Alkali Etching Research Articles

Nanoarchitectonics on Bi2MoO6 by alkali etching for enhanced photocatalytic performance

As a layered photocatalyst, the photogenerated carriers separation efficiency of bismuth molybdate (Bi2MoO6) can be improved by introduction of oxygen vacancy. The alkali etching method provides a facile structural tuning way for modifying the bismuth-based semiconductors. Defect state bismuth molybdate (Bi2MoO6) photocatalyst was synthesized by alkali etching. The structure of Bi2MoO6 was tuned by adjusting the concentration of alkali solution. The phase structure, morphology, surface chemistry, and optical property of Bi2MoO6 were characterized by kinds of characterization methods such as X-ray diffraction, scanning electron microscope, X-ray photoelectron spectroscopy, UV–vis diffuse reflectance absorption spectroscopy. The phase of Bi2MoO6 can be transformed to Bi2O3 accompanied with the change of morphology due to the etching of molybdenum atom and oxygen atom from the Bi2MoO6 body structure by sodium hydroxide solution. Meanwhile, the concentration of oxygen vacancy increased obviously as indicated by the XPS results. Therefore, the separation efficiency of photogenerated carriers was enhanced because of the promotion of oxygen vacancy. Alkali etching Bi2MoO6 by 0.075 M sodium hydroxide solution exhibited an excellent photocatalytic performance owing to the suitable vacancy concentration. Superoxide free radicals instead of hydroxyl radicals took the important role in the photodegradation process.

High performance alkaline battery-supercapacitor hybrid device based on diffusion driven double shelled CoSn(OH)6 nanocube@∝-Ni(OH)2 core-shell nanoflower

The design and fabrication of an alkaline battery-supercapacitor hybrid (BSH) device has been proposed here introducing a hierarchically orientated double hydroxide assembly of cobalt tin hydroxide@nickel hydroxide (CoSn(OH)6@∝-Ni(OH)2) as positive (CTH@NH) and nitrogen, sulfur-doped reduced graphene oxide (NSG) as negative electrode. CTH@NH was obtained through few step time-dependant processes. Double shelled CTH nanocube arrays were obtained by excess alkali etching of intermediate nanocubes that are resulted from fast co-precipitation process at earlier stage. Subsequent growth of ∝-Ni(OH)2 nanoflake upon double shelled CTH nanocube at high pressure and temperature resulted CTH@NH. Improvised core-shell nanodimensional architecture, reduced particle size, mesoporosity and ample surface charges encourage diffusion mediated charge storage with deep flow facility of electrolyte into inner core of electrode material and thus strengthening charge storage capability and capacity which assured significant specific capacity (Cm) of ∼ 1467.24 C g−1 (specific capacitance (Cs): ∼ 2445 F g−1) in the hybrid much higher than CTH (Cm ∼ 480.96 C g−1, Cs of ∼ 801.7 F g−1) at 1 A g−1. The [CTH@NH//NSG] assembled BSH acquired CASC of ∼ 151.5 F g−1 at 1 A g−1 current density with a long term cycling stability of ∼ 95% even after 8000 successive charge-discharge cycles. The concurrent battery performance of CTH@NH along with supercapacitive output of the NSG overcomes the deficiency of specific energy of supercapacitor by providing an excellent specific energy of ∼ 53.8 Wh Kg−1 at a specific power of ∼ 800 W Kg−1 and instigates the high-performance features in the as-fabricated BSH device.

High specific surface area NiTiAl layered double hydroxide derived via alkali etching for high performance supercapacitor electrode

As a simple and efficient approach, alkaline etching acquires high porosity and increases the specific surface area for Al containing layered double hydroxides Al containing to improve the specific capacity and rate performance. Herein, NiTiAl layered double hydroxide was synthesized by adding minute amounts of Al to the NiTi-LDH basal layer and then etched some Al using sodium hydroxide solution to obtain higher specific surface area, specific capacity, and rate performance of supercapacitor electrodes. The sample etched for 18 h acquires a high specific surface area up to 203 m2g-1 and the highest specific capacity of 3483 mF cm-2 under 5 mA cm-2, which is 86.58 m2g-1 and 933 mF cm-2 higher than the unetched sample, also acquires a better structural stability than NiTi layered double hydroxide. The same sample exhibits the best rate performance with the highest specific capacity of 2000 mF cm-2 at 30 mA cm-2, which is 57.4% of the specific capacity for the same sample tested at 5 mA cm-2 and 750 mF cm-2 higher than the unetched samples tested at 30 mA cm-2. A hybrid device assembled by the etched sample acquires a higher energy density of 45.1 Wh kg-1 and a power density of 16000 W kg-1.

Synergistic effect of tunable oxygen-vacancy defects and graphene on accelerating the photothermal degradation of methanol over Co3O4/rGO nanocomposites

A series of Co3O4/rGO (reduced graphene oxide) nanocomposites with tunable oxygen vacancy defect concentrations were synthesized via a simple alkali etching and applied for photothermal degradation of methanol. In the samples without alkali etching, the Co3O4/rGO nanocomposite showed higher photothermal catalytic performance than Co3O4 due to the thermal auxiliary effect of rGO. Among the Co3O4/rGO nanocomposites, the nanocomposite etched by 0.64 g NaOH (denoted as Co3O4/rGO-N0.64) exhibited the highest photothermal catalytic performance (methanol conversion of 96%, CO2 yield of 91%) as well as excellent durability under the solar irradiation. The superior activity was mainly attributed to the synergistic effect of the thermal auxiliary effect of rGO and the abundant oxygen vacancy defects. The reaction mechanism confirmed that both of gas phase oxygen and lattice oxygen were activated and participated in this reaction process. The enrichment of oxygen vacancy defects in Co3O4/rGO-N0.64 can boost the circulation of oxygen species, which played a crucial role in the continuous and effective photothermal catalytic oxidation of methanol.

Electronic Metal-Support Interactions for Electrochemiluminescence Signal Amplification.

Au nanoparticle-amplified electrochemiluminescence (ECL) signals are generally realized by nanoparticle morphology modification, functionalization, and nanoalloys formation. It remains a great challenge to utilize the intrinsic catalytic activity of spherical Au nanoparticles for ECL performance improvement. In this work, we prepared the oxygen vacancy-rich CoAl-layered double hydroxide (LDH-Ov)-supported spherical Au nanoparticles via alkali etching of LDH and electrodeposition of Au nanoparticles on the surface of LDH. It was found that the luminol ECL signals of the as-prepared system were significantly enhanced by forming the strong electronic metal-support interaction (EMSI) between Au nanoparticles and LDH-Ov. The further mechanism study demonstrated that EMSI can increase the electron density of interfacial Au atom (Auδ-) due to a redistribution of charge and promote electron transfer between Au species and LDH-Ov. This study not only introduces EMSI to the ECL field but also paves a new way to the applications of the intrinsic activity of spherical Au nanoparticles in ECL signal amplification. We anticipate that EMSI would be applied to other metal nanocatalysts for the development of highly efficient ECL systems.

Operando leaching of pre-incorporated Al and mechanism in transition-metal hybrids on carbon substrates for enhanced charge storage

Operando leaching of pre-incorporated Al and mechanism in transition-metal hybrids on carbon substrates for enhanced charge storage

Coral-like prussian blue analogues-derived bimetallic phosphide with enhanced electrocatalytic performance for hydrogen evolution reaction

Tailoring the architecture is of great significance for developing and designing highly active non-noble metal-based electrocatalysts for hydrogen evolution reaction (HER). Here, the architecture of the precursor was improved by alkali etching after the introduction of Prussian blue analogue (PBA). The coral-like precursor was transformed into bimetallic phosphide via phosphating. Detailed investigation disclares that the introduction of PBA could dramatically improves the electrochemical surface area of the material, and the abundant active site could be obtained via alkali etching. In addition, the removal of amorphous carbon may be the main reason for the enhanced conductivity.

N, S-doped silicon oxycarbide-drived carbon/amorphous ball-flower-like NiO as high performance electrode in asymmetric supercapacitors

Efficient utilization for synergistic effect from different electrode materials with optimized structure and composition is a promising strategy to enhance electrochemical performance of composite electrodes. In this work, amorphous ball-flower-like NiO loaded on N, S-doped silicon oxycarbide-drived carbon (NSCDC/NiO) composites with tunable microstructure were synthesized through an easy alkali etching method followed via a hydrothermal method. The synthesized NSCDC/NiO electrodes display a good capacitance (804 F g−1/111.7 mAh g−1 at 1 A g−1) and hopeful cyclic property. Furthermore, the asymmetric supercapacitor (ASC) device assembled using NSCDC/NiO as anode and N, S-doped silicon oxycarbide-drived carbon (NSCDC) as cathode possesses excellent energy density (100.6 Wh kg−1) when the power density reaches 850.2 W kg−1. After 5000 cycles, ASC device has 84.0% retention rate of original capacitance, showing that the NSCDC/NiO composite is a satisfying and promising electrode applied in supercapacitors. The electrochemical properties of NSCDC/NiO composites are greatly improved owing to the excellent electrical conductivity of NSCDC framework, ball-flower-like NiO with amorphous state and hierarchical porous structure, which are beneficial to transportation of ion and electron, reduction of volume change during charge-discharge process.

Preparation of flexible Pt/TiO2/γ-Al2O3 nanofiber paper for room-temperature HCHO oxidation and particulate filtration

A flexible Pt/TiO2/γ-Al2O3 nanfiber paper (PTA) was successfully prepared by an electrospinning-coating-NaBH4 reduction method. The catalytic performance for formaldehyde (HCHO) oxidation over PTA with different Pt contents was investigated at room temperature. The results show that PTA exhibits higher catalytic activity and stability than Pt/γ-Al2O3. The TiO2 coating can promote the transformation of intermediate species and the desorption of CO2. The PTA1 with 1 wt% Pt shows the best catalytic activity and good stability. In the view of activity and cost, 0.5 wt% Pt is the optimal Pt content. The TiO2 coating not only plays an important role in improving the activity and stability of PTA, but also acts as a protective shell of γ-Al2O3 fiber, which greatly reduces the risk of alkali etching during preparation and maintains the good flexibility of PTA. The dioxymethylene (DOM), formate/carbonate species, CO are the intermediate species in HCHO oxidation over PTA. In addition, PTA exhibits good filtration performance for fine particles as well as good fluid permeability. Because of the high efficiency and flexibility of PTA composite catalytic material for HCHO oxidation with good filtration performance for fine particles, it will be a promising candidate in air purification.

Metal-modified PtTe2 nanorods: Surface reconstruction for efficient methanol oxidation electrocatalysis

Nanomaterials with both one-dimensional structure and surface defect are attractive for electrocatalysis owing to the increased atomic utilization and exposed high-energetic sites. Herein, utilizing the alkali etching to drive structure reconstruction, a class of Co-modified PtTe2 nanorods (ae-PtTeCo NRs) with surface defects are designed for efficient methanol oxidation reaction (MOR). The optimized ae-Pt2Te6Co3 NRs show remarkable mass activity of 1.47 A mg−1Pt and specific activity of 8.3 mA cm−2, which are 3.4-fold and 8.1-fold higher than those of commercial Pt/C, respectively. Electrochemical measurements and physical characterizations reveal that the enhanced MOR performance of ae-Pt3Te6Co2 NRs is attributed to the abundant surface defects and optimized electronic structure, which effectively promote the oxidation of CO-like intermediates. Meanwhile, such an alkali etching strategy is universal to fabricate other metal-modified PtTe2 NRs with surface defects, achieving a class of ae-PtTeM NRs (M = Ag, Sn, Cu, Pd, Pb, Ir, In, Cd, Bi, Ni, Ru, Ga).

Capturing functional two-dimensional nanosheets from sandwich-structure vermiculite for cancer theranostics

Clay-based nanomaterials, especially 2:1 aluminosilicates such as vermiculite, biotite, and illite, have demonstrated great potential in various fields. However, their characteristic sandwiched structures and the lack of effective methods to exfoliate two-dimensional (2D) functional core layers (FCLs) greatly limit their future applications. Herein, we present a universal wet-chemical exfoliation method based on alkali etching that can intelligently “capture” the ultrathin and biocompatible FCLs (MgO and Fe2O3) sandwiched between two identical tetrahedral layers (SiO2 and Al2O3) from vermiculite. Without the sandwich structures that shielded their active sites, the obtained FCL nanosheets (NSs) exhibit a tunable and appropriate electron band structure (with the bandgap decreased from 2.0 eV to 1.4 eV), a conductive band that increased from −0.4 eV to −0.6 eV, and excellent light response characteristics. The great properties of 2D FCL NSs endow them with exciting potential in diverse applications including energy, photocatalysis, and biomedical engineering. This study specifically highlights their application in cancer theranostics as an example, potentially serving as a prelude to future extensive studies of 2D FCL NSs.

Cation vacancy driven efficient CoFe-LDH-based electrocatalysts for water splitting and Zn–air batteries

The rich cation vacancies CoFeV-LDH with excellent OER properties was synthesized by tunable alkali etching. Pt was anchored within vacancies (Pt@CoFeV-LDH) by UV reduction to optimize its HER and ORR performance.

Fabrication of a stable Ti/Pb-TiOxNWs/PbO2 anode and its application in benzoquinone degradation

To delay passivation of a titanium (Ti) substrate as well as enhance adhesion between an electrodeposited PbO2 coating and the Ti substrate, a titanium-lead composite oxide nanowire (Pb-TiOxNWs) intermediate layer was formed in situ on the surface of porous Ti by alkali etching, ion substitution, and high-temperature calcination. At the same time, Ti/PbO2 and Ti/TiO2NWs/PbO2 electrodes with porous Ti as a matrix were prepared for comparison. The surface structure and morphology of the prepared intermediate layer and the PbO2 coating were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The influences of the composite oxide intermediate layer on the electrochemical performance of the PbO2 electrode were analyzed by cyclic voltammetry (CV), linear sweep voltammetry (LSV), and AC impedance spectroscopy (EIS). Accelerated lifetime tests were performed for electrodes with and without different intermediate layers. The results showed that PbOx was incorporated into the titanium dioxide three-dimensional network structure, resulting in formation of Pb-TiOxNWs. The surface of the Ti/Pb-TiOxNWs/PbO2 electrode was denser due to the smaller particle size of PbO2. The preferred crystal orientation of β(110) was observed for PbO2 deposited on Ti/Pb-TiOxNWs. The oxygen evolution potential reached a maximum of 2.19 V for Ti/Pb-TiOxNWs/PbO2. Accelerated life tests showed that compared with Ti/PbO2 and Ti/TiO2NWs/PbO2, the electrode life of Ti/Pb-TiOxNWs/PbO2 was increased by 91.7% and 35.3%, respectively. Therefore, it can be concluded that significantly improved morphology and electrochemical performance were achieved for titanium-based PbO2 electrodes by the addition of a Pb-TiOxNWs intermediate layer. In particular, the electrochemical stability of the PbO2 coating electrodes was improved markedly by the Pb-TiOxNWs intermediate layer. The electrodes were used for electrochemical oxidation of benzoquinone in wastewater (100 mg/L). It was found that chloride ions played a critical role in improving the current efficiency of electro-oxidative degradation. Under the same conditions, the COD removal rate in the presence of NaCl was 45% higher than in the presence of sulfate. The results of HPLC analysis of the intermediate products indicated that the oxidants electro-generated by chloride ions had stronger ring-opening and mineralization capabilities than those electro-generated by sulfate ions.

Alkali etching metal silicates derived from bamboo leaves with enhanced electrochemical properties for solid-state hybrid supercapacitors

Metal silicates are demonstrated to be potential electrode materials for rechargeable batteries and supercapacitors (SCs). However, their electrochemical properties applied to SCs in the literatures are far less than the expectation. Herein, we propose a strategy of alkali etching process to improve the electrochemical properties of metal silicates. First, nickel silicate (NiSi), cobalt silicate (CoSi) and manganese silicate (MnSi) with the interconnected nanosheets are derived from bamboo leaves (BLs) by a calcination in combination with a facile hydrothermal method. Then 3 M NaOH aqueous solution was used to soak the metal silicates for alkali etching to obtain the etching metal silicates (denoted as e-NiSi, e-CoSi, e-MnSi). The alkali etching can enlarge the accessibility for electrolyte ions and improve the electrochemical performance of metal silicates. In the three-electrode system, the e-NiSi-3, e-CoSi-3 and e-MnSi-3 exhibit higher specific capacitances and cycling performances than that of NiSi-3, CoSi-3 and MnSi-3. The solid-state hybrid supercapacitor (HSC) devices are fabricated by using the metal silicates as the positive material and activated carbon (AC) as the negative material. In the two-electrode system, the e-MSi-3//AC HSC devices (M = Ni, Co and Mn) deliver higher areal specific capacitances and energy densities and better cycle performance than those of MSi-3//AC HSC devices, for instance, e-NiSi-3//AC HSC device (412 mF cm−2 at 4 mA cm−2, 3.2 Wh m−2 at 3.8 W m−2) is superior to NiSi-3//AC HSC device (207 mF cm−2 at 4 mA cm−2, 1.6 Wh m−2 at 3.8 W m−2). This work proves that the alkali etching process can enhance the electrochemical performance of metal silicates and e-MSi-3 can be hopeful and prominent electrode materials applied to high-performance SCs.

Mesoporous Fe3O4@C nanoarrays as high-performance anode for rechargeable Ni/Fe battery

Rechargeable aqueous batteries with high power density and energy density are highly desired for electrochemical energy storage. Despite the recent reports of various cathode materials with ultrahigh pseudocapacitance exceeding 3000 F g−1 (or 800 mA h g−1), the development of anode materials is relatively insufficient, which limits the whole performance of the devices far from practical applications. Herein, we report the preparation of mesoporous Fe3O4@C nanoarrays as high-performance anode for rechargeable Ni/Fe battery by a self-generated sacrificial template method. ZnO/Fe3O4 composite was first synthesized by a co-deposition process, and ZnO was subsequently removed by alkali etching to construct the mesoporous structure. A thin carbon film was introduced onto the surface of the electrode by the carbonization of glucose to increase the structural stability of the electrode. The unique mesoporous nanoarray architecture endows the electrode with larger specific surface area, faster charge/mass transport and higher utilization of Fe3O4, which shows an ultrahigh specific capacity (292.4 mA h g−1 at a current density of 5 mA cm−2) and superior stability in aqueous electrolyte (capacitance retention of 90.8% after 5000 cycles). After assembled with hierarchical mesoporous NiO nanoarray as a cathode, an optimized rechargeable Ni/Fe battery with double mesoporous nanoarray electrodes was fabricated, which provided high energy/power densities (213.3 W h kg−1 at 0.658 kW kg−1 and 20.7 kW kg−1 at 113.9 W h kg−1, based on the total mass of the active materials) in the potential window of 1.5 V with excellent cyclability (81.7% retention after 5000 charge/discharge cycles).

Reduced graphene oxide decorated CoSnO3@ZnSnO3 with multi-component double-layered hollow nanoboxes for high energy storage and capacity retention asymmetric supercapacitors

Double-layered hollow nanostructure composed of CoSn(OH)6@ZnSn(OH)6 which are derived from CoSn(OH)6 nanoboxes wrapped by ZnSn(OH)6 cover layers is prepared by a controlled alkali etching method, followed by heating treatment to prepare reduced graphene oxide (rGO) decorated CoSnO3@ZnSnO3 nanoboxes. The results revealed that the specific surface area of CoSnO3@ZnSnO3 reached 70.29 m2 g−1, which is much higher than that of CoSnO3 (44.25 m2 g−1). The enlargement in specific surface area of the double-layered hollow structure can responsible to provide sufficient buffer space for the dramatic volume change during the cycle. Moreover, rGO can effectively promote the transmission of ions or electrons and can provides a large reaction interface for electrochemical reaction. In addition, it can avoid CoSnO3@ZnSnO3 agglomeration and further improve storage capacity and stability. When the current density is 0.5 A g−1, the capacitance of material is 400.2 F g−1, which is higher than CoSnO3 and CoSnO3@ZnSnO3. Besides, the capacity retention rate of CoSnO3@ZnSnO3/rGO reaches 85% after 1100 cycles at a current density of 3 A g−1. This work demonstrates the interaction between the modification of the rGO network and the multi-layered hollow structure with the high specific surface areas can provide large spaces to effectively buffer volume changes and facilitate electron and electrolyte ion transport.

Hemishell Zeolites Synthesized by Asymmetric Modification as Biphasic Nanoreactors with Tunable Amphiphilicity for Catalysis of Cascade Reactions.

It is strongly desired to design and synthesize amphiphilic nanoreactors with tunable compatibility, which are stable at the biphasic interface in both acidic and alkaline environments. Herein, a novel amphiphilic R1-ZSM-5-R2 nanoreactor with adjustable hydrophilic-lipophilic balance (solid) (HLB(S)) values has been successfully synthesized by hydrophilic/lipophilic asymmetric modification of the surface of hemishell zeolites. The hemishell zeolites obtained by alkali etching have different surfaces for this asymmetric modification. Owing to the unique hemishell structures and asymmetric modification, the R1-ZSM-5-R2 nanoreactors with an optimized type and amount of modified organosilanes show excellent stability and emulsifying properties under extreme environments, which is important for cascade reactions in a biphasic system. The modified amino groups on the surface of the nanoreactors not only enhance the hydrophilicity of the hemishell zeolites and stabilize ultrasmall Pt nanoparticles (1.90 nm) but also used for the catalytic synthesis of trans-cinnamaldehyde. The Pt@R1-ZSM-5-R2 amphiphilic catalysts fabricated through a one-step reduction of Pt nanoparticles present outstanding performances in the biphasic cascade synthesis of cinnamic acid, achieving a very high turnover frequency (TOF) of 978 h-1. The TOF values of the catalysts correspond well to the HLB(S) values of the R1-ZSM-5-R2 nanoreactors.

AFM Observation of Etch-Pit Shapes on β-Ga<sub>2</sub>O<sub>3</sub> (001) Surface Formed by Molten Alkali Etching

In order to understand the crystal defects of beta-gallium oxide (β-Ga2O3) in more detail, we classified the crystal defects of a 2-inch substrate of β-Ga2O3 (001) single crystal. As a result of observing the etch pits formed by molten alkali etching using scanning electron microscope (SEM) and atomic force microscope (AFM), we succeeded in observing six different etch pit shapes. These etch pit shapes are categorized into “Cicada I type”, “Cicada II type”, “Cannonball type”, “Trapezoid type”, “Bar type”, and “Shell type”. We consider that “Cicada I type” and “Cicada II type” are etch pit shapes caused by planar defects, and “Cannon ball type” is etch pit shapes due to dislocations. In addition, “Trapezoid type”, “Bar type”, and “Shell type” are deduced the result of surface morphology.

Alkali Etching of Layered Double Hydroxide Nanosheets for Enhanced Photocatalytic N2 Reduction to NH3

AbstractLayered double hydroxide (LDH) nanosheets show good activity in a wide range of photoreactions, with this activity being generally attributable to an abundance of surface oxygen vacancies or coordinatively unsaturated metal cations in the nanosheets which serve as active sites for reactant adsorption and activation. Recently, LDH nanosheets have been shown to be very effective for photocatalytic N2 reduction to NH3 using water as the reducing agent. Herein, it is demonstrated that a simple pretreatment of ZnCr‐LDH, ZnAl‐LDH, and NiAl‐LDH nanosheets with aqueous NaOH can greatly enhance the concentration of oxygen vacancies and low coordination metal centers in the nanosheets, thus significantly enhancing their photocatalytic activity for N2 reduction to NH3 under UV–vis irradiation (without the need for added sacrificial agents or cocatalysts). The facile alkali etching strategy introduced here is expected to be widely adopted in the future development of high‐performance LDH photocatalysts for ammonia production and other challenging chemical transformations (e.g., CO2 reduction and water splitting).

Fluorine-free treatment of Ti3AlC2 for supercapacitor electrode material

A new supercapacitor electrode material Ti3C2TX@Al (T = OH, O) derived from Ti3AlC2 is prepared by partly removing aluminum using alkali etching method at 40 °C. The reserved aluminum can not only ensure high interlayer conductivity but also prevent the structure from collapsing. The effect of etching time is investigated, the sample Ti3C2TX@Al obtained after etching 36 h in sodium hydroxide exhibits the best electrochemical performance among the series of samples. In 1M H2SO4, the material shows a high specific capacitance of 587 F g−1 at the current density of 1 A g−1, and 173 F g−1 at 10 A g−1. A low capacitance loss can be observed (0.0025% one cycle) after 6000 cycles test at the current density of 20 A g−1. It implies that the material Ti3AlC2 which etching in sodium hydroxide for 36 h may be a promising candidate for energy storage material, and this simple synthesis method can also be applied to other MAX members.