Oxidative Polyoxometalates Modified Graphitic Carbon Nitride for

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Oxidative Polyoxometalates Modified Graphitic Carbon Nitride for Visible-Light CO2 Reduction Jie Zhou, Weichao Chen, Chunyi Sun,* Lu Han, Chao Qin, Mengmeng Chen, Xinlong Wang,* Enbo Wang, and Zhongmin Su Institute of Functional Material Chemistry, National & Local United Engineering Laboratory for Power Batteries, Northeast Normal University, Changchun, 130024 Jilin, People’s Republic of China S Supporting Information *

ABSTRACT: Developing a photocatalysis system for converting CO2 to valuable fuels or chemicals is a promising strategy to address global warming and fossil fuel consumption. Exploring photocatalysts with high-performance and low-cost has been two ultimate goals toward photoreduction of CO2. Herein, noble-metal-free polyoxometalates (Co4) with oxidative ability was first introduced into g-C3N4 resulted in inexpensive hybrid materials (Co4@g-C3N4) with staggered band alignment. The staggered composited materials show a higher activity of CO2 reduction than bare g-C3N4. An optimized Co4@g-C3N4 hybrid sample exhibited a high yield (107 μmol g−1 h−1) under visible-light irradiation (λ ≥ 420 nm), meanwhile maintaining high selectivity for CO production (94%). After 10 h of irradiation, the production of CO reached 896 μmol g−1. Mechanistic studies revealed the introduction of Co4 not only facilitate the charge transfer of g-C3N4 but greatly increased the surface catalytic oxidative ability. This work creatively combined g-C3N4 with oxidative polyoxometalates which provide novel insights into the design of low-cost photocatalytic materials for CO2 reduction. KEYWORDS: graphitic carbon nitride, composited materials, polyoxometalates, photocatalysis, reduction of carbon dioxide

1. INTRODUCTION With continually increased emissions of carbon dioxide from burning fossil fuels, the reduction of CO2 into chemical feedstocks, has been a crucial task that would bring about positively effect to the global carbon balance and energy crisis.1−4 As CO2 is extremely stable and chemically inert, converting it into chemicals is an intractable challenge that requires high energy input and appropriate catalysts. One of the best solutions to accomplish the conversion is exploring suitable photocatalysts utilizing clean, safe, and renewable solar energy as the power resource.5 The development of highperformance and low-cost photocatalysts has been two ultimate goals toward photoreduction of CO2.6 Among various systems, semiconductor catalysts built on naturally abundant elements C, H, and N represent a promising manner to cut down material cost.7 More recently, a new type of 2D metal-free polymeric semiconductor, graphitic carbon nitride (g-C3N4), has grabbed extensive attention as it has appealing electronic structure and is easily available, thermally and chemically stable, especially low cost.8−10 Unlike classical semiconductors, only being active in the UV region, g-C3N4 is a visible-light-active photocatalyst with multifaceted application, such as energy conversion, water splitting, oxygen reduction reaction, and purification of contaminated water. Despite these priorities, the efficiency of pure g-C3N4 in CO2 reduction still suffers from low efficiencies, which is largely due to the fast recombination of photoinduced electrons and holes. Fabricating semiconductor heterojunction is considered as one © XXXX American Chemical Society

of the most convenient and efficient routes to solve this issue. Up to now, various materials have been employed to construct such heterojunctions and the enhanced efficiency of hybrid materials in CO2 reduction was achieved. However, the progress in the field is still unsatisfied.9,11−14 Other factors, such as solar light absorption ability and surface catalytic redox reactivity also effect the efficiency.13 Nevertheless, the extension of visible-light absorption in the reported heterojunctions is quite limited, and the effect of the doped materials in accelerating catalytic oxidation reaction of CO2 conversion system has rarely taken into consideration. Herein, we reported a novel low-cost hybrid photocatalyst constructed from g-C3N4 and oxidative polyoxometalates (POMs), which possess highly efficiency in CO2 reduction under visible-light irradiation. POMs as a class of large cluster polyoxoanion consisting of both oxo ligand and high oxidation state transition metal ion (P, W, and V elements) have widely used in broad areas, especially in catalysis.15−17 For example, the tetra-Co-sandwiched kegging-type POM, Na10Co4(H2O)2(PW9O34)2 (denoted as Co4), makes a breakthrough in water oxidation.18 Unluckily, POM materials have rarely explored in the field of photoreduction of CO2 and the efficiency is limited around 10 μmol based on noble-metal POMs.19,20 Considering Co4 has broad range of visible light Received: February 5, 2017 Accepted: March 16, 2017

A

DOI: 10.1021/acsami.7b01721 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

counter electrode is a platinum wire. A flow of N2 was maintained over the electrolyte during the recording of electrochemical measurements. To test the LSV, a N2-saturated sodium phosphate buffer (150 mL) was used and 1 mL of TEOA was added for the experiment needed TEOA. The current density was normalized to the geometrical surface area (GSA) and the measured potentials vs Ag/AgCl were converted to a reversible hydrogen electrode (RHE) scale via the Nernst equation: ERHE = EAg/AgCl + 0.059pH + E0Ag/AgCl (E0 Ag/AgCl = +0.199 V).

absorption and high reactivity in water oxidation which might accelerate the oxidative reaction in CO2 conversion. In this work, Co4 was deliberately selected as the doping material to synthesize g-C3N4 heterojunctions. The resultant hybrid material Co4@g-C3N4 (43 wt % of Co4) exhibited high efficiency in converting CO2 into CO, up to 107 μmol g−1 h−1 under visible-light irradiation (λ ≥ 420 nm) with the selectivity of CO around 94%. Steady-state and transient photoluminescent spectroscopy and photoelectrochemical measurement was performed to investigate the rooted reasons behind the function of the structure.

3. RESULTS AND DISCUSSION For the synthesis of the composited photocatalyst, we first prepared g-C3N4 by directly calcining urea in air at 550 °C.21 Then a series of hybrids with different Co4 content were obtained through one-step hydrothermal treatment of g-C3N4 and Co4 mixtures (Figure 1a). As determined by inductively coupled plasma mass spectrometry (ICP-MS), the corresponding Co4 content in the resultant Co4@g-C3N4 composites was 28, 43, and 65 wt %, respectively, based on Co. The PXRD

2. EXPERIMENTAL SECTION 2.1. G-C3N4 and the Hybrid Co4@g-C3N4 Photocatalysts. Pure g-C3N4 was synthesized by directly calcining urea under air atmosphere. Urea powder (10 g) was put into a crucible with a cover and then heated to 550 °C at a rate of 0.5 °C/min keeping this temperature for 2.5 h. g-C3N4 was received after cooling down to 25 °C. Co4 was synthesized following ref 18. The desired amount of Co4 (0.1, 0.2, and 0.3 g) was added into 10 mL of H2O and heated to 80 °C until Co4 dissolved. The above Co4 solution was gradually added into g-C3N4 (0.3 g) suspension under ultrasonic for 30 min. The mixture was then transferred to a Teflon reactor (120 mL) and heated at 150 °C for 12 h. After cooling, the mixture was filtrated. The obtained solid was evaporated at a controlled temperature of 45 °C in an oven for 12 h. 2.2. CO2 Photoreduction. The photocatalytic test was performed in a quartz tube (50 mL) under an atmospheric pressure of CO2. In the tube, the photocatalytic CO2 reduction reaction was carried out by dispersing 50 mg catalyst powder in a solution containing acetonitrile (6 mL), triethanolamine (2 mL), CoCl2 (1 μmol), and bipyridine (15 mg). This mixture system was bubbled with pure CO2 gas for 20 min. The temperature of the reaction solution was maintained at 20 °C controlled by a flow of water during the reaction. Then, the system was irradiated with a nonfocus 300 W Xe lamp with a 420 nm cutoff filter under vigorous stirring. The produced gases (CO and H2) were detected using a gas chromatography equipped with a packed molecular sieve column (TDX-01 mesh). Argon was used as the carrier gas. An isotopic experiment was carried out under the identical photocatalytic reaction conditions using 13CO2 (99% in purity, 1 bar) as the carbon source. The photocatalytic evolution of CO was analyzed by gas chromatography−mass spectrometry (GC-MS). Control experiment for investigate the double-transfer mechanism: The condition is similar to that of CO2 photoreduction except the light source using 500 nm −800 nm. 2.3. Photocurrent Measurements. Photocurrent measurements were performed on an electrochemical workstation (CHI660e, CH Instruments, Inc., USA) with a three-electrode configuration. Our sample, Ag/AgCl electrode and Pt-wire electrode were employed as the working, reference and counter electrode, respectively. Irradiation was carried out by using a 300 W xenon lamp with a 420 nm cutoff filter. The Na2SO4 solution (0.1M) was used as the electrolyte. The working electrodes were prepared by spreading aqueous slurries of various samples on FTO glass substrate, using adhesive tapes as spaces to obtain a 1 cm × 1 cm electrode. 2.4. Linear Sweep Voltammograms Measurements. Electrode preparation. One milligram of the synthesized photocatalyst was first ultrasonically dispersed in the mixture of 1500 μL of deionized water, 500 μL of ethanol, and 20 μL of Nafion solutions (DuPont D521, 5 wt %). Ten microliters of the catalyst dispersion liquid was then transferred onto the glassy carbon electrode (3 mm diameter) via a controlled drop casting approach. The solvent was then evaporated at a controlled temperature of 25 °C in a drying oven for 3 h. The resulting electrode was then obtained to serve as working electrode. Electrochemical Characterization. Linear sweep voltammograms (LSVs) were carried out using a CHI660e (CH Instruments, Inc., USA). Reference electrode is an Ag/AgCl in saturated KCl solution (0.2 V vs standard hydrogen electrode (SHE)) and the

Figure 1. Samples of pure g-C3N4 and composites with different Co4 content. (a) From the left to right, photographs of them are corresponding to g-C3N4, Co4@g-C3N4 with 28, 43, and 56 wt % Co4, respectively. (b−e) Transmission electron micrographic images of them (TEM); (f, g) scanning electron micrographic (SEM) images of them; (h) SEM mapping of Co4@g-C3N4 with 43 wt % Co4. B


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peaks of C 1s shifts to 284.8 and 288.0 eV and the binding energy of N 1s shifts to 398.2 and 400.8 eV.23 When comparing binding energy of O 1s in Co4@g-C3N4 with that in pure Co4 POM (Figure S3), obvious shifts could be observed from 529.7, 531.8, and 533.2 eV to 528.4, 530.3, and 532.9 eV. These shifts indicate that hydrogen-bonding interactions might exist between Co4 and g-C3N4 when forming [email protected] The light absorption capability of resultant composites analyzed via UV−vis. Co4@g-C3N4 hybrids showing two absorption bands (Figure 3), the one ranging from 200 to

pattern of the as-prepared g-C3N4, Co4 and Co4@g-C3N4 hybrids showed in (Figure S1). As evident from Figure S1, the Co4@g-C3N4 hybrids exhibit characteristic PXRD peaks of both g-C3N4 and Co4. The change of surface area was revealed from N2 adsorption behavior which could be an evidence for the successful loading of Co4. The N2 adsorption−desorption isotherms (Figure S4) show the SBET of pure g-C3N4 is 82.8 m2 g−1 while the SBET of hybrid Co4@g-C3N4 is relatively low (61.5 m2 g−1). The decrease in SBET was account for the loading of Co4. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) of g-C3N4 and Co4@g-C3N4 hybrids with different Co4 content were taken to directly analyze the structures of the samples. TEM image (Figure 1b− e) indicates a sheetlike structure of g-C3N4 and this shape was reserved in hybrids. From these images, we could see that after the hydrothermal process, the Co4 particles spread on the surface of g-C3N4 randomly. With the concentration of Co4 increasing, the black clusters become intensive which further indicate the successful loading of Co4. Their sheetlike morphologies also revealed by SEM images (Figure 1f and g). Energy-dispersive X-ray (EDX) spectrum and corresponding EDX elemental mappings (Figure 1h) of Co4@g-C3N4 (65 wt %) resulting from selected area show that the C, N, O, W, and Co elements exist in the composited material. To characterize the interaction between Co4 and g-C3N4 in the hybrids, high-resolution X-ray photoelectron spectroscopy (XPS, Figure 2, Figures S2 and S3) measurements were

Figure 3. UV−vis diffuse reflectance spectra of pure g-C3N4 and composited sample with different Co4 content.

460 nm ascribes to the absorption of g-C3N4 and the other from 480 to 710 nm originates from Co4. As the content of Co4 increasing, the typical absorption of Co4 in hybrids becomes clear. Before photocatalytic experiment, the capability of Co4@g-C3N4 in CO2 adsorption was also investigated. The CO2 adsorption isotherms (Figure S5) exhibits the maximum CO2 uptake of Co4@g-C3N4 is 20.1 cm3 g−1, whereas that of bare g-C3N4 is 11.3 cm3 g−1. The enhancement of Co4@gC3N4 in CO2 adsorption may result from the increased interaction between Co4 and CO2.24 The CO2 reduction experiment was performed following the condition for bulk g-C3N4 reported by Wang and co-worker in which bipyridine (bpy) ligand and Co2+ was employed as the electron mediator, and triethanolamine (TEOA) served as the electron/proton donor.25 Our experiment was carried in MeCN and atmosphere of CO2 using Co4@g-C3N4 as photocatalysts under a visible-light-driven catalytic system (λ ≥ 420 nm). As a control, the photocatalytic performance of bare g-C3N4 investigated, as well. Figure 4 shows the results of CO2 reduction with CO and H2 gases generated as the main reaction products. The doping content of Co4 in hybrids showed considerable influence on the photocatalytic activity (Figure 4b): (i) when Co4 (28 wt %) was loaded, the photocatalytic activity in CO generation was increased from 114.4 to 216.0 μmol g−1, whereas H2 evolution was not enhanced obviously; (ii) the CO generation increases as the Co4 content rose to 43 wt %, which is around 3 times higher than that of bare g-C3N4 but the H2 amount presented a little decrease; (iii) further enhancing Co4 content (65 wt %) led to a decrease in both CO and H2 generation, which might result from agglomeration of Co4. These results evolve Co4 doping brought out superior enhancement in both the production and the selectivity of CO and the optimal doping amount of Co4 is 43 wt %.

Figure 2. XP spectrum of the composited sample with 43 wt % Co4. (a) Co4@g-C3N4, (b) C 1s, (c) N 1s, (d) O 1s, (e) W 4f.

performed. As shown in Figure S2, bare g-C3N4 exhibits C 1s and N 1s. The binding energy of C 1s peaking at 284.6 and 287.8 eV can be assigned to sp2 C−C bonds and sp2-bonded C in the triazine rings (N−CN), respectively.22 The N 1s spectrum presented peaks at 398.4 and 400.5 eV, corresponding to sp2-bonded N involved in N contained aromatic ring (C−NC).22 In Co4@g-C3N4 (Figure 2), the corresponding C


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Figure 4. Yield of CO and H2 of CO2 reduction using different sample as photocatalysts under visible light irradiation (λ ≥ 420 nm). (a) Photocatalyst of 43 wt % Co4@g-C3N4 under different time’s irradiation; (b) photocatalyst of pure g-C3N4 and composites with different Co4 content under 3 h irradiation.

Figure 5. (a) Recycling experiments using the 43 wt % Co4@g-C3N4 sample as catalyst. (b) Time course of the CO and H2 with the 43 wt % Co4@g-C3N4 sample as catalyst under 10 h irradiation.

Figure S9, the peaks at 1.91 and 13 min. Twenty-eight minutes with m/z 29 and 45 assigned to 13CO and 13CO2, respectively. No signal at m/z 28 was found. This information provided valid evidence that the produced CO derived from the reduction of CO2 rather than the decomposition of any other organic species in the photocatalytic system. To confirm role of composited materials, we measured the photocatalytic activity of CO2 reduction of mechanically mixed samples (entry 5, Table S1). The yield of CO for the mechanical mixture decreases greatly in contrast to their counterparts, implying that the loosely contact are insufficient for the reduction of CO2. Therefore, it may be reasonably deduced the formation of heterojunctions are responsible for the enhanced activity of our composite materials. Other reference experiments were implemented to demonstrate the importance of the components in photoreduction system (Table S1). In the absence of either Co4@g-C3N4 or light, no gas was detected which illuminated that the other components cannot arouse CO2 reduction, reflecting the lightto-charge pair production effect of Co4@g-C3N4. Pure Co4 sample did not show noticeable CO production which further revealed that g-C3N4 acted as the photocatalyst in this system. When replace Co4 with another isomorphic POMs, Na10Ni4(H2O)2(PW9O34)227 (named Ni4), around 110 μmol g−1 CO was produced (Figure S10) which indicated Co4 is primarily ingredient for the reductive reaction of CO2 to CO. Once CO2 gas was replaced by Ar, the generation of CO also stopped, therefore excluding degradation effects of organic species, as well. Given the high yield of Co4@g-C3N4 hybrids in CO2 photoreduction, the rooted reasons behind the function of the structure were investigated. Band alignments of hybrid

Reproducibility and durability are vital issues for the longterm use of a photocatalyst in practical application. To examine our system durability, we carried out five-run cycling experiments of Co4@g-C3N4 (43 wt %) (Figure 5a). Almost no obvious decrease in CO generation was observed within the first four runs although a slight activity loss emerged during the fifth run. After catalysis, IR spectroscopy and PXRD (Figures S6 and F7) of the sample was collected. The PXRD shows that the pattern of Co4 in Co4@g-C3N4 became weaker. This might ascribe to some leaking of Co4 from the hybrid sample after five runs and this leaking might lead to the decreased efficiency in the fifth run. On the basis of this observation, we further explored the CO2 reduction capability of Co4@g-C3N4 (43 wt %) for 10 h (Figure 5b). The result shows the production of CO up to 896.0 μmol g−1 together with the generation of H2 58.0 μmol g−1 and the selectivity of CO was around 94%. For bare g-C3N4, only 318.4 μmol g−1 of CO and 53.2 μmol g−1 of H2 was produced after irradiation for 10h and the selectivity of CO was 82.9% (Figure S8). The increased selectivity of CO may originate from the higher CO2 adsorption ability of Co4@ g-C3N4.6 Control experiment using g-C3N4 as photocatalyst presenting a yield of CO and H2 is 364 and 67 μmol g−1, respectively. The yield and selectivity of Co4@g-C3N4 is higher than most of reported hybrid g-C3N4 materials, such as the ones constructed from SnO2, GeO2, BiVO4, and SnNb2O6.8,26 This is the first example of non-noble-metal POMs used in CO2 photoreduction and represents the highest efficiency of POMs in this field.19,20 To convincingly demonstrate the source of the generated CO, we actualized an isotopic experiment using 13CO2 as gas atmosphere under the same conditions.25 As presented in D


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ACS Applied Materials & Interfaces materials are known to have direct influence on the redox reaction occurring at catalyst surface. First, band edge positions of g-C3N4 and Co4 was investigated. The conduction band (CB) energy and valence band (VB) energy (Ev) of g-C3N4 were obtained from the references28 which is −0.75 and +1.98 V vs normally reversible hydrogen electrode (RHE), respectively. As for Co4, the CB and VB were obtained from ultraviolet photoelectron spectroscopy (UPS, Figure S11a) and Tauc plot indirectly. UPS was employed to determine the ionization potential which is equivalent to the VB energy and the calculated data is 1.35 V. As calculating from Tauc plot (Figure S11b), the optical band gap (Eg) of Co4 is determined to be 2.2 eV. Thus, the CB of Co4 is obtained from Ev − Eg which is −0.85 V. Therefore, it could be concluded that the band alignment between Co4 and g-C3N4 belongs to staggered band alignment (Type II). On the basis of these results and previous literatures,29 one possible mechanism for the enhanced reactivity over Co4@gC3N4 is exhibited schematically in Figure 6. Because of the CB

Figure 7. (a) Steady-state PL spectra. (b) Transient PL decay for pure g-C3N4 and composites.

C3N4 shows only one radiative lifetime of around three ns (Figure 6b), which is related to the photoexcited electron and holes in g-C3N4 that eventually suffer from recombination.31 As for hybrids, a longer lifetime component (Table S2) emerges accompanying by the shortened lifetime of g-C3N4, indicating that some excited electrons undergo migrate between g-C3N4 and Co4.32 These evidence reveal that the hybrid materials remarkably promote the charge transfer. Transient photocurrent responses measurements were carried out under irradiation of several on−off cycles. The results (Figure 8a) show that the hybrid material of 43 wt % Co4@g-C3N4 possesses the highest photocurrent intensity (up to 7.5 uA cm−2), which is more than 3-fold higher than that of bare g-C3N4. These observations suggest hybrid materials permit more efficient separation of photogenerated electron− hole pairs as comparing with bare g-C3N4.33 What’s more, the current intensity of 56 wt % Co4@g-C3N4 is a little bit lower than that of 43 wt % sample, which might interpret the lower reactivity of 56 wt % hybrids in photocatalysis. Given Co4 is an excellent catalyst on water oxidation, we presumed this property might accelerate oxidation reaction kinetics in the system of CO2 reduction. The catalytic oxidation abilities were revealed by studying electrocatalytic oxidation activities of pure g-C3N4 and [email protected] Linear sweep voltammetry (LSV, Figure 8b) was carried out to investigate the oxidation activities with the same amount of bare g-C3N4 and Co4@g-C3N4 loaded on glassy carbon electrodes. In contrast to bare g-C3N4 possessing an onset potential of ca. 2.1 V vs RHE, Co4@g-C3N4 exhibits a cathodic shift of the onset potential to be around 1.7 V vs RHE, indicating that the surface catalytic oxidative ability of g-C3N4 was greatly increased

Figure 6. Schematic diagram of photoinduced electron−hole pairs transfer process.

edge potential of Co4 is more negative than that of g-C3N4, the photoexcited electrons on Co4 could transfer to the CB of gC3N4.30 Such a transfer process is benefit for decreasing recombination of electron−hole pairs and therefore enhancing the photocatalytic activity. According to this transfer mechanism, the CO2 reduction would proceed on CB of g-C3N4, accompanying by oxidation reaction on the VB of Co4 (Figure 6). Additional experiment was implemented to prove this transfer process. As g-C3N4 can only adsorb the light below 500 nm, this experiment was conducted following the conditions above for CO2 reduction except the light source (λ ≥ 500 nm). Around 33 μmol g−1 CO was obtained under this condition. Because under this condition, the light excites Co4 solely, the electrons in the reduction reaction should origin from the transferred ones of Co4. The enhanced separation of electron−hole pairs were investigated through steady-state and transient emission measurements and transient photocurrent responses. Figure 7a shows the steady-state emission spectra of bare g-C3N4 and composites at an excitation wavelength of 400 nm. The g-C3N4 sample exhibits a strong emission peak centered at 460 nm. In comparison, the intensity of this emission band of 28% Co4@gC3N4 dropped significantly and with Co4 content increasing, the intensity of this band decreased gradually. The lifetime of charge carriers in g-C3N4 and hybrid materials were examined using transient emission spectroscopy. The sample of bare gE


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represents the highest data of POMs in CO2 photoreduction. The studies of steady-state and transient photoluminescent spectroscopy and photoelectron-chemistry of bare g-C3N4 and hybrid materials revealed the introduction of Co4 not only facilitate the charge transfer of g-C3N4 but greatly increased the surface catalytic oxidative ability. As a direct result, hybrid Co4@g-C3N4 significantly enhances photocatalytic conversion of CO2 compared to the bare g-C3N4. This work first combined g-C3N4 with oxidative polyoxometalates which might open up an alternative method to develop a high-performance and lowcost photocatalyst for efficient CO2 photoreduction.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b01721. Materials and measurements, additional XRD pattern, infrared spectroscopy, nitrogen adsorption/desorption, etc. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID Figure 8. (a) Transient photocurrent responses for the g-C3N4 and hybrid Co4@g-C3N4. (b) Linear scan voltammogram (LSV) curves for g-C3N4 and CN/MFO-150 electrodes at an RDE (1600 rpm) in N2saturated 0.1 MKOH solution. Scan rate, 10 mV s−1.

Xinlong Wang: 0000-0002-5758-6351 Enbo Wang: 0000-0001-5236-7566 Zhongmin Su: 0000-0002-3342-1966 Notes

The authors declare no competing financial interest.

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through Co4 doping. The intense peak located around 1.75 V is ascribed to the oxidation of CoII and the resultant product could be a more efficient species for the oxidation reaction. Considering the photocatalytic reaction was actually performed in TEOA conditions, LSV was also performed after adding TEOA. From Figure 8b, we could see that the onset potentials for both g-C3N4 and Co4@g-C3N4 were greatly reduced, which could be ascribed to TEOA serving as holes sacrificial agent in photocatalysis, and it is much easier to be oxidized than OH−. In spite of this, the onset potential for TEOA oxidation in the system containing Co4@g-C3N4 has ∼0.18 V cathodic shift in comparing with that in g-C3N4 system. These results indicated that Co4 might enhance the catalytic oxidative ability of g-C3N4. Therefore, in this reaction system, g-C3N4 receives the photoinduced electrons from the CB of Co4 acting as active center of reduction while Co4 obtaining the holes from the holes from the VB of g-C3N4 accelerating oxidation.

ACKNOWLEDGMENTS This work was financially supported by the NSFC of China (21471027, 21671034), National Key Basic Research Program of China (2013CB834802), the Fundamental Research Funds for the Central Universities (2412016KJ041), Changbai Mountain Scholars of Jilin Province, Foundation of Jilin Educational Committee (2016498).

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REFERENCES

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4. CONCLUSIONS In conclusion, through creatively introducing noble-metal-free POM (Co4) into g-C3N4, an inexpensive photocatalyst for CO2 reduction were fabricated. The resultant composites exhibited extended visible light absorption ranging from 425 to 700 nm and better ability for CO2 capture. Under visible light irradiation (λ ≥ 420 nm), CO2 reduction reaction was performed. A high yield of 107 μmol g−1 h−1 and selectivity of 94% for CO production was obtained when 43 wt % Co4@ g-C3N4 was employed as photocatalyst. After 10 h of irradiation, the production of CO reached 896 μmol g−1 which are quite rare in reported hybrid g-C3N4 materials and F


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