Amorphous Arsenic Research Articles

A novel strategy for arsenic removal from acid wastewater via strong reduction processing.

Due to the high-acidic arsenic-containing wastewater pollution greatly threatening human health and ecological safety, a simple and efficient method for reducing arsenic was proposed in this paper to solve this problem. By using potassium borohydride (KBH4) as a reducing agent, the soluble arsenic was converted into the gaseous arsine (AsH3) or solid arsenic (As0) to achieve the purpose of removing arsenic in wastewater. By exploring the reaction kinetics of the arsenic removal process, it was found that the fast reaction stage (0-2min) conformed to pseudo-first-order kinetics. The removal rate of arsenic increased to over 73% in 0.5min, and reaction equilibrium was reached after 30min. Various influence factors including arsenic valence, aeration, addition method, concentrations of reducing agent, and hydrogen ion (H+) were investigated. The results showed that As(III) was easier to be removed by reduction than As(V), while adding KBH4 in multiples and aeration were both favorable to the removal of arsenic. Increased concentration of KBH4 also enhanced the removal of arsenic. Appropriate H+ concentration contributed to the arsenic removal, but excessive H+ concentration conversely has an inhibitory effect. The maximum removal rate of arsenic was 95.87%, with the maximum removal capacity of 45.50mg/g. Based on the XRD and SEM-EDS analysis of residue, amorphous arsenic (As0) with a mass ratio of more than 94.52% was generated after the reduction of soluble arsenic. Our study demonstrated that the reaction mechanism of reductive degradation is soluble arsenic with hydrogen radicals (H•) to form arsenic (As0) and arsine (AsH3) (in the molar ratio of 6:1). Although the generated solid arsenic (As0) is convenient for the soluble arsenic removal from wastewater, attention must be paid to the formation of AsH3, and strategies for AsH3 treatment should be considered.

Pressure-induced phase transformations in amorphous arsenic

The atomic structure of amorphous arsenic and its response to high pressure are explored using a constant pressure ab initio molecular dynamics technique. Different analyzing techniques reveal that amorphous arsenic has a local structure close to that of the crystalline phase. The model also presents some twofold and fourfold coordination defects. The existence of a possible amorphous to amorphous phase transition for arsenic is proposed on the bases of the observation of a gradual coordination increase with the application of pressure. Further compression of the amorphous state yields a transformation into a simple cubic crystal.

Arsenic availability in rice from a mining area: Is amorphous iron oxide-bound arsenic a source or sink?

The effect of iron (Fe) redox cycling on the mobility and bioavailability of arsenic (As) in paddy soils has attracted increasing concerns, especially in Asia, where the paddy soil is characteristic of Fe with high abundance and activity. However, whether amorphous Fe oxide-bound As acts as a source or a sink of As in natural field conditions needs to be clarified further. In this study, 73 pairs of soil and rice were collected from paddy fields contaminated by As-containing acid mining drainage. The most significant correlations between the iron fractions and As fractions suggest that Fe redox cycling can directly affect As fractionation in soils, which can then indirectly affect As bioavailability. Significantly negative correlations between amorphous Fe oxide-bound As in soil and As in rice grain were found, indicating that amorphous Fe oxide-bound As acts a sink of As.

Honoring S. R. Elliott

Honoring S. R. Elliott

Arsenic removal from aqueous solution using ferrous based red mud sludge

Ferrous based red mud sludge (FRS) which combined the iron–arsenic co-precipitation and the high arsenic adsorption features was developed aimed at low arsenic water treatment in rural areas. Arsenic removal studies shown that FRS in dosage of 0.2 or 0.3 g/l can be used effectively to remove arsenic from aqueous solutions when initial As(V) concentration was 0.2 or 0.3 mg/l. Meanwhile, turbidity of supernatant in disturbing water was lower than 2 NTU after 24 h. The pH range (4.5–8.0) for FRS in effective arsenic removal was applicable in natural circumstance. Phosphate can greatly reduce the arsenic removal efficiency while the presence of carbonate had no significant effect on arsenic removal. Arsenic fractionation experiments showed that amorphous hydrous oxide-bound arsenic was the major components. When aqueous pH was decreased from 8.0 to 4.5, arsenic in FRS was not obviously released. The high arsenic uptake capability, good settlement performance and cost-effective characteristic of FRS make it potentially attractive material for the arsenic removal in rural areas.

GaMnAs: Layers, Wires and Dots

Thin layers of GaMnAs ferromagnetic semiconductor grown by molecular beam epitaxy on GaAS(001) substrates were studied. To improve their magnetic properties the post-growth annealing procedures were applied, using the surface passivation layers of amorphous arsenic. This post-growth treatment effectively increases the ferromagnetic-to-paramagnetic phase transition temperature in GaMnAs, and provides surface-rich MnAs layer which can be used for formation of low-dimensional structures such as superlattices. If the surface rich it-InAs layer consists of MnAs dots, then it is possible to grow Mn-doped GaAs nanowires.

ToF-SIMS depth profiling of (Ga,Mn)As capped with amorphous arsenic: Effects of annealing time

The influence of annealing time on an amorphous As cap layer and the depth distribution of Mn atoms have been investigated. The results show that a ∼1600 Å thick As cap layer is completely desorbed after 3 h of annealing time. The depth distributions of Mn indicate that interstitial Mn atoms have diffused to the outer surface and being passivated. The thickness of the Mn passivation layer was around 90 Å.

Electron beam induced chemical modification of amorphous chalcogenide–metal bilayers and its application

Metal migration and surface modification have been observed in metal-doped amorphous arsenic and antimony-based chalcogenide systems such as As 2Se 3 and Sb 2S 3 under the irradiation of an electron beam. Surface expansion of the order of 5–35% of the film thickness has been observed by applying 5–30 kV electron accelerating voltages. This electron beam induced surface modification has been employed to produce submicron and nanometre dimensional patterns which will have applications in single stage processing of X-ray masks fabricated on an X-ray transparent silicon nitride (Si 3N 4) membrane. Masks with a silver deficient trough-like structure have been obtained at lower accelerating voltages (5–10 kV). Silver-rich protruding structures were obtained at higher accelerating voltages (15–30 kV). These two types of masks exhibit a different X-ray absorption behaviour by comparison with the regions unexposed to the electron beam.

Surface passivation and morphology of GaAs(1 0 0) treated in HCl-isopropanol solution

A promising chemical surface preparation technique, which consists in the treatment of GaAs(1 0 0) in HCl-isopropyl alcohol (HCl-iPA) solution under nitrogen atmosphere, is further developed. It was shown earlier [Tereshchenko et al., J. Vac. Sci. Technol. A 17 (1999) 2655] that HCl-iPA treatment and subsequent anneals in vacuum yielded atomically clean GaAs(1 0 0) surface with the whole range of surface reconstructions characteristic of this crystal face. In the present work the mechanisms of the passivation of GaAs(1 0 0) surfaces by arsenic overlayers as a result of HCl-iPA treatment are experimentally studied by X-ray photoelectron spectroscopy, low-energy electron diffraction and atomic force microscopy. The HCl-iPA treatment of clean As-stabilized GaAs(1 0 0) surfaces results in chemical passivation of the surface by submonolayer amount of excess arsenic. For the initially oxidized surfaces the treatment leads to the formation of 1–3 monolayers of amorphous arsenic on the surface, with the major part of the arsenic originating from the surface oxides dissolved in HCl-iPA. The HCl-iPA treatment preserves the atomic flatness of the GaAs(1 0 0) surface, keeping the mean roughness on a very low level of approximately ∼0.1 nm.

Spontaneous growth of arsenic oxide micro-crystals on chemically etched MnAs surfaces

Micrometer-size crystals are observed to grow spontaneously on chemically etched MnAs surfaces. The wet chemical etching leaves a nearly exclusive pile of amorphous arsenic on the surface when the MnAs layer is etched incompletely. Using Raman spectroscopy, we identify that these micro-crystals are the arsenolite crystal of arsenic oxides. The manganese in the MnAs layer is oxidized by the hydrogen peroxide in the etch solution to MnO2, which then works as the catalyst for the rapid oxidation and crystallization of the amorphous arsenic.

Preparation of High-Purity Arsenic via Hydrogen Reduction of Arsenic Trichloride

The chemical equilibria involved in the hydrogen reduction of arsenic trichloride under reaction conditions of practical interest are assessed by thermodynamic modeling. Experimental data for high-purity arsenic trichloride are used to optimize the reduction conditions. The optimal residence time of the gas mixture in the reaction zone is 30–50 s. The optimal reduction temperature is 1023 K. Condensation temperatures in the range 600–650 K make it possible to minimize the percentage of amorphous arsenic. The yield of elemental arsenic is ≃99.99%.

The formation of inas quantum dotsin an aluminum oxide matrix

The formation of InAs quantum dots in an aluminum oxide matrix by selective oxidation of AlAs layers in an InAs/AlAs heterostructure is studied. According to the Raman spectroscopy data, the process of selective oxidation leads to stress relaxation in the InAs quantum dots and to the formation of amorphous arsenic at the boundary between oxidized and unoxidized regions.

Extreme band bending at MBE-grown InAs(0 0 1) surfaces induced by in situ sulphur passivation

The passivation of InAs(0 0 1) surfaces by exposure to sulphur in ultra-high vacuum (UHV), followed by capping with amorphous arsenic, has been studied by high resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). Samples grown and passivated at NIMS were transferred to Warwick for analysis, through air and without special precautions. Well ordered and clean InAs surfaces were recovered simply by thermal annealing in UHV. A series of reconstructions were investigated using LEED and XPS by increasing the anneal temperature: a disordered S-rich (1×1), ordered S-terminated (2×1) and finally a S-free (4×2)/ c(8×2) In-terminated surface. The evolution of the HREEL spectra with annealing was also measured, and observations of the surface plasmon mode indicated a very strong surface electron accumulation layer. The downward band bending was largest for the S-rich surfaces with the surface Fermi level as high as 600 meV above the conduction band minimum.

Cluster simulations of structural transformations in yellow arsenic

Yellow arsenic (y-As) consists of tetrahedral As 4 molecules that may be packed in some amorphous and crystalline structures. Like many other arsenic structures, y-As is metastable and undergoes irreversible transitions (polymerization) under irradiation. The process of y-As polymerization, which is observed experimentally, usually leads to the formation of amorphous arsenic (a-As) possessing a continuous random network structure. Our previous quantum chemical simulation for an eight-atom cluster model performed using semi-empirical CNDO/BW approach, combined with optimization technique of cyclic coordinate descent, have shown a formation of molecular dimers due to breaking of one bond in each tetrahedral As 4 molecule accompanied with bond switching over to chair-like structure. In that case a pair of approaching As 4 molecules is positioned in a staggered “face-to-face” configuration ( D 2 d symmetry), which may be considered as a conformation with a six-membered chair-shaped ring dominating in the structure of a polymerized a-As. Two energetically preferable configurations of an As 8 cluster have been found here after careful semi-empirical optimization of the face-to-face structure: they possess either cubane configuration ( O h symmetry) or eclipsed “edge-to-edge” configuration ( D 2 h symmetry). For the first time, a two-dimensional energy surface E( z, θ) has been calculated in order to analyze possible paths of structural transformations in y-As. Analogous qualitative results have been just obtained by us when using ab initio Hartree–Fock method combined with the electron correlation corrections for eight-atom cluster model of molecular arsenic. A comparison of both quantum chemical simulations, together with data from previous experimental studies allows us to describe a possible mechanism of the initial stage of the polymerization of y-As.

Detection of As 2O 3 arsenic oxide on GaAs surface by Raman scattering

GaAs samples after wet chemical etching in nitric acid have been investigated for the first time by Raman spectroscopy. The GaAs surface prepared by this chemical etching results in a rough surface structure with an intrinsic chemically formed oxide film. The Raman scattering data suggest that the oxide layer is primarily composed of As 2O 3 in the crystalline form of arsenolite. This oxide film has been found to be subjected to a tensile stress. No experimental Raman observation of this kind of As 2O 3 arsenic oxide on oxided GaAs surfaces has been reported yet. No trace of amorphous arsenic and of elemental As has been detected.

Formation of As 8 dimers in molecular solid-state arsenic

Molecular yellow arsenic (y-As) consists of tetrahedral As 4 molecules that may be packed in various ways. All y-As modifications, both disordered and crystalline, are metastable and undergo irreversible transitions (polymerization) under action of heat and light, which cause a change in the nature of bonding in the molecules. Polymerization of y-As leads to the formation of amorphous arsenic (a-As) possessing a continuous random network structure. DTA studies show that polymerization is an activated exothermic process. The value of its enthalpy agrees satisfactorily with an estimate of the excess energy of strained ‘banana-shaped’ bonds in an As 4 molecule. Quantum chemical calculations, applying the semiempirical CNDO/BW approach, show that at the initial polymerization stage we have a formation of As 8 dimers due to cleavage of one bond in the tetrahedral As 4 molecule. Simulation of this process shows that formation of a stable As 8 cluster, possessing either D 2h or O h symmetry, may take place if the dimerization reaction path possesses D 2d symmetry. In this case a pair of approaching molecules is positioned in staggered ‘face-to-face’ configuration, which may be considered as a conformation with a six-membered chair-shaped ring dominating in the structure of polymerized a-As. The most favourable is found to be a molecular As 8 dimer with eclipsed ‘edge-to-edge’ configuration (D 2h symmetry).

In situ surface passivation of III–V semiconductors in MOVPE by amorphous As and P layers

Abstract A new process for chemical passivation of III–V semiconductor surfaces in metalorganic vapour phase epitaxy (MOVPE) is developed. A passivation layer is deposited directly after growth in the reactor. It consists of amorphous arsenic or a double-layer package of amorphous phosphorus and arsenic, which are grown by photo-decomposition of the group-V hydrides. These layers (caps) serve to protect the surfaces against contamination in air after removing the samples from the MOVPE growth reactor. Such passivation is applicable e.g. for a two-step epitaxy or for further surface characterizations.

Formation and optical properties of nanometer-sized amorphous arsenic clusters embedded in amorphous SiO 2 by ion implantation

Nanometer-sized amorphous arsenic colloid particles embedded in SiO 2 glasses were prepared by ion implantation of As + ions to a dose of 2 × 10 17 cm −2 at an acceleration voltage of 300 kV. X-ray photoelectron spectroscopy of As 3 p 3 2 levels revealed that approximately 70% of implanted As exists in a neutral state. The formation of amorphous clusters with diameter of 3–6 nm was observed by transmission electron microscopy. These particles were identified as amorphous (a-) arsenic by resonance Raman scattering measurements. The optical band gap determined from a Tauc plot was 1.7 eV, which is larger by ∼ 0.5 eV than that of bulk a-As. A red shift of ∼ 0.05 eV in the optical band gap was observed upon band-gap light illumination. This shift was tentatively ascribed to photo-induced polymerization of As 4 molecules, which are partially contained in the clusters. The nonlinear optical susceptibility, χ (3), measured by degenerate four-wave mixing using a XeCl-pumped dye laser (pulse duration: 20 ns) was 4 × 10 −8 esu. There results were compared with those for P-implanted SiO 2 glasses.

Uniformity of Arsenic Dimer and Tetramer Fluxes from a Valved Arsenic Cracking Source in MBE

AbstractWe have studied the uniformity of the arsenic flux from a commercially available valved arsenic cracker source for MBE. There are many epitaxial structures that require a uniform arsenic flux distribution for rotated epitaxial growth and, in the non-rotated case, for RHEED. For non-stoichiometric GaAs growth at low substrate temperatures, which results in arsenic precipitates being formed during the in-situ anneal process, a specific As/Ga ratio must be used to maintain high crystalline quality and to achieve the desired electrical properties. Also, in the growth of InGaAsP the As/P ratio is equally important for lattice match and quality device results. We report on the uniformity of amorphous arsenic (a-As) films condensed on the surface of 3" diameter sapphire wafers slightly below room temperature. Sapphire substrates were used in place of GaAs wafers to facilitate the use of x-ray fluorescence for the measurement of the arsenic film uniformity. Uniformity maps will be presented for rotated and non-rotated a-As depositions of As2 and As4. Results indicate the uniformity of a-As, As4, films to have a standard deviation of < 2.5% across a 3" diameter substrate for the rotated case. Characteristics of the non-rotated case will be presented.

CdTe/GaAs(100) heterojunctions: Growth modification by thin silicon interface layers

The growth of CdTe epilayers on GaAs has been explored as a potential buffer layer for the fabrication of CMT/CdTe/GaAs structures on silicon substrates. CdTe layers were deposited by molecular beam epitaxy onto GaAs(100) specimens transferred into the II–VI reactor protected by an amorphous arsenic (a-As) cap. Reflection high energy electron diffraction and double- crystal-x-ray diffraction studies show that CdTe{111} layers grow epitaxially on both As- and Ga-rich GaAs(100) surfaces. The relatively narrow x-ray linewidths associated with the layers demonstrate that the layers are of high structural quality. The detailed nature of the interface between CdTe and GaAs has been studied by x-ray photoemission spectroscopy and low energy electron diffraction. It is shown that the interface is highly reactive, leading to the formation of gallium telluride, which influences the growth orientation of the CdTe layer. In an attempt to prevent such interface chemical reactions, we have explored the use of ultrathin silicon interface layers. Thus, GaAs(100) substrates were covered with 1 ML of Si before being capped with a-As. The samples were transferred to a surface science spectrometer, where the a-As cap was removed, producing an ordered Si-(3×1)/GaAs(100) surface. The subsequent growth of CdTe on the Si/GaAs(100) samples yielded well-ordered epilayers, always displaying the (100) orientation. Possible reasons for the change in epilayer crystallographic orientation due to the presence of the Si interlayer are considered.