TiO2 nanocrystals grafted on macroporous silica: A novel hybrid organic–inorganic sol–gel approach for the synthesis of highly photoactive composite material
Graphical abstract
Highlights
► Meso/macroporous TiO2–SiO2 exhibits photoactivity comparable to TiO2 in slurry. ► TiO2 nanocrystals were functionalized with organic molecules. ► Functionalized TiO2 was grafted on macroporous SiO2 by sol–gel synthesis. ► Grafted TiO2 was dispersed with mesoporous distribution on SiO2. ► TiO2–SiO2 meso/macroporous structure guarantees high accessibility of the catalyst.
Introduction
In the last few years nanocrystalline TiO2 has been extensively studied as the photocatalyst in the oxidative degradation of organic and inorganic pollutants [1]. The interaction of this oxide with UV radiation generates electron–hole pairs which are able to activate surface reactive processes [2]. The recombination rate of charges, which affects catalyst photoactivity, strongly depends on the morphological and structural properties of the oxide, such as the different crystalline phase, surface area, particle shape and porosity [3]. Consequently, the control of the photocatalytic activity of TiO2 nanoparticles throughout the tailoring of their morphological and structural properties is a current topic of great interest.
The photodegradation of toxic compounds is usually performed by using titania nanoparticles in aqueous suspension (slurry). However, the use of nanosized powders as slurries in wastewater treatment causes difficult post-use recovery and requires expensive and time-consuming separation/recycling processes [4]. In addition, TiO2 nanoparticles, when dispersed in the surrounding environment, may be hazardous, due to their potential inflammatory and cytotoxic effects [5]. These drawbacks can be avoided by immobilizing or embedding the TiO2 nanoparticles on a support. Many inorganic or polymeric materials have been employed for this purpose: silica glass shaped as beads [6], rings [7], reactor walls [8] and fibers [9]; quartz [10]; zeolites [11]; perlite [12]; pumice [13]; alumina-based ceramics [14]; stainless steel [15]; aluminium [16]; cotton fibers [17]; polyester, acrylate [18], fluorinate [19] polymers. Nevertheless, both immobilization and embedding frequently lower the catalyst exposed area compared to that of the powder suspension [20].
Polymeric substrates show poor resistance to thermal treatments and undesired sensitivity to photooxidative processes, compared to inorganic ones [20b]. Inorganic membranes, consisting of macroporous ceramic substrates covered by micro/mesoporous active TiO2 layers, seem promising alternatives for several large scale catalytic processes [21]. In fact, the porous skeleton of the ceramic framework provides chemical and thermal stability, mechanical durability, low pressure drops and rapid mass transport of fluids, due to the extensive interconnection between the macropores. This structure guarantees high accessibility to the catalyst active sites and fast uptake/release cycles. In addition, the mesoporosity of the titania layer preserves the permeability of the ceramic matrix and provides an effective contact between the target molecules and the catalyst particles [22].
Different approaches, based on soft-chemistry routes, hydrothermal synthesis, and chemical (CVD)/physical (PVD) vapour deposition, were proposed in order to obtain oxide coatings on preformed macroporous ceramic matrices [23]. The ideal active layers should be homogeneous, chemically and thermally stable, loading large amounts of material crucial for the catalytic activity and without pore occlusion which limits the whole permeability.
In this context, we propose a novel sol–gel synthetic strategy, employing hybrid organic–inorganic reactants for the preparation of a macroporous silica matrix and the simultaneous surface grafting of preformed TiO2 nanocrystals. The goal is to design a TiO2–SiO2 (TS) composite material with the following characteristics:
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high macroporosity and UV-transparency of the silica matrix, which guarantee easy accessibility of the catalyst surface sites and allow effective interaction of TiO2 with UV radiation;
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TiO2 nanocrystals grafted on the surface of the ceramic matrix, whose well defined morphological and structural properties determine high photoactivity;
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minimum loss of photoactivity due to catalyst immobilization, in comparison to the slurry TiO2;
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improved thermal stability and durability without leaching of the grafted catalyst.
The macroporous silica network was produced by the sol–gel process based on hydrolysis and condensation of tetramethylorthosilicate (TMOS) in the presence of polyethylenglycol (PEG) as the sacrificial template [24]. Due the TMOS sol–gel transition, phase separation occurs between the silica gel network and the polymer. PEG also acts as a carrier of TiO2 particles into the macropores of silica matrix. In order to disperse TiO2 nanocrystals in the PEG phase, the oxide particles were functionalized with carboxylic acid or amine derivatives [25], which make the TiO2 surface less hydrophilic, improving the interactions with the polymeric chains. Subsequent annealing at 500 °C removes both PEG and functionalizing organic molecules and generates the interconnected macroporous silica network. Thus, the TiO2 crystals, previously dispersed in the PEG phase, graft onto the silica surface channels preserving their crystal phase and size.
TS composites were prepared by using anatase nanoparticles with known morphological and structural characteristics and high photoactivity [3], [26], which were functionalized by carboxylic acid and amine derivatives having different side chains (propionic acid, exylamine and 2-methoxyethylamine). The influence of the functionalizing agent on the dispersion of TiO2 nanoparticles in PEG and, consequently, on their final exposure and dispersion on silica macropore walls was demonstrated.
Photocatalytic activity of TS samples was tested in the degradation reaction of PhOH in aqueous solution and compared to the same anatase in slurry. It turned out that the immobilization procedure uncommonly preserves the catalytic properties of the TiO2 nanocrystals.
The proposed new synthetic strategy may be potentially applied to graft different functional oxides to ceramic macroporous substrates, keeping high functional properties eventually modulating the porous architecture of the silica matrix by changing the sol–gel synthesis parameters.
Section snippets
Chemicals
All chemicals and solvents were purchased from Sigma–Aldrich as analytical grade and used as received without further purification. Deionised water (18 MΩ cm) was used for the procedures that required water.
Functionalization of TiO2 nanoparticles
Nanocrystalline TiO2 anatase was obtained by hydrothermal synthesis, according to a previously reported procedure [26], by reacting aqueous solutions of TiOCl2 (Aldrich, 99%) and NH3 (Fluka, >25 wt %) in a teflon lined autoclave (Parr, model 4768Q). The autoclave was heated at a rate of 2.67
Characterization of pristine and functionalized TiO2 nanoparticles
The XRD pattern of pristine anatase TiO2 nanoparticles is reported in (Fig. 1). The average crystallite size, estimated from the peak of (1 0 1) reflection using Scherrer's equation, resulted 10.5 nm. The XRD patterns (not reported) of TiO2-PA, TiO2-EA, TiO2-MA, TiO2 functionalized with propionic acid, exylamine and 2-methoxyethylamine respectively, revealed the same size and phase of the pristine TiO2 nanoparticles.
TiO2-PA, TiO2-EA, TiO2-MA nanoparticles were characterized by means of solid-state
Conclusions
TiO2–SiO2 composite materials with nanostructured TiO2 particles grafted onto macroporous silica support were successfully obtained by a novel sol–gel synthetic strategy which employs hybrid organic–inorganic reactants. The materials exhibits high thermal stability and a photocatalytic activity comparable to that of powder TiO2 in slurry.
The macroporous silica network was synthesized by hydrolysis and condensation of TMOS assisted by PEG as templating agent. TiO2–SiO2 were prepared by grafting
Acknowledgements
The Milan group gratefully acknowledge Dr. Paolo Gentile for the SEM images and Dr. Angeloclaudio Nale for the XRD experiments.
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