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

Solar-driven water splitting provides a leading approach to store the abundant yet intermittent solar energy and produce hydrogen as a clean and sustainable energy carrier. A straightforward route to light-driven water splitting is to apply self-supported particulate photocatalysts, which is expected to allow solar hydrogen to be competitive with fossil-fuel-derived hydrogen on a levelized cost basis. More importantly, the powder-based systems can lend themselves to making functional panels on a large scale while retaining the intrinsic activity of the photocatalyst. However, all attempts to generate hydrogen via powder-based solar water-splitting systems to date have unfortunately fallen short of the efficiency values required for practical applications. Photocatalysis on photocatalyst particles involves three sequential steps: (i) absorption of photons with higher energies than the bandgap of the photocatalysts, leading to the excitation of electron-hole pairs in the particles, (ii) charge separation and migration of these photoexcited carriers, and (iii) surface chemical reactions based on these carriers. In this review, we focus on the challenges of each step and summarize material design strategies to overcome the obstacles and limitations. This review illustrates that it is possible to employ the fundamental principles underlying photosynthesis and the tools of chemical and materials science to design and prepare photocatalysts for overall water splitting.

Particulate Photocatalysts for Light-Driven Water Splitting: Mechanisms, Challenges, and Design Strategies

As new members of carbon material family, carbon and graphene quantum dots (CDs, GQDs) have attracted tremendous attentions for their potentials for biological, optoelectronic, and energy related applications. Among these applications, bio-imaging has been intensively studied, but optoelectronic and energy devices are rapidly rising. In this Feature Article, recent exciting progresses on CD- and GQD-based optoelectronic and energy devices, such as light emitting diodes (LEDs), solar cells (SCs), photodetctors (PDs), photocatalysis, batteries, and supercapacitors are highlighted. The recent understanding on their microstructure and optical properties are briefly introduced in the first part. Some important progresses on optoelectronic and energy devices are then addressed as the main part of this Feature Article. Finally, a brief outlook is given, pointing out that CDs and GQDs could play more important roles in communication- and energy-functional devices in the near future.

Carbon and Graphene Quantum Dots for Optoelectronic and Energy Devices: A Review

Charge kinetics is highly critical in determining the quantum efficiency of solar-to-chemical conversion in photocatalysis, and this includes, but is not limited to, the separation of photoexcited electron–hole pairs, utilization of plasmonic hot carriers and delivery of photo-induced charges to reaction sites, as well as activation of reactants by energized charges. In this review, we highlight the recent progress on probing and steering charge kinetics toward designing highly efficient photocatalysts and elucidate the fundamentals behind the combinative use of controlled synthesis, characterization techniques (with a focus on spectroscopic characterizations) and theoretical simulations in photocatalysis studies. We first introduce the principles of various processes associated with charge kinetics that account for or may affect photocatalysis, from which a set of parameters that are critical to photocatalyst design can be summarized. We then outline the design rules for photocatalyst structures and their corresponding synthetic approaches. The implementation of characterization techniques and theoretical simulations in different steps of photocatalysis, together with the associated fundamentals and working mechanisms, are also presented. Finally, we discuss the challenges and opportunities for photocatalysis research at this unique intersection as well as the potential impact on other research fields.

Steering charge kinetics in photocatalysis: intersection of materials syntheses, characterization techniques and theoretical simulations

Carbon quantum dots (CQDs) as a rising star of carbon nanomaterials, by virtue of their unique physicochemical, optical and electronic properties, have displayed tremendous momentum in numerous fields such as biosensing, bioimaging, drug delivery, optoelectronics, photovoltaics and photocatalysis. In particular, the rich optical and electronic properties of CQDs including efficient light harvesting, tunable photoluminescence (PL), extraordinary up-converted photoluminescence (UCPL) and outstanding photoinduced electron transfer have attracted considerable interest in different photocatalytic applications for the sake of full utilization of the solar spectrum. This review aims to demonstrate the recent progress in the synthesis, properties and photocatalytic applications of CQDs, particularly highlighting the fundamental multifaceted roles of CQDs in photoredox processes. Furthermore, we discuss the challenges and future direction of CQD-based materials in this booming research field, with a perspective toward the ultimate achievement of highly efficient and long-term stable CQD-based photocatalysts.

Recent progress in carbon quantum dots: synthesis, properties and applications in photocatalysis

Carbon quantum dots modified P25 TiO2 composites (CQDs/P25) with a “dyade”-like structure were prepared via a facile one-step hydrothermal reaction. CQDs/P25 exhibited improved photocatalytic H2 evolution under UV-Vis and visible light (λ > 450 nm) irradiation without loading any noble metal cocatalyst, compared to pure P25. A possible mechanism of the photocatalytic H2 production activity over CQDs/P25 was proposed based on detailed measurements of the transient photocurrent response, surface photovoltage and hydroxyl radicals. CQDs play dual roles on the improved photocatalytic activity of CQDs/P25. Under UV-Vis light irradiation CQDs act as an electron reservoir to improve the efficient separation of the photoinduced electron–hole pairs of P25. However, under visible light irradiation CQDs act as a photosensitizer to sensitize P25 into a visible light response “dyade” structure for H2 evolution.

Carbon quantum dots/TiO2 composites for efficient photocatalytic hydrogen evolution

Plasmon-mediated photocatalytic systems generally suffer from poor efficiency due to weak absorption overlap and thus limited energy transfer between the plasmonic metal and the semiconductor. Herein, a near-ideal plasmon-mediated photocatalyst system is developed. Au/CdSe nanocrystal clusters (NCs) are successfully fabricated through a facile emulsion-based self-assembly approach, containing Au nanoparticles (NPs) of size 2.8, 4.6, 7.2, or 9.0 nm and CdSe quantum dots (QDs) of size ≈3.3 nm. Under visible-light irradiation, the Au/CdSe NCs with 7.2 nm Au NPs afford very stable operation and a remarkable H2-evolution rate of 73 mmol gCdSe−1 h−1 (10× higher than bare CdSe NCs). Plasmon resonance energy transfer from the Au NPs to the CdSe QDs, which enhances charge-carrier generation in the semiconductor and suppresses bulk recombination, is responsible for the outstanding photocatalytic performance. The approach used here to fabricate the Au/CdSe NCs is suitable for the construction of other plasmon-mediated photocatalysts.

Self-Assembled Au/CdSe Nanocrystal Clusters for Plasmon-Mediated Photocatalytic Hydrogen Evolution.

Facile preparation of black Nb4+ self-doped K4Nb6O17 microspheres with high solar absorption and enhanced photocatalytic activity.

The invention discloses a silicon dioxide-filled nano-cluster composite material. The silicon dioxide-filled nano-cluster composite material comprises silicon dioxide and at least two types of inorganic nano-crystals, wherein one type is the inorganic nano-crystals with a surface plasma resonance effect, and the other type is the inorganic nano-crystals in overlapping absorption with the first type of the inorganic nano-crystals; the at least two types of the inorganic nano-crystals are assembled to obtain nano-clusters; the silicon dioxide is distributed in the whole nano-clusters to form the nano-cluster composite material; the particle size of the inorganic nano-crystals is 1-100nm; and the diameter of the nano-cluster composite material is 50-1000nm. The nano-cluster composite material has the advantages of simplicity, convenience, easiness in obtainment and adjustable size and composition, and further has important significance in actual application. The invention further discloses a method for preparing the silicon dioxide-filled nano-cluster composite material and application.

Silicon dioxide-filled nano-cluster composite material and preparation method and application thereof

Carbon quantum dots modified P25 TiO₂ composites (CQDs/P25) with a “dyade”-like structure were prepared via a facile one-step hydrothermal reaction. CQDs/P25 exhibited improved photocatalytic H₂ evolution under UV-Vis and visible light (λ > 450 nm) irradiation without loading any noble metal cocatalyst, compared to pure P25. A possible mechanism of the photocatalytic H₂ production activity over CQDs/P25 was proposed based on detailed measurements of the transient photocurrent response, surface photovoltage and hydroxyl radicals. CQDs play dual roles on the improved photocatalytic activity of CQDs/P25. Under UV-Vis light irradiation CQDs act as an electron reservoir to improve the efficient separation of the photoinduced electron–hole pairs of P25. However, under visible light irradiation CQDs act as a photosensitizer to sensitize P25 into a visible light response “dyade” structure for H₂ evolution.

Yinhu Cao

Papers

Carbon quantum dots/TiO2 composites for efficient photocatalytic hydrogen evolution

Self-Assembled Au/CdSe Nanocrystal Clusters for Plasmon-Mediated Photocatalytic Hydrogen Evolution.

Facile preparation of black Nb4+ self-doped K4Nb6O17 microspheres with high solar absorption and enhanced photocatalytic activity.

Silicon dioxide-filled nano-cluster composite material and preparation method and application thereof

Carbon quantum dots/TiO₂ composites for efficient photocatalytic hydrogen evolution