Effect of Nitrogen Doping Level on the Performance of N-Doped Carbon Quantum Dot/TiO2 Composites for Photocatalytic Hydrogen Evolution

Waste Polyethylene-Derived Carbon Dots: Administration of Metal-Free Oxidizing Agents for Tunable Properties and Photocatalytic Hyperactivity

The possibility of converting waste plastics into carbon dots (CDs) with 100% efficiencies using KMnO4 has emerged as a significant discovery in mitigating plastic pollution and upcycling. However, the lack of tunability of their properties, viz. aerial O2 harvesting, light-induced autophagy, and photoactivity using air as a free oxidant, has remained a bottleneck. Besides, the toxicity of KMnO4 makes the process less sustainable. Attempting to bridge these gaps, herein, we demonstrate the preparation of CDs using polyethylene with enormous controllability of their properties by utilizing less-toxic and metal-residue-free oxidizers, e.g., H2O2, HNO3, HClO4, and NaClO. We obtain structurally diverse CDs with controllable luminescent quantum yields (∼0.5-8%), excitonic lifetimes (1.3-2.3 ns), and binding energies (147-290 meV). These CDs exhibit a hugely extended range of molecular O2 harvesting (∼405-650 μM) with different amounts of strongly and weakly surface-bound O2 molecules within an estimated ratio of ∼0.77-2.51. Autophagy varied from 14 days to a nearly "no-autophagy" show. We efficiently utilized their oxygen harvesting and photocatalytic abilities to synthesize imine compounds from the corresponding amines in the open air (rate constant of ∼0.055 min-1), surpassing the literature efficiencies achieved using an O2 flow and noble metals. Notably, due to oxygen harvesting by CDs, no additional rate enhancement was observed after O2 purging, establishing the role of CDs in making free air an excellent oxidizing agent.

A review on carbon quantum dot/semiconductor-based nanocomposites as hydrogen production photocatalysts

Carbon quantum dots (CQDs) are discrete, quasi-spherical carbon nanoparticles with sizes below 10 nm. The properties of CQDs can be further enhanced by doping with elements such as nitrogen, phosphorous, sulphur, and boron or co-doping with heteroatoms such as nitrogen-phosphorous, nitrogen-sulphur, and nitrogen-boron. These excellent properties of CQDs can be utilized to enhance the photocatalytic performance of semiconductors. Therefore, in this review, we summarize different types of bare CQD-scaffolded semiconductors, both doped and co-doped, used for photocatalytic hydrogen production. Moreover, the detailed photocatalytic mechanism of CQD/semiconductor-based hydrogen production is reviewed. Recent progress in the design and development of CQD-based photocatalysts, along with the challenges involved, is comprehensively reviewed.

Breakthrough in High-Efficiency Photocatalytic Degradation of Acebutolol by Advanced Binary CeO2-MnO2 Oxide System

The sustainable catalytic efficacy of transition metal oxides (TMO) and rare earth element-based oxides positions them as pivotal materials for effectively treating contaminated wastewater. This study successfully synthesized a series of Ce@MnO2 photocatalysts using a straightforward hydrothermal method. These photocatalysts were thoroughly characterized for their optical properties, structural morphology, and phase purity. Among the synthesized materials, the Ce@MnO2 (40:60) exhibited the highest photocatalytic activity for the degradation of Acebutolol (ACB), achieving a remarkable degradation efficiency of 92.71% within 90 min under visible light irradiation. This superior performance is attributed to the increased presence of active species and the efficient separation of photogenerated carriers. Additionally, the photocatalytic reaction mechanism was elucidated, highlighting the catalyst's surface charge properties which significantly enhanced performance in a solution with pH 8. The outstanding photo-response in the visible spectrum renders this method not only cost-effective but also environmentally benign, presenting a promising approach for large-scale water purification.

An Emerging Trend in the Synthesis of Iron Titanate Photocatalyst Toward Water Splitting

Hydrogen gas is a prominent focus in pursuing renewable and clean alternative energy sources. The quest for maximizing hydrogen production yield involves the exploration of an ideal photocatalyst and the development of a simple, cost-effective technique for its generation. Iron titanate has garnered attention in this context due to its photocatalytic properties, affordability, and non-toxic nature. Over the years, different synthesis routes, different morphologies, and some modifications of iron titanate have been carried out to improve its photocatalytic performance by enhancing light absorption in the visible region, boosting charge carrier transfer, and decreasing recombination of electrons and holes. The use of iron titanate photocatalyst for hydrogen evolution reaction has seen an upward trend in recent times, and based on available findings, more can be done to improve the performance. This review paper provides a comprehensive overview of the fundamental principles of photocatalysis for hydrogen generation, encompassing the synthesis, morphology, and application of iron titanate-based photocatalysts. The discussion delves into the limitations of current methodologies and present and future perspectives for advancing iron titanate photocatalysts. By addressing these limitations and contemplating future directions, the aim is to enhance the properties of materials fabricated for photocatalytic water splitting.

Excellent Charge Separation of NCQDs/ZnS Nanocomposites for the Promotion of Photocatalytic H2 Evolution

Carbon Quantum dots (CQDs) are widely studied because of their good optical and electronic characteristics and because they can easily generate photocarriers. Nitrogen-doped CQDs (NCQDs) may exhibit improved hydrophilic, optical, and electron-transfer properties, which are conducive to photocatalytic hydrogen evolution. In this paper, NCQD-modified ZnS catalysts were successfully prepared. Under the irradiation of the full spectrum, the H2 evolution rate of the optimal catalyst 0.25 wt % NCQDs/ZnS achieves 5.70 mmol g-1 h-1, which is 11.88, 43.84, and 5.14 times the values of ZnS (0.48 mmol g-1 h-1), NCQDs (0.13 mmol g-1 h-1), and CQDs/ZnS (1.11 mmol g-1 h-1), respectively. Furthermore, it shows good stability, indicating that the modification of NCQDs prevents the photocorrosion and oxidation of ZnS. The enhanced performance is due to NCQD loading, which promotes the separation of photogenerated carriers, optimizes the structures, and increases the specific surface area. This work highlights the fact that NCQD-modified ZnS may afford a new strategy to synthesize ZnS-based photocatalysts with enhanced H2 production performance.

Superior white electroluminescent devices using nitrogen-doped carbon dots/TiO2nanorods heterostructures

Nitrogen-doped carbon dots (NCDs), exhibiting strong yellow emission in aqueous solution and solid matrices, have been utilized for fabricating heterostructure white electroluminescence devices. These devices consist of nitrogen-doped carbon dots as an emissive layer sandwiched between an organic hole transport layer (PEDOT:PSS) and an array of rutile TiO2nanorods, acting as an electron transport layer. Under an applied forward bias of 5 V, the device exhibits broadband electroluminescence covering the wavelength range of 390-900 nm, resulting in pure white light emission characteristics at room temperature. The result demonstrates the successful fabrication of all solution-processed, low-cost, eco-friendly NCDs-based LEDs with CIE (Commission Internationale d'Éclairage) coordinate of (0.31, 0.34) and color rendering index (CRI) > 90, which are close to ideal white light emission characteristics. The device functionalities are achieved based on defect-related NIR emission from TiO2nanorods array and visible emission from nitrogen-doped carbon dots. This result paves a new opportunity to develop low-cost, solution-processed nitrogen-doped carbon dots based on warm White light emitting diodes with high CRI for large-area display and lighting applications.

Nitrogen-doped carbon dots for efficient deep-blue light-emitting diodes with CIE closely approaching the HDTV standard color Rec.BT.709

Here, we demonstrate deep-blue carbon dots (CDs) with luminescence centered at 415 nm and PLQY exceeding 60% via nitrogen doping. A bright and high-color-purity CDs-based light-emitting diode (CLED) is achieved with an external quantum efficiency (EQE) of 1.74%, a maximum luminance of 1155.0 cd m^-2, and a colour coordinate (0.16, 0.08) closely approaching the HDTV standard color Rec.BT.709 (0.15, 0.06) specification.

Visible light-driven highly-efficient hydrogen production by a naphthalene imide derivative-sensitized TiO2 photocatalyst

Sensitizing titanium dioxide (TiO₂) with dye molecules offers a cost-effective and environmentally friendly strategy for creating powerful photocatalysts for hydrogen production by reducing the band gap and enhancing sunlight absorption. Despite the challenges of identifying a stable dye with high light harvesting efficiency and effective charge recombination, we present a 1,8-naphthalimide derivative-sensitized TiO₂ that achieves ultra-efficient photocatalytic hydrogen production (10.615 mmol g^-1 h^-1) and maintains activity after 30 hours of cycling. Our research offers valuable insights for designing optimized organic dye-sensitized photocatalysts, advancing clean and sustainable energy solutions.

Carbon Quantum Dots from Amino Acids Revisited: Survey of Renewable Precursors toward High Quantum-Yield Blue and Green Fluorescence

Carbon quantum dots (CQDs) were synthesized via a green, one-step hydrothermal method. As CQD precursors, nine amino acids of different structural descriptors (negatively/positively charged in water, polar, hydrophobic, sulfur-containing, and other/complex ones) were surveyed: Asp, Cys, Gly, His, Leu, Lys, Phe, Pro, and Ser. The reactions were performed in an autoclave in the presence of citric acid at 180 °C for 24 h and yielded core-shell CQDs. CQDs were comprehensively characterized by transmission electron microscopy, dynamic light scattering, Raman, UV/Vis, infrared, X-ray photoelectron spectroscopy, and fluorescence spectroscopy. At the excitation wavelength of λ_ex = 350 nm, Cys-, Phe-, Leu-, and Lys-based CQDs displayed the highest quantum yield blue fluorescence-90 ± 5, 90 ± 4, 87 ± 5, and 67 ± 3%, respectively-superior to the conventional fluorescent dyes. Strikingly, for Lys- and Phe-CQDs, dissimilar trends in the excitation-emission wavelength relationships were identified, that is, constantly strong red shifts versus excitation wavelength-independent emission. Cys- and Lys-CQDs were water-dispersible toward the narrow unimodal distribution of hydrodynamic diameters-0.6 and 2.5 nm, respectively. Additionally, Lys- and Cys-CQDs, with high absolute zeta potential values, formed stable aqueous colloids in a broad range of pH (2, 7, and 12). The results constitute important premises for water-based applications of CQDs, such as bioimaging or photocatalysis.

Photocatalytic removal of pharmaceutical water pollutants by TiO2 - Carbon dots nanocomposites: A review

Pharmaceuticals are becoming increasingly more relevant water contaminants, with photocatalysts (such as TiO₂) being a promising approach to remove these compounds from water. However, TiO₂ has poor sunlight-harvesting capacity, low photonic efficiency, and poor adsorption towards organic pollutants. One of the emerging strategies to enhance the photocatalytic performance of TiO₂ is by conjugating it with fluorescent carbon dots. Herein, we performed a critical review of the development of TiO₂ - carbon dots nanocomposites for the photocatalytic removal of pharmaceuticals. We found that carbon dots can improve the photocatalytic efficiency of the resulting nanocomposites, mostly due to increasing the adsorption of organic pollutants and enhancing the absorption in the visible range. However, while this approach shows significant promise, we also identified and discussed several aspects that need to be addressed before this strategy could be more widely used. We hope that this review can guide future studies aiming to the development of enhanced photocatalytic TiO₂ - carbon dots nanocomposites.

Direct Electron Transfer from Upconversion Graphene Quantum Dots to TiO2 Enabling Infrared Light-Driven Overall Water Splitting

Utilization of infrared light in photocatalytic water splitting is highly important yet challenging given its large proportion in sunlight. Although upconversion material may photogenerate electrons with sufficient energy, the electron transfer between upconversion material and semiconductor is inefficient limiting overall photocatalytic performance. In this work, a TiO₂/graphene quantum dot (GQD) hybrid system has been designed with intimate interface, which enables highly efficient transfer of photogenerated electrons from GQDs to TiO₂. The designed hybrid material with high photogenerated electron density displays photocatalytic activity under infrared light (20 mW cm⁻²) for overall water splitting (H₂: 60.4 μmol g_cat.⁻¹ h⁻¹ and O₂: 30.0 μmol g_cat.⁻¹ h⁻¹). With infrared light well harnessed, the system offers a solar-to-hydrogen (STH) efficiency of 0.80% in full solar spectrum. This work provides new insight into harnessing charge transfer between upconversion materials and semiconductor photocatalysts and opens a new avenue for designing photocatalysts toward working under infrared light.

Sensitive photoelectrochemical detection of colitoxin DNA based on NCDs@CuO/ZnO heterostructured nanocomposites with efficient separation capacity of photo-induced carriers

A metal-organic framework (MOF) of Cu-TPA (terephthalic acid) microsphere was prepared, followed by calcinating the MOF precursor of Cu-TPA/ZIF-8 mixture to obtain the CuO/ZnO. N-doped carbon dots (NCDs) were employed to combine the CuO/ZnO composite to form a tripartite heterostructured architecture of NCDs@CuO/ZnO, which led to a fierce enlargement of the photocurrent response. This  was ascribed to the thinner-shell structure of the CuO microsphere and the fact that hollow ZnO particles could sharply promote the incidence intensity of visible light. The more porous defectiveness exposed on CuO/ZnO surface was in favor of rapidly infiltrating electrolyte ions. The p-n type CuO/ZnO composite with more contact interface could abridge the transfer distance of photo-induced electron (e^-1)/hole (h⁺) pairs and repress their recombination availably. NCDs not only could boost electron transfer rate on the electrode interface but also successfully sensitized the CuO/ZnO composite, which resulted in high conversion efficiency of photon-to-electron. The probe DNA (S1) was firmly assembled on the modified ITO electrode surface (S1/NCDs@CuO/ZnO) through an amidation reaction. Under optimal conditions, the prepared DNA biosensor displayed a wide linear range of 1.0 × 10^-6 ~ 7.5 × 10^-1 nM and a low limit of detection (LOD) of 1.81 × 10^-7 nM for colitoxin DNA (S2) measure, which exhibited a better photoelectrochemistry (PEC) analysis performance than that obtained by differential pulse voltammetry techniques. The relative standard deviation (RSD) of the sensing platform for target DNA detection of 5.0 × 10^-2 nM was 6.3%. This proposed DNA biosensor also showed good selectivity, stability, and reproducibility, demonstrating that the well-designed and synthesized photoactive materials of NCDs@CuO/ZnO are promising candidates for PEC analysis.

N-Doped Graphene Quantum Dots/Titanium Dioxide Nanocomposites: A Study of ROS-Forming Mechanisms, Cytotoxicity and Photodynamic Therapy

Titanium dioxide nanoparticles (TiO2 NPs) have been proven to be potential candidates in cancer therapy, particularly photodynamic therapy (PDT). However, the application of TiO2 NPs is limited due to the fast recombination rate of the electron (e−)/hole (h+) pairs attributed to their broader bandgap energy. Thus, surface modification has been explored to shift the absorption edge to a longer wavelength with lower e−/h+ recombination rates, thereby allowing penetration into deep-seated tumors. In this study, TiO2 NPs and N-doped graphene quantum dots (QDs)/titanium dioxide nanocomposites (N-GQDs/TiO2 NCs) were synthesized via microwave-assisted synthesis and the two-pot hydrothermal method, respectively. The synthesized anatase TiO2 NPs were self-doped TiO2 (Ti3+ ions), have a small crystallite size (12.2 nm) and low bandgap energy (2.93 eV). As for the N-GQDs/TiO2 NCs, the shift to a bandgap energy of 1.53 eV was prominent as the titanium (IV) tetraisopropoxide (TTIP) loading increased, while maintaining the anatase tetragonal crystal structure with a crystallite size of 11.2 nm. Besides, the cytotoxicity assay showed that the safe concentrations of the nanomaterials were from 0.01 to 0.5 mg mL−1. Upon the photo-activation of N-GQDs/TiO2 NCs with near-infrared (NIR) light, the nanocomposites generated reactive oxygen species (ROS), mainly singlet oxygen (1O2), which caused more significant cell death in MDA-MB-231 (an epithelial, human breast cancer cells) than in HS27 (human foreskin fibroblast). An increase in the N-GQDs/TiO2 NCs concentrations elevates ROS levels, which triggered mitochondria-associated apoptotic cell death in MDA-MB-231 cells. As such, titanium dioxide-based nanocomposite upon photoactivation has a good potential as a photosensitizer in PDT for breast cancer treatment.

Enhanced photocatalytic activity of glyphosate over a combination strategy of GQDs/TNAs heterojunction composites

A photocatalytic process was used to effectively remove glyphosate, an emerging pollutant and contaminant, through advanced oxidation. For this purpose, a feasible combination strategy of two-step anodisation and electrodeposition methods were proposed to fabricate graphene quantum dots (GQDs) supported titanium dioxide nanotube arrays (TNAs). The resultant GQDs/TNAs heterojunction composite exhibited significant degradation reactivity and circulation stability for glyphosate due to its excellent photo-generated electron and hole separation ability. After the introduction of GQDs into TNAs, the photodegradation efficiency of glyphosate increased from 69.5% to 94.7% within 60 min under UV-Vis light irradiation (λ = 320-780 nm). By analysing the intermediate products and through the evolvement of heteroatoms during glyphosate photodegradation, alanine and serine were discovered for the first time, and a detailed degradation mechanism of glyphosate was proposed. This study indicates that GQDs/TNAs heterojunction composite can almost completely degrade the glyphosate into inorganics under the appropriate conditions.

Degradation of Anti-Inflammatory Drugs in Synthetic Wastewater by Solar Photocatalysis

Due to the high number of anti-inflammatory drugs (AIMDs) used by the public health sector in Iraq and distributed all over the country and due to their toxicity, there is a need for an environmental-friendly technique to degrade any wasted (AIMD) present in aquatic ecosystem. The degradation of diclofenac sodium (DCF), ibuprofen (IBN), and mefenamic acid (MFA) in synthetic hospital wastewater were investigated utilizing locally-made Cu-coated TiO2 nanoparticles in a solar-irradiated reactor. Different key variables were studied for their effects on process efficiency, such as loadings of catalyst (C CU-TiO2 = 100–500 mg/L), AIMDs (100 µg/L), pH (4–9), and hydrogen peroxide (CH2O2 = 200–800 mg/L). The results revealed that degradation percentages of 96.5, 94.2, and 82.3%, were obtained for DCF, IBN, and MFA, respectively, using our Cu-coated TiO2 catalyst within 65 min at pH = 9, while other parameters were C CU-TiO2 = 300 mg/L, and CH2O2 = 400 mg/L. The experimental results revealed coupling photocatalysis with solar irradiation as a clean energy source could be utilized for the degradation of toxic pollutants in surface water.

Highly Efficient Flower-Like Graphene Quantum Dots-Based Fuschin Photocatalyst for Selective NAD(P)H Cofactor Regeneration Under Solar Light Irradiation

To meet the energy demands artificial photosynthesis is playing the most promising role these days. Using sunlight directly in a manner that can provide useful chemicals is a sustainable means of generating energy. Studies are going on in this context to make better and effective photocatalyst. In this paper, we study the synthesis of flower-like graphene quantum dots-based fuschin (GQDF) photocatalyst from lemon_, which is created by a green and sustainable process that is effective and environment friendly. The UV-visible spectra of prepared GQDF showed a high molar extinction coefficient. A flower-like GQDF photocatalyst has better catalytic efficiency for the selective creation and regeneration of the NAD(P)H cofactor. A newly designed flower-like GQDF photocatalyst is used as one of its most effective photocatalysts as they have several additional applications in the removal of CO₂ and aqueous contaminants like heavy metals.

Cytotoxicity and cell imaging of six types of carbon nanodots prepared through carbonization and hydrothermal processing of natural plant materials

In this study we prepared six types of carbon nanodots (CNDs) from natural plant materials – through carbonization of two species of bamboo (Bamboo-I, Bamboo-II) and one type of wood (Wood), and through hydrothermal processing of the stem and root of the herb Mahonia oiwakensis Hayata (MO) and of the agricultural waste of two species of pineapple root (PA, PB). The resulting CNDs were spherical with dimensions on the nanoscale (3–7 nm); furthermore, CND-Bamboo I, CND-Wood, CND-Bamboo II, CND-MO, CND-PA, and CND-PB displayed fluorescence quantum yields of 9.63, 12.34, 0.90, 10.86, 0.35, and 0.71%, respectively. X-ray diffraction revealed that the carbon nanostructures possessed somewhat ordered and disordered lattices, as evidenced by broad signals at values of 2θ between 20 and 30°. CND-Bamboo I, CND-Wood, and CND-Bamboo II were obtained in yields of 2–3%; CND-MO, CND-PA, and CND-PB were obtained in yields of 17.64, 9.36, and 22.47%, respectively. Cytotoxicity assays for mouse macrophage RAW264.7 cells treated with the six types of CNDs and a commercial sample of Ag nanoparticles (NPs) revealed that each of our CNDs provided a cell viability of 90% at 2000 μg mL⁻¹, whereas it was only 20% after treatment with the Ag NPs at 62.5 μg mL⁻¹. The six types of CNDs also displayed low cytotoxicity toward human keratinocyte HacaT cells, human MCF-7 breast cancer cells, and HT-29 colon adenocarcinoma cells when treated at 500 μg mL⁻¹. Moreover, confocal microscopic cell imaging revealed that the fluorescent CND-Bamboo I particles were located on the MCF-7 cell membrane and inside the cells after treatment for 6 and 24 h, respectively. We have thoroughly investigated the photoluminescence properties and carbon nanostructures of these highly dispersed CNDs. Because of the facile green synthesis of these six types of CNDs and their sourcing from abundant natural plants, herbs, and agriculture waste, these materials provide a cost-effective method, with low cytotoxicity and stable fluorescence, for biolabeling and for developing cell nanocarriers.

Green nanotechnology of six types of carbon nanodots (CNDs), and their sourcing from abundant natural plants, herbs, and agriculture waste, provides a cost-effective method, with low cytotoxicity and stable fluorescence, for biolabeling and for developing cell nanocarriers.

In Situ Synthesis of Lead-Free Halide Perovskite Cs2AgBiBr6 Supported on Nitrogen-Doped Carbon for Efficient Hydrogen Evolution in Aqueous HBr Solution

Lead halide perovskites have shown great potential in photovoltaic and photocatalytic fields. However, the toxicity of lead impedes their wide application. Herein composites of lead-free halide perovskite Cs₂AgBiBr₆ supported on nitrogen-doped carbon (N-C) materials were synthesized successfully through a facile one-pot method for the first time. Without deposition of noble metals as the cocatalyst, the optimal composite Cs₂AgBiBr₆/N-C (Cs₂AgBiBr₆/N-C-140) exhibits outstanding photocatalytic performance with a high hydrogen evolution rate of 380 μmol g^-1 h^-1 under visible light irradiation (λ ≥ 420 nm), which is about 19 times faster than that of pure Cs₂AgBiBr₆ and 4 times faster than that of physically mixed Cs₂AgBiBr₆/N-C-140, respectively. The Cs₂AgBiBr₆/N-C-140 composite also displays high stability with no significant decrease after six cycles of repeated hydrogen evolution experiments. The addition of N-C with a high surface area helps to prevent aggregation of Cs₂AgBiBr₆ NPs and provides more pathways for the migration of photoinduced carriers. The nitrogen dopant can facilitate photoinduced electron transfer from Cs₂AgBiBr₆ to N-C to result in spatially separated electrons and holes with prolonged electron time and greatly enhance the photocatalytic performance. This study indicates that Cs₂AgBiBr₆-based perovskite materials are promising candidates for photocatalytic hydrogen evolution.

Broad-spectrum response NCQDs/Bi2O2CO3 heterojunction nanosheets for ciprofloxacin photodegradation: Unraveling the unique roles of NCQDs upon different light irradiation

N-doped carbon quantum dots (NCQDs) decorated Bi₂O₂CO₃ heterojunction nanosheets have been successfully constructed by a facile hydrothermal method. The obtained NCQDs/Bi₂O₂CO₃ heterojunction exhibits a wide-spectrum absorption ability and remarkably enhanced photocatalytic activities for ciprofloxacin photodegradation from ultraviolet to near-infrared region. The critical roles of NCQDs and two different charge separation and transfer processes of NCQDs/Bi₂O₂CO₃ heterojunction under different light irradiations have been elucidated. Upon UV light irradiation, NCQDs act as electron reservoirs and a Z-scheme charge transfer process between Bi₂O₂CO₃ and NCQDs promotes electrons transfer and •O₂^- reactive species generation. Under visible and NIR light irradiation, NCQDs act as photosensitizer (hole reservoirs) to harvest solar light and a type-II heterojunction leads to an efficient charge carrier separation and thus high catalytic ability. The mechanisms and pathways of ciprofloxacin degradation driven by different lights are discussed accordingly. This work provides a versatile pathway to well design an efficient wide-spectrum response photocatalyst.

Synthesis of a carbon dots modified g-C3N4/SnO2 Z-scheme photocatalyst with superior photocatalytic activity for PPCPs degradation under visible light irradiation

As an emerging carbon nanomaterial, carbon dots (CDs) have superior prospects for applications in the area of photocatalysis due to their unique optical and electronic properties. In this study, a novel CDs modified g-C₃N₄/SnO₂ photocatalyst (CDs/g-C₃N₄/ SnO₂) was successfully synthesized by the thermal polymerization. Under visible light irradiation, the resulting CDs/g-C₃N₄/SnO₂ photocatalyst exhibited excellent photocatalytic activity for the degradation of indomethacin (IDM). It was demonstrated that a 0.5 % loading content of CDs led to the highest IDM degradation rate, which was 5.62 times higher than that of pristine g-C₃N₄. This improved photocatalytic activity might have been attributed to the unique up-conversion photoluminescence (PL) properties and efficient charge separation capacities of the CDs. Moreover, the combination of g-C₃N₄ with SnO₂ improved the separation of photoinduced carriers and augmented the specific surface area. Reactive species (RSs) scavenging experiments and electron spin resonance (ESR) revealed that superoxide radical anions (O₂^·-) and photogenerated holes (h⁺) played critical roles during the photocatalytic process. The results of the detection of H₂O₂ and ESR confirmed that CDs/g-C₃N₄/ SnO₂ was a Z-scheme heterojunction photocatalyst. Further, HRAM LC-MS/MS was employed to identify the byproducts of IDM, and the major IDM degradation pathways of the CDs/g-C₃N₄/SnO₂ photocatalyst were proposed. This study provides new ideas for the design of novel CDs modified photocatalysts for environmental remediation.

Visible light-driven photocatalytic degradation of organic pollutants via carbon quantum dots/TiO2

The preparation process of carbon quantum dots and TiO2, and a diagram of the mechanism of the degradation of organic pollutants by composite materials under visible light.

Carbon quantum dot-sensitized hollow TiO2 spheres for high-performance visible light photocatalysis

TiO₂/CQD composites were synthesized through carbon quantum dots covalently attached to the surface of hollow TiO₂ spheres for visible light photocatalytic degradation of organics.

Effect of heteroatoms on the optical properties and enzymatic activity of N-doped carbon dots

Carbon dots (CDs) are attractive nanomaterials because of their facile synthesis, biocompatibility, superior physicochemical properties, and low cost of their precursors. Recent advances in CDs have particularly relied on the modulation of their properties by heteroatom doping (e.g., nitrogen). Although nitrogen-doped CDs (N-CDs) have attracted considerable attention owing to their different properties compared to those of the original CDs, the effects of the heteroatom content and types of bonding on the properties of N-doped CDs remain underexplored. In this work, we prepared N-CDs with controlled nitrogen contents, and fully examined their optical properties, enzymatic activity, and toxicity. We demonstrate that (i) the type of carbon–heteroatom bonding (i.e., carbon–oxygen and carbon–nitrogen bonds) can be altered by changing the ratio of carbon to heteroatom sources, and (ii) both the heteroatom content and the heteroatom-bonding character significantly influence the properties of the doped CDs. Notably, N-CDs exhibited higher quantum yields and peroxidase-like activities than the non-doped CDs. Furthermore, the negatively charged N-CDs exhibited negligible cytotoxicity. Such comprehensive investigations on the physicochemical properties of N-CDs are expected to guide the design of N-CDs for targeted applications.

The characteristics of N-CDs suitable for their optical applications or for use as nanozymes were demonstrated by rationalizing the relationship between the dopant content (e.g., the amount of doped N and types of chemical bonding) and physicochemical properties.

The synthesis and modification of highly fluorescent carbon quantum dots for reversible detection of water-soluble phosphonate-1-hydroxyethane-1,1-diphosphonic acid by fluorescence spectroscopy

Photoluminescent (PL) carbon quantum dots (CQDs) were prepared successfully using a facile and green procedure. They exhibited striking blue fluorescence and excellent optical properties, with a quantum yield as high as 61.44%. Due to the fluorescence quenching effect and the stronger complexing ability of the phosphoric acid group of 1-hydroxyethane-1,1-diphosphonic acid (HEDP) to Fe³⁺ , CQDs doped with Fe³⁺ were adequately constructed as an efficient and sensitive fluorescent probe for HEDP-specific sensing. The proposed fluorescent probe had a sensitive and rapid response in the range 5-70 μM. Furthermore, quantitative molecular surface (QMS) analysis based on the Multiwfn program was applied to explore the complexation mode of HEDP and metal ions. The distribution of electrostatic potential (ESP), average local ionization energy (ALIE), the minimum value points and the position of the lone pair electrons on the surface of molecular van der Waals were further determined. More strikingly, this experiment achieved the quantitative detection of water-soluble phosphonate-HEDP, for the first time using fluorescence spectrometry. It has been proved to be an effective and intuitive sensing method for the detection of HEDP in real samples.

Mg-O-Bridged Polypyrrole/g-C3 N4 Nanocomposites as Efficient Visible-Light Catalysts for Hydrogen Evolution

It is highly desired to improve the visible-light activity of g-C₃ N₄ for H₂ evolution by constructing closely contacted heterojunctions with conductive polymers. Herein, a polymer nanocomposite photocatalyst with high visible-light activity is fabricated successfully by coupling nanosized polypyrrole (NPPy) particles onto g-C₃ N₄ nanosheets through a simple wet-chemical process, and its visible-light activity is improved further by constructing Mg-O bridges between the NPPy and g-C₃ N₄ . The amount-optimized bridged nanocomposite displays an approximately ninefold improvement in visible-light activity compared with g-C₃ N₄ . On the basis of transient-state surface photovoltage responses, photoluminescence spectra, ^. OH amount evaluation, and photoelectrochemical curves, it is concluded that the exceptional photoactivity can be attributed to the significantly promoted charge transfer and separation along with visible photosensitization from NPPy. Interestingly, it is confirmed that the promoted charge separation depends mainly on the excited high-level electron transfer from g-C₃ N₄ to NPPy by single-wavelength photocurrent action spectra. This work provides a feasible strategy for designing polymer nano-heterojunction photocatalysts with exceptional visible-light activities.

Cellulose-Derived Hierarchical g-C3N4/TiO2-Nanotube Heterostructured Composites with Enhanced Visible-Light Photocatalytic Performance

A novel cellulose-derived hierarchical g-C₃N₄/TiO₂-nanotube heterostructured nanocomposite was fabricated by in situ coating thin g-C₃N₄ layers onto the surfaces of the TiO₂ nanotubes, which were synthesized by utilizing the natural cellulose substance (e.g., commercial ordinary filter paper) as the structural template. These g-C₃N₄/TiO₂-nanotube composites with varied thicknesses (ca. 3-30 nm) of the outer g-C₃N₄ layers displayed improved visible-light (λ > 420 nm)-driven photocatalytic degradation performances toward methylene blue. The optimal nanocomposite with an outer g-C₃N₄ layer of ca. 7.5 nm composed of 46 wt % g-C₃N₄ displayed an apparent rate constant of 0.0035 min^-1, which was 8.5- and 4-fold larger than those of the referential TiO₂-nanotube and g-C₃N₄ powder. The excellent and durable photocatalytic activities of these cellulose-derived g-C₃N₄/TiO₂-nanotube composites were ascribed to their hierarchically network porous structures replicated from the cellulose template, as well as the formation of close heterojunctions in-between the g-C₃N₄ and TiO₂ phases. Moreover, it was demonstrated that the photocatalytic mechanism matched with the type-II heterostructured model, while the main effective species during the photocatalytic processes of the nanocomposite were proved to be superoxide radicals.

Enhanced photoelectrochemical activity of α-Fe2O3/TiO2 nanoheterojunction by controlling hydrodynamic conditions

Interfacial heterostructuring has appeared to be an efficient strategy to address the efficiency and applicability of the photocatalysts in solar energy conversion. Herein, we developed one-dimensional (1D) α-Fe₂O₃/TiO₂ nanoheterojunction arrays for enhanced photoelectrochemical (PEC) activity. α-Fe₂O₃ nanotubes were firstly prepared via anodization under controlled hydrodynamic conditions to increase the efficiency. 1D α-Fe₂O₃/TiO₂ nanoheterojunction arrays were then prepared through a hydrothermal treatment and a subsequent annealing process. A controlled anodization by modulating the hydrodynamic conditions, added a fine coating of TiO₂ overlayer, to finally give an optimized composition and geometry for improved light absorption and spatial charge separation efficiency. Consequently, the optimized α-Fe₂O₃ generated a photocurrent of 0.07 mA cm^-2 (3.5 times higher than that of pristine α-Fe₂O₃), and the as-obtained α-Fe₂O₃/TiO₂ nanoheterojunction exhibited a photocurrent intensity of 0.12 mA cm^-2 (about 6 times higher than that of pristine α-Fe₂O₃). A long-term stability can also be ensured. The well-controlled architectures provides a guideline for synthesis of advanced nanomaterials.

TiO2 nanosheet/NiO nanorod hierarchical nanostructures: p-n heterojunctions towards efficient photocatalysis

TiO₂ nanosheet/NiO nanorod heterojunction hybrids have been developed through a hydrothermal route, where NiO nanorods (size: 5 nm in diameter and 20-40 nm in length) are deposited at the {0 0 1} facet of anatase TiO₂ nanosheets. The photocatalysts were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), N₂ adsorption-desorption analysis, UV-vis spectroscopy, X-ray photoelectron spectroscopy (XPS), photoluminescence spectroscopy and time-resolved fluorescence. The TiO₂/NiO photocatalysts exhibited good photocatalytic activities towards the degradation of methyl blue (MB) and phenol, and hydrogen generation efficiency under visible light irradiation. The maximum rate constant can be reached 0.0279 min^-1 and 0.0135 min^-1 respectively, which are about 12 and 10 times higher than that of TiO₂ nanosheets. And the hydrogen generation efficiency is 10 times higher than physical mixing of TiO₂ and NiO. Photocatalytic degradation efficiency remains more than 90% after 6 times cycle dye degradation, and the H₂ production efficiency is almost the same after four cycles, suggesting good stability and reusability. The enhanced photocatalytic activities are associated with the rational design of TiO₂/NiO hierarchical heterojunctions which ensues high photogenerated charge separation efficiency. With the improved photocatalytic performance, the TiO₂/NiO heterojunction hybrids are expected to be potential photocatalysts in environmental and energy related areas.

CO2-triggered reversible phase transfer of graphene quantum dots for visible light-promoted amine oxidation

Carbon dots, emerging as novel photoluminescent nanomaterials, have attracted increasing attention for photocatalytic applications such as hydrogen evolution, carbon dioxide reduction, pollutant degradation and organic synthesis. However, the separation of carbon dots from the reaction system is always a cumbersome process, which may limit their applications in photocatalysis. Herein, we report the synthesis of a graphene quantum dot with CO2-switchable reversible phase transfer performance via a facile surface functionalization approach. The mechanism of this hydrophilicity and hydrophobicity alteration involves the protonation-deprotonation transformation and reversible formation of hydrophilic bicarbonate salts when CO2 is bubbled and removed. Then, the obtained graphene quantum dot was utilized as a visible-light photocatalyst for the oxidative coupling of amines. Our photocatalyst demonstrates excellent catalytic efficiency with both high reaction conversion and selectivity. Furthermore, the proposed graphene quantum dot could be recycled via a simple CO2 bubbling method.

Bidirectional Photochromism via Anchoring of Carbon Dots to TiO2 Porous Films

Photochromic materials present photocontrollable properties, which is of great interest for potential applications including high-density storage and optical displays. Herein, we demonstrate a promising pathway toward smart photochromic nanocomposite exploration by anchoring of carbon dots (CDs) to titanium dioxide (TiO₂) porous films. This study reveals that the color of the CDs/TiO₂ film obtained by dropping anchoring becomes darker and that obtained by immersion anchoring becomes lighter, both under blue light irradiation. For the photobleaching material system, the spectral response is strongly dependent on wavelength and polarization of the exciting light, which provides new dimensions for optical information encryption and memory. This work lays the foundation for the materials platform in the integration of advanced information processing in the future.

Facet-dependent decoration of TiO2 mesocrystals on TiO2 microcrystals for enhanced photoactivity

Bottom-up constructions of hierarchical TiO₂ are effective to enhance their photoactivity towards degradations of organic pollutants. Thanks to highly active facets, {001} exposed anatase TiO₂ microcrystals attract much attention in photocatalysis; yet their efficiency is limited by the large crystal size. Herein, we report a facile solution approach to deposit anatase TiO₂ mesocrystals only on {101} facets of anatase TiO₂ microcrystals. The selective surface decoration enhances the photoactivity through replacing the less active {101} facets with more active TiO₂ mesocrystals; whilst the highly active {001} facets remain untouched. When utilized to assist photodegradation of phenol in water under UV light illumination, the hierarchical TiO₂ exhibited a reaction rate constant doubled that of the pristine {001} exposed TiO₂ microcrystals. The present tactic to selectively decorate TiO₂ microcrystals may give hints to other applications involving facet-dependent properties.

Photosensitive Hybrid Nanostructured Materials: The Big Challenges for Sunlight Capture

Solar radiation is becoming increasingly appreciated because of its influence on living matter and the feasibility of its application for a variety of purposes. It is an available and everlasting natural source of energy, rapidly gaining ground as a supplement and alternative to the nonrenewable energy feedstock. Actually, an increasing interest is involved in the development of efficient materials as the core of photocatalytic and photothermal processes, allowing solar energy harvesting and conversion for many technological applications, including hydrogen production, CO2 reduction, pollutants degradation, as well as organic syntheses. Particularly, photosensitive nanostructured hybrid materials synthesized coupling inorganic semiconductors with organic compounds, and polymers or carbon-based materials are attracting ever-growing research attention since their peculiar properties overcome several limitations of photocatalytic semiconductors through different approaches, including dye or charge transfer complex sensitization and heterostructures formation. The aim of this review was to describe the most promising recent advances in the field of hybrid nanostructured materials for sunlight capture and solar energy exploitation by photocatalytic processes. Beside diverse materials based on metal oxide semiconductors, emerging photoactive systems, such as metal-organic frameworks (MOFs) and hybrid perovskites, were discussed. Finally, future research opportunities and challenges associated with the design and development of highly efficient and cost-effective photosensitive nanomaterials for technological claims were outlined.

Improved CdS photocatalytic H2 evolution using Au–Ag nanoparticles with tunable plasmon-enhanced resonance energy transfer

Au–Ag hollow nanoparticles (HNPs) with tunable plasmon absorption peaks were mixed with CdS to achieve stepwise spectral overlap to enhance energy transfer and photocatalytic hydrogen evolution.

An overview of photocatalytic degradation: photocatalysts, mechanisms, and development of photocatalytic membrane

Photocatalysis is an ecofriendly technique that emerged as a promising alternative for the degradation of many organic pollutants. The weaknesses of the present photocatalytic system which limit their industrial applications include low-usage of visible light, fast charge recombination, and low migration ability of the photo-generated electrons and holes. Therefore, various elements such as noble metals and transition metals as well as non-metals and metalloids (i.e., graphene, carbon nanotube, and carbon quantum dots) are doped into the photocatalyst as co-catalysts to enhance the photodegradation performance. The incorporation of the co-catalyst which alters the photocatalytic mechanism was discussed in detail. The application of photocatalysts in treating persistent organic pollutants such as pesticide, pharmaceutical compounds, oil and grease and textile in real wastewater was also discussed. Besides, a few photocatalytic reactors in pilot scale had been designed for the effort of commercializing the system. In addition, hybrid photocatalytic system integrating with membrane filtration together with their membrane fabrication methods had also been reviewed. This review outlined various types of heterogeneous photocatalysts, mechanism, synthesis methods of biomass supported photocatalyst, photocatalytic degradation of organic substances in real wastewater, and photocatalytic reactor designs and their operating parameters as well as the latest development of photocatalyst incorporated membrane.