Hydrogenated TiO2 Nanorod Arrays Decorated with Carbon Quantum

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Hydrogenated TiO Nanorod Arrays Decorated with Carbon Quantum Dots toward Efficient Photoelectrochemical Water Splitting Zhao Liang, Huilin Hou, Zhi Fang, Fengmei Gao, Lin Wang, Ding Chen, and Weiyou Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 06 May 2019 Downloaded from http://pubs.acs.org on May 6, 2019

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ACS Applied Materials & Interfaces

Hydrogenated TiO2 Nanorod Arrays Decorated with Carbon Quantum Dots toward Efficient Photoelectrochemical Water Splitting

Zhao Liang1,2, , Huilin Hou2, , Zhi Fang2, Fengmei Gao2, Lin Wang2, Ding Chen1,3,*, and Weiyou Yang2,* 1

School of Materials Science and Engineering, Hunan University, Changsha City, 410082, P.R. China

2

3

Institute of Materials, Ningbo University of Technology, Ningbo City, 315211, P.R. China

State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, College of

Mechanical and Vehicle Engineering, Hunan University, Changsha City,410082, P.R. China

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ABSTRACT The limited light harvesting and charge collection are recognized as the grand challenge for the exploration of highly efficient TiO2 photoanodes. To overcome these intrinsic shortcomings, we reported the designed photoanode based on TiO2 nanoarrays with both hydrogenation treatment and surface decoration of carbon quantum dots (CQDs) toward efficient photoelectrochemical (PEC) water splitting. The results revealed that hydrogenation treatment could cause the formation of oxygen vacancies to suppress the recombination of photo-induced carriers. Meanwhile, the decorated CQDs could not only be played as the electron reservoirs to trap photo-induced electrons, but remarkably enhance the solar light harvesting due to their up-conversion effect. The as-fabricated photoanodes exhibited a large photocurrent density of ~3.0 mA /cm2 at 1.23 V vs. reversible hydrogen electrode (RHE) under simulated sunlight, which was the highest one among hydrogenated TiO2 photoanodes ever reported, and was ~6 times to that of pristine analogues.

KEYWORDS: TiO2, nanoarrays, carbon quantum dots, hydrogen plasma, photoelectrochemical activities.


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1. Introduction Photoelectrochemical (PEC) water splitting for hydrogen production has been recognized as one of the most prospective strategies for solar energy conversion to clean and renewable chemical fuel. Inspired by the first report of photocatalytic water splitting on a TiO2 electrode in 1972,1 variety of semiconductor materials have been developed as photoelectrode for PEC water splitting, such as ZnO,2-4 Fe2O3,5-7 WO3,8-10 BiVO4,11,12 SrTiO3,13-15 Ta3N16,17 and C3N4.18,19 Among the semiconductor families, TiO2 is recognized as one of the most representative candidates for exploring PEC devices, owing to its favorable physical characteristics, such as appropriate band-edge positions, high photocorrosion resistance, natural abundance, nontoxicity and low cost.20-22 However, the solar-to-hydrogen conversion efficiency of TiO2 is substantially limited by its relatively large band gaps (e.g., ~3.0 and ~3.2 eV for rutile and anatase phases, respectively)23,24 and fast electron-hole recombination. Thus, many efforts have been carried out to address these problems,25,26 including tailoring structures,21 incorporating dopants,27 building heterojunctions15,28,29 and introducing defects,30 and so forth. Recently, hydrogenation treatment of TiO2 (H/TiO2) has triggered intense interest.31,32 Typically, based on such technique, a large amount of oxygen vacancies (Ti3+) could be introduced with more localized states created in band gap, making a remarkably enhanced optical absorption in visible and near-infrared region. Particularly, one-dimensional (1D) H/TiO2 nanoarrays could deliver the desired large surface area and short transmission distance for photogenerated carriers. For instance, Wang et al. reported TiO2 nanorods annealed at 350 ℃ in hydrogen with an improved photocurrent density of ∼2.5 mA/cm2 at 1.23 V vs. reversible hydrogen electrode (RHE).30 Liu et al. reported TiO2 nanotube arrays treated in H2 atmosphere with a high open-circuit photocatalytic hydrogen production rate.33 Herein, we report a approach for improving the PEC performance of TiO2 photoanodes, ACS Paragon Plus Environment


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contributed from the synergetic effect of surface decoration of carbon quantum dots (CQDs) on hydrogenation treated TiO2 nanoarrays. It is found that the oxygen vacancies and Ti3+ caused by hydric-plasma could suppress the recombination of photo-generated carriers, and the introduced CQDs could be served as not only the electron reservoirs to trap photo-generated electrons with significantly reduced electron-hole recombination, but also the photosensitizer to enhance the solar light harvesting, thus leading to a significantly enhanced PEC performance.34-36 A high photocurrent density of 3.0 mA/cm2 at 1.23 V vs.RHE was achieved, which is the state-of-the-art one among the photoanodes based on hydrogen-treated TiO2 1D nanostructures.

2. Experimental methods 2.1 Materials. Titanium butoxide (TBOT), hydrochloric acid (HCl, 36~38 %), ascorbic acid (C6H8O6), ethanol (CH3CH2OH), acetone (CH3COCH3) and methylene chloride (CH2Cl2) were purchased from Adamas-beta (Shanghai, China). Purified water was attained from a water purification system (FCT1002-UF-P).

2.2 Preparation of photoanodes 2.2.1 Preparation of TiO2 nanorod arrays. TiO2 nanorod arrays were synthesized via a hydrothermal method assisted by fluorine-doped tin oxide (FTO) glass substrate. In a typical experiment, 1.2 mL TBOT was added into the mixture solution containing 30 mL HCl and 30 mL purified water under electromagnetic stirring. After that, the sealed autoclave with the located FTO substrate was heated up to 150 ℃ in a drying oven and maintained there for 12 h, followed by cooling in air condition. Subsequently, the resultant sample was subjected to be washed with deionized water and air drying, followed by air annealing at 500 ℃ for 2 h. ACS Paragon Plus Environment

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2.2.2 Preparation of H/TiO2 photoanode. The preparation of H/TiO2 photoanode was carried out in a PE ALD-100A system (Kemicro Co. Ltd) with a 200 W power supply. The samples were exposed to 10 Torr of H2 at 400 ℃ with different times to accomplish the fabrication of hydrogenated TiO2 nanoarrays, which were referred to sample H/TiO2. 2.2.3 Preparation of carbon quantum dots (CQDs). The CQDs were fabricated according to a reported method.34 In a typical experiment, a certain amount of ascorbic acid was dissolved in a mixture of 10~30 ml H2O and 10~30 ml CH3CH2OH, followed by ultrasonic treatment for a period of time. Afterwards, the resultant transparent solution was then transferred to a 100 mL autoclave, followed by hydrothermal reaction performed at 433 K for 2~6 h. Finally, the product was extracted with methylene chloride, and then dialysed using a semipermeable membrane (MWCO 1000) for obtaining pure CQDs. 2.2.4 Preparation of CQDs/TiO2 photoanode. To realize the surface decoration of CQDs, the as-grown TiO2 nanoarrays were immersed in CQDs solution, and transferred into an oven at 80 °C for 2h. Finally, the resultant samples were washed with purified water, followed by drying in a drying box. The as-prepared sample was referred to CQDs/TiO2. Similar process was used for surface decoration of CQDs on hydrogenated TiO2 nanoarrays (H/TiO2), and the as-fabricated sample was referred to CQDs-H/TiO2.

2.3 Characterization The morphology and microstructures of the samples were characterized by scanning electron microscope (SEM, S-4800, Hitachi) and X-ray diffraction (XRD, D8 Advance, Bruker). Transmission electron microscopy (TEM) analysis was carried out on JEOL TEM-2100F machine with a voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) was performed on a ESCALab250 system. EPR spectra were tested by a Bruker spectrometer (A300-10-12) operated at magnetic field modulation of ACS Paragon Plus Environment


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100 kHz. The optical absorption was collected by a UV-vis spectrophotometer (UV-3900, Hitachi) equipped with an integrating sphere using BaSO4 as the reflectance standard. The photoluminescence (PL) spectra were investigated on a fluorescence spectrophotometer (PL, Fluoro Max-4, Japan) at ambient temperature.

2.4 PEC Measurements The electrochemical analyses were performed by a conventional three-electrode system, under the irradiation of a 300 W xenon lamp (FX300, Perfect Light, China) equipped with an AM 1.5G filter. Its intensity was adjusted to 100 mW/cm2 by a spectrometer. The as-fabricated photoanodes, Ag/AgCl electrode (3.5 M KCl) and Pt plate were used as the working electrode, reference and counter electrode, respectively. 1M KOH aqueous solution (pH 13.6) was used as the electrolytes, which was purged with Ar for 30 min to remove the oxygen before the measurements. All the measured potentials were converted to RHE (ERHE = EAg/AgCl + 0.197+0.0591 × pH, EAg/AgCl = 0.1976V vs. NHE at 25 ℃). The electrochemical measurements of the photoanodes were performed using PGSTAT 302N. The incident-photon-to-current-conversion efficiency (IPCE) was measured under different monochromic lights. The electrochemical impedance was tested at the frequencies from 10000 to 0.01 Hz under a simulated solar light.

3. Results and Discussion Figure 1a provides a schematic illustration on the fabrication of CQDs-H/TiO2 nanorod arrays, which includes three typical steps, such as hydrothermal reaction with subsequent air annealing (Step I, and the obtained sample is referred to TiO2), H2 plasma treatment (Step II, and the obtained sample is referred to H/TiO2) and surface decoration of CQDs (Step III, and the obtained sample is referred to ACS Paragon Plus Environment

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CQDs-H/TiO2). Figure 1b1-b3 display the typical SEM images of the resultant sample after Step I. It seems that dense and prism-shaped nanorod arrays have perpendicularly grown on FTO substrate (the inset in Figure 1b1). Their average diameter and length are sized in ~150 nm and ~2 μm, respectively. After H2 plasma treatment (Step II), no obvious morphology and size changes are observed (Figure 1c1-c3). After that, surface decoration of CQDs (the morphologies, size distribution and optical properties of the as-synthesized CCDs are shown in Figure S1 in Supporting Information) based on H/TiO2 nanorod array (Step III) was performed. It discloses that the as-fabricated CQDs-H/TiO2 nanorod array (Figure 1d1-d3) also preserves the similar feature and size to those of pristine TiO2 sample. Figure S2 and Table S1 (Supporting Information) give their typical EDX analysis results, clarifying that they contained 0.65 wt% of C. These results confirmed that that the CQDs have been decorated onto the surface of H/TiO2 nanoarrays. Figure 2 presents the representative TEM and HRTEM images of pristine TiO2 (Figure 2a1-a3), H/TiO2 (Figure 2b1-b3) and CQDs-H/TiO2 (Figure 2c1-c3) nanoarrays. Figure 2a2 shows a HRTEM image of pristine TiO2 nanorod products, suggesting its high crystallization. The distances of two neighbored lattice fringes are of 0.32 and 0.29 nm, corresponding to the d-spacings of (110) and (001) planes of rutile TiO2, respectively. Figure 2b2-b3 reveal that an amorphous layer covers around the surface of H/TiO2 nanoarrays, as compared to that of pristine TiO2 counterparts. It was proposed that such an amorphous layer could be acted as trapping sites for photo-generated carriers and prevent them from rapid recombination, thus promoting electron transfer with enhanced photocatalytic activity.37 When the CQDs are further assembled onto the surface of H/TiO2 nanorods (Figure 2c2-c3), the nanoparticles with a typical lattice spacing of 0.321 nm can be commonly observed, which is identified as (004) crystal plane of graphite. In terms of the ultrasonic treatment for the preparation of TEM samples, it suggests that the CQDs contact solidly to the TiO2 nanorods. ACS Paragon Plus Environment


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To investigate the changes of microstructure and phase after the hydrogen plasma treatment and CQDs decoration, XRD patterns are collected from pristine TiO2, H/TiO2 and CQDs-H/TiO2 nanoarrays, as shown in Figure S3 in Supporting Information. The characteristic peaks of 2θ = 36.4° and 63.2° can be observed in the XRD patterns of all the samples. The two obvious diffraction peaks match well to the standard ones of rutile phase (JCPDS 88-1175), which respond to the diffractions from (101) and (002) crystal planes, respectively. The peak intensity at 36.4° is stronger than that at 63.2°, indicating the preferred orientation of rutile TiO2 nanorods along [001] direction. In the cases of H/TiO2 and CQDs-H/TiO2 nanoarrays, two diffraction peaks at 70.2° and 70.5° are significantly enhanced, as compared to those of pristine TiO2, implying that (301) and (112) crystal planes are exposed after hydrogen plasma treatment. Additionally, no obvious diffractions have been detected from CQDs, mainly ascribed to the tiny amount of introduced CQDs. Figure 3a presents the survey XPS spectra of as-grown TiO2, H/TiO2 and CQDs-H/TiO2 nanoarrays. The results show the existence of Ti, O and C elements in three samples. Figure 3b displays the high resolution XPS spectrum of Ti 2p, in which two peaks respond to Ti 2p1/2 and Ti 2p3/2, respectively. For pristine TiO2, both Ti 2p3/2 and Ti 2p1/2 peaks at ~459.1 and 464.8 eV are ascribed to the Ti4+ of TiO2. In the cases of two samples after hydrogen plasma treatment, namely H/TiO2 and CQDs-H/TiO2 analogues, such two characteristic peaks move negatively, suggesting that some Ti3+ has been formed on the surface. It implies the existence of oxygen vacancies (VO) after hydrogen plasma treatment. After decoration of CQDs, the binding energies for Ti 2p3/2 and Ti 2p1/2 are centered at 458.7 and 464.6 eV, with a tiny positive shift from H/TiO2 (458.5 and 564.2 eV) to CQDs-H/TiO2. This may be due to the decoration of CQDs, resulting in the reduction of Ti3+ in CQDs-H/TiO2. For XPS O1s spectra of pristine TiO2 nanoarrays, the main peak located at 530.0 eV clarifies the presence of Ti-O-Ti. After hydrogen plasma treatment, the fitted curves at 531.3±0.1 eV can be detected in H/TiO2 and ACS Paragon Plus Environment

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CQDs-H/TiO2 nanoarrays, which is assigned to the binding energy of Ti-OH.38 The deconvoluted C1s XPS spectrum of CQDs-H/TiO2 nanoarrays is presented in Figure 3d. C 1s peaks are resolved into three peaks at 284.8, 286.3 and 289.2 eV, which are ascribed to the C-C, C-O and O=C-O groups, respectively, indicating the presence of CQDs.34,39,40 EPR measurement is further used to show the formations of VO and Ti3+ after hydrogen plasma treatment. As shown in Figure 3e, there is nearly no EPR activity in pristine TiO2 nanoarrays. In contrast, after hydrogenation treatment, the H/TiO2 counterpart exhibits clears EPR signals at g=1.997, which is attributed to the presence of VO and surface Ti3+ within the surface of the sample.31,41,42 These experiments confirm the formations of VO and Ti3+, which are induced by the hydrogen plasma treatment. Figure S4 (Supporting Information) provides the UV-vis diffuse reflectance spectra of pristine TiO2, CQDs/TiO2, H/TiO2 and CQDs-H/TiO2 nanoarrays. The absorption band edge of pristine TiO2 nanoarrays (Figure S4a) is 400 nm, which is corresponding to the optical absorption band edge of rutile TiO2. The absorption of pristine TiO2 photoanode in visible region (the raising tails in Figure S4a) can be attributed to the scattered light, owing to pores or cracks in the nanorod arrays.43,44 Figure S5 (Supporting Information) shows the UV-Vis spectra of pristine TiO2 and H/TiO2 nanoarrays with different times of plasma treatment of 15, 30 and 60 mins. It discloses that all the samples have fairly strong optical absorption in UV region (< 400 nm), and the absorption decreases rapidly at ~410 nm, in accordance with the band gap of 3.0 eV of rutile TiO2. After hydrogen plasma treatment, the absorption increases in both visible (VIS) and UV regions, but the band gap of H/TiO2 has been hardly changed, which are accordance with the recent reports.45,46 In contrast, CQDs/TiO2 and CQDs-H/TiO2 nanoarrays show strong absorptions in the wavelengths ranged from 420 to 800 nm (Line c and d in Figure S4 in Supporting Information), which is due to the intrinsic absorption of CQDs. To optimize the hydrogen plasma treatment for CQDs-H/TiO2 photoanodes, the experiments with ACS Paragon Plus Environment


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different hydrogenation times (i.e., 0, 15, 30 and 60 min) were carried out (the corresponding XRD patterns are shown in Figure S6, Supporting Information). As shown in Figure S7 (Supporting Information), their dark currents are negligible from 0 to 1.6 V (vs. RHE) with respect to their respective photocurrent, indicating nearly no PEC water splitting occurred. It is noteworthy that the photocurrents of three H/TiO2 samples are all higher than that of pristine TiO2 under illumination, evidencing that the hydrogenation treatment boosts the PEC activity of TiO2. Especially, the TiO2 photoanode with 30 min hydrogenation exhibits the highest photocurrent density among these four photoanodes, which reaches 2 mA/cm2 at 1.23 V (vs. RHE), and is 400 % of pristine TiO2 (i.e., 0.5 mA/cm2 at 1.23 V vs. RHE). This means that, in current case, the hydrogenation treatment should be fixed at ~30 min. Figure 4a and Figure S8 (Supporting Information) present the PEC performance of CQDs-H/TiO2 photoanodes. Upon irradiation, it is worth noting that the photocurrent density of CQDs-H/TiO2 nanoarrays is measured to be of 3.0 mA/cm2 at 1.23 V vs. RHE, which is ~150, 210 and 600 % of H/TiO2, CQDs/TiO2 and pristine TiO2 counterparts, respectively. In addition, the open-circuit voltages of CQDs-H/TiO2, CQDs/TiO2 and H/TiO2 photoanodes are smaller than that of pristine TiO2, implying that the electrons are more easily transferred. Both the higher photocurrent density and lower onset potential represent more efficient charge separation of the photoanode, demonstrating that the hydrogenation treatment and CQDs decoration synergistically suppress the recombination of photogenerated electrons and holes, thus leading to a significant improvement in PEC performance. To explore the photoresponses of the photoanodes, the experiments are carried out under illumination with 50 s light on/off cycles at 1.23 V vs. RHE, and their transient photocurrent responses vs. time are shown in Figure 4b. The photocurrents are almost zero without illumination, and rapidly rise to steady states upon illumination for four photoanodes. The observed steady-state photocurrent of CQDs-H/TiO2 is higher than those of CQDs/TiO2, H/TiO2 and pristine TiO2 samples, which is consistent with the linear sweep ACS Paragon Plus Environment

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voltammograms as shown in Figure 4a. Furthermore, it is reproducible for several on/off cycles with almost identical photocurrent and dark current, indicating their satisfied stability. To further study this point, the photoanodes are tested by chronoamperometry at 1.23 V vs. RHE under continuous illumination in 1 M KOH electrolyte. As seen in Figure 4c, the ratio of photocurrent decay is ~27.7 % for the pristine TiO2 photoanode over 2 h irradiation, and those of H/TiO2 and CQDs/TiO2 photoanodes are dropped by 7.16 and 10.2 %, respectively. By contrast, that of CQDs-H/TiO2 sample is slightly dropped by 5.98 %. This evidences that the PEC stability of pristine TiO2 photoanode has been significantly enhanced, indicating that the synergetic effect of hydrogenation treatment and surface decoration of CQDs is effective to hinder the photocorrosion. More interestingly, the photocurrent density of the present CQDs-H/TiO2 photoanode is superior to those of most photoanodes based on one-dimensional (1D) TiO2 ever reported, as shown in Figure 4d, suggesting their excellent and stable PEC performance. The calculation on IPCE is conducted at an applied bias of 1.23 V vs. RHE, to explore the correlation between light absorption and photocurrent by using below equation:53-55 IPCE (%)  1240 

Iph

Pin

 100% …………………………………..(1)

where Iph is the measured photocurrent,  is the wavelength of input light, and Pin is the light intensity at a specific wavelength. As shown in Figure 5a, the CQDs-H/TiO2 sample exhibits the highest IPCE of ~66.8 % at 300 nm, which is ~6.8, ~1.7 and ~1.3 times to those of pristine TiO2 (~9.78 % at 360 nm), CQDs/TiO2 (~38.2 % at 300 nm) and H/TiO2 (~53.5 % at 300 nm) samples. In addition, it is observed that the photocurrent responses of pristine TiO2 and H/TiO2 samples are mainly located in the wavelength region of 300-400 nm, and decline to almost 0 above 400 nm. However, for CQDs-H/TiO2 counterpart, the photocurrent response is extended to the visible light range from 400 to 450 nm. These experimental results ACS Paragon Plus Environment


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reveal that the hydrogen treatment mainly enhances the PEC activity in UV region, while the decorated CQDs improve the response both in UV and VIS regions, which is in agreement with the extended optical absorption measurements (Figure S4 in Supporting Information). The conversion efficiencies of PEC water splitting based on oxygen evolution at the photoanode can be evaluated through following formula,56,57

η=I(1.23-Vapp)/Plight …………………………………………………..(2) where I is the photocurrent density at the measured bias, Vapp is the applied external potential vs. RHE, and Plight is the power density of illumination (AM 1.5G, 100mW/cm2). As shown in Figure 5b, it is observed that the pristine TiO2 photoanode reaches the maximum conversion efficiency of 0.14 % at 0.75 V vs. RHE, while the H/TiO2 and CQDs/TiO2 electrodes have a larger value of 0.79 % and 0.62 % at a smaller bias voltage of 0.66 and 0.58 V vs. RHE, respectively. Significantly, the CQDs-H/TiO2 photoanode holds the highest efficiency of 1.29% at 0.615 V vs. RHE, which is 8 times higher than that of pristine TiO2, implying an efficient separation of photogenerated electrons-hole pairs. The charge-transfer kinetics of the electrodes are further investigated by electrochemical impedance spectroscopy (EIS) responses at 1.23 V vs. RHE under AM 1.5 G illuminations. The recorded EIS Nyquist plots of pristine TiO2, H/TiO2, CQDs/TiO2 and CQDs-H/TiO2 electrodes are shown in Figure 5c. It is found that the CQDs-H/TiO2 sample has the smallest semicircle than the other samples, clarifying its faster interfacial charge transfer to the electron donor/acceptor.10,58 Furthermore, the EIS spectra could be fitted to a 1-RC circuit model (the inset in Figure 5c), including the external circuit resistance (Rs), surface states charge trapping resistance (Rtrap), double-layer capacitance (CPEbulk), charge-compensated capacitance (CPEss) and charge transfer resistance (Rct).58 The fitted Rs, Rtrap and Rct are summarized in Table 1. It represents that the CQDs-H/TiO2 electrode possess the smallest impedance than the other samples, confirming that it offers the fastest charge transport and longest ACS Paragon Plus Environment

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service life of charge carriers for the photoanode. Based on the above analyses, a schematic diagram of CQDs-H/TiO2 photoanode applied in PEC water splitting is showed in Figure 5d and 5e, respectively. Firstly, after hydrogenation of rutile TiO2, a portion of Ti4+ would be reduced to Ti3+ (with oxygen vacancies), which is confirmed by the XPS and EPR analyses as discussed above. In this view of point, band bending occurs in the oxygen-deficient TiO2 (Figure 5d). Upon light irradiation, the band bending would draw more photo-generated electrons toward the core to fill the shallow trap states. It means that the oxygen vacancies could be served as shallow donors, and thereby increase the donor densities of TiO2. In addition, the Ti3+ can also act as the hole traps to suppress the recombination of photo-generated carriers, consequently prolong the lifetime of electron-hole pairs, inducing a significantly improved PEC performance.59 However, the amount of the introduced Ti3+ or VO should be controlled in a certain scale, since excessive VO and Ti3+ would be, in turn, acted as the recombination centers, causing a weakened PEC activity.60 Secondly, the decorated CQDs on the surface of H/TiO2 nanorod could be played as the electron reservoirs to trap photo-generated electrons from the conduction band of TiO2 under UV light irradiation.61,62 Furthermore, the CQDs could enhance the absorption of TiO2 in visible light range, owing to their up-conversion effect caused by two-photon absorption (as shown in the UV-vis diffuse reflectance spectra).63,64 Typically, the electrons in the π orbital of CQDs could be excited and transited to the high-energy orbital (the π* orbital) under low photon energy excitation. Subsequently, the excited electrons would drop back to the lower-energy orbital (σ orbital) due to the radiative relaxation,34 leading to the emission of short wavelength light.34 It is well known that the CQDs can absorb visible light (400~760 nm) and near-infrared (NIR) light (>760 nm) (Figure 5e), thus exciting the H/TiO2 electrode to form electron/hole pairs. Consequently, the solar energy could be harvested efficiently, bringing a fundamentally enhanced light absorption and photocatalytic activity for the PEC cells. ACS Paragon Plus Environment


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Briefly, current work presents that the synergetic effect from both hydrogenation treatment and surface decoration of CQDs significantly boost the PEC performance of conventional TiO2 electrodes.

4. Conclusions In conclusion, we have reported the exploration of photoanode based on TiO2 nanoarrays with hydrogenation treatment and surface decoration of carbon quantum dots (CQDs). The hydrogenation treatment causes the formation of oxygen vacancies and Ti3+ within TiO2 photoanodes to suppress the recombination of photo-generated carriers, and the decorated CQDs remarkably enhance the solar light harvesting. The as-designed photoanodes exhibit an IPCE value of ~66.8 %, which is 6 times higher than that of pristine TiO2. Their photocurrent densities reach ~3.0 mA/cm2 at 1.23 V vs. RHE under simulated sunlight, which is the state-of-the-art one ever reported, and is ~6 times to that of pristine TiO2. This work might provide some insight on the research of advanced photoanodes with excellent PEC activities.

Supporting Information Microstructural characterizations, UV-Vis spectra and PEC performances of CQDs, pristine TiO2, CQDs/TiO2, H/TiO2 and CQDs-H/TiO2 nanoarray samples.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (D. Chen) * E-mail: [email protected] (W. Yang)  These

authors contributed equally to this work.

Notes The authors declare no competing financial interest. ACS Paragon Plus Environment

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Acknowledgements This work was sponsored by National Natural Science Foundation of China (Grant Nos. 51572133, 51602163, 51672137and 51702175), Foundation of Educational Commission in Zhejiang Province of China (Grant No. Y201533586), Natural Science Foundation of Ningbo Municipal Government (Grant Nos. 2017A610005 and 2017A610002), and Ningbo Leading and Top-notch Talent Training Project (Grant No. NBLJ201801007).

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Figure 1. (a) Schematic illustration on the fabrication of CQDs-H/TiO2 nanoarrays. (b1-d3) Typical SEM images of pristine TiO2 (b1~b3), H/TiO2 (c1~c3) and CQDs-H/TiO2 (d1~d3). The insets show the corresponding cross-sectional SEM images.



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Figure 2. Representative TEM and HRTEM images of pristine TiO2 (a1~a3), H/TiO2 (b1~b3) and CQDs-H/TiO2 single nanorod (c1~c3).


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Figure 3. (a) The full-scale XPS spectra of TiO2, H/TiO2 and CQDs-H/TiO2 nanoarrays. (b) Ti 2p XPS spectra of pristine TiO2, H/TiO2 and CQDs-H/TiO2 nanoarrays. (c) Normalized O 1s XPS spectra of TiO2, H/TiO2 and CQDs-H/TiO2 nanoarrays. (d) Normalized C 1s XPS spectra of CQDs-H/TiO2 nanoarray. (e) Typical EPR spectra of H/TiO2 nanoarray.



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Figure 4. (a) The J–V curves of pristine TiO2, CQDs/TiO2, H/TiO2 and CQDs-H/TiO2 collected at a scan rate of 10 mV/s and applied potentials from 0 to 1.6 V vs. RHE in 1 M aqueous KOH solution. (b) The chopped transient photocurrent density vs. time under 1.23 V vs. RHE. (c) The photocurrent-time profiles of pristine TiO2, CQDs/TiO2, H/TiO2 and CQDs-H/TiO2 photoanodes, measured at a potential of 1.23 V vs. RHE. (d) Brief summary of recent reports on hydrogenated TiO2 photoanodes (Since 2009).


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Figure 5. (a) IPCE spectra of pristine TiO2, CQDs/TiO2, H/TiO2 and CQDs-H/TiO2, measured at a potential of 1.23 V vs. RHE. The inset is the magnified IPCE spectra at incident wavelength ranged from 420 to 600 nm. (b) Calculated photoconversion efficiencies of pristine TiO2, CQDs/TiO2, H/TiO2 and CQDs-H/TiO2 photoanodes. (c) Nyquist plots of pristine TiO2, CQDs/TiO2, H/TiO2 and CQDs-H/TiO2 photoanodes at an open circuit potential under simulated AM 1.5 G illumination. (d-e) The schematic models for enhanced PEC behaviors within H/TiO2 and CQDs-H/TiO2 photoanodes, respectively.



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Table 1. Fitted data of EIS in pristine TiO2, CQDs/TiO2, H/TiO2 and CQDs-H/TiO2 phtoanodes Sample

TiO2

CQDs/TiO2

H/TiO2

CQDs-H/TiO2

RS(Ω)

49.64

24.52

78.65

20.47

Rtrap(Ω)

152.7

107.2

95.35

80.9

Rct(Ω)

3604

2396

1247

927.6


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Graphical Table of Contents

We report a strategy by combining hydrogen plasma treatment and surface decoration of carbon quantum dots (CQDs), for boosting the PEC activities of TiO2 nanorod arrays with high photocurrent density and stability.


Hydrogenated TiO2 Nanorod Arrays Decorated with Carbon Quantum

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