PbS Surface Research Articles

Galena surface reactivity at acidic pH and 25 °C based on flow-through and in situ AFM experiments

A study of (PbS) galena dissolution at acidic conditions was carried out by means of nonstirred flow-through experiments , in situ and ex situ Atomic Force Microscopy (AFM) experiments, X-ray photoelectron spectroscopy (XPS) surface analysis, and X-ray diffraction (XRD) patterns of the reacted surfaces. The nonstirred flow-through experiments were performed at pH 3, 25 °C, and oxygen-saturated atmosphere using both raw galena and a pyritic sludge from the Aznalcóllar mine tailing (SW Spain) with 0.8 wt.% of galena. Based on the Pb release, the steady-state dissolution rate of galena, normalized to the initial specific surface area, is 1.2±0.18×10 −10 mol m −2 s −1. The in situ AFM experiments were carried out at the pH range from 1 to 3 at 20±3 °C in a saturated O 2 atmosphere using galena fragments of known dimensions. Also based on Pb release, the galena dissolution rates were estimated by normalizing to the geometric area. A derived empirical rate law describing the dissolution rate–pH dependency at 1<pH<3 can be expressed as R Pb = 10 − 5.16 a H + 0.41 where R Pb is the galena dissolution rate in mol m −2 s −1. Moreover, using the AFM images, galena dissolution rates were estimated by carrying out a systematic section analysis of the surface microtopography variation as dissolution of the {100} cleavage surface occurred. The AFM-estimated galena dissolution rates were slower than the dissolution rates based on the Pb release probably because of lower reactivity of the area scanned by the AFM probe compared with the entire surface reactivity. The dissolution of galena appeared to be noncongruent as aqueous sulphur depletion was observed, resulting in a Pb/S ratio higher than one. We suggest that through the overall dissolution reaction, a fraction of H 2S (aq) is converted into H 2S (g) as S is detached from the PbS surface, causing the aqueous S deficit. The overall dissolution mechanism observed on the {100} galena surface is similar to the one reported by De Giudici and Zuddas (De Giudici, G., Zuddas, P., 2001. In situ investigation of galena dissolution in oxygen saturated solution: Evolution of surface features and kinetic rate. Geochimica et Cosmochimica Acta 65, 9, 1381–1389) in which surface protrusions form over the PbS surface and dissolve continuously. Furthermore, the ex situ Tapping mode images show the growth of larger protrusions on galena substrate at acidic pH. A potential oxidative effect of the reacting solution on the galena dissolution mechanism at acidic pH was also studied: (1) As the HNO 3 solution is more oxidative than HCl solution, the protrusions formed faster over the PbS surface; (2) since the Fe(III) in solution reduces to Fe(II) to oxidize sulphur to sulphate, PbSO 4 and S precipitate on the PbS surface. The XPS surface analysis and the XRD pattern of the reacted {100} PbS surfaces yield further insight into the existence of lead–sulphur phases such as anglesite (PbSO 4) and elemental sulphur on the PbS surface.

Rumpled relaxation and surface core-level shift in PbS(001)

The surface relaxation of the PbS (001) surface has been calculated by the all-electron full potential linear augmented-plane-wave method. Surface core level shifts have been estimated at different atomic sites of the rumpled surface of the crystal. The rumpling is such that the cations are slightly below the anions in the surface layer. Comparison with recent experimental measurements by x-ray photoemission spectroscopy shows good agreement. The initial state approximation slightly overestimates the value of the surface core level shifts (SCLS) whereas the final state effect reduces the SCLS. It is shown that the Madelung potential cannot explain the shifts.

The geometric and electronic properties of the PbS, PbSe and PbTe (0 0 1) surfaces

The geometric and electronic structures of the PbS, PbSe and PbTe(0 0 1) surfaces were investigated by using the density functional theory. The calculations show that the (0 0 1) surfaces of these semiconductors do not reconstruct but exhibit oscillatory geometric relaxation to some extent. The interlayer distance shift and the intralayer rumpling have some obvious regularity. The calculations also show that the electronic structures of these semiconductors are very similar. The direct surface band gap locates at the X point and there are no new surface states appearing in the fundamental band gap. Some p characters surface resonance states appear mainly in the valence and the conduction band, and the other surface state with the anions s character also appears in the deep band gap below −10.0 eV.

The adsorption interaction of anionic sulfhydryl collectors on different PbS(1 0 0) surface sites

Computational ab initio methods were used to study the local adsorption interaction of diethyl dithiocarbamate ((CH 3CH 2) 2NCS 2 −), ethyl xanthate (CH 3CH 2OCS 2 −) and ethyl trithiocarbonate (CH 3CH 2SCS 2 −) collectors with the PbS(1 0 0) surface using cluster models. The study showed that ethyl xanthate and ethyl trithiocarbonate adsorb monodentatively, and diethyl dithiocarbamate forms bidentate coordination on the lead atom site of galena. The interaction energy of diethyl dithiocarbamate is stronger than that of ethyl xanthate or ethyl trithiocarbonate. The interaction of the collectors was also detected on the sulfur atom site of the PbS(1 0 0) surface. The results provide information on the effect of the collector structure on the adsorption on the PbS surface.

First principles studies of the surface of galena PbS

Ab initio calculations of the surface structure and properties of the (001) surface of galena (PbS) have been performed using two complementary approaches. Both Gaussian and numerical basis functions have been utilized, as embodied within the programs CRYSTAL98 and SIESTA, with their associated implementations, in order to determine to what extent computational factors are influencing the results. Relaxations are presented based on calculations employing the HF, B3LYP, LDA, and GGA Hamiltonians. Significant relaxations (up to about 0.2 Å) are found to occur at the surface which decay with increasing depth into the bulk. The magnitude of the relaxation oscillates between layers, leading to pairing, with Pb ions being displaced outwards (by approximately 0.05 Å) relative to the S ions. We find that the LDA, GGA, and B3LYP Hamiltonians give results in reasonable agreement with each other, but that the structures computed with HF theory are vastly different. The present calculations also predict a surface core level shift of 0.3 eV for the sulphur 2 p level at the surface itself, in excellent agreement with a recent experimental determination.

Magnetic and Structural Properties of EuS-PbS Multilayers Grown on n-PbS (100) Substrates

Magnetic properties of semiconductor EuS(t)-PbS(d)-EuS(t) ferromagnetic trilayers (t = 30 ÷ 300 A and d = 7.5 ÷ 70 A) grown on n-type monocrystalline PbS (100) substrate were studied by SQUID magnetometry and ferromagnetic resonance technique yielding, in particular, the dependence of the ferromagnetic Curie temperature on the thickness of the EuS layer. Structural parameters of layers were examined by X-ray powder diffraction analysis. A high structural quality of the substrate and the multilayer was verified by the measurements of the X-ray rocking curve width indicating the values of the order of 100 arcsec and by atomic force microscopy revealing the presence on the cleft PbS surface regions practically flat in the atomic scale over the area of 1 x 0.1 μm 2 .

Field-effect-assisted photoconductivity in PbS films deposited on silicon dioxide

Lead sulfide (PbS) thin films were deposited from a chemical bath onto SiO2/Si (n-type) substrates. Pseudo-metal–oxide–semiconductor devices were obtained by evaporating source and drain gold electrodes on a PbS surface and aluminum gate electrode on a Si substrate. Field-effect-assisted photoconductivity in the PbS layer was investigated at room temperature, in the 800–2700-nm-wavelength domain for different values and polarities of the drain and gate voltages. The best results were obtained for a positive gate, when both semiconductors are in depletion. An enhancement of about 25% of the photoconductive signal is obtained compared with the case when the gate electrode is absent or is not used. A simple model is proposed that explains the behavior of the dark current and photoconductive signal in PbS film with changing the gate voltage.

Photocurrent instability of PbS-sensitized TiO 2 electrodes in S 2− and SO 32− solution

The mechanism of photocurrent decay with time of illumination of PbS-coated TiO 2 (PbS/TiO 2) photoelectrochemical cells in S 2− and SO 3 2− solution was investigated. The photocurrent decay at short circuit can primarily be attributed to the formation of PbSO 4 on the surface of PbS, followed by the dissolution of PbSO 4 into the aqueous solution. In the presence of pyridine or ethanol in the solution, the J sc decay with time of illumination is reduced due to the suppression of the PbSO 4 formation.

Step edges on galena (100): Probing the basis for defect driven surface reactivity at the atomic scale

Scanning tunneling microscopy (STM) images of the PbS (100) surface with a step and several kinks were obtained with atomic resolution. These images show an increased tunneling current at step edge sites and an apparent deformation of the lattice near the step. The experimental images are compared with theoretical ab initio calculations for which we developed a hybrid method of constant current and constant height mode STM image simulation. With these calculations, we find that the apparent deformation is mainly an electronic effect rather than relaxation of atoms. In addition, with the help of these calculations, we can identify the changes of individual terrace-like and step-like orbitals that are observed using the STM in terms of the energy, density and shape of these states. This detailed knowledge of the electronic behavior of the PbS surface near a step can be used as a basis for explaining adsorption, acid/base, and redox behavior on PbS terraces and at steps, and the differences between the two.

Electrochemical quartz crystal microbalance study on cyclic electrodeposition of PbS thin-films

Electrochemical quartz crystal microbalance combined with cyclic voltammetry was used to study the electrodeposition of PbS thin-films on Au surface. The electrodeposition was carried out by cycling between two potentials with voltage scan rates of 0.1 and 0.01 V/s. In addition, the electrochemical behaviour of PbS film and the related precursors, PbEDTA 2− and HS − were studied separately. It was found that the reduction of PbEDTA 2− occurred at a more positive potential on the PbS surface than on the Au surface. Similarly, HS − oxidised at a more negative potential on the PbS than on the Au surface. Therefore, the deposition of PbS was proposed to occur by two mechanisms. The first mechanism includes the reduction of PbEDTA 2− to elemental lead which during the oxidation reacts with HS −. In the other mechanism, PbS surface induces the chemical reaction between PbEDTA 2− and HS. The electrodeposited PbS is stable in the potential range of −1.2 to −0.2 V vs. Ag/AgCl.

Field Effect Controlled Photoresistors Based on Chemically Deposited PbS Films

AbstractMOS-like structures were obtained by chemical deposition of a polycrystalline PbS thin film on top of a silicon dioxide/Si substrate. Gold ohmic electrodes in coplanar configuration were subsequently deposited by vacuum evaporation on PbS surface (drain and source electrodes). The gate aluminum electrode was deposited on the back of the Si substrate. The dependence of the photoconductive signal, generated in the PbS film, on the gate voltage was studied for wavelengths ranging between 800 nm and 3000 nm at room temperature as well as at low temperatures. It was found that the relative variation of the signal could be as high as 50 % for gate voltages ranging between −30 V and +30 V. Two possible mechanisms are proposed to explain the signal variation with the gate voltage: 1) Variation of the depleted region's thickness in the PbS film, that leads to a variation of the conduction channel's resistance (the reference resistance called, also, the dark resistance), 2) The possible variation of the majority carriers (holes) life-time due to the electron blocking at the PbS/oxide interface when positive gate voltages are applied on the back electrode. Integrated IR detectors with controlled sensitivity in the 800–3000 nm range can be manufactured at a relatively low cost using the PbS/oxide/Si MOS-like structure.

Surface structure and core-level shifts in lead chalcogenide (001) surfaces

We study the geometrical structure, the relaxation, and the surface core-level shifts of PbS and PbTe (001) surfaces via ab initio pseudopotential calculations. In both systems, an oscillatory relaxation of the surface-layer separations is predicted and this feature is expected to be present in other polarizable ionic rocksalt compounds. In agreement with experiment, PbTe displays a large surface rumpling that is absent in PbS. Reasons for these similarities and differences are addressed. Our results compare well with most available experimental data. Partial disagreement with a recent x-ray standing wave investigation in PbS is discussed. Core-level shifts in PbS are very small and are not sensitive to surface relaxation. Therefore, they are not a useful tool to investigate surface geometry in these compounds.

Atomic and electronic structure of epitaxial PbS on InP(110) and InP(001)

Thin films of PbS have been deposited on InP(110) and InP(001) surfaces. Epitaxial growth of the films was obtained already at room temperature (RT). Low-energy electron diffraction (LEED) patterns according to the PbS(001) surface were obtained on both substrate orientations. This result indicates a rather good lattice match on InP(001), while an incommensurate lattice matching was observed along [001] on InP(110). Core level photoemission studies have demonstrated that an abrupt interface forms at the (110) substrate orientation, while an interface reaction occurs on the In-terminated InP(001)-2×4 superstructure after PbS deposition. The electronic structure of PbS films was studied experimentally by using angle-resolved ultraviolet photoemission spectroscopy (ARUPS) and theoretically by applying supercell calculations on the density functional theory level in the local density approximation (DFT-LDA). The supercell calculations indicate surface rumpling and inward relaxation as well as the existence of up to three surface-related states on PbS(001). The experimental data for PbS bulk and thicker PbS films coincide well and agree reasonably with the simulations. The dispersion of the electronic states in ultrathin PbS films (2 nm) is significantly suppressed in comparison to bulk, indicating the incomplete development of the bulk-derived electronic states.

SIMS studies of oxidation mechanisms and polysulfide formation in reacted sulfide surfaces

The surface oxidation of metal sulfides in air and aqueous solution is of central importance in mineral separation and environmental control of acid mine drainage. Mechanisms of oxidation, dissolution and surface restructuring have been extensively studied using XPS. High binding energy components in S 2p XPS spectra have been attributed to metal-deficiency, formation of polysulfide S n 2−, elemental sulfur and electronic defect structures (ie Cu(I)/ZnS). The assignment of these components in S 2p XPS spectra has, however, left significant uncertainties particularly in the formation of SS bonding in polysulfide species requiring confirmation from other surface analytical techniques. The use of static ToF-SIMS has provided a new avenue for identification of these species and their development in oxidation of the sulfide surfaces. For the iron sulfides, there is a systematic change in the FeS 2/FeS fragment ratio from troilite (FeS) through pyrrhotite (Fe 1−xS) to pyrite (FeS 2) with ratios varying from 0.59, 1.2 to 32 respectively. Similarly, high ratios for FeS n/FeS are found for pyrite compared with pyrrhotite and troilite mirrored in the S n/S fragment ratios. Changes in surface oxidation, represented in atomic concentrations and S 2p XPS spectra, are seen in the ToF-SIMS signals for S n/SO n ratios in the same iron sulfide sequence. These mass markers, reflecting increased SS bonding, increase in surfaces after oxidation giving further confidence in XPS assignment to polysulfide species. Freshly cleaved galena PbS surfaces reacted in pH8 aqueous solution for increasing periods of time have also shown a systematic increase in S n/S ratios with increasing at.% of oxidised S n 2− species from XPS spectra. Statistical analysis of oxidised galena has shown that the ratios 206PbO +/ 206Pb + and 208PbOH +/ 208Pb + directly reflect the degree of oxidation of the surface lead species whilst the O −/S −, S −/total — ion yield and SO 3 −/S − are the best measures for following the oxidation of sulfur species. Results from these ratios suggest that initial air oxidation takes place predominantly on the S sites rather than Pb sites but, in solution at pH9, both sites are oxidised. The ToF-SIMS results appear to directly reflect the surface chemistry of the metal and sulfur species and are not consistent with recombination or fragmentation of secondary neutral or ionic species. The results strongly suggest increasing polymerisation of SS species with increasing oxidation in accord with the XPS assignment to polysulfide of increasing chain length.

Structure and stability of galena (PbS) at the interface with aqueous solution: a combined embedded cluster/reaction field study

We have explored the adsorption and reactivity of molecular water on the surface of PbS (galena) and its interface with aqueous solution, employing the recently proposed CECILIA method [E.V. Stefanovich, T.N. Truong, J. Chem. Phys. 106 (1997) 7700]. Using an embedded cluster model of the solid and including the effects of bulk water via a reaction field, our results suggest that physisorption occurs at the perfect (001) surface, with a barrier to molecular adsorption at around 5Å above the surface. At a step defect, chemisorption of the water is found. The barrier to dissociation is increased upon inclusion of the effect of aqueous solution. Reasons for these findings are discussed.

Role of zinc sulfate in depression of lead-activated sphalerite

The results of flotation of lead-activated sphalerite, in presence and absence of zinc sulfate, are presented in this paper. Zeta potential and adsorption measurements as well as thermodynamic calculations, substantiate that activation of sphalerite by lead nitrate is due to a replacement reaction: ZnS+Pb ++=PbS (surface)+Zn ++ Depression of lead-activated sphalerite by zinc sulfate occurs by the reaction: PbS (surface)+Zn ++=ZnS+Pb ++ The equilibrium constant for this reaction is modified to suit the conditions at the solution/mineral interface.

Cluster models of the dissociation of water on the surface of galena (PbS)

Cluster models have been used to investigate the reaction of water on the surface of PbS (galena). The results from both semi-empirical and ab initio levels of theory suggest that on a perfect {001} surface, water is a stable species and dissociation does not occur. However, at a small step-like feature the reaction PbS+H 2O→Pb(OH) ++HS − is exothermic with a sufficiently low barrier that a facile reaction occurs at ambient temperature.

Temperature Controled Photoelectron Spectroscopic Investigation of Volatile Species on PbS(100) Surface

A special sample precooling technique has been utilised to preserve thin layers of volatile chemical species on the sample surface in ultra high vacuum conditions for investigation with conventional and synchrotron excited photoelectron spectroscopy. The precooling system has been constructed for the Scienta 300 electron spectrometer at the beam line 51 at MAX I synchrotron laboratory in Lund (Sweden) and it will be transferred to beam line 411 at the new MAX II storage ring. The spectrometer also includes a possibility for continuous temperature control of the sample during the measurement at temperature range 70 K – 450 K. In this work, temperature controlled measuring technique has been applied in a study of electrochemically treated PbS (100) surface. The measured sulfur 2p core level spectra clearly reveal a layer structure of elemental sulfur and metal polysulfides formed during anodic oxidation of PbS in acidic aqueous solution. Similar measuring technique can also be applied, e.g., in studies of adsorption layers on various substrates.

Surface core-level shift and phonon broadening in PbS(100)

In situ cleaved PbS(100) surfaces were studied by means of X-ray photoelectron spectroscopy (XPS) using synchrotron radiation. A significant narrowing of the S 2p, Pb 5d and Pb 4f lines was found at low temperatures, indicating strong dependence of phonon broadening on temperature. The narrowing was found to be strongest for the S 2p surface peak. This phenomenon made it possible to observe, for the first time, a surface core-level shift (SCLS) in the S 2p lines of PbS. No SCLS was observed for the Pb 5d and Pb 4f lines, which is in agreement with Paolucci and Prince, Phys. Rev. B 41 (1990) 3851. Also, the inelastic mean free paths (IMFP's) for the photoelectrons were determined.

Interaction of sodium overlayers with the PbS(100) (galena) surface: evidence for a Na↔Pb exchange reaction

X-ray standing wave (XSW) and core level photoemission methods were used to study the atomic and electronic structure of sodium overlayers on cleaved (100) surfaces of the mineral galena (PbS). Both techniques provide clear evidence of an exchange reaction in which, at least, a portion of the surface lead ions (Pb2+) is replaced by sodium ions (Na+). The replacement results in a new component in the photoemission spectra associated with reduced lead, while a direct change in the lead surface atom position relative to the 200 bulk diffraction planes is observed in the XSW signa. Changes in the surface geometry of sulfur ions are smaller. The interface of the reacted PbS surface is strongly disordered, as indicated by a small coherent fraction (0.37–0.48) for sodium and rapid quenching of the substrate (1×1) low energy electron diffraction pattern.