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Article

Photoreduction of CO2 into CH4 Using Novel Composite of Triangular Silver Nanoplates on Graphene-BiVO4

by
Zhen Zhu
1,
Bo-Xun Jiang
2,
Ren-Jang Wu
2,*,
Cheng-Liang Huang
3,* and
You Chang
4
1
School of Environmental Science and Safety Engineering, Tianjin University of Technology, Tianjin 300384, China
2
Department of Applied Chemistry, Providence University, Taichung 43301, Taiwan
3
Department of Applied Chemistry, National Chiayi University, Chiayi City 600355, Taiwan
4
Chiou Shang Co., Ltd., Taichung 43301, Taiwan
*
Authors to whom correspondence should be addressed.
Catalysts 2022, 12(7), 750; https://doi.org/10.3390/catal12070750
Submission received: 13 June 2022 / Revised: 30 June 2022 / Accepted: 5 July 2022 / Published: 7 July 2022
(This article belongs to the Special Issue Graphene in Photocatalysis/Electrocatalysis)
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Abstract

:
Plasmonic photocatalysis, combing noble metal nanoparticles (NMNPs) with semiconductors, has been widely studied and proven to perform better than pure semiconductors. The plasmonic effects are mainly based on the localized surface plasmon resonance (LSPR) of NMNPs. The LSPR wavelength depends on several parameters, such as size, shape, the surrounding media, and the interdistance of the NMNPs. In this study, graphene-modified plate-like BiVO4 composites, combined with silver nanoplates (AgNPts), were successfully prepared and used as a photocatalyst for CO2 photoconversion. Triangular silver nanoplates (TAgNPts), icosahedral silver nanoparticles (I-AgNPs), and decahedra silver nanoparticles (D-AgNPs) were synthesized using photochemical methods and introduced to the nanocomposites to compare the shape-dependent plasmonic effect. Among them, T-AgNPts/graphene/BiVO4 exhibited the highest photoreduction efficiency of CO2 to CH4, at 18.1 μmolg−1h−1, which is 5.03 times higher than that of pure BiVO4 under the irradiation of a Hg lamp. A possible CO2 photoreduction mechanism was proposed to explain the synergetic effect of each component in TAgNPts/graphene/BiVO4. This high efficiency reveals the importance of considering the compositions of photocatalysts for converting CO2 to solar fuels.

Graphical Abstract

1. Introduction

Carbon dioxide is a major greenhouse gas that is considered the main reason for global warming [1]. Recently, owing to the rapid depletion of fossil energy, carbon dioxide emissions have significantly increased, leading to serious environmental problems. Thus, many efforts have been made to deal with this problem in recent years [2,3,4,5,6]. To this end, CO2 photoreduction is one of the most promising green methods to alleviate current environmental issues and energy crises, which converts carbon dioxide into valuable hydrocarbon fuel [5,6].
It is important to design and prepare highly effective photocatalysts to achieve optimal CO2 photoreduction performance. At this point, a number of photocatalysts have been exploited for CO2 photoreduction, such as TiO2 [7,8], NiCO2O4 [9], MnS [10], g-C3N4 [11], Bi2WO6 [12], WO3 [13], Bi4Ti3O12 [14], MoS2 [15], and so on. Due to its chemical stability, low production cost [16,17], environmental-friendliness, abundance, and appropriate bandgap, bismuth vanadate (BiVO4) has also been studied as a candidate material for CO2 photoreduction [16,17,18,19,20]. However, it has some shortcomings, such as the recombination of photoinduced charge carriers and pure BiVO4 exhibits relatively low CO2 photoreduction efficiency, which restricts the photocatalytic application of pure BiVO4 [20]. To solve the above shortcomings, several methods have been developed to improve the CO2 photoreduction performance of BiVO4, including surface modification, element doping, and metal deposition [3,16,18,19]. Table 1 lists conversion rates of CO2 to CH4 on several kinds of BiVO4 photocatalysts, drawing on information from previous studies and from this work [16,18,19]. The constructed series of CdS/BiVO4 nanocomposites was applied for the CO2 photoreduction reaction [18]. The optimal photocatalytic reduction rate was measured as 2.1 μmolg−1h−1 of the activity from converted CO2 into CH4 [18]. Flower-shaped g-C3N4/Ag/AgCl/BiVO4 microstructures were fabricated for the conversion of CO2 to CH4 [19], which exhibited a CH4 production rate of 5.3 μmolg−1h−1. A photocatalyst Cu/BiVO4 (0.5 wt% Cu) has the photocatalytic activity for CO2 photoreduction, with a maximum CH4 production rate of 7.4 μmolg−1h−1 [16]. Another parameter used to evaluate photoactivity was the quantum efficiency (QE) [15]. The QE of the catalysts was determined as the ratio of the effective electrons used for gas production, such as the CH4 molecule, to the total input photon flux [15]. The corresponding QE of all photocatalytic activity of samples is summarized in Table 1.
Plasmonic photocatalysis, combing noble metal nanoparticles (NMNPs) with semiconductors, has been widely studied and proven to perform better than pure semiconductors [21,22,23,24,25]. The plasmonic effects are mainly based on the phenomenon called localized surface plasmon resonance (LSPR) of NMNPs. The LSPR is the collective oscillation of conduction electrons on a nanoparticle surface. The LSPR wavelength depends on several parameters, such as size, shape, the surrounding media, and the interdistance of the NMNPs. Triangular silver nanoplates (TAgNPts) are particularly intriguing among all kinds of NMNPs, because the LSPR bands of TAgNPs can be easily adjusted using several physical and chemical methods [26,27,28,29]. To compare the plasmonic effects, icosahedral silver nanoparticles (I-AgNPs), decahedra silver nanoparticles (D-AgNPs), and TAgNPts were synthesized using photochemical methods in this study.
In the present investigation, we successfully synthesized TAgNPts/graphene/BiVO4, and some instrument characterizations were performed. The obtained TAgNPts/graphene/BiVO4 exhibited about five times higher activity in CO2 photoreduction than pure BiVO4. Additionally, we proposed a possible photocatalytic mechanism for CO2 photoreduction on TAgNPts/graphene/BiVO4 based on our experimental results.

2. Results and Discussion

2.1. Characterization of Composite Photocatalysts

The chemical composition and crystal structure parameters of materials can be determined using XRD spectra. Figure 1a shows the XRD spectrum of BiVO4 with characteristic diffraction peaks (2θ) at 18.6°, 28.2°, 30.6°, 34.4°, 35.1°, 39.7°, 42.5°, 46.3°, 46.8°, 50.1°, 53.3°, 58.1°, and 58.4°, which we attributed to the (110), (121), (040), (200), (002), (211), (051), (132), (240), (202), (161), (312), and (016) planes of monoclinic BiVO4, respectively [30]. Additionally, a small peak at around 21.5° could be indexed to the crystal (002) plane of graphene [31]. Furthermore, the prominent diffraction peak was observed, which is attributable to the (200) plane of Ag [30], as shown in Figure 1b. Using Scherer’s equation and XRD data, the mean crystallite sizes of all as-synthesized BiVO4 samples could be calculated within the range of 24.2–36.5 nm.
Figure 2a shows the SEM image of pure BiVO4. The plate-like microstructures, with a dimension of 0.4–1.5 µm, indicate that the nano-size BiVO4 tended to aggregate, possibly due to the shrinkage of the electrical double layer in the sample-preparation drying process for SEM measurement. Figure 2b,c show the SEM images of 7 wt% graphene/BiVO4 and TAgNPt/graphene/BiVO4 with 0.003 wt% of TAgNPt and 7 wt% of graphene, respectively. These two images also show the plate-like structures, similar to pure BiVO4, as shown in Figure 2a. This observation indicates that the addition of graphene and TAgNPt hardly changed the surface properties of composites. In addition, the composites heavily aggregated, such that some AgNPts (marked with red circles) were still observed on the surface of photocatalysts.
More detailed structure information of the photocatalysts was obtained using TEM measurements. As illustrated in Figure 3a,b, it can be seen that BiVO4 and graphene/BiVO4 presented lamellar-shaped nanoparticles. The distances between lattice fringes were 0.29 and 0.312 nm, consistent with the d-spacings of (040) and (121) planes of BiVO4, respectively [32,33]. Figure 3c depicts the triangular silver nanoplates embedded on the surface of graphene/BiVO4. In addition, the lattice spacing of about 0.204 nm corresponds to the d-spacing of the Ag (200) plane, as displayed in Figure 3d. Energy-dispersive X-ray spectroscopy (EDS) was carried out to further understand the chemical composition of as-synthesized TAgNPts/graphene/BiVO4. As shown in Figure 4, the EDS spectrum revealed the existence of V, Bi C, Ag, and O in TAgNPts/graphene/BiVO4.
The optical absorption properties of the as-prepared photocatalysts were investigated using UV–Vis spectroscopy. Compared with bare BiVO4, the addition of graphene and AgNPs enhanced light absorption in the visible region of the spectrum. Figure 5a,b show the increase in visible region absorption. This was also detected for TAgNPts/graphene/BiVO4, as shown in Figure 5b. According to Tauc’s equation (ahυ = K(hυ − Eg)n/2 with n = 1), the bandgaps of the as-prepared photocatalysts were estimated with similar values, and are shown in Table 2.
The photoluminescence intensities can indicate the changes in charge recombination rates of different photocatalysts. In Figure 6, it can be observed that the PL spectra of the as-obtained photocatalysts were quite broad, with the phonon band structure corresponding to the transitions from the VB to different phonon levels in the conduction band (CB) of the photocatalysts. Furthermore, the PL intensity of the as-synthesized TAgNPts/graphene/BiVO4 remarkably decreased compared with that of bulk BiVO4, suggesting that the recombination rate of photogenerated holes and electrons pairs was suppressed, leading to the improved photocatalytic performance of TAgNPts/graphene/BiVO4. Figure 7a–e show the XPS spectra of the 0.1% TAgNPt/7% graphene/BiVO4, which exhibited several peaks that we attributed to the binding energies of Bi 4f, V 2p, C 1s, O 1s, and Ag 3d. Figure 7a shows two peaks with 158 and 164 eV binding energies, which were attributed to Bi 4f7/2 and Bi 4f5/2, respectively. The fact that these two peaks are symmetrical in shape, without shoulders, indicates that only one oxidation state of Bi, i.e., Bi3+, was present in the as-prepared catalyst. Figure 7b shows two peaks at 516 and 524 eV, which were assigned to the binding energies of V 2p3/2 and V 2p1/2, respectively. Although the observation that peak separation (8 eV) and peak area ratio (close to 2:1) of V 2p3/2 and V 2p1/2 was consistent with the prediction of spin-orbital coupling, it remains challenging to understand why the peak width of V 2p1/2 was much broader than that of V 2p1/2. Figure 7c shows a peak at 529 eV, which we assigned to the binding energy of O 1s. Figure 7d shows one peak at 284 eV, which we assigned to C 1s. Figure 7e shows two peaks at 367  and 374 eV, which we assigned to the binding energies of the 3d5/2 and 3d3/2 of Ag, respectively. Because these two peaks were symmetrical in shape without the presence of shoulder peaks, most of the silver elements in the catalyst were metallic silver. Figure 7f shows the XPS valence band spectrum of BiVO4 and 0.1 wt% TAgNPt/7% graphene/BiVO4. By linearly extrapolating the low binding energy edge intersecting with the XPS background, the valence band energies (EVB) were calculated to be 1.50 and 1.25 eV for BiVO4 and 0.1 wt% TAgNPt/7% graphene/BiVO4, respectively. The band gaps (EBG) of BiVO4 and 0.1 wt% TAgNPt/7% graphene/BiVO4 were estimated to be 2.40 and 2.38 eV, respectively, from the UV–Vis spectra. Their conduction band energies (ECB) of BiVO4 and 0.1 wt% TAgNPt/7% graphene/BiVO4 were calculated to be −0.90 and −1.13 eV, respectively, using the equation: ECB = EVB − EBG. These band structure data of the different photocatalysts in this study are shown in Table 2.

2.2. CO2 Reduction Activity of Photocatalysts

Figure 8a shows the CH4 production yields of graphene/BiVO4 photocatalysts with different amounts of graphene under visible light irradiation. Pure BiVO4 exhibited the lowest photocatalytic activity, with a CH4 yield of 28.8 μmolg−1 after 8 h of irradiation. The CH4 production rate of BiVO4 was calculated as 3.6 μmolg−1h−1 (Table 3). Through the addition of graphene to BiVO4, the CH4 yield and production rate increased, which are shown in Figure 8a and Table 3, respectively. This result occurred due to the increase in visible light absorption intensity caused by the addition of graphene (Figure 5). Because the PL intensity of graphene/BiVO4 is lower than that of BiVO4 (Figure 6), another reason might be that the electron in the BiVO4 conduction band could transfer to graphene, thus decreasing the charge recombination rate and increasing the CH4 production rate. The CH4 production rates of graphene/BiVO4 with different loading amounts of graphene are shown in Table 3. Graphene/BiVO4 with an optimized loading amount of 7 wt% graphene reached a maximum CH4 production rate of 7.7 μmolg−1h−1.
Figure 8b reveals that the CH4 production yield could be further promoted by adding TAgNPts to 7% graphene/BiVO4. For the TAgNPts’ unique optical properties, the enhancing absorption of visible light was studied, as shown in Figure 5. Interestingly, the CH4 yield first increased with the introduction of AgNPts, and then decreased with increasing loading amounts of AgNPts. After 8 h of irradiation, the production yield of CH4 reached 144.8 μmolg−1. The optimal loading amount of AgNPts was 0.003 wt%, which is shown in Table 4 and Figure 8b.
Table 4 shows the CH4 production rates of various photocatalysts. Loading 7 wt% graphene/BiVO4 with decahedral silver nanoparticles (D-AgNPs) and triangular silver nanoplates (TAgNPts) improved the CH4 production rates. In contrast, loading icosahedral silver nanoparticles (I-AgNPs) depressed the production rate. This is because AgNPs can not only act as electron acceptors to block the electron–hole recombination on BiVO4, but can also exhibit plasmonic effects. AgNPs with quite different LSPR bands (or shapes) can be synthesized by choosing light with different wavelengths, even in the absence of surfactants. In order to compare the plasmonic effect of AgNPs with different shapes, I-AgNPs (λLSPR = 410 nm), D-AgNPs (λLSPR = 490 nm), and TAgNPts (λLSPR = 680 nm) were prepared using the photochemical method. Figure 9 shows the extinction spectra and the morphologies of these three kinds of AgNP colloids. The extinction spectra show that the LSPR bands of TAgNPts were broader or more intense in the visible range than those of D-AgNPs and I-AgNPs. In particular, TAgNPts generated stronger local fields under visible light irradiation than D-AgNPs and I-AgNPs. Additionally, TAgNPts with sharper corners could help generate stronger local fields than D-AgNPs and I-AgNPs [33]. After performing a series of experiments, an optimal loading amount (0.003 wt%) of TAgNPts was found on 7 wt% graphene/BiVO4 to achieve the maximum CH4 production rate. Through calculations, the corresponding quantum efficiency [15] of 0.003 wt% TAgNPts/7 wt% graphene/BiVO4 was determined to be 0.49%.
Stability is one of the primary parameters used for evaluating the performance of photocatalysts. Recycling experiments were conducted, and the results are displayed in Figure 10. After each run, the photocatalysts were recovered by drying in an oven at 373 K and reused for the next run. We observed that 0.003% TAgNPts/7% graphene/BiVO4 exhibited only a slight decrease in CH4 yield after three consecutive runs (maintained at approximately 97% from the first cycle), indicating that the as-prepared TAgNPts/graphene/BiVO4 demonstrated outstanding stability for CO2 photoreduction.

2.3. Mechanism of the CO2 Reduction Reaction

According to the above results and the corresponding analysis, a possible mechanism for the as-prepared TAgNPts/graphene/BiVO4 on the photocatalytic CO2 reduction process is proposed by Equations (1) to (6) and described in Figure 11. The electrons in the valence band (VB) were excited through light absorption to the conduction band (CB) of BiVO4 and generated electron–hole pairs (Equation (1)). Subsequently, a portion of electrons on the CB of BiVO4 can transfer to graphene and TAgNPts (Equations (3) and (4)) [30,34].
These energetic electrons can reduce CO2 to form CH4 (Equation (5)). There are many possible pathways in this CO2 photoconversion process, which are not discussed in this paper, but have been described in more detail in previous studies [8,35,36,37]. Here, graphene and TAgNPts can act as excellent electron acceptors [38,39], successfully preventing the recombination of the charge pairs on BiVO4. These remaining holes can oxidate H2O to form O2 and H+ (Equation (6)) [35,36,37].
Hot electrons and hot holes can also be generated from the decay of excited LSPR of TAgNPts (Equation (2)) [40,41]. In addition, the strong localized electric fields, produced by LSPRs of TAgNPts, may possibly play an important role in enhancing the probability of charge transfer [42].
BiVO4 + hv → BiVO4 (h+ + e−)
TAgNPt + hv → TAgNPt (h+ + e−)
BiVO4 (e−) + graphene→ BiVO4 + graphene (e−)
BiVO4 (e−) + TAgNPt→ BiVO4 + TAgNPt (e−)
CO2 + 8H+ + 8e−→ CH4 + 2H2O
2H2O + 4 h+ → O2 + 4H+

3. Experiment

3.1. Materials

Bismuth(III)nitrate pentahydrate (Bi(NO3)3•5H2O) from Alfa chemical company with 97% purity; ammonium metavanadate (NH4VO3), ethanol (C2H5OH), and ethylenediaminetetraacetic acid (EDTA) were obtained from Sigma Aldrich (>99%). Silver nitrate, sodium citrate, and sodium borohydride were purchased from Sigma-Aldrich (St. Louis, MO, USA). Graphene samples were provided by Chiou Shang Co., Ltd. Sodium hydroxide (NaOH, AR grade) was purchased from J. T. Baker (Phillipsburg, NJ, USA). All chemicals were used without any further purification. Milli-Q grade water (>18 M) was used in all of the experiments.

3.2. Preparation of Graphene/BiVO4 Catalyst and Ag Nanomaterials

As is typical, 5 mmol of Bi(NO3)3·5H2O and a certain amount of graphene were dissolved in 20 mL of HNO3 (2 M) with the aid of ultrasound for 1 h to form solution A, while 5 mmol of NH4VO3 and 3.4 mmol of EDTA were dissolved in 15 mL of NaOH (2 M) to form solution B. Then, the above two solutions were mixed, and the pH value of the solution was adjusted to 6.0 using a NaOH solution (2 M) under magnetic stirring for 0.5 h. Subsequently, the solution was poured into a Teflon-lined hydrothermal autoclave, and then baked in the oven at 180 °C for 6 h, followed by filtering and washing with deionized (DI) water and ethanol several times. Then, the solution was mixed with different amounts of graphene and dried at 80 °C for 12 h to harvest a serial of graphene/BiVO4.
For the purpose of comparing the plasmonic effect of the AgNPs with different morphologies, three kinds of AgNPs, including decahedral silver nanoparticles (D-AgNPs), icosahedral silver nanoparticles (I-AgNPs), and triangular silver nanoplates (TAgNPts), were synthesized using the plasmon-mediated process or photochemical methods according to previous studies [43,44,45]. A solution was prepared using a mixture of 1 mL of 1.0 × 10−2 M silver nitrate (AgNO3), 1 mL of 3.0 × 10−2 M sodium citrate, and 97.0 mL DI water. Subsequently, 1 mL of 5.0 × 10−3 M sodium borohydride (NaBH4) was added dropwise to the ice-cold mixture under magnetic stirring. The colorless solution immediately turned bright yellow after the addition of NaBH4. The bright yellow color indicated the formation of silver nanoseeds. Subsequently, the silver seed solution was irradiated for 90 min with a 400 W sodium lamp (Philips, average intensity per area ≈ 120 mW/cm2). The solution changed color from yellow to blue, indicating the formation of triangular silver nanoplates (T-AgNPts). D-AgNPs and I-AgNPs were obtained using a seed-free photo-assisted citrate reduction method under the irradiation of blue and violet LEDs, respectively [46]. As in a typical synthesis, 0.5 mL of sodium citrate (4.5 × 10−1 M) and 0.5 mL of AgNO3 (1.0 × 10−2 M) were mixed with 49.0 mL of pure water. Subsequently, the mixture was irradiated with blue LEDs (λmax = 476 nm, average intensity per area ≈ 80 mW/cm2) or violet LEDs (λmax = 405 nm, average intensity per area ≈ 60 mW/cm2). The colloidal solutions colored orange and yellow corresponded to the formations of D-AgNPs and I-AgNPs, respectively, after 90 min of LED irradiation.

3.3. Fabrication of Ag/Graphene/BiVO4 Composites

The as-obtained graphene/BiVO4 was added to the aqueous solution with different amounts of nano-Ag colloids (I-AgNPs, D-AgNPs, and TAgNPts) under continuous stirring for 0.5 h in the dark, before further irradiation under UV light for 2 h. The colloidal suspension was then centrifuged at 5000 g for 30 min, and the sediment was washed with DI water and dried at 80 °C for 12 h to obtain a series of AgNP/graphene/BiVO4.

3.4. XRD, SEM, TEM/EDS, UV–Vis, PL and XPS

The structural properties of as-prepared photocatalysts were characterized by the powder X-ray diffraction (XRD) method. The XRD spectra of all samples were measured using a powder XRD diffractometer (XRD-6000, Shimadzu Corp.) with Cu Kα radiation (λ = 1.5404 Å) at a scanning rate of 2°/min in the 2θ range of 10° to 80°. The average crystalline size (grain size) of the as-obtained photocatalysts was estimated using the Debye–Scherrer formula, D = κλ/βcosθ, where κ is 0.9, λ is the wavelength of the X-ray, θ is the Bragg angle, and β is the full width at half the maximum intensity (FWHM) of the diffraction peak. TEM images were obtained using a JEOL-JEM2100F operated at 200 kV. Samples for TEM measurements were dispersed in ethanol, and then a drop was placed on carbon-coated copper grids, followed by drying in ambient conditions. Energy dispersive X-ray spectroscopy (EDS) was used for elemental analysis. Extinction spectra were recorded using a T90+ UV–Vis spectrometer (PG Instrument Ltd.). The photoluminescence spectra (PL) were measured using an Edinburgh FS5 fluorescence spectrometer assembled with an ozone-free xenon lamp. The electronic structures of nanocomposites were analyzed by X-ray photoelectron spectroscopy (XPS) using a PHI 5000 versaprobe/scanning ESCA microprobe photoelectron spectroscope with Mono Al Kα radiation.

3.5. Photocatalytic Reduction of CO2

Figure 12 shows the experimental setup for the CO2 photoreduction and analysis system. Generally, 0.05 g of the as-prepared photocatalyst was placed and evenly distributed in a Pyrex glass container. High-purity CO2, regulated using a mass flow controller (15 mL min−1, 35 min), passed through a water bubbler to obtain a H2O/CO2 mixture. The glass container was irradiated under a 400 W high-pressure Hg lamp. The products (gas phase) were collected and analyzed using an online gas chromatograph (Gas Chromatography Personal GC 1000, Taipei, Taiwan) equipped with a Carboxen 1000 packed column at various times during the photoreaction. Each experiment was performed at least three times.

4. Conclusions

In summary, pure BiVO4, graphene/BiVO4, and AgNPs/graphene/BiVO4 photocatalysts with different loading ratios were synthesized to enhance the efficiency of CO2 conversion to CH4 under light irradiation. Our experimental results demonstrated that the as-obtained TAgNPts/graphene/BiVO4 with 7 wt% graphene and 0.003 wt% Ag exhibited optimum photocatalytic activity, about 5.03 times higher than that of pure BiVO4. Moreover, a possible mechanism for CO2 photoreduction was proposed based on the experimental data and results of this study. Therefore, this work introduced a feasible strategy to potentially design an effective photocatalyst for CO2 photoreduction.

Author Contributions

Conceptualization, C.-L.H. and R.-J.W.; methodology, R.-J.W.; software, B.-X.J.; validation, B.-X.J.; formal analysis, B.-X.J.; investigation, B.-X.J.; resources, Y.C.; data curation, B.-X.J.; writing—original draft preparation, Z.Z.; writing—review and editing, C.-L.H. and R.-J.W.; supervision, R.-J.W.; project administration, R.-J.W.; funding acquisition, R.-J.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Ministry of Science and Technology (Grant No.: MOST 110-2113-M-126-001), Taiwan.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The authors would like to thank the Ministry of Science and Technology (grant No.: MOST 110-2113-M-126-001), Taiwan, R.O.C., for the financial support that it provided for this study.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 12 00750 g001a 550Catalysts 12 00750 g001b 550
Figure 1. XRD patterns of the obtained photocatalysts (a) in the range of 2θ = 10°–80° and (b) in the range of 2θ of 40°–50°.
Figure 1. XRD patterns of the obtained photocatalysts (a) in the range of 2θ = 10°–80° and (b) in the range of 2θ of 40°–50°.
Catalysts 12 00750 g001aCatalysts 12 00750 g001b
Catalysts 12 00750 g002a 550Catalysts 12 00750 g002b 550
Figure 2. SEM images of the as-prepared (a) BiVO4, (b) 7% graphene/BiVO4, and (c) TAgNPt/Graphene/BiVO4 with 0.003 wt% of TAgNPt and 7 wt% of graphene.
Figure 2. SEM images of the as-prepared (a) BiVO4, (b) 7% graphene/BiVO4, and (c) TAgNPt/Graphene/BiVO4 with 0.003 wt% of TAgNPt and 7 wt% of graphene.
Catalysts 12 00750 g002aCatalysts 12 00750 g002b
Catalysts 12 00750 g003a 550Catalysts 12 00750 g003b 550
Figure 3. TEM images of the as-prepared photocatalysts. (a) HR-TEM image of pure BiVO4; (b) HR-TEM image of 7 wt% graphene/BiVO4; (c) low-resolution TEM image; (d) HR-TEM image TAgNPt/graphene/BiVO4 with 0.003 wt% TAgNPts and 7 wt% graphene.
Figure 3. TEM images of the as-prepared photocatalysts. (a) HR-TEM image of pure BiVO4; (b) HR-TEM image of 7 wt% graphene/BiVO4; (c) low-resolution TEM image; (d) HR-TEM image TAgNPt/graphene/BiVO4 with 0.003 wt% TAgNPts and 7 wt% graphene.
Catalysts 12 00750 g003aCatalysts 12 00750 g003b
Catalysts 12 00750 g004 550
Figure 4. EDS spectrum of 0.003% TAgNPt/7% graphene/BiVO4.
Figure 4. EDS spectrum of 0.003% TAgNPt/7% graphene/BiVO4.
Catalysts 12 00750 g004
Catalysts 12 00750 g005 550
Figure 5. UV–Vis absorption spectra of the obtained photocatalysts (a) by varioue loading amounts of graphene to BiVO4 (b) by various kinds of silver on 7% graphene/BiVO4.
Figure 5. UV–Vis absorption spectra of the obtained photocatalysts (a) by varioue loading amounts of graphene to BiVO4 (b) by various kinds of silver on 7% graphene/BiVO4.
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Catalysts 12 00750 g006 550
Figure 6. PL spectra of the as-obtained photocatalysts.
Figure 6. PL spectra of the as-obtained photocatalysts.
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Catalysts 12 00750 g007 550
Figure 7. XPS spectra of 0.1%TAgNPt/7% graphene/BiVO4; (a) Bi 4f spectrum, (b) V 2p spectrum, (c) O 1s spectrum, (d) C 1s spectrum, (e) Ag 3d spectrum, and (f) valence band spectra of BiVO4 (black) and 0.1 wt% TAgNPt/BiVO4 (red).
Figure 7. XPS spectra of 0.1%TAgNPt/7% graphene/BiVO4; (a) Bi 4f spectrum, (b) V 2p spectrum, (c) O 1s spectrum, (d) C 1s spectrum, (e) Ag 3d spectrum, and (f) valence band spectra of BiVO4 (black) and 0.1 wt% TAgNPt/BiVO4 (red).
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Catalysts 12 00750 g008 550
Figure 8. Production yields of CH4 over the as-obtained photocatalysts.
Figure 8. Production yields of CH4 over the as-obtained photocatalysts.
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Catalysts 12 00750 g009 550
Figure 9. The extinction spectra and the morphologies of I-AgNP, D-AgNP, and TAgNPt colloids.
Figure 9. The extinction spectra and the morphologies of I-AgNP, D-AgNP, and TAgNPt colloids.
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Catalysts 12 00750 g010 550
Figure 10. Recycling stability tests over the as-obtained 0.003%TAgNPt/7% graphene/BiVO4.
Figure 10. Recycling stability tests over the as-obtained 0.003%TAgNPt/7% graphene/BiVO4.
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Catalysts 12 00750 g011 550
Figure 11. Proposed mechanism for CO2 photoreduction over the as-obtained TAgNPt/Graphene/BiVO4.
Figure 11. Proposed mechanism for CO2 photoreduction over the as-obtained TAgNPt/Graphene/BiVO4.
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Catalysts 12 00750 g012 550
Figure 12. Schematic of the experimental setup for CO2 photoreduction.
Figure 12. Schematic of the experimental setup for CO2 photoreduction.
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Table
Table 1. Production rate of CH4 on various BiVO4 catalysts.
Table 1. Production rate of CH4 on various BiVO4 catalysts.
Year/ReferencePhotocatalysts
/Light Source		Rate of CH4 (μmol g−1 h−1)Quantum Efficiency (%)
2018 [18]CdS/BiVO4
300 W xenon lamp		2.1--
2018 [19]g-C3N4/Ag/AgCl/BiVO4
64 W fluorescent lamps		5.3--
2020 [16]0.5% Cu/BiVO4
400 W Hg light		7.40.19
2021 [17]0.3% Pd/MgO/BiVO4
400 W Hg light		12.80.34
2021 [20]ZnIn2S4/BiVO40.3--
2021 [15]Ultrathin MoS2 film1.38 nmol/cm20.00007
This work0.003%AgNPts/
7% graphene/BiVO4
400 W Hg light		18.10.49
-- No measurement.
Table
Table 2. Band gap calculation of the obtained photocatalysts.
Table 2. Band gap calculation of the obtained photocatalysts.
PhotocatalystBand Gap (eV)Valence Band (eV)Conduction Band (eV)
BiVO42.401.50−0.90
7% graphene/BiVO42.40----
0.1% TAgNPt/7% graphene
/BiVO4		2.381.25−1.13
0.5 wt% I-AgNP
/BiVO4		2.40----
-- No measurement.
Table
Table 3. Production rate of CH4 on various loading of graphene with BiVO4.
Table 3. Production rate of CH4 on various loading of graphene with BiVO4.
PhotocatalystsRate of CH4 (μmol g−1 h−1)
TiO2 (P25)0.4
BiVO43.6
1 wt% graphene/BiVO44.4
2 wt% graphene/BiVO45.2
3.5 wt% graphene/BiVO45.6
5 wt% graphene/BiVO46.4
7 wt% graphene/BiVO47.7
8.5 wt% graphene/BiVO45.7
10 wt% graphene/BiVO45.3
Table
Table 4. Production rate of CH4 on various photocatalysts.
Table 4. Production rate of CH4 on various photocatalysts.
PhotocatalystsRate of CH4 (μmol g−1 h−1)
BiVO43.6
7 wt% graphene/BiVO47.7
0.001 wt% TAgNPts/7 wt% graphene/BiVO412.7
0.003 wt% TAgNPts/7 wt% graphene/BiVO418.1
0.0075 wt% TAgNPts/7 wt% graphene/BiVO48.8
0.003 wt% D-AgNPs/7 wt% graphene/BiVO48.4
0.003 wt% I-AgNPs/7 wt% graphene/BiVO47.0
0.5 wt% I-AgNPts/7 wt% graphene/BiVO44.5
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Zhu, Z.; Jiang, B.-X.; Wu, R.-J.; Huang, C.-L.; Chang, Y. Photoreduction of CO2 into CH4 Using Novel Composite of Triangular Silver Nanoplates on Graphene-BiVO4. Catalysts 2022, 12, 750. https://doi.org/10.3390/catal12070750

AMA Style

Zhu Z, Jiang B-X, Wu R-J, Huang C-L, Chang Y. Photoreduction of CO2 into CH4 Using Novel Composite of Triangular Silver Nanoplates on Graphene-BiVO4. Catalysts. 2022; 12(7):750. https://doi.org/10.3390/catal12070750

Chicago/Turabian Style

Zhu, Zhen, Bo-Xun Jiang, Ren-Jang Wu, Cheng-Liang Huang, and You Chang. 2022. "Photoreduction of CO2 into CH4 Using Novel Composite of Triangular Silver Nanoplates on Graphene-BiVO4" Catalysts 12, no. 7: 750. https://doi.org/10.3390/catal12070750

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