Role of Cl Ion Desorption in Photocurrent ... - ACS Publications

Aug 14, 2017 - ABSTRACT: TiO2 nanorods arrays (NRAs) have been consid- ered as very promising photoanode materials in photoelec- trochemical (PEC) ...
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Role of Cl Ion Desorption in Photocurrent Enhancement of the Annealed Rutile Single-Crystalline TiO2 Nanorod Arrays Chao Huang,† Juncao Bian,† and Rui-Qin Zhang* Department of Physics and Centre for Functional Photonics (CFP), City University of Hong Kong, No 83, Tat Chee Avenue, Hong Kong S.A.R., China Shenzhen Research Institute, City University of Hong Kong, Shenzhen, China

J. Phys. Chem. C 2017.121:18892-18899. Downloaded from pubs.acs.org by UNIV OF SASKATCHEWAN on 01/01/19. For personal use only.

S Supporting Information *

ABSTRACT: TiO2 nanorods arrays (NRAs) have been considered as very promising photoanode materials in photoelectrochemical (PEC) cells. However, the performance of TiO2 NRAs still requires substantial improvement in order to reach the goal of practical applications. Annealing treatment of TiO2 NRAs can help to improve the PEC performance, but the mechanism is still not yet fully understood. In this work, we systematically investigated the optical and electronic properties, as well as the PEC performance of the thermally treated rutile singlecrystalline TiO2 NRAs. Surprisingly, we recorded a maximum photocurrent density of 1.38 mA/cm2 at 1.3 V versus reversible hydrogen electrode for TiO2 NRAs annealed in O2, which is about 28 times higher than that of the pristine TiO2 NRAs. We further revealed that the surface adsorbed Cl ions largely suppress the photoresponse of the TiO2 NRAs as they serve as recombination centers and block the adsorption of water molecules to the surface of TiO2 NRAs. The enhancement in photocurrent after annealing in O2 is due to the desorption of the Cl ions, filling of the surface Vo, expansion of the depletion layer, and increase of the grain size. Our results shed light on the effect of annealing on the PEC performance of TiO2 NRAs and offer guidance for annealing of other semiconductor materials.

1. INTRODUCTION Since the first discovery of photoelectrochemical (PEC) water splitting on a TiO2 thin film by Fujishima and Honda in 1972,1 solar water splitting for H2 evolution has drawn much attention due to its potential for large scale, cost-effective, and sustainable production of H2.2−6 Compared with the powder based water splitting systems, PEC water splitting can suppress the backward reactions as water oxidation and reduction happen on different electrodes, facilitating the collection of the charge carriers and the system efficiency enhancement via applying a bias.7 To date, numerous semiconductors (e.g., TiO2,8−10 ZnO,11 Fe2O3,12 WO3,13 and graphitic carbon nitride14,15) have been explored as photocatalysts for such a purpose. Among them, TiO2 is regarded as an ideal material for water splitting due to its excellent stability, appropriate band edge position, elemental abundance, and nontoxicity.16,17 Recently, singlecrystalline TiO2 NRAs on fluorine-doped tin oxide (FTO) glass are widely investigated as they exhibit superior charge transportation properties and offer direct electrical pathways for the excited electrons.18 Currently, TiO2 RNAs are mainly prepared by hydrothermal method. In addition to the intrinsic shortcomings of TiO2, such as large band gap and poor conductivity, the hydrothermally grown TiO2 NRAs generally contain high-concentration defects, which always act as trapping centers for charge carriers and weaken the overall catalytic performance.19 Several © 2017 American Chemical Society

approaches have been applied to overcome the shortcomings, such as doping with other elements, forming heterogeneous structures, and adding sensitizers.20−23 Postannealing is usually applied to remove the surface defects and the surface adsorbed molecules. Yang et al. reported that thermal annealing can remove some functional groups related to carbon, nitrogen, and hydrogen from the surface of nanorods.24 Mahajan et al. found that the oxygen vacancy (Vo) on the surface of TiO2 nanotubes can be healed after annealing in oxygen atmosphere.25 The same healing effect on the surface of ZnO nanorods was also reported by Zhao et al.26 Thus, it is essential to understand the influence of annealing atmosphere on the surface state of single-crystalline TiO2 NRAs, which is still not yet fully understood. In this work, we systematically studied the effect of annealing atmosphere on the electronic and PEC properties of the TiO2 NRAs. X-ray photoelectron spectroscopy (XPS) spectra suggest that the residual chlorine (Cl) on the surface of TiO2 NRAs can be removed after annealing in N2 or O2 flow. A maximum photocurrent density was detected from the TiO2 NRAs annealed in O2 (O2−TiO2 NRAs), which is due to the removal of residual Cl ions, healing of O defects on the surface, and Received: April 30, 2017 Revised: August 14, 2017 Published: August 14, 2017 18892

DOI: 10.1021/acs.jpcc.7b04071 J. Phys. Chem. C 2017, 121, 18892−18899

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The Journal of Physical Chemistry C

Figure 1. Top-view and cross-sectional SEM images of the pristine TiO2 NRAs (a,d), N2−TiO2 NRAs (b,e), and O2−TiO2 NRAs (e,f). The growth time of TiO2 RNAs is 4 h. N2−TiO2 and O2−TiO2 NRAs were postannealed in N2 and O2 flows at 500 °C for 1 h, respectively. The scale bars are 1 μm.

expansion of the depletion layer. N2 annealed TiO2 (N2−TiO2) NRAs showed a slightly lower current compared to those thermally treated in O2 flow, which is possibly due to the lack of O to heal the Vo.

scanning electron microscopy (SEM, JEOL JSM-6335F). UV− vis absorption spectra of TiO2 NRAs were measured on a UV− vis-NIR spectrophotometer (Shimadzu, UV 3600) combined with integrating sphere. XPS characterizations were performed on a PHI 5802 XPS with Al Kα radiation. No additional sputtering treatment was applied before XPS analysis. The binding energy of the XPS was calibrated by C 1s (284.6 eV). 2.4. PEC Measurements. PEC properties were characterized in a three-electrode cell and were recorded by an electrochemical workstation (CHI 760E) with Pt plate, Ag/ AgCl, and TiO2 NRAs being used as counter, reference, and working electrodes, respectively. An aqueous Na2SO4 solution (0.2 M, pH = 7) was applied as the electrolyte. Prior to the measurement, the electrolyte was purged with N2 flow for 20 min to remove the dissolved O2. All of the potentials were converted to those versus (vs) reversible hydrogen electrode (RHE) according to the Nernst equation. Photoresponses of the samples were measured under a simulated solar light, which was generated by a 300 W xenon lamp equipped with an air mass 1.5 global (AM 1.5G) filter (Beijing NBET Technology Co., Ltd.). Electrochemical impedance spectroscopy (EIS) was measured in both dark and illuminated conditions from 0.1 Hz to 100 000 Hz at an applied potential equal to open circuit potential and with an alternating current (AC) amplitude of 10 mV. The EIS measurements for the Mott−Schottky plots were carried out at the frequency of 1000 Hz with an AC amplitude of 10 mV.

2. EXPERIMENTAL SECTION 2.1. Material Preparation. Rutile TiO2 NRAs on FTO glass were prepared by a hydrothermal approach as previously reported.18 FTO glass was ultrasonically cleaned in the mixture of isopropanol, acetone, and deionized (DI) water (18.2 MΩ· cm) with the ratio of 1:1:1 for 1 h. Then it was further cleaned by DI water for another 0.5 h and dried by N2. A total of 15 mL of concentrated HCl solution (37%, Sigma) was mixed with 15 mL of DI water, and the mixture was stirred for 10 min. Then 0.45 mL of titanium butoxide (Acros) was added as titanium precursor. The as-prepared solution was stirred for another 10 min to form a transparent solution before it was finally introduced in a 100 mL Teflon liner. A piece of clean FTO glass (4 cm × 5 cm x 0.16 cm, square resistance = 7 Ω/cm2, Zhuhai Kaivo Optoelectronic Technology Co., Ltd.) was placed with conducting side facing downward against the inside wall of the Teflon liner. After that, the Teflon liner was transferred into a stainless steel autoclave and sealed. The autoclave was placed in an oven kept at 150 °C for 4 h to obtain the highly ordered rutile single-crystalline TiO2 NRAs. After hydrothermal reaction, the autoclave was cooled to room temperature by water. To remove the surface residuals, the as-synthesized TiO2 NRAs were then ultrasonically cleaned by DI water for 5 min and dried under N2. 2.2. Annealing of TiO2 NRAs. The as-prepared sample was cut into four even pieces (1 × 5 cm), which are considered to possess the same physical and geometric properties. Different pieces were annealed in N2 and O2 flows (referred to as N2− TiO2 NRAs and O2−TiO2 NRAs), respectively. In a typical annealing process, a piece of the TiO2 NRAs was placed in a rapid annealing furnace (OTF-1200X). These samples were annealed at 500 °C for 1 h with a heating rate of 10 °C/s and cooled naturally. 2.3. Characterization. The crystal structure of the TiO2 NRAs was analyzed by an X-ray diffractometer (XRD, Rigaku SmartLab) with Cu Kα monochromatic radiation. Surface and cross-section morphology of TiO2 NRAs were characterized by

3. RESULTS AND DISCUSSION Figure 1 shows the top-view and cross-sectional morphologies of the pristine TiO2, N2−TiO2, and O2−TiO2 NRAs, separately. All of the TiO2 NRAs were prepared through a 4 h hydrothermal reaction. N2−TiO2 and O2−TiO2 NRAs were additionally annealed in N2 and O2 flows, respectively, at 500 °C for 1 h. Figure 1a,d reveal that the pristine TiO2 NRAs are uniformly distributed and perpendicular to the FTO substrate. No obvious change in morphology of the TiO2 NRAs is observed after annealing in N2 (Figure 1b,e) and O2 (Figure 1c,f) atmospheres. As the reaction time was elongated, the prepared TiO2 NRAs are longer with an increase in diameter, as shown in Figure S1. The average lengths of the prepared TiO2 NRAs grown for 2, 4, 6, and 8 h were measured to be 0.97 ± 18893

DOI: 10.1021/acs.jpcc.7b04071 J. Phys. Chem. C 2017, 121, 18892−18899

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The Journal of Physical Chemistry C

Figure 2. (a) XRD patterns of the pristine and the annealed TiO2 NRAs. (b−d) XPS spectra of the Ti 2p, O 1s, and Cl 2p core levels of pristine and annealed TiO2 NRAs, respectively.

Figure 3. (a) UV−vis absorption spectra of the pristine and the annealed TiO2 NRAs. (b) Tauc plot of the pristine and annealed TiO2 NRAs.

0.05, 1.16 ± 0.05, 1.42 ± 0.05, and 1.82 ± 0.05 μm, respectively. Figure 2a displays the XRD patterns of the pristine and annealed TiO2 NRAs. Strong diffraction peaks in (101) crystal face of rutile TiO2 (JCPDS No. 88-1175) as well as weak diffraction in (002) and (112) faces can be observed, indicating that the TiO2 NRAs are orientated in the (101) direction.18 In comparison to the pristine TiO2 NRAs, no more diffraction peaks are found from the annealed samples, indicating that no phase transformation occurs during the annealing in various atmospheres. It should be noted that the (101) diffraction peak becomes sharper and the line breadth at half-maximum decreases after annealing. The Scherrer equation (τ = Kλ/(β cos θ)) was applied to determine the size of TiO2 crystals. Herein, τ, K, λ, β, and θ represent the grain size, shape factor, X-ray wavelength, line breadth at half-maximum, and Bragg angle, respectively. After annealing, the grain size was increased from 28.2 nm for pristine TiO2 NRAs to 32.4 and 35.0 nm for those of N2−TiO2 and O2−TiO2 NRAs, respectively. The improved crystallinity can be attributed to the thermal treatment induced lattice reconstruction of TiO2 NRAs.

XPS was performed to investigate the effect of annealing under different atmospheres on the chemical oxidation state and the change in the surface bonding of TiO2 NRAs. As shown in Figure 2b, the pristine TiO2 NRAs exhibit two main peaks centered at about 464.7 and 458.9 eV, which correspond to the characteristic Ti 2p1/2 and Ti 2p3/2. This observation confirms the existence of Ti4+−O bonds.27 N2−TiO2 NRAs show two typical peaks with smaller binding energy compared to pristine TiO2 NRAs. The shift of two main peaks toward lower binding energy is due to the presence of the Vo induced surface state.28 In contrast, these two peaks slightly shift to higher binding energies for O2−TiO2 NRAs, which is due to the filling up of Vo. The same shift is also observed from the O 1s XPS profiles, as shown in Figure 2c. The peaks centered at 530.1 and 531.9 eV are attributed to the O within the crystal lattice (Ti−O−Ti) and the hydroxyl groups (Ti−OH).29 It should be noted that the annealed samples show higher shoulder peaks compared to the pristine TiO2 NRAs, indicating that the water molecules prefer to adsorb on the surface of annealed TiO2 NRAs, which is possibly due to the removal of residual Cl ions from the surface of TiO2 NRAs after annealing. 18894

DOI: 10.1021/acs.jpcc.7b04071 J. Phys. Chem. C 2017, 121, 18892−18899

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Figure 4. (a) Linear sweep voltammograms curves measured with a scan rate of 5 mV/s in the dark (upper) and under 100 mW/cm2 illumination (lower). (b) Transient photocurrent responses of pristine (black), N2 (red), and O2 (blue) annealed TiO2 NRAs measured at 1.3 V vs RHE in 0.2 M Na2SO4 solution. All of the TiO2 NRAs used in panels a and b were prepared by 4 h of hydrothermal reaction at 150 °C.

1.38 mA/cm2, as displayed in Figure S2b. Further increase of the annealing temperature to 600 °C can lead to a decrease in photocurrent density, which is due to the increase of electrical resistance of the FTO conducting layer.32 Therefore, the photoelectrochemical properties of TiO2 NRAs grown for 4 h and annealed at 500 °C for 1 h are investigated in detail in the following part. In order to study the PEC properties of the prepared samples, linear sweep voltammograms (LSV) and transient photocurrent were measured. As shown in Figure 4a, the dark currents (upper) are negligible compared to those under illumination (lower), indicating that no electrocatalytic oxygen evolution occurred.33 In the dark, pristine TiO2 NRAs possess slightly larger current than that of N2−TiO2 and O2−TiO2 NRAs. For an n-type semiconductor such as the prepared TiO2 NRAs, the enhanced current implies higher donor (electron) density as the charge transfer in the dark is dominated by the major carrier (electron).34 Photocurrent densities of the annealed samples are remarkably increased compared to that of the pristine TiO2 NRAs. O2−TiO2 NRAs exhibit the highest photocurrent density at different bias potentials, suggesting more efficient charge carrier collection in O2−TiO2 NRAs.17 N2−TiO2 NRAs show a little smaller photocurrent density compared to O2 annealed sample. The reason for this phenomenon will be discussed in the following part. Figure 4b displays the real time photoresponse of the pristine TiO2, N2−TiO2, and O2−TiO2 NRAs at 1.3 V vs RHE in 0.2 M Na2SO4 solution when the irradiation light was switched on and off. All of the samples showed a rapid response to the light with negligible dark current. The maximum photocurrent densities of O2−TiO2, N2−TiO2, and pristine TiO2 NRAs were measured to be 1.38, 1.19, and 0.05 mA/cm2, respectively. The highest photocurrent of O2−TiO2 NRAs is about 28 times higher than that of pristine TiO2 NRAs, indicating that a large amount of photogenerated carriers are rapidly separated and collected. This is further confirmed by the IPCE spectra shown in Figure S3. As expected, the highest IPCE value was recorded to be 29% for the O2−TiO2 NRAs and the lowest value to be 15% for the pristine-TiO2 NRAs. N2−TiO2 exhibited a moderate IPCE of 22%, in good accordance with LSV results. In order to uncover the mechanism of photoresponse enhancement upon annealing, EIS was used to study the charge transport along the nanorods and the charge transfer process at the interface between semiconductor and electrolyte. The diameter of semicircle in the Nyquist plots conveys the

Since the TiO2 NRAS were grown in a solution containing a high concentration of HCl, in which Cl ions were used to restrain the growth of the TiO2 (110) surface,18 high-resolution XPS spectra of Cl 2p were therefore performed to investigate the residual Cl products on the surface of TiO2 NRAs. Figure 2d compares the Cl 2p XPS spectra of all three samples. Two peaks of pristine TiO2 NRAs centered at 198.4 and 200.0 eV are ascribed to the Cl 2p3/2 and Cl 2p1/2, respectively. The binding energy values of Cl 2p3/2 and Cl 2p1/2 are downshifted, compared to that of physisorbed Cl, ascribing to the chemisorption induced dramatic change in chemical environment of Cl ions.30 However, no obvious peaks can be distinguished from either of the annealed TiO2 NRAs, indicating that the annealing treatment at 500 °C can remove the surface adsorbed Cl ions. Figure 3a shows the UV−vis spectra of the pristine and the annealed TiO2 NRAs. It can be seen that the light absorption of N2−TiO2 NRAs is slightly enhanced in the visible range, which may be attributed to the generation of Vo.31 The absorption edge of N2−TiO2 NRAs shows a slight red-shift, indicating a decrease of the optical band gap. No enhancement in light absorption was observed for the O2−TiO2 NRAs. The calculated Tauc plots were used to determine the band gap of these samples. As shown in Figure 3b, the band gaps of O2− TiO2 and N2−TiO2 and pristine TiO2 NRAs are 2.99, 2.95, and 2.97 eV, respectively. Annealing treatment in O2 can heal the defects and eliminate the grain boundaries in the as-prepared TiO2 NRAs.26 The reduction of the optical band gap of the N2−TiO2 is due to the Vo induced surface states within the band gap. It has been reported that TiO2 NRAs exhibit a length dependent photoresponse due to the compromise between the charge carrier generation and collection.3,4 Normally, the photocurrent density of TiO2 NRAs can be enhanced at first with an increase of the length and reaches an optimum value. The photoresponse becomes poorer with further elongation of the length of TiO2 NRAs because of the severe recombination of the charge carrier due to the sharp difference between the length of the TiO2 NRAs and the diffusion length of charge carriers. It is found that the TiO2 NRAs grown for 4 h show the best photoresponse, as shown in Figure S2a. Hence, we chose this sample for further investigation of the effect of annealing temperature on the photoresponse. By increasing the annealing temperature from 200 to 500 °C, the photocurrent was continuously improved from 0.03 mA/cm2 to a maximum of 18895

DOI: 10.1021/acs.jpcc.7b04071 J. Phys. Chem. C 2017, 121, 18892−18899

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Figure 5. Nyquist plots of the pristine and the 500 °C annealed TiO2 NRAs (a) in the dark and (b) under AM 1.5G. (c) Nyquist plots of O2−TiO2 NRAs annealed at 200, 300, 400, 500, and 600 °C for 1 h, respectively. All of the plots in panel b were collected under AM 1.5G. The impedance measurements were carried out at open circuit potential.

Figure 6. (a) Valence-band XPS spectra of pristine (black), N2 (red), and O2 (blue) annealed TiO2 NRAs. (b) DOS diagram of pristine (left), N2− TiO2 (middle), and O2−TiO2 (right) NRAs. (c) Mott−Schottky plots of the pristine and the annealed TiO2 NRAs.

information on charge transfer resistance.35 The larger diameter of semicircles, the higher resistance for the charge transport and transfer. Figure 5a shows the typical Nyquist plots of the samples in dark and under irradiation. It is unanticipated that the O2−TiO2 NRAs, which have the best PEC performance, displayed the largest diameter, different from the previous report.36 A magnified EIS spectrum of the high frequency region is displayed in Figure S4. The pristine TiO2 NRAs show the smallest diameter, indicating the smallest resistance for charge carrier transport and transfer, among all three samples. (More detailed analysis of transport data are provided in the Supporting Information.) This may be due to the adsorbed Cl ions at the interface of TiO2 NRAs and electrolyte, which can lead to increased charge transfer.37 In order to verify this, we further performed another group of comparative measurements by varying the annealing temperature of TiO2 NRAs (Figure 5c). We found that the diameter of semicircles become larger as the annealing temperature gradually increases to 600 °C. It has been previously reported that adsorbed Cl ions can be gradually removed by increasing the annealing temperature.36 Thus, the increase in the diameters of semicircles is attributed to the desorption of Cl ions when the annealing temperature becomes

higher. The surface adsorbed Cl ions play the role of dopant, which increases the conductivity of the TiO2 NRAs, similar to the surface doping of Si nanowires by H atoms.38 However, in a PEC cell, the separation and transportation of photogenerated charge carriers are the two main factors that affect the overall PEC performance of the semiconductors. Although the surface adsorbed Cl ions increase the transportation of the charge carriers, they also become the recombination centers and severely decrease the charge carrier separation efficiency. Moreover, the residual Cl ions block the water molecules from reaching to the active sites of TiO2 NRAs.36 Hence, the pristine TiO2 NRAs exhibited much worse photoresponse compared with those of O2−TiO2 and N2−TiO2 NRAs. To investigate the mechanism of the photoresponse enhancement, XPS spectra of valence band maximum (VBM) were performed. As shown in Figure 6a, the VBM evaluated for the pristine TiO2, N2−TiO2, and O2−TiO2 are 1.98, 2.06, and 2.14 eV, respectively, below the Fermi level,39 indicating the downshift of the VBM. Furthermore, both the spectra of pristine and N2 annealed TiO2 NRAs exhibit band tails toward the vacuum level at about 1.78 and 1.58 eV, respectively. These additional energy levels are attributed to the annealing-induced 18896

DOI: 10.1021/acs.jpcc.7b04071 J. Phys. Chem. C 2017, 121, 18892−18899

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The Journal of Physical Chemistry C Vo on N2−TiO2 and the intrinsic defects of the pristine TiO2 NRAs.40 The Vo induced localized states of N2−TiO2 NRAs are responsible for the enhanced absorption in UV−vis spectra, as shown in Figure 3a. Taken into account the optical band gaps of O2−TiO2 (2.99 eV) and N2−TiO2 (2.95 eV) obtained from the UV−vis spectra, the conduction band minimum (CBM) is inferred to occur at −0.85 and −0.89 eV for O2−TiO2 and N2− TiO2, which is 0.14 and 0.10 eV lower than that of the pristine TiO2 NRAs. A schematic illustration of the DOS of pristine TiO2, N2−TiO2, and O2−TiO2 is shown in Figure 6b. It is wellknown that higher conduction band with respect to hydrogen evolution potential would facilitate the charge transfer. The lowered band edge states increase the charge transfer resistance, which coincides with the results of EIS measurements, indicating that there should be other factors contributing to the final performance. In order to further investigate the underlying reason for the difference in photocurrent densities, Mott−Schottky plots were measured. For the purpose of determining the properties at the interface of TiO2 NRAs and electrolyte, the frequency in the measurement for Mott−Schottky analysis must be neither too high nor too low. A capacitance spectrum is displayed in Figure S5 to illustrate the selection of proper measurement frequency. As shown in Figure 6c, all of the curves show positive slopes, suggesting that this set of samples are n-type semiconductors. It can be found that the slope of the linear part of the Mott− Schottky plots of the annealed TiO2 NRAs increased compared to the pristine TiO2 NRAs, indicating that the donor densities of O2−TiO2 and N2−TiO2 are lower than that of the pristine TiO2 NRAs. The carrier densities are calculated following the eq 1: ⎡ 2 ⎢ dE Nd = eε0ε ⎢⎢ d 12 ⎣ C

( )

⎤ ⎥ ⎥ ⎥⎦

determine its influence on the PEC performance following eq 2: W=

2εε0|ϕSC| eNd

(2)

where ϕSC ≡ E − Efb is the maximum potential drop in the depletion region; Nd, e, ε0, and ε represent the donor density, elemental charge value, the permittivity of a vacuum, and dielectric constant of the semiconductor (170 for rutile TiO2), respectively. Accordingly, the depletion widths of the pristine TiO2, N2−TiO2, and O2−TiO2 NRAs are 13.4, 21.1, and 22.2 nm, respectively. It is known that the collisions and recombination rates of photoexcited electrons and holes within the depletion region would be suppressed. Therefore, a wider depletion area existing in TiO2 NRAs is beneficial for more efficient charge separation. Moreover, it should be noted that the crystallite sizes of the annealed TiO2 NRAs are larger than that of the pristine TiO2 RNAs. It means that the number of the grain boundaries inside the TiO2 NRAs decrease. This will suppress the recombination of charge carriers at the grain boundaries. On the basis of the above discussion, the enhancement in the photoresponse of the O2−TiO2 NRAs are due to the desorption of the Cl ions, filling of surface Vo, expansion of depletion layer, and increase of grain size.

4. CONCLUSIONS In conclusion, we have uncovered the mechanism of the enhancement in PEC performance of TiO2 NRAs after thermal treatment. Annealing of TiO2 in O2 yields the highest photocurrent of 1.38 mA/cm2 under AM 1.5G irradiation with an applied bias of 1.3 V (vs RHE). The annealing treatment can desorb the Cl atoms from TiO2 NRAs and increase the grain size. Removal of Cl ions after annealing downshifts the energy bands of TiO2 NRAs, increasing the resistance for charge carrier transport and transfer. Annealing in O2 can fill the surface Vo and expand depletion layer. The findings in this work facilitate a profound understanding of the properties of annealed TiO2 NRAs, which is beneficial for the optimization of TiO2 NRAs for potential applications.

(1)

where C, Nd, e, ε0, and ε represent the space charge capacitance, donor density, elemental charge value, the permittivity of a vacuum, and dielectric constant of the semiconductor (170 for rutile TiO2), respectively. The donor densities of pristine TiO2, N2−TiO2 and O2−TiO2 NRAs were calculated to be 6.5 × 1018, 1.6 × 1018, and 1.3 × 1018 cm−3, respectively. The decrease of donor densities for the annealed TiO2 NRAs is due to desorption of the Cl ions from the surface of TiO2 NRAs. Moreover, a larger donor density usually results in an increased band bending, which can be reflected in the flat band potentials. The flat band value can be obtained through extension of the longest linear portion of the M-S plots to 1/C2 to the intercept. The flat band potentials of pristine, N2, and O2 annealed TiO2 NRAs were measured to be −0.02, +0.22, and +0.26 eV (vs RHE) correspondingly. O2−TiO2 NRAs showed positive shift of Efb, indicating a smaller bending degree of the conduction band.41 In general, a larger band bending could generate a stronger built-in internal electric field that facilitates the separation and transportation of electrons and holes.42 However, the pristine TiO2 NRAs conversely exhibit the poorest photocurrent. It is worth noting that O2−TiO2 NRAs contain the lowest donor density, because thermal annealing in an O2 environment makes TiO2 NRAs more stoichiometric. Nevertheless, the maximum photocurrent density was detected from the O2−TiO2 NRAs. In order to further probe the underneath fundamental mechanism, the width of the depletion region was calculated to

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b04071. Scanning electron microscopy (SEM) images of TiO2 NRAs growing for 2, 4, 6, and 8 h. Furthermore, the photocurrent densities of above-mentioned samples as well as different temperatures (200, 300, 400, 500, and 600 °C) annealed TiO2 NRAs can be found there. Some other results including IPCE plots, magnified Nyquist posts and capacitance spectrum are also provided to show the conversion efficiency, charge transfer, and transport properties and capacitance change of TiO2 NRAs at different frequencies, respectively. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Rui-Qin Zhang: 0000-0001-6897-4010 18897

DOI: 10.1021/acs.jpcc.7b04071 J. Phys. Chem. C 2017, 121, 18892−18899

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The Journal of Physical Chemistry C Author Contributions

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C.H. and J.B. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the grants from the Research Grants Council of the Hong Kong SAR [Project No. CityU 11334716], Basic Research Program in Shenzhen, China [Project No. JCYJ20150601102053060], and Centre for Functional Photonics of the City University of Hong Kong.

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DOI: 10.1021/acs.jpcc.7b04071 J. Phys. Chem. C 2017, 121, 18892−18899

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DOI: 10.1021/acs.jpcc.7b04071 J. Phys. Chem. C 2017, 121, 18892−18899