Effect of Temperature Variation on Band Gap Value in Thin Layers of Nano Photocatalyst Fe2O3/CuO/MnO2

Water is one of the primary needs of all living things in daily life. However, the availability of clean water is currently starting to decrease along with the decline in the quality and quantity of water in the environment. Industrialization and rapid economic development have caused concern due to the habit of disposing of waste without proper management. Attempts to get clean water free from pollution are to utilize technology with photocatalytic properties that can reduce liquid pollutants with Infra-Red light. The purpose of this study was to determine the optimum conditions of the band gap to photocatalyst by temperature variation. Fe2O3, CuO, and MnO2 are doped semiconductor materials for the application of photocatalyst properties. The materials were milled using a High Energy Milling (HEM-3ED) to obtain nanoparticle sizes. The synthesis was carried out by sol-gel and spin-coating methods to make a thin film in enhancing photocatalytic activity in industrial applications. Characterization was conducted using ultraviolet-visible (Uv-Vis) spectrophotometry analysis. The results showed that temperature affects photocatalyst properties and the band gap value obtained at a temperature variation of 400°C is 1.36 eV which is the most optimum semiconductor band gap energy.


Introduction
Water is one of the main needs of all organisms in everyday life.The fulfillment of clean water needs is currently starting to decrease due to the decline in the quality and quantity of water in our environment [1].Industrial activities such as textiles, dyes, and laundry services that have increased rapidly to help the community have created many environmental problems.These activities have the potential to generate harmful waste, such as oxidizing agents that flows untreated.This is very concerning, besides being able to interfere with human health, it also causes scarcity of clean water and triggers ecosystem damage [2].
Efforts are made to obtain clean water that is free from pollution is to utilizing technology that has photocatalyst properties.Photocatalysis is generally defined as a chemical reaction in a solid catalytic material activated by the presence of photon energy, one of which is sunlight.Photocatalyst is a combination process between photochemistry and catalyst which is a process of chemical transformation involving light as a trigger, while the catalyst accelerates the transformation process [3,4].Photocatalyst properties consist of semiconductor materials in the form of solid crystalline materials that are electrically conductive and have a higher electrical resistance than metals in the range of 10 -3 to 10 -8 ohms.Photocatalytic decomposition degrades organic and inorganic pollutants near the surface of photocatalytic nanoparticles by converting light energy from absorbed light into chemical energy for redox reactions [5,6].
The energy band gap requires electrons to be excited from the valence band to the conduction band.The greater the gap of the band gap, the more difficult the electrons are excited, and the smaller the photocatalytic effect.The magnitude of the energy of this band gap can be measured using the wavelength of light that excites electrons [7].The larger the gap of the semiconductor tape, the greater the energy required for excitation.Photocatalytic process semiconductor materials have the following advantages: low cost, the process is relatively fast, non-toxic, the ability to use long-term, and can degrade many pollutants [8].The advanced heterogeneous oxidation process uses heterogeneous oxidation catalysts such as Mn, Cu, Fe metal oxides, and mixed metal oxides to reduce organic contaminants and achieve the best properties of each metal used to improve the catalyst [3,9].According to Ratnawulan & Ramli (2021), CuO-Fe2O3 and calcination temperature play a large and effective role in large contact angles.The maximum contact angle obtained is 125.46° for a 3:1 composition, and the combustion temperature is 200 • C. The composite found PS/CuO-Fe 2O3 is hydrophobic.However, the activity of photocatalysts is very low, namely: 0.24%.
Fe2O3 is one of the type-n semiconductors with a band gap energy of 2.3 eV which has the most stable phase among iron oxides, corrosion resistance, high efficiency, non-toxic properties, cheap, and environmentally friendly [10].The combination of CuO and Fe2O3 being nanocomposites causes the physical properties of CuO-Fe2O3 to be better than CuO and Fe2O3 alone [4,11].CuO is a type-p semiconductor with a narrow band gap energy of 1.9-2.1 eV.CuO can absorb visible light to ultraviolet light.This ability is superior to TiO2, ZnO, SiO2, and Fe2O3 which are only able to absorb ultraviolet rays [12].Recently, the conductivity of many conductor polymers doped with metal oxides has been widely investigated.MnO2 can coat metals and protect them from rust and has tough properties that can be used for other metal reinforcement [13].The band gap energy of MnO2 is 0.25 eV so it can be used as a doping material for Fe2O3 and CuO.It promises heterogeneous photocatalysts as a new generation of environmentally friendly catalysts.This is due to its high specific surface area, crystallinity, the ability to crush water molecules into hydrogen and hydroxide ions [3].The article describes the fabrication process of a thin layer on a glass substrate using nanocomposites Fe2O3/CuO/MnO2.Synthesis is carried out at various temperature variations: 200 o C, 250 o C, 300 o C, 350 o C, and 400 o C.

Method
This research was experimental research that aims to investigate the effect of temperature variations on band gap values and optical properties in a thin layer of nano-photocatalysts Fe2O3/CuO/MnO2.The materials were more effective in degrading organic pollutants.This research uses the sol-gel method for material synthesis and the spin-coating method for the coating process.This research examined the effect of temperature nano photocatalyst Fe2O3/CuO/MnO2 on photocatalyst activity in the environment.The tools used in this study are High Energy Milling (HEM-3ED), magnetic stirrer, digital scales, 1 ml measuring pipette, spin coating, oven, furnace, and Uv-Vis Spectrophotometer.The materials used in this study were Fe2O3 milling powder for 5 hours, CuO milling for 20 hours, MnO2 milling for 20 hours [4], nitric acid (HNO3) used as a mixture of Cu(CH3COO).H2O, acetic acid (CH3COOH) is used as a mixture of precursor Fe(NO3)2•9H2O, ethanol is used as a solvent in the initial manufacture of sols from a mixture of precursors, ethylene glycol (C2H6O2) is used as a reagent in the formation of gel Fe2O3/CuO/MnO2, substrate glass measuring 2 x 3 centimeter is used as a thin layer substrate, and aquades to prevent dirt and fat particles from sticking so as not to reduce the evenness of the layer.The materials used in this study were Merck Pudak, Indonesia, and Loba Chemie PVP.LTD, Germany.
This research was carried out in several stages, sample preparation and synthesis techniques, characterization techniques, and data analysis.Fe2O3, CuO, and MnO2 powders were weighed as much as 10 grams each, then milled using High Energy Milling (HEM-E3D) for 5 hours for Fe2O3, for 20 hours for CuO and MnO 2 powders to obtain particles in nano-size to speed up the photocatalyst process [14].Fe2O3/CuO/MnO2 was synthesized using the sol-gel method [10] as follows: Manufacture of precursors Fe(NO3)2•9H2O obtained by mixing 1.25 grams of Fe2O3 with 0.18 grams (Nitric Acid) HNO3, then precursors Cu(CH3 COO)2•H2O was obtained by mixing 3.7 grams of CuO with 0.54 grams (Acetic Acid) CH3COOH.Precursors Fe(NO3)2•9H2O and Cu(CH3COO)2• H2O were reacted with ethanol of 5 ml each with a concentration of 0.3 M. Furthermore, stirring was carried out at room temperature (25 o C) until completely mixed [3].After 15 minutes, 1 gram of MnO2 powder was added little by little to the precursor mixture.Stirring is carried out for 20 hours, and the resulting solution is prepared at the beginning of the sole.The addition of ethylene glycol (10% of the solution composition) is added and stirred for 30 minutes then a nanocomposite gel of Fe2O3/CuO/MnO2 will be formed.The step for sol-gel synthesis can be seen in Figure 1. Figure 1 can be seen the results of the synthesis that will be placed on the thin layers by continuing the spin coating method.The substrate used in this study is a glass preparation that is 2 x 3 centimeters for samples to be tested with a Uv-Vis Spectrophotometer tool.The cut glass is cleaned with alcohol using an Ultrasonic Cleaner for 1 hour, then the glass is oven-dried at a temperature of 60 o C for 30 minutes.The coating of nanocomposites with a spin coating technique is carried out using a substrate dripped with a nanocomposite gel of 3 drops to cover the entire glass surface.The substrate is then rotated at a spin-coating rate of 1000 rpm for 60 seconds.The sample was heated for 1 hour using a furnace at temperatures of 200 o C, 250 o C, 300 o C, 350 o C, and 400 o C.
Energy band gaps based on a crystal's optical absorption of energy are divided into two types: direct band gaps and indirect band gaps [15].In the direct band gap, the photon energy absorbed by the crystal produces an electron and hole without any change in momentum.The minimum energy band (Eg) gap can be defined by equation 1.
Eg = ħ ɷg (1) In the indirect band gap, the optical absorption process involves the formation of electrons and holes separated by a k wave vector, so that the minimum energy band gap (Eg) of the indirect band gap is expressed by equation 2.
Eg = ħ ɷg ± ħΩ (2) By Ω is the erroneous frequency of the emitted phonon of the k wave vector.The thin layer energy band gap is obtained by plotting the absorbance data using a direct transition equation (direct bandgap) such as equation 3.
αhv = K (hv -Eg) 1/2  (3) With α being the absorption coefficient, hν is the photon energy (eV) and K is the constant [16].The absorption coefficient (α) is determined based on absorbance or transmittance data for each wavelength through fundamental as shown in equation 4.
I = I0 exp (-αt) (4) Where I the intensity of the light transmitted through the sample layer, I0 is the intensity of the light coming and t is the thickness of the sample layer.The absorbance is written in Equation 5.
So, the coefficient of absorption (α) is defined by equation 6. Α = 2,303   (6) Where A is the absorbance, c is the concentration of the solution (gL -1 ), and L is the length of the circulation line (L=1 cm).Plot 2 (αhv) 2 vs. hv by extrapolating the linear part of the curve to the zero absorption line gives the energy band gap value for the direct transition [17].Data obtained from the test results of the Uv-Vis Spectrophotometer function in determining the energy band gap from photocatalyst or semiconductors that can identify quantitatively and qualitatively the absorption properties in the wavelength range of UV rays, visible light, and Infra-Red [18].This method begins by determining the maximum and minimum transmission values of the Fe2O3/CuO/MnO2 layer obtained from measurements using the Uv-Vis Spectrophotometer.Determination of band gap energy using the Tauc Plot method in Origin Pro 2022 Software.

Results and Discussion
This study used five temperature variations: 200 o C, 250 o C, 300 o C, 350 o C, and 400 o C to determine the effect of temperature on band gap value in the photocatalyst process.Sample characterization using Uv-Vis Spectrophotometer for optical analysis of absorption spectra and determination of band gaps in a thin layer of nanocomposite Fe2O3/CuO/MnO2.The sample used was a Fe2O3/CuO/MnO2 sol-gel that was dropped on the substrate glass, then leveled the layer through the spin coating method, speed 1000 rpm, for 60 seconds.Spin coating is done to grow a thin layer on the glass subtract.The following is a thin layer of Fe2O3/CuO/MnO2 nanocomposites on the glass substrate shown in Figure 2. Based on Figure 2 can be seen as the result of a thin layer of Fe2O3/CuO/MnO2 on a glass substrate that has been calcined using a furnace.Furthermore, the characterization of the thin layer seen in Figure 2 is carried out using the Uv-Vis spectrophotometer.The results of characterization using the Uv-Vis Spectrophotometer can be seen in several temperature variations.

Characterized Results of the Uv-Vis Spectrophotometer
The results of characterization using the Uv-Vis Spectrophotometer tool can be seen in several temperature variations.The results of the optic band gap directly into the nanocomposite layer Fe2O3/CuO/MnO2 can be shown in Figure 3.The influence of temperature variations on the band gap value plotted through the slump of the curve formed.The higher the temperature, the lower the band gap value of the semiconductor material.The energy band gap of a thin layer of Fe2O3/CuO/MnO2 nanocomposites with temperatures of 200 o C, 250 o C, 300 o C, 350 o C, and 400 o C respectively as shown in Table 1.  1 is the result of processing the Uv-Vis Spectrophotometer of 5 temperature variations.Samples in semiconductor materials ranged from band gap values of 1.36 eV to 1.64 eV.The results showed that the optimal band gap value was obtained at a temperature of 400 o C, which was 1.36 eV.The effect of temperature variation on the band gap value will increase with the increase in the temperature used.Based on the research conducted a thin layer of Fe2O3/CuO/MnO2 nanocomposites is in the wavelength range of 742-911 nm.The photocatalyst properties of these semiconductor materials can reduce liquid pollutants by utilizing light in the Infra-Red (IR) ray area.
Semiconductors can undergo an excitatory photo process when absorbing energy that matches or is greater than the energy of the band gap.At the moment when the valence band hole interacts with the nucleophilic molecule, it will form a radical (oxidation process).While the electrons in the conduction band when interacting with electrophilic molecules produce radicals (reduction process).If this process continues, then it causes the occurrence of the photocatalytic process [19].The oxidation reaction will occur when the excited electrons diffuse to the surface of the catalyst.Specifically, (h + ) can react with the surface of H2O or OH -to produce hydroxyl radicals (·OH) and reduction occurs when (e -) reacts with oxygen (O2) to produce superoxide radical anions (·O2 -).The following schematic of photocatalyst activity is shown in Figure 4. Figure 4 shows the scheme of photocatalyst activity that occurs so that the energy of a good semiconductor band needs to be around 1 electron volt.If the space between the colors in a substance is far apart, the ability of the substance to convert light into energy becomes weaker and it becomes harder to activate electrons.Electrons are hard to get excited because the energy needed to excite these electrons is getting bigger.Figure 5 shows the band gap value decreasing with increasing such used.However, at a temperature of 350•C, there is an increase in the band gap value, then at a temperature of 400 • C, the band gap value is obtained smaller.A thin layer of Nanocomposites Fe2O3/CuO/MnO2 with a temperature of 400 • C has an optimum energy gap to be applied as a photocatalyst material capable of reducing liquid pollutants with Infra-Red rays, namely: 1.36 eV.The shift in the energy gap is caused by quantum effects and the presence of amorphous phases in thin layers [20] The amorphous phase in thin layers can be reduced by increasing the calcination temperature to the optimal calcination point.Increased calcination gives more energy to atoms to form crystals.

Optical Properties on Thin Layers Fe2O3/CuO/MnO2
The results of testing the optical properties of thin layers of Fe2O3/CuO/MnO2 generally show the relationship between absorbance and transmittance.The temperature difference can affect the increase in the absorbance value as shown in Figure 6.The increase in transmittance occurs when a thin layer of Fe2O3/CuO/MnO2 is subjected to the visible light spectrum at a wavelength of 306 nm.So that, at that wavelength the UV light emitted will be slightly passed by the layer.When the light of various wavelengths (polychromatic light) hits a substance, then light with a certain wavelength will be absorbed.Fe2O3/CuO/MnO2 acts as an electron acceptor for molecules in the surrounding medium.The mechanism of contaminant degradation above the surface of Fe2O3/CuO/MnO2 begins with the absorption of photons equal to or exceeding the energy of its band gap of 1.67 to 1.36 eV resulting in pairs of e -and h + electron holes.When e -cb is excited or changes places fill the room on the conduction tape, it is occupied by empty holes (h + vb) so that it oxidizes and h + vb is trapped on the surface of Fe2O3/CuO/MnO2 or reacts with adsorbed species such as water, hydroxide ions, contaminants compounds, and oxygen.

Conclusion
Fe2O3/CuO/MnO2 acts as an electron acceptor for molecules in the surrounding medium.The band gap value is obtained <1.7 eV so that this catalyst can reduce liquid pollutants with Infra-Red (IR) rays.The MnO2 doping in this research succeeded in reducing the band gap value in the Fe2O3/CuO/MnO2 catalyst, which is 1.36 eV of the optimum band gap produced at a temperature of 400 o C so that it can require smaller energy.The effect of temperature variation on the gap band value will increase with the increase in the temperature used.

Figure 4 .
Figure 4. Photocatalyst Activity Scheme [2] Fe2O3/CuO/MnO2 Uv-Vis is used to determine the optical properties and band gap pada samples with variations in the calcination temperature used: 200 o C, 250 o C, 300 o C, 350 o C, and 400 o C. A graph of Uv-Vis test results on a thin layer of Fe2O3/CuO/MnO2 can be seen in Figure 5 below:

Figure 5 .
Figure 5.The graph on Uv-Vis Test Result of Thin Layer Fe2O3/CuO/MnO2

Table 1 .
Average Band Gap Value from Uv-Vis Characterization Test Results