g-C3N4 Quantum Dots Research Articles

Quantum-chemical calculations on graphitic carbon nitride (g-C3N4) single-layer nanostructures: polymeric slab vs. quantum dot

Graphitic carbon nitride (g-C3N4) has been the focus of enormous attention in recent years for its fantastic in-plane and surface properties. Several periodic and cluster models of g-C3N4 including a quantum dot have been investigated using density functional theory (DFT) at the HSE06/Def2-TZVP level. The quantum dot with side triazine rings in nearly perpendicular alignment to the central ring was (by 98.40 kcal/mol) more stable than any other cluster, including its planar analogue—a metastable phase of carbon nitride. The g-C3N4 quantum dot showed the largest deviation (3.27 eV, 7.9%) from the bandgap of the polymeric material. On the other hand, the unrelaxed symmetrical cluster had the smallest deviation (+ 0.03 eV, 1.0%) from the reference bandgap (and also in terms of global hardness), indicating that it could be taken as a replacement cluster for modeling of a polymeric surface in such explorations. The plots of the density of states (DOS) revealed the inherent instabilities of the planar models compared to the quantum dot. Furthermore, the g-C3N4 quantum dot showed the highest chemical hardness among the models investigated. The electronic band structures of the g-C3N4 quantum dot implied its relatively better photoabsorption ability referenced to the polymeric surface. However, the structural changes had significant effects on the orbital and charge distributions in the C3N4 models.

A Review on Quantum Dots Modified g-C3N4-Based Photocatalysts with Improved Photocatalytic Activity

In the 21st century, the development of sustainable energy and advanced technologies to cope with energy shortages and environmental pollution has become vital. Semiconductor photocatalysis is a promising technology that can directly convert solar energy to chemical energy and is extensively used for its environmentally-friendly properties. In the field of photocatalysis, graphitic carbon nitride (g-C3N4) has obtained increasing interest due to its unique physicochemical properties. Therefore, numerous researchers have attempted to integrate quantum dots (QDs) with g-C3N4 to optimize the photocatalytic activity. In this review, recent progress in combining g-C3N4 with QDs for synthesizing new photocatalysts was introduced. The methods of QDs/g-C3N4-based photocatalysts synthesis are summarized. Recent studies assessing the application of photocatalytic performance and mechanism of modification of g-C3N4 with carbon quantum dots (CQDs), graphene quantum dots (GQDs), and g-C3N4 QDs are herein discussed. Lastly, challenges and future perspectives of QDs modified g-C3N4-based photocatalysts in photocatalytic applications are discussed. We hope that this review will provide a valuable overview and insight for the promotion of applications of QDs modified g-C3N4 based-photocatalysts.

Construction of efficient “on-off-on” fluorescence aptasensor for ultrasensitive detection of prostate specific antigen via covalent energy transfer between g-C3N4 quantum dots and palladium triangular plates

Development of ultra-sensitive and high specific aptasensors is important for early diagnosis of prostate cancer. Herein, ultrasensitive detection of prostate specific antigen (PSA) aptasensor was realized based on the “on-off-on” model via fluorescence (FL) covalent energy transfer between g-C3N4 quantum dot (g-CNQDs) and palladium triangular plates (Pd TPs). Specifically, the Pd TPs were primarily linked with PSA aptamer (PA) as the reporter probe, followed by attaching them onto the g-CNQDs surfaces, causing the highly enlarged FL quenching rate (ca. 75%). After the introduction of PSA, the FL intensities recovered again because of the distinctively stronger affinity of PA to PSA than that of g-CNQDs. The bond of pyridine N with Pd was identified as efficient energy transfer pathway through the X-ray photoelectron spectroscopy (XPS) and FL measurements. The surface plasmon resonance (SPR) experiments certified the remarkably different affinity of PA towards g-CNQDs and PSA. The as-constructed FL aptasensor can accurately quantify PSA with wide linear range of 10 pg mL−1-50 ng mL−1 and ultra-low limit of detection (LOD, 4.2 pg mL−1), indicating the promising applications in clinical assay and biological detection.

The g-C3N4 Quantum Dot Decorated g-C3N4 Sheet/Reduced Graphene Oxide Composite as Efficient Metal-Free Electrocatalyst for Oxygen Reduction Reaction

To develop feasible oxygen reduction reaction (ORR) for fuel cell technology and metal-air battery, it is crucial to find suitable replacement for platinum-based noble metal catalysts. N-doped carbon materials specially for graphene have been intensively investigated and proved to be efficient for ORR. In this work, to enhance performance of graphene-based electrocatalyst, we develop a facile method to prepare g-C3N4 quantum dot (QD) decorated g-C3N4 sheet/reduced graphene oxide (rGO) composite for efficient ORR. At first, a hydrothermal reaction gives a 3D g-C3N4 sheet/rGO matrix. Then, the g-C3N4 QDs are attached to the 3D g-C3N4 sheet/rGO matrix by simple stirring. The obtained product has high nitrogen content and provides sufficient active sites for ORR, showing excellent catalytic performance compared with Pt/C, which is attributed to as follows: the 3D honeycomb structure provides larger specific surface area for ORR; the addition of g-C3N4 QDs provides sufficient active sites to increase the reaction efficiency; high nitrogen content plays an important role in the progress for the oxygen reduction reaction. Present work provides possibility for the commercial application of non-metallic catalysts for ORR.

Bi2O3 and g-C3N4 quantum dot modified anatase TiO2 heterojunction system for degradation of dyes under sunlight irradiation.

A facile and feasible method was successfully utilized to incorporate Bi2O3 and g-C3N4 quantum dots on TiO2 surface to synthesize a novel composite g-C3N4/TiO2/Bi2O3. The photocatalytic activity of the composite g-C3N4/TiO2/Bi2O3 for degradation of dyes under sunlight and UV light irradiation was evaluated. It possessed the higher photocatalytic performance than that of pristine TiO2 or g-C3N4 under the same conditions. Under sunlight irradiation, the reaction rate constants of the g-C3N4/TiO2/Bi2O3 was about 4.2 times and 3.3 times higher than that of TiO2 and g-C3N4, respectively. The promising photocatalytic performance was attributed to the broader light absorption range and efficient separation of photoinduced carriers. Moreover, based on the TEM, XPS, XRD, UV-vis spectrum, radicals scavenging test and Mott–Schottky analysis systematic mechanism for photodegradation process was proposed. This work provides a promising strategy for the modification of TiO2-based semiconductors by incorporating different quantum dots and promoting the efficiency of the photocatalysts in practical application.

Graphene quantum dot-decorated carbon electrodes for energy storage in vanadium redox flow batteries.

Nitrogen-doped graphene quantum dots (GQDs) and graphitic carbon nitride (g-C3N4) quantum dots are synthesized via a solid-phase microwave-assisted (SPMA) technique. The resulting GQDs are deposited on graphite felt (GF) and are employed as high-performance electrodes for all-vanadium redox flow batteries (VRFBs). The SPMA method is capable of synthesizing highly oxidized and amidized GQDs using citric acid and urea as the precursor. The as-prepared GQDs contain an ultrahigh O/C (56-61%) and N/C (34-66%) atomic ratio, much higher than the values reported for other carbon-based nano-materials (e.g. oxidized activated carbon, carbon nanotubes, and graphene oxide). Three types of quantum dots, having an average particle size of 2.8-4.2 nm, are homogeneously dispersed onto GF electrodes, forming GQD/GF composite electrodes. Through deposition of GQDs onto the electrode structure, the catalytic activity, equivalent series resistance, durability, and voltage efficiency are improved. The capacity utilization using GQD/GF electrode is substantially enhanced (∼69% increase within 40 cycles). The improved performance is attributed to the synergistic effect of GQDs containing oxygen functionalities (epoxy, phenolic and carboxylic groups) and lattice N atoms (quaternary, pyrrolic and pyridinic N) which result in enhanced wettability and increased electrochemical surface area providing increased reaction sites.

Non enzymatic fluorometric determination of glucose by using quenchable g-C3N4 quantum dots.

A non-enzymatic fluorometric assay is described for the determination of glucose. The method is based on the use of g-C3N4 quantum dots (QDs) that have good water solubility. The QDs were synthesized by a one-step solvothermal process using formamide as precursor. The QDs possess an average size of ~5nm, a band gap of 3.0~3.5eV, and strongblue fluorescence (with excitation/emission maxima at 400/447nm). Fluorescence is quenched by glucose (which acts as the electron acceptor) via an electron transfer mechanism. Comprehensive spectroscopy and density functional theory calculations show that the selectivity of the fluorescent probe can be attributed to the presence of N-H bonds that are formed between the QDs (mainly at plane edges) and glucose. The interaction forces lead to the formation of localized states for capturing hot electrons. This results in a decrease in the band gap and a reduction in fluorescence intensity. The probe is selective over some typical interfering species (such as cysteine and albumin) which often are present in the urine of diabetics. The method has a linear response in the 0.2 to 5.0mM glucose concentration range and a 0.2mM detection limit. Graphical abstractSchematic representation of the synthesis of g-C3N4 quantum dots (QDs) as afluorescent nanoprobe for selective detection of glucose.

Constructing g-C3N4 quantum dots modified g-C3N4/GO nanosheet aerogel for UV-Vis-NIR driven highly efficient photocatalytic H2 production

The development of ultraviolet to near-infrared (UV-Vis-NIR) responsive photocatalysts offers a unique opportunity for the full use of solar energy to solve the energy and the environmental problems. Here, successful preparation of a three-dimensional (3D) porous photocatalyst of graphitic carbon nitride quantum dot (CNQDs) modified g-C3N4/graphene oxide composite aerogel (CNGO/CNQDs) via hydrothermal and vacuum injection method was reported. In this unique ternary 3D photocatalyst, graphene oxide could improve the separation of photogenerated electrons and holes and promote the charge separation, while the aerogel's 3D network structure provided a rich active site. Simultaneously, due to the appropriate up-conversion performance of the nitrogen carbide quantum dots, CNGO/CNQDs achieved a light response from ultraviolet (UV) to near-infrared (NIR). These properties endow it with a good photocatalytic performance. The hydrogen production efficiency of CNGO/CNQDs reached 1231 μmol h−1, which was 16 times more than that of matrix material. In addition, the apparent quantum yields (AQY) of CNGO/CNQDs at wavelengths of 420 nm and 700 nm were 13% and 0.116%, respectively.

Dual quantum dots decorated TiO2 nanorod arrays for efficient CO2 reduction

To meet the demand of CO2 reduction, we designed a Z-type composite photocatalyst based on vertically aligned rutile TiO2 (r-TiO2) nanorod arrays. The composite photocatalyst was obtained by decorating g-C3N4 and Au quantum dots on the r-TiO2 nanorods. Benefiting from the high charge separation efficiency of C3N4/TiO2 heterojunction and the electron capture effect of Au quantum dots, the composite catalyst indicates a significantly enhanced hydrocarbon yield efficiency. The yield rate of CO can increase to 0.138 μmol cm−2 h−1 and the yield rate of CH4 can reach 0.032 μmol cm−2 h−1 without any sacrificial agents, which is almost 5 times than that of pristine r-TiO2 nanorod arrays. This work provides an efficient strategy to rationally design high-performance photocatalysts by using quantum dots and nanorods (or nanowires) as building blocks.

The adsorption of small size Pd clusters on a g-C3N4 quantum dot: DFT and TD-DFT study

In this study, the adsorption of Pdn cluster with different size (n = 2–8) on graphitized carbon nitride (g-C3N4) quantum dot has been studied using the density functional theory (DFT). The Pd2, Pd4 and Pd6 clusters showed tendency for landing at a corner of the six-fold cavity of g-C3N4 while the Pd8 cluster was landed on the top of the cavity of g-C3N4 for efficient interaction with all pyridine like nitrogen atoms of the cavity, simultaneously. The charge of Pdn cluster in Pdn/g-C3N4 (n = 2–6) complexes was positive showing the charge transfer from the metal cluster to g-C3N4 while the net charge of Pd8 in Pd8/g-C3N4 became negative showing an opposite direction of charge transfer in this complex. Moreover, it was found that by increasing the size of Pdn cluster, the energy band gap of Pdn/g-C3N4 complex becomes smaller compared with the bare g-C3N4. The time-dependent density functional theory (TD-DFT) was employed to calculate the absorption spectra of the bare g-C3N4 and Pdn/g-C3N4 complexes. It was observed that the presence of Pdn clusters on the g-C3N4 shifted the absorption peaks of g-C3N4 to the lower wavelengths. Also, it increased the intensity of the absorption bands in the range of 400 to 800 nm depending on the Pdn cluster size which could lead to the increase in the photo-catalytic activity of Pdn/g-C3N4 complexes compared to bare g-C3N4 in visible region. Most of the UV–vis absorption bands of the Pdn/g-C3N4 complexes in region of the 400–800 nm were related to the excitation of electron from the Pdn cluster to the g-C3N4 which decreases the recombination of electron-hole in the complexes compared to the bare g-C3N4. Mixing of the states of g-C3N4 with the states of Pdn clusters causes the decrease in the intensity of absorption bands of the g-C3N4 in the region of 200–300 nm.

Metal-free visible light photocatalytic carbon nitride quantum dots as efficient antibacterial agents: An insight study

Graphitic carbon nitride (g-CN, commonly called as g-C3N4) quantum dots exhibiting excellent photoluminescence behaviour is explored as metal-free visible light photocatalytic antibacterial agents. The g-CNQDs were synthesized by two-step thermal polymerization and selective dialysis approach yielding particle size in the range of 2–7 nm with a large specific surface area of 264 m2/g. The fluorescent g-CNQDs could efficiently generate superoxide and hydroxyl radicals under visible light irradiation, inhibiting∼99% of Gram negative E.coli and ∼90% of Gram positive S.aureus at a concentration of 0.1 mg/mL. A strong correlation between the TEM, SEM and confocal microscopic analysis is observed demonstrating that g-CNQDs cause light induced photo toxicity. This toxicity is caused by the physical interaction (between the QDs and cells) and due to the generation of ROS in presence of light. The g-CNQDs were found to be non-toxic to fibroblast cells up to a concentration of 3.2 mg/mL. A comprehensive comparison of g-CNQDs with bulk non-porous g-CN, mesoporous CN, silver loaded mesoporous CN and bare silver nanoparticles indicated them to be promising photo induced bactericidal material.

Facile and efficient fabrication of g-C3N4 quantum dots for fluorescent analysis of trace copper(II) in environmental samples

The facile preparation of g-C3N4 QDs with high fluorescent performance has become an important direction in the last decade. Herein, we develop a facile, rapid approach to synthesize highly fluorescent QDs based on recrystallization and ultrasonic exfoliation. Size-controllable graphitic carbon nitride (g-C3N4) QDs can be obtained from the precursor of recrystallized dicyandiamide, only 90 min is needed and the size of QDs is adjusted from 5 nm to 200 nm by controlling the ultrasonic time. Moreover, better fluorescent efficiency is also obtained comparing to traditional g-C3N4 QDs. The obtained g-C3N4 QDs responds to Cu(II) in the 0.5 nmol/L to 30 μmol/L concentration range, with a 0.3 nmol/L detection limit. The method was applied to the determination of Cu(II) in different environmental water samples.

0D/2D Z-scheme heterojunctions of g-C3N4 quantum dots/ZnO nanosheets as a highly efficient visible-light photocatalyst

Constructing Z-scheme heterojunctions comprising of constituents with different dimensionality is an effective strategy to spatially separate electron and hole. To fully utilize the synergistic coupling effect of dimensionality, herein, we first immobilize g-C3N4 quantum dots (CNQDs) onto ZnO nanosheets with oxygen vacancies (OV-ZnO) to create a 0D/2D hybrid via a facile and cost-effective approach. The CNQDs/OV-ZnO heterojunctions display CNQDs content-dependent performance in visible-light photocatalytic activity. The optimal CNQDs/OV-ZnO heterojunction exhibits high photocatalytic activity for degradation of methyl blue and bisphenol A, where the kinetic constant is 11.4 and 32.5 fold of pure OV-ZnO, respectively. Photoluminescence, electrochemical impedance spectroscopy and photocurrent verify that the photogenerated electron-hole pairs in this 0D/2D Z-scheme heterojunction have been effectively separated. The enhanced photocatalytic activity could be attributed to the synergistic effect of efficient Z-scheme charge separation, highly dispersed 0D CNQDs, coordinating sites of 2D OV-ZnO nanosheets and the strong coupling between them. In addition, the 3D flower-like structure constructed by 2D nanosheets greatly inhibits the leaching and loss of the photocatalyst in the recycling process, and ensures the high recycling ability of CNQDs/OV-ZnO. This work paves the way toward designing novel visible-light 0D/2D photocatalysts in the application of solar energy.

Modification of g-C3N4 quantum dots by Ni–Ni3C@C nanoparticles for hydrogen production

A rational design of noble-metal-free cocatalyst plays an important role in the potential application of g-C3N4 for hydrogen production using solar energy. In this work, graphitic carbon covered Ni–Ni3C (Ni–Ni3C@C) nanoparticles were used as cocatalyst for visible light-driven H2 production over g-C3N4 quantum dots (CNQD). Ni–Ni3C@C/CNQD composite can promote the separation of photogenerated electron-hole pairs, thus bringing about a significantly enhanced photocatalytic performance. The H2-production rate of the optimal Ni–Ni3C@C/CNQD composite is approximately 86 times higher than that of pure CNQD. Our work will provide new insights for the application of metal carbide as a noble-metal-free cocatalyst in photocatalytic system.

Fabrication of Fluorescence Turn-off-on Sensor Based on g-C3N4 Quantum Dots and MgFe Layered Double Hydroxide for the Detection of Citrate.

We fabricated a new and selective fluorescent sensor for the detection of citrate by employing g-C3N4 quantum dots (g-CNQDs) and MgFe layered double hydroxide (MgFe-LDH). The g-CNQDs interacted with MgFe-LDH via electrostatic interaction and the fluorescence of g-CNQDs was effectively quenched by MgFe-LDH due to inner filter effect. Upon addition of citrate, the fluorescence of the g-CNQDs was significantly enhanced, arising from the replacement of g-CNQDs by citrate because citrate competed with g-CNQDs to form more stable complexes with MgFe-LDH. Therefore, we developed a "turn-off-on" fluorescence assay method for the detection of citrate. This method enabled the selective detection of citrate with linear range of 0.5-3.0μM and 3.0-10.0μM with a 12.3nM limit of detection. This method exhibited advantages including easy preparation, environmentally friendly process and rapid response toward citrate.

TiO2 nanotubes/g-C3N4 quantum dots/rGO Schottky heterojunction nanocomposites as sensors for ppb-level detection of NO2

As a strong oxidizing pollutant, NO2 can cause fire or even explosion. People living in atmosphere containing NO2 for a long time will significantly affects human health. In this work, we developed a Schottky heterojunction sensor modified by g-C3N4 quantum dots (g-C3N4QDs) and rGO deposited on TiO2 nanotubes (TNTs) arrays. This sensor showed high response and extremely fast response/recovery time as well as excellent detection of ppb level of NO2 at room temperature. TNTs were obtained using a one-step anodic oxidation process. TNTs were modified with g-C3N4QDs and rGO using quasi-CVD method and cyclic voltammetry during in situ electrodeposition, respectively. TNTs/g-C3N4QDs/rGO Schottky heterojunction sensor exhibited high sensitivity to 10 ppm of NO2 (response equal to 15982) at room temperature. Below 15 ppb, sensing response also can reach 127. Sensor response was very fast and increased to 15982 in just 2 s when exposed to 10 ppm of NO2 after which it recovers 90% within 1.16 s. This work clarified the influence of abundant oxygen vacancies (VO·) in TNTs and photogenerated electrons on TNTs/g-C3N4QDs/rGO nanostructures as well as their sensing performances. Our experimental details demonstrated that Schottky barrier was established between TNTs and rGO, which was very beneficial for ppb-level NO2 detection at room temperature.

Specific fluorometric assay for direct determination of amikacin by molecularly imprinting polymer on high fluorescent g-C3N4 quantum dots.

Here, a specific and reliable fluorometric method for the rapid determination of amikacin was developed based on the molecularly imprinting polymer (MIP) capped g-C3N4 quantum dots (QDs). g-C3N4 QDs were obtained by facile and one-spot ethanol-thermal treatment of bulk g-C3N4 powder and showed a high yield fluorescence emission under UV irradiation. The MIP layer was also created on the surface on QDs, via usual self-assembly process of 3-aminopropyl triethoxysilane (APTES) functional monomers and tetraethyl ortho-silicate (TEOS) cross linker in the presence of amikacin as template molecules. The synthesized MIP-QDs composite showed an improved tendency toward the amikacin molecules. In this state, amikacin molecules located adjacent to the g-C3N4 QDs caused a remarkable quenching effect on the fluorescence emission intensity of QDs. This effect has a linear relationship with amikacin concentration and so, formed the basis of a selective assay to recognize amikacin. Under optimized experimental conditions, a linear calibration graph was obtained as the quenched emission and amikacin concentration, in the range of 3-400 ng mL-1 (4.4-585.1 nM) with a detection limit of 1.2 ng mL-1 (1.8 nM). The high selectivity of MIP sites as well as individual fluorescence properties of g-C3N4 QDs offers a high specific and sensitive monitoring method for drug detection. The method was acceptably applied for the measurement of amikacin in biological samples.

Photocatalytic removal using g-C3N4 quantum dots/Bi2Ti2O7 composites.

In this work, a simple method to load of g-C3N4 quantum dots (CN QDs) onto Bi2Ti2O7 (BTO) microsphere with the amount of CN QDs (3, 7, 10 and 15%). The photocatalyst was used for the treatment of water pollutants under visible-light illumination, which proved that CNBTO composites showed improved photocatalytic activity matched up to pure BTO. Reformation of BTO with CN QDs enhanced the light assimilation capacity, and promoted the isolation of photo-induced electron-hole pairs. The trapping experiments and ESR were announced the holes (h+) and superoxide oxide (O2-) played the key role, and the relative mechanism of the photocatalytic process was proposed. Meanwhile, the effects of CN QDs content, pH and initial pollutant concentration on the removal efficiency of ciprofloxacin (CIP) were studied. Results showed that the CN QDs loaded on BTO presented higher photocatalytic efficiency, and an optimum value for the dosage of photocatalytic in pH 8.0.

Enhanced performance of flexible ultraviolet photodetectors based on carbon nitride quantum dot/ZnO nanowire nanocomposites

While the ZnO based ultraviolet (UV) photodetectors are generally restrained by the slow response and the weak photosensitivity, present work develops a facile method to fabricate flexible UV photodetectors with high device performance based on graphitic carbon nitride (g-C3N4) quantum dot/ZnO nanowire nanocomposites. The sensors are prepared by spin-coating method followed by annealing on flexible substrates, which exhibit remarkable responsivity of 355 mA W−1 under UV irradiation, a 1000% improvement over ZnO nanowire sensors (35.4 mA W−1). Both on/off ratio and photoresponse current are significantly improved by introducing g-C3N4 quantum dots compared with pure ZnO nanowire sensors. Current report provides a new choice for UV material system to efficiently improve sensor performance, and it could be easily scaled up for large scale applications.

Ultrathin Bi2WO6 nanosheets loaded g-C3N4 quantum dots: A direct Z-scheme photocatalyst with enhanced photocatalytic activity towards degradation of organic pollutants under wide spectrum light irradiation

A novel ultrathin Bi2WO6 nanosheets loaded g-C3N4 quantum dots (CNQDs/BWO) photocatalyst was successfully fabricated, and used to catalyze two representative organic pollutants, rhodamine B (RhB) and tetracycline (TC) under wide spectrum light irradiation. The degradation experiments showed that CNQDs/BWO exhibited enhanced photocatalytic activities towards degradation of organic pollutants. Under visible light irradiation, the 5% CNQDs/BWO exhibited the best degradation efficiency with 87% and 92.51% removal of TC and RhB within 60 min, respectively. And under near-infrared (NIR) light, the 5% CNQDs/BWO still showed the best performance, its degradation efficiency to TC were ∼2 times than pure BWO. The upconversion behaviors of CNQDs might contribute to the enhanced photocatalysis. According to similar degradation trend, it is inferred that the catalytic mechanism in NIR light is consistent with that in visible light. The enhanced photocatalytic activity of CNQDs/BWO under wide spectrum light irradiation can be ascribed to a Z-scheme mechanism based on the calculated the lowest unoccupied molecular orbital (LUMO) of CNQDs and CB position of BWO, the free radical quenching experiment, and ESR characterization results. The composites have prominent light absorption, high stability and excellent photocatalysis efficiency, which would be used as a promising strategy for organic pollutants degradation.