Steven Firth

Graphitic carbon nitride compounds were prepared by thermal treatment of C–N–H precursor mixtures (melamine C3N6H9, dicyandiamide C2N4H4). This led to solids based on polymerized heptazine or triazine ring units linked by −N═ or −NH– groups. The H content decreased, and the C/N ratio varied between 0.59 and 0.70 with preparation temperatures between 550 and 650 °C due to increased layer condensation. The UV–vis spectra exhibited a strong π–π* transition near 400 nm with a semiconductor-like band edge extending into the visible range. Samples synthesized at 600–650 °C showed an additional absorption near 500 nm that is assigned to n−π* electronic transitions involving the N lone pairs. These are forbidden for planar symmetric s-triazine or heptazine structures but become allowed as increased condensation causes distortion of the polymeric units. Photocatalysis studies showed there was no correlation between the increased visible absorption due to this feature and H2 evolution from methanol used for the anodic reaction. In the absence of any cocatalyst the sample synthesized at 550 °C showed 1.5 μmol h–1 H2 evolution with UV–vis irradiation, but this dropped to ∼0.23 μmol h–1 when the UV spectrum was blocked. Use of a Pt cocatalyst was required to observe H2 evolution from the other samples. Using a more powerful (300 W) lamp led to higher H2 production rates (31.5 μmol h–1) with visible illumination. We suggest the distorted N sites caused by increased polymerization result in electron/hole traps that counter the photocatalytic efficiency. Issues concerning sample porosity are also present. Photocatalytic O2 evolution was determined for RuO2-coated samples using the 300 W lamp with aqueous AgNO3 solution as the sacrificial agent. The materials all showed production rates ∼9 μmol h–1. A highly crystalline compound containing polytriazine structural units ((C3N3)2(NH)3·LiCl) prepared in this study did not show measurable photocatalytic activity.

The Raman spectra of plattnerite [lead(IV) oxide, PbO2] and of the lead pigments red lead (Pb3O4), lead monoxide [PbO, litharge (tetragonal) and massicot (orthorhombic)], lead white [basic lead carbonate, 2PbCO3·Pb(OH)2] and of their laser-induced degradation products were recorded using a range of different excitation lines, spectrometer systems and experimental conditions. The degradation of PbO2 is more extensive along the pathway PbO2 → Pb3O4 → PbO (litharge) → PbO (massicot) the shorter the wavelength of the excitation line and the higher its power. The Raman spectrum of PbO2, which is black and of the rutile structure, is particularly difficult to obtain but three bands, at 653, 515 and 424 cm−1, were identified as arising from the b2g, a1g and eg modes respectively, by analogy with the corresponding modes of isostructural SnO2 (776, 634 and 475 cm−1). A further oxide was identified, PbO1.55, the Raman spectrum of which does not correspond to that of any of the laser-induced degradation products of PbO2 at any of the wavelengths used. The Raman results are critical to the future use of Raman microscopy for the identification of lead pigments on artworks.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTMillimeter-wave spectroscopy of biomolecules: alanineP. D. Godfrey, S. Firth, L. D. Hatherley, R. D. Brown, and A. P. PierlotCite this: J. Am. Chem. Soc. 1993, 115, 21, 9687–9691Publication Date (Print):October 1, 1993Publication History Published online1 May 2002Published inissue 1 October 1993https://pubs.acs.org/doi/10.1021/ja00074a039https://doi.org/10.1021/ja00074a039research-articleACS PublicationsRequest reuse permissionsArticle Views438Altmetric-Citations168LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose Get e-Alerts

Raman spectra of cleaved, millimeter-sized single crystals of mineral galena (PbS) have been measured between 100 and 1200 cm−1 over the temperature range 80 to 373 K. Three low wave number bands are observed which can be reconciled with the results of earlier reports. However, changes in the peak wave numbers and in the relative intensities of these bands in response to changes in temperature do not conform with previous assignments for the observed bands. Detailed assignments are proposed which are based on resonance Raman processes. The observed bands are assigned to forbidden longitudinal optical excitations, which are allowed under resonance conditions by a Fröhlich interaction mechanism, and two-phonon excitations.

When nanotubes form in a carbon arc, the presence of boron results in long boron-doped carbon nanotubes which are generated as dominant zigzags. Metallic behaviour is observed, in contrast to carbon nanotubes, which are semi-conducting.