Haifeng Zhao

Formation mechanism and optimization of highly luminescent N-doped graphene quantum dots

Photoluminescent graphene quantum dots (GQDs) have received enormous attention because of their unique chemical, electronic and optical properties. Here a series of GQDs were synthesized under hydrothermal processes in order to investigate the formation process and optical properties of N-doped GQDs. Citric acid (CA) was used as a carbon precursor and self-assembled into sheet structure in a basic condition and formed N-free GQD graphite framework through intermolecular dehydrolysis reaction. N-doped GQDs were prepared using a series of N-containing bases such as urea. Detailed structural and property studies demonstrated the formation mechanism of N-doped GQDs for tunable optical emissions. Hydrothermal conditions promote formation of amide between –NH2 and –COOH with the presence of amine in the reaction. The intramoleculur dehydrolysis between neighbour amide and COOH groups led to formation of pyrrolic N in the graphene framework. Further, the pyrrolic N transformed to graphite N under hydrothermal conditions. N-doping results in a great improvement of PL quantum yield (QY) of GQDs. By optimized reaction conditions, the highest PL QY (94%) of N-doped GQDs was obtained using CA as a carbon source and ethylene diamine as a N source. The obtained N-doped GQDs exhibit an excitation-independent blue emission with single exponential lifetime decay.

Integrating Oxaliplatin with Highly Luminescent Carbon Dots: An Unprecedented Theranostic Agent for Personalized Medicine

A theranostic nanomedicine (CD-Oxa) is synthesized by means of the condensation reaction between the amino groups on the surface of fluorescent carbon dots (CDs) and the carboxyl group of the oxaliplatin derivative Oxa(IV)-COOH. CD-Oxa, which integrates the optical properties of CDs and the anticancer function of oxaliplatin, could be used for simultaneous drug delivery and fluorescent tracking.

Self-Targeting Fluorescent Carbon Dots for Diagnosis of Brain Cancer Cells

A new type of carbon dots (CD-Asp) with targeting function toward brain cancer glioma was synthesized via a straightforward pyrolysis route by using D-glucose and L-aspartic acid as starting materials. The as-prepared CD-Asp exhibits not only excellent biocompatibility and tunable full-color emission, but also significant capability of targeting C6 glioma cells without the aid of any extra targeting molecules. In vivo fluorescence images showed high-contrast biodistribution of CD-Asp 15 min after tail vein injection. A much stronger fluorescent signal was detected in the glioma site than that in normal brain, indicating their ability to freely penetrate the blood-brain barrier and precisely targeting glioma tissue. However, its counterparts, the CDs synthesized from D-glucose (CD-G), L-asparic acid (CD-A), or D-glucose and L-glutamic acid (CD-Glu) have no or low selectivity for glioma. Therefore, CD-Asp could act as a fluorescence imaging and targeting agent for noninvasive glioma diagnosis. This work highlights the potential application of CDs for constructing an intelligent nanomedicine with integration of diagnostic, targeting, and therapeutic functions.

Phase control of hierarchically structured mesoporous anatase TiO2 microspheres covered with {001} facets

The controlled synthesis of anatase titanium dioxide (TiO2) with both high surface area and high energy facets is technologically important for its application in photocatalysis, photoelectrochemical cells, and solar cells. Here we report a simple and fluorine free hydrothermal method to synthesize hierarchically nanostructured mesoporous anatase TiO2 spheres (MATS), which were covered with {001} facets. Mild H2SO4 was used as both a phase-inducer for the formation of the anatase phase and a capping agent to promote oriented growth and formation of {001} facets. Detailed XRD and SEM studies suggested that formation of MATS follows a typical nucleation and growth process. The refining or reconstruction of TiO2 crystal structure during growth resulted in a mesoporous crystalline framework that exhibits enhanced adsorption and photocatalytic degradation of rhodamine B in comparison with that of commercial Degussa P25 TiO2.

Reduced TiO2 rutile nanorods with well-defined facets and their visible-light photocatalytic activity

Stable reduced TiO2 rutile nanorods with well-defined facets were prepared by a solvothermal route in the presence of Zn powder. The oxygen vacancy in the TiO2 nanorods, which can be tuned by the amount of Zn, results in a narrow band gap and visible-light photocatalytic activity.

Orientated anatase TiO2 nanocrystal array thin films for self-cleaning coating

We developed a simple method to synthesize TiO2 nanowire arrays with nearly 100% exposed {001} facets. The coating exhibits good transparency. The thin films of TiO2 nanowire arrays display a very good photocatalytic degradation of dye molecules and good durability. Based on the above features, the TiO2 nanowire array coating is advantageous for self-cleaning coating.

Constructing bulk defective perovskite SrTiO3 nanocubes for high performance photocatalysts

Defects (Ti3+ or oxygen vacancies) have been demonstrated to promote the charge separation process in TiO2 based photocatalysts. Particularly, the bulk defects within a certain concentration can give a great enhancement for both light absorption and charge separation efficiency. In this report, we explored a one-step molten salts route to synthesize SrTiO3 nanocubes with bulk defects (Ti3+ doped) by using SrCO3 as a Sr source, and TiO2 and Ti powder as Ti sources. The amount of defects can be tuned by changing the molar ratio of Ti/TiO2. The corresponding bandgap of SrTiO3 can be changed from 3.29 to 2.73 eV with the increase of defects. X-ray diffraction and electron microscopy disclose that SrTiO3 is highly crystalline and has a cubic morphology. X-ray photoelectron spectroscopy and electron paramagnetic resonance indicate that the as-prepared SrTiO3 is close to the Ti3+ doped SrTiO3. Surface photovoltage spectroscopy (SPS) and field-induced SPS confirm that Ti3+ doping in the SrTiO3 turns it from an n-type semiconductor to p-type. The SrTiO3 with an optimal amount of defects exhibits highly enhanced photocatalytic performance. An excess amount of defects results in a weak SPS response and photocatalytic performance.

An annealing-free anatase TiO2 nanocrystal film as an electron collection layer in organic solar cells

Anatase TiO2 film, which is traditionally fabricated by high-temperature annealing of TiOx precursors, has been widely used as an electron collection layer in photovoltaics. In order to avoid the undesired high temperature treatment, in this work, we developed a convenient and moderate procedure to fabricate anatase TiO2 nanocrystal films at room temperature. Ultrafine, clean and high-quality anatase TiO2 nanocrystals have been pre-prepared in a simple solvothermal route and subsequent spin-coating of the nanocrystal dispersion provides a well formed TiO2 nanocrystal film without any further thermal treatment. The characteristics of the TiO2 nanocrystal film, in terms of crystallization phase, film morphology, optical and electronic properties, have been carefully studied, and then a typical [ITO/TiO2/P3HT:PCBM/MoO3/Ag] inverted photovoltaic device using TiO2 nanocrystals as the ECL has been fabricated and exhibits a comparable power conversion efficiency (PCE) of 3.35% to that of the conventional device (3.39% PCE).

CORRIGENDUM: Formation mechanism and optimization of highly luminescent N-doped graphene quantum dots

CORRIGENDUM: Formation mechanism and optimization of highly luminescent N-doped graphene quantum dots

Hierarchical TiO2 spheres decorated with Au nanoparticles for visible light hydrogen production

Hierarchical TiO2 spheres composed of nanosheets are successfully synthesized via a simple solvothermal route. TiO2 spheres with high surface area (∼100 m2 g−1) exhibit excellent photocatalytic activity. Au nanoparticles are loaded on the surface of TiO2 nanosheets through anchor molecules – thiolglycolic acid. The LSPR absorption band at ∼550 nm of Au nanoparticles is clearly observed in the diffusion reflective UV-Vis spectra. H2 production results show that the TiO2 spheres have higher photocatalytic activity than commercial P25 TiO2. After loading with Au nanoparticles, TiO2–Au spheres display a 27.6 μmol (g−1 h−1) H2 production rate under visible light irradiation (λ > 420 nm) because the localized surface plasmon resonance (LSPR) of Au nanoparticles enhances the visible light absorption. Furthermore, the H2 production rate could be improved to 92.4 μmol (g−1 h−1) for TiO2 spheres loaded with both Au and Pt nanoparticles. Based on these results, we propose a possible mechanism. Under UV light, TiO2 absorbs UV light and generates excited electrons, passing to Au nanoparticles for H2 production. In the case of visible light irradiation, the hot electrons are generated from Au nanoparticles due to the LSPR effect. And then the hot electrons are transferred from the Au nanoparticles to TiO2 and cocatalyst Pt nanoparticles for H2 generation.