Retraction of ‘Synthesis and facet-dependent photocatalytic activity of BiOBr single-crystalline nanosheets’ by Wenwen Lin et al., Chem. Commun., 2014, DOI: 10.1039/c3cc41498a.

A facile and novel method has been developed to prepare ultrathin PbI2 single crystals with good mechanical flexibility and bendability for high sensitivity photodetectors. The prototype lateral metal–semiconductor–metal photodetector based on the as-prepared crystal exhibits a responsivity of up to 11.3 A W−1, a photogain of 31.84, and a high-speed photoresponse with a time constant of 354 μs.

Zinc oxide, a wide band-gap semiconductor, has shown extensive potential applications in high-efficiency semiconductor photoelectronic devices, semiconductor photocatalysis, and diluted magnetic semiconductors. Due to the undisputed lattice integrity, ZnO single crystals are essential for the fabrication of high-quality ZnO-based photoelectronic devices, and also believed to be ideal research subjects for understanding the underlying mechanisms of semiconductor photocatalysis and diluted magnetic semiconductors. This review, which is organized in two main parts, introduces the recent progress in growth, basic characterization, and device development of ZnO single crystals, and some related works in our group. The first part begins from the growth of ZnO single crystal, and summarizes the fundamental and applied investigations based on ZnO single crystals. These works are composed of the fabrication of homoepitaxial ZnO-based photoelectronic devices, the research on the photocatalysis mechanism, and dilute magnetic mechanism. The second part describes the fabrication of highly thermostable n-type ZnO with high mobility and high electron concentration through intentional doping. More importantly, in this part, a conceptual approach for fabricating highly thermostable p-type ZnO materials with high mobility through an integrated three-step treatment is proposed on the basis of the preliminary research.

Due to the large refractive index of ZnO material, light extraction efficiency (LEE) of ZnO based light-emitting diode (LED) is limited by the total reflection. We present a combined experimental and computational approach to study the effects of integrated surface textures on the efficiency of light extraction from the ZnO material. Hexagonal pyramids and pits textures were obtained by selective and anisotropic etching of O-polarity ZnO single crystal in HCl and KOH solutions, respectively. Dense hexagonal pyramids were integrated into the hexagonal pits via a two-step wet etching process. Ray-tracing simulations indicate that the substrate texturing can improve the performance of the ZnO based LEDs by enhancing the LEE and improving the light field uniformity. The integrated photoluminescence extraction was improved by a factor of 117% with respect to that of a planar ZnO single crystal through integrating the pyramids textures into the hexagonal pits.

Au-nanoparticle-decorated ZnS nanoarchitectures were fabricated by a simple hydrothermal approach combined with a deposition-precipitation method. After the deposition-precipitation process, 5-nm Au nanoparticles were homogeneously dispersed on the ZnS surface. In addition, the band gap of ZnS was also narrowed by the incorporation of a small amount of Au(I) ions. The photocatalytic hydrogen production activities of all the samples were evaluated by using Na2S and Na2SO3 as sacrificial reagents in water under a 350 W xenon arc lamp. The results show that the photocatalytic hydrogen production rate of ZnS nanoarchitectures can be significantly improved by loading Au cocatalysts and reaches an optimal value (3306 μmol h–1 g–1) at the Au content of 4% wt. Although strong surface plasmon resonance (SPR) absorption of the Au nanoparticles was found in the Au-loaded samples, all of these samples exhibit no activities in the visible light region (λ > 420 nm). On the basis of this Au/ZnS system, the possible roles of Au deposition in improving the photocatalytic hydrogen production activity, especially the necessary condition for SPR effect of metal nanostructures to function in the visible-light photocatalysis, are critically discussed.Keywords: charge transfer; gold; hydrogen production; photocatalysis; surface plasmon resonance; ZnS nanoarchitecture;

CdS/g-C3N4 core/shell nanowires with different g-C3N4 contents were fabricated by a combined solvothermal and chemisorption method and characterized by X-ray powder diffraction, scanning electronic microscopy, transmission electron microscopy, and UV–vis diffuse reflection spectroscopy. The photocatalytic hydrogen-production activities of these samples were evaluated using Na2S and Na2SO3 as sacrificial reagents in water under visible-light illumination (λ ≥ 420 nm). The results show that after a spontaneous adsorption process g-C3N4 is successfully coated on CdS nanowires with intimate contact and can significantly improve the photocatalytic hydrogen-production rate of CdS nanowires, which reaches an optimal value of up to 4152 μmol h–1 g–1 at the g-C3N4 content of 2 wt %. More importantly, g-C3N4 coating can substantially reinforce the photostability of CdS nanowires even in a nonsacrificial system. The synergic effect between g-C3N4 and CdS, which can effectively accelerate the charge separation and transfer corrosive holes from CdS to robust C3N4, was proposed to be responsible for the enhancement of the photocatalytic activity and photostability. The possible conditions necessary for the synergic effect to work in a CdS/g-C3N4 core/shell configuration is also discussed.Keywords: carbon nitride; CdS nanowire; charge transfer; hydrogen production; photocatalysis; solar fuels;

Ni(OH)2-modified graphitic carbon nitride (Ni(OH)2–g-C3N4) composite photocatalysts were prepared by a simple precipitation method using g-C3N4 as support and nickel nitrate as precursor. The effect of Ni(OH)2 content on the rate of visible-light photocatalytic H2-production was studied for a series of Ni(OH)2–g-C3N4 composite samples in triethanolamine aqueous solutions. The results demonstrated that Ni(OH)2 was an efficient co-catalyst for the photocatalytic H2 production of g-C3N4. The optimal Ni(OH)2 loading was found to be 0.5 mol%, giving a H2-production rate of 7.6 μmol h−1 (with an apparent quantum efficiency (QE) of 1.1% at 420 nm), which approached that of optimal 1.0 wt% Pt/g-C3N4 (8.2 μmol h−1). This work showed the possibility for the utilization of low cost Ni(OH)2 as a substitute for noble metals (such as Pt) in the photocatalytic H2 production for g-C3N4.

Mg0.50Zn0.50O thin films were grown on MgO, sapphire and quartz substrates by a home-made magnetron sputtering system under the same deposition conditions. The Mg compositions in all these MgxZn1−xO films are around 50%, independent of the growth substrates. X-ray diffraction measurements reveal that the lattice structures of these Mg0.50Zn0.50O films depend on the lattice structures of growth substrates and are cubic (C), wurtzite (W) and mixed phase, respectively. Transmission spectra show that the cutoff wavelength of C-Mg0.50Zn0.50O is located in the UV-C (200–280 nm) region, while that of W-Mg0.50Zn0.50O is located in the UV-B (280–320 nm) region. Though Mg compositions of the Mg0.50Zn0.50O films are the same, the energy-band gap of the C-Mg0.50Zn0.50O film (5.1 eV) is much larger than that of the W-Mg0.50Zn0.50O film (4.3 eV) due to the different electronic structure between the cubic and wurtzite structures. Metal–semiconductor–metal structure solar-blind and visible-blind UV photodetectors were fabricated based on the C-Mg0.50Zn0.50O and W-Mg0.50Zn0.50O films, respectively.

Ambient S annealing was adopted to regulate the crystallinity of a ZnS microsphere, which resulted in a significant improvement in the photocatalytic hydrogen production activity (PHPA). Moreover, with S ambient treatment, wurtzite ZnS showed better PHPA than sphalerite ZnS, possibly because the inter-polar electric field of the wurtzite phase could promote the separation of photo-excited electron–hole pairs.

Via the investigation of the photocatalytic activities of a set of low temperature annealed ZnS microsphere materials, the influences of both the crystallinity and the phase composition on the photocatalytic efficiency were discussed.

Based on a novel Al-doped ZnO thin film/ZnO single crystal system, the effects of interface electric field on photocatalysis were investigated in detail. Both the position and intensity of the electric field are found to have decisive influence on the photocatalytic activities.

ZnO:Ga (GZO) materials with high thermal stability and high carrier mobility are essential for the development of transparent conductive electrodes (TCE). Theoretically, only the substitution doping of Ga could result in the main donor with very high thermal stability. For achieving this goal, we adopted a hydrothermal method to acquire GZO single crystals, based on the fact that the hydrothermal method can possibly provide an approximate thermodynamic equilibrium growth environment to grow GZO materials with perfect substitution of Ga for Zn. A centimetre-sized GZO single crystal with Ga dosage amount of 0.053 wt% (2.19 × 1019 cm−3) was grown by the hydrothermal method. A precise measurement of the lattice constants reveals that the cell volume of the GZO crystal slightly shrinks by 0.09% after doping with Ga, indicating that the Ga atoms have substituted for Zn atoms, instead of existing as interstitial sites to expand the lattice parameters. A sharp X-ray rocking curve with the full width at half-maximum of the (002) reflection around 46.4 arc sec shows the high crystallinity of the GZO crystal. As expected, the as-grown GZO crystal exhibits a high RT carrier concentration (1.07 × 1019 cm−3) and the highest value of Hall mobility (81.5 cm2/(V s)) among current GZO materials. High thermal-stability is indicated by the little variation of these two parameters after being annealed at 1100 °C for 24 h. The carrier concentration is nearly independent of temperature (77–300 K).

Ideal Al-doped ZnO (AZO) thin films should have high carrier mobility and carrier concentration, as well as high thermal and chemical stability. To achieve these properties, ZnO should be heavily doped with Al and perfectly crystallized. Through analyzing the possible valence state of the elements and local lattice structures of AZO films during the gas-phase deposition process, we find that the current gas-phase deposition method may encounter an intrinsic obstacle that heavy doping of Al, high thermal stability, and high mobility (perfect crystallinity) cannot be achieved simultaneously. However, based on the understanding that an AZO thin film prepared in oxidizing atmosphere is actually accompanied with a high concentration of zinc vacancy, we propose a strategy to obtain an AZO film with ideal characteristics. Under an oxidizing atmosphere, a heavily doped AZO film with a high concentration of zinc vacancy is prepared using a gas-phase deposition method. Then a zinc vapor annealing treatment is employed to improve the crystallinity and conductivity of the film by filling the zinc vacancies with zinc atoms. The prepared AZO films possess the highest mobility (36.8 cm2 V−1 s−1) ever reported. Moreover, the films also show remarkable stability in carrier concentration, mobility, and resistivity under damp heat treatment (85 °C) over months.

Water-soluble mercaptoacetic acid-coated 3.1 nm CdS quantum dots (QDs) with two concentrations were selected for studying the correlation between the photoluminescence and the crystal growth mechanism. By achieving the classic Ostwald ripening mechanism and oriented attachment (OA) growth mechanism, we have shown that the evolution of the emission spectra were obviously different. The change in both the surface and internal defects during OA crystal growth were responsible for the specific variation of the photoluminescence of CdS QDs. Strategies for obtaining QDs with different luminescent properties are suggested.

The crystal growth mechanism, kinetics, and microstructure development play a fundamental role in tailoring the materials with controllable sizes and morphologies. The classical crystal growth kinetics—Ostwald ripening (OR) theory is usually used to explain the diffusion-controlled crystal growth process, in which larger particles grow at the expense of smaller particles. In nanoscale systems, another significant mechanism named “oriented attachment (OA)” was found, where nanoparticles with common crystallographic orientations directly combine together to form larger ones. Comparing with the classical atom/molecular-mediated crystallization pathway, the OA mechanism shows its specific characteristics and roles in the process of nanocrystal growth. In recent years, the OA mechanism has been widely reported in preparing low-dimension nanostructural materials and reveals remarkable effects on directing and mediating the self-assembly of nanocrystals. Currently, the interests are more focused on the investigation of its role rather than the comprehensive insight of the mechanism and kinetics. The inner complicacy of crystal growth and the occurrence of coexisting mechanisms lead to the difficulty and lack of understanding this growth process by the OA mechanism. In this context, we review the progress of the OA mechanism and its impact on materials science, and especially highlight the OA-based growth kinetics aiming to achieve a further understanding of this crystal growth route. To explore the OA-limited growth, the influence of the OR mechanism needs to be eliminated. The introduction of strong surface adsorption was reported as the effective solution to hinder OR from occurring and facilitate the exclusive OA growth stage. A detailed survey of the nanocrystal growth kinetics under the effect of surface adsorption was presented and summarized. Moreover, the development of OA kinetic models was systematically generalized, in which the “molecular-like” kinetic models were built to take the OA nanocrystal growth behavior as the collision and reaction between molecules. The development of OA growth kinetics can provide a sufficient understanding of crystal growth, and the awareness of underlying factors in the growth will offer promising guidance on how to control the size distribution and shape development of nanostructural materials.

Acquiring stable binary wide band-gap semiconductor (WBS) materials with high p-type mobility is essential for the development of WBS optoelectronic devices. CuI is a p-type WBS material with a large band gap (3.1 eV) and high exciton binding energy (62 meV). However, the semiconductor characteristics of the CuI single crystal are unknown due to the lack of a large sized and high quality crystal. Our approach focuses on the design of the mineralizer for the hydrothermal method to effectively control the growth habit and the impurity concentration in the crystal. A large size (15 mm × 10 mm × 1 mm) and high quality CuI single crystal is obtained by using a new mineralizer (NH4I + KI). The crystal shows high p-type mobility (43.9 cm2·V−1·S1−). The strong and sharp band-edge emission at 410 nm indicates that the interband excitonic transition dominates the optical response in the spectrum. Such a binary crystalline material may open the way to new applications in optoelectronic devices.

In this article, we report a novel molten hydroxide strategy to prepare bulk-sized complexes of three-dimensional (3D) nanostructures at a large scale. Taking cadmium hydroxide as an example, we revealed that the bulk-sized architectures were composed by cross-linked nanoplates, which could be fabricated by a one-step hydrothermal route in the presence of molten composite-hydroxide (NaOH + KOH) at 200 °C. Powder X-ray diffraction, scanning electron microscopy, and transmission electron microscopy analysis show that the growth of the well-defined nanostructures actually undergoes two stages. Initially, with the mixture of cadmium chloride and a large excess of composite-hydroxide, a prestructure of aggregating nanoparticles is formed. Afterward, the prestructure recrystallizes into nanoplates of 150 nm thickness and with the (001) plane as exposed surface. Meanwhile, these nanoplates are interweaved and organized into a globe-like structure and 3D aggregates. We speculate that the molten hydroxide plays a critical role not only in providing a strong surface effect to allow recrystallization and anisotropic growth but also in realizing the self-assembly of nanoparticles in inorganic solution. The molten composite-hydroxide solution strategy was further extended into a calcium hydroxide system and the same assembling nanoplate architectures were successfully achieved.

The crystallizations of citrate-metal complexes with diverse structures are greatly affected by the deprotonated states of the citrate ion, counter cations and chemical nature of the metal ions. Herein we studied the ternary system Zn-cit-L2 (cit = citrate acid, L2 = 2,2-bipyridine (bipy) or 1,10-phenanthroline (phen)) and obtained four novel compounds Zn2(H2cit)2(bipy)2·H2O (1), Zn2(H2cit)2(phen)2·1.5H2O (2), Zn2(Hcit)(bipy)Cl (3), and Zn2(Hcit)(phen)Cl·H2O (4). All the four compounds were characterized by elemental analysis, FT-IR spectroscopy, and X-ray crystallography. Complexes 1 and 2 with the citrate ion doubly deprotonated are dinuclear species isolated in the Zn-cit system. Complexes 3 and 4 exhibit 1D chains in which the metal ions are connected by the triply deprotonated citrate. It shows that the final solid-state species is much dependent on the pH value of the solution. That is the dinuclear compounds of 1 and 2 are precipitated at pH 3–4 whereas the 1D chain of 3 and 4 at a lower acidity of pH 5–6.

By comparing the photocatalytic bactericidal effect on different crystal faces of bulk ZnO crystal, we found that an electron degradation mechanism dominates the photocatalytic processes of ZnO material.

On the basis of our previous work on the investigation of the subsolidus phase relation of the system ZnO- Li2O-MoO3, we for the first time successfully prepared single crystals of Li2Zn2(MoO4)3 (I) with dimensions up to 10 × 4 × 3 mm3 and Co-doped Li2Zn2(MoO4)3 (II) with a size of 15 × 10 × 3 mm3 by using the top-seeded solution growth method. The X-ray diffraction analysis shows that the Li+ cation and transition metal ion fill in the octahedral and trigonal prism interstice composed of surrounding MoO4 tetrahedrons. The UV−vis spectra show that by doping with Co2+, there is an obvious red shift of the absorption band at 276 nm with the steep absorption edge changing from 370 nm (I) to 430 nm (II). The band gaps of I and II calculated with CASTEP are ≪ 2.4 eV and ≪ 1.9 eV, respectively, indicating that Co2+ play roles in the change of the band gap of the host crystal. The X-ray powder analysis of the system Li2−2xZn2+x(MoO4)3 suggests the framework is stable over a range of composition for −0.1 ≤ x ≤ 0.3. The magnetic experiments indicate that the Co-doped Li2Zn2(MoO4)3 is paramagnetic with a rather large μeff (5.37 μB). The crystal Li2Zn2(MoO4)3 may have at least two feasible applications: (i) the absorption in the ultraviolet band suggested that they can be used as a light filter; (ii) since the framework of Li2−2xZn2+x(MoO4)3 is stable over a range of composition for −0.1 ≤ x ≤ 0.3, the refractive index of the crystal may vary with different ratios of Li/Zn, suggesting that the crystals can be used as tunable refractive index materials in the middle- and near-ultraviolet region.

Thirty millimeter ZnO single crystals have been grown by the hydrothermal method with new mineralizers and a low-cost liner. A sharp X-ray rocking curve with the full width at half-maximum of 36 arcsec has been obtained for the (002) reflection, indicating a good crystallinity of the crystal. Low temperature photoluminescence spectrum revealed the crystal has a narrow and strong free exciton emission band (3.359 eV) at 10 K, with the absence of a green-yellow emission band that generally is induced by the impurities or lattice vacancies. Compared to ZnO crystals hydrothermally grown from conventional mineralizers (LiOH and KOH), this crystal has a unique feature that the room-temperature carrier mobility is close to the intrinsic value, with the carrier concentration maintained as high as 4.09 × 1016 cm−3. Two main donors which may relate to the H impurity were indicated. Analysis revealed that the diffusion of H in the ZnO lattice might be responsible for the decrease in conductivity of the ZnO crystal after annealing. The reason why the new hydrothermal method yielded low-resistance ZnO crystals could be interpreted as a much higher concentration of H impurity and much lower concentration of Li impurity incorporated into the ZnO lattice, compared with samples yielded by the conventional hydrothermal method. We anticipate the reported high-quality ZnO single crystals can not only be suitable objects for studying the intrinsic properties of ZnO, but also be potential substrates for fabricating ZnO light emitting diodes devices.

The subsolidus phase relations of the ternary system ZnO–V2O5–WO3 was investigated by means of X-ray diffraction (XRD) analysis. The phase relations had been constructed. Five binary compounds, five tie lines and six three-phase regions were determined in this system. The possible component regions for ZnO single crystal flux growth were discussed. The phase diagram of Zn4V2O9–ZnWO4 pseudo-binary system from 740 °C to 1300 °C was also determined through XRD and differential thermal analysis (DTA) methods. DTA results indicated this system was a eutectic system, and the eutectic temperature was 880 °C and eutectic point component was 78 mol% Zn4V2O9 and 22 mol% ZnWO4. The compounds composed of 78 mol% Zn4V2O9 and 22 mol% ZnWO4 might be suitable flux for ZnO crystal growth.

Under hydrothermal conditions and extremely high NaOH activity, ZnS forms nanostructures with complex morphologies that are based upon individual or interpenetrating nanosheets. Nanostructure morphology is independent of the size of the ZnS precursor (3 nm, 20 nm, or bulk) but varies systematically with NaOH concentration, producing compact microspheres, open nest- and flowerlike structures, and finally, individual nanosheets. The observations are consistent with a concept of nanostructure morphology controlled by a single parameter—the interfacial free energy of the ZnS (001) face. The synthesis of thermodynamically stable nanostructures open opportunities for new synthetic routes of materials with complex architectures.

Based on the principle of thermodynamically stable nanophase, we design a strategy for fast and mass preparation of ZnS nanostructures. Under concentrated NaOH environment, where ZnS nanoarchitecture can be thermodynamically stable, ZnS nanoarchitecture was rapidly grown by using equal stoichiometric proportions of ZnO and Na2S as reaction precursors. We demonstrate that the precursors of higher free energy can react quickly and produce the same ZnS nanoarchitecture as from slow thermodynamic transformation. The idea of thermodynamic design can be extended to a series of NaOH concentration and is insusceptible to the small changes in chemical components. We propose that the direct growth of thermodynamic stable nanomaterials has potential advantage in industrial mass production.

The subsolidus phase relations of the systems K2O–ZnO–AO3 (A = Mo, W) have been investigated by X-ray diffraction (XRD) analyses. The phase diagrams have been constructed. There are six binary compounds and two ternary compounds in the K2O–ZnO–MoO3 system, it can be divided into 11 three-phase regions. The K2O–ZnO–WO3 system consists of six binary compounds and one ternary compound. This system can be divided into 9 three-phase regions. DTA results indicated the compounds K2MoO4 and K2WO4 are not suitable to be fluxes for ZnO crystal growth.

The subsolidus phase relationships of ternary system Na2O–ZnO–WO3 have been investigated by X-ray diffraction (XRD) and differential thermal analyzer (DTA). All the samples were synthesized in the temperature range from 530 to 850 °C in air. There are one ternary compound and five binary compounds in the Na2O–ZnO–WO3 system, which can be divided into eight three-phase regions. The crystal structure of the ternary compound Na3.6Zn1.2(WO4)3 is determined by single-crystal structure analysis method. It belongs to triclinic system with space group P1¯ and lattice constants a = 7.237 (5) Å, b = 9.172 (6) Å, c = 9.339 (6) Å and α = 94.920 (4)°, β = 105.772 (9)°, γ = 103.531 (8)°, Z = 2. DTA analyses indicate that the compound Na2WO4 is not suitable to be the flux for ZnO crystal growth below 1250 °C, since no liquidus was observed in the system before 1250 °C.

The subsolidus phase relations in the ZnO–MoO3–B2O3, ZnO–MoO3–WO3 and ZnO–WO3–B2O3 ternary systems have been investigated by the means of X-ray powder diffraction (XRD). There is no ternary compound in all the systems. There are five binary compounds and five tie lines in the ZnO–MoO3–B2O3 system. This system can be divided into six 3-phase regions. There are three binary compounds and three tie lines in the ZnO–MoO3–WO3 system. This system can be divided into four 3-phase regions. There are four binary compounds and four tie lines in the ZnO–WO3–B2O3 system. This system can be divided into five 3-phase regions. The possible component regions for ZnO single crystal flux growth were discussed. The phase diagram of Zn3B2O6–ZnWO4 pseudo-binary system has been constructed, and the result reveals this system is eutectic system. The eutectic temperature is 1007 °C and eutectic point component is 70 mol% Zn3B2O6.

A facile and novel method has been developed to prepare ultrathin PbI2 single crystals with good mechanical flexibility and bendability for high sensitivity photodetectors. The prototype lateral metal–semiconductor–metal photodetector based on the as-prepared crystal exhibits a responsivity of up to 11.3 A W−1, a photogain of 31.84, and a high-speed photoresponse with a time constant of 354 μs.

Retraction of ‘Synthesis and facet-dependent photocatalytic activity of BiOBr single-crystalline nanosheets’ by Wenwen Lin et al., Chem. Commun., 2014, DOI: 10.1039/c3cc41498a.

Ni(OH)2-modified graphitic carbon nitride (Ni(OH)2–g-C3N4) composite photocatalysts were prepared by a simple precipitation method using g-C3N4 as support and nickel nitrate as precursor. The effect of Ni(OH)2 content on the rate of visible-light photocatalytic H2-production was studied for a series of Ni(OH)2–g-C3N4 composite samples in triethanolamine aqueous solutions. The results demonstrated that Ni(OH)2 was an efficient co-catalyst for the photocatalytic H2 production of g-C3N4. The optimal Ni(OH)2 loading was found to be 0.5 mol%, giving a H2-production rate of 7.6 μmol h−1 (with an apparent quantum efficiency (QE) of 1.1% at 420 nm), which approached that of optimal 1.0 wt% Pt/g-C3N4 (8.2 μmol h−1). This work showed the possibility for the utilization of low cost Ni(OH)2 as a substitute for noble metals (such as Pt) in the photocatalytic H2 production for g-C3N4.

Based on a novel Al-doped ZnO thin film/ZnO single crystal system, the effects of interface electric field on photocatalysis were investigated in detail. Both the position and intensity of the electric field are found to have decisive influence on the photocatalytic activities.

By comparing the photocatalytic bactericidal effect on different crystal faces of bulk ZnO crystal, we found that an electron degradation mechanism dominates the photocatalytic processes of ZnO material.