PH3-Treated TiO2 Nanorods with Dual-Doping Effect for

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PH3-treated TiO2 Nanorods with Dual-doping Effect for Photoelectrochemical Oxidation of Water Dong-Dong Qin, Xue-Huai Wang, Yang Li, Jing Gu, Xingming Ning, Jing Chen, Xiao-Quan Lu, and Chun-Lan Tao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06903 • Publication Date (Web): 20 Sep 2016 Downloaded from http://pubs.acs.org on September 22, 2016

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The Journal of Physical Chemistry

PH3-treated TiO2 Nanorods with Dual-doping Effect for Photoelectrochemical Oxidation of Water Dong-Dong Qin,*aXue-Huai Wang,a Yang Li,a Jing Gu,b Xing-Ming Ning,a Jing Chen,a XiaoQuan Lu*a and Chun-Lan Tao*c a.

Key Lab of Bioelectrochemistry and Environmental Analysis of Gansu, College of Chemistry

and Chemical Engineering, Northwest Normal University, Lanzhou,730070, People's Republic of China. b.

c.

Department of Chemistry, San Diego State University, San Diego, CA 92182-1030, USA

School of Physical Science and Technology, Lanzhou University, Lanzhou, Gansu 730000,

People's Republic of China.

ACS Paragon Plus Environment

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ABSTRACT :

Elemental doping is an effective way to suppress charge recombination and modify the band gap of TiO2, therefore enhancing its photocatalytic activity. Here, we report a dual-doping method induced by an one step novel low temperature (300 °C) PH3 annealing method for improving the photoelectrochemical performance of TiO2 nanorods grown on transparent conducting substrates. X-ray photoelectron spectroscopy (XPS) indicates that Ti4+ is completely converted to Ti3+ within a surface layer of TiO2 to a depth of about 20 nm following this treatment. In addition to Ti3+ self doping, phosphorous ions in two different oxidation states (P5+ and P3-) are observed. Incorporation of these ions into TiO2 leads to an increase of one order of magnitude in the carrier density, resulting in faster transport and longer lifetimes of photo-generated electrons. Additionally, the valence band maximums of the PH3-treated rutile and anatase TiO2 shift towards the direction of the Fermi level by 0.92 and 0.42 eV , respectively, together with absorption change indicating successful band gap narrowing. This doping effect give rise to extension of absorption to the longer wavelength and enhancement of the photoactivity of TiO2 photoelectrodes under visible light. Although the PH3 treatment increases the density of surface states and thus lead to a positive shift in the photocurrent onset potential and a lower charge injection efficiency, a greatly improved photocurrent of 1.8 mA·cm-2at 1.23 V vs RHE (AM 1.5G, 100 mW·cm-2) for rutile TiO2 is seen. This is five times higher than the photocurrent observed for undoped control samples (as compared with 0.35 mA·cm-2 of the undoped control samples). The PH3 annealing strategy seems to be quite general and should have applications in improving the visible light photon absorptivity of other oxide semiconductors, especially those with wide band gaps.


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1. INTRODUCTION Over the past two decades, extensive efforts have been devoted to optimizing the efficiency of TiO2 for solar-driven photoelectrochemical hydrogen evolution. TiO2 is widely studied because of its excellent photostability, high photocatalytic activity, and low toxicity.1-4 However, the relatively large band gap of TiO2 (3.0 eV for rutile TiO2 and 3.2 eV for anatase TiO2) requires the absorption of UV light, which is not ideal for solar hydrogen production since UV light only covers less than 5% of the energy in the solar spectrum.5-8 Additionally, the high resistance and high carrier recombination rate limit the efficiency of TiO2 photoelectrodes.9 To date, two strategies have been widely used to overcome these drawbacks. One is nanostructuring of TiO2. In this context, photoanodes composed of highly oriented one-dimensional nanocrystals are attractive since they have a low concentration of grain boundaries and exhibit fast charge transport rate.10-14Another strategy is self- or foreign elemental doping with metal or nonmetal ions. Metal and nonmetal cations with oxidation states higher than four can act as electron donors and lead to improved photolectrochemicalperformance.15 However, this strategy generally leaves the absorption edge unchanged or blue shifted rather than extends it into the visible range. A few exceptions have been reported, for example, Nb5+-doped TiO2 prepared by ultrasonic spray pyrolysis is yellow due to incorporation of Ti3+ defects.16 Nb5+ by itself is unlikely to be capable of shrinking the band gap of TiO2 since the defect states induced by Nb5+ substitution are located above the conduction band minimum. Direct Ti3+ self-doping by annealing TiO2 in a reducing atmosphere (e.g., in H2)17 or a one-step combustion method18 creates oxygen vacancy sites and consequently, improves light absorption and charge transport. Doping with transition metal ions such as Fe3+ can increase visible light absorbance by creating


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additional d states below the conduction band minimum,19 but these deep gap states usually behave as recombination centers. Unlike metal ions, nonmetal anion dopants can directly shift the valence band maximum of TiO2 by mixing p-orbitals of O and the impurity anions. Nonmetal anion doping (by N 20-24 or S25, 26

) has achieved significant success in extending the absorption of TiO2 into the visible light

region through shifting the valence band position. Mullins et al. reported N-modified rutile TiO2 nanowires with feature sizes of 5 nm. This material exhibited a visible light photocurrent density of 0.23 mA·cm-2 at 1.23 V vs RHE.27 Subsequently, Ti3+ and N co-doped TiO2 was made by the same group by treating with H2 followed by NH3 and was shown to enhance the photoactivity under visible light.28 Recently, Duan et al. observed significant enhancement of visible light photoactivity in N-implanted TiO2 nanowires, for which a photocurrent of about 0.4 mA·cm-2 at 1.23 V vs RHE was obtained.22 S-doped TiO2 nanotubes also have exhibited enhanced visible light photocurrent.25 Inspired by the success of N and S-doping in narrowing the band gap and enhancing the visible light photoactiviy of TiO2, a few studies have explored phosphorus doping.29, 30 When phosphate is used as the source of phosphorus in these studies, only the P5+ oxidation state is observed in the resulting materials. Doping with P5+ does increase the conductivity of TiO2 but does not significantly narrow the band gap. Recently, a case of doped TiO2 containing both P5+ and P3- was reported,31 but this work focused on powdered TiO2 and did not demonstrate the band gap narrowing even though P3- was successfully introduced. Therefore, the goal of this study is to prepare P3--doped TiO2 and to study the band structure as well as fundamental factors influencing the photoelectrochemistry as a function of doping level. So far, there have been no studies of Pdoped TiO2 films that used PH3 as the P source. Here, we report a facile one step PH3 annealing


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method at the relatively low temperature of 300 °C for preparing P-doped rutile and anatase TiO2 nanorod arrays on transparent conducting substrates. This temperature is substantially lower than that required for incorporating N or S via NH3 or H2S annealing (500 °C). In PH3-treated samples, three doping ions (Ti3+, P5+ and P3-) are found. Compared with pristine TiO2 nanorod arrays, the PH3-treated samples exhibit a narrowed band gap and dramatically increased photocurrent due to band gap narrowing, as well as faster charge separation and transport as a result of increased the carrier concentration. 2. EXPERIMENTAL SECTION

Figure 1. Schematic diagrams of the preparation of PH3-treated TiO2 nanorods and the photoelectrochemical water splitting cell. 2.1 Synthesis of rutile TiO2 nanorod arrays10, 11(Fig. 1) 0.25 mL titanium butoxide (97% Aldrich) was added to a solution of 7.5 mL concentrated aqueous HCl (36.5%-38% by weight) and 7.5 mL deionized water. After stirring for 30 min, the


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solution was poured into a 25 mL Teflon-lined stainless steel autoclave, into which a FTO (fluorine-doped tin oxide)-coated glass slide was loaded with the FTO side facing down. After reacting at 150 °C for 5 h, the sample was taken out and rinsed with deionized water. 2.2 Synthesis of Anatase TiO2 nanorod arrays12 7.82 mL of concentrated aqueous HCl (36.5%-38% by weight) and 0.435 mL H2SO4 (98% by weight) were added to 10 mL of deionized water. After the mixture cooled to room temperature, 0.5 mL titanium butoxide (97% Aldrich) was added. The resulting solution was poured into a 25 mL Teflon-lined stainless steel autoclave, into which a FTO-coated glass slide coated with a TiO2 seed layer had been loaded in advance, with the FTO side facing down. After reacting at 180 °C for 4 h under hydrothermal conditions, the sample was taken out and rinsed with deionized water. The seed layer was prepared as follows: 0.2 mL concentrated HCl (36.5%-38% by weight) and 5 mL titanium butoxide were added to 60 mL absolute ethanol in a 100 mL round bottom flask. The mixture was refluxed under constant stirring for 12 h to form a TiO2 sol. The TiO2 seed layer was prepared by the dip coating method, and the sample was then heated at 500 °C for 1 h. 2.3 Preparation of control and PH3-treated samples32 Control samples were prepared by annealing pristine rutile and anatase TiO2 nanorod arrays in N2 at 300 °C for 1 h. For PH3 annealing, 1.0 g sodium hypophosphite (NaH2PO2) was placed at the center of a tube furnace and the TiO2 samples were located in the downstream side of the furnace, at distance of about 7 cm. The furnace was heated to 300 °C for 1 h at a heating rate of 2 °C/ min. under N2 flow.


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2.4 Equipment and measurements The morphology of samples was characterized by using a field-emission scanning electron microscope (Zeiss ULTRA Plus) operated at an accelerating voltage of 5 kV. X-ray diffraction patterns (XRD) were recorded on a PANalytical X'Pert PRO instrument using Cu Kα radiation (40 kV, λ = 1.5406 Å) with 2θ from 20 to 80° at a scanning rate of 0.067°/s. UV-visible diffuse reflectance spectra were measured on a UV-2550 (Shimadzu) spectrometer by using BaSO4 as the reference. The elemental composition was determined by X-ray photoelectron spectroscopy and

the

valence

band

maximum

potential

was

determined

by

ultraviolet

photoelectron spectroscopy (Kratos Axis Ultra DLD). Photoelectrochemical measurements were made in 1 M NaOH under AM 1.5G illumination (100 mW·cm-2) in a three-electrode configuration with the TiO2 film as the working photoelectrode, saturated calomel electrode (SCE) as the reference electrode, and platinum foil as the counter electrode. Sunlight was simulated with a 300W Xenon lamp and an AM 1.5G filter (HSX-F300, Beijing NBeT Technology Co., Ltd). The light intensity was set using a calibrated crystalline silicon solar cell. Photocurrent response, open circult voltage decay and electrochemical impedance spectroscopy (EIS) were recorded using a CHI-660D potentiostat. The impedance data was fit to an equivalent circuit model using Zview software. During measurements for Mott-Schottky plots, the superimposed alternating current (AC) signal was maintained at 5 mV, while the frequency was scanned between 100 kHz and 0.1 Hz at potentials between -0.8 and 0.9 V versus SCE in the dark in an electrolyte of 1 M NaOH, with Pt as the counter electrode and saturated calomel electrode(SCE) as the reference electrode. For EIS testing, a fixed bias potential of 1.5 V vs RHE was used and the light intensity was set to 50 mW·cm-2. The capacitance was extracted from the EIS spectra by use of an equivalent circuit Rs(CPE-Rp), where Rs is the ohmic contribution,


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CPE is aconstant phase element that takes into account non-idealities in the capacitance of the Helmholtz layer, and Rp is the charge-transfer resistance. IPCE spectra were measured using light from a 300 W xenon lamp that was focused by a parabolic reflector and passed through a monochromator, at 0.2 V bias versus SCE. Samples were measured using a TiO2 film as the working photoelectrode and platinum foil as the counter electrode in 1 M NaOH. The intensity modulated photocurrent spectroscopy (IMPS) response was measured using the light source of a LED array (470 nm) driven by the output current of the Autolab LED Driver. The dc output of the LED Driver was controlled by the DAC164 of the Autolab, and the ac output of the LED Driver was controlled by the FRA32 M module. The ac amplitude was set to 25% of the dc output. The IMPS response was examined over a frequency range from 0.1 Hz to 10 kHz at an applied potential of 1.2 V vs RHE in 1.0 M NaOH solution. The transfer function, H, was monitored using the external inputs of the FRA module. The potentials of the photoelectrodes are reported against RHE, converting between the SCE and RHE potential scales using , where pH is 13.6 (1.0 M NaOH) and

= 0.2415 V (25 ºC).

3. RESULTS AND DISCUSSION


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Figure 2. SEM images of rutile (a) and anatase (c) TiO2 nanorod arrays; (b) and (d) are (a) and (c) after PH3 treatment; (e) and (f) are XRD patterns. Insets of (a)-(d) are cross sectional SEM images and insets of (e) and (f) are crystal structures of rutile and anatase TiO2, respectively. The pristine rutile and anatase TiO2 nanorod arrays are uniform and perpendicular to the substrate，as can be seen from SEM images in Fig. 2. The average diameter and length are around 100-160 nm and 1.3 µm for rutile nanorods, and 100-150 nm and 0.9 µm for anatase nanorods. After PH3 annealing, the rutile nanorods aggregate and the cross section of the anatase


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nanorods becomes larger, but the film thickness remains unchanged. X-ray diffraction (XRD) patterns were collected and are shown in Fig. 2e and f. In Fig. 2e, two peaks appeared at 36.2 and 63.2°2θ, which can be indexed to the (101) and (002) reflections of tetragonal rutile TiO2 (JCPDS No. 88-1175). However, compared with previously reported XRD data for rutile TiO2 nanorods annealed at higher temperature in oxygen atmosphere,10 the intensity of the (002) diffraction peak is relatively weak. This might be due to the high concentration of defects in the TiO2 nanorods, which can not be efficiently eliminated at 300 °C under nitrogen atmosphere. In Fig. 2f, only one sharp peak appears at 38.0°2θ after subtracting the diffraction signals arising from the FTO substrate. This peak can be indexed to the (004) reflection of anatase TiO2 (JCPDS No. 1-562) and implies oriented growth of nanorods along the (001) direction.12 Similar diffraction patterns before and after PH3 annealing for both rutile and anatase TiO2 indicate that the main features of the crystal polymorph and orientation are not changed during this treatment.


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Figure 3. Absorption spectra of rutile TiO2 nanorod arrays before and after PH3 treatment (a), VB UPS (b) and XPS of samples (c and d). R and A signify rutile and anatase, respectively. Optical spectra in Fig. 3a, Fig. S1 and S2 indicate that the band gap of pristine rutile and anatase nanorods are approximately 3.1 and 3.2 eV, respectively, consistent with reported values for TiO2.1-9 An additional tail absorption extending into visible are seen for treated samples. The shift of onset of optical absorption combined with the color change from white to yellow may imply band gap narrowing. In order to confirm this hypothesis, valence band UPS (Ultraviolet Photoelectron Spectroscopy) experiments were done and the results are shown in Fig. 3b. The intercepts on the energy axis in the UPS spectra characterize the valence band energy with respect to the Fermi level. The valence band maxima for pristine rutile and anatase TiO2 samples lie at about 1.26 and 1.24 eV. Given the band gaps of 3.1 and 3.2 eV for rutile and anatase TiO2 from the optical spectra, the conduction band minima would lie at about -1.84 and -1.96 eV, respectively. For PH3 treated samples, the valence band maximum shifts in the direction of the Fermi level, to about 0.34 and 0.82 eV for rutile and anatase TiO2, respectively. Assuming that the conduction band edge does not change in the case of anion doping,20-24 the UPS spectra demonstrate that band gap narrowing occurs for the PH3-treated samples. X-ray photoelectron spectroscopy (XPS) shown in Fig. 3c reveals the existence of P5+ on the surface of the treated samples, with a binding energy of 133.8-134.2 eV for the P 2p line.29-31 The fitting curves of Ti 2p XPS spectra in Fig. 3d show peaks at 458.8 and 464.7 eV for rutile as well as 458.8 and 464.4 eV for anatase TiO2, suggesting the existence of Ti3+.12, 17 However, it has been reported that cations with oxidation states higher than four plus do not induce band gap narrowing of TiO2 (as in W6+, 33 or Ta5+, 34 doped TiO2), excluding the contribution of P5+ as color center. At high concentrations, Ti3+ can act as a color center due to the formation of oxygen


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vacancy sites, which usually form donor states below the conduction band and thus lead to increased visible light absorption in the red and near-infrared. However, this effect depends on the doping method used and the concentration of Ti3+. For example, hydrogen-treated TiO2 nanorods17 show the existence of Ti3+ ions, but Ti3+ has not been detected in hydrogen-treated TiO2 quantum dots35 even though both samples are black and visible light active. Ti3+ selfdoping induced by electrochemical reduction of anatase TiO2 does not result in a color change or a shift of onset absorption.12,

36

Because of the low concentration of PH3 and annealing

temperature utilized in this study, the concentration of Ti3+ should be much lower than that it is in hydrogen-annealed TiO2. However, considering the similarity of N and P in chemical properties, it is likely that P3- is present in the treated samples. The absence of a P3- signal in XPS at the surface of the samples may be caused by air oxidation due to the high reactivity of P3-. To check this hypothesis, the surfaces of the treated samples were etched to a depth of about 20 nm by argon sputtering prior to XPS measurements. These etched samples, for both treated rutile and anatase TiO2, show new peaks centered at about 130 eV, which are in good agreement with the position of P3- anions.31 The 3p orbitals of P3- may hybridize with O 2p orbitals, or they could possibly form an isolated intermediate gap state. Orbital mixing should shift the valence band maximum of TiO2, as it does in the case of N3- doping, in turn narrowing the band gap.35 We note that only Ti3+ is detected in the etched samples, implying that Ti4+ is completely converted to Ti3+ within a shell thickness of about 20 nm. Ti4+ on the surface should thus be the oxidation product of Ti3+.


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Figure 4. Photocurrent response of rutile TiO2 nanorod arrays (a), long-term stability testing of PH3-treated rutile TiO2 nanorod arrays (b), visible light photocurrent (>420 nm) (c) and IPCE (d) of rutile TiO2 nanorod arrays before and after PH3 treatment. As shown in Fig. 4a and Fig. S3, pristine TiO2 shows negligible photocurrent. Controlled rutile and anatase samples demonstrate a slightly increased photocurrents of 0.35 and 0.2 mA·cm-2, respectively, at 1.23 V vs RHE, likely due to the increased crystallinity after annealing at 300 °C in nitrogen. Interestingly, dramatically increased photocurrent is observed for the PH3 treated samples. Note that a photocurrent of 1.8 mA·cm-2 at 1.23 V vs RHE for treated rutile TiO2 is obtained, which is 5 times higher than that of the corresponding control sample. Compared with the anatase TiO2 nanorods, the higher photocurrent for rutile can be attributed to a greater film thickness and larger gaps between the individual rods, which might facilitate PH3 penetration and the overall reaction. Long-term stability testing of an hour of PH3 treated rutile TiO2 (Fig. 4b) shows that almost 100% of photocurrent is retained and no observable color


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change is observed on the material during the electrolysis, suggesting good stability of this photoanode. The large band gap of TiO2 allows it to absorb only UV light and consequently results in negligible photocurrent under visible light (>420 nm), as evidenced by the results in Figs. 4c and S4-5. Enhanced visible light photocurrent for treated samples likely benefits from the narrowed band gap. Although this photocurrent is smaller than that obtained with nitrogen-implanted23 or with H2 and NH3 co-treated28 TiO2 nanorods, it suggest at least that PH3 treatment is an effective way to achieve enhanced visible light photoactivity with TiO2. Higher visible light photocurrent might be expected if higher concentrations of PH3 or higher annealing temperatures are used, but the side reaction between PH3 and SnO2 restricts this possibility with FTO electrodes. One alternative would be to replace FTO by other transparent conductor with that is chemically inert to PH3, but this may lead to difficulty in growing TiO2 nanorod arrays. We note from Fig. 4c that the treated samples in chopped light exhibit both positive and negative photocurrent transients when the light is turned on and off. The positive transient in photocurrent might be caused by surface state trapping and thus slow water oxidation kinetics of holes at the surface of the photoanode. Similarly, negative transients of the photocurrent reflect the recombination of photogenerated electrons with accumulated holes. These behaviours suggest that the surface extraction of holes is slow relative to the arrival charge carriers at the TiO2-electrolyte interface as a result of surface states introduced by PH3 annealing. IPCE (incident photo-to-current conversion efficiency) measurements were carried out in order to characterize the interplay between photoelectrochemical performance and light absorption. As shown in Figs. 4d and S6, enhanced IPCE is observed for treated TiO2 and can be mainly assigned to the efficient conversion of UV light because the contribution of visible light is small.


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Hence, the increased photocurrent under full light illumination may be due to suppressed recombination and accelerated charge transport as a result of the increased carrier density created by PH3 treatment.

Figure 5. Mott-Schottky plots of rutile (a) and anatase (b) TiO2 nanorod arrays measured in the dark in 1.0 M NaOH. Electrochemical impedance spectroscopy of ruitle (c) and anatase (d) TiO2 nanorod arrays collected under illumination (50 mW·cm-2) in 1.0 M NaOH with a bias potential of 1.5 V vs RHE. In order to check the electronic properties of the samples, electrochemical impedance spectroscopy was carried out. As seen in Fig. 5a and b, the positive slope of the Mott-Schottky plots indicates the n-type doping for PH3-treated TiO2. The carrier density of the samples tested is found to be in the range of 1020-1021 cm-3, which is comparable to the previously reported values.17 The carrier density of PH3-treated samples (1021 cm-3) is similar to that of H2S-treated


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TiO217 but lower than that of H2-treated TiO2 (1022 cm-3).9 This can be explained by high concentration of oxygen vacancy sites in H2-treated TiO2 because of more complete reduction by H2 at elevated temperature. Additionally, it is found that the carrier density of both treated-rutile and anatase TiO2 are one order of magnitude higher than those of corresponding pristine and control samples, likely from a combination of electron donor doping by P5+ and oxygen vacancies induced by Ti3+ self-doping.17, 31 The higher carrier concentration can undoubtedly lower the resistance of TiO2 and shift the Fermi level toward conduction band edge, thus promoting charge separation as a result of increased band bending.12 In order to supply more evidences for more efficient charge separation occurring in the samples with higher carrier density, Nyquist plots were measured under illumination and fitted by using a simple Rs(CPERp) equivalent circuit. In this model, the response at low frequency is attributed to the charge transfer resistance. Thus, the smaller the low-frequency semicircle is, the faster the charge transfer of photogenerated carriers. As seen in Fig. 5c and d, faster charge transfer for treated samples is confirmed by the smaller semicircle.


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Figure 6. Voc decay (a and b) and IMPS (c and d) measurements of the samples. To further investigate the charge separation efficiency and electron lifetime, open circuit voltage (Voc) decays were recorded and are shown in Fig. 6a and b. The positive change of open circuit voltage indicates n-type electronic conduction in PH3-treated TiO2,37 supporting the results of the Mott-Schottky measurements. The lifetime of photoelectrons can be estimated by conversion of Voc decay using the equation below38: (

)-1

Where is the lifetime of photoexcited electrons, temperature, and

is Boltzmann’s constant,

is the absolute

is the elementary charge. Under open circuit conditions, faster decay kinetics

should be seen if the recombination rate is high, which in turn leads to shortening of electron lifetime. As seen in the inset of Fig. 6a and b, longer electron lifetimes are observed for treated TiO2 over the entire voltage range studied, relative to the control samples. This is indicative of


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efficient charge separation. Interestingly, PH3-treated rutile and anatase TiO2 have lower electron lifetimes than the pristine samples. This maybe caused by a higher concentration of surface states, as supported by the observation of a transient photocurrent response under chopped light. Surface states can facilitate the carrier recombination,28 thus, resulting in a faster charge recombination and shorter electron lifetime. Intensity modulated photocurrent spectroscopy (IMPS) was employed to investigate the charge transport properties of samples.39 The average lifetime for photoexcited electrons to diffuse to electrode can be estimated by the equation of time and

=

)-1, where

is the transit

is the frequency at which the minimum in the IMPS plot occurs. As shown in Fig.

6c and d, the electron transport time for rutile and anatase TiO2 nanorod control samples are 0.44 and 0.2 ms, respectively. For PH3-treated samples, these values shorten to 0.3 and 0.16 ms, indicating faster transport of photoelectrons along the nanorods. The origin of superior electron transport for the treated samples is most likely the improved conductivity arising from higher carrier concentration. Comparing reported electron transport times of 5-7 ms for TiO2 nanoparticulate and nanotubular film,13 the

for TiO2 nanorods are relatively short, and can be

ascribed to the lower density of grain boundaries and recombination sites in TiO2 nanorods.


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Figure 7. Photocurrent onset potential measurements for rutile (a) and anatase (b) TiO2 nanorod arrays with AM 1.5G (100 mW·cm-2) illumination in 1.0 M NaOH. Photocurrent of control samples (c) and treated (d) rutile TiO2 nanorod arrays with and without the addition of 0.1 M H2O2; the inset is the calculated charge injection efficiency. It is noted from Fig. 7a and b that the onset potentials of both treated rutile and anatse TiO2 shift positively compared with control samples. This phenomenon has also been observed by Mullins et al. in NH3 and NH3-H2 treated rutile TiO2 nanorod films.28 They claimed two reasons for this onset potential shift: 1) large band bending required for separation of photogenerated electrons and holes due to material’s poorer charge transport properties after treatment; 2) slower water oxidation kinetics caused by an increased density of surface states. In our case, the PH3treated sample obviously has improved charge separation and transport properties as confirmed by the results obtained from EIS, Voc decay and IMPS testing, which explains the origin of


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enhanced photocurrent for treated samples under full light illumination. Therefore, we postulate that the shift in onset potential of PH3-treated TiO2 maybe caused by surface states and slower water oxidation kinetics.27, 28 As H2O2 can be used as hole scavenger with nearly 100% charge collection efficiency, it is possible to determine the charge injection efficiency by calculating the photocurrent ratio measured without/with H2O2. It can be seen from Fig. 7c and d that both control samples and treated rutile TiO2 display increased photocurrent in the presence of H2O2. This indicates that the transfer of photogenerated holes from TiO2 to water is not as efficient as hole transfer from TiO2 to H2O2. Comparing the charge injection efficiencies shown in the insets of Fig. 7c and d, the relatively lower efficiency at the treated TiO2/electrolyte interface suggests slower surface kinetics of treated TiO2, leading to a positive shift in onset potential and supporting our hypothesis.23 4. CONCLUSION In summary, a facile low temperature PH3 annealing method has been developed to improve the photoelectrochemical performance of both rutile and anatase TiO2 nanorod arrays. Following this treatment, Ti3+, P5+ and P3- doping as well as band gap narrowing are observed. Compared with control sample, dramatically increased photocurrents are obtained with treated TiO2 due to efficient charge separation and transport as a result of increased carrier density. Band gap narrowing broadens the working spectrum of TiO2, in turn improving the photocurrent and IPCE in the visible light region. This treatment scheme allows a mild reaction condition and dualdoping effects for the underlying large band gap semiconductors, which would permit further improvement by individually optimization either on semiconductor materials or reaction conditions.


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ASSOCIATED CONTENT Supporting Information. Absorption spectra, photocurrent response and IPCE of anatase TiO2 nanorods, i-t curves of both rutile and anatase TiO2 nanorods under illumination with LED light source (470 nm) are supplied as Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Email: [email protected] ( D. D. Qin), [email protected] (X. Q. Lu) and [email protected] (C. L. Tao) ACKNOWLEDGMENT We acknowledge funding support of National Natural Science Foundation of China for Young Scholars (No. 21401150, 21501083), National Natural Science Foundation of China (No. 51562034, 21565022, 21327005, 61574070), The Program for Chang Jiang Scholars and Innovative Research Team, Ministry of Education of China (No. IRT1283). ABBREVIATIONS FTO, fluorine-doped tin oxide; UPS, ultraviolet photoelectron spectroscopy; XPS, X-ray photoelectron spectroscopy; IPCE, incident photo-to-current conversion efficiency; Voc, open circuit voltage; IMPS, intensity modulated photocurrent spectroscopy. REFERENCES


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Table of Contents Graphic


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PH3-Treated TiO2 Nanorods with Dual-Doping Effect for

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