Photoproduction of iodine with nanoparticulate semiconductors and insulators
© Karunakaran et al 2010
Received: 11 May 2011
Accepted: 16 June 2011
Published: 16 June 2011
The crystal structures of different forms of TiO2 and those of BaTiO3, ZnO, SnO2, WO3, CuO, Fe2O3, Fe3O4, ZrO2 and Al2O3 nanoparticles have been deduced by powder X-ray diffraction. Their optical edges have been obtained by UV-visible diffuse reflectance spectra. The photocatalytic activities of these oxides and also those of SiO2 and SiO2 porous to oxidize iodide ion have been determined and compared. The relationships between the photocatalytic activities of the studied oxides and the illumination time, wavelength of illumination, concentration of iodide ion, airflow rate, photon flux, pH, etc., have been obtained. Use of acetonitrile as medium favors the photogeneration of iodine.
Nanoparticles exhibit physical properties distinctively different from that of bulk. They possess a large fraction of surface atoms or ions or molecules in unit volume. The very large surface area provides a huge surface energy. Further, the electronic structures of semiconductor nanocrystals differ from those of bulk materials. Band gap-illumination of semiconductor results in formation of electron-hole pairs; electron in the conduction band (CB) and hole in the valence band (VB) . While most of the electron-hole pairs recombine, some of the charge carriers diffuse to the crystal surface and react with the adsorbed electron donors and acceptors leading to photocatalysis. Here we compare the photocatalytic efficiencies of nanocrystalline semiconductors. Iodide ion is the test substrate taken up for the study. Production of energy bearing chemicals through thermodynamically uphill reactions is the objective of solar energy conversion and storage and iodide ion-oxidation is such a reaction (ΔG° = +51.6 kJ mol-1). In addition, it is well known that degradation of organic molecules involve photogenerated reactive oxygen species (ROS) and the major active oxidizing species is hydroxyl radical . The capacity to photogenerate hydroxyl radical is also taken as a measure of the photocatalytic activity of photocatalyst . More importantly, the photocatalytic mineralization of organics is complicated by the formation of a number of stable intermediates. But the iodide ion oxidation is a simple electron transfer process [4–7]. Further, unlike iodide ion the organic molecules such as phenols and dyes may have chemical affinity to the oxide surface and enter into some sort of bond formation with the oxides. These factors led to the selection of iodide ion as the test substrate for this investigation. The present photocatalytic results on iodide ion oxidation show that some of the nanocrystalline semiconductors are less efficient photocatalysts than insulators such as Al2O3 and SiO2. Recently, we have reported photodegradation of carboxylic acids on Al2O3 and SiO2 nanoparticles .
2. Results and Discussion
2.1. Crystal structure
The recorded diffractogram of ZnO confirms its crystal structure. The peak fitting is highly satisfactory (JCPDS 89-7102) and the deduced crystal parameters are: primitive hexagonal, a 3.2526 Å, b 3.2526 Å, c 5.1888 Å, α 90°, β 90°, γ 120°. The diffraction pattern of SnO2 is in accordance with its structure. It is in complete agreement with that of JCPDS 88-0287. The cell constants are: primitive tetragonal, a 4.7355 Å, b 4.7355 Å, c 3.1703 Å, α 90°, β 90°, γ 90°. The WO3 crystals provide an XRD pattern that belongs to primitive monoclinic system (JCPDS 89-4476). The crystal constants are: a 7.3291 Å, b 7.5006 Å, c 7.6718 Å, α 90°, β 88.18 ± 2.89°, γ 90°. The XRD pattern of CuO matches with JCPDS 89-2529 pattern and confirms the crystal structure as end centered monoclinic with crystal constants as: a 4.6977 Å, b 3.4193 Å, c 5.1285 Å, α 90°, β 81.20 ± 3.76°, γ 90°. The recorded XRD pattern of Fe2O3 shows the oxide as maghemite (γ-Fe2O3). The XRD is in total agreement with JCPDS 39-1346 and the crystals belong to cubic system with unit cell length as 8.3515 Å. The Fe3O4 used is of face centered cubic system. The recorded XRD pattern is in agreement with JCPDS 89-4319. The crystal parameters are: a 8.3381 Å, b 8.3381 Å, c 8.3381 Å, α 90°, β 90°, γ 90°.
Size (D) and surface area (S) of the oxides with rates of iodide ion-photooxidation*
Iodine-formation (nM s-1)
TiO2 P25 (anatase:rutile::81:19)
TiO2 Hombikat (anatase:rutile::69:31)
160 ± 20
640 ± 50
2.2. Optical edge
2.3. Photocatalytic oxidation of iodide
In aqueous suspension, anatase TiO2 catalyzes iodide ion oxidation more effectively whereas Hombikat TiO2 and TiO2 P25 effectively, Al2O3, SiO2, BaTiO3, and ZnO moderately and ZrO2, rutile TiO2, SnO2, WO3, CuO, Fe2O3, and Fe3O4 feebly under UVA light. The UV-visible spectrum of KI solution illuminated with any of the said oxides reveals iodine formation (λmax 350 nm); the spectra are akin to that of the authentic iodine-iodide solution (not given). Chemical tests also confirm the formation of iodine; the solution turns purple with starch and discharged by thiosulfate. The iodine liberation does not occur in dark. Also, the photogeneration of iodine in absence of the oxides is insignificant (data not presented).
Iodide-oxidation at different wavelength of illumination and in tubular and immersion reactors*
Iodine-formation (nM s-1)
254 nm-illumination a
365 nm- illuminationb
Tubular reactor c
Comparison of the photocatalytic efficiencies of the nanomaterials reveals TiO2 anatase as the most efficient photocatalyst. Even the benchmark photocatalyst TiO2 P25 Degussa, which is a blend of anatase and rutile, is found to be less effective than the anatase studied. TiO2 rutile shows poor photocatalytic activity. Many semiconductors such as BaTiO3, SnO2, ZnO, WO3, CuO and Fe2O3 fail to display better photocatalytic efficiency than the insulators Al2O3 and SiO2. One of the possible reasons is the unabated rapid recombination of the photogenerated electron-hole pairs in these semiconductors. Another reason could be the large surface area of SiO2. The mechanism of photocatalytic oxidation of iodide ion and also that of iodide ion-photooxidation on Al2O3 and SiO2 surfaces have been discussed elsewhere in detail [4–6, 8].
Improving the photocatalytic efficiency, particularly that of generation of energy bearing chemicals via thermodynamically uphill reactions, is of prime concern in solar energy conversion and storage. The listed oxides show improved photoformation of iodine in acetonitrile and Table 1 displays the results. Among the effective semiconductors, on moving from aqueous to acetonitrile medium the photocatalysis by TiO2 anatase improves by about 65% whereas those by TiO2 P25 and Hombikat increases by about 6- and 4-folds, respectively. On switching from aqueous to acetonitrile medium, among moderates catalysis, ZnO improves its efficiency by about 13-fold whereas BaTiO3, Al2O3 and SiO2 could do so only by about 4-fold. Among the feebly active catalyst, the least active CuO and Fe3O4 improve their efficiencies in acetonitrile by about 55 and 25%, respectively. Rutile TiO2 and SnO2 efficiencies go up by 8-fold, whereas that of ZrO2 and Fe2O3 is by about 15%. However, WO3 efficiency is increased by about 35%. General analysis of Table 1 shows that the efficiencies of the less active catalysts are improved many fold on using acetonitrile as medium instead of water. A possible reason for the larger photocatalytic activity in acetonitrile is the absence of hole-capture by hydroxyl ion and water molecule. One of the plausible explanations for the enhanced formation of iodine in acetonitrile on insulator surface may be the efficient transfer of excited electron from the adsorbed iodide ion to the neighboring adsorbed oxygen molecule. In aqueous suspension, adsorption of water molecule and hydroxide ion on the insulator surface may reduce the probability of adjacent adsorption of iodide ion and oxygen molecule. The photocatalytic efficiencies of the nanoparticles are of the order: TiO2 anatase > TiO2 P25 ≈ TiO2 Hombikat > ZnO > WO3 > Al2O3 ≈ SiO2 ≈ SnO2 ≈ BaTiO3 ≈ CuO ≈ Fe2O3 ≈ Fe3O4 ≈ ZrO2. These efficiencies are not in accordance with their band gap energies.
The photocatalytic efficiency of anatase TiO2 to generate iodine is much larger than those of TiO2 P25, TiO2 Hombikat, TiO2 rutile, BaTiO3, ZnO, SnO2, WO3, CuO, Fe2O3, Fe3O4, ZrO2, Al2O3 and SiO2 nanoparticles. Some of the studied nanocrystalline semiconductors are less efficient than Al2O3 and SiO2 nanoparticles. The photocatalysis conforms to the Langmuir-Hinshelwood kinetic model. Use of acetonitrile as medium favors the photogeneration of iodine.
TiO2 anatase, TiO2 rutile, ZnO, SnO2, WO3, CuO, Fe2O3, Fe3O4, ZrO2, BaTiO3, Al2O3 and SiO2 nanopowders used were those supplied by Sigma Aldrich. TiO2 Hombikat was supplied by Fluka. TiO2 P25 was a gift from Degussa.
The powder X-ray diffractograms were recorded with a Bruker D8 system using Cu Kα radiation of wavelength 1.5406 Å in a 2θ range of 10-70° at a scan rate of 0.05° s-1 with a tube current of 30 mA at 40 kV. Rich. Siefert model 3000 X-ray diffractrometer was also employed to obtain the diffraction pattern. A PerkinElmer Lambda 35 or Varian-Cary 5E or Shimadzu UV-2450 spectrophotometer was used to record the UV-visible diffuse reflectance spectra (DRS) of the oxides.
A photoreactor fitted with eight 8-W mercury lamps of wavelength 365 nm (Sankyo Denki, Japan) and highly polished aluminum reflector was used for the detailed photocatalytic study. The reactor was cooled by fans mounted at the bottom. Borosilicate glass tube of 15-mm inner diameter was employed as the reaction vessel. Immersion type photoreactor with 125-W medium pressure mercury lamp emitting at 365 nm, surrounded by highly polished anodized aluminum reflector, was also used. The reaction vessel was a 500-mL double walled borosilicate immersion well with inlet and outlet for water circulation. Micro photoreactor with 6-W, 254-nm low pressure mercury lamp and 6-W, 365-nm medium pressure mercury lamp was employed to study photocatalysis under UVC and UVA light.
4.4. Photocatalytic study
KI-solutions of required concentrations were prepared afresh and used. The volume of solution employed in multilamp, immersion and micro photoreactors were 25, 250 and 10 mL, respectively. Air was bubbled through the reaction solution using a micro air pump which kept the added nanoparticles under suspension and at constant motion. The airflow rate was determined by soap bubble method, the dissolved oxygen was measured using Elico dissolved oxygen analyzer PE 135 and the light intensity was found out by ferrioxalate actinometry.
Financial support through research grant no. F.12-64/2003 (SR) by the University Grants Commission (UGC), New Delhi, is thankfully acknowledged. P.A. is grateful to UGC for PF and PG to CSIR for SRF.
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