The effect of Mg2+ on hydrothermal formation of α-CaSO4·0.5H2O whiskers with high aspect ratios was investigated in this paper. α-CaSO4·0.5H2O whiskers with a preferential growth along the c axis and an average aspect ratio up to 370 were synthesized using hydrothermal treatment of CaSO4·2H2O precursor in the presence of 1.97 × 10–3 mol·L–1 MgCl2. The preferential adsorption of Mg2+ on the negative (200), (400), and (020) facets was confirmed by EDS, XPS, and zeta potential measurements. ATR-FTIR analysis revealed the ligand adsorption of Mg2+ on the surface of α-CaSO4·0.5H2O. The doping of Mg2+ in α-CaSO4·0.5H2O whiskers was confirmed by the XRD analysis. The experimental results indicated that the adsorption and doping of Mg2+ promoted the 1-D growth of α-CaSO4·0.5H2O whiskers, leading to the formation of whiskers with high aspect ratios.

•rGO/flower-like MoS2 hybrid film was obtain by a simple self-assembly method.•rGO/MoS2 hybrid film exhibited fast response to HCHO at RT.•Two-step electron transfer plays roles in the enhanced effects of hybrid films.A hybrid film composed of reduced graphene oxide (rGO) and molybdenum disulfide (MoS2) nanosheet-assemblies has been fabricated for high-performance formaldehyde (HCHO) detection, by a simple layer-by-layer self-assembly (LBL SA) method. The rGO/MoS2 hybrid films not only show improved sensitivity to HCHO at room temperature compared to solely rGO or MoS2 films, but also exhibit fast response characteristics and good reproducibility. The synergetic effects of rGO and MoS2 on HCHO sensing enhancement of the hybrid films are attributed to the energy level matching among rGO, MoS2 and HCHO, which lead to a two-stepped efficient electron transfer from HCHO to rGO.

•g-C3N4/oxygen-defective ZnO heterojunction photocatalysts were fabricated.•Oxygen vacancies improved the light absorption and mediated the Z-scheme mechanism.•Z-scheme charge transfer enhanced the charge separation efficiency.•Nanocomposite enhanced visible-light degradation of 4-chlorophenol and H2 evolution.g-C3N4 nanosheets were coupled with oxygen-defective ZnO nanorods (OD-ZnO) to form a heterojunction photocatalyst with a core-shell structure. Multiple optical and electrochemical analysis including electrochemical impedance spectroscopy, photocurrent response and steady/transient photoluminescence spectroscopy revealed that the g-C3N4/OD-ZnO heterojunction exhibited increased visible-light absorption, improved charge generation/separation efficiency as well as prolonged lifetime, leading to the enhanced photocatalytic activities for the degradation of 4-chlorophenol under visible-light illumination (λ > 420 nm). An oxygen defects-mediated Z-scheme mechanism was proposed for the charge separation in the heterojunction, which involved the recombining of photoinduced electrons that were trapped in the oxygen defects-level of OD-ZnO directly with the holes in the valence band of g-C3N4 at the heterojunction interface. The detection of surface generated reactive species including O2− and OH clearly supported the Z-scheme mechanism. Moreover, the g-C3N4/OD-ZnO photocatalysts also exhibited enhanced visible-light Z-scheme H2 evolution activity, with an optimal H2 evolution rate of about 5 times than that of pure g-C3N4. The present work not only provided an alternative strategy for construction of novel visible-light-driven Z-scheme photocatalysts, but also gained some new insights into the role of oxygen-defects of semiconductors in mediating the Z-scheme charge separation.Download high-res image (95KB)Download full-size image

•ZnO nanowires sensitive film utilizing (m-SWCNT) electrode was obtain by spray deposition method.•ZnO/m-SWCNT sensing device exhibited excellent p-type response to NO2 at RT.•Schottky junction barrier profile at interface plays roles in the enhanced NO2-sensing properties.A novel room-temperature (RT) resistive-type NO2 gas sensor was developed by utilizing ZnO nanowires as sensitive materials and metallic single-walled carbon nanotubes (m-SWCNT) as electrodes where both of them were fabricated by spray deposition process. The ZnO/m-SWCNT sensing devices showed better sensing response to NO2, as compared to traditional ZnO/Au sensing devices, which possess opposite sensing response. This can be attributed to different Schottky junction barrier characteristics at ZnO/m-SWCNT interface. This work paves the way to explore effective sensing and electrodes for future novel NO2 gas sensors in practical and rigid gas sensing system applications.

•The conversion of Li2CO3 to LiOH at difference temperatures was studied.•The in-situ ion-exchange and dissolution-precipitation mechanism co-existed.•The in-situ ion-exchange route dominated at lower temperature.•Increase of temperature accelerated the dissolution and conversion of Li2CO3 to LiOH.The effects of temperature on the conversion of Li2CO3 to LiOH in a Ca(OH)2 suspension were investigated. Li2CO3 microplates were used as the Li source. The results showed that Li2CO3 was converted to LiOH via in situ ion-exchange and dissolution–precipitation routes. The formation of mixed CaxLi2−2xCO3 intermediate species confirmed that at 25 °C needle-like CaCO3 was formed heterogeneously on the Li2CO3 surface via in situ ion-exchange. At 60–100 °C, isolated CaCO3 agglomerates were formed homogeneously via dissolution–precipitation. Temperature increases accelerated the dissolution and conversion of Li2CO3 to LiOH, producing solutions with high [CO32−]/[Ca2+] ratios; this favored homogeneous precipitation of isolated CaCO3 agglomerates.Download high-res image (140KB)Download full-size image

Ultralong ZnO nanowires with lengths of 20–80 μm and aspect ratios of 200–500 were synthesized within 15 minutes via a low-temperature hydrothermal method. With the assistance of sodium dodecyl sulfonate (SDSN) as the capping ligand, ZnO nanowires were formed by the initial nucleation of nanocrystals followed by the ligand-directed oriented attachment. Head-to-head attachment, side-by-side coalescence and nanocrystals attached to the surfaces were observed at different growth stages. ZnO microrods (lengths: 2–10 μm, diameters: 0.5 to 5 μm) were formed in the absence of SDSN. FT-IR spectra, XPS analysis and molecular dynamics simulations revealed that SDSN molecules were preferentially adsorbed onto the (100) planes rather than polar (001) planes, with their sulfonate groups coordinating with the surface zinc ions and possibly forming Zn–SO3 complexes. Such selective adsorption not only protected the initially nucleated ZnO nanocrystals from rapid aggregation, but also directed their subsequent self-assembly into highly-anisotropic nanowires. The as-prepared ZnO nanowires exhibited improved photoluminescence properties compared to the microrods. I–V characteristics indicated that the ZnO nanowires exhibited a much lower dark current, while an enhanced photocurrent upon 360 nm light illumination compared to the microrods. In addition, UV photodetectors using ZnO nanowires showed 27 times higher photo-sensitivity and 15.4/13.8 times higher rise/decay rates compared to those using ZnO microrods, which were attributed to the morphological effects in addition to the improved optical properties.

A facile one-step solution method has been developed here to fabricate hierarchical ZnO nanosheet–nanorod architectures for compositing with poly(3-hexylthiophene) (P3HT) for fabricating a hybrid NO2 sensor. The hierarchical ZnO nanosheet–nanorod architectures were controllably synthesized by aging the solutions containing 0.05 mol·L–1 Zn2+ and 0.33 mol·L–1 OH– at 60 °C through a metastable phase-directed mechanism. The concentration of OH– played a huge role on the morphology evolution. When the [OH–] concentration was decreased from 0.5 to 0.3 mol·L–1, the morphology of the ZnO nanostructures changed gradually from monodispersed nanorods (NR) to nanorod assemblies (NRA), and then to nanosheet–nanorod architectures (NS-NR) and nanosheet assemblies (NSA), depending on the formation of various metastable, intermediate phases. The formation of NS-NR included the initial formation of ZnO nanosheets/γ-Zn(OH)2 mixed intermediates, followed by the dissolution of Zn(OH)2, which served as soluble zinc source. Soluble Zn(OH)2 facilitated the dislocation-driven secondary growth of ZnO nanorod arrays on the primary defect-rich nanosheet substrates. Hybrid sensors based on composite films composed of P3HT and the as-prepared ZnO nanostructures were fabricated for the detection of NO2 at room temperature. The P3HT/ZnO NS-NR bilayer film exhibited not only the highest sensitivity but also good reproducibility and selectivity to NO2 at room temperature. The enhanced sensing performance was attributed to the formation of the P3HT/ZnO heterojunction in addition to the enhanced adsorption of NO2 by NS-NR ZnO rich in oxygen-vacancy defects.Keywords: metastable phase-directed synthesis; nanosheet−nanorod architectures; P3HT/ZnO heterojunction film; room-temperature NO2 sensor; ZnO

Here we demonstrate high-performance room-temperature NO2 sensors based on ultrathin ZnO nanorods/reduced graphene oxide (rGO) mesoporous nanocomposites. Ultrathin ZnO nanorods were loaded on rGO nanosheets by a facile two-step additive-free solution synthesis involving anchored seeding followed by oriented growth. The ZnO nanorod diameters were simply controlled by the seed diameters associated with the spatial confinement effects of graphene oxide (GO) nanosheets. Compared to the solely ZnO nanorods and rGO-based sensors, the optimal sensor based on ultrathin ZnO nanorods/rGO nanocomposites exhibited higher sensitivity and quicker p-type response to parts per million level of NO2 at room temperature, and the sensitivity to 1 ppm of NO2 was 119% with the response and recovery time being 75 and 132 s. Moreover, the sensor exhibited full reversibility, excellent selectivity, and a low detection limit (50 ppb) to NO2 at room temperature. In addition to the high transport capability of rGO as well as excellent NO2 adsorption ability derived from ultrathin ZnO nanorods and mesoporous structures, the superior sensing performance of the nanocomposites was attributed to the synergetic effect of ZnO and rGO, which was realized by the electron transfer across the ZnO–rGO interfaces through band energy alignment.Keywords: NO2; reduced graphene oxide; room-temperature sensor; spatial confinement growth; ZnO;

The supersaturation-induced hydrothermal formation of α-CaSO4·0.5H2O whiskers from the CaSO4·2H2O precursor at 115–150 °C was investigated in this paper. The experimental results indicated that the conversion processes were carried out via dissolution–precipitation and homogeneous nucleation routes and connected with the critical supersaturation (abbreviated as Scrit, lnScrit = a(γ/T)3/2) of the system. In the temperature range of 115–150 °C, α-CaSO4·0.5H2O whiskers with a length of 200–500 μm and a diameter of 0.1–0.5 μm formed quickly within 2.0–5.0 minutes once the supersaturation (abbreviated as S) reached Scrit. The supersaturation-induced formation of α-CaSO4·0.5H2O is well consistent with the first-order reaction model, and the corresponding activation energy (Ea) and pre-exponential factor (A0) were 161.7 kJ mol−1 and 2.601 × 1021 h−1, respectively.

•ZnO nanorods were synthesized from ε-Zn(OH)2 in 0–2.0 mol L−1 NaOH solutions.•The aspect ratios of the ZnO nanorods could be tuned from 5 to 40.•The in-situ crystallization and the dissolution–precipitation mechanisms coexisted.•The effects of NaOH on the two competitive mechanisms were demonstrated.ZnO nanorods with varying aspect ratios were synthesized by aging ε-Zn(OH)2 precursors in 0–2.0 mol L−1 NaOH solutions at 80 °C, via competing in-situ crystallization and dissolution–precipitation routes. The effects of NaOH on the growth process and morphological evolution of ZnO nanorods were investigated. An increase of NaOH concentration from 0 to 2 mol L−1 led to increased average aspect ratio of the ZnO nanorods from 5 to 40, and decrease of conversion time from 420 min to 40 min. ε-Zn(OH)2 was converted to ZnO nanorods primarily by in-situ crystallization in water, while for NaOH solutions they were converted mainly by dissolution–precipitation.

•High-aspect-ratio ZnO whiskers were hydrothermally synthesized in NaOH–Na2SO4 solutions.•The influence of Na2SO4 on the formation of ZnO whiskers was investigated.•Adsorption of SO42− mainly on (1 0 0) plane promoted 1D growth of ZnO whiskers along c-axis.The influence of Na2SO4 on the formation of ZnO whiskers was investigated in this paper. ZnO whiskers with aspect ratios of up to 50 were synthesized by dissolving ɛ-Zn(OH)2 precursor in NaOH/Na2SO4 solution at room temperature, followed by aging of the resulting solution at 140 °C for 6 h. Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy analyses revealed that SO42− ions were primarily adsorbed on the (1 0 0) plane of the ZnO whiskers via an outer-sphere complex configuration (OH···O), thereby promoting the one-dimensional growth of ZnO whiskers along the c-axis.

A facile additive-free solution method has been developed to fabricate ZnO star-like assemblies, nanosheet-based microspheres and nanorod assemblies by rapid dilution of zinc-bearing alkaline solution with water at 60 °C. The shape of the nanoscale building blocks of the hierarchical structures depends on the supersaturations, which can be adjusted by varying the dilution ratios or the [OH−] to [Zn2+] ratios. The experimental results indicate that with the increase of supersaturation, the building blocks of the hierarchical structures evolve from nanorods to nanosheets and then to nanoparticles. The formation of ZnO nanorod assemblies involves the initial precipitation of the ε-Zn(OH)2 intermediate followed by a relatively slow phase transformation process, while ZnO nanosheet-based microspheres and star-like assemblies are constructed within 30 seconds due to the rapid increase of supersaturation induced by dilution, involving the oriented attachment and the random aggregation mechanisms, respectively. Photoluminescence results reveal that the hierarchical structures are rich with singly and doubly ionized oxygen vacancies. The photocatalytic activities of the ZnO hierarchical structures were also evaluated by the degradation of rhodamine B under ultraviolet light. The photocatalytic performance depends on the content of oxygen vacancies as well as the specific surface area.

•High-aspect-ratios CaSO4·0.5H2O were synthesized via a Na2HPO4·12H2O-assisted hydrothermal method.•High super-saturation achieved in the presence of Na2HPO4·12H2O favored formation of CaSO4·0.5H2O.•Na2HPO4·12H2O promoted formation of CaSO4·0.5H2O with thinner diameter and shorter length.The influence of Na2HPO4·12H2O on the hydrothermal formation of hemihydrate calcium sulfate (CaSO4·0.5H2O) whiskers from dihydrate calcium sulfate (CaSO4·2H2O) at 135 °C was investigated. Experimental results indicate that the addition of phosphorus accelerates the hydrothermal conversion of CaSO4·2H2O to CaSO4·0.5H2O via the formation of Ca3(PO4)2 and produces CaSO4·0.5H2O whiskers with thinner diameters and shorter lengths. Compared with the blank experiment without Na2HPO4·12H2O, the existence of minor amounts (8.65 × 10−4–4.36 × 10−3 mol/L) of Na2HPO4·12H2O led to a decrease in the diameter of CaSO4·0.5H2O whiskers from 1.0–10.0 to 0.5–2.0 μm and lengths from 70–300 to 50–200 μm.

The effect of Mg2+ on hydrothermal formation of α-CaSO4·0.5H2O whiskers with high aspect ratios was investigated in this paper. α-CaSO4·0.5H2O whiskers with a preferential growth along the c axis and an average aspect ratio up to 370 were synthesized using hydrothermal treatment of CaSO4·2H2O precursor in the presence of 1.97 × 10–3 mol·L–1 MgCl2. The preferential adsorption of Mg2+ on the negative (200), (400), and (020) facets was confirmed by EDS, XPS, and zeta potential measurements. ATR-FTIR analysis revealed the ligand adsorption of Mg2+ on the surface of α-CaSO4·0.5H2O. The doping of Mg2+ in α-CaSO4·0.5H2O whiskers was confirmed by the XRD analysis. The experimental results indicated that the adsorption and doping of Mg2+ promoted the 1-D growth of α-CaSO4·0.5H2O whiskers, leading to the formation of whiskers with high aspect ratios.

This paper reports the zinc interstitial-induced room temperature ferromagnetism (RT-FM) in undoped ZnO nanorods synthesized by aging ε-Zn(OH)2 precursor in 0–2 mol·L–1 NaOH at 80 °C for 10.0 h. The variations of the defect states and ferromagnetism of the ZnO nanorods with NaOH concentration were investigated by X-ray diffraction, Raman scattering, photoluminescence, electron spin resonance, X-ray photoelectron spectroscopy, and superconducting quantum interference device so as to identify the origin of RT-FM. The experimental results revealed that the increase of the NaOH concentration led to the increase of the oxygen-related defects but the decrease of the zinc interstitials in association with the magnetization value. A direct correlation between the ferromagnetism and the relative concentration of the zinc interstitials was established, which indicated that the zinc interstitials may play an important role in mediating the RT-FM in the undoped ZnO nanorods.

Hydrothermal synthesis (and other soft chemistry based thermal conversion routes) has emerged as one of the thriving methods for high quality one-dimensional (1D) nanostructures. However, it is still a great challenge to repair the pores and preserve the 1D morphology during the thermal conversion. The effects of the heating procedures, especially the flux agent on the calcined Mg2B2O5 nanowhiskers based on the hydrothermally synthesized MgBO2(OH) nanowhiskers, have been investigated. By using the CCD camera aided in situ dynamic observation, the thermal decomposition of MgBO2(OH) nanowhiskers is discovered for the first time. With the dehydration going on, pores are generated, coalesced, ruptured, migrated, and finally evaporated out of the bulk of the nanowhiskers. Meanwhile, the molten flux agent penetrates into the bulk of the nanowhiskers through the microchannels exposed onto the surfaces and further flows to each point to which the pores migrate. During the recrystallization, the flux is extruded out to the surfaces and finally washed out via post-treatment. The flux agent serves as the ideal liquid medium for the rearrangement of Mg2B2O5, through which the pores formed during the dehydration are repaired and the overall 1D morphology is preserved. This is helpful to further understand the fantastic formation and thus contributes to the state-of-the-art controllable synthesis of pore-free high crystallinity 1D anhydrous nanostructures via the hydrothermal or other soft chemistry based thermal conversion routes.

Developing green approaches to micro-/nanomaterials is becoming more and more important in constructing a more sustainable society for chemical as well as materials related community. Here, a byproduct-assisted thermal conversion (BATC) route to submicron Mg2B2O5 whiskers (diameter: 145–350 nm, length: 560–3490 nm) is developed. Room temperature co-precipitation of the reactants leads to precursor slurries, followed by filtration and washing. The resultant precursor containing residual byproduct NaCl is calcined at 800–900 °C for 2 h, giving rise to final submicron Mg2B2O5 whiskers. Compared with the conventional methods for Mg2B2O5 micro-/nanostructures, the present BATC route exhibits distinct advantages, such as improvement of size/morphology uniformity and dispersion of product particles, no need of additionally introducing abundant flux agent, recycling and reuse of byproduct NaCl as flux, no need of boiling water in flux separation and product purification, reduction of waste water rich of NaCl, energy saving and low cost. The present BATC route is high-pressure-free, environmental benign, and thus can be extended for designing novel sustainable approaches to other micro-/nanostructures especially those traditionally acquired by high temperature molten salt synthesis or high pressure supercritical/hydrothermal method.

Dispersive ZnO nanoparticles with a primary particle size of about 70 nm and an average agglomerate size of about 2.0 μm were synthesized via the precipitation-thermal decomposition route using ZnSO4 and Na2CO3 as the reactants and sodium dodecyl sulfate (SDS) as the surface modification agent. The presence of minor amounts of SDS in the formation of hydrozicite (Zn5(CO3)2(OH)6) precursor changed the agglomeration size of ZnO from 9.7 to 2.0 μm and the primary particle size of ZnO from about 45 to 70 nm. Molecular simulation based on the DISCOVER model and COMPASS force field indicated that SDS was adsorbed on the surface of Zn5(CO3)2(OH)6 mainly via the coulomb and hydrogen bond interactions.

Herein we report a simple emulsion-phase route for the synthesis of honeycomb-like basic magnesium carbonate (BMC, Mg5(OH)2(CO3)4·4H2O) micro-spheres at 80 °C. Magnesium(II) salts in water are precipitated by sodium carbonate in the presence of cetyltrimethylammonium bromide (CTAB). Scanning electron microscopy shows the obtained BMC samples are composed of a lot of micro-spheres (diameter ranging from 8 to 10 μm) which are interweaved by a lot of nano-sized thin sheets (thickness of 20–30 nm and length >1 μm). The BMC micro-spheres prepared by this approach are porous and appear to be hollow structures. The size and shape of BMC are related to the CTAB concentration and temperature. The lower concentration of CTAB resulted in the decrease of the micro-spheres sizes. When the temperature was elevated to 110 °C, hexagonal tablets (thickness of 20 nm, length of each side varies from 400 to 600 nm) can be prepared. After the calcinations for BMC at 600 °C for 2 h, BMC are almost completely converted to MgO. Transmission electron microscopy indicates that the obtained MgO samples have a poly-crystalline feature. The possible formation mechanism of BMC micro-spheres has been discussed.

Mass production of one-dimensional (1D) nanomaterials has emerged as one of the most significant challenges in powder technology. In this contribution, MgBO2(OH) nanowhiskers were hydrothermally produced at a kilogram scale in a 150 L stainless steel autoclave at 200 °C for 12.0 h by using MgCl2·6H2O, H3BO3 and NaOH as the raw materials. The subsequent thermal conversion of the MgBO2(OH) nanowhiskers at 700 °C for 6 h led to 3.75 kg of high crystallinity monoclinic Mg2B2O5 nanorods, with a length of 0.47–1.3 µm, a diameter of 55–160 nm, and an aspect ratio of 3–15. After the nanorods have been surface modified with the silane coupling agent KH-550, the reinforcing and toughening effects of the Mg2B2O5 nanorods on the biaxially oriented polypropylene resins (BOPP-D1) were evaluated. The filling of the Mg2B2O5 nanorods into the resins resulted in the increase in the tensile strength, the impact strength, and the melt flow index of the BOPP-D1 composites. The appropriate ratio of coupling agent to fillers (Mg2B2O5 nanorods) and the ratio of fillers to resins were determined within the range of 0.6–1.2 wt.% and 8–15 wt.%, respectively. The optimal ratio of fillers to resins was ca. 10 wt.%. The present mass production of MgBO2(OH) nanowhiskers and Mg2B2O5 nanorods is believed to be helpful for enlarging and propelling the applications of the 1D magnesium borate nanostructures in the near future.Kilograms of MgBO2(OH) nanowhiskers were hydrothermally produced in a 150 L stainless autoclave at 200 ºC for 12.0 h by using MgCl2·6H2O, H3BO3 and NaOH as raw materials. A subsequent thermal conversion at 700 ºC for 6.0 h led to the formation of Mg2B2O5 nanorods. The Mg2B2O5 nanorods demonstrated good reinforcement performance on biaxially oriented polypropylene resins.

Calcium sulfates (anhydrite and hydrates) were synthesized by mixing CaCl2 and Na2SO4 solutions at room temperature followed by aging the resulting slurries at elevated temperatures. The variation of the morphology and structure of the calcium sulfates with aging temperature was investigated. Experimental results indicated that CaSO4·2H2O plates, CaSO4·0.5H2O whiskers and CaSO4 spindles were formed at ≤100 °C, 130–160 °C and ≥170 °C, respectively. The formation and conversion of the calcium sulfates were discussed on the basis of characterization of the products and chemical analysis of the solutions. Compared to NaCl solution, pure water favors one-dimensional hydrothermal growth of CaSO4·0.5H2O whiskers owing to lower supersaturation.

The successive effects of rolling up, oriented attachment and Ostwald ripening on the hydrothermal formation of szaibelyite MgBO2(OH) nanowhiskers (diameter: 20–60 nm, aspect ratio: 10–70) from amorphous precursor obtained at room temperature are investigated in this paper, and the MgBO2(OH) nanowhiskers successively experience three various stages in the course of the hydrothermal treatment, dominated via rolling up at an early heating stage, head-to-head overlapped and side-by-side oriented attachment at medium-term crystal growth, and Ostwald ripening at late hydrothermal coarsening, which lead to rudimental 1D MgBO2(OH), lotus root-like MgBO2(OH) with a concavo-convex surface, and uniform MgBO2(OH) nanowhiskers with a smooth surface and a high aspect ratio, respectively.

Boehmite (γ-AlOOH) nanocrystals with varying morphologies were synthesized by hydrothermal treatment of the freshly precipitated Al(OH)3 gel at 240 °C for 16 h in different solutions. Under acidic conditions (initial pH 4.0), boehmite nanorods with an aspect ratio of about 2 and 4, respectively, were formed in the presence of nitrate and chloride, and boehmite nanowires with an aspect ratio of about 80 were formed in the case of sulfate. Under alkaline conditions (initial pH 10.5), boehmite nanoplates with a size of about 100 nm were produced, irrespective of the anion types. Further analyses indicated that the adsorption of the anions on a boehmite surface may be responsible for the shape formation of boehmite. Under acidic conditions the adsorption order was as follows: nitrate < chloride < sulfate, in agreement with the preferential growth tendency order: nitrate < chloride < sulfate, while little anions can be adsorbed under alkaline conditions. The selective adsorption of anions on the (010) and (001) facets of boehmite is suggested to be responsible for the preferential growth of boehmite along [100] direction.

Calcium carbonate with various structures and morphologies were prepared by double injection of the CaCl2 and NH4HCO3 solutions with molar ratio of 1:1 at 30-80 °C. The lamellar vaterite particles, the mixture composed of vaterite, aragonite and calcite, and the aragonite whiskers were formed at 30-40 °C, 50-70 °C and 80 °C, respectively. Thermodynamic calculation showed that the value of [CO32−]/[Ca2+] decreased with the increase of temperature, which may be one of the reasons for the formation of the lamellar vaterite at 30-40 °C and the aragonite whiskers at 80 °C.Lamellar vaterite aggregates and aragonite whiskers were synthesized via double injection of the CaCl2 and NH4HCO3 solutions at 30-40 °C and 80 °C, respectively. Thermodynamic calculation showed that the value of [CO32−]/[Ca2+] decreased with the increase of temperature, which may be one of the reasons for the formation of lamellar vaterite and aragonite whiskers at 30-40 °C and 80 °C respectively.

The significant effect of the feeding mode on the morphology and size distribution of the hydrothermal synthesized MgBO2(OH) is investigated, which indicates that, slow dropping rate (0.5 drop s−1) and small droplet size (0.02 mL d−1) of the dropwise added NaOH solution are favorable for promoting the one-dimensional (1D) preferential growth and thus enlarging the aspect ratio of the 1D MgBO2(OH) nanostructures. The joint effect of the low concentration of the reactants and feeding mode on the hydrothermal product results in the head-to-head coalesced MgBO2(OH) nanowires with a length of 0.5–9.0 μm, a diameter of 20–70 nm, and an aspect ratio of 20–300 in absence of any capping reagents/surfactants or seeds.

A flux-assisted thermal conversion route to the pore-free high crystallinity magnesium borate (Mg2B2O5) nanowhiskers with a length of 0.47−3.0 μm, diameter of 50−240 nm, and aspect ratio of 5−36 at a relatively low temperature of 650−700 °C (200−350 °C lower than that needed via the traditional method) was developed in this paper. Magnesium borate hydroxide [MgBO2(OH)] nanowhiskers were first prepared by a coprecipitation−hydrothermal approach at 240 °C for 18 h by using MgCl2·6H2O, H3BO3 and NaOH (or KOH) as the raw materials and then calcined to produce Mg2B2O5 nanowhiskers. The resultant NaCl (or KCl) in the coprecipitation served as the flux and played a key role in the thermal conversion of MgBO2(OH) nanowhiskers as the transport medium for the rearrangement of structural units of Mg2B2O5, leading to the final formation of the pore-free Mg2B2O5 nanowhiskers with uniform one-dimensional morphology, high crystallinity, and twin crystal structures.

A facile hydrothermal method was developed to synthesize boehmite nanorods with a length of 50−2000 nm, a diameter of 6−20 nm, and a preferential growth along [100] by treating the Al(OH)3 gel in acidified sulfate solutions at 240 °C. Studies on the hydrothermal treatment of Al(OH)3 gel in sulfate solutions showed that the morphology and the composition of the hydrothermal products were connected with the sulfate concentration and the pH of the hydrothermal solution. The aspect ratio of the boehmite nanorods increased to 300 as the initial H2SO4 concentration increased to 0.043 mol·L−1, whereas boehmite nanorods and (H3O)Al3(SO4)2(OH)6 cubic particles coexisted in the case of the initial H2SO4 concentration ≥ 0.054 mol·L−1. Sole boehmite nanoflakes with a diameter of about 50 nm were formed under alkaline conditions (pH 10.5) despite the existence of the sulfate. The chemical and Raman analyses indicated that SO42− in acidified solutions adsorbed on the boehmite surface via H-bonds. On the basis of the above results, the growth of boehmite along the [100] direction was attributed to the selective adsorption of SO42− on the (010) and (001) planes of boehmite.

One-dimensional magnesium oxysulfate 5Mg(OH)2 · MgSO4 · 3H2O (abbreviated as 513MOS) with high aspect ratio has attracted much attention because of its distinctive properties from those of the conventional bulk materials. 513MOS nanowires with different morphologies were formed by varying the mixing ways of MgSO4 · 7H2O and NH4OH solutions at room temperature followed by hydrothermal treatment of the slurries at 150 °C for 12 h with or without EDTA. 513MOS nanowires with a length of 20–60 μm and a diameter of 60–300 nm were prepared in the case of double injection (adding MgSO4 · 7H2O and NH4OH solutions simultaneously into water), compared with the 513MOS with a length of 20–30 μm and a diameter of 0.3–1.7 μm in the case of the single injection (adding MgSO4 · 7H2O solution into NH4OH solution). The presence of minor amount of EDTA in the single injection method led to the formation of 513MOS nanowires with a length of 100–200 μm, a diameter of 80–200 nm, and an aspect ratio of up to 1000. The analysis of the experimental results indicated that the hydrothermal solutions with a lower supersaturation were favorable for the preferential growth of 513MOS nanowires along b axis.

•ZnO rods with varying diameters were synthesized from ε-Zn(OH)2 precursor.•The influence of temperature on the formation of ZnO from ε-Zn(OH)2 was investigated.•Formation mechanism of ZnO rods from ε-Zn(OH)2 was discussed.•A facile two-stage route was developed to synthesize ZnO nanorods with thin diameters and high photo-degradation activity.The influence of temperature on the formation of one-dimensional (1D) ZnO from ε-Zn(OH)2 via solution route was studied in this paper, using ZnSO4 and NaOH as the raw materials. The experimental results indicated that the increase of temperature from 25 °C to 80 °C accelerated the conversion of ε-Zn(OH)2 to ZnO, leading to the formation of 1D ZnO with comparative big diameters. A two-stage route was then developed to synthesize ZnO nanorods with diameters of 20–100 nm and lengths of 1–3 μm by aging Zn(OH)2 in 1.0 mol L−1 NaOH at 25 °C for 72 h followed by aging the slurry at 80 °C for 2 h. Compared with the ZnO submicron-rods (diameters: 100–500 nm, lengths: 1–3 μm) formed at 80 °C, the ZnO nanorods formed via the two-stage route exhibited a thinner diameter and a higher photo-degradation activity for Rhodamine B owing to the pre-formation of ZnO nanorods (diameters: 10–50 nm) on ε-Zn(OH)2 surface at 25 °C as well as the existence of more oxygen defects.

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 (VO). The VO-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 VO-enhanced photocatalytic activity was attributed to the generation of VO-isolated level in the band structure. Under UV light, the VO-level could promote charge separation by trapping the photoinduced electrons, while under visible-light, the VO-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.

Ultralong ZnO nanowires with lengths of 20–80 μm and aspect ratios of 200–500 were synthesized within 15 minutes via a low-temperature hydrothermal method. With the assistance of sodium dodecyl sulfonate (SDSN) as the capping ligand, ZnO nanowires were formed by the initial nucleation of nanocrystals followed by the ligand-directed oriented attachment. Head-to-head attachment, side-by-side coalescence and nanocrystals attached to the surfaces were observed at different growth stages. ZnO microrods (lengths: 2–10 μm, diameters: 0.5 to 5 μm) were formed in the absence of SDSN. FT-IR spectra, XPS analysis and molecular dynamics simulations revealed that SDSN molecules were preferentially adsorbed onto the (100) planes rather than polar (001) planes, with their sulfonate groups coordinating with the surface zinc ions and possibly forming Zn–SO3 complexes. Such selective adsorption not only protected the initially nucleated ZnO nanocrystals from rapid aggregation, but also directed their subsequent self-assembly into highly-anisotropic nanowires. The as-prepared ZnO nanowires exhibited improved photoluminescence properties compared to the microrods. I–V characteristics indicated that the ZnO nanowires exhibited a much lower dark current, while an enhanced photocurrent upon 360 nm light illumination compared to the microrods. In addition, UV photodetectors using ZnO nanowires showed 27 times higher photo-sensitivity and 15.4/13.8 times higher rise/decay rates compared to those using ZnO microrods, which were attributed to the morphological effects in addition to the improved optical properties.