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Article

Defect Engineering and Surface Polarization of TiO2 Nanorod Arrays toward Efficient Photoelectrochemical Oxygen Evolution

by
Yueying Li
1,*,†,
Shiyu Liang
2,†,
Huanhuan Sun
2,
Wei Hua
2 and
Jian-Gan Wang
2,*
1
New Energy (Photovoltaic) Industry Research Center, Qinghai University, No. 251, Ningda Road, Xi’ning 810016, China
2
State Key Laboratory of Solidification Processing, Center for Nano Energy Materials, School of Materials Science and Engineering, Northwestern Polytechnical University and Shaanxi Joint Lab of Graphene (NPU), No. 127, Youyi West Road, Xi’an 710072, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Catalysts 2022, 12(9), 1021; https://doi.org/10.3390/catal12091021
Submission received: 18 August 2022 / Revised: 4 September 2022 / Accepted: 6 September 2022 / Published: 8 September 2022
(This article belongs to the Special Issue Heterogeneous Electrocatalysis: Fundamentals and Applications II)
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Abstract

:
The relatively low photo-conversion efficiencies of semiconductors greatly restrict their real-world practices toward photoelectrochemical water splitting. In this work, we demonstrate the fabrication of TiO2-x nanorod arrays enriched with oxygen defects and surface-polarized hydroxyl groups by a facile surface reduction method. The oxygen defects located in the bulk/surface of TiO2-x enable fast charge transport and act as catalytically active sites to accelerate the water oxidation kinetics. Meanwhile, the hydroxyl groups could establish a surface electric field by polarization, for efficient charge separation. The as-optimized TiO2-x nanorod photoanode achieves a high photocurrent density of 2.62 mA cm−2 without any cocatalyst loading at 1.23 VRHE under 100 mW cm−2, which is almost double that of the bare TiO2 counterpart. Notably, the surface charge separation and injection efficiency of the TiO2-x photoanode reach as high as 80% and 97% at 1.23 VRHE, respectively, and the maximum incident photon-to-current efficiency reaches 90% at 400 nm. This work provides a new surface treatment strategy for the development of high-performance photoanodes in photoelectrochemical water splitting.

1. Introduction

Solar-driven photoelectrochemical (PEC) water splitting, a sustainable strategy to generate clean hydrogen fuels, is deemed a major breakthrough to alleviate the current energy crisis and environmental deterioration [1,2,3,4,5]. Recently, various semiconductor photoelectrodes, such as TiO2, Fe2O3, BiVO4, and WO3, have attracted extensive attention to enhance the PEC performance. In particular, TiO2 is still regarded as a benchmark photocatalyst thanks to its low cost, appropriate energy band structure, and excellent chemical stability [6,7]. However, the PEC water splitting performance of TiO2 is severely limited by its poor light utilization efficiency [8], the short diffusion length of minority carriers, and its sluggish surface catalytic reaction [9,10,11]. Therefore, detailed investigations on the optical–electrical conversion properties of TiO2 are necessary to realize its promising water splitting ability.
Nowadays, the strategies to promote PEC performance mainly include heteroatom doping [12,13,14], deposited cocatalysts [11,15,16], and defect engineering. Heteroatom doping can positively affect the light harvesting property through modulating the band structure, but it fails to boost the surface oxidation reaction. In contrast, cocatalyst modification will enhance the surface catalytic reaction kinetics, but at the cost of reducing the light absorption efficiency of the semiconductor. Notably, defect engineering is capable of simultaneously optimizing the optical and electrical properties, resulting in enhanced light harvesting, fast charge transportation, and suppressed bulk recombination. Meanwhile, the generated oxygen defects are reported to be capable of self-catalyzing water oxidation and decreasing the surface adsorption energy of water, thus boosting the catalytic reaction kinetics [17,18]. Nevertheless, excessive surface defects may act as the centers of charge recombination, inhibiting the separation of carriers on the surfaces of catalysts.
Polarization, including macroscopic polarization, piezoelectric polarization, ferroelectric polarization, and surface polarization, possesses the ability to promote the separation of photo-generated carriers via establishing a polarization electric field [19]. In particular, surface polarization that is obtained by introducing surface terminals (such as hydroxy groups and halogen ions) can drastically facilitate surface charge separation [20]. For instance, Ma and co-workers [21] recently reported a rational approach to building an efficient electric field via modifying the surface of TiO2 with hydroxy groups, which promoted the water oxidation performance up to 1.41 mA cm−2 at 1.23 VRHE. Despite this substantial progress, the PEC activity of TiO2 is still not satisfactory. Developing a complementary method that can significantly improve the efficiency of TiO2 photoanodes still remains a great challenge.
Herein, we demonstrate the preparation of TiO2-x nanorod arrays by a simple NaBH4 reduction treatment, which can simultaneously induce the generation of oxygen vacancies and hydroxy groups. Under PEC operation, the vacancies enable fast charge transport, enhanced charge injection, and accelerated surface catalytic kinetics. Meanwhile, the enriched hydroxy groups bring about a polarized electric field to promote surface charge migration and separation. Additionally, the ordered nanorod arrays facilitate electrolyte diffusion and provide a large accessible surface area for electrocatalytic active sites. As a consequence, the optimized TiO2-x photoanode exhibits a highly enhanced photocurrent density of 2.62 mA cm−2 at 1.23 VRHE in the absence of surface cocatalysts and hole scavengers, which is approximately two times higher than the pristine TiO2. Moreover, the remarkably enhanced charge separation efficiency to 80% and charge injection efficiency to 97% at 1.23 VRHE are accomplished. Our work demonstrates the possibility of using the synergistic effect of defect engineering and surface polarization in enhancing PEC water splitting performance, and this strategy can be extended to other solar utilization-related research areas.

2. Results and Discussion

Figure 1a schematically illustrates the synthesis procedures of the TiO2-x photoanode. TiO2-x NRs were firstly fabricated via a hydrothermal reduction process. As shown in Figure 1b and Figure S1, densely aligned and tetragonal rutile TiO2 nanorods with a single-crystalline structure were successfully grown on the FTO substrate. These nanorod bundles were composed of many tiny single TiO2 crystals. After the NaBH4 reduction, the radially grown TiO2 NRs were well maintained (Figure 1d), indicating a negligible effect on the morphology. From the cross-section image (inset of Figure 1b,d), the nanorods were estimated to be ~2.5 μm in length and ~100 nm in diameter. From the TEM images, it can be seen that the TiO2-x (Figure 1f) showed a defined nanorod with an average diameter of approximately 100 nm. The lattice spacing (Figure 1g) of 0.325 nm corresponded to the (110) crystal plane of the rutile TiO2 phase [22]. Interestingly, the surface wettability of TiO2 was greatly changed with reduced treatment, as evidenced by the decreased contact angle from 27.3° to 16° (Figure 1c,e). The enhanced hydrophilicity is of great benefit for the water absorption process, thus favoring PEC performance [23].
The crystal structures of pristine TiO2 and TiO2-x were investigated by XRD. As displayed in Figure 2a, for the pristine TiO2 NRs, the characteristic peaks located at 36°, 63°, and 70° were well indexed to the (101), (002), and (112) planes of tetragonal rutile TiO2 (JCPDS No. 88-1175) [24]. To further determine the structure of the as-prepared samples, Raman spectroscopy was performed, and the results are presented in Figure 2b. The three Raman peaks appearing at 147, 447, and 607 cm−1 correspond to typical B1g, Eg, and A1g vibrational modes of rutile TiO2, respectively. The broad band at 244 cm−1 is related to the multi-photon process of rutile TiO2 [25,26,27]. It is worth noting that the intensity of all the Raman peaks for TiO2-x is weaker than that of pristine TiO2, arising from the reduced crystallinity induced by abundant oxygen vacancies [28,29].
XPS characterization was conducted to probe the surface chemical environment of TiO2-x. As displayed in Figure 2c, the peaks centered at the binding energies (BE) of 458.4 and 464.2 eV are assigned to Ti 2p3/2 and Ti 2p1/2, respectively, coinciding with the Ti4+ values in the TiO2 lattice [30,31,32]. After NaBH4 reduction, the Ti 2p peaks are shifted to a lower BE in comparison to TiO2, indicating the formation of Ti3+ [33]. The O 1s spectra (Figure 2d) of TiO2 and TiO2-x are all deconvoluted into four peaks centered at 529.8, 530.9, 531.5, and 532.1 eV, corresponding to the O2− species in the lattice (OL), oxygen vacancy (OV), the hydroxyl oxygen (Ti-OH), and surface-absorbed water (OA), respectively [11,34,35]. Accordingly, the relative content of these three O species (i.e., OA, OV, Ti-OH) was calculated and is shown in Figure 2e. It is important to note that the calculated OV content in the TiO2-x (8.9%) is much higher than that in the pristine TiO2 (6.5%), suggesting the existence of abundant OV. Meanwhile, the amount of hydroxyl oxygen also increased from 7.5% of TiO2 to 11.9% of TiO2-x. These enriched -OH groups with dipole moment can polarize the surface of TiO2, leading to a drastic improvement in the charge separation efficiency [21,36]. To further characterize the OV, EPR was carried out. As shown in Figure 2f, an obvious EPR signal at g = 2.001 appeared in TiO2-x, which is the typical signal of OV, suggesting the existence of OV in TiO2-x [31,37]. It was encouraging that the intensity of the EPR signal for TiO2-x was much stronger than that of the TiO2 signal, indicating the enrichment of Ov in TiO2-x, which was consistent with the XPS results (Figure 2d). The simultaneous existence of oxygen defects and hydroxy groups is expected to accelerate the charge separation, transport, and surface reaction activity.
The PEC properties of the TiO2 and TiO2-x electrodes were monitored in a typical three-electrode system using 1 M NaOH as the electrolyte. The photocurrent densities of TiO2-x with different NaBH4 reduction times were compared to investigate the effect of the reduction degree on the PEC performance (Figure 3a). When TiO2 was reduced by NaBH4, the photocurrent density increased and surmounted at a reduction time of 40 min. Therefore, hereinafter, the discussion will focus on a comparison of the TiO2 and TiO2-x (40 min) to explore the underlying mechanism of the outstanding PEC performance of TiO2-x. As shown in Figure 3b, the TiO2-x exhibits a substantially increased photocurrent density of 2.6 mA cm−2, which is approximately two times higher than that of pristine TiO2. Encouragingly, such excellent performance of TiO2-x surpasses that of most of the recently reported TiO2-based photoelectrodes, such as TiO2/NH2-MIL(Fe0.25Ni0.75)-88 (1.56 mA cm−2) [38], Gly-TiO2 (1.41 mA cm−2) [21], Sn3O4/TiO2/Au (2.5 mA cm−2) [39], and MnO2/N-TiO2 NTs (1.95 mA cm−2) [40] (Table S1). Moreover, the ABPE was calculated based on the LSV curves in Figure 3b. As shown in Figure 3c, for the TiO2-x photoanode, the maximum ABPE value reaches 1.6% at 0.5 VRHE, which is much higher than that of pristine TiO2 (0.75% at 0.52 VRHE). Additionally, the first derivatives of the photocurrent densities versus voltages, which can efficiently reflect the change rate of photocurrent density, are depicted in Figure 3d. Interestingly, the TiO2-x demonstrates larger dJ/dV than TiO2 at a low bias of <0.5 VRHE, indicating that the charge transfer dynamics near the surface of the photoanode are expedited [41].
The transient photocurrent response curves (Figure 3e) were obtained via switching on/off the incident light for five consecutive cycles at 1.23 VRHE, which demonstrates the same enhancement as that of the LSV curves. The gentle photocurrent spikes of the TiO2-x suggest that the accumulation of photo-generated holes at the solid/liquid interface is inhibited thanks to the accelerated surface reaction kinetics [42]. To evaluate the photoactivity and light absorption of the catalysts, the IPCE was analyzed at 1.23 VRHE (Figure 3f). The TiO2-x photoanode displayed remarkably higher IPCE than bare TiO2 in the light absorption range of 350–440 nm. It achieves a maximum IPCE value of 90% at approximately 400 nm, which is approximately 1.8 times that of TiO2 (50%). The results indicate the efficient utilization of incident light (350–440 nm) for PEC water splitting. Furthermore, the durability of TiO2-x and TiO2 was measured at 1.23 VRHE under continuous illumination for 10 h (Figure 3g). The TiO2-x exhibited a high photocurrent density of around 2.5 mA cm−2 without obvious degradation, demonstrating its excellent electrochemically stable property during the PEC water splitting process.
The PEC performance of a photoanode is theoretically related to the light harvesting efficiency (ηabsorbance), charge injection efficiency (ηinjection), and separation efficiency (ηseparation). Thereby, to explore the underlying mechanism of the excellent performance of TiO2-x, we systematically studied the optical property and charge behavior of TiO2-x. Firstly, the optical properties of TiO2-x and TiO2 were characterized with UV-vis diffuse reflection spectra (DRS). As shown in Figure 4a, the absorbance edge of TiO2-x shows a negligible shift compared with that of TiO2. Meanwhile, the bandgaps of TiO2-x derived from the DRS spectra based on the Kubelka-Munk equation also reveal little difference with TiO2 (the inset of Figure 4a), indicative that the ηabsorbance is not the main reason for the enhanced PEC activity.
Then, the ηinjection was quantified using the LSV data measured in 1.0 M NaOH electrolyte containing 0.5 M Na2SO3 (Figure 4b). Since sulfite oxidation is a very fast process, assuming that all the photogenerated holes can be immediately consumed once they reach the surface [43], the ηinjection with Na2SO3 can be regarded as 100%. The charge injection efficiency (ηinjection) can be calculated by the equation of ηinjection (%) = JH2O/JNa2SO3 × 100, in which JH2O and JNa2SO3 are the photocurrent density without and with Na2SO3 as hole scavengers, respectively. As shown in Figure 4c, the ηinjection of TiO2-x is sharply increased to 97% in comparison to that of TiO2 (80%) at 1.23 VRHE, implying that the TiO2-x photoanode possesses superior intrinsic electrocatalytic activity for water oxidation, which was further validated by the oxygen evolution reaction performance (Figure 4d). To drive the current density of 1 mA cm−2, TiO2-x requires an overpotential of 450 mV, which is approximately 150 mV lower than that of TiO2. Furthermore, from the anodic/cathodic current densities in the CV curves measured under different scan rates (insets of Figure 4e,f), we could calculate the relative electrochemical surface areas (ECSA) of TiO2 and TiO2-x. The electrochemically active area of TiO2-x was clearly much higher than that of TiO2. This is consistent with the higher ηinjection value of TiO2-x. The faster charge injection efficiency and improved number of active sites could be related to the abundant oxygen defects in TiO2-x caused by NaBH4 reduction, contributing to the enhanced PEC performance.
Finally, the charge separation efficiency (ηseparation) was calculated using the formula of ηseparation (%) = JNa2SO3 /Jabs × 100, where Jabs can be obtained by assuming that all the photons absorbed by the semiconductor are converted into a current. As shown in Figure 5a, TiO2-x achieves a ηseparation of approximately 80% at 1.23 VRHE, which is ~two times higher than that of TiO2. The improved charge separation is also verified by the PL spectra in Figure 5b. The PL intensity of TiO2-x is weaker than that of TiO2 throughout the measured wavelength, demonstrating the efficient separation of electron-hole pairs [44,45]. An EIS test was carried out to study the charge transfer kinetics. The Nyquist plots were analyzed by the equivalent circuit model (inset), in which Rs, Rct1, and Rct2 correspond to the external circuit resistance, charge transport resistance of the photoanode, and electron transfer resistance of the PEC reaction at the solid electrode/electrolyte interface [46]. The obtained fitting parameters are shown in Table S2. Notably, the TiO2-x exhibited lower Rs, Rct1, and Rct2 values than the TiO2, indicating enhanced charge transfer efficiency.
Additionally, the M-S plots were obtained under dark conditions to further clarify the charge transfer behavior of the two samples. As presented in Figure 5d, the positive slopes of both curves suggest that the n-type semiconductor essence of TiO2 is not changed by NaBH4 reduction [47]. Meanwhile, TiO2-x displays smaller slopes than TiO2, suggesting that reduction increases the carrier concentration. The carrier concentration (Nd) can be calculated according to the equation of Nd= [2/(e0εε0)] [d(1/C2)/dE]−1, where e0, ε, ε0, and E correspond to the elemental charge (1.6 × 10−19 C), dielectric constant of rutile TiO2, permittivity of vacuum, and applied electrode potential. The calculated result of TiO2-x is 1.48 × 1019 cm−3, which is 2.4 times larger than that of the pristine TiO2 (6.21 × 1018 cm−3). The increased carrier concentration can be attributed to the existence of oxygen vacancies, which can enhance the bulk electrical conductivity of TiO2. It is worth noting that the flat band potential of TiO2-x experiences a positive shift, which can be attributed to the polarized surface induced by hydroxyl groups. This polarization builds a polarized electric field near the surface, facilitating charge transfer to the surface for the catalytic reaction [20,48,49].
Based on the above results, a rational mechanism for the enhanced PEC performance of TiO2-x is proposed in Figure 6. Upon light illumination, the photo-generated holes rapidly migrate to the photoanode surface thanks to the enhanced conductivity and the formation of a surface-polarized electric field, which could increase the driving force to facilitate charge separation and transport. Moreover, the oxygen vacancies on the surface could act as the active sites for the water oxidation reaction. Meanwhile, the separated electrons located in TiO2 migrate to the counter electrode for the reduction reaction. Thus, the coexistence of oxygen vacancies and hydroxy groups in TiO2-x not only efficiently suppresses surface and bulk charge recombination, but also supplies the active sites for the catalytic reaction.

3. Conclusions

In conclusion, a TiO2-x photoanode with an ordered nanorod array architecture has been successfully fabricated by an in situ chemical reduction method. Thanks to the coexistence of oxygen vacancies and hydroxy groups, the as-obtained TiO2-x photoanode enables a significant enhancement in the PEC performance by accelerating the charge transport/separation and surface reaction kinetics. A highly increased photocurrent density of 2.6 mA cm−2 with outstanding durability is achieved at the potential of 1.23 V versus RHE. It is believed that the present work will provide an innovative design and pave the way towards introducing oxygen vacancies for various semiconductors in PEC systems.

4. Materials and Methods

4.1. Materials Synthesis

The TiO2 nanorod arrays (TiO2 NRs) were synthesized by a hydrothermal process. Typically, 0.4 mL of tetrabutyl titanate was added to 24 mL HCl aqueous solution (12 mL of deionized water and 12 mL of hydrochloric acid) and stirred for 30 min. Then, the mixture was transferred into a 50 mL Teflon-lined autoclave with a piece of FTO substrate. The sealed autoclave was placed in an oven at 180 °C for 8 h. Afterward, the cooled TiO2 NRs were rinsed with DI water. Finally, the TiO2 NRs were placed in a muffle furnace and calcined at 450 °C for 2 h in air, and then left to cool naturally to increase the crystallinity of the TiO2 nanorod arrays.
The as-prepared TiO2 NRs were immersed in 0.1 M of NaBH4 for 40 min at room temperature, followed by rinsing with DI water and drying in N2 flow. The obtained sample is referred to as TiO2-x.

4.2. Material Characterization

The morphology and composition were examined by field emission scanning electron microscopy (SEM, FEI Nano SEM 450) and transmission electronic microscopy (TEM, FEI Talos F200X). The crystalline structure was characterized by X-ray diffraction (XRD, Bruker D8, Karlsruhe, Germany) with a Cu Kα (λ = 0.15406 nm) source. The light absorption spectra were recorded with a UV-vis spectrophotometer, with high-purity BaSO4 as a reference (UH4150). The surface elemental compositions and states were analyzed by X-ray photoelectron spectroscopy (XPS, Kratons AXIS Ultra DLD) with an Al Kα source (hν = 1486.6 eV). The electron paramagnetic resonance (EPR) measurements were recorded using a JES-FA200 spectrometer at room temperature. Photoluminescence (PL) intensity was measured by a fluorescence spectrometer (FLS980) at room temperature.

4.3. Photoelectrochemical Measurements

All of the photoelectrochemical measurements were performed on a CHI760E electrochemical workstation via a typical three-electrode system. In a typical test system, the as-prepared TiO2-x photoanode (1 cm2 working area), Pt plate (1 cm × 1 cm), and Ag/AgCl (saturated KCl) were employed as the working electrodes, counter electrode, and reference electrode, respectively, as shown in Figure S2. Moreover, 1 M NaOH (pH = 13.7) was used as the electrolyte. A 300 W Xe lamp was chosen to provide a light source, with an incident light intensity of 100 mW cm−2. The typical linear sweep voltammetry (LSV) curves were measured under a scan rate of 10 mV s−1 in the forward direction. The applied potential vs. Ag/AgCl was converted to the reference hydrogen electrode (RHE) using the following equation [24]: ERHE = EAg⁄AgCl + E0Ag/AgCl +0.0591 × pH (E0Ag/AgCl = 0.1976 V vs. NHE at 25 °C), where ERHE and EAg⁄AgCl refer to the potentials vs. RHE and experimentally applied potential. The electrochemical impedance spectroscopy (EIS) was measured at the open-circuit voltage under illumination, sweeping in the frequency range from 105 Hz to 10−2 Hz. The applied bias photon-to-current efficiency (ABPE) of the electrodes could be quantized by the photocurrent densities at different potentials using the formula below [50]:
ABPE(%) =Jph × (1.23 − Vbias)/Pin × 100
where Jph, Vbias, and Pin are the current density (mA cm−2) obtained from the LSV curve, applied potential (V vs. RHE), and the power density of incident light (mW cm−2), respectively.
The incident photon-to-current conversion efficiency (IPCE) of the samples was calculated on the basis of the following equation:
IPCE(%) =1240 × Jph /Pin × λ × 100
where Jph refers to the current density (mA cm−2) at a certain wavelength, Pin is the power density of the incident light at the corresponding wavelength, and λ represents the wavelength of the corresponding incident light.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal12091021/s1. Figure S1: (a) HRTEM and (b) SAED pattern of TiO2; Figure S2: Digital images of (a) the electrochemical cell and (b) as-prepared TiO2 and TiO2-x samples; Table S1: Comparison of PEC performance for TiO2-based photoelectrodes [11,21,33,38,39,40,50,51,52,53,54,55,56,57,58]; Table S2: The fitted values of Nyquist plots according to the equivalent circuit in Figure 5c.

Author Contributions

Y.L. and S.L.: writing—original draft preparation; J.-G.W. and Y.L.: conceptualization, supervision, funding, writing—review and editing; H.S. and W.H.: investigation, resources. All authors have read and agreed to the published version of the manuscript.

Funding

This work is financially supported by the National Natural Science Foundation of China (52272239), Fundamental Research Program of Qinghai Province (2021-ZJ-941Q), Youth Research Foundation Project of Qinghai University (2020-QGY-5) and Fundamental Research Funds for the Central Universities (D5000210894).

Data Availability Statement

Not applicable.

Acknowledgments

The authors appreciate the support of the Analytical & Testing Center of Northwestern Polytechnical University during the TEM analysis.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Synthesis illustration of TiO2-x photoanode; SEM of (b)TiO2 and (d) TiO2-x; (f) and (g) TEM images of TiO2-x; contact angles of water droplets on (c) TiO2 and (e) TiO2-x.
Figure 1. (a) Synthesis illustration of TiO2-x photoanode; SEM of (b)TiO2 and (d) TiO2-x; (f) and (g) TEM images of TiO2-x; contact angles of water droplets on (c) TiO2 and (e) TiO2-x.
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Figure 2. (a) XRD patterns and (b) Raman spectra of TiO2 and TiO2-x; XPS spectra of (c) Ti 2p, (d) O 1s of TiO2 and TiO2-x; (e) relative content of oxygen vacancy, Ti−OH, and adsorbed oxygen groups in the TiO2-x photoanodes; (f) EPR curves of TiO2 and TiO2-x.
Figure 2. (a) XRD patterns and (b) Raman spectra of TiO2 and TiO2-x; XPS spectra of (c) Ti 2p, (d) O 1s of TiO2 and TiO2-x; (e) relative content of oxygen vacancy, Ti−OH, and adsorbed oxygen groups in the TiO2-x photoanodes; (f) EPR curves of TiO2 and TiO2-x.
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Figure 3. (a) LSV curves of TiO2-x photoanodes with different NaBH4 reduction times; (b) LSV curves; (c) ABPE values; (d) derivative curves of current with respect to voltage; (e) chopped current-time curves; (f) IPCE plots; (g) stability test at 1.23 V vs. RHE of TiO2 and TiO2-x photoanodes.
Figure 3. (a) LSV curves of TiO2-x photoanodes with different NaBH4 reduction times; (b) LSV curves; (c) ABPE values; (d) derivative curves of current with respect to voltage; (e) chopped current-time curves; (f) IPCE plots; (g) stability test at 1.23 V vs. RHE of TiO2 and TiO2-x photoanodes.
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Figure 4. (a) UV-vis diffuse reflectance spectra; the inset shows the Tauc plots of TiO2 and TiO2-x; (b) LSV curves of TiO2 and TiO2-x photoanodes in the presence of sacrifice agents (0.5 M Na2SO3); (c) charge injection efficiencies; (d) LSV curves in dark of TiO2 and TiO2-x; relative electrochemical surface areas (ECSA) of (e) TiO2 and (f) TiO2-x; linear relationship between the capacitive current and scan rate; the insets show the CV curves of TiO2 and TiO2-x under different scan rates, respectively.
Figure 4. (a) UV-vis diffuse reflectance spectra; the inset shows the Tauc plots of TiO2 and TiO2-x; (b) LSV curves of TiO2 and TiO2-x photoanodes in the presence of sacrifice agents (0.5 M Na2SO3); (c) charge injection efficiencies; (d) LSV curves in dark of TiO2 and TiO2-x; relative electrochemical surface areas (ECSA) of (e) TiO2 and (f) TiO2-x; linear relationship between the capacitive current and scan rate; the insets show the CV curves of TiO2 and TiO2-x under different scan rates, respectively.
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Figure 5. (a) Bulk charge separation efficiency; (b) PL plots; (c) Nyquist plots measured at open circuit; (d) M-S plots under dark of TiO2 and TiO2-x.
Figure 5. (a) Bulk charge separation efficiency; (b) PL plots; (c) Nyquist plots measured at open circuit; (d) M-S plots under dark of TiO2 and TiO2-x.
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Figure 6. Schematic representation of PEC water oxidation mechanism on TiO2-x photoelectrode.
Figure 6. Schematic representation of PEC water oxidation mechanism on TiO2-x photoelectrode.
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Li, Y.; Liang, S.; Sun, H.; Hua, W.; Wang, J.-G. Defect Engineering and Surface Polarization of TiO2 Nanorod Arrays toward Efficient Photoelectrochemical Oxygen Evolution. Catalysts 2022, 12, 1021. https://doi.org/10.3390/catal12091021

AMA Style

Li Y, Liang S, Sun H, Hua W, Wang J-G. Defect Engineering and Surface Polarization of TiO2 Nanorod Arrays toward Efficient Photoelectrochemical Oxygen Evolution. Catalysts. 2022; 12(9):1021. https://doi.org/10.3390/catal12091021

Chicago/Turabian Style

Li, Yueying, Shiyu Liang, Huanhuan Sun, Wei Hua, and Jian-Gan Wang. 2022. "Defect Engineering and Surface Polarization of TiO2 Nanorod Arrays toward Efficient Photoelectrochemical Oxygen Evolution" Catalysts 12, no. 9: 1021. https://doi.org/10.3390/catal12091021

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