An elliptical nanoantenna array plasmonic metasurface for efficient solar energy harvesting.

Plasmonic metasurfaces with subwavelength nanoantenna arrays have attracted much attention for their ability to control and manage optical properties. Solar absorbers are potential candidates for effectively converting photons into heat and electricity. This study introduces a novel ultrathin metasurface solar absorber employing elliptical-shaped nanoantenna arrays. We theoretically and numerically demonstrate a near-perfect broadband absorber with over 90% absorption efficiency in a wide range of wavelengths of 300-2500 nm, using finite element (FEM) and finite-difference time-domain (FDTD) methods. The proposed nanostructure configuration enhances light absorption by exciting localized surface plasmon resonances (LSPRs) between elliptical-shaped nanoantenna gaps at many wavelengths, maintaining stability at wide incident angles and insensitivity to light polarization. Compared to other state-of-the-art absorbers with a thickness of less than 300 nm, the designed nanostructure with 260 nm thickness achieves over 90% optical absorption across a broad range of wavelengths of 300-1116 nm in air (or vacuum) environments and performs effectively under water conditions for solar energy harvesting in a range of wavelengths of 300-1436 nm, and therefore can serve as a solar evaporator. Combining refractory plasmonic titanium nitride (TiN) and semiconductor gallium nitride (GaN) nanostructures holds great potential for efficient optoelectronic and photocatalytic applications, especially in harsh and high-temperature environments like thermophotovoltaic systems. The TiN-based metasurface absorber, with its ultrathin nanostructure and suitable spectral absorption in ultraviolet-visible-infrared spectra, offers scalability and cost-effectiveness. The findings in this work will deepen our understanding of LSPRs and pave a novel path for efficient solar energy conversion.

Shape-Controlled Photochemical Synthesis of Noble Metal Nanocrystals Based on Reduced Graphene Oxide.

The fabrication of supported noble metal nanocrystals (NCs) with well-controlled morphologies have been attracted considerable interests due to their merits in a wide variety of applications. Photodeposition is a facile and effective method to load metals over semiconductors in a simple slurry reactor under irradiation. By optimizing the photodeposition process, the size, chemical states, and the geometrical distribution of metal NCs have been successfully tuned. However, metal NCs with well-controlled shapes through the photodeposition process have not been reported until now. Here, we report our important advances in the controlled photodeposition process to load regular noble metal NCs. Reduced graphene oxide (rGO) is introduced as a reservoir for the fast transfer of photoelectrons to avoid the fast accumulation of photogenerated electrons on the noble metals which makes the growth process uncontrollable. Meanwhile, rGO also provides stable surface for the controlled nucleation and oriented growth. Noble metal NCs with regular morphologies are then evenly deposited on rGO. This strategy has been demonstrated feasible for different precious metals (Pd, Au, and Pt) and semiconductors (TiO2, ZnO, ZrO2, CeO2, and g-C3N4). In the prototype application of electrochemical hydrogen evolution reaction, regular Pd NCs with enclosed {111} facets showed much better performance compared with that of irregular Pd NCs.

Magnetically Recyclable Fe3O4@TMU-32 Metal-Organic Framework Photocatalyst for Tetracycline Degradation Under Visible Light.

Metal-organic frameworks (MOFs) are a new class of porous crystalline materials being used as photocatalysts for efficient pollutant removal and environmental remediation. In this study, the TMU-32 MOF was synthesized as an effective photocatalyst for the photodegradation of tetracycline (TC) with 96% efficiency in 60 min under visible light. The high photocatalytic activity of the TMU-32 MOF is mainly due to its large specific surface area, which is beneficial for promoting both the adsorption of TC and the separation of the photoinduced charges. Moreover, its desired crystallinity makes it a semiconductor with an appropriate band gap energy. Next, a composite of the TMU-32 MOF with Fe3O4 nanoparticles (as Fe3O4@TMU-32) was prepared as a magnetically recyclable photocatalyst. The results showed that the photocatalytic activity of the Fe3O4@TMU-32 nanocomposite is slightly lower (68% degradation of TC within 60 min) than that of TMU-32 toward TC degradation since Fe3O4 nanoparticles are not acting as a photocatalyst and are used only to make the host photocatalyst (here, TMU-32) magnetically separable. The effects of the photocatalyst concentration and recyclability on the photodegradation of TC were studied under similar conditions. We found that the Fe3O4@TMU-32 composite is easily recycled without a significant loss of photocatalytic activity after being used several times, indicating the stability of the photocatalyst. Finally, a density functional theory study was also conducted to investigate the structural and electronic properties such as the band gap energy and density of states of the TMU-32 MOF and the Fe3O4@TMU-32 composite. Our computational results are in good agreement with the experimental ones. A photocatalytic degradation mechanism was finally proposed under visible-light photoirradiation.

Band alignment tuning of heptazine-g-C3N4/g-ZnO vdW heterostructure as a promising water-splitting photocatalyst.

Van der Waals (vdW) heterostructures of two-dimensional monolayers are a relatively new class of materials with highly tunable band alignment, bandgap energy, and bandgap transition type. In this study, we performed density functional theory calculations to investigate how a vdW heterostructure of heptazine-based graphitic carbon nitride (hg-C3N4) and graphitic zinc oxide (g-ZnO) monolayers is formed (hg-C3N4/g-ZnO). This heterostructure is a potential solar-driven photocatalyst for the water-splitting reaction. Upon the formation of the heterostructure, a type-I indirect bandgap (Eg = 2.08 eV) is created with appropriate conduction band minimum and valence band maximum levels relative to the oxidation/reduction potentials for the water-splitting reaction. In addition, a very large electrostatic potential difference of 11.18 eV is generated across the heterostructure, leading to a large, naturally-formed, built-in electric field directing from hg-C3N4 to g-ZnO. The produced electric field forces photogenerated electrons in g-ZnO to transfer toward hg-C3N4, leading to a decrease in the electron-hole recombination rate. We also found that both g-ZnO and hg-C3N4 synergistically lead to higher light absorption of the heterostructure (λmax = 387 nm). Furthermore, band alignment, bandgap energy, and transition type of the heterostructure can be tuned by applying external perpendicular electric fields and biaxial strains. It was found that a strain of +2% leads to a Z-scheme band alignment (Eg = 2.34 eV, direct) and an electric field of 1 V Å-1 leads to a type-II heterostructure (Eg = 2.29 eV, indirect), which are both beneficial for efficient water-splitting photocatalysis.

Towards developing efficient metalloporphyrin-based hybrid photocatalysts for CO2 reduction; an ab initio study.

A series of thiophene-based donor-acceptor-donor (D-A-D) oligomer substituted metalloporphyrins (MPors) with different 3d central metal-ions (M = Co, Ni, Cu, and Zn) were systematically investigated to screen efficient hybrid photocatalysts for CO2 reduction based on density functional theory (DFT) and time-dependent DFT simulations. Compared with base MPors, the newly designed hybrid photocatalysts have a lower bandgap energy, stronger and broader absorption spectra, and enhanced intermolecular charge transfer, exciton lifetime, and light-harvesting efficiency. Then, the introduction of D-A-D electron donor (ED) groups into the meso-positions of MPors is a promising method for the construction of efficient photocatalysts. According to the calculated adsorption distance, adsorption energy, Hirshfeld charge and electrostatic potential analysis, it was revealed that CO2 physically adsorbed on the designed photocatalyst surface. In addition, among the studied model systems the ZnPor(ED)4 catalyst with four D-A-D electron donors exhibits the best photocatalytic performance due to its broadest absorption spectra with λmax = 500.12 nm and the highest adsorption energy of about 26 kJ mol-1. Finally, the sensing ability of the ZnPor(ED)4-based multi-terminal molecular junction for CO2 gas detection is determined using Green's functions. The transmission plots of this molecular junction are barely changed due to the physical adsorption of CO2 on the molecular surface, leading to the low sensitivity of the device. We believe that such a theoretical design can provide a general approach for further experimental and computational studies of photocatalysts used in the CO2 reduction process.

Dependence of electron transfer dynamics on the number of graphene layers in π-stacked 2D materials: insights from ab initio nonadiabatic molecular dynamics.

Recent time-resolved transient absorption studies demonstrated that the rate of photoinduced interfacial charge transfer (CT) from Zn-phthalocyanine (ZnPc) to single-layer graphene (SLG) is faster than to double-layer graphene (DLG), in contrast to the expectation from Fermi's golden rule. We present the first time-domain non-adiabatic molecular dynamics (NA-MD) study of the electron injection process from photoexcited ZnPc molecules into SLG and DLG substrates. Our calculations suggest that CT occurs faster in the ZnPc/SLG system than in the ZnPc/DLG system, with 580 fs and 810 fs being the fastest components of the observed CT timescales, respectively. The computed timescales are in close agreement with those reported in the experiment. The computed CT timescales are determined largely by the magnitudes of the non-adiabatic couplings (NAC), which we find to be 4 meV and 2 meV, for the ZnPc/SLG and ZnPc/DLG systems, respectively. The transitions are driven mainly by the ZnPc out-of-plane bending mode at 1100 cm-1 and an overtone of fundamental modes in graphene at 2450 cm-1. We find that dephasing occurs on the timescale of 20 fs and is similar in both systems, so decoherence does not notably change the qualitative trends in the CT timescales. We highlight the importance of proper energy level alignment for capturing the qualitative trends in the CT dynamics observed in experiment. In addition, we illustrate several methodological points that are important for accurately modeling nonadiabatic dynamics in the ZnPc/FLG systems, such as the choice of surface hopping methodology, the use of phase corrections, NAC scaling, and the inclusion of Hubbard terms in the density functional and molecular dynamics calculations.

Well-designed Ag/ZnO/3D graphene structure for dye removal: Adsorption, photocatalysis and physical separation capabilities.

In this research, adsorption and photocatalytic degradation process were utilized to remove organic dye from wastewater. To accomplish that, a newly-designed ternary nanostructure based on Ag nanoparticles/ZnO nanorods/three-dimensional graphene network (Ag NPs/ZnO NRs/3DG) was prepared using a combined hydrothermal-photodeposition method. The three-dimensional structure of graphene hydrogel as a support for growth of ZnO nanorods was characterized using field emission scanning electron microscopy (FESEM). In addition, diameter of silver nanoparticles grown on the ZnO nanorods with the average aspect ratio of 5 was determined in the range of 30-80 nm by using transmission electron microscopy (TEM). The X-ray diffraction (XRD) pattern was revealed hexagonal Wurtzite structure of ZnO nanorods and the (1 1 1) lattice plane of the face-centered cubic (FCC) of the silver nanoparticles. The dye adsorption capacity of the synthesized 3DG was evaluated at about 300 mg/g using kinetic study. The photocatalytic dye degradation under both UV and visible light irradiation exhibited an enhanced activity of the prepared ternary Ag/ZnO/3DG sample in comparison to ZnO/3DG and 3DG structures. Different charge-carrier scavengers were utilized to elucidate the synergistic effect of adsorption and visible-light photocatalytic degradation mechanism for dye removal. The facile photocatalyst recovery as well as the high elimination rate of dye is promising for future applications such as efficient removal of organic contaminants from industrial wastewater under solar irradiation.

Group 6 transition metal dichalcogenide nanomaterials: synthesis, applications and future perspectives.

Group 6 transition metal dichalcogenides (G6-TMDs), most notably MoS2, MoSe2, MoTe2, WS2 and WSe2, constitute an important class of materials with a layered crystal structure. Various types of G6-TMD nanomaterials, such as nanosheets, nanotubes and quantum dot nano-objects and flower-like nanostructures, have been synthesized. High thermodynamic stability under ambient conditions, even in atomically thin form, made nanosheets of these inorganic semiconductors a valuable asset in the existing library of two-dimensional (2D) materials, along with the well-known semimetallic graphene and insulating hexagonal boron nitride. G6-TMDs generally possess an appropriate bandgap (1-2 eV) which is tunable by size and dimensionality and changes from indirect to direct in monolayer nanosheets, intriguing for (opto)electronic, sensing, and solar energy harvesting applications. Moreover, rich intercalation chemistry and abundance of catalytically active edge sites make them promising for fabrication of novel energy storage devices and advanced catalysts. In this review, we provide an overview on all aspects of the basic science, physicochemical properties and characterization techniques as well as all existing production methods and applications of G6-TMD nanomaterials in a comprehensive yet concise treatment. Particular emphasis is placed on establishing a linkage between the features of production methods and the specific needs of rapidly growing applications of G6-TMDs to develop a production-application selection guide. Based on this selection guide, a framework is suggested for future research on how to bridge existing knowledge gaps and improve current production methods towards technological application of G6-TMD nanomaterials.

A facile approach for reducing the working voltage of Au/TiO2/Au nanostructured memristors by enhancing the local electric field.

Memristor devices have attracted tremendous interest due to different applications ranging from nonvolatile data storage to neuromorphic computing units. Exploring the role of surface roughness of the bottom electrode (BE)/active layer interface provides useful guidelines for the optimization of the memristor switching performance. This study focuses on the effect of surface roughness of the BE electrode on the switching characteristics of Au/TiO2/Au three-layer memristor devices. An optimized wet-etching treatment condition was found to modify the surface roughness of the Au BE where the measurement results indicate that the roughness of the Au BE is affected by both duration time and solution concentrations of the wet-etching process. Then we fabricated arrays of TiO2-based nanostructured memristors sandwiched between two sets of cross-bar Au electrode lines (junction area 900 µm2). The results revealed a reduction in the working voltages in current-voltage characteristic of the device performance when increasing the surface roughness at the Au(BE)/TiO2 active layer interface. The set voltage of the device (Vset) significantly decreased from 2.26-1.93 V when we increased the interface roughness from 4.2-13.1 nm. The present work provides information for better understanding the switching mechanism of titanium-dioxide-based devices, and it can be inferred that enhancing the roughness of the Au BE/TiO2 active layer interface leads to a localized non-uniform electric field distribution that plays a vital role in reducing the energy consumption of the device.

Unexpected optical limiting properties from MoS2 nanosheets modified by a semiconductive polymer.

Direct solvent exfoliation of bulk MoS2 with the assistance of poly(3-hexylthiophene) (P3HT) produces a novel two-dimensional organic/inorganic semiconductor hetero-junction. The obtained P3HT-MoS2 nanohybrid exhibits unexpected optical limiting properties in contrast to the saturated absorption behavior of both P3HT and MoS2, showing potential in future photoelectric applications.

Hierarchical Co3O4/Co(OH)2 Nanoflakes as a Supercapacitor Electrode: Experimental and Semi-Empirical Model.

In this research, facile and low cost synthesis methods, electrodeposition at constant current density and anodization at various applied voltages, were used to produce hierarchical cobalt oxide/hydroxide nanoflakes on top of porous anodized cobalt layer. The maximum electrochemical capacitance of 601 mF cm(-2) at scan rate of 2 mV s(-1) was achieved for 30 V optimized anodization applied voltage with high stability. Morphology and surface chemical composition were determined by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) analysis. The size, thickness, and density of nanoflakes, as well as length of the porous anodized Co layer were measured about 460±45 nm, 52±5 nm, 22±3 µm(-2), and 3.4±0.3 µm for the optimized anodization voltage, respectively. Moreover, the effect of anodization voltage on the resulting supercapacitance was modeled by using the Butler-Volmer formalism. The behavior of the modeled capacitance in different anodization voltages was in good agreement with the measured experimental data, and it was found that the role and contribution of the porous morphology was more decisive than structure of nanoflakes in the supercapacitance application.