Photocatalytic Activity Of ZnO Nanoparticles Research Articles

Pure and Mg-doped self-assembled ZnO nano-particles for the enhanced photocatalytic degradation of 4-chlorophenol

A novel self-assembled pure and Mg doped ZnO nano-particles (NPs) were successfully synthesized by a simple low temperature co-precipitation method. The prepared photocatalysts were characterized by X-ray diffraction, high resolution scanning electron microscopy, high resolution transmission electron microscopy, diffuse reflectance spectroscopy and photoluminescence (PL) spectroscopy. The results indicated that the prepared photocatalysts showed high crystallinity with a uniform size distribution of the NPS. The degradation of cholorphenols is highly mandatory in today's scenario as they are affecting the environment adversely. Thus, the photocatalytic degradation of 4-chlorophenol (4-CP), a potent endocrine disrupting chemical in aqueous medium was investigated by both pure and Mg-doped ZnO NPs under UV-light irradiation in the present study. The influence of the Mg content on the structure, morphology, PL character and photocatalytic activity of ZnO NPs were investigated systematically. Furthermore, the effect of different parameters such as 4-CP concentration, photocatalyst amount, pH and UV-light wavelength on the resulting photocatalytic activity was investigated.

Effect of starting properties and annealing on photocatalytic activity of ZnO nanoparticles

Eight commercial ZnO samples were investigated, as-received and after annealing in the flow of oxygen or forming gas. We found that ZnO nanoparticles with higher photoluminescence (PL) intensity (fewer nonradiative defects) exhibited a high photocatalytic activity. Samples exhibiting high PL intensity presented lower concentrations of surface adsorbates, which is consistent with lower native defect density in those samples. It was observed that annealing did not result in an improvement in photocatalytic activity of the samples with low defect density (high PL intensity). For the samples with low PL intensity, the same annealing condition could result in an improvement or worsening of photocatalytic activity, depending on the starting properties of the samples. Variable effects of annealing on samples with different properties indicate that the effect of a post-growth treatment cannot be generalized for all ZnO samples. The improvements in the photocatalytic activity were observed for a cationic dye methylene blue, while for an anionic dye acid red 27 annealing resulted in the changes in the amount of adsorbed dye, but no improvement in the photocatalytic activity for dye degradation.

Photochemical Behavior of Polylactide/ZnO Nanocomposite Films

This study reports the effect of light on PLA/ZnO nanocomposites films produced by melt-extrusion. The attention focused on the discrimination between the photocatalytic degradation of PLA provoked by ZnO and the UV screening effect of the ZnO nanoparticles. The chemical modifications of PLA induced by UV light irradiation were analyzed using infrared spectroscopy and completed through the analysis of the low-molecular-weight photoproducts using IC and SPME and the characterization of chain scissions with SEC. A comprehensive mechanism for the photooxidation of PLA was then proposed. The results indicated that the photocatalytic activity of ZnO nanoparticles induces the oxidation of PLA. Because ZnO limits the penetration of light inside the samples, this effect mainly concerns the first micrometers at the surface of the exposed samples. Cross-sectional analysis using micro-IR and ATR-IR spectroscopies was performed to highlight the degradation profile in the PLA/ZnO nanocomposites.

Enhancing Stability and Photocatalytic Activity of ZnO Nanoparticles by Surface Modification of Graphene Oxide

This work reports a simple method for the preparation of high-quality GO-ZnO nanocomposite materials. Transmission electron microscopy (TEM) revealed that the ZnO nanoparticles are uniformly distributed on the GO sheets and the diameter of the ZnO nanoparticles falls in 5-8 nm. Further experimental results imply that involving GO sheets into the system could remarkably prevent the aggregation of ZnO nanoparticles compared to pure ZnO. The photocatalytic activity and stability of the prepared GO-ZnO composite for the degradation of Acid Orange 7 (AO 7) under UV light irradiation is significantly enhanced in comparison to the as-synthesized pristine ZnO nanoparticles. Considering the high photocatalytic acitivity and relative stability, this high-quality GO-ZnO nanocomposite is beneficial for the applications in environmental engineering and other fields.

Reduction of the photocatalytic activity of ZnO nanoparticles for UV protection applications

The detrimental effects of UV radiation are having a significant impact on our life and environment. The development of effective UV shielding agents is therefore of great importance to our society. ZnO nanoparticles are considered to be one of the most effective UV blocking agents. However, the development of ZnO–based UV shielding products is currently hindered due to the adverse effects of the inherent photocatalytic activity exhibited by ZnO. This paper reports our recent study on the possibility of reducing the photoactivity of ZnO nanoparticles via surface modification and impurity doping. It was found that the photoactivity was drastically reduced by SiO2–coatings that were applied to ZnO quantum dots using the Stöber method and a microemulsion technique. The effect of transition metal doping on the photoactivity was also studied using mechanochemical processing and a co–precipitation method. Cobalt doping reduced the photoactivity, while manganese doping led to mixed results, possibly due to the difference in the location of dopant ions derived from the difference in the synthesis methods.

Size-Controlled Electron Transfer and Photocatalytic Activity of ZnO–Au Nanoparticle Composites

This Letter describes size-controlled photocatalytic activity of ZnO nanoparticles coated with glutathione-protected gold nanoparticles with diameters of 1.1, 1.6, and 2.8 nm. The photocatalytic activity of the ZnO–Au composites was found to increase with increasing gold size for both oxidative and reductive catalytic reactions. Photoluminescence decay dynamics of the composites showed that the electron-transfer rate from the photoexcited ZnO to gold nanoparticle also increased as the gold size increased. These results demonstrate that the photogenerated electron transfer and the resulting catalytic activity of the composites can be controlled by the size of the mediating gold capacitors.

Controlled reduction of the deleterious effects of photocatalytic activity of ZnO nanoparticles by PVA capping

Nanoparticles of uncapped and PVA (poly vinyl alcohol) capped zinc oxide were synthesized by precipitation method. The synthesized ZnO nanoparticles were characterized by fourier transform infrared spectroscopy, X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy and thermogravimetric-differential thermal analysis. The photocatalytic activity of bare and modified ZnO nanoparticles was studied by monitoring the degradation of Rhodamine B. The results show that PVA capped ZnO nanoparticles has reduced photocatalytic activity than the bare ZnO nanoparticles. The reduction in the chemical oxygen demand and total organic carbon results also revealed the reduced photocatalytic activity of PVA capped ZnO. The UV-shielding property was evaluated by measuring the transmittance which shows that both bare and PVA capped ZnO nanoparticles possess good UV-shielding ability.

Photocatalytic oxidation of organic dyes with nanostructured zinc dioxide modified with silver metals

Silver-doped nano-ZnO samples with different Ag loadings were prepared by a one-spot solvothermal method. The structure, physico-chemical and optical properties of the products are characterized by using X-ray diffraction (XRD), transmission electron microscope (TEM), energy dispersive X-ray analysis (EDS), diffuse reflectance spectroscopy (DRS) and photoluminescence spectra (PL). The photocatalytic activity of the as-prepared samples was examined by using photocatalytic oxidation of methyl orange (MO) as a model reaction, and the effects of the noble metal content on the photocatalytic activity were investigated. The results indicated that the photocatalytic activity of ZnO nanoparticles can be greatly improved by depositing appropriate amount of noble metal on their surface as electron scavengers. In addition, a key mechanism was proposed in order to account for the enhanced activity. The enhancement for the photocatalytic activities can be attributed to the interaction between Ag particles and ZnO, which made Ag particles act as electron traps to effectively separate the excited electron-hole pairs.

Effects of synthesis conditions and annealing post-treatment on the photocatalytic activities of ZnO nanoparticles in the degradation of methylene blue dye

The effects of synthesis conditions (i.e., metal concentration, precursor flowrate, and O 2 dispersion) during flame-spray pyrolysis (FSP) and annealing post-treatment on the characteristics and photocatalytic activities of ZnO nanoparticles have been investigated. The average particle size of ZnO powder prepared by one-step FSP method were in the range of 8.8–47.0 nm and were found to be increased with increasing the enthalpy density, flame height, and high-temperature residence time during FSP synthesis. The larger particle size FSP-derived ZnO nanoparticles exhibited higher photocatalytic activities in the degradation of methylene blue (MB) dye. The degradation rate over FSP-ZnO-47.0 nm was 1.7 and 7.2 times higher than those of the commercially available photocatalysts Degussa P-25 and JRC-TiO 2, respectively. The better photocatalytic performance of the FSP-ZnO was correlated well with the improved crystalline quality of ZnO nanoparticles as revealed by the X-ray diffraction (XRD) and the photoluminescence (PL) results. Further increase of FSP-ZnO particle size to 52.6–103.5 nm by annealing post-treatment at high temperatures (750–900 °C), however, gradually decreased their photocatalytic activities. Our results in this study suggest a balance between high crystalline quality that enhanced photo phenomena and the surface area available for substrate adsorption in order to obtain high photocatalytic activity of ZnO nanoparticles.

Rapid synthesis, characterization and optical properties of TiO2 coated ZnO nanocomposite particles by a novel microwave irradiation method

Abstract A novel and rapid microwave method was used to prepare TiO 2 coated ZnO nanocomposite particles. The resulted particles were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HR-TEM) and X-ray photoelectron spectroscopy (XPS). Results show that ZnO nanoparticles were coated with 6–10 nm amorphous TiO 2 layers. In addition, zeta potential analysis demonstrated the presence of TiO 2 layer on the surface of ZnO nanoparticles. Photoluminescence (PL) spectroscopy and UV–visible spectroscopy were used to investigate the optical properties of the nanoparticles. Compared to uncoated ZnO nanoparticles, the TiO 2 coated ZnO nanoparticles showed enhanced UV emission. The UV–visible diffuse reflectance study revealed the significant UV shielding characteristics of the nanocomposite particles. Moreover, amorphous TiO 2 coating effectively reduced the photocatalytic activity of ZnO nanoparticles as evidenced by the photodegradation of Orange G with uncoated and TiO 2 coated ZnO nanoparticles under UV radiation.

Photocatalytic activity of ZnO nanoparticles prepared via submerged arc discharge method

ZnO nanostructures were synthesized through arc discharge of zinc electrodes in deionized (DI) water. X-ray diffraction (XRD) analysis of the prepared nanostructures indicates formation of crystalline ZnO of hexagonal lattice structures. Transmission electron microscopy (TEM) images illustrate rod-like as well as semi spherical ZnO nanoparticles with 15–20 nm diameter range, which were formed during the discharge process with 5 A arc current. The average particle size was found to increase with the increasing arc current. X-ray photoelectron spectroscopy (XPS) analysis confirms formation of ZnO at the surface of the nanoparticles. Surface area of the sample prepared at 5 A arc current, measured by BET analysis, was 34 m2/g. Photodegradation of Rhodamine B (Rh. B) shows that the prepared samples at lower currents have a higher photocatalytic activity due to larger surface area and smaller particle size.

Enhancement photocatalytic activity of ZnO nanoparticles by silver doping with optimization of photodeposition method parameters

The photocatalytic activity of silver-doped ZnO was tested by photocatalytic degradation of C.I. Acid Red 88 (AR88) as a model contaminant from monoazo textile dyes. Silver-doped ZnO was prepared by photodeposition (PD) method. Results show that silver-doped ZnO is more efficient than undoped ZnO at photocatalytic degradation of AR88. The PD method parameters such as irradiation time, calcination temperature and silver content of doping were effective on the photoactivity of silver-doped ZnO. Silver content had an optimum value of 0.5% for achieving high photocatalytic activity. In the real wastewater from a textile-dyeing factory color removal with pure and silver-doped ZnO after 18 min of irradiation were 45 and 90%, respectively.

Microwave-assisted silica coating and photocatalytic activities of ZnO nanoparticles

A new and rapid method for silica coating of ZnO nanoparticles by the simple microwave irradiation technique is reported. Silica-coated ZnO nanoparticles were characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), high-resolution transmission electron microscopy (HR-TEM), CHN elemental analysis and zeta potential measurements. The FT-IR spectra and XPS clearly confirmed the silica coating on ZnO nanoparticles. The results of XPS analysis showed that the elements in the coating at the surface of the ZnO nanoparticles were Zn, O and Si. HR-TEM micrographs revealed a continuous and uniform dense silica coating layer of about 3 nm in thickness on the surface of ZnO nanoparticles. In addition, the silica coating on the ZnO nanoparticles was confirmed by the agreement in the zeta potential of the silica-coated ZnO nanoparticles with that of SiO 2. The results of the photocatalytic degradation of methylene blue (MB) in aqueous solution showed that silica coating effectively reduced the photocatalytic activity of ZnO nanoparticles. Silica-coated ZnO nanoparticles showed excellent UV shielding ability and visible light transparency.

The effects of oxygen partial pressure on the microstructures and photocatalytic property of ZnO nanoparticles

The effects of oxygen partial pressure on the microstructures and photocatalytic activity of ZnO nanoparticles prepared by evaporation–condensation method in a flow of Ar and O 2 have been studied by field-emission scanning electron microscopy (FESEM), XRD, XSM800 photoelectron spectrometer (XPS) and UV–Vis spectrometer. The size and shape of ZnO nanoparticles prepared at the oxygen partial pressure (700–800 Pa) show slight difference from that prepared at the oxygen partial pressure ranging from 1500 to 2300 Pa. However, the relative percentage of surface oxygen vacancies decreases from 40.6% (prepared at 700–800 Pa) to 29.3% (2200–2300 Pa) in the condition of similar ratio of Zn atoms to total oxygen species by the qualitative calculation of XPS data. ZnO nanoparticles were examined as photocatalysts for the UV-induced degradation of methyl orange in water solution. The photodegradation results directly demonstrate that the oxygen partial pressure influences the photoactivity of ZnO nanoparticles greatly.

Effects of noble metal modification on surface oxygen composition, charge separation and photocatalytic activity of ZnO nanoparticles

In this paper, ZnO nanoparticles were modified by depositing different amount of noble metal Ag or Pd on their surfaces with a photoreduction method. The as-prepared ZnO samples were principally characterized by X-ray photoelectron spectroscopy (XPS) and surface photovoltage spectroscopy (SPS), and their activity was evaluated by a photocatalytic degradation reaction of phenol solution. The effects of noble metal modification on surface composition, photoinduced charge transfer behavior and photocatalytic activity of ZnO samples were mainly investigated. The results show that, after an appropriate amount of Ag or Pd is deposited on the ZnO surfaces, the O 1s XPS spectrum shifts to higher binding energy, indicating that the content of surface hydroxyl oxygen (O H) increases. And the SPS intensity greatly decreases, indicating that the photoinduced electrons are easily captured by adsorbed oxygen via noble metal clusters so that the recombination of photoinduced electron and hole pairs is effectively inhibited. These are responsible for the enhancement in the photocatalytic activity. Moreover, the effects of Pd modification on photocatalytic activity is greater than that of Ag, which can be explained by means of surface hydroxyl content as well as charge separation situation.

Investigations on the surface modification of ZnO nanoparticle photocatalyst by depositing Pd

In this paper, ZnO nanoparticle photocatalysts were modified by depositing Pd on their surfaces with a photoreduction method. We mainly investigated the modification mechanisms as well as the effects on the photocatalytic activity of ZnO nanoparticles of deposited Pd by means of XPS and SPS (Surface Photovoltage Spectroscopy), and the effects of Pd content on SPS responses were also discussed from the point of the electronic energy level. The results showed that the content of crystal lattice oxygen on the surface of ZnO nanoparticle decreased after an appropriate amount of Pd was deposited, while that of adsorbed oxygen increased, indicating that Pd was mainly deposited on the crystal lattice oxygen. At the same time, the intensity of SPS responses of ZnO nanoparticles remarkably decreased. In addition, the activity of ZnO nanoparticles could be greatly improved by depositing an appropriate amount of Pd in the gas phase photocatalytic oxidation of n-C 7H 16. Thus, it could be concluded that the increase in surface content of adsorbed oxygen could facilitate the photocatalytic reaction, and there were close relationships between the SPS response and photocatalytic activity, i.e. the weaker the SPS response, the higher the photocatalytic activity, of Pd-deposited ZnO nanoparticles.