Melem: A metal-free unit for photocatalytic hydrogen evolution

Photophysics and Photocatalysis of Melem: A Spectroscopic Reinvestigation

“…XRD, FTIR Spectroscopy,and ElementalAnalysis Characterization of Melem Figure 1s hows the XRD patterns of as-synthesized melem, hot-water-washed melem,a nd melem, alongw ith those of melaminea nd g-CN for reference. The XRD pattern of as-synthesized melem in this work is consistent with the reported XRD pattern of as-synthesized melem, [14,18,19] and the XRD pattern of hot-water-washed melem is consistent with that of DMSO-soluble melem [14] and dehydrated melem ( Figure 1). [21] The clear difference in the XRD patternsb etween as-synthesized and hot-water-washed melem indicates that the as-synthesized melem contains ac ertain amount of melamine and that hot water can remove unreacted melamine from the assynthesized melem.A ccording to the reported method,t he hot-water-washed melem was further boiled with HCl and then with as olution of NaOH, [22] and finally melem powerw as obtained.…”

“…All FTIR spectra of as-synthesized melem, hot-water-washed melem,a nd melem present similar features, which agree with the reported FTIR datao f melem. [14,18,19,21,23] In general, the FTIR spectra of all melem samples presentc haracteristic bands of aromatic CN heterocycles in the range of ñ = 1200-1700 cm À1 and one breathing mode of tri-s-triazine unit at ñ = 805 cm À1 .T he broad bands in the range of ñ = 3000-3700 cm À1 are attributed to the stretching modes of secondary and primary amines, or their intermolecular hydrogen-bonding interactions. [24] By comparing the FTIR spectra of the as-synthesizedm elem,h ot-water-washed melem,a nd melem shown in Figure 2, the FTIR spectrum of melem is more structured than those of as-synthesized and hot-water-washedm elem in the range of ñ = 3000-3700 cm À1 .…”

“…Lotsch et al report- ed the UVDRS of DMSO-solublem elem powder with am aximum at l % 350 nm, [14] and Zou et al reported the UVDRS of melem powder maximized at l % 390 nm. [18] According to the DRS bandso fa s-synthesized melem, hot-water-washed melem, and melem shown in Figure 3, inconsistences in UVDRS data for melem recorded by different groups are due to the different constituents of the obtainedm elem.A ss hown in Figure 3, the UVDRS band of the prepared melem presentsm onomer characteristics, whichi ndicates that the melem prepared in this work is monomer melem, whereas the as-synthesized melem and the hot-water-washed melem contains both monomer melem and higherc ondensates. Figure 4s hows the fluorescences pectra of melamine,a ssynthesized melem, hot-water-washed melem,m elem, and g-CN under l = 250 nm light excitation.T he fluorescencee mission of melem has am aximum at l % 395 nm, which is consistent with the report from the group of Shigesato (l = 390 nm [25] ), but is inconsistent with the reports from the groups of Ricci (l = 415 nm [20] )a nd Brückner (l % 420 nm [17] ).…”

“…[17] Later,Z ou et al studied the photocatalytic H 2 evolution of melem and they concluded that melem was am etal-free unit for photocatalytic H 2 evolution. [18] Almost at the same time, Lotsch et al explored the photocatalytic H 2 evolution of melem andi ts higherc ondensates and claimed that the oligomers of melem were the most active species. [14] Understanding of the photocatalytic reaction mechanism of ap hotocatalyst closely depends on knowledge of its photophysical properties.…”

“…For the photophysical properties of melem,t here are inconsistencies, such as its fluorescenced ecay lifetime and fluorescence spectrum. [12,[16][17][18][19][20] Therefore, as ystematic investigation into the photophysical properties of melem is still in need to correctly understand its photocatalytic mechanism and related heptazine-based materials.…”