Back Electron Transfer at TiO2 Nanotube Photoanodes in the

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Back Electron Transfer at TiO2 Nanotube Photoanodes in the Presence of a H2O2 Hole Scavenger Heng Zhu, Shicheng Yan, Zhaosheng Li, and Zhigang Zou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09827 • Publication Date (Web): 13 Sep 2017 Downloaded from http://pubs.acs.org on September 14, 2017

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ACS Applied Materials & Interfaces

Back Electron Transfer at TiO2 Nanotube Photoanodes in the Presence of a H2O2 Hole Scavenger Heng Zhu,† Shicheng Yan,*,† Zhaosheng Li,† Zhigang Zou†,‡

†

Eco-materials and Renewable Energy Research Center (ERERC), Collaborative Innovation

Center of Advanced Microstructures, College of Engineering and Applied Sciences, Nanjing University, No. 22, Hankou Road, Nanjing, Jiangsu 210093, P. R. China ‡

Jiangsu Key Laboratory for Nano Technology, National Laboratory of Solid State

Microstructures, Department of Physics, Nanjing University, No. 22, Hankou Road, Nanjing, Jiangsu 210093, P. R. China

ACS Paragon Plus Environment

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Abstract Adding charge scavengers, which usually are more unstable than water, is an effective method to quantify the quantum efficiency loss of photoelectrode during the charge separation, transfer, and injection processes of the water splitting reaction. Here, we detected, on TiO2 nanotube photoanodes after using hydrogen peroxide (H2O2) as a hole scavenger, a nearly 40% saturated photocurrent decrease in alkaline electrolyte and a negligible saturated photocurrent difference in acid electrolyte. We found that the photoelectrons were trapped in the surface states of TiO2 with the nearly same storage capacity of electrons in a wide range of pH values from 1.0 to 13.6. However, kinetics of a back reaction, H2O2 reduction by the photoelectrons trapped in surface states, is about 10 times higher for that in alkaline electrolyte than in acid electrolyte. As a result, the pH-dependent kinetic difference in H2O2 reduction induced the negative effects on the saturated photocurrent. Our results offer a new insight to understand the effects of back electron transfer on electrochemical behaviors of surface states and charge scavengers. Keywords:

photoelectrocatalysis, TiO2 nanotube, photoelectron transfer, surface states, H2O2

reduction


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Introduction Photoelectrochemical (PEC) water splitting is currently receiving significant interest as it offers a potential route to convert intermittent sunlight into fuels and chemicals. To achieve large-scale PEC water splitting, an efficient photoanode is needed due to that water oxidation with four-electron transfer is the rate-determining step of overall water splitting.1 One of the key factors that affect the performance of semiconductor photoanodes is carrier loss in their separation, transfer, and injection processes. Hole scavengers, such as SO32-, Fe(CN)64-, I- and H2O2, are often used to quantify the carrier loss of a given photoanode due to that they are easily oxidized by hole with high reaction rate constant.2-10 The carrier surface recombination is expected to be completely suppressed in the presence of the hole scavengers, then the carrier bulk recombination can be determined by subtracting the observed photocurrent from the maximum theoretical photocurrent. Surface recombination is quantified by comparing the difference between photocurrents with and without a hole scavenger. The effort of such an analysis is that it allows us to know the carrier loss in photoanode clearly and contributes to the rational design of efficient photoelectrodes.8 However, among the existing hole scavengers, chemical stabilities of SO32- and Fe(CN)64are pH-dependent. SO32- ion with Eo = +0.17 VRHE (Reversible hydrogen electrode) for the SO42/SO32- couple does not work effectively as a hole scavenger in acid electrolyte due to its decomposition. Fe(CN)64- (Eo = +0.36 VRHE for the Fe(CN)64-/Fe(CN)63- couple) is not stable in alkaline electrolyte. H2O2 (Eo = +0.68 VRHE for the O2/H2O2 couple) is relatively chemical stable in wide pH range from pH 0 to 14.8, 11 This means that the electrolyte properties affect the effectiveness of hole scavenger. In addition, for a given photoanode, some hole scavengers do


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not work effectively even if they are used in appropriate electrolyte. For example, the photocurrent measurements indicated that compared to the H2O2, Fe(CN)64- and I-, the SO32- is the more effective hole scavenger for LaTiO2N photoanode,2,3 and the oxygen species-free Fe(CN)64- is more efficient in receiving the holes than H2O2 and stabilizing the Ta3N5 photoanode.10 The effectiveness of hole scavenger is probably relating to its reaction kinetics and the interaction between hole scavenger species and surface of photoanode. H2O2 has an oxidation rate constant that is 10 times higher than water oxidation, and was well used as efficient hole scavenger for Fe2O3 photoanodes.8 More importantly, the H2O2 can exhibit oxidizing and reducing properties (Eo = +0.68 VRHE for the O2/H2O2 couple, Eo = +1.78 VRHE for the H2O2/H2O couple).11 As a result, the electrochemical current produced with H2O2 is due to the following reactions:8,12-14 H2O2 + e- → ·OH + OH-

(1)

·

OH + e- → OH- or ·OH → OH- + h+

(2)

H2O2 + 2h+ → 2H+ + O2

(3)

Reactions (1) and (2) give rise to a cathodic dark current and can induce a current doubling effect on some p-type semiconductor electrodes due to the hole injection into the valence band of semiconductor.12, 13 In reaction (3), anodic dark- and photocurrents result from hole oxidization of H2O2 into oxygen molecule, making H2O2 can be used as hole scavenger. 8,14 Such two reaction routes imply that after adding H2O2 into electrolyte the H2O2 reduction in place of its hole scavenger role may occur on the surface of photoanode when there is a back electron transfer. This means that the H2O2 is an ideal electron-transfer-indicator to disclose the electron trapping of surface states and the effect of such electron trapping on the PEC performance of photoanodes. Here we use TiO2 nanotubes photoanode as the working electrode,


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due to its easy preparation by anodic oxidation method with reliable repeatability and abundant surface states15-17. We found photoelectrons trapped in the surface states of TiO2 can induce reduction reaction of H2O2 to occur, partially in place of its role of hole scavenger, and hence decrease the PEC performance of TiO2 nanotube photoanodes. More importantly, back-transfer electron inducing H2O2 reduction is dependent strongly on the pH value of electrolyte due to the pH-dependent H2O2 reduction reaction kinetics. Our findings offer a new insight to understand the effects of surface states on charge transfer of PEC devices. Experimental Methods Preparation and characterization of TiO2 nanotube array electrodes. The TiO2 nanotube arrays film was fabricated by an electrochemical anodic oxidation of titanium sheet (0.2 mm thickness, 99.5% purity).15 The titanium sheet was first chemically polished in a solution containing HF, HNO3 and deionized water (1:2:7 in volume) for 30 s to remove the compact oxidation layer. The resulting titanium sheets were ultrasonically cleaned with acetone, ethanol and deionized water, and then were anodized in 250 mL NH4F-containing ethylene glycol (EG) solution (1g NH4F, 5mL deionized water) at 60 V for 15 min at room temperature and the counter electrode was a platinum plate9. The as-anodized samples were thoroughly cleaned with deionized water and then annealed in air at 450 oC for 2 h with a heating rate of 10 o

C/min. The morphologies of the TiO2 nanotube photoanodes were observed by field-emission

scanning electron microscope (FE-SEM; Nova NanoSEM 230, FEI) and the transmission electron microscopy (TEM; JEM-200CX, JOLE). Crystal phases of these samples were determined using an X-ray diffractometer (XRD, Rigaku Ultima Ⅲ, Japan) operated at 40 kV and 40 mA using Cu Ka radiation.


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Electrochemical Studies. A CHI760E electrochemical workstation (Shanghai Chenhua Company, Shanghai, China) was used to conduct the electrochemical and photoelectrochemical measurements, which were performed in a standard three-electrode cell at room temperature. To obtain ohmic contact, a copper wire was attached on back side of TiO2 nanotube arrays electrode using conductive silver adhesive, and then the electrode except the interface between semiconductor and electrolyte was thoroughly sealed with silica gel. A platinum plate and saturated Ag/AgCl were used as counter and reference electrodes, respectively. All the potentials are described by referring to the Ag/AgCl reference electrode. A 500 W Xe lamp was used as the light source for photocurrent measurements. The irradiated area was circular with area of 0.28 cm2 and photocurrent densities were normalized to 1 cm2. Reagents (Chemically pure), NaOH, H2SO4, K2HPO4, KH2PO4, and Milli-Q water (18 MΩ) were used to prepare different pH electrolytes: 1 M NaOH solution (pH 13.6), 0.5 M H2SO4 solution (pH 1.0) and phosphate buffer solution (0.2 M KH2PO4, pH 4.6; a mixture of 0.1 M K2HPO4 and 0.1 M KH2PO4, pH 7.0; 0.2 M KH2PO4, pH 9.4). The electrochemical impedance spectra (EIS) were measured using an electrochemical analyzer (Solartron 1260 + 1287, AMETEK, USA) with a 10 mV amplitude perturbation and frequencies between 100 kHz and 0.1 Hz. The Mott-Schottky curves were measured using an electrochemical analyzer (2273, Princeton Applied Research, AMETEK, USA).

Results and discussion XRD patterns of the TiO2 nanotubes electrode before and after annealed in air are shown in Figure 1a and S1 in Supporting information. we can get that the samples obtained by anodic oxidation without annealing in air is essentially amorphous because all the diffraction peaks are


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assigned to Ti substrate. After heating in air at 450 oC for 2 h, the amorphous film became crystallized single-phase TiO2 with anatase structure (JCPDS Card No. 21-1272). The diffraction peaks are assigned to (101), (004), (200) and (105) planes of anatase phase. Scanning electron microscope (SEM) observations indicated that the TiO2 film is composed of uniform nanotubes with 70-90 nm in diameter, about 10 nm in thickness of tube wall and 5-6 µm in length (Figure 1b). High-resolution TEM image exhibited that the TiO2 nanotube formed by aggregation of TiO2 nanocrystals with a particle size of about 20 nm (Figure 1c). The average d spacing estimated from the lattice fringes in high-resolution TEM image is 0.33 nm, well consistent with the (101) plane of anatase TiO2 (Figure 1d).

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Figure 1. Structural and morphology properties of TiO2 nanotube arrays film. (a) XRD patterns of the as-grown TiO2 nanotube arrays before and after annealing in the air. (b) Cross-section


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SEM image of the TiO2 nanotube arrays. The inset shows a top-view SEM image. (c) TEM image of the TiO2 nanotube arrays. (d) High-resolution TEM lattice image for TiO2 nanotube. The inset shows the image of a nanotube. The current density versus applied potential curves were obtained in different pH electrolytes with and without H2O2. Obviously, the saturated photocurrent from water oxidation in 1 M NaOH solution (pH 13.6) is 0.35 mA cm-2, which is almost the same as that in 0.5 M H2SO4 solution (pH 1.0) and about 0.1 mA cm-2 higher than that in phosphate buffer solution (pH 7.0), respectively (Figure 2). In this study, to avoid a local pH change near surface of photoanode, which is usually detected in a buffer-free electrolyte,18, 19 the pH-stable phosphate buffer solution was used as a neutral electrolyte. In a nonbuffered electrolyte, water molecules are the major species to be adsorbed and then be decomposed on the surface of electrodes under illumination, which process makes an important contribution to photocurrent density. As confirmed in many reports, in phosphate buffer solution, the hydrogen phosphate ions in phosphate buffer electrolyte were easily adsorbed on surface of TiO2, hampering water molecules from being adsorbed.18,20 The active sites on the surface of TiO2 adsorbed hydrogenophosphate species become passive. As a result, the photocurrent density measured in phosphate buffer solution is slightly smaller than that in 1 M NaOH or 0.5 M H2SO4.

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0.2 0.1 0.0 Dark current

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Figure 2. Effect of hydrogen peroxide on photocurrent density for TiO2 nanotube electrodes. Electrolyte: (a) phosphate buffer solution, pH 7.0, (b) 0.5 M H2SO4 solution, pH 1.0, (c) 1 M


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NaOH solution, pH 13.6. The photo/electrochemical tests were performed by positive-going potential sweep with a scan rate of 20 mV s-1. After adding H2O2 (Figure 2a), about 30% increase in saturated photocurrent was probed in phosphate buffer electrolyte, similar to the addition of Na2SO3, confirming that the H2O2 in place of water was oxidized by photogenerated holes (Figure S2, SI). No obvious change in photocurrent was observed in 0.5 M H2SO4 electrolyte before and after adding H2O2 (Figure 2b), indicating an approximately 100% hole injection efficiency or the ineffectiveness of hole scavenger.8 However, in the presence of H2O2, saturated photocurrent in 1 M NaOH electrolyte was decreased by 40% (Figure 2c). This abnormal phenomenon means that the H2O2 does not only act as a hole scavenger in the strong basic electrolyte. Obviously, after adding H2O2 into all the electrolytes used in this study, the onset potential of cathodic currents both under dark and irradiation shifted positively. It indicates that a strong reduction reaction occurs when H2O2 existed in the electrolytes. When n-type semiconducting TiO2 film electrode was introduced into electrolyte, an upward surface band bending forms at the semiconductor-liquid junction (SCLJ) region after reaching thermodynamic equilibrium.21 The upward band bending is responsible to obstruct the conduction-band electron injection into electrolyte and the SCLJ electric field forces the electron drift forward the conductive substrate. The barrier for electron injection into electrolyte is positive correlation with the increase of external applied potentials. Thus it is expected to completely suppress the back conduction-band electron transport at high potential.8 Taking into consideration that the hydrogen peroxide can exhibit both oxidizing and reducing properties (Eo = +0.68 VRHE for the O2/H2O2 couple, Eo = +1.78 VRHE for the H2O2/H2O couple (Table S1, SI).11 And a cathodic photocurrent on TiO2 nanocrystalline film electrodes was observed and attributed to oxygen reduction.22 In our case, after adding H2O2, the positive shift


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of dark cathodic current onset potential would result from the H2O2 reduction reaction on the surface of TiO2 photoanode. In particular, in the alkaline electrolyte, the 40% decrease in saturated photocurrent means that a back electron transport occurs on the TiO2 photoanode even if at a high applied potential. Such a H2O2-related decrease in saturated photocurrent was also observed in the compact TiO2 plate electrodes (Figure S3, SI), indicating that this phenomenon is not dependent on the microstructure of electrodes. It was well known that surface irregularities of TiO2 naturally originate from the reduced Ti species, oxygen vacancies and external adsorbates.23-27 The irregularities will induce the formation of surface electron states that fall within the band gap of the semiconductor. The photogenerated electrons can be trapped in such states.19, 25, 28 Therefore, a possible route is the photogenerated electrons trapped in surface states to reduce the H2O2, leading to the decrease in saturated photocurrent of the electrode. We accordingly investigated the dark transient current response on TiO2 nanotube arrays electrode in alkaline and acidic electrolytes (Figure 3), for tracing the trapping and filling process of the electrons by muti-potential steps method.29 The reported flat band potential (Ufb) of the TiO2 electrode determined by Mott-Schottky equation was -1.14 V at pH 13.6 and -0.40 V at pH 1.0.

30, 31

To completely exhaust the charges possibly

stored in both the space charge region and the surface states, the TiO2 nanotube arrays electrode was first polarized at a potential more positive than its Ufb for 1 min, that is, the electrode polarization was respectively performed at 0 V in 1 M NaOH (pH 13.6) and 0.7 V in 0.5 M H2SO4 (pH 1.0). And then the potential was immediately shifted to a preset value positive than Ufb of TiO2 nanotube electrodes and the transient current was recorded under dark.


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-0.3 to -0.5 V

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Figure 3. Transient currents of the TiO2 electrodes when varied the potentials with a potential step of 0.1 V in different electrolytes. (a) 1 M NaOH, (b) 1 M NaOH with 0.1 M H2O2, (c) 0.5 M Na2SO4, (d) 0.5 M H2SO4 with 0.1 M H2O2. The insets in Figure 3a and 3c show the integrated charge of the transient current-time curves. Figure 3a shows the dark transient current of the TiO2 nanotube electrodes in 1 M NaOH solution. When the potential shifted from 0 V to a negative one, a sharp cathodic current was observed and was limited to a maximum value of 1.0 mA cm-2 for comparison. Obviously, the current decays depend on the applied potentials. At potentials -0.3 to -0.5 V, the transient currents decrease to almost zero within a few milliseconds. The decay time of the transient current significantly increased when the applied potential is more negative than -0.6 V. Usually,


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after changing the applied potential, a new equilibrium state was reached via a fast charge redistribution in space charge region due to its high built-in potential effectively sweeping the charges from this region, exhibiting the neglectable current decay time. However, the occurrence of electrons injection into surface states will prolong the current decay time, depending on if Fermi level of semiconductor passes through the surface states.29 Therefore, the cathodic current decay from initial maximum to near zero is mainly attributed to the gradual electron injection process of surface states before reaching a new equilibrium state. At potentials more positive than -0.6 V, the fast current decay would be attributed to the injection of a small number of electrons into surface states to build a new equilibrium state. In contrast, the prolonged current decay at potential more negative than -0.6 V would result in the migration of a large amount of electrons into the surface states that process requires a long filling time for reaching new equilibrium state. The potential-dependent electron injection behavior would reflect that the Fermi level of semiconductor was gradually driven across the surface states by external applied potentials. The integrated charge from the transient current-time curves (inset of Figure 3a) also revealed that the accumulated charge starts to increase sharply at -0.6 V. The accumulated charge-potential curves would reflect that the surface states surpassed by Fermi level of semiconductor depends on the bias potentials. We can get that the surface states are described as an exponential distribution from the conduction band of TiO2 into its in-gap states. At more positive bias potentials, the unoccupied surface states below the Fermi level of the semiconductor increased and would contribute to the enlarged demand for electrons and filling time to reach a new equilibrium state. Given that a potential for reduction of H2O2 to H2O is 0.78 V vs. Ag/AgCl at pH 13.6, the applied potentials between -0.3 to -0.5 V is enough to drive such a reduction reaction to occur.


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However, adding H2O2 into the electrolyte, a steady cathodic current was only observed when the potentials are more negative than -0.6 V, consistent with the potential where the surface states start to receive electrons (Figure 3b). This evidence may mean that the steady cathodic current results from the H2O2 reduction by electrons trapped in surface states. Indeed, the steady cathodic current significantly raises and surpasses -0.9 mA cm-2 when potential is more negative than -0.7 V, at which potentials more surface states can trap electrons. The significantly positive correlation between steady cathodic current and electron occupancy of surface states make us believe that the steady cathodic current is a result of electrons trapped in surface states to reduce H2O2 into H2O. When replaced the 1 M NaOH (pH 13.6) by 0.5 M H2SO4 (pH 1.0) as electrolyte, a similar phenomenon was also observed (Figure 3c). The transient cathodic current measurements indicated that the filling of electrons into surface states occurred when the bias potential is more negative than 0.4 V. Correspondingly, the charging capacity of the surface states determined by integrated the charge-potential relation from 0.4 V to the flat band potential of TiO2 (-0.40 V), confirming that the transient cathodic current originates from the surface state charging (inset of Figure 3c). However, in presence of H2O2, the steady cathodic current is observed to be initialed at 0 V (Figure 3d), which is 0.4 V positive than the initial potential for occurrence of surface states (0.4 V), probably due to the decreased reaction kinetics of H2O2 reduction in acidic electrolyte (will be discussed later).


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Figure 4. Electrochemical impedance diagrams (Bode representation) of the TiO2 nanotube electrodes (a) in 1 M NaOH, pH 13.6 and (b) in 0.5 M H2SO4, pH 1.0. (c, d) Equivalent circuits used to simulate the EIS data with and without surface states. Next, the electron occupancy of surface states was further investigated by electrochemical impedance spectroscopy (EIS).32-34 The Bode plots of EIS were shown in Figure 4 to find the possible interface capacitance, depending on the applied potentials. According to a typical Randle’s equivalent circuit, the resistance (Rs) from electrolyte and semiconductor-substrate electrical connection is far lower than the interfacial charge transfer resistance between semiconductor and electrolyte (Rct,bulk). Therefore, the circuit capacitance (C) can be determined by equation of C=(2πRsfc)-1, where fc is characteristic frequencies at the phase angle 45° for Bode


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plot.34 Clearly, the fc is dependent on circuit capacitance due to that the Rs is a constant for a given electrolyte and semiconductor electrode. In both the 1 M NaOH (pH 13.6) and 0.5 M H2SO4 (pH 1) electrolytes (Figure 4a and b), the TiO2 nanotube electrode exhibited about two orders of magnitude increase in fc for that was measured at high potential than at low potential, indicative of the significantly enlarged capacitance with decreasing the applied potentials. Interestingly, initial potential for increment of the capacitance is the same as the potential where the surface states start to receive electrons. This evidence may indicate that the small capacitance at high potential and large capacitance at low potential can be assigned to the space charge layer capacitance (Csc) and surface state capacitance (Ctrap), respectively. Additionally, taking the 1 M NaOH electrolyte as example, two equivalent circuits at a more positive and more negative potential than -0.6 V were used to simulate the Nyquist plots of EIS (Figure S4, SI). As shown in Figure 4c and d, the equivalent circuit at more negative than -0.6 V is composed of Rs and resistance for charge trapping in the surface states (Rtrap), resistance for charge transfer from the surface states to electrolyte (Rct,trap) and surface state capacitance (Ctrap). And the equivalent circuit at more positive potential than -0.6 V consists of Rs, Csc , and Rct,bulk. To obtain a better fitting of the EIS data, we used a constant phase element (CPE) instead of a pure capacitance. The two proposed equivalent circuits are similar to the classic Randle’s equivalent circuit (Figure 4c and 4d).35 The equivalent circuit analysis indicated that Ctrap is about two orders of magnitude higher than Csc. And the Rct,trap is much larger than Rs+Rtrap (Table S2, SI). These results confirmed that the injection of electrons into the surface states is dependent strongly on the applied potentials, which adjust the relative energy levels between the Fermi level of semiconductor and the surface states.


15


1

0.6

3.0

2

2.0

0.8

0.4 0.2 0.0 -0.8 -0.6 -0.4 -0.2 0.0 Potential (V vs. Ag/AgCl)

0.3

Space charge layer

Ufb (pH 1.0)

-0.4 0.0 0.4 Potential (V vs. Ag/AgCl)

Surface states

0.9

0.0 -0.8

0.6

2.0

Surface states

1.5

-0.6 -0.4 -0.2 Potential (V vs. Ag/AgCl)

0.0

1.5 1.0 0.5

1.0

0.0 0.0 0.2 0.4 0.6 0.8 Potential (V vs. Ag/AgCl)

0.5

Space charge layer

0.0

1 M NaOH

-0.8

2.5

2

1.2

Capacitance (mF/cm )

2

2

0 -1.2

(c) 2.5

2

3

10

-2

4

2


4

Ufb (pH 13.6)

0.8


(b) 1.5

1 M NaOH 0.5 M H2SO4


(a) 5 1/C (10 F cm )

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0.5 M H2SO4

0.0

0.2 0.4 0.6 Potential (V vs. Ag/AgCl)

0.8

Figure 5. (a) Mott-Schottky plots for TiO2 electrodes in the dark in 1 M NaOH (black squares) and 0.5 M H2SO4 solution (red circles) at 1000 Hz. (b, c) The capacitance obtained by fitting the EIS data measured (b) in 1 M NaOH and (c) in 0.5 M H2SO4. The insets show the capacitance calculated based on Mott-Schottky data in Figure 5a. When considered the surface state capacitance (Css), after neglecting the huge Hemholtz layer capacitance (CH) and Gouy layer capacitance (CG), the total semiconductor-electrolyte interface capacitance (C) can be described as C-1 = (CSC + CSS)-1.30, 36 Correspondingly, the MottSchottky (MS) equation should be modified as:

(

1 2 kT )2 = 2 (V − U fb − B ) where Csc +Css A qκε 0 N D q ,

V is the

applied voltage, Ufb is the flat band potential, Nd is the donor density, kB is the Boltzmann’s constant, T is the absolute temperature, q is the elementary charge, κ is the dielectric constant of the semiconductor, ε0 is the vacuum permittivity and A is the surface area of the electrodes. According to the modified MS equation, the MS plots for the TiO2 nanotube electrodes in 1 M NaOH and 0.5 M H2SO4 are shown in Figure 5a. Comparing with the theoretical Ufb,30, 31 both the Ufb of TiO2 nanotube electrode derived from the MS plots in the two electrolytes move positively. The Ufb of TiO2 nanotube electrodes in 1 M NaOH (pH 13.6) is -0.59 V, about 0.55 V positive than the theoretical Ufb of -1.14 V. The 0.55 V potential gap can be attributed to the Fermi level pinning effect by surface electron states. This evidence also means that the surface states may start at -0.59 V. Similarly, in 0.5 M H2SO4, the initial surface state of TiO2 nanotube


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electrode is at 0.32 V, about 0.72 V positive than the theoretical Ufb of -0.40 V. Further checking the effect of pH on surface states indicated that the distribution region of surface states increased with the decrease of pH (Figure S5, SI). The increment step of initial potential for occurrence of surface states is about 74 mV per pH, is slightly higher than the 59 mV per pH described by Nernst equation, probably resulting from the effect of surface states on the potential-determining ions. We further compared the capacitance derived based on MS and EIS data in different electrolytes. As shown in Figure 5b and c, the calculated capacitances by the two measurement methods present the completely consistent variation tend with the change of applied potentials. The initial increment of capacitance occurred at -0.6 V in 1 M NaOH and at 0.32 V in 0.5 M H2SO4. At the potentials more negative than the critical potential, the capacitances exhibited significant increase with the negatively shifting of potentials, reflecting a filling process of electron injection into surface states. And a small capacitance was found when the potential is more positive than the critical potential, which can be assigned to the space charge layer capacitance. 0 2


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-5

pH 1.0 pH 4.6 pH 7.0 pH 9.4 pH 13.6

-10 -15 -20 Ufb (pH 1.0)

Ufb

-25

(pH 13.6)

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

Potential (V vs. Ag/AgCl)

Figure 6. Effect of hydrogen peroxide (0.1 M) on dark linear sweep voltammetry curves for TiO2 nanotube electrodes in different pH-value electrolytes. Negative-going potential sweep with


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a scan rate of 50 mV s-1. In addition to effect of the surface states charging on the H2O2 reduction, in the presence of H2O2, change of cathodic current density in acidic electrolyte with the bias potential is clearly smaller than that in alkaline electrolyte (Figure 3b and 3d). It may indicate that the H2O2 was reduced easily in alkaline solution. The electrochemical behavior of H2O2 on TiO2 nanotube electrodes in different pH-value electrolytes was further studied by linear sweep voltammetry (Figure 6). Obviously, the dark cathodic current gradually enlarged during raising the pH of electrolyte from 1.0 to 13.6, meaning that the kinetics of H2O2 reduction reaction is pHdependent. A similar effect of pH-promoted O2 reduction was well demonstrated on TiO2 nanocrystalline electrode.22 The kinetics of H2O2 reduction reaction may depend on the adsorbed states of H2O2 on TiO2. It was well known that, because the pK value of H2O2 is 12, in alkaline electrolyte the H2O2 tends to dissociate into OOH-, which is more easily reduced than the indissociated molecule itself.12 In addition, internal reflection Fourier transform infrared spectroscopy analyses have confirmed that the Ti-O- species formed on the surface of TiO2 in the alkaline electrolyte.37 In the presence of H2O2, the interaction between OOH- and Ti-O- species may further promote the reduction of OOH-. Similarly, H2O2 is easily reduced on GaP electrodes in alkaline solution, probably meaning that the OOH- reduction is achieved via the interaction between OOH- and surface cation species of semiconductor.12 However, the H2O2 reduction on the Pt electrode does not exhibit the dependence on pH, indicating that there is completely different mechanism for H2O2 reduction on the metal and semiconductor electrode (Figure S6, SI). Now, we can conclude that high-density surface states existed on the surface of TiO2 nanotube and can be charged, depending on the applied potentials. The H2O2 can be reduced by


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electrons trapped in surface states and its reduction reaction kinetics is pH-dependent. On the basis of these conclusions, we can well understand after adding H2O2 the nature of the obvious difference in photocurrent of the TiO2 nanotube photoanodes in alkaline, neutral and acidic electrolytes. As described in Figure 7, according to the evidences from exponentially enlarged surface state capacitance and cathodic currents, the surface states on TiO2 nanotube electrode are believed to exhibit the exponential distributions, well consistent with the previous conclusions.38, 39

Evidently, at the flat band potential, the surface states distributed exponentially from a specific

band gap potential to the conduction band (Figure 7a), which can be filled by electrons to the most extent for both under irradiation or dark. At flat band, applying a positive bias potential on the TiO2 nanotube electrodes under dark (Figure 7b), the electrons in surface states will be first extracted and collected by conductive substrate via conduction band of semiconductor, and subsequent thermodynamic equilibrium at semiconductor-electrolyte interface was achieved by sweeping the charges from space charge layer to form an upward surface band bending.40 Under irradiation (Figure 7c), the upward surface band bending is a barrier to obstruct the injection of photoelectrons in conduction band into the electrolyte. The higher positive potential will induce a bigger barrier, thus completely suppressing the back electron transfer process. Our experimental evidences have demonstrated that the H2O2 can be reduced even if the TiO2 nanotube photoanode is held at a very high bias potential. This means that the photogenerated hot electrons were injected into the surface states, which induced the H2O2 reduction.


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Figure 7. (a) The surface states distribution from -0.59 V to Ufb of TiO2 (-1.14 V) at Ufb potential. (b) An anodic potential empties the surface states in the dark. (c) At positive bias potential in H2O2-containing 1 M NaOH under light, photogenerated electrons trapped in surface states inject into conduction band of semiconductor or electrolyte. As described above, as a result of application of positive bias potential, the Fermi level of semiconductor is in part or fully below the surface states. Therefore, there is a competition process for injection of electrons in surface states into electrolyte or conduction band. The competitive electron injection process would depend on the electron transfer barrier or electrondriven chemical reaction kinetics. Moreover, given the pH dependence of reaction kinetics of H2O2, the measured photocurrents on TiO2 nanotube electrodes in H2O2-containing electrolytes with different pH values were attributed to the following reasons: (1) In alkaline electrolyte, due to the fast kinetics of H2O2 reduction reaction, photoelectrons in surface states reducing H2O2 is a more favorable process, rather than the injection of electrons into conduction band of semiconductor, thus contributing to 40% decrease in photocurrent from water oxidation. (2) In acidic electrolyte, the slow kinetics of H2O2 reduction reaction makes the photoelectrons in


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surface states tend to relax into conduction band, thus exhibiting the nearly same photocurrent before and after adding H2O2. (3) An increase of photocurrent in H2O2 reduction-slow neutral electrolyte can be attributed to that the hydrogen phosphate species in electrolyte adsorbed on TiO2 prevent from the adsorption of H2O and do not affect the adsorption of H2O2, thus making the H2O2 as a hole scavenger to improve the photocurrent. Indeed, the saturated photocurrent in H2O2-containing neutral electrolyte is nearly close to those in both acidic and alkaline electrolytes from water oxidation, due to the same amounts of photogenerated holes at a highly positive potential. 1 M NaOH pH 13.6

-0.4 -0.5

with 0.1M H2O2

-0.6 -0.7

without H2O2

-0.8

light off

-0.9

buffer solution pH 7.0

0.1

light on

Potential (V vs Ag/AgCl)

light on

(b) 0.2

with 0.1M H2O2

-0.2 without H2O2

-0.3

0

50

100

150 200 Time (s)

250

300

350

0.5 M H2SO4 pH 1.0

0.5

0.0 -0.1

-0.4

light on

0.4 0.3 with 0.1M H2O2

0.2 0.1

light off

0.0

without H2O2

-0.1 -0.2

light off

-0.5

-1.0

(c) 0.6 Potential (V vs Ag/AgCl)

(a) -0.3 Potential (V vs Ag/AgCl)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0.3

0

50

100

150 200 Time (s)

250

300

350

0

50

100

150 200 Time (s)

250

300

350

Figure 8. The open-circuit potential under light on and off in different pH electrolytes with and without H2O2. (a) 1 M NaOH, (b) phosphate buffer solution, and (c) 0.5 M H2SO4. To further validate our conclusion, we probed the open-circuit potential of the TiO2 nanotube electrodes in different pH-value electrolytes (Figure 8). In the dark, the position of Fermi level can be measured by open-circuit potential which reflects the equilibrium potential between the semiconductor and the electrolyte interface. Under ideal condition, the open circuit potential equals to the redox potential of O2/H2O (0.23 V versus Ag/AgCl at pH 13.6).41 The measured open circuit potential was about -0.45 V in 1 M NaOH, indicating 0.68 V difference to the ideal equilibrium potential due to the surface state pinning effect. Under illumination, the electrons leap from valence band into conduction band of semiconductor, which process will make the quasi-Fermi level of semiconductor shifts negatively. From Figure 8a, we can find that under


21


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illumination the open circuit potential shifts negatively to -0.87 V. And when the light off, the relaxation time of the open circuit potential is very long, due to that large amount of electrons transfer from the surface states on the TiO2 nanotube electrodes. Adding H2O2 into the alkaline electrolyte, the open circuit potential was almost no change in the dark, but under illumination shifted negatively to -0.69 V. This suggests that the decrease in injection of electrons into the conduction band is a result of some electrons trapped in the surface states to reduce H2O2. In neutral and acidic solution with H2O2, although the kinetics of H2O2 reduction reaction is very slow, the open circuit potentials under irradiation shift negatively, indicating that the electrons trapped in surface states are energetically enough to reduce the H2O2.

Conclusions In this work, we have found that photogenerated electrons trapped in surface states of TiO2 nanotube electrodes can reduce the H2O2 in place of its well-known hole scavenger role. A competition process for injecting photogenerated electrons into the electrolytes or conduction band of semiconductor is dependent on the kinetics of H2O2 reduction reaction. The reduction of H2O2 in alkaline solution is easier than that in acidic solution at the surface of TiO2 nanotube electrodes. The surface states-related back electron transfer behavior provides a new perspective on understanding the role of the surface states in the photoelectrochemical or photocatalytic reaction, especially for the photoelectrons transfer mechanism in photoanodes.


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ASSOCIATED CONTENT Supporting Information Detailed XRD patterns, photoelelctrochemical measurements, values of standard electrode potentials and fitting parameters of ESI data.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. Acknowledgements This work was supported primarily by the National Basic Research Program of China (2013CB632404), the National Natural Science Foundation of China (51572121, 21603098 and 21633004), the Natural Science Foundation of Jiangsu Province (BK20151265 and BK20150580) and the Fundamental Research Funds for the Central Universities (021314380084).


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Table of Contents Graphic

Photogenerated electrons trapped in surface states of TiO2 nanotube electrodes can reduce the hydrogen peroxide (H2O2).


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Back Electron Transfer at TiO2 Nanotube Photoanodes in the

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