Heterogeneous Semiconductors Research Articles

Semiheterogeneous Photoredox Catalysis: Synthesis of 3-Sulfonylquinolines via Radical Multicomponent Cascade Annulation.

An efficient photocatalytic protocol combining heterogeneous semiconductors as photocatalysts and NHPI as a redox reagent was disclosed. Under this protocol, 3-sulfonylquinolines were formed in up to 92% yield via a multicomponent radical cascade annulation. Good substituent tolerance and gram-scale reaction showed the potential in fine chemicals modification and pharmaceutical synthesis.

Direct Z-Scheme Heterojunction Catalysts Constructed by Graphitic-C3N4 and Photosensitive Metal-Organic Cages for Efficient Photocatalytic Hydrogen Evolution.

The demand for improving the activity, durability, and recyclability of metal-organic cages (MOCs) that work as photocatalytic molecular devices in a homogeneous system has promoted research to combine them with other solid materials. An M2L4 type photosensitive metal-organic cage MOC-Q2 with light-harvesting ligands and catalytic Pd2+ centers has been synthesized and further heterogenized with graphitic carbon nitride to prepare a robust direct Z-scheme heterojunction photocatalyst for visible-light-driven hydrogen generation. The optimized g-C3N4/MOC-Q2 (0.7 wt%) sample exhibits a high H2 evolution activity of 6423 μmol g−1 h−1 in 5 h, and a total turnover number of 39,695 after 10 h, significantly superior to the bare MOC-Q2 used in the homogeneous solution and the comparison sample Pd/g-C3N4/L-4. The enhanced performances of g-C3N4/MOC-Q2 can be ascribed to its direct Z-scheme heterostructure, which effectively improves the charge separation and transfer efficiency. This work presents a rational approach of designing a binary photocatalytic system through combing micromolecular MOCs with heterogeneous semiconductors for water splitting.

Inorganometallic Photocatalyst for CO2 Reduction.

ConspectusDuring the last few decades, the design of catalytic systems for CO2 reduction has been extensively researched and generally involves (1) traditional approaches using molecular organic/organometallic materials and heterogeneous inorganic semiconductors and (2) combinatory approaches wherein these materials are combined as needed. Recently, we have devised a number of new TiO2-mediated multicomponent hybrid systems that synergistically integrate the intrinsic merits of various materials, namely, molecular photosensitizers/catalysts and n-type TiO2 semiconductors, and lower the energetic and kinetic barriers between components. We have termed such multicomponent hybrid systems assembled from the hybridization of various organic/inorganic/organometallic units in a single platform inorganometallic photocatalysts. The multicomponent inorganometallic (MIOM) hybrid system onto which the photosensitizer and catalyst are coadsorbed efficiently eliminates the need for bulk-phase diffusion of the components and avoids the accumulation of radical intermediates that invokes a degradation pathway, in contrast to the homogeneous system, in which the free reactive species are concentrated in a confined reaction space. In particular, in energetic terms, we discovered that in nonaqueous media, the conduction band (CB) levels of reduced TiO2 (TiO2(e-)) are positioned at a higher level (in the range -1.5 to -1.9 V vs SCE). This energetic benefit of reduced TiO2 allows smooth electron transfer (ET) from injected electrons (TiO2(e-)) to the coadsorbed CO2 reduction catalyst, which requires relatively high reducing power (at least more than -1.1 V vs SCE). On the other hand, the existence of various shallow surface trapping sites and surface bands, which are 0.3-1.0 eV below the CB of TiO2, efficiently facilitates electron injection from any photosensitizer (including dyes having low excited energy levels) to TiO2 without energetic limitation. This is contrasted with most photocatalytic systems, wherein successive absorption of single high-energy photons is required to produce excited states with enough energy to fulfill photocatalytic reaction, which may allow unwanted side reactions during photocatalysis. In this Account, we present our recent research efforts toward advancing these MIOM hybrid systems for photochemical CO2 reduction and discuss their working mechanisms in detail. Basic ET processes within the MIOM system, including intervalence ET in organic/organometallic redox systems, metal-to-ligand charge transfer of organometallic complexes, and interfacial/outer-sphere charge transfer between components, were investigated by conducting serial photophysical and electrochemical analyses. Because such ET events occur primarily at the interface between the components, the efficiency of interfacial ET between the molecular components (organic/organometallic photosensitizers and molecular reduction catalysts) and the bulk inorganic solid (mainly n-type TiO2 semiconductors) has a significant influence on the overall photochemical reaction kinetics and mechanism. In some TiO2-mediated MIOM hybrids, the chemical attachment of organic or organometallic photosensitizing units onto TiO2 semiconductors efficiently eliminates the step of diffusion/collision-controlled ET between components and prevents the accumulation of reactive species (oxidatively quenched cations or reductively quenched anions) in the reaction solution, ensuring steady photosensitization over an extended reaction period. The site isolation of a single-site organometallic catalyst employing TiO2 immobilization promotes the monomeric catalytic pathway during the CO2 reduction process, resulting in enhanced product selectivity and catalytic performance, including lifetime extension. In addition, as an alternative inorganic solid scaffold, the introduction of a host porphyrin matrix (interlinked in a metal-organic framework (MOF) material) led to efficient and durable photocatalytic CO2 conversion by the new MOF-Re(I) hybrid as a result of efficient light harvesting/exciton migration in the porphyrinic MOF and rapid quenching of the photogenerated electrons by the doped Re(I) catalytic sites. Overall, the case studies presented herein provide valuable insights for the rational design of advanced multicomponent hybrid systems for artificial photosynthesis involving CO2 reduction.

Surface and Interface Investigations of Matrix–Fillers in Heterogeneous Amorphous Semiconductors

Phosphate glass-based composites are prone to be as effective as amorphous semiconductors, enhancing the glass properties by the addition of a small amount of metallic fillers (Cr, Co, Ni, and Zn) and leading to the creation of composite materials where the conductive particles can be distributed in the glass matrix PbO–P2O5 (PbP) at the micrometer level. This paper deals with scanning electron microscopy (SEM) examination and the wetting behavior of the phosphate glass–metal powder composites. We focused on the filler effect on wetting characteristics, such as interfacial free energy. The change in the contact angle of water on the glass surface was also measured. Scanning electron microscopy images of the composites showed a good dispersion in the fillers within the glass matrix. The contact angles of the composites with water and three polar and apolar solvents were calculated. The total solid surface free energy was analyzed. The interaction parameter between the composites and the liquid has been calculated using Owens–Wendt equation.

Engineering BiVO4@Bi2S3 heterojunction by cosharing bismuth atoms toward boosted photocatalytic Cr(VI) reduction

The photocatalytic efficiency is limited by poor charge separation efficiency and high carrier transport activation energy (CTAE) of photogenerated electron/hole pairs than traditional semiconductor. Hybridizing nanostructure with two staggered alignment band structure is proved as an effective strategy to mitigate these two challenges but still suffers a strong coulomb electrostatic repulsive force between two heterogeneous semiconductors. Here, we steer a friendly sulfurization process to construct BiVO4@Bi2S3 heterojunction with a scenario of cosharing Bi atoms. The intimate atomic-level contact between BiVO4 and Bi2S3 not only enhances the visible-light absorption and lowers CTAE, but also accelerate carrier’s separation efficiency, which enables it to deliver the best photocatalytic performance toward reduction of Cr(VI). BiVO4@Bi2S3 only needs less than 40 min to completely reduce 50 ppm Cr(VI) solution. The type II heterojunction photocatalytic mechanism is systematically studied to decipher the carriers’ transfer track between BiVO4 and Bi2S3. Our new finding of engineering inorganic heterojunction by cosharing atoms opens a new avenue to other similar materials for potential applications.

Heterogeneous Photocatalysis in Organic Synthesis

AbstractVisible light photocatalysis relies mainly on expensive noble metal complexes and organic dyes that are not recyclable. Heterogeneous semiconductors, which are mainly applied for artificial photosynthesis and wastewater treatment, are a promising sustainable alternative and gain increasing attention. Inorganic and organic semiconductors with suitable bandgaps are among the most widely studied heterogeneous photocatalysts due to their high stability and recyclability. More recently, microporous materials, such as conjugated organic polymers, covalent organic frameworks, and metal organic frameworks that can be tuned and designed on a molecular level showed promising results. This review provides an overview of the most common heterogeneous photocatalysts with a focus on their applicability in organic synthesis.

Fabrication of BiOCl/ZnO/CN Nanocomposite for Visible‐Light Photocatalytic Degradation of Dyes

AbstractThe zinc oxide (ZnO) based heterogeneous semiconductors has acknowledged extensive attention but its property was radically repressed because of diminutive visible light photocatalytic activity. Herein, metal‐nonmetal hetero‐structure coupling is adopted to encompass the visible light harvesting capability and endorses the charge separation and transferring efficiency. Graphitic‐carbon‐nitride (CN) and bismuth oxychloride (BiOCl) semiconductors were effectively incorporated into the ZnO crystal structure by hydrothermal and calcination treatments. The obtained BiOCl/ZnO/CN nanocomposite has improved visible light capability with efficient separation and low recombination rate of photo generated charges. The synergistic effect of semiconductors coupling endues BiOCl/ZnO/CN with superior photocatalytic activity up to 98.6% for Rhodamine B (RhB) and 93.5% for Methyl Orange (MO) with rate constant (k)=0.2134 min−1 and 0.138 min−1 respectively. Furthermore, BiOCl/ZnO/CN sample has outstanding stability and reusability. Scavengers trapping experiments validate that superoxide and holes are the main active species in the photocatalytic reaction.

Semi-heterogeneous Dual Nickel/Photocatalysis using Carbon Nitrides: Esterification of Carboxylic Acids with Aryl Halides.

Cross-coupling reactions mediated by dual nickel/photocatalysis are synthetically attractive but rely mainly on expensive, non-recyclable noble-metal complexes as photocatalysts. Heterogeneous semiconductors, which are commonly used for artificial photosynthesis and wastewater treatment, are a sustainable alternative. Graphitic carbon nitrides, a class of metal-free polymers that can be easily prepared from bulk chemicals, are heterogeneous semiconductors with high potential for photocatalytic organic transformations. Here, we demonstrate that graphitic carbon nitrides in combination with nickel catalysis can induce selective C-O cross-couplings of carboxylic acids with aryl halides, yielding the respective aryl esters in excellent yield and selectivity. The heterogeneous organic photocatalyst exhibits a broad substrate scope, is able to harvest green light, and can be recycled multiple times. In situ FTIR was used to track the reaction progress to study this transformation at different irradiation wavelengths and reaction scales.

Covalent Organic Frameworks: A New Class of Porous Organic Frameworks for Supercapacitor Electrodes

AbstractCovalent organic frameworks (COFs) are a new class of porous organic materials, which are constructed with periodic organic units comprised entirely of light elements (typically C, H, O, N and B) and linked by strong covalent bonds. COFs have been applied in extensive fields, owing to their extraordinary properties in areas such as gas storage and separation, heterogeneous catalysis, sensors, semiconductors, drug delivery, and photoconduction. In particular, the ordered micropore or mesopore structures, high surface areas, and designable structures have enabled COFs to become new candidates for supercapacitor electrode materials. This Minireview focuses on the major progress of COFs as electrode materials for supercapacitors.

WO3 nanospheres with improved catalytic activity for visible light induced cross dehydrogenative coupling reactions

Three tungsten oxides (WO3-W1 to WO3-W3) with different morphologies were prepared and characterized by XRD, SEM, TEM and HR-TEM measurements. The as-synthesized tungsten oxides were screened as photocatalysts for visible-light-driven cross dehydrogenative coupling (CDC) reactions along with commercially available WO3-W4. Preliminary studies showed that WO3-W1 with hollow sphere morphology efficiently photocatalyzed oxidative CH functionalization as compared to other tested samples, by using molecular oxygen as a benign oxidant. The superior photocatalytic performance of WO3-W1 can be attributed to its larger surface area and pore structure which was supported by nitrogen adsorption-desorption measurements. Further, this protocol was used to synthesize α-functionalized tertiary amines in good to excellent yields by irradiating a mixture of WO3-W1, tertiary amine, and nucleophiles (nitromethane or ketones) to visible light under aerobic conditions. Moreover, WO3-W1 can be recycled and reused with no obvious change in catalytic activity, indicating that this is an environmentally friendly and economical protocol and also underlines the robustness of the catalysts in light mediated cross dehydrogenative coupling reactions. It is hoped that our results could offer useful information for designing of new heterogeneous semiconductors for photoredox catalytic organic reactions.

High-resolution charge carrier mobility mapping of heterogeneous organic semiconductors

Organic electronic device performance is contingent on charge transport across a heterogeneous landscape of structural features. Methods are therefore needed to unravel the effects of local structure on overall electrical performance. Using conductive atomic force microscopy, we construct high-resolution out-of-plane hole mobility maps from arrays of 5000 to 16 000 current-voltage curves. To demonstrate the efficacy of this non-invasive approach for quantifying and mapping local differences in electrical performance due to structural heterogeneities, we investigate two thin film test systems, one bearing a heterogeneous crystal structure [solvent vapor annealed 5,11-Bis(triethylsilylethynyl)anthradithiophene (TES-ADT)—a small molecule organic semiconductor] and one bearing a heterogeneous chemical composition [p-DTS(FBTTh2)2:PC71BM—a high-performance organic photovoltaic active layer]. TES-ADT shows nearly an order of magnitude difference in hole mobility between semicrystalline and crystalline areas, along with a distinct boundary between the two regions, while p-DTS(FBTTh2)2:PC71BM exhibits subtle local variations in hole mobility and a nanoscale domain structure with features below 10 nm in size. We also demonstrate mapping of the built-in potential, which plays a significant role in organic light emitting diode and organic solar cell operation.

Dehydrogenative Transformations of Imines Using a Heterogeneous Photocatalyst.

Heterogeneous semiconductors are underexploited as photoredox catalysts in organic synthesis relative to their homogeneous, molecular counterparts. Here, we report the use of metal/TiO2 particles as catalysts for light-induced dehydrogenative imine transformations. The highly oxophilic nature of the TiO2 surface promotes the selective binding and dehydrogenation of alcohols in the presence of other oxidizable and Lewis basic functional groups. This feature enables the clean photogeneration of aldehyde equivalents that can be utilized in multicomponent couplings.

Perga's electrical characteristics

The aim of this study was to measure the electrical properties of perga. We measured the capacitance, resistance and impedance. Each property was measured at all frequencies three times. Average value and standard deviation has been computed from these ones. The resistance, impedance, and capacitance decreases with frequency according to power function. The impedance has a slightly higher value as resistance, which is caused by the capacitance of perga. It was determined that the dielectric constant of perga increases when the moisture content of perga increases. The minimal decrease is at higher frequencies. The measurements indicate that perga must be included in the organic heterogeneous multi-component dielectrics or semi-conductors.

Recent advances in dye-sensitized semiconductor systems for photocatalytic hydrogen production

This review especially focuses on the advances in heterogeneous dye-sensitized semiconductors for H2 production during the past 30 years.

Tricrystalline TiO2 with enhanced photocatalytic activity and durability for removing volatile organic compounds from indoor air

It is important to develop efficient and economic techniques for removing volatile organic compounds (VOCs) in indoor air. Heterogeneous TiO2-based semiconductors are a promising technology for achieving this goal. Anatase/brookite/rutile tricrystalline TiO2 with mesoporous structure was synthesized by a low-temperature hydrothermal route in the presence of HNO3. The obtained samples were characterized by X-ray diffraction and N2 adsorption–desorption isotherm. The photocatalytic activity was evaluated by photocatalytic decomposition of toluene in air under UV light illumination. The results show that tricrystalline TiO2 exhibited higher photocatalytic activity and durability toward gaseous toluene than bicrystalline TiO2, due to the synergistic effects of high surface area, uniform mesoporous structure and junctions among mixed phases. The tricrystalline TiO2 prepared at RHNO3=0.8, containing 80.7% anatase, 15.6% brookite and 3.7% rutile, exhibited the highest photocatalytic activity, about 3.85-fold higher than that of P25. The high activity did not significantly degrade even after five reuse cycles. In conclusion, it is expected that our study regarding gas-phase degradation of toluene over tricrystalline TiO2 will enrich the chemistry of the TiO2-based materials as photocatalysts for environmental remediation and stimulate further research interest on this intriguing topic.

Preparation and characterization of perovskite Cu doped LaFeO3 semiconductor ceramics

Orthorhombic structured polycrystalline Cu doped LaFeO3 perovskite ceramics were prepared by sol-gel route and characterized by X-ray diffraction (XRD), differential thermal analysis (DTA), Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopic (FT-IR) techniques. The complex dielectric properties for these ceramic samples investigated as function of temperature, in the range 80K – 300K, and frequency, in the range 20 Hz -1MHz. These samples exhibiting colossal dielectric constant value of 104 near room temperature at low frequencies. The response is similar to that observed for relaxor ferroelectrics. Impedance spectroscopy (IS) data analysis indicates the ceramics to be electrically heterogeneous semiconductors consisting of semiconducting grains with dielectric constant below 100. We conclude, therefore that grain boundary effects is the primary source for the high effective dielectric constant in Cu doped LaFeO3 ceramics.

Engineering heterogeneous semiconductors for solar water splitting

Recent progress and strategies toward solar water splitting over heterogeneous semiconductors are reviewed and the challenges and future perspectives are suggested.

Photocatalytic degradation of phenol in aqueous solution by rare earth-doped SnO2 nanoparticles

The influence of heterogeneous semiconductors on the photodegradation of phenol in water was investigated using doped tin dioxide (SnO2) nanoparticles. Photocatalysts of SnO2 were synthesized with ...

Visible‐Light‐Promoted Stereoselective Alkylation by Combining Heterogeneous Photocatalysis with Organocatalysis

Dream team: Heterogeneous inorganic semiconductors and chiral organocatalysts team up for the stereoselective photocatalytic formation of carbon–carbon bonds. However, the connection between the organic and inorganic catalysts should not be too tight: Covalent immobilization inactivates the system.

Relaxor ferroelectric-like high effective permittivity in leaky dielectrics/oxide semiconductors induced by electrode effects: A case study of CuO ceramics

The electrical behavior of copper oxide (CuO) ceramics sintered at 920 °C has been characterized by a combination of fixed, radio frequency (rf) capacitance measurements, and impedance spectroscopy (IS). Fixed rf capacitance measurements on ceramics with sputtered Au electrodes revealed a temperature- and frequency-dependent high effective permittivity of ∼104 in the temperature range of 150–320 K. The response is similar to that observed for relaxor-ferroelectrics, however, the magnitude of the effect can be suppressed by thermal annealing of the ceramics with Au electrodes in air at 300 °C or by changing the work function of the electrode material by using In–Ga as opposed to Au. IS data analysis revealed the ceramics to be electrically heterogeneous semiconductors with a room temperature dc resistivity <104 Ω cm, consisting of semiconducting grains with relative permittivity, εr, <10 and slightly more resistive grain boundaries with “effective” permittivity, εeff, of ∼110. Samples with Au electrodes exhibited an additional low frequency response with εeff∼104. dc bias experiments showed the capacitance behavior of this additional response to obey the Mott–Schottky law and thus confirm it to be a non-Ohmic electrode contact. We conclude, therefore, that an electrode rather than a grain boundary effect is the primary source for the high effective permittivity in CuO ceramics, although the latter is also present and does give additional effective permittivity. This work demonstrates how an extrinsic effect associated with non-Ohmic electrode contacts can; (i) dominate the rf capacitance spectra of leaky dielectrics/oxide semiconductors over a wide temperature and frequency range; and, (ii) manifest a dielectric response more typically associated with relaxor-ferroelectrics.