Visible Light Absorbing TiO2 Nanotube Arrays by Sulfur Treatment for

May 27, 2015 - Department of Chemical Engineering, Yeungnam University, ... Convergence Research Center, Chonnam National University, Gwangju...
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Visible Light Absorbing TiO2 Nanotube Arrays by Sulfur Treatment for Photoelectrochemical Water Splitting Seung Wook Shin,†,⊥ Jeong Yong Lee,† Kwang-Soon Ahn,‡ Soon Hyung Kang,*,§ and Jin Hyeok Kim*,∥ †

Center for Nanomaterials and Chemical Reactions, Institute for Basic Science, Daejeon 305-701, South Korea Department of Chemical Engineering, Yeungnam University, Gyeongsan 712-749, South Korea § Department of Chemistry Education and Optoelectronics Convergence Research Center, Chonnam National University, Gwangju 500-757, South Korea ∥ Department of Materials Science and Engineering, Chonnam National University, Gwangju 500-757, South Korea ‡

ABSTRACT: Herein, we report the preparation and characterizations of the sulfur (S)-doped TiO2 nanotube (TONT) arrays prepared by a sulfurization process of TONT arrays via electrochemical anodization on a Ti substrate with pure TONT arrays. The S-doped TONT arrays were prepared with the annealing temperature from 450 to 550 °C under H2S gas for 10 min, and these reaction conditions corresponded to no modification of the morphological features relative to that of the TONT arrays. Furthermore, the 500 °C annealed S-doped TONT arrays showed enhanced visible light absorption and high electric conductivity, thus resulting in the most improved photocurrent density (2.92 mA cm−2 at 1.0 V vs sat. Ag/AgCl) in the 0.1 M KOH solution as compared with that (0.965 mA cm−2 at 1.0 V vs sat. Ag/AgCl) of TONT arrays. In addition, the incident photon-to-electron conversion efficiency (IPCE) of S-doped TONT arrays exhibited approximately 43% in the UV region, whereas TONT arrays had 32% IPCE in the UV region. In addition, the small photoactivity in the visible light region for the S-doped TONT arrays was observed up to a 600 nm wavelength, where IPCE value of 2.4% at 500 nm was achieved in the Sdoped TONT arrays, in contrast to the negligible IPCE values for the TONT arrays. However, the relatively reduced photocurrent density (2.04 mA cm−2 at 1.0 V vs sat. Ag/AgCl) was achieved at further sulfurization temperature at 550 °C for the S-doped TONT arrays; this value is attributed to the rough tube shape and atomic level defects in the edge region for the excessively S-doped TONT array, which indicated a role as the light scattering centers and the electron−hole trap sites.

1. INTRODUCTION Anatase titania (TiO2) is a well-studied photoanode material due to its good charge transport properties and high chemical stability in solution and is regarded as a promising solar-driven photocatalyst for hydrogen generation and water cleaning.1−3 However, its band gap energy of 3.2 eV limits its solar-tohydrogen (STH) efficiency for photoelectrochemical (PEC) applications. Much effort has been focused on improving its ultraviolet (UV) and visible (Vis) light harvesting capability by modulating its microstructure (morphology, size, crystallinity, facets, etc.).4−6 Research has also focused on improving its UV and Vis light harvesting capability by the introduction of lightabsorbing materials (sensitizer, quantum dots, and dyes) and by tuning its electronic properties from the addition of external impurities, thus causing the change of the position of the maximum valence band and the minimum conduction band.7−9 Among these strategies, the shift of its absorption from the UV region into the visible light region allows for more photons to be absorbed and utilized in splitting the water. Much progress has been made in the area of Vis-light-active TiO2 by introducing various dopants into its crystal lattice structure; these dopants include metal and nonmetal elements.10−15 In © XXXX American Chemical Society

particular, anatase TiO2 was reported to have better solar absorption and photocatalytic activity from nonmetal doping such as N, C, and S due to certain disadvantages of metal doping such as low thermal stability and enhanced recombination of charge carriers.13−15 Also, it was reported that these dopants give positive or negative effects on the overall photocurrent depending on the experimental conditions such as electronic properties of the materials, their concentration, and the method for incorporating these materials into the host lattice. These results imply that the optimal doping conditions can be varied depending on the material and its preparation method. In the case of S dopant into the TiO2 crystal system, it has been suggested that the band gap energy is narrowed because of the new formation of the S 3s state above the maximum valence band, therefore resulting in the increase of visible light absorption by the transition from the S 3s state to the minimum conduction band.16,17 Several groups have already reported that S-doped TiO2 exhibits the enhanced Received: February 3, 2015 Revised: May 6, 2015

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three-step (acetone-ethanol-deionized (DI) water) ultrasonic cleaning process for 10 min. The constant conditions (60 V for 30 min) and the distance (∼3 cm) between the working (Ti foil) and counter electrode (Pt mesh) were used for the electrochemical anodization. As reported elsewhere,26 the electrolyte consisted of 0.25 wt % NH4F in ethylene glycol containing an extremely small amount of water. After the reaction was complete, the as-anodic TONT arrays were cleaned with ethanol solution, followed by drying using nitrogen gas. Afterward, the high-temperature thermal treatment was performed at 450 °C for 3 h under ambient air in order to improve the crystallinity. Furthermore, in order to fabricate the S-doped TONT arrays, the TONT arrays were annealed in a tubular furnace system in a mixed N2 (95%) + H2S (5%) atmosphere for 10 min at annealing temperatures including 450, 500, and 550 °C. The heating rate of the annealing process was 10 °C/min. After the annealing process, the annealed thin films were cooled naturally for 3 h. 2.2. Characterizations. The PEC measurements were performed in a three-electrode cell under light illumination using a potentiostat (CHI Instruments, USA). TONT arrays were used as the working electrodes with an active area of 0.785 cm2. The Pt sheet (area: 10 cm2) and an Ag/AgCl electrode with saturated KCl were used as the counter and reference electrodes, respectively. After nitrogen bubbling, an aqueous electrolyte containing 0.1 M KOH (pH 13.5) was used to remove the dissolved oxygen gas. The PEC responses were measured using a Xe lamp (150 W) with a light intensity of 100 mW/cm2. The current−voltage (J−V) performances under the chopped light on/off illumination were measured at a scan rate of 5 mV/s during the potential sweep. The incident photon-tocurrent conversion efficiency (IPCE) was measured from a 300 to 650 nm wavelength at a potential of 1 V vs sat. Ag/AgCl using a specially designed IPCE system for PEC water splitting. Then, a 150 W xenon lamp was used as a light source for generating the monochromatic beam. Calibration was performed using a silicon photodiode, which was certified by NREL. Also, in order to assess the flat-band potential (VFB) and donor concentration of TONT and S-doped TONT arrays, Mott−Schottky plots (AUTOLAB/PGSTAT, 128N) with a frequency of 1 kHz were measured using a standard potentiostat equipped with an impedance spectra analyzer (Nova) in the same electrochemical configuration and electrolyte under the light illumination controlling the voltage bias. The thickness and morphology of TONT and S-doped TONT arrays were confirmed by field emission scanning electron microscopy (FE-SEM, S4800, HITACHI Inc.) operating at 10 kV and 20 mA. High-resolution transmission electron microscopy (HR-TEM, JSM-200FXII, JEOL, Japan) was used to identify the modification of crystalline properties in the S-doped TONT arrays. The crystalline properties of TONT and S-doped TONT arrays were identified using the highpower X-ray diffraction (HP-XRD, PANalytical, X′Pert PRO) operating at 60 kV and 55 mA. Furthermore, the absorbance of each sample was evaluated using ultraviolet−visible (UV−vis) spectroscopy (PerkinElmer LAMBDA-900 UV/VB/IR Spectrometer). In addition, the chemical bonding states of each element in the TONT and S-doped TONT arrays were examined by X-ray photoemission spectroscopy (XPS, PHI 5200 mode) using an Al Kα X-ray source with a chamber base pressure of ∼10−10 Torr.

photocatalytic activity under visible light. In particular, since the presence of both anionic S2− and cationic S4+/6+ species was experimentally identified on S-doped TiO2, anionic and cationic doping was accordingly proposed via the substitution of S2− and S4+/6+ for O and Ti ions in the TiO2 lattice, respectively.18 This substitution would lead to the intragap impurity states between the valence and conduction band and narrow the TiO2 band gap energy. In both cases, the optical absorption was found to shift to the narrow energies promoting the response of the doped material into the visible range. Recently, one-dimensional (1-D) TiO2 nanotubes (TONTs) on Ti substrate have been widely studied19−21 due to their outstanding charge transport properties and the increase of photoexcited charge carrier lifetime by more than an order of magnitude in the circumstance where the photogenerated charge carrier on the surface of TONT arrays can directly pass through the 1-D nanostructure in comparison to the randomly oriented nanoparticles. Also, the porous nanostructure promotes ion diffusion in the TiO2/electrolyte interface, thus suppressing the photogenerated electron−hole pairs from the charge recombination. Furthermore, the pore size and length of TONT arrays are easily controlled by the experimental parameters such as applied potential, time, and electrolyte composition in order to provide the multiplex functionality for the photoelectrochemical and photocatalytic water splitting, the degradation of organic pollutant, and dye-sensitized solar cells.22−24 However, the absorption of solar light for anatase TONT arrays is still poor due to the wide band gap energy (3.2 eV) needed for the doping treatment. Therefore, it is expected that a combination of S doping and TONT arrays may offer the synergetic effects including the favorable charge transport and the increased carrier lifetime from the TONT arrays and the enhanced Vis light absorption and the increased donor density from S doping. To the authors’ knowledge, no detailed studies on the fabrication and PEC properties of S-doped TONT arrays have been reported. However, Li’s group reported the S doping of TONT arrays in the low temperature (380 °C) thermal treatment under the H2S ambient and demonstrated the effect of S doping on the TONT arrays.25 Then, the improved photocurrent density of Sdoped TONT arrays is ranged from 0.1 to 0.2 mA/cm2 in the potential range 0.4−1.0 V (vs saturated calomel electrode), compared with that (below 0.1 mA/cm2) of anatase TONT arrays, corresponding with the photocurrent density in the range of ∼μA/cm2 under the Vis light. However, it is still needed to enhance the PEC performance to control the morphology and the method for S treatment through the more precise control of S content as well as their sufficient photoand electrochemical characterizations. Therefore, in this paper, we report the fabrication of highly ordered TONT arrays with the thickness of approximately 10 μm by electrochemical anodization, which is subsequently followed by S treatment from the thermal treatment in a H2S atmosphere under various temperatures from 450 to 550 °C for 10 min. The color of Sdoped TONT arrays turned to the dark brown color, thus indirectly showing that the doped S ions effectively contributed to the narrowing of TiO2 band gap energy, leading to the enhanced PEC activity under full sunlight and visible light illumination.

2. EXPERIMENTAL SECTION 2.1. Preparation of S-Doped TONT Arrays. Titanium foil (99.9%, 0.25 μm), 1.5 cm × 2 cm in size, was prepared after a B

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Figure 1. FE-SEM images of (a) the 10 μm thick TONT film, (b) the S-doped (450 °C) TONT film, (c) the S-doped (500 °C) TONT film, and (d) the S-doped (550 °C) TONT film. The inset of part a indicates the cross-sectional view of the TONT film.

3. RESULTS AND DISCUSSION 3.1. Morphological Characterization of TONT and SDoped TONT Arrays. Figure 1 shows the top FE-SEM images of (a) TONT, (b) S-doped TONT (450 °C), (c) S-doped TONT (500 °C), and (d) S-doped TONT (550 °C) arrays. An average length of TONT arrays was approximately 10 μm, as shown in the inset of Figure 1a, and an inner pore diameter of about 100 nm was observed. The presence of well-aligned TONT arrays vertically oriented from the Ti substrate promoted any directional charge transport due to the 1-D features of the tubes. The S treatment from annealing by H2S atmosphere briefly denoted as the sulfurization process kept their nanotubular structure independent of the sulfurization temperature, thus indicating that the sulfurization process did not destroy the nanotubular structure of TONT arrays. To identify the crystalline properties of each sample, the XRD measurements were carried out as shown in Figure 2. All samples exhibited the highly polycrystalline anatase TiO2 phase with the (101), (103), (004), (200), (105), (211), (204), and (116) planes. The intensity of the (004) plane associated with the growth toward the c-axis direction was similar when using the same thick TONT arrays. The additional sulfurization process induced no modification of the full width at halfmaximum, and the calculated average grain sizes of TONT arrays using Scherrer’s equation27 were 32.7, 34.2, 33.7, and 29.5 nm for TONT, S-doped TONT (450 °C), S-doped TONT (500 °C), and S-doped TONT (550 °C) arrays, respectively. It was noticed that the grain size slightly decreased in the case of S-doped TONT (550 °C) arrays after the sulfurization process. Furthermore, in order to examine the modification of the crystalline properties by sulfur treatment, TEM measurements were carried out as shown in Figure 3. Low magnification and HR-TEM images were observed for the TONT (a and b), S-doped TONT (500 °C, c and d), and S-doped TONT (550 °C, e and f) arrays. The TONT arrays were highly ordered and vertically aligned. Parts a, c, and e of Figure 3 show the nanotube shape with a length of several micrometers and a diameter of 100 ± 11.4 nm. Remarkably, it

Figure 2. XRD patterns of (a) the TONT film, (b) the S-doped (450 °C) TONT film, (c) the S-doped (500 °C) TONT film, and (d) the S-doped (550 °C) TONT film. Asterisks indicate the peaks from the Ti substrate.

was seen that the edge of TONT arrays was smoother than that of S-doped TONT arrays. These results are attributed to the element rearrangement and recrystallization at the edge region in the TONT arrays though the S annealing process. The interplanar spaces of 0.351 and 0.125 nm in the HR-TEM image of TONT arrays (Figure 3b) with clear lattice fringes corresponded to the anatase (101) and (215) planes of the TiO2 crystal system. However, HR-TEM images of S-doped TONT arrays showed the slightly larger interplanar spaces of 0.353 nm (500 °C) and 0.357 nm (550 °C) as compared to that of the anatase (101) plane of the TiO2 crystal system. The slightly larger interplanar spaces in the S-doped TONT arrays are attributed to the difference in the ionic radius between S2− (0.184 nm) and O2− (0.136 nm), which indicated the partial replacement of the O element in the edge region of TONT arrays by the larger S element. In addition, the atomic level defects including vacancies, stacking faults, and twin boundaries were observed in the S-doped TONT arrays (550 °C) as C

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Figure 3. Low magnification and HR-TEM images of (a and b) the TONT film, (c and d) the S-doped (500 °C) TONT film, (e and f) the S-doped (550 °C) TONT film, and TEM EDS spectra of (g) the TONT film and (h) the S-doped (550 °C) TONT film. The Cu peaks in the EDS spectra resulted from Cu TEM mesh grids.

compared to that of TONT and S-doped TONT (500 °C) arrays, as represented in Figure 3f. The TEM EDS characterizations were carried out in order to prove the doping of S elements in the TONT and S-doped TONT (500 °C) arrays, as shown in Figure 3g and h under Cu mesh TEM grids. The Ti and O element peaks were only observed in the TONT arrays,

whereas that of S-doped TONT showed Ti, O, and S element peaks. Figure 4 compares the diffused reflection absorbance of all samples. On the basis of the opaque Ti substrate, the TONT arrays were explored as a reference sample and displayed the maximum absorbance at a wavelength of 360 nm with a D

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Figure 4. UV−vis absorbance spectra of (a) the TONT film, (b) the S-doped (450 °C) TONT film, (c) the S-doped (500 °C) TONT film, and (d) the S-doped (550 °C) TONT film.

threshold wavelength of 388 nm (Eg = 3.195 eV). The TONT arrays showed a sharp absorption edge, indicating that the TONT arrays had a highly uniform morphological characteristic. The enhanced absorbance of TONT arrays in the visible region was attributed to the scattering of light resulting from the pores or cracks in the nanotube arrays.28−30 Meanwhile, a clear shift of the band onset in the direction of the visible region of the spectrum was observed for S-doped TONT arrays. The order of the shift from left to right is 450, 500, and 550 °C. In the case of S-doped TONT arrays, the band onset was approximately 530 nm, whereas the band onset of S-doped (500 °C) TONT arrays exhibited two sites around 530 and 623 nm. However, it was observed that the light absorption near 623 nm was weak compared to that of the 530 nm region. Conversely, the S-doped TONT (550 °C) arrays showed strong visible light absorption starting from the 623 nm region, thus revealing the shift of the maximum light absorption wavelength from 365 nm (anatase TiO2) to 425 nm. In the case of the onset band gap energy of 530 nm, it was obvious that the narrowing of the band gap energy happened in the S-doped TONT arrays due to the formation of an impurity state of S 3p above the valence band, which was responsible for the red-shift absorption edge in the substitutional S- and O- doped anatase TiO2. However, the higher doping levels may induce the excessive formation of the point defects (e.g., interstitial S doping in the TiO2 atomic structure), influencing the light absorption in the visible wavelength. That is to say, the excessive formation of the point defects can induce the formation of a new sub-band gap energy, 0.26, 0.49, and 0.75 eV below the minimum conduction band in the electronic band gap of TiO2, resulting in the narrowing of the electronic band gap of TiO2 and finally increasing the visible light absorption by the transition from the S 3s to the minimum conduction band of TiO2.16,17,31 In particular, in the case of S-doped TONT (550 °C) arrays, the shift of the maximum light absorption wavelength indirectly indicates that the contribution of the other defect states is more than the photoresponsive doping treatment. XPS is a powerful tool to investigate the change of surface chemical bonding as well as the electronic valence band position, as shown in Figure 5. Ar+ etching was further applied on the S-doped TONT arrays for 10 min in order to clean the

Figure 5. XPS data of (a) the Ti 2p core-level peak, (b) the O 1s corelevel peak, and (c) the S 2p core-level peak of the TONT (●) and the S-doped (550 °C) TONT film (○).

film surface by removing the topmost atomic layers and explore the incorporation of sulfur into the TiO2 lattice. The Ti 2p and O 1s XPS spectra (Figure 5a and b) of TONT arrays were in good agreement with the reported anatase titania with peaks at 458 eV (Ti 2p3/2) and 529.1 eV, respectively.32 The shoulder at the higher binding energy (BE) on the main O 1s peak (denoted as the dotted circle) was observed in many transition metal oxides and was attributed to either oxygen vacancies sites or hydroxide on the surface. The sulfurization at 500 °C caused the downshift of the Ti 2p and O 1s peaks to approximately 0.3 and 0.18 eV, respectively. The different electronic interactions of Ti with S by a doping process induced a partial electron transformation from S to Ti and an increase of the electron density on Ti because of the lower electron negativity of S (2.58) compared with O (3.44). Furthermore, the shifts of O 1s E

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Figure 6. Mott−Schottky plot of (a) the TONT film and the S-doped (450, 500, and 550 °C) TONT films in 0.1 M KOH solution and (b) the enlarged Mott−Schottky plot of the S-doped (450, 500, and 550 °C) TONT films denoted as the red circle.

donor density, and V is the applied voltage. Figure 6 shows the Mott−Schottky plots of TONT and S-doped TONT arrays in 0.1 M KOH solution. From the positive slope, it was evident that the TONT and S-doped TONT arrays were n-type semiconductors, and from the magnitude of the slope, the calculated electron densities of TONT and S-doped TONT arrays at annealing temperatures of 450, 500, and 550 °C were 4 × 1017, 1.23 × 1021, 1.47 × 1021, and 2.1 × 1021 cm−3, respectively. The S-doped TONT arrays showed about 3 orders of magnitude higher donor densities than that of TONT arrays due to the S doping and the presence of oxygen vacancies, which can contribute to the improvement of the electrical conductivity of S-doped TONT arrays. It can be known that the S-doped TONT arrays exhibit similar donor densities independent of the sulfurization temperature. The Efb of the TONT and the S-doped TONT (450 °C), S-doped TONT (500 °C), and S-doped TONT (550 °C) arrays were about −0.79, −0.68, −0.67, and −0.65 V vs saturated Ag/AgCl, respectively. Compared with TONT arrays, the Efb of S-doped TONT arrays shifted to about 0.12 V positively, which suggested that a lower quasi-Fermi level could be obtained by doping S. The PEC activity of TONT and S-doped TONT arrays was evaluated in a 0.1 M KOH solution using the prepared electrodes as photoanodes for the water-photoelectrolysis reaction. Figure 7a shows a set of a linear-sweep voltammogram recorded on the TONT and S-doped TONT arrays annealed at 450, 500, and 550 °C under H2S ambient. These arrays were illuminated with chopped AM 1.5 light of 100 mW cm−2 intensity. Upon sweeping the potential from −1.0 to 1.5 V under illumination, all samples exhibited similar photoresponses through the scanned potential region. In particular, the TONT arrays as a reference sample exhibited a photocurrent density of 0.965 mA cm−2 at 1.0 V vs sat. Ag/AgCl. Notably, the photocurrent density of S-doped TONT (500 °C) arrays showed ∼3 times enhancement up to 2.92 mA cm−2 at the same applied potential. Even in the case of 450 and 550 °C annealed S-doped TONT arrays, the improvement of photocurrent densities approached up to ∼2 times enhancement with 2.12 and 2.04 mA cm−2, respectively. Accordingly, it can be concluded that the H2 S treatment induced the facile modification of TONT electronic structure. Thus, this treatment resulted in the betterment of the PEC activity, fairly independent of the annealing temperature. However, the optimal enhancement of the PEC activity in this experimental series was found to be the 500 °C annealed TONT array under

peaks were due to the introduction of oxygen vacancies into the TiO2 lattices, which favorably contributed to the S doping. Figure 5c compares in detail the S 2p core level spectra of TONT and S-doped TONT arrays, where TONT arrays revealed no S related peaks; in contrast, two broad peaks were observed at 162.9 eV (0.62 at. %) and 167.6 eV (0.83 at. %) in the S-doped TONT arrays. In general, the XPS peaks in the S 2p core-level region with a BE of 160−163 eV were attributed to the formation of Ti−S bonds, since O2− was substituted for S2−. The XPS peaks in the range of 167−170 eV area were due to the presence of S6+ and S4+ species.33 These results occurred due to two possible routes: the substitution of Ti4+ ions by S6+/ S4+ cations or the presence of sulfate/sulfide groups (SO42−/ SO32−) coordinated on TiO2 through bidentate bonds with surface Ti4+ ions, similar to the sulfated titania. Herein, the color change of S-doped TONT arrays after PEC tests from a strong brown to light brown color indicated that the latter route can be adapted in this case. Also, Han’s group34 reported the effect of Ar+ etching on the S-doped TiO2 particles, thus resulting in a marked decrease of the S 2p peak at higher binding energy. They verified that the XPS peaks with a BE of 167.6 eV were primarily due to the presence of the sulfate groups anchored on the surface of S-doped TONT arrays. Furthermore, it was also noticed that S-doped TONT (550 °C) arrays showed two broad peaks at 162.9 eV (0.71 at. %) and 167.6 eV (1.39 at. %), where the excessive S doping can contribute to the formation of the unnecessary defect or trap sites, not shown here. It is broadly thought that band bending is negligible for nanoparticles with very small size below 6 nm, but band bending may exist in nanoparticles with diameters above 10 nm. The nanocrystalline sizes of TONT and S-doped TONT arrays, herein, were in the range of ∼30 nm, thus enabling them to form the space charge region. Actually, the Mott−Schottky plot (M−S plot) involved measuring the capacitance (C) of the space charge region as a function of electrode potential under depletion conditions and was based on the Mott−Schottky relationship of a semiconductor film. Thus, these results proved the information on carrier densities through the gradient dV/ d(1/C2) of Mott−Schottky plots and flat band potential (Efb) determined by extrapolating C = 035 Nd =

2 ⎡ dV ⎤ ⎥ ⎢ e0εε0 ⎣ d(1/C 2) ⎦

where e0 is the electronic charge, ε is the dielectric constant (170) of TiO2, ε0 is the permittivity of the vacuum, Nd is the F

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basis of the above-mentioned reasons, the dark current was reduced in the S-doped TONT (550 °C) arrays.36 To examine the absorption of visible light in the S-doped TONT arrays, the chopped J−V curves were measured exploring the cutoff filter of 420 nm under the dark and visible light illumination, as shown in Figure 7b. In the case of TONT arrays, no meaningful photocurrent was generated through the scanned potential region. In contrast, the S-doped TONT arrays showed significantly enhanced photocurrent densities accompanied by the dark current. With the baseline based on the dark current, the photocurrent densities under the visible light illumination were quantitatively compared at 1.0 V (vs sat. Ag/AgCl), thus displaying values of 0.026, 0.25, and 0.26 mA cm−2 for 450, 500, and 550 °C treated S-doped TONT arrays, respectively. When increasing the annealing temperature under H2S atmosphere, the slightly increased photocurrent densities corresponded. In particular, it can be known that the abrupt increase of visible light absorption was remarkably attained at the H2S annealing temperature above 500 °C. Herein, the S treatment on the TONT arrays also induced not only the doping effect of the TiO2 crystal system to narrow the band gap energy but also the rough surface morphology and atomic level defects. Consequently, the stable photocurrent occurred with the less dark current in the S treatment above 550 °C. For the 450 and 500 °C treatments, the generation of dark current was noticeably observed, probably elucidated by the less defective TiOx materials as well as the partial collapse of crystalline TiO2 materials forming the new dark current pathways.36 Further detailed studies on optimizing the sulfurization process and characterizations of defective regions in the edge for TONT arrays are currently underway in order to fully understand the loss mechanism of the photocurrent and the improved PEC ability. Furthermore, in order to figure out the relationship between the photoactivity and the light absorption of S-doped TONT arrays, we quantitatively investigated the photoactivity as a function of the wavelength of incident light, referred to as the incident photon-to-current conversion efficiency (IPCE). IPCE measurements were performed on the TONT and S-doped TONT (500 °C) arrays at 1.0 V vs sat. Ag/AgCl (Figure 7c). IPCE can be expressed by the following equation IPCE = (1240J )/(λ ·Jlight )

Figure 7. (a) LSV, (b) chopped LSV under full sun on/off cycles, (c) chopped LSV under visible sun on/off cycles, and (d) IPCE of the TONT and the S-doped (500 °C) TONT films in 0.1 M KOH solution.

where J is the measured photocurrent density at a specific wavelength, λ is the wavelength of incident light, and Jlight is the measured irradiance at a specific wavelength. In comparison to the TONT arrays, the S-doped TONT arrays exhibited significantly enhanced photoactivity over the entire UV and visible light region. Particularly, the S-doped TONT arrays exhibited an IPCE of 43%, whereas the TONT arrays had an IPCE of 32%. These results indicated that the UV light was effectively used for the PEC water splitting. The separation and transportation of the photoexcited charge carriers were very efficient in the S-doped TONT arrays. Additionally, the small photoactivity in the visible light region for the S-doped TONT arrays was observed up to a 600 nm wavelength, where the IPCE values of 2.4% at 500 nm were achieved in the S-doped TONT arrays, in contrast to the negligible IPCE values for TONT arrays. These results are direct evidence for showing the visible light photoresponse of S-doped TONT arrays as a result of S doping process. Therefore, these results are mainly

H2S gas, and it is believed that the formation of S-doped TONT arrays enabled absorption of the more visible light and increased the electron densities, finally contributing to the increase of the photocurrent densities. However, the excessive S treatment can also generate the rough tube shape and atomic level defects including vacancies, stacking faults, and twin boundaries in the edge region for the S-doped TONT array (550 °C) (Figure 3f), and these atomic level defects played a role in the light scattering centers and the electron−hole trap sites on the S-doped TONT array. In addition, the electrical resistivity of the Ti(S,O)2 materials was higher than that of the TiO2. This difference indicates that the excess S-doped Ti(S,O)2 nanotube array played a passivation role in the interface between water and the S-doped TONT arrays. On the G

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The Journal of Physical Chemistry C

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attributed to the improved IPCE in the UV region as well as the newly contributing visible light absorption.

4. CONCLUSIONS We successfully demonstrated the enhanced PEC ability of Sdoped TONT arrays prepared by a facile electrochemical anodization, subsequently followed by the thermal treatment under S atmosphere on the Ti substrate in comparison with that of pure TONT arrays. The findings based on this research revealed the considerably beneficial effect of S doping at 500 °C and showed the larger interplanar spaces in the crystal structure, the enhanced absorbing property in the visible region, and the modified electronic band structure (narrow band gap energy). These changes in properties and structure finally improved the photocurrent (2.92 mA cm−2 at 1.0 V vs sat. Ag/AgCl) as compared with that (0.965 mA cm−2 at 1.0 V vs sat. Ag/AgCl) of TONT arrays. However, the excess S doping in the TONT arrays reversely induced the reduced photocurrent (2.04 mA cm−2 at 1.0 V vs sat. Ag/AgCl) due to the formation of rough tube morphological characteristics, defective regions in the edge of TONT arrays, and the passivation role in the interface between water and S-doped TONT arrays. Therefore, the optimal introduction of external impurities in the TONT arrays resulted in the enhancement of the PEC performance in terms of the modification of the electronic, optical, and crystalline properties.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +82-62-530-2497. *E-mail: [email protected]. Phone: +82-62-530-1709. Present Address ⊥

Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USA. Author Contributions

Profs. S. H. Kang and J. H. Kim contributed equally to this work as the corresponding authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and future Planning (Grant No. 2014R1A2A2A04004950) and by Human Resources Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy (No.: 20124010203180). Also, K.-S. Ahn thanks financial support by Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2014R1A6A1031189).

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DOI: 10.1021/acs.jpcc.5b01104 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.5b01104 J. Phys. Chem. C XXXX, XXX, XXX−XXX