Highly blue fluorescent graphitic carbon nitride quantum dots (g-CNQDs) were synthesized by a simple microwave mediated method from formamide (HCONH 2 ). The fluorescence emission of g-CNQDs is found to be strongly dependent on solvent, pH as well as on excitation wavelengths; the life time of the emission decreases with increasing polarity of the solvent. These quantum dots are highly sensitive and selective florescent probes for mercuric ions in aqueous media due to the ''superquenching'' of fluorescence. The complex formation of Hg 2+ ions with the CN x sheet involving p delocalized electron moieties of the latter, in fact, is responsible for the quenching of fluorescence. The addition of iodide ions abstracts the bound Hg 2+ forming HgI 2 and gives back the fluorescence characteristic of g-CNQDs. Thus, g-CNQDs can play a dual role for selective and sensitive detection of mercuric ions as well as iodide ions in aqueous media via ''ON-OFF-ON'' fluorescence response.

The hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) in aqueous medium are two fundamental reactions for the development of non-fossil energy storage and conversion devices. In the polymer electrolyte membrane fuel cell (PEMFC) carbon supported platinum (Pt/C) based catalysts are universally used in cathodes and anodes; however, the poor durability of Pt/C due to degradation of the catalyst in the strongly oxidizing environment prevents its widespread applications. It remains a great challenge to develop new electrocatalysts with superior activity and very high durability for the HER/HOR. Here, we report the synthesis of a porous palladium nanoparticle− carbon nitride composite (Pd-CN x ) for its superior activity and high durability toward the HER/HOR in acidic and alkaline media. The Pd-CN x composites exhibited high catalytic activity for hydrogen evolution in acidic media with a small onset potential of −12 mV and a Tafel slope of 35 mV dec −1 . At a small Pd loading of 0.043 mg cm −2 , this catalyst also exhibits a current density of 10 mA cm −2 at a low overpotential of −55 mV with an excellent stability. The HER activity on Pd-CN x composite is comparable to that of commercial Pt/C in acid media. The stability tests of this catalyst were done through a large number of repeated potential cycles and long-term electrolysis. These confirm the exceptional durability of this catalyst, which is much better than that of Pt/C catalysts. Furthermore, this catalyst has also displayed superior HOR activity, measured by a rotating-disk experiment with a broad range of pH (0−14) in different buffer solutions. The HER/HOR activities of porous Pd-CN x composite in different buffer solutions were correlated with the hydrogen binding energy (HBE) of the catalyst surface. The HER/HOR activity gradually decreases with an increase in the HBE as the solution pH increases. The superior HER/HOR activities and very high durability at porous Pd-CN x composite are due to strong bonding between Pd and carbon (Pd−C bond), the porous morphology, and synergistic interactions between Pd-NPs and the carbon nitride (CN x ) support.

Development of highly efficient and durable bifunctional electrocatalyst for hydrogen and oxygen evolution reactions (HER and OER) is essential for efficient solar fuel generation. The commercial RuO or RuO-based catalysts are highly active toward OER, but their poor stability under different operating conditions is the main obstacle for their commercialization. Herein, we report growth of one-dimensional highly crystalline RuO nanowires on carbon nitride (1D-RuO-CN) for their applications in HER and OER at all pH values. The 1D-RuO-CN, as an OER catalyst, exhibits a low onset overpotential of ∼200 mV in both acidic and basic media, whereas Tafel slopes are 52 and 56 mV/dec in acidic and basic media, respectively. This catalyst requires a low overpotential of 250 and 260 mV to drive the current density of 10 mA cm in acidic and basic media, respectively. The mass activity of 1D-RuO-CN catalyst is 352 mA mg, which is ∼14 times higher than that of commercial RuO. Most importantly, the 1D-RuO-CN catalyst has remarkably higher stability compared to commercial RuO. This catalyst also exhibits superior HER activity with a current density of 10 mAcm at ∼93 and 95 mV in acidic and basic media. The HER Tafel slopes of this catalyst are 40 mV/dec in acidic condition and 70 mV/dec in basic condition. The HER activity of this catalyst is slightly lower than Pt/C in acidic media, whereas in basic media it is comparable or even better than that of Pt/C at higher overpotentials. The HER stability of this catalyst is also better than that of Pt/C in all pH solutions. This superior catalytic activity of 1D-RuO-CN composite can be attributed to catalyst-support interaction, enhanced mass and electron transport, one-dimensional morphology, and highly crystalline rutile RuO structure.

The single perovskite slab alkylammonium lead iodides (C n H2 n +1NH3)2PbI4, n = 12, 16, 18, display two phase transitions, just above room temperature, associated with changes in the alkylammonium chains. We have followed these two phase transitions using scanning calorimetry, X-ray powder diffraction, and IR and Raman spectroscopies. We find the first phase transition to be associated with symmetry changes arising from a dynamic rotational disordering of the ammonium headgroup of the chain whereas the second transition, the melting of the chains in two dimensions, is characterized by an increased conformational disorder of the methylene units of the alkyl chains. We examine these phase transitions in light of the interesting optical properties of these materials, as well as the relevance of these systems as models for phase transitions in lipid bilayers.

Molecular electronic junctions consisting of a 20 nm thick layer of polypyrrole (PPy) and 10 nm of TiO2 between conducting layers of carbon and gold were investigated as potential nonvolatile memory devices. By making the polymer layer much thinner than conventional polymer electronic devices, it is possible to dynamically oxidize and reduce the polypyrrole layer by an applied bias. When the electrode in contact with the PPy is biased positive, oxidation of the PPy occurs to yield a conducting polaron state. The junctions exhibit a large increase in conductance in response to the positive bias, which is reversed by a subsequent negatively biased pulse. Switching between the conducting and nonconducting state can occur for pulses at least as short as 10 micros, and the conducting state persists after a positive bias pulse for at least 1 week. The read/write/read/erase cycle may be repeated for at least 1700 cycles, although with an error rate of approximately 3% due mainly to an incomplete "erase" step. The speed and retention of the PPy/TiO2 junctions are far superior to those of the analogous fluorene/TiO2 devices lacking the polymer, and the conductance changes are absent if SiO2 is substituted for TiO2. The observations are consistent with "dynamic doping" of the solid-state polymer layer, with the possible involvement of adventitious mobile ions. Although the speed of the current polymer/TiO2 junctions is slower than commercial dynamic random access memory, their retention is approximately 5 orders of magnitude longer.