Anomalous reduction in the band gap of zirconium titanate based composites Wasan Awin Eranezhuth a , Subhendra Keshari Sahoo b , Ravi Kumar a,⇑ a Laboratory for High Performance Ceramics, Department of Metallurgical and Materials Engineering, Indian Institute of Technology-Madras (IIT Madras), Chennai 600036, India b Department of Metallurgical and Materials Engineering, National Institute of Technology, Rourkela, Odisha 769008, India article info Article history: Received 1 February 2019 Received in revised form 12 May 2019 Accepted 15 May 2019 Available online 15 May 2019 Keywords: Microstructure Heterojunctions Composite materials abstract In this work, heterojunctioned TiO 2 /ZrO 2 – zirconium titanate phases have been designed through a pre- cursor derived approach. The X-ray powder diffractograms (XRD) revealed that the synthesized powder contained phase pure rutile, a mixture of rutile/zirconia and zirconium titanate or monoclinic zirconia (m-ZrO 2 ) as we vary the amount of initial precursors. The scanning electron micrographs (SEM) and transmission electron micrographs (TEM) showed a decrease in crystallite size with increase in the amount of Zr to Ti precursor and formation of heterojunctions, respectively. An anomalous reduction in band gap was observed for 70T-30Z (2.54 eV) and 60T-40Z (2.71 eV) samples when compared to their parent oxides, TiO 2 (2.86 eV) and ZrO 2 (4.5 eV) and this could be attributed to the formation of hetero- junctions between the two phases. Ó 2019 Elsevier B.V. All rights reserved. 1. Introduction Titania (TiO 2 ) and zirconia (ZrO 2 ), possessing a wide band gap of 3.1 eV and 4.5 eV, respectively restricts its applications to UV regime, which constitutes just 4–5% of solar spectrum. This results in poor photoelectric conversion efficiency which has prompted researchers to develop materials with lower band gap materials. The designing of heterojunctioned material systems comprising of distinct crystallite phases not only narrows the band gap but also reduces the recombination rate [1]. The intermediate stable phases of TiO 2 and ZrO 2 i. e., ZrTiO 4 and ZrTi 2 O 6 (ZT) arouse consid- erable interest due to the proximal band gap values of the ZT phases when compared to that of TiO 2 /ZrO 2 [2,3]. The formation of heterojunctions in ZrTiO 6 /TiO 2 and ZrTiO 4 / ZrTi 2 O 6 /TiO 2 material systems was found to efficiently separate the photogenerated electron-hole pairs and resulted in enhanced photocatalytic activity [4,5]. The incorporation of ZrO 2 into TiO 2 often leads to an intermediate band gap values between the parent oxides (TiO 2 , ZrO 2 ). A slight decrease in band gap values was observed when Zr was doped into TiO 2 due to the formation of ZrTiO 4 phase [6]. The band gap of ZTs was found to be increasing with decrease in crystallite and particle size [6,7]. A slight increase in band gap value was observed with increase in calcination tem- perature [9]. However, Alonso et al. [10] observed an insignificant change in band gap upon incorporation of zirconium (Zr) into the TiO 2 lattice. The band gap value of nitrogen doped ZrTiO 4 was also found to be unaffected when compared to the undoped material [11]. Nevertheless, the role of rutile/ZrTiO 4 , rutile/ZrTi 2 O 6 , m- ZrO 2 /ZrTiO 4 phases towards the engineering of band gap are yet to be fully understood. The possibility to engineer the band gap by the introduction of nanoscaled heterojunctions at the interface of TiO 2 and ZT phases forms the crux of the present work. The pre- cursor derived approach used to design the heterojunctioned sys- tems could alter the microstructure and fine tune the band gap so as to harness the light in the visible wavelength regime and is expected to enhance the overall photocatalytic efficiency. Here in, we report a facile approach towards the synthesis of heterojunctioned ZT-TiO 2 /ZrO 2 composites and analyze the influ- ence of ZT phases on the band gap of the synthesized materials. The crystallization behavior and structural evolution was studied and TEM confirmed the existence of heterojunctions between ZT and rutile phase. The optical band gap of the prepared samples were determined and compared with its parent oxides (TiO 2 and ZrO 2 ). 2. Experimental Titanium butoxide (TB) and zirconium butoxide (ZB) were cho- sen as initial precursors for the synthesis of zirconium-titanium based composites. The ZTs have been reported to exhibit two types of stable ordered structures with different stoichiometries below https://doi.org/10.1016/j.matlet.2019.05.063 0167-577X/Ó 2019 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: nvrk@iitm.ac.in (R. Kumar). Materials Letters 251 (2019) 230–233 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/mlblue