Tianxing Wu

Adsorption of Hg2+ by thiol functionalized hollow mesoporous silica microspheres with magnetic cores

Novel hollow mesoporous silica spheres with magnetic cores (HMSMCs) were successfully synthesized by using hybrid magnetic carbon (Fe3O4/C) spheres as templates. The microspheres were further functionalized with (3-mercaptopropyl)trimethoxysilane (MPTS) to produce thiol functionalized HMSMCs (SH-HMSMCs), and their ability to absorb traces of toxic Hg2+ was evaluated. The characterization results revealed that the hollow microspheres were 250–300 nm in diameter. The thickness of the shell was about 50 nm, in which contained an inner core of Fe3O4 crystallites with a size of about 10 nm. It was also found that the saturation magnetization of the sample was 62.5 emu g−1 and the BET surface area was 421 m2 g−1. These magnetic hybrid silica microspheres with thiol functional groups were found to have a high affinity to Hg2+, and were able to reduce even a low concentration of Hg2+ (<1 mg L−1) down to about 0.53 μg L−1, which was less than the Hg2+ content in the drinking water standard. The super strong affinity towards Hg2+ was attributed to the synergistic effect of the thiol groups and the unique structure of the microspheres. Moreover, the microspheres as adsorbents could be easily separated by an external magnetic field, and the adsorbed Hg2+ on the adsorbents could be removed by using hydrochloric acid, thus the adsorbents are readily reusable.

Co9S8@N,P-doped porous carbon electrocatalyst using biomass-derived carbon nanodots as a precursor for overall water splitting in alkaline media

In this study, we first synthesized Co9S8@N-doped porous carbon (Co9S8@NC) using shrimp-shell derived carbon nanodots as a carbon/nitrogen source in the presence of CoSO4 by a one-step molten-salt calcination method. This was followed by low-temperature phosphorization in the presence of NaH2PO2, whereby Co9S8@N,P-doped porous carbon (Co9S8@NPC) was finally obtained using the Co9S8@NC as a precursor. The results demonstrated that the molten-salt calcination approach can effectively create a pyrolytic product with a porous structure and improve the material’s surface area, which is favourable for electrocatalysis-related mass transport and the exposure of catalytic active sites during electrocatalysis. As an electrocatalyst, Co9S8@NPC exhibits higher catalytic activity for the hydrogen evolution reaction (HER) than Co9S8@NC in an alkaline medium. Among all the investigated Co9S8@NPC catalysts, Co9S8@NPC-10 (mass ratio of NaH2PO2 to Co9S8@NC = 10:1) displays the best HER activity with an overpotential of 261 mV at 10 mA cm−2 in the alkaline medium. Interestingly, Co9S8@NPC-10 also displays good catalytic activity for the oxygen evolution reaction (OER) in this study. Owing to its bifunctional catalytic activity towards the HER and OER, the fabricated Co9S8@NPC-10 was simultaneously used as an anode and cathode material to generate O2 and H2 from overall water splitting in the alkaline medium, exhibiting a nearly 100% faradaic yield. This study would be helpful to the design and development of high performance non-precious metal electrocatalysts to be applied in overall water splitting to produce H2 and O2.

Two-dimensional CoNi nanoparticles@S,N-doped carbon composites derived from S,N-containing Co/Ni MOFs for high performance supercapacitors

Due to their controllable morphologies, tunable porous structures, diverse compositions and easy fabrication, metal–organic frameworks (MOFs) are an ideal class of precursor material to develop high performance carbon-based materials for energy applications. In this work, two-dimensional (2D) Co/Ni MOFs nanosheets with a molar ratio of Co2+ to Ni2+ of 1 : 1 were first synthesized at room temperature using thiophene-2,5-dicarboxylate (Tdc) and 4,4′-bipyridine (4,4′-Bpy) as organic linkers. As a precursor material, the as-synthesized 2D Co/Ni MOFs nanosheets were further pyrolized at 550 °C in N2 atmosphere to incorporate 2D CoNi alloy nanoparticles into S, N-doped carbon nanosheets (CoNi@SNC) with a surface area of 224 m2 g−1, a porous structure, and good conductivity. Interestingly, it was found that the 2D Co/Ni MOFs nanosheets can be directly used as electrode materials for supercapacitors, delivering a specific capacitance of 312 F g−1 at 1 A g−1, whereas CoNi@SNC derived from its MOFs precursor as an electrode material for supercapacitors exhibits a much higher specific capacitance (1970, 1897 and 1730 F g−1 at 1, 2, 5 A g−1, respectively) with long cycling life (retaining 95.1% of the value at 10 A g−1 after 3000 cycles) and excellent rate capability at a high charge/discharge current. Further, an asymmetric supercapacitor device was constructed with CoNi@SNC as the positive electrode and active carbon as the negative electrode, exhibiting an energy density of 55.7 W h kg−1 at a power density of 0.8 kW kg−1 with lifetime stability up to 4000 charge–discharge cycles (capacitance retention of ∼90.6%). The results demonstrate that electrochemical activation-generated CoNi oxides/oxyhydroxides on the surface of the CoNi alloy nanoparticles in alkaline electrolyte during electrochemical measurements are the electrochemical active species of the CoNi@SNC-constructed supercapacitor. Additionally, the high performance of the CoNi@SNC-constructed supercapacitor can be collectively attributed to its relatively high surface area, which is favourable for the exposure of electrochemical active sites; its porous structure, which promotes redox-related mass transport; and the combination of CoNi alloy nanoparticles with graphitic carbon, which functions as an electron collector to improve electron transfer.

Growth and in situ transformation of TiO2 and HTiOF3 crystals on chitosan-polyvinyl alcohol co-polymer substrates under vapor phase hydrothermal conditions

A chitosan-polyvinyl alcohol (CS/PVA) co-polymer substrate possessing a large number of amino and hydroxyl groups is used as a substrate to induce the direct growth and in situ sequential transformation of titanate crystals under HF vapor phase hydrothermal conditions. The process involves four distinct formation/transformation stages. HTiOF3 crystals with well-defined hexagonal shapes are formed during stage I, and are subsequently transformed into {001} faceted anatase TiO2 crystal nanosheets during stage II. Interestingly, the formed anatase TiO2 crystals are further transformed into cross-shaped and hollow squareshaped HTiOF3 crystals during stages III and IV, respectively. Although TiO2 crystal phases and facet transformations under hydrothermal conditions have been previously reported, in situ crystal transformations between different titanate compounds have not been widely reported. Such crystal formation/transformations are likely due to the presence of large numbers of amino groups in the CS/PVA substrate. When celluloses possessing only hydroxyl groups are used as a substrate, the direct formation of {001} faceted TiO2 nanocrystal sheets is observed (rather than any sequential crystal transformations). This substrate organic functional group-induced crystal formation/transformation approach could be applicable to other material systems.

Vapor-phase hydrothermal growth of single crystalline NiS2 nanostructure film on carbon fiber cloth for electrocatalytic oxidation of alcohols to ketones and simultaneous H2 evolution

Electrocatalytic synthesis of value-added chemicals is attracting significant research attention owing to its mild reaction conditions, environmental benignity, and potentially scalable application to organic synthetic chemistry. Herein, we report the preparation of a single-crystalline NiS2 nanostructure film of ∼ 50 nm thickness grown directly on a carbon fiber cloth (NiS2/CFC) by a facile vapor-phase hydrothermal (VPH) method. NiS2/CFC as an electrocatalyst exhibits activity for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) in alkaline media. Furthermore, a series of alcohols (2-propanol, 2-butanol, 2-pentanol, and cyclohexanol) were electrocatalytically converted to the corresponding ketones with high selectivity, efficiency, and durability using the NiS2/CFC electrode in alkaline media. In the presence of 0.45 M alcohol, a remarkably decreased overpotential (∼ 150 mV, vs. RHE) at the NiS2/CFC anode compared with that for water oxidation to generate O2, i.e., the OER, in alkaline media leads to significantly improved H2 generation. For instance, the H2 generation rate in the presence of 0.45 M 2-propanol is almost 1.2-times of that obtained for pure water splitting, but in a system that employs an applied voltage at least 280 mV lower than that required for water splitting to achieve the same current density (20 mA·cm–2). Thus, our results demonstrate the applicability of our bifunctional non-precious-metal electrocatalyst for organic synthesis and simultaneous H2 production.

Determination of mercury in aquatic systems by DGT device using thiol-modified carbon nanoparticle suspension as the liquid binding phase

A diffusive gradients in thin films technique (DGT) device using thiol-modified carbon nanoparticle (SH-CNP) suspension as the liquid binding phase and cellulose acetate membrane as the diffusive layer was evaluated for determination of Hg2+ in water. Laboratory DGT validation experiments gave linear mass uptake over time (R2 ≥ 0.99) for Hg2+ in solutions of different concentrations. The effect of pH, ionic strength and potential interfering ions on Hg2+ binding with DGT devices was investigated. The results showed that the gathering amount of SH-CNPs-DGT for Hg2+ reached the maximum when the pH of solution was close to neutral and the ionic strength of solution and co-existing potential interfering ions such as Cd2+, Cr3+, Cu2+ and Pb2+ had no significant effect on gathering of SH-CNPs-DGT for Hg2+. Finally, validation of the SH-CNPs-DGT devices was undertaken for Hg2+ in spiked local water systems (Dongpu Reservoir and Nanfei River). For in situ measurements in Nanfei River water, the average labile Hg concentrations were 0.091 ± 0.009, 0.053 ± 0.003, and 0.071 ± 0.006 μg L−1 for three, six and seven days, respectively, which were lower than the value obtained by using ICP-MS, as DGT only measures ionic mercury and labile mercury species but direct measurement measures total mercury including inert organic species and large colloids.

Zirconium metal organic frameworks-based DGT technique for in situ measurement of dissolved reactive phosphorus in waters

In an effort to provide early warnings for the occurrence of eutrophication, it is highly desirable to develop an accurate and efficient technique to ensure continuous monitoring of dissolved reactive phosphorus (DRP) in the aquatic environment from the viewpoint of environmental management. Herein, a new diffusive gradient in thin film (DGT) technique was developed and evaluated for in situ measurement of DRP in waters, in which Zr-based metal organic frameworks (MOFs, UiO-66) were utilized as aqueous binding agent (abbreviated as UiO-66 DGT). As expected, the UiO-66 DGT demonstrated high uptake capacity towards phosphorus (20.8 μg P cm-2). Meanwhile, an excellent linearity between the accumulated DRP mass and deployment time over 5 d (R2 = 0.996) was obtained regardless of high or low phosphate solution. In addition, effective diffusion coefficients (D) of DRP increased exponentially with increasing ionic strengths (R2 = 0.99). Based on the rectified D, the performance of the UiO-66 DGT was independent of solution pH (6.5-8.5) and ionic strengths (ranging from 0.01 to 100 mmol L-1). Furthermore, field deployments of the UiO-66 DGT were undertaken in a natural eutrophic lake (Lake Chaohu, China). It was noteworthy that DRP could be continually accumulated by the UiO-66 DGT for more than 14 d and good agreements were obtained between the concentrations measured by DGT (CDGT) and those by ex situ chemical extraction method in solution (Csol), as reflected by CDGT/Csol of 0.9-1.1. In situ determination of DRP speciation was also carried out at different sites across Lake Chaohu. Overall, this study contributed to a better constructing of liquid binding phase DGT for the measurement of DRP in waters, facilitating the widespread application of the UiO-66 DGT as a routine monitoring technique and for large-scale environmental analysis.

Structure-enhanced removal of Cr(VI) in aqueous solutions using MoS2 ultrathin nanosheets

Molybdenum disulfide (MoS2) was successfully synthesized via a facile one-step hydrothermal method without addition of templates or surfactants. The obtained MoS2 product was characterized via various techniques including FESEM, HRTEM, XRD, BET and XPS analyses. Their application in capturing Cr(VI) ions in water was investigated. The results showed that the MoS2 nanosheets were ultrathin with a thickness of 5–10 nm and had a uniform lateral size of 100–200 nm. Impressively, these MoS2 nanosheets not only possessed enlarged interlayer spacing, but also had multiple defects on the basal planes, which were proved to be crucial in the removal of Cr(VI). The kinetics of chromium adsorption was found to follow the pseudo-second-order rate equation and the adsorption isotherm data were well fitted to the Langmuir model with a chromium in-take capacity of about 84.03 mg g−1. This structure-enhanced removal was likely related to the synergism of adsorption and reduction, in which the anionic Cr(VI) was adsorbed on the surface of D-MoS2via electrostatic interaction. At the same time, some fraction of the adsorbed Cr(VI) was reduced to low toxic Cr(III) and the generated Cr(III) species were adsorbed by the adsorbent. More significantly, the adsorption and reduction processes not only transferred the Cr(VI) from the aqueous environment to the surface of solid adsorbents but also alleviated the toxicity of Cr(VI). It was evident that this study may promote the application of other transition metal dichalcogenides in environmental remediation.

Electrochemical deposition of Pt on carbon fiber cloth utilizing Pt mesh counter electrode during hydrogen evolution reaction for electrocatalytic hydrogenation reduction of p-nitrophenol

Through the slight dissolution of a Pt mesh counter electrode, Pt-deposited carbon fiber cloth (Pt/CFC) was successfully fabricated by successive cyclic voltammetry (CV) scanning from −0.77 V to 0.20 V (vs. RHE) in 0.5 M H2SO4 electrolyte. In the process of successive CV scanning, the electrocatalytic activity of CFC toward the hydrogen evolution reaction (HER) becomes better and better owing to Pt deposition on CFC, and after 1000 CV cycles, the onset potential of CFC is identical to that of the commercial Pt/C catalyst (0 V, vs. RHE) with high current densities at low overpotentials (η10, η100, and η170 = 4.5, 34.5, and 43.5 mV at current densities of 10, 100 and 170 mA cm−2). Interestingly, it was found that as-prepared Pt/CFC exhibits decreased HER activity with an onset potential of −0.26 V (vs. RHE) when Pt mesh was changed to bare CFC as the counter electrode, mainly due to the lower Pt loading amount (0.5 mg cm−2) on CFC compared to commercial Pt/C (Pt loading amount of 1.0 mg cm−2) and the inert CFC counter electrode (not like the Pt mesh counter electrode with slight dissolution in acidic media during CV scanning). Generally, there is great competition between the HER from water splitting and hydrogenation reduction of organic matter using the same HER-active electrocatalyst under identical experimental conditions. In this work, Pt/CFC was, therefore, evaluated for hydrogenation reduction of p-nitrophenol (PNP) to p-aminophenol (PAP) using water as the hydrogen donor in a CFC counter electrode configurated two-compartment reaction system. The results demonstrate that PNP can be effectively converted into PAP with a conversion efficiency of 83.1% and a Faradaic efficiency of 9.9% after electrocatalytic reaction for 12 h at an overpotential of −0.023 V. The findings of this work indicate that Pt counter electrodes are prone to dissolving, resulting in Pt deposition on the working electrode in acidic media, readily causing the false appearance of a non-Pt working electrode with high HER activity. Additionally, the Pt-deposited working electrode produced by this electrochemical approach is efficient for hydrogenation reduction of organic substances into high value-added chemicals using water as the hydrogenation donor.