Facet-Dependent Electrical Conductivity Properties of Silver Oxide Crystals

Growth of Size-Tunable Ag2O Polyhedra and Revelation of Their Bulk and Surface Lattices

By primarily adjusting the reagent amounts, particularly the volume of AgNO3 solution introduced, Ag2O cubes with decreasing sizes from 440 to 79 nm, octahedra from 714 to 106 nm, and rhombic dodecahedra from 644 to 168 nm are synthesized. 733 nm cuboctahedra are also prepared for structural analysis. With in-house X-ray diffraction (XRD) peak calibration, shape-related peak shifts are recognizable. Synchrotron XRD measurements at 100 K reveal the presence of bulk and surface layer lattices. Bulk cell constants also deviate slightly. They show a negative thermal expansion behavior with shrinking cell constants at higher temperatures. The Ag2O crystals exhibit size- and facet-dependent optical properties. Bandgaps red-shift continuously with increasing particle sizes. Optical facet effect is also observable. Moreover, synchrotron XRD peaks of a mixture of Cu2O rhombicuboctahedra and edge- and corner-truncated cubes exposing all three crystal faces can be deconvoluted into three components with the bulk and the [111] microstrain phase as the major component. Interestingly, while the unheated Cu2O sample shows clear diffraction peak asymmetry, annealing the sample to 450 K yields nearly symmetric peaks even when returning the sample to room temperature, meaning even moderately high temperatures can permanently change the crystal lattice.

Experimental Revelation of Surface and Bulk Lattices in Faceted Cu2 O Crystals

Semiconductor crystals have generally shown facet-dependent electrical, photocatalytic, and optical properties. These phenomena have been proposed to result from the presence of a surface layer with bond-level deviations. To provide experimental evidence of this structural feature, synchrotron X-ray sources are used to obtain X-ray diffraction (XRD) patterns of polyhedral cuprous oxide crystals. Cu2 O rhombic dodecahedra display two distinct cell constants from peak splitting. Peak disappearance during slow Cu2 O reduction to Cu with ammonia borane differentiates bulk and surface layer lattices. Cubes and octahedra also show two peak components, while diffraction peaks of cuboctahedra are comprised of three components. Temperature-varying lattice changes in the bulk and surface regions also show shape dependence. From transmission electron microscopy (TEM) images, slight plane spacing deviations in surface and inner crystal regions are measured. Image processing provides visualization of the surface layer with depths of about 1.5-4 nm giving dashed lattice points instead of dots from atomic position deviations. Close TEM examination reveals considerable variation in lattice spot size and shape for different particle morphologies, explaining why facet-dependent properties are emerged. Raman spectrum reflects the large bulk and surface lattice difference in rhombic dodecahedra. Surface lattice difference can change the particle bandgap.

Procedure to Obtain Cu2O Isolate Films, Structural, Electrical, and Morphological Characterization, and Its Use as an Electrical Isolator to Build a New Tube Furnace

Copper oxide is a widely studied compound in wastewater decontamination, hydrogen production, solar cell production, and sensor fabrication. In recent years, many architectures and structures with the potential for developing clean technologies have been synthesized. A procedure by thermal oxidation to grow electrical insolate Cu2O films on copper surfaces in an air atmosphere was developed. The results of the morphological and structural characterization of the copper oxide layers evidence the presence of Cu2O polycrystalline films. The films have polyhedral architectures of approximately 1.4 μm thickness and are electrically insulating. A novel copper resistive furnace was built using this copper oxide film which was used as an electrical insulator between the electrical resistance of the heater and the surface of the copper thermal block. The application improves the efficiency of the resistive furnace in terms of the temperature reached and the thermal coupling response time relative to the performance of conventional furnaces using ceramic insulation. Over the entire operating temperature range explored for the same power supply, the copper oxide-coated furnace achieved higher temperatures and faster response times than the traditionally coated furnace.

Optoelectronic Properties Prediction of Lead-Free Methylammonium Alkaline-Earth Perovskite Based on DFT Calculations

Dynamical stability plays an essential role in phase transition and structure, and it could be a fundamental method of discovering new lead-free perovskite materials. The perovskite materials are well-known for their excellent optoelectronic properties, but the lead element inside could be a hindrance to future development. This research is trying to predict the promising cation candidates in the high-temperature application for lead-free perovskite materials from the replacement of lead in MAPbCl₃ (MA = methylammonium) with alkaline-earth cations. The alkaline-earth cations are of a stable positive divalent sort, which is the same as Pb, and most of them are abundant in nature. Therefore, by improving the dynamical stability, the Mg²⁺, Ca²⁺, and Sr²⁺ cations replacement of lead ions could stabilize the perovskite structure by decreasing the imaginary part of phonon density of states. Finally, the density functional theory results show that the MACaCl₃ could be a dynamic stable lead-free methylammonium perovskite material with an ultrawide band gap (5.96 eV).

Surface-dependent band structure variations and bond deviations of GaN

Density functional theory (DFT) calculations on a tunable number of GaN (0001) planes give an invariant band structure, density of states (DOS) diagram, and band gap of the GaN unit cell. Dissimilar band structures and DOS diagrams are obtained for 1, 3, 5, 7, and 9 layers of GaN (101̄0) planes, but the same band structure as that of the (0001) plane returns for 2, 4, 6, and 8 (101̄0) planes. Furthermore, 1 to 4 layers of GaN (101̄1) planes exhibit dissimilar band structures, but the GaN unit cell band structure is obtained for 5 (101̄1) planes. While there are no changes to the Ga-N bond length and bond geometry for the (0001) planes, the (101̄0) planes present bond length variation and bond distortion with odd numbers of layers. Bond length and bond direction deviations are also obtained for 1 to 4 (101̄1) planes. These results suggest that slight structural deviations may be present near the GaN surface to produce facet-dependent properties, and such atomic position deviations in the surface layer can be observed in various semiconductors.

Transition Metal Ions in Methylammonium Chloride Perovskites

Organic-inorganic perovskite materials have become star materials for future wide band gap optoelectronics due to their excellent optical and electrical properties. However, the lead ions inside perovskites have become a crucial environmental issue in the commercialization of wide band gap perovskite devices . This research tries to find the structure and properties of lead-free perovskite materials by screening Sn2+ and transition-metal ions to replace Pb2+ within the methylammonium (MA)-based chloride perovskite and find out a new two-dimensional structure of MA-based transition-metal ion chlorides. Overall, MAZnCl3 may be a potential ultraviolet-C luminescent material with a stable two-dimensional structure with a wide band gap of 5.64 eV, which is suitable for ultraviolet-C luminescence applications.

Current Rectification and Photo-Responsive Current Achieved through Interfacial Facet Control of Cu2O-Si Wafer Heterojunctions

Conductive atomic force microscopy (C-AFM) was employed to perform conductivity measurements on a facet-specific Cu₂O cube, octahedron, and rhombic dodecahedron and intrinsic Si {100}, {111}, and {110} wafers. Similar I-V curves to those recorded previously using a nanomanipulator were obtained with the exception of high conductivity for the Si {110} wafer. Next, I-V curves of different Cu₂O-Si heterostructures were evaluated. Among the nine possible arrangements, Cu₂O octahedron/Si {100} wafer and Cu₂O octahedron/Si {110} wafer combinations show good current rectification behaviors. Under white light illumination, Cu₂O cube/Si {110} wafer and Cu₂O rhombic dodecahedron/Si {111} wafer combinations exhibit the largest degrees of photocurrent, so such interfacial plane-controlled semiconductor heterojunctions with light sensitivity can be applied to make photodetectors. Adjusted band diagrams are presented highlighting different interfacial band bending situations to facilitate or inhibit current flow for different Cu₂O-Si junctions. More importantly, the observation of clear current-rectifying effects produced at the semiconductor heterojunctions with properly selected contacting faces or planes implies that novel field-effect transistors (FETs) can be fabricated using this design strategy, which should integrate well with current chip manufacturing processes.

Surface-dependent band structure variations and bond-level deviations in Cu2O

New DFT calculations revealed notable variations in the band gap, bond length and bond geometry in different planes of Cu2O, which helped in understanding its various facet-dependent behaviors.

Facet-Specific Photocatalytic Activity Enhancement of Cu2O Polyhedra Functionalized with 4-Ethynylanaline Resulting from Band Structure Tuning

Cu₂O rhombic dodecahedra, octahedra, and cubes were densely modified with conjugated 4-ethynylaniline (4-EA) for facet-dependent photocatalytic activity examination. Infrared spectroscopy affirms bonding of the acetylenic group of 4-EA onto the surface copper atoms. The photocatalytically inactive Cu₂O cubes showed surprisingly high activity toward methyl orange photodegradation after 4-EA modification, while the already active Cu₂O rhombic dodecahedra and octahedra exhibited a photocatalytic activity enhancement. Electron, hole, and radical scavenger experiments prove that the photocatalytic charge transport processes have occurred in the functionalized Cu₂O cubes. Electrochemical impedance spectroscopy also indicates reduced charge transfer resistance of the functionalized Cu₂O crystals. A band diagram constructed from UV-vis spectral and Mott-Schottky measurements reveals significant band energy shifts in all Cu₂O samples after decorating with 4-EA. From density functional theory (DFT) calculations, a new band has emerged slightly above the valence band maximum within the band gap of Cu₂O, which has been found to originate from 4-EA through band-decomposed charge density analysis. The increased charge density localized on the 4-EA molecule and the smallest electron transition energy to reach the 4-EA-generated band are factors making {100}-bound Cu₂O cubes photocatalytically active. Proper molecular decoration represents a powerful approach to improving the photocatalytic efficiency of semiconductors.

Biogenesis of Prism-Like Silver Oxide Nanoparticles Using Nappa Cabbage Extract and Their p-Nitrophenol Sensing Activity

The present study aimed to explore the eco-friendly synthesis of prism-like silver oxide nanoparticles (Ag₂ONPs) from nappa cabbage extract and its p-nitrophenol sensing activity. The prepared Ag₂ONPs were characterized by X-ray diffraction (XRD), field-emission scanning spectroscopy (FESEM), energy-dispersive spectroscopy (EDS), transmission electron microscopy (TEM), and ultraviolet (UV)-visible light spectral analysis (UV-Vis). p-Nitrophenol sensing properties of the prepared nanoparticles were also determined using a simple I-V method. The results showed that the as-prepared Ag₂ONPs have a face-centered cubic (fcc) crystalline nature and a prism-like morphology with particle size in the range 21.61-92.26 nm. The result also showed a high intensity of the (111) facet, making the Ag₂ONP-carbon black/nickel foam electrode (Ag₂ONP-C/NFE) exhibit a high-performance response to p-nitrophenol spanning a wide range of concentrations from 1.0 mM to 0.1 pM and a response time of around 5 s, indicating a high potential for water treatment applications.

Formation of Silver Rhombic Dodecahedra, Octahedra, and Cubes through Pseudomorphic Conversion of Ag2O Crystals with Nitroarene Reduction Activity

Using ethanol as a co-solvent, relatively small-sized Ag₂O octahedra (∼645 nm in opposite corner distance) and rhombic dodecahedra (∼540 and 655 nm in opposite face distance) were synthesized in aqueous solutions. Ag₂O cubes synthesized in an aqueous solution have an edge length of ∼425 nm. Band gaps of these crystals have been obtained, revealing the presence of facet-dependent light absorption properties. The Ag₂O rhombic dodecahedra, octahedra, and cubes were treated with ammonia borane in ethanol at 50 °C for just 10 min to pseudomorphically convert to Ag polyhedra of the corresponding morphologies. Transmission electron microscopy characterization confirms that the Ag cubes, octahedra, and rhombic dodecahedra are bound by the {100}, {111}, and {110} faces, respectively. The Ag rhombic dodecahedra, available for the first time, showed more superior catalytic activity toward 4-nitroaniline reduction at 50 °C than Ag octahedra and cubes, and gave 100% product yield after 1 h of reaction. This work demonstrates the value of forming Ag rhombic dodecahedra, exposing {110} surfaces that may be useful in other organic transformations.

Facet-Dependent Optical Properties of Semiconductor Nanocrystals

Recent observations of facet-dependent electrical conductivity and photocatalytic activity of various semiconductor crystals are presented. Then, the discovery of facet-dependent surface plasmon resonance absorption of metal-Cu₂ O core-shell nanocrystals with tunable sizes and shapes is discussed. The Cu₂ O shells also exhibit a facet-specific optical absorption feature. The facet-dependent electrical conductivity, photocatalytic activity, and optical properties are related phenomena, resulting from the presence of an ultrathin surface layer with different band structures and thus varying degrees of band bending for the {100}, {110}, and {111} faces of Cu₂ O to absorb light of somewhat different wavelengths. Recently, it is shown that the light absorption and photoluminescence properties of pure Cu₂ O cubes, octahedra, and rhombic dodecahedra also display size and facet effects because of their tunable band gaps. A modified band diagram of Cu₂ O can be constructed to incorporate these optical effects. Literature also provides examples of facet-dependent optical behaviors of semiconductor nanostructures, indicating that optical properties of nanoscale semiconductor materials are intrinsically facet-dependent. Some applications of semiconductor optical size and facet effects are considered.

Facet-Dependent Photocatalytic Behaviors of ZnS-Decorated Cu2O Polyhedra Arising from Tunable Interfacial Band Alignment

ZnS particles were grown over Cu₂O cubes, octahedra, and rhombic dodecahedra for examination of their facet-dependent photocatalytic behaviors. After ZnS growth, Cu₂O cubes stay photocatalytically inactive. ZnS-decorated Cu₂O octahedra show enhanced photocatalytic activity, resulting from better charge carrier separation upon photoexcitation. Surprisingly, Cu₂O rhombic dodecahedra give greatly suppressed photocatalytic activity after ZnS deposition. Electron paramagnetic resonance spectra agree with these experimental observations. Time-resolved photoluminescence profiles provide charge-transfer insights. The decrease in the photocatalytic activity is attributed to an unfavorable band alignment caused by significant band bending within the Cu₂O(110)/ZnS(200) plane interface. A modified Cu₂O-ZnS band diagram is presented. Density functional theory calculations generating plane-specific band energy diagrams of Cu₂O and ZnS match well with the experimental results, showing that charge transfer across the Cu₂O(110)/ZnS(200) plane interface would not happen. This example further illustrates that the actual photocatalysis outcome for semiconductor heterojunctions cannot be assumed because interfacial charge transfer is strongly facet-dependent.

Aqueous-Phase Synthesis of Size-Tunable PbSe Nanocubes at Room Temperature for Optical Property Characterization

A simple aqueous-phase synthesis of PbSe nanocubes with tunable sizes has been developed by first preparing a Na₂ SeSO₃ stock solution through dissolution of selenium powder in a solution of Na₂ SO₃ at 90-100 °C for 30 min, and adding part of this solution to a mixture of lead acetate and acetic acid at room temperature with stirring for only 5-8 min to complete the nanocrystal growth. Adjusting the volume of acetic acid and Na₂ SeSO₃ solution added enabled the size of the nanocrystals to be tuned, with average edge lengths of 13 to 121 nm attained. Changes in solution color revealed very different crystal growth rates for the 13 and 121 nm nanocubes. The PbSe cubes exhibit size-dependent absorption bands in the ultraviolet and visible-light region; the band positions show progressive redshifts with increasing particle size. Slight photocatalytic activity upon 532 nm laser irradiation of the nanocubes suggests the presence of higher energy levels in the band structure of PbSe. The synthetic conditions can be easily scaled up to obtain a large quantity of PbSe nanocubes for applications.

Switching of Current Rectification Ratios within a Single Nanocrystal by Facet-Resolved Electrical Wiring

Here we show that within a single polyhedral metal oxide nanoparticle a nanometer-scale lateral or vertical sliding of a small metal top contact ( e. g., <50 nm) leads to a 10-fold change in current rectification ratios. Electron tunneling imaging and constant-force current-potential analysis in atomic force microscopy demonstrate that within an individual p-n rectifier (a Cu₂O nanocrystal on silicon) the degree of current asymmetry can be modulated predictably by a set of geometric considerations. We demonstrate the concept of a single nanoscale entity displaying an in-built range of discrete electrical signatures and address fundamental questions in the direction of "landing" contacts in single-particle diodes. This concept is scalable to large 2D arrays, up to millimeters in size, with implications in the design and understanding of nanoparticle circuitry.

Density Functional Theory Calculations Revealing Metal-like Band Structures for Ultrathin Germanium (111) and (211) Surface Layers

To find out if germanium possesses facet-dependent electrical-conductivity properties, surface-state density functional theory (DFT) calculations were performed on one to six layers of germanium (100), (110), (111), and (211) planes. Tunable Ge(100) and Ge(110) planes always present the same semiconducting band structure with a band gap of 0.67 eV expected of bulk germanium. In contrast, one, two, four, and five layers of Ge(111) and Ge(211) plane models show metal-like band structures with continuous density of states (DOS) throughout the entire band. For three and six layers of Ge(111) and Ge(211) plane models, the normal semiconducting band structure was obtained. The plane layers with metal-like band structures also show Ge-Ge bond-length deviations and bond distortions, as well as significantly different 4s and 4p frontier-orbital electron counts and relative percentages integrated over the valence and conduction bands from those of the semiconducting state. These differences should contribute to strikingly dissimilar band structures. The calculation results suggest the observation of facet-dependent electrical-conductivity properties of germanium materials; when making transistors from germanium, the facet effects with shrinking dimensions approaching 3 nm may also need to be considered.

Facet-Dependent Electrical, Photocatalytic, and Optical Properties of Semiconductor Crystals and Their Implications for Applications

Recent studies on the electrical conductivity and photocatalytic activity properties of semiconductor nanocrystals such as Cu₂O, Ag₂O, TiO₂, PbS, and Ag₃PO₄ exposing well-defined surfaces have revealed strong facet effects. For example, the electrical conductivity of Cu₂O crystals can vary from highly conductive to nonconductive, and they can be highly photocatalytically active or inactive depending on the exposed faces. The crystal surfaces can even tune their light absorption wavelengths. Our understanding is that the emergence of these unusual phenomena can be explained in terms of the presence of an ultrathin surface layer having different band structures and degrees of band bending for different surfaces, which affects charge transport and photons into and out of the crystals. This review uses primarily results from our research on this frontier area of semiconductor properties to illustrate the existence of semiconductor facet effects. A simple adjustment to normal semiconductor band diagram allows good understanding of the observed phenomena. Recognizing that facet-dependent behaviors are intrinsic semiconductor properties, we should pay attention to their influence in the explanation of the measured photocatalytic properties, and consider ways to enhance photocatalytic efficiency or design electrical components utilizing the facet effects. There should be many opportunities to advance applications of semiconductor nanocrystals and nanostructures with continued research on the facet-dependent properties of various semiconductor materials.

Synthesis of Ag3PO4 Crystals with Tunable Shapes for Facet-Dependent Optical Property, Photocatalytic Activity, and Electrical Conductivity Examinations

This work has developed conditions for the synthesis of Ag₃PO₄ cubes, rhombic dodecahedra, {100}-truncated rhombic dodecahedra, tetrahedra, and tetrapods by tuning the amount of NH₄NO₃, NaOH, AgNO₃, and K₂HPO₄ solutions added. Use of a minimal amount of AgNO₃ solution can form much smaller rhombic dodecahedra and tetrahedra. Submicrometer-sized Ag₃PO₄ cubes and rhombic dodecahedra with sizes larger than 300 nm do not exhibit the optical size effect, but ∼290 nm rhombic dodecahedra show a smaller band gap value than larger cubes, and tetrahedra show the most blue-shifted absorption edge. The optical facet effect is present in Ag₃PO₄ crystals. Ag₃PO₄ cubes are more photocatalytically active than rhombic dodecahedra toward photodegradation of methyl orange, but tetrahedra are inactive, showing clear presence of photocatalytic facet effects. Electron paramagnetic resonance results confirm much higher production of hydroxyl radicals from photoirradiated Ag₃PO₄ cubes than from rhombic dodecahedra, while tetrahedra yield essentially no radicals. A modified band diagram showing different degrees of band edge bending can explain these observations. All these Ag₃PO₄ crystals show poor electrical conductivity properties, but the {110} faces are slightly more conductive than the {100} faces. As a result, current rectifying I-V curves have been obtained, demonstrating that facet-dependent electrical properties are broadly observable in many semiconductor materials. This work reveals again that facet-dependent optical, photocatalytic, and electrical conductivity properties are intrinsic semiconductor properties.

Enhancing the Photocatalytic Hydrogen Evolution Performance of a Metal/Semiconductor Catalyst through Modulation of the Schottky Barrier Height by Controlling the Orientation of the Interface

Construction of a metal-semiconductor heterojunction is a promising method to improve heterogeneous photocatalysis for various reactions. Although the structure and photocatalytic performance of such a catalyst system have been extensively studied, few reports have demonstrated the effect of interface orientation at the metal-semiconductor junction on junction-barrier bending and the electronic transport properties. Here, we construct a Pt/PbS heterojunction, in which Pt nanoparticles are used as highly active catalysts and PbS nanocrystals (NCs) with well-controlled shapes are used as light-harvesting supports. Experimental results show that the photoelectrocatalytic activities of the Pt/PbS catalyst are strongly dependent on the contacting facets of PbS at the junction. Pt/octahedral PbS NCs with exposed PbS(111) facets show the highest photoinduced enhancement of hydrogen evolution reaction activity, which is ∼14.38 times higher than that of the ones with only PbS(100) facets (Pt/cubic PbS NCs). This enhancement can further be rationalized by the different energy barriers of the Pt/PbS Schottky junction due to the specific band structure and electron affinity, which is also confirmed by the calculations based on density functional theory. Therefore, controlling the contacting interfaces of a metal/semiconductor material may offer an effective approach to form the desired heterojunction for optimization of the catalytic performance.