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Nanostructure morphology influences in electrical properties of titanium dioxide thin films Patrick Pires Conti1,2, Eupídio Scopel1,3, Edson Roberto Leite2,b), Cleocir José Dalmaschio1,a) 1

Department of Chemistry, Federal University of Espírito Santo (UFES), Vitória 29075-910, Espírito Santo, Brazil Department of Chemistry, Federal University of São Carlos (UFSCar), 13565-905, São Carlos, São Paulo, Brazil 3 Department of Physical Chemistry, Institute of Chemistry, University of Campinas (Unicamp), 13083-970, Campinas, São Paulo, Brazil a) Address all correspondence to this author. e-mail: [email protected] b) Contributing Editor: Edson leite 2

Received: 17 April 2020; accepted: 3 August 2020

Titanium dioxide (TiO2) is a semiconductor that can be applied in different technological areas. In this work, we investigated the modifications of the electrical properties of thin films composed of TiO2 nanoparticles produced with different morphologies. The solvothermal route used for the synthesis allowed the production of nanoparticles with functionalized surfaces due to oleate groups. It was possible to modulate nanocrystals shape and size due to the detachment crystal growth mechanism, by changing the reaction time. Nanorods were obtained using 4 h of synthesis, and an increase in the reaction time to 64 h led to a bipyramidal morphology. The functionalization by the organic ligand allowed the preparation of stable colloidal solutions, which were used to prepare thin films by the dip-coating method. The films presented a homogeneous surface, an average thickness around 100 nm, and no agglomerations were observed. The electrical resistance measurements indicated a typical behavior of semiconductors, and they were dependent on the nanoparticle morphology. An exploratory test indicated that the thin films prepared using nanorod particles presented a higher electrical response compared with isotropic particles, when exposed in a liquefied petroleum gas vapor atmosphere. Therefore, the morphology of the nanoparticles is a key factor for the further application of these thin films in gas sensing. Employing an easy methodology which required simple apparatus, and by using reaction time modulation only, it was possible to prepare homogeneous thin films with a tunable electrical response.

Introduction Titanium dioxide (TiO2) is a nontoxic, low cost, and photocorrosion-free semiconductor that is used in several technological applications, such as dye-sensitized solar cells [1, 2], photoelectrochemistry [3], photocatalysis [4, 5, 6], and sensors [7, 8]. These applications are achievable mainly due to the electrical conductivity and the photocatalytic properties [9, 10] of TiO2 which are enhanced at the nanometric scale. The outstanding properties of TiO2 make it an interesting material for use as a sensing element in electronic devices, such as gas sensors [11]. An electronic sensor consists of a substrate, an electrode, and a sensing element, which can be a thin film of semiconductor material, such as TiO2. The inte