Photoproduction of iodine with nanoparticulate semiconductors and insulators

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.


Background
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) [1]. 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 [2]. The capacity to photogenerate hydroxyl radical is also taken as a measure of the photocatalytic activity of photocatalyst [3]. 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][5][6][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 Al 2 O 3 and SiO 2 . Recently, we have reported photodegradation of carboxylic acids on Al 2 O 3 and SiO 2 nanoparticles [8].

Optical edge
The diffuse reflectance spectra (DRS) of the employed oxides are shown in Figure 3. The reflectance data are presented as F(R) value, obtained by the application of where R is the reflectance. The DRS clearly show that SiO 2 , Al 2 O 3 and ZrO 2 do not absorb UVA light. Figure  3 also displays the band gap excitation of TiO 2 anatase, TiO 2 P25, TiO 2 Hombikat, TiO 2 rutile, BaTiO 3 , ZnO and SnO 2 under UVA radiation. The DRS further reveals that blue light is capable of effecting band gap excitation of WO 3 . In addition, the DRS of Fe 2 O 3 displays the commencement of light absorption at about 600 nm itself. Also, the DRS of CuO shows that the oxide is susceptible to photoexcitation by the entire spectrum of visible light. The DRS of Fe 3 O 4 does not show any significant variation in the measured reflectance with visible and UVA light. This is because of its reported band gap of about 0.1 eV [9]. The displayed K-M plots are in total agreement with the expected band gaps of the studied oxides [9]. The band gap of ZrO 2 is very wide (about 5 eV) and Al 2 O 3 and SiO 2 are insulators and hence do not absorb in the visible and UVA region.

Photocatalytic oxidation of iodide
In aqueous suspension, anatase TiO 2 catalyzes iodide ion oxidation more effectively whereas Hombikat TiO 2 and TiO 2 P25 effectively, Al 2 O 3 , SiO 2 , BaTiO 3 , and ZnO moderately and ZrO 2 , rutile TiO 2 , SnO 2 , WO 3 , CuO, Fe 2 O 3 , and Fe 3 O 4 feebly under UVA light. The UV-visible spectrum of KI solution illuminated with any of the said oxides reveals iodine formation (λ max 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). Figure 4 is the time profile of photoformation of iodine. It shows that iodine-generation on TiO 2 anatase, TiO 2 Hombikat and TiO 2 P25 slackens in 15 min whereas that on ZnO and WO 3 does so at 30 and 60 min, respectively. The other oxides exhibit sustainable photocatalysis at least up to 2 h of illumination. The slackening of iodine-formation with illumination time is not unknown. Photoformation of iodine on Ag-TiO 2 [7], Pt-TiO 2 [10], phthalocyanine sensitized TiO 2 [11] and immobilized TiO 2 [5] or ZnO [5,6] show such behavior. Since the iodine generation on TiO 2 anatase, TiO 2 Hombikat and TiO 2 P25 are not slackened at least up to 15 min and on the other oxides at least up to 30 min, the reaction rates have been obtained by measuring the iodine formed in 15 and 30 min on anatase, Hombikat and P25 TiO 2 and the rest of the oxides, respectively. All the nanooxides show sustainable photocatalysis. The recycled oxides without any pre-treatment provide identical results (results not listed).   kinetic model [4,7]. The generation of iodine at different airflow rates is displayed in Figure 5. Iodine-formation is enhanced with increased airflow and the variation conforms to Langmuir-Hinshelwood kinetics. Moreover, oxygen is essential for the photoformation of iodine. Iodine is not formed in nitrogen-purged iodide ion solution illuminated with any of the studied oxide (data not listed). The dependence of generation of iodine on the light intensity is also displayed in Figure 5. The photocatalysis lacks linear dependence on photon flux. Less than first power dependence of rates of surface-photocatalyzed reactions on light intensity at high photon flux is known [4,7]. The dependence of photocatalytic iodine generation on the pH of the medium is shown in Figure  6. The pH of the slurry was adjusted by the addition of small volume of NaOH or HCl solution. Except TiO 2 rutile and BaTiO 3 all other oxides slow down the iodine generation with increase of pH. Rutile TiO 2 and BaTiO 3 are less sensitive to pH variation. The adsorption of ionic species on the semiconductor depends also on the surface excess charge on the semiconductor crystals. At pH higher than the point of zero charge (PZC), the semiconductor surface is negatively charged resulting in electrostatic repulsion between iodide ion and the semiconductor crystal. Hence, the concentration of iodide ion at the surface and in the double layer is likely to be lesser than that in the bulk of the solution. The adsorption isotherm turns linear leading to a first order kinetics of photocatalysis.  [9]. Examination of Figure 6 reveals, for some oxides at least (TiO 2 , SnO 2 , Fe 2 O 3 , Fe 3 O 4 and ZrO 2 ), uniform trend in the photocatalysis at pH higher as well as lower than the PZC. A possible explanation is the modification of the PZC values by the ions present in the solution [12,13]. For example, the PZC of TiO 2 is reported to change from 6.4 to 4.5 [12]. UVC light is more effective than UVA light to generate iodine ( Table 2). A possible reason for the larger formation of iodine under UVC radiation than UVA radiation is that the generated iodine also absorbs at 365 nm. That is, the liberated iodine may act as an inner filter by absorbing part of the UVA illumination thereby decreasing the intensity of impinging radiation on the nanoparticles. In the case of ZrO 2 , 254 nm-illumination will bring in band gap excitation. This may lead to the larger iodine-formation. Table 2 also shows that with majority of oxides studied the photogeneration of iodine is more in the immersion reactor than in the tubular reactor. BaTiO 3 , CuO, Fe 3 O 4 , ZrO 2 and SiO 2 porous are the exceptions. These oxides fail to disperse uniformly throughout the volume of the KI solution (250 mL) in the immersion reactor. It is evident from Table 2 that baring the said five oxides the process is not limited to micro-level.
Comparison of the photocatalytic efficiencies of the nanomaterials reveals TiO 2 anatase as the most efficient photocatalyst. Even the benchmark photocatalyst TiO 2 P25 Degussa, which is a blend of anatase and rutile, is found to be less effective than the anatase studied. TiO 2 rutile shows poor photocatalytic activity. Many semiconductors such as BaTiO 3 , SnO 2 , ZnO, WO 3 , CuO and Fe 2 O 3 fail to display better photocatalytic efficiency than the insulators Al 2 O 3 and SiO 2 . 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 SiO 2 . The mechanism of photocatalytic oxidation of iodide ion and also that of iodide ion-photooxidation on Al 2 O 3 and SiO 2 surfaces have been discussed elsewhere in detail [4][5][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 TiO 2 anatase improves by about 65% whereas those by TiO 2 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 BaTiO 3 , Al 2 O 3 and SiO 2 could do so only by about 4-fold. Among the feebly active catalyst, the least active CuO and Fe 3 O 4 improve their efficiencies in acetonitrile by about 55 and 25%, respectively. Rutile TiO 2 and SnO 2 efficiencies go up by 8-fold, whereas that of ZrO 2 and Fe 2 O 3 is by about 15%. However, WO 3 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

Conclusions
The photocatalytic efficiency of anatase TiO 2 to generate iodine is much larger than those of

Characterization
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.

Photoreactors
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 15mm 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, 254nm low pressure mercury lamp and 6-W, 365-nm medium pressure mercury lamp was employed to study photocatalysis under UVC and UVA light.

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.