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

Round 1

Reviewer 1 Report

In this manuscript, graphene, BiVO4 and silver nanoplates has been composed as a photocatalyst for photocatalytic CO2 reduction to CH4, which performs much better than pure BiVO4. This work proposes an effective method to improve the photocatalytic CO2 reduction property of BiVO4, but major revision is needed before considering the manuscript for publication.

(1)   In the abstract, the meaningless expression” Characterization techniques used for TAgNPts/Graphene/BiVO4 include X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), UV-vis diffuse reflectance spectroscopy (DRS), and photoluminescence spectroscopy (PL) ”  should be replaced by the main results from these characterizations.

(2)   There are extensive reports on photocatalytically CO2 reduction using BiVO4-baed photocatalysts. More references should be added in table 1.

(3)   How did you confirm that the small particles in Figure 2(c) are Ag nanoparticles instead of debris of BiVO4?

(4)   In Figure 8, the CH4 yields with irradiation time should be presented using the form of “the data plus line” instead of line.

(5)   In table 4, the performance of 0.003 wt% I-AgNPts/7wt% Graphene/BiVO4 should be added, to make a comparison with 0.003 wt% D-AgNPs/7 wt% Graphene/BiVO4 and 0.003 wt% AgNPts/7 wt% Graphene/BiVO4.

(6)   The authors stated” the hot hole cans also transfer from BiVO4 to TAgNPts [Eq. (6)“ How was hot holes generated on BiVO4?

(7)   The authors stated that “These energetic electrons can reduce CO2 to form CH4 [Eq. (7)]”, Here, [Eq. (7)] should be [Eq. (8)].

(8)   The description of the photocatalytic mechanism is confusing. Please describe the equations in order. Furthermore, [Eq. (7)] was not described.

Author Response

Reviewer 1

Comments and Suggestions for Authors

In this manuscript, graphene, BiVO4 and silver nanoplates has been composed as a photocatalyst for photocatalytic CO2 reduction to CH4, which performs much better than pure BiVO4. This work proposes an effective method to improve the photocatalytic CO2 reduction property of BiVO4, but major revision is needed before considering the manuscript for publication.

• In the abstract, the meaningless expression” Characterization techniques used for TAgNPts/Graphene/BiVO4 include X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), UV-vis diffuse reflectance spectroscopy (DRS), and photoluminescence spectroscopy (PL) ”  should be replaced by the main results from these characterizations.

Response: We are thankful for the reviewer’s thoughtful comments. We have rewritten the abstract by following.

Abstract: “Plasmonic photocatalysis, combing Noble metal nanoparticles (NMNPs) with semiconductors, has been studied widely 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 the size, shape, the surrounding media, and the inter distance 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 the 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, 18.1 μmolg^-1h^-1, which is 5.03 times higher than pure BiVO₄ under the irradiation of a Hg lamp. A possible CO₂ photoreduction mechanism has been proposed to explain the synergetic effect of each component in TAgNPts/Graphene/BiVO₄. This high efficiency gives a hint for considering the compositions of photocatalysts for converting CO₂ to solar fuels.”

• There are extensive reports on photocatalytically CO2 reduction using BiVO4-baed photocatalysts. More references should be added in table 1.

Response: We are thankful for the reviewer’s valuable comments. We have rewritten the Table 1 and added some references in the revised manuscript by following.

“Another parameter to evaluation the photo-activity is used 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 are summarized in Table 1.”

Table 1. Production rate of CH₄ on various BiVO₄ catalysts.

         --No measurement

• How did you confirm that the small particles in Figure 2(c) are Ag nanoparticles instead of debris of BiVO4?

Response: We are thankful for the reviewer’s thoughtful comments. Indeed, due to the resolution, it is not easy to point out the images belonging to T-AgNPs in the SEM images. In contrast, we can find the T-AgNPs on the nanocomposites in the TEM images using the information obtained from the lattice fringe. TEM images are formed when the electrons penetrate the sample, while SEM images are formed when the sample scatters the electrons. Thus, the samples should be thin enough, i.e., less than several tens nm, to be detected well by TEM; however, it is easier to obtain good SEM images when the sample is thick enough or when the sample is aggregated. We thoroughly compared several SEM images of TAgNPt/Graphene/BiVO₄ and Graphene/BiVO₄. Due to the size of triangular shape ghosts (indicated with the red circle in Figure 2c) on BiVO₄ being almost the same as the triangular silver nanoplates in TEM images, it is straightforward to assign this image to TAgNPts.

Figure 2. (c) TAgNPt/Graphene/BiVO₄ with 0.003 wt% of TAgNPt and 7 wt% of graphene.

• In Figure 8, the CH4 yields with irradiation time should be presented using the form of “the data plus line” instead of line.

Response: Thanks for this thoughtful comment. We have changed Figure 8 using “the data plus line.”

Figure 8. Production yields of CH₄ over the as-obtained photocatalysts.

• In table 4, the performance of 0.003 wt% I-AgNPts/7wt% Graphene/BiVO4 should be added, to make a comparison with 0.003 wt% D-AgNPs/7 wt% Graphene/BiVO4 and 0.003 wt% AgNPts/7 wt% Graphene/BiVO4.

Response: Thanks for this thoughtful comment. We have changed Table 8 and related texts by addition the performance of 0.003 wt% I-AgNPts/7wt% Graphene/BiVO₄ photocatalysts.

“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) can improve CH₄ production rates. In contrast, loading icosahedral silver nanoparticles (I-AgNPs) depress the production rate.”

Table 4. Production rate of CH₄ on various photocatalysts

• The authors stated” the hot hole cans also transfer from BiVO4 to TAgNPts [Eq. (6)“ How was hot holes generated on BiVO4?

Response: Thanks for these thoughtful comments. We have rewritten the paragraph (with blue color words) in the 2.3 section.

• The authors stated that “These energetic electrons can reduce CO2 to form CH4 [Eq. (7)]”, Here, [Eq. (7)] should be [Eq. (8)].

Response: Thanks for this thorough opinion. We have corrected the number of the equation and rewritten the paragraph (with blue color words) in the 2.3 section.

• The description of the photocatalytic mechanism is confusing. Please describe the equations in order. Furthermore, [Eq. (7)] was not described.

Response: Thanks for this thoughtful comments. We have rewritten the paragraph in the 2.3 section.

“These energetic electrons can reduce CO₂ to form CH₄ [Eq. (5)]. There are many possible pathways in this CO₂ photoconversion process, which are not discussed in this work and are described in more detail in previous studies [8, 35-37]. 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⁺ [Eq. (6)][35-37].

   Hot electrons and hot holes can also be generated from the decay of excited LSPR of TAgNPts [Eq. (2)][40, 41]. In addition, the strong localized electric fields produced by LSPRs of TAgNPts may possibly play an important role to enhance the probability of charge transfer [42].“

BiVO₄ + hv → BiVO₄ (h⁺ + e⁻)                                                             (1)

TAgNPt + hv → TAgNPt (h⁺ + e⁻)                                                         (2)

BiVO₄ (e⁻) + graphene→ BiVO₄ + graphene (e⁻)                             (3)

BiVO₄ (e⁻) + TAgNPt→ BiVO₄ + TAgNPt (e⁻)                                          (4)

CO₂ + 8H⁺ + 8e⁻→ CH₄ + 2H₂O                                                           (5)

2H₂O + 4 h⁺ → O₂ + 4H⁺                                                                         (6)

Author Response File: Author Response.pdf

Reviewer 2 Report

The work submitted by Zhu et al. devoted to the investigation of promising system TAgNPs supported on Graphene-BiVO4 in photoreduction of CO₂. The introduction clearly lays out reasons for studying of such system. The results are quite interesting. Authors used a fruitful combination of different techniques for the catalysts investigation, including XRD, XPS, SEM, DRS, PL and TEM accompanied by EDS. But despite to the overall positive impression there are some improving of data presentation should be done. I would recommend the article presented for publication in the Catalysts journal only after the following major issues are taken into account in a revised version.

Comments for authors listed below:

1)    Overall impression – the results discussed in a really thesis. For example when authors discuss the XRD data – in order to understand which exactly measurements were done the reader have to look on the Figure 1. While even you look on the Figure still don’t clear what is on the Figure 1b, the legend says that it is the samples of something containing the different amount of TAgNPs. What is triangle plates? As I understand the Fig 1a and 1b show the XRD data measured for completely different samples, but you could not find the information concerning this not in text of the manuscript nor at the Figure caption. In principal you can only guess it from the legends. And more or less similar situation with the discussion of rest results.

2)    Table 1. The comparison of the reaction rates is not correct due to the fact that the values directly depend on the conditions of the catalytic tests, i.e. type of reactor, amount of the catalysts etc. Usually TOF values are used to do so.

3)    The Experimental part the description of XPS measurements is missing.

4)    The Binding energies values of XPS spectra defined with accuracy 1 eV. Usually shifts in 0.2-0.3 eV could already pointing on the change in chemical or charging state. Unclear which line was used as internal standard for the peaks position correction? Usually the discussion of the chemical states of the elements from BE values should include links on the relevant articles.

5)    Are the VB spectra were measured at the same conditions as the core-levels peaks?

6)    Fig.8. As I understand the data presented were obtained after a certain period of time point by point. From my opinion it would be better to add the dots to the lines presented on the Figure.

Author Response

Reviewer 2

Comments and Suggestions for Authors

The work submitted by Zhu et al. devoted to the investigation of promising system TAgNPs supported on Graphene-BiVO4 in photoreduction of CO₂. The introduction clearly lays out reasons for studying of such system. The results are quite interesting. Authors used a fruitful combination of different techniques for the catalysts investigation, including XRD, XPS, SEM, DRS, PL and TEM accompanied by EDS. But despite to the overall positive impression there are some improving of data presentation should be done. I would recommend the article presented for publication in the Catalysts journal only after the following major issues are taken into account in a revised version.

Comments for authors listed below:

• Overall impression – the results discussed in a really thesis. For example when authors discuss the XRD data – in order to understand which exactly measurements were done the reader have to look on the Figure 1. While even you look on the Figure still don’t clear what is on the Figure 1b, the legend says that it is the samples of something containing the different amount of TAgNPs. What is triangle plates? As I understand the Fig 1a and 1b show the XRD data measured for completely different samples, but you could not find the information concerning this not in text of the manuscript nor at the Figure caption. In principal you can only guess it from the legends. And more or less similar situation with the discussion of rest results.

Response: Thanks for this thorough opinion. The XRD spectra with the indication were redraw in Figures 1a and 1b. We hope these figures clearly show that the signal corresponding to 200 plane of Ag in the XRD spectra increases as more TAgNPts are added into the nanocomposites.

Figure 1. XRD patterns of the obtained photocatalysts (a) in the range of 2θ = 10^。-80^。 degree (b) in the range of 2θ from = 40^。-50^。

• Table 1. The comparison of the reaction rates is not correct due to the fact that the values directly depend on the conditions of the catalytic tests, i.e. type of reactor, amount of the catalysts etc. Usually TOF values are used to do so.

Response: We are thankful for the reviewer’s valuable comments. We have rewritten the Table 1 and related texts in the revised manuscript by following.

“Another parameter to evaluation the photo-activity is used 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 are summarized in Table 1.”

Table 1. Production rate of CH₄ on various BiVO₄ catalysts.

         --No measurement

• The Experimental part the description of XPS measurements is missing.

Response: Thanks for the kind suggestion. We have added the description (with red color) about XPS measurement in the 3.5 section.

“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.”

• The Binding energies values of XPS spectra defined with accuracy 1 eV. Usually shifts in 0.2-0.3 eV could already pointing on the change in chemical or charging state. Unclear which line was used as internal standard for the peaks position correction? Usually the discussion of the chemical states of the elements from BE values should include links on the relevant articles.

Response: Thanks for the kind suggestion. Because the resolutions of XPS spectra of these nanocomposites are not good enough for us to determine the oxidation state, we tend to identify the elements using these spectra without discussing the change of their charges.

• Are the VB spectra measured at the same conditions as the core-levels peaks?

Response: Yes, they were measured in the same condition.

6)    Fig.8. As I understand the data presented were obtained after a certain period of time point by point. From my opinion it would be better to add the dots to the lines presented on the Figure.

Response: Thanks for the kind suggestion. We have added the dots to the lines in Figure 8.

Figure 8. Production yields of CH₄ over the as-obtained photocatalysts.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

the manuscript has been revised according to my comments and can be accepted for publication now.

Reviewer 2 Report

The authors anwsered the questions adequately.