Standalone Highly Efficient Graphene Oxide as an Emerging Visible Light-Driven Photocatalyst and Recyclable Adsorbent for the Sustainable Removal of Organic Pollutants.

Carbon-based nanostructures are promising eco-friendly multifunctional nanomaterials because of their tunable surface and optoelectronic properties for a variety of energy and environmental applications. The present study focuses on the synthesis of graphene oxide (GO) with particular emphasis on engineering its surface and optical properties for making it an excellent adsorbent as well as a visible light-active photocatalyst. It was achieved by modifying the improved Hummers method through optimizing the synthesis parameters involved in the oxidation process. This controlled synthesis allows for systematic tailoring of structural, optical, and surface functionality, leading to improved adsorption and photocatalytic properties for the sustainable removal of organic pollutants in water treatment. Several spectroscopic and microscopic characterization techniques, such as XRD, SEM, Raman, UV-visible, FTIR, TEM, XPS, BET, etc. were employed to analyze the degree of oxidation, surface chemistry/functionalization, morphological, optical, and structural properties of the synthesized GO nanostructures. The analyses showed excellent surface functionality with surface active sites for better adsorptive removal and a tunable band gap from 2.51 to 2.76 eV exhibiting excellent natural sunlight activity (>99%) for photocatalytic removal of the organic pollutant. Various adsorption isotherms have been studied with excellent adsorption capability (Qmax = 454.54 mg/g) as compared to the literature. The study introduces GO both as a proficient stand-alone (sole) nanoadsorbent as well as a nanophotocatalyst for the efficient removal of organic dye pollutants in water treatment. Additionally, the article highlights the sustainable solar light-induced green chemistry aspects of GO as an excellent recyclable adsorbent as a result of its self-cleaning ability under natural sunlight, demonstrating its potential in real eco-friendly environmental and practical applications.

Intrinsic point-defect balance in self-ion-implanted ZnO.

The role of excess intrinsic atoms for residual point defect balance has been discriminated by implanting Zn or O ions into Li-containing ZnO and monitoring Li redistribution and electrical resistivity after postimplant anneals. Strongly Li-depleted regions were detected in the Zn-implanted samples at depths beyond the projected range (R(p)) upon annealing ≥ 600 °C, correlating with a resistivity decrease. In contrast, similar anneals of the O-implanted samples resulted in Li accumulation at R(p) and an increased resistivity. Control samples implanted with Ar or Ne ions, yielding similar defect production as for the Zn or O implants but with no surplus of intrinsic atoms, revealed no Li depletion. Thus, the depletion of Li shows evidence of excess Zn interstitials (Zn(I)) being released during annealing of the Zn-implanted samples. These Zn(I)'s convert substitutional Li atoms (Li(Zn)) into highly mobile interstitial ones leading to the strongly Li-depleted regions. In the O-implanted samples, the high resistivity provides evidence of stable O(I)-related acceptors.

Influence of graphene synthesizing techniques on the photocatalytic performance of graphene-TiO2 nanocomposites.

Model photocatalysts composed of TiO2-graphene nanocomposites are prepared to address the effect of graphene quality on their photocatalytic performance. Graphene is synthesized by catalyst-assisted chemical vapor deposition (CVD), catalyst-free CVD and solution processing methods. TiO2 is prepared by reactive magnetron sputtering and subsequent annealing. Fabricated model photocatalysts have different morphology and physical properties, as revealed using spectrophotometry, atomic force microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, photoluminescence, and four-probe electrical measurements. All graphene-containing composites have significantly higher photocatalytic activity compared to bare TiO2 films in the gas phase methanol photooxidation tests. Their activity is proportional to the electrical conductivity and surface roughness of the respective carbon structure, which in turn depends on the preparation methods. The mechanisms of enhancement are further assessed by comparison with the performance of reference TiO2-graphitic-carbon and TiO2-Au thin films.

Nanostructure formation and passivation of large-area black silicon for solar cell applications.

Nanoscale textured silicon and its passivation are explored by simple low-cost metal-assisted chemical etching and thermal oxidation, and large-area black silicon was fabricated both on single-crystalline Si and multicrystalline Si for solar cell applications. When the Si surface was etched by HF/AgNO(3) solution for 4 or 5 min, nanopores formed in the Si surface, 50-100 nm in diameter and 200-300 nm deep. The nanoscale textured silicon surface turns into an effective medium with a gradually varying refractive index, which leads to the low reflectivity and black appearance of the samples. Mean reflectance was reduced to as low as 2% for crystalline Si and 4% for multicrystalline Si from 300 to 1000 nm, with no antireflective (AR) coating. A black-etched multicrystalline-Si of 156 mm × 156 mm was used to fabricate a primary solar cell with no surface passivation or AR coating. Its conversion efficiency (η) was 11.5%. The cell conversion efficiency was increased greatly by using surface passivation process, which proved very useful in suppressing excess carrier recombination on the nanostructured surface. Finally, a black m-Si cell with efficiency of 15.8% was achieved by using SiO(2) and SiN(X) bilayer passivation structure, indicating that passivation plays a key role in large-scale manufacture of black silicon solar cells.

Identification and Suppression of Majority Surface States in the Dry-Etched ß-Ga2O3.

Surface treatment after dry etching is vital to enhance the surface quality of the material and thus improve device performance. In this Letter, we identified the majority surface states induced by the dry etching of ß-Ga2O3 and optimized surface treatments to suppress these electrically active defects with the improved performance of Schottky barrier diodes. Transient spectroscopies suggested that the majority traps (EC-0.75 eV) related to divacancies (VGa-VO) were enhanced in the concentration of 3.37 × 1014 cm-3 by dry etching and reduced to 0.90 × 1014 cm-3 by the combined means of oxygen annealing and piranha solution treatment. The trap evolution is supported by the suppressed donor-acceptor pair radiative recombination related to oxygen vacancies, the improved carrier transport (negligible hysteresis current-voltage and unity ideality factor), and the reduced surface band bending. These findings provide a straightforward strategy to improve surface quality for the further performance improvement of Ga2O3 power diodes.

Band gap maps beyond the delocalization limit: correlation between optical band gaps and plasmon energies at the nanoscale.

Recent progresses in nanoscale semiconductor technology have heightened the need for measurements of band gaps with high spatial resolution. Band gap mapping can be performed through a combination of probe-corrected scanning transmission electron microscopy (STEM) and monochromated electron energy-loss spectroscopy (EELS), but are rare owing to the complexity of the experiments and the data analysis. Furthermore, although this method is far superior in terms of spatial resolution to any other techniques, it is still fundamentally resolution-limited due to inelastic delocalization of the EELS signal. In this work we have established a quantitative correlation between optical band gaps and plasmon energies using the Zn1-xCd x O/ZnO system as an example, thereby side-stepping the fundamental resolution limits of band gap measurements, and providing a simple and convenient approach to achieve band gap maps with unprecedented spatial resolution.