Investigation of the electrochemical and photoelectrochemical properties of Ni – Al LDH photocatalysts †

Layered double hydroxide (LDH) photocatalysts, including Ni–Al LDH, are active for the photocatalytic conversion of CO2 in water under UV light irradiation. In this study, we found that a series of LDHs exhibited anodic photocurrent which is a characteristic feature corresponding to n-type materials. Also, we estimated the potentials of photogenerated electrons and holes for LDHs, which are responsible for the photocatalytic reactions, using electrochemical techniques. The flat band potential of the Ni–Al LDH photocatalyst was estimated to be 0.40 V vs. NHE (pH = 0), indicating that the potential of the photogenerated electron is sufficient to reduce CO2 to CO. Moreover, we revealed that the flat band potentials of M–M LDH are clearly influenced by the type of trivalent metal (M) components.


Introduction
Layered double hydroxides (LDHs), also known as anionic clays, [1][2][3] have the general formula [M 1Àx 2+ -M x 3+ (OH) 2 ](A nÀ ) x/n ÁmH 2 O, where M 2+ and M 3+ are divalent and trivalent cations, respectively; A nÀ is an interlayer anion of valence n; and x denotes the molar ratio of M 3+ /(M 2+ + M 3+ ).The metal cations occupy the centers of edge-sharing octahedra, whose vertexes contain hydroxide ions that connect to form infinite two-dimensional sheets.3][24][25] We developed a photocatalytic system for the conversion of CO 2 in aqueous solution using Ni-Al LDH, 26,27 which enables the selective formation of CO (as the reduction product of CO 2 ) because of the complete suppression of H 2 evolution via the reduction of protons (H + ).The hydroxyl groups on the surface of LDH might act as an adsorption site for the dissolved CO 2 molecules in the reaction solution.In such a case, photoirradiation through Pyrex glass did not induce any photocatalytic reaction over Ni-Al LDH, whereas the use of quartz glass allowed the photocatalytic conversion of CO 2 , indicating that UV light irradiation (l o 300 nm) is necessary for this photocatalytic reaction. 28In contrast, Garcı ´a et al. reported that Zn-Cr LDH was an active photocatalyst for the oxidation of H 2 O into O 2 under visible light irradiation. 21Because LDHs show photocatalytic activities for several kinds of reactions as reported by us and others (as cited above), there must be an energy gap that can induce photocatalytic reactions under illumination.However, only a few studies have investigated the electrochemical properties of these LDHs, such as energy gap measurement, charge separation induced by photoirradiation, and potentials of photogenerated electrons and holes.Concerning the reduction of CO 2 in aqueous media, the relationship between the standard potential for CO 2 (i.e., E1(CO 2 /CO) = À0.11V vs. NHE) 29 and the potential of the photoexcited electron in the photocatalyst material must be considered carefully because it is difficult to reduce CO 2 , one of the most stable molecules, particularly in comparison with the reduction of a proton (E1(H + /H 2 ) = 0.0 V vs. NHE). 29lectrochemical and photoelectrochemical measurements are useful techniques that are widely used in the field of photocatalysis to understand the photocatalytic properties of materials, such as the distinction between n-type and p-type semiconductors, photon-to-current conversion efficiencies, energy levels of the This journal is © the Owner Societies 2016 bottom of the conduction band (hereinafter ''E CB '') and the top of the valence band (hereinafter ''E VB ''), and the energy gap of the band structure (hereinafter ''E BG ''). 30Electrochemical properties can be investigated by observing the interface between the semiconductor electrode and the electrolyte solution while applying an external bias to the former. 31,32When a certain potential is applied, the Fermi level of the semiconductor lies at the same energy as the redox potential of the electrolyte solution, and there is no charge transfer between them; thus, no band bending occurs.This potential is referred to as the flat band potential (hereinafter ''E FB ''). 33,34For n-type semiconductors, 31 the cathodic dark current is detected when the applied potential is more negative than E FB because the accumulation layer, which appears under cathodic polarization, enables electron transfer from the electrode to the electrolyte solution, similar to that of a metallic electrode.In contrast, n-type semiconductors can act as photoanodes under anodic polarization.The photogenerated hole (h + ) in the depletion layer can move toward the interface, in accordance with band bending, and can easily receive electrons from the electrolyte solution.This permits electron transfer from the semiconductor to the counter electrode through a conductor cable, which is regarded as the anodic photocurrent.The values of E FB and E BG for semiconductor photocatalysts can be determined experimentally by using the Mott-Schottky and Davis-Mott equations; the details are presented in the Experimental section.
Recently, Wei et al. reported the theoretical studies on the band edge placement of M 2+ -M 3+ LDHs (M 2+ = Mg 2+ , Co 2+ , Ni 2+ , and Zn 2+ ; M 3+ = Al 3+ and Ga 3+ ), and the relationship between the band position and the photocatalytic activity for water oxidation was discussed. 35In the present study, we have demonstrated properties such as the photoresponse character, energy gap, and potential of the photogenerated electron and hole for LDHs including Ni-Al LDH, which show the photocatalytic activities for the conversion of CO 2 in water, using experimental techniques based on photoelectrochemical considerations.

Electrode preparation
Ni-Al LDH was synthesized using a typical coprecipitation method.An aqueous solution containing NiCl 2 Á6H 2 O (99.9%, Wako) and AlCl 3 (98.0%,Wako) was slowly dropped into an aqueous Na 2 CO 3 (99.5%,Wako) solution at room temperature.The pH of the solution was kept stable between 9.9 and 10.1 by adding an aqueous NaOH (97.0%,Wako) solution using a liquid feed pump (NRP-76, NISSHIN RIKA).The resulting suspension was aged at 383 K under hydrothermal conditions using a stainless steel autoclave equipped with an inner Teflon vessel.The solid cake was collected by filtration and washed with ultra-pure water, and then dried at 383 K in air.Other combinations of metal components in LDH, such as Mg-Al, Zn-Al, Ni-Ga, and Ni-In, were fabricated via the same procedure as Ni-Al LDH using chloride salts as precursors.
For electrochemical and a photoelectrochemical measurements, LDH electrodes were prepared on a fluorine doped tin oxide (FTO) glass via an electrophoresis deposition method.100 mg of LDH powder was added into 50 mL of an acetone solution containing 10 mg of iodine as an electrolyte, and then the LDH powder was dispersed thoroughly by ultrasonication.Prior to use, FTO glass (AGC fabritech Co., Ltd) was washed with acetone and 2-propanol solution in turn.Two FTO glasses were immersed in the solution facing each other, and direct current (DC) was applied between the two FTO glasses by using an electrochemical measurement system (HZ-5000, Hokuto Denko Corp.), at 0.1 mA stable current (2 min) for measurements in an aqueous solution, and at 10.0 V stable voltage (5 min) for those in an organic solution.After drying at room temperature in air, the prepared LDH electrode (hereinafter, called the X LDH/FTO electrode, X = Ni-Al, Mg-Al, Zn-Al, Ni-Ga, and Ni-In) was heated at 473 K for 2 h in order to remove the residual iodine.As reference electrodes, TiO 2 /FTO and Ta 2 O 5 /FTO electrodes were prepared via the same procedure using TiO 2 (ST-01, Ishihara Sangyo Kaisha, Ltd) and Ta 2 O 5 (Kojundo Chemical Laboratory Co., Ltd) as powder samples.Characterization X-ray diffraction (XRD) patterns of the prepared electrodes were collected by using an X-ray diffractometer (Multiflex, Rigaku) using Cu K a radiation at an acceleration voltage of 40 kV with 0.021 per step.Scanning electron microscopy images for a sectioned view of the electrodes were captured by using a Field Emission Scanning Electron Microscope (SU-8220, Hitachi High-Technologies) at an acceleration voltage of 3.0 kV.UV/Vis diffuse reflectance spectra of LDH powder samples were measured using a UV-Visible spectrometer (V-670, JASCO) equipped with an integrated sphere accessory.The energy gap (hereinafter ''E g '') of a series of LDHs and the E BG of TiO 2 and Ta 2 O 5 were determined by the Davis-Mott equation 36 using the Kubelka-Munk function F(R N ) obtained from the diffuse reflectance spectrum, where h, n, and A are Planck's constant, frequency of vibration, and proportional constant, respectively.

Electrochemical measurements
Electrochemical measurements were performed using a three-electrode electrochemical cell consisting of the prepared LDH/FTO electrode, Ag/AgCl electrode, and Pt wire as the working electrode, reference electrode, and counter electrode, respectively.Prior to the measurements, the dissolved air in the electrolyte solution was completely removed by He gas flow.For a cyclic voltammetry, acetonitrile solution was used as a solvent, which contains 0.05 M of LiClO 4 as the electrolyte.The current value was recorded as the applied voltage was varied from À2.0 to 1.0 V vs. Ag/AgCl at a sweep rate of 10 mV s À1 using an electrochemical measurement system (HZ-5000, Hokuto Denko Corp.).For impedance measurements, an aqueous Na 2 SO 4 solution (0.1 M) was used as an electrolyte solution.The imaginary component of the impedance (Z 00 ) of the equivalent circuit including the X LDH/FTO electrode (X = Ni-Al, Mg-Al, Zn-Al, Ni-Ga, or Ni-In) was evaluated at an alternating current frequency of 72.0, 52.0, and 37.3 kHz with a sweeping applied voltage from 0.5 to À0.5 V vs. Ag/AgCl using an electrochemical measurement system (HZ-5000, Hokuto Denko Corp.).The capacitance (C) of the circuit was calculated from the imaginary component of the impedance (Z 00 ) using the relation, where p and f denote the circumference ratio and the frequency of the alternating current.The value of E FB for the working electrode was estimated by using the resulting value of C in accordance with the Mott-Schottky equation, where C and A are the interfacial capacitance and area, respectively, N D the number of donors, E the applied potential, k B Boltzmann's constant, T the absolute temperature, e the dielectric constant of the semiconductor, e 0 the permittivity of free space, and e is the electronic charge.Therefore, the value of E FB should be obtained from the intercept of the x-axis in the plot of C À2 versus the applied potential E. In the present study, the energy gap, the potential of the photogenerated electron and hole are hereinafter represented as E g , E e and E h for a series of LDHs, and the band gap energy, the potential of the bottom of the conduction band and the top of the valence band are called as E BG , E CB and E VB for TiO 2 and Ta 2 O 5 typical semiconductor photocatalysts.
The difference of potential (E) derived from the value of pH of the electrolyte solution was revised in accordance with the Nernst equation as below, where E (pH=a) and E (pH=b) are the values of potential at pH = a and b.For example, 0.00 V at pH = 0.0 should be converted into 0.41 V at pH = 7.0.Furthermore, the difference of potential with kinds of standard electrodes was converted to the normal hydrogen electrode (NHE) by using relationships as presented below for the silver-silver chloride standard electrode (Ag/AgCl) 37,38 and the standard calomel electrode (SCE). 37,38vs.NHE) À E(vs.Ag/AgCl) = 0.199 V ( 5)

Photoelectrochemical measurements
Photoelectrochemical measurements were conducted using a three-electrode electrochemical cell equipped with a quartz window.The prepared LDH/FTO electrode, the Ag/AgCl electrode, and Pt wire were used as the working electrode, reference electrode, and counter electrode, respectively.For the measurement in an organic solvent, LiClO 4 acetonitrile solution (0.05 M) containing 5 vol% methanol was used as the electrolyte solution.
In contrast, an aqueous Na 2 SO 4 solution which contains 5 vol% methanol was used for the measurement in an aqueous solution.
In both cases, the solution was thoroughly degassed by He gas flow.The working electrode (X LDH/FTO, X = Ni-Al, Mg-Al, and Zn-Al) was irradiated using a 200 W Hg-Xe lamp (SAN-EI Electric Co., Ltd) through the quartz window.The photocurrent value was collected using an electrochemical measurement system (HZ-5000, Hokuto Denko Corp.) without an external bias.For linear sweep voltammetry under photoirradiation, the applied bias was swept from À1.5 to 1.0 V vs. Ag/AgCl.shows mixed diffraction peaks of the above two samples.Two low angle peaks, 111 and 231, in the powder sample are indexed as (0 0 3) and (0 0 6) planes, respectively, which define the characteristic layer structure of the LDH powders. 1 The peak positions of these reflections correspond to the layer-to-layer thickness; that is, the interlayer distance is equal to d (0 0 3) with respect to hexagonal axes.As shown in Fig. 1B, Ni-Al LDH/FTO exhibited small peaks around the same position as those in the powder sample, indicating that Ni-Al LDH is fixed onto the FTO glass and retains its layer structure.Moreover, the interlayer distance was conserved after the deposition onto FTO, because the positions of the (0 0 3) and (0 0 6) reflections were unchanged.As previously reported, 26,27 other LDHs (Mg-Al, Zn-Al, Ni-Ga, and Ni-In LDH) were successfully synthesized (XRD patterns of powder samples of these prepared LDHs are presented in Fig. S1, ESI †).Moreover, the specific surface area of the Ni-Al LDH powder sample was estimated to be ca.100 m 2 g À1 .Fig. 2A shows the SEM images of Ni-Al LDH/FTO prepared using the electrophoresis deposition method.Regions X and Y mark the deposited Ni-Al LDH region and the FTO glass, respectively.We found that Ni-Al LDH particles with a thickness of several micrometers were deposited on the FTO glass.Regarding the enlarged views shown in Fig. 2B, flower-petal-like morphologies, which are characteristic of LDH-type materials, 1,39 were also maintained.These SEM images are consistent with the results of XRD measurements of Ni-Al LDH/FTO.Based on the XRD patterns and SEM images, the state of the Ni-Al LDH deposited on FTO glass is considered to be similar to that of the powder sample, which shows photocatalytic activity for the conversion of CO 2 in aqueous solution. 26,28ig. 3 shows the cyclic voltammogram recorded using a threeelectrode electrochemical cell.The prepared Ni-Al LDH/FTO was used as the working electrode under a He atmosphere.When the working electrode was set under cathodic polarization, the cathodic dark current was determined, indicating that electron transfer from the working electrode to the electrolyte solution occurred because of the formation of an accumulation layer at the interface of the electrode, making the electron flow possible without an electric barrier.The cathodic dark current under cathodic polarization is known to be a characteristic feature of n-type semiconductor materials.In contrast, anodic current should not be found under dark conditions for n-type electrodes because of the band bending in the depletion layer.However, two anodic current peaks were found.Because the standard potential was reported as E1(IO 3 À /I À ) = 1.08 V vs. NHE (pH = 0), 29 the first peak, at ca. 0.4 V vs. Ag/AgCl, was assigned to the oxidation of I À to IO 3 À , derived from I 2 that was used in the electrophoresis process.However, we were unable to assign the other peak, at ca. À0.2 V, to any reaction.Fig. 4A shows the photocurrent values of the Ni-Al LDH/FTO working electrode under illumination cycling through the quartz window with no applied external bias.The anodic photocurrent increased upon illumination and diminished when the illumination stopped.The use of acetonitrile solution containing LiClO 4 resulted in a higher photocurrent than that compared to the use of an aqueous Na 2 SO 4 solution.In both cases, the addition of methanol into the electrolyte solution was necessary to induce the anodic photocurrent, indicating that photogenerated holes (h + )   can oxidize methanol in the electrolyte solution and photoexcited electrons (e À ) can transfer to the counter electrode as an anodic current.As shown in Fig. 4B, the substitution of Ni 2+ in Ni-Al LDH by either Mg 2+ or Zn 2+ affected the photocurrent value, and the order of the initial photocurrent intensity was (e) Zn-Al LDH 4, (d) Mg-Al LDH 4 and (c) Ni-Al LDH.This agrees with the quantity of electrons consumed in the photocatalytic of CO 2 in water using these LDHs (powder samples) as previously shown; 26 specifically, the amount of electrons used to reduce CO 2 and H + (CO and H 2 were produced as reduction products) during 8 h of photoirradiation was 254, 110, and 68 mmol for Zn-Al LDH, Mg-Al LDH, and Ni-Al LDH, respectively (see the ESI † for the details of powder reaction: Table S1).Although Ni-Al LDH led to the evolution of the largest amount of CO and showed the highest selectivity among these M 2+ -Al LDHs, the total amount of reduction products was largest in the Zn-Al LDH because, in that case, a large amount of H 2 was evolved.The photocurrent values measured in the present study are considered to reflect the total photocatalytic activities of the LDH photocatalysts; that is, the metal components of the hydroxide sheets alter the photocatalytic activity of a series of LDH materials, and Zn-Al LDH is a good photocatalyst based on the results of the photoelectrochemical measurements.Nevertheless the photocurrent value was smaller than others; Ni-Al LDH can reduce CO 2 into CO with high selectivity (Table S1, ESI †).However, it is possible that self-oxidation of Zn-Al LDH by the photogenerated holes led to a rapid decrease in the photocurrent intensity, as reported for the case of ZnO. 40Fig. 4C shows the photocurrent value of Ni-Al LDH/FTO under cycled illumination in a photoelectrochemical cell using an aqueous Na 2 SO 4 solution as the electrolyte.In regions (f) and (k), photoirradiation was performed using a Hg-Xe lamp through a quartz window.For regions (g), (h), (i), and (j), long-pass cut-off filters UV-29, UV-31, UV-33, and UV-35, respectively, were set to control the photoirradiation wavelengths.The photocurrent intensity under photoirradiation through the UV-29 filter was much smaller than that through the quartz window.Moreover, the use of UV-31, UV-33, or UV-35 significantly diminished the photocurrent value during illumination.This result indicates that UV light irradiation of o270 nm is necessary to induce photocatalytic reactions over Ni-Al LDH, because a very low percentage of light at 270 nm can be transmitted through the UV-29 filter.Based on a Davis-Mott plot of Ni-Al LDH shown in Fig. S2 (ESI †), the band gap energy of Ni-Al LDH is estimated at 4.8 eV (=258 nm).This value is consistent with the results of the wavelength dependence measurements on the photocurrent intensity.

Results and discussion
Fig. 5 shows the linear sweep voltammograms measured while the light source was cycled on and off under a He atmosphere.Under anodic polarization, the anodic photocurrent flowed when photoirradiation started.The presence of a depletion layer at the interface of the electrode under anodic polarization enables the separation of the photogenerated charge carriers.Accordingly, for an n-type material at the positive potential, the band edges curve upwards, the hole moves toward the interface, and the electron moves to the interior of the material. 31,32Holes are high-energy species that can extract an electron from the electrolyte solution; that is, the n-type material acts as a photoanode under anodic polarization.Based on this, it can be inferred that Ni-Al LDH has electrochemical properties corresponding to an n-type material.In contrast, a small cathodic photocurrent was observed under cathodic polarization, which generally results from the p-type character.We presumed that the cathodic photocurrent was due to the self-reduction of Ni species in Ni-Al LDH: from Ni 2+ to Ni 0 (E1(Ni 2+ /Ni) = À0.26V vs. NHE (pH = 0)) 29 or from NiOOH to Ni(OH) 2 (E1(NiOOH/Ni(OH) 2 ) = 0.52 V vs. NHE (pH = 0)), 29 because it can be estimated that Ni 2+ incoporated into Ni-Al LDH was oxidized by the anodic applied potential during the electrophoresis method.Therefore, Mg-Al LDH did not show the cathodic photocurrent at that region.Moreover, the onset potential for the anodic photocurrent, based on the shape of the photocurrent profile during photoirradiation cycling, was estimated to be ca.À0.75 V vs. Ag/AgCl in this study.6A.Despite that, the E FB of Ni-Al LDH/FTO was estimated to range from À0.9 to À0.7 V vs. NHE (pH = 6.2).Moreover, the incline of the fitting line for the Mott-Schottky plot was positive (rightward inclining), indicating that the Ni-Al LDH shows n-type semiconductor-like properties, 31 as we determined before from the photoelectrochemical measurements.Fig. 6B displays the Mott-Schottky plots of (d) TiO 2 /FTO, (e) Ni-Al LDH/FTO, and (f) Ta 2 O 5 /FTO, recorded using an aqueous Na 2 SO 4 solution (pH = 6.2) as an electrolyte under 52.0 kHz of the alternating current frequency.From this result, the E FB of anatase TiO 2 was considered to be À0.55 V vs. NHE, and the value of E FB matched with those of previous reports that described the energy position of the flat band for anatase TiO 2 . 41,42imilarly, the estimated value of E FB of Ta 2 O 5 in this study was À0.92 V vs. NHE (pH = 6.2), similar to those reported in the  previous literature. 43Compared to these reference samples, the position of E FB in Ni-Al LDH was between those of TiO 2 and Ta 2 O 5 , and was determined to be À0.77V vs. NHE (pH = 6.2).This value was slightly shifted from the onset potential obtained from the photocurrent observation by sweeping the applied bias (Fig. 5).Bolts et al. and Sprunken et al. have pointed out the dependence of the onset potential of photocurrent on experimental conditions such as surface coverage with reducible adsorbates, illumination intensity, wavelength, and pH of the electrolyte solution. 44,45Concerning the present study, the results of the impedance measurements in accordance with the Mott-Schottky equation were used to determine the value of E FB .Generally, the difference in the value of E FB for n-type metal oxide semiconductors is considered to be due to the metal components, which form the conduction band.The conduction band of TiO 2 and Ta 2 O 5 mainly comprise the Ti 3d and Ta 5d orbitals, respectively.As shown in Fig. 6C, a change in the estimated E FB value was found upon the substitution of metal species of hydroxide sheets within the LDH structure.The incorporation of Ga 3+ or In 3+ instead of Al 3+ into the Ni-Al LDH for the formation of Ni-Ga LDH or Ni-In LDH clearly influenced the value of E FB , whereas the substitution of Ni 2+ with Mg 2+ or Zn 2+ did not affect the value of E FB , as shown in Fig. S3 (ESI †).Based on these results, we conclude that the potential of the photogenerated electrons for a series of M 2+ -M 3+ LDH group materials are regulated by M 3+ species within the hydroxide sheets, rather than M 2+ species.Moreover, the Mott-Schottky plot for Ni-Al LDH shifted to negative potential with increasing pH of the electrolyte solution in accordance with the Nernst equation (eqn (4)), as shown in Fig. S4 (ESI †).Fig. 7 summarizes the results of the investigation of the electrochemical properties for X LDHs (X = Ni-Al, Mg-Al, Zn-Al, Ni-Ga, and Ni-In) and reference samples (TiO 2 and Ta 2 O 5 ).Unless otherwise noted, the units of potential in this paragraph are quoted vs. NHE (pH = 0).Because the Fermi level for electrons of n-type character materials is considered to merge with the lowest potential of the conduction band, 30 the value of E FB obtained through the impedance measurements in this study is read as that of E e for the LDH photocatalysts, and that of E CB for the TiO 2 and Ta 2 O 5 typical semiconductor photocatalysts.The value of energy gap (E g ) between E e and E h for a series of LDHs, and the band gap (E BG ) between E CB and E VB for TiO 2 and Ta 2 O 5 were estimated from UV/vis diffuse reflectance spectra based on the Davis-Mott plot, as shown in Fig. S2 (ESI †).In addition, the potential of the top of the valence band (E VB ) was calculated using the values of E CB and E BG .For the case of TiO 2 , E CB and E BG were estimated to be À0.19 eV and 3.4 eV, respectively.The value of E CB was adapted to that previously reported because À0.40 V vs. SCE (pH = 0) in the literature presented by Gra ¨tzel et al. 42 must be converted to À0.16 V vs. NHE (pH = 0) in accordance with eqn (6).The E BG value determined in this study was slightly larger than the general value.For Ta 2 O 5 , the estimated values of E CB and E VB were, respectively, À0.56 V and 3.46 V, which are consistent with the results of electrochemical measurements in the literature reported by Domen and co-authors. 43Based on these results for the reference samples, we suggest that the electrochemical character of X LDH (X = Ni-Al, Mg-Al, Zn-Al, Ni-Ga, and Ni-In) photocatalysts is as follows.For Ni-Al LDH, which has been reported as a photocatalyst for the selective conversion of CO 2 into CO in our previous studies, 26,28 E e was À0.40 V, thereby satisfying the required potential for the reduction of CO 2 to CO; however, it was not enough for the oneelectron reduction of CO 2 (E1(CO 2 /CO 2 À ) E À2.0 V vs. NHE). 46om this result, it can be inferred that the Ni-Al LDH photocatalyst might reduce CO 2 into CO in a two-electron process.
On the other hand, E h of Ni-Al LDH was found to be 4.39 V, significantly more positive than the value of E VB for typical oxides such as TiO 2 and Ta 2 O 5 .Our experimental results for electrochemical properties of LDH photocatalysts were not fully consistent with the results of the theoretical studies. 35As is often the case, the disagreement between experimental and theoretical results caused by the differences in limitations of the two approaches should be considered in the near future.However, this potential is sufficient to oxidize Cl À into Cl 2 , as shown in Fig. 7 (E1(Cl 2 /Cl À ) = 1.39 V). 29 We previously insisted that Cl À in the reaction solution can act as an effective hole scavenger for the photocatalytic conversion of CO 2 in an aqueous solution and Cl À should be oxidized into Cl 2 , which is immediately converted into hypochlorous acid (HClO). 28By carrying out electrochemical investigations, we inferred that photogenerated electrons and holes in Ni-Al LDH are capable of reducing CO 2 into CO and oxidizing Cl À into Cl 2 .The changes in the value of E h were determined by substituting Ni 2+ in Ni-Al LDH with Mg 2+ or Zn 2+ , whereas the position of E e was not influenced by the type of divalent metal (M 2+ ) in M 2+ -Al LDH, as mentioned above.In contrast, the change of trivalent metal clearly affected the position of E e ; for example, the incorporation of In 3+ shifted to a negative value compared to Al 3+ , while the E of Ni-Ga LDH was found to have a positive potential, similar to the value of E CB for TiO 2 .By considering thermodynamic aspects, the reduction of protons to H 2 (E1(H + /H 2 ) = 0.0 V), 29 which competitively takes place in the photocatalytic conversion of CO 2 in an aqueous media, is relatively favorable in comparison to the reduction of CO 2 into CO (E1(CO 2 /CO) = À0.11V) 29,46 because of their reduction potentials.The overpotential of the photoexcited electron toward the reduction of CO 2 into CO (DE OP ) is presented in accordance with eqn (7), as shown below.
where E1(CO 2 /CO) and E e are the standard potential of CO 2 reduction to CO and the potential of the photogenerated electron, estimated from E FB , respectively.Hence, Ni-In LDH, whose photoexcited electron has a large overpotential in the reduction of CO 2 into CO (DE OP = 0.55 V) should be more advantageous for the selective reduction of CO 2 than Ni-Al LDH (DE OP = 0.29 V).However, the results of the photocatalytic conversion of CO 2 in water using powder samples indicated that Ni-Al LDH, whose E e is more positive than that of Ni-In LDH, shows a higher selectivity toward CO 2 reduction among the reduction products (65.7% of the selectivity toward CO 2 reduction among the total reduction products; CO and H 2 ) rather than Ni-In LDH (less than 5%).In such a case, Ni-In LDH evolved large amounts of H 2 as the reduction product of H + , indicating that the photogenerated electrons were mainly consumed in the reduction of H + .In other words, Ni-Al LDH enabled the selective conversion of CO 2 in spite of its relatively positive E e .This correlation clearly indicates that the selectivity toward CO 2 reduction and the inhibition of proton reduction to H 2 were determined by not only the overpotential of photogenerated electron but also the chemical properties and morphologies.Herein, we have clarified the photocurrent character and electrochemical properties such as potentials of photogenerated electrons and holes for a series of LDHs, including Ni-Al LDH, through electrochemical and photoelectrochemical measurements.

Conclusion
A series of LDHs (Ni-Al, Mg-Al, and Zn-Al LDH) were found to produce anodic photocurrent under no external bias, and the order of the initial photocurrent intensities were in accordance with the amount of electrons consumed in the photocatalytic conversion of CO 2 in water using these LDH powder samples.Ni-Al LDH had electrochemical properties corresponding to n-type materials; that is, a cathodic dark current under cathodic polarization and an anodic photocurrent under anodic polarization were determined by electrochemical measurements.The value of E e for the Ni-Al LDH photocatalyst was estimated to be À0.40 V vs. NHE (pH = 0), which is enough to induce the twoelectron reduction of CO 2 into CO.The change of M 3+ metal in Ni-M 3+ LDH clearly influenced E e ; E e changed from negative to positive in the following order: Ni-In LDH, Ni-Al LDH, and Ni-Ga LDH.We concluded that the selectivity toward CO 2 reduction and the inhibition of proton reduction to H 2 are regulated not only by the overpotential of photogenerated electrons but also by the chemical properties of LDH photocatalysts.

Fig. 1A shows
Fig.1Ashows the X-ray diffraction (XRD) pattern of the prepared Ni-Al LDH/FTO composite.Compared to those of the Ni-Al LDH powder sample (a) and bare FTO glass (c), the XRD pattern of Ni-Al LDH/FTO (b) shows mixed diffraction peaks of the above two samples.Two low angle peaks, 111 and 231, in the powder sample are indexed as (0 0 3) and (0 0 6) planes, respectively, which define the characteristic layer structure of the LDH powders.1 The peak positions of these reflections correspond to the layer-to-layer thickness; that is, the interlayer distance is equal to d (0 0 3) with respect to hexagonal axes.As shown in Fig.1B, Ni-Al LDH/FTO exhibited small peaks around the same position as those in the powder sample, indicating that Ni-Al LDH is fixed onto the FTO glass and retains its layer structure.Moreover, the interlayer distance was conserved after the deposition onto FTO, because the positions of the (0 0 3) and (0 0 6) reflections were unchanged.As previously reported,26,27 other LDHs (Mg-Al, Zn-Al, Ni-Ga, and Ni-In LDH) were successfully synthesized (XRD patterns of powder samples of these prepared LDHs are presented in Fig.S1, ESI †).Moreover, the specific surface area of the Ni-Al LDH powder sample was estimated to be ca.100 m 2 g À1 .Fig.2Ashows the SEM images of Ni-Al LDH/FTO prepared using the electrophoresis deposition method.Regions X and Y mark the deposited Ni-Al LDH region and the FTO glass, respectively.We found that Ni-Al LDH particles with a thickness of several micrometers were deposited on the FTO glass.Regarding the enlarged views shown in Fig.2B, flower-petal-like morphologies, which are characteristic of LDH-type materials,1,39 were also maintained.These SEM images are consistent with the results of XRD measurements of Ni-Al LDH/FTO.Based on the XRD patterns and SEM images, the state of the Ni-Al LDH deposited on FTO glass is considered to be similar to that of the powder sample, which shows photocatalytic activity for the conversion of CO 2 in aqueous solution.26,28Fig.3shows the cyclic voltammogram recorded using a threeelectrode electrochemical cell.The prepared Ni-Al LDH/FTO was used as the working electrode under a He atmosphere.When the working electrode was set under cathodic polarization, the cathodic dark current was determined, indicating that electron transfer from the working electrode to the electrolyte solution occurred because of the formation of an accumulation layer at the interface of the electrode, making the electron flow possible without an electric barrier.The cathodic dark current under cathodic polarization is known to be a characteristic feature of n-type semiconductor materials.In contrast, anodic current should not be found under dark conditions for n-type electrodes because of the band bending in the depletion layer.However, two anodic current peaks were found.Because the standard potential was reported as E1(IO 3 À /I À ) = 1.08 V vs. NHE (pH = 0),29

Fig. 2
Fig. 2 Sectioned view of SEM images of Ni-Al LDH/FTO captured at a magnification of (A) 40.0k, (B-a) 50.0k, (B-b) 100k, (B-c) 200k, and (B-d) 300k.The regions X and Y in the image (A) indicate the deposited Ni-Al LDH layer and the FTO substrate, respectively.

Fig. 1
Fig. 1 XRD patterns of (a) Ni-Al LDH powder sample, (b) Ni-Al LDH/FTO, and (c) bare FTO collected in a (A) wide range scan and (B) narrow range scan.
Fig. 6A-C show the Mott-Schottky plots of X LDH/FTO (X = Ni-Al, Mg-Al, Zn-Al, Ni-Ga, and Ni-In), TiO 2 /FTO, and Ta 2 O 5 /FTO based on the impedance measurements using these materials as working electrodes.As mentioned above, the flat band potential (E FB ) can be determined by extrapolation to C À2 = 0 (C: capacitance), in accordance with the Mott-Schottky equation.When impedance measurements are conducted at variable frequencies, the fitted line for each Mott-Schottky plot should converge at E FB (C À2 = 0).In the case of Ni-Al LDH/FTO, the fitting lines of the Mott-Schottky plots for frequencies of 37.3, 52.0, and 72.0 kHz crossed at a point shifted from the x-axis intercept, as shown in Fig.

Fig. 7
Fig. 7 Summary of the investigation of the electrochemical properties of X LDHs (X = Ni-Al, Mg-Al, Zn-Al, Ni-Ga, and Ni-In), and references (TiO 2 and Ta 2 O 5 ).Standard potentials of redox couples are cited from the literature. 29,45Potentials of the photogenerated electron and hole are, respectively, presented in red and blue color.