Photocatalytic production of hydrogen peroxide (H2O2) on semiconductor catalysts with alcohol as a hydrogen source and molecular oxygen (O2) as an oxygen source is a potential method for safe H2O2 synthesis because the reaction can be carried out without the use of explosive H2/O2 mixed gases. Early reported photocatalytic systems, however, produce H2O2 with significantly low selectivity (∼1%). We found that visible light irradiation (λ > 420 nm) of graphitic carbon nitride (g-C3N4), a polymeric semiconductor, in an alcohol/water mixture with O2 efficiently produces H2O2 with very high selectivity (∼90%). Raman spectroscopy and electron spin resonance analysis revealed that the high H2O2 selectivity is due to the efficient formation of 1,4-endoperoxide species on the g-C3N4 surface. This suppresses one-electron reduction of O2 (superoxide radical formation), resulting in selective promotion of two-electron reduction of O2 (H2O2 formation).

http://pubs.acs.org/doi/pdf/10.1021/cs401208c

Highly Selective Production of Hydrogen Peroxide on Graphitic Carbon Nitride (g-C3N4) Photocatalyst Activated by Visible Light

Herein, we demonstrate the synthesis of highly efficient Fe-doped graphitic carbon nitride (g-C3N4) nanosheets via a facile and cost effective method. The synthesized Fe-doped g-C3N4 nanosheets were well characterized by various analytical techniques. The results revealed that the Fe exists mainly in the +3 oxidation state in the Fe-doped g-C3N4 nanosheets. Fe doping of g-C3N4 nanosheets has a great influence on the electronic and optical properties. The diffuse reflectance spectra of Fe-doped g-C3N4 nanosheets exhibit red shift and increased absorption in the visible light range, which is highly beneficial for absorbing the visible light in the solar spectrum. More significantly, the Fe-doped g-C3N4 nanosheets exhibit greatly enhanced photocatalytic activity for the degradation of Rhodamine B under sunlight irradiation. The photocatalytic activity of 2 mol% Fe-doped g-C3N4 nanosheets is almost 7 times higher than that of bulk g-C3N4 and 4.5 times higher than that of pure g-C3N4 nanosheets. A proposed mechanism for the enhanced photocatalytic activity of Fe-doped g-C3N4 nanosheets was investigated by trapping experiments. The synthesized photocatalysts are highly stable even after five successive experimental runs. The enhanced photocatalytic performance of Fe-doped g-C3N4 nanosheets is due to high visible light response, large surface area, high charge separation and charge transfer. Therefore, the Fe-doped g-C3N4 photocatalyst is a promising candidate for energy conversion and environmental remediation.

Fe-doped and -mediated graphitic carbon nitride nanosheets for enhanced photocatalytic performance under natural sunlight

Pressure is a fundamental thermodynamic variable that can be used to control the properties of materials, because it reduces interatomic distances and profoundly modifies electronic orbitals and bonding patterns. It is thus a versatile tool for the creation of exotic materials not accessible at ambient conditions. Recently developed static and dynamic high-pressure experimental techniques have led to the synthesis of many functional materials with excellent performance: for example, superconductors, superhard materials and high-energy-density materials. Some of these advances have been aided and accelerated by first-principles crystal-structure searching simulations. In this Review, we discuss recent progress in high-pressure materials discovery, placing particular emphasis on the record high-temperature superconductivity in hydrogen sulfide and on nanotwinned cubic boron nitride and diamond, the hardest known materials. Energy materials and exotic chemical materials obtained under high pressures are also discussed. The main drawback of high-pressure materials is their destabilization after pressure release; this problem and its possible solutions are surveyed in the conclusions, which also provide an outlook on the future developments in the field. High pressure offers a unique degree of freedom for the creation of new materials, leading to new superconductors, superhard materials, high-energy-density materials and exotic chemical materials with unprecedented properties. This Review discusses these materials, along with recently developed theoretical and experimental methods for materials discovery at high pressures.

Materials discovery at high pressures

Design of green, safe, and sustainable process for the synthesis of hydrogen peroxide (H2O2) is a very important subject. Early reported processes, however, require hydrogen (H2) and palladium-based catalysts. Herein we propose a photocatalytic process for H2O2 synthesis driven by metal-free catalysts with earth-abundant water and molecular oxygen (O2) as resources under sunlight irradiation (λ>400 nm). We use graphitic carbon nitride (g-C3N4) containing electron-deficient aromatic diimide units as catalysts. Incorporating the diimide units positively shifts the valence-band potential of the catalysts, while maintaining sufficient conduction-band potential for O2 reduction. Visible light irradiation of the catalysts in pure water with O2 successfully produces H2O2 by oxidation of water by the photoformed valence-band holes and selective two-electron reduction of O2 by the conduction band electrons.

Sunlight-Driven Hydrogen Peroxide Production from Water and Molecular Oxygen by Metal-Free Photocatalysts†

The g-C3N4@TiO2 core-shell structure photocatalysts with controlled ultrathin g-C3N4 layer (0 nm, 1.0 nm, 1.5 nm, 3.0 nm) were prepared by a new method of the sol-gel approaches in situ coating re-assembled. The g-C3N4@TiO2 sample with 1.0 nm thickness of shell layers has the highest visible light photocatalytic degradation phenol activity which is almost 7.2 times as high as that of bulk g-C3N4. The highest photocurrent response intensity is increased by ten times higher than that of g-C3N4 and five orders of magnitude compare to TiO2. The removal rate of phenol using g-C3N4@TiO2 core-shell catalyst is 30% and the degree of mineralization by the same catalyst is 19.8%, which dramatically increase compared with that of g-C3N4 and TiO2. The enhanced performance of the degradation phenol and the mineralization is owing to effective charge separation revealed by the photoluminescence (PL), electrochemical impedance spectroscopy (EIS) and density functional theory calculations (DFT), superoxide radicals as the main oxidative species proved by electron spin resonance spectroscopy (ESR). And the core-shell structure could effectively promote the electron transfer from g-C3N4 to TiO2 during the catalytic process. The results of repetitive experiment and cycle experiment show that the g-C3N4@TiO2 has a strong binding force between the core and shell, which is stable, without secondary pollution and convenient for recovery. What's more, the results revealed the law between the different g-C3N4 shell layers (0 nm, 1.0 nm, 1.5 nm, 3.0 nm) over the g-C3N4@TiO2 samples and the corresponding catalytic activity, which successfully established the structure-activity relationship. A new catalytic concept namely layer-dependent effect was found, that is number of layers over g-C3N4 of the core-shell structure determines photocatalytic activity.

Photocatalytic activity enhancement of core-shell structure g-C3N4@TiO2 via controlled ultrathin g-C3N4 layer

Ultraviolet and near-infrared Raman spectroscopy of graphitic C3N4 phase

Near-infrared Raman spectroscopy is a powerful analytical tool for detecting critical differences in biological samples with minimum interference in the Raman spectra from the native fluorescence of the samples. The technique is often suggested as a potential screening tool for cancer. In this article we report in vitro Raman spectra of squamous cells in normal and cancerous cervical human tissue from seven patients, which have good signal-to-noise ratio and which were found to be reproducible. These preliminary results show that several Raman features in these spectra could be used to distinguish cancerous cervical squamous cells from normal cervical squamous cells. In general, the Raman spectra of cervical cancer cells show intensity differences compared to those of normal squamous cell spectra. For example, several well-defined Raman peaks of collagen in the 775 to 975 cm−1 region are observed in the case of normal squamous cells, but these are below the detection limit of normal Raman spectroscopy in the spectra of invasive cervical cancer cells. In the high frequency 2800 to 3100 cm−1 region, it is found that the peak area under the CH stretching band is lower by a factor of approximately six in the spectra of cervical cancer cells as compared with that of the normal cells. The Raman chemical maps of regions of cancer and normal cells in the cervical epithelium made from the spectral features in the 775 to 975 cm−1 and 2800 to 3100 cm−1 regions are also found to show good correlation with each other.

/pdf/near-infrared-micro-raman-spectroscopy-for-in-vitro-3or4bfsklb.pdf

Near-Infrared Micro-Raman Spectroscopy for in Vitro Detection of Cervical Cancer

We report time-resolved (TR) remote Raman spectra of minerals under supercritical CO(2) (approx. 95 atm pressure and 423 K) and under atmospheric pressure and high temperature up to 1003 K at distances of 1.5 and 9 m, respectively. The TR Raman spectra of hydrous and anhydrous sulphates, carbonate and silicate minerals (e.g. talc, olivine, pyroxenes and feldspars) under supercritical CO(2) (approx. 95 atm pressure and 423 K) clearly show the well-defined Raman fingerprints of each mineral along with the Fermi resonance doublet of CO(2). Besides the CO(2) doublet and the effect of the viewing window, the main differences in the Raman spectra under Venus conditions are the phase transitions, the dehydration and decarbonation of various minerals, along with a slight shift in the peak positions and an increase in line-widths. The dehydration of melanterite (FeSO(4).7H(2)O) at 423 K under approximately 95 atm CO(2) is detected by the presence of the Raman fingerprints of rozenite (FeSO(4).4H(2)O) in the spectrum. Similarly, the high-temperature Raman spectra under ambient pressure of gypsum (CaSO(4).2H(2)O) and talc (Mg(3)Si(4)O(10)(OH)(2)) indicate that gypsum dehydrates at 518 K, but talc remains stable up to 1003 K. Partial dissociation of dolomite (CaMg(CO(3))(2)) is observed at 973 K. The TR remote Raman spectra of olivine, alpha-spodumene (LiAlSi(2)O(6)) and clino-enstatite (MgSiO(3)) pyroxenes and of albite (NaAlSi(3)O(8)) and microcline (KAlSi(3)O(8)) feldspars at high temperatures also show that the Raman lines remain sharp and well defined in the high-temperature spectra. The results of this study show that TR remote Raman spectroscopy could be a potential tool for exploring the surface mineralogy of Venus during both daytime and nighttime at short and long distances.

Time-resolved remote Raman study of minerals under supercritical CO2 and high temperatures relevant to Venus exploration

We synthesized a cubic BC3 (c-BC3) phase, by direct transformation from graphitic phases at a pressure of 39 GPa and temperature of 2200 K in a laser-heated diamond anvil cell. A combination of x-ray diffraction, electron diffraction, transmission electron microscopy (TEM) imaging, and electron energy loss spectroscopy (EELS) measurements lead us to conclude that the obtained phase is hetero-nano-diamond, c-BC3. High-resolution TEM imaging of the c-BC3 specimen recovered at ambient conditions demonstrates that the c-BC3 is a single, uniform, nanocrystalline phase with a grain size of about 3–5 nm. The EELS measurements show that the atoms inside the cubic structure are bonded by sp3 bonds. The zero-pressure lattice parameter of the c-BC3 calculated from diffraction peaks was found to be a = 3.589 ± 0.007 A. The composition of the c-BC3 is determined from EELS measurements. The ratio of carbon to boron, C/B, is approximately 3 (2.8 ± 0.7).

/pdf/phase-transition-in-bcx-system-under-high-pressure-and-high-4jhctuc9ls.pdf

Phase Transition in BCx system under High-Pressure and High-Temperature: Synthesis of Cubic Dense BC3 Nanostructured Phase

The capability to analyze and detect the composition of distant samples (minerals, organics, and chemicals) in real time is of interest for various fields including detecting explosives, geological surveying, and pollution mapping. For the past 10 years, the University of Hawaii has been developing standoff Raman systems suitable for measuring Raman spectra of various chemicals in daytime or nighttime. In this article we present standoff Raman spectra of various minerals and chemicals obtained from a distance of 120 m using single laser pulse excitation during daytime. The standoff Raman system utilizes an 8-inch Meade telescope as collection optics and a frequency-doubled 532 nm Nd : YAG laser with pulse energy of 100 mJ/pulse and pulse width of 10 ns. A gated intensified charge-coupled device (ICCD) detector is used to measure time-resolved Raman spectra in daytime with detection time of 100 ns. A gate delay of 800 ns (equivalent to target placed at 120 m distance) was used to minimize interference from the atmospheric gases along the laser beam path and near-field scattering. Reproducible, good quality single-shot Raman spectra of various inorganic and organic chemicals and minerals such as ammonium nitrate, potassium perchlorate, sulfur, gypsum, calcite, benzene, nitrobenzene, etc., were obtained through sealed glass vials during daytime. The data indicate that various chemicals could easily be identified from their Raman fingerprint spectra from a far standoff distance in real time using single-shot laser excitation.

Tayro E. Acosta

Papers

Ultraviolet and near-infrared Raman spectroscopy of graphitic C3N4 phase

Near-Infrared Micro-Raman Spectroscopy for in Vitro Detection of Cervical Cancer

Time-resolved remote Raman study of minerals under supercritical CO2 and high temperatures relevant to Venus exploration

Phase Transition in BCx system under High-Pressure and High-Temperature: Synthesis of Cubic Dense BC3 Nanostructured Phase

Single-Pulse Standoff Raman Detection of Chemicals from 120 m Distance During Daytime