Accelerated Charge Transfer through Interface Chemical Bonds in MoS2/TiO2 for Photocatalytic Conversion of Lignocellulosic Biomass to H2.

Solar photocatalytic H2 production from lignocellulosic biomass has attracted great interest, but it suffers from low photocatalytic efficiency owing to the absence of highly efficient photocatalysts. Herein, we designed and constructed ultrathin MoS2-modified porous TiO2 microspheres (MT) with abundant interface Ti-S bonds as photocatalysts for photocatalytic H2 generation from lignocellulosic biomass. Owing to the accelerated charge transfer related to Ti-S bonds, as well as the abundant active sites for both H2 and ●OH generation, respectively, related to the high exposed edge of MoS2 and the large specific surface area of TiO2, MT photocatalysts demonstrate good performance in the photocatalytic conversion of α-cellulose and lignocellulosic biomass to H2. The highest H2 generation rate of 849 µmol·g-1·h-1 and apparent quantum yield of 4.45% at 380 nm was achieved in α-cellulose aqueous solution for the optimized MT photocatalyst. More importantly, lignocellulosic biomass of corncob, rice hull, bamboo, polar wood chip, and wheat straw were successfully converted to H2 over MT photocatalysts with H2 generation rate of 10, 19, 36, 29, and 8 µmol·g-1·h-1, respectively. This work provides a guiding design approach to develop highly active photocatalysts via interface engineering for solar H2 production from lignocellulosic biomass.

Comparative Analysis of the Interfacial Structure and Properties of BiOX/BiOY (X, Y = F, Cl, Br, and I) Heterostructures through DFT Calculations.

This study focuses on the systematic investigation of the microstructure, interfacial energy, and electronic structure of six BiOX/BiOY heterostructures constructed using four bismuth oxyhalide materials. Utilizing density functional theory (DFT) calculations, the study provides fundamental insights into the interfacial structure and properties of these heterostructures. The results indicate that the formation energies of BiOX/BiOY heterostructures decrease in the order of BiOF/BiOI, BiOF/BiOBr, BiOF/BiOCl, BiOCl/BiOBr, BiOBr/BiOI, and BiOCl/BiOI. BiOCl/BiBr heterostructures were found to have the lowest formation energy and were the most easily formed. Conversely, the formation of BiOF/BiOY heterostructures was observed to be unstable and difficult to achieve. Furthermore, the interfacial electronic structure analysis revealed that BiOCl/BiOBr, BiOCl/BiOI, and BiOBr/BiOI displayed opposite electric fields that facilitated electron-hole pair separation. Therefore, these research findings provide a comprehensive understanding of the mechanisms underlying the formation of BiOX/BiOY heterostructures and present theoretical guidance for the design of innovative and efficient photocatalytic heterostructures, with an emphasis on BiOCl/BiOBr heterostructures. This study highlights the advantages of distinctively layered BiOX materials and their heterostructures, which offer a wide range of band gap values, and demonstrates their potential for various research and practical applications.

Role of the Polar Electric Field in Bismuth Oxyhalides for Photocatalytic Water Splitting.

The built-in electric field generated by polar materials is one of the most effective strategies to promote the separation of photogenerated electron-hole pairs in the field of photocatalysis. However, because of the complexity and diversity of the built-in electric field in polar materials, it is not clear how to enhance the photocatalytic performance and how to control the polar electric field effectively. To this end, four-layered bismuth oxyhalides, BiOX, and BiOXO3 (X = Br, I) were synthesized by a simple hydrothermal method. X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy analysis confirmed that they all have the structure characteristics of a sillenite phase. Scanning electron microscopy images show that they all have the morphology of nanosheets. Among them, BiOBrO3 was successfully synthesized and characterized for the first time in the present work. The order of photocatalytic performance (including carrier's lifetime, photocurrent density, and H2 evolution rate) of the four compounds is listed as follows: BiOBrO3 > BiOI > BiOIO3 > BiOBr. In the bulk of the BiOXO3 photocatalyst, the spontaneous polar built-in electric field along the [001] direction is the crucial factor to inhibit the recombination of photogenerated electron-hole pairs, while the surface polar electric field in BiOI can outstandingly inhibit the recombination of photogenerated electron-hole pairs due to the breaking of the mirror symmetry. Therefore, regulating the microstructure and composition of the structure unit, which generates the built-in electric field, can indeed control the magnitude, direction, and effects of built-in electric fields. In practice, we should carefully adjust the strategy according to the actual situation so as to reasonably design and use the polar electric field, giving full play to its role and enhancing the photocatalytic performance.

Effects of the Preparation Process on the Photocatalytic Performance of Delafossite CuCrO2.

Hydrothermal and solid-state reaction methods are commonly used to prepare the delafossite CuCrO2 photocatalyst. It has been reported that the photocatalytic performances of CuCrO2 samples prepared by these methods are quite different. In order to explore the possible influence of different preparation processes on the photocatalytic performance and the corresponding improvement strategies, this work compares the microstructure and physicochemical properties of the samples prepared by these two methods on the basis of optimizing the process conditions. A CuCrO2 sample prepared by a hydrothermal method is characterized by high purity, low crystallinity, small grain size, and relatively higher photocatalytic activity. A CuCrO2 sample prepared by a solid-state reaction method is characterized by low purity, high crystallinity, large grain size, and relatively lower photocatalytic activity. In combination with DFT calculations, it is confirmed that the CuCrO2 sample prepared by a solid-state reaction method contains a certain amount of interstitial oxygens. Due to the presence of interstitial oxygens, CuCrO2 has strong light absorption in the visible region, presents semimetallic ferromagnetism, and changes the carrier transport, reaction process, and rate on the electrode surface. These findings will contribute to the further development of efficient CuCrO2-based photocatalysts.

Spontaneous Polarization Effect and Photocatalytic Activity of Layered Compound of BiOIO3.

Internal polarized electric field is found to be an effective and available strategy to separate photogenerated electron-hole pairs. By this method, the efficiency of photocatalytic reactions can be obviously enhanced. Here, the layered compound of BiOIO3 with spontaneous polarization was synthesized by a simple hydrothermal method. Taking another bismuth compound BiOI as a counterpart, which has a similar layered structure, the spontaneous polarization effects of BiOIO3 were analyzed and confirmed. The photocatalytic activity of BiOIO3 and BiOI were evaluated by the degradation of methyl orange. Methyl orange was almost completely photocatalytically decomposed by BiOIO3 and BiOI in 40 and 90 min, respectively. The separation and transfer behaviors of photogenerated electron-hole pairs were investigated by a series of photoelectrochemical characterizations. It is further proved the separation and transmission efficiency of BiOIO3 are higher than those of BiOI. According to the results of density of theory calculations, the internal polarized electric field in BiOIO3 is ascribed to the spatial asymmetry of the IO3 group, which is estimated to ∼1.5 × 1010 V/m. Under the action of this internal polarized electric field, the photogenerated electrons and holes would transfer along opposite directions, i.e., photogenerated electrons and holes respectively gather at the Bi/I side and O side. Additionally, superoxide radicals (•O2-) and holes (h+) are produced during the degradation process, which are responsible for the high visible-light photocatalytic activity. Finally, the cyclic degradation test proves that its photocatalytic performance has long-term stability. Therefore, BiOIO3 polar material can be used as one of the alternative materials for efficient photocatalytic reaction.

Effects of crystal structure and composition on the photocatalytic performance of Ta-O-N functional materials.

For photocatalytic applications, the response of a material to the solar spectrum and its redox capabilities are two important factors determined by the band gap and band edge position of the electronic structure of the material. The crystal structure and composition of the photocatalyst are fundamental for determining the above factors. In this article, we examine the functional material Ta-O-N as an example of how to discuss relationships among these factors in detail with the use of theoretical calculations. To explore how the crystal structure and composition influence the photocatalytic performance, two groups of Ta-O-N materials were considered: the first group included ε-Ta2O5, TaON, and Ta3N5; the second group included ß-Ta2O5, δ-Ta2O5, ε-Ta2O5, and amorphous-Ta2O5. Calculation results indicated that the band gap and band edge position are determined by interactions between the atomic core and valence electrons, the overlap of valence electronic states, and the localization of valence states. Ta3N5 and TaON are suitable candidates for efficient photocatalysts owing to their photocatalytic water-splitting ability and good utilization efficiency of solar energy. δ-Ta2O5 has a strong oxidation potential and a band gap suitable for absorbing visible light. Thus, it can be applied to photocatalytic degradation of most pollutants. Although a-Ta2O5, ε-Ta2O5, and ß-Ta2O5 cannot be directly used as photocatalysts, they can still be applied to modify conventional Ta-O-N photocatalysts, owing to their similar composition and structure. These calculation results will be helpful as reference data for analyzing the photocatalytic performance of more complicated Ta-O-N functional materials. On the basis of these findings, one could design novel Ta-O-N functional materials for specific photocatalytic applications by tuning the composition and crystal structure.

Density functional theory study on the metal-support interaction between a Au9 cluster and an anatase TiO2(001) surface.

Noble metals supported on TiO2 surfaces have shown extraordinary photocatalytic properties in many important processes such as hydrogenation, water splitting, degradation of hazards, and so on. Using density functional theory calculations, this work has systematically investigated the microstructure and electronic structure of three different Au9 isomers loaded on anatase TiO2(001) surface. The calculated results show that the interaction between the Au9 cluster and the TiO2 support is closely related to the adsorption site and the stability of the Au9 cluster in the gas phase. The adsorption energy of the 2D configuration is larger than that of the 3D configuration of the Au9 cluster, owing to the stronger interactions between more adsorption sites. The stable adsorption site for Au9 clusters deposited on the anatase TiO2(001) surface tends to be the O2c-O2c hollow site. The presentation of the MIGS of the Au9 cluster, the disappearance of surface states of the TiO2(001) surface, and the shifting of the Fermi level from the top of the valence band to the bottom of the conduction band suggest strong interactions between the Au9 clusters and the TiO2(001) surface. Importantly, the electron transfer from the Au9 clusters to the TiO2 support occurs mainly through Au-O2c interactions, which are mainly localized at the contact layer of the Au9 clusters. These conclusions are useful to understand various physical and chemical properties of noble metal clusters loaded onto an oxide surface, and helpful to design novel metal/semiconductor functional composite materials and devices.

Investigation of energy band alignments and interfacial properties of rutile NMO2/TiO2 (NM = Ru, Rh, Os, and Ir) by first-principles calculations.

In the field of photocatalysis, constructing hetero-structures is an efficient strategy to improve quantum efficiency. However, a lattice mismatch often induces unfavorable interfacial states that can act as recombination centers for photo-generated electron-hole pairs. If the hetero-structure's components have the same crystal structure, this disadvantage can be easily avoided. Conversely, in the process of loading a noble metal co-catalyst onto the TiO2 surface, a transition layer of noble metal oxides is often formed between the TiO2 layer and the noble metal layer. In this article, interfacial properties of hetero-structures composed of a noble metal dioxide and TiO2 with a rutile crystal structure have been systematically investigated using first-principles calculations. In particular, the Schottky barrier height, band bending, and energy band alignments are studied to provide evidence for practical applications. In all cases, no interfacial states exist in the forbidden band of TiO2, and the interfacial formation energy is very small. A strong internal electric field generated by interfacial electron transfer leads to an efficient separation of photo-generated carriers and band bending. Because of the differences in the atomic properties of the components, RuO2/TiO2 and OsO2/TiO2 hetero-structures demonstrate band dividing, while RhO2/TiO2 and IrO2/TiO2 hetero-structures have a pseudo-gap near the Fermi energy level. Furthermore, NMO2/TiO2 hetero-structures show upward band bending. Conversely, RuO2/TiO2 and OsO2/TiO2 hetero-structures present a relatively strong infrared light absorption, while RhO2/TiO2 and IrO2/TiO2 hetero-structures show an obvious absorption edge in the visible light region. Overall, considering all aspects of their properties, RuO2/TiO2 and OsO2/TiO2 hetero-structures are more suitable than others for improving the photocatalytic performance of TiO2. These findings will provide useful information for understanding the role and effects of a noble metal dioxide as a transition layer between a noble metal co-catalyst and a TiO2 photocatalyst.

DFT study on the interfacial properties of vertical and in-plane BiOI/BiOIO3 hetero-structures.

Composite photocatalysts with hetero-structures usually favor the effective separation of photo-generated carriers. In this study, BiOIO3 was chosen to form a hetero-structure with BiOI, due to its internal polar field and good lattice matching with BiOI. The interfacial properties and band offsets were focused on and analyzed in detail by DFT calculations. The results show that the charge depletion and accumulation mainly occur in the region near the interface. This effect leads to an interfacial electric field and thus, the photo-generated electron-hole pairs can be easily separated and transferred along opposite directions at the interface, which is significant for the enhancement of the photocatalytic activity. Moreover, according to the analysis of band offsets, the vertical BiOI/BiOIO3 belongs to the type-II hetero-structure, while the in-plane BiOI/BiOIO3 belongs to the type-I hetero-structure. The former type of hetero-structure has more favorable effects to enhance the photocatalytic activity of BiOI than that of the latter type of hetero-structure. In the case of the vertical BiOI/BiOIO3 hetero-structure, photo-generated electrons can move from the conduction band of BiOI to that of BiOIO3, while holes can move from the valence band of BiOIO3 to that of BiOI under solar radiation. In addition, the introduced internal electric field functions as a selector that can promote the separation of photo-generated carriers, resulting in the higher photocatalytic quantum efficiency. These findings illustrate the underlying mechanism for the reported experiments, and can be used as a basis for the design of novel highly efficient composite photocatalysts with hetero-structures.

Electronic Structure and Optical Properties of BiOI Ultrathin Films for Photocatalytic Water Splitting.

As a promising photocatalyst driven by visible light, BiOI suffers from its lower conduction band edge position, which leads to its inability to produce hydrogen from photocatalytic water splitting. However, BiOI has an open layered intergrowth structure, which makes it easily cleavable along (001) plane. Thus, inspired by the progress of graphene-like two-dimensional nanomaterials, researchers believe that single-layer BiOI presents excellent photocatalytic activity for water splitting. To further explore the relationship between intrinsic properties and photocatalytic performance of BiOI ultrathin film, its electronic structure and optical properties as a function of layer thickness are systematically investigated by using first-principle calculations. The calculated results indicate that the quantum confinement effects can cause the following variations: band gap increasing, band edge position upshifting, and built-in electric field strengthening, which are very favorable for enhancement of photocatalytic performance. Importantly, if the layer thickness is less than 3 nm, the conduction band edge position will be higher than the reduction potential of H(+)/H2 and thus appropriate for the overall photocatalytic water-splitting reaction. However, layer thickness also caused disadvantageous reduction of sunlight absorption, which is noticed and avoided in practice.

Synergistic effects of nonmetal co-doping with sulfur in anatase TiO2: a DFT + U study.

Using DFT + U calculations, the crystal structure and electronic properties of nonmetal co-doping with sulfur in anatase TiO2 are systematically investigated. The initial purpose of this work is to improve the photocatalytic performance of S mono-doped TiO2, in which S occupies the lattice Ti site and acts as a recombination center. Among eight nonmetal impurities that occupy the interstitial site of a TiO6 octahedron, the synergistic effects of B, C, and O with S could achieve this purpose: suppressing the recombination of photogenerated electron-hole pairs by inducing a local inner built-in electric field and eliminating the deep impurity energy bands of S mono-doped TiO2. Furthermore, the photon absorption could be extended to the visible-light region, owing to the overlap of impurity energy bands with the top of the valence band or the bottom of the conduction band. Thus, Ti1-xO2SxBy, Ti1-xO2SxCy and Ti1-xO2SxOy could be considered as promising efficient photocatalysts. Furthermore, the underlying mechanism and tendency of these synergistic effects have been discussed, according to the relationship between the photocatalytic performance and the crystal or electronic structure.

Modification mechanism of praseodymium doping for the photocatalytic performance of TiO2: a combined experimental and theoretical study.

Impurity doping is a simple and efficient modification method to improve the photocatalytic performance of wide band gap photocatalysts. However, some basic and important issues about the mechanism of impurity doping modification still need to be further confirmed and explained. In the present work, Pr-doped TiO2 with a mono-phase crystal structure was prepared by a sol-gel method. Then, the crystal structure, binding information, optical absorption, and photocatalytic activity were systematically investigated. The experimental results show that Pr doping could significantly enhance the photocatalytic activity of TiO2, and the effects of modification on rutile TiO2 are more obvious than for anatase TiO2. In order to understand the underlying mechanism, density functional theory was utilized to calculate the crystal structure and electronic structure of pure and Pr-doped TiO2. The differences in electronic structure between anatase and rutile phases lead to the above photocatalytic performance. The experimental measurements and theoretical calculations mutually support each other in the present work. Two points are confirmed: the position of the band edge determines the redox activity of the photocatalyst, and the shallow energy bands induced by impurity doping could improve the photocatalytic performance.

Structural, electronic, and optical properties of Eu-doped BiOX (X = F, Cl, Br, I): a DFT+U study.

In order to understand the photophysical properties and explain the experimental observations of Eu-doped BiOX (X = F, Cl, Br, I), the crystal structure, electronic structure, and optical properties of pure BiOX and Eu-doped BiOX have been calculated using the DFT+U method. By Eu doping, the band gap of BiOI is slightly narrowed, while the band gaps of others (BiOF, BiOCl, and BiOBr) are slightly broadened. Importantly, there is an isolated impurity energy band in the middle of the band gap, which is formed by seven spin-up energy levels of Eu-4f states. Furthermore, Eu doping enhances the internal electric fields of BiOX and makes the variation of band gaps and band widths become more outstanding, especially for the band widths of X-ns, O-2s, and Bi-6s related bands. Taking into account the energy level matching and band edge position, Eu-doped BiOCl is favorable not only for the photoluminescence application but also for the photocatalysis application. The findings in the present work could well explain the experimental observations in the literature and are helpful for the development of novel optoelectronic applications of BiOX-based materials.

First-principles study on doping effects of sodium in kesterite Cu2ZnSnS4.

A sodium impurity is inevitable for Cu2ZnSnS4 on a substrate of soda-lime glass during high-temperature processing. Recently, it was found that a sodium impurity could improve the photovoltaic properties of Cu2ZnSnS4-based thin film solar cells (including influencing crystallinity, affecting grain growth, increasing hole density, shifting the acceptor level closer to the conduction band, increasing carrier concentration, elongating minority carrier lifetime, and so on). Thus, sodium doping becomes an effective modification means for Cu2ZnSnS4 on the flexible substrate. However, there are some examples available in the literature that discuss the underlying physical mechanism. In the present work, the crystal structure, electronic structure, and optical properties of sodium occupying different lattice sites or interstitial sites of kesterite Cu2ZnSnS4 were systematically calculated by density functional theory within the GGA+U method. Na impurity favors occupation of the interstitial sites. If Na impurity occupies the cation lattice sites, the band gap of Cu2ZnSnS4 will be broadened, which is opposite to the situation of an Na impurity occupying the interstitial sites. The doping effects of Na in Cu2ZnSnS4 are mainly exhibited by the following aspects: energy band shifting, energy band broadening or narrowing, and effective mass of holes on the top of valence band reduction. The calculated results in the present work not only confirm experimental observationa in published articles but also provide an in-depth understanding of them. Thus, the findings could help to promote novel, high-efficiency Cu2ZnSnS4-based thin-film solar cells.

Insight into insulator-to-metal transition of sulfur-doped silicon by DFT calculations.

Using density functional theory calculations, the mechanism of insulator-to-metal transition of S-doped Si has been systematically investigated. The calculated crystal structure indicates that the gentle lattice distortion is caused by sulfur doping, and this doping effect is gradually weakened with the increase of sulfur concentration. Two distinct impurity energy levels in the band gap are induced by sulfur doping, and their position and width are linearly varying along with the increase of sulfur concentration. Owing to the overlap and dispersion of these impurity energy levels, the insulator-to-metal transition occurs at the sulfur concentration of 2.095 × 10(20) cm(-3), which is consistent with the experimental measurement. Moreover, the defect states related with sulfur doping show delocalization features and are more outstanding at the higher sulfur concentration. The calculated results suggest that S-hyperdoped Si is a suitable candidate for intermediate band solar cells.

Tuning electronic structure for enhanced photocatalytic performance: theoretical and experimental investigation of CuM1-xM'xO2 (M, M' = B, Al, Ga, In) solid solutions.

This study introduces robust screening methodology for the efficient design of delafossite CuM1-xM'xO2 solid-solution photocatalysts using band-structure engineering. The investigation not only reveals the formation rules for various CuM1-xM'xO2 solid solutions but also highlights the dependence on both lattice compatibility and thermodynamic stability. Moreover, the study uncovers the nonlinear relationship between composition and band gaps in these solid solutions, with the bowing coefficient determined by the substitution constituents. By optimizing the constituent elements of the conduction band edge and adjusting solubility, the band structure of CuM1-xM'xO2 samples can be fine-tuned to the visible light region. Among the examined photocatalysts, CuAl0.5Ga0.5O2 exhibits the highest H2 evolution rate by striking a balance between visible-light absorption and sufficient reduction potential, showing improvements of 28.8 and 6.9 times those of CuAlO2 and CuGaO2, respectively. Additionally, CuGa0.9In0.1O2 demonstrates enhanced electron migration and surpasses CuGaO2 in H2 evolution due to a reduction in the effective mass of photogenerated electrons. These findings emphasize the pivotal role of theoretical predictions in synthesizing CuM1-xM'xO2 solid solutions and underscore the importance of rational substitution constituents in optimizing light absorption, reduction potentials, and effective mass for efficient hydrogen production.

Stability and fundamental properties of Cu x O1-x as optoelectronic functional materials.

Binary Cu x O1-x compounds have some advantages as optoelectronic functional materials, but their further development has encountered some bottlenecks, such as inaccurate bandgap values and slow improvement of photoelectric conversion efficiency. In this work, all possible stoichiometric ratios and crystal structures of binary Cu x O1-x compounds were comprehensively analyzed based on a high-throughput computing database. Stable and metastable phases with different stoichiometric ratios were obtained. Their stability in different chemical environments was further analyzed according to the component phase diagram and chemical potential phase diagram. The calculation results show that Cu, Cu2O and CuO have obvious advantages in thermodynamics. The comparison and analysis of crystal microstructure show that the stable phase of Cu x O1-x compounds contains the following two motifs: planar square with Cu atoms as the center and four O atoms as the vertices; regular tetrahedron with O atoms as the center and four Cu atoms as the vertices. In different stoichiometric ratio regions, the electron transfer and interaction modes between Cu and O atoms are different. This effect causes energy differences between bonding and antibonding states, resulting in the different conductivity of binary Cu x O1-x compounds: semi-metallic ferromagnetic, semiconducting, and metallicity. This is the root of the inconsistent and inaccurate bandgap values of Cu x O1-x compounds. These compositional, structural, and property variations provide greater freedom and scope for the development of binary Cu x O1-x compounds as optoelectronic functional materials.

Rational Design of a Two-Dimensional Janus CuFeO2+δ Single Layer as a Photocatalyst and Photoelectrode.

The exfoliation of 2D nanomaterials from 3D multimetal oxides with a stable structure is a great challenge. Herein, a delafossite CuFeO2+δ nanosheet becomes an open-layered structure by introducing excess oxygen so that the 2D Janus CuFeO2+δ single layer can be further obtained by aqueous ultrasonic exfoliation. The 2D Janus CuFeO2+δ single layer breaks the limitation of mirror symmetry, which is very beneficial to the effective separation of photogenerated electron-hole pairs. Serving as both a photoelectrode and a photocatalyst, the 2D Janus CuFeO2+δ single layer/few layer remarkably enhances the photocatalytic activity with long-term stability: the photocurrent density is increased by 2-fold, and the rate of H2 evolution is increased by 1.5-fold, in comparison with the counterpart of unexfoliated CuFeO2+δ nanosheets. This work demonstrates that 2D nanomaterials can be directly exfoliated from 3D nanomaterials by rational composition and microstructure design, which is helpful in promoting the development of bimetallic-oxide-ene (BMOene) as a novel functional material.

Effect of Ag alloying and trace precipitation on corrosion resistance of Ti-Ta-Ag ternary alloy.

This work systematically analysed the electrochemical and corrosion behaviour of Ti-Ta-Ag ternary alloy samples in Hank's solution. For the samples with 1.5% and 3% Ag content, the sintering temperature increased from 750 to 950°C, and the corresponding corrosion resistance increased by 100 times due to the increased alloying of Ag; meanwhile for the sample with 4.5% Ag content, the sintering temperature increased from 750 to 950°C, and the corresponding corrosion resistance decreased by six times due to the increased precipitation of Ag. These tests prove that the Ag alloying is beneficial to the enhancement of the corrosion resistance of Ti-Ta-Ag ternary alloy, but the Ag trace precipitation has the opposite effect. A series of electrochemical characterizations and density functional theory calculations explain the mechanism of the above phenomenon. Ag alloying can promote the formation of uniform, complete, dense, stable and thick passivation layer on the surface of Ti-Ta-Ag ternary alloy, which makes Ti-Ta-Ag ternary alloy uniformly corroded without pitting. In addition, Ag alloying can effectively reduce the contact resistance of the solid-liquid interface. However, the trace precipitation of Ag plays the opposite role to the above effect.

Study of Ag precipitation and mechanical properties of Ti-Ta-Ag ternary alloy.

Ti-25Ta-xAg alloy samples with different content of Ag were prepared by spark plasma sintering method. X-ray diffraction, microscopic metallographic, scanning electron microscopy, and transmission electron microscopy were used to analyze the phase structure and morphology of the alloy samples. Ti-Ta-Ag can form a stable ternary alloy system. Furthermore, with the increase of Ag content and sintering temperature, Ag will be precipitated at the grain boundary. In order to explore the precipitation mechanism of Ag in the alloy and its influence on the mechanical properties, the crystal structure, electronic structure, and elastic constant under different Ag solid solubility were calculated systematically by using first-principles calculations. The results show that the critical temperature of Ag in Ti-Ta-Ag ternary alloy is about 2200 K, and the high temperature is favorable for the aging precipitation of Ag. The lattice constants and mechanical properties of (Ti1-x Ag x )3Ta solid solution suddenly change when the Ag solid solubility x value is equal to 0.8, and their changes will follow different rules. The internal mechanism of this phenomenon is that the 4d10 electronic states of Ag have changed from obvious local electronic states to mixed local and non-local electronic states. These results provide theoretical guidance for the application of Ti-Ta-Ag ternary alloys in biomedicine.