Rutile Surface Research Articles

Influence of SHMP Concentration on Rutile Flotation in BHA System: Guiding the Separation of Rutile and Hornblende.

Sodium hexametaphosphate (SHMP) is a common depressant used in rutile flotation, especially for silicate gangue minerals such as amphibole. However, the experiment found that in the benzyl hydroxamic acid (BHA) system, SHMP of small concentrations affected the flotation recovery of rutile. In this work, the influence mechanism of SHMP concentration on the flotation of rutile in the BHA system was studied by flotation test, adsorption capacity determination, ζ potential determination, infrared spectrum analysis, and X-ray photoelectron spectroscopy (XPS) analysis. The test results show that in the BHA system, SHMP chemically adsorbs on the rutile surface, hindering the effect of BHA on the rutile surface, and hydrolyzes in the pulp to form hydrophilic colloids. When the concentration of BHA is 40 mg/L and that of SHMP is 6 mg/L, the effect of SHMP is the most obvious. However, with the increase of SHMP concentration to 8-10 mg/L, its effect on the recovery of rutile became very weak. When the concentration of SHMP is lower than 6 mg/L, the SHMP anion-metaphosphate ion (SHMP-) interacts with the hydrophilic colloidal particles on the surface of rutile, which affects the combination of BHA and the active sites on the surface of rutile, thus affecting the recovery of rutile. When the concentration of SHMP continues to increase, the SHMP- adsorbed on the rutile surface is removed from the rutile surface due to the charge repulsion and the competitive adsorption of BHA.

Modeling adsorption reactions of ammonium perchlorate on rutile and anatase surfaces.

In this work, the effects of two TiO2 polymorphs on the decomposition of ammonium perchlorate (NH4ClO4) were studied experimentally and theoretically. The interactions between AP and various surfaces of TiO2 were modeled using density functional theory (DFT) calculations. Specifically, the adsorption of AP on three rutile surfaces (1 1 0), (1 0 0), and (0 0 1), as well as two anatase surfaces (1 0 1), and (0 0 1) were modeled using cluster models, along with the decomposition of adsorbed AP into small molecules. The optimized complexes of the AP molecule on TiO2 surfaces were very stable, indicating strong covalent and hydrogen bonding interactions, leading to highly energetic adsorption reactions. The calculated energy of adsorption (ΔEads) ranged from -120.23 to -301.98 kJ/mol, with highly exergonic calculated Gibbs free energy (ΔGads) of reaction, and highly exothermic enthalpy of reaction (ΔHads). The decomposition of adsorbed AP was also found to have very negative ΔEdec values between -199.08 and -380.73 kJ/mol. The values of ΔGdec and ΔHdec reveal exergonic and exothermic reactions. The adsorption of AP on TiO2 surfaces anticipates the heat release of decomposition, in agreement with experimental results. The most common anatase surface, (1 0 1), was predicted to be more reactive for AP decomposition than the most stable rutile surface, (1 1 0), which was confirmed by experiments. DFT calculations show the mechanism for activation of the two TiO2 polymorphs is entropy driven.

Trends in the oxygen adsorption energy derived from the thermodynamic potential model for the oxygen evolution reaction

Across the last years, substantial efforts were done to understand the limitations of the oxygen evolution reaction (OER) catalysts and to develop robust ones such as to solve the efficiency problem in water splitting. Fundamental understanding represents a pathway towards designing superior catalysts. In this direction, several descriptors (ηTD, ESSI, Gmax(η)) have been derived based on the adsorption energies of the OER intermediate moieties (∆EHO*/∆EO*/∆EHOO*) for faster screening of the materials. A universal scaling between the adsorption energies of HO* and HOO* was established for a wide range of materials and in many cases was shown to govern the minimum theoretical overpotential. On the other hand, the scaling between the adsorption energies of HO* and O* fragments have a large scattering along the trendline. In this work we derive five trends for the O* adsorption energies based on theoretical overpotential intervals, using data collected from the published studies employing DFT calculations for OER. It is shown that the best materials have an adsorption energy for HO* placed within a limited interval, yet sufficiently large for a pool of promising catalysts (-0.5,1.5 eV), and that the adsorption energies of O* scales with that of HO* at a slope of one and an intercept of 1.81 eV. Furthermore, a decrease of the intercept for the scaling between HOO* and HO* from 3.14 (valid for all data analyzed) to 3.03 eV is found. For three of the other four trends, the slope of the adsorption energies of O*/HO*scaling is one with intercepts closer either to HOO* (2.2/2.78) or to HO*(1.43/0.78) adsorption energies. For each set of data, the scaling between HOO* and HO* adsorption energies varies from 3 to 3.37 eV. Separately, the trends for O* adsorption energies were analyzed for the TiO2(110) semiconducting rutile surface when doped with transition metals and modified (i) with HO* or H* co-adsorbed fragments that act as charge donor/acceptor moieties or (ii) when the surface was provided with an excess or a lack of electrons. The analysis was performed using the data collected from the published literature, supplemented by additional calculations. Clear trends based on the amount of charge were obtained when standard GGA functionals were used. An understanding for the origin of the large variation of the oxygen adsorption energies for the systems that have similar adsorption energies for the HO*/HOO* is provided. However, when the Hubbard corrected GGA functional is used, some trends change. Clearly, it is still a challenge describing accurately and in a cost-effective manner the adsorption of OER moieties on the undoped or doped transition metal oxides, especially for O*. A strategy to get further insight into the origin of large variations of oxygen adsorption energies for the materials that have similar adsorption energies of HO* and HOO* is discussed.

How to construct the most stable structure of (110) surface from rutile TiO2 bulk?

XRD pattern of rutile TiO2 bulk and surface energy of possible terminated (110) planes were investigated using the DFT+U method. The (110) surface was demonstrated to be the most popular facet of rutile TiO2, which is in good agreement with data of the JCPDS card No. 21-1276. The difference in surface energy among possible terminated (110) planes is attributed to structure of top and bottom atomic layers. We have found that the P5 plane is the most stable. It represents structure of (110) surface. Rutile TiO2 (110) surface has calculated surface energy of 0.98 J/m2. The value compares well with previous publications. Besides, DFT calculations were also performed. In comparison with DFT+U, surface energy obtained from DFT calculation for (110) surface is very small, about 0.48 J/m2.

Effect of Iron(III) ions on rutile flotation: Surface properties and adsorption mechanism

Various studies have shown that the presence of metal ions in the slurry can significantly impact flotation. However, it was still uncertain how iron, a typical metal ion, affected rutile flotation. Micro-flotation experiments, x-ray photoelectron spectroscopy, icp-oes, solution chemistry calculation, zeta potential and contact angle test were used to explore the effect of iron ions on the surface properties and floatability of rutile in this investigation. Micro-flotation results showed that iron ions not only substantially increased the rutile flotation recovery from 42.27 % to 75.69 % at pH = 4, but also converted the optimal pH of flotation from a point (pH = 4) to a platform (pH = 4–8). Solution chemistry calculations indicated that iron ions could form complexes with hydroxyl to produce FeOH2+, which reacted with benzohydroxamic acid (BHA) to create [Fe(C7H6NO2)2]+ at pH=4–7. The analysis conducted through X-ray photoelectron spectroscopy, coupled with the results obtained from contact angle measurements, revealed that the complex [Fe(C7H6NO2)2]+ had the capability to engage with Ti-OH groups, improving the hydrophobicity of rutile surface. Zeta potential analysis results further illustrated that Fe3+/BHA complexes reinforce the adsorption of BHA and iron ions onto rutile surface by the formation of [Fe(C7H6NO2)2]+. In addition, the O1s peak fitting results and ICP-OES analysis results proved that at the same pH, 60 mg/L was the optimal concentration for iron ion activation, which may be limited by Saturated adsorption capacity of [Fe(C7H6NO2)2]+.

Electronic and Structural Properties of Antibacterial Ag–Ti-Based Surfaces: An Ab Initio Theoretical Study

Coatings with tunable multifunctional features are important for several technological applications. Ti-based materials have been used in diverse applications ranging from metallic diodes in electronic devices up to medical implants. This work uses ab initio calculations to achieve a more fundamental understanding of the structural and electronic properties of β-TiNb and its passive TiO2 film surfaces upon Ag addition, investigating the alterations in the electronic band gap and the stability of the antibacterial coating. We find that Ag’s 4d electrons introduce localized electron states, characterized by bonding features with the favoured Ti first neighbour atoms, approximately −5 eV below the fermi level in both β-TiNb bulk and surface. Ag’s binding energy on β-TiNb(110) depends on the local environment (the lattice site and the type of bonded surface atoms) ranging from −2.70 eV up to −4.21 eV for the adatom on a four-fold Ti site, offering a variety of options for the design of a stable coating or for Ag ion release. In Ti–O terminated anatase and rutile (001) surfaces, surface states are introduced altering the TiO2 band gap. Silver is bonded more strongly, and therefore creates a more stable antibacterial coat on rutile than on anatase. In addition, the Ag coating exhibits enhanced 4d electron states at the highest occupied state on anatase (001),which are extended from −5 eV up to the Fermi level on rutile (001), which might be altered depending on the coat structural features, thus creating systems with tunable electronic band gap that can be used for the design of thin film semiconductors.

Enhanced CO2 methanation activity of Ni-Rutile-based catalyst by tuning the metal−support interaction with Fe doping

CO2 methanation is an effective way for CO2 recycling, and nickel-based catalysts have attracted much attention in this field due to their excellent CH4 selectivity and cost-effectiveness; however, their applications are limited by their suboptimal catalytic activity. In this work, the catalytic activity of Ni-rutile-based catalysts was increased by almost an order of magnitude simply by doping the rutile with Fe. Series characterizations show that three factors that mainly contributed to the improved catalytic performance of the Ni/Fe-Rutile, which are the reduction of the Ni size, the increase of the basic sites, and the inhibition of the encapsulation of the Ni particles by the support. With Fe doping, the OH and oxygen vacancies on the rutile surface were reduced, which made the support difficult to reduce and thus suppressed the encapsulation of Ni by the support. As a result, the number of exposed active sites on the Ni/Fe-Rutile which with a smaller Ni size was further successfully increased, and the catalytic activity was significantly improved. This study provides new ideas for the design of activity-enhanced supported catalysts from the perspective of tuning the metal-support interaction and may be applicable to other reducible oxide-supported metal catalysts.

Effect of different particle size fractions on flotation separation of fine rutile from garnet

The flotation separation of fine minerals, including rutile, has long been a challenging research focus. This study selected rutile and garnet samples with size fractions of −15, −38, and −74 μm to investigate the impact of particle size on flotation separation. Infrared spectroscopy, Zeta potential analysis, adsorption capacity measurements, and high-speed camera were utilized to analyze the mechanism. Results indicate that a high-grade concentrate with recovery of 92.33 % was achieved by mixing −74 μm rutile and −15 μm garnet in a 1:1 ratio. Benzohydroxamic acid (BHA) collector primarily chemisorbs onto the surface of rutile particles. BHA's adsorption capacity on each particle size's rutile surface is significantly higher than that of garnet. Meanwhile, sodium fluorosilicate (SSF) exhibits the strongest inhibitory effect on −15 μm garnet particles. Additionally, adhesion between bubbles and −74 μm rutile is much stronger than that between bubbles and fine particles overall. These findings have significant reference value for improving eclogite-type rutile ore's fine particle separation.

The Role of Water Molecules on Polaron Behavior at Rutile (110) Surface: A Constrained Density Functional Theory Study.

Understanding the behavior of a polaron in contact with water is of significant importance for many photocatalytic applications. We investigated the influence of water on the localization and transport properties of polarons at the rutile (110) surface by constrained density functional theory. An excess electron at a dry surface favors the formation of a small polaron at the subsurface Ti site, with a preferred transport direction along the [001] axis. As the surface is covered by water, the preferred spatial localization of the polarons is moved from the subsurface to the surface. When the water coverage exceeds half a monolayer, the preferred direction of polaron hopping is changed to the [110] direction toward the surface. This characteristic behavior is related to the Ti3d-orbital occupations and crystal field splitting induced by different distorted structures under water coverage. Our work describes the reduced sites that might eventually play a role in photocatalysis for rutile (110) surfaces in a water environment.

Geometrical Stabilities and Electronic Structures of Rh5 Nanoclusters on Rutile TiO2 (110) for Green Hydrogen Production.

Addressing the urgent need for sustainable energy sources, this study investigates the intricate relationship between rhodium (Rh5) nanoclusters and TiO2 rutile (110) surfaces, aiming to advance photocatalytic water splitting for green hydrogen production. Motivated by the imperative to transition from conventional fossil fuels, this study employs density functional theory (DFT) with DFT-D3 and HSE06 hybrid functionals to analyse the geometrical stabilities and electronic structures of Rh5 nanoclusters on TiO2 rutile (110). TiO2, a prominent photocatalyst, faces challenges such as limited visible light absorption, leading researchers to explore noble metals like Rh as cocatalysts. Our results show that bipyramidal Rh5 nanoclusters exhibit enhanced stability and charge transfer when adsorbed on TiO2 rutile (110) compared to trapezoidal configurations. The most stable adsorption induces the oxidation of the nanocluster, altering the electronic structure of TiO2. Extending the analysis to defective TiO2 surfaces, this study explores the impact of Rh5 nanoclusters on oxygen vacancy formation, revealing the stabilisation of TiO2 and increased oxygen vacancy formation energy. This theoretical exploration contributes insights into the potential of Rh5 nanoclusters as efficient cocatalysts for TiO2-based photocatalytic systems, laying the foundation for experimental validations and the rational design of highly efficient photocatalysts for sustainable hydrogen production. The observed effects on electronic structures and oxygen vacancy formation emphasize the complex interactions between Rh5 nanoclusters and the TiO2 surface, guiding future research in the quest for clean energy alternatives.

Surface sensitization of TiO2 via Pd/Rb2O cocatalysts: Mechanistic insights to the arsenic elimination from ground drinking water†

In order to abide the regulations put forth by World Health Organization (WHO), it is imperative to employ innovative technique that can eliminate arsenic from ground drinking water. The effective sorbents are desperately needed to remove arsenic from the water. In this work, rutile TiO2 surfaces were tuned with nominal ratios of palladium and rubidium oxide cocatalysts via co-precipitation method. Firstly, As (III) contents were converted to As (V) by photooxidation reaction (POR) and then As (V) contents were removed by adsorption over Pd/Rb2O@TiO2 surfaces. The morphology, optical characteristics and structural features were obtained by XRD, Raman, UV-Vis/DRS, SEM, PL and TGA analysis. The surface characteristics and chemical compositions of Pd/Rb2O@TiO2 were obtained using TEM and XPS analysis. Arsenic contaminated water samples were collected from Hasilpur Tehsil (Punjab/Pakistan). The mechanism of As (III) to As (V) conversion, adsorption kinetic for pseudo-second-order (R2꞊0.99919), and the Langmuir adsorption isotherm kinetic model (R2꞊0.98906) provided the significant clarification. Spontaneous arsenic adsorption has been determined (ΔG=−12.52 KJ/mol), which addressed the substantial columbic interactions between adsorbate and substrate. Due to non-toxic nature, dual feautres (photocatalysis and adsorption), easy accesability and low cost, the utilization of titania based sorbents appear to be viable options for arsenic removal technologies. The results of this study depict that, Pd/Rb2O@TiO2 can effectively remove 99% arsenic from contaminated water.

Mechanistic insight on water dissociation on pristine low-index TiO2 surfaces from machine learning molecular dynamics simulations

Water adsorption and dissociation processes on pristine low-index TiO2 interfaces are important but poorly understood outside the well-studied anatase (101) and rutile (110). To understand these, we construct three sets of machine learning potentials that are simultaneously applicable to various TiO2 surfaces, based on three density-functional-theory approximations. Here we show the water dissociation free energies on seven pristine TiO2 surfaces, and predict that anatase (100), anatase (110), rutile (001), and rutile (011) favor water dissociation, anatase (101) and rutile (100) have mostly molecular adsorption, while the simulations of rutile (110) sensitively depend on the slab thickness and molecular adsorption is preferred with thick slabs. Moreover, using an automated algorithm, we reveal that these surfaces follow different types of atomistic mechanisms for proton transfer and water dissociation: one-step, two-step, or both. These mechanisms can be rationalized based on the arrangements of water molecules on the different surfaces. Our finding thus demonstrates that the different pristine TiO2 surfaces react with water in distinct ways, and cannot be represented using just the low-energy anatase (101) and rutile (110) surfaces.

Tip-activated single-atom catalysis: CO oxidation on Au adatom on oxidized rutile TiO2 surface.

Single-atom catalysis of carbon monoxide oxidation on metal-oxide surfaces is crucial for greenhouse recycling, automotive catalysis, and beyond, but reports of the atomic-scale mechanism are still scarce. Here, using scanning probe microscopy, we show that charging single gold atoms on oxidized rutile titanium dioxide surface, both positively and negatively, considerably promotes adsorption of carbon monoxide. No carbon monoxide adsorption is observed on neutral gold atoms. Two different carbon monoxide adsorption geometries on gold atoms are identified. We demonstrate full control over the redox state of adsorbed gold single atoms, carbon monoxide adsorption geometry, and carbon monoxide adsorption/desorption by the atomic force microscopy tip. On charged gold atoms, we activate Eley-Rideal oxidation reaction between carbon monoxide and a neighboring oxygen adatom by the tip. Our results provide unprecedented insights into carbon monoxide adsorption and suggest that the gold dual activity for carbon monoxide oxidation after electron or hole attachment is also the key ingredient in photocatalysis under realistic conditions.

Comparison of H2O Adsorption and Dissociation Behaviors on Rutile (110) and Anatase (101) Surfaces Based on ReaxFF Molecular Dynamics Simulation.

The relationship between structure and reactivity plays a dominant role in water dissociation on the various TiO2 crystallines. To observe the adsorption and dissociation behavior of H2O, the reaction force field (ReaxFF) is used to investigate the dynamic behavior of H2O on rutile (110) and anatase (101) surfaces in an aqueous environment. Simulation results show that there is a direct proton transfer between the adsorbed H2O (H2Oad) and the bridging oxygen (Obr) on the rutile (110) surface. Compared with that on the rutile (110) surface, an indirect proton transfer occurs on the anatase (101) surface along the H-bond network from the second layer of water. This different mechanism of water dissociation is determined by the distance between the 5-fold coordinated Ti (Ti5c) and Obr of the rutile and anatase TiO2 surfaces, resulting in the direct or indirect proton transfer. Additionally, the hydrogen bond (H-bond) network plays a crucial role in the adsorption and dissociation of H2O on the TiO2 surface. To describe interfacial water structures between TiO2 and bulk water, the double-layer model is proposed. The first layer is the dissociated H2O on the rutile (110) and anatase (101) surfaces. The second layer forms an ordered water structure adsorbed to the surface Obr or terminal OH group through strong hydrogen bonding (H-bonding). Affected by the H-bond network, the H2O dissociation on the rutile (110) surface is inhibited but that on the anatase (101) surface is promoted.

Interaction of biomolecules with anatase, rutile and amorphous TiO2 surfaces: A molecular dynamics study.

The adhesion of biomolecules to dental and orthopedic implants is a fundamental step in the process of osseointegration. Short peptide motifs, such as RGD or KRSR, carried by extracellular matrix proteins or coated onto implant surfaces, accelerate cell adhesion and tissue formation. For this reason, understanding the binding mechanisms of adhesive peptides to oxidized surfaces of titanium implants is of paramount importance. We performed molecular dynamics simulations to compare the adhesion properties of 6 peptides, including the tripeptide RGD, its variants KGD and LGD, as well as the tetrapeptide KRSR, its variant LRSR and its truncated version RSR, on anatase, rutile, and amorphous titanium dioxide (TiO2) surfaces. The migration of these molecules from the water phase to the surface was simulated in an aqueous environment. Based on these simulations, we calculated the residence time of each peptide bound to the three different TiO2 structures. It was found that the presence of an N-terminal lysine or arginine amino acid residue resulted in more efficient surface binding. A pulling simulation was performed to detach the adhered molecules. The maximum pulling force and the binding energy were determined from the results of these simulations. The tri- and tetrapeptides had slightly greater adhesion affinity to the amorphous and anatase structure than to rutile in general, however specific surface and peptide binding characters could be detected. The binding energies obtained from our simulations allowed us to rank the adhesion strengths of the studied peptides.

Toward Quantum Chemical Free Energy Simulations of Platinum Nanoparticles on Titania Support.

Platinum nanoparticles (Pt-NPs) supported on titania surfaces are costly but indispensable heterogeneous catalysts because of their highly effective and selective catalytic properties. Therefore, it is vital to understand their physicochemical processes during catalysis to optimize their use and to further develop better catalysts. However, simulating these dynamic processes is challenging due to the need for a reliable quantum chemical method to describe chemical bond breaking and bond formation during the processes but, at the same time, fast enough to sample a large number of configurations required to compute the corresponding free energy surfaces. Density functional theory (DFT) is often used to explore Pt-NPs; nonetheless, it is usually limited to some minimum-energy reaction pathways on static potential energy surfaces because of its high computational cost. We report here a combination of the density functional tight binding (DFTB) method as a fast but reliable approximation to DFT, the steered molecular dynamics (SMD) technique, and the Jarzynski equality to construct free energy surfaces of the temperature-dependent diffusion and growth of platinum particles on a titania surface. In particular, we present the parametrization for Pt-X (X = Pt, Ti, or O) interactions in the framework of the second-order DFTB method, using a previous parametrization for titania as a basis. The optimized parameter set was used to simulate the surface diffusion of a single platinum atom (Pt1) and the growth of Pt6 from Pt5 and Pt1 on the rutile (110) surface at three different temperatures (T = 400, 600, 800 K). The free energy profile was constructed by using over a hundred SMD trajectories for each process. We found that increasing the temperature has a minimal effect on the formation free energy; nevertheless, it significantly reduces the free energy barrier of Pt atom migration on the TiO2 surface and the transition state (TS) of its deposition. In a concluding remark, the methodology opens the pathway to quantum chemical free energy simulations of Pt-NPs' temperature-dependent growth and other transformation processes on the titania support.

Facet-Dependent Selectivity of Rutile IrO2 for Oxygen and Chlorine Evolution Reactions

Water electrolysis is a promising route for sustainable production of hydrogen as an energy storage medium and valuable precursor for industrial chemical syntheses such as ammonia and methanol. Direct electrolysis of seawater circumvents costly desalination and purification steps to reduce the price of renewable hydrogen to achieve cost parity with carbon-intensive steam reforming. However, the significant concentration of chloride salts in seawater poses a challenge to selectivity of seawater electrolysis. In aqueous chloride electrolytes, the chlorine evolution reaction (CER) is kinetically favored over the oxygen evolution reaction (OER). Rutile-type iridium dioxide (IrO2) is a state-of-the-art electrocatalyst for water electrolysis but is also a benchmark chlorine evolution electrocatalyst in the industrial chlor-alkali process. Understanding OER/CER selectivity is needed to make seawater electrolysis a viable energy conversion technology in the future.In this work, we seek to understand the relationship between crystallographic facet and competitive reaction pathway between OER and CER on epitaxial, rutile IrO2 thin films. We investigate facet-dependent OER and CER activity on a series of single-crystalline IrO2 thin films using a rotating disk electrode geometry. The OER/CER selectivity, reaction rate order, and reaction intermediate electroadsorption affinities are explored to add fundamental insight into the reactivity of rutile IrO2 surface. To gain further insight into OER intermediate adsorption affinities, we paired electrochemical measurements with surface-sensitive ambient pressure x-ray photoelectron spectroscopy (AP-XPS). Moving forward, our facet-dependent study of OER/CER selectivity of rutile IrO2 can be used to design selective seawater electrocatalysts for cost-effective and sustainable hydrogen production.

A PN-type CuO@TiO2 nanorods heterojunction for efficient PEC water splitting: DFT model and experimental investigation on the effect of calcination temperature

Heterojunction formation between two dissimilar semiconductors is a promising strategy to improve the harvesting of the visible photons and separation of the electron-hole pairs in semiconductor-based photocatalysts. Herein, the interfacial characteristics of TiO2 rutile (011) surface and Cu-MOF derived cupric oxide (CuO) were examined via simulation and experimental approaches for solar hydrogen production. The results illustrate the formation of van der Waals type heterojunction (Type-II) with higher visible light absorption and lesser charge transfer resistance. This was attributed to significant charge transfer from CuO to the TiO2 surface, establishing a PN-type heterojunction. The valence band maxima and conduction band minima of the designed heterostructure matched well with redox potential of water even with the considerable bandgap reduction around 1.66 eV. Furthermore, the hybrid photocatalyst calcined at 450 °C produced the highest hydrogen production at 32,766 μmol g−1 h−1. The outcome thus provides a valuable insight in developing suitable photocatalysts for practical production of hydrogen through photo-electrolysis technique.

Multi‐Phase Sputtered TiO2‐Induced Current–Voltage Distortion in Sb2Se3 Solar Cells

AbstractDespite the recent success of CdS/Sb2Se3 heterojunction devices, cadmium toxicity, parasitic absorption from the relatively narrow CdS band gap (2.4 eV) and multiple reports of inter‐diffusion at the interface forming Cd(S,Se) and Sb2(S,Se)3 phases, present significant limitations to this device architecture. Among the options for alternative partner layers in antimony chalcogenide solar cells, the wide band gap, non‐toxic titanium dioxide (TiO2) has demonstrated the most promise. It is generally accepted that the anatase phase of the polymorphic TiO2 is preferred, although there is currently an absence of analysis with regard to phase influence on device performance. This work reports approaches to distinguish between TiO2 phases using both surface and bulk characterization methods. A device fabricated with a radio frequency (RF) magnetron sputtered rutile‐TiO2 window layer (FTO/TiO2/Sb2Se3/P3HT/Au) achieved an efficiency of 6.88% and near‐record short–circuit current density (Jsc) of 32.44 mA cm−2, which is comparable to established solution based TiO2 fabrication methods that produced a highly anatase‐TiO2 partner layer and a 6.91% efficiency device. The sputtered method introduces reproducibility challenges via the enhancement of interfacial charge barriers in multi‐phase TiO2 films with a rutile surface and anatase bulk. This is shown to introduce severe S‐shaped current–voltage (J–V) distortion and a drastic fill–factor (FF) reduction in these devices.

Thermochemical oxidation of commercially pure titanium; controlled formation of robust white titanium oxide layers for biomedical applications.

This study addresses controlled formation of white rutile surface layers on commercially pure (CP) titanium by gaseous thermochemical oxidation. The formed oxide layers were investigated with light optical microscopy (LOM), scanning electron microscopy (SEM), X-ray diffraction (XRD), glow discharge optical emission spectroscopy (GDOES), transmission electron microscopy - energy-dispersive X-ray spectroscopy (TEM-EDXs), spectrophotometry, thermogravimetric analysis (TGA), and Vickers micro-indentation. The oxidation response of CP titanium in different single gas systems, O2, N2O, or CO2, at temperatures ranging from 750 °C to 1000 °C, showed that the formed oxide scales exhibit oxide stratification, irrespective of the applied gas. Additionally, a two-step oxidation process was found to result in controlled growth of robust, dense, adherent white titanium oxide layers. The two-step process entails a first oxidation step in an atmosphere of CO/CO2 at 750 °C; a second oxidation step is performed in N2/N2O at 650 °C. The oxidation in a CO/CO2 atmosphere results in the incorporation of carbon in the forming oxide layer. TEM-EDXs analysis after oxidation in CO/CO2 revealed that carbon resides in thin interlayers between slightly stratified 200-300 nm layers of rutile. This unique “composite” oxide layer containing carbon was found to be robust and densely adhering to the substrate. After the second oxidation step in N2O carbon was “retracted” (oxidized) from the oxide‑carbon composite layer, resulting in an aesthetically pleasing, adherent white oxide layer. Finally, this two-step process, leading to robust white oxide layer formation, is show-cased on a biomedical demonstrator part for dental applications.