Efficiency Improvement of Photoelectrochemical Solar Cell Applications by Using Ternary Hybrid 𝑴𝒐𝑺

. In this paper, molybdenum disulfide (𝑀𝑜𝑆 2 ) was hybridized with graphene carbon nitrite ( 𝑔 − 𝐶 3 𝑁 4 ) and Cu 2 O in order to enhance the photoelectrochemical (PEC) activity and increase the light absorption range of Cu 2 O thin film. The melamine powder was poured in an empty container and then heated in a furnace to attain the 𝑔 − 𝐶 3 𝑁 4 powder. The ternary hetero-epitaxial growth was achieved by growing of 𝑀𝑜𝑆 2 / 𝑔 − 𝐶 3 𝑁 4 on the 𝐶𝑢 2 𝑂 hybrid by a partial thermal oxidation process. The characteristics of 𝑀𝑜𝑆 2 / 𝑔 − 𝐶 3 𝑁 4 / 𝐶𝑢 2 𝑂 hybrid film were investigated through XRD, FT-IR and photoelectronchemistry-related measurements. The PEC behavior of the ternary hybrid electrode was investigated using current-voltage test under illumination. The efficiency calculated from current-voltage test under illumination shows that the presence of graphene carbon nitrite and molybdenum disulphide within the film networks, despite its low content, could stimulate substantial improvement in maximum photoconversion efficiency from 0.036% to 0.33%. This improvement is attributable to the enhancement of the electron-transferring proficiency upon the insertion of 𝑔 − 𝐶 3 𝑁 4 and 𝑀𝑜𝑆 2 , as confirmed by X-Ray Diffraction Analysis (XRD). The PEC test results signify that the photoelectrochemical activity of the 𝑀𝑜𝑆 2 / 𝑔 − 𝐶 3 𝑁 4 / 𝐶𝑢 2 𝑂 ternary hybrid is much higher than that of 𝐶𝑢 2 𝑂 subtrate. The mechanisms accountable for the enhanced PEC behavior of the 𝑀𝑜𝑆 2 / 𝑔 − 𝐶 3 𝑁 4 / 𝐶𝑢 2 𝑂 ternary hybrid are discussed in detail.


Introduction
The Industrial rebellion has brought about a fabulous increase in the world's population, and the ultimatum for energy is increasing [1].It is anticipated that about 9 billion people will live on the planet by 2050, and about 30 TW of energy will be required to sustain this population.Nevertheless, more than 70% of our energy needs are currently met by finite fossil fuels, which will soon be exhausted [2][3][4][5].Therefore, using a green backup energy source has become a big challenge for people.As a nearly limitless source of green energy, solar energy has received substantial attention in recent years.To this end, photoelectrochemical (PEC) cells are considered to be efficient devices for converting solar energy into hydrogen chemical energy by water splitting [6][7].Photoelectrodes, usually made of semiconductors, play the most important roles in light absorption, electron-hole pairing and charge transfer in PEC cells [8][9].However, due to their large bandgaps, most semiconductor materials can only absorb a small fraction of sunlight in the UV range, which severely limits their potential applications [10].
In order to improve the PEC efficiency of photoelectrodes, doping with compounds or elements and fabrication of semiconductors with heterostructures have been investigated in detail due to the different interactions between different semiconductor materials [11][12].Among them, cuprous oxide (Cu2O) has been considered as the state-of-the-art candidate of photocathode [13][14].Cu2O is a typical p-type semiconductor with a band gap of ~ 2 eV, with which it could achieve a theoretical photocurrent of -14.7 mAcm -2 for water splitting and a solar to hydrogen conversion efficiency of 18.1 % on the AM 1.5 spectrum [14].
Moreover, it is scalable, earth-abundant, environmentally benevolent and compatible with inexpensive fabrication processes, which are important requirements to placate the terawatt-scale global energy demand [15].The practical application of Cu2O in the PEC process is still limited by two major drawbacks despite the aforementioned advantages: (1) sky-scraping recombination rate of photogenerated electron-hole carriers, partly ascribed to its mismatched electron diffusion length (usually 20-100 nm) with the light absorption depth (about 10 μm) [16]; (2) Deprived photostability because of self-photocorrosion in electrolyte solution [17].
Structure engineering has been reported to effectively address the above limitations of Cu2O.Currently, owing to the simple preparation process, most of Cu2O-based photocathode for PEC water splitting is built basing on Cu2O film, usually showing a low photoelectric conversion efficiency [18][19].In contrast, its Cu2O nanowire/nanorod-based counterparts show significantly improved efficiency.This is mainly attributed to more efficient light harvesting, more efficient separation and transport of photogenerated charge carriers, larger surface area for fast interfacial charge transfer, and electrochemical reactions [20][21].
In addition to structure engineering, heterojunction engineering is widely considered as another effective strategy to improve PEC water splitting performance of Cu2O through efficient separation of photogenerated charge carriers.[22][23].The Cu2O-bound semiconductor is not only a key element in forming the pn junction, but in some cases also acts as a protective layer that slows down the corrosion of the latter [24][25].
Owing to its layered structure, MoS2 possesses highly exposed catalytic edge sites, while, g-C3N4 possess good visible-light absorption property and relatively high surface area [26].However, the photocatalytic activity of pristine g-C3N4 has been found to be low owing to possible recombination of electron-hole pair.Therefore, for enhanced H2 generation activity, g-C3N4 is normally coupled with other materials for effective charge separation [27][28].
Despite tremendous efforts, Cu2O-based photocathode challenges still remain, and the combination of the above two strategies will lead to the development of a novel and efficient Cu2Obased photocathode with potential applications in PEC water splitting.It indicates that the continuing need to further explore the photocathode could go further.This work deals with his PEC investigation of his Cu2O films produced by thermal oxidation.Copper forms two different oxides, Cu2O, with a direct bandgap of 2.1 eV [29][30], so it strongly absorbs only at wavelengths below 600 nm, whereas CuO with a bandgap of 1.21-1.51eV [31][32] absorbs the entire visible range.In addition, other reasons for choosing Cu2O as a material in this study are (a) its abundance in nature, (b) non-toxicity, (c) low cost of production, (d) stability and (e) fairly good electrical properties.The effects of ternary hybrid deposition of  2 / −  3  4 / 2  film's PEC behavior were investigated.In addition, the films were also characterized in terms of structure and phase discrimination using XRD, I-V characteristic curve and FTIR, respectively.

Synthesis of Cu2O thin film
Commercial pure copper (99.98%) in foil form (0.1 mm thick) was cut into standard size wafers of 2 cm x 2 cm.The sample was pickled on the rim of the bottle to make it smooth, immersed in dilute nitric acid, rinsed thoroughly with distilled water several times, and then dried to remove impurities on the film surface.After cleaning, the copper film was thermally oxidized by furnace annealing in air.The oxidation temperature was controlled over a wide range from room temperature (RT) to 450 °C.The heating rate was about 10 °C./min, and once the preferred maximum temperature was reached, it was held for 30 minutes to allow copper oxide to form.After oxide formation, the furnace was allowed to cool for 2 hours.Slow cooling was maintained to minimize possible thermal stress and film cracking.

Preparation of g-C3N4
In a typical synthesis, the g-C3N4 photocatalyst was synthesized separately from melamine [33].3.0 g of melamine was taken in alumina crucibles with cover and calcined at 500 °C with a heating rate of 3 °C min -1 .It is further heated to 520 °C for 2 h at a heating rate of 2 °C min -1 and allowed to cool down to room temperature.A yellow product was collected and ground into fine powder.The sample was named as CN.

Synthesis of 𝑴𝒐𝑺 𝟐 /𝒈 − 𝑪 𝟑 𝑵 𝟒 /𝑪𝒖 𝟐 𝑶 hybrid film
The method used to grow MoS2 was chemical vapor deposition (CVD) [34][35].The substrate was placed under specific temperature and pressure conditions and one or more precursors were chemically reacted on the surface of the substrate to produce a high-quality large-area thin film.The application of CVD in the preparation of single-layer TMDs starts with MoS2 growth.The heating rate was set to 10 °C per min, the growth temperature was 650 °C, and the temperature was held for 30 min.Once the temperature was dropped to 400 °C, the lid was opened.The MoS2 structure was obtained when the temperature decreases to room temperature.The experimental process was shown in Figure 1.CVD can effectively produce monolayer and multilayer MoS 2 /g − C 3 N 4 .It was able to grow high-quality single-crystal materials and produce thin films uniformly distributed over large areas, which were useful in later fabrication of optoelectronic components.

Photoelectrochemical tests
For this purpose,  2 / −  3  4 / 2  and Cu electrode were arranged and dipped into transparent plastic container.To prepare the electrolyte, the 1 g of NaCl powders were mixed with 25 ml of distilled water, stirrer gently until the electrolyte was dissolved completely.The PEC performance of the hybrid electrodes was evaluated using Current-Voltage measurements (Figure 2).The photoelectrochemical tests were performed via a two-electrode electrochemical system.A working electrode ( 2 / −  3  4 / 2 ) and a copper plate were employed as the counter electrodes, respectively.The multimeter was employed to accomplished electrical route of the photocurrent density and photo voltage of electrodes under illumination (AM 1.5 G) within the potential window using 1 g of NaCl electrolyte as a mediator between the two electrodes.The approach described in this study provides a simple and novel method to synthesize thin film materials, ready for practical applications such as the photoelectrochemical solar cell and hydrogen production.

FT-IR spectra analysis
FT-IR analysis (Figure 5) was performed to analyze the chemical and structural properties of bare g-C3N4, MoS2, and Cu2O thin films.Pure g-C3N4 exhibits a characteristic IR peak at 1632 cm -1 , whereas peaks at 1245, 1320 and 1417 cm -1 are assigned to the C-N heterocyclic stretch of g-C3N4 [45][46].Broad shoulder bands in the regions of 3150, 3320 and 3350 cm -1 correspond to the stretching modes of the terminal NH groups at the aromatic ring defects [44][45].The presence of oxygen-related bonds was due to the presence of sharp bonds about 3620 cm -1 [47][48].The peak at 3405 cm -1 is attributed to O-H groups [47][48], and 839 cm -1 and 893.39 cm -1 are broad absorption bands attributed to MoS2 [49][50].The peak at 3506 cm -1 is due to the oxygen related compound [47][48].A peak of about 1730 cm -1 indicates the presence of C = O bonds in the sample.The 2325-2425 cm -1 band represents the P-H stretch.2998-2959 cm -1 is assigned to symmetric C-H stretch vibration.From Figure 5 it is observed that, the characteristic peaks for Cu2O (630 cm -1 ) shift (towards lower wavenumber) after the incorporation of g-C3N4 and MoS2, which indicated that there was an interaction between g-C3N4, Cu2O and MoS2.From the spectra, all the characteristic peaks of both g-C3N4, Cu2O and MoS2 appeared in the MoS2/g-C3N4/Cu2O, which is in accordance with the Fourier infrared spectra.

I-V Curve analysis
The efficiency, maximum power, photo voltage and photocurrent was obtained under illumination as outline in Table 2 and the Cu- 2 / −  3  4 / 2  PEC solar cell showed characteristic curves of a number of external parameters followed by transition power efficiency deduce from Figure 6 and Figure 7.When analyzing the prototype, the calculated external parameters of the prepared samples Cu-Cu2O and  2 / −  3  4 / 2  are stated as shown in Table 2. Two different readings are recorded using a multimeter and numerous solar irradiances in order to study the solar cell parameters, two dissimilar graphs are studied for two different samples for testing photo response and photo voltage of the electrode under illumination.In Table 2 it can be seen that for the synthesized  2 / −  3  4 / 2 , the deposited of 2D materials increases the photo response at the same time increasing the efficiency of the sample.It also works as an absorber layer to generate charge carriers (electrons and holes) under solar light irradiation.A Comparison of some Findings of Isc, Voc and η with other Works in Literature displayed in Table 3 below.The first row in Table 2 is for Cu-Cu2O synthesized using thermal oxidation method while the subsequent row is for - 2 / −  3  4 / 2  layer deposited by partially thermal oxidation method.One can see from Figure 6 and Figure 7 that there is change in all the solar cells external parameters can be enhancement which is attained for lengthy wavelength light, when majority of electron-hole pairs are generated outside of the space charge region (SCR) and high lifetime values are required for participation of carriers in PEC reactions.On the other hand, the similar enhancement of power conversion efficiency due to deposition of  2 / −  3  4 adding up to the thermal oxidation is observed also for a short wavelength light, which is absorbed mainly in the thin near-surface part of film as shown in Table 2 and Table 3. .19184/cerimre.v6i2.38493 .19184/cerimre.v6i2.38493

Figure 4 .
Figure 4. Analysis report of the synthesized sample

Table 3 .
Comparison of Findings with other Works in Literature