Cationic Defect Engineering for Controlling the Infrared Absorption of Hexagonal Cesium Tungsten Bronze Nanoparticles Shuhei Nakakura, †,‡ Aditya Farhan Arif, † Keisuke Machida, ‡ Kenji Adachi, ‡ and Takashi Ogi* ,† † Department of Chemical Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashihiroshima, Hiroshima 739-8527, Japan ‡ Ichikawa Research Center, Sumitomo Metal Mining Co., Ltd, 3-18-5 Nakakokubun, Ichikawa, Chiba 272-8588, Japan * S Supporting Information ABSTRACT: Cesium tungsten bronzes (Cs 0.32 WO 3 ) have attracted much attention as a near-infrared absorbing material. We report the successful synthesis of highly crystalline and high purity Cs 0.32 WO 3 nanoparticles through a spray pyrolysis route. Careful analyses disclosed the presence of cationic defects, that is, a tungsten deficiency and insufficient Cs doping in the Cs 0.32 WO 3 nanoparticles. These cationic defects can be controlled by facile heat treatment in a mildly reducing atmosphere. In particular, we clarify that the tungsten deficiency is a key factor among the cationic defects to obtain high near-infrared absorption properties. Furthermore, this study clearly demonstrates the precise tunability of the optical properties by means of the lattice constants of the Cs 0.32 WO 3 crystal. The realized range of lattice constants is significantly wider than those previously reported. These findings should contribute to the engineering of Cs 0.32 WO 3 structure and properties. 1. INTRODUCTION Structural defects play an important role in regulating the properties of metal oxides. Controllably introduced defects alter the density of localized charges, which affect the charge mobility and the response to an incident electromagnetic field and other chemicals. 1 Based on this principle, structural defects have been engineered to control the electronic, 2,3 optical, 4 ionic transport, 5 adsorption, 6 and catalytic properties 7 of metal oxides. Controlling the stoichiometry of metal oxides by creating an oxygen vacancy (V O ) has been popular in defect engineering. 8 For example, V O can be introduced to the structure of tungsten trioxide (WO 3 ), which creates a suboxide WO 3-x . A recent study reported that this structure has a high chemical adsorption ability owing to the formation of active sites by V O s. 9 Another study found that the electrons provided from V O s and interstitial dopants, such as Rb + and Cs + , facilitated the absorption of near-infrared (NIR) light in hexagonal tungsten bronzes. 10-13 Besides anionic defects (i.e., V O ), Li et al. suggested that a cationic defect can also greatly influence the electrical conductivity and catalytic activity of metal oxides. 14 Similarly, Zhao et al. synthesized the chalcogenide copper-deficient Cu 2-x S for optical materials. 15 In titania, the incorporation of fluorine as an interstitial dopant determines the number of titanium vacancies, which suggests a correlation between cationic defects and an interstitial dopant. 14 Among the tungsten bronzes, nanometer-sized cesium tungsten bronzes (Cs 0.32 WO 3 ), in particular, have attracted much attention as a NIR shielding material owing to its excellent NIR absorption capability and high transmittance of visible light. 10,16 These properties are strongly correlated with the electrons in the hybridized orbitals of W 5d and O 2p in the conduction band, which are derived from V O s and alkali dopants. 17,12,13 In a recent study, Okada et al. 18,12 observed a coordinated linear structural change in lattice dimensions, which were interpreted as arising from the relaxation of the pseudo-Jahn-Teller (PJT) distortion by decreasing Cs defects and/or increasing V O s in Cs 0.32 WO 3 . However, the effect of cationic defects, which includes W deficiency in Cs 0.32 WO 3 , remains a concern that should be investigated. On the basis of the above background, the current study focuses on the nanoscale engineering of cationic defects in the Cs 0.32 WO 3 crystal. Specifically, we focus on W deficiency and doping states of Cs ions with their effects on the NIR absorption properties. Cs 0.32 WO 3 nanoparticles including cationic defects were synthesized using spray pyrolysis. This process has been proven to be effective for the synthesis of various nanostructured and heteroatom-doped particles. 19-22 It features a rapid quenching, which increases the possibility of defect formation. A control of the cationic defects is achieved by heat treatment in a mildly reducing atmosphere. Close attention is given to the spatial aspect of the crystal structure through observations of lattice constants, atomic arrangements, and interatomic distances to construct a hypothesis for the Received: March 5, 2019 Article pubs.acs.org/IC Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.inorgchem.9b00642 Inorg. Chem. XXXX, XXX, XXX-XXX Downloaded by AMERICAN UNIV OF BEIRUT at 05:49:05:238 on June 27, 2019 from https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b00642.