CoO Nanocomposites with Significantly Enhanced

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Novel g-CN/CoO Nanocomposites with Significantly Enhanced Visible-Light Photocatalytic Activity for H Evolution 2

Zhiyong Mao, Jingjing Chen, Yanfang Yang, Dajian Wang, Li-Jian Bie, and Bradley D. Fahlman ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 22 Mar 2017 Downloaded from http://pubs.acs.org on March 24, 2017

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ACS Applied Materials & Interfaces

Novel g-C3N4/CoO Nanocomposites with Significantly Enhanced Visible-Light Photocatalytic Activity for H2 Evolution Zhiyong Mao,*, , ⊥ Jingjing Chen, , ⊥ Yanfang Yang, Dajian Wang,*, Lijian Bie, and Bradley D. § Fahlman †

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School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, PR. China.

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Tianjin Key Laboratory for Photoelectric Materials and Devices, Tianjin University of Technology, Tianjin 300384, PR. China. §

Department of Chemistry & Biochemistry and Science of Advanced Materials Program, Central Michigan University, Mount Pleasant, MI USA 48859. ABSTRACT: Novel g-C3N4/CoO nanocomposites application for photocatalytic H2 evolution were designed and fab-

ricated for the first time in this work. The structure and morphology of g-C3N4/CoO were investigated by a wide range of characterization methods. The obtained g-C3N4/CoO composites exhibited more efficiency utilization of solar energy than pure g-C3N4, resulting in higher photocatalytic activity for H2 evolution. The optimum photoactivity in H2 evolution under visible light irradiation for g-C3N4/CoO composites with a CoO mass content of 0.5 wt% -1 -1 -1 -1 (651.3 µmol h g ) was up to 3 times as high as that of pure g-C3N4 (220.16 µmol h g ). The remarkably increased photocatalytic performance of g-C3N4/CoO composites was mainly attributed to the synergistic effect of the junction/interface formed between g-C3N4 and CoO. KEYWORDS: Photocatalyst, g-C3N4/CoO nanocomposite, Visible light, H2 evolution, Heterostructure, Cocatalyst ing the solar light absorption, promoting the charge separation and transportation, lowering the redox overpotential.17 Consequently, it has been stimulated enormous exploration to synthesize g-C3N4-based composite photocatalysts. The construction of heterojunction nanohybirds has elicited a lot of attetions in the field of g-C3N4-based composite photocatalysis. The formed heterojunction between semiconductor A and semiconductor B can direct impel the photogenerated electrons and holes to transfer in opposite directions, beneficial to enhance the charge separation efficiency resulted in increasing photocatalytic performance.18-19 Many important and interesting findings on the g-C3N4-based heterojunction photocatalysts have been widely reported. 20-29 In Addition, the employment of cocatalysts, including platinum group metals or their oxides or MS2 (M= Mo, W), can result in an increase in overall photocatalytic performances, including activity, selectivity, and stability of g-C3N4. 30

INTRODUCTION To address energy crisis and environmental deterioration, water splitting into hydrogen utilizing solar energy has emerged as a hot research area. The traditional TiO2 is the most widely used semiconductor, but the large band gap and the low solar energy utilization efficiency have enormously restricted practical application.1-3 Very recently, the design of visible-light-responsive photocatalysts is vastly pursued by researchers.4-7 Polymeric graphitic carbon nitride (g-C3N4), with a graphitic π-conjugated stacked structure consisting of tri-s-triazine repeating units, process many advantages such as good physicochemical stability, and an appealing adjustable electronic properties combined with a medium band gap (Eg~2.7 eV).7-8 These unique merits make g-C3N4 serve as a promising candidate for visible-light responsive photocatalytic application. However, pristine g-C3N4 still suffers from some obstacles, such as high charge carries recombination rate, small specific surface area, low electrical conductivity, as well as low visible light utilization efficiency. Several approaches, including synthesis techniques,8 nanostructure design,9-12 electronic structure modulation,13-16 have been systematically performed to optimize the photoactivity. Apart from above mentioned methods, g-C3N4-based composite photocatalysts demonstrate a great potential to promote the photoactivity for practical application, owing to the potential merits in strengthen-

Various metal oxides semiconductors, such as TiO2 (Eg~3.2 eV), ZnO (Eg~3.2 eV), and WO3 (Eg~2.6-2.8 eV), have been extensively and preferentially acted as a cocatalyst for constructing g-C3N4 based composites to enhance the photoactivity and the utilization of solar energy. Yan et al.15 demonstrated TiO2/g-C3N4 heterojunction by a one-step hydrothermal process using bulk g-C3N4 and titanium tetrachloride as the precursors, accompanied by a remarkable enhancement of photocatalytic capability in hydrogen evolution. Kumar et al.23 reported the synthesis

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of N-doped ZnO/g-C3N4 hybrid core-shell nanoplates via a facile, cost-effective ultrasonic dispersion method. As a result, the band gap in the hybrid nanocomposites were remarkably reduced, thereby enhanced the utilization of solar energy resulted in improved visible-light photodegradation of Rhodamine B (RhB). Hou et al. 30 disclosed that the H2 production performance of mesoporous gC3N4 under visible light could be significantly improved by loading WS2 cocatalyst. Huang et al.31 obtained WO3/gC3N4 composites for the application in degradation of methylene blue, showing the highly photocatalytic performance with respect to bare g-C3N4 and WO3.

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Industrial Corporation (Shanghai, China), and were used without purification. Firstly, g-C3N4 powder was prepared by the polycondensation of urea as reported in our previous work.37 Typically, 10 g urea was put into a covered crucible and calcined in a muffle furnace at 550 ℃ for 6 h using a heating rate of 2.5 ℃/min and cooling naturally to room temperature. Then, 0.50 g g-C3N4 powder was dispersed in 50 mL of ethanol and stirred 1 h to form homogeneous solution, followed by addition suitable amount of cobalt acetate. Subsequently, the prepared solution was dried in oven at 80℃. Finally, the dried powder was sintered at 400 ℃ for 4h in an argon atmosphere, with a heating rate of 10 ℃/min and cooling naturally to room temperature. In the experiment, the mass ratio of CoO in g-C3N4/CoO composite is tuned from 0.125, 0.25, 0.5, 1.0, 10 wt%, respectively. As a comparison, pure CoO nanoparticles were synthesized by the similar processes without the presence of g-C3N4 powder.

Apart from the above precedents, earth-abundant cobalt oxides were also employed to construct g-C3N4 composites for enhancement of catalyst performance. Such as, Zhang et al. demonstrated the increasing oxygen evolution reaction rates by loading flower-like Co(OH)2/Co3O4 onto g-C3N4.32 Lee et al. assembled cobalt-oxidephosphate (CoPi) on the surface of mesoporous g-C3N4, resulting in enhanced photocatalytic activity in water oxidation.33 CoO nanostructure with character of visiblelight responsive, shows strong photocatalytic ability to decompose water into H2 and O2. Liao et al.34 exploited an efficient CoO nanocrystalline photocatalyst to drive water-splitting with a solar-to-hydrogen efficiency of around 5%. Nevertheless, there are only a very few papers focusing on the investigation of CoO photocatalyst. One of these is above mentioned CoO nanocrystalline synthesized via CoO micropowders ablated by femtosecond laser pulses;34 Another paper reported CoO nanowire arrays via a facile hydrothermal and subsequent annealing method for efficient water-splitting, with the advantages of easy fabrication, recyclability, and high stability.35 The reason for this is that CoO photocatalystis easily deactivated after a short time reaction, arising from the seriously aggregation of the CoO nanoparticles. Alternatively, compositing CoO nanoparticles with other suitable semiconductors might be an available route to repulse its aggregation. For example, Jin et al. developed a novel graphene-supported g-C3N4/CoO core-shell hybrid electrocatalyst, which exhibited outstanding electrocatalytic activity, long-term stability and excellent resistance.36 Thus, further efforts are needed to be paid on the exploitation of CoO-based photocatalyst. Especially, there has no works reported the fabrication of g-C3N4/CoO composites for H2-evoluted from water under visible-light radiation.

Sample characterization. The crystalline structure of the prepared samples was analyzed by an X-ray diffractometer (XRD) (Rigaku, RINT Ultima-III, Japan) with CuKα radiation. The morphology and elemental analysis was measured by a scanning electron microscope (SEM) equipped with energy-dispersive spectroscopy (EDS) (Hitachi, S-4800), and a transmission electron microscope (TEM, JEM-6700F, Japan). X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy (Thermo scientific, ESCALAB 250Xi, America) was used to characterize the chemical speciation and the redox potential of samples. The chemical bonding status was assessed by Fourier transform infrared spectroscopy (FTIR, Bruker, WQE-410). Ultraviolet-visible (UV-Vis) absorption spectra were measured by a UV-Visible spectrometer (TU-1901, China) with a 60-mm diameter integration sphere. Photoluminescence spectra (PL) were measured using a fluorescence spectrometer (Hitachi F-4600, Japan). Specific surface area measurements were taken using the BET method (N2 absorption, Kangta AUTOSORB-1, America). Electrochemistry impedance spectroscopy (EIS) and photoelectric current (PC) response measurements were performed on an electrochemical workstation (CHI600E, China) based on a conventional threeelectrode system with the as-prepared photocatalyst and PVDF (mass ratio 3:1) coated on FTO glass as a working electrode, platinum foil as a counter electrode, and saturated calomel electrode (SCE) as a reference electrode, respectively. The electrode was immersed in 1 M Na2SO4 aqueous solution, the frequency range was from 0.01 Hz to 100 kHz, and the amplitude of the applied sine wave potential in each case was 5 mV for the EIS measurements. The photocurrent was measured at a bias voltage of 0.02 V under chopped illumination with 40 s light on/off cycles. Incident light was obtained from a 300 W xenon lamp (PLS-SXE 300C, Perfectlight, Beijing).

In this work, g-C3N4/CoO nanocomposite photocatalyst were fabricated using a facile thermal annealing route under an argon atmosphere for the first time. The phase structure, chemical composition, morphology, and photocatalytic performance for H2 generation were investigated in detail. A comprehensive comparison between pure gC3N4 and g-C3N4/CoO composite photocatalysts also provided to discuss the enhancement mechanism of photocatalytic activity for as-prepared g-C3N4/CoO composite.

Photocatalytic H2 generation testing. Photocatalytic hydrogen production studies were carried out in a Pyrex top-irradiation reaction vessel connected to a closed glass gas-circulation system (Lab-Solar-Ⅲ , AG, Perfectlight, Beijing). A 300-W xenon lamp (PLS-SXE 300C,

EXPERIMENTAL SECTION Sample preparation. Urea, triethanolamine (TEOA), and cobalt acetate were purchased from Aladdin


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Perfectlight, Beijing) with a 400-nm cut-off filter was chosen as a visible light source, and the light intensity was 100 mW/cm2 (determined by PL-MW 2000, Perfectlight, Beijing). 100 mg of photocatalyst was added to an aqueous solution that contained 90 mL water and 10 mL triethanolamine. Then, 3 wt% of Pt nanoparticles were loaded onto the surface of the catalystby insitu photodeposition using H2PtCl6.6H2O as the precursor. Next, the reactant solution was evacuated several times to remove air prior to the irradiation experiments. The temperature of the reaction solution was maintained at 7 ℃ by a flow of cooling water during the photocatalytic reaction. The evolved gases were analyzed by gas chromatography (GC 7900, Shanghai) equipped with a thermal conductivity detector (TCD) and a 5 Å molecular sieve column, using argon as the carrier gas.

To investigate the chemical state of the the as-synthesized pure g-C3N4, CoO and g-C3N4/CoO nanocomposites, FTIR analysis was performed as shown in Fig. 2. The broad absorption peak 3000-3600 cm-1 is assigned to the stretching vibrational modes of N-H or O-H group. For pure g-C3N4, the skeletal vibrations in the region 12001700 cm-1 for aromatic CN heterocycles and the breathing vibration at 810 cm-1 for triazine units can be observed distinctly. Considering a very limited amount of CoO, no obvious change for the characteristic absorption peaks of g-C3N4 can be detected, revealing the major chemical structures of g-C3N4 was retained in g-C3N4/CoO composites, consistent with the result of XRD patterns.

RESULTS AND DISCUSSION The crystal structure and the phase component for assynthesized g-C3N4/CoO samples were studied by XRD. As shown in Fig. 1(a), pure g-C3N4 gives two typical diffraction peaks at 2θ=27.4ºand 13.1º, which are ascribed to the layer stacking of conjugated aromatic rings, and the in-plane trigonal nitrogen linkage of tri-s-triazine motifs on g-C3N4, respectively.7 For g-C3N4/CoO composites, the intrisic diffraction peaks of g-C3N4 were observed clearly. This indicated that these composites maitain the basic structure of g-C3N4. Compared to pure g-C3N4, diffraction files of g-C3N4/CoO composites displayed an unconspicuous reduced intensity. This may be attributed to the reduced crystallinity of g-C3N4 during the posttreatment with argon gas. As the composite content of CoO increased to 10 wt%, obviously diffraction peaks (marked by ♦) pertaining to crystalline CoO were detected. These peaks were well agreeable with the diffraction pattern of pure CoO (Fig. S1). The characteristic XRD peaks of both g-C3N4 and CoO in XRD profile revealed the successful fabrication of g-C3N4/CoO composites. No CoO diffraction peaks could be discerned for other composite samples, owing to the low content of CoO.

Fig. 2 FTIR of pure g-C3N4, CoO and g-C3N4/CoO composites with variable CoO mass contents (0.125, 0.25, 0.5, 1.0, 10 wt%).

The morphologies of pure g-C3N4 and g-C3N4/CoO composites were investigated by SEM and TEM images, as shown in Fig. 3. Fig. 3(a) and 3(b) showed the SEM images of pure g-C3N4 and g-C3N4/CoO-0.125 wt%, respectively. We could find that the obtained g-C3N4-based samples are cotton-like particles aggregated by irregular nanosheets with thickness of 30-60 nm. A large number of pores with nanoscale diameters were observed in the nanosheets. EDS element anlysis (Table S1) revealed that pure g-C3N4 is dominant composed of C and N elements, accomparing with slight of O element. Comparing with pure g-C3N4, the presence of Co element and the increased atomic percentage of O element for gC3N4/CoO-0.125 wt% implied the formation of gC3N4/CoO nanocomposites. Furtherly, TEM and HRTEM images (Fig. 3(c)-(f)) were performed to show the overview of g-C3N4/CoO composites. As shown in Fig. 3(e), CoO nanoparticles are uniformly deposited on the surface of g-C3N4 nanosheets, illustrating the existence of junction/interface between g-C3N4 and CoO. The lattice fringes (Fig. 3(f)), allowing identification of crystallographic sapaceing of d = 0.24 nm, matches well with the (111) plane of CoO. High magnification image (inset in Fig. 3(e)) reveals that the CoO nanoparticle is aggregated by abundant nanocrystals with size of 30-50 nm, which is smaller than that of the pure CoO (~150 nm) (Fig. S2).

Fig. 1 XRD patterns of g-C3N4 and g-C3N4/CoO composites with variable CoO contents (0.125, 0.25, 0.5, 1.0, 10 wt%).



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Fig. 3 SEM images of (a) pure g-C3N4; and (b) g-C3N4/CoO-0.125 wt% samples; TEM image (c) and HRTEM image (d) of pure gC3N4 sample; TEM image (e) and HRTEM image (f) of g-C3N4/CoO-10 wt% composite sample.

To further investigate the surface microstructure of CoO nanoparticles on the g-C3N4 nanosheets, X-ray photoelectron spectroscopy (XPS) was performed, as shown in Fig. 4(a)-(d). There are three sharp peaks at 286 eV, 398 eV, and 530 eV in pure g-C3N4 and g-C3N4/CoO-10 wt% composite samples, which are assigned to C 1s, N 1s, and O 1s signals, respectively. Comparing with the pure gC3N4 sample, a new peak positioned at 782 eV is observed obviously for g-C3N4/CoO-10 wt% composite sample, which is attributed to the presence of Co atoms. Notably, the peak of O 1s in g-C3N4/CoO-10 wt% is an obvious stronger than that of the pure g-C3N4. Quantitative analysis from XPS spectra reveals that the oxygen content in pure g-C3N4 and g-C3N4/CoO-10 wt% is 3.54 at.% and 8.20 at.% respectively. This reveals the introduciton of a higher density of oxygen-containing species on the pure g-C3N4 surface. From high-resolution XPS analysis, the Co 2p peak (Fig. 4(b)) can be deconvoluted into two pairs of individual peaks centered at 780.3 eV and 796.2 eV, respectively, which are identified as the major binding energies of Co2+ in CoO, suggesting the presence of CoO in g-C3N4/CoO composites.34 The C 1s spectrum of pure gC3N4 sample (Fig. 4(c)) can be fitted by Gaussian curves with dominant components centered at 282.6 and 286.0 eV, which are respectively attributed to C=C and N-C=N bonds. The N 1s peak of pure g-C3N4 sample (Fig. 4(d)) can be deconvoluted into three peaks centered at 396.2, 397.2 and 398.3 eV, corresponding to C=N-C, N-[C]3, and C-NHx bonds, respectively. Compared with the pure gC3N4 sample, the spectra of C 1s and N 1s in g-C3N4/CoO10 wt% composite sample both have a binding energy shift, which may be caused by the interaction between CoO nanoparticles and g-C3N4 nanosheets. Therefore, with the combination of the XRD, FTIR, SEM-EDS, TEM,

and XPS investigation, these results confirmed that there were both CoO and g-C3N4 species in the composites structure. The pore structure and surface area of the prepared samples were obtained from N2 absorption-desorption mesurements. As shown in Fig. 5, pure g-C3N4 and gC3N4/CoO-0.5 wt% composite samples exhibit a type-Ⅳ isotherm, suggesting the presence of large mesopores and small macropores in these materials. The specific surface area of pure g-C3N4 and g-C3N4/CoO-0.5 wt% composite samples was calculated to 58.5 m2 g-1 and 111.0 m2 g-1, respectively. The larger surface area for g-C3N4/CoO-0.5 wt% composite sample was attributed to thermal exfoliation of g-C3N4 into thiner nanosheets during the compositing process. In addition, the presence of CoO nanoparticles on g-C3N4 nanosheets might be another reason for the increased surface area. It was well recognized that the large surface area contributes to absorb more active species and reactants on its surface, which is beneficial to effectively promote the photocatalytic reaction. Thus, it was assumed that the gC3N4/CoO composite sample may display more efficiency photocatalytic activity than pure g-C3N4 sample. The pore size distributions (the inset of Fig. 5) of g-C3N4/CoO-0.5 wt% composite sample are very broad (15-80 nm), compared with that of pure g-C3N4 sample (20-60 nm), arising from gases released at elevated temperatures. However, smaller pore volume (0.27 cc g-1) was recorded for the g-C3N4/CoO-0.5 wt% composite than pure g-C3N4 (0.33 cc g-1) at P/P0=0.99. This might be ascirbed to the tamping of pores by the loading CoO nanoparticles on gC3N4 nanosheets.


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Fig. 4(a) XPS survey spectra for pure g-C3N4 and g-C3N4/CoO-10 wt% composite samples; (b) high resolution of Co 2p for gC3N4/CoO-10 wt% sample; (c) high resolution C 1s; and (d) high resolution N 1s for pure g-C3N4 and g-C3N4/CoO-10 wt% composite samples.

Fig. 6(a), one could see that the pure g-C3N4 absorbs lights from the UV through the visible range up to 450 nm, corresponding to the intrisic absorption from valence to conduction band. As compared with g-C3N4, UV-vis spectra indicates that the absorption edge of g-C3N4/CoO composite dispplays a remarkable red shift (~470 to ~500 nm) with the CoO content increased, due to the interaction between CoO and g-C3N4 in the composites. Especially, the light absorption in the visible light region (450 nm to 800) was enhanced significantly. The enhanced visible light absorption indicated that the composite phocatalysts could utilize more visible light, which will be favorable for a photocatalytic reaction. The photoluminescence (PL) spectra for as-obtained g-C3N4 and g-C3N4/CoO composites under an excitation wavelength of 375 nm were demonstrated in Fig. 6(b). Pure g-C3N4 shows a strong intrinsic emission band with a peak at 470 nm. The optical energy of this emission band was close to the bandgap energy of g-C3N4, indicating that this intrisic emission is attributed to direct electron-hole recombination of band transition. Compared with pure gC3N4, g-C3N4/CoO composite shows much weaker emission profile with the compositing CoO content increasing. This is a good illustration that the recombination rate of the photogenerated charge carrier is enormously restrained due to the development of an adaptive junction/interface structure between g-C3N4 and CoO.

Fig. 5 Nitrogen adsorption-desorption isotherms and the corresponding pore size distribution curves for pure g-C3N4 and g-C3N4/CoO-0.5 wt% composite samples.

To identify the electronic structure and photoelectric properties of the as-synthesized pure g-C3N4 and gC3N4/CoO composite samples, UV-Vis diffuse reflectance spectroscopy (DRS) and photoluminescence (PL) were measured, as shown in Fig. 6(a) and 6(b), respectively. In



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and g-C3N4 is largely improved. The EIS and photocurrent analysis are consistent with the results of PL result, indicating g-C3N4/CoO composites might exhibit better photocatalytic performance.

Fig. 6(a) UV-Vis diffuse reflectance spectroscopy (DRS); and (b) Photoluminescence (PL) spectra (λex = 375 nm) for assynthesized pure g-C3N4 and g-C3N4/CoO composites with variable CoO mass contents (0.125, 0.25, 0.5, 1.0, 10 wt%).

To further study the photogenerated charge separation and transfer properties, electrochemical impedance spectroscopy (EIS) and photocurrents were measured. The experimental Nyquist impedance plots for pure gC3N4 and g-C3N4/CoO-0.5 wt% samples without light illumination were shown in Fig. 7(a). The arc radius on the EIS Nyquist plot can reflect the reaction rate on the surface of the electrode. A smaller arc radius corresponds to a more effective separation of photogenerated electronhole pairs and a higher efficiency of charge immigration across the electrode/electrolyte interface.38 Obviously, the arc radii of g-C3N4/CoO-0.5 wt% electrode is smaller than that of the pure g-C3N4 electrode, suggesting that the gC3N4/CoO composite has a stronger electronic conductivity in the non-photoexcited state and would be very beneficial for photoexcited charge separation. Fig.7 (b) shows the transient photocurrent responses for pure g-C3N4 and g-C3N4/CoO-0.5 wt% samples under visible light irradiation in an on-and-off cycle mode. It is clearly seen than the photocurrent of g-C3N4/CoO-0.5 wt% composite is higher than that of pure g-C3N4 sample, which suggests that the recombination of electrons and holes is greatly retarded, and the separation of photogenerated charge carriers at the interface between CoO

Fig. 7(a) EIS; (b) transient photocurrents of g-C3N4 and gC3N4/CoO-0.5 wt% samples in a 1 M Na2SO4 aqueous solution.

The photocatalytic performance of the prepared samples was evaluated with H2 production from water under visible irradiation, as shown in Fig. 8. From Fig. 8(a), one can see that the photocatalytic activity in H2 evolution rate for g-C3N4/CoO composites with 0.125, 0.25, 0.5, 1.0 and 10 wt% CoO content is recorded to be 581.3, 619.9, 651.3, 436.6, 240.9 µmol h-1 g-1, respectively, which are all higher than that of pure g-C3N4 (220.16 µmol h-1 g-1). These results clearly demonstrate the photocatalytic activity of g-C3N4/CoO composites have been significantly improved compared to that of the pure g-C3N4, which may be attributed to the enhanced visible-light harvesting, and effective charge separation. g-C3N4/CoO-0.5 wt% sample has a favorable photocatalytic activity with an average H2 evolution rate of 651.3 µmol h-1 g-1, which is about ~3 times as high as that of pure g-C3N4. And it can be found that the photoactivity of g-C3N4/CoO composites decrease if the CoO loaded content larger than 0.5 wt%. This decrease can be ascribed to the loading of excessive CoO leading to the shielding (or decrease) of the g-C3N4 surface active sites. The stability of H2 evolution for g-


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C3N4/CoO-0.5 wt% sample is determined by three cycling photocatalytic experiments under the same condition (Fig. 8(b)). After three recycling runs, the photocatalytic activity of g-C3N4/CoO-0.5 wt% photocatalyst weakened a little, which means this composites basically stable. The gC3N4/CoO-0.5 wt% photocatalyst before and after recycling experiments was submitted to XRD and SEManalyses. XRD diffraction patterns (Fig. S3) revealed that the photocatalyst maintained crystalline structure of gC3N4 after the photocatalytic H2 evolution. Comparing with the unused sample, the recycling sample features more uniform nanosheets without aggregation (Fig. S4). This is ascribed to the strong stirring during the photocatalytic H2 evolution test. Meanwhile, some CoO nanoparticles might detach from g-C3N4 nanosheets during the strong stirring procedure, then resulting in the slight decrease of photocatalytic performance.

the ultraviolet photoelectron spectroscopy (UPS) was employed to measure the conduction band and valence band levels of pure g-C3N4 and CoO, as shown in Fig. 9(a). The valence band energy (Ev) (equivalent to ionization potential) of g-C3N4 and CoO is determined to be 6.92 and 6.13 eV by subtracting the width of the He I UPS spectra from the excitation energy (21.22 eV). The conduction band energy Ec is thus estimated at 4.06 and 3.93 eV for gC3N4 and CoO from formula Ev-Eg. The Eg values for gC3N4 (Eg=2.86 eV) and CoO (Eg=2.20 eV) was determined by the DRS (Fig. S3). The values of Ev and Ec in electron volts are converted to electrochemical energy potentials in volts according to the reference standard for which 0 versus RHE (reversible hydrogen electrode) equals -4.44 eV versus evac (vacuum level). The evaluated Ev and Ec potentials of g-C3N4 and CoO are capable to construct a heterojunction between g-C3N4 and CoO. Fig. 9(b) shows the schematic energy band illustration of the heterojunction for g-C3N4/CoO composites with the redox potentials of the photocatalytic reaction. The reduction level for H2 is positioned below the Ec of g-C3N4 and CoO, and the oxidation level for H2O to O2 is above the Ev of g-C3N4 and CoO. These bands are properly positioned to permit transfer of electrons and holes for water splitting. In addition, the Ec of CoO (∼-0.51 eV vs. RHE) is more negative than that of g-C3N4 (∼-0.38 eV vs. RHE) and the Ev of gC3N4 (∼2.48 eV vs. RHE) is more positive than that of CoO (∼1.69 eV vs. RHE). Considering the inner electric field and energy band structure of the heterojunction, the photoexcited electrons on the Ec of CoO can rapidly transfer to the Ec of g-C3N4, while the photogenerated holes on the Ev of g-C3N4 can migrate to the Ev of CoO. As a result, the photogenerated electron-hole pairs can be separated efficiently in the photocatalytic system of gC3N4/CoO heterostructure, leading to a significantly enhanced photocatalytic H2 evolution than the pure g-C3N4. This proposed heterojunction mechanism is deduced basing on the unchangeableness of band potentials for CoO in g-C3N4/CoO composites. As discussed above, the CoO loading in g-C3N4/CoO composites is aggregated by abundant nanocrystals with size of 30-50 nm, which is smaller than that of the pure CoO (~150 nm). The decreasing size and the small content of CoO in g-C3N4/CoO composites could change the band potentials of CoO due to the well-known quantum effect. If the change is huge, the formed heterojunction between the coupling g-C3N4 and CoO would be destroyed. In this case, the CoO loading served as a cocatalyst in g-C3N4/CoO composite system might be assigned to the enhanced photocatalyic mechanism, as depicted in Fig. 9(c). Upon photoexcitation, electrons and holes are generated in the conduction band and valence band of g-C3N4. The photogenerated electros move toward the surface of catalyst and reduce protons to H2. The presence of CoO cocatalyst facilitates the transfer of the photo-generated electrons from g-C3N4 to CoO, and protons can be efficiently reduced to produce H2 because CoO can serve as active sites for H2 evolution. Moreover, the formed junction/interface between g-C3N4 and CoO promote the electrons transfer from g-C3N4 to CoO. Thus, the separation efficiency of

Fig. 8(a) the photocatalytic H2 evolution with 10 vol % TEOA, 3 wt % Pt, and 100 mg g-C3N4/CoO-0.5 wt % photocatalysts under visible light(≥ 400 nm); (b) recyclability of gC3N4/CoO-0.5 wt % photocatalyst for the photocatalytic H2 evolution under visible-light irradiation.

Based on the results above, the enhanced photocatalytic activity could be mainly attributed to the improved separation of photogenerated charge carriers at the interface between g-C3N4 and CoO. The formation of heterojunction between g-C3N4 and CoO is first proposed for the enhanced photocatalytic mechanism. To estimate the formation of heterojunction in g-C3N4/CoO composites,



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photogenerated charge carriers is promoted substantially due to the synergistic effect of the junction/interface formed between g-C3N4 and CoO, no matter heterojunction or cocatalyst is formed in g-C3N4/CoO composite system. It can be concluded that the enhanced photocatalytic activity of the as-prepared g-C3N4/CoO composites can be ascribed to the enhanced visible-light harvesting and high effective charge separation rate, resulting from the synergistic effect of the junction/interface formed between g-C3N4 and CoO.

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0.5 wt% composites showed the highest photocatalytic activity under visible light irradiation with up to 3 times as high as that of the pure g-C3N4. The remarkably increased photocatalytic performance of g-C3N4/CoO nanocomposites was mainly attributed to the synergistic effect of the junction/interface formed between g-C3N4 and CoO, resulting in enhanced optical absorption in visible light region and high separation rate of the photogenerated carriers.

ASSOCIATED CONTENT Supporting Information. XRD and SEM of pure CoO, EDS element analysis for pure gC3N4 and g-C3N4/CoO-0.125 wt% composite, Eg evaluation of pure g-C3N4 and pure CoO, XRD and SEM analyses for recycling experiment. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Corresponding authors: E-mail address: Prof. Mao, [email protected]. E-mail address: Prof. Wang, [email protected].

Author Contributions ⊥

Zhiyong Mao and Jingjing Chen contributed equally to this work and should be considered co-first authors. The manuscript was written and revised by Zhiyong Mao and Jingjing Chen through contributions of all authors.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We gratefully acknowledge the financial support by the National Natural Science Foundation of China (nos. 51102265 and 50872091) and Program of Discipline Leader of Colleges and Universities (Tianjin, China) and “Foreign Experts” Thousand Talents Program (Tianjin, China).

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Fig. 9(a) UPS spectra of pure g-C3N4 and CoO; (b) proposed band structure diagram of heterojunction for g-C3N4/CoO composites; (c) proposed cocatalyst machanism for enhanced photocatalytic H2 evolution in g-C3N4/CoO composites.

CONCLUSIONS In summary, g-C3N4/CoO nanocomposites photocatalysts were prepared for H2 evolution from water by one-pot calcination under an argon atmosphere with different mass contents of CoO nanoparticles on the g-C3N4 nanosheets. Experimental data verified that g-C3N4/CoO-


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CoO Nanocomposites with Significantly Enhanced

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