Hypergolic Synthesis of Inorganic Materials by the Reaction of Metallocene Dichlorides with Fuming Nitric Acid at Ambient Conditions: The Case of Photocatalytic Titania

Nikolaos Chalmpes,Georgios Asimakopoulos,Maria Baikousi,Dimitrios Gournis,Athanasios B Bourlinos,Michael A Karakassides

doi:10.3390/sci3040046

Abstract

Hypergolic materials synthesis is a new preparative technique in materials science that allows a wide range of carbon or inorganic solids with useful properties to be obtained. Previously we have demonstrated that metallocenes are versatile reagents in the hypergolic synthesis of inorganic materials, such as γ-Fe2O3, Cr2O3, Co, Ni and alloy CoNi. Here, we go one step further by using metallocene dichlorides as precursors for the hypergolic synthesis of additional inorganic phases, such as photocatalytic titania. Metallocene dichlorides are closely related to metallocenes, thus expanding the arsenal of organometallic compounds that can be used in hypergolic materials synthesis. In the present case, we show that hypergolic ignition of the titanocene dichloride–fuming nitric acid pair results in the fast and spontaneous formation of titania nanoparticles at ambient conditions in the form of anatase–rutile mixed phases. The obtained titania shows good photocatalytic activity towards Cr(VI) removal (100% within 9 h), with the latter being dramatically enhanced after calcination of the powder at 500 °C (100% within 3 h). Notably, this performance was found to be comparable to that of commercially available P25 TiO2 under identical conditions. The cases of zirconocene, hafnocene and molybdocene dichlorides are discussed in this work, which aims to show the wider applicability of metallocene dichlorides in the hypergolic synthesis of inorganic materials (ZrO2, HfO2, MoO2).

Highlights

Progress in materials science largely depends on new synthesis methods and techniques
The titanium-containing organometallic compound serves as the source of titania, whereas the attached cyclopentadienyl groups serve as the source of residual carbon [9]
In an effort to explore more chemical options in this context, we have presented the hypergolic ignition of titanocene, zirconocene, hafnocene and molybdocene dichlorides by fuming nitric acid for the fast and spontaneous formation of the corresponding TiO2, ZrO2, HfO2 and MoO2 phases at ambient conditions

Summary

Introduction

Progress in materials science largely depends on new synthesis methods and techniques. These typically include solid–state, ball–milling, arc–discharge, plasma, pyrolytic, template synthesis, nanolithography, high pressure–high temperature (HPHT), sol–gel, freeze–drying, microemulsion, precipitation, borohydride reduction, thermolysis, hotinjection, sonochemical, hydrothermal, chemical vapor deposition, sputtering, flame spray pyrolysis, electrochemical and microwave synthesis [1,2,3]. In spite of the large variety of existing techniques today, there is an ever-increasing demand for new synthesis methods in materials science that can deal with needs not met by the previous ones. Our group introduced the synthesis of hypergolic materials as a radically new preparative method in materials science [4,5,6,7,8,9,10,11,12]. Spontaneity results from the exothermic character of hypergolic reactions (∆H < 0) as well as the release of large amounts of gaseous products

Methods

Results

Conclusion