Multielectrocatalysis by layer-by-layer films based on pararosaniline and vanadium-substituted phosphomolybdate

Efficient electrocatalytic CO2 reduction to ethanol through the proton coupled electron transfer process of PVnMo(12-n) (n = 1, 2, 3) over indium electrode

The multistep proton-coupled electron transfer (PCET) processes are beneficial for products distribution and selectivity of the electrocatalytic CO2 reduction reaction (CO2RR), which are affected by the nature of the catalyst and electrolyte at electrode-electrolyte interface. Polyoxometalates (POMs) are electron regulators of PCET processes, which can catalyze CO2RR effectively. Accordingly, the commercial indium electrodes are combined in this work with a series of Keggin-type POMs (PVnMo(12-n)O40)(n+3)-, n = 1, 2, 3) to process CO2RR with Faradaic efficiency toward ethanol reaching 93.4% at -0.3 V (vs. RHE). The cyclic voltammetry and X-ray photoelectron spectroscopy results reveal the activation of CO2 molecules by the first PCET process of the VⅤ/Ⅳ in POM. Subsequently, the PCET process of MoⅥ/Ⅴ results the oxidation of the electrode, causing the loss of In0 active sites. Electrochemical in-situ infrared spectroscopy confirms the weak adsorption of *CO at the later stage of electrolysis due to the oxidation of the In0 active sites. The indium electrode in PV3Mo9 system retains more In0 active sites owing to the highest V-substitution ratio, thereby ensuring a high adsorption ratio of *CO and CC coupling. In sum, the regulation of the interface microenvironment by POM electrolyte additives can be used to boost the performance of CO2RR.

Hourglass-Type Polyoxometalate-Based Crystalline Material as an Efficient Proton-Conducting Solid Electrolyte

Proton exchange membrane fuel cells are limited because they are limited to working temperatures and are susceptible to damage by dramatic electrochemical environments such as hydrogen peroxide/radicals. It is necessary to develop new proton-conducting materials that are water-stable and can operate at high temperatures. The hourglass reduced molybdophosphate-based compound (H₂bimb)₃[Zn₃(H₆P₄Mo₆O₃₁)₂] (bimb = 1,4-bis[(1H-imidazol-1-yl)methyl]benzene) was designed and synthesized under solvothermal conditions. Single-crystal X-ray diffraction analyses demonstrated noticeably that CUST-571 was composed of an hourglass {Zn[P₄Mo₆]₂} structure, which consisted of two fully reduced half-units {P₄Mo₆}. It was found that CUST-571 possessed an excellent proton conductivity of 4.54 × 10^-3 S cm^-1 at 85 °C and 98% RH (relative humidity). In addition, CUST-571 is capable of an excellent catalytic decomposition of H₂O₂, which is beneficial to increase the life of fuel cells. On the basis of the aforementioned results, CUST-571 may be a promising proton-conducting polyoxometalate hybrid material in the future.

α-γ-Type [Mo8O26]4--Containing Metal-Organic Complex Possessing Efficient Catalytic Activity toward the Oxidation of Thioether Derivatives

In this work, a new α-γ-type [Mo₈O₂₆]^4- anion was first synthesized and characterized by single-crystal X-ray diffraction analysis and was obtained by introducing molybdate to the synthesis of metal-organic complex (MOC) under hydrothermal conditions. An octamolybdate-based MOC, namely, {[Cu₈(H₂O)₆](dpyh)₄(α-γ-Mo₈O₂₆) }·(β-Mo₈O₂₆)·8.5H₂O (H₂dpyh = N,N-bis(3-pyrazolamide)-1,2-hexahydrobenzene), was obtained. The α-γ-type [Mo₈O₂₆]^4- anion was composed of four MoO₆ octahedra and four MoO₅ trigonal bipyramids by sharing their edges and corners. The title complex exhibited a 1D structure in which an α-γ-type [Mo₈O₂₆]^4- anion was connected with [Cu₄(dpyh)₂] units in a staggered manner. Under optimized conditions, complex 1 as the catalyst can achieve a highly efficient conversion (more than 99%) of thioanisole within 30 min and above 99% selectivity toward sulfoxide. Furthermore, efficient catalytic oxidation of thioether derivatives was also performed with 1 as the catalyst. In addition, the stable electrochemical sensing performance and adsorption capacity toward organic dyes were tested.

Polyoxometalates in Analytical Sciences

Polyoxometalates (POMs) have been used for spectrophotometric determinations of silicon and phosphorus under acidic conditions, referred to as the molybdenum yellow method and molybdenum blue method, respectively. Many POMs are redox active and exhibit fascinating but complicated voltammetric responses. These compounds can reversibly accommodate and release many electrons without exhibiting structural changes, implying that POMs can function as excellent mediators and can be applied to sensitive determination methods based on catalytic electrochemical reactions. In addition, some rare-earth-metal-incorporated POMs exhibit fluorescence, which enables sensitive determination by the enhancement and quenching of fluorescence intensities. In this review, various analytical applications of POMs are introduced, mainly focusing on papers published after 2000, except for the molybdenum yellow method and molybdenum blue method.

The rational design of four multifunctional octamolybdate-based complexes for detecting different ions and removing organic dyes from aqueous solution

Four new octamolybdate-based complexes were obtained and used not only as electrochemical sensors for sensing Cr(vi), Fe(iii), BrO₃⁻, and NO₂⁻ ions, but also as adsorbents to adsorb organic dyes.

Octamolybdate-based hybrid constructed by flexible bis-triazole ligands: synthesis, photocatalytic and electrochemical properties

A novel polyoxometalate-based organic–inorganic hybrid compound was synthesized via a facile in situ hydrothermal reaction. 1 exhibits selective photocatalytic degradation of dyes and the great property to electrocatalytically reduce H₂O₂ and NaNO₂.

Various types of isopolymolybdate-based metal–organic complexes formed in different conditions: synthesis, structures, luminescence, electrochemical, and photocatalytic performances

A series of isopolymolybdate-based complexes of different metals and ligands were prepared, which exhibited different luminescence behaviors, current sensing activities, and photocatalytic MB performances.

Paper-Based Electrochemical Detection of Chlorate

We describe the use of a paper-based probe impregnated with a vanadium-containing polyoxometalate anion, [PMo₁₁VO₄₀]⁵⁻, on screen-printed carbon electrodes for the electrochemical determination of chlorate. Cyclic voltammetry (CV) and chronocoulometry were used to characterize the ClO₃⁻ response in a pH = 2.5 solution of 100 mM sodium acetate. A linear CV current response was observed between 0.156 and 1.25 mg/mL with a detection limit of 0.083 mg/mL (S/N > 3). This performance was reproducible using [PMo₁₁VO₄₀]⁵⁻-impregnated filter paper stored under ambient conditions for as long as 8 months prior to use. At high concentration of chlorate, an additional catalytic cathodic peak was seen in the reverse scan of the CVs, which was digitally simulated using a simple model. For chronocoulometry, the charge measured after 5 min gave a linear response from 0.625 to 2.5 mg/mL with a detection limit of 0.31 mg/mL (S/N > 3). In addition, the slope of charge vs. time also gave a linear response. In this case the linear range was from 0.312 to 2.5 mg/mL with a detection limit of 0.15 mg/mL (S/N > 3). Simple assays were conducted using three types of soil, and recovery measurements reported.

Various polyoxomolybdate-based hybrids induced by pH and solvents: structures, adsorption activities for dyes and bifunctional electrocatalytic properties

By changing the pH values and solvents, four different polyoxomolybdate hybrids were synthesized, which display good adsorption activities for different organic dyes and bifunctional electrocatalytic properties.

Innovation in Layer-by-Layer Assembly

Methods for depositing thin films are important in generating functional materials for diverse applications in a wide variety of fields. Over the last half-century, the layer-by-layer assembly of nanoscale films has received intense and growing interest. This has been fueled by innovation in the available materials and assembly technologies, as well as the film-characterization techniques. In this Review, we explore, discuss, and detail innovation in layer-by-layer assembly in terms of past and present developments, and we highlight how these might guide future advances. A particular focus is on conventional and early developments that have only recently regained interest in the layer-by-layer assembly field. We then review unconventional assemblies and approaches that have been gaining popularity, which include inorganic/organic hybrid materials, cells and tissues, and the use of stereocomplexation, patterning, and dip-pen lithography, to name a few. A relatively recent development is the use of layer-by-layer assembly materials and techniques to assemble films in a single continuous step. We name this "quasi"-layer-by-layer assembly and discuss the impacts and innovations surrounding this approach. Finally, the application of characterization methods to monitor and evaluate layer-by-layer assembly is discussed, as innovation in this area is often overlooked but is essential for development of the field. While we intend for this Review to be easily accessible and act as a guide to researchers new to layer-by-layer assembly, we also believe it will provide insight to current researchers in the field and help guide future developments and innovation.

A chitosan–Pt nanoparticles/carbon nanotubes-doped phosphomolybdate nanocomposite as a platform for the sensitive detection of nitrite in tap water

A polyoxometalate-based composite film decorated with CNTs and Pt–CHIT NPs was constructed on an electrode using the LBL self-assembly method. It acted as an electrochemical nitrite sensor with greatly enhanced electron transfer ability and sensing performance.