NiO Quantum Dots Modified TiO2 toward Robust Hydrogen Production

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NiO Quantum Dots Modified TiO2 toward Robust Hydrogen Production Performance Weizhao Hong, Yansong Zhou, Chade Lv, Zhonghui Han, and Gang Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03250 • Publication Date (Web): 24 Nov 2017 Downloaded from http://pubs.acs.org on November 26, 2017

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ACS Sustainable Chemistry & Engineering

NiO Quantum Dots Modified TiO2 toward Robust Hydrogen Production Performance Weizhao Hong, Yansong Zhou, Chade Lv, Zhonghui Han, 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 Harbin, P. R. China. ∗Corresponding

author:

Gang

Chen.

E-mail:

[email protected].

Fax:

(+86)-451-86413753. Author: Weizhao Hong. E-mail: [email protected] Author: Yansong Zhou. E-mail: [email protected] Author: Chade Lv. E-mail: [email protected] Author: Zhonghui Han. E-mail: [email protected] Abstract: Quantum dots (QDs) are of great potential to build heterojunctions for efficiently 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/TiO2 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 NiO QDs/TiO2 sample is achieved, which is 37 times higher than NiO/TiO2 and 56 times higher than pure TiO2. Furthermore, this is one of the best performance reported in heterojunctions made of NiO and TiO2. Keywords: TiO2, quantum dots,

heterojunction, sacrificial coatings, photocatalysis

Introduction Splitting water directly into hydrogen by solar energy has been regarded as one of the 1

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most promising way 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 hydrogen energy system is to produce H2 with a low cost, stable and high-efficient photocatalyst.2-6 However, the current efficiency for photocatalytic water-splitting is limited by the highly preferred recombination of the 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 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 valance band edges than the bulk materials because of the quantum size effects have been regarded as one of the most promising structure 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 these wet chemistry methods are performed at low temperature, leading to low crystallinity with more centers for carriers recombination.31-33 Photocatalysts with high crystallinity have fewer centers for carriers recombination, which can suppress the carriers recombination. With the purpose of realizing QDs 2


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decorated heterojunction with high crystallinity, heat treatment at high temperature is necessary. Nevertheless, aggregation of QDs will emerge accompanied with 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 confine effect of carbon materials.34 Inspired by this work, we conjecture QDs with high crystallinity can be fabricated by introducing carbon materials as coating to prevent from aggregation. However, to the best of our knowledge, the achievement of QDs for constructing heterojunction by introducing carbon materials as sacrificial coating lacks of 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 photocatalysts combined with NiO QDs have been reported owing to its rigorous preparation condition. Herein, highly crystalline NiO QDs modified TiO2 was synthesized via a sacrificial coating strategy at high temperature. The in-situ introduced graphite N-doped carbon derived from urea furnished space confine effect for realizing NiO QDs with high crystallinity. The as-synthesized NiO QDs/TiO2 showed an excellent performance at 1.35 mmol h-1 g-1, which was 37 times higher than NiO/TiO2 and 56 times higher than pure TiO2, even comparable with Pt loaded TiO2 photocatalyst, for hydrogen production. Significantly, it is one of the highest hydrogen production performance among the reported heterojunctions consisted from TiO2 and NiO (Table S1). This work provides 3



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a new insight for crafting QDs modified photocatalysts with high crystallinity to achieve highly efficient photocatalytic water-splitting performance. Experimental 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. NiO QDs/TiO2 heterojunction was synthesized by temperature-programmed method. At first, commercial TiO2 (0.25 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 grinding 30 min, the uniform compounds were heated up under static N2 atmosphere at 773 K for 2 hours with a heating rate of 5 K/min and followed by annealing at 1073 K for 2 hours with the same heating rate as above to fabricate the N-doped carbon coating. To remove the sacrificing carbon coatings, the temperature was to be maintained at 1073 K for 2 hours in air. TiO2 in this work was only treated by temperature-programmed process as NiO QDs/TiO2. NiO/TiO2 was prepared by the same method as NiO QDs/TiO2, except that urea was absence. Pt/TiO2: TiO2 adding K2[PtCl6]·6H2O (2.5wt% calculated with Pt) during photocatalytic performance test was denoted as Pt/TiO2. Nb2O5, NiO QDs/Nb2O5, Pt/Nb2O5, Ta2O5, NiO QDs/Ta2O5 and Pt/Ta2O5 were 4


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prepared with Nb2O5 and Ta2O5, respectively, instead of TiO2.

Working electrode preparation

In brief, 10 mg of catalyst powder was dispersed in 1 mL terpineol, and then ultrasonic 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 as-prepared catalyst film was dried at 80 °C in vacuum for 12 h.

Materials characterization

The structure of the obtained samples were confirmed by X-ray diffraction (XRD) on 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 were observed by a HELIOS NanoLab 600i field emission scanning electron micro-scope (FE-SEM). The operating voltage was set to 20 kV and the samples were prepared by 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 by 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 convertional three-electrode cell by using a Ag/AgCl (sat. KCl) electrode as the reference electrode, a Pt electrode 5



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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 enclosing photoreactor evacuated with N2, using a PLS-SXE300 300 W Xe lamp. 50 mg of the photocatalyst was dispersed in 300 mL 10 vol% methanol solution and ultrasonic for 20 min. The system was injected the nitrogen for about 5 min to remove air. The H2 evolution quantity was analyzed by 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. Results and Discussion As illustrated in Figure 1, a sacrificial coating strategy was utilized to synthesize 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 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 aggregation (Figure S1). After this step, NiO QDs decorated TiO2 coated with N-doped carbon (NiO QDs/TiO2@NC) was obtained. Finally, the as-prepared NiO QDs/TiO2@NC was further calcinated at 1073K in air 6


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and the graphite N-doped carbon was removed to achieve NiO QDs/TiO2 (step III).

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

The composition of NiO QDs/TiO2 is confirmed by X-ray diffraction (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 can not be observed which could be due to the NiO possesses ultra small size.15 To determine the specific morphology of NiO, transmission electron microscopy (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 high-resolution TEM (HRTEM) image (Figure 2d), the lattice fringe spacing of particle is 0.35 nm, indexing to the (101) plane of anatase. Interestingly, the particle is covered by

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numerous QDs with a diameter of approximately 2 nm (Figure 2c), which show clear lattice fringe spacing of 0.24 nm assigned to (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 and Figure 2f are shown in Figure 2g-i. According to the elemental distribution of Ti, O and Ni, the bulk of this particle is anatase and Ni element disperses on the surface sparsely, indicating that NiO QDs are well-dispersed on the surface of TiO2. Especially, 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 X-ray photoelectron spectroscopy (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, i.e. C, Ti, O (Figure S4a). 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 doping carbon in TiO2 is not observed, suggesting that carbon is not doped into TiO2. In Figure 3b, Ti 2p region of NiO QDs/TiO2 has no shift comparing with TiO2, excluding the anion-doping.43 The peaks at 855.8 and 872.3 eV can be attributed to Ni 2p3/2 and Ni 2p1/2, respectively, manifesting that Ni element possesses +2 oxidation state.44 Furthermore, 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 N-doped carbon. 8


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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 which is decomposed 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 above-mentioned coating (Figure 3d).

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 (HAADF) images of NiO QDs/TiO2 and elemental 9



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mapping results of (g) Ti, (h) O, (i) Ni

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, elucidating that the carbon is removed successfully after annealing in air.46 Otherwise, NiO cannot be detected by Raman spectra because of its low amount as well as the small size.47-49

Figure 3. High-resolution XPS spectra (HRXPS) of TiO2, NiO QDs/TiO2@NC and

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NiO QDs/TiO2. (a) C 1s, (b) Ti 2p, (c) Ni 2p and (d) N 1s regions

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 prevent from the aggregation of NiO QDs but also can be facile to be removed by annealing.

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

Photocatalysts combined with carbon materials have been regarded as a fantastic strategy for the enhanced carrier separation efficiency owing to the prior 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 compensating 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 consuming should be clear to determine whether or not removing the N-doped carbon. Photoelectrochemical measurements have been identified as efficient approaches to 11



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obtain the information of carrier transport and consuming since photocatalysts could be regarded as a tiny electrolyzer.52 The transient photocurrent responses of NiO QDs/TiO2@NC are measured at potentials 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 and followed by dramatically decrease after several second in accordance with several previous studies.19,

53

Interestingly, the decrease amplitude of photocurrents is

increasing while the bias accumulates. Noting that the bias can improve the separation efficiency of carriers, we realize that the decrease amplitude of photocurrents could be concerned with carrier density. Thus, further measurements of NiO QDs/TiO2@NC are carried out under weaker irradiation intensity, in which situation photocatalysts can generate fewer carriers. Consequently, in the case of weaker irradiation intensity (Figure 5b), the drop amplitude of photocurrents are slighter 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 inverse electric field (Figure 5c). N2 adsorption-desorption measurements show that the Brunauer-Emmett-Teller (BET) surface area of NiO QDs/TiO2 is 16.5% more than 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 12


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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 consuming 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 set up which 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 Na2SO4 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 possess stronger active sites than coated N-doped carbon. Thus, the removal of N-doped carbon is imperative to eliminate the obstruction of ion diffusion.

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(b)

(a)

100 mW/m2, 0.1 M

150 mW/m 2, 0.1 M

(c)

(d)

150 mW/m2, 0.2 V

Figure 5. Photocurrent vs. time of NiO QDs/TiO2@NC measurements under different condition. (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 hamper of ion transport by coating and (d) Photocurrent vs. Time of NiO QDs/TiO2@NC different concentration of Na2SO4 solution under 15 W/cm2 irradiation intensity and 0.2 V bias.

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 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 14


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the carrier-transfer and carrier-consuming ability of NiO QDs are significantly better than 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 photoluminescence (PL) spectroscopy of TiO2, NiO/TiO2 and NiO QDs/TiO2 are carried out as shown in Figure 5b. These samples exhibit a strong emission peaks 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 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 generate without consuming by any reaction in PL measurement so that carriers-transport-balance builds quickly and offsets the internal electric field with such low amount of NiO.

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

Figure 7a shows the hydrogen production rate of TiO2, TiO2@NC, NiO/TiO2, NiO

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QDs/TiO2@NC, NiO QDs/TiO2 and Pt/TiO2. The hydrogen production rate of TiO2 is 1.2 µmol h-1, while 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 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 NiO QDs/TiO2@NC as expected and reach the same level as Pt/TiO2. This excellent hydrogen performance could be attributed to the small size 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 photocatalytic Hydrogen production activity of NiO QDs/Ta2O5 and NiO QDs/Nb2O5 are 3.2 and 1.8 times more than 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 are carried out, and each run last for 3 hours (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.

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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 4 repeated cycles (3 h/cycle) of NiO QDs/TiO2

Conclusion In summary, NiO quantum dots 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 NiO/TiO2 and 56 times higher than pure TiO2, and even approach to 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 heterojunction for photocatalytic hydrogen production.

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Supporting information for publication Performance of NiO/TiO2 heterojunction in recent report (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, HAADF image, elemental mapping results of Ti and Ni (Figure S3); X-ray photoelectron spectroscopy (XPS) measurements 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 QDs/TiO2@NC and NiO QDs/TiO2 (Figure S5); BET 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); The stable 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, Pt/Ta2O5 (Figure S9) The authors declare no competing financial interest. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21471040).

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Synopsis As a photocatalyst, NiO QDs/TiO2 can efficiently convert the water into hydrogen by harvesting solar energy for sustainable energy engineering.

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NiO Quantum Dots Modified TiO2 toward Robust Hydrogen Production

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