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Materials and Interfaces
Flame Reduced TiO2 Nanorod Arrays with Ag Nanoparticle Decoration for Efficient Solar Water Splitting Biyi Chen, Xue Chen, Ruoyuan Li, Weiqiang Fan, Fagen Wang, Baodong Mao, and Weidong Shi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b06171 • Publication Date (Web): 01 Mar 2019 Downloaded from http://pubs.acs.org on March 1, 2019
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Flame Reduced TiO2 Nanorod Arrays with Ag Nanoparticle Decoration for Efficient Solar Water Splitting Biyi Chen†, Xue Chen†, Ruoyuan Li†, Weiqiang Fan†, Fagen Wang†, Baodong Mao†, Weidong Shi†* †
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, P. R.
ABSTRACT: Low photogenerated charge density and fast surface charge recombination of TiO2
photoelectrochemical water splitting. Herein, we demonstrate an efficient, and facile flame reduction method to produce rich oxygen vacancies in single-crystal rutile TiO2 nanorod arrays without destroying the catalyst and conductive substrate at ambient conditions. The oxygen vacancies improve the conductivity of TiO2 and act as the role that intermediate electron donor increases the charge density. We further construct a Schottky junction by depositing Ag nanoparticles on the flame reduced TiO2 to enhance surface charge separation efficiency. The optimal TiO2 photoelectrodes exhibit an astonishing surface charge separation efficiency of 91%, as well as photocurrent density as high as 1.52 mA cm-2 (at 1.23 V, vs. reversible hydrogen electrode) which is approximately 7.2 times that of the pristine rutile TiO2 (0.21 mA cm-2). This work demonstrates that facile flame reduction method combined with Schottky junction construction exhibits significant application prospect for the enhanced solar conversion efficiency of metal oxide photoelectrodes.
INTRODUCTION TiO2 photoanode, which has been extensively investigated for photoelectrochemical (PEC) water splitting, has many intrinsic merits, such as non-toxicity, affordability, and stability.1 However, low photogenerated charge density caused by larger band gap (Eg = 3.03 eV for rutile) is an obstacle that hinders photoelectrochemical (PEC) performance. Moreover, the higher average effective masses cause the sluggish migration process from the bulk to the surface and severe surface recombination of photogenerated carriers.2 Up to date, various efforts were devoted to improving the PEC performance of TiO2 photoanodes, such as ions doping,3 morphology/structure regulating,4 heterojunctions and band engineering.5 In many strategies, surface defects play a class of nonnegligible roles and give crucial effects on the catalytic capability of metallic oxide semiconductor materials. Typically, surface defects can narrow band gap,6 serve as photogenerated carrier trapping sites preventing the meaningless recombination,7 increase the density of active sites and boost adsorption and desorption.8-9 Particularly, oxygen vacancy, an important surface defect, is recognized as an electron donor and helpful to boost photogenerated charge density as well as carrier transport in semiconductor photoelectrode.10 It can directly affect the surface property,11 electron structure,12 charge transfer13 and intermediate reaction of semiconductor material.14 Those are significant impacts in obtaining a higher solar energy conversing efficiency in PEC water splitting. So, for PEC water splitting, it is desired that oxygen vacancies can enhance the conductivity and charge
density of metal oxide semiconductors. The generation of rich oxygen vacancies can be achieved by several traditional methods, including solvents chemical reduction (NaBH4, thermites),15-16 annealing metal oxides under reducing atmosphere (H2, CO),17-18 electrochemical reduction by applying bias19 and particle bombardment with high kinetic energy (High energy electron or argon ion).20-21 These methods are effective for generating oxygen vacancies. However, they are sophisticated, time-consuming and expensive, which hinders macroscopic quantity preparation. For instances, solvent chemical reduction takes a certain amount of time (dozens of minutes) and may bring about negative changes on crystallinity and electronic structure. The reductive gas annealing is risky and needs a long process of heating and cooling. High cost and low yield are two prominent shortcomings in high-energy particle bombardment. Compared with the above methods, intuitively, there are many unique advantages in using a flame to reduce metal oxides, such as high temperature (>1000 °C), controllable reducibility and ultrafast heating/cooling rates. These characteristics allow abundant oxygen vacancies to be implanted onto metal oxides within a very short time, while preserving the morphology and structure of the catalyst. Although high photogenerated charge density of photocatalyst is a prerequisite for excellent performance, the sufficient surface charge separation is essential for obtaining ideal photoelectrodes. Loading noble metal nanoparticles (NPs) is an ideal way to separate surface charge for improved PEC performance. Ag NPs, as a typical representative, are widely used in photocatalysis and PEC water splitting field because it can form Schottky barrier with semiconductor materials, which promote interfacial charge separation, boosting photocatalysis performance.22-23
Here, we demonstrated an efficient, and facile flame reduction strategy to introduce oxygen vacancies in single-crystal TiO2 NR arrays. The rich oxygen vacancies can be introduced to TiO2 in the times scale of second (7 s) and the flame reduction process does not damage the TiO2 NR arrays morphology, crystallinity and FTO substrates. Moreover, the flame reduction does not lead to Sn doping TiO2 or carbon deposition, owing to very short reduction time. Further, Ag NPs were deposited onto the peak of TiO2 NRs via PEC cathodic reduction in a few minutes (5 min). This Schottky junctions formed by Ag NPs and TiO2 improved the surface charge separation greatly and 91% surface charge separation efficiency value was obtained. Further research showed that the oxygen vacancies and Ag NPs could simultaneously optimize light absorption and surface charge separation. With rationally modulating the time of flame reduction and PEC cathodic reduction, photoelectrodes with gradient PEC activity have been attained, proving the synergistic promotion of the oxygen vacancies and Ag NPs in the rutile TiO2 NR array for solar energy conversion.
EXPERIMENTAL SECTION Flame reduction experiment parameters. The flame reduction was conducted in the ambient using a commercial flame gun (CAMPSOR-918) with a 1.8 cm diameter, using premixture gases containing air (oxidizer) and fuel (n-butane). The air intake is the maximum allowable value of the flame gun, and fuel intake is 1/3 of the maximum allowable value. The FTO substrate was fixed in 2 cm ahead of the flame gun, ensuring that the FTO was fully covered with inner flame. A simplified preparation process diagram is shown in Figure 1.
Figure 1. A diagrammatic sketch of the flame reduction method to introduce oxygen vacancies to TiO2.
Sample preparation. Single-crystal rutile TiO2 NR arrays coated-FTO glass (Fluorinedoped tin oxide, 1.5 cm × 3 cm, 14 Ω, light transmittance ≥90%, Geao, Japan) were synthesized using the hydrothermal method with some modifications.24 Briefly, 0.6 mL of tetrabutyl titanate C16H36O4Ti (Adamas-beta®, ≥98.5%) was added into 50 mL of an aqueous HCl solution (25 mL of deionized water and 25 mL of concentrated HCl (Sinopharm, 38%) under mild magnetic stirring for ten minutes. Then, the mixing liquid was transferred to a Teflon lined stainless steel autoclave, where a clean FTO has been placed. The autoclave was placed in an electronic oven at 170 °C for 7 h. Next, the autoclave was opened and the FTO was cleaned with deionized water and absolute alcohol before cooling to room temperature, following annealing at 450 °C in muffle furnace for 3 h. The TiO2 NR arrays were reduced by flame with varying burning time. For simplicity, the flame reduced TiO2 photoanodes were abbreviated as TiO2-FR. The Ag NPs were deposited onto the peak of TiO2 NR arrays by PEC cathodic reduction of AgNO33 solution. Typically, nanoporous BiVO4 electrode (preparation see supporting information), Ag/AgCl (in saturated potassium chloride solution) and TiO2-FR 7 s were employed as a photoanode, reference electrode, and counter electrode, respectively. The electrolyte is made up of the mixed solution containing 49 mL of 0.5 M Na2SO4 and1 mL 50
mM of AgNO3. A commercial 300 W xenon lamp (Beijing Zhong Jiao Jin Yuan Technology Co., Ltd., LS-SXE300CUV) was used to provide light irradiation. The light intensity of the irradiation to BiVO4 photoanode was 100 mW cm-2. A constant voltage of 1.0 V vs. Ag/AgCl was used to promote charge separation of TiO2 photoanode in the process of reduction of AgNO3 with varying deposition time. The resulting Ag NPs loaded to flame reduced TiO2 electrodes were rinsed with deionized water and ethanol. Correspondingly, the Ag NPs loaded to TiO2 photoanodes with the flame reduction were labeled as Ag-TiO2–FR. The preparation process of Ag NPs loaded to TiO2 samples was the same as above except the flame reduction.
Characterization. The crystal structure was explored by using a D8 ADVANCE X-ray diffraction (XRD) equipment (10 – 80°, 5.0° min−1) and Tecnai G2 F30 S-TWIN transmission electron microscope (TEM). The acquisition of information on surface morphology was accomplished by using a JSM 7001F field emission scanning electron microscopy (FESEM). An ESCALAB 250Xi X-ray photoelectron spectroscopy (XPS) system was employed to explore surface electronic structure, using standard C1s peaks (284.4 eV) supported by adventitious carbon to calibrate samples. Raman spectra were conducted using DRX Lase Raman Spectrometer to confirm the crystal structure and surface carbon deposition. Electronic paramagnetic resonance (EPR) spectra were measured to explore defects using A300-10/12 under the condition of operating at the specific frequencies (9.8 GHz). A UV2450 UV integrated optical measurement system was employed to explore the light absorption. Element analysis was investigated by using Inductively coupled plasma optical emission spectrometer
Photoelectrochemical tests. Photoelectrochemical tests were carried out in a typical three-electrode device under simulated 1 sun irradiation (100 mW/cm2) supplied by 300 W Xe lamp solar simulator (Beijing Zhong Jiao Jin Yuan Technology Co., Ltd., LS-SXE300CUV), using an electrochemical workstation (CHI852C). The as-prepared photoelectrodes, Pt wire and Ag/AgCl (soaked in saturated KCl solution) were used as working electrodes, a counter electrode, and a reference electrode, respectively. The electrolyte was 0.5 M Na2SO4 solution (pH = 6.85). This conversion of applied bias to reversible hydrogen electrode (RHE) was carried out by using the equation ERHE =EAg/AgCl + 0.059 pH + 0.197, where ERHE, EAg/AgCl, and pH are the RHE (V), the applied potential (V) vs the Ag/AgCl electrode and the value of the electrolyte. All the J–V curves were measured with a scan rate of 50 mV s-1. A constant voltage of 0.62 V vs. Ag/AgCl was used to test Incident Photon to Current Efficiency (IPCE) values. IPCE (%) =1240J/Pλ
where J (mA cm-2) is the current density at monochromatic light, λ (nm) is the wavelength of the monochromatic light, and P (mW cm-2) is the illumination power intensity, respectively. Supposing 100% Faradaic efficiencies, the applied bias photon-to-current efficiency (ABPE) was derived according to the equation: ABPE (%) = J × (1.23 - V)(V)/P
where J (mA cm-2) is the measured current density, V is the applied bias (vs. RHE) and P (mW cm-2) is the same as meanings above. The surface charge separation sufficiency (ηsur) was calculated using the equation:
where J (Na2SO4) (mA cm-2) and J (Na2SO3) (mA cm-2) are current densities with or without 0.5 M Na2SO3 as a hole scavenger., respectively. The frequency range of Electrochemical impedance spectroscopy (EIS) was from 0.01 Hz to 1000 kHz under Xe lamp irradiation and a ZSimpWin software was used to perform fitting data. Mott–Schottky analyses were carried out with the scan rate of 10 mV s−1 at a frequency of 1 kHz under the condition of shielding illumination.
RESULTS AND DISCUSSION XRD, FESEM, HRTEM, Raman spectra and other technologies were employed to explore the effects of flame reduction on the as-prepared samples. The XRD was used to detect the effects of flame reduction on the crystallinity of TiO2 NRs and FTO substrates. As shown in Figure 2a, no detectable transformations have been observed in the (101) crystal surface of TiO2 (JCPSD No.21-1267) and FTO substrate, suggesting that no destruction of the crystal structure of TiO2 and SnO2 conductive layer after flame reduction 7 s. The diffraction peak in 38.18°, 44.25°, 64.72°, and 77.40° can be attributed to the (111), (200), (220) and (311) crystal planes of Ag NPs (JCPDS No. 01-087-0719) respectively, which confirmed that Ag NPs were successfully introduced to TiO2. In order to further achieve information on surface species and crystallinity of TiO2-FR, we have employed the Raman spectrum to characterize all flame reduced TiO2. As demonstrated in Figure 2b, it can be obviously observed that three typical Raman characteristics of B1g (230 cm-1), Eg (445 cm-1) and A1g (610 cm-1) belong to rutile TiO2.25 No
Figure 2. (a) The XRD patterns of (A) Pure FTO substrate, (B) TiO2 NR arrays coated FTO, (C) TiO2-FR 7 s coated FTO, (D) Ag-TiO2-FR 7 s coated FTO; (b) Raman spectra of TiO2-FR at 0, 3, 5, 7, 9, 11 s; The XPS of the Sn 3d (c) and Ag 3d (d) region of Ag-TiO2-FR 7 s powder samples scraped down from the FTO; (e) The relation of the mass fraction of Ag vs. reduction time for TiO2-FR 7 s; (f) The EPR spectra of pristine TiO2, TiO2-FR 7 s and Ag-TiO2-FR 7 s.
shift and disappearance of the peak indicate that the rutile structure is well preserved after flame reduction. Significantly, there are no characteristic peaks in the range of 1300 to 1600 cm-1, which means the flame reduction strategy is clean and do not deposit carbon on the surface of
TiO2.26 Therefore, carbon deposition is not responsible for enhanced solar energy conversion efficiency of TiO2 NR arrays. Considering the fact is that the SnO2 on FTO substrates is also rutile phase and the lattice parameters of rutile TiO2 (a, b = 0.4517 nm, c = 0.2940 nm) and SnO2 (a, b = 0.4687 nm) are very close.24 It is necessary to explore whether flame reduction led to Sn doping TiO2. Hence, we collected the TiO2-FR-7 s powder samples scraped down from the FTO to carry out XPS characterization. That the signals of Sn are disorderly forcefully demonstrated that the flame reduction did not lead to Sn doping TiO2 NRs (Figure 2c). Therefore, the improved PEC performance of TiO2 NR arrays can be ruled out by Sn doping effect. The peaks of 367.63 and 373.68 eV can be classified as Ag 3d3/2 and Ag 3d5/2 orbits (Figure 2d), demonstrating that Ag species are zero valences.27 We employed ICP-OES to investigate the content of Ag nanoparticles. It can be clearly seen that the mass fraction of Ag increases with the prolongation of cathodic deposition (Figure 2e). In order to verify the oxygen vacancy, we have employed EPR spectra to confirm the formation of rich oxygen vacancies. The strong signal (g = 2.002) can be ascribed to the oxygen vacancy species,28 which further confirms that the rich oxygen vacancies have been introduced to TiO2 (Figure 2f). The AgTiO2-FR 7s exhibits a weaker EPR signal than TiO2-FR 7s, which may be concerned with that the diamagnetism of Ag shields magnetic moment signal generated by the unpaired electrons in oxygen vacancies.29-30 In addition, the EPR spectra of TiO2-FR at different reduction time is revealed in Figure S1 (see supporting information). Based on the observations of EPR, it can draw a reasonable conclusion that the flame reduction can introduce rich oxygen vacancies to TiO2 within seconds of time scale.
Figure 3. High-resolution O 1s XPS spectra of (a) Ag-TiO2-FR 7 s; (b) TiO2-FR 7 s; (c) pristine TiO2.
The XPS spectrum of the O 1s was used to further verify the oxygen vacancy. In Figure 3, the binding energy at 529.7 and 531.2 eV can be ascribed to lattice oxygen and oxygen species at the vacancy sites, respectively.8, 31 The peak at 532 eV is external hydroxyl groups adhering onto the surface of TiO2.32 Additionally, the O 1s spectra shows that the Ag-TiO2-FR 7 s and TiO2-FR 7 s sample possess an obviously higher intensity of the shoulder peak around 531.2 eV than the pristine TiO2, indicating that an increased density of oxygen vacancy was generated
after flame reduction. The observations of XPS and EPR spectrum verify that the rich oxygen vacancies can be introduced in TiO2 by flame reduction method in time scale of second level. FESEM and HRTEM were used to investigate the changes of flame reduction on the surface morphology and crystallinity of TiO2 NRs. Figure 4a shows a typical FESEM image of the TiO2 NR arrays. The NRs are very smooth and cover some trimmed edges. In addition, crosssectional view reveals that the NRs are aligned vertically on FTO and its average length of NRs is about 3 µm (Figure 4a inset). The TiO2 NR arrays still maintain its former pattern after flame reduction 7 s (Figure 4b), namely, high temperature flame reduction did not change the
Figure 4. Top-view images of the (a) TiO2 NR arrays, inset is cross-sectional view; (b) TiO2-FR 7 s arrays; (c) Ag-TiO2-FR 7 s arrays, inset is high magnification image, scale bar, 1 μm; HRTEM and corresponding selected area electron diffraction patterns of the (d) pristine rutile TiO2 NR, (e) the TiO2-FR 7 s; (f) HRTEM image of the Ag-TiO2-FR 7 s, inset is TEM view.
morphology of TiO2 NR arrays on a modest time scale. Figure 4c reveals that most Ag+ are reduced to Ag NPs that are attached to the top of TiO2 nanorods, which is beneficial to the improvement of PEC performance. The reason for the improvement is that the migration direction of photogenerated carriers is orthogonal to the long axis of one-dimensional NR
structure, which can avoid the disordered movement and conduce to hole collection. The HRTEM was used to explore the change of the flame reduction on the crystallinity. As shown in Figure 4d and 4e, the well-defined lattice distance was about 0.242 and 0.240 nm respectively, showing that the flame reduction did not change the high crystallinity of TiO2 NRs. In addition, the observations of selected area electron diffraction of inset of Figure 4d and 4e show typical single crystal diffraction pattern, demonstrating that the flame reduction did not damage the single-crystal structure of TiO2. Figure 4f clearly reveals that the Ag NPs are attached to the surface TiO2 NR and the corresponding spacing of the lattice fringe (0.235 nm) corresponds to Ag (111) crystal surface, in accordance with corresponding FESEM views. For the sake of obtaining the size distribution of Ag NPs, we counted 200 Ag NPs and showed the corresponding results in Figure S2 (see supporting information). Some Ag nanoparticles with serious aggregation were excluded from the statistical results. Judging from the results of Figure S2, the particle size of Ag NPs is mainly concentrated in the range of 5-15 nm, indicating that the Ag NPs prepared by cathodic reduction are monodispersed to a certain degree. To sum up, the flame reduction can effectively produce rich oxygen vacancies to TiO2 NRs, without destroying the NRs morphology, crystallinity, FTO conductive substrate, and leading carbon deposition as well as Sn doping. The size of Ag NPs obtained by cathodic reduction is mainly concentrated in the range of 5-15 nm. For comparison, we introduced the data of Ag NPs loaded to TiO2 in linear sweep voltammetry (LSV), transient photocurrent and ABPE. A series of results of LSV to examine the PEC properties is shown in Figure 5a. All photoelectrodes show an extremely low dark current density, suggesting that the electrocatalytic water splitting is negligible. Obviously,
compared with the pristine TiO2, the TiO2-FR 7 s exhibits higher photocurrent density. In addition, all the TiO2 containing Ag NPs exhibit an obvious cathodic shift of onset potential (ca. 300 mV). The reason for that is the Schottky junction formed by Ag NPs and TiO2 promotes the surface charge separation, thereby increasing the photovoltage and decreasing the on-set potential directly.33 Significantly, the Ag-TiO2-FR 7 s exhibits an obviously enhanced photocurrent density (1.52 mA cm-2 at 1.23 V vs. RHE), which is 6.2 times higher than that of the pristine TiO2. The J-V diagrams of photoanodes of optimized flame reduction and PEC cathodic reduction are revealed in Figure S3 and S4 (supporting information). The curves of the transient photocurrent are revealed in Figure 5b. The steady-state photocurrent of Ag-TiO2FR 7 s (1.33 mA cm-2) is more than 6 times higher than of pristine TiO2 (0.18 mA cm-2). Moreover, both the TiO2-FR and Ag-TiO2 show higher photocurrent density, according to the LSV. The ABPE, an important evaluation indicator for PEC water splitting, can represent the solar-to-hydrogen efficiency to a certain extent. The ABPE for the Ag-TiO2-FR 7 s could reach up to 0.37% at 0.83 V (Figure 5c), while only 0.03% at 0.92 V for pristine TiO2 was obtained. Further, IPCE measurements were performed to explore energy conversion (Figure 5d). Compared with the pristine TiO2, the flame reduction and Ag NPs lead to the obviously improved IPCE values and a shift in absorption edges (ca. 430 nm). Moreover, the peak values of flame reduced samples both display a clear red-shift from 380 to 400 nm, which derives from the oxygen vacancies narrowing band gap.34 The IPCE values are further increased because of the enhanced surface charge separation efficiency induced by the Ag NPs. The results of IPCE agree with the trend of ABPE. To confirm this conjecture, we compared the JV behaviors of the three sets of samples with 0.5M Na2SO3 as a hole scavenger (Figure S5).
Figure 5. (a) The J–V curves for pristine TiO2, TiO2-FR-7 s, Ag-TiO2 and Ag-TiO2-FR 7 s; (b) Transient photocurrent responses of the pristine TiO2 (black line), TiO2-FR 7 s (red line), Ag NPs loaded to TiO2 (green line) and Ag-TiO2-FR 7 s (blue line); (c) ABPE curves; (d) IPCE curves; (e) Surface charge separation efficiency of pristine TiO2 (black line), TiO2-FR-7 s (red line) and Ag-TiO2-FR 7 s (blue line); (f) Stability of the Ag-TiO2-FR 7 s photoanode measured at 1.23 V vs. RHE in 0.5 M Na2SO4 under 100 mW cm-2 simulated sunlight illumination.
Based on the J–V curves, the surface charge separation efficiency of the pristine TiO2 (21%), TiO2-FR 7 s (55%) and Ag-TiO2-FR 7 s (91%) at 1.23 V vs. RHE are calculated (Figure 5e).
The great improvement of ηsur corroborates our expectations that constructing the Schottky barrier is an effective method to promote surface charge separation, obtaining enhanced PEC performances. Considering the fact that stability and reliability are prerequisites for commercial application, a stability test was performed at 1.23 V vs. RHE. The photocurrent density of the Ag-TiO2-FR 7 s remained at approximately 1.24 mA/cm2 over 10 h, which is 95.4% of the initial value (1.30 mA/cm2), indicating its excellent stability for PEC water splitting (Figure 5f). Figure 6a reveals the optical absorption of as-papered samples. The absorbance of visible and partial infrared regions was enhanced significantly, indicating that the oxygen vacancies and Ag NPs both can boost the absorption of TiO2. The results arise from that intermediate level effects of oxygen vacancy and surface plasmon resonance of Ag NPs.35-36 Additionally, a slight red-shift in absorption edge indicates the oxygen vacancy changes the thermodynamic migration direction of photogenerated electrons, which is due to the oxygen vacancy as the electron donor.28,
To explore the photoinduced-charge transfer actions of
semiconductor/electrolyte interface, Mott-Schottky and EIS analysis were carried out. The Mott-Schottky analysis is a method to calculate donor density in the circuit models presented above. The Mott-Schottky equation is: 𝐴 2
( ) = 𝐶
2 ⅇ𝜀0 𝜀𝑟 𝑁𝐷
(𝐸 − 𝐸fb −
, where C (F) is the capacitance of the space charge layer at the interface between semiconductor and electrolyte, A is the tested area (cm2), ε0 is the relative dielectric constant of TiO2 (114 for rutile)38, εr is the permittivity of free space (8.854 × 10-12 F m-1), e is the elementary charge quantity (1.6 × 10−19 C), ND is the surface charge carrier density, E is the
applied voltage (V vs. RHE), Efb is the flat-band potential (V), kB is the Boltzmann constant (1.38064852 × 10-23 J K-1), and T is the system temperature (298 K). Conversion of MottSchottky equation yields mathematic relation of 1/C2 to applied potential (Figure 6b inset), the flat-band potential is the value of the intersection of the straight line and the x-axis, giving 0.28, 0.30 and 0.34 V vs. RHE for pristine TiO2, TiO2-FR 7 s and Ag-TiO2-FR 7 s, respectively. Obviously, compared to pristine TiO2, a positive shift of Efb occurs in TiO2-FR 7 s and Ag-
Figure 6. (a) Absorption spectra of the pristine TiO2, TiO2-FR 7 s and Ag-TiO2-FR 7 s; (b) Mott–Schottky plots of the pristine TiO2 (black line), TiO2-FR 7 s (red line) and Ag-TiO2-FR 7 s (blue line); (c) Nyquist plots of electrochemical impedance spectra under 100 mW cm-2 simulated solar light irradiation; (d) Hamann equivalent circuit.
TiO2-FR 7 s, which means an increase in band bending, thereby facilitating the surface charge separation.39 Moreover, the surface charge density (ND) can be acquired by the equation:
, where the ND values of pristine TiO2, TiO2-FR 7 s and Ag-TiO2-FR 7 s were calculated to be 1.06 × 1017, 2.54 × 1017 and 1.28 × 1018 cm-3, respectively. Typically, the higher ND signifies a more optimized charge transfer process and is more beneficial to interfacial water oxidation reaction.40-41 The EIS analysis was further conducted to explore charge transfer behavior (Figure 6c). Figure 6c clearly shows two circular arcs in TiO2, indicating two types of capacitances with different properties.42 Thus, the typical Hamann circuit diagram is used as an equivalent analog circuit (Figure 6d), which contains impedance of the bulk and semiconductor/electrolyte interface. The Rs, Rbulk, and Rss represent the impedance between conductive glass and photocatalyst, the charge transfer impedance of bulk phase and electrolyte interface, respectively. The fitting results of Rbulk for the pristine TiO2 and TiO2-FR 7 s photoanodes are 1246 and 622 Ω, which confirms that the oxygen vacancies can improve the conductivity of TiO2. The Rss values for the pristine TiO2, TiO2-FR 7 s and Ag-TiO2-FR 7 s photoanodes are 4025, 2635, 1163 Ω, respectively. The value of Rss reduces dramatically when Ag NPs are loaded, which is due to the increase upward band bending, boosting surface charge separation.43 When the oxygen vacancies and Ag NPs are introduced to TiO2, the value of Rbulk and Rss decreases further, demonstrating their synergistic promotion. Stability is an important factor that must be considered when products are going to practical application. As shown in Figure 7, neglectable transformations can be observed on the morphology, crystalline phase and chemical valence of catalysts, suggesting that the Ag-TiO2FR 7 s photoanodes are very stable for the PEC water splitting.
Figure 7. Top-view FESEM images of the Ag- TiO2-FR 7 s (a) Before stability test; (b) After stability test, scale bar, 1 μm; (c) The XRD patterns of Ag- TiO2-FR 7 s before and after stability test; The XPS spectra of (d) Ag 3d, (e) Ti 2p and (f) O 1s. Black and red lines represent the signals of catalysts before and after the stability test, respectively.
Based on the above results, a possible energy band structures and charge transfer mechanism is proposed in Scheme 1. According to the principles of semiconductor physics, electron flow will occur in interface between Ag NPs and TiO2, where the free electrons will flow from the TiO2 to the Ag NPs because Ag have higher work function (4.35 eV) than TiO2 (3.87 eV).44 There will be no net charge exchange between semiconductor and metal until their Fermi levels reach equilibrium. Under equilibrium, a space charge region will be formed, where negative charges are driven to the surface of Ag NPs and positive charge to TiO2 due to electrostatic induction.45 For n-type semiconductors, the space charge region is also known as the depletion layer. The depletion layer will induce the upward shift of the energy band edges in the TiO2, which is called band bending.46 Under illumination, electrons could be easy to jump to the oxygen vacancy level due to relatively lower excitation energy, and the corresponding holes in the valence band move toward the semiconductor surface.47-54 The absorption range of TiO2 is
Scheme 1. The band energetics of a semiconductor/liquid contact and photogenerated charge transfer process.
extended to the infrared region due to the shorten band gap caused by oxygen vacancy.55 On the other hand, the band bending and its associated space electric fields can not only promote the separation of electrons and holes but also accelerate the diffusion of ions.56 The electrons and holes were driven to move to FTO substrates and semiconductor/electrolyte interface, respectively.
CONCLUSIONS In summary, we have demonstrated a simple but efficient flame reduction method to introduce rich oxygen vacancies in TiO2 nanorod arrays. The oxygen vacancies not only enhance the conductivity of TiO2 but also act as the electron donor to increase the photogenerated density. Further, Ag NPs enhance light absorption and surface charge separation greatly. Compared to pristine TiO2 photoanodes, the optimized Ag NPs loaded to the flame reduced TiO2 photoanodes exhibit enhanced photogenerated charge density and surface charge separation efficiency. The optimal TiO2 photoelectrodes exhibit a surprising surface charge separation efficiency of 91% as well as photocurrent density as high as 1.52 mA cm-2 at 1.23 V (vs.
reversible hydrogen electrode) which is approximately 6.2 times higher than that of pristine rutile TiO2. We firmly believe that the facile flame reduction method combined with Schottky junction construction is an ideal approach for obtaining the enhanced solar conversion efficiency of metal oxide photoelectrodes.
AUTHOR INFORMATION Supporting Information The preparation of nanoporous BiVO4 photoelectrodes. Figure. S1 shows that The EPR spectra of TiO2-FR at different reduction time (3 – 11 s). Figure S2 shows that the TEM views of AgTiO2-FR 7 s and the size distribution of Ag nanoparticles. The J-V diagrams of photoanodes of optimized flame reduction and PEC cathode reduction are shown in Figure S3 and S4 respectively. Figure S5 shows that the J–V curves under the conditions of containing a 0.5 M Na2SO3 as the hole scavenger for (a) pristine TiO2, (b) TiO2-FR 7 s, (c) Ag- TiO2-FR 7 s.
AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] Notes The authors declare no competing financial interest.
ACKNOWLEDGMENTS We sincerely thank the financial support from the National Natural Science Foundation of
China (21477050, 21671083 and 21522603), the Excellent Youth Foundation of Jiangsu Scientiﬁc Committee (BK20140011), the Program for New Century Excellent Talents in University (NCET-13-0835), the Henry Fok Education Foundation (141068), as well as the Innovation/Entrepreneurship Program of Jiangsu Province (Surencaiban  32).
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