g-C3N4 nanosheets

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Kinetics, Catalysis, and Reaction Engineering

Indirect Z-scheme assembly of 2D ZnV2O6/RGO/gC3N4 nanosheets with RGO/pCN as solid-state electron mediators toward visible-light enhanced CO2 reduction Abdullah Salem Mohammed Bafaqeer, Muhammad Tahir, Azmat Ali khan, and Nor Aishah Saidina Amin Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b06053 • Publication Date (Web): 12 Apr 2019 Downloaded from http://pubs.acs.org on April 12, 2019

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Industrial & Engineering Chemistry Research

Indirect Z-scheme assembly of 2D ZnV2O6/RGO/g-C3N4 nanosheets with RGO/pCN as solid-state electron mediators toward visible-light enhanced CO2 reduction Abdullah Bafaqeer, Muhammad Tahir *, Azmat Ali Khan, Nor Aishah Saidina Amin Chemical Reaction Engineering Group (GREG), School of Chemical and Energy Engineering, Universiti Teknologi Malaysia, 81310, UTM, Johor Bahru, Johor, Malaysia. *Corresponding

author email address: [email protected]

Abstract Indirect Z-scheme assembly of graphene bridged 2D ZnV2O6/pCN nanosheets composite has been fabricated by one step solvothermal process and tested for photo-induced CO2 conversion under visible light irradiations. Highest CH3OH production of 3488 μmole g-cat-1 was obtained over ZnV2O6/RGO/g-C3N4 composite, 1.02 and 1.25 times higher comparing to ZnV2O6/RGO and ZnV2O6/g-C3N4 samples, respectively. This enhanced efficiency can be ascribed to welldesigned ternary heterojunction with hierarchical structure and efficient charges separation by RGO. More importantly, CH3OH yield was further improved by introducing RGO/pCN as electrons sink which was 1.07 times higher than using only RGO. This reveals, ternary 2D ZnV2O6/RGO/pCN nanostructure has higher visible light absorption, improved charges separation and enhanced photocatalytic efficiency due to RGO/pCN as multiple mediators. The stability of composite catalyst also prevailed for 32 hours for continuous CH3OH production. Therefore, structured Z-scheme composite with multiple electron mediators enables efficient CO2 conversion under visible light irradiations. Keywords: 2D ZnV2O6 nanosheets; RGO; Functionalized g-C3N4; RGO/pCN electron mediators; CO2 photo-reduction; Methanol 1. Introduction Climate change because of global warming arising from the emission of greenhouse gas carbon dioxide (CO2) poses a sever threat to our future. Meanwhile, the exhaustion of fossil resources is another issue that has forced the development of sustainable energy solutions 1-4.

1

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From this perspective, the photocatalytic CO2 reduction to valuable fuels such as CH3OH, HCOOH, CH4 and CO is one of the most promising areas of research that can solve both the energy crisis and the environmental issues 5-7. Generally, semiconducting materials like ZnO 8,

TiO2 9-10, CdS 11 and WO3 12 are commonly utilized for CO2 reduction applications. However,

these semiconducting photo-catalysts suffer from very low CO2 photo-reduction efficiency due to rapid charges recombination, low conduction band position, small surface area and lower visible light absorption. Thus, it is crucial to develop highly effective semiconductor materials to reduce CO2 efficiently while utilizing solar energy. Recently, 2D layered graphitic carbon nitride (g-C3N4) has received much attentions because of many advantages such as elemental abundance, high chemical and thermal stability, appropriate band gap energy and eco-friendly nature 13. However, the performance of g-C3N4 is still restricted because of its high recombination rate of photo-produced electron and hole pairs

14-17.

The efficiency of g-C3N4 can be improved by exfoliation and functionalization.

Several efforts were made to promote the efficiency of g-C3N4 like doping with metal and nonmetal elements 18, engineering approach 19 and coupling with other semiconductors 20-23. RGO with two-dimensional structure is one of the most promising mediator/modifier due to its excellent electron mobility and higher light absorption 24-25. Modification of g-C3N4 with RGO has exhibited considerable progress due to it promotes separation of charges. Therefore, numerous research efforts were made to improve g-C3N4 efficiency such as RGO/ protonated g-C3N4 26, RGO/g-C3N4 27, RGO/g-C3N4 28 and g-C3N4/GO 29. In the current development, ternary nanostructures like nanoplates, nanosheets, microspheres and nanorods have gained much attention and exhibited magnificent performances in photo-induced CO2 reduction and other energy applications

30-34.

Moreover,

the construction of Z-scheme photo-induced system has received much attention because of its ideal effectiveness in improving the photo-induced efficiency

2


35.

In this viewpoint, the

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construction

of

Z-scheme

semiconductors

such

as

g-C3N4/RGO/Bi2WO6

36,

g-

C3N4/RGO/BiVO4 20, g-C3N4/CNTs/Bi2WO6 37, g-C3N4/RGO/FeWO4 38, BiVO4/RGO/Bi2O3 39 and Cd0.5Zn0.5S/RGO/g-C3N4 40 have displayed significantly promoted photo-induced activity towards CO2 reduction to fuels. Recently, we reported ZnV2O6 nanosheets as a favourable photo-catalyst for photocatalytic conversion of CO2 to CH3OH under visible-light with enhanced activity and stability. This was due to special 2D ZnV2O6 interfacial structure which enhances the transfer rate of the photo-produced holes and electrons, appropriate band structure and strong visible absorption leading to a higher photocatalytic activity

41.

Combining 2D

ZnV2O6 nanosheets with rGO/g-C3N4 composite would develop indirect Z-scheme heterojunction which may not only increase charges separation performance but also provides good redox potential for selective CO2 reduction under visible light. Thus, it is extremely desirable to construct functionalized g-C3N4 modified RGO/ZnV2O6 composite to improve photo-induced conversion of CO2 into fuels under visible light illuminations. Herein, the construction of indirect Z-scheme assembly of two dimensional ZnV2O6/RGO/pCN heterojunction nanostructures was synthesized via a simple one step solvothermal method for efficient photo-conversion of CO2 to CH3OH under visible light illuminations. The samples were systematically characterized by N2 sorption, TEM, XRD, SEM, XPS, UV-Visible, PL and RAMAN techniques. The efficiency of ZnV2O6/RGO/g-C3N4 was specifically investigated and compared by employing RGO as a single mediator and pCN/RGO as multiple mediators under the same operating conditions. The stability of the samples was investigated under visible light illuminations. Finally, possible mechanism for photocatalytic CO2 reduction based on RGO/pCN as multiple mediators has been proposed. 2. Experimental 2.1

Materials

3



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Melamine (99.0%), N,N-dimethyl formamide (DMF), zinc acetate and zinc oxide were purchased from Sigma-Aldrich. Ammonium metavanadate and NH3 solution were supplied by Merck, Germany. Oxalic acid dehydrated was supplied by Qrec, New Zealand. All other reagents used in this work are of analytical grade and were used as received without any further purification. 2.2

Synthesis of ZnO/V2O5 composite ZnO/V2O5 was synthesized by a sol-gel process as reported previously

41.

The typical

synthesis of ZnO/V2O5 as follows: 0.5 g of NH4VO3 was first dissolved in 40 ml DI water and the solution pH was adjusted to 9 utilizing 2 M NH3. After stirring for about 15 min, zinc oxide (ZnO) was added and the solution was stirred for 1 h. The collected precipitates were washed repeatedly with DI water and dried in oven at 80 °C for 12 h. Finally, ZnO/V2O5 was calcined at 550 °C for 3 h. The V2O5 sample was synthesized utilizing the same method excluding the ZnO addition. 2.3

Synthesis of ZnV2O6 nanosheets ZnV2O6 was synthesized utilizing one-step solvothermal technique as reported

previously 30. Specifically, 2.052 mmol NH4VO3 was added into a glass bottle with 20 ml DMF and stirred vigorously for 10 min. Then, Zn(O2CCH3)2 was dissolved in the above solution and vigorously stirred until a colloidal solution was obtained. After that, Oxalic acid dehydrated [H2C2O4·2H2O] was added to the solution of ammonium monovanadate in the ratio of (NH4VO3/Oxalic acid) 3:1. After being stirred for 30 min, the mixture was transferred to a 75 ml Teflon-lined autoclave and heated at 200 °C for 24 h. The product was collected after washing for several times with absolute ethanol and dried at 80 °C for 12 h. The fabricated ZnV2O6 was calcined at 550 °C for 3 h and the effectiveness was compared between calcined and as-synthesized samples. Using the same procedure, various ZnV2O6 nanosheets were

4


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fabricated by varying heating exposure times of 12, 48 and 72 hours while keeping temperature of 200 oC. 2.4

Preparation of functionalized g-C3N4 (pCN) Bulk g-C3N4 was fabricated by annealing melamine in a muffle furnace. Typically, 4 g

melamine powder was placed in a crucible with a cover and then heated to 550 °C for 2 h. After cooling to ambient temperature, the obtained yellow colour product was ground carefully to a fine powder. After that, 2 g of bulk g-C3N4 was put into a 200 mL beaker with 0.1 M nitric acid (HNO3) aqueous solution and stirred for 4 h at room temperature h to get functionalized gC3N4 sample. Afterwards, the washing of mixture was conducted with deionized water for several times for the removal of superfluous HNO3 to achieve the neutral solution. Finally, slurry was dried in air flow oven at 80 °C for 12 h and grinded into fine powder. 2.5

Preparation of ZnV2O6/RGO/pCN nanosheets In a typical synthesis, 2.052 mmol ammonium monovanadate (NH4VO3) was added

into a glass bottle with 20 ml DMF and stirred vigorously for 10 min. Then, Zn(O2CCH3)2 was dissolved in the above solution and vigorously stirred until a colloidal solution was obtained. After that, Oxalic acid dehydrated [H2C2O4·2H2O] was added to the solution of ammonium monovanadate in the ratio of (NH4VO3/Oxalic acid) 3:1. At the same time, 4 % of graphene oxide and 20 ml of DMF are mixed for 10 min to produce a graphene oxide suspension. Also, 100 % of pCN is mixed with 20 ml of DMF and stirred for 10 min. Next, suspensions of graphene oxide and pCN were added to above solution and stirred together for 30 min. Finally, the mixture was transferred to a 75 ml Teflon-lined autoclave and heated at 200 °C for 24 h. The product was collected after washing for several times with absolute ethanol and drying at 80 °C for 12 h. For comparison, ZnV2O6/RGO/pCN composite fabricated without functionalization of g-C3N4 was synthesised under the similar experimental conditions.

5



ZnV2O6/RGO and ZnV2O6/pCN samples were synthesized utilizing the same method except the addition of pCN and/or RGO. 2.6

Structure Characterization The crystal phase of the obtained samples was examined by Rigaku X-ray

diffractometer with Cu-Kα radiation (λ = 0.154178 nm, 40 kV, 30 Ma), recorded with 2θ ranging from 3 to 100°. UV-vis diffuse reflectance spectra (DRS) of the samples were determined in the wavelength range of 200-800 nm using Agilent, Cary 100 UV-vis spectrophotometer (Model G9821A) equipped with an integrated sphere. The morphology and the corresponding elemental mapping images of all the samples was characterized by Zeiss Crossbeam 340 field-emission scanning electron microscopy (FE-SEM). The particle size and d-spacing of the samples were measured using HITACHI-HT7700 high-resolution transmission electron microscope (HRTEM). The compositions and elemental chemical states were analysed through X-ray photoelectron spectroscopy (XPS) using a Kratos-Axis Ultra DLD apparatus. The binding energy was referenced to the C1s signal at 284.60 eV of the surface adventitious carbon. The Brunauer-Emmett-Teller (BET) surface area and pore size distribution of the samples were determined by nitrogen-adsorption-desorption isotherms at 77 K using a Micrometrics ASAP 2020 surface area and porosity analyser. The Raman spectra of samples were analysed using Raman Spectrometer (Lab RAM HR Evolution, HORIBA) with 532 nm laser excitation. Photoluminescence (PL) spectra of the products were obtained by fluorescence spectrometer (HORIBA) at room temperature.

2.7

Photocatalytic testing and analysis Photocatalytic reduction of CO2 with H2O in a liquid phase was performed using 150

mL quartz glass solar photo-reactor 42. A 35W Xe-lamp was used as the light source and the

6


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focused light intensity at the surface of the photocatalyst was 20 mW cm−2 43. First, 100 mg powder photo-catalysts and 100 mL H2O containing 0.1 M sodium hydroxide (NaOH) aqueous solution were placed in a 150 mL quartz photoreactor. Compressed CO2 (99.99%) was injected into the solution for 30 min at a flow rate of 20 ml/min utilizing mass flow controller (MFC). Then, the irradiation source was turned on and samples were being withdrawn at the interval of 2 h utilizing a syringe. The products were filtered utilizing syringe Filter (diameter 33 mm, pore size 0.45 μm) to eliminate catalyst particles. During the photo-catalytic process, the reaction system was stirred by a magnetic stirrer. The photocatalytic conversion products were detected using a gas chromatograph (GC) equipped with a flame ionization detector. 3. Results and discussion 3.1

Characterization of catalysts The XRD patterns of the pure ZnV2O6 and ZnV2O6 calcined are displayed in Figure 1

(a). The XRD patterns of the ZnV2O6 nanosheets (blue curve) exhibits characteristics peaks located at 2θ of 20.9°, 28.9°, 29.2o, 29.7o, 35.4° and 47.3o, corresponds to (201), (110), (202), (111), (111) and (311) planes, respectively. The diffraction patterns matched well with JCPDS card# 01-074-1262, confirming successful fabrication of ZnV2O6. Another peak appeared in the XRD patterns of ZnV2O6 at 2θ=10.0° was probably due to impurity present in the final product and can be ascribed to metal alkoxide. However, a pure ZnV2O6 with high degree of crystallinity was obtained after calcining at 550 °C for 3 h. Previously, similar observations have been reported in the literature 44-45. All the specified peaks (red curve) can be allocated to ZnV2O6 (JCPDS card 01-074-1262). In addition, weak peaks of V2O5 (JCPDS card 01-0720433) appeared, which indicated that a small quantity of V2O5 was produced. The XRD patterns of ZnV2O6, RGO, pCN and ZnV2O6/RGO/pCN composite are exhibited in Figure 1 (b). For pure RGO, a representatively broad peak at 2θ=25.0° is indexed

7



Page 8 of 44

to the (002) plane of RGO material 46. The XRD pattern of pCN displays two distinct peaks at 13.11° and 27.3°. The weak peak at 13.11° reflects the in-planar ordering of tri-s-triazine units, whereas the stronger peak at 27.3° can be assigned to the interlayer stacking of the conjugated aromatic systems

26.

The XRD patterns of the RGO modified ZnV2O6 sample displays

comparable diffraction patterns as ZnV2O6. The XRD patterns of the ZnV2O6/pCN composite shows blending of the two sets of diffraction patterns for ZnV2O6 and pCN. The diffraction peaks of ZnV2O6/pCN were not influenced with the increase in content of pCN. The characteristic RGO peak is not appeared in the ZnV2O6/RGO/pCN composite because of its low content. Surface area and physical properties of all photocatalysts are presented in Table 1. BET specific surface areas (SBET) of 11.57 and 10.98 m2 g-1 were obtained for ZnV2O6 and pCN, respectively. However, in the case of ZnV2O6/pCN (100%) nanosheets, the BET surface is slightly reduced to 11.26 m2 g-1. With RGO (4%) modified ZnV2O6 nanosheets, there is no significant effect on BET surface area. On the other hand, the BET surface area was increased to 12.13 m2 g-1 when 4 wt. % RGO and 100 wt. % pCN were combined to ZnV2O6 nanosheets. The BJH pore volume (VP) increased with ZnV2O6 combined pCN, possibly because of pCN distributed over the ZnV2O6, having higher pore volumes. The BJH surface area (SBJH) of pCN and RGO combined ZnV2O6 was increased. This approving pCN and RGO combined ZnV2O6 composites have higher mesoporosity than in pure ZnV2O6 nanosheets. Table 1: Physical properties of ZnV2O6 and pCN/RGO modified ZnV2O6 samples. Photocatalysts

SBET

SBJH

VP

Ebg

(m2 g-1)

(m2 g-1)

(cm3 g-1)

(eV)

ZnV2O6

11.57

3.80

0.014

2.02

pCN

10.98

11.56

0.069

2.6

ZnV2O6/RGO (4%)

11.62

10.48

0.017

1.97

8


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ZnV2O6/pCN 100%

11.26

4.8

0.047

2.21

ZnV2O6/RGO/pCN

12.13

8.93

0.058

2.16

Figure 2 exhibits FESEM analysis of pCN, ZnV2O6, ZnV2O6/RGO, ZnV2O6/pCN and ZnV2O6/RGO/pCN samples. It can be found that the functionalized g-C3N4 (pCN) layers are stacked together with irregular folding (Figure 2 a). Figure 2 (b and c) displays ZnV2O6 images obtained at different reaction stages. The ZnV2O6 prepared for 12 h reaction time consists of intermediate products which contains numerous compressed sheets as displayed in Figure 2 (b). It can be seen in figure 2 (c), many nanosheets were formed when the reaction period for catalysis synthesis was increased to 24 h. Furthermore, in the case of ZnV2O6 fabricated for 24 h reaction and calcined for 3 h at 550 ◦C, smooth and obvious nanosheets were obtained as exhibited in Figure 2 (d). Figure 2 (e) exhibits the ZnV2O6 nanosheets was partially covered by the RGO. It is obvious from Figure 2 (f) that for ZnV2O6/pCN sample, the pCN nanosheets spread uniformly on the surface of ZnV2O6 nanosheets to develop heterojunction composite structures. Figure 2 (g and h) are the images of ZnV2O6/RGO/pCN composite demonstrating that RGO inter layer is anchored between ZnV2O6 with pCN to produce 2D structured samples with higher interaction among the components. EDX mapping of RGO/pCN combined with ZnV2O6 sample are displayed in Figure 3. The EDX analysis in Figure 3 (a and b) shows the existence of RGO/pCN over the ZnV2O6 surface. The EDX plot of elements in Figure 3(c) confirms the presence of Vanadium, zinc, carbon, nitrogen and oxygen elements in ZnV2O6/RGO/pCN composite. The existence of platinum (Pt) was because of samples coated with Pt before FE-SEM analysis. These findings assured successful development of ZnV2O6/RGO/pCN composite heterojunction. The schematic of growth mechanism for Z-scheme ZnV2O6/RGO/pCN nanosheets is illustrated in Figure 4. The early reaction consists of intermediate products which contains 9



numerous compressed sheets 47. Additionally, RGO/pCN sheets are dispersed regularly on the ZnV2O6 nanostructure through the growth of nanosheets. When the reaction period was proceeded further, many ZnV2O6/RGO/pCN nanosheets were produced via in situ reduction and persistent disbanding of the nanosheets intermediate product, and concurrently gathered to nanosheets to decrease surface energy. As the reaction continued in prolonged time, the nanosheets totally gathered to uniform nanosheets with comparatively smooth surface of structured ZnV2O6/RGO/pCN composite with 2D nanosheets. TEM images obviously demonstrate the morphology and structure of the prepared samples as illustrated in Figure 5. The functionalized g-C3N4 (pCN) displayed a thin layered structure as shown in Figure 5 (a). It can be seen in Figure 5 (b), the ZnV2O6 (24 h reaction) composed of abundant compress nanosheets. Figure 5 (c) displays the ultrathin RGO nanosheet has an extremely transparent 2D nanostructure with clear folds and wrinkles. The RGO are distributed on the surface of ZnV2O6 sample as shown in Figure 5 (d), which discloses a decent mixture of RGO and ZnV2O6 to create heterojunction. From Figure 5 (e), the light parts relate to pCN while the dim parts ought to be ZnV2O6, which further demonstrates that the surface of ZnV2O6 has covered by pCN. This uncovers a decent blend of ZnV2O6 and pCN to create heterojunction. The RGO and pCN are effectively deposited over the ZnV2O6 surface in the sheets superstructure as exhibited in Figure 5 (f and g). The lattices fringes in the ZnV2O6 sample is around 0.48 nm, corresponding to plane (111) of orthorhombic stage. The nanosheets on the surface of ZnV2O6/RGO/pCN sample have the lattices fringes of 0.32 nm, corresponding to plane (002) of the graphite like carbon nitride 36. The SAED pattern of the ZnV2O6/RGO and ZnV2O6/RGO/pCN composite in Figure 5 (h and i), respectively, displays a clear crystalline ring due to great crystallization of ZnV2O6. The surface chemical compositions of ZnV2O6/RGO/pCN nanosheets were analyzed using XPS analysis as displayed in Figure 6. According to the survey XPS analysis (Figure.

10


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6a), V, Zn, O, N and C can be detected. The high resolution V2p XPS spectra (Figure 6b) displays two clear signals at 517 and 524.3 eV, corresponds to V2p3/2 and V2p1/2, respectively, which is typical characteristics of V5+ 48. Figure 6 (c) shows two peaks at 1021.5 and 1044.6 eV corresponding to Zn 2p3/2 and Zn 2p1/2, respectively, confirming the presence of Zn2+ 49. The high-resolution O1s XPS spectrum (Figure 6d) can be resolved into two peaks at 529.9 and 531.4 eV. The former is ascribed to the lattice-oxygen and the latter is ascribed to the adsorbedoxygen 44. The XPS of C1s for ZnV2O6/RGO/pCN composite is shown in Figure 6 (e). Two peaks positioned at 284.7 and 288.1 eV belong to sp2-hybridised carbons and the sp2-bonded C in N-containing aromatic rings (N-C=N), respectively

50-51.

N 1s XPS spectra of

ZnV2O6/RGO/pCN composite can be fitted into four peaks at 398.7 eV (sp2-bonded N atoms, C-N=C), 400.6 eV (ternary N groups, N-(C)3) and 401.8 eV (side N-H groups) 52-53 as displayed in Figure 6 (f). The small peak at 404.5 eV was owing to the successful functionalization of gC3N4 (positive charge) 26. The acid pre-treatment with nitric acid (HNO3) can simply alter the g-C3N4 to possess positive charge that can serve as a mediator and trap for photoexcited electrons. The optical properties of ZnO/V2O5, pCN, ZnV2O6, ZnV2O6 calcined, ZnV2O6/RGO (4%), ZnV2O6/pCN (50%), ZnV2O6/pCN (100%), and ZnV2O6/RGO/pCN composite were investigated by UV–Vis diffuse reflectance spectra as shown in Figure 7 (a). The RGO modified ZnV2O6/pCN composite can clearly improve the absorbance of pCN towards visible light illuminations. The wavelengths of ZnO/V2O5, pCN, ZnV2O6, ZnV2O6 calcined, ZnV2O6/RGO (4%), ZnV2O6/pCN (50%), ZnV2O6/pCN (100%) and ZnV2O6/RGO/pCN photo-catalysts are 447, 477, 613, 685, 630, 541, 560 and 574 nm, respectively. Clearly, ZnV2O6 sample exhibits higher absorption than ZnO/V2O5 in the visible-light zone. The ZnV2O6 calcined shifted absorbance of ZnV2O6 sample to the higher wavelength in the visible region. With RGO and pCN to ZnV2O6, the absorption wavelengths were shifted to higher

11



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value in the visible region. The band gap energies (Ebg) of photo-catalysts were calculated using Tauc equation (equation 1). E bg (eV) 

1240

(1)



The band gap (Ebg) of all the samples were calculated and results are presented in Table 1. The valence band energies (EVB) of ZnV2O6 and pCN samples were measured by x‐ray photoemission spectroscopy (XPS). Figure 7 (b and c) shows the valence band energies of ZnV2O6 and pCN were located at ∼1.15 and ∼1.48 eV, respectively. The conduction band energies (ECB) of ZnV2O6 and pCN samples were calculated using equation (2).

E VB  E CB  E bg

(2)

Therefore, the conduction band energies of ZnV2O6 and pCN were calculated to be -0.87 and -1.12 eV, respectively. Thus, it could be concluded that the valence band of ZnV2O6 is higher than pCN, and the conduction band of pCN is higher than ZnV2O6. This combination of band structures would be suitable for enhanced CO2 reduction to CH3OH under visible light irradiations. The samples are additionally characterized by Raman spectra as illustrated in Figure 8 (a). RGO displays a Raman shift at 1323 and 1581 cm-1 corresponding to the D and G bands, respectively. The Raman spectra of the bare pCN exhibits the presence of peaks at 476, 707, 763, 986, 1236 and 1310 cm−1 36. Also, the peaks of ZnV2O6 at 81, 276, 502, 698, 876, 900 and 986 cm−1 are found in the Raman spectra. For the ZnV2O6/RGO composite, all the peaks of ZnV2O6 are obtained in the Raman spectra while the D and G bands were not obtained because of its low contents. The characteristic peaks of the bare ZnV2O6 and pCN are found in the Raman spectra of the ZnV2O6/pCN and ZnV2O6/RGO/pCN samples. However, some peaks of ZnV2O6 and pCN are hard to identify in the Raman spectra of the ZnV2O6/pCN and

12


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ZnV2O6/RGO/pCN composite, which revealed pCN has high dispersion over the ZnV2O6 surface. Figure 8 (b) exhibits PL emission spectra of the RGO, ZnV2O6, pCN, ZnV2O6 calcined, ZnV2O6/RGO (4%), ZnV2O6/pCN (50%), ZnV2O6/pCN (100%) and ZnV2O6/RGO/pCN photo-catalysts excited at a wavelength of 325 nm. The ZnV2O6 shows lower peak intensity compared with the pCN and ZnV2O6 calcined samples. The calcined ZnV2O6 has higher peak intensity in comparison to ZnV2O6 due to difference in morphology and structure. This was also because of impurities present in pristine ZnV2O6 sample which help in trapping electrons, resulting in lower PL intensity. However, all the impurities were were disappeared after calcination of ZnV2O6 as evidenced by XRD analysis. After the pCN or RGO was introduced, the hetero-structured samples display lower peak intensity in comparison to the bare ZnV2O6. This was obviously due to trapping of electrons by RGO in the composite sample. Additionally, the peak of ZnV2O6/RGO/pCN composite displays lowest intensity. This shows, pCN/ZnV2O6 have good interaction to transfer electrons, while addition of RGO further improved electron transfer efficiency, thus lowest PL intensity was obtained. The current consequences demonstrate that RGO inserted Z-scheme photo-catalyst of ZnV2O6/RGO/pCN composite could contribute more active charge separation than pCN/ZnV2O6 nanosheets. Besides, functionalization of g-C3N4 and RGO could serve as multiple mediators to promote the separation of photo-generated carriers. 3.2

Photocatalytic activity and stability The experiments for photo-induced reduction of CO2 were performed at feed flow rate

20 ml min-1, room temperature and atmospheric pressure under visible-light illuminations. Without light or reactants, the carbon containing compounds were not produced in the reaction system. Nonetheless, a significant amount of methanol was obtained when CO2 was reduced in the presence of photo-catalyst and light illuminations. This demonstrates that reduction of 13



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CO2 requires both catalyst and light energy for photo-catalysis reaction. These observations evidenced that CH3OH was produced during CO2 reduction process instead from contamination in the catalyst if any The production of CH3OH over various photo-catalysts in different reaction medium has been summarized in Table 2. The photo-activity of g-C3N4 for CH3OH production was examined using CO2 reduction with water and sodium hydroxide (NaOH) solutions. It can be seen that the CO2 reduction over g-C3N4 in 0.1 M NaOH solution gave CH3OH yield of 732.9 µmol g-cat-1, 1.013 folds higher than using only H2O. More importantly, production of CH3OH over pCN of 753.7 µmol g-cat-1 was obtained, 1.03 times higher than its production with gC3N4 in a 0.1 M NaOH solution. Besides, using pure ZnO, V2O5 and ZnO/V2O5 photo-catalysts production of CH3OH was not much appreciable, however, CH3OH production was greatly improved with 2D ZnV2O6 nanosheets. This was because of hierarchical structure, efficient charge transfer property and effective visible light absorption. More significantly, production of CH3OH over ZnV2O6 clacined at 550 °C for 3 h was much lower compared to as prepared ZnV2O6 sample. According to PL spectrum, the calcined ZnV2O6 nanosheets shows higher peak intensity compared with the pure ZnV2O6. This shows as prepared ZnV2O6 has impurities which help in trapping and transportation of charge carrier as evidenced by XRD and PL analysis. Recently, Zhang et al. 45 reported similar observations while examining H2 evolution during water splitting over BiFeO3, Bi2Fe4O9 and BiFeO3/BiFe4O9 photo-catalysts under visible-light illuminations. Table 2: Summary of CH3OH production during photo-catalytic CO2 reduction using various reaction medium and photo-catalysts (Irradiation time 2 h, flow rate 20 mL min-1, atmospheric pressure and room temperature). Photocatalysts

Reaction medium

Yield (μmol g-cat-1)* CH3OH

g-C3N4

H2O

724.2

14


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g-C3N4

NaOH

732.9

pCN

NaOH

753.7

ZnO

NaOH

703.8

V2O5

NaOH

708.2

ZnO/V2O5

NaOH

767.8

ZnV2O6

NaOH

2250.5

ZnV2O6 (12 h)

NaOH

1531.3

ZnV2O6 (24 h)

NaOH

2250.5

ZnV2O6 (48 h)

NaOH

2294.9

ZnV2O6 (72 h)

NaOH

2339.2

ZnV2O6 (Calcined, 24 h)

NaOH

874.2

* Methanol

yield calculated at 2 h irradiation time.

The production of CH3OH over ZnV2O6 with various exposure times of heating for 12, 24, 48 and 72 h at 200 ◦C for photo-reduction of CO2 with H2O in NaOH solution are also summarized in Table 2. Comparatively, exposure time of heating for 24 h for synthesis of ZnV2O6 nanosheets shows higher CH3OH yield than heating time for 12 h. However, photocatalyst elaboration time of 48 and 72 h have no significant effect on CH3OH production rate. Thus, ZnV2O6 nanosheets synthesized at exposure time of 24 h were further tested to examine the influence of irradiation time and stability analysis. Figure 9 (a) shows performances of various samples like g-C3N4, pCN, ZnV2O6, ZnV2O6/pCN (100%), ZnV2O6/RGO (4%), ZnV2O6/2% RGO/pCN, ZnV2O6/4% RGO/g-C3N4, ZnV2O6/6% RGO/g-C3N4 and ZnV2O6/4% RGO/g-C3N4 on the photo-activity for dynamic CO2 reduction to CH3OH under visible-light illuminations. It is clear that the yield of CH3OH over g-C3N4 and pCN was lower than using ZnV2O6, which could be attributed to the better photo-absorption efficiency, hierarchical structure and efficient charge transfer property of

15



ZnV2O6 compared with g-C3N4 and pCN. The performance of ZnV2O6 was further improved when RGO and pCN were loaded to get ZnV2O6/RGO, ZnV2O6/pCN and ZnV2O6/RGO/pCN composite samples. Combined pCN with ZnV2O6 nanosheets has significantly improved photoreduction of CO2 and composite ratios of 1:1 gives highest yield of CH3OH. This improved in photoactivity was due to development of type II heterojunction to transfer electrons from pCN to ZnV2O6 and also because of appropriate band structure for enhanced CO2 reduction to CH3OH. More interestingly, addition of RGO into ZnV2O6/pCN develop Zscheme heterojunction enables efficient trapping and transport of electrons 55. 0.4 wt. % RGO was the optimal loading amounts at which highest methanol production (3724.4 μmole g-cat1)

was achieved. Moreover, yields of CH3OH was declined when RGO loading was exceeded

from 0.4 to 0.6 wt. %, probably due to shielding influence of higher RGO contents that decreased light illuminations striking the catalyst surface and perhaps due to developing charges recombination centres. Generally, significantly improved photo-activity of RGO modified ZnV2O6/pCN composite towards the reduction of CO2 to CH3OH was because of Zscheme heterojunction nanostructures which enhances separation of electron-hole pairs and inhibits charge carrier recombination in ZnV2O6/RGO/pCN photo-catalysts. The performance comparison of composite catalysts with pristine samples at different irradiation times is presented in Figure 9 (b). The continuous production of CH3OH was obtained over the entire irradiation time until reached to steady state. It can be noticed that without pCN or RGO, bare ZnO/V2O5 and ZnV2O6 photo-catalysts have lesser photoactivity for CH3OH production during photo-induced CO2 conversion compared with ZnV2O6/RGO, ZnV2O6/pCN and ZnV2O6/RGO/pCN composite samples. The photocatalytic activity of ZnV2O6 can be enhanced by increasing pCN content to an optimum loading of 100 wt. %. Beyond this loading amount, the CH3OH production was declined. This was probably because of lower activity of pCN, inappropriate band structure and increased in recombination rate of

16


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charge carriers. The CH3OH yield of 4% RGO modified ZnV2O6/100%pCN was around 5.74 and 1.67 times higher than that of bare ZnO/V2O5 and ZnV2O6, respectively. This higher production of CH3OH over ZnV2O6/RGO(4%)/pCN(100%) Z-scheme heterojunction composite was because of suitable band structure, effective visible light absorption and synergistic effects of pCN/RGO as mediators for faster separation and transportation of charges in the composite structure. The performance of photocatalysts for photo-reduction of CO2 with H2O was further evaluated based on quantum yield (QY). Herein, quantum yield (%) for generating CH3OH was calculated according to equation (3).

QY (%) 

n production rate (mol/s) 100 photon flux (mol/s)

(3)

The quantum yield was calculated based on 6 electrons (n) consumed for CH3OH production and moles of photon input energy over the photocatalyst surface. The quantum yield of CH3OH production over different photocatalyst samples is listed in Table 3. The QE of 0.2830 % for of H3OH production was obtained over ZnV2O6/RGO/pCN, which was 28.9, 8.3, 7.3 and 5.3 folds higher than ZnO/V2O5, ZnV2O6, ZnV2O6/pCN (100%) and ZnV2O6/RGO (4%) samples, respectively. The significantly improved QE over ZnV2O6/RGO/pCN was obviously due to more production of electrons due to structured composite catalyst, higher electron mobility and efficient charge transfer rate by RGO/pCN due to their function as multiple electron mediators. Table 3: Summary of methanol yield rates and quantum yield over various photocatalysts. Production rate (μmol g-cat.−1 h-1)

Quantum yield (%)

CH3OH

CH3OH

ZnO/V2O5

94.53

0.0098

ZnV2O6

325.38

0.0339

Catalysts

17



Page 18 of 44

ZnV2O6/pCN (100%)

374.22

0.0389

ZnV2O6/RGO (4%)

515.39

0.0537

ZnV2O6/RGO/pCN

542.92

0.2830

Yield rates calculated using 10 h irradiation, Lamp power 35 W, Intensity 20 mW cm−2 and wavelength 450 nm. Figure 10 illustrates schematic of heterojunction structures to understand the role of each component in the composite for photo-activity enhancement. The 2D ZnV2O6 nanosheets deliver lower photo-efficiency for improved reduction of CO2 to CH3OH due to faster charges recombination (Figure 10 a). When g-C3N4 sheets were loaded over the ZnV2O6 nanosheets to develop 2D/2D heterojunction structure, the photoactivity was enhanced due to faster charges transportation in nano-channels and adjustment of band structure for CH3OH production in the ZnV2O6/g-C3N4 composite sample as illustrated in Figure 10 (b). Additionally, the regularly dispersed functionalized g-C3N4 (pCN) nanosheets on ZnV2O6 nanosheets with heterojunction structure further increase the quantity of charge transfer nanochannels as illustrated in Figure 10 (c). Using RGO and uniformly distributed functionalized g-C3N4 (pCN) nanosheets on ZnV2O6 nanosheets with 2D heterojunction structures can build abundant high velocity charge transport nano-channels (Figure 10 d), which powerfully hasten the high-activity separation and relocation for the photo-generated charges. The

stability

of

ZnV2O6, ZnV2O6/pCN

(100%),

ZnV2O6/RGO

(4%)

and

ZnV2O6/RGO/pCN composites photocatalyst was additionally examined to assess the life of photo-catalysts under visible-light irradiations. In order to examine the stability of these photocatalysts, experiments were conducted with an accumulative of 32 h reaction time. The influences of RGO or pCN modified ZnV2O6 for photo-induced conversion of CO2 with H2O to CH3OH utilizing various irradiation times are illustrated in Figure 11. It can be seen that combining pCN with ZnV2O6 nanosheets provides higher photoactivity and stability for

18


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enhanced CO2 reduction to CH3OH than using pure ZnV2O6 nanosheets. The addition of RGO into ZnV2O6/pCN further improved the photoactivity and stability because of higher mobility of charges with hindered charges recombination rate by RGO due to its function as a mediator. 3.3

Reaction Mechanism for methanol production In this study, CH3OH was identified as the main photo-reduction products during photo-

induced CO2 reduction with H2O over Z-scheme ZnV2O6/RGO/pCN nanostructures. During the reduction process, reaction steps have been explained by the equations (4) to (10). hv ZnV2O6 / g- C3 N 4   ZnV2O6 (e- + h + ) / g- C3 N 4 (e- + h + )

(4)

Z-scheme ZnV2O6 (e- + h + ) / g- C3 N 4 (e- + h + )   ZnV2O6 (h + ) / g- C3 N 4 (e- )

(5)

pCN+ e-  pCN  e

(6)

RGO  e    RGO  e 

(7)

H 2O+ h +  • OH+ H +

(8)

CO 2 + e-  CO•2

(9)

CO 2 + 6 H + + 6e-  CH3OH+ H 2O

(10)

Equations (4) to (7) reveals photoexcited electron and hole pairs generation and their trapping via RGO and pCN. The CO2 reduction occurs at the conduction band by the electrons while H2O is oxidized by holes at the valence band and this is explained in equations (8) and (9). The generation of CH3OH through the reduction of CO2 is displayed in equation (10). The schematic presentation of photocatalytic CO2 reduction with H2O over ZnV2O6/RGO/pCN composite is presented in Figure 12. The ZnV2O6 and pCN are excited to produce the holes and electrons under visible-light illuminations. The pCN in the hetero-

19



structured photo-catalysts works as a sensitizer to absorb photons as well as excite electronhole pairs upon light irradiation. Since, the CB of pCN (-1.12 eV) is more negative than that of ZnV2O6 (-0.87 eV), the photo-excited electrons on pCN can transfer to the CB of ZnV2O6. ZnV2O6/pCN hybrids feature the type-II band alignment charge carrier transfer process, reduces the redox capacity of the systems, thus unavoidably lowering the photo-induced activity of the ZnV2O6/pCN hybrid photo-catalysts. However, loading RGO into ZnV2O6/pCN composite provides Z-scheme system which further improved CH3OH production due to faster charges separation and more appropriate band structure for oxidation and reduction reactions. This is because RGO provides conductive layer at the interface of two semiconductors to promote excellent interfacial charge transfer during the photo-induced conversion. It indicates that the RGO as a mediator clearly provides efficient channels to promote electrons transport with high mobility between ZnV2O6 and pCN. The stronger ability of electron transfers of RGO from ZnV2O6 conduction band to pCN valence band for the ZnV2O6/RGO/pCN than the ZnV2O6/pCN may be because of smaller electrical resistances of the contact interfaces of ZnV2O6/RGO and RGO/pCN than ZnV2O6/pCN 56. RGO and functionalization of g-C3N4 can serve as multiple mediators and trapped electrons; hence, the photo-induced electrons could be rapidly transported to the pCN nanosheets. Holes in the VB of ZnV2O6 react with H2O generating H+ and O2. However, in this Z-Scheme photo-catalytic system, the photo-generated electrons in the CB of ZnV2O6 can be transferred faster to the VB of pCN through RGO and recombine with photo-generate holes, which effectively reduce the recombination of photogenerated charge carriers. Therefore, the Z-scheme ZnV2O6/RGO/pCN composite displayed an improved photo-induced CO2 reduction activity because of the positive synergistic effect of ZnV2O6, RGO and pCN. 4.

Conclusions

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In summary, efficient photocatalytic reduction of CO2 to CH3OH under visible light irradiation is achieved using Z-scheme ZnV2O6/RGO/pCN nanostructures, which was prepared by one-step solvothermal method. RGO in the ZnV2O6/RGO/pCN serves as a charge transference bridge between ZnV2O6 and pCN, which could be the principal of gathering holes in the VB of ZnV2O6 and the electrons in the CB of pCN. The enhanced photo-activity and efficiency of the composite can be ascribed to the arrangement of the composite constituents which influenced the mechanism of the photo-reaction along with multiple mediators for trapping electrons. The maximum yield of CH3OH over ZnV2O6/RGO/pCN nanocatalyst was 5429 μmol g-cat-1, meaningfully higher than utilizing ZnV2O6 and pCN samples. Stability experiment assured prolonged life of ZnV2O6/RGO/pCN composite for selective CH3OH generation under visible light illuminations. The current study delivers a reduced graphene oxide inserted Z-scheme heterojunction with high photo-induced efficiency for effective CO2 photo-reduction, and it can be expanded to the design of a diversity of reduced graphene oxidebased Z-scheme photocatalysts for environmental applications. Acknowledgements The Ministry of Education (MOE) Malaysia is highly appreciated for providing financial support for this research work under Fundamental Research Grant Scheme (FRGS, Vot 4F876 and Vot 4F988) and Universiti Teknologi Malaysia under Research University Grant (RUG, Vot 17H06). References (1) Zhou, M.; Wang, S.; Yang, P.; Huang, C.; Wang, X. Boron Carbon Nitride Semiconductors Decorated with CdS Nanoparticles for Photocatalytic Reduction of CO2. ACS Catal. 2018, 8, 4928. (2) Bao, Y.; Chen, K. A novel Z-scheme visible light driven Cu2O/Cu/g-C3N4 photocatalyst using metallic copper as a charge transfer mediator. Mol. Catal. 2017, 432, 187. 21



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(46) Lv, H.; Wu, X.; Liu, Y.; Cao, Y.; Ren, H. In situ synthesis of ternary Zn0.5Cd0.5S (0D)/RGO (2D)/g-C3N4 (2D) heterostructures with efficient photocatalytic H2 generation activity. Mater. Lett. 2019, 236, 690. (47) Duan, F.; Dong, W.; Shi, D.; Chen, M. Template-free synthesis of ZnV2O4 hollow spheres and their application for organic dye removal. Appl. Surf. Sci. 2011, 258, 189. (48) Bai, J.; Li, X.; Liu, G.; Qian, Y.; Xiong, S. Unusual Formation of ZnCo2O4 3D Hierarchical Twin Microspheres as a High‐Rate and Ultralong‐Life Lithium‐Ion Battery Anode Material. Adv. Funct. Mater. 2014, 24, 3012. (49) Shahid, M.; Liu, J.; Ali, Z.; Shakir, I.; Warsi, M. F. Structural and electrochemical properties of single crystalline MoV2O8 nanowires for energy storage devices. J. Power Sources 2013, 230, 277. (50) Bao, Y.; Chen, K. Novel Z-scheme BiOBr/reduced graphene oxide/protonated g-C3N4 photocatalyst: Synthesis, characterization, visible light photocatalytic activity and mechanism. Appl. Surf. Sci. 2018, 437, 51. (51) Jo, W.-K.; Kumar, S.; Eslava, S.; Tonda, S. Construction of Bi2WO6/RGO/g-C3N4 2D/2D/2D hybrid Z-scheme heterojunctions with large interfacial contact area for efficient charge separation and high-performance photoreduction of CO2 and H2O into solar fuels. Appl. Catal., B 2018, 239, 586. (52) Lu, X.; Xu, K.; Chen, P.; Jia, K.; Liu, S.; Wu, C. Facile one step method realizing scalable production of g-C3N4 nanosheets and study of their photocatalytic H 2 evolution activity. J. Mater. Chem. A 2014, 2, 18924. (53) Hou, Y.; Wen, Z.; Cui, S.; Guo, X.; Chen, J. Constructing 2D porous graphitic C3N4 nanosheets/nitrogen‐doped graphene/layered MoS2 ternary nanojunction with enhanced photoelectrochemical activity. Adv. Mater. 2013, 25, 6291.

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Figure Captions Figure 1

XRD pattern of (a) ZnV2O6 and ZnV2O6 calcined and (b) RGO, pCN, ZnV2O6, ZnV2O6/RGO (4%), ZnV2O6/pCN (50%), ZnV2O6/pCN (100%) and ZnV2O6/RGO(4%)/pCN(100%) samples.

Figure 2

FE-SEM images of (a) pCN; (b) ZnV2O6 after 12 h; (c) ZnV2O6 after 24 h; (d) Calcined ZnV2O6 after 24 h; (e) ZnV2O6/RGO (4%); (f) ZnV2O6/pCN (100%); (g and h) ZnV2O6/RGO(4%)/pCN(100%).

Figure 3

SEM micrographs of RGO modified ZnV2O6/pCN nanosheets: (a) ZnV2O6/RGO(4%)/pCN(100%) samples; (b and c) EDX mapping of ZnV2O6/RGO(4%)/pCN(100%).

Figure 4

Schematic illustration for the ZnV2O6/RGO/pCN nanostructures.

Figure 5

TEM images of (a) pCN; (b) 2D ZnV2O6; (c) RGO; (d) ZnV2O6/RGO (4%) nanosheets; (e) ZnV2O6/pCN (100%) nanosheets; (f) ZnV2O6/RGO(4%)/pCN(100%) sample; (g) d-spacing of ZnV2O6/RGO(4%)/pCN(100%) sample; (h) SAED pattern of the ZnV2O6/RGO (4%) sample; (i) SAED pattern of the ZnV2O6/RGO(4%)/pCN(100%) sample.

Figure 6

XPS spectra of ZnV2O6/RGO(4%)/pCN(100%): (a) survey spectrum; (b) V 2p; (c) Zn 2p; (d) O 1s; (e) C 1s and (f) N 1s.

Figure 7

(a) UV–vis diffuse reflectance absorbance spectra of pCN, 2D ZnV2O6, calcined ZnV2O6, ZnO/V2O5, ZnV2O6/pCN (50%), ZnV2O6/pCN (100%), ZnV2O6/RGO (4%) and ZnV2O6/RGO(4%)/pCN(100%) samples; (b) Valence band (VB) XPS spectra of ZnV2O6; (c) Valence band (VB) XPS spectra of pCN.

Figure 8

(a) Raman spectra of pCN, ZnV2O6, RGO, ZnV2O6/pCN (50%), ZnV2O6/pCN (100%), ZnV2O6/RGO (4%) and ZnV2O6/RGO(4%)/pCN(100%) samples; (b) Photoluminescence (PL) spectra for pCN, ZnV2O6, calcined ZnV2O6, RGO, ZnV2O6/pCN (50%), ZnV2O6/pCN (100%), ZnV2O6/RGO (4%) and ZnV2O6/RGO(4%)/pCN(100%) samples.

Figure 9

(a) Yield of methanol over various photo-catalysts: reaction parameters (Room temperature, atmospheric pressure, feed flow rate 20 ml/min and irradiation time 2 h); (b) Effect of irradiation time on photocatalytic CO2 reduction to methanol over various photo-catalysts.

Figure 10

Schematic illustration of heterojunction structures for (a) 2D ZnV2O6 nanosheets; (b) ZnV2O6/g-C3N4 heterojunction; (c) ZnV2O6/g-C3N4 heterojunction with functionalization (HNO3) as a mediator; and (d)

29

formation


process

of

Z-scheme


ZnV2O6/RGO/g-C3N4 heterojunction with RGO and functionalization (HNO3) as multiple mediators. Figure 11

Stability study of ZnV2O6, ZnV2O6/pCN (100%), ZnV2O6/RGO (4%) and ZnV2O6/RGO(4%)/pCN(100%) samples for CO2 conversion to CH3OH.

Figure 12

Schematic diagram of the separation and transfer of photo-generated charges in ZnV2O6/RGO/pCN composite under visible light irradiation.

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pCN

ZnV2O6

(b)

RGO

ZnV2O6/RGO(4%)/pCN(100%)

Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ZnV2O6/pCN (100%) ZnV2O6/pCN (50%) ZnV2O6/RGO (4%) ZnV2O6 pCN RGO

10

20

30 40 50 2-theta (degree)

Figure 1

31


60

70

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Figure 2

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Figure 3

33



Figure 4

34


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Figure 5

35



Figure 6

36


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Figure 7

37



pCN,

ZnV2O6/pCN (100%), ZnV2O6/RGO(4%),

ZnV2O6, ZnV2O6/pCN (50%),

81

707

876

989

311

707

876

986

311

707

876

276

502

698

876

502

698

876

476

400

707 763

600

(a) 1553 1553 1546

986

900

ZnV2O6, ZnV2O6/pCN (50%),

900

ZnV2O6/pCN (100%),

1656 1656 1648

996

ZnV2O6/RGO(4%) ZnV2O6/RGO(4%)/pCN(100%) ZnV2O6 Calcined RGO

(b)

986

986

1236 1314 D band (1323)

200

pCN,

ZnV2O6/RGO(4%)/pCN(100%)

311

84 278

RGO

Intensity (a.u)

Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

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800 1000 1200 Raman Shift (cm-1)

G band (1581)

1400

1600

1800

200 240 280 320 360 400 440 480 520 560 600 640 680 720 Wavelength (nm)

Figure 8

38


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Yield of methanol (µmol/g cat )

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8000 7500 7000 6500 6000 5500 5000 4500 4000 3500 3000 2500 2000 1500 1000 500 0

ZnO/V2O5, ZnV2O6,

ZnV2O6/pCN (150%) ZnV2O6/RGO (4%)

ZnV2O6/pCN (50%), ZnV2O6/pCN (100%)

ZnV2O6/(4%)RGO/(100%)pCN

(b)

0

60

120

180

240

300

360

Time (min)

Figure 9 39


420

480

540

600


Figure 10

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Figure 11

41



Figure 12

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Highlights 

Self-assembly of Z-scheme ZnV2O6/RGO/g-C3N4 constructed by one-pot solvothermal method.



RGO and g-C3N4 (pCN) functions as multiple mediators to get higher yield and selectivity.



ZnV2O6/RGO/pCN exhibited superior activity toward reduction of CO2 to CH3OH.



CH3OH yield over Z-scheme ZnV2O6/RGO/pCN was 1.67 times of ZnV2O6 and 7.2-fold of pCN.



Z-scheme ZnV2O6/RGO/pCN sustained stability for CH3OH production even after 32 h irradiation time.

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Graphical Abstract

44


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g-C3N4 nanosheets

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