S-doped ZnO Research Articles

New insight into the enhanced photocatalytic activity of N-, C- and S-doped ZnO photocatalysts

In general, N-, C- and S-doped ZnO exhibit much higher phototcatalytic activity than the pure ZnO. However, the essential factors and underlying mechanism regarding the enhancement of photocatalytic activity are still unclear. In this work, the electronic structures, optical properties and effective masses of charge carriers are investigated by first-principle density functional theory calculation. Due to the nature of p-type doping, N and C doping can generate vacant states above the Fermi level and shift the conduction band into lower energy region, resulting in narrowing of band gap. Thus, N- and C-doped ZnO demonstrate much stronger light absorption in both visible and ultraviolet region. In contrast, because of the absence of vacant states, only limited enhancement of light absorption is observed for S-doped ZnO whose improved photocatalytic performance can only be attributed to the direct reduction of band gap. The calculation of the effective masses show that ZnO typically possess light electrons and heavy holes, confirming its intrinsic character of n-type semiconductor, while N, C and S doping can generally render electrons lighter and holes heavier, resulting in slower recombination rate of photogenerated electron–hole pairs. Noticeably, C doping can discourage such recombination to the greatest extent and separate electron–hole pairs most efficiently compared with N and S doping, serving as a potentially promising pathway to increase the quantum efficiency of ZnO-based photocatalysts. This work will provide some new insights into the understanding of doping effect over the enhancement of photocatalytic activity of N-, C- and S-doped ZnO.

Synthesis of S-doped ZnO by the interaction of sulfur with zinc salt in PEG200

S-doped ZnO nanoparticles (S-ZnO) were synthesized via a one-step and green method. In this method, zinc acetate dihydrate was used as a precursor and sulfur was the dopant. The reaction between zinc salt and S occurred in PEG200 media. X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, UV–Vis spectroscopy and room temperature photoluminescence were used to characterize the products. The results show that the molar ratio of Zn(II) and S determines the composition, structure, surface morphology, and luminescence properties of the products greatly. When the number of moles of sulfur (nS) is smaller than that of zinc salt (nZn), the products are S-ZnO with diameters of 40–55nm and they have ultraviolet absorption peaks at 363nm. The incorporation of S into ZnO is supported by broadening and lower Bragg angle shift in XRD pattern. FTIR spectra show that PEG200 adsorbs onto the surfaces of the S-ZnO as a capping agent. PL spectra show that the effective sulfur doping enhances the green emission and suppresses the near band gap emission. The possible reaction and growth mechanism is also developed.

Electronic structures and energy band properties of Be- and S-doped wurtzite ZnO

The energy band properties, density of states, and band alignment of the BexZn1−xO1−ySy alloy (Be- and S-doped wurtzite ZnO) are investigated by the first-principles method. BexZn1−xO1−ySy alloy is a direct band gap semiconductor, the valence band maximum (VBM) and the conduction band minimum (CBM) of BexZn1−xO1−ySy are dominated by S 3p and Zn 4s states, respectively. The band gap and lattice constant of BexZn1−xO1−ySy alloy can be modulated by changing the doped content values x and y. With the increase in Be content value x in the BexZn1−xO1−ySy alloy, the band gap increases and the lattice constant reduces, but the situation is just the opposite when increasing the S content value y in the BexZn1−xO1−ySy alloy. Because the lattice constant of Be0.375Zn0.625O0.75S0.25 alloy is well matched with that of ZnO and its energy gap is large compared with that of ZnO, so the Be0.375Zn0.625O0.75S0.25 alloy is suitable for serving as the blocking material for a high-quality ZnO-based device.

S-doped ZnO nanorods on stainless-steel wire mesh as immobilized hierarchical photocatalysts for photocatalytic H2 production

S-doped ZnO nanorods were grown on stainless steel mesh as immobilized hierarchical photocatalysts for hydrogen production. Properties of the photocatalysts were investigated by field-emission scanning electron microscope (FESEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), photoinduced current, and photocatalytic hydrogen evolution test. Effects of polymer additive and doping on the surface texture, surface property, and H2 production performance of the photocatalysts were studied. Polyethyleneimine helps the growth of nanorods on the entire surface of wire mesh. Photocatalytic H2 production activity of the photocatalysts changes with dopant content and surface texture modification. Due to increased surface area of the hierarchical photocatalyst, enhanced light trapping and liquid flow among wire-mesh, the highest hydrogen evolution rate of 3640 μmol g−1 h−1 is obtained. The photocatalytic activity of photocatalyst remained at 87% of its original performance after five cycles.

A green process for efficient lignin (biomass) degradation and hydrogen production via water splitting using nanostructured C, N, S-doped ZnO under solar light

Simultaneous photocatalytic hydrogen production (water splitting) and waste lignin (biomass) degradation under visible light has been demonstrated using C, N, S-doped ZnO/ZnS.

S掺杂纤锌矿ZnO的光催化性质的第一性原理研究

S掺杂纤锌矿ZnO的光催化性质的第一性原理研究

Recombination-tunneling conduction in Cu- and S-doped ZnO nanorods’ core–shell junction: dependence of diode parameters on thermal annealing temperature and role of interfacial defects

We have investigated the effect of thermal annealing on the diode characteristics of solution-processed novel core–shell coaxial n–p nanorods heterojunction consisting of n-type ZnO as a core and p-type CuS as a shell material. The values of turn-on-voltage, rectification ratio, reverse saturation current density, barrier height, and ideality factors have been improved as the as-prepared heterojunction is annealed at higher temperatures owing to the improvement in the interface between ZnO and CuS as evidenced from the high-resolution transmission electron microscopy. The X-ray diffraction results also confirm the improvement in the crystalline quality of ZnO and CuS through annealing. The experimental current–voltage data are consistent with the presence of dominating recombination-tunneling conduction occurring through the interface defect states between ZnO and CuS. The results demonstrate that an annealing process plays a dominant role in the interfacial defects which helps to modify the diode performance paving a way to cheaper electronic nanodevices.

Fabrication and properties of sulfur (S)-doped ZnO nanorods

Highly-aligned sulfur (S)-doped ZnO nanorods have been grown using the hydrothermal approach at 90 °C for 2 h onto quartz substrate pre-coated with ZnO seed layer deposited by radio frequency magnetron sputtering system. The morphology, crystal structure, and transmittance of the S-doped ZnO nanorods grown with varied sulfur concentration have been investigated. The scanning electron microscope images showed that the S-doped ZnO nanorods dimension is affected by sulfur doping. The nanorods doped with sulfur concentration of ~1, 1.5, and 2 at.% found to show nanorods with an average diameter of ~130, 170, and 270 nm respectively. X-ray diffraction measurements revealed that the sulfur-doped ZnO nanorods have hexagonal-wurtzite crystal structure and grown vertically in the (002) plane along c-axis perpendicular to the substrate. The nanorods doped with 1 at.% sulfur showed ~70 % transmittance in the visible region while the nanorods doped with 2 at.% sulfur showed transmittance of ~77 % and exhibited blue shift in the fundamental absorption edge.

Hybrid functional study of structural, electronic and magnetic properties of S-doped ZnO with and without neutral vacancy

Abstract The structural and electronic properties of S-doped ZnO are investigated by density functional theory (DFT) and empirical pseudopotential method (EPM). Using the Heyd–Scuseria–Ernzerhof (HSE) hybrid functional with an adjusted mixing coefficient α , we obtain a good agreement on lattice parameters and band gap energy with the available experimental data. We have also investigate the Zn-vacancy effects on the electronic and magnetic properties of S-doped ZnO. Our calculations demonstrate that S impurity prefers to be close to the cation vacancy in the apical position. The magnetic analysis with the HSE functional shows a triplet state character with a total magnetic moment of 1.81 μ B , which is mainly arises from the p -orbitals of the atoms around the Zn-vacancy (15% from S, 12% from Zn and 73% from O-atoms). The substitution of S by an isovalent atom decreases the total magnetic moments of the system and weakens the local triplet state without destroying it.

Spatial-resolved cathode luminescence study of S-doped ZnO particles for the luminescence of UV, green and orange band emission

The hexagonal bi-pillar structures of undoped and sulfur-doped ZnO particles have been synthesized by hydrothermal method to study the relationship between UV and long wavelength emissions, green emission (GE) and orange emission (OE) bands. A room temperature photoluminescence comparison between undoped and sulfur-doped ZnO shows an increase of long wavelength emission, that is presumably due to an increase of defects in doped ZnO. Spatial cathode luminescence (CL) analysis on a single ZnO particle shows a competitive behavior between GE and OE bands. The long wavelength emissions in ZnO are all related to Zn vacancies. GE will dominate if the Zn vacancy-related oxygen stays at lattice point; otherwise OE will dominate if this oxygen atom leaves lattice point to form a interstitial atom.

First-principles study of dopants and defects in S-doped ZnO and its effect on photocatalytic activity

Electronic structure, optical properties and photocatalytic activity of S-doped ZnO are investigated by density functional theory (DFT). The configurations with the substitution of O by one and two S atoms in different positions (SO, far-2SO and near-2SO) are considered, and zinc and oxygen vacancies introduced by SO doping (SO–VZn and SO–VO) are also considered. For SO-doped ZnO, S3p states locate above the top of the valence band and mix with O2p states, leading to band gap narrowing, but the influence is slight with the increase of doping concentration. The imaginary parts of dielectric functions and absorption coefficients display that S atoms doping is not the critical factor for improving the photocatalytic activity. The absorption coefficient of introduced native defect (VO and VZn) shows different features in low energy range. The existence of VO in SO-doped ZnO leads to a strong absorption in UV-light range and VZn plays a critical role in visible-light absorption, which may improve the photocatalytic activity of ZnO in the UV-visible light range, corporately.

Prediction of band gap reduction and magnetism in (Cu, S)-codoped ZnO

By employing a density functional theory plane-wave pseudopotential method, we investigated band gap reduction and magnetism as well as electronic structures of (Cu, S)-codoped ZnO. Our calculations indicated that Cu and/or S-doped ZnO can reduce the band gap of ZnO. The (Cu, S)-codoped ZnO has a large band gap reduction of 0.37eV, two times larger than that in Cu-doped ZnO. S atom has no contribution for the total magnetic moment of (Cu, S)-codoped ZnO, whereas it plays a central role in spin-polarizing of both Cu and S dopants due to strong coupling between Cu 3d and S 3p states. This would offer a new strategy for designing narrow band gap devices with magnetism.

Structural and optical properties of the S-doped ZnO particles synthesized by hydrothermal method

Sulfur -doped ZnO particles have been synthesized by hydrothermal method. The structural and optical properties were studied systematically by XRD, scanning electron microscopy (SEM), and photoluminescence. SEM results show that the particle is hexagonal and the average size decreases with increasing sulfur doping, which means a retardant effect of sulfur on the growth of S-doped ZnO. XRD results show that the lattice parameters increase with more sulfur, which means an effective sulfur doping and increasing strain. Optical characterization also shows that the effective sulfur doping will enhance the green emission and suppress the near bandgap emissions.

Ecofriendly synthesis and solar photocatalytic activity of S-doped ZnO

The S-doped ZnO was prepared by new ecofriendly method, which involves simple mechanochemical synthesis followed by thermal decomposition of bisthiourea zinc oxalate (BTZO) powders. The BTZO was characterized by FTIR and TG–DTA analysis while S-doped ZnO crystallite was characterized by XRD, XPS, SEM, EDXS, and photoluminescence (PL) spectra. X-ray diffraction data suggest the single phase wurtzite structure for S-doped ZnO and the incorporation of sulfur expand the lattice constants of ZnO. Room temperature PL spectra show more number of oxygen vacancies in S-doped ZnO as compare to that of pure ZnO. Photocatalytic activity of S-doped ZnO was checked by means of solar photocatalytic degradation (PCD) of resorcinol, using a batch photoreactor. The PCD efficiency of S-doped ZnO was found to be 2 times greater than that of pure ZnO. The inherent relationship between PL intensity and photocatalytic activity of S-doped ZnO was discussed.

The effects of annealing temperature on structural and optical properties of S-doped ZnO nanobelts

In this study, the effects of thermal annealing temperature on the structural and optical properties of S-doped ZnO nanobelts were investigated. The XRD pattern shows that the crystallinity of S-doped ZnO nanobelts improves with increased annealing temperature. Room temperature photoluminescence spectroscopy of the as-grown S-doped ZnO nanobelts shows no detectable ultraviolet peak with the broad peaks in the visible emission region at 480, 505, and 518 nm. A weak peak in the ultraviolet region at 383 nm appears after annealing at 400 and 600 °C. Raman spectroscopy of the sample also shows a significant change with an increase in the annealing temperature.

Effect of S- and Sn-doping to the optical properties of ZnO nanobelts

In this study, the optical properties of S- and Sn-doped ZnO nanobelts, grown by thermal evaporation, were investigated. The sulfur and tin contents in the nanobelts were about 12% and 8% (atomic), respectively. The average widths of the S- and Sn-doped ZnO nanobelts were 73 and 121 nm, respectively. Room temperature photoluminescence (PL) spectroscopy exhibits significantly different optical properties for the two types of nanobelts. The PL result of the S-doped ZnO nanobelts shows the broad visible emission with no detectable ultraviolet (UV) peak, while the PL result of the Sn-doped sample shows two emission bands, one related to UV emission with a strong peak at 376 nm that is blue-shifted by 4 nm in comparison to pure ZnO nanobelts, and another related to green emission with a weak peak. A weak peak in the UV region at 383 nm appeared after annealing the S-doped ZnO nanobelts at 600 °C. Additionally, the annealed S-doped nanobelts show a stronger peak in the visible emission region in comparison to that observed prior to annealing. The Sn-doped ZnO nanobelts are also affected by annealing, as the UV emission peak is blue-shifted to 372 nm after annealing.

Simple solvothermal route to synthesize S-doped ZnO nanonails and ZnS/ZnO core/shell nanorods

A simple solvothermal route was successfully utilized to fabricate S-doped ZnO nanonails with a hexagonal cap followed by a comparatively thin long needle shaped stem portion. Diameters of the hexagonal caps varied within 10–25 nm and lengths of the stem portions varied within 50–80 nm. These nanonails were found to exhibit intense green light emission property at room temperature. Similar synthesis protocol was employed to fabricate ZnS/ZnO core/shell nanorods by suitably tailoring the concentration of the sulfur source in the synthesis environment.

Structure and photoluminescence of S-doped ZnO nanorod arrays

Vertically aligned S-doped ZnO nanorod arrays have been successfully synthesized by hydrothermal method at 90 °C for 2 h. The obtained nanorod is ∼ 70 nm in diameter and 1.2 μm in length. The XRD pattern and the Raman spectra indicate that the S-doped nanorod arrays are orientated at [001] and are single crystals with hexagonal wurtzite structure. The photoluminescence (PL) spectra show that S-doped ZnO nanorod arrays exhibit a relative weak ultraviolet (UV) emission, a violet emission and a strong green emission. The effects of S-doping on the structure and photoluminescence of ZnO nanorod arrays are discussed in detail.

Synthesis and optical properties of heterostructured ZnO:S/ZnO nanosaws

Heterostructured ZnO : S/ZnO (S-doped ZnO ribbon with ZnO nanoteeth) nanosaws were synthesized by a one-step catalyst-free process through the chemical vapour transport and condensation method. Optimum selectivity and maximum nanosaw densities are obtained for growth temperatures in the range 400–450 °C. Our study suggests that the ribbons were ZnO : S while the teeth were only ZnO. Nanorods, nanoairplane, nanobelts and smooth surface nanotip-like structures were also obtained in our experiment at different growth temperatures. The photoluminescence spectrum of the nanosaws showed a strong visible emission band compared with undoped ZnO powder at room temperature. This strong visible emission in the hetrostructured nanosaws might be useful as a future UV-excited phosphor for producing bright and broad visible-wavelength light.

Synthesis and optical properties of partially S-doped ZnO symmetric three-sidedfeather-like nanostructures

Partially S-doped ZnO symmetric three-sided feather-like nanostructures havebeen synthesized via a thermal evaporation and transport method. The productswere grown in a one-step catalyst-free process. The synthesized nanostructures ofboth samples have nearly the same morphology although these contain differentconcentrations of sulfur (1.55 and 10.48 at.%). Our study suggests that the stemswere S-doped while the teeth were not. These synthesized nanostructures weresingle-crystalline wurtzite structures. Room temperature photoluminescence (PL)spectra of the synthesized products showed three PL peaks in the ultraviolet,blue and green emission regions. The peaks were shifted towards high energyby sulfur doping. These symmetric three-sided nanostructures may be used inthree-dimensional nanolasers in optoelectronics and nanorotors in nanomechanical systems.