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High photoelectrochemical water splitting performance on nitrogen doped double-wall TiO2 nanotube array electrodes
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 6 ( 2 0 1 1 ) 1 3 4 8 1 e1 3 4 8 7
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High photoelectrochemical water splitting performance on nitrogen doped double-wall TiO2 nanotube array electrodes Hongjun Wu a,b, Zhonghai Zhang b,c,* a
College of Chemistry and Chemical Engineering, Northeast Petroleum University, Daqing 163318, China Institute of Basic Energy Science and Technology, George Washington University, VA 20147, USA c Graduate School of Science and Engineering for Research, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan b
article info
abstract
Article history:
The nitrogen doped double-wall TiO2 nanotube arrays (N-DW-TiO2 NTs) have been
Received 12 May 2011
prepared by a facile two steps electrochemical anodization method, and the nitrogen has
Received in revised form
been successfully incorporated into the nanotubes in situ anodization process. The unique
30 July 2011
double walls tubular surface morphology has been achieved by conducting higher anodic
Accepted 3 August 2011
voltage in second anodization process than that in the first anodized step. The nitrogen
Available online 1 September 2011
doping and following annealed process in nitrogen atmosphere did not damage the unique, ordered, and vertically aligned structures. Under illumination of simulated solar light (AM
Keywords:
1.5, 100 mW/cm2), the N-DW-TiO2 NTs presented a high photoelectrochemical water
Nitrogen doped
splitting performance, which mainly ascribed to the high surface areas and expended
Double walls
optical absorbance to visible light region. The high surface areas, ordered structure for
TiO2 nanotube
facilitating electron transfer, and visible light absorbance present the new avenue for
Photoelectrochemical
improving the solar light application in photoelectrochemical water splitting process for
Hydrogen generation
practical hydrogen generation. The N-DW-TiO2 NTs can be one of promising prototype nanomaterials, and much higher photoconversion efficiency can be expected for the codoped or sensitized on the N-DW-TiO2 NTs. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
To solve energy and environmental issues, a clean and renewable energy system is necessary to construct in near future in global level [1]. Hydrogen can play an important role in this system due to its high energy capacity and environmental friendliness as well it can be used in fuel cells [2]. The most hydrogen on earth is conserved in form of water by combining with oxygen. The decomposition of water to form hydrogen and oxygen is a highly endothermic and endergonic process with DH ¼ 285.9 kJ/mol and DG ¼ 237.2 kJ/mol [3]. This
water splitting reaction can be driven via solar energy for hydrogen generation by photoelectrochemical (PEC) process on interface of semiconductor and water, which is a green, carbon dioxide free, and renewable process [4]. Since the pioneering work of PEC water splitting on n-type TiO2 electrode [5], lots of nanomaterials have been extensively studied in this field, such as TiO2 [6e13], ZnO [14,15], Fe2O3 [16e19], and WO3 [20e22] et al with different morphologies and constructions. Among them, the TiO2 is the most promising materials due to the powerful oxidation properties, superior charge transport properties and chemical and optical
* Corresponding author. Institute of Basic Energy Science and Technology, George Washington University, 20101 Academic way Ashburn, VA 20147, USA. Tel./fax: þ1 703 726 8256. E-mail address: [email protected] (Z. Zhang). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.08.014
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stability. Recently, the highly ordered TiO2 nanotube arrays (TiO2 NTs) electrodes have attracted intensive attention in photocatalytic and photovoltaic fields [23e25]. The surface area of TiO2 NTs is an essential factor for maximizing photoconversion efficiency. The tubular structure is regards as the best way to enlarge the surface area without increasing the geometric area [26]. The TiO2 NTs showed very high surface areas, comparable to porous titania nanoparticle films, and has been proven efficient photocatalytic properties [27]. In order to get higher surface area and consequent higher photoconversion efficiency, the double-wall TiO2 NTs maybe one of promising options [28,29]. While the wide band gap of 3.0 and 3.2 eV for rutile and anatase, respectively, restricts the excitation to ultraviolet light, thus limits the optical absorption to visible light region. Many efforts have been used to increase the optical absorption of TiO2 by varying the chemical composition with metal or non-metal doping or selfimpurity [11,30e33]. For example, the optical absorption of TiO2 can be modified by non-metal dopants, which create the electronic transitions from the dopant 2p or 3p orbital to the Ti 3d orbital [34]. At present research, the nitrogen doping showed the greatest optical response [35]. In this paper, the nitrogen doped double-wall TiO2 nanotube arrays (N-DW-TiO2 NTs) have been prepared by a facile two steps anodization method, nitrogen has been successfully incorporated into the nanotubes in situ anodization process. N-DW-TiO2 NTs was used in a PEC water splitting process for hydrogen generation. The photoconversion efficiency of 1.26% was achieved on the N-DW-TiO2 NTs under illumination of simulated solar light (AM 1.5G, 100 mA/cm2).
2.
Experimental section
2.1.
Materials and chemicals
Pure Ti foils (99.6% purity, 0.2 mm thick) were purchased from Nilaco Corp., Japan. Ethylene glycol (EG), ammonium fluoride (NH4F), potassium hydroxide, and ethanol of analytical grade were obtained from Wako Chemicals (Japan) without further purification. All solutions were prepared with doubly distilled deionized water.
2.2.
Preparation of N-DW-TiO2 NTs electrodes
A two-step electrochemical anodization method was used to fabricate the nitrogen-doped TiO2 NTs electrode. Prior to anodization, the Ti foils were first degreased by sonicating in ethanol and cold distilled water in turn, followed by drying in pure nitrogen stream. The anodization experiments were carried out using a conventional two-electrode system with Ti foil as anode and Pt foil as cathode, respectively. The timedependent current behavior under constant potential was recorded using a computer controlled Keithley 2400 sourcemeter. All electrolytes consisted of 0.3 wt% NH4F in an aqueous EG solution (2 vol% water in EG). All the anodization experiments were carried out at room temperature. In the first-step anodization, the Ti foil was anodized at 50 V for 1 h, and the nanotube layer was grown on the foil surface. The nanotube layer was removed by sonicating in deionized
water, and a mirror surface of Ti was exposed. Then, the pretreated Ti was used as anode again for the second-step anodization at 75 V for 30 min. The debris from the anodization bath redepositing on top of the original nanotubes was removed by ultrasonic agitation. The nitrogen incorporated within the nanotubes during the anodization is primarily supplied by the NH4F. After the anodization processes, the sample was cleaned with distilled water and dried off with N2 gas. The as-anodized TiO2 nanotubes have an amorphous structure, and the phase was converted into anatase through annealing process. However, annealing the nitrogen-doped sample at high temperatures in an oxygen-rich environment generally removes the nitrogen by oxidation. The incorporated nitrogen was retained by annealing in dry nitrogen environment at 450 C for 1 h with heating and cooling rate of 2 C/min.
2.3.
Characterization of N-DW-TiO2 NTs
The surface morphologies were observed using field-emission scanning electron microscopy (FE-SEM) (JEOL, FE-SEM 6700), and the elemental composition was estimated by energy dispersive X-ray spectroscopy (EDS, JED-2200, fitted to the JEOL FE-SEM 6700) analysis. The transmission electron microscopy (TEM) by EM-002B (TOPCON Co. Ltd.) has been used to observe the details of morphology about double walls. The crystal structures were characterized by grazing incidence X-ray diffraction (GIXRD) analysis on an X-ray diffractometer (Shimadzu XRD-6000) using Cu Ka source (l ¼ 1.54060 nm) with 40 kV and 20 mA. The GIXRD data was recorded in the range of 2q ¼ 20e70 C with a step of 0.02 and incident angle of 0.5 . The UVevisible absorbance spectrum of the TiO2 NTs sample was measured using diffuse reflection mode spectrophotometer (Shimadazu UVeVISeNIR 3100) at room temperature within the wavelength of 300e900 nm. Surface composition of N-DW-TiO2 NTs was detected using X-ray photoelectron spectroscopy (XPS) (ESCA Lab 220i) with a monochromatic Al anode X-ray gun.
2.4.
Photoelectrochemical measurements
The PEC water splitting experiments were carried out in 1 M KOH (Wako, 25 C, pH ¼ 13.6) electrolyte. Photocurrent was measured in a standard three-electrode configuration with TiO2 NTs as working electrode, Ag/AgCl in saturated KCl as reference electrode, and platinum foil as counter electrode, respectively. The potential and photocurrent of the photoelectrode were controlled by a potentiostat and was reported against the reversible hydrogen electrode (RHE): ERHE ¼ EAg=AgCl þ 0:059 pH þ EAg=AgCl
¼ 0:1976 V at 25 C
with EAg=AgCl (1)
The samples were illuminated by an artificial sunlight simulator, consisting of a SOLAX lamp (model: SET-140F, SERIC Ltd.) and an AM 1.5 filter (100 mW/cm2, thermopile detector from Hamamatsu was used for the measurements). The photocurrent action spectra were obtained under illumination through a monochromator (SG-80, Yokogawa).
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 6 ( 2 0 1 1 ) 1 3 4 8 1 e1 3 4 8 7
3.
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Results and discussion
The top-view SEM images of N-DW-TiO2 NTs are shown in Fig. 1a,b, and obvious double walls are observed with inner pore diameter of 33 2 nm, outer pore diameter of 130 4 nm, and thinner outer wall thickness of 16 1 nm and thicker inner wall thickness of 35 2 nm. The Fig. 1c shows the bottom SEM image of N-DW-TiO2 NTs, the evidence of double walls can be identified from some broken parts of TiO2 NTs, and the tube-in-tube structures have been clearly observed. The cross-sectional view of TiO2 NTs is shown in Fig. 1d, and the as-prepared samples were found to have a length of approximately 9 mm. The N-DW-TiO2 NTs are further characterized by TEM, as shown in Fig. 1e, more evidence of double-wall structures have been found in the individual TiO2 NTs. The high-resolution TEM (HRTEM) image of Fig. 1f indicates single crystallinity with a neighboring lattice fringe distance of 0.35 nm, which corresponds to the distance of the (101) plane in the anatase TiO2 phase. The elemental composition was estimated by EDS analysis, as shown in Fig. 2. The EDS result indicated the chemical
Fig. 2 e EDS of N-DW-TiO2 NTs. The inset shows the highresolution EDS of N peak.
Fig. 1 e SEM and TEM images of N-DW-TiO2 NTs: (a,b) Top-view SEM image; (c) Bottom-view SEM image; (d) cross-sectional SEM image; (e)TEM image; (f) HRTEM image, the inset showed the lattice of anatase.
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Fig. 3 e GIXRD pattern of N-DW-TiO2 NTs.
structure of TiO2, with a chemical composition of Ti:O ¼ 27.42:48.66, and nitrogen element was detected at 0.392 keV (shown in inset of Fig. 2), with atom ratio of 4.32%. Fig. 3 shows the GIXRD pattern of the annealed N-DW-TiO2 NTs, and a strong preferential orientation of (101) is observed. The average crystallite size has been estimated from the DebyeeScherrer equation from the (101) crystal plane as follows [36]: D ¼ 0:9l=bcosq
(2)
where D is the crystalline dimension, l is the wavelength of the X-ray radiation (Cu Ka ¼ 0.15406 nm), q is the diffraction angle, and b is the full width at half maximum. According to the DebyeeScherrer equation, the mean crystallite sizes is 10.5 nm for the (101) plane. The crystallite size is an important factor to determine the stability of TiO2 NTs crystalline phases. A smaller crystallite size implies more stability for the anatase phase. The diffuse reflectance spectra of N-DW-TiO2 NTs were measured using a spectrophotometer equipped with an integrating sphere. As shown the DRS in Fig. 4a, the N-DWTiO2 NTs not only showed the typically initial absorption edge at near ultraviolet region, but also showed visible light absorption peak due to the nitrogen incorporating. The relationship of the absorption coefficient and the incident photon
Fig. 5 e High-resolution XPS spectra of (a) Ti 2p and (b) N 1s.
energy of semiconductors were given by the following equation [37]: ðahnÞ ¼ A hn Eg
n
(3)
Where a is the absorption coefficient, A is a constant, and n is 0.5 and 2.0 for direct transition semiconductor and indirect transition semiconductor, respectively. The TiO2 is the direct transition metal oxide, so the n ¼ 0.5 is used in the above Eq. (3). The direct process based on regions of linear fit in
Fig. 4 e (a) Spectral absorbance of N-DW-TiO2 NTs; (b) the band gap determination of N-DW-TiO2 NTs by (ahn) 2 vs hn curves.
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 6 ( 2 0 1 1 ) 1 3 4 8 1 e1 3 4 8 7
(ahn)2 versus hn was shown in Fig. 4b. Extrapolated direct optical main band gaps of 3.19 eV and sub-band gap of 1.44 eV, which indicated the optical gap of the N-DW-TiO2 NTs is significantly narrowed by the nitrogen incorporated. The high-resolution XPS spectra of Ti 2p and N 1s were recorded to determine the chemical components, existing of nitrogen and the oxidation states of Ti at the surface of the N-DW-TiO2 NTs sample, as shown in Fig. 5. In Fig. 5a, the spectrum indicated the existence of doublet Ti 2p3/2 and Ti 2p1/2 with binding energies of 458.5 eV and 464.2 eV, respectively. The XPS spectrum of N 1s was shown in Fig. 5b with binding energy of 399.8 eV, which confirmed the existence of substitutional nitrogen element. Systematic photoelectrochemical measurements were carried out on the photoelectrodes fabricated from N-DWTiO2 NTs and nitrogen doped single wall TiO2 NTs (N-SWTiO2 NTs). A set of linear-sweep voltammagrams (LSV) were recorded in dark and under illumination of 100 mW/cm2 (AM 1.5), as shown in Fig. 6a. In dark condition, both the N-DWTiO2 NTs and N-SW-TiO2 NTs photoelectrodes showed insignificant current of less than 106 A/cm2, which indicated that no electrocatalytic oxygen evolution occurred. Under illumination, at 1.23 VRHE (corresponding to the potential of the reversible oxygen electrode), the current density is 0.64 mA/cm2 on N-SW-TiO2 NTs. The N-DW-TiO2 NTs showed higher photocurrent density throughout the potential window, which suggested efficient charge separation, and the
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photocurrent reached a high value of 1.51 mA/cm2 at 1.23 VRHE, 136% higher than that on N-SW-TiO2 NTs. These results implied that the N-DW-TiO2 NTs significantly increased the surface area, and enhanced the PEC performance. Amperometric I-t measurements were performed to examine the photoresponse of N-DW-TiO2 NTs over time. As shown in Fig. 6b, good photoresponses were recorded on N-DWTiO2 NTs under conditions of light on and light off, and this photocurrent pattern was highly reproducible for numerous light on/off cycles. The photon-to-hydrogen efficiency was calculated via the following equation [38]: (4) hð%Þ ¼ Jp Erev Eapp Ilight 100 where h is the photoconversion efficiency, Jp is the photocurrent density (mA/cm2), Ilight is the incident light irradiance, E rev is the standard reversible potential which is 1.23 V RHE, and Eapp is the applied potential which is Eapp ¼ Emeas Eaoc, where Emeas is the electrode potential (vs Ag/AgCl) of the working electrode and Eaoc is the electrode potential (vs Ag/ AgCl) of the same working electrode under open circuit condition under illumination. Plots of photoconversion efficiency vs applied potential were shown in Fig. 6c. The SWTiO2 NTs and N-DW-TiO2 NTs showed maximum efficiencies of 0.47% and 1.26% at 0.5 V vs RHE. In order to estimate the quantitative correlation of light absorption on TiO2 NTs, incident-photon-to-current conversion
Fig. 6 e Photoelectrochemical properties of N-SW-TiO2 NTs and N-DW-TiO2 NTs electrodes: (a) linear-sweep voltammagrams collected with a scan rate of 5 mV/s in dark and under illumination; (b) amperometric I-t curves at an applied potential of 1.23 VRHE under illumination with 60 s light on/off cycles; (c) photoconversion efficiency as a function of applied potential; (d) IPCE spectra in the region of 300e500 nm at 1.23 VRHE.
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generation. We think that this result can serve important contribution to the researchers in the fields of inorganic and nanomaterials, physical chemistry, and energy-related sciences.
Acknowledgment This work was supported in part by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (JSPS). Z. Z acknowledges also the support of this work by the venture business laboratory of Toyama University.
references Fig. 7 e Amperometric I-t curves of the N-DW-TiO2 NTs collected at a potential of D1.23 V RHE for 3000 s.
efficiency (IPCE) measurements were performed to study the photoresponse wavelength region for N-SW-TiO2 NTs and NDW-TiO2 NTs. IPCE was calculated using the following equation [39]: IPCE ¼ 1240 Jp
l Ilight
(5)
where Jp is the photocurrent density, l is the incident light wavelength, and Ilight is the measured irradiance. The N-SWTiO2 NTs and N-DW-TiO2 NTs behaved in a similar manner, but the N-DW-TiO2 NTs showed a much higher IPCE value. The N-SW-TiO2 NTs showed a maximum IPCE value of 14% and the N-DW-TiO2 NTs showed a maximum IPCE value of 56% at 390 nm, as shown in Fig. 6d. The N-DW-TiO2 NTs showed much higher photoconversion conversion than NSW-TiO2 NTs under illumination of solar light, which caused not only the significant increase of surface area, but also the better structural smoothness and orderliness of TiO2 NTs [40], another possibility is the decrease of Ti3þ due to the high anodized potentials [41]. In addition, the stability of the photoelectrodes was also an important factor for practical application of PEC water splitting. A small decay of photocurrent density was observed after continuous running for 3000 s under illumination, which indicated that the N-DWTiO2 NTs electrodes were relatively stable in the PEC process, as shown in Fig. 7.
4.
Conclusion
In summary, we prepared N-DWTiO2 NTs by a facile twostep anodization method. The surface area, smoothness and orderliness of the TiO2 NTs were improved considerably in comparison with the conventional N-SWTiO2 NTs. The N-DWTiO2 NTs can be promising prototype nanomaterials, much higher photoconversion efficiency can be expected for the doped and sensitized of TiO2 NTs, and these researches are ongoing. The highly surface and ordered structure present the new avenue for improving the efficiency of other photocatalysts in PEC water splitting for practical hydrogen
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Available at www.sciencedirect.com
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High photoelectrochemical water splitting performance on nitrogen doped double-wall TiO2 nanotube array electrodes Hongjun Wu a,b, Zhonghai Zhang b,c,* a
College of Chemistry and Chemical Engineering, Northeast Petroleum University, Daqing 163318, China Institute of Basic Energy Science and Technology, George Washington University, VA 20147, USA c Graduate School of Science and Engineering for Research, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan b
article info
abstract
Article history:
The nitrogen doped double-wall TiO2 nanotube arrays (N-DW-TiO2 NTs) have been
Received 12 May 2011
prepared by a facile two steps electrochemical anodization method, and the nitrogen has
Received in revised form
been successfully incorporated into the nanotubes in situ anodization process. The unique
30 July 2011
double walls tubular surface morphology has been achieved by conducting higher anodic
Accepted 3 August 2011
voltage in second anodization process than that in the first anodized step. The nitrogen
Available online 1 September 2011
doping and following annealed process in nitrogen atmosphere did not damage the unique, ordered, and vertically aligned structures. Under illumination of simulated solar light (AM
Keywords:
1.5, 100 mW/cm2), the N-DW-TiO2 NTs presented a high photoelectrochemical water
Nitrogen doped
splitting performance, which mainly ascribed to the high surface areas and expended
Double walls
optical absorbance to visible light region. The high surface areas, ordered structure for
TiO2 nanotube
facilitating electron transfer, and visible light absorbance present the new avenue for
Photoelectrochemical
improving the solar light application in photoelectrochemical water splitting process for
Hydrogen generation
practical hydrogen generation. The N-DW-TiO2 NTs can be one of promising prototype nanomaterials, and much higher photoconversion efficiency can be expected for the codoped or sensitized on the N-DW-TiO2 NTs. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
To solve energy and environmental issues, a clean and renewable energy system is necessary to construct in near future in global level [1]. Hydrogen can play an important role in this system due to its high energy capacity and environmental friendliness as well it can be used in fuel cells [2]. The most hydrogen on earth is conserved in form of water by combining with oxygen. The decomposition of water to form hydrogen and oxygen is a highly endothermic and endergonic process with DH ¼ 285.9 kJ/mol and DG ¼ 237.2 kJ/mol [3]. This
water splitting reaction can be driven via solar energy for hydrogen generation by photoelectrochemical (PEC) process on interface of semiconductor and water, which is a green, carbon dioxide free, and renewable process [4]. Since the pioneering work of PEC water splitting on n-type TiO2 electrode [5], lots of nanomaterials have been extensively studied in this field, such as TiO2 [6e13], ZnO [14,15], Fe2O3 [16e19], and WO3 [20e22] et al with different morphologies and constructions. Among them, the TiO2 is the most promising materials due to the powerful oxidation properties, superior charge transport properties and chemical and optical
* Corresponding author. Institute of Basic Energy Science and Technology, George Washington University, 20101 Academic way Ashburn, VA 20147, USA. Tel./fax: þ1 703 726 8256. E-mail address: [email protected] (Z. Zhang). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.08.014
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stability. Recently, the highly ordered TiO2 nanotube arrays (TiO2 NTs) electrodes have attracted intensive attention in photocatalytic and photovoltaic fields [23e25]. The surface area of TiO2 NTs is an essential factor for maximizing photoconversion efficiency. The tubular structure is regards as the best way to enlarge the surface area without increasing the geometric area [26]. The TiO2 NTs showed very high surface areas, comparable to porous titania nanoparticle films, and has been proven efficient photocatalytic properties [27]. In order to get higher surface area and consequent higher photoconversion efficiency, the double-wall TiO2 NTs maybe one of promising options [28,29]. While the wide band gap of 3.0 and 3.2 eV for rutile and anatase, respectively, restricts the excitation to ultraviolet light, thus limits the optical absorption to visible light region. Many efforts have been used to increase the optical absorption of TiO2 by varying the chemical composition with metal or non-metal doping or selfimpurity [11,30e33]. For example, the optical absorption of TiO2 can be modified by non-metal dopants, which create the electronic transitions from the dopant 2p or 3p orbital to the Ti 3d orbital [34]. At present research, the nitrogen doping showed the greatest optical response [35]. In this paper, the nitrogen doped double-wall TiO2 nanotube arrays (N-DW-TiO2 NTs) have been prepared by a facile two steps anodization method, nitrogen has been successfully incorporated into the nanotubes in situ anodization process. N-DW-TiO2 NTs was used in a PEC water splitting process for hydrogen generation. The photoconversion efficiency of 1.26% was achieved on the N-DW-TiO2 NTs under illumination of simulated solar light (AM 1.5G, 100 mA/cm2).
2.
Experimental section
2.1.
Materials and chemicals
Pure Ti foils (99.6% purity, 0.2 mm thick) were purchased from Nilaco Corp., Japan. Ethylene glycol (EG), ammonium fluoride (NH4F), potassium hydroxide, and ethanol of analytical grade were obtained from Wako Chemicals (Japan) without further purification. All solutions were prepared with doubly distilled deionized water.
2.2.
Preparation of N-DW-TiO2 NTs electrodes
A two-step electrochemical anodization method was used to fabricate the nitrogen-doped TiO2 NTs electrode. Prior to anodization, the Ti foils were first degreased by sonicating in ethanol and cold distilled water in turn, followed by drying in pure nitrogen stream. The anodization experiments were carried out using a conventional two-electrode system with Ti foil as anode and Pt foil as cathode, respectively. The timedependent current behavior under constant potential was recorded using a computer controlled Keithley 2400 sourcemeter. All electrolytes consisted of 0.3 wt% NH4F in an aqueous EG solution (2 vol% water in EG). All the anodization experiments were carried out at room temperature. In the first-step anodization, the Ti foil was anodized at 50 V for 1 h, and the nanotube layer was grown on the foil surface. The nanotube layer was removed by sonicating in deionized
water, and a mirror surface of Ti was exposed. Then, the pretreated Ti was used as anode again for the second-step anodization at 75 V for 30 min. The debris from the anodization bath redepositing on top of the original nanotubes was removed by ultrasonic agitation. The nitrogen incorporated within the nanotubes during the anodization is primarily supplied by the NH4F. After the anodization processes, the sample was cleaned with distilled water and dried off with N2 gas. The as-anodized TiO2 nanotubes have an amorphous structure, and the phase was converted into anatase through annealing process. However, annealing the nitrogen-doped sample at high temperatures in an oxygen-rich environment generally removes the nitrogen by oxidation. The incorporated nitrogen was retained by annealing in dry nitrogen environment at 450 C for 1 h with heating and cooling rate of 2 C/min.
2.3.
Characterization of N-DW-TiO2 NTs
The surface morphologies were observed using field-emission scanning electron microscopy (FE-SEM) (JEOL, FE-SEM 6700), and the elemental composition was estimated by energy dispersive X-ray spectroscopy (EDS, JED-2200, fitted to the JEOL FE-SEM 6700) analysis. The transmission electron microscopy (TEM) by EM-002B (TOPCON Co. Ltd.) has been used to observe the details of morphology about double walls. The crystal structures were characterized by grazing incidence X-ray diffraction (GIXRD) analysis on an X-ray diffractometer (Shimadzu XRD-6000) using Cu Ka source (l ¼ 1.54060 nm) with 40 kV and 20 mA. The GIXRD data was recorded in the range of 2q ¼ 20e70 C with a step of 0.02 and incident angle of 0.5 . The UVevisible absorbance spectrum of the TiO2 NTs sample was measured using diffuse reflection mode spectrophotometer (Shimadazu UVeVISeNIR 3100) at room temperature within the wavelength of 300e900 nm. Surface composition of N-DW-TiO2 NTs was detected using X-ray photoelectron spectroscopy (XPS) (ESCA Lab 220i) with a monochromatic Al anode X-ray gun.
2.4.
Photoelectrochemical measurements
The PEC water splitting experiments were carried out in 1 M KOH (Wako, 25 C, pH ¼ 13.6) electrolyte. Photocurrent was measured in a standard three-electrode configuration with TiO2 NTs as working electrode, Ag/AgCl in saturated KCl as reference electrode, and platinum foil as counter electrode, respectively. The potential and photocurrent of the photoelectrode were controlled by a potentiostat and was reported against the reversible hydrogen electrode (RHE): ERHE ¼ EAg=AgCl þ 0:059 pH þ EAg=AgCl
¼ 0:1976 V at 25 C
with EAg=AgCl (1)
The samples were illuminated by an artificial sunlight simulator, consisting of a SOLAX lamp (model: SET-140F, SERIC Ltd.) and an AM 1.5 filter (100 mW/cm2, thermopile detector from Hamamatsu was used for the measurements). The photocurrent action spectra were obtained under illumination through a monochromator (SG-80, Yokogawa).
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3.
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Results and discussion
The top-view SEM images of N-DW-TiO2 NTs are shown in Fig. 1a,b, and obvious double walls are observed with inner pore diameter of 33 2 nm, outer pore diameter of 130 4 nm, and thinner outer wall thickness of 16 1 nm and thicker inner wall thickness of 35 2 nm. The Fig. 1c shows the bottom SEM image of N-DW-TiO2 NTs, the evidence of double walls can be identified from some broken parts of TiO2 NTs, and the tube-in-tube structures have been clearly observed. The cross-sectional view of TiO2 NTs is shown in Fig. 1d, and the as-prepared samples were found to have a length of approximately 9 mm. The N-DW-TiO2 NTs are further characterized by TEM, as shown in Fig. 1e, more evidence of double-wall structures have been found in the individual TiO2 NTs. The high-resolution TEM (HRTEM) image of Fig. 1f indicates single crystallinity with a neighboring lattice fringe distance of 0.35 nm, which corresponds to the distance of the (101) plane in the anatase TiO2 phase. The elemental composition was estimated by EDS analysis, as shown in Fig. 2. The EDS result indicated the chemical
Fig. 2 e EDS of N-DW-TiO2 NTs. The inset shows the highresolution EDS of N peak.
Fig. 1 e SEM and TEM images of N-DW-TiO2 NTs: (a,b) Top-view SEM image; (c) Bottom-view SEM image; (d) cross-sectional SEM image; (e)TEM image; (f) HRTEM image, the inset showed the lattice of anatase.
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Fig. 3 e GIXRD pattern of N-DW-TiO2 NTs.
structure of TiO2, with a chemical composition of Ti:O ¼ 27.42:48.66, and nitrogen element was detected at 0.392 keV (shown in inset of Fig. 2), with atom ratio of 4.32%. Fig. 3 shows the GIXRD pattern of the annealed N-DW-TiO2 NTs, and a strong preferential orientation of (101) is observed. The average crystallite size has been estimated from the DebyeeScherrer equation from the (101) crystal plane as follows [36]: D ¼ 0:9l=bcosq
(2)
where D is the crystalline dimension, l is the wavelength of the X-ray radiation (Cu Ka ¼ 0.15406 nm), q is the diffraction angle, and b is the full width at half maximum. According to the DebyeeScherrer equation, the mean crystallite sizes is 10.5 nm for the (101) plane. The crystallite size is an important factor to determine the stability of TiO2 NTs crystalline phases. A smaller crystallite size implies more stability for the anatase phase. The diffuse reflectance spectra of N-DW-TiO2 NTs were measured using a spectrophotometer equipped with an integrating sphere. As shown the DRS in Fig. 4a, the N-DWTiO2 NTs not only showed the typically initial absorption edge at near ultraviolet region, but also showed visible light absorption peak due to the nitrogen incorporating. The relationship of the absorption coefficient and the incident photon
Fig. 5 e High-resolution XPS spectra of (a) Ti 2p and (b) N 1s.
energy of semiconductors were given by the following equation [37]: ðahnÞ ¼ A hn Eg
n
(3)
Where a is the absorption coefficient, A is a constant, and n is 0.5 and 2.0 for direct transition semiconductor and indirect transition semiconductor, respectively. The TiO2 is the direct transition metal oxide, so the n ¼ 0.5 is used in the above Eq. (3). The direct process based on regions of linear fit in
Fig. 4 e (a) Spectral absorbance of N-DW-TiO2 NTs; (b) the band gap determination of N-DW-TiO2 NTs by (ahn) 2 vs hn curves.
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(ahn)2 versus hn was shown in Fig. 4b. Extrapolated direct optical main band gaps of 3.19 eV and sub-band gap of 1.44 eV, which indicated the optical gap of the N-DW-TiO2 NTs is significantly narrowed by the nitrogen incorporated. The high-resolution XPS spectra of Ti 2p and N 1s were recorded to determine the chemical components, existing of nitrogen and the oxidation states of Ti at the surface of the N-DW-TiO2 NTs sample, as shown in Fig. 5. In Fig. 5a, the spectrum indicated the existence of doublet Ti 2p3/2 and Ti 2p1/2 with binding energies of 458.5 eV and 464.2 eV, respectively. The XPS spectrum of N 1s was shown in Fig. 5b with binding energy of 399.8 eV, which confirmed the existence of substitutional nitrogen element. Systematic photoelectrochemical measurements were carried out on the photoelectrodes fabricated from N-DWTiO2 NTs and nitrogen doped single wall TiO2 NTs (N-SWTiO2 NTs). A set of linear-sweep voltammagrams (LSV) were recorded in dark and under illumination of 100 mW/cm2 (AM 1.5), as shown in Fig. 6a. In dark condition, both the N-DWTiO2 NTs and N-SW-TiO2 NTs photoelectrodes showed insignificant current of less than 106 A/cm2, which indicated that no electrocatalytic oxygen evolution occurred. Under illumination, at 1.23 VRHE (corresponding to the potential of the reversible oxygen electrode), the current density is 0.64 mA/cm2 on N-SW-TiO2 NTs. The N-DW-TiO2 NTs showed higher photocurrent density throughout the potential window, which suggested efficient charge separation, and the
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photocurrent reached a high value of 1.51 mA/cm2 at 1.23 VRHE, 136% higher than that on N-SW-TiO2 NTs. These results implied that the N-DW-TiO2 NTs significantly increased the surface area, and enhanced the PEC performance. Amperometric I-t measurements were performed to examine the photoresponse of N-DW-TiO2 NTs over time. As shown in Fig. 6b, good photoresponses were recorded on N-DWTiO2 NTs under conditions of light on and light off, and this photocurrent pattern was highly reproducible for numerous light on/off cycles. The photon-to-hydrogen efficiency was calculated via the following equation [38]: (4) hð%Þ ¼ Jp Erev Eapp Ilight 100 where h is the photoconversion efficiency, Jp is the photocurrent density (mA/cm2), Ilight is the incident light irradiance, E rev is the standard reversible potential which is 1.23 V RHE, and Eapp is the applied potential which is Eapp ¼ Emeas Eaoc, where Emeas is the electrode potential (vs Ag/AgCl) of the working electrode and Eaoc is the electrode potential (vs Ag/ AgCl) of the same working electrode under open circuit condition under illumination. Plots of photoconversion efficiency vs applied potential were shown in Fig. 6c. The SWTiO2 NTs and N-DW-TiO2 NTs showed maximum efficiencies of 0.47% and 1.26% at 0.5 V vs RHE. In order to estimate the quantitative correlation of light absorption on TiO2 NTs, incident-photon-to-current conversion
Fig. 6 e Photoelectrochemical properties of N-SW-TiO2 NTs and N-DW-TiO2 NTs electrodes: (a) linear-sweep voltammagrams collected with a scan rate of 5 mV/s in dark and under illumination; (b) amperometric I-t curves at an applied potential of 1.23 VRHE under illumination with 60 s light on/off cycles; (c) photoconversion efficiency as a function of applied potential; (d) IPCE spectra in the region of 300e500 nm at 1.23 VRHE.
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generation. We think that this result can serve important contribution to the researchers in the fields of inorganic and nanomaterials, physical chemistry, and energy-related sciences.
Acknowledgment This work was supported in part by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (JSPS). Z. Z acknowledges also the support of this work by the venture business laboratory of Toyama University.
references Fig. 7 e Amperometric I-t curves of the N-DW-TiO2 NTs collected at a potential of D1.23 V RHE for 3000 s.
efficiency (IPCE) measurements were performed to study the photoresponse wavelength region for N-SW-TiO2 NTs and NDW-TiO2 NTs. IPCE was calculated using the following equation [39]: IPCE ¼ 1240 Jp
l Ilight
(5)
where Jp is the photocurrent density, l is the incident light wavelength, and Ilight is the measured irradiance. The N-SWTiO2 NTs and N-DW-TiO2 NTs behaved in a similar manner, but the N-DW-TiO2 NTs showed a much higher IPCE value. The N-SW-TiO2 NTs showed a maximum IPCE value of 14% and the N-DW-TiO2 NTs showed a maximum IPCE value of 56% at 390 nm, as shown in Fig. 6d. The N-DW-TiO2 NTs showed much higher photoconversion conversion than NSW-TiO2 NTs under illumination of solar light, which caused not only the significant increase of surface area, but also the better structural smoothness and orderliness of TiO2 NTs [40], another possibility is the decrease of Ti3þ due to the high anodized potentials [41]. In addition, the stability of the photoelectrodes was also an important factor for practical application of PEC water splitting. A small decay of photocurrent density was observed after continuous running for 3000 s under illumination, which indicated that the N-DWTiO2 NTs electrodes were relatively stable in the PEC process, as shown in Fig. 7.
4.
Conclusion
In summary, we prepared N-DWTiO2 NTs by a facile twostep anodization method. The surface area, smoothness and orderliness of the TiO2 NTs were improved considerably in comparison with the conventional N-SWTiO2 NTs. The N-DWTiO2 NTs can be promising prototype nanomaterials, much higher photoconversion efficiency can be expected for the doped and sensitized of TiO2 NTs, and these researches are ongoing. The highly surface and ordered structure present the new avenue for improving the efficiency of other photocatalysts in PEC water splitting for practical hydrogen
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