{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,4,1]],"date-time":"2026-04-01T20:43:00Z","timestamp":1775076180456,"version":"3.50.1"},"reference-count":30,"publisher":"MDPI AG","issue":"1","license":[{"start":{"date-parts":[[2024,12,28]],"date-time":"2024-12-28T00:00:00Z","timestamp":1735344000000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Coatings"],"abstract":"<jats:p>This study investigates the structural and electronic transitions of sol\u2013gel derived titanium dioxide (TiO2) thin films using vacuum ultraviolet (VUV) spectroscopy, to elucidate the impact of annealing-induced phase evolution. As the annealing temperature increased from 400 \u00b0C to 800 \u00b0C, the films transitioned from amorphous to anatase, mixed anatase\u2013rutile, and finally rutile phases. VUV spectroscopy revealed distinct absorption features: a high-energy \u03c3 \u2192 \u03c0* transition below 150 nm, associated with bonding to antibonding orbital excitations, and lower-energy absorption bands in the range 175\u2013180 nm and near 280 nm, attributed to \u03c0 \u2192 \u03c0* and t2g(\u03c0) \u2192 t*2g(\u03c0*) transitions, respectively. These spectral features highlight the material\u2019s intrinsic electronic states and defect-related transitions. A slight redshift of the absorption band from 176 nm to 177 nm with annealing reflects bandgap narrowing, attributed to increased rutile content, crystallite growth, and defect-induced effects. Broadening and additional absorption features around 280 nm were attributed to oxygen vacancies and reduced titanium oxidation states (Ti3\u207a), as corroborated by X-ray photoelectron spectroscopy (XPS). XPS further confirmed the presence of Ti3\u207a species and oxygen vacancies, providing complementary evidence of defect-mediated transitions observed in the VUV spectra. While complementary techniques such as X-ray diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR) confirmed phase transitions and the reduction of hydroxyl groups, respectively, VUV spectroscopy uniquely captured the dynamic interplay between structural defects, phase evolution, and optical properties. This study underscores the utility of VUV spectroscopy as a powerful tool for probing the electronic structure of TiO2 films, offering insights critical for tailoring their functional properties in advanced applications.<\/jats:p>","DOI":"10.3390\/coatings15010019","type":"journal-article","created":{"date-parts":[[2024,12,31]],"date-time":"2024-12-31T07:34:19Z","timestamp":1735630459000},"page":"19","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":7,"title":["Vacuum Ultraviolet Spectroscopic Analysis of Structural Phases in TiO2 Sol\u2013Gel Thin Films"],"prefix":"10.3390","volume":"15","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-5935-870X","authenticated-orcid":false,"given":"Helena Cristina","family":"Vasconcelos","sequence":"first","affiliation":[{"name":"Faculty of Science and Technology, University of the Azores, Ponta Delgada, S. Miguel, 9500-321 Azores, Portugal"},{"name":"Laboratory of Instrumentation, Biomedical Engineering and Radiation Physics (LIBPhys, UNL), Department of Physics, NOVA School of Science and Technology, 2829-516 Caparica, Portugal"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-9356-3045","authenticated-orcid":false,"given":"Maria","family":"Meirelles","sequence":"additional","affiliation":[{"name":"Faculty of Science and Technology, University of the Azores, Ponta Delgada, S. Miguel, 9500-321 Azores, Portugal"},{"name":"Research Institute of Marine Sciences of the University of the Azores (OKEANOS), Horta, Faial, 9901-862 Azores, Portugal"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-5893-0660","authenticated-orcid":false,"given":"Re\u015fit","family":"\u00d6zmente\u015f","sequence":"additional","affiliation":[{"name":"Vocational School of Health Services, Bitlis Eren University, 13100 Bitlis, Turkey"}]},{"given":"Abdulkadir","family":"Korkut","sequence":"additional","affiliation":[{"name":"Faculty of Science, Department of Physics, Van Yuzuncu Yil University, 65080 Van, Turkey"}]}],"member":"1968","published-online":{"date-parts":[[2024,12,28]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"37","DOI":"10.1038\/238037a0","article-title":"Electrochemical Photolysis of Water at a Semiconductor Electrode","volume":"238","author":"Fujishima","year":"1972","journal-title":"Nature"},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"2891","DOI":"10.1021\/cr0500535","article-title":"Titanium dioxide nanomaterials: Synthesis, properties, modifications, and applications","volume":"107","author":"Chen","year":"2007","journal-title":"Chem. Rev."},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"53","DOI":"10.1016\/S0167-5729(02)00100-0","article-title":"The surface science of titanium dioxide","volume":"48","author":"Diebold","year":"2003","journal-title":"Surf. Sci. Rep."},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"9281","DOI":"10.1021\/cr500422r","article-title":"Introduction: Titanium Dioxide (TiO2) Nanomaterials","volume":"114","author":"Chen","year":"2014","journal-title":"Chem. Rev."},{"key":"ref_5","doi-asserted-by":"crossref","unstructured":"Gon\u00e7alves, M.C., Pereira, J.C., Matos, J.C., and Vasconcelos, H.C. (2018). Photonic Band Gap and Bactericide Performance of Amorphous Sol-Gel Titania: An Alternative to Crystalline TiO2. Molecules, 23.","DOI":"10.3390\/molecules23071677"},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"13669","DOI":"10.1021\/j100102a038","article-title":"The Role of Metal Ion Dopants in Quantum-Sized TiO2: Correlation between Photoreactivity and Charge Carrier Recombination Dynamics","volume":"98","author":"Choi","year":"1994","journal-title":"J. Phys. Chem."},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"5606","DOI":"10.1103\/PhysRevB.18.5606","article-title":"Fine structure in the intrinsic absorption edge of TiO2","volume":"18","author":"Pascual","year":"1978","journal-title":"Phys. Rev. B"},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"267","DOI":"10.1016\/0038-1098(94)90889-3","article-title":"Optical properties of anatase (TiO2)","volume":"92","author":"Tang","year":"1994","journal-title":"Solid State Commun."},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"71","DOI":"10.1016\/j.cplett.2011.04.002","article-title":"Spectroscopic studies of anatase TiO2 thin films prepared by DC reactive magnetron sputtering","volume":"508","author":"Jorge","year":"2011","journal-title":"Chem. Phys. Lett."},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"75","DOI":"10.1016\/S0368-2048(96)80030-2","article-title":"UV reflection spectra of anatase TiO2","volume":"78","author":"Hosaka","year":"1996","journal-title":"J. Electron Spectrosc. Relat. Phenom."},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"877","DOI":"10.1143\/JPSJ.66.877","article-title":"Optical Properties of Single-Crystal Anatase TiO2","volume":"66","author":"Hosaka","year":"1997","journal-title":"J. Phys. Soc. Jpn."},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"7706","DOI":"10.1039\/C6TC02368A","article-title":"Far- and deep-ultraviolet spectroscopic investigations for titanium dioxide: Electronic absorption, Rayleigh scattering, and Raman spectroscopy","volume":"4","author":"Tanabe","year":"2016","journal-title":"J. Mater. Chem. C"},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"1300","DOI":"10.1016\/j.jnoncrysol.2010.03.008","article-title":"Optical properties of sol\u2013gel processed TiO2 thin films up to the vacuum ultraviolet energy region","volume":"356","author":"Sato","year":"2010","journal-title":"J. Non-Cryst. Solids"},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"2759","DOI":"10.1016\/j.apsusc.2008.08.051","article-title":"Surface defect (Ti3+) controlling in the first step on the anatase TiO2 nanocrystal by using sol\u2013gel technique","volume":"255","author":"Suriye","year":"2008","journal-title":"Appl. Surf. Sci."},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"5254","DOI":"10.1021\/cg101105t","article-title":"Nucleation and Crystal Growth of Nanocrystalline Anatase and Rutile Phase TiO2 from a Water-Soluble Precursor","volume":"10","author":"Kinsinger","year":"2010","journal-title":"Cryst. Growth Des."},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"236","DOI":"10.2138\/am.2009.3050","article-title":"Heat capacities and thermodynamic functions of TiO2 anatase and rutile: Analysis of phase stability","volume":"94","author":"Smith","year":"2009","journal-title":"Am. Miner."},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"4603","DOI":"10.1016\/j.tsf.2007.05.079","article-title":"Effects of internal stress on photocatalytic properties of TiO2 films","volume":"516","author":"Miyamura","year":"2008","journal-title":"Thin Solid Films"},{"key":"ref_18","unstructured":"(2024, November 21). XRD Crystallite (Grain) Size Calculator (Scherrer Equation)\u2014InstaNANO. Available online: https:\/\/instanano.com\/all\/characterization\/xrd\/crystallite-size\/."},{"key":"ref_19","doi-asserted-by":"crossref","first-page":"7459","DOI":"10.1103\/PhysRevB.61.7459","article-title":"Electronic and Optical Properties of Anatase TiO2","volume":"61","author":"Asahi","year":"2000","journal-title":"J. Phys. Rev. B"},{"key":"ref_20","doi-asserted-by":"crossref","unstructured":"Li, J., Wei, S., Dong, Y., Zhang, Y., and Wang, L. (2024). Theoretical Study on Photocatalytic Reduction of CO2 on Anatase\/Rutile Mixed-Phase TiO2. Molecules, 29.","DOI":"10.3390\/molecules29174105"},{"key":"ref_21","doi-asserted-by":"crossref","first-page":"13023","DOI":"10.1103\/PhysRevB.51.13023","article-title":"Electronic and optical properties of three phases of titanium dioxide: Rutile, anatase, and brookite","volume":"51","author":"Mo","year":"1995","journal-title":"Phys. Rev. B"},{"key":"ref_22","doi-asserted-by":"crossref","unstructured":"M\u00eendroiu, V.M., Stoian, A.B., Irodia, R., Tru\u0219c\u0103, R., and Vasile, E. (2023). Titanium Dioxide Thin Films Produced on FTO Substrate Using the Sol\u2013Gel Process: The Effect of the Dispersant on Optical, Surface and Electrochemical Features. Materials, 16.","DOI":"10.3390\/ma16083147"},{"key":"ref_23","doi-asserted-by":"crossref","unstructured":"Vasconcelos, H.C., and Pinto, A.S. (2017). Fluorescence Properties of Rare-Earth-Doped Sol-Gel Glasses. Recent Applications in Sol-Gel Synthesis, InTech.","DOI":"10.5772\/intechopen.68534"},{"key":"ref_24","doi-asserted-by":"crossref","first-page":"16176","DOI":"10.1021\/acsomega.0c01756","article-title":"Nonhazardous Process for Extracting Pure Titanium Dioxide Nanorods from Geogenic Ilmenite","volume":"5","author":"Rajakaruna","year":"2020","journal-title":"ACS Omega"},{"key":"ref_25","doi-asserted-by":"crossref","first-page":"7","DOI":"10.1016\/S0040-6090(00)01542-X","article-title":"Effect of surface structure on photocatalytic activity of TiO2 thin films prepared by sol-gel method","volume":"379","author":"Yu","year":"2000","journal-title":"Thin Solid Films"},{"key":"ref_26","doi-asserted-by":"crossref","first-page":"5275","DOI":"10.1021\/jp077275m","article-title":"Defect Chemistry of Titanium Dioxide. Application of Defect Engineering in Processing of TiO2-Based Photocatalysts","volume":"112","author":"Nowotny","year":"2008","journal-title":"J. Phys. Chem. C"},{"key":"ref_27","doi-asserted-by":"crossref","first-page":"7142","DOI":"10.1039\/C7RA13523H","article-title":"In situ green oxidation synthesis of Ti3+ and N self-doped SrTiOxNy nanoparticles with enhanced photocatalytic activity under visible light","volume":"8","author":"Liu","year":"2018","journal-title":"RSC Adv."},{"key":"ref_28","first-page":"163","article-title":"XRD and XPS analysis of TiO2 thin films annealed in different environments","volume":"4","author":"Potlog","year":"2014","journal-title":"J. Mater. Sci. Eng. B"},{"key":"ref_29","doi-asserted-by":"crossref","unstructured":"Bharti, B., Kumar, S., Lee, H.-N., and Kumar, R. (2016). Formation of oxygen vacancies and Ti3+ state in TiO2 thin film and enhanced optical properties by air plasma treatment. Sci. Rep., 6.","DOI":"10.1038\/srep32355"},{"key":"ref_30","doi-asserted-by":"crossref","first-page":"23773","DOI":"10.1021\/acs.jpcc.0c07568","article-title":"Defects in the Amorphous\u2013Crystalline Evolution of Gel-Derived TiO2","volume":"124","author":"Koral","year":"2020","journal-title":"J. Phys. Chem. C"}],"container-title":["Coatings"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/2079-6412\/15\/1\/19\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,10]],"date-time":"2025-10-10T16:55:17Z","timestamp":1760115317000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/2079-6412\/15\/1\/19"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2024,12,28]]},"references-count":30,"journal-issue":{"issue":"1","published-online":{"date-parts":[[2025,1]]}},"alternative-id":["coatings15010019"],"URL":"https:\/\/doi.org\/10.3390\/coatings15010019","relation":{},"ISSN":["2079-6412"],"issn-type":[{"value":"2079-6412","type":"electronic"}],"subject":[],"published":{"date-parts":[[2024,12,28]]}}}