Article pubs.acs.org/IECR
Low Temperature Fabrication of Photoactive Anatase TiO2 Coating and Phosphor from Water−Alcohol Dispersible Nanopowder Manish Kumar Mishra, Shreyasi Chattopadhyay, Anuradha Mitra, and Goutam De* Nano-Structured Materials Division, CSIR-Central Glass and Ceramic Research Institute, 196, Raja S. C. Mullick Road, Kolkata 700032, India S Supporting Information *
ABSTRACT: Low temperature fabrication of durable photoactive anatase TiO2 coatings (on glass and plastic substrates) has been accomplished using the synthesized water and water-EtOH dispersible organic-free TiO2 nanopowder. This nanopowder has been synthesized by refluxing the mixture of Ti(OiPr)4, EtOH, and H2O in the presence of excess NO3− ion as a stabilizer at 80 °C for 24 h. The nanopowder has a small crystallite size (∼5 nm) and a high specific surface area of about 268 m2 g−1. It is highly dispersible in water (up to 20 wt %), and the water−EtOH mixture (10 wt %) and the resulting dispersions are very stable. The water−EtOH (1:3 w/w) dispersion of the TiO2 nanopowder (4−5 wt %) has been used to prepare transparent coatings on glass and flexible plastic (polypropylene and polycarbonate) substrates with a surface hardness of ∼2−3 and 1−2 H, respectively, and good adhesion (5B; high quality). The reasons behind good adhesion and hardness of these coatings have been discussed. Such coatings on plastic and glass substrates have been used as reusable photocatalysts for degradation of toxic dye (methylene blue) to show the self-cleaning property under UV (365 nm) and visible light (Xenon source; 1 sun) sources. Further, by using this TiO2 powder, a fluorescent ZnS:Mn/TiO2 phosphor can be easily prepared at a much lower temperature.
1. INTRODUCTION Studies and research on TiO2 nanoparticles (NPs) are still flourishing like a newly discovered field just by virtue of its genuine multifunctional applications in the field of photocatalysis,1−4 photovoltaics,5−8 photochromic devices,9,10 pigments,11−13 gas sensor,14−17 water purification,18−20 ceramics,21,22 crystal structure tailoring,23 and so on. Among the three most abundant crystallographic forms of TiO2 (anatase, rutile, and brookite), the anatase form is mainly used in photocatalysis and solar energy conversion owing to its higher charge-carrier mobility and an increased density of surface hydroxyls.24−27 To date many research groups have come forward with different synthetic routes to prepare nanocrystalline anatase TiO2 using a wet chemical route like hydrothermal, sol−gel, and reverse micelle.28−32 However, reports on the solution based low temperature synthesis of high pure anatase nanopowders highly dispersible in water as well as in alcohol are quite few and limited to the ligand exchange method or use of organic capping as a stabilizer.30,33,34 Moreover, several reports on dispersibility of TiO2 powder in water is available, but quantitative data regarding the same are unavailable. Further, the requirements of organic-free transparent anatase TiO2 films for applications as coatings are mainly stuck to the sol−gel method.35−38 Such fabrications of TiO2 coatings from the conventional sol−gel method yield amorphous TiO2 below 400 °C and require high temperature to remove organics and achieve crystallization in anatase form. For this reason, syntheses of coatings are restricted to materials having stability at high temperature (like glass or metallic substrates). Primary criteria for the fabrication of crystalline TiO2 coating at low temperature is solubility of organic free TiO2 powder in alcohols and high surface area of the powder. Most of the water-soluble organic free dispersible powders are © 2014 American Chemical Society
insoluble in ethanol or bigger chain alcohols and have low surface area. Xi et al. reported the solution based process to prepare TiO2 coatings on glass substrate.39 Owing to the presense of organics they were unable to attain the desirable hardness and adhesion of the film at low temperature. Coincidentally, most of the reports on anatase TiO2 film fabrication do not provide any analysis for adhesion quality of films. However, the low temperature fabrication of anatase TiO2 films on soft and flexible plastics (like polypropylene and polycarbonate) as well as on hard substrate (like glass) having good adhesion by a general reliable process of dip-coating is quite elusive and a challenging task. If successfully executed, it can save large amounts of energy and fill up many gaps in several application areas. Very recently Jing et al.40 synthesized water dispersible TiO2 using isopropyl alcohol, a small amount of HNO3, and an excess volume of water. However, the wt % of powder which can be dispersed in water was not available in their report, and the synthesized powder cannot be directly usable for dip-coating. They mentioned that crystalline TiO2 (anatase) cannot be produced at low temperature by heating the ethanolic solution of titanium alkoxide in the presence of nitric acid and less amount of water.40 Hence a maximum number of authors have reported synthesis of anatase TiO2 at relatively high temperature to achieve crystallization.41 It is also noteworthy to mention here that the formation of quantum dots (QDs)/anatase TiO2 composite phosphor requires high annealing temperature which destroys the photoluminescence (PL) of QDs, while at low temperature Received: Revised: Accepted: Published: 928
August 19, 2014 December 23, 2014 December 26, 2014 December 26, 2014 DOI: 10.1021/ie5033028 Ind. Eng. Chem. Res. 2015, 54, 928−937
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Industrial & Engineering Chemistry Research
2. EXPERIMENTAL SECTION 2.1. Materials. All chemicals were used as received. Titanium(IV) isopropoxide (97%), pluronic P123 (EO70PO20EO70; Mav = 5800), and Zn(ac)2·2H2O (98%) were purchased from Sigma-Aldrich. Mn(ac)2·4H2O and Na2S (purified) were supplied by Merck. Absolute ethanol (AR; Tedia Company Inc. USA), HNO3 (70% AR grade; Rankem), and water (18 MΩ; Milli-Q) were used. Commercially available TiO2 NPs (HNO3 stabilized; size ≈4−8 nm and P25 grade; sizeav 21 ± 10 nm) were purchased from Reinste Nano Ventures for control experiments. 2.2. Characterizations. X-ray diffraction (XRD) pattern of the powder and films were recorded on a Rigaku Smart lab diffractometer operating at 9 kW (200 mA × 45 kV) with Cu Kα radiation (λ = 1.5405 Å). In the case of film a 0.5° grazing incidence angle was maintained for all the measurements. XRD studies of the products formed at different time intervals of refluxing was done by drop casting the reaction mixture on a glass plate followed by drying at 70 °C for 5 min. Plasma etching of polypropylene substrate was done in a Gatan advanced plasma system (Solarus 950) in order to generate surface CO groups.46 While a plasma etching vacuum was maintained at 460 mTorr, argon and oxygen gas flow rates were maintained at 11.5 and 34 sccm, respectively, using a mass flow controller. Attenuated total reflection (ATR) and Fourier transformed infrared (FTIR) spectra of the samples were recorded using alpha FTIR spectrometer (Bruker) and Nicolet 380 FTIR spectrometer, respectively. X-ray photoelectron spectroscopy (XPS) measurements were done on a PHI 5000 Versaprobe II XPS system with Al Kα source and a charge neutralizer at room temperature, maintaining a base pressure about 6 × 10−10 mbar and an energy resolution of 0.6 eV. Raman spectra were obtained by using Renishaw InVia Reflex micro Raman spectrometer with excitation of argon ion (514 nm) laser. The laser power was kept sufficiently low to avoid heating of the samples, and spectra were collected with a resolution of 1 cm−1. Transmission electron microscopic (TEM) measurements were carried out using JEOL JEM 2100F operating at 200 kV attached with an energy dispersive X-ray scattering (EDX) unit. A small drop of TiO2 dispersion was placed on a carbon coated Cu grid underlying a tissue paper, dried at 70 °C, and analyzed by TEM. In the case of FESEM, film was deposited on the single side polished silicon wafer and analyzed by ZEISS SUPRA 35VP field-emission scanning electron microscope. The thickness of the films were measured ellipsometrically by fitting the refractive index (n), extinction coefficient (k) curves in the nonabsorbing region. Emission spectra of the films were recorded with PTI QM-30 spectrometers. The thermogravimetry analysis (TGA) was done by the Netzsch TG 209 F3 Tarsus thermal analyzer using a dynamic heating rate of 5 °C min−1 in static air atmosphere. The specific surface area was estimated by the BET method using Quantachrome Autosorb 1. For BET measurements samples were degassed at 60 °C under 10−2 T vacuum. The pencil hardness of coatings was measured by ASTM D3363 specifications using a BYK Gardner pencil hardness tester. Adhesion of the films on plastic substrates was evaluated using the method as per ASTM D3359 cross-cut tape test. 2.3. Synthesis of Water Dispersible TiO2 Powder. In a round-bottom flask containing a mixture of 200 g of EtOH and 22 g of HNO3 (70%), 22 g of TTIP was slowly added with constant stirring (1450 rpm), and stirring was continued for
the PL of QDs can be preserved but it inhibits the high crystallinity of TiO2.42 Additionally, at higher temperature (≥500 °C) when crystallization of anatase TiO2 takes place, there is a high chance of reaction between metal salts (or QDs) and the TiO2 to form the corresponding titanates.43,44 Hence for the synthesis of aforesaid crystalline TiO2/QDs based phosphors, the basic requirement should be a low temperature (
