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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.

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.

The exfoliation of 2D nanomaterials from 3D multimetal oxides with a stable structure is a great challenge. Herein, a delafossite CuFeO

Developing new optoelectronic devices based on two-dimensional materials is an important research topic at present. In these developments, to construct GaAs/Graphene hetero-structure is becoming research hotspot, which can significantly improve the performance of GaAs-based optoelectronic devices. For the application of optoelectronic devices, the interfacial properties and effects are the key factors. However, these issues are not yet fully explored and understood so far, especially the atom diffusion from GaAs substrate to Graphene active layer. In the present work, the electronic structures and atom transportation of GaAs/Graphene hetero-structures were systematically investigated by First-principle calculations. The Ga or As atom, which diffuses from GaAs substrate to Graphene active layer, can open the zero-band-gap of Graphene. Furthermore, Ga atoms adsorbed onto Graphene is the most obvious to realize the purpose of opening the zero-band-gap of Graphene. Moreover, owing to the existence of interfacial built-in electric field, the electrons can efficiently transfer from GaAs substrate to Graphene active layer. This may be the underlying physical mechanism, by which to improve the optoelectronic performance of GaAs-based devices. The findings in this paper can explain the experimental phenomena reported in some literatures, and are helpful for the development of novel GaAs-based optoelectronic devices in the future.

To design and develop high-performance optoelectronic functional materials, the crystal structures and compositions should be fully considered. Copper-based chalcogenides as the core materials of o...

Composite photocatalyst with heterostructure is often recognized as an effective strategy to achieve efficient charge separation and increase carrier concentration. In this study, the (1 1 0) and (1 0 1) surfaces of TiO2 and SnO2 were chosen to construct the abrupt and graded TiO2/SnO2 hetero-structures. The interface properties and band offsets of four heterostructures are calculated and analyzed in detail. It was found that the graded heterostructures are more stable and easier to preparation than the abrupt heterostructures, the charge depletion and accumulation occur at both ends of the transition layer. The interfacial electric field caused by this effect allows the photo-generated electron-hole pairs to be easily separated and transferred in the opposite direction at the interface, which is important for improving the photocatalytic activity. In addition, the intrinsic mechanism of the four hetero-structures is discussed according to the analysis of the band offset, which belongs to the type II heterostructures. For TiO2/SnO2-(1 1 0) heterostructure, the photo-generated electrons transfer from the conduction band of TiO2 to the conduction band of SnO2, while the photo-generated holes transfer from the SnO2 to the valence band of TiO2. This phenomenon leads to the accumulation of the photo-generated electrons in the conduction band of TiO2, accumulation of the photo-generated holes in the valence band of SnO2. In short, the graded heterostructure has more favorable effects to enhance the photocatalytic activity than that of the abrupt heterostructure. The internal electric field at the TiO2/SnO2 interface acts as a selector for photo-generated carriers, which may lead to higher photocatalytic quantum efficiency and carrier concentration under the same external conditions. These intrinsic photocatalytic mechanisms and the interpretation of the interfacial microscopic properties of heterostructure can be used as a basis for the design of the sensing devices and composite photocatalysts with heterostructures.

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 BiOIO

As a typical representative of layered transition metal dichalcogenides, MoS2 nanosheets have been widely studied in both experimental and theoretical works, and have a wide range of applications in the fields of nanotechnology and microelectronics. Based on hybrid functional within density functional theory calculations, the microstructure, electronic structures, and optical properties of MoS2 nanosheets with different crystal phases are investigated. Based on the analysis of phonon dispersion, it can be determined that the 2H-MoS2 monolayer is thermodynamically stable. In the case of 2H-MoS2 nanosheets, the band gap monotonically decreases from 2.219 to 1.441 eV by the exponential form, as the layer number increases from 1 to infinite. Moreover, the 2H-MoS2 monolayer is a direct band gap semiconductor. On the other hand, the 3R-MoS2 monolayer still exhibits metallic characteristics, while the 1T′-MoS2 monolayer has a very narrow band gap. The main features of the electronic structure of MoS2 nanosheets are contributed by the intra-layer interaction, and the inter-layer interaction only induces slight perturbation. But the latter has an important influence on the electronic structure of MoS2 ultrathin nanosheets, especially the monolayer. Furthermore, some fitting equations about optical properties are also provided, which not only contribute to better understanding of the variation of the electronic structure with respect to change in the layer number, but also provide a convenient method to determine the layer number of MoS2 nanosheets in practice. These results indicate that MoS2 nanosheets may serve as promising candidates for photo-electric applications.

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 e-Ta2O5, TaON, and Ta3N5; the second group included β-Ta2O5, δ-Ta2O5, e-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, e-Ta2O5, and β-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.

A new area of simple and effective wurtzite gallium nitride (GaN) electrodes for silver (I) detection using anodic stripping voltammetry (ASV) has been demonstrated. GaN electrode has exhibited obvious advantages over mercury (Hg) in silver ion evolution for its nontoxicity and wide range of anodic potential. ASV tests confirm GaN electrodes exhibit desirable stability and very sensitive response to Ag(I) in aqueous solution. Comparing with GaN nanostructures, bulky GaN thin film shows superior electrochemical sensitivity with Ag(I) detection limit as low as 10 ppb. However, modified GaN nanostructure electrodes with better conductivity are supposed to have more promising applications in trace silver ion detection.

As an alternative to high-temperature oxidation, the present study demonstrates a new approach to the intercalation of excess O into 3R-delafossite CuFeO2, based on a non-equilibrium hydrothermal synthesis. At lower temperatures, or during the initial stage of this hydrothermal reaction, a large amount of excess O remains between the self-assembled block layers, owing to the incomplete reduction of propionaldehyde in the hydrothermal process. This work not only confirmed the presence of this excess O and quantified the excess O concentrations, but also established the mechanism by which non-stoichiometric CuFeO2+δ is generated, on the basis of experimental observations and theoretical calculations. Normally, highly crystalline CuFeO2+δ prepared using the conventional hydrothermal process only contains low levels of excess O, but the present work shows that this problem can be mitigated by employing a microwave-assisted hydrothermal reaction. This technique can rapidly synthesize high quality 3R-delafossite CuFeO2+δ containing elevated concentrations of excess O. Electronic structures calculated based on density functional theory showed that CuFeO2+δ should exhibit half-metallic ferromagnetism and multiple band gaps. Systematic experimental characterization and theoretical calculations were used to elucidate the manner in which excess O significantly affects the microstructure of this material, while enhancing the electrical, optical and electrochemical properties of CuFeO2. As a result of these enhancements, this material is suitable for the improvement of solar energy conversion. Photoelectrochemical trials and data from the photocatalytic performances confirm these properties, suggesting that non-stoichiometric 3R-delafossite CuFeO2+δ compounds have potential applications in solar energy conversion.

The localized surface plasmon resonance (LSPR) is considered as one of the effective strategies to broaden the spectral absorption range and improve quantum conversion efficiency or photocatalyst. Because of low-cost and LSPR absorption peaks in the visible-light region, Cu-based plasmonic photocatalysts have attracted concern in recent years. However, the mechanisms of generation, modulation, and application of LSPR effects of Cu nanoparticles are still insufficient. To this regard, by using finite-difference time-domain simulations and density functional theory calculations, the intrinsic mechanism of LSPR in the system of Cu nanosphere loaded onto TiO2 nanosphere has been systematically analyzed. When Cu nanosphere is gradually sinking into TiO2 nanosphere to form core-shell configuration, the LSPR absorption peaks is gradually red-shifting and separated from the interband region of metallic Cu nanosphere. The interfacial electronic states are the root of enhancement and red-shifting of LSPR absorption peaks in Cu/TiO2 nanospheres. What's more, the LSPR effects of different Cu/TiO2 nanosphere configurations are highly susceptible to the dielectric media with high refractive index, the direction of incidence light, and environmental media. The embedding configuration of Cu/TiO2 nanospheres is predicted to present outstanding photocatalytic performance, owing to less affected by the incident direction of light, effectively excited LSPR wavelength and local electric field, and more reaction sites for photo-redox reaction. The findings in the present work provide a convenient and efficient approach to screen plasmonic photocatalysts for efficient solar energy conversion.

Photocatalysis technology can meet the two global issues of energy shortage and environmental pollution at the same time. Due to the unique layered quasi-two-dimensional superlattice structure, and the specific functions of structural motifs, delafossite compounds have become a popular candidate family for photoelectric conversion applications. In this article, 3R-delafossite CuFeO2 microcrystals with complete stoichiometry, high crystallinity and purity, uniform composition, cubic size of ∼1.47 μm, single crystal phase were obtain by the optimized hydrothermal process. Combined with optical measurement and density functional theory calculation, the electron transition mechanism of CuFeO2 was analyzed in depth and systematically. CuFeO2 has obvious multi-band spectral absorption characteristics, which are helpful to clarify the controversy about band gap values of CuFeO2. CuFeO2 microcrystals exhibit potential hydrogen production from photocatalytic water-splitting to and photocatalytic degradation of pollutants. Tetracycline hydrochloride degradation testing was adopted to further verify the potential of CuFeO2 microcrystals for wastewater purification. CuFeO2 microcrystals not only exhibit excellent photocatalytic degradation performance, but also present Fenton reaction activity in dark conditions after adding H2O2. It is particularly important that the removal rate of tetracycline hydrochloride was significantly enhanced by the coupling of photocatalytic reaction and Fenton reaction, suggesting that CuFeO2 emerges excellent Photo-Fenton reaction activity. Finally, this article not only provides the fundamental physical chemical properties of CuFeO2 for solar energy conversion, but also points out the development direction of further improving the efficiency.

Localized surface plasmon resonance (LSPR) plays a significant role in the fields of photocatalysis and solar cells. It can not only broaden the spectral response range of materials, but also improve the separation probability of photo-generated electron-hole pairs through local field enhancement or hot electron injection. In this article, the LSPR effects of Au/TiO2 composite photocatalyst, with different sizes and shapes, have been simulated by the finite difference time domain (FDTD) method. The variation tendency of the resonance-absorption peaks and the intensity of enhanced local enhanced electric field were systematically compared and emphasized. When the location of Au nanosphere is gradually immersed into the TiO2 substrate, the local enhanced electric field of the boundary is gradually enhanced. When Au nanoshperes are covered by TiO2 at 100 nm depths, the local enhanced electric field intensities reach the maximum value. However, when Au nanorods are loaded on the surface of the TiO2 substrate, the intensity of the corresponding enhanced local enhanced electric field is the maximum. Au nanospheres produce two strong absorption peaks in the visible light region, which are induced by the LSPR effect and interband transitions between Au nanoparticles and the TiO2 substrate. For the LSPR resonance-absorption peaks, the corresponding position is red-shifted by about 100 nm, as the location of Au nanospheres are gradually immersed into the TiO2 substrate. On the other hand, the size change of the Au nanorods do not lead to a similar variation of the LSPR resonance-absorption peaks, except to change the length-diameter ratio. Meanwhile, the LSPR effects are obviously interfered with by the interband transitions between the Au nanorods and TiO2 substrate. At the end of this article, three photo-generated carrier separation mechanisms are proposed. Among them, the existence of direct electron transfer between Au nanoparticles and the TiO2 substrate leads to the enhanced local enhanced electric field at the boundaries, which is favorable for the improvement of photocatalytic performance of TiO2. These findings could explain the underlying mechanism of some experimental observations in published experimental works, and helpful to design highly efficient composite photocatalysts that contain noble metal co-catalyst nanoparticles.

In order to achieve the purpose of further improving the photocatalytic performance of sulfur (S)-doped anatatse titanium dioxide (TiO2) and providing an example to design an efficient photocatalyst, the electronic and optical properties of pure, S-doped, tantalum (Ta)-doped and S+Ta-codoped anatase TiO2 were systematically investigated by using the density functional theory within a GGA+U method. The results indicate that Ta codoping could improve the S incorporated into TiO2 lattice under reducing conditions. And the incorporation of Ta into S-doped TiO2 further increases the photocatalytic activity under visible-light irradiation compared with that of the S-doped TiO2: the distance from S-related impurity energy levels to the top of valence band is obviously decreased in S+Ta-codoped TiO2; and the calculated band gap of S+Ta-codoped TiO2 is smaller than that of S-doped TiO2; furthermore, through the synergistic effects between S and Ta, S+Ta-codoped TiO2 not only has the similar high absorption rate to S-doped TiO2 in the UV-light region, but also has higher absorption rate than S/Ta-doped TiO2 in the visible-light region.

The microstructures and electronic structures of Pt mono- and bi-doped anatase TiO2 (101) surfaces were investigated by density functional theory calculations to elucidate the surface doping effects and the initial stages of Pt cocatalyst formation on a TiO2 photocatalyst surface. Several substitutional and interstitial configurations for the Pt impurity on the surface were studied, and the relative stability of different doping configurations was compared by the impurity formation energy. Under reducing conditions, surface interstitial (in other words, surface adsorbed) doping for Pt is more stable than any competitors (substitutional, bulk or subsurface doping) on an anatase TiO2 (101) surface. Compared with bulk doping, the metal induced gap states were very localized and outstanding in the case of Pt surface interstitial doping onto a TiO2 (101) surface. In the case of Pt substitutional doping, the surface doping effects were harmful for photocatalysis due to the metal induced gap states in the middle of the band gap. However, in the case of Pt interstitial doping, the surface doping effects were very favorable for photocatalysis due to the overlapping of metal induced gap states with the top of the valance band. Systematic calculations revealed that Pt doping is prone to occupy the interstitial sites on the (101) surface. In particular, in the case of Pt bi-doping, two Pt impurity atoms can be co-adsorbed on the surface to form a stable configuration due to the strong Pt–Pt atomic interaction. Therefore, Pt mono-/bi-doping on an anatase TiO2 (101) surface can be considered as the initial stage (nucleation process) of Pt cocatalyst loading onto a TiO2 photocatalyst surface. Therefore, the calculated results can form the basis for further investigations about Ptn cluster loadings or Pt/TiO2 interface formation as well as water molecule adsorption or decomposition on a Pt/TiO2 composite photocatalyst.

As promising solar cell absorb materials, detailed density functional theory calculations of the structural, electronic, and optical properties of Cu 2 ZnSn(S 1 − x Se x ) 4 alloys with different crystal structures (including: zincblende-derived kesterite, zincblende-derived stannite, wurtzite-derived kesterite, and wurtzite-derived stannite) over the whole range of Se composition from x = 0 to x = 1 were systematically investigated in the present work. The calculated results revealed that the lattice constants variation of Cu 2 ZnSn(S 1 − x Se x ) 4 alloys obey the Vegard’s law; and the wurtzite-derived alloys have better alloy solubility and component-uniform compared with zincblende-derived alloys. In the whole range of x, the calculated lattice constants, band gaps, dielectric constants, and refractive index are nearly linear varying with Se compositions. Using the fitting functional relationships, one not only designs suitable Cu 2 ZnSn(S 1 − x Se x ) 4 alloys as absorber materials for solar cells, but also determines the composition of Cu 2 ZnSn(S 1 − x Se x ) 4 alloys by conventional optical measurements in practice.

Six BiOX1−xYx (X, Y = F, Cl, Br and I) solid solutions have been systematically investigated by density functional theory calculations. BiOCl1−xBrx, BiOBr1−xIx and BiOCl1−xIx solid solutions have very small bowing parameters; as such, some of their properties increase almost linearly with increasing x. For BiOF1−xYx solid solutions, the bowing parameters are very large and it is extremely difficult to fit the related calculated data by a single equation. Consequently, BiOX1−xYx (X, Y = Cl, Br and I) solid solutions are highly miscible, while BiOF1−xYx (Y = Cl, Br and I) solid solutions are partially miscible. In other words, BiOF1−xYx solid solutions have miscibility gaps or high miscibility temperature, resulting in phase separation and F/Y inhomogeneity. Comparison and analysis of the calculated results and the related physical–chemical properties with different halogen compositions indicates that the parameters of BiOX1−xYx solid solutions are determined by the differences of the physical–chemical properties of the two halogen compositions. In this way, the large deviation of some BiOX1−xYx solid solutions from Vegard’s law observed in experiments can be explained. Moreover, the composition ratio of BiOX1−xYx solid solutions can be measured or monitored using optical measurements.

As important functional materials, the electronic structure and physical properties of (GaAs)m(AlAs)n superlattices (SLs) have been extensively studied. However, due to limitations of computational methods and computational resources, it is sometimes difficult to thoroughly understand how and why the modification of their structural parameters affects their electronic structure and physical properties. In this article, a high-throughput study based on density functional theory calculations has been carried out to obtain detailed information and to further provide the underlying intrinsic mechanisms. The band gap variations of (GaAs)m(AlAs)n superlattices have been systematically investigated and summarized. They are very consistent with the available reported experimental measurements. Furthermore, the direct-to-indirect-gap transition of (GaAs)m(AlAs)n superlattices has been predicted and explained. For certain thicknesses of the GaAs well (m), the band gap value of (GaAs)m(AlAs)n SLs exponentially increases (increasing n), while for certain thicknesses of the AlAs barrier (n), the band gap value of (GaAs)m(AlAs)n SLs exponentially decreases (increasing m). In both cases, the band gap values converge to certain values. Furthermore, owing to the energy eigenvalues at different k-points showing different variation trends, (GaAs)m(AlAs)n SLs transform from a &Gamma, &Gamma, direct band gap to &Gamma, M indirect band gap when the AlAs barrier is thick enough. The intrinsic reason for these variations is that the contributions and positions of the electronic states of the GaAs well and the AlAs barrier change under altered thickness conditions. Moreover, we have found that the binding energy can be used as a detector to estimate the band gap value in the design of (GaAs)m(AlAs)n devices. Our findings are useful for the design of novel (GaAs)m(AlAs)n superlattices-based optoelectronic devices.

In order to confirm the doping effects of sulfur on the photocatalytic performance of TiO2, the crystal structure, electronic structure, and optical properties of S-doped anatase and rutile TiO2, in which sulfur impurity occupies the lattice oxygen or titanium sites and the interstitial sites, were calculated by density functional theory within GGA + U method. If sulfur impurity incorporates into anatase and rutile lattice by the same doping mode, it exhibits similar doping effects (including lattice distortion, variation of band gap, localized defect states, and type and position of impurity energy levels). However, there are some subtle different doping effects between different crystal structures or different doping modes. According to the calculated results and basic relationship between electronic structure and photocatalytic performance, it is determined that sulfur interstitial doping in anatase TiO2 processes optimal modification effects due to its shallow impurity energy levels, located at the top of the valence and the bottom of the conduction band, which could not only improve the absorption of visible light, but also suppress the recombination of photo-generated electron–hole pairs.