Synthesis of (001) oriented BiOI thin films by a dual-port ultrasonic spray pyrolysis system J. Alejandro Arana-Trenado a,b , Vinoth Kumar Jayaraman a , Monserrat Bizarro a,⇑ a Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, Circuito Exterior S/N, Ciudad Universitaria, Coyoacán, 04510 Mexico City, Mexico b Posgrado en Ciencia e Ingeniería de Materiales, Universidad Nacional Autónoma de México, A.P. 70-360, Coyoacán, 04510 Mexico City, Mexico article info Article history: Received 13 December 2019 Accepted 11 March 2020 Available online 12 March 2020 Keywords: Spray pyrolysis Atomization Bismuth oxyhalides Thin film Indigo carmine abstract A dual-port ultrasonic spray pyrolysis (DP-USP) system was developed to obtain bismuth oxyiodide (BiOI) thin films. Independent bismuth and iodine precursor solutions were separately atomized and mixed in a chamber just above the heated substrate where reacted to form the film. This configuration allowed a complete reaction and uniform substrate coverage. Crystalline BiOI films were tetragonal with the typical layered structure; their band gap was around 1.6 eV and showed photocatalytic response under simulated sunlight. Photocatalytic activity, tested with indigo carmine (IC) dye molecule, was pH dependent, reaching the highest degradation at pH = 3. A careful evaluation of IC’s absorbance spectra indicated its decomposition into two degradation products. This was confirmed by comparison with the- oretical calculations of the absorbance spectrum of IC and its degradation products. Ó 2020 Elsevier B.V. All rights reserved. 1. Introduction Bismuth oxyiodide (BiOI) is a promising semiconductor mate- rial, which has recently attracted attention for its visible-light pho- tocatalytic activity. It is a ternary oxide compound from groups V, VI and VII having a matlockite-type layered structure where each bismuth atom is surrounded by four oxygen and four iodine atoms. Its structure can be seen as layers of [Bi 2 O 2 ] 2+ alternating by a dou- ble I - slabs interacting by Van der Waals forces. This arrangement promotes an internal electric field oriented to c direction and it is believed to be responsible for an improved charge carrier separa- tion [1]. Its bandgap is reported between 1.7 and 2.0 eV [2,3], mak- ing it ideal for visible light absorption. Additionally, BiOI is a material with an indirect bandgap transition, thus, the recombina- tion of photogenerated e  h + pairs is low. Despite all good charac- teristics, BiOI has not yet been widely applied as photocatalyst. Still some aspects limit its application such as the short lifetime of car- riers, low mobility and limited conductivity [4]. BiOI has been prepared as powder form by ionic liquid-assisted precipitation, solvothermal or hydrothermal methods and similar, reporting the formation of different nanostructures such as nano- plates, nanosheets, microspheres, nanoflakes and so on [5]. How- ever, the synthesis of BiOI as thin film is peculiarly difficult, due to the incompatibility of most of the precursors and solvents typi- cally used. For example, KI salt is easily dissolved in water, but in contact with a bismuth nitrate aqueous solution leads to precipita- tion. To avoid precipitation strong acids should be used [6], but this can damage some deposition systems by corrosion. Moreover, many of the deposition processes produce a mixture of phases in addition to BiOI, such as BiI, BiO x I y , BiI x [6], or even metallic Bi, not a pure BiOI film. Hahn et al. achieved BiOI films using Bi (NO 3 ) 3 and NH 4 I in ethylene glycol, but report that other iodine sources led to unsuccessful results. For example, KI produced a large postassium contamination, whereas elemental iodine did not react to form BiOI under the attempted conditions and only Bi 2 O 3 was obtained [7]. For many chemical deposition methods, an essential require- ment is to obtain a homogeneous precursor solution. Without this condition the uniformity of the deposition cannot be guaranteed. This is the case of the spray pyrolysis technique for thin film depo- sition, where the generated droplets must have the same composi- tion in order to produce equivalent chemical reactions to form the film. In our experience, the conventional ultrasonic spray pyrolysis (USP) system – where a precursor solution containing all the ele- ments wanted in the film is atomized over a heated substrate – was not suitable for the deposition of uniform BiOI films, even after trying with different combination of precursor salts and solvents. In a previous work, Gómez et al. [8] used bismuth nitrate and HN 4 I dissolved in ethylene glycol that was manually atomized at 275 °C, as the solution could not be nebulized by USP. In this work we present a quite effective way to prepare BiOI films by the mod- https://doi.org/10.1016/j.matlet.2020.127655 0167-577X/Ó 2020 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: monserrat@iim.unam.mx (M. Bizarro). Materials Letters 269 (2020) 127655 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/mlblue