Defective Potassium Poly(Heptazine Imide) Preventing Spin Delocalization and Hole Transfer Deactivation for Efficient Solar Energy Conversion and Storage.

Anti-site defective potassium poly(heptazine imide) (KPHI) with the central nitrogen atoms partially replaced by graphitic carbon atoms in the flawed heptazine rings is prepared by direct ionothermal treatment of the rationally designed supramolecular complex in KSCN salt molten. Compared to the KPHIs without the anti-site defect, the anti-site defective KPHI demonstrates significantly improved photocatalytic and dark photocatalytic performances for H2 evolution reaction (HER). In the presence of the hole scavenger, the anti-site defective KPHI exhibits superior photocatalytic stability for HER lasting 20 h, whereas the deactivation is observed from the ordinary KHPIs after 3 h HER. Moreover, the H2 yield in the dark by the stored photoelectrons in the anti-site defective KPHI increases by more than an order of magnitude. Density functional theory calculations reveal that the anti-site defective unit in KPHI not only prevents spin delocalization but also inhibits the deactivation of hole transfer, which are beneficial to photoelectron storage and photocatalytic activity. The findings in this study provide insight into the photophysical and catalytic properties of KPHI, which conclude a strategy to improve the performances for solar energy conversion and storage by incorporating intrinsic anti-site defects in KPHI.

Ionization of Volatile Organics and Nonvolatile Biomolecules Directly from a Titanium Slab for Mass Spectrometric Analysis.

Atmospheric pressure chemical ionization (APCI)-mass spectrometry (MS) and electrospray ionization (ESI)-MS can cover the analysis of analytes from low to high polarities. Thus, an ion source that possesses these two ionization functions is useful. Atmospheric surface-assisted ionization (ASAI), which can be used to ionize polar and nonpolar analytes in vapor, liquid, and solid forms, was demonstrated in this study. The ionization of analytes through APCI or ESI was induced from the surface of a metal substrate such as a titanium slab. ASAI is a contactless approach operated at atmospheric pressure. No electric contacts nor any voltages were required to be applied on the metal substrate during ionization. When placing samples with high vapor pressure in condensed phase underneath a titanium slab close to the inlet of the mass spectrometer, analytes can be readily ionized and detected by the mass spectrometer. Furthermore, a sample droplet (~2 µL) containing high-polarity analytes, including polar organics and biomolecules, was ionized using the titanium slab. One titanium slab is sufficient to induce the ionization of analytes occurring in front of a mass spectrometer applied with a high voltage. Moreover, this ionization method can be used to detect high volatile or polar analytes through APCI-like or ESI-like processes, respectively.

Porphyrin Dye-Sensitized Zinc Oxide Aggregated Anodes for Use in Solar Cells.

Porphyrin YD2-o-C8-based dyes were employed to sensitize room-temperature (RT) chemical-assembled ZnO aggregated anodes for use in dye-sensitized solar cells (DSSCs). To reduce the acidity of the YD2-o-C8 dye solution, the proton in the carboxyl group of a porphyrin dye was replaced with tetrabuthyl ammonium (TBA⁺) in this work. The short-circuit current density (Jsc) of the YD2-o-C8-TBA-sensitized ZnO DSSCs is higher than that of the YD2-o-C8-sensitized cells, resulting in the improvement of the efficiency of the YD2-o-C8-based ZnO DSSCs. With an appropriate incorporation of chenodeoxycholic acid (CDCA) as coadsorbate, the Jsc and efficiency of the YD2-o-C8-TBA-sensitized ZnO DSSC are enhanced due to the improvement of the incident-photon-to-current efficiency (IPCE) values in the wavelength range of 400-450 nm. Moreover, a considerable increase in Jsc is achieved by the addition of a light scattering layer in the YD2-o-C8-TBA-sensitized ZnO photoanodes. Significant IPCE enhancement in the range 475-600 nm is not attainable by tuning the YD2-o-C8-TBA sensitization processes for the anodes without light scattering layers. Using the RT chemical-assembled ZnO aggregated anode with a light scattering layer, an efficiency of 3.43% was achieved in the YD2-o-C8-TBA-sensitized ZnO DSSC.

Charge collection enhancement by incorporation of gold-silica core-shell nanoparticles into P3HT:PCBM/ZnO nanorod array hybrid solar cells.

In this work, gold-silica core-shell (Au@silica) nanoparticles (NPs) with various silica-shell thicknesses are incorporated into P3HT:PCBM/ZnO nanorod (NR) hybrid solar cells. Enhancement in the short-circuit current density and the efficiency of the hybrid solar cells is attained with the appropriate addition of Au@silica NPs regardless of the silica-shell thickness. Compared to the P3HT:PCBM/ZnO NR hybrid solar cell, a 63% enhancement in the efficiency is achieved by the P3HT:PCBM/Au@silica NP/ZnO NR hybrid solar cell. The finite difference time domain simulations indicate that the strength of the Fano resonance, i.e., the electric field of the quasi-static asymmetric quadrupole, on the surface of Au@silica NPs in the P3HT:PCBM/ZnO NR hybrid significantly decreases with increasing thickness of the silica shell. Raman characterization reveals that the degree of P3HT order increases when Au@silica NPs are incorporated into the P3HT:PCBM/ZnO NR hybrid. The charge separation at the interface between P3HT and PCBM as well as the electron transport in the active layer are retarded by the electric field of the Fano resonance. Nevertheless, the prolongation of the electron lifetime and the reduction of the electron transit time in the P3HT:PCBM/ZnO NR hybrid solar cells, which result in an enhancement of electron collection, are achieved by the addition of Au@silica NPs. This may be attributed to the improvement in the degree of P3HT order and connectivity of PCBM when Au@silica NPs are incorporated into the P3HT:PCBM active layer.

Dual-Vacancy-Engineered ZnIn2S4 Nanosheets for Harnessing Low-Frequency Vibration Induced Piezoelectric Polarization Coupled with Static Dipole Field to Enhance Photocatalytic H2 Evolution.

This study investigates the impact of In- and S-vacancy concentrations on the photocatalytic activity of non-centrosymmetric zinc indium sulfide (ZIS) nanosheets for the hydrogen evolution reaction (HER). A positive correlation between the concentrations of dual In and S vacancies and the photocatalytic HER rate over ZIS nanosheets is observed. The piezoelectric polarization, stimulated by low-frequency vortex vibration to ensure the well-dispersion of ZIS nanosheets in solution, plays a crucial role in enhancing photocatalytic HER over the dual-vacancy engineered ZIS nanosheets. The piezoelectric characteristic of the defective ZIS nanosheets is confirmed through the piezopotential response measured using piezoelectric force microscopy. Piezophotocatalytic H2 evolution over the ZIS nanosheets is boosted under accelerated vortex vibrations. The research explores how vacancies alter ZIS's dipole moment and piezoelectric properties, thereby increasing electric potential gradient and improving charge-separation efficiency, through multi-scale simulations, including Density Functional Theory and Finite Element Analysis, and a machine-learning interatomic potential for defect identification. Increased In and S vacancies lead to higher electric potential gradients in ZIS along [100] and [010] directions, attributing to dipole moment and the piezoelectric effect. This research provides a comprehensive exploration of vacancy engineering in ZIS nanosheets, leveraging the piezopotential and dipole field to enhance photocatalytic performances.

Investigation of charge transfer in Au nanoparticle-ZnO nanosheet composite photocatalysts.

Ohmic contact formation at the interface of the Au nanoparticle (NP)-ZnO nanosheet (NS), which facilitates photoelectron transfer from ZnO NSs to Au NPs, is determined by scanning Kelvin microscopy for the first time. Reduction of charge recombination in the ZnO NSs confirmed by the quench of green band emission results in the enhancement of photocatalytic activity of the Au NP-ZnO NS composite.

Efficacious CO2 Photoconversion to C2 and C3 Hydrocarbons on Upright SnS-SnS2 Heterojunction Nanosheet Frameworks.

In this work, SnS-SnS2 heterostructured upright nanosheet frameworks are constructed on FTO substrates, which demonstrate promising photocatalytic performances for the conversion of CO2 and water to C2 (acetaldehyde) and C3 (acetone) hydrocarbons without H2 formation. With post annealing in designated atmospheres, the photocatalytic activity of the SnS-SnS2 heterostructured nanosheet framework is critically enhanced by increasing the fraction of crystalline SnS in nanosheets through partial transformation of the SnS2 matrix to SnS but not obviously influenced by improving the crystallinity of the SnS2 matrix. DFT calculations indicate that transformed SnS possesses the CO2 adsorption sites with significantly lower activation energy for the rate-determining step to drive efficient CO2 conversion catalysis. The experimental results and DFT calculations suggest that the SnS-SnS2 heterojunction nanosheet framework photocatalyst experiences Z-scheme charge transfer dynamic to allow the water oxidation and CO2 reduction reactions occurring on the surfaces of SnS2 and SnS, respectively. The Z-scheme SnS-SnS2 heterostructured nanosheet framework photocatalyst exhibits not only efficient charge separation but also highly catalytic active sites to boost the photocatalytic activity for CO2 conversion to C2 and C3 hydrocarbons.

Role of Interfacial Defects in Photoelectrochemical Properties of BiVO4 Coated on ZnO Nanodendrites: X-ray Spectroscopic and Microscopic Investigation.

Synchrotron-based X-ray spectroscopic and microscopic techniques are used to identify the origin of enhancement of the photoelectrochemical (PEC) properties of BiVO4 (BVO) that is coated on ZnO nanodendrites (hereafter referred to as BVO/ZnO). The atomic and electronic structures of core-shell BVO/ZnO nanodendrites have been well-characterized, and the heterojunction has been determined to favor the migration of charge carriers under the PEC condition. The variation of charge density between ZnO and BVO in core-shell BVO/ZnO nanodendrites with many unpaired O 2p-derived states at the interface forms interfacial oxygen defects and yields a band gap of approximately 2.6 eV in BVO/ZnO nanocomposites. Atomic structural distortions at the interface of BVO/ZnO nanodendrites, which support the fact that there are many interfacial oxygen defects, affect the O 2p-V 3d hybridization and reduce the crystal field energy 10Dq ∼2.1 eV. Such an interfacial atomic/electronic structure and band gap modulation increase the efficiency of absorption of solar light and electron-hole separation. This study provides evidence that the interfacial oxygen defects act as a trapping center and are critical for the charge transfer, retarding electron-hole recombination, and high absorption of visible light, which can result in favorable PEC properties of a nanostructured core-shell BVO/ZnO heterojunction. Insights into the local atomic and electronic structures of the BVO/ZnO heterojunction support the fabrication of semiconductor heterojunctions with optimal compositions and an optimal interface, which are sought to maximize solar light utilization and the transportation of charge carriers for PEC water splitting and related applications.

An efficient route to nanostructured tungsten oxide films with improved electrochromic properties.

An effective chemical route to nanostructured tungsten oxide films derived from a peroxopolytungstic acid (PTA)/thiourea precursor solution is demonstrated. The conventional procedure of preparing the precursor needs more than 24 h for well-mixing and refluxing the PTA-based solution, while the thiourea-assisted approach takes less than 1 h to prepare the precursor solution because the excess hydrogen peroxide can be efficiently eliminated by oxidation of thiourea. With the precursor solution, tungsten oxide films are deposited by spin coating followed by high temperature annealing. The film annealed at 400 °C possesses a porous nanostructure of nanocrystalline tungsten oxide embedded in an amorphous tungsten oxide matrix, which arises from the gaseous species released through decomposition of thiourea oxides during annealing. The 400 °C-annealed, thiourea-assisted tungsten oxide film exhibits electrochromic (EC) properties superior to those of the film prepared without thiourea, including large transmittance modulation and coloration efficiency, fast response time and adequate reliability. When increasing the annealing temperature to 450 °C, the thiourea-assisted tungsten oxide film is also porous but well-crystallized and shows inferior EC properties. Electrochemical impedance spectroscopy analysis indicates that, in addition to the porous structure, a fast charge-transport rate within the solid portion of the 400 °C-annealed nanostructured film plays a crucial role in enhancing EC performances of the thiourea-assisted tungsten oxide film.

Enhancing electron collection efficiency and effective diffusion length in dye-sensitized solar cells.

Intensity-modulated photocurrent spectroscopy and intensity-modulated photovoltage spectroscopy are employed to measure the dynamics of electron transport and recombination in the ZnO nanowire (NW) array-ZnO/layered basic zinc acetate (LBZA) nanoparticle (NP) composite dye-sensitized solar cells (DSSCs). The roles of the vertical ZnO NWs and insulating LBZA in the electron collection and transport in DSSCs are investigated by comparing the results to those in the TiO(2)-NP, horizontal TiO(2)-NW and vertical ZnO-NW-array DSSCs. The electron transport rate and electron lifetime in the ZnO NW/NP composite DSSC are superior to those in the conventional TiO(2)-NP cell due to the existence of the vertical ZnO NWs and insulating LBZA. It indicates that the ZnO NW/NP composite anode is able to sustain efficient electron collection over much greater thickness than the TiO(2)-NP cell does. Consequently, a larger effective electron diffusion length is available in the ZnO composite DSSC.

Outperformed electrochromic behavior of poly(ethylene glycol)-template nanostructured tungsten oxide films with enhanced charge transfer/transport characteristics.

Porous tungsten oxide films of nanocrystalline tungsten oxide embedded in an amorphous tungsten oxide matrix have been synthesized via poly(ethylene glycol) (PEG)-template sol-gel technique with peroxopolytungstic acid precursor. The effects of PEG addition on the microstructure and electrochromic performance of the tungsten oxide films are investigated. Charge transfer/transport properties in the tungsten oxide films are studied by electrochemical impedance spectroscopy (EIS) as well. Triclinic tungsten oxide film is formed in the absence of PEG. The PEG-template tungsten oxide film demonstrates an electrochromic performance superior to that of the crystalline tungsten oxide film, including larger transmittance modulation and coloration/bleaching efficiency as well as faster response times. EIS measurements indicate that faster charge-transfer rates at the tungsten oxide/electrolyte interface and larger Li(+) diffusion coefficients in tungsten oxide are achieved in the PEG-template film. We suggest that the PEG-template tungsten oxide film with a porous crystalline/amorphous nanostructure provides an effective means for charge transfer/transport to encourage its superior electrochromic performance.

Microscopic Revelation of Charge-Trapping Sites in Polymeric Carbon Nitrides for Enhanced Photocatalytic Activity by Correlating with Chemical and Electronic Structures.

The influences of chemical and electronic structures on the photophysical properties of polymeric carbon nitrides (PCNs) photocatalysts, which govern the microscopic mechanisms of the superior photocatalytic activity under visible-light irradiation, have been resolved in this work. Time-resolved photoluminescence and in situ electron paramagnetic resonance measurements indicate that the photoexcited electrons in the fractured PCNs swiftly transfer to the C2p-localized states where the trapped photoelectrons exhibit longer lifetime compared to those in the ordinary PCNs. Moreover, the structure deviation at the carbon (Cb) atoms around the bridging sites of heptazine ring units, where trapped photoelectrons are localized, has been determined in the fractured PCNs based on the 13C solid-state nuclear magnetic resonance spectra and the density functional theory calculations. Accordingly, the formation of fractured PCNs by breaking the in-plane hydrogen bonds at a high temperature is a promising strategy for the enhancement of photocatalytic activity.

Toward Eco-Friendly and Highly Efficient Solar Water Splitting Using In2S3/Anatase/Rutile TiO2 Dual-Staggered-Heterojunction Nanodendrite Array Photoanode.

The TiO2-based heterojunction nanodendrite (ND) array composed of anatase nanoparticles (ANPs) on the surface of the rutile ND (RND) array is selected as the model photoanode to demonstrate the strategies toward eco-friendly and efficient solar water splitting using neutral electrolyte and seawater. Compared with the performances in alkaline electrolyte, a non-negligible potential drop across the electrolyte as well as impeded charge injection and charge separation is monitored in the ANP/RND array photoanode with neutral electrolyte, which are, respectively, ascribed to the series resistance of neutral electrolyte, the fundamentally pH-dependent water oxidation mechanism on TiO2 surface, as well as the less band bending at the interface of TiO2 and neutral electrolyte. Accordingly, a TiO2-based dual-staggered heterojunction ND array photoanode is further designed in this work to overcome the issue of less band bending with the neutral electrolyte. The improvement of charge separation efficiency is realized by the deposition of a transparent In2S3 layer on the ANP/RND array photoanode for constructing additional staggered heterojunction. Under illumination of AM 1.5G (100 mW cm-2), the improved photocurrent densities acquired both in neutral electrolyte and seawater at 1.23 V vs reversible hydrogen electrode (RHE), which approach the theoretical value for rutile TiO2, are demonstrated in the dual-staggered-heterojunction ND array photoanode. Faradaic efficiencies of ∼95 and ∼32% for solar water oxidation in neutral electrolyte and solar seawater oxidation for 2 h are acquired at 1.23 V vs RHE, respectively.

Aqueous solution route to high-aspect-ratio zinc oxide nanostructures on indium tin oxide substrates.

High-aspect-ratio ZnO nanowires and nanotubes are formed on indium tin oxide (ITO) substrates using a three-step route at low temperatures. The three steps, including successive ionic layer absorption and reaction (SILAR) deposition of the ZnO seed layer, hydrothermal annealing of the seed layer, and chemical bath deposition (CBD) of the one-dimensional (1D) ZnO nanostructures, are all conducted in aqueous solutions at temperatures below 120 degrees C. Both the hydrothermal annealing of the SILAR seed layer and the low-concentration precursor solution employed in the CBD process are crucial in order to synthesize the uniform and high-aspect-ratio ZnO nanostructures on the ITO substrate. TEM analyses reveal that both the nanowire and the nanotube possess the single-crystal structure and are grown along [001] direction. Room-temperature cathodoluminescence spectrum of the 1D ZnO nanostructures shows a sharp ultraviolet emission at 375 nm and a broad green-band emission.

Growth and magnetic properties of oriented alpha-Fe2O3 nanorods.

alpha-Fe(2)O(3) nanorods have been deposited on Si substrates using the metal-organic chemical vapor deposition method. Structural analyses indicated that alpha-Fe(2)O(3) nanorods are preferentially oriented in the [104] direction on Si(100) substrates, and the nanorod possesses the single-crystalline structure. MFM image suggests that a spin domain is formed in the alpha-Fe(2)O(3) nanorod. Anisotropic magnetic property of the alpha-Fe(2)O(3) nanorods, i.e., the discrepancy of the saturation magnetization, is observed from SQUID measurements when the magnetic field are applied parallel and perpendicular to the substrate. A lower Morin temperature than that of the macroscopically crystalline hematite is observed when the magnetic field is applied parallel to the substrate.

Fabrication of an Efficient BiVO4-TiO2 Heterojunction Photoanode for Photoelectrochemical Water Oxidation.

In this work, a simple planar BiVO4/TiO2 heterojunction photoanode was prepared on a fluorine-doped tin oxide (FTO) substrate for photoelectrochemical (PEC) water oxidation. The measurements of surface photovoltage, photocurrent transient behavior, and hole-scavenger-assisted PEC performance indicate that charge separation efficiency is improved compared to that of the BiVO4/FTO photoanode. This improvement is caused by the formation of the staggered BiVO4/TiO2 heterojunction. However, the photocurrent densities of the BiVO4/TiO2/FTO photoanode are higher than those of the BiVO4/FTO one only at potentials >1.2 V vs reversible hydrogen electrode, although the two BiVO4 layers with comparable light harvesting efficiencies were prepared by the same method. The hole-scavenger-assisted PEC measurements reveal that the hole injection efficiency of the BiVO4/TiO2/FTO photoanode is inferior to that of the bare BiVO4/FTO anode for oxygen evolution. It shows that the surface property of the BiVO4 layers is altered as they are deposited on different substrates. On the basis of these characterizations, the cocatalyst cobalt phosphate (Co-Pi) was further deposited on the surface of BiVO4/TiO2/FTO photoanode to improve the hole injection efficiency. Subsequently, the photocurrent density and stability of the Co-Pi/BiVO4/TiO2/FTO photoanode were significantly improved compared to those of the bare BiVO4/FTO photoanode.

Formation of well-aligned ZnGa(2)O(4) nanowires from Ga(2)O(3)/ZnO core-shell nanowires via a Ga(2)O(3)/ZnGa(2)O(4) epitaxial relationship.

Formation of well-aligned and single-crystalline ZnGa(2)O(4) nanowires on sapphire (0001) substrates has been achieved via annealing of the Ga(2)O(3)/ZnO core-shell nanowires. Ga(2)O(3)/ZnO core-shell nanowires were prepared using a two-step method. The thickness of the original ZnO shell and the thermal budget of the annealing process play crucial roles for preparing single-crystalline ZnGa(2)O(4) nanowires. Structural analyses of the annealed nanowires reveal the existence of an epitaxial relationship between ZnGa(2)O(4) and Ga(2)O(3) phases during the solid-state reaction. A strong CL emission band centered at 360 nm and a small tail at 680 nm are obtained at room temperature from the single-crystalline ZnGa(2)O(4) nanowires.

Photoreduction of graphene oxide enhanced by sacrificial agents.

In this work, the photoreduction of graphene oxides (GOs) was carried out in the presence of a sacrificial agent of Na2S/Na2SO3 and triethanolamine (TEA) separately in the solution. The photoreduction of GOs was enhanced with the addition of the sacrificial agent, which was examined in terms of reduction extent and needed reduction period. The quench of the GO emission was observed in the photoluminescence spectra of both GO solutions with Na2S/Na2SO3 and TEA. Although both sacrificial agents facilitated the charge transfer in the irradiated GO solutions, the aggregation of GO/reduced GO (RGO) occurred in the Na2S/Na2SO3-contained solution during photoreduction, which limited further photoreduction of GOs with the assistance of Na2S/Na2SO3. By keeping good dispersion characteristic during the whole process, the photoreduction efficiency of GO in the presence of TEA was therefore superior to that with the assistance of Na2S/Na2SO3.

n-Fe2O3 to N⁺-TiO2Heterojunction Photoanode for Photoelectrochemical Water Oxidation.

To improve the performance of the thin hematite photoanode for photoelectrochemical water oxidation, in this work, an nN(+) α-Fe2O3 (hematite)-TiO2 heterojunction photoanode is constructed on fluorine-doped tin oxide substrate to establish a built-in field in the space charge region for facilitating the charge separation in the hematite layer. Charge distribution in the hematite-TiO2 heterostructure is investigated using Kelvin probe force microscopy, which confirms the improvement of charge separation in hematite layer by the formation of energy-matched nN(+) α-Fe2O3-TiO2 heterojunction. Compared to the hematite photoanode, an eightfold enhancement of the photocurrent density at 1.23 V versus reversible hydrogen electrode is measured in the hematite-TiO2 heterojunction photoanode. By using hydrogen peroxide as a hole scavenger, it demonstrates that both charge separation and charge injection efficiencies in the hematite-TiO2 heterojunction photoanode are superior to those in the hematite photoanode. It results from the significant suppressions of the charge recombinations occurring within the hematite layer as well as at the interface of photoelectrode and electrolyte by the formation of the nN(+) α-Fe2O3-TiO2 heterojunction.

Room-temperature chemical integration of ZnO nanoarchitectures on plastic substrates for flexible dye-sensitized solar cells.

ZnO nanoarchitectured anodes composed of the ZnO nanocactus array and the top ZnO particle layer are chemically integrated on ITO-PET substrates using a facile room-temperature chemical bath deposition method for dye-sensitized solar cells (DSSCs). In the absence of high-temperature post-treatment and mechanical compression, a notable efficiency of 5.24% is simply achieved in the flexible ZnO DSSC.