Platinum/zinc oxide nanoparticles: Enhanced photocatalysts degrade malachite green dye under visible light conditions

Zinc oxide nanoparticles (ZnO) were prepared via a sol–gel method, and a photo-assisted deposition method was used to prepare platinum on zinc oxide nanoparticles (Pt/ZnO). Several techniques were used to characterize these enhanced photocatalysts: XRD, TEM, UV–vis spectra, PL spectra, XPS, and BET surface area analysis. As-prepared samples’ photocatalytic performances were studied via degradation of malachite green dye under various visible-light-only irradiation scenarios. Results demonstrated the following: platinum (Pt) was well dispersed on and in ZnO's surfaces and pores; as such, Pt/ZnO had less surface area than pure ZnO due to pore blockage; however, advantages gained from enhanced electron-hole separation and decreased band gap width more than made up for this negative effect; moreover, Pt/ZnO prepared with 0.3 wt% Pt exhibited the lowest band gap and the highest photocatalytic activity of the various samples with a solids loading of 0.8 g/l; finally, such samples were recyclable, i.e., photocatalytic performance remained stable even after five uses.     

Influence of Er doping on the structural,optical and luminescence properties of pulsed laser deposited Er: BaZrO3 thin films

Pure and erbium doped (1, 2, 3 and 5 at%) Barium zirconate (BZE) thin films have been deposited on Si (0 0 1) substrate via pulsed laser deposition using 100 mJ Nd: YAG laser operated at second harmonics (532 nm). Er doping significantly affects the surface morphology, microstructure and optical properties of grown thin films. All the films exhibit cubic BaZrO₃ structure and are polycrystalline in nature as extracted from XRD data. The optical band gap energies (3.75–3.63 eV) of doped (1, 2, 3, and 5 at%) BZE thin films are found to be less than that of pure BZO film (4.03 eV). PL spectra, excited at 328 nm, mainly consist of violet-blue (412 nm) and green (523–543 nm) emissions for all the doped films. The green emission increases with the increase in Er doping upto 3 at% and then concentration quenching effect appears at 5 at%. It is noted that the relative intensity of PL emission and the optical band gap can be tuned by varying Er concentration to alter the properties of the phosphor. The emission peaks in photoluminescence spectra makes the Er: BZO films potential candidates to be used in optoelectronic devices such as light emitting diodes (LEDs).     

Cost-effective large-scale synthesis of oxygen-defective ZnO photocatalyst with superior activities under UV and visible light

A cost-effective solution method was developed to produce ZnO photocatalyst in large quantity, through the conversion of ε-Zn(OH)₂ to ZnO in NaOH solutions. Experimental results indicated that the concentrated NaOH solution (4 mol L⁻¹) promoted the rapid formation of ZnO owing to the enhanced dissolution-precipitation reactions. The large-scale synthesis was also achieved with high-yield and solvent-recyclability. Structural analysis based on X-ray photoelectron spectroscopy, electron spin resonance and photoluminescence revealed that the as-prepared ZnO photocatalyst was rich in oxygen vacancies (V_O). The V_O-rich ZnO photocatalyst exhibited improved visible-light absorption, higher photocurrent responses and superior activities toward the degradation of rhodamine B under both UV (λ~254 nm) and visible-light illumination (λ>420 nm) compared to commercial ZnO and P25 TiO₂ powders, as well as good cycle stability. Based on the results of photoluminescence and active species detection, the V_O-enhanced photocatalytic activity was attributed to the generation of V_O-isolated level in the band structure. Under UV light, the V_O-level could promote charge separation by trapping the photoinduced electrons, while under visible-light, the V_O-level improved visible-light absorption and facilitated the charge generation. The presently developed synthesis may potentially benefit the large-scale production and low-cost application of ZnO photocatalyst for solar energy utilization.     

Microstructural and optical properties of Ta-doped ZnO films prepared by radio frequency magnetron sputtering

Ta-doped ZnO films with different doping levels (0–5.02 at%) were prepared by radio frequency magnetron sputtering. The effects of the doping amount on the microstructure and the optical properties of the films were investigated. The grain size and surface roughness first significantly decrease and then slowly increase with the increase of Ta doping concentration. Both the grain size and the root mean square (RMS) roughness reach their minimum values at the doping content of 3.32 at%. X-ray Diffraction (XRD) patterns confirmed that the prepared Ta-doped ZnO films are polycrystalline with hexagonal wurtzite structure and a preferred orientation along the (002) plane. X-ray photoelectron spectroscopy (XPS) analysis reveals that Ta exists in the ZnO film in the Ta⁵⁺ and Ta⁴⁺ states. The average optical transmission values of the Ta-doped ZnO films are higher than those of the un-doped ZnO film in the visible region. The band gap energy extracted from the absorption edge of transmission spectra becomes large and the near band edge (NBE) emission energy obtained from PL spectra blueshifts to high energy when the Ta doping content grows from 0 at% to 5.02 at%, which can be explained by the Burstein–Moss shift.     

The O2-dependent growth of ZnO nanowires and their photoluminescence properties

ZnO nanowires were massively synthesized on a Ni(NO₃)₂-coated silicon substrate under oxygen-containing argon atmosphere by a simple chemical vapor deposition method. The average diameter of the ZnO nanowires was about 50 nm and the average length was about 20 μm. The morphologies of the ZnO nanowires strongly depended on oxygen content in the growth atmosphere. At low oxygen concentration (about 5–10 ppm), ZnO nanocones and nanoneedles were obtained, while at high oxygen concentration (about ∼250 ppm), ZnO nanoparticles deposited on the substrate. The room temperature photoluminescence (PL) spectrum of the ZnO nanowires revealed that a strong UV band at 384 nm dominated the whole spectrum. These results indicate that the ZnO nanowires grown under oxygen-containing atmosphere possess better crystalline quality and UV luminescence properties than those grown in reducing hydrogen atmosphere. Based on the analysis of oxygen effect on the ZnO nanostructures, a vapor–liquid–solid mechanism assisted by the redox growth mode was proposed to understand the growth of the ZnO nanowires.     

Increase of Mn solubility with decreasing grain size in ZnO

Nanograined (grain size 20 nm) ZnO films with various Mn content (from 0 to 47 at%) were synthesized by the novel wet chemistry method. The solubility limit for Mn was determined at 550 °C. The lattice parameter c of the ZnO-based solid solution with wurzite structure ceases to grow at 30 at% Mn. The peaks of the second phase (Mn₃O₄ with cubic lattice) become visible in the X-rays diffraction spectra at 30 at% Mn. The same second phase appears in the bulk ZnO already at 12 at% Mn. The recently published papers on the structure and magnetic behaviour of Mn-doped ZnO allowed us to obtain the size-dependence of Mn solubility in ZnO for the polycrystals and small single-crystalline particles. The overall Mn solubility drastically increases with decreasing grain size. The quantitative estimation leads to the conclusion that, close to the bulk solubility limit, the thickness of an Mn-enriched layer is several monolayers in GBs and at least two monolayers in the free surfaces.     

Vitis labruska skin extract assisted green synthesis of ZnO super structures for multifunctional applications

The formation of unique morphologies of zinc oxide (ZnO) super structured frameworks were reported via a simple and eco-friendly route employing Vitis labruska fruit Black Grape Skin (BGS) extract as a fuel. XRD, FTIR, UV–vis, PL, SEM and TEM studies are performed to analyse the formation and characterization of ZnO. XRD confirmed the crystalline nature of the material with hexagonal Wurtzite structure having average crystallite size of ~50 nm. FTIR spectrum shows a band at 532 cm⁻¹ due to the vibrational mode of Zn-O bending. The band gap of the ZnO was found to be 3.26 eV. SEM images confirm the formation of different morphologies like Mysore pak (a popular Indian dessert), canine teeth, hollow pyramids and gems were obtained by varying the weight of BGS. These superstructures were obtained due to the interaction of Zn²⁺ with BGS extract. TEM images clearly shows lattice spacing of 0.29 nm corresponding to the (002) plane of ZnO. Photoluminescence (PL) spectrum shows strong yellow light emission upon excitation at 320 nm due to the Zn-O defects. Synthesized ZnO nanoparticles (Nps) exhibited good photocatalytic activity for the degradation of Methylene blue (MB) dye. The photocatalytic activity was due to the production of OH˙ radicals during photo irradiation on ZnO Nps. ZnO Nps also exhibited superior antibacterial activity against Staphylococcus aureus and Escherichia coli bacteria. Further, ZnO Nps were also used in the development of novel electrochemical sensing platform towards the electrochemical detection of hydrazine as a model system at very low concentrations having a detection limit of 0.3 µM.     

Spectrophotometric studies of photo-induced degradation of Tertrodirect Light Blue (TLB) using a nanostructure zinc zirconate composite

Zinc zirconate nanopowder (ZZN) photocatalyst was prepared by sol–gel method using zinc acetate and zirconium acetylacetonate as precursors. The optimal calcination temperature was 800 °C and ZnZrO₃ phase was formed. The structural and morphology properties of the nanocomposite were characterized using X-ray diffraction (XRD), scanning electron microscope (SEM), Energy-dispersive X-ray spectroscopy (EDAX) and ultraviolet visible diffuse reflectance (UV-DRS). The SEM observation for ZZN showed the average grain size of 26 nm. UV–vis diffuse reflectance spectra (DRS) of the nanocomposite showed an absorption edge at 355 nm. The catalytic activity of ZZN was investigated by degradation of Tertrodirect Light Blue (TLB) dye in water using UV–vis spectroscopy (UV–vis) with an initial concentration of 20 mg/L dye. The influence of the catalyst concentration, time of irradiation and pH on photodegradation of dye was investigated. The results showed that degradation of TLB dye can be conducted in the photocatalytic process. Accordingly, a degradation of more than 97% of dye was achieved by applying the optimal operational parameters with 30 mg/L of catalyst, pH 9 at 1 h irradiation.     

Electrochemical degradation of malachite green with BDD electrodes: Effect of electrochemical parameters

The dye malachite green (MG) has found extensive use all over the world in the fish farming industry as a fungicide. It has been suspected to be genotoxic and carcinogenic and it has now become a highly controversial compound due to the risks it poses to the consumers of treated fish. In this context, electrochemical oxidation of malachite green (MG) has been studied on a one-compartment batch reactor employing a boron-doped diamond (BDD) as anode and a stainless steel as cathode. The effects of several experimental parameters, such as current density, pH initial and supporting electrolyte were investigated. The optimal conditions were determined such as a current intensity of 32 mA cm^− 2, strongly acidic pH (pH = 3), and Na₂SO₄ as a good supporting electrolyte. Under these conditions, a degradation efficiency of 98% was obtained after 60 min of electrolysis and 91% of chemical oxygen demand (COD) removal after 180 min of electrolysis.     

The changes in structural and optical properties of (ZnO:AlN) thin films fabricated at different RF powers of ZnO target

AlN doped ZnO thin films were prepared on glass and Si (100) substrates by RF sputtering. The ratio of nitrogen (N₂) to Argon (Ar) used to prepare the films was 80:20. The films were deposited at different RF powers of 150 W, 175 W, 200 W, 225 W and 250 W for ZnO target and 200 W for AlN target. XRD results revealed the existence of (002) ZnO phase for RF power of ZnO target above 175 W. However, at the RF power of 150 W, the film exhibited amorphous properties. The prepared films showed transmission values above 70% in the visible range. The average calculated value of energy band gap and the refractive index were 3.43 eV and 2.29 respectively. The green and UV emission peaks were observed from PL spectra. Raman Peaks at 275.49 cm^?1 and 580.17 cm^?1 corresponding to ZnO:N and ZnO:AlN were also observed.     

Influence of sprayed nanocrystalline Zn-doped TiO2 photoelectrode with the dye extracted from Hibiscus Surattensis as sensitizer in dye-sensitized solar cell

Highly oriented Zn doped TiO₂ thin films (0, 2, 4, 6 and 8 at%) were deposited by spray pyrolysis technique. X-ray diffraction analysis showed a strong orientation along (101) direction for 6 at% Zn with polycrystalline tetragonal anatase phase. Scanning electron microscopy observations revealed uniform distribution of spherical-shaped grains, whereas columnar arrangement of tetragonal-shaped grains with porous nature was revealed from atomic force microscopy. Transmittance spectra indicated a decrease in the energy band gap with increasing doping concentration; i.e. 3.55 up to 3.21 eV, attributed to grain refinement to the nanoscale regime. The optical constants such as refractive index and extinction coefficient as a function of wavelength, were determined; the low extinction coefficient values confirmed the good quality of the thin films. Photoluminescence spectra showed strong emissions at 423 and 437 nm with a weak emission at 505 nm, which confirmed the lesser defect density in 6 at% Zn film. The electrical properties studied by Hall Effect measurements revealed that the 6 at% Zn led to an increase in the carrier concentration, as well as an increase in the mobility with a least resistivity. The efficiency of dye sensitized solar cells, assembled by using natural dye extracted from Hibiscus Surattensis as sensitizer and Zn-doped TiO₂ nanocrystalline thin films as a photoelectrode, was found to be around 1.22%.     

Preparation of nanostructure CuO/ZnO mixed oxide by sol–gel thermal decomposition of a CuCO3 and ZnCO3: TG,DTG, XRD,FESEM and DRS investigations

Nanostructure CuO/ZnO mixed oxide was systematically prepared via the sol–gel route using zinc and copper carbonates as precursors (molar ratio of 2:1) under thermal decomposition. The zinc and copper carbonates precursors have been synthesized by a simple chemical reaction in high yield and characterized by its melting point, FT-IR and thermal analysis (TG/DTG). The TG/DTG analysis proved that the thermal decomposition of zinc and copper carbonates precursors at 255 °C and 289 °C respectively. Thermo-gravimetric analysis (TG-DTG), X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and diffuse reflectance spectroscopy (DRS) studies were undertaken to investigate the thermal properties and electronic structure of the CuO/ZnO mixed oxide catalysts. XRD data of the samples proved the formation of the nano-crystalline CuO/ZnO mixed oxide. Scanning electron microscopy (SEM) showed that the spherical-like particles have a diameter in the range 35–45 nm. Optical spectra of the nanostructure show a band peaked at 1.35 eV which is associated to near band gap transitions of CuO and a band centered at about 3.00 eV related to band gap transitions of ZnO nanostructures.     

Photocatalytic decolorization of methyl green using Fe(II)-o-phenanthroline as supported onto zeolite Y

The potential of Fe(II)-orthophenantroline photocatalyst, doped on zeolite Na-Y via complexation process, was studied in decolorization of methyl green. The characterization of samples was studied using XRD, FT-IR, TG/DTG and SEM methods. The best photodecolorization efficiency was obtained at: 1 g L⁻¹ catalyst, 40 ppm MG, pH: 9 using catalyst containing 7 mg per gram catalyst. The degradation process obeyed first-order kinetics. The reusability of the intended catalyst was also investigated. Homogeneous photodecolorization using unsupported complex does not significant role in photodecolorization of the dye, which confirms the importance of zeolitic support to prevent the aggregation of complex particles.     

Photocatalytic degradation of 4-nitrophenol with ZnO supported nano-clinoptilolite zeolite

Photocatalytic degradation of 4-nitrophenol aqueous solution was investigated using ZnO/nano-clinoptilolite zeolite under UV irradiation. Nano-powders of clinoptilolite were prepared using a planetary ball mill mechanically method. The photocatalyst was prepared by ion exchanging of nano-clinoptilolite in a 0.1 M zinc nitrate aqueous solution for 24 h followed by calcination at 450 °C for 12 h. Samples were characterized by XRD, FT-IR, DRS, AAS, TG/DTG, SEM, TEM and BET. Effects of various key operating parameters on the degradation rate were studied. The COD and HPLC tests were also used to confirm the degradation extent of the pollutant.     

Synthesizing vertical porous ZnO nanowires arrays on Si/ITO substrate for enhanced photocatalysis

In this work, porous ZnO nanowires arrays (NWAs) which are perpendicular to the substrate were synthesized using facile and simple hydrothermal process approaches. The surface of ZnO nanowires (NWs) was increased by chemical electroless etching many macropores on them, which results in better photocatalytic activity, which is 1.73 times in terms of the rate constant for methyl orange (MO) degradation for porous ZnO NWAs compared with ZnO NWAs. The absorption of the vertical porous ZnO NWAs was considerably enhanced in the visible region. The optical band-gap became slightly narrower from 3.24 eV (ZnO NWAs) to 3.22 eV (porous ZnO NWAs). Thus, the vertical porous ZnO NWAs can be employed for various applications, such as gas sensor, photocatalyst, and solar cell.     

Growth kinetics and surface properties of single-crystalline aluminum-doped zinc oxide nanowires on silicon substrates

We present here the results from a systematic investigation on the growth kinetics and surface properties of Al-doped ZnO (AZO) nanowires synthesized on (0 0 1)Si substrates under different hydrothermal conditions. The as-synthesized vertical AZO nanowires exhibited a hydrophilic characteristic and their crystal structures were determined to be perfectly single crystalline with the axis of the wire parallel to the [0 0 0 1] direction. TEM and EDS results revealed that the as-synthesized AZO nanowires have tapered tips, and the Al-doped concentration in the AZO nanowires was about 1.6 at%. After a series of SEM examinations, the average length of AZO nanowires synthesized at each temperature studied was found to follow a linear relationship with the reaction time, indicating that the hydrothermal growth of AZO nanowires was a reaction-controlled process. The activation energy for linear growth of AZO nanowires on Si substrate, as obtained from an Arrhenius plot, was found to be about 46 kJ/mol. From UV–vis spectroscopic measurements, it was found that the Si substrate coated with vertically-aligned AZO nanowire arrays exhibited remarkably reduced reflectance (10–12%) over a wide range of visible wavelengths (400–800 nm) and angles of light incidence (8–60°). The good broadband and omnidirectional antireflection characteristics can be attributed to the light trapping effect and the graded refractive index resulting from the tapered AZO nanowire structures.     

Structural,optical and electrical properties of low temperature grown undoped and (Al,Ga) co-doped ZnO thin films by spray pyrolysis

Aluminum and gallium co-doped ZnO (AGZO) thin films were grown by simple, flexible and cost-effective spray pyrolysis method on glass substrates at a temperature of 230 °C. Effects of equal co-doping with aluminum (Al) and gallium (Ga) on structural, optical and electrical properties were investigated by X-ray diffraction (XRD), UV–vis–NIR spectrophotometry and Current–Voltage (I–V) measurements, respectively. XRD patterns showed a successful growth with high quality polycrystalline films on glass substrates. The predominant orientation of the films is (002) at dopant concentrations ≤2 at% and (101) at higher dopant concentrations. Incorporation of Al and Ga to the ZnO crystal structure decreased the crystallite size and increased residual stress of the thin films. All films were highly transparent in the visible region with average transmittance of 80%. Increasing doping concentrations increased the optical band gap, from 3.12 to 3.30 eV. A blue shift of the optical band gap was observed from 400 nm to 380 nm with increase in equal co-doping. Co-doping improved the electrical conductivity of ZnO thin films. It has been found from the electrical measurements that films with dopant concentration of 2 at% have lowest resistivity of 1.621×10⁻⁴ Ω cm.     

Synthesis of the hexagonal pillar shaped ZnOs using a hydrothermal method

This study focused on the stable synthesis to obtaining nanometer sized hexagonal pillar-like ZnO, which can be utilized as LED (light emission device) materials. Various zinc oxides were successfully synthesized by hydrothermal treatment at 150, 200 and 250 °C for 8 h, with their morphologies controlled at pHs 9, 11 and 13, by the addition of ammonium hydroxide. The SEM (scanning electron microscopy) results reveal that the as-prepared particles at pH 13 and 200 °C for 8 h were a complete hexagonal pillar shaped. On the other hand, the morphology was flower shaped at lower pH and temperature. This result indicates that hexagonal pillar shaped ZnO can be easily formed higher pH and temperature reaction conditions. Two types of emitting band for all ZnO samples were observed at around 400 nm (violet) and above 550 nm (green) in the photoluminescence (PL) spectra.     

Synthesis of Na-doped ZnO nanowires and their antibacterial properties

Na-doped ZnO nanowires with an average diameter of ~ 40 nm have been fabricated by a thermal decomposition route at temperatures around 400 °C. Their properties have been investigated using X-ray diffraction, field emission scanning electron microscope, Raman spectra, photoluminescence spectra and X-ray photoelectron spectra. Photoluminescence spectra showed that the as-synthesized ZnO samples exhibited strong visible emission with a major peak at 420 nm. Furthermore, intensity of the visible emission at 420 nm enhanced as the increase of Na concentration. The improvement of visible emission in the Na-doped ZnO samples should be a result of the surface defects increased by doping of Na in ZnO. Their antibacterial activities were also evaluated by determination of minimum inhibitory concentration (MIC) against Escherichia coli 25922 in vitro. Antibacterial tests indicated these nanomaterials showed good antibacterial properties after UV illumination for a short time.     

Synthesis and characterization of ZnO:Ni thin films grown by spray-deposition

In the present study, nickel-doped zinc oxide thin films (ZnO:Ni) at different percentages (0–10%) were deposited on glass substrates by using a chemical spray technique. The effect of Ni concentration on the structural and optical properties of the ZnO:Ni thin films was investigated. The effect of Ni contents on the crystalline structure and optical properties of the films was systematically investigated by X-ray diffraction (XRD), scanning electronic microscopy (SEM), UV–vis, Photoluminescence spectra PL, and Raman spectrometry. The XRD analysis showed that both the undoped and Ni-doped ZnO films were crystallized in the hexagonal structure with a preferred orientation of the crystallites along the [002] direction perpendicular to the substrate. The XRD analysis also showed that the films were well crystallized in würtzite phase with the crystallites preferentially oriented towards (002) direction parallel to the c-axis. SEM study reveals the surface of NiZnO to be made of nanocrystalline particles. The SEM images showed a relatively dense surface structure composed of crystallites in the spherical form whose average size decreases when the [Ni]/[Zn] ratio increases. The optical study showed that all the films were highly transparent. The band gap decreased up to the 7 at% Ni doping level, but the band gap increased after 10 at% Ni doping level. All thin films exhibited approximately 80% and above transmittance in the visible region. PL spectra of undoped and Ni-doped ZnO thin films showed some marked peaks at 376, 389, 494, and 515 nm. The obtained results revealed that the structures and optical properties of the films were greatly affected by doping levels. These films are useful as conducting layers in electro chromic and photovoltaic devices. Finally, all results were discussed in terms of the nickel doping concentration.