The present study reports the investigation of polyoxometalate catalyzed electron transfer from the conduction band of photoexcited TiO2 to molecular oxygen. The oxidation of 1,2-dichlorobenzene (DCB) was used as an index reaction for evaluating the photocatalyst systems TiO2-PW12O40(3-), TiO2-SiW12O40,4- and TiO2-W10O32(4-) in oxygenated aqueous solution. Addition of these polyoxometalate (POM) anions to TiO2 suspensions resulted in significant rate enhancement for DCB oxidation. Photodegradation kinetics exhibited [POM] dependence, experiencing different maximum (k = 0.0318 min(-1), 0.0108 min(-1), and 0.0066 min(-1)) for each POM at different [POM] (0.1 mM PW12O40(3-), 0.07 mM SiW12O40,4- and 1 mM W10O32,4-respectively). The probability that the difference in the adsorption affinity of POMs on TiO2 surface could account for the observed ranking of photodegradation rates was ruled out by adsorption isotherm experiments that revealed similar binding constants for each POM (467 M(-1), 459 M(-1), and 417 M(-1) for PW12O40(3-), SiW12O40(4-), and W10O32(4-), respectively). DCB degradation over TiO2 with O2 or POM+O2 systems can be modeled by the Langmuir-Hinshelwood (saturation kinetics) model. The concentration-independent rate constants (kL-H) for TiO2-O2, TiO2-W10O32(4-), TiO2-SiW12O40(4-), and TiO2-PW12O40(3-) were 0.0818, 0.152, 0.421, and 0.638 min(-1), respectively. An analysis of deltaG for electron transfer from the conduction band of TiO2 to POMs in this study shows that the electron transfer takes place even when it is endothermic.

This article reports the first systematic study on the quantitative relationship between the process parameters of solution concentration ratio, structure, and physical and optical properties of ZnO nanowires grown on cotton surfaces. To develop a fundamental understanding concerning the process-structure-activity relations, we grew a series of well-defined, radially oriented, highly dense, and uniform single-crystalline ZnO nanorods and nanoneedles on cotton surfaces by a simple and inexpensive two-step optimized hydrothermal process at a relatively low temperature. This process involves seed treatment of a cotton substrate with ZnO nanocrystals that will serve as the nucleation sites for subsequent anisotropic growth of single crystalline ZnO nanowires. All of the ZnO nanowires exhibit wurtzite crystal structure oriented along the c-axis. For investigating structure-controlled properties, seed-to-growth solutions concentrations ratio ([S]/[G]) of the synthesis process was varied over six different values. Superhydrophobicity was achieved for all morphologies after 1-dodecanethiol modification, which was highly durable after prolonged UV irradiation. Durability of the ZnO materials under laundry condition was also verified. Variation of the [S]/[G] ratio resulted in a morphological transform from nanorods to needle-like structures in conjunction with a drastic change in the physical and optical properties of the ZnO modified cotton surfaces. Higher [S]/[G] ratios yielded formation of ZnO nanoneedles with high degree of crystallinity and higher aspect ratio compared to nanorods. Increasing [S]/[G] ratio resulted in the amount of ZnO grown on the cotton surface to drop significantly, which also caused a decrease in the surface hydrophobicity and UV absorption. In addition, room temperature photoluminescence measurements revealed that the band gap of ZnO widened and the structural defects were reduced as the morphology changed from nanorods to nanoneedles. A similar trend was observed in the UV-vis absorption of nanorods and nanoneedles, the onset of the latter exhibiting a blue-shift that correlates with the widening of band gap with nanoneedle formation.

In this work, we report the effectiveness of the NaY zeolite as a solid support for several photocatalytically active polyoxometalate (POM) salts (H2NaPW12O40, H4SiW12O40, or H3PMo12O40) as indicated by the photodegradation kinetics of the probe compound 1,2-dichlorobenzene (DCB) in water. The photooxidation of DCB was carried out in illuminated (254−370 nm), oxygenated solution at pH 1.0. NaY zeolite-supported POMs exhibited photocatalytic activity over the wavelength range 254−350 nm. Photocatalytic activity did not drop off significantly until λ > 350 nm. At 350 nm, k obs values at an optimum NaY zeolite/POM combination were 1.02 × 10-2 , 6.5 × 10-3 , and 5.1 × 10-3 min-1 for PW12O403 -, SiW12O404 -, and PMo12O40 3-, respectively. Unsupported POMs exhibited no detectable photocatalytic activity at λ > 340 nm. At all wavelengths, photooxidation of DCB in the presence of 0.1% NaY zeolite with each of three different 0.5 mM polyoxometalates (POMs) was first order in DCB. The optimum POM/NaY zeolite ratios at 254 nm for DCB oxidation were 0.5 mM PW12O40 3-/0.2 wt % NaY zeolite (k obs = 1.69 × 10-2 min-1), 0.75 mM SiW12O40 4-/0.1 wt % NaY zeolite (k obs = 8.1 × 10-3 min-1), and 0.5 mM PMo12O40 3-/0.1 wt % NaY zeolite (k obs= 6.8 × 10-3 min-1). The k obs values are a factor of 4−8 times higher than those with PW12O40 3-, SiW12O40 4-, and PMo12O40 3- alone, respectively. Although the system is clearly complex, with oxidation rates dependent on [POM], [DCB]o, zeolite loading, and λ, the results indicate a promising approach for enhancing polyoxometalate photochemistry and for developing new photocatalytic materials.

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We report a simple and effective route for fabricating branched hierarchical nanostructures of TiO(2)/ZnO by combining electrospinning and the low-temperature hydrothermal growth technique. First, TiO(2) nanofibers were prepared by electrospinning polystyrene (PS)/titanium tetraisopropoxide (Ti(OiPr)(4)) solutions onto glass substrates followed by calcination at 500 °C. The electrospun TiO(2) nanofibers served as a 3D primary platform upon which the branched, highly uniform, and dense secondary ZnO nanorods were hydrothermally grown. We observed that the concentration of Ti(OiPr)(4) in the polystyrene solution has a significant effect on the surface roughness and areal material ratio of the electrospun fibers. Most significantly, the morphology of the branched secondary ZnO nanorods and the overall charge transfer capacity of the nanoheterostructured systems are controlled by the density of the TiO(2) platform. This study demonstrates that, by properly choosing the synthesis parameters, it is possible to fine-tune the microscopic and macroscopic properties of branched hierarchical metal-oxide systems. The presented approach can be applied to the development of controlled, reproducible, miniaturized, and robust high-performance metal-oxide photovoltaic and photocatalytic systems.