Effect of pH in the hydrothermal preparation of monoclinic tungsten oxide

Teodóra Nagyné-Kovács,István Endre Lukács,Anna Szabó,Klara Hernadi,Tamás Igricz,Krisztina László,Imre M Szilágyi,György Pokol

doi:10.1016/j.jssc.2019.121044

Abstract

This paper presents the preparation of monoclinic WO3 by a one-step hydrothermal method. The effect of very acidic pH (0.1) and the significance of various additives (CH3COOH, NaClO4, Na2SO4) were investigated. To clarify the role of pH on the obtained crystal structure and morphology, every synthesis using pH 1 were repeated, and the effect of temperature, using 180 and 200 °C, was also studied. All samples prepared at pH 0.1 were pure, well crystallized monoclinic WO3 independently from the temperature, the presence and the quality of the additives. At 180 and 200 °C, applying CH3COOH and NaClO4 resulted nanosheets similar in size. With Na2SO4 additive at 180 °C sheets, at 200 °C sheets and also rods formed indicating that SO42− was a capping agent only at 200 °C. For comparison, at pH 1 at both temperatures the crystalline phases and the morphologies varied depending on the type of the additive.

Highlights

Tungsten oxides are widely studied materials due to their versatile possibilities in everyday use
Samples obtained at 180 C, pH 0.1 Based on X-ray powder diffraction (XRD) patterns, samples 1–7 are identified as pure m-WO3 (ICDD 04-005-4272) independently on the used additive (Fig. 1)
We investigated the effect of various additives such as CH3COOH in different volumes (1.25/2.5/5.0/7.5 mL), as well as NaClO4, Na2SO4 and changed the temperature from 180 to 200 C

Summary

Introduction

Tungsten oxides are widely studied materials due to their versatile possibilities in everyday use They can catalyse many reactions e.g. CO oxidation [1], isomerization of alkenes [2], dehydrogenation of alcohols [3], hydrodesulphurization and hydrocracking [4,5], epoxidation of cycloocta-1,5-diene [6], Knoevenagel condensation [7] but can be used as gas sensors [8,9,10,11,12,13] or photoanodes in electrochemical cells [14,15,16]. Only some papers have reported about its one-step hydrothermal synthesis [48,49,50,51,52] In these works the authors studied the effect of various parameters such as temperature, time, pH in the acidic range or concentration of the acid, the role of pH is still not clear. Their specific surface area and band gaps were determined

Hydrothermal synthesis

Characterization

Crystal structure and morphology

Specific surface area

Conclusion