146 © 2024 The Author(s). Published by College of Education for Pure Science (Ibn Al-Haitham), University of Baghdad. This is an open-access article distributed under the terms of the Creative Commons Attribution 4.0 International License The Effect of Annealing Process on Surface Plasmon Resonances in Tungsten Trioxide Films Mohammed H. Mustafa1,* and Aliyah A. Shihab2 1,2 Department of Physics, College of Education for Pure Sciences (Ibn Al-Haitham), University of Baghdad, Baghdad, Iraq. *Corresponding Author Received: 26 June 2023 Accepted: 1 August 2023 Published: 20 October 2024 doi.org/10.30526/37.4.3627 Abstract In this study, after pyrolyzing the prepared solution, we made films from transitional tungsten oxide metal doped with gold nanoparticles and deposited them using a spray deposition technique on glass substrates at a substrate temperature of 320°C. We then annealed the prepared films at various temperatures (673,773 K) for one hour, in which we detected the band of localized surface plasmon resonance (LSPR) in gold-doped tungsten oxide films around the wavelength of 595 nm, and thermal treatment increased its intensity to near the wavelength of 580 nm. The produced and annealed thin films also demonstrated an indirect energy gap smaller than (2.86- 2.61) eV in the UV-visible spectrum. The structural characteristics of the manufactured and annealed thin films reveal an amorphous structure at the substrate temperature of 320°C, but a polycrystalline structure at the annealing temperature, where researchers discovered monoclinic tungsten trioxide structures. (AFM), which achieved the maximum particle size of 75.93 nm after it was 47.85 nm, allowing researchers to see that the thin film of all the samples has a nanostructure. Keywords: Structural, optical properties, surface plasmon resonances, WO3 thin films. 1. Introduction Due to their exceptional electrical characteristics, tungsten oxide films have recently attracted much scientific attention. Films can take various forms depending on the deposition conditions and methods [1-3]. The structural, optical, and electrical behaviors are very different. Additionally, there are other uses for these films, including electrocatalysis and gas sensors [4,5]. Various methods such as thermal evaporation, laser deposition, radio-frequency spattering, sol- gel, hydrothermal, spray pyrolysis, and others can create WO3 thin films[6-8]. With a large band gap (Eg) of 2.5–3.2 eV, WO3 has a monoclinic crystal structure [9-13]. This substance is exceptional for several reasons, including its low cost, chemical stability, non-toxicity, and mechanical properties. It is also thought to be a catalyst for semiconductors [14-17]. The current https://creativecommons.org/licenses/by/4.0/ https://creativecommons.org/licenses/by/4.0/ https://orcid.org/0000-0002-9256-8562 mailto:mohammed.h.m@ihcoedu.uobaghdad.edu.iq https://orcid.org/0000-0003-0793-1943 mailto::alia.a.s@ihcoedu.uobaghdad.edu.iq IHJPAS. 2024, 37 (4) 147 study used spray pyrolysis to generate WO3 thin films doped with gold nanoparticles. We periodically change the spray solution in the spray pyrolysis process to allow the adatoms enough time to break down. Avoid nozzle sprays because typical pyrolysis spray deposits frequently experience massive drop precipitation. The main purpose of this study is to investigate how annealing changes the crystal structure, optical properties, and surface properties of tungsten trioxide films created by spray pyrolysis. 2. 2. Methods and Material This study used a molar concentration of 0.06 M and nano-gold doping to make tungsten trioxide thin films on glass substrates at 320°C. We used distilled water to dissolve H2WO4 powder, a source of tungsten oxide, in the presence of a small amount of ammonia. To create WO3: Au films, dissolve them in water [18,19] and thoroughly agitate with a magnetic stirrer until the solution is clear. Then, add HAuCl4, which represents the source of gold nanoparticles at a concentration of 6 mM, to the previously prepared solution. Using the pyrolysis spray approach, we deposited films with a thickness of 250 nm, spraying at a velocity of 0.15 ml/sec and maintaining 25 cm between the glass substrates and the spray nozzle. We anneal the prepared samples at temperatures of 673 K and 773 K. We examined the structural characteristics of all the prepared and annealed thin films using an (XRD). We examined the surface morphological properties of the film using atomic force spectrometry (AFM). We used a UV-Vis spectrophotometer with wavelengths between 300 and 1000 nm to examine the optical characteristics of the thin films. 3. Results and Discussion 3.1. Structural Analysis 3.1.1. XRD Analysis Figure 1 illustrates the X-ray diffraction patterns of the WO3 films without annealing and with annealing at different temperatures. at 2Ɵ ~ 24 o in the film without annealing indicates an amorphous structure with short-range atomic organization in the crystal lattice. This feature might include dispersion from the glass and film substrates. According to the figure, tungsten trioxide exhibits an amorphous structure at 320 °C, and the peak at 2Ɵ=38.213° shows the cubic phase development of gold in (111) Orientation [20]. Using Scherrer's equation, the average crystal size of gold nanoparticles was calculated [21,22]. For the doped samples containing 6 mM of gold, the average size of the Au crystal was determined to be roughly 10.05 nm. C.S = (0.94 λ) / β cos Ɵ (1) Where:(λ) is the wavelength of XRD photons; is the full width at half maximum (FWHM); and (Ɵ) is the Brage diffraction angle in degrees. After heating samples to 673K Figure 1. and Table 1., we recognized reflections at the angles mentioned in Table 1. as the monoclinic WO3 phase's reflections [13,23]. This film’s recommended orientation is 200. According to Figure 1. and Table 1., samples that were annealed IHJPAS. 2024, 37 (4) 148 at 773 K likewise showed evidence of the WO3 phase, but with a preferred orientation of (002). Based on CAS Number (1314-35-8), peak position and plane indices are also displayed. Table 1. Peak positions (2Ɵ), spacing (d), planes (hkl), and crystallite size (C.S), of WO3:Au as-deposited films and annealed at 673 K and 773 K. C.S (nm) Planes ( hkl ) Phase d (Å) Exp. d (Å) ASTM 2θ(deg.) Exp. Ta (k) 18.1206 (002) Mon. WO3 3.8428 3.8440 23.202 673 14.5837 (020) Mon. WO3 3.7673 3.7690 23.619 16.5344 (200) Mon. WO3 3.6469 3.6480 24.419 10.6557 (111) Cub. Au 2.3547 2.3550 38.208 9.06666 (200) Cub. Au 2.0383 2.0390 44.337 18.8330 (002) Mon. WO3 3.8433 3.8440 23.179 773 15.8988 (020) Mon. WO3 3.7690 3.7679 23.597 17.4157 (200) Mon. WO3 3.6474 3.6480 24.399 14.4891 (209) Mon. WO3 2.6223 2.6230 34.172 11.3229 (111) Cub. Au 2.3550 2.3550 38.198 12.5702 (200) Cub. Au 2.0395 2.0390 44.363 Figure 1. X-ray diffraction pattern of WO3: Au films without annealing and with annealing at different temperatures. 3.1.2. The Surface Morphology Analysis Figure 2. displays the AFM pictures of the deposited and annealed WO3 thin films. The films' surfaces are smooth and have grains between (47 and 76) nm in size. The deposited films did not clearly show the grain boundaries, but annealing made them more obvious. IHJPAS. 2024, 37 (4) 149 Figure 2. AFM images of WO3: Au thin films without annealing and with annealing. The films' particle sizes grow as the annealing temperature rises. We discovered that the RMS value changed, and the surface remained smooth continuously. Surface roughness increases only slightly because of annealing. Some deep valleys developed, and the structure became more distinct. Films produced and annealed at 673 K and 773 K have smooth surfaces and contain nanoscale particles. Table 2. displays the roughness and typical grain size of synthetic and annealed films. According to Figure 2., the films' grain size increased as the annealing temperature rose. High temperatures caused the grains to grow and become clearer. The surface's continuous structure changed into a distinct one. As-deposited Annealed at 673 K Annealed at 773 K IHJPAS. 2024, 37 (4) 150 Table 2. The roughness and average grain size thin films of WO3:Au as-deposited sample and when annealed at two temperatures 673 K and 773 K. Root mean square (nm) Roughness Average (nm) Average diameter (nm) Annealing Temperature (K) 16.39 11.71 47.85 As-deposited 51.59 40.58 59.01 673 69.85 56.38 75.93 773 3.2. Optical Analysis The optical transmittance plots of the WO3 films are displayed in Figure 3. All films exhibit high visible-range transmittance, which denotes a low level of oxygen ion vacancies [24,25]. Below λ = 400 nm, the transmittance values decrease dramatically, indicating significant band-to- band absorption [26]. The total transmittance of the films was not considerably altered by raising the annealing temperature, it caused the absorption edges to move to lower energy ranges, resulting in a reduction in band gaps. Figure 3. The transmission spectrum for WO3:Au films as a function of wavelength. The band gap values of WO3 doped with gold nanoparticles and annealed with different temperature films can be obtained by extrapolating the straight-line section of the (αhʋ)1/2 versus (hʋ). Figure 4. and Table 3. show that after the annealing process, the energy gap for tungsten oxide films doped with gold nanoparticles dropped from 2.86 eV to 2.61 eV. The films' decreased oxygen content due to their annealing at high temperatures is thought to be the drop in the value of Eg opt. This conclusion is in line with [13, 23, 27, 28]. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 250 350 450 550 650 750 850 950 1050 T λ (nm) as-deposited annealing at 673 K annealing at 773 K IHJPAS. 2024, 37 (4) 151 Figure 4. The plot of (αhυ)1/2 as a function of (hυ) for WO3:Au thin films. Table 3 displays data from various WO3 probes, including Au at full width at half maximum (FWHM) and LSPR peak position (SPR). The transmittance spectra of the gold-embedded film show a reduction at about 595 nm, possibly due to absorption from gold nanoparticles' surface plasmon resonance (SPR). The main reason why the LSPR absorption band gets stronger with temperature is that Au nanoparticles get bigger [29, 30]. According to figure 3, as the annealing temperature changes, the position of the LSPR peak also shifts, with the orange hue reverting to yellow at the 773 K LSPR at 580 nm after moving away from the SPRAD at 595 nm. This behavior is comparable to that seen by the researcher [3] after annealing doped tungsten oxide films to various annealing intensities. Table 3. Energy gap (Eg), full width at half maximum (FWHM), and LSPR peak location (λSPR) for WO3:Au as- deposited and at the different annealing temperatures. Annealing temperature (K) Eg (eV) FWHM (nm) λ SPR (nm) As deposited 2.86 327 595 673 2.67 255 585 773 2.61 236 580 4. Conclusion Electron beam evaporation deposited WO3:Au sheets with a thickness of 250 nm on glass substrates. We demonstrate both the amorphous WO3:Au films and the crystalline films produced following the annealing temperature have surfaces with homogeneous shapes and no surface cracks. 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