In situ growing of CoO nanoparticles on g-C3N4 composites with highly improved photocatalytic activity for hydrogen evolution

CoO/g-C3N4 hybrid catalyst is facilely prepared for application to photocatalytic H2 evolution from water splitting by the vacuum rotation–evaporation and in situ thermal method. The physical and chemical properties of CoO/g-C3N4 are determined by a series of characterization methods. The g-C3N4 with 0.6 wt% Co loading exhibits superior photocatalytic hydrogen evolution activity with an H2 evolution amount of 23.25 mmol g−1 after 5 h. The obtained 0.6 wt% CoO/g-C3N4 can split water to generate 0.39 mmol g−1 H2 without sacrificial agent and noble metal, while the pure g-C3N4 is inactive under the same reaction conditions. The remarkable enhancement of photocatalytic H2 evolution activity of CoO/g-C3N4 composites is mainly ascribed to the effective separation of electron–hole pairs and charge transfer. The work creates new opportunities for the design of low-cost g-C3N4-based photocatalysts with high photocatalytic H2 evolution activity from overall water splitting.

Review form: Reviewer 2

Recommendation?
Major revision is needed (please make suggestions in comments)

Comments to the Author(s)
The novelty of the work is not reflected in the introduction. Additionallly, the the result and discussion for hydrogen generation is not compared with respect to the bare CoO to highlight or to show the novelty of their work. However, the paper can be published after careful observation the reply of the major revision. 1. Why the peak (532.2) of 5 wt% CoO/g-C3N4 is shifted to the lower binding energy position? It is necessary to explain the trend of binding energy to the lower energy level with the increasing the CoO wt%. 2. The author need to give information of CoO/C3N4 composite prepared by the other groups? 3. Is the synthesis process of CoO, C3N4 and CoO/C3N4 is novel or other researcher already prepared the materials by the same method? 4. The author need to give the data of photocatalytic hydrogen evolution the bare CoO catalyst. It is necessary to compare it(bare CoO) with the CoO/C3N4 in Figure 6a (hydrogen generation), Figure 7 (hydrogen generation), and in Figure 8 (impedance).

Are the interpretations and conclusions justified by the results? Yes
Is the language acceptable? Yes

Do you have any ethical concerns with this paper? No
Have you any concerns about statistical analyses in this paper? No

Recommendation?
Major revision is needed (please make suggestions in comments)

Comments to the Author(s)
In this work, CoO nanoparticles are in-situ growing on the g-C3N4 to prepare CoO/g-C3N4 composite photocatalyst by facile thermal annealing method under nitrogen atmosphere.The enhancement mechanism of photocatalytic overall water splitting for hydrogen evolution of assynthetized CoO/g-C3N4 nanocomposite is also discussed. There are still some shortcomings in the paper. I believe the paper may be accepted for publication after carefully addressing the following points. 1. CoO nanoparticles are in-situ growing on the g-C 3 N 4 in this paper. What is the novelty or advantage over other literatures? The reason or characteristic for this method need to be emphasized in Introduction. 2. The results from this paper could be compared with the data from previous literatures. 3. Some expressions need to improved, such as "can't generate H 2 under" 4. The photocatalytic mechanisms of composites for water need to be discussed further, some reference could be referred. Ceramics International, 44 (2018), 1711-1718, Journal of Membrane Science, 520 (2016Ceramics International, 42 (2016), 15012-15022. 5. In the introduction, you should note the problems with traditional CoO/g-C3N4 combination methods, which can make the reader know what is unique about your work. 6. There are many previous works about the in-situ synthesis of nanoparticles, such as DOi:10.1016DOi:10. /j.electacta.2018DOi:10. .10.039, 10.1016DOi:10. /j.snb.2019.026 and so on. Is there improvement for in-situ synthesis? These need to be explained in Introduction.

24-Apr-2019
Dear Dr Liu: Title: In-situ growing of CoO nanoparticles on g-C3N4 composites with highly improved photocatalytic activity for hydrogen evolution Manuscript ID: RSOS-190433 Thank you for your submission to Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
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Once again, thank you for submitting your manuscript to Royal Society Open Science and I look forward to receiving your revision. If you have any questions at all, please do not hesitate to get in touch. ********************************************** RSC Associate Editor: Comments to the Author: Please address the reviewers' comments in full, in particular addressing their concerns about the advance in scientific knowledge presented in this work.
Major comments 1. The introduction of this manuscript is missing the important information on CoO as a photocatalyst for water splitting, and H2O2 poisoning of catalyst surface and challenges to overcome the drawbacks with citing the relevant Ref. 2. Authors mentioned (presumed) in the Page 3 Line 14,… "Therefore, the combination of g-C3N4 and CoO could achieve......... sacrificial agent. However, the combination of g-C3N4 and CoO NPs has already known to be improved photocatalysts for water splitting in the literature. (see Ref. Han et al. Inorg. Chem. Front., 2017, 4, 1691-1696Guo et al. Applied Catalysis B: Environmental, 226, 2018, 412-420;Wang et al., Inorganic Chemistry Communications, 92, 2018, 14-17). This phrase should be rewritten by including the above Ref.'s 3. Thermal annealing method for preparation of CoO@g-C3N4 composites is known in the literature (Ref, Han et al. Inorg. Chem. Front., 2017, 4, 1691-1696. Authors should justify how their approach is different or improved process than the reported Ref.
4. The main focus of present work is to explore the catalytic potential of CoO/g-C3N4 composites without Pt co-catalyst. The present draft shows more emphasis on Pt co-catalyst along with CoO/g-C3N4 in Fig 6a. Authors should move these controls into the Supp Information, and change the samples notation, for example, g-C3N4@Pt, 0.3%CoO/g-C3N4@Pt...........etc inset of Fog 6a..Include controlled catalysis of CoO alone and various wt% of CoO on g-C3N4 composites for water splitting in the main manuscript.
5. The photocurrent studies of the composites in the presence of light and dark need to be included in the manuscript.
6. The XPS characterization of recovered 0.6 wt% CoO/g-C3N4 catalyst need to be included to see the changes on surfaces. Comments to the Author(s) The novelty of the work is not reflected in the introduction. Additionallly, the the result and discussion for hydrogen generation is not compared with respect to the bare CoO to highlight or to show the novelty of their work. However, the paper can be published after careful observation the reply of the major revision. 1. Why the peak (532.2) of 5 wt% CoO/g-C3N4 is shifted to the lower binding energy position? It is necessary to explain the trend of binding energy to the lower energy level with the increasing the CoO wt%. 2. The author need to give information of CoO/C3N4 composite prepared by the other groups? 3. Is the synthesis process of CoO, C3N4 and CoO/C3N4 is novel or other researcher already prepared the materials by the same method? 4. The author need to give the data of photocatalytic hydrogen evolution the bare CoO catalyst. It is necessary to compare it(bare CoO) with the CoO/C3N4 in Figure 6a (hydrogen generation), Figure 7 (hydrogen generation), and in Figure 8 (impedance).

Reviewer: 3
Comments to the Author(s) In this work, CoO nanoparticles are in-situ growing on the g-C3N4 to prepare CoO/g-C3N4 composite photocatalyst by facile thermal annealing method under nitrogen atmosphere.The enhancement mechanism of photocatalytic overall water splitting for hydrogen evolution of assynthetized CoO/g-C3N4 nanocomposite is also discussed. There are still some shortcomings in the paper. I believe the paper may be accepted for publication after carefully addressing the following points. 1. CoO nanoparticles are in-situ growing on the g-C 3 N 4 in this paper. What is the novelty or advantage over other literatures? The reason or characteristic for this method need to be emphasized in Introduction. 2. The results from this paper could be compared with the data from previous literatures. 3. Some expressions need to improved, such as "can't generate H 2 under" 4. The photocatalytic mechanisms of composites for water need to be discussed further, some reference could be referred. Ceramics International, 44 (2018)

Recommendation?
Accept as is 3. Thermal annealing method for preparation of CoO@g-C3N4 composites is known in the literature (Ref, Han et al. Inorg. Chem. Front., 2017, 4, 1691-1696. Authors should justify how their approach is different or improved process than the reported Ref.
Response to Reviewer comment No.3: It is well known that the nanoparticles could be highly dispersed by rotation-evaporation method. Therefore, photocatalytic hydrogen generation performance of CoO/g-C3N4 composite will be improved by facile thermal annealing and rotation-evaporation method compared with the reported references.
Corresponding changes have been in the revised manuscript at line 72 as follow.
4. The main focus of present work is to explore the catalytic potential of CoO/g-C3N4 composites without Pt co-catalyst. The present draft shows more emphasis on Pt co-catalyst along with CoO/g-C3N4 in Fig 6a. Authors should move these controls into the Supp Information, and change the samples notation, for example, g-C3N4@Pt, 0.3%CoO/g-C3N4@Pt...........etc inset of Fog 6a. Include controlled catalysis of CoO alone and various wt% of CoO on g-C3N4 composites for water splitting in the main manuscript.
Response to Reviewer comment No. 4: In this paper, CoO/g-C3N4 composites with Pt co-catalyst are investigated to explore CoO/g-C3N4 composite with well-defined characteristics for overall water splitting. In the caption of figure 6a, the reaction condition with 1.5wt% Pt has been specified for the reader's better understanding.
The photocatalytic overall water-splitting performance of pure CoO has been studied in the Figure 7. It is found that 0.6wt% CoO/g-C3N4 can split pure water to generate H2 without sacrificial agent and noble metal Pt, while the pure g-C3N4 and bulk CoO exhibit negligible photocatalytic activity towards H2 generation under the same reaction condition. Corresponding changes have been in the revised manuscript. 6. The XPS characterization of recovered 0.6 wt% CoO/g-C3N4 catalyst need to be included to see the changes on surfaces.
Response to Reviewer comment No. 6: The XPS characterization of recovered 0.6 wt% CoO/g-C3N4 catalyst has been carried on to see the changes on surfaces. The following figure R1 shows XPS profiles of O 1s and Co 2p of 0.6 wt% CoO/g-C3N4 and recovered 0.6 wt% CoO/g-C3N4 samples. After being recovered, the signal of Co 2p has no significant change. The O 1s spectra with two peaks at about 529 eV and 532 eV are shown in figure R1. The binding energy at 529 eV is ascribed to the Co-O bond in the CoO phase, while the strong peak at about 532 eV corresponds to the Co-O-C bond, indicating that a strong interaction exists between CoO and g-C3N4. It can be seen that the signal of Co-O-C bond in the recovered 0.6 wt% CoO/g-C3N4 catalyst becomes much bigger than pure 0.6 wt% CoO/g-C3N4 because of the change of electronic state of adsorbed oxygen species by formed H2O2.   [25][26][27] . As the nanoparticles could be highly dispersed by vacuum rotation-evaporation method, in this paper, CoO/g-C3N4 composite is improved by vacuum rotation-evaporation and thermal annealing method compared with these reported references.
4. The author need to give the data of photocatalytic hydrogen evolution the bare CoO catalyst. It is necessary to compare it (bare CoO) with the CoO/C3N4 in Figure 6a figure   6(a), it can be found that the photocatalytic H2 evolution amount for CoO/g-C3N4 composite with 0, 0.3, 0.6, 1, 5 and 100 wt% Co loading content is recorded to be 14. 79, 17.19, 23.25, 13.02, 1.90 and 0.019 mmol g -1 after 5h, respectively. It is found that 0.6wt% CoO/g-C3N4 can split pure water to generate H2 without sacrificial agent and noble metal Pt, while the pure g-C3N4 and bulk CoO exhibit negligible photocatalytic activity towards H2 generation under the same reaction condition.
Reviewer: 3 1. CoO nanoparticles are in-situ growing on the g-C3N4 in this paper. What is the novelty or advantage over other literatures? The reason or characteristic for this method need to be emphasized in Introduction.
Response to Reviewer comment No. 1: It is still a challenge to seek suitable structure of CoO based catalyst with high activity and stability. CoO/C3N4 composite prepared by thermal annealing method has been reported by other literatures. As the nanoparticles could be highly dispersed by vacuum rotation-evaporation method, in this paper, CoO/g-C3N4 composite is improved by vacuum rotation-evaporation and thermal annealing method compared with these reported references. Corresponding changes have been in the revised manuscript at line 68-71 as follow.
2. The results from this paper could be compared with the data from previous literatures.
found that 0.6wt% CoO/g-C3N4 can split pure water to generate H2 without sacrificial agent and noble metal Pt, while the pure g-C3N4 and bulk CoO exhibit negligible photocatalytic activity towards H2 generation under the same reaction condition.
Corresponding changes have been in the revised manuscript at line 244-245 as follow.
4. The photocatalytic mechanisms of composites for water need to be discussed further, some reference could be referred. Ceramics International, 44 (2018) 5. In the introduction, you should note the problems with traditional CoO/g-C3N4 combination methods, which can make the reader know what is unique about your work.
Response to Reviewer comment No. 5: The problems with traditional CoO based materials have been noted in the introduction section in the revised manuscript. CoO with efficient photo-induced electrons separation can be used as an effectively co-catalyst to improve the photocatalytic water splitting activity for hydrogen evolution. It is reported that the combination of g-C3N4 and CoO can be improved photocatalysts for water splitting. But poor stability of the CoO catalyst is caused by H2O2 poisoning to hinder its further development. It is still a challenge to seek suitable structure of CoO based catalyst with high activity and stability. The particles could be well dispersed on the carrier by vacuum rotation-evaporation method. 28 In this work, CoO nanoparticles are in-situ growing on the g-C3N4 to prepare well-dispersed CoO/g-C3N4 composite photocatalyst by vacuum rotation-evaporation and thermal annealing method under nitrogen atmosphere. Corresponding changes have been in the revised manuscript at line 66-74 as follow.
6. There are many previous works about the in-situ synthesis of nanoparticles, such as DOi:10.1016/j.electacta.2018.10.039, 10.1016/j.snb.2019.02.026 and so on. Is there improvement for in-situ synthesis? These need to be explained in Introduction.
Response to Reviewer comment No. 6: The vacuum rotation-evaporation method was used to improve the in-situ synthesis. The particles could be well dispersed on the carrier by vacuum rotation-evaporation method. In this work, CoO nanoparticles are in-situ growing on the g-C3N4 to prepare well-dispersed CoO/g-C3N4 composite photocatalyst by vacuum rotation-evaporation and thermal annealing method under nitrogen atmosphere. Corresponding changes have been in the revised manuscript at line 70-74 as follow.
Finally, thanks very much for your kind work and precious comments of our paper.
On behalf of my co-authors, we would like to express our great appreciation to editor and reviewers.