Synthesis of ZnO/Cu2S core/shell nanorods and their enhanced photoelectric performance

In this work, two kinds of ZnO/Cu₂S core/shell nanorods (NRs) have been successfully synthesized from ZnO NRs for photoelectrochemical (PEC) water splitting by a versatile hydrothermal chemical conversion method (H-ZnO/Cu₂S core/shell NRs) and successive ionic layer adsorption and reaction method (S-ZnO/Cu₂S core/shell NRs), respectively. The photoelectrode is composed of a core/shell structure where the core portion is ZnO NRs and the shell portion is Cu₂S nanoparticles sequentially located on the surface. The ZnO NRs array provides a fast electron transport pathway due to its high electron mobility properties. The optical property of both two kinds of core/shell NRs was characterized, and enhanced absorption spectrum was discovered. Our PEC system produced very high photocurrent density and photoconversion efficiency under 1.5 AM irradiation for hydrogen generation. On the basis of a versatile chemical conversion process based on the ion-by-ion growth mechanism, H-ZnO/Cu₂S core/shell NRs exhibit a much higher photocatalytic activity than S-ZnO/Cu₂S core/shell NRs. The photocurrent density and photoconversion efficiency of H-ZnO/Cu₂S core/shell NRs are up to 20.12 mA cm^?2 at 0.85 V versus SCE and 12.81 % at 0.40 V versus SCE, respectively.     

Room temperature fabrication of hollow ZnS and ZnO architectures by a sacrificial template route

Hollow ZnS and ZnO architectures are fabricated by employing Zn(5)(CO(3))(2)(OH)(6) microspheres as the sacrificial template. Zn(5)(CO(3))(2)(OH)(6) microspheres can be effectively converted into the core/shell structured ZnO/ZnS composites (in the Na(2)S solution) and hollow ZnO architectures (in the KOH solution), by a spontaneous ion replacement reaction at room temperature. Removing the core by the KOH treatment of core/shell structured ZnO/ZnS, hollow ZnS spheres with different shell thicknesses can be effectively achieved. The obtained hollow ZnO architectures exhibit unique geometrical shapes, and their walls are composed of nanocrystals, which are connected to each other to form their hemispherical or circular shape. A possible formation process from Zn(5)(CO(3))(2)(OH)(6) microspheres to core/shell structured ZnO/ZnS composites is proposed by arresting a series of intermediate morphologies.     

Photodegradation of Rhodamine B on Sulfur Doped ZnO/TiO2 Nanocomposite Photocatalyst under Visible-light Irradiation

Sulfur doped ZnO/TiO₂ nanocomposite photocatalysts were synthesized by a facile sol‐gel method. The structure and properties of catalysts were characterized by X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), UV‐vis diffusive reflectance spectroscopy (DRS) and N₂ desorption‐adsorption isotherm. The XRD study showed that TiO₂ was anatase phase and there was no obvious difference in crystal composition of various S‐ZnO/TiO₂. The XPS study showed that the Zn element exists as ZnO and S atoms form SO^2?₄. The prepared samples had mesoporosity revealed by N₂ desorption‐adsorption isotherm result. The degradation of Rhodamine B dye under visible light irradiation was chosen as probe reaction to evaluate the photocatalytic activity of the ZnO/TiO₂ nanocomposite. The commercial TiO₂ photocatalyst (Degussa P25) was taken as standard photocatalyst to contrast the prepared different photocatalyst in current work. The improvement of the photocatalytic activity of S‐ZnO/TiO₂ composite photocatalyst can be attributed to the suitable energetic positions between ZnO and TiO₂, the acidity site caused by sulfur doping and the enlargement of the specific area. S‐3.0ZnO/TiO₂ exhibited the highest photocatalytic activity under visible light irradiation after Zn amount was optimized, which was 2.6 times higher than P25.     

Influence of the cathodic polarization on the stability of Ti surface in concentrated KOH solutions

The stability of spontaneous thin layers and thin layers formed upon cathodical polarization of Ti in KOH solutions have been studied by potentiostatic and ellipsometric methods. At open circuit potential (OCP) the strongly adherent films, whose thickness depends on the concentration of the KOH solution, were formed. During the cathodic polarization the transformation of these films to weakly adsorbed precipitated layers on the electrode surface was observed. Comparing the theoretically computed curves with the experimental Ψ vs Δ loci measured ellipsometrically, the complex indices of refraction and the thickness of the generated films, from 3.6 to 60 nm in 1 M KOH and from 36 to 105 nm in 5 M KOH (adherent to the electrode surface), were determined. At OCP the rate of film growth increases with increasing the concentration of KOH solution. Cathodic polarizations change the chemical composition and retard the rate of film growth. Based on the ellipsometric and electrochemical data the chemical compositions of the formed films consisted of TiO₂, Ti₂O₃, TiO₂·H₂O, Ti(OH)₃ and TiOOH·nH₂O.     

Photovoltaic properties of oriented ZnO nanowires arrays decorated with TiO<Subscript>2</Subscript> shell layer for dye-sensitized solar cell application

Almost vertically aligned ZnO nanowires have been grown on Silicon substrates via a simple hydrothermal method. In order to improve the photoelectric conversion efficiency for fabricated dye-sensitized solar cells (DSSCs), an easily-operated immersing method was employed to fabricate a TiO₂/ZnO nanowires array heterojunction, which has advantage of high aspect ratio, low recombination rate and high absorption of visible light. The structure and surface morphology of the samples were characterized by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM), respectively. The photovoltaic properties of TiO₂/ZnO based DSCCs were measured by considering the power efficiency (η), photocurrent density (J_sc), open-circuit voltage (V_oc), and fill factor (FF). An efficiency of 0.559% is achieved for the composite cell, increasing 0.426 and 0.185% for the ZnO nanowires cell and TiO₂ cell, respectively. The short-circuit current and open-circuit voltage are also enhancing. The improvements are because of high surface are of TiO₂ shell layer, as well as fast electron transport and light scattering effect of ZnO nanowires.     

TiO2@CdS core–shell nanorods films: Fabrication and dramatically enhanced photoelectrochemical properties

In this paper, we prepared TiO₂@CdS core–shell nanorods films electrodes using a simple and low-cost chemical bath deposition method. The core–shell nanorods films electrodes were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and UV–vis spectrometry techniques. After applying these TiO₂@CdS core–shell nanorods electrodes in photovoltaic cells, we found that the photocurrent was dramatically enhanced, comparing with those of bare TiO₂ nanorods and CdS films electrodes. Moreover, TiO₂@CdS core–shell nanorods film electrode showed better cell performance than CdS nanoparticles deposited TiO₂ nanoparticles (P25) film electrode. A photocurrent of 1.31 mA/cm², a fill factor of 0.43, an open circuit photovoltage of 0.44 V, and a conversion efficiency of 0.8% were obtained under an illumination of 32 mW/cm², when the CdS nanoparticles deposited on TiO₂ nanorods film for about 20 min. The maximum quantum efficiency of 5.0% was obtained at an incident wavelength of 500 nm. We believe that TiO₂@CdS core–shell heterostructured nanorods are excellent candidates for studying some fundamental aspects on charge separation and transfer in the fields of photovoltaic cells and photocatalysis.     

ZnO纳米薄膜的制备及其气敏性质研究

纳米氧化锌薄膜具有光电、压电、压敏和气敏等多种性质,使其在透明导体、发光元件、太阳能电池窗口、光波导器、单色场发射显示器、高频压电转换器、微传感器等方面具有广泛的用途.     

Preparation and properties of amorphous titania-coated zinc oxide nanoparticles

Amorphous TiO₂-coated ZnO nanoparticles were prepared by the solvothermal synthesis of ZnO nanoparticles in ethanol and the followed by sol-gel coating of TiO₂ nanolayer. The analyses of X-ray diffraction (XRD) and transmission electron microscopy (TEM) revealed that the resultant ZnO nanoparticles were hexagonal with a wurtzite structure and a mean diameter of about 60 nm. Also, after TiO₂ coating, the TEM images clearly indicated the darker ZnO nanoparticles being surrounded by the lighter amorphous TiO₂ layers. The zeta potential analysis revealed the pH dependence of zeta potentials for ZnO nanoparticles shifted completely to that for TiO₂ nanoparticles after TiO₂ coating, confirming the formation of core-shell structure and suggesting the coating of TiO₂ was achieved via the adhesion of the hydrolyzed species Ti-O⁻ to the positively charged surface of ZnO nanoparticles. Furthermore, the analyses of Fourier transform infrared (FTIR) and Raman spectra were also conducted to confirm that amorphous TiO₂ were indeed coated on the surface of ZnO nanoparticles. In addition, the analyses of ultraviolet-visible (UV-VIS) and photoluminescence (PL) spectra revealed that the absorbance of amorphous TiO₂-coated ZnO nanoparticles at 375 nm gradually decreased with an increase in the Ti/Zn molar ratio and the time for TiO₂ coating, and the emission intensity of ZnO cores could be significantly enhanced by the amorphous TiO₂ shell.     

Turning Redundant Ligands into Treasure: A New Strategy for Constructing MIL‐53(Al)@Nanoscale TiO2 Layers

A strategy for in situ fabrication of nanoscale‐thin layers of anatase TiO₂ coated on the metal–organic framework (MOF) material, MIL‐53(Al), is developed. The preparation conditions for crystallized TiO₂ are normally incompatible with the thermal and chemical stability of MOFs. Based on our strategy, we found that the redundant organic ligands (1,4‐benzenedicarboxylic acid, H₂BDC) within the pores of the as‐synthesized MOF play a key function in the protection and support of the framework during hydrothermal loading of the TiO₂ precursor, as well as in preventing the infiltration of the precursor into the pores. After annealing, a nanoscale‐thin layer of highly crystalline anatase TiO₂, with a thickness of 6–10 nm, was successfully attached to the external surface of the MIL‐53(Al) crystals, while the porous framework remains intact. The core–shell structure of the MOF@TiO₂ nanocomposite endows the resulting materials with additional optical response and enhanced moisture and chemical stability.     

Biochar of Spent Coffee Grounds as Per Se and Impregnated with TiO2: Promising Waste-Derived Adsorbents for Balofloxacin

Biochars (BC) of spent coffee grounds, both pristine (SCBC) and impregnated with titanium oxide (TiO₂@SCBC) were exploited as environmentally friendly and economical sorbents for the fluroquinolone antibiotic balofloxacin (BALX). Surface morphology, functional moieties, and thermal stabilities of both adsorbents were scrutinized using SEM, EDS, TEM, BET, FTIR, Raman, and TG/dT analyses. BET analysis indicated that the impregnation with TiO₂ has increased the surface area (50.54 m²/g) and decreased the pore size and volume. Batch adsorption experiments were completed in lights of the experimental set-up of Plackett-Burman design (PBD). Two responses were maximized; the % removal (%R) and the adsorption capacity (q_e, mg/g) as a function of four variables: pH, adsorbent dosage (AD), BALX concentration ([BALX]), and contact time (CT). %R of 68.34% and 91.78% were accomplished using the pristine and TiO₂@SCBC, respectively. Equilibrium isotherms indicated that Freundlich model was of a perfect fit for adsorption of BALX onto both adsorbents. Maximum adsorption capacity (q_max) of 142.55 mg/g for SCBC and 196.73 mg/g for the TiO₂@SCBC. Kinetics of the adsorption process were best demonstrated using the pseudo-second order (PSO) model. The adsorption-desorption studies showed that both adsorbents could be restored with the adsorption efficiency being conserved up to 66.32% after the fifth cycles.     

可移动金核的二氧化钛空心球合成及光催化性质

Uniform hollow Au@TiO₂ core shell spheres with moveable Au nanoparticles were synthesized based on templating against Au@carbon spheres. The diameter of the shell of the Au@TiO₂ spheres could be controlled by adjusting the Ti(OC₄H₉)₄ hydrolyzing reaction time or the ratio of Ti(OC₄H₉)₄ to Au@carbon spheres, and the shell thickness of the core-shell spheres can be varied from 25 nm to 40 nm. As prepared hollow Au@TiO₂ core-shell spheres display enhanced photocatalytic activity in the initial stage of photocatalytic degradation of methylene blue compared with pure hollow TiO₂ spheres and the commercial photocatalyst TiO₂(P-25).     

MAA-modified and luminescence properties of ZnO quantum dots

The precursor of ZnO has been prepared by refluxing in ethanol at 70°C for 4 h using ZnAc₂·H₂O as the initial agent. Then ZnO has been prepared from the reaction of LiOH·H₂O and the precursor. The ZnO is modified by mercaptoacetic acid (MAA) and detected by SEM and XRD. These ZnO particles have single phase, like-sphere and size of 4.6 nm. The modified effectivity of MAA for the quantum dots (QDs) has been investigated with UV-visible and fluorescence analysis, and the mechanism and property of ZnO light emitting have been discussed under given conditions. The reasons why the fluorescent emission peak of surface defects disappear and the exciton emission peak increase are that MAA effectively covers the surface defects of ZnO and stably coats ZnO particles. At the same time, the effect of the added amount of the MAA, temperature and electrolyte on light-emitting properties of modified product has also been studied. The result shows that the modified ZnO QDs have good fluorescence property, stability and suitable capability of resisting electrolyte. These results are important for biological analysis.     

Ammonia Gas Sensor Using Polypyrrole‐Coated TiO2/ZnO Nanofibers

Highly porous polypyrrole (PPy)‐coated TiO₂/ZnO nanofibrous mat has been successfully synthesized. The core TiO₂/ZnO nanofibers have an average diameter of ca. 100 nm and the shell of ultrathin PPy layer has a thickness of ca. 7 nm. The NH₃ gas sensor using the as‐prepared material exhibited a fast response over a wide dynamic range and high sensitivity with a detection limit of 60 ppb (S/N=3). Compared to conventional pristine PPy film, the improved performance in NH₃ detection can be attributed to the free access of NH₃ to PPy and a minimized gas diffusion resistance through the ultrathin PPy layer.     

Magnetically Recoverable Core–Shell Nanocomposites with Enhanced Photocatalytic Activity

Core–shell structured Fe₃O₄/SiO₂/TiO₂ nanocomposites with enhanced photocatalytic activity that are capable of fast magnetic separation have been successfully synthesized by combining two steps of a sol–gel process with calcination. The as‐obtained core–shell structure is composed of a central magnetite core with a strong response to external fields, an interlayer of SiO₂, and an outer layer of TiO₂ nanocrystals with a tunable average size. The convenient control over the size and crystallinity of the TiO₂ nanocatalysts makes it possible to achieve higher photocatalytic efficiency than that of commercial photocatalyst Degussa P25. The photocatalytic activity increases as the thickness of the TiO₂ nanocrystal shell decreases. The presence of SiO₂ interlayer helps to enhance the photocatalytic efficiency of the TiO₂ nanocrystal shell as well as the chemical and thermal stability of Fe₃O₄ core. In addition, the TiO₂ nanocrystals strongly adhere to the magnetic supports through covalent bonds. We demonstrate that this photocatalyst can be easily recycled by applying an external magnetic field while maintaining their photocatalytic activity during at least eighteen cycles of use.     

New insights into the ZnO/a-SiC:H(B) interface using XPS analysis

A poor electric contact between the ZnO and the p-type a-SiC:H(B) layer limits the fill factor of amorphous silicon solar cells using ZnO as front electrode. To gain a deeper understanding of the chemical and electronic properties of the ZnO/p interface, in-situ XPS measurements were applied to thin a-SiC:H(B) films deposited on ZnO. The effects of H₂ plasma pretreatments on clean ZnO surfaces and the influence of deposition conditions on the ZnO/a-SiC:H interface were investigated. Upon H₂ plasma treatment, a shift of all core levels towards higher binding energies indicates the formation of an accumulation layer. Depth profiling across the ZnO/a-SiC:H interface shows SiO₂ formation on ZnO. The depth profile of the ZnO related core levels exhibits two features: a reduction of the ZnO at the interface and a hydrogen induced accumulation layer in the n-type ZnO. The latter causes a depletion of the p-layer resulting in an enhanced series resistance and diminished fill factor.     

Reactivity of nanoparticles; interaction between zinc and copper oxide nanoparticles and iron ions in an alkaline medium

The reactivity of zinc and copper oxide nanoparticles was investigated upon their interaction with iron oxides. It was ascertained that, depending on the reaction conditions, nanoparticles of zinc and copper ferrites (ZnFe₂O₄ and CuFe₂O₄) or core/shell nanoparticles (Fe₃O₄/ZnO) are produced. Size, composition, and structure of the resulting nanoparticles were determined by transmission electron microscopy and X-ray diffraction analysis. The average size of zinc and copper ferrite nanoparticles was ascertained to be 9–10 and 2–3 nm, respectively. For core/shell Fe₃O₄/ZnO nanoparticles, the average size is 20 nm. It was experimentally proved that the photoluminescence radiative characteristics of ZnO nanoparticles are retained in core/shell Fe₃O₄/ZnO nanoparticles.     

Effect of Cl− anions on photocatalytic decomposition of gaseous ozone over Au @ Ag/TiO2 catalyst

Au core Ag shell composite structure nanoparticles were prepared using a sol method. The Au core Ag shell composite nanoparticles were loaded on TiO₂ nanoparticles as support using a modified powder–sol method, enabling the generation of Au @ Ag/TiO₂ photocatalysts for photocatalytic decomposition and elimination of ozone. The sols were characterized by means of ultraviolet–visible light (UV–Vis) reflection spectrometry, X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The activity of the Au @ Ag/TiO₂ photocatalysts for photocatalytic decomposition and elimination of ozone was evaluated and the effect of Cl^? anions on the photocatalytic activity of the catalysts was highlighted. Results showed that Au @ Ag/TiO₂ prepared via the modified powder–sol route in the presence of an appropriate amount of NaCl solid as demulsifier had better activity in the photocatalytic decomposition and elimination of ozone. At the same time, Au @ Ag/TiO₂ catalysts had better ability to resist poisonous Cl^? anions than conventional Au/TiO₂ catalyst. The reasons could be, first, that NaCl was capable of reducing the concentration of free Ag⁺ by adsorption on the surface of Ag particles forming AgCl and enhancing the formation of Au core Ag shell particles, leading to a better resistance to Cl^? anions of the catalysts, and, second, AgCl took part in the photocatalytic decomposition of ozone together with Au @ Ag/TiO₂ catalysts and had a synergistic effect on the latter, resulting in better photocatalytic activity of Au @ Ag/TiO₂ catalysts.     

Highly Efficient Low‐Temperature Plasma‐Assisted Modification of TiO2 Nanosheets with Exposed {001} Facets for Enhanced Visible‐Light Photocatalytic Activity

Anatase TiO₂ nanosheets with exposed {001} facets have been controllably modified under non‐thermal dielectric barrier discharge (DBD) plasma with various working gas, including Ar, H₂, and NH₃. The obtained TiO₂ nanosheets possess a unique crystalline core/amorphous shell structure (TiO₂@TiO_2?x), which exhibit the improved visible and near‐infrared light absorption. The types of dopants (oxygen vacancy/surface Ti³⁺/substituted N) in oxygen‐deficient TiO₂ can be tuned by controlling the working gases during plasma discharge. Both surface Ti³⁺ and substituted N were doped into the lattice of TiO₂ through NH₃ plasma discharge, whereas the oxygen vacancy or Ti³⁺ (along with the oxygen vacancy) was obtained after Ar or H₂ plasma treatment. The TiO₂@TiO_2?x from NH₃ plasma with a green color shows the highest photocatalytic activity under visible‐light irradiation compared with the products from Ar plasma or H₂ plasma due to the synergistic effect of reduction and simultaneous nitridation in the NH₃ plasma.     

Use of catalase and superoxide dismutase to assess the roles of hydrogen peroxide and superoxide in the TiO2 or ZnO photocatalytic destruction of 1,2-dimethoxybenzene in water

The effects of two antioxidant enzymes on the rate of disappearance, r, of the pollutant 1,2-dimethoxybenzene (1,2-DMB) in UV-irradiated (λ > 340 nm) TiO₂ or ZnO aqueous suspensions have been determined. Catalase, which catalyzes the overall reaction 2H₂O₂ →> 2H₂O + O₂, caused a relatively moderate decrease in r in the case of TiO₂ and no effect in the case of ZnO, showing that hydrogen peroxide formed in situ is not essential for the pollutant removal. The effect of H₂O₂ added to ZnO was negative and in the case of TiO₂ was either favorable or unfavorable depending on the initial ratio [H₂O₂]/[1,2-DMB] as a result of competition between these compounds for the adsorption sites and/or the photoproduced holes, the formation of additional OH^? radicals and the detrimental modification of the TiO₂ surface. Favorable and unfavorable effects of added H₂O₂ were suppressed by catalase. The detrimental effect on r of superoxide dismutase (SOD), which catalyzes the overall reaction 2O₂ ^?? + 2H⁺ →O₂ + H₂O₂, was very important for both TiO₂ and ZnO. It is inferred that it stems from the catalytic action of SOD and not from competitive photocatalytic destruction of 1,2-DMB and SOD or from the formation of H₂O₂. Therefore, these results point to the essential role of the O₂ ^?? radical-anion as an active species in the photocatalytic degradation of the pollutant and this role is tentatively discussed, in particular with respect to the formation of the 1,2-DMB^?+ radical-cation.     

Synthesis of Multifunctional Metal‐ and Metal Oxide Core@Mesoporous Silica Shell Structures by Using a Wet Chemical Approach

We demonstrate a facile wet chemical approach for fabricating spherical metal/metal‐oxide core@mesoporous silica shell hybrid nanoparticles with different core and shell thicknesses. Vertically aligned mesoporous silica (mSiO₂) shells were fabricated over the pre‐synthesized spherical SiO₂ nanoparticles through a three‐step strategy: 1) synthesis of core materials, 2) covering the core with an organic–inorganic composite layer, and 3) removing the organic template through calcinations in air. The mechanisms of hybrid structure formation are proposed. The multifunctional nature of the hybrid structures could be induced by incorporating guest ions/molecules, such as Ag, Mn, and TiO₂, into the pores of an mSiO₂ shell. Mn and TiO₂ cluster‐ incorporated composite structures have been tested to be antioxidizing agents and effective photocatalysts through electron spin resonance, radical scavenging tests, and the photocatalytic degradation of rhodamine B. The possibility of incorporating several hetero‐element guest clusters in these mesoporous composite particles makes them highly attractive for multifunctional applications.