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Optimizing the preparation parameters of mesoporous nanocrystalline titania and its photocatalytic activity in water: Physical properties and growth mechanisms
Accepted Manuscript Title: Optimizing the preparation parameters of mesoporous nanocrystalline titania and its photocatalytic activity in water: physical properties and growth mechanisms Author: H.A. Hamad M.M. Abd El-latif A.B. Kashyout W.A. Sadik M.Y. Feteha PII: DOI: Reference:
S0957-5820(15)00174-3 http://dx.doi.org/doi:10.1016/j.psep.2015.09.011 PSEP 625
To appear in:
Process Safety and Environment Protection
Received date: Revised date: Accepted date:
9-4-2015 28-8-2015 20-9-2015
Please cite this article as: Hamad, H.A., Abd El-latif, M.M., Kashyout, A.B., Sadik, W.A., Feteha, M.Y.,Optimizing the preparation parameters of mesoporous nanocrystalline titania and its photocatalytic activity in water: physical properties and growth mechanisms, Process Safety and Environment Protection (2015), http://dx.doi.org/10.1016/j.psep.2015.09.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Optimizing
the
preparation
parameters
of
mesoporous
nanocrystalline titania and its photocatalytic activity in water:
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physical properties and growth mechanisms. H. A. Hamad 1,*, M. M. Abd El-latif 1, A. B. Kashyout2, W.A. Sadik3, M. Y. Feteha3 Fabrication Technology Research Department, Advanced Technology and New Materials
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1
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Research Institute (ATNMRI), City of Scientific Research and Technological Applications (SRTA-City), New Borg El-Arab City, P.O. Box 21934, Alexandria, Egypt. Electronic Materials Department, Advanced Technology and New Materials Research Institute
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2
(ATNMRI), City of Scientific Research and Technological Applications (SRTA-City), New
Materials Science Department , Institute of Graduate Studies and Research (IGSR), Alexandria
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3
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Borg El-Arab City, P.O. Box 21934, Alexandria, Egypt.
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University, 163 Horrya Avenue, P.O. Box 832, Shatby , 21526 Alexandria, Egypt. *Corresponding author: Tel: 002-03-4593414
Fax: 002-03-4593414
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E-mail: [email protected]
High lights
Mesoporus anatase TiO2 nanoparticles were successfully prepared by acid treatmentsolgel/ hydrothermal method . The effects of preparation parameters on crystallite size and phase composition of prepared titania nanoparticles. Mesoporous TiO2 show superior photocatalytic activity and selectivity due to large surface area. Study the reaction mechanism of the prepared titania nanoparticles
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Abstract
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Titaniananomaterial with an anatase structure and 5.6 nm crystallite size and 280.7 m2/g specific surface areas had been successfully prepared by sol-gel/hydrothermal route. The effect of pH as a type of autoclave and calcination was studied. Crystallite size and phase composition of the
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prepared samples were identified. X-ray diffraction analyses showed the presence of anatase with little or no rutile phases. The crystallite size of the prepared TiO2 with acidic catalyst was
cr
both smaller than that prepared with basic catalyst, and was increasing after acidic calcinations
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by a factor 4-5. Basic calcinations produced a specific increase of 1.5. Rutile ratio and the particle size were increased after calcination at 500ºC. However, TiO2 powder synthesized using
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a basic catalyst persisted the anatase phase and a loosely aggregation of particles. Anatase TiO2 as prepared with acidic catalyst in Teflon lined stainless steel autoclave demonstrated the highest
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photocatalytic activity fordegradation of 2,6 dichlorophenolindophenolunder ultraviolet
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irradiation with t½ 0.8 min.
Photocatalytic activity.
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1. Introduction
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Keywords: nanostructures, sol-gel growth, Hydrothermal, Titania, X-ray Diffraction,
Nanosizedtitania has attracted increasing attention in the scientific community for its broad applications in photocatalysts (Hoffmann et al. 1995, Ohno, 2004, Xu, 2014, Hamad et al. 2015), bacterial disinfection (Kashyout et al.2006), solar cells (Hong, 2014), gas sensors (Zakrzewska et al. 2001), electro coagulation /electrofiltration (Yang & Chaung 2005) and optoelectronic devices (Kalyanasundaram et al. 1998) . TiO2 was intensively investigated for photocatalysis because of insolubility, its biological, and chemical inertness, strong oxidizing power, cost effectiveness, non-toxicity and long-term stability against photo and chemical corrosion under the reaction conditions (Nakataet al . 2012; Akhavan et al . 2009; Rachel, 2002). Anatase has
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been reported to be the most effective phase for photocatalysis since higher crystallinity means fewer defects for the recombination of photogenerated electrons and holes (Bahnemann et al. 1993; Li, 2009). One of the methods is the sol-gel which had emerged as a promising processing
factors as
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route for synthesis of nano sized TiO2 with high photocatalytic activity due to the reasons and inexpensive operation procedures, simplicity, low temperatures, and the
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homogeneous and the highly pure resulting product (You, 2004). The hydrothermal method is
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widely used for the production of small particles in the ceramic industry. Usually, this process is conducted in steel pressure vessels called autoclaves under controlled temperature and /or
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pressure with the reactions occurring in aqueous solution (Sohrabi et al.2007). Hydrothermal processing of either amorphous titania and titanium-containing precursor had been shown to be
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an ideal method for obtaining titania with small grain size (often 10 nm or less), high specific
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surface area, and high crystallinity (Sangchay, 2012). The properties of titania nanoparticles
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were controlled using preparation conditions such as water contents, autoclave time and temperatures and calcinations temperatures (Kashyout et al. 2010).
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The study under investigation using a novel method for the synthesis of TiO2 nanoparticles was being carried out by sol-gel/ hydrothermal methods from titanium butoxide as a precursor and acetyl acetone was used as an inhibitor to slowdown the rate of hydrolysis and polycondensation. The microstructure of the as-prepared photocatalyst was studied using XRD, SEM, EDAX, and TEM. The formation of all the prepared photocatalysts and the TiO2 Degussa P-25 and their photo degradation activities for 2,6 dichlorophenol-indophenol (DCPIP) under UV illumination were investigated.
2. Materials and Methods. 2.1. Materials.
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Titanium butoxide (Ti(OBu)4, 97%) was obtained from Aldrich chemicals (Germany), acetylacetone (CH3(COCH3)2, 99%) was supplied from Alfa asear (Germany), Ammonia solution (NH4OH, 30%) was obtained from Sigma Aldrich chemicals (E.U), Nitric Acid (HNO3
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,65%) was supplied from Pharos (Egypt), 2-Propanol ((CH3)2CHOH , 98%) was obtained from Panreac (E.U) , Hydrochloric acid (HCl, 37%) and Absolute ethanol ([C2H5OH] , 99.9%) were
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supplied from Sigma-Aldrich (E.U) , TiO2 (Degusaa P25 ) (99.5%) was supplied from Degussa
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(Germany). The dye was investigated for photocatlysis was 2,6 dichloro indophenol – sodium salt (DCPIP) (C12H6Cl2NO2.Na, 97%) and was obtained from Sigma (USA).
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2.2. Preparation of highly active TiO2 nanoparticles.
Nano Titania was prepared by sol-gel/hydrothermal method under both basic and acidic
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conditions. Different samples of titania were prepared by adding 30 % NH4OH or 10 % HNO3 to
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a solution of Ti(OBu)4 , an acetyl acetone (acac) and De- ionized water (18.2 MΩ.cm, TKA-
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Genpure). The molar ratios of the ingredients were as follows: Titanium precursor / acac / 30% NH4OH or 10% HNO3 / De-ionized water = 1:1:10 or 3:20. The pH values of the resulting
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solutions were 10 or 3, respectively. The final solution was stirred using amagnetic stirrer (Selecta, Model Agiamatic-ND, Spain) for 4 hrs at room temperature. The resulting solution was placed in 100 mL Teflon autoclave (BERGhof Product Instrument –Germany) or Teflon lined stainless steel autoclave and then placed in an oven (Nabertherm, Model TR60, Germany) for hydrothermal treatment at 170 C for 72 hrs. Then, the autoclave was naturally cooled to room temperature and the obtained product was washed several times with HCl , 2-propanol and distilled water to remove the residues of organics placed in solution and centrifuged by Ultracentrifuge (Hettichzentrifuge –Model Rotofix32 , Germany). Then the samples were dried at 100 C for 12 hrs. Finally, the samples were calcined at 500C in a muffle furnace (Barnstead
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International, Model F4800, USA) for 2 hrs. Schematic flow chart of various preparation parameters of TiO2 nanocrystals were described as shown in Fig. 1. 2.3. Sample Characterization.
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Crystal phases and crystallite size of as prepared samples were obtained by X-ray diffractometer (XRD- Shimadzu 7000, Japan) that operated at a voltage of 30 kV and a current
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of 30 mA with CuKα radiation (=1.54 Å) The average crystallite size was estimated by applying
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the Scherrer equation (L = K λ / (β cos θ) where L is the crystallite size (Å), λ is the wavelength of the X-ray radiation (Å), K is usually taken as 0.89, β is the line width at half-maximum height
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(radians) and θ is the diffraction angle (degree).The phase composition of the samples can be calculated from such equation as XR = 1 – ( 1/ (1+1.26 IR/IA ); where XR is the weight fraction of
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rutile in the mixture, and IR and IA are the relative intensities of the strongest diffractions peaks
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of rutile and anatase, respectively. The size and morphology of TiO2 were determined by
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scanning electron microscopy (JEOL, Model JSM 6360LA, Japan) and transmission electron microscopy (JEOL JEM 1230, Japan). The compositional and elemental analyses of TiO2
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nanoparticle were measured by EDAX.UV-Vis Spectroscopy (Labomed, Inc) to indicate the effect that processes had on an incident beam when the radiation source was in the ultravioletvisible wavelengths (190-1100 nm).
2.4. Measurement of Photocatalytic Activity. The photocatalytic performance of the prepared TiO2 was investigated by using the photocatalytic decomposition of anionic dichlorophenol-indophenol (DCPIP) dyefrom aqueous solution under ultraviolet irradiation in a slurry reactor at constant pH (natural 6.7) and room temperature (253C). The slurry reactor included a glass container (1 liter). The contents (1104
M DCPIP dye solution and 1g/l of catalyst) were maintained as a suspension by means of a
5 Page 5 of 26
magnetic stirrer. Then, dry air was fed into the solution (source of oxygen) with rate 4 liters/min. The reaction was conducted under vigorous agitation to ensure uniform distribution throughout the reacting medium. A tubular low pressure mercury vapor source (total rating 43 watts, total
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UV output at 254 nm, 13.4 watts, length 120 cm, VALBER LOURMAT , Germany) was used to irradiate the solution which was located 10 cm away from the surface of the source . The total
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intensity reaching the slurry solutions was measured by using a UVX radiometer (UV products
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Ltd., Cambridge) equipped with a sensor with peak sensitivity at 254 nm was 4 mWcm-2. The local volumetric rate of energy reaching the solutions was 2.1μ Einstein's-1 L-1at 254 nm.
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Samples were withdrawn at periodic intervals from the reactor with the help of a syringe and then filtered through a Millipore sterile syringe filter of 0.2 μ m (Corning, NY 14831, Germany).
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Then, they were analyzed for decolorization and degradation using spectrophotometer (model
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Hitachi UV-2000, double beam, Japan) at max, i.e., 600 nm for DCPIP dye. The Photodegrdation
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efficiency of 2,6dichloroindophenol dye could be calculated according to this equation (%
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Degradation = C-C/C100% = A- A/A100% ) ; where C is the initial concentration of dye solution (mg/L), C is the concentration of dye solution after photoirradiation (mg/L) at time t (minute). Ais the value of absorbance of dye aqueous solution after adsorption in the dark, and A is the value of absorbance of dye aqueous solution after reaction, and where A and Awas unitless.
3. Results and discussion.
3.1. Crystalline Size and Phase Composition study. XRD patterns of the as uncalcined and calcined TiO2 nanoparticles under Teflon autoclave and teflon lined stainless steel in acidic and basic media were shown in Figure 2. The asprepared and calcined nanoparticles were exhibited both anatase and rutile structures (JCPDS 6 Page 6 of 26
Card No. 021-1272 and 021-1276), respectively. It was clear that the samples contained both the anatase and the rutile phases and their percentages change with Teflon type, pH, and calcination. In case of Teflon and stainless steel Teflon autoclaves at acidic medium, the as prepared samples
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contained the anatase phase. For basic medium, both the anatase and the rutile were obtained but the dominate phase was the anatase. But in case of Teflon lined stainless steel autoclave, the
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anatase phase only occurred before calcination and after calcination rutile phase began to appear.
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The phase composition ranged from zero to 3.6 % of rutile percentage in acidic and basic media respectively for the nanoparticles prepared under Teflon autoclave, and from 5.9 to 3.6 % of
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rutile percentage in acidic and basic media respectively for the nanoparticles calcined at 500ºC under Teflon lined stainless steel. Table 1 indicated the increase as in the crystallite size from 5.6
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to 22.5 nm in acidic media due to calcination, and as from 8.2 to 12.8 nm in basic media in
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Teflon autoclave and 6 to 23.2 nm in acidic media, from 7.2 to 18.9 nm in basic media in Teflon
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lined stainless steel for anatase phase. This may be due to the original bondings in uncalcined which may be broken to form new bondings for rutile structure and condensation of free OH
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groups on the surface of the nano TiO2 particles (Watson, 2004). Before the calcination for both types of autoclave, the crystallite size of anatase phase in basic medium was greater than that of the crystallite size of anatase phase in acidic medium. This may be attributed to the TiO2 particles catalyzed by acid which were more closely packed than those treated by base catalyst where particles that had more tendencies to retain hydroxyls and water molecules and their particle sizes after calcinations became smaller than acid-treated particles. Therefore, it seemed that a more closely packed structure of primary particles of acid-treated TiO2 could provide larger secondary particles during heat treatment. After calcination, the crystallite size of anatase phase in a basic medium was smaller than in an acidic medium. This may be due to the base-
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treated TiO2 particles which were loosely packed during calcinations resulting in a relatively smaller secondary particle size (Song et al., 2001). The calcination under acidic media increased the crystallite size (by factor 4-5) in comparison to that under basic media (by factor 1.5).
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3.2. Morphology Study.
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SEM micrographs of several samples that were successfully prepared at different conditions were shown in Figure 3 and 4. All samples have ordered arrays for TiO2 nanocrystals whose
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predominant shape was in a spherical shape. In the as-prepared sample (acidic and alkaline media) for both autoclaves, the strong repulsive charge among particles reduced the probability
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to coalesce and more stable sol could be formed with little or no aggregates (Chang, 1995). In
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the calcined samples, there was a little increase in crystallite size and more aggregate. Scanning electron micrograph of TiO2 nanoparticles prepared under Teflon lined stainless steel autoclave
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and calcined in basic catalyst medium presented the preferred sample due to its relatively small
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grain size, homogeneity and less aggregation as shown in Figure 4c. The size of the particles at higher pH values was smaller with less agglomerate than in a basic
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medium. The variety of TiO2 surface charge was pH dependent. TiO2 in sols possessed electrical charge due to the absorption of H+ or OH- in aqueous suspension. The surface charges of TiO2 could be determined by chemisorption (Polanams et al. 2006 ; Labbe et al. 2008) (equation 4 and 5):
For H+,
TiO2 + nH+ ↔TiO2Hnn+ for pH < 3.5
(4)
TiO2 + nOH- ↔TiO2 (OH) nn- for pH >3.5
(5)
For OH-,
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In acidic and alkaline media, the strong repulsive charge among particles reduced the probability to coalesce and more stable sol could be formed. Studies by (Su et al., 2004) Sue group had indicated that the pH for isoelectric TiO2 powder was between 5and 6.8.
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The as prepared sample under acidic conditions with Teflon lined stainless steel (4C) was favored due to many reasons; its purity and homogeneity with a very little organic residues, low
cr
crystal size and low aggregation. This was clear from the study of XRD and SEM.
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The EDAX of the optimized TiO2 (4C) (Figure not included).No vague peaks associated with other crystal structures were observed. The as-prepared sample was expected to contain only Ti
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and O elements. The percentage of oxygen is 70.65 wt % in the titania. The EDAX spectrum of the titania indicated a strong Ti signal at 4.5 eV with atomic percentage of 29.35%, confirming
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that the external shell was principally composed of TiO2.
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TEM images of the optimum TiO2 nanoparticles were shown in Fig.5. The distribution indicated
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the particles of sizes 5-8 nm for this sample. This was in agreement with the crystallite size calculated from XRD. Most of the particles appeared to be in a spherical shape as shown in the
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TEM images.
The large specific surface area of the preferred sample of titania at 280.7 m2g-1 with mesoporous structure, total pore volume at 0.193 cm3g-1, and high volume fraction of atoms were located both on the surface and at the grain boundaries result in an increased surface energy.
3.4. Photocatalytic Properties. The photodegradation of dichlorophenol-indophenol (DCPIP) in aqueous solution over different powder samples was plotted against UV irradiation time. The obtained results matched with the Langmuir–Hinshelwood (L–H) kinetic model (equation 6): 1 / r = 1/ kr + 1 / (krKC)
(6)
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where r was the decomposition rate of the reactant (mg L-1 min.-1), C was the concentration of the reactant (mg L-1), K represented the equilibrium constant for adsorption of the dye on TiO2 particles (Langmuir constant) and kr represented the specific reaction rate constant for the
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oxidation of the reactant (mg L-1 min.-1) (Machado et al., 2003).
When the chemical concentration C is the order of millimolar, a first order equation (7) of
(7)
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ln (C/C) = krK t = kapp t
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integrated form will be produced.
Where kapp represented the apparent first order rate constant of the photocatalytic degradation The time dependence of ln (C/C) could be represented by a straight line of slope to
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(min-1).
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equal the apparent first order rate constant kapp as illustrated in Table 2. Generally, first order kinetics was appropriate for several studies and fitted well with this kinetic model.
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equation 8 and as shown in table 2.
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The relation between the half life time (t½) of the first order reaction and kapp was given by
t½ = 0.693/ kapp
(8)
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The Degradation efficiency of samples with different crystal sizes - as prepared TiO2 powder (4A, 4B, 4B-1 and 4C) and Degussa P-25 under UV light irradiation were shown in Fig. 6. It could be noted that the most effective sample for degradation of DCPIP dye was sample 4C. A comparison between kapp values for all types of TiO2 revealed that sample 4C was the most effective for degradation of DCPIP dye. This may be due to several aspects such as its high degree of crystallinity, pure anatase phase, and homogeneity which could be consistent with the more efficient electron–hole utilization, the decrease in the content of amorphous TiO2, the high degree and quality of crystallization which indicated the fewer TiO2 bulk defects presence
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leading to recombination of photoexcited electron and holes (Table 2). The photodegradation of these photocatalysts obeyed the following order: 4C > 4B-1 > 4B > 4A > Degussa P-25.
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The highest value of the obtained apparent rate constant kapp is obtained to be 0,86 min-1 for the 4C sample while the lowest value was 0.046 min-1 for Degussa P-25. The apparent rate constant
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kapp were obtained to be 0.36 min-1 for sample 4A and 0.43 min-1 for sample 4B. Both were
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smaller than those of sample 4B-1 (0.79 min-1). The lower photoactivity of the rutile form in sample 4A could be attributed to many aspects such as the higher tendency for electron-hole
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recombination and also to its lesser ability to absorb oxidizing species such as O2 and its dependence on the preparation method (Carp et al. 2004). The photocatalytic activity of sample
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influence their photocatalytic activity.
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4B-1 was closer to 4C due to the crystal size of TiO2 particles which has been demonstrated to
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Therefore, it was obvious that the photocatalytic activity was increased with decreasing crystal size, although there appeared to be some disagreement with (Rivera, 2004). It had been observed
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that about the linear increase of photocatalytic degradation rate went in line with increasing anatase crystallite size (Table 2). Some reports (Anpo et al. 1987) mentioned that the anatase particles in the range of 4–50 nm, photocatalytic activity increased with decreasing TiO2 particle diameter which might be attributed to size quantization. As the particle size was lowered below a certain limit, most of the electrons and hole are generated close to the surface. Therefore, surface recombination was faster than interfacial charge-carrier transfer processes. Therefore, it could be concluded that the lower activity of Degussa P-25 as compared with other samples was more likely attributable to an increase in rutile content in the Degussa sample rather than the reduction in surface area. Pigmentory grade particles were in the micron range and their
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photoactivity of Degussa P-25 was decreasing due the decrease of the ratio between surface area and volume. This result contradicted the increased heterogeneous catalytic activity observed with increasing surface to volume ratio for nano-sized particles. The efficiency of the surface trapping
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of photo-generated holes was increasing in smaller particle sizes due to the availability of more active sites. This reflected their ability to better promote photocatalyzed processes on the surface
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of the catalyst (Porter et al., 1999).
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3.5. Reaction mechanism.
One of the alternatives of calcinations which yielded finer and less aggregated nanocrystals was
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the hydrolysis of titanium containing precursor. Acids were not only catalyzing the formation of
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crystalline TiO2 structure but also were acting as a peptizing agent that stabilized individual nanocrystal surface and consequently preventing aggregation, discouraging particle growth, and
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allowing the particles to exist in the form of a colloidal sol.
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Accordingly, it could be noted that the possible preparation mechanism of TiO2 nanoparticles at basic and acidic conditions might follow two paths a and b during synthesis.
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(a) Formation of Ti-OH bond by hydrolysis mechanism. Through this process of synthesizing, the electrophillic and the nucleophillic mechanisms had depended upon on acidic and basic catalyst: Firstly, under acidic conditions, it was likely that the butoxide group was protonated in a rapid first step. Electron density was withdrawn from titanium atom, making it more electrophilic and thus more susceptible to be attacked by water. This resulted in the formation of a penta-coordinated transition state (Parra et al. 2008). This transition state was decayed by the displacement of an alcohol and a conversion to titanium tetrahedron as shown in Figure 7a.
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Secondly, under basic conditions, the titanium butoxide which was known the basic alkoxide oxygen tended to repel the nucleophilic,-OH. No sooner had the initial hydrolysis occurred than the reaction proceeded stepwise with each subsequent alkoxide group and was more easily
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removed from the monomer. Therefore, more highly hydrolyzed titanium was more prone to attack. In addition, the hydrolysis of the forming polymer had been much more hindered than the
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hydrolysis of monomer. Although hydrolysis in basic medium was slow and tends to be
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completed, it is irreversible. Under basic conditions, water was likely dissociated rapidly to produce hydroxyl anions during the first step (Jung, 1999). The hydroxyl anion then attacked the
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titanium atom where –OH replaces –OR formed the titanium tetrahedron, as shown in Figure 7. (b)Crystallization of TiO2 from Ti-OH as result of condensation.
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Crystallization of TiO2 was the result of condensation reactions among these Ti (IV) (3d) co-
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ordination complexes, i.e, the reaction between OH- ligand on different complexes resulting in
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the elimination of water to form Ti-O-Ti.
Another path for crystallization was that, there were two types of linkage involved in the
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formation of TiO2 crystals, edge sharing , and corner sharing that to be depending on the linkage of (TiO62−) octahedra (Chang, 1995) as shown in Figure 8. Two opposite edges of each octahedron were edge shared forming a linear chain with other chains via corner oxygen atoms. Anatase, however, did not involve corner sharing, but had four edges shared per octahedron as were shown in Figure 8. The anatase chains were linked together through shared edges. The anatase phase was statistically more favorable because of the existence of more edges available to form a bond. The configuration of rutile was thermodynamically more favorable due to the linear arrangement which minimized the electrostatic repulsive energy. Moreover, the formation of corner sharing bonds was more difficult than edge sharing bonds (Diebold et al. 2003).
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It was found that as the number of OH - ligands in the coordination was increasing the probability of edge shared bonding (and thus anatase formation) among [TiO6] was increased. At the same time, the corner shared bonding (and thus rutile formation) was favored when the number of OH-
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ligands was decreased. The existence of relatively large OH- ligands under acidic condition would encourage the formation of the anatase phase and would suppress the formation of the
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rutile phase. The higher the acidic concentration, the more strongly anatase was favored.
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Accordingly, the pH of the medium was neutral or basic as that result in rutile production which was the favored product (Chang, 1995).
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4. Conclusions
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TiO2 nanoparticles with anatase phase, 5.6 nm average crystallite size was successfully synthesized via sol-gel/hydrothermal route with acidic condition under Teflon autoclave. Pure
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TiO2 and anatase phase with high crystallite size were obtained under basic medium in Teflon
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autoclave in comparison to Teflon pressure autoclave. The crystallite size with as prepared with acidic catalyst was smaller than that one prepared with basic catalyst. By calcination at 500 C,
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the crystallite size and grain growth were increasing agglomeration and rutile content. UV/ TiO2 (4C) was the most effective catalytic system for the degradation of DCPIP dye with a degradation efficiency of 90.77% and that could be applied for a wide range of organic compounds.
Acknowledgment
This work was done under the project funded by the Science and Technology Development Fund (STDF), Ministry of Scientific Research, Project ID: 1414, “Quantum Dots Nanomaterials Dye Sensitized Solar Cells”. Also, the authors would like to thank Dr. Marwa Fathy, Researcher at SRTA-City, for her helpful assistance in the writing stage of the reaction mechanism. 14 Page 14 of 26
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of TiO2 nanoparticles for dye sensitized solar cells. Renewable Energy 35,2914-2920.
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Labbe, M., 2008. Photocatalytic Degradation of Select Drinking Water Pollutants Using Nano-
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TiO2 Catalyst, Ph.D Dissertation, University of Windsor, Ontario, Canada. Machado, H., Miranda, A., Freitas, F., Duarte, M., Ferreira, F., Albuquerque, T., Ruggiero, R., Sattler, C., Oliveira, L., 2003.Destruction of the organic matter present in effluent from a cellulose and paper industry using photocatalysis. J PhotochemPhotobiol A –Chem 155, 231 – 241.
Nag, M., Basak, P., Manorama, S., 2007.Low-temperature hydrothermal synthesis of phase-pure rutile titania nanocrystals: Time temperature tuning of morphology and photocatalyst activity. Mater Res Bull42,1691–1704.
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Nakata, K., Fujishima, A., 2012.TiO2photocatalysis: Design and applications, Review. J PhotochemPhotobiol C 13, 169– 189. Ohno, T., 2004. Preparation of visible light active S-doped TiO2photocatalysts and their
ip t
photocatalytic activitiesWater Sci&Tech49 (4), 159 - 163 Parra, R., Góes, S., Castrom, S., Longo, E., Bueno, R., Varela, A., 2008.Reaction pathway to the
cr
synthesis of anatase via the chemical modification of titanium isopropoxide with acetic acid. Chem Mater20,143 – 150.
us
Polanams, J. T., 2006. Size Control in Nanoparticles Synthesis, Characterization and
an
Photocatalytsis, Ph. D Dissertation, University of New Mexico, Albuquerque, New Mexico. Porter. J.F., Yu-Guang, L., Chak, K. C.,1999. The effect of calcination on the microstructural
M
characteristics and photoreactivity of Degussa P-25 TiO2. J Mater Sci34, 1523 – 1531. Rachel, A., Suhrahmanyam, M., Boule, P., 2002. Comparison of photocatalytic efficiencies
d
of TiO2 in suspended and immobilised form for the photocatalytic degradation of
Ac ce p
te
nitrobenzenesulfonic acids. ApplCatal B- Environ37, 301 – 308.
Rivera, P., Tanaka, K., Hisanaga, T., 1993.Photocatalytic Degradation of Pollutant over TiO2 in Different Crystal Structures. ApplCatal B- Environ3,37-44. Sangchay, W., Sikong, L., Kooptarnond, K.,2012.Comparison of photocatalytic reaction of commercial P25 and synthetic TiO2-AgCl nanoparticles. Procedia Eng.32, 590 – 596. Song, C., Pratsinis, E., 2001.Packaging of Sol-Gel made Porous Nanostructured Titania Particles by Spray Drying. J AmerChemSoc 84, 92-98. Su, C., Hong, B., Tseng, C., 2004.Sol-gel preparation and photocatalysis of titanium dioxide. Catal Today96,119–126. Tavakoli, A., Sohrabi, M., Kargari, A., 2007.A review of methods for synthesis of nanostructured metals with emphasis on iron compounds. Chem Pap 61,151 – 170. 17 Page 17 of 26
Watson, S., Beydoun, D., Scott, J., Amal, R., 2004.Preparation of nanosized crystalline TiO2 particles at low temperature for photocatalysis. J Nanopart Res6,193 – 207. Xu, C.,
Yang, W., Guo,
Q.,
Dai, D., Chen, M,, Yang, X., 2014. Molecular Hydrogen
ip t
Formation from Photocatalysis of Methanol on Anatase-TiO2 (101). J Am ChemSoc136 (2),602–
cr
605.
Yang, G.C.C., Chuang, C. C., 2005. Treatment of nanosized TiO2-containing wastewater by
us
simultaneous electrocoagulation/electrofiltration. Water Sci&Tech 52 (10-11), 377-381
an
Yanting, L., Xiuguo, S., Huiwan, L., Wang, S., Wei, Y., 2009. Preparation of anatase TiO2 nanoparticles with high thermal stability and specific surface area by alcohothermal method.
M
Powder Techn. 194,149 – 152.
Ac ce p
te
d
Zakrzewska, K.2001 Mixed oxides as gas sensors. Thin Solid Films 391,229–238
18 Page 18 of 26
Tables Table 1 Crystallite size of TiO2 samples prepared in acidic and basic medium under Teflon and
Calcination
Autoclave
temperature(°C)
Crystallite size (nm)
cr
Type of
ip t
Teflon lined stainless steel autoclave. A: Anatase , R:Rutile
basic
R
0
5.6
0
500
25.5
Teflon lined
0
6.0
stainless steel
500
23.2
A
R
8.2
1.3
3.6
12.8
26.7
0
7.1
----
5.9
18.9
----
Ac ce p
te
d
Teflon
M
A
an
us
Acidic
Table 2: Kinetic parameters, effect of crystallinity on DCPIP photodegrdation using slurry reactor. Catalyst Crystal size Degradation % Apparent rate t½ (nm) constant (min.) kapp (min-1) 4A 8.2 64.85 0.36 1.93 19 Page 19 of 26
7.1 5.6 6.0
71.27 89.53 90.77
0.43 0.79 0.86
1.6 0.88 0.81
Degussa P-25
18.2
16.11
0.046
15.1
Ac ce p
te
d
M
an
us
cr
ip t
4B 4B-1 4C
20 Page 20 of 26
Figures
ip t
Nanocrystalline titania
pH = 3
cr
pH = 10
us
pH
Teflon
lined
Teflon
Stainless
Autoclave
lined
Teflon
Stainless
Autoclave
Steel
500
S.N
4C
40C
0
500
0
500
0
500
4B-1
40B-1
4B
40B
4A
40A
te
0
Ac ce p
C.T
d
M
Steel
an
Teflon
Au. T
Fig. 1: Schematic flow chart representation for preparation of TiO2 nanoparticles. Au.T: Autoclave Type, C.T: Calcination Temperature, S.N: Sample Number
21 Page 21 of 26
0
0
cr
500 C
0 10
20
30
40
50
60
70
80
10
2 Degree
us
0
0C
ip t
Shifted Relative Intensity (a.u)
A(215)
A(116) A(220)
A(204)
A(105) A(211)
A(004)
A(200)
A(101)
Shifted Relative Intensity (a.u)
0
500 C
20
30
40
50
0
0C
60
70
80
2 Degree
an
(
(b)
0
0 10
Ac ce p
te
500 C
20
30
40
50
60
70
Shifted Relative Intensity (a.u)
d
Shifted Relative Intensity (a.u)
M
a)
0
500 C
0
0C
0
0C 0 10
80
20
30
40
50
60
70
80
2 Degree
2 Degree
(d)
(c)
Fig. 2: Wide angle XRD patterns of TiO2 nanoparticles under Teflon autoclave a) under basic, and b) acidic catalyst and under Teflon lined stainless steel autoclave; c) under basic, and d) acidic catalyst with calcined and uncalcined temperature .
22 Page 22 of 26
ip t cr
(a)
(d)
te
(c)
d
M
an
us
(b)
Ac ce p
Fig. 3: Scanning electron micrograph of TiO2 nanoparticles under Teflon autoclave with calcined and uncalcined ; a) under basic, and b) acidic catalyst.
23 Page 23 of 26
ip t cr
(a)
(d)
te
(c)
d
M
an
us
(b)
Ac ce p
Fig. 4: Scanning electron micrograph of TiO2 nanoparticles under Teflon lined stainless steel autoclave with calcined and uncalcined ; a) under basic, and b) acidic catalyst.
Fig. 5: TEM micrograph of the the uncalcined TiO2 nanopowder as prepared in acidic medium under Teflon lined stainless steel autoclave.
24 Page 24 of 26
ip t cr us an
Fig. 6: Photodegrdation efficiency under UV irradiation, initial dye concentration 110-4
Ac ce p
te
d
M
M, pH 6.7 (natural), and 1g/l of TiO2 at 3 min using different samples.
(a)
25 Page 25 of 26
Ac ce p
te
d
M
an
us
cr
ip t
(b) Figure 7: Hydrolysis mechanism for (a) acidic and (b) basic catalytisis.
Figure 8: Influence of OH– ligands on the crystallization of TiO2 from hydrolytic reactions.
26 Page 26 of 26
S0957-5820(15)00174-3 http://dx.doi.org/doi:10.1016/j.psep.2015.09.011 PSEP 625
To appear in:
Process Safety and Environment Protection
Received date: Revised date: Accepted date:
9-4-2015 28-8-2015 20-9-2015
Please cite this article as: Hamad, H.A., Abd El-latif, M.M., Kashyout, A.B., Sadik, W.A., Feteha, M.Y.,Optimizing the preparation parameters of mesoporous nanocrystalline titania and its photocatalytic activity in water: physical properties and growth mechanisms, Process Safety and Environment Protection (2015), http://dx.doi.org/10.1016/j.psep.2015.09.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Optimizing
the
preparation
parameters
of
mesoporous
nanocrystalline titania and its photocatalytic activity in water:
ip t
physical properties and growth mechanisms. H. A. Hamad 1,*, M. M. Abd El-latif 1, A. B. Kashyout2, W.A. Sadik3, M. Y. Feteha3 Fabrication Technology Research Department, Advanced Technology and New Materials
cr
1
us
Research Institute (ATNMRI), City of Scientific Research and Technological Applications (SRTA-City), New Borg El-Arab City, P.O. Box 21934, Alexandria, Egypt. Electronic Materials Department, Advanced Technology and New Materials Research Institute
an
2
(ATNMRI), City of Scientific Research and Technological Applications (SRTA-City), New
Materials Science Department , Institute of Graduate Studies and Research (IGSR), Alexandria
d
3
M
Borg El-Arab City, P.O. Box 21934, Alexandria, Egypt.
te
University, 163 Horrya Avenue, P.O. Box 832, Shatby , 21526 Alexandria, Egypt. *Corresponding author: Tel: 002-03-4593414
Fax: 002-03-4593414
Ac ce p
E-mail: [email protected]
High lights
Mesoporus anatase TiO2 nanoparticles were successfully prepared by acid treatmentsolgel/ hydrothermal method . The effects of preparation parameters on crystallite size and phase composition of prepared titania nanoparticles. Mesoporous TiO2 show superior photocatalytic activity and selectivity due to large surface area. Study the reaction mechanism of the prepared titania nanoparticles
1234-
Abstract
1 Page 1 of 26
Titaniananomaterial with an anatase structure and 5.6 nm crystallite size and 280.7 m2/g specific surface areas had been successfully prepared by sol-gel/hydrothermal route. The effect of pH as a type of autoclave and calcination was studied. Crystallite size and phase composition of the
ip t
prepared samples were identified. X-ray diffraction analyses showed the presence of anatase with little or no rutile phases. The crystallite size of the prepared TiO2 with acidic catalyst was
cr
both smaller than that prepared with basic catalyst, and was increasing after acidic calcinations
us
by a factor 4-5. Basic calcinations produced a specific increase of 1.5. Rutile ratio and the particle size were increased after calcination at 500ºC. However, TiO2 powder synthesized using
an
a basic catalyst persisted the anatase phase and a loosely aggregation of particles. Anatase TiO2 as prepared with acidic catalyst in Teflon lined stainless steel autoclave demonstrated the highest
M
photocatalytic activity fordegradation of 2,6 dichlorophenolindophenolunder ultraviolet
d
irradiation with t½ 0.8 min.
Photocatalytic activity.
Ac ce p
1. Introduction
te
Keywords: nanostructures, sol-gel growth, Hydrothermal, Titania, X-ray Diffraction,
Nanosizedtitania has attracted increasing attention in the scientific community for its broad applications in photocatalysts (Hoffmann et al. 1995, Ohno, 2004, Xu, 2014, Hamad et al. 2015), bacterial disinfection (Kashyout et al.2006), solar cells (Hong, 2014), gas sensors (Zakrzewska et al. 2001), electro coagulation /electrofiltration (Yang & Chaung 2005) and optoelectronic devices (Kalyanasundaram et al. 1998) . TiO2 was intensively investigated for photocatalysis because of insolubility, its biological, and chemical inertness, strong oxidizing power, cost effectiveness, non-toxicity and long-term stability against photo and chemical corrosion under the reaction conditions (Nakataet al . 2012; Akhavan et al . 2009; Rachel, 2002). Anatase has
2 Page 2 of 26
been reported to be the most effective phase for photocatalysis since higher crystallinity means fewer defects for the recombination of photogenerated electrons and holes (Bahnemann et al. 1993; Li, 2009). One of the methods is the sol-gel which had emerged as a promising processing
factors as
ip t
route for synthesis of nano sized TiO2 with high photocatalytic activity due to the reasons and inexpensive operation procedures, simplicity, low temperatures, and the
cr
homogeneous and the highly pure resulting product (You, 2004). The hydrothermal method is
us
widely used for the production of small particles in the ceramic industry. Usually, this process is conducted in steel pressure vessels called autoclaves under controlled temperature and /or
an
pressure with the reactions occurring in aqueous solution (Sohrabi et al.2007). Hydrothermal processing of either amorphous titania and titanium-containing precursor had been shown to be
M
an ideal method for obtaining titania with small grain size (often 10 nm or less), high specific
d
surface area, and high crystallinity (Sangchay, 2012). The properties of titania nanoparticles
te
were controlled using preparation conditions such as water contents, autoclave time and temperatures and calcinations temperatures (Kashyout et al. 2010).
Ac ce p
The study under investigation using a novel method for the synthesis of TiO2 nanoparticles was being carried out by sol-gel/ hydrothermal methods from titanium butoxide as a precursor and acetyl acetone was used as an inhibitor to slowdown the rate of hydrolysis and polycondensation. The microstructure of the as-prepared photocatalyst was studied using XRD, SEM, EDAX, and TEM. The formation of all the prepared photocatalysts and the TiO2 Degussa P-25 and their photo degradation activities for 2,6 dichlorophenol-indophenol (DCPIP) under UV illumination were investigated.
2. Materials and Methods. 2.1. Materials.
3 Page 3 of 26
Titanium butoxide (Ti(OBu)4, 97%) was obtained from Aldrich chemicals (Germany), acetylacetone (CH3(COCH3)2, 99%) was supplied from Alfa asear (Germany), Ammonia solution (NH4OH, 30%) was obtained from Sigma Aldrich chemicals (E.U), Nitric Acid (HNO3
ip t
,65%) was supplied from Pharos (Egypt), 2-Propanol ((CH3)2CHOH , 98%) was obtained from Panreac (E.U) , Hydrochloric acid (HCl, 37%) and Absolute ethanol ([C2H5OH] , 99.9%) were
cr
supplied from Sigma-Aldrich (E.U) , TiO2 (Degusaa P25 ) (99.5%) was supplied from Degussa
us
(Germany). The dye was investigated for photocatlysis was 2,6 dichloro indophenol – sodium salt (DCPIP) (C12H6Cl2NO2.Na, 97%) and was obtained from Sigma (USA).
an
2.2. Preparation of highly active TiO2 nanoparticles.
Nano Titania was prepared by sol-gel/hydrothermal method under both basic and acidic
M
conditions. Different samples of titania were prepared by adding 30 % NH4OH or 10 % HNO3 to
d
a solution of Ti(OBu)4 , an acetyl acetone (acac) and De- ionized water (18.2 MΩ.cm, TKA-
te
Genpure). The molar ratios of the ingredients were as follows: Titanium precursor / acac / 30% NH4OH or 10% HNO3 / De-ionized water = 1:1:10 or 3:20. The pH values of the resulting
Ac ce p
solutions were 10 or 3, respectively. The final solution was stirred using amagnetic stirrer (Selecta, Model Agiamatic-ND, Spain) for 4 hrs at room temperature. The resulting solution was placed in 100 mL Teflon autoclave (BERGhof Product Instrument –Germany) or Teflon lined stainless steel autoclave and then placed in an oven (Nabertherm, Model TR60, Germany) for hydrothermal treatment at 170 C for 72 hrs. Then, the autoclave was naturally cooled to room temperature and the obtained product was washed several times with HCl , 2-propanol and distilled water to remove the residues of organics placed in solution and centrifuged by Ultracentrifuge (Hettichzentrifuge –Model Rotofix32 , Germany). Then the samples were dried at 100 C for 12 hrs. Finally, the samples were calcined at 500C in a muffle furnace (Barnstead
4 Page 4 of 26
International, Model F4800, USA) for 2 hrs. Schematic flow chart of various preparation parameters of TiO2 nanocrystals were described as shown in Fig. 1. 2.3. Sample Characterization.
ip t
Crystal phases and crystallite size of as prepared samples were obtained by X-ray diffractometer (XRD- Shimadzu 7000, Japan) that operated at a voltage of 30 kV and a current
cr
of 30 mA with CuKα radiation (=1.54 Å) The average crystallite size was estimated by applying
us
the Scherrer equation (L = K λ / (β cos θ) where L is the crystallite size (Å), λ is the wavelength of the X-ray radiation (Å), K is usually taken as 0.89, β is the line width at half-maximum height
an
(radians) and θ is the diffraction angle (degree).The phase composition of the samples can be calculated from such equation as XR = 1 – ( 1/ (1+1.26 IR/IA ); where XR is the weight fraction of
M
rutile in the mixture, and IR and IA are the relative intensities of the strongest diffractions peaks
d
of rutile and anatase, respectively. The size and morphology of TiO2 were determined by
te
scanning electron microscopy (JEOL, Model JSM 6360LA, Japan) and transmission electron microscopy (JEOL JEM 1230, Japan). The compositional and elemental analyses of TiO2
Ac ce p
nanoparticle were measured by EDAX.UV-Vis Spectroscopy (Labomed, Inc) to indicate the effect that processes had on an incident beam when the radiation source was in the ultravioletvisible wavelengths (190-1100 nm).
2.4. Measurement of Photocatalytic Activity. The photocatalytic performance of the prepared TiO2 was investigated by using the photocatalytic decomposition of anionic dichlorophenol-indophenol (DCPIP) dyefrom aqueous solution under ultraviolet irradiation in a slurry reactor at constant pH (natural 6.7) and room temperature (253C). The slurry reactor included a glass container (1 liter). The contents (1104
M DCPIP dye solution and 1g/l of catalyst) were maintained as a suspension by means of a
5 Page 5 of 26
magnetic stirrer. Then, dry air was fed into the solution (source of oxygen) with rate 4 liters/min. The reaction was conducted under vigorous agitation to ensure uniform distribution throughout the reacting medium. A tubular low pressure mercury vapor source (total rating 43 watts, total
ip t
UV output at 254 nm, 13.4 watts, length 120 cm, VALBER LOURMAT , Germany) was used to irradiate the solution which was located 10 cm away from the surface of the source . The total
cr
intensity reaching the slurry solutions was measured by using a UVX radiometer (UV products
us
Ltd., Cambridge) equipped with a sensor with peak sensitivity at 254 nm was 4 mWcm-2. The local volumetric rate of energy reaching the solutions was 2.1μ Einstein's-1 L-1at 254 nm.
an
Samples were withdrawn at periodic intervals from the reactor with the help of a syringe and then filtered through a Millipore sterile syringe filter of 0.2 μ m (Corning, NY 14831, Germany).
M
Then, they were analyzed for decolorization and degradation using spectrophotometer (model
d
Hitachi UV-2000, double beam, Japan) at max, i.e., 600 nm for DCPIP dye. The Photodegrdation
te
efficiency of 2,6dichloroindophenol dye could be calculated according to this equation (%
Ac ce p
Degradation = C-C/C100% = A- A/A100% ) ; where C is the initial concentration of dye solution (mg/L), C is the concentration of dye solution after photoirradiation (mg/L) at time t (minute). Ais the value of absorbance of dye aqueous solution after adsorption in the dark, and A is the value of absorbance of dye aqueous solution after reaction, and where A and Awas unitless.
3. Results and discussion.
3.1. Crystalline Size and Phase Composition study. XRD patterns of the as uncalcined and calcined TiO2 nanoparticles under Teflon autoclave and teflon lined stainless steel in acidic and basic media were shown in Figure 2. The asprepared and calcined nanoparticles were exhibited both anatase and rutile structures (JCPDS 6 Page 6 of 26
Card No. 021-1272 and 021-1276), respectively. It was clear that the samples contained both the anatase and the rutile phases and their percentages change with Teflon type, pH, and calcination. In case of Teflon and stainless steel Teflon autoclaves at acidic medium, the as prepared samples
ip t
contained the anatase phase. For basic medium, both the anatase and the rutile were obtained but the dominate phase was the anatase. But in case of Teflon lined stainless steel autoclave, the
cr
anatase phase only occurred before calcination and after calcination rutile phase began to appear.
us
The phase composition ranged from zero to 3.6 % of rutile percentage in acidic and basic media respectively for the nanoparticles prepared under Teflon autoclave, and from 5.9 to 3.6 % of
an
rutile percentage in acidic and basic media respectively for the nanoparticles calcined at 500ºC under Teflon lined stainless steel. Table 1 indicated the increase as in the crystallite size from 5.6
M
to 22.5 nm in acidic media due to calcination, and as from 8.2 to 12.8 nm in basic media in
d
Teflon autoclave and 6 to 23.2 nm in acidic media, from 7.2 to 18.9 nm in basic media in Teflon
te
lined stainless steel for anatase phase. This may be due to the original bondings in uncalcined which may be broken to form new bondings for rutile structure and condensation of free OH
Ac ce p
groups on the surface of the nano TiO2 particles (Watson, 2004). Before the calcination for both types of autoclave, the crystallite size of anatase phase in basic medium was greater than that of the crystallite size of anatase phase in acidic medium. This may be attributed to the TiO2 particles catalyzed by acid which were more closely packed than those treated by base catalyst where particles that had more tendencies to retain hydroxyls and water molecules and their particle sizes after calcinations became smaller than acid-treated particles. Therefore, it seemed that a more closely packed structure of primary particles of acid-treated TiO2 could provide larger secondary particles during heat treatment. After calcination, the crystallite size of anatase phase in a basic medium was smaller than in an acidic medium. This may be due to the base-
7 Page 7 of 26
treated TiO2 particles which were loosely packed during calcinations resulting in a relatively smaller secondary particle size (Song et al., 2001). The calcination under acidic media increased the crystallite size (by factor 4-5) in comparison to that under basic media (by factor 1.5).
ip t
3.2. Morphology Study.
cr
SEM micrographs of several samples that were successfully prepared at different conditions were shown in Figure 3 and 4. All samples have ordered arrays for TiO2 nanocrystals whose
us
predominant shape was in a spherical shape. In the as-prepared sample (acidic and alkaline media) for both autoclaves, the strong repulsive charge among particles reduced the probability
an
to coalesce and more stable sol could be formed with little or no aggregates (Chang, 1995). In
M
the calcined samples, there was a little increase in crystallite size and more aggregate. Scanning electron micrograph of TiO2 nanoparticles prepared under Teflon lined stainless steel autoclave
d
and calcined in basic catalyst medium presented the preferred sample due to its relatively small
te
grain size, homogeneity and less aggregation as shown in Figure 4c. The size of the particles at higher pH values was smaller with less agglomerate than in a basic
Ac ce p
medium. The variety of TiO2 surface charge was pH dependent. TiO2 in sols possessed electrical charge due to the absorption of H+ or OH- in aqueous suspension. The surface charges of TiO2 could be determined by chemisorption (Polanams et al. 2006 ; Labbe et al. 2008) (equation 4 and 5):
For H+,
TiO2 + nH+ ↔TiO2Hnn+ for pH < 3.5
(4)
TiO2 + nOH- ↔TiO2 (OH) nn- for pH >3.5
(5)
For OH-,
8 Page 8 of 26
In acidic and alkaline media, the strong repulsive charge among particles reduced the probability to coalesce and more stable sol could be formed. Studies by (Su et al., 2004) Sue group had indicated that the pH for isoelectric TiO2 powder was between 5and 6.8.
ip t
The as prepared sample under acidic conditions with Teflon lined stainless steel (4C) was favored due to many reasons; its purity and homogeneity with a very little organic residues, low
cr
crystal size and low aggregation. This was clear from the study of XRD and SEM.
us
The EDAX of the optimized TiO2 (4C) (Figure not included).No vague peaks associated with other crystal structures were observed. The as-prepared sample was expected to contain only Ti
an
and O elements. The percentage of oxygen is 70.65 wt % in the titania. The EDAX spectrum of the titania indicated a strong Ti signal at 4.5 eV with atomic percentage of 29.35%, confirming
M
that the external shell was principally composed of TiO2.
d
TEM images of the optimum TiO2 nanoparticles were shown in Fig.5. The distribution indicated
te
the particles of sizes 5-8 nm for this sample. This was in agreement with the crystallite size calculated from XRD. Most of the particles appeared to be in a spherical shape as shown in the
Ac ce p
TEM images.
The large specific surface area of the preferred sample of titania at 280.7 m2g-1 with mesoporous structure, total pore volume at 0.193 cm3g-1, and high volume fraction of atoms were located both on the surface and at the grain boundaries result in an increased surface energy.
3.4. Photocatalytic Properties. The photodegradation of dichlorophenol-indophenol (DCPIP) in aqueous solution over different powder samples was plotted against UV irradiation time. The obtained results matched with the Langmuir–Hinshelwood (L–H) kinetic model (equation 6): 1 / r = 1/ kr + 1 / (krKC)
(6)
9 Page 9 of 26
where r was the decomposition rate of the reactant (mg L-1 min.-1), C was the concentration of the reactant (mg L-1), K represented the equilibrium constant for adsorption of the dye on TiO2 particles (Langmuir constant) and kr represented the specific reaction rate constant for the
ip t
oxidation of the reactant (mg L-1 min.-1) (Machado et al., 2003).
When the chemical concentration C is the order of millimolar, a first order equation (7) of
(7)
us
ln (C/C) = krK t = kapp t
cr
integrated form will be produced.
Where kapp represented the apparent first order rate constant of the photocatalytic degradation The time dependence of ln (C/C) could be represented by a straight line of slope to
an
(min-1).
M
equal the apparent first order rate constant kapp as illustrated in Table 2. Generally, first order kinetics was appropriate for several studies and fitted well with this kinetic model.
te
equation 8 and as shown in table 2.
d
The relation between the half life time (t½) of the first order reaction and kapp was given by
t½ = 0.693/ kapp
(8)
Ac ce p
The Degradation efficiency of samples with different crystal sizes - as prepared TiO2 powder (4A, 4B, 4B-1 and 4C) and Degussa P-25 under UV light irradiation were shown in Fig. 6. It could be noted that the most effective sample for degradation of DCPIP dye was sample 4C. A comparison between kapp values for all types of TiO2 revealed that sample 4C was the most effective for degradation of DCPIP dye. This may be due to several aspects such as its high degree of crystallinity, pure anatase phase, and homogeneity which could be consistent with the more efficient electron–hole utilization, the decrease in the content of amorphous TiO2, the high degree and quality of crystallization which indicated the fewer TiO2 bulk defects presence
10 Page 10 of 26
leading to recombination of photoexcited electron and holes (Table 2). The photodegradation of these photocatalysts obeyed the following order: 4C > 4B-1 > 4B > 4A > Degussa P-25.
ip t
The highest value of the obtained apparent rate constant kapp is obtained to be 0,86 min-1 for the 4C sample while the lowest value was 0.046 min-1 for Degussa P-25. The apparent rate constant
cr
kapp were obtained to be 0.36 min-1 for sample 4A and 0.43 min-1 for sample 4B. Both were
us
smaller than those of sample 4B-1 (0.79 min-1). The lower photoactivity of the rutile form in sample 4A could be attributed to many aspects such as the higher tendency for electron-hole
an
recombination and also to its lesser ability to absorb oxidizing species such as O2 and its dependence on the preparation method (Carp et al. 2004). The photocatalytic activity of sample
d
influence their photocatalytic activity.
M
4B-1 was closer to 4C due to the crystal size of TiO2 particles which has been demonstrated to
te
Therefore, it was obvious that the photocatalytic activity was increased with decreasing crystal size, although there appeared to be some disagreement with (Rivera, 2004). It had been observed
Ac ce p
that about the linear increase of photocatalytic degradation rate went in line with increasing anatase crystallite size (Table 2). Some reports (Anpo et al. 1987) mentioned that the anatase particles in the range of 4–50 nm, photocatalytic activity increased with decreasing TiO2 particle diameter which might be attributed to size quantization. As the particle size was lowered below a certain limit, most of the electrons and hole are generated close to the surface. Therefore, surface recombination was faster than interfacial charge-carrier transfer processes. Therefore, it could be concluded that the lower activity of Degussa P-25 as compared with other samples was more likely attributable to an increase in rutile content in the Degussa sample rather than the reduction in surface area. Pigmentory grade particles were in the micron range and their
11 Page 11 of 26
photoactivity of Degussa P-25 was decreasing due the decrease of the ratio between surface area and volume. This result contradicted the increased heterogeneous catalytic activity observed with increasing surface to volume ratio for nano-sized particles. The efficiency of the surface trapping
ip t
of photo-generated holes was increasing in smaller particle sizes due to the availability of more active sites. This reflected their ability to better promote photocatalyzed processes on the surface
cr
of the catalyst (Porter et al., 1999).
us
3.5. Reaction mechanism.
One of the alternatives of calcinations which yielded finer and less aggregated nanocrystals was
an
the hydrolysis of titanium containing precursor. Acids were not only catalyzing the formation of
M
crystalline TiO2 structure but also were acting as a peptizing agent that stabilized individual nanocrystal surface and consequently preventing aggregation, discouraging particle growth, and
d
allowing the particles to exist in the form of a colloidal sol.
te
Accordingly, it could be noted that the possible preparation mechanism of TiO2 nanoparticles at basic and acidic conditions might follow two paths a and b during synthesis.
Ac ce p
(a) Formation of Ti-OH bond by hydrolysis mechanism. Through this process of synthesizing, the electrophillic and the nucleophillic mechanisms had depended upon on acidic and basic catalyst: Firstly, under acidic conditions, it was likely that the butoxide group was protonated in a rapid first step. Electron density was withdrawn from titanium atom, making it more electrophilic and thus more susceptible to be attacked by water. This resulted in the formation of a penta-coordinated transition state (Parra et al. 2008). This transition state was decayed by the displacement of an alcohol and a conversion to titanium tetrahedron as shown in Figure 7a.
12 Page 12 of 26
Secondly, under basic conditions, the titanium butoxide which was known the basic alkoxide oxygen tended to repel the nucleophilic,-OH. No sooner had the initial hydrolysis occurred than the reaction proceeded stepwise with each subsequent alkoxide group and was more easily
ip t
removed from the monomer. Therefore, more highly hydrolyzed titanium was more prone to attack. In addition, the hydrolysis of the forming polymer had been much more hindered than the
cr
hydrolysis of monomer. Although hydrolysis in basic medium was slow and tends to be
us
completed, it is irreversible. Under basic conditions, water was likely dissociated rapidly to produce hydroxyl anions during the first step (Jung, 1999). The hydroxyl anion then attacked the
an
titanium atom where –OH replaces –OR formed the titanium tetrahedron, as shown in Figure 7. (b)Crystallization of TiO2 from Ti-OH as result of condensation.
M
Crystallization of TiO2 was the result of condensation reactions among these Ti (IV) (3d) co-
d
ordination complexes, i.e, the reaction between OH- ligand on different complexes resulting in
te
the elimination of water to form Ti-O-Ti.
Another path for crystallization was that, there were two types of linkage involved in the
Ac ce p
formation of TiO2 crystals, edge sharing , and corner sharing that to be depending on the linkage of (TiO62−) octahedra (Chang, 1995) as shown in Figure 8. Two opposite edges of each octahedron were edge shared forming a linear chain with other chains via corner oxygen atoms. Anatase, however, did not involve corner sharing, but had four edges shared per octahedron as were shown in Figure 8. The anatase chains were linked together through shared edges. The anatase phase was statistically more favorable because of the existence of more edges available to form a bond. The configuration of rutile was thermodynamically more favorable due to the linear arrangement which minimized the electrostatic repulsive energy. Moreover, the formation of corner sharing bonds was more difficult than edge sharing bonds (Diebold et al. 2003).
13 Page 13 of 26
It was found that as the number of OH - ligands in the coordination was increasing the probability of edge shared bonding (and thus anatase formation) among [TiO6] was increased. At the same time, the corner shared bonding (and thus rutile formation) was favored when the number of OH-
ip t
ligands was decreased. The existence of relatively large OH- ligands under acidic condition would encourage the formation of the anatase phase and would suppress the formation of the
cr
rutile phase. The higher the acidic concentration, the more strongly anatase was favored.
us
Accordingly, the pH of the medium was neutral or basic as that result in rutile production which was the favored product (Chang, 1995).
an
4. Conclusions
M
TiO2 nanoparticles with anatase phase, 5.6 nm average crystallite size was successfully synthesized via sol-gel/hydrothermal route with acidic condition under Teflon autoclave. Pure
d
TiO2 and anatase phase with high crystallite size were obtained under basic medium in Teflon
te
autoclave in comparison to Teflon pressure autoclave. The crystallite size with as prepared with acidic catalyst was smaller than that one prepared with basic catalyst. By calcination at 500 C,
Ac ce p
the crystallite size and grain growth were increasing agglomeration and rutile content. UV/ TiO2 (4C) was the most effective catalytic system for the degradation of DCPIP dye with a degradation efficiency of 90.77% and that could be applied for a wide range of organic compounds.
Acknowledgment
This work was done under the project funded by the Science and Technology Development Fund (STDF), Ministry of Scientific Research, Project ID: 1414, “Quantum Dots Nanomaterials Dye Sensitized Solar Cells”. Also, the authors would like to thank Dr. Marwa Fathy, Researcher at SRTA-City, for her helpful assistance in the writing stage of the reaction mechanism. 14 Page 14 of 26
5. References. Akhavan, O., Azimirad, R., 2009.Photocatalytic property of Fe2O3nanograin chains coated by TiO2nanolayer in visible light irradiation. ApplCatal A- Gener., 369,77 – 82.
ip t
Anpo, M., Shima, T., Kodama, S., Kubokawa, Y., 1987.Photocatalytic hydrogenation of propyne
cr
with water on small-particle titania: size quantization effects and reaction intermediates. J
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PhysChem 91,4305-4310.
Bahnemann, D., Bockelemann, D., Goslich, R., Hilgendorff, M., Weichgrebe, D.,
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1993.Photocatalytic detoxification: Novel catalysis, mechanisms and solar applications. Elsevier Science Publishers, New York, 301 – 319.
Prog Solid State Chem32(1-2),33 – 177.
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Carp, O., Huisman, C., Reller, A.,2004.Photoinduced reactivity of titanium dioxide : Review.
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d
Chang, H., Ma, J., Zhao, Z., Qi, L., 1995.Hydrothermal preparation of Uniform nanosize
Ac ce p
rutile and anatase particles. Chem Mater 7, 663-671. Diebold, U., 2003. The surface science of titanium dioxide. Surf Sci Rep 48 (5–8), 53–229. Hamad, H.A., Abd El-latif , M. M., Kashyout , A. B., Sadik , W.A., Feteha, M. Y., 2015. Synthesis
and
characterization
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core–shell–shell magnetic
(CoFe2O4–SiO2–TiO2)
nanocomposites and TiO2 nanoparticles for the evaluation of photocatalytic activity under UV and visible irradiation. New J Chem 39, 3116-3128. Hoffmann, M., Martin, S., Choi, W., Bahnemann, D., 1995. Environmental applications of semiconductor photocatalysis. Chem Rev95, 69–96.
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Hong, C. K., Jung, Y. H., Kim, H. J., Park, K. H., 2014. Electrochemical properties of TiO2 nanoparticle/nanorod composite photoanode for dye-sensitized solar cells. CurrApplPhy14 (3): 294-299. Jung, Y., Park, B.,1999.Anatase-phase titania: preparation by embedding silica and
ip t
phtocatlytic activity for the decomposition of trichloroethylene. J.PhotochemPhotobiol A-Chem
cr
127, 117 – 122.
Kalyanasundaram, K., Grätzel, M., 1998. Applications of functionalized transition metal
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complexes in photonic and optoelectronic devices. CoordChem Rev 177,347–414.
Kashyout, A., Soliman, M., El-Haleem, D., 2006.Disinfection of bacterial suspensions by
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photocatalytic oxidation using TiO2 nanoparticles under ultraviolet illumination. Alex Eng J
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45,367-371.
Kashyout. A., Soliman, M., Fathy, M., 2010. Effect of preparation parameters on the properties
d
of TiO2 nanoparticles for dye sensitized solar cells. Renewable Energy 35,2914-2920.
te
Labbe, M., 2008. Photocatalytic Degradation of Select Drinking Water Pollutants Using Nano-
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TiO2 Catalyst, Ph.D Dissertation, University of Windsor, Ontario, Canada. Machado, H., Miranda, A., Freitas, F., Duarte, M., Ferreira, F., Albuquerque, T., Ruggiero, R., Sattler, C., Oliveira, L., 2003.Destruction of the organic matter present in effluent from a cellulose and paper industry using photocatalysis. J PhotochemPhotobiol A –Chem 155, 231 – 241.
Nag, M., Basak, P., Manorama, S., 2007.Low-temperature hydrothermal synthesis of phase-pure rutile titania nanocrystals: Time temperature tuning of morphology and photocatalyst activity. Mater Res Bull42,1691–1704.
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Nakata, K., Fujishima, A., 2012.TiO2photocatalysis: Design and applications, Review. J PhotochemPhotobiol C 13, 169– 189. Ohno, T., 2004. Preparation of visible light active S-doped TiO2photocatalysts and their
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photocatalytic activitiesWater Sci&Tech49 (4), 159 - 163 Parra, R., Góes, S., Castrom, S., Longo, E., Bueno, R., Varela, A., 2008.Reaction pathway to the
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synthesis of anatase via the chemical modification of titanium isopropoxide with acetic acid. Chem Mater20,143 – 150.
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Polanams, J. T., 2006. Size Control in Nanoparticles Synthesis, Characterization and
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Photocatalytsis, Ph. D Dissertation, University of New Mexico, Albuquerque, New Mexico. Porter. J.F., Yu-Guang, L., Chak, K. C.,1999. The effect of calcination on the microstructural
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characteristics and photoreactivity of Degussa P-25 TiO2. J Mater Sci34, 1523 – 1531. Rachel, A., Suhrahmanyam, M., Boule, P., 2002. Comparison of photocatalytic efficiencies
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ip t
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Powder Techn. 194,149 – 152.
Ac ce p
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d
Zakrzewska, K.2001 Mixed oxides as gas sensors. Thin Solid Films 391,229–238
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Tables Table 1 Crystallite size of TiO2 samples prepared in acidic and basic medium under Teflon and
Calcination
Autoclave
temperature(°C)
Crystallite size (nm)
cr
Type of
ip t
Teflon lined stainless steel autoclave. A: Anatase , R:Rutile
basic
R
0
5.6
0
500
25.5
Teflon lined
0
6.0
stainless steel
500
23.2
A
R
8.2
1.3
3.6
12.8
26.7
0
7.1
----
5.9
18.9
----
Ac ce p
te
d
Teflon
M
A
an
us
Acidic
Table 2: Kinetic parameters, effect of crystallinity on DCPIP photodegrdation using slurry reactor. Catalyst Crystal size Degradation % Apparent rate t½ (nm) constant (min.) kapp (min-1) 4A 8.2 64.85 0.36 1.93 19 Page 19 of 26
7.1 5.6 6.0
71.27 89.53 90.77
0.43 0.79 0.86
1.6 0.88 0.81
Degussa P-25
18.2
16.11
0.046
15.1
Ac ce p
te
d
M
an
us
cr
ip t
4B 4B-1 4C
20 Page 20 of 26
Figures
ip t
Nanocrystalline titania
pH = 3
cr
pH = 10
us
pH
Teflon
lined
Teflon
Stainless
Autoclave
lined
Teflon
Stainless
Autoclave
Steel
500
S.N
4C
40C
0
500
0
500
0
500
4B-1
40B-1
4B
40B
4A
40A
te
0
Ac ce p
C.T
d
M
Steel
an
Teflon
Au. T
Fig. 1: Schematic flow chart representation for preparation of TiO2 nanoparticles. Au.T: Autoclave Type, C.T: Calcination Temperature, S.N: Sample Number
21 Page 21 of 26
0
0
cr
500 C
0 10
20
30
40
50
60
70
80
10
2 Degree
us
0
0C
ip t
Shifted Relative Intensity (a.u)
A(215)
A(116) A(220)
A(204)
A(105) A(211)
A(004)
A(200)
A(101)
Shifted Relative Intensity (a.u)
0
500 C
20
30
40
50
0
0C
60
70
80
2 Degree
an
(
(b)
0
0 10
Ac ce p
te
500 C
20
30
40
50
60
70
Shifted Relative Intensity (a.u)
d
Shifted Relative Intensity (a.u)
M
a)
0
500 C
0
0C
0
0C 0 10
80
20
30
40
50
60
70
80
2 Degree
2 Degree
(d)
(c)
Fig. 2: Wide angle XRD patterns of TiO2 nanoparticles under Teflon autoclave a) under basic, and b) acidic catalyst and under Teflon lined stainless steel autoclave; c) under basic, and d) acidic catalyst with calcined and uncalcined temperature .
22 Page 22 of 26
ip t cr
(a)
(d)
te
(c)
d
M
an
us
(b)
Ac ce p
Fig. 3: Scanning electron micrograph of TiO2 nanoparticles under Teflon autoclave with calcined and uncalcined ; a) under basic, and b) acidic catalyst.
23 Page 23 of 26
ip t cr
(a)
(d)
te
(c)
d
M
an
us
(b)
Ac ce p
Fig. 4: Scanning electron micrograph of TiO2 nanoparticles under Teflon lined stainless steel autoclave with calcined and uncalcined ; a) under basic, and b) acidic catalyst.
Fig. 5: TEM micrograph of the the uncalcined TiO2 nanopowder as prepared in acidic medium under Teflon lined stainless steel autoclave.
24 Page 24 of 26
ip t cr us an
Fig. 6: Photodegrdation efficiency under UV irradiation, initial dye concentration 110-4
Ac ce p
te
d
M
M, pH 6.7 (natural), and 1g/l of TiO2 at 3 min using different samples.
(a)
25 Page 25 of 26
Ac ce p
te
d
M
an
us
cr
ip t
(b) Figure 7: Hydrolysis mechanism for (a) acidic and (b) basic catalytisis.
Figure 8: Influence of OH– ligands on the crystallization of TiO2 from hydrolytic reactions.
26 Page 26 of 26