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Effect of anodization voltage on performance of TiO2 nanotube arrays for hydrogen generation in a two-compartment photoelectrochemical cell
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 1 3 6 8 e1 1 3 7 5
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Effect of anodization voltage on performance of TiO2 nanotube arrays for hydrogen generation in a two-compartment photoelectrochemical cell Yan Sun a, Kang-Ping Yan b,* a b
School of Industrial Manufacturing, Chengdu University, Chengdu 610106, PR China College of Chemical Engineering, Sichuan University, Chengdu 610065, PR China
article info
abstract
Article history:
Highly ordered TiO2 nanotube arrays were prepared by anodic oxidation of Ti foil under
Received 5 October 2013
different anodization voltages in ethylene glycol electrolyte. The morphology and photo-
Received in revised form
electrochemical performance of the TiO2 nanotubes (NTs) samples were characterized by
2 May 2014
FESEM and electrochemical working station. Hydrogen production was measured by
Accepted 18 May 2014
splitting water in the two-compartment photoelectrochemical (PEC) cell without any
Available online 20 June 2014
external applied voltage or sacrificial agent. The results indicated that anodization voltage significantly affects morphology structures, photoelectrochemical properties and hydrogen
Keywords:
production of TiO2 NTs. The pore diameter and layer thickness of TiO2 samples increased
TiO2 nanotubes
linearly with the anodization voltage, which led to the enhancement of active surface area.
Anodization voltage
Accordingly, the photocurrent response, photoconversion efficiency and hydrogen pro-
Photoelectrochemical
duction of TiO2 nanotubes were also linearly correlated with the anodization voltage.
Hydrogen production
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In recent years, with the development of industry, energy shortage and environmental pollution have become the two critical problems of human society. Hence it is necessary to develop clean and sustainable energy sources. Hydrogen, a green and renewable energy carrier [1,2], is considered as an ideal energy to replace the fossil fuel. Converting solar energy into hydrogen by photocatalytic water splitting may provide a promising way to resolve the energy and environment problems [3e6]. Since Fujishima and Honda [7] successfully obtained hydrogen via photoelectrochemical water splitting with TiO2 photoelectrode in 1972, TiO2 has attracted much
attention as an efficient photocatalyst due to its unique properties, such as low cost, nontoxicity and high photochemical stability [8e11]. Currently, TiO2 powder suspension system has been extensively studied for photocatalytic water splitting [12e14]. However, there are still several problems which limit its practical application. Firstly, the recovery of photocatalysts in the aqueous solution is very difficult [15]. Secondly, the electron transporting time is long in the bulk of TiO2 particles, resulting in more recombination [16]. Furthermore, H2 and O2 produced at the same time in the liquid, leading to the backreaction [17]. Therefore, more researches focus on TiO2 photoelectrode in the form of thin film for hydrogen generation in PEC cell [18e22], in which TiO2 film as photoanode connects
* Corresponding author. Tel.: þ86 28 85406192. E-mail address: [email protected] (K.-P. Yan). http://dx.doi.org/10.1016/j.ijhydene.2014.05.115 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 1 3 6 8 e1 1 3 7 5
with the cathode, typically made of Pt. As the open-circuit voltage of this conventional PEC system is not high enough for the potential of water splitting, applying external voltage across the two electrodes is essential to ensure hydrogen production process, which greatly increases energy consumption and hydrogen production cost. In this study, the above problems can be resolved by using a two-compartment PEC cell, which demonstrates an efficient and economical technique of converting solar energy into hydrogen energy. On the other hand, in order to enhance the efficiency of hydrogen generation, TiO2 as a photoanode requires a suitable structure that minimizes recombination of electron-hole pairs and maximizes photo absorption [23]. Thus, one-dimensional TiO2 nanotube arrays have been widely investigated owing to their large surface area for photo absorption and unidirectional electrical channel for charge transfer. Among various fabrication methods of TiO2 nanotubes, anodic oxidation of titanium sheet is considered as the simplest and most feasible technique. The geometry parameters of TiO2 NTs can be easily controlled by varying experimental conditions such as electrolyte composition, anodization voltage, anodization time and annealing temperature [24e27]. Many researches have focused on the morphologies of TiO2 nanotubes so far [28e31]. However, the influence of various preparation parameters on the hydrogen generation performance is rarely reported. As we know, anodization voltage plays an important role on the TiO2 nanotubes formation, which determines the pore diameter and tube length. In this work, a series of TiO2 nanotubes were synthesized under different anodization voltages. The effect of anodization voltage on the morphology structures, photoelectrochemical properties and efficient hydrogen generation in the two-compartment photoelectrochemical cell was systematically discussed.
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electron microscope (FESEM, Hitachi S4800). The thickness of TiO2 NTs was determined directly from cross-sectional images by bending the TiO2 samples and examining the cracked layers. The photoelectrochemical properties of TiO2 NTs were measured using potentiostat/galvanostat (Model PAR 273A) in a three-electrode PEC cell with TiO2 nanotubes as the working electrode, platinum electrode as the counter electrode, and saturated calomel electrode (SCE) as the reference electrode, respectively. The working electrode (active area: 0.5 cm 2.0 cm) was illuminated by a 350 W xenon lamp with the light intensity of 150 mW/cm2.
Hydrogen generation performance The evolution of hydrogen was performed in a twocompartment PEC cell without any external applied voltage or sacrificial agent. As shown in Fig. 1, the PEC cell was separated into an anodic compartment and a cathodic compartment by a Nafion membrane, which were filled with 1 M KOH and 0.5 M H2SO4 electrolytes, respectively. The synthesized TiO2 NTs used as photoanode and a platinum electrode served as cathode were connected with a copper wire. So that a small chemical bias produced by two different electrolytes could assist the electron to transfer from TiO2 NTs into the Pt side. An inverted burette with displacing electrolyte in the burette column was employed to collect hydrogen gas generated at Pt electrode. The volume of H2 was directly determined by reading the variation of electrolyte level.
Results and discussion Electrical response during anodization
Experimental Preparation of TiO2 nanotube arrays
Fig. 2 shows the current density-time curve recorded by a milliammeter during the anodization of Ti samples in the ethylene glycol electrolytes. As can be seen clearly, similar
Highly ordered TiO2 nanotube arrays were synthesized by a rapid anodization process reported in the literature [32]. Before the electrochemical anodization, Ti foils (0.2 mm thick, 99.6% purity) were degreased at room temperature by sonicating in acetone and ethanol for 30 min, respectively, then rinsed with deionized water and dried in air. All anodization experiments were carried out in a two-electrode electrochemical cell at room temperature. Ti foil and Pt foil were used as the anode and cathode, respectively. Ethylene glycol solution containing 0.25 wt% NH4F and 2 vol% H2O was used as electrolyte. The anodization process was performed with a direct current (DC) power supply for 30 min. The anodization voltage was varied from 10 V to 50 V in this study. After electrochemical treatment, the samples were rinsed with deionized water, dried at 80 C and subsequently annealed in air at 450 C for 1 h [33].
Characterization and photoelectrochemical measurements The surface and cross-sectional morphologies of TiO2 nanotube arrays were observed by a field emission scanning
Fig. 1 e Scheme of the two-compartment photoelectrochemical cell.
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Fig. 2 e Current densityetime curve for TiO2 formation process under different anodization voltages.
trend is evident for anodization under different voltages, which is the typical behavior of Ti anodization process. At the initial stage of the anodization, the current density rises to a maximum during the anodization voltage sweeping up to the desired value. Then a rapid decrease to a minimum is observed, which is due to the formation of a barrier layer on the surface of titanium foil (Eq. (1)). The barrier layer is a nonconductive layer. Therefore, it can increase the resistance and reduce the current significantly. At the next stage, as the newly formed pores grow (Eq. (2)), the current begins to increase. When the rate of electrochemical oxidization of Ti and chemical dissolution of TiO2 reaches a dynamic equilibrium, the current gradually approaches to a steady state value. Besides, it is observable that the current density increases with increasing anodization voltage due to the enhancement of electric field intensity. Ti þ 2H2O / TiO2 þ 4Hþ þ 4e
(1)
TiO2 þ 6F þ 4Hþ / TiF2 6 þ 2H2O
(2)
Morphological characterization The surface and cross-sectional morphologies of TiO2 NTs samples anodized under different voltages of 10 V, 20 V, 30 V, 40 V and 50 V are shown in Fig. 3(a)e(e) and (f)e(j), respectively. At 10 V of anodization voltage, a homogeneous nanopores layer is observed clearly from Fig. 3 (a) and (f), which can be explained that the anodization voltage of 10 V is not high enough to reach the formation voltage of TiO2 nanotube arrays in the ethylene glycol electrolyte. According to Eq. (2), fluorine iron plays a key role in the formation of TiO2 NTs, which controls the chemical dissolution rate of TiO2 oxide film. As we know, comparing to the inorganic electrolytes [34,35], organic electrolytes with high viscous severely restrict the diffusion of fluorine iron. Hence the formation of TiO2 NTs needs a higher electric field in ethylene glycol electrolyte. With the increase of anodization voltage, a mixed
structureda top nanoporous layer and an underneath highly ordered nanotube layer is appeared. The mixed structure is attributed to the fact that the thin nanoporous layer on the top surface of TiO2 NTs is not completely dissolved during the anodization process for 30 min in this experiment. Furthermore, it is found that both the average inner diameter and layer thickness of TiO2 NTs increase with increasing anodization voltage. As anodization voltage increases from 10 V to 50 V, the pore diameter ranges from 15 nm to 55 nm while layer thickness ranges 0.4 mme2.7 mm. The effect of anodization voltage on the average pore diameter and thickness of TiO2 samples is summarized in Fig. 4. Obliviously the pore diameter and layer thickness depend linearly on the anodization voltage, which is consistent with other previous reports [36e39]. Based on the TiO2 NTs growth mechanism, anodization process is a competition of electrochemical oxidization (Eq. (1)) and chemical dissolution (Eq. (2)). As the barrier layer formed at the initial anodization, small pits originate in this oxide layer due to the localized dissolution of the oxide. Then the increase in electric field intensity improves electrochemical etching rate, resulting in further pore growth. The deeper pores grow, the higher the electric field intensity is required in these metallic regions, which enhances field-assisted oxide growth and chemical dissolution of the oxide [40]. Therefore, well-defined interpore voids begin forming. After this, both the voids and tubes grow in equilibrium to form the final tubular structure. The tube length increases gradually until the electrochemical oxidization rate and chemical dissolution rate reach a dynamic equilibrium. Thus, improving anodization voltage could promote the growth of pores and nanotube layers simultaneously.
Photoelectrochemical properties The transient photocurrent response of TiO2 nanotubes was measured in the two-compartment PEC cell under light interruption at zero bias. As shown in Fig. 5(a), the current value is approximately 0 mA/cm2 in dark condition while the photocurrent rapidly rises to a constant value upon illumination. All photocurrent patterns of TiO2 samples anodized under different voltages are highly reproducible for several light oneoff cycles. This result indicates that TiO2 nanotubes prepared in this study exhibit good photoresponse and fast charge transfer. The average photocurrent measured during the 3 light oneoff cycles as a function of anodization voltage is shown in Fig. 5(b). The result that the photocurrent increases linearly with increasing anodization voltage is in agreement with Fig. 4(b). It is known that TiO2 NTs with longer tube length in a certain range could provide a larger active surface area and higher absorption of incident photons, which leads to more photogenerated electron-hole pairs [38]. Hence the improvement of photocurrent density is achieved by increasing anodization voltage of TiO2 NTs to obtain longer tube length. Further study of photoelectrochemical behavior of TiO2 NTs anodized under various voltages was determined by linear sweep voltammetry at a scan rate of 10 mV/s from 1.0 V to 1.0 V (vs SCE). As all the dark currents of TiO2 NTs samples are very small (around 103 mA/cm2) and the dark
Fig. 3 e FESEM images of TiO2 nanotube arrays anodized under various voltages (10 V～50 V): (a)e(e) top surface views; (f)e(j) cross-section views.
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Fig. 4 e Dependence of average inner diameter (a) and average layer thickness (b) of TiO2 nanotube arrays on the anodization voltage.
IeV curves are superposed practically, Fig. 6(a) only shows the dark current of TiO2 NTs anodized at 20 V. However, with the illumination, the photocurrent density increases with the increase of applied potential and gradually reaches a plateau value, which demonstrates typical behavior of n-type semiconductor. The result clearly shows that anodization voltage significantly affect the photoelectrochemical performance of TiO2 NTs. The photocurrent increases with increasing anodization voltage and the maximum photocurrent is obtained of TiO2 NTs anodized at 50 V. Corresponding photoconversion efficiency of TiO2 NTs could be calculated by the following equation [41]: " # E0 Eapp 100 hð%Þ ¼ jp rev I0
(3)
where jp is the photocurrent density (mA/cm2), jp E0rev is the total power output, jp Eapp is the electrical power input, and I0 is the power density of incident light. E0rev is the standard reversible potential of 1.23 V/NHE and the applied potential is Eapp ¼ Emeas Eaoc, where Emeas is the electrode potential (vs SCE) of the working electrode and Eaoc is the electrode potential (vs SCE) of the same working electrode under open
Fig. 5 e The transient photocurrent response of TiO2 nanotubes anodized under different voltages (a) and average photocurrent as a function of anodization voltage (b).
circuit condition in the same electrolyte. As expected in Fig. 6(b), TiO2 NTs anodized at 50 V for 30 min exhibits a maximum photoconversion efficiency of 3.51% at 0.61 V vs SCE. Moreover, a linear relationship between the maximum photoconversion efficiency of TiO2 NTs samples and anodization voltage is obtained in Fig. 6(c), which is in line with the layer thickness result (Fig. 4).
Photocatalytic activity The photocatalytic activity of TiO2 NTs was evaluated by hydrogen production in the two-compartment PEC cell without any external applied voltage or sacrificial agent. Efficient separation of H2 generated on Pt side and O2 produced on TiO2 NTs side is achieved via the two-compartment PEC device. Moreover, the small chemical bias produced by the two different electrolytes could accelerate charge transfer and reduce the recombination of electrons and holes. Consequently, there is no need to apply external voltage across the two electrodes or add sacrificial agent in the electrolyte to realize high efficiency of hydrogen generation.
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7
light
6
50V
5
40V
4
30V
3
20V
2
10V
2
Photocurrent density (mA/cm )
(a)
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1 dark
0 -1
-0.8
-0.4
0.0 0.4 Potential (V/vs SCE)
0.8
1.2
Photoconversion efficiency (%)
(b) 4.0
50V
3.5 3.0
40V
2.5
30V
2.0
20V
1.5
10V
1.0 0.5 0.0
-0.8
-0.6
-0.4 -0.2 0.0 Potential (V/vs SCE)
0.2
0.4
Fig. 6 e Photoelectrochemical properties of TiO2 nanotubes with various anodization voltages: (a) the IeV curves (b) corresponding photoconversion efficiency (c) photoconversion efficiency as a function of anodization voltage.
Fig. 7 e Hydrogen generation in the two-compartment PEC cell without any external applied voltage: (a) hydrogen evolution of TiO2 nanotubes anodized under 40 V as a function of irradiation time (b) Effect of anodization voltage on the hydrogen production.
Hydrogen production of TiO2 NTs anodized under 40 V as a function of irradiation time is shown in Fig. 7(a). The amount of evolved H2 is strictly linear with the reaction time, which indicates that TiO2 nanotubes synthesized in this work have excellent stability during the photocatalytic reaction process. The influence of anodization voltage on the hydrogen production is discussed in Fig. 7(b). It can be seen clearly that the amount of hydrogen increases linearly with increasing anodization voltage. A maximum hydrogen production rate of 93.6 mmol h1cm2 is achieved of TiO2 NTs anodized under 50 V. This result is consistent with transient photocurrent response and photoconversion efficiency, which all depend on the layer thickness of TiO2. Previously we reported [32] that the two important factors influencing hydrogen production were the charge carrier generation and their recombination of TiO2. TiO2 samples anodized under a higher voltage with longer tube length could offer a larger active surface area and contact area with the electrolyte, which benefit generating of photoelectron-hole
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pairs and photoelectrochemical reactions. Thus the evolution of hydrogen can be improved efficiently. However, continuous increasing of tube length doesn't mean the continual enhancement of hydrogen production. The charge transfer process should be considered. TiO2 NTs with longer length may generate more electron-hole pairs and provide a longer pathway for charge transfer simultaneously, where a competitive relationship exists consequentially. As the layer thickness beyond a range, the probability of charge recombination would be increased.
Conclusion Self-organized TiO2 nanotube arrays were prepared by electrochemical anodization of Ti foil in ethylene glycol electrolyte. By varying anodization voltage from 10 V to 50 V, the pore diameter of TiO2 nanotubes ranges from 15 nm to 55 nm while the layer thickness ranges from 0.4 mm to 2.7 mm. Both pore diameter and layer thickness increase linearly with anodization voltage, which provides a theoretical basis for controlling growth of TiO2 nanotubes. Efficient hydrogen generation can be achieved by splitting water in the two-compartment PEC cell without any external applied voltage or sacrificial agent. The photocurrent response, photoconversion efficiency and hydrogen production of TiO2 nanotubes are also linearly correlated with the anodization voltage, which are consistent with the result of layer thickness. In this work, TiO2 NTs with the largest active surface area anodized under 50 V for 30 min demonstrates a maximum hydrogen production rate of 93.6 mmol h1cm2 (2.24 mL h1cm2) with the corresponding photoconversion efficiency of 3.51%.
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Effect of anodization voltage on performance of TiO2 nanotube arrays for hydrogen generation in a two-compartment photoelectrochemical cell Yan Sun a, Kang-Ping Yan b,* a b
School of Industrial Manufacturing, Chengdu University, Chengdu 610106, PR China College of Chemical Engineering, Sichuan University, Chengdu 610065, PR China
article info
abstract
Article history:
Highly ordered TiO2 nanotube arrays were prepared by anodic oxidation of Ti foil under
Received 5 October 2013
different anodization voltages in ethylene glycol electrolyte. The morphology and photo-
Received in revised form
electrochemical performance of the TiO2 nanotubes (NTs) samples were characterized by
2 May 2014
FESEM and electrochemical working station. Hydrogen production was measured by
Accepted 18 May 2014
splitting water in the two-compartment photoelectrochemical (PEC) cell without any
Available online 20 June 2014
external applied voltage or sacrificial agent. The results indicated that anodization voltage significantly affects morphology structures, photoelectrochemical properties and hydrogen
Keywords:
production of TiO2 NTs. The pore diameter and layer thickness of TiO2 samples increased
TiO2 nanotubes
linearly with the anodization voltage, which led to the enhancement of active surface area.
Anodization voltage
Accordingly, the photocurrent response, photoconversion efficiency and hydrogen pro-
Photoelectrochemical
duction of TiO2 nanotubes were also linearly correlated with the anodization voltage.
Hydrogen production
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In recent years, with the development of industry, energy shortage and environmental pollution have become the two critical problems of human society. Hence it is necessary to develop clean and sustainable energy sources. Hydrogen, a green and renewable energy carrier [1,2], is considered as an ideal energy to replace the fossil fuel. Converting solar energy into hydrogen by photocatalytic water splitting may provide a promising way to resolve the energy and environment problems [3e6]. Since Fujishima and Honda [7] successfully obtained hydrogen via photoelectrochemical water splitting with TiO2 photoelectrode in 1972, TiO2 has attracted much
attention as an efficient photocatalyst due to its unique properties, such as low cost, nontoxicity and high photochemical stability [8e11]. Currently, TiO2 powder suspension system has been extensively studied for photocatalytic water splitting [12e14]. However, there are still several problems which limit its practical application. Firstly, the recovery of photocatalysts in the aqueous solution is very difficult [15]. Secondly, the electron transporting time is long in the bulk of TiO2 particles, resulting in more recombination [16]. Furthermore, H2 and O2 produced at the same time in the liquid, leading to the backreaction [17]. Therefore, more researches focus on TiO2 photoelectrode in the form of thin film for hydrogen generation in PEC cell [18e22], in which TiO2 film as photoanode connects
* Corresponding author. Tel.: þ86 28 85406192. E-mail address: [email protected] (K.-P. Yan). http://dx.doi.org/10.1016/j.ijhydene.2014.05.115 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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with the cathode, typically made of Pt. As the open-circuit voltage of this conventional PEC system is not high enough for the potential of water splitting, applying external voltage across the two electrodes is essential to ensure hydrogen production process, which greatly increases energy consumption and hydrogen production cost. In this study, the above problems can be resolved by using a two-compartment PEC cell, which demonstrates an efficient and economical technique of converting solar energy into hydrogen energy. On the other hand, in order to enhance the efficiency of hydrogen generation, TiO2 as a photoanode requires a suitable structure that minimizes recombination of electron-hole pairs and maximizes photo absorption [23]. Thus, one-dimensional TiO2 nanotube arrays have been widely investigated owing to their large surface area for photo absorption and unidirectional electrical channel for charge transfer. Among various fabrication methods of TiO2 nanotubes, anodic oxidation of titanium sheet is considered as the simplest and most feasible technique. The geometry parameters of TiO2 NTs can be easily controlled by varying experimental conditions such as electrolyte composition, anodization voltage, anodization time and annealing temperature [24e27]. Many researches have focused on the morphologies of TiO2 nanotubes so far [28e31]. However, the influence of various preparation parameters on the hydrogen generation performance is rarely reported. As we know, anodization voltage plays an important role on the TiO2 nanotubes formation, which determines the pore diameter and tube length. In this work, a series of TiO2 nanotubes were synthesized under different anodization voltages. The effect of anodization voltage on the morphology structures, photoelectrochemical properties and efficient hydrogen generation in the two-compartment photoelectrochemical cell was systematically discussed.
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electron microscope (FESEM, Hitachi S4800). The thickness of TiO2 NTs was determined directly from cross-sectional images by bending the TiO2 samples and examining the cracked layers. The photoelectrochemical properties of TiO2 NTs were measured using potentiostat/galvanostat (Model PAR 273A) in a three-electrode PEC cell with TiO2 nanotubes as the working electrode, platinum electrode as the counter electrode, and saturated calomel electrode (SCE) as the reference electrode, respectively. The working electrode (active area: 0.5 cm 2.0 cm) was illuminated by a 350 W xenon lamp with the light intensity of 150 mW/cm2.
Hydrogen generation performance The evolution of hydrogen was performed in a twocompartment PEC cell without any external applied voltage or sacrificial agent. As shown in Fig. 1, the PEC cell was separated into an anodic compartment and a cathodic compartment by a Nafion membrane, which were filled with 1 M KOH and 0.5 M H2SO4 electrolytes, respectively. The synthesized TiO2 NTs used as photoanode and a platinum electrode served as cathode were connected with a copper wire. So that a small chemical bias produced by two different electrolytes could assist the electron to transfer from TiO2 NTs into the Pt side. An inverted burette with displacing electrolyte in the burette column was employed to collect hydrogen gas generated at Pt electrode. The volume of H2 was directly determined by reading the variation of electrolyte level.
Results and discussion Electrical response during anodization
Experimental Preparation of TiO2 nanotube arrays
Fig. 2 shows the current density-time curve recorded by a milliammeter during the anodization of Ti samples in the ethylene glycol electrolytes. As can be seen clearly, similar
Highly ordered TiO2 nanotube arrays were synthesized by a rapid anodization process reported in the literature [32]. Before the electrochemical anodization, Ti foils (0.2 mm thick, 99.6% purity) were degreased at room temperature by sonicating in acetone and ethanol for 30 min, respectively, then rinsed with deionized water and dried in air. All anodization experiments were carried out in a two-electrode electrochemical cell at room temperature. Ti foil and Pt foil were used as the anode and cathode, respectively. Ethylene glycol solution containing 0.25 wt% NH4F and 2 vol% H2O was used as electrolyte. The anodization process was performed with a direct current (DC) power supply for 30 min. The anodization voltage was varied from 10 V to 50 V in this study. After electrochemical treatment, the samples were rinsed with deionized water, dried at 80 C and subsequently annealed in air at 450 C for 1 h [33].
Characterization and photoelectrochemical measurements The surface and cross-sectional morphologies of TiO2 nanotube arrays were observed by a field emission scanning
Fig. 1 e Scheme of the two-compartment photoelectrochemical cell.
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Fig. 2 e Current densityetime curve for TiO2 formation process under different anodization voltages.
trend is evident for anodization under different voltages, which is the typical behavior of Ti anodization process. At the initial stage of the anodization, the current density rises to a maximum during the anodization voltage sweeping up to the desired value. Then a rapid decrease to a minimum is observed, which is due to the formation of a barrier layer on the surface of titanium foil (Eq. (1)). The barrier layer is a nonconductive layer. Therefore, it can increase the resistance and reduce the current significantly. At the next stage, as the newly formed pores grow (Eq. (2)), the current begins to increase. When the rate of electrochemical oxidization of Ti and chemical dissolution of TiO2 reaches a dynamic equilibrium, the current gradually approaches to a steady state value. Besides, it is observable that the current density increases with increasing anodization voltage due to the enhancement of electric field intensity. Ti þ 2H2O / TiO2 þ 4Hþ þ 4e
(1)
TiO2 þ 6F þ 4Hþ / TiF2 6 þ 2H2O
(2)
Morphological characterization The surface and cross-sectional morphologies of TiO2 NTs samples anodized under different voltages of 10 V, 20 V, 30 V, 40 V and 50 V are shown in Fig. 3(a)e(e) and (f)e(j), respectively. At 10 V of anodization voltage, a homogeneous nanopores layer is observed clearly from Fig. 3 (a) and (f), which can be explained that the anodization voltage of 10 V is not high enough to reach the formation voltage of TiO2 nanotube arrays in the ethylene glycol electrolyte. According to Eq. (2), fluorine iron plays a key role in the formation of TiO2 NTs, which controls the chemical dissolution rate of TiO2 oxide film. As we know, comparing to the inorganic electrolytes [34,35], organic electrolytes with high viscous severely restrict the diffusion of fluorine iron. Hence the formation of TiO2 NTs needs a higher electric field in ethylene glycol electrolyte. With the increase of anodization voltage, a mixed
structureda top nanoporous layer and an underneath highly ordered nanotube layer is appeared. The mixed structure is attributed to the fact that the thin nanoporous layer on the top surface of TiO2 NTs is not completely dissolved during the anodization process for 30 min in this experiment. Furthermore, it is found that both the average inner diameter and layer thickness of TiO2 NTs increase with increasing anodization voltage. As anodization voltage increases from 10 V to 50 V, the pore diameter ranges from 15 nm to 55 nm while layer thickness ranges 0.4 mme2.7 mm. The effect of anodization voltage on the average pore diameter and thickness of TiO2 samples is summarized in Fig. 4. Obliviously the pore diameter and layer thickness depend linearly on the anodization voltage, which is consistent with other previous reports [36e39]. Based on the TiO2 NTs growth mechanism, anodization process is a competition of electrochemical oxidization (Eq. (1)) and chemical dissolution (Eq. (2)). As the barrier layer formed at the initial anodization, small pits originate in this oxide layer due to the localized dissolution of the oxide. Then the increase in electric field intensity improves electrochemical etching rate, resulting in further pore growth. The deeper pores grow, the higher the electric field intensity is required in these metallic regions, which enhances field-assisted oxide growth and chemical dissolution of the oxide [40]. Therefore, well-defined interpore voids begin forming. After this, both the voids and tubes grow in equilibrium to form the final tubular structure. The tube length increases gradually until the electrochemical oxidization rate and chemical dissolution rate reach a dynamic equilibrium. Thus, improving anodization voltage could promote the growth of pores and nanotube layers simultaneously.
Photoelectrochemical properties The transient photocurrent response of TiO2 nanotubes was measured in the two-compartment PEC cell under light interruption at zero bias. As shown in Fig. 5(a), the current value is approximately 0 mA/cm2 in dark condition while the photocurrent rapidly rises to a constant value upon illumination. All photocurrent patterns of TiO2 samples anodized under different voltages are highly reproducible for several light oneoff cycles. This result indicates that TiO2 nanotubes prepared in this study exhibit good photoresponse and fast charge transfer. The average photocurrent measured during the 3 light oneoff cycles as a function of anodization voltage is shown in Fig. 5(b). The result that the photocurrent increases linearly with increasing anodization voltage is in agreement with Fig. 4(b). It is known that TiO2 NTs with longer tube length in a certain range could provide a larger active surface area and higher absorption of incident photons, which leads to more photogenerated electron-hole pairs [38]. Hence the improvement of photocurrent density is achieved by increasing anodization voltage of TiO2 NTs to obtain longer tube length. Further study of photoelectrochemical behavior of TiO2 NTs anodized under various voltages was determined by linear sweep voltammetry at a scan rate of 10 mV/s from 1.0 V to 1.0 V (vs SCE). As all the dark currents of TiO2 NTs samples are very small (around 103 mA/cm2) and the dark
Fig. 3 e FESEM images of TiO2 nanotube arrays anodized under various voltages (10 V～50 V): (a)e(e) top surface views; (f)e(j) cross-section views.
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Fig. 4 e Dependence of average inner diameter (a) and average layer thickness (b) of TiO2 nanotube arrays on the anodization voltage.
IeV curves are superposed practically, Fig. 6(a) only shows the dark current of TiO2 NTs anodized at 20 V. However, with the illumination, the photocurrent density increases with the increase of applied potential and gradually reaches a plateau value, which demonstrates typical behavior of n-type semiconductor. The result clearly shows that anodization voltage significantly affect the photoelectrochemical performance of TiO2 NTs. The photocurrent increases with increasing anodization voltage and the maximum photocurrent is obtained of TiO2 NTs anodized at 50 V. Corresponding photoconversion efficiency of TiO2 NTs could be calculated by the following equation [41]: " # E0 Eapp 100 hð%Þ ¼ jp rev I0
(3)
where jp is the photocurrent density (mA/cm2), jp E0rev is the total power output, jp Eapp is the electrical power input, and I0 is the power density of incident light. E0rev is the standard reversible potential of 1.23 V/NHE and the applied potential is Eapp ¼ Emeas Eaoc, where Emeas is the electrode potential (vs SCE) of the working electrode and Eaoc is the electrode potential (vs SCE) of the same working electrode under open
Fig. 5 e The transient photocurrent response of TiO2 nanotubes anodized under different voltages (a) and average photocurrent as a function of anodization voltage (b).
circuit condition in the same electrolyte. As expected in Fig. 6(b), TiO2 NTs anodized at 50 V for 30 min exhibits a maximum photoconversion efficiency of 3.51% at 0.61 V vs SCE. Moreover, a linear relationship between the maximum photoconversion efficiency of TiO2 NTs samples and anodization voltage is obtained in Fig. 6(c), which is in line with the layer thickness result (Fig. 4).
Photocatalytic activity The photocatalytic activity of TiO2 NTs was evaluated by hydrogen production in the two-compartment PEC cell without any external applied voltage or sacrificial agent. Efficient separation of H2 generated on Pt side and O2 produced on TiO2 NTs side is achieved via the two-compartment PEC device. Moreover, the small chemical bias produced by the two different electrolytes could accelerate charge transfer and reduce the recombination of electrons and holes. Consequently, there is no need to apply external voltage across the two electrodes or add sacrificial agent in the electrolyte to realize high efficiency of hydrogen generation.
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7
light
6
50V
5
40V
4
30V
3
20V
2
10V
2
Photocurrent density (mA/cm )
(a)
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1 dark
0 -1
-0.8
-0.4
0.0 0.4 Potential (V/vs SCE)
0.8
1.2
Photoconversion efficiency (%)
(b) 4.0
50V
3.5 3.0
40V
2.5
30V
2.0
20V
1.5
10V
1.0 0.5 0.0
-0.8
-0.6
-0.4 -0.2 0.0 Potential (V/vs SCE)
0.2
0.4
Fig. 6 e Photoelectrochemical properties of TiO2 nanotubes with various anodization voltages: (a) the IeV curves (b) corresponding photoconversion efficiency (c) photoconversion efficiency as a function of anodization voltage.
Fig. 7 e Hydrogen generation in the two-compartment PEC cell without any external applied voltage: (a) hydrogen evolution of TiO2 nanotubes anodized under 40 V as a function of irradiation time (b) Effect of anodization voltage on the hydrogen production.
Hydrogen production of TiO2 NTs anodized under 40 V as a function of irradiation time is shown in Fig. 7(a). The amount of evolved H2 is strictly linear with the reaction time, which indicates that TiO2 nanotubes synthesized in this work have excellent stability during the photocatalytic reaction process. The influence of anodization voltage on the hydrogen production is discussed in Fig. 7(b). It can be seen clearly that the amount of hydrogen increases linearly with increasing anodization voltage. A maximum hydrogen production rate of 93.6 mmol h1cm2 is achieved of TiO2 NTs anodized under 50 V. This result is consistent with transient photocurrent response and photoconversion efficiency, which all depend on the layer thickness of TiO2. Previously we reported [32] that the two important factors influencing hydrogen production were the charge carrier generation and their recombination of TiO2. TiO2 samples anodized under a higher voltage with longer tube length could offer a larger active surface area and contact area with the electrolyte, which benefit generating of photoelectron-hole
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pairs and photoelectrochemical reactions. Thus the evolution of hydrogen can be improved efficiently. However, continuous increasing of tube length doesn't mean the continual enhancement of hydrogen production. The charge transfer process should be considered. TiO2 NTs with longer length may generate more electron-hole pairs and provide a longer pathway for charge transfer simultaneously, where a competitive relationship exists consequentially. As the layer thickness beyond a range, the probability of charge recombination would be increased.
Conclusion Self-organized TiO2 nanotube arrays were prepared by electrochemical anodization of Ti foil in ethylene glycol electrolyte. By varying anodization voltage from 10 V to 50 V, the pore diameter of TiO2 nanotubes ranges from 15 nm to 55 nm while the layer thickness ranges from 0.4 mm to 2.7 mm. Both pore diameter and layer thickness increase linearly with anodization voltage, which provides a theoretical basis for controlling growth of TiO2 nanotubes. Efficient hydrogen generation can be achieved by splitting water in the two-compartment PEC cell without any external applied voltage or sacrificial agent. The photocurrent response, photoconversion efficiency and hydrogen production of TiO2 nanotubes are also linearly correlated with the anodization voltage, which are consistent with the result of layer thickness. In this work, TiO2 NTs with the largest active surface area anodized under 50 V for 30 min demonstrates a maximum hydrogen production rate of 93.6 mmol h1cm2 (2.24 mL h1cm2) with the corresponding photoconversion efficiency of 3.51%.
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