NiO Quantum Dot Modified TiO2 toward Robust Hydrogen Production

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Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 889−896

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NiO Quantum Dot Modified TiO2 toward Robust Hydrogen Production Performance Weizhao Hong, Yansong Zhou, Chade Lv, Zhonghui Han, and Gang Chen* MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, 150001 Harbin, P.R. China S Supporting Information *

ABSTRACT: Quantum dots (QDs) have great potential to build heterojunctions for efficient photocatalytic hydrogen production because of the quantum size effects, whereas they are limited by the rigorous preparation. Herein, we report a facile sacrificial coating strategy toward the construction of the NiO QDs/TiO 2 heterojunction. Shorter transmission distance of carriers and higher crystallinity between NiO QDs and TiO2 substrate are achieved to minimize the recombination of photoinduced electrons and holes. As a result, excellent performance of 1.35 mmol h−1 g−1 over the NiO QDs/TiO2 sample is achieved, which is 37 times higher than that of NiO/TiO2 and 56 times higher than that of pure TiO2. Furthermore, this is one of the best performances reported in heterojunctions made of NiO and TiO2. KEYWORDS: TiO2, Quantum dots, Heterojunction, Sacrificial coatings, Photocatalysis

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INTRODUCTION Splitting water directly into hydrogen by solar energy has been regarded as one of the most promising ways to solve the energy crisis since the first work reported by Fujishima and Honda in 1972.1 The significant challenge leading to extensive use of the hydrogen energy system is to produce H2 with a low-cost, stable, and highly efficient photocatalyst.2−6 However, the current efficiency for photocatalytic water splitting is limited by the highly preferred recombination of photogenerated carriers in photocatalysts. Currently, constructing semiconductor heterojunctions has been demonstrated to be one of the most important approaches to promoting the hydrogen production efficiency because the the heterojunction can significantly improve the separation efficiency of photogenerated carriers.7−18 Quantum dots (QDs) which possess more negative conduction band edges and more positive valence band edges than the bulk materials because of the quantum size effects have been regarded as one of the most promising structures to enhance the separation efficiency of photogenerated carriers.8,19−23 The small size of QDs leads to larger specific surface area with more active sites for water splitting and shortens the path length of charge transmission to the surface of QDs.24−27 Generally, various wet synthesis methods have been utilized to obtain QDs for the construction of heterojunctions, such as hydrothermal/solvothermal methods, stabilizer-assisted deposition methods, and solution growth techniques, etc.8,28−30 Almost all of these wet chemistry methods are performed at low temperature, leading to low crystallinity with more centers for carrier recombination.31−33 © 2017 American Chemical Society

Photocatalysts with high crystallinity have fewer centers for carriers recombination, which can suppress the carrier recombination. With the purpose of realizing QD decorated heterojunctions with high crystallinity, heat treatment at high temperature is necessary. Nevertheless, aggregation of QDs will emerge accompanied by treatment at high temperature. Thus, a feasible strategy is urgently desired to suppress the aggregation of QDs at high temperature. As reported, small size metal nanoparticles stabilized at high temperature have been achieved via the space confinement effect of carbon materials.34 Inspired by this work, we conjecture QDs with high crystallinity can be fabricated by introducing carbon materials as a coating to prevent aggregation. However, to the best of our knowledge, the achievement of QDs for constructing a heterojunction by introducing carbon materials as a sacrificial coating lacks exploration. As a cocatalyst, NiO can dramatically improve the photocatalytic hydrogen production performance of various photocatalysts, such as SrTiO3, TiO2, Nb2O5, Ga2O3, and so on, in water splitting.35−41 However, rarely have photocatalysts combined with NiO QDs been reported owing to their rigorous preparation condition. Herein, highly crystalline NiO QD modified TiO2 was synthesized via a sacrificial coating strategy at high temperature. The in situ introduced graphite Ndoped carbon derived from urea furnished a space confinement effect for realizing NiO QDs with high crystallinity. The asReceived: September 13, 2017 Revised: November 9, 2017 Published: November 24, 2017 889

DOI: 10.1021/acssuschemeng.7b03250 ACS Sustainable Chem. Eng. 2018, 6, 889−896

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ACS Sustainable Chemistry & Engineering

Figure 1. Schematic illustration of the fabrication process of the NiO QDs/TiO2.

Figure 2. Composition and morphology characterization of NiO QDs/TiO2. (a) XRD pattern, (b) TEM image of NiO QDs/TiO2, (c,d) HRTEM image of NiO QDs/TiO2, (e,f) high-angle annular dark-field images of NiO QDs/TiO2 and elemental mapping results of (g) Ti, (h) O, and (i) Ni.

photocatalysts with high crystallinity to achieve highly efficient photocatalytic water splitting performance.

synthesized NiO QDs/TiO2 showed an excellent performance at 1.35 mmol h−1 g−1, which was 37 times higher than that of NiO/TiO2 and 56 times higher than that of pure TiO2, even comparable with that of Pt-loaded TiO2 photocatalyst, for hydrogen production. Significantly, it is one of the highest hydrogen production performances among the reported heterojunctions consisting of TiO2 and NiO (Table S1). This work provides a new insight for crafting QD modified

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EXPERIMENTAL SECTION

Catalyst Synthesis. Nickel(II) nitrate hexahydrate (AR), TiO2 (AR), urea (AR), methanol (AR), potassium hexachloroplatinate (IV) (AR), Na2SO4 (AR), Nb2O5 (SP), and Ta2O5 (SP) were purchased from Aladdin. All of the reagents were used as received. The NiO QDs/TiO2 heterojunction was synthesized by a temperature-programmed method. At first, commercial TiO2 (0.25 890

DOI: 10.1021/acssuschemeng.7b03250 ACS Sustainable Chem. Eng. 2018, 6, 889−896

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ACS Sustainable Chemistry & Engineering

Figure 3. High-resolution XPS spectra of TiO2, NiO QDs/TiO2@NC, and NiO QDs/TiO2. (a) C 1s, (b) Ti 2p, (c) Ni 2p, and (d) N 1s regions. g), urea (2 g), and different weight percent of Ni(NO3)2·6H2O (Ni/ TiO2 = 0, 1.5, 2, 2.5, 3, 3.5%) were mixed. After being ground for 30 min, the uniform compounds were heated under static N2 atmosphere at 773 K for 2 h with a heating rate of 5 K/min, followed by annealing at 1073 K for 2 h with the same heating rate as above to fabricate the N-doped carbon coating. To remove the sacrificial carbon coatings, the temperature was maintained at 1073 K for 2 h in air. TiO2 in this work was only treated by a temperature-programmed process as NiO QDs/TiO2. NiO/TiO2 was prepared by the same method as NiO QDs/TiO2, except that urea was absent. For Pt/TiO2, TiO2 addition to K2[PtCl6]·6H2O (2.5 wt % calculated with Pt) during the photocatalytic performance test was denoted as Pt/TiO2. Nb2O5, NiO QDs/Nb2O5, Pt/Nb2O5, Ta2O5, NiO QDs/Ta2O5, and Pt/Ta2O5 were prepared with Nb2O5 and Ta2O5 instead of TiO2. Working Electrode Preparation. In brief, 10 mg of catalyst powder was dispersed in 1 mL of terpineol and then ultrasonically vibrated for about 30 min to generate a homogeneous ink. Next, 0.1 mL of dispersion was transferred onto FTO conducting glass (2.25 cm2), leading to a catalyst loading of ∼1 mg cm−2. Finally, the asprepared catalyst film was dried at 80 °C in vacuum for 12 h. Material Characterization. The structures of the obtained samples were confirmed by X-ray diffraction (XRD) on a Rigaku D/ max-2000 diffractometer with Cu Kα radiation (λ = 0.15406 nm). Diffraction patterns were collected from 10 to 90° at a speed of 4° min−1 with a scan width of 0.02°. The morphology of the samples was observed with a HELIOS NanoLab 600i field emission SEM. The operating voltage was set to 20 kV, and the samples were prepared with the pre-ultrasonic-dispersed (10 min) ethanol turbid liquid onto the chip of silicon. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were carried out on FEI Tecnai G2 S-Twin operating at 300 kV. UV−vis diffuse reflectance spectra were acquired with a spectrophotometer (HITACHI UH-4150). X-ray photoelectron spectroscopy (XPS) was accomplished using a Thermo Scientific ESCALAB 250Xi X-ray photoelectron spectrometer with a pass energy of 20.00 eV and an Al Kα excitation source (1486.6 eV). The photoluminescence (PL) spectra were obtained on HORIBA FluoroMax-4, and the excitation wavelength was determined as 240 nm. Electrochemical measurements were performed with a Autolab PGSTAT302N (ECO-Chemie) electrochemical workstation. All measurements were carried out in 0.5 M Na2SO4 solution and

conducted in a conventional three-electrode cell by using a Ag/AgCl (saturated KCl) electrode as the reference electrode, a Pt electrode as the counter electrode, and the sample on the FTO conducting glass as the working electrode. Photocatalytic Performance Test. The photocatalytic hydrogen evolution tests were carried out in a 500 mL enclosed photoreactor evacuated with N2, using a PLS-SXE300 300 W Xe lamp. Fifty milligrams of the photocatalyst was dispersed in 300 mL of 10 vol % methanol solution and ultrasonically vibrated for 20 min. The system was injected with the nitrogen for about 5 min to remove air. The H2 evolution quantity was analyzed with an Agilent 7890A GC. Magnetic stirring (450 rpm) was used during the water splitting experiments to ensure homogeneity of the suspension. To evaluate the H2 evolution stability, after each run of 4 h, the photocatalytic reaction system was evacuated with N2 again, and the next run was continued.

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RESULTS AND DISCUSSION As illustrated in Figure 1, a sacrificial coating strategy was utilized to synthesize the NiO QDs/TiO2 heterojunction. First, TiO2, Ni(NO3)2·6H2O, and urea were mechanically mixed by grinding (step I). Subsequently, the mixture was annealed in a tube furnace with programmed temperature method under N2 atmosphere (step II). While the Ni(NO3)2 was decomposed into NiO QDs during the heat treatment at high temperature, the urea was converted into graphite N-doped carbon, which could prevent the NiO QDs from aggregating (Figure S1). After this step, NiO QD decorated TiO2 coated with N-doped carbon (NiO QDs/TiO2@NC) was obtained. Finally, the asprepared NiO QDs/TiO2@NC was further calcinated at 1073 K in air, and the graphite N-doped carbon was removed to achieve NiO QDs/TiO2 (step III). The composition of NiO QDs/TiO2 is confirmed by XRD (Figure 2a). It reveals that the sample mainly consists of anatase phases. Meanwhile, no impurity peaks or peak shift of NiO QDs/TiO2 arise, stating that C or N are not doped into TiO2. However, the peaks of NiO cannot be observed, which could be because NiO is ultrasmall.15 891

DOI: 10.1021/acssuschemeng.7b03250 ACS Sustainable Chem. Eng. 2018, 6, 889−896

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Figure 4. Raman spectra of pure TiO2, NiO QDs/TiO2@NC, and NiO QDs/TiO2. Raman shift range from (a) 200 to 1000 cm−1 and (b) 1000 to 2000 cm−1.

Figure 5. Photocurrent vs time of NiO QDs/TiO2@NC measurements under different conditions: (a) diffenrent bias under 15 W/cm2 irradiation intensity and 0.1 mol/L Na2SO4 solution, (c) diffenrent bias under 10 W/cm2 irradiation intensity and 0.1 mol/L Na2SO4 solution. (c) Schematic illustration of the hampering of ion transport by coating and (d) photocurrent vs time of NiO QDs/TiO2@NC with different concentrations of Na2SO4 solution under 15 W/cm2 irradiation intensity and 0.2 V bias.

disperses on the surface sparsely, indicating that NiO QDs are well-dispersed on the surface of TiO2. In particular, in the region where Ti (Figure 2g) vanishes, O (Figure 2h) and Ni (Figure 2i) are still abundant, further verifying that the QDs are NiO. To investigate the surface compositions and oxidation states of the elements, the XPS measurements are carried out for TiO2, NiO QDs/TiO2@NC, and NiO QDs/TiO2 (Figure 3). These samples are mainly composed of several principal elements, such as C, Ti, and O (Figure S4a). The C 1s peak has been calibrated with 284.6 eV (Figure 3a).42 In addition, the peak at 288.7 eV corresponding to the species of doped carbon in TiO2 is not observed, suggesting that carbon is not doped into TiO2. In Figure 3b, the Ti 2p region of NiO QDs/TiO2 has no shift compared to TiO2, excluding the anion doping.43 The peaks at 855.8 and 872.3 eV can be attributed to Ni 2p3/2

To determine the specific morphology of NiO, TEM is carried out for NiO QDs/TiO2. The sample mainly consists of nanoparticles with a diameter of 50−200 nm (Figure 2b), which is in agreement with scanning electron microscope (SEM) images (Figure S2). According to HRTEM image (Figure 2d), the lattice fringe spacing of the particle is 0.35 nm, indexing to the (101) plane of anatase. Interestingly, the particle is covered by numerous QDs with a diameter of approximately 2 nm (Figure 2c), which show clear lattice fringe spacing of 0.24 nm assigned to the (200) plane of NiO (Figure 2d), confirming the high crystallinity of NiO QDs. By contrast, NiO/TiO2 is constructed by unevenly blended NiO and TiO2 (Figure S3). The elemental mapping results of NiO QDs/TiO2 corresponding to the area marked in Figure 2e,f are shown in Figure 2g−i. According to the elemental distribution of Ti, O, and Ni, the bulk of this particle is anatase, and the Ni element 892

DOI: 10.1021/acssuschemeng.7b03250 ACS Sustainable Chem. Eng. 2018, 6, 889−896

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Figure 6. Separation efficiency of carriers measurement of TiO2, NiO/TiO2, and NiO QDs/TiO2. (a) Photocurrent vs time, (b) PL spectra.

of 0, 0.2, and 0.8 V vs Ag/AgCl via several on−off cycles of irradiation under periodical light on−off cycles (Figure 5a). Interestingly, NiO QDs/TiO2@NC shows high photocurrent at the beginning, followed by a dramatic decrease after several seconds in accordance with several previous studies.19,53 Interestingly, the decreased amplitude of photocurrents increases while the bias accumulates. Noting that the bias can improve the separation efficiency of carriers, we realize that the decreased amplitude of photocurrents could be due to carrier density. Thus, further measurements of NiO QDs/TiO2@NC are carried out under weaker irradiation intensity, in which situation the photocatalysts can generate fewer carriers. Consequently, in the case of weaker irradiation intensity (Figure 5b), the drop amplitudes of photocurrents are smaller than that under higher irradiation intensity and vary depending on bias as before. Considering that promoting the density of carriers can accelerate the electrode reaction, we make a reasonable deduction that coated N-doped carbon possesses weaker active sites than NiO QDs/TiO2 so that the coating would hamper the ion diffusion, resulting in an inverse electric field (Figure 5c). N2 adsorption−desorption measurements show that the Brunauer−Emmett−Teller surface area of NiO QDs/TiO2 is 16.5% more than that of NiO QDs/TiO2@NC (Table S2), which is in agreement with our assumption. The coating absorbs on the surface of active sites with weak van der Waals force so that a transitional region appears. While the reaction consumes the cations in the transitional region, in a general way, cations in solution will be transported to compensate for the decrease of cations. However, if the consumption of cations is compensated completely in a rapid reaction, the transitional region would have a low density of cations and a concentration electric field would be setup that might be the direct reason for the observed drop of photocurrents. To further verify our hypothesis, a further test to increase the ion diffusion has been carried out (Figure 5d). When promoting the ability of ion transport with a higher concentration of the Na2SO4 solution, the drop of photocurrent disappears as excepted. Furthermore, the hydrogen production performance of NiO QDs/TiO2 increases with the concentrations of Na 2 SO 4 in low concentrations (Figure S6), which agrees with our assumption. As mentioned above, carbon coating might hamper the ion diffusion to the active sites on NiO QDs, which is one of the determined steps as NiO QDs/TiO2 possesses stronger active sites than coated N-doped carbon. Thus, the removal of Ndoped carbon is imperative to eliminate the obstruction of ion diffusion.

and Ni 2p1/2, respectively, manifesting that Ni possesses a +2 oxidation state.44 Furthermore, the Ti 2p region of NiO QDs/ TiO2@NC shows a shift of 0.1 eV toward low energy in the position corresponding to Ti 2p3/2, as shown in Figure 3b, which could be due to the interaction between TiO2 and Ndoped carbon. For Ni 2p region, the peak intensity of NiO QDs/TiO2 increases in contrast with that of NiO QDs/TiO2@ NC (Figure 3c), affirming NiO QDs/TiO2@NC is covered by coating that decomposes after annealing in air. The small peak of NiO QDs/TiO2@NC at 398.6 eV can be attributed to N 1s, while it cannot be observed in the XPS results of NiO QDs/ TiO2, indicating that N-doped carbon is the above-mentioned coating (Figure 3d). Raman spectra of TiO2, NiO QDs/TiO2@NC, and NiO QDs/TiO2 are collected to investigate the change of N-doped carbon (Figure 4). All samples show the characteristic bands at 399 (B1g), 516 (A1g + B1g), and 639 cm−1 (Eg) for anatase, which is in agreement with XRD patterns (Figure S5).45 Furthermore, the characteristic bands of carbon materials appear at 1340 (D-band) and 1600 cm−1 (G-band) in the NiO QDs/TiO2@NC, whereas the peaks disappear in the NiO QDs/TiO2, indicating that the carbon is removed successfully after being annealed in air.46 Otherwise, NiO cannot be detected by Raman spectra because of its low amount as well as the small size.47−49 According to the results of TEM, XPS, and Raman spectra, we confirm that the NiO QDs/TiO2 has been synthesized successfully with the limitation of N-doped carbon derived from urea, and the N-doped carbon not only prevents the aggregation of NiO QDs but also can be facile to remove by annealing. Photocatalysts combined with carbon materials have been regarded as a fantastic strategy for the enhanced carrier separation efficiency owing to the superior conductivity of carbon materials.50−52 Nevertheless, it should be noted that the carbon materials would cover up the photocatalytic active sites of photocatalysts despite being compensated with the active sites of carbon material. As a matter of course, the modification of carbon materials would make no sense when the photocatalysts possess stronger active sites. Accordingly, the details of carrier transport and consumption should be clear to determine whether or not the N-doped carbon is removed. Photoelectrochemical measurements have been identified as efficient approaches to obtain the information on carrier transport and consumption because photocatalysts could be regarded as a tiny electrolyzer.52 The transient photocurrent responses of NiO QDs/TiO2@NC are measured at potentials 893

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Figure 7. Photocatalytic performance: (a) hydrogen production rate of TiO2, TiO2@NC, NiO/TiO2, NiO QDs/TiO2@NC, NiO QDs/TiO2, and Pt/TiO2; (b) hydrogen production in four repeated cycles (3 h/cycle) of NiO QDs/TiO2.

photocatalytic hydrogen production activities of NiO QDs/ Ta2O5 and NiO QDs/Nb2O5 are 3.2 and 1.8 times more than that of Pt/Ta2O5 and Pt/Nb2O5 (Figure S9), respectively, confirming it could be a general method to improve the performance of photocatalysts. Furthermore, to evaluate the durability and photocatalytic stability of the NiO QDs/TiO2 photocatalyst, the photocatalytic cycling tests were carried out, and each run lasted for 3 h (Figure 6b). It can be seen clearly that the NiO QDs/TiO2 does not exhibit any obvious loss in the activity after three cycles, suggesting that the NiO QDs/TiO2 is highly stable during the photocatalytic process.

To shed light on the promotion effect of NiO QDs on TiO2, the transient photocurrent responses of TiO2, NiO/TiO2, and NiO QDs/TiO2 are measured at a fixed bias potential of 0.2 V vs Ag/AgCl via several on−off cycles of irradiation under periodical light (Figure 5a). As expected, the photocurrent of NiO QDs/TiO2 shows a stable photocurrent of 13.0 μA/cm2, which is about 2.5 times over that of bare TiO2 (5.0 μA/cm2) and NiO/TiO2. This obviously enhanced transient photocurrent indicates that the carrier transfer and carrier-consuming ability of NiO QDs are significantly better than that of others, owing to the strong photocatalytic active sites of QDs. To explore the change of efficiency of charge carrier trapping and transfer of the photogenerated electron−hole pairs, the PL spectroscopy of TiO2, NiO/TiO2, and NiO QDs/TiO2 is carried out as shown in Figure 6b. These samples exhibit a strong emission peak at around 388 nm corresponding to the band gap characterized by UV−vis spectra (Figure S7). The PL intensity of NiO QDs/TiO2 is lower than that of TiO2 and NiO/TiO2, indicating the efficient inhibition of photogenerated electron−hole pair recombination by the NiO QDs/TiO2 heterojunction. However, the amplitude of PL intensity decreases slightly, which could be due to the photogenerated carriers only being generated without being consumed by any reaction in PL measurement so that carrier transport/balance builds quickly and offsets the internal electric field with such a low amount of NiO. Figure 7a shows the hydrogen production rate of TiO2, TiO2@NC, NiO/TiO2, NiO QDs/TiO2@NC, NiO QDs/ TiO2, and Pt/TiO2. The hydrogen production rate of TiO2 is 1.2 μmol h−1, whereas for TiO2@NC and NiO/TiO2, they are approximately 1.7 and 1.8 μmol h−1, respectively. This enhanced activity highlights the role played by the good separation efficiency of heterojunction with N-doped carbon and NiO. By contrast, the hydrogen production performance of NiO QDs/TiO2@NC is up to 13.7 μmol h−1, which is quite better than that of TiO2@NC and NiO/TiO2. Interestingly, NiO QDs/TiO2 with the optimum loading amount of Ni at 2.5% (Figure S8) shows an excellent hydrogen production rate at 67.8 μmol h−1, about 1.35 mmol h−1 g−1, which is far more than that of NiO QDs/TiO2@NC, as expected, and reaches the same level as that of Pt/TiO2. This excellent hydrogen performance could be attributed to the small sizes of NiO QDs, which possess shorter transfer distance of photogenerated carriers and highly active sites for photocatalysis. Moreover, we have investigated the photocatalytic hydrogen production activity of Ta2O5 and Nb2O5 modified with NiO QDs. The

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CONCLUSION In summary, NiO quantum dot modified TiO2 with robust hydrogen production performance was synthesized via a sacrificial coating strategy. NiO quantum dots with diameters around 2 nm dispersed homogeneously on the surface of TiO2. The as-synthesized NiO QDs/TiO2 showed an excellent performance for hydrogen production of 1.35 mmol h−1 g−1, which is 37 times higher than that of NiO/TiO2 and 56 times higher than that of pure TiO2 and even approached the performance of Pt-loaded TiO2. To the best of our knowledge, it was the highest hydrogen production performance among the TiO2/NiO heterojunction photocatalysts. This work opens a new insight for preparing QDs and shows the advantages of QDs in heterojunctions for photocatalytic hydrogen production.

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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/acssuschemeng.7b03250. Performance of NiO/TiO2 heterojunction (Table S1); TEM images of NiO QDs/TiO2@NC (Figure S1); SEM micrographs of TiO2@NC, NiO/TiO2, NiO QDs/ TiO2@NC, and NiO QDs/TiO2 (Figure S2); morphology and composition characterization of NiO/TiO2, TEM image, HRTEM image, high-angle annular darkfield image, elemental mapping results of Ti and Ni (Figure S3); XPS measurement results, wide scan survey XPS spectra of TiO2, NiO QDs/TiO2@NC, and NiO QDs/TiO2, high-resolution XPS spectra of O 1s (Figure S4); XRD pattern of TiO2, TiO2@NC, NiO/TiO2, NiO 894

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QDs/TiO2@NC, and NiO QDs/TiO2 (Figure S5); Brunauer−Emmett−Teller surface area of NiO QDs/ TiO2@NC and NiO QDs/TiO2 (Table S2); hydrogen production performance of NiO QDs/TiO2 with different concentrations of Na2SO4 (Figure S6); UV−vis absorbance spectra of TiO2, NiO/TiO2, and NiO QDs/ TiO2 (Figure S7); stability rates of hydrogen production of NiO QDs/TiO2 with different amount of NiO QDs (Figure S8); hydrogen production of Nb2O5, NiO QDs/ Nb2O5, Pt/Nb2O5, Ta2O5, NiO QDs/Ta2O5, and Pt/ Ta2O5 (Figure S9) (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (+86)-451-86413753. ORCID

Gang Chen: 0000-0003-1502-0330 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21471040). REFERENCES

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DOI: 10.1021/acssuschemeng.7b03250 ACS Sustainable Chem. Eng. 2018, 6, 889−896