catalysts Fabrication of Six Manganese Containing Polyoxometalate Modiﬁed Graphite C 3 N 4 Nanosheets Catalysts Used to Catalyze Water Decomposition

: 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 C 3 N 4 nanosheets for the construction of composite ﬁlm (Mn 6 SiW/g-C 3 N 4 NSs) 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 µ A/cm 2 (at 1.23 V vs. Ag/AgCl) and 239 µ A/cm 2 (at 1.5 V vs. Ag/AgCl). Compared with the g-C 3 N 4 NSs electrode alone, the photoelectric density of the composite electrode increased by 1 time. The reason is attributed to the fact that Mn 6 SiW has a similar structure characteristic to photosynthesizing PSII and high electron transferability. The construction of the composite ﬁlm containing low-cost Mn 6 SiW to modify g-C 3 N 4 NSs 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.


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 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). 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.
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, highefficiency 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. The bond lengths are given in Å; Mn, purple; Ca, green; O, orange; All H atoms and CH 3 groups have been omitted for clarity [16]. Reprinted with permission from ref. [16]. Copyright 2015 The American Association for the Advancement of Science.
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-C 3 N 4 ) 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-C 3 N 4 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-C 3 N 4 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, [ Figure 2, with a similar structure characteristic to photosynthesizing PSII, and fabricated with graphite C 3 N 4 nanosheets to construct a composite film (Mn 6 SiW/g-C 3 N 4 NSs) modified electrode by layer by layer self-assembly technology (LBL), via reasonable design and band gap matching. The obtained composite material Mn 6 SiW/g-C 3 N 4 NSs 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.
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, [Mn III 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.  Mn 6 SiW; WO 6 polyhedra, purple; Mn, orange and brown; Si, green and O, red [33]. Reprinted with permission from ref. [33]. Copyright 2011 John Wiley and Sons.

Infrared Spectrum (IR) Test
In order to verify the compounds Mn 6 SiW and g-C 3 N 4 NSs 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 Mn 6 SiW in the literature [33]. In addition, in Figure 3b, the IR spectrum of g-C 3 N 4 NSs shows obvious peaks at 809, 1246-1634, 2990-3550 cm −1 , which are consistent with that of g-C 3 N 4 NSs 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 Mn 6 SiW and g-C 3 N 4 NSs have been successfully prepared. See the Figure S1 in supporting information for a detailed description of the feature band.  [33]. Reprinted with permission from ref. [33]. Copyright 2011 John Wiley and Sons.

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.   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.  Figure 4 shows the SEM image of g-C 3 N 4 NSs. In Figure 4a, the image presents a thin sheet structure of g-C 3 N 4 NSs. 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-C 3 N 4 NSs is about 500 nm after acidic treatment. Based on the images above, it can be found that the g-C 3 N 4 NSs possess multilayer sheet structure, similar to graphene, so they are called graphite carbon nitride nanosheets.

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

UV-Vis Absorption Spectrum Test
From Figure 5a, it can be seen that the g-C 3 N 4 NSs has a certain absorption peak in the visible region, while Mn 6 SiW has an absorption peak at the 253 nm ultraviolet range. The UV-vis spectrum of the mixture of g-C 3 N 4 NSs and Mn 6 SiW presents a wide absorption range in the ultraviolet and visible region, which is the reason why the g-C 3 N 4 NSs and Mn 6 SiW were chosen to construct the composite material. Additionally, according to the reports in the literature, the surface of components g-C 3 N 4 NSs and Mn 6 SiW have positive and negative charges, respectively. Therefore, the composite films based on g-C 3 N 4 NSs and Mn 6 SiW can be directly constructed by electrostatic interaction. The growth process of [g-C 3 N 4 NSs/Mn 6 SiW]n was monitored by UV-Vis spectroscopy. As shown in Figure 5b, when comparing the absorption spectra of the components g-C 3 N 4 NSs and Mn 6 SiW in the composite film with that in the solution, it can be observed that the characteristic absorption peaks of g-C 3 N 4 NSs and Mn 6 SiW appear in the spectra of each layer, indicating that g-C 3 N 4 NSs and Mn 6 SiW have been assembled on the composite film. In addition, as shown in Figure 5c, the absorbance values at 253 nm for Mn 6 SiWand 400 nm for g-C 3 N 4 NSs 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-C 3 N 4 NSs and Mn 6 SiW grew uniformly on the composite film.

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.

Electrochemical Characteristics
The electrochemical behaviors of Mn 6 SiW in solution and composite film were studied, and the obtained cyclic voltammograms (CVs) were recorded. In Figure 6a, it is found that Mn 6 SiW exhibits electrocatalytic activity towards water oxidation, although there is no light. This is because the Mn cluster in Mn 6 SiW contains four divalent Mn and two trivalent Mn, which is similar with the component of OEC in nature, oxidizing OHin water to generate oxygen. Thus, the fabrication of the Mn 6 SiW and g-C 3 N 4 NSs composite film modified the electrode and would play a synergistic role in photoelectrocatalytic water oxidation. Figure 6b displays the CVs of Mn 6 SiW 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 Mn 6 SiW 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 Mn II/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.

PEC Oxygen Evolution Reaction (OER)
In order to study the PEC catalytic performance of Mn 6 SiW/g-C 3 N 4 NSs composite film electrode for water oxidation, the PEC tests were carried out. Figure 7a is the LSV curve of the Mn 6 SiW/g-C 3 N 4 NSs 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 Mn 6 SiW/g-C 3 N 4 NSs composite exhibits a certain catalytic effect on water oxidation. When irradiated by light, g-C 3 N 4 NSs 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 Mn 6 SiW, 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.
C3N4NSs and Mn6SiW grew uniformly on the composite film.

Electrochemical Characteristics (a) (b)
Catalysts 2021, 11, x FOR PEER REVIEW 6 of 12 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 OHin 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 Mn II / 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 UVvis measurement.

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.

PDDA/PSS/(g-C 3 N 4 NSs/Mn 6 SiW) 2 -ITO E (V) on
Light on 1.100 Light off 1.146 In order to compare the PEC performance of three electrodes, (g-C 3 N 4 NSs/PSS) 2 film (a), (PDDA/Mn 6 SiW) 2 film (b), and (g-C 3 N 4 NSs/Mn 6 SiW) 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-C 3 N 4 NSs is the lowest in light on and dark, which is mainly due to the rapid recombination of photogenerated electrons and holes in g-C 3 N 4 NSs. The photocurrent of Mn 6 SiW electrode is higher than that of g-C 3 N 4 NSs. The photocurrent of Mn 6 SiW/g-C 3 N 4 NSs electrode at 1.23 V is twice as high as that of g-C 3 N 4 NSs. The main reason is that Mn 6 SiW 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 Mn 6 SiW/g-C 3 N 4 NSs electrode has the strongest photoelectric response, which further indicates that Mn 6 SiW promotes the photoelectric conversion efficiency of g-C 3 N 4 NSs. These results show that the Mn 6 SiW/g-C 3 N 4 NSs 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 Mn 6 SiW/g-C 3 N 4 NSs 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.  In Table 3, we compared with other reported polyoxometalate modified C 3 N 4 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. 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 (Na 2 WO 4 ·2H 2 O), sodium metasilicate (Na 2 SiO 3 ·5H 2 O), sodium hydroxide (NaOH), manganese chloride (MnCl 2 ), potassium carbonate (K 2 CO 3 ), 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).

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).  [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 MnCl 2 was added by stirring. The pH value of the solution was adjusted to 8.40 with 2 M K 2 CO 3 solution. The pH value of the solution was adjusted from 8.40 to 8.50 with 0.2 M K 2 CO 3 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.

Preparation of Electrode
3.2.4. Synthesis of Protonated Graphitic Carbon Nitride Nanosheets (g-C 3 N 4 NSs) The g-C 3 N 4 NSs 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-C 3 N 4 was obtained. Then, g-C 3 N 4 was dispersed into 30 mL of 5M HNO 3 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).

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: CH 3 CH 2 OH (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-C 3 N 4 NSs (1.0 mg mL −1 ) aqueous solution for 20 min. Fourth, the substrate was immersed in 0.5 mM Mn 6 SiW 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-C 3 N 4 NSs/Mn 6 SiW] composite films were prepared. Different layers of [PDDA/PSS/g-C 3 N 4 NSs/Mn 6 SiW]n-ITO (n = 1-8) composite film modified electrodes were prepared by repeating steps 3 and 4.

UV-Vis Absorption Spectra
The fabrication process of the samples is the same as that of the LBL assembling t multilayer film modified electrode in Section 3.3.5. The only difference is that ITO is r placed by a quartz slide. Firstly, the quartz glass was cleaned according to the literatu method (different from ITO) [40], and then assembled according to the film assemb steps, monitoring the assembly process of composite films by UV-vis absorption spe trum.

Photo-Electrochemical Test
A series of experiments were performed to study the PEC catalytic activity for wat oxidation, ITO/PDDA/PSS/[g-C3N4NSs/Mn6SiW]n was used as the working electrode, u ing Ag/AgCl as a reference electrode, and a platinum wire as a counter electrode. T buffer solution was obtained from 0.5 M pH 7 CH3COONa (NaAc) solutions adjusted CH3COOH (HAc). Before the electrochemical experiments, the oxygen in the electroly was removed by purging high-purity nitrogen. All the electrochemical measuremen Scheme 1. The assembly process diagram of Mn 6 SiW and g-C 3 N 4 NSs composite films by LBL.

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.

Photo-Electrochemical Test
A series of experiments were performed to study the PEC catalytic activity for water oxidation, ITO/PDDA/PSS/[g-C 3 N 4 NSs/Mn 6 SiW]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 CH 3 COONa (NaAc) solutions adjusted by CH 3 COOH (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., Bee Cave, TX, USA) 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.

Conclusions
In summary, Mn 6 SiW/g-C 3 N 4 NSs 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/cm 2 (E = 1.5V vs. Ag/AgCl). Compared to the simple g-C 3 N 4 NSs electrode, the photoelectric density of the binary composite electrode is two times stronger. Therefore, using low-cost Mn 6 SiW to modify g-C 3 N 4 NSs can effectively improve the photoelectrocatalytic performance of C 3 N 4 for water decomposition. Therefore, this study provides valuable reference information for the development of low-cost and high-performance photoelectrocatalytic materials.

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.