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TiO2 supported carbon nanosheets for photocatalytic hydrogen production
Colloids and Surfaces A 587 (2020) 124365
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Salt templated synthesis of NiO/TiO2 supported carbon nanosheets for photocatalytic hydrogen production
T
Xueying Zhaoa, Wenyu Xieb, Zhibo Dengc, Gan Wanga, Aihui Caoa, Huamei Chena, Bo Yanga, Zhuan Wanga, Xintai Suc,*, Chao Yanga,* a
Ministry Key Laboratory of Oil and Gas Fine Chemicals, College of the Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046, China Faculty of Environmental Science and Engineering, Guangdong University of Petrochemical Technology, Maoming 525000, PR China c The Key Laboratory of Pollution Control and Ecosystem Restoration in Industry Clusters (Ministry of Education), School of Environment and Energy, South China University of Technology, Guangzhou 510006, China b
G R A P H I C A L A B S T R A C T
NiO modified TiO2 photocatalyst was prepared by phase transfer and molten salt calcination method. The combined action of NiO as co-catalyst and C film carrier reduced TiO2 to TiO2-x, which provided stronger active center and UV response for the photocatalytic H2 evolution.
A R T I C LE I N FO
A B S T R A C T
Keywords: Defect TiO2 Co-catalyst TiO2/C Photocatalytic H2 production Molten salt method
NiO-TiO2-x/C composites for photo-catalytic hydrogen production have been prepared by a molten salt templateassisted pyrolysis of Ni/Ti-oleylamine precursors at different temperatures. X-ray diffraction (XRD) analysis shows that it is brookite structure, and X-ray photoelectron spectrum (XPS) determines the chemical state of the elements. Carbon nanosheets improve the dispersion and stability of TiO2, making the later fully close to each other. NiO facilitates the separation of photo-generated carriers and enhances the photocatalytic hydrogen production activity of TiO2. Due to the efficient synergy of carbon nanosheets, NiO, and TiO2, the optimized (1 wt%)NiO-TiO2-x/C-T650 nanocomposites show enhanced activity toward photocatalytic hydrogen production from water, outperforming 18-fold in activity higher than pristine TiO2/C. Moreover, the (1 wt%)NiO-TiO2-x/CT650 catalyst exhibits good stability without obvious decrease in activity under continuous cycling. This work will lead to the optimization of carbon-supported multi-component catalysts for photocatalytic hydrogen production.
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Corresponding authors. E-mail addresses: [email protected] (X. Su), [email protected] (C. Yang).
https://doi.org/10.1016/j.colsurfa.2019.124365 Received 12 October 2019; Received in revised form 14 December 2019; Accepted 16 December 2019 Available online 17 December 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.
Colloids and Surfaces A 587 (2020) 124365
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1. Introduction
solution containing 30 mL of n-hexane and 20 mL of ethanol. Subsequently, 0.0229 g of NiCl2·6H2O (M =238 g/mol, equivalent to NiO containing 0.0072 g) was dissolved in 7 g of distilled water, and added to the above solution drop by drop. In reactants, the mass ratio of NiO to TiO2 was 1 and the reaction was refluxed at 70 °C for 2 h under continuous stirring. The obtained precursor was dried at 80 °C for 24 h. Finally, the precursor was mixed with Na2SO4 (mass ratio of 1:10) and ground to form a homogeneous mixture. The mixture was annealed at 500 °C, 550 °C, 600 °C, 650 °C and 700 °C respectively for 3 h with a heating ramping rate (10 °C/min) in Ar atmosphere to obtain the productions. After being cooled, the products were washed with distilled water and ethanol and dried at 60 °C overnight. The obtained sample denoted as (1 wt%)NiO-TiO2-x/C-T500, (1 wt%)NiO-TiO2-x/C-T550, (1 wt%)NiO-TiO2-x/C-T600, (1 wt%)NiO-TiO2-x/C-T650 and (1 wt%)NiOTiO2-x/C-T700. Under the optimum temperature of 650 °C, the desired content of NiO to TiO2 in the catalyst was adjusted through changing the amount of NiCl2·6H2O. The procedure of preparing the NiO-TiO2-x/C nanocomposites with NiO to TiO2 mass ratios of 0.50 and 0.20 was the same as that of (1 wt%)NiO-TiO2-x/C-T650, added amount of NiCl2·6H2O was 0.01147 g and 0.04589 g. The obtained samples denoted as (0.5 wt%) NiO-TiO2-x/C-T650, and (2 wt%)NiO-TiO2-x/C-T650, respectively. The procedure of preparing TiO2/C was the same as that of (1 wt%) NiO-TiO2-x/C-T650 except for no addition of NiCl2·6H2O.
In recent years, the world's rapidly growing population and industrialization have led to an increasing global energy demand, forcing people to find sustainable and efficient clean energy source [1–5]. Considering that solar energy is the most available and sustainable natural energy source, the technology of photocatalytic water splitting using sunlight is considered to be an effective way to solve the energy problem in the future [6–8]. So far, titanium dioxide (TiO2), as a chemically stable, environmentally friendly, innocuity, rich and cost-effective photocatalyst, have attracted wide attention in the field of photocatalysis. However, this semiconductor only absorb UV light and exhibit low quantum efficiency [9–13]. In addition, the pure TiO2 has small specific surface area and narrow spectral absorption range, which limits its photocatalytic activity and utilization efficiency [14–16]. Therefore, it is necessary to explore a method to optimize TiO2 material structure to improve the separation efficiency and inhibit the recombination efficiency, so that the TiO2 material can catalyze water splitting reaction to produce more H2 production. It has been reported that the doping of non-metallic elements [17–19] (such as C, S, N) and the loading/deposition of noble metal nanoparticles [20–25] (such as Ag, Pt, Au) have been proved to be very effective strategies to enhance the photocatalytic activity of TiO2. However, the doping process of non-metallic elements suffers from complex manipulations under harsh conditions and uncontrollable heteroatom contents. Noble metals for the loading/deposition are facing the problems of high price and scarce sources, which restrict their applications in hydrogen production. In contrast, the incorporation of non-noble transition metal oxides, in particular of NiO as co-catalysts, have proven to provide another low-cost but effective way to enhance photocatalytic activity of TiO2 in the hydrogen production [26–29]. Previous reports have shown that the incorporation of a small amount of NiO into TiO2 could significantly suppress the recombination of photo-generated carriers, reduce the hydrogen production overpotential, and leads to an enhancement of photocatalytic activity of TiO2 [30–36]. Nevertheless, NiO/TiO2 nanoparticles tend to agglomerate into large clusters, impeding efficient photo-generated carriers transfer, which leaves a lot of room for further enhancing the photocatalytic activity of NiO/TiO2 p-n heterojunctions. Herein, we demonstrate that the incorporation of carbon nanosheets from oleylamine pyrolysis into NiO/TiO2 nanoparticles can significantly improve the dispersion and stability, therefore the photocatalytic activity of p-n heterojunction. Specifically, NiO-TiO2-x/C nanocomposites with good photocatalytic hydrogen production performance were prepared by the molten salt template-assisted pyrolysis, through adjusting the calcination temperature and the loading of NiO.
2.3. Characterization Scanning electron microscope (SEM), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM) (Tecnai F30 microscope, Holland) carried out the structure analysis of the as-prepared samples. The crystal structure of the samples was investigated by X-ray diffraction (XRD) using a Bruker D8 X-ray diffractometer (Germany) with Cu-Kα1radiation (λ =1.5406 Å). X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo XPS ESCALAB 250Xi instrument equipped with an Al Kα (1486.6 eV) X-ray source. The UV–vis diffuse reflectance spectra can obtained by a Hitachi spectrophotometer using BaSO4 as a reference. Ra`man spectrum was conducted on Bruker Senterra with 532 nm wave-length laser source. Photoluminescence (PL) spectra of the samples were measured with a fluorescence spectrophotometer (Hitachi F-4500) with the exciting wavelength of 255 nm. 2.4. Photocatalytic activity for water splitting In the process of water splitting under UV light (300 W Xeon-lamp), the photocatalytic activity of samples was determined. The reaction was carried out in a sealed circular quartz glass reactor. For each reaction, using a magnetic stirrer, the specified amount of photocatalyst (20 mg) was suspended in an aqueous methanol solution (80 mL of distilled water, 20 mL of methanol sacrificial agent) by means of a closed-gas circulation reaction cell. Before the reaction, the system was completely vacuumed to remove O2 and CO2 from the water. The amount of hydrogen production and area count of record peak were determined by gas chromatography, and gas chromatography (TCD) and Ar gas as the carrier gas.
2. Materials and methods 2.1. Materials All reagents and solvents were of analytical grade and used as received from commercial suppliers. Butyl titanate (C16H36O4Ti, 99%), ethanol and n-hexane was purchased from Tianjin Zhiyuan Chemical Reagents Co., Ltd. NiCl2·6H2O and Na2SO4 Tianjin Hedong district Hongyan reagent factory. Oleylamine (C18H35NH2, 80–90%) was supplied from Aladdin Reagents Company. Deionized water was used for all experimental steps.
3. Results and discussion 3.1. Characterization of NiO-TiO2-x/C nanocomposites The synthesis of NiO-TiO2-x/C nanocomposites involves the combination of phase transfer and molten salt template-assisted pyrolysis, as shown in Fig.1. Firstly, Ti4+ and Ni2+ species were transferred from water phase to oil phase by adsorbing or coordinating with oleylamine molecules. Then, the Ti4+/Ni2+-oleylamine precursor was uniformly coated on the surface of Na2SO4 by grinding. Thirdly, upon calcination
2.2. Synthesis methods 2.2.1. Synthesis of NiO-TiO2-x/C The procedure for preparing NiO-TiO2-x/C nanocomposites as follows. 9 mmol of butyl titanate (M =340 g/mol, equivalent to TiO2 containing 0.72 g) and 38 mmol of oleylamine were mixed in the 2
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Fig. 1. Schematic illustration of the synthesis of NiO-TiO2-x/C nanocomposites.
observed in the XRD pattern. Fig. 3(a–e) shows the SEM, TEM, SAED, HR-TEM and STEM-EDX elemental mapping images of (1 wt%)NiO-TiO2-x/C-T650, respectively. From Fig. 3(a–c), it was evidenced that the (1 wt%)NiO-TiO2-x/C-T650 sample was mainly composed of TiO2 nanoparticles doped with NiO immobilized on carbon nanosheets. EDS analysis of the chemical composition of the material also confirmed this point (see Fig. S1). The HRTEM image of (1 wt%)NiO-TiO2-x/C-T650 (Fig. 3d) presents the lattice spacing was 0.292 nm, which belongs to the (121) plane of brookite TiO2. According to the STEM-EDX elemental mapping of (1 wt %)NiO-TiO2-x/C-T650 (Fig. 3e), we can observe that the elements Ti, Ni, O and C were evenly distributed in the layer, respectively, indicated that carbon nanosheets promoted the dispersion and stability of TiO2 and NiO.
under argon atmosphere, the precursor of Ti was transformed into TiO2, and oleylamine molecules were pyrolyzed into carbon layer around TiO2 nanoparticles. Meanwhile, the pyrolysis of oleylamine molecules help TiO2 to create defect TiO2-x. In the whole synthetic process, oleylamine serves as an organic ligand in the precursor preparation process and then was converted into carbon nanosheets during the high-temperature pyrolysis. 3.2. Structure and morphology analysis The phase analysis of NiO-TiO2-x/C nanocomposites prepared by calcination is shown in Fig. 2. The XRD results show that the NiO-TiO2x/C nanocomposites in Fig. 2(a–c) was composed of amorphous structure. At the same time, Raman spectra further confirmed the existence of amorphous carbon in NiO-TiO2-x/C nanocomposites. This was due to the complete decomposition of oleylamine molecules into amorphous carbon after calcination under argon conditions [37]. The carbonization degree in Fig. 2(d–g) samples has a great relationship with the calcination temperature. The characteristic peaks corresponding to brookite in Fig. 2(d–g) were at 32.3, 34.5, 42.3, 49 and 62.5° (JCPDS 29–1360), respectively [38,39]. Due to the low doping content of NiO in the NiO-TiO2-x/C nanocomposites, no obvious absorption peak was
3.3. Spectroscopic analysis Fig. 4 exhibits the XPS spectra of the TiO2/C and (1 wt%)NiO-TiO2As shown in Fig. 4a, the fully scanned spectra can be assigned to Ti, Ni, C, and O elements. Accordingly, the Ti 2p high-resolution XPS spectra of TiO2/C and (1 wt%)NiO-TiO2-x/C-T650 can be observed in Fig. 4b. For TiO2/C, Ti 2p was composed of where two characteristic peaks at binding energies of 464.7 eV (2p1/2) and 459 eV (2p3/2), with spin-orbit splitting of Δ=Ti 2p1/2-Ti 2p3/2 = 5.7 eV associated with TiO2 [40]. The Ti 2p binding energy of (1 wt%)NiO-TiO2-x/C-T650 shifts to high binding energy, compared with the value of TiO2/C. The Ti 2p peaks were well de-convoluted into four peaks as Ti3+ 2p3/2, Ti4+ 2p3/2, Ti3+ 2p1/2, and Ti4+ 2p1/2 consecutively around 458.8, 459.3, 464.3, and 464.9 eV. The above results indicated that the presence of Ti3+ and Ti4+ species in (1 wt%)NiO-TiO2-x/C-T650. The existence of Ti3+ species can inhibit the recombination of electron-hole pairs and promote catalytic activity. The results indicated that there exist a strong interaction between NiO co-catalyst and TiO2-x nanoparticles [40–45]. Fig. 4c shows the high-resolution XPS spectrum of C 1s. the results showed that the C 1s composed of three kinds of C, including sp2 hybrid C, C-O and C = O, and the binding energies were located at 284.9, 286 and 287.9, respectively [46]. As shown in Fig. 4d, the peaks of TiO2/C at 530.7 eV, 532.9 eV and 534.5 eV correspond to lattice oxygen, Ti-OH and chemisorbed water, respectively. The O 1s scan peak of (1wt%) NiO-TiO2-x/C-T650 at 529.9 eV was the superposition of NiO and TiO2 The peak at 531.6 eV corresponds to the deficient oxygen and the peak at 533.1 eV belongs to the chemisorbed. Note that for the two samples with different conditions, each characteristic peak of (1 wt%)NiO-TiO2x/C-T650 has shift [47–52]. With respect to the XPS spectra of Ni 2p in x/C-T650.
Fig. 2. XRD patterns of different samples: (a) (1 wt%)NiO-TiO2-x/C-T500, (b) (1 wt%)NiO-TiO2-x/C-T550, (c) (1 wt%)NiO-TiO2-x/C-T600, (d) (1 wt%)NiO-TiO2x/C-T650, (e) (1 wt%)NiO-TiO2-x/C-T700, (f) (0.5 wt%)NiO-TiO2-x/C-T650, (g) (2 wt%)NiO-TiO2-x/C-T650. 3
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Fig. 3. (a, b) SEM images, (c) TEM images and SAED patterns (inset), (d) HR-TEM images and (e) STEM-EDX elemental mapping of (1 wt%)NiO-TiO2-x/C-T650.
than other NiO-TiO2-x/C nanocomposites, and BET surface analysis displayed that (1 wt%)NiO-TiO2-x/C-T650 has a higher surface area (81 m2 g−1) than TiO2/C sample (62 m2 g−1) (see Fig. S2). Under UV irradiation, (1 wt%)NiO-TiO2-x/C-T650 exhibited the highest enhanced photocatalytic activity among those samples.
Fig. 4e, the peaks at 856.5 eV (862 eV corresponds to the satellite of the Ni 2p3/2) and 873.8 eV (879 eV corresponds to the satellite of the Ni 2p1/2) the product present were mainly assigned to Ni2+ in nickel oxide [53–55]{Qi, 2019 #62}. Raman analysis was performed to reveal the graphitization of carbon in the nanocomposites. As shown in Fig. 5(a, b), two obvious vibration peaks appear at 1342 and 1589 cm−1, respectively, which can be assigned to the typical D and G peaks of carbon. In Fig. 5a, the ID/IG value of the sample increases with increasing calcination temperature, with the best degree of carbonization of (1 wt%)NiO-TiO2-x/C-T700. However, the ID/IG ratio of the sample containing different content of NiO was nearly the same (in Fig. 5b), indicating that the introduction of NiO has little effect on the graphitization degree of carbon [56,57]. UV-vis absorption spectra of NiO-TiO2-x/C nanocomposites are shown in Fig. 6(a, b). It showed that the samples all show the absorption ability of TiO2 in the UV region. Notably, the absorption of (1 wt%) NiO-TiO2-x/C-T500 was obviously highest than that of the other NiOTiO2-x/C nanocomposites in Fig. 6a, and sample (2 wt%)NiO-TiO2-x/CT650 absorption (Fig. 6b) was best than other NiO-TiO2-x/C nanocomposites. Fig. 6(c, d) shown the PL spectra of NiO-TiO2-x/C nanocomposites. In the photocatalytic process, the higher intensity of fluorescence scattering indicates the weakest catalytic activity. Conversely, Low fluorescence intensity means more efficient migration and separation of photogenerated electron-hole pairs, which provides strong evidence for efficient photocatalytic activity. As can be seen from Fig. 6(c, d), (1 wt%)NiO-TiO2-x/C-T650 has a lower PL intensity
3.4. Photocatalytic hydrogen evolution of samples Fig. 7(a, c) shows the photocatalytic hydrogen production rates of P25 and (1 wt%)NiO-TiO2-x/C-TX (X = 500, 550, 600, 650, and 700 °C) under UV light. An increase and then decrease in hydrogen production rate appears over the (1 wt%)NiO-TiO2-x/C catalysts synthesized at different temperatures. Furthermore, the contrast experiments shown in Fig. 7(b, d) reveals that the catalytic activity of the NiO-TiO2-x/C-T650 nanocomposites was dependent on the loading content of NiO. The (1 wt%)NiO-TiO2-x/C-T650 catalyst achieves the maximum hydrogen production rate of 1.55 mmol h−1 g−1. As a comparison, the commercial P25 catalyst displays a poor photocatalytic activity in the absence of any co-catalyst. Meanwhile, we compared the photocatalytic hydrogen production activity of (1 wt%)NiO-TiO2-x/C-T650 nanocomposites at different pH values (see Fig. S3). The (1 wt%)NiO-TiO2-x/ C-T650 catalyst also shows a good stability toward photocatalytic hydrogen production, as confirmed by the recycling test in Fig. S4. To evaluate interfacial charge separation efficiency, transient photocurrent responses of NiO-TiO2-x/C nanocomposite were recorded for five on-off cycles in Fig. 7(e, f). The photocurrent density of (1 wt%) 4
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Fig. 4. XPS spectra of full scan survey (a), Ti 2p (b), C 1s (c), O 1s (d) and Ni 2p (e) in the TiO2/C and (1 wt%)NiO-TiO2-x/C-T650 samples.
synergistic catalytic activity and high photocatalytic hydrogen production actively [56,58–61].
NiO-TiO2-x/C-T650 was obviously higher than that of other NiO-TiO2-x/ C nanocomposites prepared at different calcination temperature and NiO content. Combining the results from photocurrent and aforementioned PL tests, (1 wt%)NiO-TiO2-x/C-T650 also showed significant separation efficiency. Based on the above experimental results and analysis, the mechanism of enhanced photocatalytic activity of NiO-TiO2-x/C nanocomposites sample was shown in Fig. 8. When NiO was deposited on the surface of TiO2, abundant p-n heterojunction were formed at the interface. Once excited by light for NiO-TiO2-x/C nanocomposites, photogenerated electron-hole pairs were created. Subsequently, the photogenerated electrons transfer to the conduction band of n-type TiO2-x and the photogenerated holes transfer to the valence band of p-type NiO. The internal electric field formed at the p-n heterojunction interface was effectively suppress electron-hole recombination. In addition, Raman spectra showed that the carbonization degree of NiO-TiO2-x/C nanocomposites was good, and the unique properties of carbon nanosheets play an important role in improving the efficiency of electron charge transfer in the process of light irradiation. The close contact between carbon layer and TiO2-x can effectively inhibit electron-hole recombination. The electrons in the conduction band of TiO2-x can be further transferred to the carbon, while the exhausted holes by sacrificial agent (CH3OH). NiO-TiO2-x/C nanocomposite have good
4. Conclusions In summary, NiO-TiO2-x nanoparticles were grown on carbon nanosheets by a molten salt template-assisted pyrolytic method and used as catalysts for photocatalytic hydrogen production. The pyrolysis of oleylamine produced carbon nanosheets and led to the creation of defected TiO2. The resulting carbon nanosheets promoted the dispersion and stability of TiO2-x nanoparticles. The introduction of NiO into TiO2x facilitated the separation of photogenerated electron-hole pairs under UV irradiation. These combined advantages allow for NiO-TiO2-x/C composites performing as highly active catalyst for photocatalytic hydrogen production. This work offers an facile preparation route to NiO modified TiO2 supported on carbon nanosheets with high photocatalytic activity. Author contributions section Xueying Zhao, Wenyu Xie, Xintai Su and Chao Yang conceived and designed the project. Xueying Zhao conducted the preparation, characterizations and performance test of all samples. Zhibo Deng, Gan
Fig. 5. Raman Spectra of NiO-TiO2-x/C nanocomposites synthesized at (a) different temperatures and (b) different NiO content. 5
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Fig. 6. (a, b) UV–vis diffuse reflectance spectra and (c, d) PL spectra of NiO-TiO2-x/C nanocomposites.
Declaration of Competing Interest
Wang, Aihui Cao, Huamei Chen, Bo Yang, Zhuan Wang, Xintai Su and Chao Yang made discussions of the results. Xueying Zhao wrote the manuscript. Xintai Su and Chao Yang supervised the work.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Fig. 7. (a–d) Comparison of the photocatalytic H2-production activities between P25 and NiO-TiO2-x/C nanocomposites samples; (e), (f) the transient photocurrent response for NiO-TiO2-x/C production at different temperatures and ratios in 0.5 mol L−1 Na2SO4 aqueous solution under UV light irradiation, respectively. 6
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properties, Nanoscale 3 (2011) 2943–2949. [17] X. Chen, C. Burda, The electronic origin of the visible-light absorption properties of C-, N-and S-doped TiO2 nanomaterials, J. Am. Chem. Soc. 130 (2008) 5018–5019. [18] I.C. Kang, Q. Zhang, S. Yin, T. Sato, F. Saito, Improvement in photocatalytic activity of TiO2 under visible irradiation through addition of N-TiO2, Environ. Sci. Technol. 42 (2008) 3622–3626. [19] J.H. Pan, X. Zhang, A.J. Du, D.D. Sun, J.O. Leckie, Self-etching reconstruction of hierarchically mesoporous F-TiO2 hollow microspherical photocatalyst for concurrent membrane water purifications, J. Am. Chem. Soc. 130 (2008) 11256–11257. [20] M.V. Sofianou, N. Boukos, T. Vaimakis, C. Trapalis, Decoration of TiO2 anatase nanoplates with silver nanoparticles on the {101} crystal facets and their photocatalytic behaviour, Appl. Catal. B 158–159 (2014) 91–95. [21] Z. Cai, Z. Xiong, X. Lu, J. Teng, In situ gold-loaded titania photonic crystals with enhanced photocatalytic activity, J. Mater. Chem. A 2 (2014) 545–553. [22] A. Iwase, H. Kato, A. Kudo, Nanosized Au particles as an efficient cocatalyst for photocatalytic overall water splitting, Catal. Letters 108 (2006) 7–10. [23] D. Wodka, E. Bielanska, R.P. Socha, M. Elzbieciak-Wodka, J. Gurgul, P. Nowak, P. Warszynski, I. Kumakiri, Photocatalytic activity of titanium dioxide modified by silver nanoparticles, ACS Appl. Mater. Interfaces 2 (2010) 1945–1953. [24] N. Zhang, S. Liu, X. Fu, Y.-J. Xu, Synthesis of M@TiO2 (M = Au, Pd, Pt) core–shell nanocomposites with tunable photoreactivity, J. Phys. Chem. C 115 (2011) 9136–9145. [25] J. Yu, L. Qi, M. Jaroniec, Hydrogen production by photocatalytic water splitting over Pt/TiO2 nanosheets with exposed (001) facets, J. Phys. Chem. C 114 (2010) 13118–13125. [26] C.V. Reddy, I.N. Reddy, B. Akkinepally, V.V.N. Harish, K.R. Reddy, S. Jaesool, Mndoped ZrO2 nanoparticles prepared by a template-free method for electrochemical energy storage and abatement of dye degradation, Ceram. Int. 45 (2019) 15298–15306. [27] C.V. Reddy, I.N. Reddy, B. Akkinepally, K.R. Reddy, J. Shim, Synthesis and photoelectrochemical water oxidation of (Y, Cu) codoped α-Fe2O3 nanostructure photoanode, J. Alloys. Compd. 814 (2020) 152349. [28] C.V. Reddy, I.N. Reddy, K.R. Reddy, S. Jaesool, K. Yoo, Template-free synthesis of tetragonal Co-doped ZrO2 nanoparticles for applications in electrochemical energy storage and water treatment, Electrochim. Acta 317 (2019) 416–426. [29] S.A. Rawool, M.R. Pai, A.M. Banerjee, A. Arya, R.S. Ningthoujam, R. Tewari, R. Rao, B. Chalke, P. Ayyub, A.K. Tripathi, S.R. Bharadwaj, Pn Heterojunctions in NiO:TiO2 composites with type-II band alignment assisting sunlight driven photocatalytic H2 generation, Appl. Catal. B 221 (2018) 443–458. [30] J. Liu, Q. Jia, J. Long, X. Wang, Z. Gao, Q. Gu, Amorphous NiO as co-catalyst for enhanced visible-light-driven hydrogen generation over g-C3N4 photocatalyst, Appl. Catal. B 222 (2018) 35–43. [31] Z. Khan, M. Khannam, N. Vinothkumar, M. De, M. Qureshi, Hierarchical 3D NiO–CdS heteroarchitecture for efficient visible light photocatalytic hydrogen generation, J. Mater. Chem. A 22 (2012) 12090–12095. [32] T. Sreethawong, Y. Suzuki, S. Yoshikawa, Photocatalytic evolution of hydrogen over mesoporous TiO2 supported NiO photocatalyst prepared by single-step sol–gel process with surfactant template, Int. J. Hydrogen Energy 30 (2005) 1053–1062. [33] C.J. Chen, C.H. Liao, K.C. Hsu, Y.T. Wu, J.C.S. Wu, P–N junction mechanism on improved NiO/TiO2 photocatalyst, Catal. Commun. 12 (2011) 1307–1310. [34] C.C. Hu, H. Teng, Structural features of p-type semiconducting NiO as a co-catalyst for photocatalytic water splitting, J. Catal. 272 (2010) 1–8. [35] L. Li, B. Cheng, Y. Wang, J. Yu, Enhanced photocatalytic H2-production activity of bicomponent NiO/TiO2 composite nanofibers, J. Colloid Interface Sci. 449 (2015) 115–121. [36] S. Fujita, H. Kawamori, D. Honda, H. Yoshida, M. Arai, Photocatalytic hydrogen production from aqueous glycerol solution using NiO/TiO2 catalysts: effects of preparation and reaction conditions, Appl. Catal. B 181 (2016) 818–824. [37] B. Yan, J. Zhou, X. Liang, K. Song, X. Su, Facile synthesis of flake-like TiO2/C nanocomposites for photocatalytic H2 evolution under visible-light irradiation, Appl. Surf. Sci. 392 (2017) 889–896. [38] K. Haouemi, F. Touati, N. Gharbi, Characterization of a new TiO2 nanoflower prepared by the sol–gel process in a reverse microemulsion, J. Inorg. Organomet. Polym. Mater. 21 (2011) 929–936. [39] J.G. Li, C. Tang, D. Li, H. Haneda, T. Ishigaki, Monodispersed spherical particles of brookite‐type TiO2: synthesis, characterization, and photocatalytic property, J. Am. Ceram. Soc. 87 (2004) 1358–1361. [40] S. Naseem, I.V. Pinchuk, Y.K. Luo, R.K. Kawakami, S. Khan, S. Husain, W. Khan, Epitaxial growth of cobalt doped TiO2 thin films on LaAlO3(100) substrate by molecular beam epitaxy and their opto-magnetic based applications, Appl. Surf. Sci. 493 (2019) 691–702. [41] Z. Zhang, C. Shao, X. Li, C. Wang, M. Zhang, Y. Liu, Electrospun nanofibers of p-type NiO/n-type ZnO heterojunctions with enhanced photocatalytic activity, ACS Appl. Mater. Interfaces 2 (2010) 2915–2923. [42] H. Tan, Z. Zhao, M. Niu, C. Mao, D. Cao, D. Cheng, P. Feng, Z. Sun, A facile and versatile method for preparation of colored TiO2 with enhanced solar-driven photocatalytic activity, Nanoscale 6 (2014) 10216–10223. [43] J. Shao, W. Sheng, M. Wang, S. Li, J. Chen, Y. Zhang, S. Cao, In situ synthesis of carbon-doped TiO2 single-crystal nanorods with a remarkably photocatalytic efficiency, Appl. Catal. B 209 (2017) 311–319. [44] W. Zhou, W. Li, J.Q. Wang, Y. Qu, Y. Yang, Y. Xie, K. Zhang, L. Wang, H. Fu, D. Zhao, Ordered mesoporous black TiO2 as highly efficient hydrogen evolution photocatalyst, J. Am. Chem. Soc. 136 (2014) 9280-9083. [45] M. Lu, C. Shao, K. Wang, N. Lu, X. Zhang, P. Zhang, M. Zhang, X. li, Y. Liu, p-MoO3 nanostructures/n-TiO2 nanofiber heterojunctions: controlled fabrication and
Fig. 8. Schematic diagram illustrating the possible photocatalytic mechanism of the NiO-TiO2-x/C nanocomposites under xenon lamp irradiation.
Acknowledgment We appreciate the financial support of the National Natural Science Foundation of China (U1503391). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.colsurfa.2019.124365. References [1] S.S. Lee, H. Bai, Z. Liu, D.D. Sun, Green approach for photocatalytic Cu(II)-EDTA degradation over TiO2: toward environmental sustainability, Environ. Sci. Technol. 49 (2015) 2541–2548. [2] J. Zhang, Y. Wang, J. Jin, J. Zhang, L. Zhang, F. Huang, J. Yu, Efficient visible-light photocatalytic hydrogen evolution and enhanced photostability of core/shell CdS/ g-C3N4 nanowires, ACS Appl. Mater. Interfaces 5 (2013) 10317–10324. [3] A. Mishra, A. Mehta, S. Basu, N.P. Shetti, K.R. Reddy, T.M. Aminabhavi, Graphitic carbon nitride (g–C3N4)–based metal-free photocatalysts for water splitting: a review, Carbon 149 (2019) 693–721. [4] Z. Hu, Y. Mi, Y. Ji, R. Wang, W. Zhou, X. Qiu, X. Liu, Z. Fang, X. Wu, Multiplasmon modes for enhancing the photocatalytic activity of Au/Ag/Cu2O core-shell nanorods, Nanoscale 11 (2019) 16445–16454. [5] A. Kudo, Y. Miseki, Heterogeneous photocatalyst materials for water splitting, Chem. Soc. Rev. 38 (2009) 253–278. [6] K.R. Reddy, C. Venkata Reddy, M.N. Nadagouda, N.P. Shetti, S. Jaesool, T.M. Aminabhavi, Polymeric graphitic carbon nitride (g-C3N4)-based semiconducting nanostructured materials: synthesis methods, properties and photocatalytic applications, J. Environ. Manage. 238 (2019) 25–40. [7] N.L. Reddy, V.N. Rao, M. Vijayakumar, R. Santhosh, S. Anandan, M. Karthik, M.V. Shankar, K.R. Reddy, N.P. Shetti, M.N. Nadagouda, T.M. Aminabhavi, A review on frontiers in plasmonic nano-photocatalysts for hydrogen production, Int. J. Hydrogen Energy 44 (2019) 10453–10472. [8] V.N. Rao, N.L. Reddy, M.M. Kumari, P. Ravi, M. Sathish, K. Kuruvilla, V. Preethi, K.R. Reddy, N.P. Shetti, T.M. Aminabhavi, Photocatalytic recovery of H2from H2S containing wastewater: Surface and interface control of photo-excitons in Cu2S@TiO2 core-shell nanostructures, Appl. Catal. B 254 (2019) 174–185. [9] M. Ni, M.K.H. Leung, D.Y.C. Leung, K. Sumathy, A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production, Renewable Sustainable Energy Rev. 11 (2007) 401–425. [10] J. Wen, X. Li, W. Liu, Y. Fang, J. Xie, Y. Xu, Photocatalysis fundamentals and surface modification of TiO2 nanomaterials, Chinese J. Catal. 36 (2015) 2049–2070. [11] A. Mehta, A. Mishra, S. Basu, N.P. Shetti, K.R. Reddy, T.A. Saleh, T.M. Aminabhavi, Band gap tuning and surface modification of carbon dots for sustainable environmental remediation and photocatalytic hydrogen production - A review, J. Environ. Manage. 250 (2019) 109486. [12] X. Chen, S.S. Mao, Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications, Chem. Rev. 107 (2007) 2891–2959. [13] P.S. Basavarajappa, B.N.H. Seethya, N. Ganganagappa, K.B. Eshwaraswamy, R.R. Kakarla, Enhanced photocatalytic activity and biosensing of gadolinium substituted BiFeO3 nanoparticles, ChemistrySelect 3 (2018) 9025–9033. [14] D. Chen, F. Huang, Y.B. Cheng, R.A. Caruso, Mesoporous anatase TiO2 beads with high surface areas and controllable pore sizes: a superior candidate for high-performance dye-sensitized solar cells, Adv. Mater. 21 (2009) 2206–2210. [15] L.C. Sim, K.H. Leong, P. Saravanan, S. Ibrahim, Rapid thermal reduced graphene oxide/Pt–TiO2 nanotube arrays for enhanced visible-light-driven photocatalytic reduction of CO2, Appl. Surf. Sci. 358 (2015) 122–129. [16] P. Zhang, C. Shao, Z. Zhang, M. Zhang, J. Mu, Z. Guo, Y. Liu, TiO2@carbon core/ shell nanofibers: controllable preparation and enhanced visible photocatalytic
7
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X. Zhao, et al.
[46]
[47]
[48]
[49]
[50]
[51]
[52]
[53]
enhanced photocatalytic properties, ACS Appl. Mater. Interfaces 6 (2014) 9004–9012. Z. Yang, G. Du, Q. Meng, Z. Guo, X. Yu, Z. Chen, T. Guo, R. Zeng, Synthesis of uniform TiO2@carbon composite nanofibers as anode for lithium ion batteries with enhanced electrochemical performance, J. Mater. Chem. A 22 (2012) 5848–5854. Y. Gao, J. Xu, S. Shi, H. Dong, Y. Cheng, C. Wei, X. Zhang, S. Yin, L. Li, TiO2 nanorod arrays based self-powered UV photodetector: heterojunction with NiO nanoflakes and enhanced UV photoresponse, ACS Appl. Mater. Interfaces 10 (2018) 11269–11279. C. Hu, K. Chu, Y. Zhao, W.Y. Teoh, Efficient photoelectrochemical water splitting over anodized p-type NiO porous films, ACS Appl. Mater. Interfaces 6 (2014) 18558–18568. Y. Ku, C.N. Lin, W.M. Hou, Characterization of coupled NiO/TiO2 photocatalyst for the photocatalytic reduction of Cr(VI) in aqueous solution, J. Mol. Catal. A Chem. 349 (2011) 20–27. P.K. Prajapati, H. Singh, R. Yadav, A.K. Sinha, S. Szunerits, R. Boukherroub, S.L. Jain, Core-shell Ni/NiO grafted cobalt (II) complex: an efficient inorganic nanocomposite for photocatalytic reduction of CO2 under visible light irradiation, Appl. Surf. Sci. 467–468 (2019) 370–381. T.V. Thi, A.K. Rai, J. Gim, J. Kim, High performance of Co-doped NiO nanoparticle anode material for rechargeable lithium ion batteries, J. Power Sources 292 (2015) 23–30. H. Tian, H. Fan, G. Dong, L. Ma, J. Ma, NiO/ZnO p–n heterostructures and their gas sensing properties for reduced operating temperature, RSC Adv. 6 (2016) 109091–109098. L. Bi, X. Gao, L. Zhang, D. Wang, X. Zou, T. Xie, Enhanced photocatalytic hydrogen
[54]
[55]
[56]
[57]
[58]
[59]
[60]
[61]
8
evolution of NiCoP/g-C3N4 with improved separation efficiency and charge transfer efficiency, ChemSusChem 11 (2018) 276–284. Y. Qi, J. Xu, M. Zhang, H. Lin, L. Wang, In situ metal–organic framework-derived cdoped Ni3S4/Ni2P hybrid co-catalysts for photocatalytic H2 production over g-C3N4 via dye sensitization, Int. J. Hydrogen Energy 44 (2019) 16336–16347. J. Xu, Y. Qi, C. Wang, L. Wang, NH2-MIL-101(Fe)/Ni(OH)2-derived C,N-codoped Fe2P/Ni2P cocatalyst modified g-C3N4 for enhanced photocatalytic hydrogen evolution from water splitting, Appl. Catal. B 241 (2019) 178–186. M. Wang, J. Han, Y. Hu, R. Guo, Y. Yin, Carbon-incorporated NiO/TiO2 mesoporous shells with p-n heterojunctions for efficient visible light photocatalysis, ACS Appl. Mater. Interfaces 8 (2016) 29511–29521. R. M, Š. Z, J. I.A, Ć.-M. G, A. S.P, Č. M.I, Improvements to the photocatalytic efficiency of polyaniline modified TiO2 nanoparticles, Appl. Catal. B 136 (2013) 133–139. F. Zuo, L. Wang, T. Wu, Z. Zhang, D. Borchardt, P. Feng, Self-doped Ti3+ enhanced photocatalyst for hydrogen production under visible light, J. Am. Chem. Soc. 41 (2010) 11856–11857. Z. Huang, Z. Gao, S. Gao, Q. Wang, Z. Wang, B. Huang, Y. Dai, Facile synthesis of Sdoped reduced TiO2-x with enhanced visible-light photocatalytic performance, Chinese J. Catal. 38 (2017) 821–830. L. Kong, X. Zhang, C. Wang, J. Xu, X. Du, L. Li, Ti3+ defect mediated g-C3N4/TiO2 Zscheme system for enhanced photocatalytic redox performance, Appl. Surf. Sci. 448 (2018) 288–296. Y. Zhou, Y. Liu, P. Liu, W. Zhang, M. Xing, J. Zhang, A facile approach to further improve the substitution of nitrogen into reduced TiO2−x with an enhanced photocatalytic activity, Appl. Catal. B 170–171 (2015) 66–73.
Contents lists available at ScienceDirect
Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Salt templated synthesis of NiO/TiO2 supported carbon nanosheets for photocatalytic hydrogen production
T
Xueying Zhaoa, Wenyu Xieb, Zhibo Dengc, Gan Wanga, Aihui Caoa, Huamei Chena, Bo Yanga, Zhuan Wanga, Xintai Suc,*, Chao Yanga,* a
Ministry Key Laboratory of Oil and Gas Fine Chemicals, College of the Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046, China Faculty of Environmental Science and Engineering, Guangdong University of Petrochemical Technology, Maoming 525000, PR China c The Key Laboratory of Pollution Control and Ecosystem Restoration in Industry Clusters (Ministry of Education), School of Environment and Energy, South China University of Technology, Guangzhou 510006, China b
G R A P H I C A L A B S T R A C T
NiO modified TiO2 photocatalyst was prepared by phase transfer and molten salt calcination method. The combined action of NiO as co-catalyst and C film carrier reduced TiO2 to TiO2-x, which provided stronger active center and UV response for the photocatalytic H2 evolution.
A R T I C LE I N FO
A B S T R A C T
Keywords: Defect TiO2 Co-catalyst TiO2/C Photocatalytic H2 production Molten salt method
NiO-TiO2-x/C composites for photo-catalytic hydrogen production have been prepared by a molten salt templateassisted pyrolysis of Ni/Ti-oleylamine precursors at different temperatures. X-ray diffraction (XRD) analysis shows that it is brookite structure, and X-ray photoelectron spectrum (XPS) determines the chemical state of the elements. Carbon nanosheets improve the dispersion and stability of TiO2, making the later fully close to each other. NiO facilitates the separation of photo-generated carriers and enhances the photocatalytic hydrogen production activity of TiO2. Due to the efficient synergy of carbon nanosheets, NiO, and TiO2, the optimized (1 wt%)NiO-TiO2-x/C-T650 nanocomposites show enhanced activity toward photocatalytic hydrogen production from water, outperforming 18-fold in activity higher than pristine TiO2/C. Moreover, the (1 wt%)NiO-TiO2-x/CT650 catalyst exhibits good stability without obvious decrease in activity under continuous cycling. This work will lead to the optimization of carbon-supported multi-component catalysts for photocatalytic hydrogen production.
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Corresponding authors. E-mail addresses: [email protected] (X. Su), [email protected] (C. Yang).
https://doi.org/10.1016/j.colsurfa.2019.124365 Received 12 October 2019; Received in revised form 14 December 2019; Accepted 16 December 2019 Available online 17 December 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.
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1. Introduction
solution containing 30 mL of n-hexane and 20 mL of ethanol. Subsequently, 0.0229 g of NiCl2·6H2O (M =238 g/mol, equivalent to NiO containing 0.0072 g) was dissolved in 7 g of distilled water, and added to the above solution drop by drop. In reactants, the mass ratio of NiO to TiO2 was 1 and the reaction was refluxed at 70 °C for 2 h under continuous stirring. The obtained precursor was dried at 80 °C for 24 h. Finally, the precursor was mixed with Na2SO4 (mass ratio of 1:10) and ground to form a homogeneous mixture. The mixture was annealed at 500 °C, 550 °C, 600 °C, 650 °C and 700 °C respectively for 3 h with a heating ramping rate (10 °C/min) in Ar atmosphere to obtain the productions. After being cooled, the products were washed with distilled water and ethanol and dried at 60 °C overnight. The obtained sample denoted as (1 wt%)NiO-TiO2-x/C-T500, (1 wt%)NiO-TiO2-x/C-T550, (1 wt%)NiO-TiO2-x/C-T600, (1 wt%)NiO-TiO2-x/C-T650 and (1 wt%)NiOTiO2-x/C-T700. Under the optimum temperature of 650 °C, the desired content of NiO to TiO2 in the catalyst was adjusted through changing the amount of NiCl2·6H2O. The procedure of preparing the NiO-TiO2-x/C nanocomposites with NiO to TiO2 mass ratios of 0.50 and 0.20 was the same as that of (1 wt%)NiO-TiO2-x/C-T650, added amount of NiCl2·6H2O was 0.01147 g and 0.04589 g. The obtained samples denoted as (0.5 wt%) NiO-TiO2-x/C-T650, and (2 wt%)NiO-TiO2-x/C-T650, respectively. The procedure of preparing TiO2/C was the same as that of (1 wt%) NiO-TiO2-x/C-T650 except for no addition of NiCl2·6H2O.
In recent years, the world's rapidly growing population and industrialization have led to an increasing global energy demand, forcing people to find sustainable and efficient clean energy source [1–5]. Considering that solar energy is the most available and sustainable natural energy source, the technology of photocatalytic water splitting using sunlight is considered to be an effective way to solve the energy problem in the future [6–8]. So far, titanium dioxide (TiO2), as a chemically stable, environmentally friendly, innocuity, rich and cost-effective photocatalyst, have attracted wide attention in the field of photocatalysis. However, this semiconductor only absorb UV light and exhibit low quantum efficiency [9–13]. In addition, the pure TiO2 has small specific surface area and narrow spectral absorption range, which limits its photocatalytic activity and utilization efficiency [14–16]. Therefore, it is necessary to explore a method to optimize TiO2 material structure to improve the separation efficiency and inhibit the recombination efficiency, so that the TiO2 material can catalyze water splitting reaction to produce more H2 production. It has been reported that the doping of non-metallic elements [17–19] (such as C, S, N) and the loading/deposition of noble metal nanoparticles [20–25] (such as Ag, Pt, Au) have been proved to be very effective strategies to enhance the photocatalytic activity of TiO2. However, the doping process of non-metallic elements suffers from complex manipulations under harsh conditions and uncontrollable heteroatom contents. Noble metals for the loading/deposition are facing the problems of high price and scarce sources, which restrict their applications in hydrogen production. In contrast, the incorporation of non-noble transition metal oxides, in particular of NiO as co-catalysts, have proven to provide another low-cost but effective way to enhance photocatalytic activity of TiO2 in the hydrogen production [26–29]. Previous reports have shown that the incorporation of a small amount of NiO into TiO2 could significantly suppress the recombination of photo-generated carriers, reduce the hydrogen production overpotential, and leads to an enhancement of photocatalytic activity of TiO2 [30–36]. Nevertheless, NiO/TiO2 nanoparticles tend to agglomerate into large clusters, impeding efficient photo-generated carriers transfer, which leaves a lot of room for further enhancing the photocatalytic activity of NiO/TiO2 p-n heterojunctions. Herein, we demonstrate that the incorporation of carbon nanosheets from oleylamine pyrolysis into NiO/TiO2 nanoparticles can significantly improve the dispersion and stability, therefore the photocatalytic activity of p-n heterojunction. Specifically, NiO-TiO2-x/C nanocomposites with good photocatalytic hydrogen production performance were prepared by the molten salt template-assisted pyrolysis, through adjusting the calcination temperature and the loading of NiO.
2.3. Characterization Scanning electron microscope (SEM), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM) (Tecnai F30 microscope, Holland) carried out the structure analysis of the as-prepared samples. The crystal structure of the samples was investigated by X-ray diffraction (XRD) using a Bruker D8 X-ray diffractometer (Germany) with Cu-Kα1radiation (λ =1.5406 Å). X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo XPS ESCALAB 250Xi instrument equipped with an Al Kα (1486.6 eV) X-ray source. The UV–vis diffuse reflectance spectra can obtained by a Hitachi spectrophotometer using BaSO4 as a reference. Ra`man spectrum was conducted on Bruker Senterra with 532 nm wave-length laser source. Photoluminescence (PL) spectra of the samples were measured with a fluorescence spectrophotometer (Hitachi F-4500) with the exciting wavelength of 255 nm. 2.4. Photocatalytic activity for water splitting In the process of water splitting under UV light (300 W Xeon-lamp), the photocatalytic activity of samples was determined. The reaction was carried out in a sealed circular quartz glass reactor. For each reaction, using a magnetic stirrer, the specified amount of photocatalyst (20 mg) was suspended in an aqueous methanol solution (80 mL of distilled water, 20 mL of methanol sacrificial agent) by means of a closed-gas circulation reaction cell. Before the reaction, the system was completely vacuumed to remove O2 and CO2 from the water. The amount of hydrogen production and area count of record peak were determined by gas chromatography, and gas chromatography (TCD) and Ar gas as the carrier gas.
2. Materials and methods 2.1. Materials All reagents and solvents were of analytical grade and used as received from commercial suppliers. Butyl titanate (C16H36O4Ti, 99%), ethanol and n-hexane was purchased from Tianjin Zhiyuan Chemical Reagents Co., Ltd. NiCl2·6H2O and Na2SO4 Tianjin Hedong district Hongyan reagent factory. Oleylamine (C18H35NH2, 80–90%) was supplied from Aladdin Reagents Company. Deionized water was used for all experimental steps.
3. Results and discussion 3.1. Characterization of NiO-TiO2-x/C nanocomposites The synthesis of NiO-TiO2-x/C nanocomposites involves the combination of phase transfer and molten salt template-assisted pyrolysis, as shown in Fig.1. Firstly, Ti4+ and Ni2+ species were transferred from water phase to oil phase by adsorbing or coordinating with oleylamine molecules. Then, the Ti4+/Ni2+-oleylamine precursor was uniformly coated on the surface of Na2SO4 by grinding. Thirdly, upon calcination
2.2. Synthesis methods 2.2.1. Synthesis of NiO-TiO2-x/C The procedure for preparing NiO-TiO2-x/C nanocomposites as follows. 9 mmol of butyl titanate (M =340 g/mol, equivalent to TiO2 containing 0.72 g) and 38 mmol of oleylamine were mixed in the 2
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Fig. 1. Schematic illustration of the synthesis of NiO-TiO2-x/C nanocomposites.
observed in the XRD pattern. Fig. 3(a–e) shows the SEM, TEM, SAED, HR-TEM and STEM-EDX elemental mapping images of (1 wt%)NiO-TiO2-x/C-T650, respectively. From Fig. 3(a–c), it was evidenced that the (1 wt%)NiO-TiO2-x/C-T650 sample was mainly composed of TiO2 nanoparticles doped with NiO immobilized on carbon nanosheets. EDS analysis of the chemical composition of the material also confirmed this point (see Fig. S1). The HRTEM image of (1 wt%)NiO-TiO2-x/C-T650 (Fig. 3d) presents the lattice spacing was 0.292 nm, which belongs to the (121) plane of brookite TiO2. According to the STEM-EDX elemental mapping of (1 wt %)NiO-TiO2-x/C-T650 (Fig. 3e), we can observe that the elements Ti, Ni, O and C were evenly distributed in the layer, respectively, indicated that carbon nanosheets promoted the dispersion and stability of TiO2 and NiO.
under argon atmosphere, the precursor of Ti was transformed into TiO2, and oleylamine molecules were pyrolyzed into carbon layer around TiO2 nanoparticles. Meanwhile, the pyrolysis of oleylamine molecules help TiO2 to create defect TiO2-x. In the whole synthetic process, oleylamine serves as an organic ligand in the precursor preparation process and then was converted into carbon nanosheets during the high-temperature pyrolysis. 3.2. Structure and morphology analysis The phase analysis of NiO-TiO2-x/C nanocomposites prepared by calcination is shown in Fig. 2. The XRD results show that the NiO-TiO2x/C nanocomposites in Fig. 2(a–c) was composed of amorphous structure. At the same time, Raman spectra further confirmed the existence of amorphous carbon in NiO-TiO2-x/C nanocomposites. This was due to the complete decomposition of oleylamine molecules into amorphous carbon after calcination under argon conditions [37]. The carbonization degree in Fig. 2(d–g) samples has a great relationship with the calcination temperature. The characteristic peaks corresponding to brookite in Fig. 2(d–g) were at 32.3, 34.5, 42.3, 49 and 62.5° (JCPDS 29–1360), respectively [38,39]. Due to the low doping content of NiO in the NiO-TiO2-x/C nanocomposites, no obvious absorption peak was
3.3. Spectroscopic analysis Fig. 4 exhibits the XPS spectra of the TiO2/C and (1 wt%)NiO-TiO2As shown in Fig. 4a, the fully scanned spectra can be assigned to Ti, Ni, C, and O elements. Accordingly, the Ti 2p high-resolution XPS spectra of TiO2/C and (1 wt%)NiO-TiO2-x/C-T650 can be observed in Fig. 4b. For TiO2/C, Ti 2p was composed of where two characteristic peaks at binding energies of 464.7 eV (2p1/2) and 459 eV (2p3/2), with spin-orbit splitting of Δ=Ti 2p1/2-Ti 2p3/2 = 5.7 eV associated with TiO2 [40]. The Ti 2p binding energy of (1 wt%)NiO-TiO2-x/C-T650 shifts to high binding energy, compared with the value of TiO2/C. The Ti 2p peaks were well de-convoluted into four peaks as Ti3+ 2p3/2, Ti4+ 2p3/2, Ti3+ 2p1/2, and Ti4+ 2p1/2 consecutively around 458.8, 459.3, 464.3, and 464.9 eV. The above results indicated that the presence of Ti3+ and Ti4+ species in (1 wt%)NiO-TiO2-x/C-T650. The existence of Ti3+ species can inhibit the recombination of electron-hole pairs and promote catalytic activity. The results indicated that there exist a strong interaction between NiO co-catalyst and TiO2-x nanoparticles [40–45]. Fig. 4c shows the high-resolution XPS spectrum of C 1s. the results showed that the C 1s composed of three kinds of C, including sp2 hybrid C, C-O and C = O, and the binding energies were located at 284.9, 286 and 287.9, respectively [46]. As shown in Fig. 4d, the peaks of TiO2/C at 530.7 eV, 532.9 eV and 534.5 eV correspond to lattice oxygen, Ti-OH and chemisorbed water, respectively. The O 1s scan peak of (1wt%) NiO-TiO2-x/C-T650 at 529.9 eV was the superposition of NiO and TiO2 The peak at 531.6 eV corresponds to the deficient oxygen and the peak at 533.1 eV belongs to the chemisorbed. Note that for the two samples with different conditions, each characteristic peak of (1 wt%)NiO-TiO2x/C-T650 has shift [47–52]. With respect to the XPS spectra of Ni 2p in x/C-T650.
Fig. 2. XRD patterns of different samples: (a) (1 wt%)NiO-TiO2-x/C-T500, (b) (1 wt%)NiO-TiO2-x/C-T550, (c) (1 wt%)NiO-TiO2-x/C-T600, (d) (1 wt%)NiO-TiO2x/C-T650, (e) (1 wt%)NiO-TiO2-x/C-T700, (f) (0.5 wt%)NiO-TiO2-x/C-T650, (g) (2 wt%)NiO-TiO2-x/C-T650. 3
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Fig. 3. (a, b) SEM images, (c) TEM images and SAED patterns (inset), (d) HR-TEM images and (e) STEM-EDX elemental mapping of (1 wt%)NiO-TiO2-x/C-T650.
than other NiO-TiO2-x/C nanocomposites, and BET surface analysis displayed that (1 wt%)NiO-TiO2-x/C-T650 has a higher surface area (81 m2 g−1) than TiO2/C sample (62 m2 g−1) (see Fig. S2). Under UV irradiation, (1 wt%)NiO-TiO2-x/C-T650 exhibited the highest enhanced photocatalytic activity among those samples.
Fig. 4e, the peaks at 856.5 eV (862 eV corresponds to the satellite of the Ni 2p3/2) and 873.8 eV (879 eV corresponds to the satellite of the Ni 2p1/2) the product present were mainly assigned to Ni2+ in nickel oxide [53–55]{Qi, 2019 #62}. Raman analysis was performed to reveal the graphitization of carbon in the nanocomposites. As shown in Fig. 5(a, b), two obvious vibration peaks appear at 1342 and 1589 cm−1, respectively, which can be assigned to the typical D and G peaks of carbon. In Fig. 5a, the ID/IG value of the sample increases with increasing calcination temperature, with the best degree of carbonization of (1 wt%)NiO-TiO2-x/C-T700. However, the ID/IG ratio of the sample containing different content of NiO was nearly the same (in Fig. 5b), indicating that the introduction of NiO has little effect on the graphitization degree of carbon [56,57]. UV-vis absorption spectra of NiO-TiO2-x/C nanocomposites are shown in Fig. 6(a, b). It showed that the samples all show the absorption ability of TiO2 in the UV region. Notably, the absorption of (1 wt%) NiO-TiO2-x/C-T500 was obviously highest than that of the other NiOTiO2-x/C nanocomposites in Fig. 6a, and sample (2 wt%)NiO-TiO2-x/CT650 absorption (Fig. 6b) was best than other NiO-TiO2-x/C nanocomposites. Fig. 6(c, d) shown the PL spectra of NiO-TiO2-x/C nanocomposites. In the photocatalytic process, the higher intensity of fluorescence scattering indicates the weakest catalytic activity. Conversely, Low fluorescence intensity means more efficient migration and separation of photogenerated electron-hole pairs, which provides strong evidence for efficient photocatalytic activity. As can be seen from Fig. 6(c, d), (1 wt%)NiO-TiO2-x/C-T650 has a lower PL intensity
3.4. Photocatalytic hydrogen evolution of samples Fig. 7(a, c) shows the photocatalytic hydrogen production rates of P25 and (1 wt%)NiO-TiO2-x/C-TX (X = 500, 550, 600, 650, and 700 °C) under UV light. An increase and then decrease in hydrogen production rate appears over the (1 wt%)NiO-TiO2-x/C catalysts synthesized at different temperatures. Furthermore, the contrast experiments shown in Fig. 7(b, d) reveals that the catalytic activity of the NiO-TiO2-x/C-T650 nanocomposites was dependent on the loading content of NiO. The (1 wt%)NiO-TiO2-x/C-T650 catalyst achieves the maximum hydrogen production rate of 1.55 mmol h−1 g−1. As a comparison, the commercial P25 catalyst displays a poor photocatalytic activity in the absence of any co-catalyst. Meanwhile, we compared the photocatalytic hydrogen production activity of (1 wt%)NiO-TiO2-x/C-T650 nanocomposites at different pH values (see Fig. S3). The (1 wt%)NiO-TiO2-x/ C-T650 catalyst also shows a good stability toward photocatalytic hydrogen production, as confirmed by the recycling test in Fig. S4. To evaluate interfacial charge separation efficiency, transient photocurrent responses of NiO-TiO2-x/C nanocomposite were recorded for five on-off cycles in Fig. 7(e, f). The photocurrent density of (1 wt%) 4
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Fig. 4. XPS spectra of full scan survey (a), Ti 2p (b), C 1s (c), O 1s (d) and Ni 2p (e) in the TiO2/C and (1 wt%)NiO-TiO2-x/C-T650 samples.
synergistic catalytic activity and high photocatalytic hydrogen production actively [56,58–61].
NiO-TiO2-x/C-T650 was obviously higher than that of other NiO-TiO2-x/ C nanocomposites prepared at different calcination temperature and NiO content. Combining the results from photocurrent and aforementioned PL tests, (1 wt%)NiO-TiO2-x/C-T650 also showed significant separation efficiency. Based on the above experimental results and analysis, the mechanism of enhanced photocatalytic activity of NiO-TiO2-x/C nanocomposites sample was shown in Fig. 8. When NiO was deposited on the surface of TiO2, abundant p-n heterojunction were formed at the interface. Once excited by light for NiO-TiO2-x/C nanocomposites, photogenerated electron-hole pairs were created. Subsequently, the photogenerated electrons transfer to the conduction band of n-type TiO2-x and the photogenerated holes transfer to the valence band of p-type NiO. The internal electric field formed at the p-n heterojunction interface was effectively suppress electron-hole recombination. In addition, Raman spectra showed that the carbonization degree of NiO-TiO2-x/C nanocomposites was good, and the unique properties of carbon nanosheets play an important role in improving the efficiency of electron charge transfer in the process of light irradiation. The close contact between carbon layer and TiO2-x can effectively inhibit electron-hole recombination. The electrons in the conduction band of TiO2-x can be further transferred to the carbon, while the exhausted holes by sacrificial agent (CH3OH). NiO-TiO2-x/C nanocomposite have good
4. Conclusions In summary, NiO-TiO2-x nanoparticles were grown on carbon nanosheets by a molten salt template-assisted pyrolytic method and used as catalysts for photocatalytic hydrogen production. The pyrolysis of oleylamine produced carbon nanosheets and led to the creation of defected TiO2. The resulting carbon nanosheets promoted the dispersion and stability of TiO2-x nanoparticles. The introduction of NiO into TiO2x facilitated the separation of photogenerated electron-hole pairs under UV irradiation. These combined advantages allow for NiO-TiO2-x/C composites performing as highly active catalyst for photocatalytic hydrogen production. This work offers an facile preparation route to NiO modified TiO2 supported on carbon nanosheets with high photocatalytic activity. Author contributions section Xueying Zhao, Wenyu Xie, Xintai Su and Chao Yang conceived and designed the project. Xueying Zhao conducted the preparation, characterizations and performance test of all samples. Zhibo Deng, Gan
Fig. 5. Raman Spectra of NiO-TiO2-x/C nanocomposites synthesized at (a) different temperatures and (b) different NiO content. 5
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Fig. 6. (a, b) UV–vis diffuse reflectance spectra and (c, d) PL spectra of NiO-TiO2-x/C nanocomposites.
Declaration of Competing Interest
Wang, Aihui Cao, Huamei Chen, Bo Yang, Zhuan Wang, Xintai Su and Chao Yang made discussions of the results. Xueying Zhao wrote the manuscript. Xintai Su and Chao Yang supervised the work.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Fig. 7. (a–d) Comparison of the photocatalytic H2-production activities between P25 and NiO-TiO2-x/C nanocomposites samples; (e), (f) the transient photocurrent response for NiO-TiO2-x/C production at different temperatures and ratios in 0.5 mol L−1 Na2SO4 aqueous solution under UV light irradiation, respectively. 6
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properties, Nanoscale 3 (2011) 2943–2949. [17] X. Chen, C. Burda, The electronic origin of the visible-light absorption properties of C-, N-and S-doped TiO2 nanomaterials, J. Am. Chem. Soc. 130 (2008) 5018–5019. [18] I.C. Kang, Q. Zhang, S. Yin, T. Sato, F. Saito, Improvement in photocatalytic activity of TiO2 under visible irradiation through addition of N-TiO2, Environ. Sci. Technol. 42 (2008) 3622–3626. [19] J.H. Pan, X. Zhang, A.J. Du, D.D. Sun, J.O. Leckie, Self-etching reconstruction of hierarchically mesoporous F-TiO2 hollow microspherical photocatalyst for concurrent membrane water purifications, J. Am. Chem. Soc. 130 (2008) 11256–11257. [20] M.V. Sofianou, N. Boukos, T. Vaimakis, C. Trapalis, Decoration of TiO2 anatase nanoplates with silver nanoparticles on the {101} crystal facets and their photocatalytic behaviour, Appl. Catal. B 158–159 (2014) 91–95. [21] Z. Cai, Z. Xiong, X. Lu, J. Teng, In situ gold-loaded titania photonic crystals with enhanced photocatalytic activity, J. Mater. Chem. A 2 (2014) 545–553. [22] A. Iwase, H. Kato, A. Kudo, Nanosized Au particles as an efficient cocatalyst for photocatalytic overall water splitting, Catal. Letters 108 (2006) 7–10. [23] D. Wodka, E. Bielanska, R.P. Socha, M. Elzbieciak-Wodka, J. Gurgul, P. Nowak, P. Warszynski, I. Kumakiri, Photocatalytic activity of titanium dioxide modified by silver nanoparticles, ACS Appl. Mater. Interfaces 2 (2010) 1945–1953. [24] N. Zhang, S. Liu, X. Fu, Y.-J. Xu, Synthesis of M@TiO2 (M = Au, Pd, Pt) core–shell nanocomposites with tunable photoreactivity, J. Phys. Chem. C 115 (2011) 9136–9145. [25] J. Yu, L. Qi, M. Jaroniec, Hydrogen production by photocatalytic water splitting over Pt/TiO2 nanosheets with exposed (001) facets, J. Phys. Chem. C 114 (2010) 13118–13125. [26] C.V. Reddy, I.N. Reddy, B. Akkinepally, V.V.N. Harish, K.R. Reddy, S. Jaesool, Mndoped ZrO2 nanoparticles prepared by a template-free method for electrochemical energy storage and abatement of dye degradation, Ceram. Int. 45 (2019) 15298–15306. [27] C.V. Reddy, I.N. Reddy, B. Akkinepally, K.R. Reddy, J. Shim, Synthesis and photoelectrochemical water oxidation of (Y, Cu) codoped α-Fe2O3 nanostructure photoanode, J. Alloys. Compd. 814 (2020) 152349. [28] C.V. Reddy, I.N. Reddy, K.R. Reddy, S. Jaesool, K. Yoo, Template-free synthesis of tetragonal Co-doped ZrO2 nanoparticles for applications in electrochemical energy storage and water treatment, Electrochim. Acta 317 (2019) 416–426. [29] S.A. Rawool, M.R. Pai, A.M. Banerjee, A. Arya, R.S. Ningthoujam, R. Tewari, R. Rao, B. Chalke, P. Ayyub, A.K. Tripathi, S.R. Bharadwaj, Pn Heterojunctions in NiO:TiO2 composites with type-II band alignment assisting sunlight driven photocatalytic H2 generation, Appl. Catal. B 221 (2018) 443–458. [30] J. Liu, Q. Jia, J. Long, X. Wang, Z. Gao, Q. Gu, Amorphous NiO as co-catalyst for enhanced visible-light-driven hydrogen generation over g-C3N4 photocatalyst, Appl. Catal. B 222 (2018) 35–43. [31] Z. Khan, M. Khannam, N. Vinothkumar, M. De, M. Qureshi, Hierarchical 3D NiO–CdS heteroarchitecture for efficient visible light photocatalytic hydrogen generation, J. Mater. Chem. A 22 (2012) 12090–12095. [32] T. Sreethawong, Y. Suzuki, S. Yoshikawa, Photocatalytic evolution of hydrogen over mesoporous TiO2 supported NiO photocatalyst prepared by single-step sol–gel process with surfactant template, Int. J. Hydrogen Energy 30 (2005) 1053–1062. [33] C.J. Chen, C.H. Liao, K.C. Hsu, Y.T. Wu, J.C.S. Wu, P–N junction mechanism on improved NiO/TiO2 photocatalyst, Catal. Commun. 12 (2011) 1307–1310. [34] C.C. Hu, H. Teng, Structural features of p-type semiconducting NiO as a co-catalyst for photocatalytic water splitting, J. Catal. 272 (2010) 1–8. [35] L. Li, B. Cheng, Y. Wang, J. Yu, Enhanced photocatalytic H2-production activity of bicomponent NiO/TiO2 composite nanofibers, J. Colloid Interface Sci. 449 (2015) 115–121. [36] S. Fujita, H. Kawamori, D. Honda, H. Yoshida, M. Arai, Photocatalytic hydrogen production from aqueous glycerol solution using NiO/TiO2 catalysts: effects of preparation and reaction conditions, Appl. Catal. B 181 (2016) 818–824. [37] B. Yan, J. Zhou, X. Liang, K. Song, X. Su, Facile synthesis of flake-like TiO2/C nanocomposites for photocatalytic H2 evolution under visible-light irradiation, Appl. Surf. Sci. 392 (2017) 889–896. [38] K. Haouemi, F. Touati, N. Gharbi, Characterization of a new TiO2 nanoflower prepared by the sol–gel process in a reverse microemulsion, J. Inorg. Organomet. Polym. Mater. 21 (2011) 929–936. [39] J.G. Li, C. Tang, D. Li, H. Haneda, T. Ishigaki, Monodispersed spherical particles of brookite‐type TiO2: synthesis, characterization, and photocatalytic property, J. Am. Ceram. Soc. 87 (2004) 1358–1361. [40] S. Naseem, I.V. Pinchuk, Y.K. Luo, R.K. Kawakami, S. Khan, S. Husain, W. Khan, Epitaxial growth of cobalt doped TiO2 thin films on LaAlO3(100) substrate by molecular beam epitaxy and their opto-magnetic based applications, Appl. Surf. Sci. 493 (2019) 691–702. [41] Z. Zhang, C. Shao, X. Li, C. Wang, M. Zhang, Y. Liu, Electrospun nanofibers of p-type NiO/n-type ZnO heterojunctions with enhanced photocatalytic activity, ACS Appl. Mater. Interfaces 2 (2010) 2915–2923. [42] H. Tan, Z. Zhao, M. Niu, C. Mao, D. Cao, D. Cheng, P. Feng, Z. Sun, A facile and versatile method for preparation of colored TiO2 with enhanced solar-driven photocatalytic activity, Nanoscale 6 (2014) 10216–10223. [43] J. Shao, W. Sheng, M. Wang, S. Li, J. Chen, Y. Zhang, S. Cao, In situ synthesis of carbon-doped TiO2 single-crystal nanorods with a remarkably photocatalytic efficiency, Appl. Catal. B 209 (2017) 311–319. [44] W. Zhou, W. Li, J.Q. Wang, Y. Qu, Y. Yang, Y. Xie, K. Zhang, L. Wang, H. Fu, D. Zhao, Ordered mesoporous black TiO2 as highly efficient hydrogen evolution photocatalyst, J. Am. Chem. Soc. 136 (2014) 9280-9083. [45] M. Lu, C. Shao, K. Wang, N. Lu, X. Zhang, P. Zhang, M. Zhang, X. li, Y. Liu, p-MoO3 nanostructures/n-TiO2 nanofiber heterojunctions: controlled fabrication and
Fig. 8. Schematic diagram illustrating the possible photocatalytic mechanism of the NiO-TiO2-x/C nanocomposites under xenon lamp irradiation.
Acknowledgment We appreciate the financial support of the National Natural Science Foundation of China (U1503391). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.colsurfa.2019.124365. References [1] S.S. Lee, H. Bai, Z. Liu, D.D. Sun, Green approach for photocatalytic Cu(II)-EDTA degradation over TiO2: toward environmental sustainability, Environ. Sci. Technol. 49 (2015) 2541–2548. [2] J. Zhang, Y. Wang, J. Jin, J. Zhang, L. Zhang, F. Huang, J. Yu, Efficient visible-light photocatalytic hydrogen evolution and enhanced photostability of core/shell CdS/ g-C3N4 nanowires, ACS Appl. Mater. Interfaces 5 (2013) 10317–10324. [3] A. Mishra, A. Mehta, S. Basu, N.P. Shetti, K.R. Reddy, T.M. Aminabhavi, Graphitic carbon nitride (g–C3N4)–based metal-free photocatalysts for water splitting: a review, Carbon 149 (2019) 693–721. [4] Z. Hu, Y. Mi, Y. Ji, R. Wang, W. Zhou, X. Qiu, X. Liu, Z. Fang, X. Wu, Multiplasmon modes for enhancing the photocatalytic activity of Au/Ag/Cu2O core-shell nanorods, Nanoscale 11 (2019) 16445–16454. [5] A. Kudo, Y. Miseki, Heterogeneous photocatalyst materials for water splitting, Chem. Soc. Rev. 38 (2009) 253–278. [6] K.R. Reddy, C. Venkata Reddy, M.N. Nadagouda, N.P. Shetti, S. Jaesool, T.M. Aminabhavi, Polymeric graphitic carbon nitride (g-C3N4)-based semiconducting nanostructured materials: synthesis methods, properties and photocatalytic applications, J. Environ. Manage. 238 (2019) 25–40. [7] N.L. Reddy, V.N. Rao, M. Vijayakumar, R. Santhosh, S. Anandan, M. Karthik, M.V. Shankar, K.R. Reddy, N.P. Shetti, M.N. Nadagouda, T.M. Aminabhavi, A review on frontiers in plasmonic nano-photocatalysts for hydrogen production, Int. J. Hydrogen Energy 44 (2019) 10453–10472. [8] V.N. Rao, N.L. Reddy, M.M. Kumari, P. Ravi, M. Sathish, K. Kuruvilla, V. Preethi, K.R. Reddy, N.P. Shetti, T.M. Aminabhavi, Photocatalytic recovery of H2from H2S containing wastewater: Surface and interface control of photo-excitons in Cu2S@TiO2 core-shell nanostructures, Appl. Catal. B 254 (2019) 174–185. [9] M. Ni, M.K.H. Leung, D.Y.C. Leung, K. Sumathy, A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production, Renewable Sustainable Energy Rev. 11 (2007) 401–425. [10] J. Wen, X. Li, W. Liu, Y. Fang, J. Xie, Y. Xu, Photocatalysis fundamentals and surface modification of TiO2 nanomaterials, Chinese J. Catal. 36 (2015) 2049–2070. [11] A. Mehta, A. Mishra, S. Basu, N.P. Shetti, K.R. Reddy, T.A. Saleh, T.M. Aminabhavi, Band gap tuning and surface modification of carbon dots for sustainable environmental remediation and photocatalytic hydrogen production - A review, J. Environ. Manage. 250 (2019) 109486. [12] X. Chen, S.S. Mao, Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications, Chem. Rev. 107 (2007) 2891–2959. [13] P.S. Basavarajappa, B.N.H. Seethya, N. Ganganagappa, K.B. Eshwaraswamy, R.R. Kakarla, Enhanced photocatalytic activity and biosensing of gadolinium substituted BiFeO3 nanoparticles, ChemistrySelect 3 (2018) 9025–9033. [14] D. Chen, F. Huang, Y.B. Cheng, R.A. Caruso, Mesoporous anatase TiO2 beads with high surface areas and controllable pore sizes: a superior candidate for high-performance dye-sensitized solar cells, Adv. Mater. 21 (2009) 2206–2210. [15] L.C. Sim, K.H. Leong, P. Saravanan, S. Ibrahim, Rapid thermal reduced graphene oxide/Pt–TiO2 nanotube arrays for enhanced visible-light-driven photocatalytic reduction of CO2, Appl. Surf. Sci. 358 (2015) 122–129. [16] P. Zhang, C. Shao, Z. Zhang, M. Zhang, J. Mu, Z. Guo, Y. Liu, TiO2@carbon core/ shell nanofibers: controllable preparation and enhanced visible photocatalytic
7
Colloids and Surfaces A 587 (2020) 124365
X. Zhao, et al.
[46]
[47]
[48]
[49]
[50]
[51]
[52]
[53]
enhanced photocatalytic properties, ACS Appl. Mater. Interfaces 6 (2014) 9004–9012. Z. Yang, G. Du, Q. Meng, Z. Guo, X. Yu, Z. Chen, T. Guo, R. Zeng, Synthesis of uniform TiO2@carbon composite nanofibers as anode for lithium ion batteries with enhanced electrochemical performance, J. Mater. Chem. A 22 (2012) 5848–5854. Y. Gao, J. Xu, S. Shi, H. Dong, Y. Cheng, C. Wei, X. Zhang, S. Yin, L. Li, TiO2 nanorod arrays based self-powered UV photodetector: heterojunction with NiO nanoflakes and enhanced UV photoresponse, ACS Appl. Mater. Interfaces 10 (2018) 11269–11279. C. Hu, K. Chu, Y. Zhao, W.Y. Teoh, Efficient photoelectrochemical water splitting over anodized p-type NiO porous films, ACS Appl. Mater. Interfaces 6 (2014) 18558–18568. Y. Ku, C.N. Lin, W.M. Hou, Characterization of coupled NiO/TiO2 photocatalyst for the photocatalytic reduction of Cr(VI) in aqueous solution, J. Mol. Catal. A Chem. 349 (2011) 20–27. P.K. Prajapati, H. Singh, R. Yadav, A.K. Sinha, S. Szunerits, R. Boukherroub, S.L. Jain, Core-shell Ni/NiO grafted cobalt (II) complex: an efficient inorganic nanocomposite for photocatalytic reduction of CO2 under visible light irradiation, Appl. Surf. Sci. 467–468 (2019) 370–381. T.V. Thi, A.K. Rai, J. Gim, J. Kim, High performance of Co-doped NiO nanoparticle anode material for rechargeable lithium ion batteries, J. Power Sources 292 (2015) 23–30. H. Tian, H. Fan, G. Dong, L. Ma, J. Ma, NiO/ZnO p–n heterostructures and their gas sensing properties for reduced operating temperature, RSC Adv. 6 (2016) 109091–109098. L. Bi, X. Gao, L. Zhang, D. Wang, X. Zou, T. Xie, Enhanced photocatalytic hydrogen
[54]
[55]
[56]
[57]
[58]
[59]
[60]
[61]
8
evolution of NiCoP/g-C3N4 with improved separation efficiency and charge transfer efficiency, ChemSusChem 11 (2018) 276–284. Y. Qi, J. Xu, M. Zhang, H. Lin, L. Wang, In situ metal–organic framework-derived cdoped Ni3S4/Ni2P hybrid co-catalysts for photocatalytic H2 production over g-C3N4 via dye sensitization, Int. J. Hydrogen Energy 44 (2019) 16336–16347. J. Xu, Y. Qi, C. Wang, L. Wang, NH2-MIL-101(Fe)/Ni(OH)2-derived C,N-codoped Fe2P/Ni2P cocatalyst modified g-C3N4 for enhanced photocatalytic hydrogen evolution from water splitting, Appl. Catal. B 241 (2019) 178–186. M. Wang, J. Han, Y. Hu, R. Guo, Y. Yin, Carbon-incorporated NiO/TiO2 mesoporous shells with p-n heterojunctions for efficient visible light photocatalysis, ACS Appl. Mater. Interfaces 8 (2016) 29511–29521. R. M, Š. Z, J. I.A, Ć.-M. G, A. S.P, Č. M.I, Improvements to the photocatalytic efficiency of polyaniline modified TiO2 nanoparticles, Appl. Catal. B 136 (2013) 133–139. F. Zuo, L. Wang, T. Wu, Z. Zhang, D. Borchardt, P. Feng, Self-doped Ti3+ enhanced photocatalyst for hydrogen production under visible light, J. Am. Chem. Soc. 41 (2010) 11856–11857. Z. Huang, Z. Gao, S. Gao, Q. Wang, Z. Wang, B. Huang, Y. Dai, Facile synthesis of Sdoped reduced TiO2-x with enhanced visible-light photocatalytic performance, Chinese J. Catal. 38 (2017) 821–830. L. Kong, X. Zhang, C. Wang, J. Xu, X. Du, L. Li, Ti3+ defect mediated g-C3N4/TiO2 Zscheme system for enhanced photocatalytic redox performance, Appl. Surf. Sci. 448 (2018) 288–296. Y. Zhou, Y. Liu, P. Liu, W. Zhang, M. Xing, J. Zhang, A facile approach to further improve the substitution of nitrogen into reduced TiO2−x with an enhanced photocatalytic activity, Appl. Catal. B 170–171 (2015) 66–73.