Similar to its popular older cousins the fullerene, the carbon nanotube, and graphene, the latest form of nanocarbon, the carbon nanodot, is inspiring intensive research efforts in its own right. These surface-passivated carbonaceous quantum dots, so-called C-dots, combine several favorable attributes of traditional semiconductor-based quantum dots (namely, size- and wavelength-dependent luminescence emission, resistance to photobleaching, ease of bioconjugation) without incurring the burden of intrinsic toxicity or elemental scarcity and without the need for stringent, intricate, tedious, costly, or inefficient preparation steps. C-dots can be produced inexpensively and on a large scale (frequently using a one-step pathway and potentially from biomass waste-derived sources) by many approaches, ranging from simple candle burning to in situ dehydration reactions to laser ablation methods. In this Review, we summarize recent advances in the synthesis and characterization of C-dots. We also speculate on their future and discuss potential developments for their use in energy conversion/storage, bioimaging, drug delivery, sensors, diagnostics, and composites.

Luminescent Carbon Nanodots: Emergent Nanolights

Carbon nanodots (C-dots) have generated enormous excitement because of their superiority in water solubility, chemical inertness, low toxicity, ease of functionalization and resistance to photobleaching. In this review, by introducing the synthesis and photo- and electron-properties of C-dots, we hope to provide further insight into their controversial emission origin (particularly the upconverted photoluminescence) and to stimulate further research into their potential applications, especially in photocatalysis, energy conversion, optoelectronics, and sensing.

Carbon nanodots: synthesis, properties and applications

Graphene quantum dots: emergent nanolights for bioimaging, sensors, catalysis and photovoltaic devices

Carbon quantum dots (CQDs, C-dots or CDs), which are generally small carbon nanoparticles (less than 10 nm in size) with various unique properties, have found wide use in more and more fields during the last few years. In this feature article, we describe the recent progress in the field of CQDs, focusing on their synthetic methods, size control, modification strategies, photoelectric properties, luminescent mechanism, and applications in biomedicine, optronics, catalysis and sensor issues.

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Carbon quantum dots: synthesis, properties and applications

There is a growing interest in the conversion of water and solar energy into clean and renewable H2 fuels using earth-abundant materials due to the depletion of fossil fuel and its serious environmental impact. This critical review highlights some key factors influencing the efficiency of heterogeneous semiconductors for solar water splitting (i.e. improved charge separation and transfer, promoted optical absorption, optimized band gap position, lowered cost and toxicity, and enhanced stability and water splitting kinetics). Moreover, different engineering strategies, such as band structure engineering, micro/nano engineering, bionic engineering, co-catalyst engineering, surface/interface engineering of heterogeneous semiconductors are summarized and discussed thoroughly. The synergistic effects of the different engineering strategies, especially for the combination of co-catalyst loading and other strategies seem to be more promising for the development of highly efficient photocatalysts. A thorough understanding of electron and hole transfer thermodynamics and kinetics at the fundamental level is also important for elucidating the key efficiency-limiting step and designing highly efficient solar-to-fuel conversion systems. In this review, we provide not only a summary of the recent progress in the different engineering strategies of heterogeneous semiconductors for solar water splitting, but also some potential opportunities for designing and optimizing solar cells, photocatalysts for the reduction of CO2 and pollutant degradation, and electrocatalysts for water splitting.

Engineering heterogeneous semiconductors for solar water splitting

Carbon nanoparticles were prepared by refluxing the combustion soot of natural gas in nitric acid. Transmission Electron Microscopy measurements showed that the resulting particles exhibited an average diameter of 4.8 ± 0.6 nm, and the crystalline lattices were consistent with graphitic carbons. 13C NMR and FTIR spectroscopic measurements further confirmed the presence of sp2 carbons in the form of aryl and carboxylic/carbonyl moieties. The resulting carbon nanoparticles were found to emit photoluminescence with a quantum yield of approximately 0.43%. Additionally, the emission band energy of the carbon nanoparticle was very similar to that of much smaller carbon nanoparticles obtained from candle soot, suggesting that the photoluminescence might arise from particle surface states, analogous to the behaviors of semiconductor quantum dots with an indirect bandgap. In electrochemical measurements, two pairs of well-defined voltammetric waves were observed, which might be ascribed to the peripheral functiona...

Nanosized Carbon Particles From Natural Gas Soot

Au−TiO2 snowman-like heterodimer nanoparticles were prepared by a surface sol−gel process based on gold Janus nanoparticles whose surface-protecting monolayers consisted of a hemisphere of hydrophobic 1-hexanethiolates and the other of hydrophilic 2-(2-mercaptoethoxy)ethanol. Transmission electron microscopic measurements showed that the resulting TiO2 nanoparticles (diameter 6 nm) exhibited well-defined lattice fringes that were consistent with the (101) diffraction planes of anatase TiO2. The heterodimer nanoparticles displayed apparent photoluminescence that was ascribed to electronic transitions involving trap states of TiO2 particles, and the photocatalytic activity was manifested by the oxidative conversion of methanol into formaldehyde, which was detected quantitatively by the Nash method. The enhanced photocatalytic performance, as compared to that of the TiO2 nanoparticles alone, was ascribed to the charge separation of photogenerated electrons and holes at the Au−TiO2 interface that was facilita...

https://chen.chemistry.ucsc.edu/am900425v.pdf

Janus Nanostructures Based on Au−TiO2 Heterodimers and Their Photocatalytic Activity in the Oxidation of Methanol

Tin oxide nanoparticles were synthesized by electrochemical oxidation of a tin metal sheet in a non-aqueous electrolyte containing NH4F. The as-prepared nanoparticles were then thermally annealed at 700 °C for 6 h. The resulting particles were characterized by a variety of experimental techniques, including X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), Raman, UV-visible, and photoluminescence (PL) spectroscopy. The XRD patterns clearly showed that the amorphous phase of the as-synthesized SnO2 particles was transformed into a rutile-type crystalline structure after thermal treatment; and from the line broadening of the XRD peaks, the average size of the annealed particles was found to be 15.4, 12.5, 11.8 nm for the particles initially synthesized at 20, 30, and 40 V, respectively. Consistent results were also observed in HRTEM measurements which showed clear crystalline lattice fringes of the calcined nanoparticles, as compared to the featureless profiles of the as-produced counterparts. In Raman spectroscopic studies, three dominant peaks were observed at 480, 640, and 780 cm−1 which were ascribed to the E1g, A1g, and B2g Raman active vibration modes, respectively, and the wavenumbers of these peaks blue-shifted with decreasing particle size. Additionally, a broad strong emission band was observed in room-temperature photoluminescence measurements.

Large-scale electrochemical synthesis of SnO2 nanoparticles

Stable ruthenium nanoparticles protected by 1-octynyl fragments were synthesized by a wet chemical method. Transmission electron microscopic measurements showed that the resulting particles exhibited an average core diameter of 2.55 ± 0.15 nm with well-defined Ru crystalline lattice fringes. Because of the formation of Ru—C≡ bonds, the C≡C vibrational stretch was found in FTIR measurements to red-shift to 1936 cm−1 from 2119 cm−1 that was observed for monomeric 1-octyne. Interestingly, the nanoparticles underwent ligand exchange reactions with alkynyl lithium (e.g., 5-phenyl-1-pentynyl lithium) for further surface functionalization, as manifested in FTIR as well as 1H and 13C NMR measurements. Optically, whereas UV−vis absorption measurements exhibited only a featureless profile, the Ru nanoparticles displayed apparent photoluminescence with an emission peak at 428 nm, which was accounted for by intraparticle charge delocalization as a consequence of the strong Ru—C≡ bonds and the conducting Ru metal core...

Alkyne-Protected Ruthenium Nanoparticles†

Stable palladium nanoparticles were prepared by the passivation of palladium–carbon covalent linkages by the reduction of diazonium derivatives. The resulting particles were characterized by TEM, UV-vis, FTIR, and NMR measurements. Interestingly, in electrochemical studies of the electronic conductivity of the particle solid ensembles, Pd–biphenyl particles exhibited metallic temperature dependence within the temperature range of 80 to 320 K; whereas the conductivity of Pd–decylphenyl particles showed a transition from semiconductor to metal at 180 K, manifested by their distinctly different temperature dependence. Control experiments with alkanethiolate-protected palladium nanoparticles in the same temperature range exhibited only semiconductor-like conductivity which increased with increasing temperature. The discrepancy was interpreted using Mott's model for metal–insulator transition and ascribed to the strong Pd–C interactions and low contact resistance, which facilitated the interparticle charge transfer.

Debraj Ghosh

Papers

Nanosized Carbon Particles From Natural Gas Soot

Janus Nanostructures Based on Au−TiO2 Heterodimers and Their Photocatalytic Activity in the Oxidation of Methanol

Large-scale electrochemical synthesis of SnO2 nanoparticles

Alkyne-Protected Ruthenium Nanoparticles†

Palladium nanoparticles passivated by metal–carbon covalent linkages