5 eV provided a reasonable description of the band gap and a better lattice constant compared to experiment.24 In this study, a 30-atom (12 Fe atoms and 18 O atoms) hematite supercell and Monkhorst−Pack k-points grid of 5 × 5 × 5 were employed.

In this work, we have synthesized hematite nanotube with NixFe2−xO3 overlayer using a combination of sonoelectrochemical anodization with electrochemical plating methods. It is found that Ni modification on hematite nanotube could enhance the photocarriers migration ability to match the wall thickness of nanotubes and avoid holes recombination at SCLJ, yielding a 1.37% photoconversion efficiency that is ∼280% higher than the efficiency of pristine Fe2O3 (0.36%). In combination with electrochemical measurements and density functional theory (DFT) calculations, the underlying mechanism for the improvement on PEC performance of Ni-doped hematite nanotube is explored.

■

EXPERIMENTAL SECTION Hematite nanotube arrays were prepared by sonoelectrochemical anodization method reported in the prior work13 and then annealed at 400 °C in oxygen for 1 h (marked as the pristine sample). To prepare Ni-doped sample, a three-electrode system was used with the pristine sample as cathode, platinum mesh as anode, and Ag/AgCl as reference electrode. As the working electrode, the pristine sample was submerged into a solution of 5 mM nickel nitrate at 1.3 V vs Ag/AgCl for 20 min (marked as the Ni-deposited sample) and then annealed at 450 °C in air for 2 h to become Ni-doped sample. Photocurrent measurements were carried out using an electrochemical workstation (Model CHI760D, CH instruments, Inc., Austin, TX) in 1 M KOH (pH 13.6) under 100 mW/cm2 AM1.5 radiation with a 300 W xenon lamp as the white light source. The monochromatic light is filtered by filter plates of different wavelengths with bandwidth of 10 nm and transmittance of 80%, and the monochromatic light power density is measured by a UV−vis irradiatometer with the accuracy of 1 μW/cm2. Mott−Schottky analysis was performed in the linear region of the C−2 plot from −0.6 to 0.4 V vs Ag/AgCl with frequency of 24061

dx.doi.org/10.1021/jp306738e | J. Phys. Chem. C 2012, 116, 24060−24067

The Journal of Physical Chemistry C

Article

Figure 2. (a) Top-view FESEM image of the Ni-deposited sample and (b) TEM image of the Ni-doped sample. Corresponding insets of (a) and (b) show the cross-sectional image of hematite nanotube arrays and HRTEM image for the marked area. (c) XRD patterns of pristine, Ni-deposited, and Ni-doped samples. (d) k2χ(k) curves (left) and FT curves (right) of Ni K-edge of NiO, Ni-deposited, Ni-doped samples, and Fe K-edge of Nideposited and Ni-doped samples. (e) Calculated DOS of Ni-doped hematite.

■

distinct than the results of other experiments11,29,30 and also higher than that of the UV region (∼3). The extra hump of Nidoped sample in Figure 1b is originated from the Ni-derived intermediate level which does not contribute much to the total energy conversion efficiency and is discussed later in XAFS and DOS analysis. The violent increase of IPCE in visible light region indicates that the structure of NixFe2−xO3/hematite nanotube not only increases the carrier migration ability but also brings advantage to utilization of carriers at low energy, significantly enhancing the utilization rate of visible light. As a matter of course, the quantum yield increasing for Ni-doped sample results in a 3-fold increase of photocurrent to 3.3 mA/ cm2 at 0.45 V vs Ag/AgCl with sufficient photoelectrochemical stability (inset of Figure 1c). Accordingly, the photoconversion efficiency could be estimated to be as high as 1.37%, almost 3fold higher than the overall energy conversion efficiency (0.36%) of the pristine sample, as shown in Figure 1d. The structural details of this sample are exhibited by a variety of detection methods to confirm the realization of our design. The SEM images in Figure 2a demonstrate a nanotube structure with an average pore diameter of 42 nm, wall thickness of 5−7 nm, and length exceeding 1 μm. According to the calculation formula in our previous work,31 the porosity of the nanotube arrays is estimated to be ∼49.6%, indicating the effective thickness of around 550 nm which guarantees sufficient light absorption. Such a structure is propitious for photogenerated electron−hole separation because of the short holes transfer path from bulk to surface and the smooth electron transport route along nanotube to the substrate. To further probe the morphology and architecture of Ni-doped hematite, the TEM and HRTEM images are performed as

RESULTS AND DISCUSSION To study the performance of Ni-modified Fe2O3 nanotube samples, IPCE and photocurrent measurements were carried out. It is of high interest that uniformly high IPCE values are observed for Ni-doped hematite nanotube sample, with the value close to 40% until 430 nm and remaining 10% at 500 nm at 0.45 V vs Ag/AgCl as shown in Figure 1a. In contrast, the pristine and Ni-deposited samples exhibit much weaker photocurrent response. The IPCEs in the whole region are all under 12%, about 10% between 350 and 400 nm, and sharply decreasing to ∼2% at wavelength >500 nm. The absorption region of hematite ranges from 200 to 600 nm as shown in Figure 1b, which covers one-third of the solar energy. However, as mentioned above, hematite is suffered from a short hole diffusion length and a high ratio of surface recombination, and the latter factor has a greater influence on the carriers with low energy, resulting in a sharp decrease of the IPCEs in the visible light region. In view of the highest power density of the visible light region in solar energy, it is easy understood that hematite suffers from an awful energy conversion efficiency even with an excellent light absorption property. Element doping such as Si,25,26 Ti,27 and Sn11 markedly increase the IPCEs of hematite in UV region, but their influences on that of visible light region are weaker. On the other hand, some of the surface treatments like deposition of Al2O316,17 or Co-Pi28 make the cathodic translation of the photocurrent without significant enhancement of photoelectroconversion efficiency. In spite of the similar absorption in Figure 1b, the IPCE of Nidoped sample is greatly enhanced compared with the pristine sample. Especially in the wavelength region of 400−550 nm, a relative ratio of ∼4.5 is observed (inset of Figure 1a), more 24062

dx.doi.org/10.1021/jp306738e | J. Phys. Chem. C 2012, 116, 24060−24067

The Journal of Physical Chemistry C

Article

decrease and intensity of coordination peak increase for the identical phase. Thus, clusters of NiO disappear after annealing. Taking into account of the thin thickness of ∼1 nm for the NixFe2−xO3 layer covering the nanotube surface, the weak peak of the second-nearest shell of Ni-doped sample may be resulted from the decreased coordination number and increased disorder of second coordination shell, suggesting that a Fe2O3-like local structure is formed on the thin NixFe2−xO3 overlayer. The Fe2O3-like local structure of thin NixFe2−xO3 overlayer is corresponding to the HRTEM image that no obviously distinct crystalline phase is observed near the surface but some structure disorder. This thin NixFe2−xO3 overlayer is conformed to our design and deemed to play an important role in improving visible light utilization. According to our DFT calculation and previous theoretical work,24 a half-filled intermediate band is formed in the band gap of hematite via Ni doping, consistent with the UV−vis absorption measurement that a new absorption peak appears at near 680 nm.24 This result also excludes the possibility of NiO and NiFe2O4 to be the main phase of the overlay, since their band gap energies are 3.6 and 2.5 eV, respectively.36,37 This intermediate band is derived from Ni atoms and provides another transfer path for photocarriers which is more conductive than in the pristine band since its transitions are permitted. The half-filled energy level will increase the carriers density due to the uplifted Fermi level which is defined as the highest occupies state,24 resulting in lower charge transfer barrier for Ni-doped sample. Covered on the surface of hematite nanotube, the NixFe2−xO3 layer will affect the conductivity of nanotube via charge exchange. Considering the depletion layer formula W = [2εε0(V − Vfb)/e0Nd]−1/2, a depletion layer of Ni-doped sample can be formed only with a thickness of material larger than 10 nm. This strongly indicates that the charge injecting length is larger than half of the thickness of nanotube wall (5−7 nm), realizing the electron injection effect of NixFe2−xO3 layer in the whole nanotube. According to the hydrogen-like model, the orbital radius of Ni-binding holes is near 1 nm, and the distance enabled to attract holes is supposed to be less than 2 nm. Thereby, a thickness of overlay of at least 1 nm is considered to fit the requirements to fully affect the pristine hematite and promote the migration of carriers. However, a larger thickness of overlay is unfavorable, since injecting a large amount of Ni into Fe compounding together can easily forms NiFe2O4, which is insulating for bulk material.38 Thus, the thickness of overlay in our design is properly modulated to about 1 nm via the variation of deposition process. On the other hand, this overlay is also expected to drive the oxygen evolution reaction and avoid surface recombination, since Ni hydr(oxy)oxide surface structure exhibits the highest oxygen evolution reactivity among 3d transition metals18 and the deposition of NiFe oxide on photoanode is recently found to be helpful for the oxygen evolution reaction.39 Compared with surface deposited of Al2O3 and Co-Pi to avoid surface recombination,15−17 the external overlay is supposed to prevent the issues of nonexcited light absorption and interfacial transfer problem. The substantial increase in the energy conversion efficiency and the effective use of low-energy visible light in this photoanode confirm that the purpose of our design via the overlay structure is basically achieved. The details of the change of electrical properties via Ni doping are displayed by electrochemical impedance measurement. The Mott−Schottky method and Nyquist plots were

shown in Figure 2b. The morphology of nanotube is wellpreserved after Ni incorporation, and no obvious boundary or adhesional particles are observed on the nanotube surface. This indicates the enhancement of photochemical activity is not owing to the morphology change of nanotube or adhesional metal oxide particles. The HRTEM image of Ni-doped hematite (inset of Figure 2b) shows something extraordinary near the surface which is not observed for pristine hematite. The atomic arrangement is uniform along the face of (110) in the internal, but this array gradually fades while approaching surface which should be originated from the influence of NixFe2−xO3 layer just like the image of titanium dioxide nanocrystals of Chen’s work32 that the missing of lattice identifies a disordered outer layer. The crystalline structures of the samples were determined with XRD patterns as shown in Figure 2c. All the XRD peaks can be well indexed to the characteristic peaks of hematite and Fe metal substrate,33 and no peaks corresponding to nickel oxide are observed for Nideposited or Ni-doped samples. The similar XRD patterns of pristine and Ni-doped samples prove that the crystalline structure has little change after Ni doping. The width of disordered layer seems to be about 2 nm in the HRTEM images, but this is probably overestimated because the bulk lattice is always covered by the disordered surface layer. In this work, we estimate the thickness of NixFe2−xO3 layer based on X-ray photoelectron spectroscopy (XPS) and XAFS fluorescence measurement. The fluorescence measurement of Nidoped hematite is performed with incident angle of 88°−89° to make sure that only the signal of nanotube arrays is collected, and calibration objects of a series of mixtures of Ni oxide and Fe oxide are measured under the same condition. By comparing the fluorescence intensity, the Ni:Fe ratio of the whole nanotube is calibrated to be 1:30. Although the XPS measurement is not suitable to clarify the Ni:Fe ratio on the surface here because of the nonplanar interfaces and may underestimate the Ni:Fe ratio by including pure hematite signal. Nevertheless, it can still provide the minimum of Ni proportion in the NixFe2−xO3. The determined ratio of Ni:Fe by XPS is 3:1, suggesting a strong Ni-rich distribution in the surface. Taking the homogeneous overlayer covered on the surface of nanotube and the thickness of the nanotube wall of 7 nm into account, the thickness of the NixFe2−xO3 layer is estimated as [(1/31)/(1/4)] × (7 nm), which is about 1 nm. Although the thickness of this layer might be thinner if the actually Ni:Fe ratio is larger, the thickness of about 1 nm is considered to be more credible refer to the HRTEM image. To clarify the atomic structure of Ni dopants, the XAFS technique was used as a sensitive local structure probe34,35 and the Ni and Fe K-edges XAFS spectra for the Ni-deposited and Ni-doped samples are shown in Figure 2d. NiO exhibits a high symmetrical cubic structure with 6 nearest Ni−O bonds and 12 next-nearest Ni−Ni bonds at distances of 2.08 and 2.95 Å, respectively. On the contrary, in Fe2O3 the six O and four Fe neighbors of the central Fe atom are divided into two groups, with Fe−O and Fe−Fe distances at 1.94 Å (3), 2.11 Å (3) for Fe−O and 2.89 Å (1), 2.97 Å (3) for Fe−Fe, respectively. Thereby, the FTs peak of the second-nearest shell of Fe2O3 is evidently lower than that of NiO. It is obvious that the Nideposited sample is in the cubic structure of NiO due to its extremely analogous EXAFS spectrum with that of NiO. Surprisingly, for the Ni-doped sample the peak intensity of the second-shell is only one-fifth of that of the Ni-deposited sample. In general, annealing makes the disorder degree 24063

dx.doi.org/10.1021/jp306738e | J. Phys. Chem. C 2012, 116, 24060−24067

The Journal of Physical Chemistry C

Article

Figure 3. (a) Mott−Schottky plots and (b) Nyquist plots of the pristine and Ni-doped samples (inset shows the equivalent circuit). (c) Photocurrent of the pristine and Ni-doped sample measured in 1 M KOH contained 0.5 M H2O2. (d) Current density (inset is the Tafel plots) and (e) chopped light chronoamperometry measurement results of pristine, Ni-doped, and catalyst/pristine samples in 1 M KOH. (f) Photoluminescence spectra of the pristine and Ni-doped samples. The pink dashed line shown in (c , d) corresponds to the standard potential of O2/H2O2 and O2/H2O couples, respectively.

the Helmholtz capacitance. For this reason, an equivalent circuit consisted of two RC elements in series, which account for the semiconductor and surface process,42 was used when fitting the impedance data of the pristine sample (see inset of Figure 3b). Charge transfers in the bulk is normally faster than that in semiconductor/liquid interface, so the low-frequency response is assigned to the semiconductor−electrolyte charge transfer resistance (R2) together with constant phase element (CPE) CPE2 while the high-frequency response is accordingly designated to electronic processes in the semiconductor with a resistance (R1) and its accompanying capacitance (CPE1). The equivalent circuit of two RC in series can fit the Nyquist plot of the pristine sample very well. Interestingly, this equivalent circuit is also most accordant among several models for the case of the Ni-doped sample because the NixFe2−xO3 layer (∼1 nm) is too thin to form space electric field which could result in another capacitance contribution. For the pristine sample, the fitted values of resistances are 559 and 8540 ohm for R1 and R2 in sequence and those of capacitance are 4.11 × 10−5 and 2 × 10−6 F for CPE1 and CPE2, respectively. For the Ni-doped sample, corresponding fitted values are 119 and 1593 ohm for R1 and R2 and 8.32 × 10−5 and 9 × 10−6 F for CPE1 and CPE2,

used to determine the carrier density, capacitance, and impedance of the samples, and the results are shown in Figure 3a,b. Carrier density can be calculated from the slope of Mott− Schottky plots using the equation Nd = (2/e0εε0)[d(1/C 2)/dV ]−1

where e0 is the electron charge, ε the dielectric constant of Fe2O3 (ε = 80),9,40 ε0 the permittivity of vacuum, Nd the donor density, and V the applied bias at the electrode. The positive slopes indicate that both pristine and Ni-doped samples show n-type behavior. The calculated electron density of pristine sample is 5.45 × 1019 cm−3, consistent with the reported results.41 The calculated electron density of Ni-doped sample is 2.39 × 1020 cm−3, 3-fold higher than that of pristine Fe2O3, confirming the conjecture of increased carriers density of matrix via Ni doping. The enhanced carrier density, which is linearly proportional to conductance according to the formula σ = ne0μ0, leads to lower impedance for Ni-doped samples. The Nyquist plots of the pristine and Ni-doped samples are fitted to reveal more details. Because of the large surface areas and high carrier density in hematite nanotube electrodes, the capacitance of space region is supposed to be comparable with 24064

dx.doi.org/10.1021/jp306738e | J. Phys. Chem. C 2012, 116, 24060−24067

The Journal of Physical Chemistry C

Article

Figure 4. Schematic of OER process. As a sketch map, it only shows the OER process taking place on the outside surface of nanotube wall covered with NixFe2−xO3 layer, and actually the same process also occurs on the inside surface.

respectively. It is notable that the fitted value of space capacitance of the Ni-doped sample is near 2 times as much as that of the pristine sample, coinciding with the Mott− Schottky analysis. Usually, the lower resistance (R) and higher CPE values represent better charge transport in bulk and at interfaces. Compared with the pristine sample, the resistance corresponding to charge transport in bulk and at surface becomes lower and the space capacitance and Helmholtz capacitance are higher via Ni doping which indicates the NixFe2−xO3 layer facilitates charge transfer in bulk and at SCLJ, ultimately resulting in increased photocurrent. Furthermore, according to the recent work of Jing group,43 a negative electrostatic field formed in the surface layers could significantly increase the lifetime of photocarriers and then facilitate the transfer of photocarriers from bulk to surface. To quantify the influence of NixFe2−xO3 layer on the matrix, 0.5 M hydrogen peroxide was added into the electrolyte solution for photocurrent measurement. It has been reported that hydrogen peroxide is an easily oxidized holes scavenger to completely suppress surface recombination.44 Therefore, the 70% raise of photocurrent of Ni-doped sample presented in Figure 3c directly corresponds to the increased quantity of carriers that successfully arrive at surface, owing to the increased carrier diffusion length. It is also remarkable that the onset potential of photocurrent of Ni-doped sample cathodically shifts by ∼100 mV, and the slope of photocurrent of Ni-doped sample is 2−3 times as much as that of the pristine sample (see Figure 1c). Since the quantity of arrival carriers is enhanced by a factor of 70%, we deduce that the oxygen evolution reaction rate is doubled. To further illuminate the surface acceleration action of NixFe2−xO3, Ni-based catalyst is deposited on the surface of pristine sample as a reference sample since it can accelerate the OER kinetics.45 Though the cathodic shift range of onset potential of dark current for Ni-doped sample is less than that of catalyst/pristine sample, the dark current increases rapidly and catches up with

that of catalyst/pristine sample soon, indicating higher OER rate and fewer holes accumulation at SCLJ for Ni-doped sample. The Tafel plot of pristine hematite and Ni-doped hematite is performed to further identify the accelerated surface reaction of Ni-doped hematite with scan rate of 1 mV/s within the overpotential range of 0.35−0.45 V in dark, and the result is plotted in the inset of Figure 3d. The Tafel relationship describes well the catalytic performance of electrodes where the Tafel slope is sensitive and accounts for changes in the mechanism of different photoanodes.46 The slope of the line for the Ni-doped sample is ∼110 mV/decade, which is much lower than that of the pristine sample (∼500 mV/decade) and is close to the usual value for catalyst (50−120 mV/decade), strongly indicating the redox activity of surface Ni. The reduced overpotential makes the cutoff wavelength of photoresponse red-shift by ∼80 nm for the Ni-doped sample as presented in Figure 1a, leading to the efficient utilization of the low-energy visible light. To further make clear the role of Ni dopants at surface, chopped light chronoamperometry measurement was carried out (see Figure 3e). There are distinct spikes of transient photocurrent at potentials lower than 0.4 V vs Ag/ AgCl for pristine sample while turning on/off light, corresponding to holes accumulation and recombination at SCLJ.16 In contrast, spikes of transient photocurrent of Nidoped sample disappear at low potentials similar to that of catalyst/pristine sample, indicating that Ni dopants cause fewer holes accumulation at the SCLJ. The lessened holes recombination at surface may be due to the passivated surface trap states by NixFe2−xO3 layer as well as by accelerated OER process, considering amount of trap states of Fe2+ presented on the surface of pure hematite.47 This passivation effect is exemplified by an increased photoluminescence quantum yield of Ni-doped sample. On account of the absorption depth of hematite (∼100 nm) at photon wavelengths around 500 nm,48 the photoluminescence spectra reveal characteristics of whole hematite nanotube because of thin nanotube wall of 5−7 nm. 24065

dx.doi.org/10.1021/jp306738e | J. Phys. Chem. C 2012, 116, 24060−24067

The Journal of Physical Chemistry C

Article

0.45 V vs Ag/AgCl, corresponding to the photoconversion efficiency of 1.37%. The enhancement of photocurrent is attributed to the extension structure of NixFe2−xO3 layer as well as the nanotube structure. Electron injection of the thin NixFe2−xO3 layer improves photocarriers migration ability in bulk, resulting in ∼70% raised arrival photocarriers at surface. Simultaneously, surface catalysis of this thin layer is useful for accelerating OER kinetics at SCLJ, leading to full utilization of photocarriers with low energy. Our design principle provides some useful information to the future exploit of a well performed tandem structured photoanodes.

The absorption of a photon with energy larger than the band gap creates an excitonic state in hematite, and then deexcitation process proceeds.49 Since surface trap states play a role in the nonradiative recombination of excitons, photoluminescence is generally not observed in pure hematite photoanode.50 While the pristine sample shows no presence of photoluminescence when excited at 520 nm in air without applied bias, the Nidoped sample shows a weak and broad emission beginning at 600 nm and tailing off at 675 nm (Figure 3f), indicating the passivation of surface trap states by the NixFe2−xO3 layer. Both the passivating of surface trap states and the accelerating of OER ensure efficient utilization of the photocarriers that migrate to the surface, especially the low-energy carriers derived from visible light absorption. For a long time, the photoresponse of hematite is mainly in the ultraviolet region, wasting its good absorption property. However, there are close ties between the ratio of photocurrent enhancement and visible light utilization improvement. As our experiments illuminate, the increase factor of photocurrent of Ni-doped sample is similar to that of IPCEs in the visible light region, but it is significantly higher than that in the UV light region. The uniformly high IPCE values in the visible light region benefit from the structure presented in Figure 4. Along the nanotube wall of hematite, there forms a thin NixFe2−xO3 layer of ∼1 nm for Ni-doped sample with a large mass of Ni dopants occupying the surface sites. Electron injecting by NixFe2−xO3 layer markedly increases the carrier density, leading to lower carrier transfer impedance and matching the carrier diffusion length with thickness of nanotube wall, so more photogenerated holes successfully transfer from bulk to surface by hopping through FeIII/FeIV valence change.10 The half-filled Ni-derived band in the middle of the bandgap (see Figure 2e) also provides a transfer path for the photocarriers. When arriving at the surface, holes are captured by NiIII sites immediately and caught in the intermediate band, avoiding recombination with the trap states of Fe2+ or electron at the surface. This intermediate band also shows the ability to drive the OER process, since the IPCEs results exhibit that irradiation of ∼680 nm, which corresponds to photocarriers excited from intermediate band to conduction band, brings in photoresponse for hematite as well. Thanks to the accelerated OER kinetics by NixFe2−xO3, the excited carriers quickly transform to chemical energy, effectively avoiding holes accumulation. Furthermore, this OER acceleration effect is more obvious on holes with low energy; thus, the increase of utilization ratio in the visible region is more significant than that in the UV region. In brief, this structure lowers the barrier during carrier transfer and conversion processes and makes full use of the carriers with low energy for hematite, so the utilization ratio of visible light increases. The design of forming an extension structure on the nanotube surface is helpful to make the full use of solar energy and provides some guidance for the future design and optimization of tandem structure photoanodes of other photoelectric materials.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Q. H. Liu); [email protected] (S. Q. Wei). Author Contributions †

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 11135008, 11105151, 10979044, and 10979041) and Knowledge Innovative Program of The Chinese Academy of Sciences (KJCX2-YW-N40). The authors would like to thank NSRL and SSRF for the synchrotron beamtime.

■

REFERENCES

(1) Crabtree, G. W.; Lewis, N. S. Phys. Today 2007, 60, 37. (2) Gratzël, M. Nature 2001, 414, 338−344. (3) Kudo, A.; Kato, H.; Tsuji, I. Chem. Lett. 2004, 33, 1534−1539. (4) Murphy, A. B.; Barnes, P. R. F.; Randeniya, L. K.; Plumb, I. C.; Grey, I. E.; Horne, M. D.; Glasscock, J. A. Int. J. Hydrogen Energy 2006, 31, 1999−2017. (5) Merchant, P.; Collins, R.; Kershaw, R.; Dwight, K.; Wold, A. J. Solid State Chem. 1979, 27, 307−315. (6) Lin, Y. J.; Yuan, G. B.; Sheehan, S.; Zhou, S.; Wang, D. W. Energy Environ. Sci. 2011, 4, 4862−4869. (7) Kennedy, J. H.; Frese, K. W. J. Electrochem. Soc. 1978, 125, 709− 714. (8) Sivula, K.; Formal, F. L.; Gratzël, M. ChemSusChem 2011, 4, 432−449. (9) Cesar, I.; Sivula, K.; Kay, A.; Zboril, R.; Gratzël, M. J. Phys. Chem. C 2009, 113, 772−782. (10) Kay, A.; Cesar, I.; Gratzël, M. J. Am. Chem. Soc. 2006, 128, 15714−15721. (11) Ling, Y. C.; Wang, G. M.; Wheeler, D. A.; Zhang, J. Z.; Li, Y. Nano Lett. 2011, 11, 2119−2125. (12) Miller, E. L.; Rocheleau, R. E.; Khan, S. Int. J. Hydrogen Energy 2004, 29, 907−914. (13) LaTempa, T. J.; Feng, X. J.; Paulose, M.; Grimes, C. A. J. Phys. Chem. C 2009, 113, 16293−16298. (14) Mohapatra, S. K.; John, S. E.; Banerjee, S.; Misra, M. Chem. Mater. 2009, 21, 3048−3055. (15) Zhong, D. K.; Gamelin, D. R. J. Am. Chem. Soc. 2010, 132, 4202−4207. (16) Formal, F. L.; Tetreault, N.; Cornuz, M.; Moehl, T.; Gratzel, M.; Sivula, K. Chem. Sci. 2011, 2, 737−742. (17) Zhong, D. K.; Cornuz, M.; Sivula, K.; Gratzel, M.; Gamelin, D. R. Energy Environ. Sci. 2011, 4, 1759−1764. (18) Subbaraman, R.; Tripkovic, D.; Chang, K. C.; Strmcnik, D.; Paulikas, A. P.; Hirunsit, P.; Chan, M.; Greeley, J.; Stamenkovic, V.; Markovic, N. M. Nat. Mater. 2012, 11, 550−557.

■

CONCLUSIONS In summary, we have synthesized hematite nanotube arrays with a NixFe2−xO3 overlayer by anodic oxidation and subsequent processing. The IPCEs analysis shows that covering the thin NixFe2−xO3 layer leads to greatly enhanced IPCEs in the 400−550 nm wavelength range due to the full utilization of visible light. The uniformly high IPCEs of Ni-doped sample results in a pronounced photocurrent density of 3.3 mA/cm2 at 24066

dx.doi.org/10.1021/jp306738e | J. Phys. Chem. C 2012, 116, 24060−24067

The Journal of Physical Chemistry C

Article

(19) Blochl, P. E. Phys. Rev. B 1994, 50, 17953−17979. (20) Kresse, G.; Hafner, J. Phys. Rev. B 1993, 48, 13115−13118. (21) Joubert, G. K. D. Phys. Rev. B 1999, 59, 1758−1775. (22) Liao, P. L.; Toroker, M. C.; Carter, E. A. Nano Lett. 2011, 11, 1775−1781. (23) He, J. F.; Liu, Q. H.; Sun, Z. H.; Yan, W. S.; Zhang, G. B.; Qi, Z. M.; Xu, P. S.; Wu, Z. Y.; Wei, S. Q. J. Phys. Chem. C 2010, 114, 6035− 6038. (24) Huda, M. N.; Walsh, A.; Yan, Y. F.; Wei, S. H.; Al-Jassim, M. M. J. Appl. Phys. 2010, 107, 123712. (25) Formal, L. F.; Gratzël, M.; Sivula, K. Adv. Funct. Mater. 2010, 20, 1099−1107. (26) Zhang, M. L.; Luo, W. J.; Li, Z. S.; Yu, T.; Zou, Z. G. Appl. Phys. Lett. 2010, 97, 042105. (27) Glasscock, J. A.; Barnes, P. R. F.; Plumb, I. C.; Savvides, N. J. Phys. Chem. C 2007, 111, 16477−16488. (28) Zhong, D. K.; Sun, J. W.; Inumaru, H.; Gamelin, D. R. J. Am. Chem. Soc. 2009, 131, 6086−6087. (29) Kleiman-Shwarsctein, A.; Huda, M. N.; Walsh, A.; Yan, Y. F.; Stucky, G. D.; Hu, Y. S.; Al-Jassim, M. M.; McFarland, E. W. Chem. Mater. 2010, 22, 510−517. (30) Kleiman-Shwarsctein, A.; Hu, Y. S.; Forman, A. J.; Stucky, G. D.; McFarland, E. W. J. Phys. Chem. C 2008, 112, 15900−15907. (31) Liang, S. Z.; He, J. F.; Sun, Z. H.; Liu, Q. H.; Jiang, Y.; Cheng, H.; He, B.; Xie, Z.; Wei, S. Q. J. Phys. Chem. C 2012, 116, 9049−9053. (32) Chen, X. B.; Liu, L.; Yu, P. Y.; Mao, S. S. Science 2011, 331, 746−749. (33) Zhang, Z. H.; Hossain, M. F.; Takahashi, T. Appl. Catal. B: Environ. 2010, l95, 423−429. (34) Yao, T.; Sun, Z. H.; Li, Y. Y.; Pan, Z. Y.; Wei, H.; Xie, Y.; Nomura, M.; Niwa, Y.; Yan, W. S.; Wu, Z. Y.; et al. J. Am. Chem. Soc. 2010, 132, 7696. (35) Yao, T.; Zhang, X. D.; Sun, Z. H.; Liu, S. J.; Huang, Y. Y.; Xie, Y.; Wu, C. Z.; Yuan, X.; Zhang, W. Q.; Wu, Z. Y.; et al. Phys. Rev. Lett. 2010, 105, 226405. (36) Dolia, S. N.; Sharma, R.; Sharma, M. P.; Saxena, N. S. J. Pure Appl. Phys. 2006, 44, 774−776. (37) Mohanty, P.; Rath, C.; Mallick, P.; Biswal, R.; Mishra, N. C. Phys. B 2010, 405, 2711−2714. (38) Lüders, U.; Bibes, M.; Bobo, J. F.; Cantoni, M.; Bertacco, R.; Fontcuberta, J. Phys. Rev. B 2005, 71, 134419. (39) Kleiman-Shwarsctein, A.; Hu, Y. S.; Stucky, G. D.; McFarland, E. W. Electrochem. Commun. 2009, 11, 1150−1153. (40) Gonc-alves, R. H.; Lima, B. H. R.; Leite, E. R. J. Am. Chem. Soc. 2011, 133, 6012−6019. (41) Ling, Y. C.; Wang, G. M.; Wheeler, D. A.; Zhang, J. Z.; Li, Y. Nano Lett. 2011, 11, 2119−2125. (42) Jun, H. C.; Im, B.; Kim, J. Y.; Im, Y. O.; Jang, J. W.; Kim, E. S.; Kim, J. Y.; Kang, H. J.; Hong, S. J.; Lee, J. S. Energy Environ. Sci. 2012, 5, 6375−6382. (43) Jing, L. Q.; Zhou, J.; Durrant, J. R.; Tang, J. W.; Liu, D. N.; Fu, H. G. Energy Environ. Sci. 2012, 5, 6552−6558. (44) Dotan, H.; Sivula, K.; Gratzël, M.; Rothschild, A.; Warren, S. C. Energy Environ. Sci. 2011, 4, 958−964. (45) Dinc, M.; Surendranath, Y.; Nocera, D. G. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 10337−10341. (46) James, B. G.; McAlpin, J. G.; Jamie, Y. C. C.; Matthew, L. R.; William, H. C.; Britt, R. D.; Shannon, S. S. J. Am. Chem. Soc. 2011, 133, 14431−14442. (47) Rosso, K. M.; Smith, D. M. A.; Dupuis, M. J. Chem. Phys. 2003, 118, 6455−6466. (48) Gardner, R. F. G.; Tanner, D. W.; Sweett, F. J. Phys. Chem. Solids 1963, 24, 1183. (49) Sherman, D. M.; Waite, T. D. Am. Mineral. 1985, 70, 1262− 1269. (50) Zou, B. S.; Volkov, V. J. Phys. Chem. Solids 2000, 61, 757−764.

24067

dx.doi.org/10.1021/jp306738e | J. Phys. Chem. C 2012, 116, 24060−24067