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Article

Fabrication of Six Manganese Containing Polyoxometalate Modified Graphite C3N4 Nanosheets Catalysts Used to Catalyze Water Decomposition

by
Yue Wu
1,*,
Xiaoxia Yu
1,
Zhijing Fu
2,
Jianye Pei
1 and
Lihua Bi
1,*
1
College of Chemistry, Jilin University, Changchun 130012, China
2
College of Water Resources and Environment, Hebei GEO University, Shijiazhuang 050031, China
*
Authors to whom correspondence should be addressed.
Catalysts 2021, 11(7), 856; https://doi.org/10.3390/catal11070856
Submission received: 16 June 2021 / Revised: 6 July 2021 / Accepted: 14 July 2021 / Published: 17 July 2021
(This article belongs to the Special Issue 10th Anniversary of Catalysts—Feature Papers in Photocatalysis)
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Versions Notes

Abstract

:
With the increase in gas population, the demand for clean and renewable energy is increasing. Hydrogen energy has a high combustion conversion energy while water is its combustion product. In recent years, a way to convert water into hydrogen and oxygen has been found by human beings inspired by plant photosynthesis. However, water decomposition consumes a significant amount of energy and is expensive. People expect to obtain a water decomposition catalyst with low cost and high efficiency. This work selected a six-manganese containing polyoxometalate with a similar structure characteristic to photosynthesizing PSII to fabricate with graphite C3N4 nanosheets for the construction of composite film (Mn6SiW/g-C3N4NSs) electrode via layer by layer self-assembly technology, which was used for the photo-electrochemical decomposition of water under visible light conditions. The binary composite film electrode displayed good catalytic efficiency. The photoelectric density of the composite electrode is 46 μA/cm2 (at 1.23 V vs. Ag/AgCl) and 239 μA/cm2 (at 1.5 V vs. Ag/AgCl). Compared with the g-C3N4NSs electrode alone, the photoelectric density of the composite electrode increased by 1 time. The reason is attributed to the fact that Mn6SiW has a similar structure characteristic to photosynthesizing PSII and high electron transferability. The construction of the composite film containing low-cost Mn6SiW to modify g-C3N4NSs can effectively improve the photocatalytic decomposition of water, thus this study provides valuable reference information for the development of low-cost and high-performance photo-electrocatalytic materials.

1. Introduction

Currently, due to the influence of the greenhouse effect, global warming is a reality that causes serious damage to the earth’s ecology. Glaciers in the polar regions are melting, and the average forest temperature in the tropical regions is increasing, which results in frequent fires and tens of thousands of animals losing their lives. Many studies have confirmed that the greenhouse effect is related to the carbon dioxide produced by the combustion of fossil fuels [1,2]. At the same time, fossil fuels are running out [3]. Mankind expects to eliminate its dependence on fossil fuels, and is seeking cleaner and renewable energy, such as electric energy [4], nuclear energy [5], solar energy [6], and biomass energy [7]. Solar energy refers to the light energy transmitted from the sun to the earth. As long as the sun and the earth exist, solar energy can be used all the time. Therefore, in the limited life of human beings, solar energy can be said to be inexhaustible. At present, reasonable utilization of solar energy includes solar cells [8], solar water heaters [9], photocatalytic organic pollutant degradation [10], photocatalytic water decomposition [11,12], and photosynthesis in nature [13,14]. The main purpose of green plant photosynthesis in nature is to produce oxygen and hydrogen through photocatalysis. The oxygen produced is released to the outside of the body, and hydrogen combines with carbon dioxide to form organic molecules (e.g., glucose, etc.) [15]. The structure of photosystem II (PSII) has been determined by Ferreira et al. [16] (as shown in Figure 1).
Human beings, inspired by plant photosynthesis, have found a way to convert water into hydrogen and oxygen. Hydrogen is a kind of clean energy with high conversion energy, and its combustion product is water. At present, hydrogen has not been widely used commercially because of two reasons: first, the molecular weight of hydrogen is small, and the storage and transportation technology are not perfect; second, the generation of hydrogen is mainly from methane [11], but the byproduct carbon dioxide produced during the conversion process still plagues us. Water, as the main storage resource of hydrogen atoms, has a huge quantity, but it consumes a lot of energy, and it is expensive to convert it to hydrogen. Therefore, the development of a low-cost, high-efficiency method of hydrogen production from water decomposition is a research hotspot for current researchers. At present, the research on catalytic water decomposition has made progress in various fields, including organic catalysis [17], enzyme catalysis [18], inorganic semiconductor catalysis [19], coordination compound catalysis [20], etc. Each kind of catalyst has its own advantages and disadvantages. The synthesis cycle of organic catalyst is long, and the cost is high. The efficiency of a biocatalyst is high in a short time, but the stability of the biocatalyst is not very good because of the possibility of enzyme deactivation. Semiconductor catalysts are mainly used in the production of hydrogen by photocatalysis, but they display high recombination rate of photogenerated electron-holes and low photoelectric conversion efficiency. For example, graphite carbonitride (g-C3N4) is a two-dimensional semiconductor with an appropriate band gap (2.7 eV at 460 nm) [21]. It has a series of advantages, such as low cost, convenient synthesis and adjustable morphology, size and pore size distribution [22]. However, bulk g-C3N4 has a small specific surface area and a low carrier mobility, which can inhibit photoinduced reaction [23], whereas adjusting the surface morphology and oxygen doping can effectively improve the photocatalytic activity.
Coordination compound catalysts have various structures and excellent properties. As a kind of coordination compound, polyoxometalates (POMs) have been studied since 1912. POMs are the general term for a kind of metal oxygen cluster complex. They possess stable structures, good solubility in water, and an outstanding redox property. The charge number of POMs can be adjusted from −3 to −14. They can be synthesized with more than 70 elements, so they have more regulated properties. The catalytic activity of POMs for water decomposition has been reported [24]. These POMs usually have the structural characteristics of reversible continuous redox [25], exhibiting the ability of multi-electron storage and electron migration. These characteristics enable POMs to combine with semiconductor catalysts, so as to overcome the shortcomings of a high recombination rate of photogenerated electrons in the semiconductor, improve the photoelectric efficiency of the semiconductor, and improve the catalytic efficiency of the water decomposition. This method has been proved by Orlandi [24], Zhang [26], Lan [27], Yan [28] and Bi [29] et al. Among the POMs, there is a kind of POM with a similar structure to PSII [30,31], which usually contains transition metals, called a transition metal-substituted POM. Some manganese containing polyoxometalates (Mn–POMs) [32] not only have similar structures with OEC in nature, but also have the same manganese and oxygen elements. Therefore, it is expected that if the Mn-POMs are combined with the semiconductor g-C3N4 to form a cocatalyst, the catalytic efficiency of water decomposition under sunlight would be improved. Water decomposition includes oxygen and hydrogen evolution. As a critical step of water decomposition, oxygen evolution is more difficult to be completed due to the four-electron transfer. Accordingly, the development of the catalyst for the oxygen evolution reaction is required urgently. In this study, we selected a six-manganese containing polyoxometalate, [MnIII2MnII4(μ3-O)2-(H2O)4(B-β-SiW8O31)(B-β-SiW9O34)(γ-SiW10O36)]18− (Mn6SiW), as shown in Figure 2, with a similar structure characteristic to photosynthesizing PSII, and fabricated with graphite C3N4 nanosheets to construct a composite film (Mn6SiW/g-C3N4NSs) modified electrode by layer by layer self-assembly technology (LBL), via reasonable design and band gap matching. The obtained composite material Mn6SiW/g-C3N4NSs was used as photoelectrochemical (PEC) for water decomposition. Through the detailed characterization of the PEC catalyst and the catalysis experiments under visible light, it is proved that the PEC catalyst has good catalytic performance for water decomposition, can effectively promote the separation and transfer of photo generated electron hole pairs, and improve the rate of oxygen evolution. Hence, this study provides a low-cost and high-performance binary catalyst for a visible light driven photoelectrocatalytic system of water decomposition.

2. Results and Discussion

2.1. Infrared Spectrum (IR) Test

In order to verify the compounds Mn6SiW and g-C3N4NSs were synthesized, their IR spectra were measured, as shown in Figure 3. It is clear in Figure 3a that the main characteristic vibration frequencies at 1628, 943, 854, 782, and 723 cm−1, and two weak peaks at 455 and 437 cm−1 (the inset in Figure 3a) are observed, which are consistent with that of Mn6SiW in the literature [33]. In addition, in Figure 3b, the IR spectrum of g-C3N4NSs shows obvious peaks at 809, 1246–1634, 2990–3550 cm−1, which are consistent with that of g-C3N4NSs reported in the literature [34,35]. There are some small differences between the measured and reported values, which may be caused by the different instrument used. The observation above suggests that the compound Mn6SiW and g-C3N4NSs have been successfully prepared. See the Figure S1 in supporting information for a detailed description of the feature band.

2.2. Scanning Electron Micrograph (SEM)

Figure 4 shows the SEM image of g-C3N4NSs. In Figure 4a, the image presents a thin sheet structure of g-C3N4NSs. It can be seen from the color difference that the white part in the image means that the thickness of the nanosheet is very thin, so it is relatively transparent, whilst the darker part in the image is caused by the overlapping of multilayer nanosheets. Moreover, Figure 4b indicates the size of g-C3N4NSs is about 500 nm after acidic treatment. Based on the images above, it can be found that the g-C3N4NSs possess multilayer sheet structure, similar to graphene, so they are called graphite carbon nitride nanosheets.

2.3. UV-Vis Absorption Spectrum Test

From Figure 5a, it can be seen that the g-C3N4NSs has a certain absorption peak in the visible region, while Mn6SiW has an absorption peak at the 253 nm ultraviolet range. The UV-vis spectrum of the mixture of g-C3N4NSs and Mn6SiW presents a wide absorption range in the ultraviolet and visible region, which is the reason why the g-C3N4NSs and Mn6SiW were chosen to construct the composite material. Additionally, according to the reports in the literature, the surface of components g-C3N4NSs and Mn6SiW have positive and negative charges, respectively. Therefore, the composite films based on g-C3N4NSs and Mn6SiW can be directly constructed by electrostatic interaction. The growth process of [g-C3N4NSs/Mn6SiW]n was monitored by UV-Vis spectroscopy. As shown in Figure 5b, when comparing the absorption spectra of the components g-C3N4NSs and Mn6SiW in the composite film with that in the solution, it can be observed that the characteristic absorption peaks of g-C3N4NSs and Mn6SiW appear in the spectra of each layer, indicating that g-C3N4NSs and Mn6SiW have been assembled on the composite film. In addition, as shown in Figure 5c, the absorbance values at 253 nm for Mn6SiWand 400 nm for g-C3N4NSs were taken to plot the relationship with layer numbers of the composite film, and a high linear relationship was found between them, which proved that g-C3N4NSs and Mn6SiW grew uniformly on the composite film.

2.4. Electrochemical Characteristics

The electrochemical behaviors of Mn6SiW in solution and composite film were studied, and the obtained cyclic voltammograms (CVs) were recorded. In Figure 6a, it is found that Mn6SiW exhibits electrocatalytic activity towards water oxidation, although there is no light. This is because the Mn cluster in Mn6SiW contains four divalent Mn and two trivalent Mn, which is similar with the component of OEC in nature, oxidizing OH- in water to generate oxygen. Thus, the fabrication of the Mn6SiW and g-C3N4NSs composite film modified the electrode and would play a synergistic role in photoelectrocatalytic water oxidation. Figure 6b displays the CVs of Mn6SiW in different pH solutions. It can be seen that the redox peak of Mn is reversible, and it shifts negatively with the increase in pH, indicating that Mn6SiW can be stable in neutral condition and create high redox activity, thereby pH 7.0 was chosen as the condition of the follow-up test. Figure 6c shows that the oxidation peak current of MnII/IV increases with the increase in the number of assembled layers, and a high linearly proportional relationship was observed in Figure 6d, indicating that the film assembly is homogeneous, which is consistent with that in UV-vis measurement.

2.5. PEC Oxygen Evolution Reaction (OER)

In order to study the PEC catalytic performance of Mn6SiW/g-C3N4NSs composite film electrode for water oxidation, the PEC tests were carried out. Figure 7a is the LSV curve of the Mn6SiW/g-C3N4NSs composite film electrode in NaAc/HAc (0.5 M, pH = 7.0) electrolyte. It can be observed that at the same potential, the current density of the composite film increases from 160.86 to 239.65 μA cm−2 after irradiation, which is 1.49 times of that in the dark (E = 1.5 V vs. Ag/AgCl). When the potential is at 1.23 V (vs. Ag/AgCl), the current density is increased from 33.16 to 46.22 μA cm−2 after irradiation, which is 1.39 times of that in the dark (as shown in Table 1). It can be seen from Figure 7b that under simulated visible light, the initial catalytic potential of the composite film shifted negatively from 1.146 V to 1.100 V (Table 2). The reason is that under dark conditions, the Mn6SiW/g-C3N4NSs composite exhibits a certain catalytic effect on water oxidation. When irradiated by light, g-C3N4NSs absorbs light energy, leading to separation of the electrons and holes, and then the electrons jump to the conduction band. In the composite film, due to the promotion of Mn6SiW, the electron migration is accelerated, so that the catalytic current is increased, and the initial catalytic potential is shifted negatively, meaning that the overpotential of oxygen evolution is reduced, promoting water decomposition reaction.
In order to compare the PEC performance of three electrodes, (g-C3N4NSs/PSS)2 film (a), (PDDA/Mn6SiW)2 film (b), and (g-C3N4NSs/Mn6SiW)2 film (c) for water oxidation, their i–t curves were measured at the applied voltage of 1.5 V (vs. Ag/AgCl). As shown in Figure 7c, the photocurrents of all electrodes are stable and repeatable. It can be seen that the photocurrent response of g-C3N4NSs is the lowest in light on and dark, which is mainly due to the rapid recombination of photogenerated electrons and holes in g-C3N4NSs. The photocurrent of Mn6SiW electrode is higher than that of g-C3N4NSs. The photocurrent of Mn6SiW/g-C3N4NSs electrode at 1.23 V is twice as high as that of g-C3N4NSs. The main reason is that Mn6SiW cannot only effectively promote the electron transfer at the interface, but can also improve the photocurrent response, owing to the characteristics of the semiconductor. It is worth noting that the Mn6SiW/g-C3N4NSs electrode has the strongest photoelectric response, which further indicates that Mn6SiW promotes the photoelectric conversion efficiency of g-C3N4NSs. These results show that the Mn6SiW/g-C3N4NSs composite electrode has good performance for photoelectrocatalytic oxidation of water.
In order to further verify the effect of the layer number of the composite film electrode on the PEC water oxidation, we assembled a multilayer film electrode with different layer numbers and tested their photocurrent response. It can be seen from Figure 8 that with the increase in the layer number of the composite film electrode, the photocurrent density also increases, showing a linear relationship between the photocurrent density and layer number, suggesting that PEC activity of the Mn6SiW/g-C3N4NSs film electrode for water oxidation is adjustable.
In Table 3, we compared with other reported polyoxometalate modified C3N4 catalysts for catalytic water decomposition. It can be seen that, although using energy-saving lamps with low energy, our designed composite film catalyst still displays high PEC activity for water oxidation.

3. Materials and Methods

3.1. Materials and Apparatus

3.1.1. Materials

Poly (diallyldimethylammonium chloride) (PDDA) (Mw = 10,000–20,000) and poly (4-styrene sulfonic acid) sodium salt (PSS) were obtained from Sigma Aldrich (Merck KGaA, Darmstadt, Germany). Sodium tungstate (Na2WO4·2H2O), sodium metasilicate (Na2SiO3·5H2O), sodium hydroxide (NaOH), manganese chloride (MnCl2), potassium carbonate (K2CO3), hydrochloric acid (HCl), potassium chloride (KCl), and ethanol were analytical grade and used as received. Water was purified by Millipore Milli-Q (Merck Millipore, Darmstadt, Germany).

3.1.2. Apparatus

PHS–25B digital acidimeter (Yuechen, Shanghai, China), determined pH value. The infrared spectra (IR) were measured on a spectrometer (Bruker Vertex 80V, Bruker, Karlsruhe, Germany). Absorption measurements (UV-vis) were completed with UV-2700 UV-Visible spectrophotometer (Shimadzu, Kyoto, Japan). The cyclic voltammetry curve (CV), the linear sweep voltammetry curve (LSV), and the current–time relationship (I–T) were recorded using CHI660e electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China).

3.2. Preparation of Electrode Modified Materials

3.2.1. Synthesis of K8[β2-SiW11O39]·14H2O (1a)

The precursors K8[β2-SiW11O39]·14H2O and K8[γ-SiW10O39]·12H2O (1b) were synthesized according to the published procedure [38]. An amount of 18.2 g Na2WO4·2H2O was dissolved in 30 mL water, and 16.5 mL 4 M HCl was slowly added to the solution and stirred for 10 min. An amount of 1.1 g of Na2SiO3·5H2O was dissolved in 10 mL of water, then poured into the above solution and adjusted from pH 5 to 6 with 4 M HCl solution. An amount of 9 g KCl was added to the solution by stirring. After 15 min, the precipitate was filtered, washed with 2 M KCl and dried.

3.2.2. Synthesis of K8[γ-SiW10O39]·12H2O (1b)

At room temperature, 3 g 1a was dissolved in 30 mL water to remove the insoluble matter quickly. Using 2 M K2CO3 aqueous solution, the pH value of the solution was quickly adjusted to 9.1, and maintained for 16 min. An amount of 8 g KCl was added to the solution for precipitation while maintaining the pH value. After 10 min, the solution was filtered, washed with 1 M KCl solution, and air dried.

3.2.3. Synthesis of K18[MnIII2MnII4(μ3-O)2-(H2O)4(B-β-SiW8O31)(B-β-SiW9O34)(γ-SiW10O36)]·40H2O (1)

The mixed-valence Mn6-containing polyoxometalates were synthesized according to the literature reported by Mitchell et al. [33]. An amount of 1.45 g 1b was dissolved in 25 mL water and heat to 40 °C. When the temperature reached 38 °C, 0.13 g MnCl2 was added by stirring. The pH value of the solution was adjusted to 8.40 with 2 M K2CO3 solution. The pH value of the solution was adjusted from 8.40 to 8.50 with 0.2 M K2CO3 solution and maintained for 40 min. The solution turned dark brown. The stirring was then continued at 40 °C for 4 h, before the solution was cooled to room temperature. The brown precipitates were removed by centrifugation (5 min at 4400 rpm) to obtain a dark brown solution, which was poured into a wide necked conical flask and crystallized overnight at 18 °C.

3.2.4. Synthesis of Protonated Graphitic Carbon Nitride Nanosheets (g-C3N4NSs)

The g-C3N4NSs was synthesized according to the literature of Fu et al. [34]. An amount of 5.0 g melamine was put in a crucible, heated in a muffle furnace to 500 °C for 2 h, and then raised to 550 °C for 4 h. The yellow powder g-C3N4 was obtained. Then, g-C3N4 was dispersed into 30 mL of 5M HNO3 solution, stirred for 3 h, and then refluxed at 125 °C for 24 h. After centrifugation, the solution was washed repeatedly with water until pH = 7, and then dried in vacuum for 12 h (60 °C).

3.2.5. LBL Assembled Composite Film Modified Electrode

Before assembly, the substrate (ITO electrode) needed to be cleaned according to the literature [39]. The ITO was immersed in 1 M NaOH: CH3CH2OH (V:V = 1:1) mixed solution, ultrasonically for 20 min, then cleaned with water, and then dried with nitrogen for standby. Then, the composite film was assembled on the clean substrate by the LbL assembly method (as shown in Scheme 1). First, the substrate was immersed in the positively charged PDDA (8%) solution and left to stand for 20 min. Second, the substrate was immersed in the negatively charged PSS solution for 20 min. Third, the substrate was immersed in the g-C3N4NSs (1.0 mg mL−1) aqueous solution for 20 min. Fourth, the substrate was immersed in 0.5 mM Mn6SiW solution with negative charge for 20 min. After each assembly, the substrate was washed with water 3–5 times to remove the physical adsorption substances, and then dried with nitrogen flow. After the above steps, the monolayer [PDDA/PSS/g-C3N4NSs/Mn6SiW] composite films were prepared. Different layers of [PDDA/PSS/g-C3N4NSs/Mn6SiW]n-ITO (n = 1–8) composite film modified electrodes were prepared by repeating steps 3 and 4.

3.3. UV-Vis Absorption Spectra

The fabrication process of the samples is the same as that of the LBL assembling the multilayer film modified electrode in Section 3.2.5. The only difference is that ITO is replaced by a quartz slide. Firstly, the quartz glass was cleaned according to the literature method (different from ITO) [40], and then assembled according to the film assembly steps, monitoring the assembly process of composite films by UV-vis absorption spectrum.

3.4. Photo-Electrochemical Test

A series of experiments were performed to study the PEC catalytic activity for water oxidation, ITO/PDDA/PSS/[g-C3N4NSs/Mn6SiW]n was used as the working electrode, using Ag/AgCl as a reference electrode, and a platinum wire as a counter electrode. The buffer solution was obtained from 0.5 M pH 7 CH3COONa (NaAc) solutions adjusted by CH3COOH (HAc). Before the electrochemical experiments, the oxygen in the electrolyte was removed by purging high-purity nitrogen. All the electrochemical measurements were performed on an electrochemical workstation (CHI 611E, CH Instruments Co.) at room temperature. An ordinary fluorescent energy-saving lamp was used as visible light source (wavelength range ≥380, 150 W). A quartz cell was used as a reaction tank, about 10 cm from the light source.

4. Conclusions

In summary, Mn6SiW/g-C3N4NSs composite film electrode was prepared by using layer by layer self-assembly technology and low-cost inorganic materials of non-precious metals. It was used for PEC catalytic water oxidation to generate oxygen. The results proved that the binary composite film electrode displayed high catalytic efficiency. The photoelectric density of the binary composite electrode reached 239 μA/cm2(E = 1.5V vs. Ag/AgCl). Compared to the simple g-C3N4NSs electrode, the photoelectric density of the binary composite electrode is two times stronger. Therefore, using low-cost Mn6SiW to modify g-C3N4NSs can effectively improve the photoelectrocatalytic performance of C3N4 for water decomposition. Therefore, this study provides valuable reference information for the development of low-cost and high-performance photoelectrocatalytic materials.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/catal11070856/s1, Figure S1: The IR spectra of g-C3N4NSs.

Author Contributions

Conceptualization. Y.W. and L.B.; validation, Y.W., X.Y. and. J.P.; formal analysis, Y.W.; investigation, Y.W., X.Y. and. J.P.; resources, Y.W. and Z.F.; data curation, Y.W.; writing—original draft preparation, Y.W.; writing—review and editing, Y.W. and L.B.; visualization, Y.W.; supervision, L.B.; All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to the need of follow-up research.

Acknowledgments

Thank the professor and co-author for their help in writing this article. Thank the reviewers for their comments and valuable opinions, and the editors for their help in publishing this article.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 11 00856 g001 550
Figure 1. (a) Mn4CaO5 core of oxygen evolution catalyst (OEC) in nature; (b) Structure of the native OEC, involving ligating protein side-chains and water molecules. The bond lengths are given in Å; Mn, purple; Ca, green; O, orange; All H atoms and CH3 groups have been omitted for clarity [16]. Reprinted with permission from ref. [16]. Copyright 2015 The American Association for the Advancement of Science.
Figure 1. (a) Mn4CaO5 core of oxygen evolution catalyst (OEC) in nature; (b) Structure of the native OEC, involving ligating protein side-chains and water molecules. The bond lengths are given in Å; Mn, purple; Ca, green; O, orange; All H atoms and CH3 groups have been omitted for clarity [16]. Reprinted with permission from ref. [16]. Copyright 2015 The American Association for the Advancement of Science.
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Figure 2. Structure of Mn6SiW; WO6 polyhedra, purple; Mn, orange and brown; Si, green and O, red [33]. Reprinted with permission from ref. [33]. Copyright 2011 John Wiley and Sons.
Figure 2. Structure of Mn6SiW; WO6 polyhedra, purple; Mn, orange and brown; Si, green and O, red [33]. Reprinted with permission from ref. [33]. Copyright 2011 John Wiley and Sons.
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Figure 3. The IR spectra of Mn6SiW (a) and (b) g-C3N4NSs.
Figure 3. The IR spectra of Mn6SiW (a) and (b) g-C3N4NSs.
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Figure 4. (a) and (b) SEM images of g-C3N4NSs.
Figure 4. (a) and (b) SEM images of g-C3N4NSs.
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Figure 5. (a) UV-vis absorption spectra of g-C3N4NSs (0.6 mM), Mn6SiW (5 μM) and g-C3N4NSs-Mn6SiW (0.6 mM: 3 μM) mixture in the aqueous solutions; (b) UV-vis spectra for the [g-C3N4NSs/Mn6SiW]n film on quartz slide; (c) relationship of the absorbance value and layer number at λ = 253 nm and λ = 400 nm.
Figure 5. (a) UV-vis absorption spectra of g-C3N4NSs (0.6 mM), Mn6SiW (5 μM) and g-C3N4NSs-Mn6SiW (0.6 mM: 3 μM) mixture in the aqueous solutions; (b) UV-vis spectra for the [g-C3N4NSs/Mn6SiW]n film on quartz slide; (c) relationship of the absorbance value and layer number at λ = 253 nm and λ = 400 nm.
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Catalysts 11 00856 g006a 550Catalysts 11 00856 g006b 550
Figure 6. (a) Cyclic voltammogram (CV) curves of Mn6SiW (0.6 mM, curve b) and blank solution (curve a) in NaAc/HAc buffer (0.5 M, pH = 7.0). (b) CV curves of Mn6SiW in NaAc/HAc buffer (0.5 M) at different pH (5/6/7); (c) CV curves of the film (g-C3N4NSs/Mn6SiW)n in NaAc/HAc buffer (0.5 M, pH 7), n = 1, 2, 4, 8; at 50 mV/s; (d) the relationship between the oxidation peak current of MnII/IV and the number of assembled layers.
Figure 6. (a) Cyclic voltammogram (CV) curves of Mn6SiW (0.6 mM, curve b) and blank solution (curve a) in NaAc/HAc buffer (0.5 M, pH = 7.0). (b) CV curves of Mn6SiW in NaAc/HAc buffer (0.5 M) at different pH (5/6/7); (c) CV curves of the film (g-C3N4NSs/Mn6SiW)n in NaAc/HAc buffer (0.5 M, pH 7), n = 1, 2, 4, 8; at 50 mV/s; (d) the relationship between the oxidation peak current of MnII/IV and the number of assembled layers.
Catalysts 11 00856 g006aCatalysts 11 00856 g006b
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Figure 7. (a) LSV of (g-C3N4NSs/Mn6SiW)2 film electrode in light on(red) and light off (black); (b) The locally enlarged graph of (a); (c) amperometric i–t Curve of the (g-C3N4/PSS)2 film (a), (PDDA/Mn6SiW)2 film (b), and (g-C3N4NSs/Mn6SiW)2 film (c) under incandescent lamp irradiation (450–650 nm).
Figure 7. (a) LSV of (g-C3N4NSs/Mn6SiW)2 film electrode in light on(red) and light off (black); (b) The locally enlarged graph of (a); (c) amperometric i–t Curve of the (g-C3N4/PSS)2 film (a), (PDDA/Mn6SiW)2 film (b), and (g-C3N4NSs/Mn6SiW)2 film (c) under incandescent lamp irradiation (450–650 nm).
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Figure 8. (a) Amperometric i–t curves of the PDDA/PSS/(g-C3N4NSs/Mn6SiW)n film (n = 1–5) under incandescent lamp irradiation (450–650 nm); (b) the relationship between average of photocurrent and number of film electrode assembly layers.
Figure 8. (a) Amperometric i–t curves of the PDDA/PSS/(g-C3N4NSs/Mn6SiW)n film (n = 1–5) under incandescent lamp irradiation (450–650 nm); (b) the relationship between average of photocurrent and number of film electrode assembly layers.
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Scheme 1. The assembly process diagram of Mn6SiW and g-C3N4NSs composite films by LBL.
Scheme 1. The assembly process diagram of Mn6SiW and g-C3N4NSs composite films by LBL.
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Table
Table 1. The current density of PDDA/PSS/(g-C3N4NSs/Mn6SiW)2-ITO electrode.
Table 1. The current density of PDDA/PSS/(g-C3N4NSs/Mn6SiW)2-ITO electrode.
E (V vs. Ag/AgCl)Current Density (μA cm−2)
Light OffLight on
1.2333.1646.22
1.50160.86239.65
Table
Table 2. The catalytic potential of PDDA/PSS/(g-C3N4NSs/Mn6SiW)2-ITO electrode.
Table 2. The catalytic potential of PDDA/PSS/(g-C3N4NSs/Mn6SiW)2-ITO electrode.
PDDA/PSS/(g-C3N4NSs/Mn6SiW)2-ITOE (V)on
Light on1.100
Light off1.146
Table
Table 3. Reported polyoxometalate modified C3N4 catalysts used to catalyze water decomposition.
Table 3. Reported polyoxometalate modified C3N4 catalysts used to catalyze water decomposition.
PEC CatalystsLight Source Current DensityGrowth Multiple (vs. C3N4)Ref.
SiW12/C3N4Xe lamp (300 W)2.1 × 10−4 μA/cm2
(E = 1.23 vs. RHE) 		1.4[36]
P5W30/g-C3N4Xe lamp (400 W)44 μA/cm2
(E = 1.23 V vs. RHE)		2.4[37]
Mn6SiW/g-C3N4NSsEnergy-saving lamps (150 W) 46 μA/cm2
(E = 1.23 V vs. Ag/AgCl)
239 μA/cm2
(E = 1.5 V vs. Ag/AgCl)		2Our work
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Wu, Y.; Yu, X.; Fu, Z.; Pei, J.; Bi, L. Fabrication of Six Manganese Containing Polyoxometalate Modified Graphite C3N4 Nanosheets Catalysts Used to Catalyze Water Decomposition. Catalysts 2021, 11, 856. https://doi.org/10.3390/catal11070856

AMA Style

Wu Y, Yu X, Fu Z, Pei J, Bi L. Fabrication of Six Manganese Containing Polyoxometalate Modified Graphite C3N4 Nanosheets Catalysts Used to Catalyze Water Decomposition. Catalysts. 2021; 11(7):856. https://doi.org/10.3390/catal11070856

Chicago/Turabian Style

Wu, Yue, Xiaoxia Yu, Zhijing Fu, Jianye Pei, and Lihua Bi. 2021. "Fabrication of Six Manganese Containing Polyoxometalate Modified Graphite C3N4 Nanosheets Catalysts Used to Catalyze Water Decomposition" Catalysts 11, no. 7: 856. https://doi.org/10.3390/catal11070856

APA Style

Wu, Y., Yu, X., Fu, Z., Pei, J., & Bi, L. (2021). Fabrication of Six Manganese Containing Polyoxometalate Modified Graphite C3N4 Nanosheets Catalysts Used to Catalyze Water Decomposition. Catalysts, 11(7), 856. https://doi.org/10.3390/catal11070856

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