TiO2 photocatalyst for H2 evolution

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Efficient noble-metal-free Co-NG/TiO2 photocatalyst for H2 evolution: synergistic effect between single-atom Co and N-doped graphene for enhanced photocatalytic activity Lanhua Yi, Fujun Lan, Jinge Li, and Caixian Zhao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02001 • Publication Date (Web): 10 Sep 2018 Downloaded from http://pubs.acs.org on September 11, 2018

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ACS Sustainable Chemistry & Engineering

Efficient noble-metal-free Co-NG/TiO2 photocatalyst for H2 evolution: synergistic effect between single-atom Co and N-doped graphene for enhanced photocatalytic activity Lanhua Yi a,*, Fujun Lan a, Jinge Li b, Caixian Zhao b,* a

Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of

Education, School of Chemistry, Xiangtan University, Xiangtan 411105, China b

College of Chemical Engineering, Xiangtan University, Xiangtan 411105, China

∗ Corresponding author. Tel.: +86 731 58292477; fax: +86 731 58292477. E-mail address: [email protected] (L. Yi), [email protected] (C. Zhao) ABSTRACT: Photocatalytic water splitting for H2 evolution is appealing for transforming solar energy into clean chemical fuel. This technique generally requires noble metals as H2 evolution cocatalysts to facilitate efficiently cleavage of water. Herein, we report a noble-metal-free TiO2 nanobelts composite photocatalyst, which TiO2 nanobelts are supported by nitrogen-doped graphene coordinated with single Co atom (Co-NG). The results show that Co-NG is an efficient cocatalytst for the photocatalytic H2 production over TiO2. The optimal amount of Co-NG loading is found to be 3.5 wt %, showing a H2 evolution rate of 677.44 µmol h-1 g-1 under illumination of AM 1.5 G simulated sunlight, which is close to that of platinized TiO2 nanobelts and, 2.6 and 31.2 times greater than that of 3.5 wt % NG/TiO2 nanobelts composite and pure TiO2 nanobelts, respectively. The significantly improved photocatalytic activity is ascribed to 1 ACS Paragon Plus Environment

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the remarkable synergistic effect between single-atom Co and N-doped graphene, which serve as effective H+ reduction sites and efficient electrons acceptor, respectively. Besides, the Co-NG cocatalyst also effectively retards the recombination of photoinduced charge carriers and leads to a prolonged lifetime of charge pairs. This study reveals that Co-NG is an up-and-coming candidate as cocatalyst for development of cost-effective photocatalysts for efficient solar-driven H2 evolution. KEYWORDS: Cobalt single-atom; Low-cost cocatalyst; Water splitting; Photocatalysis; Synergistic effect

INTRODUCTION The progressively severe energy resources shortage and environmental deterioration brought by the combustion of fossil fuels appeal us to seek a low-cost, environment-friendly, and sustainable source of energy.1 Hydrogen is well recognized as a clean energy carrier for the future.2 Since Honda and Fujishima first reported cleavage of water over TiO2 in 1972,3 solar-driven photocatalytic water splitting has aroused increasing attention and become a fascinating pathway for low-cost and eco-friendly achievement of hydrogen by utilizing renewable sources, solar power and water. As a famous semiconductor photocatalyst, TiO2 has been extensively investigated for a myriad of photocatalytic applications due to its merits of strong oxidizing ability, superior chemical stability, high cost-effectiveness, and nontoxicity.4,5 However, the low quantum efficiency for hydrogen evolution reaction (HER) caused by the ultrafast recombination 2 ACS Paragon Plus Environment

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of charge carriers limits its application in water splitting to a large extent.6 As well known, the semiconductor photocatalytic reaction is triggered on the photocatalyst surface, where the photogenerated charge pairs transfer and many other chemical and physical processes (e.g., adsorption of reactant molecules, desorption of product molecules, etc.) happen.7 Therefore, the quantum efficiency of HER largely relies on the microstructure of photocatalysts, and considerable efforts have been devoted to engineer TiO2 semiconductor. Extensive studies have shown that in comparison with 0D and 2D materials, 1D TiO2 nanostructures (such as nanobelts, nanotubes, and nanowires) exhibit enhanced photocatalytic performance due to its unique advantages of specially fast carrier transportation and effective charge surface transfer.8-11 Therefore, it is important to fundamentally optimize the nanostructure of TiO2 for effective H2 evolution. Loading cocatalyst on TiO2 surface is another valid strategy to enhance its H2 evolution performance. An appropriate cocatalyst can accelerate electrons-holes separation, restrain charge carrier recombination, and provide assigned redox reaction active sites to promote carrier transfer and retard back reaction.12-15 Generally, precious metals like Pt,16,17 Pd,18 Ru,19,20 and Ag21 are highly effective HER catalysts. However, the costly price and natural low-abundance of those noble metals greatly limit their wide application in photocatalytic water splitting. Hence, it is extremely imperative to seek efficient and low-cost cocatalyst for the hydrogen economy goal. Recently, considering the drastic cost-reduction, super-activity and high-selectivity, single-metal-atom catalysts 3 ACS Paragon Plus Environment


(SMACs) have been generally considered as very promising materials.22-24 For example, Gao et al. have found that g-C3N4 decorated with Pd and Pt single atoms was effective in converting CO2 to formic acid or methane.25 Li et al. reported that single-atom Pt supported g-C3N4 could greatly enhance photocatalytic activity, which was 9.6 folds as high as that of Pt nanoparticles.26 Liu et al. have found that Co1-P4 single site anchored on g-C3N4 nanosheets exhibited high water splitting activity.27 However, unlike organic C3N4 photocatalysts, there is a lack of anchor atoms or groups for inorganic semiconductor materials. Thus, it is still a big challenge to coordinate metal atom on the inorganic semiconductor photocatalysts. As a novel category of 2D materials with atom-thick feature, graphene has been extensively utilized in multifarious fields because of its tremendous specific surface area, superior electric conductivity, and high electron mobility.28-30 There have been numerous literature reports regarding the improved photocatalytic performances by coupling graphene or the reduced graphene oxide (rGO) with semiconductor because graphene could act as not only an eligible supportor for semiconductor but also an efficient reservoir for electron to retard the recombination of photogenerated carrier for improved catalytic activity.31-34 For instance, Xiang et al. synthesized graphene-decorated TiO2 nanosheets composite and found the H2 evolution performance was enhanced considerably in the graphene/TiO2 nanocomposites.35 Using rGO as an efficient electron transfer channel, Li et al. found the enhanced photocatalytic H2 evolution activity of TiO2 4 ACS Paragon Plus Environment

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microspheres.36 Whereas, on atom-thick graphene sheets, there are abundant defects and remanent oxygen-containing groups, which could not only impede the transport of charge carriers but also act as the recombination center of electron and hole pairs.37 Heteroatom doping (e.g., nitrogen, sulfur, boron, phosphorus) has been proved to be a convenient strategy to modulate the electronic property and catalytic activity of graphene.38-42 Mou et al. found that TiO2 nanoparticles combined with N-doped graphene exhibited high H2 evolution performance.43 Besides, N-doped graphene coordinated with transition metal has been found to possess good activity in electrocatalytic reactions. Very recently, Fei et al. have found that nitrogen-doped graphene coordinated single-atom Co (Co-NG) could act as highly active, stable and cost-effective electrocatalyst for H2 production in both acid and base media.44 Considering the similarity between electrocatalytic and photocatalytic HER process to some extent, Co-NG may be an efficient cocatalyst for the enhancement of photocatalytic H2 evolution activity. However, to the best of our knowledge, there are few reports on Co-NG as photocatalytic HER cocatalyst. In this paper, we make attempts to use Co-NG as an effective noble-metal-free cocatalyst to retard the recombination of photogenerated electron-hole pairs. Assembling Co-NG with TiO2 nanobelts yields a series of composites, Co-NG/TiO2 nanobelts, where Co-NG can effectively promote the charge separation and H+ reduction reaction by taking advantage of NG serving as a desired substrate for anchoring Co atoms as well as an electron transfer bridge between TiO2 nanobelts semiconductor and Co single-atom. The 5 ACS Paragon Plus Environment


photocatalytic activity of this novel Co-NG/TiO2 nanobelts composites for HER was studied. Additionally, the mechanism for the enhanced photocatalytic activity was also investigated in detail.

EXPERIMENTAL METHORDS Materials. Graphite powder (99.8%), sulfuric acid (H2SO4, 95.0%), hydrogen peroxide (H2O2, 30%), sodium hydroxide (NaOH, 98.0%), sodium nitrate (NaNO3, 99.0%), hydrochloric acid (HCl, 38%) were obtained from Shanghai Sinopharm Chemical Reagent Co. Ltd. Cobalt chloride hexahydrate (CoCl2·6H2O, 99.0%) was purchased from Shanghai Aladdin Bio-Chem Technology Co. Ltd. Potassium permanganate (KMnO4, 98.0%) was supplied by Tianjin Kemiou Chemical Reagent Co. Ltd. The commercial P25 TiO2 was purchased from Degussa Corporation. All chemicals utilized in the experiments were analytical reagents. Graphene oxide (GO) was prepared from graphite powder by the improved Hummers’ method.45 Synthesis of TiO2 nanobelts. The anatase TiO2 nanobelts were obtained via an alkaline hydrothermal treatment method. In a typical preparation, 1.0 g TiO2 powder (P25, Degussa) was dispersed into 100 mL of 10 mol L-1 NaOH aqueous solution with vigorous stirring to obtain a homogeneous mixture. Subsequently, the mixture was moved to a Teflon-lined stainless steel autoclave. After subjected to hydrothermal treatment at 150 ºC for 48 h, the autoclave was cooled down to room temperature naturally. After that, the white precipitate was obtained by centrifugalizing and washing with 0.1 mol L-1 6 ACS Paragon Plus Environment

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hydrochloric acid for 3 times. The white resultant was dried in a 60 ºC oven overnight, and then calcined at 550 ºC under air atmosphere for 2 h in a tube furnace with heating ramp rate of 2 ºC min-1 to obtain the ultimate TiO2 nanobelts. Synthesis of Co-NG/TiO2 nanobelts composites. The atomic cobalt dispersed on N-doped graphene (Co-NG) was synthesized based on the method provided by Fei et al.44 Typically, GO suspension and a certain amount of CoCl2·6H2O (weight ratio Co/GO=1:135) were sonicated in water to generate the precursor solution. The aqueous suspension of GO was then freeze-dried to produce a slightly brown powder. To obtain the final Co-NG catalyst, the dried powder was calcined in gaseous NH3 to dope nitrogen into GO. The control sample of NG was synthesized with an identical treatment except for adding CoCl2·6H2O into the precursor solution. In order to precisely control the loading amount of Co-NG in Co-NG/TiO2 nanobelts composites, the Co-NG was exfoliated in deionized water by ultrasonic processing to get a metastable blackish suspension with the concentration of around 0.4 mg mL-1. Meanwhile, 50 mg TiO2 nanobelts was dispersed into 20 mL of deionized water and then a certain volume of Co-NG suspension was added drop by drop under magnetic stirring. Next, the mixture was stirred continuously for additional 5 h at room temperature. The as-prepared samples were then freeze-dried for around 48 h to produce a series of colored powders from light gray to dark black. To improve the interaction between Co-NG and TiO2 nanobelts, the dried composites were further treated by calcination at 500 ºC for 10 min under argon 7 ACS Paragon Plus Environment


atmosphere. The synthetic procedure was illustrated as Figure 1. Similarly, the composites with various mass ratio of Co-NG to TiO2 (0:1, 0.0025:1, 0.0075:1, 0.015:1, 0.0275:1, 0.035:1, and 0.05:1) were prepared according to the same approach for the control experiments. For comparison, we made platinized TiO2 nanobelts (Pt/TiO2) composites by a photodeposition method.

Figure 1. Schematic diagram for fabrication of Co-NG/TiO2 nanobelts composites. Characterization. Powder X-ray diffraction (XRD) patterns data was obtained by a Rigaku D/max-2550 diffractometer employing Cu Kα (λ=0.15418 nm) irradiation over the 2θ range of 5 to 80º at a scan rate of 0.05º s-1, the operation voltage and the current applied were 40 kV and 40 mA, respectively. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) photographs were 8 ACS Paragon Plus Environment

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collected via a JEOL-3010 electron microscope operated at voltage of 300 kV. Atomic-resolution and aberration-corrected high-angle-annular-dark-field scanning transmission electron microscopy (HAADF-STEM) characterization was conducted on a 200-KeV JEOL JEM-ARM200F instrument equipped with a spherical aberration corrector, with a sub-angstrom resolution of 0.08 nm. X-ray photoelectron spectroscopy (XPS) was obtained by using an ESCALab250Xi electron spectrometer and the binding energies were corrected by a C 1s at 284.6 eV. UV-vis diffuse reflectance spectra (DRS) of the simples were carried out on a Perkin-Elmer Lambda 950 spectrophotometer with BaSO4 as a standard reference sample. Fourier transform infrared spectroscopy (FTIR) was conducted by utilizing a Nicolet 6700 IR spectrometer over the region of 400-4000 cm-1 in transmittance mode. Based on the nitrogen adsorption isotherms in a Micromeritics ASAP 2020 system, the specific surface area (SBET) of the photocatalysts was measured by the Brunauer-Emmett-Teller (BET) method. The photoluminescence (PL) spectra of the samples were performed by a Perkin-Elmer LS55 spectrophotometer using 310 nm as excitation wavelength. Raman spectra were carried out on a Renishaw 1000NR microspectrometer with a 532 nm laser excitation. Photocatalytic and photoelectrochemical performance test. The photocatalytic H2 evolution experimentations were carried out in a quartz reactor connected to the vacuum closed circulation system (Labsolar-III AG System, Perfectlight Co. Ltd., China). A 300 W Xenon lamp (PLS-SXE 300, Perfectlight Co. Ltd., China) equipped with AM 1.5 G 9 ACS Paragon Plus Environment


filter served as the irradiation light source providing about 1 sun power density. In a typical experiment, 25 mg of the photocatalyst was diffused in 120 mL of an aqueous solution containing 50 mL ethanol homogeneously. Before illumination, the system was vacuumized for removal of the oxygen contained in the reaction solution. The concentration of hydrogen was periodically detected by online GC7890 gas chromatography with the equipment of a thermal conductivity detector as well as a 0.5 nm molecular sieve column, using N2 gas as the protective atmosphere. Photocurrents and electrochemical impedance measuring experiments were carried out via a CHI660E electrochemical workstation (Shanghai Chenhua Co. Ltd., China), using a three-electrode system with photocatalyst coated on FTO glass as the working electrode, Pt net as the counter electrode, and commercial Hg/Hg2Cl2 electrode as the reference electrode. The liquid electrolyte was 0.5 mol L-1 Na2SO4 aqueous solution, and the light source was provided by a 300 W Xenon lamp. For preparing the working electrodes, 10 mg of the photocatalyst combined with 1 mL of ethanol was sonicated, and kept stirred overnight to make slurry. Subsequently, the obtained slurry was spread onto a FTO glass substrate with 2 cm × 3 cm in size. The prepared electrodes were dried in a 60 ºC vacuum oven, and treated by calcination at 300 ºC for 30 min under Ar.

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RESULTS AND DISCUSSION Structure and morphology of Co-NG/TiO2 nanobelts composite. Figure 2 displays the XRD spectra of TiO2 nanobelts and 3.5 wt % Co-NG/TiO2 nanobelts composite. Obviously, the 3.5 wt % Co-NG/TiO2 nanobelts composite exhibits similar XRD patterns to that of TiO2 nanobelts. The peaks located at 2θ angles 25.3, 37.7, 47.9, 53.8, 55.0, 62.7, 68.6, 70.1, and 74.9º correspond to the (101), (004), (200), (105), (211), (204), (116), (220), and (215) crystal face of anatase TiO2 (JCPDS card no. 21-1272), respectively. It has to be noted that there is no additional characteristic diffraction peaks related to Co species due to its trace amount. Furthermore, no distinct diffraction peaks corresponding to graphene could be discovered. Possibly, the diffraction intensity of graphene was relatively low and shielded by the strong diffraction patterns of TiO2 nanobelts. The results reveal that the existence of Co-NG has no effect on the crystal structure of TiO2 nanobelts.



Figure 2. XRD patterns for TiO2 nanobelts and 3.5 wt % Co-NG/TiO2 nanobelts composite. The morphology and microstructure of Co-NG/TiO2 nanobelts composites were examined by TEM and HRTEM. It is clearly seen that for Co-NG/TiO2 nanobelts composites (Figure 3a and b), the synthesized TiO2 nanobelts with 20-50 nm wide and several micrometers long are well dispersed on the large single Co-NG sheets. Additionally, the as-synthesized TiO2 nanobelts are thin and almost transparent. The HRTEM images in Figure 3d and inset reveal that a perfectly crystallized nanobelt has clear lattice fringes of 0.39 nm, which belongs to the (100) facet of anatase TiO2.


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Figure 3. (a, b, and c) TEM and (d) HRTEM images for 3.5 wt % Co-NG/TiO2 nanobelts composite. To

determine the distribution

of Co single-atom

in N-doped

graphene,

aberration-corrected HAADF-STEM was carried out (Figure 4). In low-resolution HAADF-STEM image, no visible Co clusters or particles are observed (Figure 4a inset), implying that Co species may exist as well-dispersed weeny clusters or individual atoms, which are indiscernible by normal STEM technique.46 As shown in Figure 4a and b, 13 ACS Paragon Plus Environment


highly dense Co single atoms are evenly decentralized in N-doped graphene. Accounting to the Z-contrast distinction between the larger Co atoms and the other atoms, the islanded bright dots highlighted with white circles in Figure 4b are indisputably attributed to Co atoms. Figure 4c is the enlarged HAADF-STEM image in correspondence to the area of selected square in Figure 4b, the presence of individual Co atom was ulteriorly confirmed by the intensity profiles along the lines X-Y. The profiles in Figure 4d clearly display the result characteristic for isolated Co atom.27 Therefore, the small amount of cobalt salt was clearly dispersed in N-doped graphene as individual Co atoms by annealing in NH3.


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Figure 4. (a, b) Typical HAADF-STEM images of Co-NG, individual Co atoms are highlighted with white circles. The inset of (a) is the low resolution TEM image of Co-NG. (c) Enlarged HAADF-STEM image of Co-NG in correspondence to the area of selected square in (b). (d) Intensity profiles along the lines X-Y in the HAADF-STEM image (c).



Raman spectrum was conducted to study the local structure of Co-NG/TiO2 nanobelts composite. For the Raman spectrum of pristine TiO2 nanobelts (Figure S1a, Supporting Information), the representative optical modes of anatase TiO2 are distinctly observed, namely Eg (144, 196, 639 cm-1), B1g (397 cm-1), and A1g+B1g (about 517 cm-1), respectively.47,48 Significantly, the Co-NG/TiO2 nanobelts sample exhibits not only typical optical modes of anatase but also shows characteristic peaks centering at 1370 cm-1 and 1597 cm-1 assigned to D band and G band for the graphitized structures, respectively, revealing the existence of graphene in the Co-NG/TiO2 nanobelts composites. Moreover, the observed Eg peaks (145, 199 cm-1) in Co-NG/TiO2 nanobelts composite are slightly shifted to longer wavenumbers with respect to that of pristine TiO2 nanobelts, indicating an intense interaction occurred between Co-NG and TiO2.43,49 Also, the D band (1370 cm-1) and G band (1597 cm-1) in the Co-NG/TiO2 nanobelts sample are slightly blue-shift in comparison with that of NG reported in literature,50 which could be attributed to the close contact between Co-NG and TiO2 nanobelts, leading to a change of surface strain.51 Figure S1b (Supporting Information) displays the FTIR spectra of Co-NG/TiO2 nanobelts composite (curve 1) and TiO2 nanobelts (curve 2). For the spectrum of Co-NG/TiO2 nanobelts sample, the absorption below 1000 cm-1 can be assigned to the Ti-O-Ti bond vibration in TiO2, which is as like as that of the pristine TiO2 nanobelts. In addition, an absorption band at 1630 cm-1, which belongs to the vibration of graphene sheets, emerged in the spectrum of Co-NG/TiO2 nanobelts sample 16 ACS Paragon Plus Environment

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suggesting that strong interactions exist between titanium dioxide and graphene. All the results verify that TiO2 nanobelts are combinated intimately with Co-NG in the Co-NG/TiO2 nanobelts composites.52 As for XPS of Co-NG/TiO2 nanobelts composite (Figure 5), in the survey spectrum, obvious peaks at 284.6, 399.1, 458.1, and 530.1 eV corresponding to the binding energy for C 1s, N 1s, Ti 2p, and O 1s, respectively, can be evidently detected, whereas no significant peaks were observed at the Co region because of the trace amount of Co. The XPS fine spectrum of Co 2p showed two weak peaks centering at 779.3 and 794.4 eV (Figure S2a, Supporting Information), which could be assigned to the Co 2p3/2 and Co 2p1/2 levels, respectively. The energy separation of 15.1 eV between those two peaks implies the existence of Co(III).27,44 Moreover, the presence of Co element was further confirmed by SEM-EDS measurement (Figure S2b, Supporting Information). The XPS detailed scan of Ti 2p showed two peaks with the binding energies at 458.7 and 464.4 eV (Figure 5a inset), which could be ascribed to the spin-orbital splitting photoelectrons of Ti 2p3/2 and Ti 2p1/2, respectively.43,53 The chemical shift between the two Ti-bands presented typical 5.7 eV, which indicated the chemical state of Ti4+ in the Co-NG/TiO2 nanobelts composite. To study the states of carbon in the hybrid structure, the XPS fine spectrum of C 1s was collected and shown in Figure 5b. The deconvoluted peaks at binding energies of 284.6, 285.4, and 287.2 eV were assigned to C-C, sp2 C-N, and sp3 C-N bonds, respectively.54 The XPS deconvoluted spectrum of N 1s peak shown in 17 ACS Paragon Plus Environment


Figure 5c displayed four types of nitrogen, namely pyridinic and N-Co, pyrrolic, graphitic and N-oxide, which centered at binding energies of 397.8, 399.5, 400.7, and 402.0 eV, respectively.44,55 The O 1s peak of Co-NG/TiO2 nanobelts composite shown in Figure 5d at about 530.1 eV may be attributed to O in Ti-O-C bonds.48 The results indicate the successful integration between Co-NG and TiO2 nanobelts, in line with the Raman results.

Figure 5. (a) XPS whole spectrum of the Co-NG/TiO2 nanobelts composite. Inset is XPS fine spectrum of Ti 2p. (b, c, and d) XPS fine spectra of C 1s, N 1s and O 1s, respectively. The BET specific surface area of sole TiO2 nanobelts, 3.5 wt % NG/TiO2 nanobelts, and 3.5 wt % Co-NG/TiO2 nanobelts composite is 68.0, 71.7, and 73.6 m2 g-1 (Table S1, 18 ACS Paragon Plus Environment

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Supporting Information), respectively. Clearly, the addition of graphene did not change the BET surface area significantly. The optical properties of Co-NG/TiO2 nanobelts composites were investigated by UV-vis DRS (Figure 6). Bare TiO2 nanobelts display absorption edge at ~390 nm, which is in agree with the absorption characters for antanse TiO2. Compared with sole TiO2 nanobelts, all Co-NG/TiO2 nanobelts composites show a slight redshift to higher wavelength in optical absorption band edge, which is possibly caused by the interfacial interaction between Co-NG and TiO2.48 Besides, over the visible light region from 400 to 800 nm, Co-NG/TiO2 nanobelts composites exhibit a significantly enhanced absorption, which is consistent with the color variation of the samples from white to dark gray, as displayed in the inset of Figure 6. The increase of absorption in visible light is resulted from the presence of carbon in Co-NG/TiO2 nanobelts composites.56



Figure 6. The UV-vis DRS of pure TiO2 nanobelts and Co-NG/TiO2 nanobelts composites. Inset is the photographs of Co-NG/TiO2 nanobelts samples with different Co-NG content. Improved photocatalytic H2 evolution performance. The photocatalytic H2 evolution performances were evaluated under the irradiation of AM 1.5 G simulated sunlight utilizing ethanol as sacrificial reagent (Figure 7). The sole TiO2 nanobelts showed negligible photocatalytic H2 evolution activity (21.71 µmol h-1 g-1) because of the ultrafast recombination of photogenerated charge carriers. Loading precious metal Pt as cocatalyst, platinized TiO2 nanobelts (1.0 wt % Pt/TiO2) gave a H2 evolution rate of 741.54 µmol h-1 g-1, and the photocatalytic activities of Pt/TiO2 with various Pt loading contents were shown in Figure S3 (Supporting Information). Surprisingly, under the same condition, 3.5 wt % Co-NG/TiO2 nanobelts exhibited a remarkable improvement in 20 ACS Paragon Plus Environment

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photocatalytic performance. The H2 evolution rate was as high as 677.44 µmol h-1 g-1, which was comparable with that of platinized TiO2 nanobelts and, 2.6 and 31.2 times higher than the rate of 3.5 wt % NG/TiO2 nanobelts composite and sole TiO2 nanobelts, respectively. Furthermore, in four successive photocatalytic reaction cycles, no noticeable decrease of the photocatalytic activity could be observed (Figure S4, Supporting Information), indicating the Co-NG/TiO2 nanobelts composites were durable for renewable solar-driven hydrogen production. All these significant results suggest that Co-NG can be a cost-effective and promising cocatalyst for H2 evolution. The effect of amount of Co-NG loading on the photocatalytic activity of composite photocatalysts was investigated, and the results are shown in Figure S5 (Supporting Information). Compared with sole TiO2 nanobelts, Co-NG/TiO2 nanobelts composites exhibited obvious enhancement of photocatalytic H2 evolution activities because of simultaneously improved visible-light absorption, accelerated charge separation and appreciated H2 evolution reaction sites. According to the photocatalytic H2 production activities, the optimal loading amount of Co-NG was 3.5 wt %, a H2 evolution rate as high as 677.44 µmol h-1 g-1 obtained. However, a further increase of Co-NG content, the photocatalytic activity of Co-NG/TiO2 composite would decrease. This is due to lower content of Co-NG result in insufficiently support TiO2 nanobelts and relatively less active sites for H2 production, whereas excess amount of Co-NG reduce incident light reaching to the semiconductor surface because of the “shielding effect”.1,29 21 ACS Paragon Plus Environment


Additionally, the Co content in Co-NG has an obvious influence on the photocatalytic H2 evolution performance of Co-NG/TiO2 nanobelts composites, and the results had been shown in Figure S6 (Supporting Information). From which, it can be seen that the H2 evolution increases with increase of Co content in Co-NG, because more Co single atoms can act as H2 evolution reaction active sites. However, when the amount of Co in Co-NG exceeds 2.47 wt %, the solar-driven H2 evolution rate decrease, resulting from the aggregation of Co atoms.44

Figure 7. (a) Photocatalytic H2 evolution over TiO2 nanobelts, 3.5 wt % NG/TiO2 nanobelts, 3.5 wt % Co-NG/TiO2 nanobelts, and 1.0 wt % Pt/TiO2 nanobelts composites in ethanol aqueous solution with suspended photocatalysts (0.025 g) under AM 1.5 G simulated sunlight irradiation. (b) Corresponding H2 evolution rate of TiO2 nanobelts, 3.5 wt % NG/TiO2 nanobelts, 3.5 wt % Co-NG/TiO2 nanobelts, and 1.0 wt % Pt/TiO2 nanobelts composites. Mechanism of improved photocatalytic performance. Efficient charge separation is pivotal in achieving high photocatalytic activity. As expected, the recombination of 22 ACS Paragon Plus Environment

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photoinduced hole-electron pairs is dramatically suppressed on 3.5 wt % Co-NG/TiO2 nanobelts composite, as confirmed by PL spectroscopy. Figure 8 displays the room-temperature PL spectra of TiO2 nanobelts, 3.5 wt % NG/TiO2 nanobelts, and 3.5 wt % Co-NG/TiO2 nanobelts composites in the wavelength region ranging from 340 nm to 600 nm under excitation at 310 nm. It is noteworthy that the intensity of the peaks belonging to 3.5 wt % Co-NG/TiO2 nanobelts composite decreases dramatically than that of pure TiO2 nanobelts and 3.5 wt % NG/TiO2 nanobelts composite. Generally speaking, the weaker PL emission intensity of the peaks indicates the lower recombination rate of photogenerated hole-electron pairs, and therefore the enhanced photocatalytic performance.57 Hence, the almost quenched PL intensity implies 3.5 wt % Co-NG/TiO2 nanobelts composite owns the longest lifespan of charge carriers and the highest separation efficiency of photoinduced charge pairs among the three samples, which is due to the photogenerated electrons rapidly transfer from TiO2 to Co atoms through NG as “electron high way” and therefore resulting a spatially separated electrons and holes between the single Co site and TiO2 nanobelts.



Figure 8. Steady-state fluorescence spectra of pure TiO2 nanobelts, 3.5 wt % NG/TiO2 nanobelts, and 3.5 wt % Co-NG/TiO2 nanobelts composites. To further confirm the improvement of charge separation efficiency, photocurrent and electrochemical impedance spectroscopy (EIS) measuring experiments were carried out. The transient photocurrent response of pristine TiO2 nanobelts, 3.5 wt % NG/TiO2 nanobelts, and 3.5 wt % Co-NG/TiO2 nanobelts composite electrodes was collected at 0.2 V versus saturated Hg/Hg2Cl2 electrode. As shown in Figure 9, a fast and quick photocurrent response was obtained for each simulated sun-light switched on and off round in the three electrodes. As expected, 3.5 wt % Co-NG/TiO2 nanobelts composite has the best optoelectronic properties among the three samples. The photocurrent density of 3.5 wt % Co-NG/TiO2 nanobelts composite was 83.8 µA cm-2, which was two times higher than that of TiO2 nanobelts (27.8 µA cm-2) and about two folds as high as that of 24 ACS Paragon Plus Environment

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3.5 wt % NG/TiO2 nanobelts composite (43.3 µA cm-2), suggesting that a considerable improvement of photo-generated charge separation.

Figure 9. Photocurrent response of TiO2 nanobelts, 3.5 wt % NG/TiO2 nanobelts, and 3.5 wt % Co-NG/TiO2 composites. EIS is usually applied to study the interfacial charge transfer rate and the separation efficiency of photoinduced hole-electron pairs.58 As Figure 10 shows, on the EIS Nyquist plot, the arc radius of 3.5 wt % Co-NG/TiO2 nanobelts composite is the smallest among the three electrodes no matter with or without AM 1.5 G simulated sunlight irradiation. The radius of arc reflects the rate of reaction occurring at the interface.43,56 Therefore, the smallest diameter of the semicircles for 3.5 wt % Co-NG/TiO2 nanobelts composite implies the fastest interfacial charge transfer to electron acceptor and the most efficient



separation of photoinduced charge carriers among the three samples. All the results agree very well with the photocatalytic water splitting activity.

Figure 10. EIS Nyquist plots of TiO2 nanobelts, 3.5 wt % NG/TiO2 nanobelts, and 3.5 wt % Co-NG/TiO2 nanobelts composites (a) without and (b) with AM 1.5 G simulated sunlight irradiation. Based on the above discussion, a plausible mechanism accounting for the enhanced photocatalytic performance of Co-NG/TiO2 nanobelts composites can be devised (Figure 11). Under the irradiation of simulated sunlight, the electrons in the valence band (VB) of TiO2 are excited and jump to the conduction band (CB), leaving holes in the VB. Due to the Fermi energy level of graphene (-0.08 V vs. NHE) is lower than the CB of TiO2 (-0.39 V vs. NHE),58,59 the transportation of photogenerated electrons from the CB of TiO2 to Co-NG is energetically favorable. Additionally, NG with huge specific surface area and excellent electrical conductivity, contacts intimately with TiO2, works as “freeway” for electron transportation, which can deliver smoothly electron from semiconductor TiO2 to Co single-atom. Furthermore, the isolated and uniformly 26 ACS Paragon Plus Environment

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dispersed Co atoms in NG can trap electrons and act as highly active sites catalyzing H+ reduction to form H2 because a lower overpotential needed for Co-NG compared with NG.44,60 Therefore, the presence of Co-NG cocatalyst in Co-NG/TiO2 composite can simultaneously facilitate effectively the separation of charge carriers, accelerate electron transport, and boost H+ reduction reaction, as evidenced by the significantly decreased PL intensity (Figure 8), enhanced photocurrent (Figure 9), and reduced arc radius of the Nyquist plot for the Co-NG/TiO2 composite (Figure 10). In summary, the notably improved photocatalytic H2 evolution activity over Co-NG/TiO2 nanobelts composite is attributed to the remarkable synergetic effect between single-atom Co and N-doped graphene.

Figure 11. (a) Schematic diagram of photocharge transfer in the Co-NG/TiO2 nanobelts composites irradiated by simulated sunlight. (b) The proposed mechanism accounting for the improved electron transfer in the Co-NG/TiO2 system for H2 evolution under simulated sunlight irradiation.



CONCLUSIONS In summary, Co-NG/TiO2 nanobelts composites were fabricated successfully by dispersing non-precious, earth-abundant metal cobalt on N-doped graphene as cocatalyst to support TiO2 nanobelts. The as-prepared Co-NG/TiO2 nanobelts composites have been demonstrated to be efficient and stable photocatalysts for H2 evolution. Under identical conditions, the maximum H2 evolution rate for the composite photocatalyst containing 3.5 wt % Co-NG is as high as 677.44 µmol h-1 g-1, which approaches to that of Pt cocatalyst, and exceeds that of the sole TiO2 nanobelts and 3.5 wt % NG/TiO2 nanobelts composite by 31.2 and 2.6 times, respectively. The improved photocatalytic activity of Co-NG/TiO2 nanobelts is believed to arise from the synergistic effect between single-atom Co and N-doped graphene, which results in an improved charge separation efficiency, a prolonged carrier lifespan, and an accelerated surface transfer reaction of electron-hole pairs simultaneously. Therefore, the Co-NG/TiO2 nanobelts composite is a promising photocatalyst for efficient solar-driven H2 production.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Raman and FTIR spectra of TiO2, and Co-NG/TiO2 composite. XPS fine spectrum of Co 2p, and SEM image of Co-NG. Photocatalytic H2 evolution activity of Pt/TiO2 samples. 28 ACS Paragon Plus Environment

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The stability of Co-NG/TiO2 composite photocatalyst. The effects of Co-NG loading amount, and Co content in Co-NG on the photocatalytic performance of Co-NG/TiO2 composites. BET data of different samples.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (21878254, 21875203, U1462121, 21203161), the Natural Science Foundation of Hunan Province (2016JJ2128), the Scientific Research Fund of Hunan Provincial Education Department (17B254), the Collaborative Innovation Center of New Chemical Technologies for Environmental Benignity and Efficient Resource Utilization.

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TOC graphic:

Synopsis: The synthesized Co-NG/TiO2 nanobelts composites exhibit remarkably enhanced photocatalytic performance for water splitting.


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TiO2 photocatalyst for H2 evolution

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