Anisotropic Heavy-Metal-Free Semiconductor Nanocrystals: Synthesis, Properties, and Applications

Heavy-metal (Cd, Hg, and Pb)-containing semiconductor nanocrystals (NCs) have been explored widely due to their unique optical and electrical properties. However, the toxicity risks of heavy metals can be a drawback of heavy-metal-containing NCs in some applications. Anisotropic heavy-metal-free semiconductor NCs are desirable replacements and can be realized following the establishment of anisotropic growth mechanisms. These anisotropic heavy-metal-free semiconductor NCs can possess lower toxicity risks, while still exhibiting unique optical and electrical properties originating from both the morphological and compositional anisotropy. As a result, they are promising light-emitting materials in use various applications. In this review, we provide an overview on the syntheses, properties, and applications of anisotropic heavy-metal-free semiconductor NCs. In the first section, we discuss hazards of heavy metals and introduce the typical heavy-metal-containing and heavy-metal-free NCs. In the next section, we discuss anisotropic growth mechanisms, including solution-liquid-solid (SLS), oriented attachment, ripening, templated-assisted growth, and others. We discuss mechanisms leading both to morphological anisotropy and to compositional anisotropy. Examples of morphological anisotropy include growth of nanorods (NRs)/nanowires (NWs), nanotubes, nanoplatelets (NPLs)/nanosheets, nanocubes, and branched structures. Examples of compositional anisotropy, including heterostructures and core/shell structures, are summarized. Third, we provide insights into the properties of anisotropic heavy-metal-free NCs including optical polarization, fast electron transfer, localized surface plasmon resonances (LSPR), and so on, which originate from the NCs' anisotropic morphologies and compositions. Finally, we summarize some applications of anisotropic heavy-metal-free NCs including catalysis, solar cells, photodetectors, lighting-emitting diodes (LEDs), and biological applications. Despite the huge progress on the syntheses and applications of anisotropic heavy-metal-free NCs, some issues still exist in the novel anisotropic heavy-metal-free NCs and the corresponding energy conversion applications. Therefore, we also discuss the challenges of this field and provide possible solutions to tackle these challenges in the future.

Crystal Facet Dependent Energy Band Structures of Polyhedral Cu2O Nanocrystals and Their Application in Solar Fuel Production

We demonstrated a facile hydrothermal method to synthesize the (100)-, (110)- and (111)-oriented Cu₂O nanocrystals (NCs) by controlling the concentration of the incorporated anions (CO₃^2- and SO₃^2-). The crystal facet dependent activity of the orientation controlled Cu₂O NCs in the rhodamine B (RhB) photodegradation and photocatalytic hydrogen (H₂) evolution was found to follow the trend: (111) > (110) > (100). The mechanism was investigated by characterizing the optical property, energy band structure, interfacial charge carrier dynamics and reducing ability. The results indicated that the (111)-oriented Cu₂O NCs exhibit the higher conduction band (CB) potential as compared with the (110)-oriented and (100)-oriented Cu₂O NCs, which resulted in the largest driving force of interfacial electron transfer for (111)-oriented Cu₂O NCs to carry out solar fuel generation. The current study offers an easy strategy for crystal facet engineering of semiconductors and provides important physical insights into their electronic properties for the desired solar energy conversions.

Room Temperature Engineering Crystal Facet of Cu2O for Photocatalytic Degradation of Methyl Orange

Cuprous oxide (Cu₂O) has received enormous interest for photocatalysis owing to its narrow band gap of 2.17 eV, which is beneficial for visible-light absorption. In this work, we succeeded in synthesizing Cu₂O nanocrystals with two morphologies, cube and sphere, at room temperature via a simple wet-chemistry strategy. The morphologies of Cu₂O change from cube to sphere when adding PVP from 0 g to 4 g and the mainly exposed crystal faces of cubic and spherical Cu₂O are (100) and (111), respectively. The photocatalytic properties of the as-prepared Cu₂O were evaluated by the photocatalytic degradation of methyl orange (MO). Cubic Cu₂O(100) showed excellent photocatalytic activity. After the optical and photoelectric properties were investigated, we found that cubic Cu₂O(100) has better photoelectric separation efficiency than spherical Cu₂O(111). Finally, the possible mechanism was proposed for cubic Cu₂O(100) degrading MO under visible light.

A carbon nanowire-promoted Cu2O/TiO2 nanocomposite for enhanced photoelectrochemical performance

We started with earth abundant materials to design a heterojunction nanocomposite with excellent visible light photoelectrochemical performance and enhanced durability.

Multi-tailoring of a modified MOF-derived CuxO electrochemical transducer for enhanced hydrogen peroxide sensing

Reasonable control of the redox states within the catalytic units together with the interconnection degrees of the substrate is of great significance in the modulation of a well-performing transducer. Herein, a novel carbon black (CB)-modified copper metal-organic framework nanomaterial (CB@Cu-MOF) prepared at room temperature was utilized as a precursor to synthesize mixed-valent copper-oxide composite catalysts (NC/Cu_xO-T). By tuning the carbonization process of the precursor at different temperatures (T = 100 °C, 200 °C, 300 °C and 400 °C), the different ratio configurations of the redox-alternated Cu_xO portions were successfully controlled with the simultaneous effective tailoring of the defect abundance in the N-doped carbon substrate. As a result, an optimized NC/Cu_xO-300 electrochemical H₂O₂ sensor was able to present a low detection limit (0.26 μM) and decent linear ranges (0.02-1.79 mM and 2.29-9.29 mM). Our strategy using easily available initial materials with mild preparation conditions is expected to promote the practical application of the star materials in laboratories.

Glycine–Nitrate Combustion Synthesis of Cu-Based Nanoparticles for NP9EO Degradation Applications

Copper-based nanoparticles were synthesized using the glycine–nitrate process (GNP) by using copper nitrate trihydrate [Cu(NO3)2·3H2O] as the main starting material, and glycine [C2H5NO2] as the complexing and incendiary agent. The as-prepared powders were characterized through X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy analysis. Using Cu(NO3)2·3H2O as the oxidizer (N) and glycine as fuel (G), we obtained CuO, mixed-valence copper oxides (CuO + Cu2O, G/N = 0.3–0.5), and metallic Cu (G/N = 0.7). The XRD and BET results indicated that increasing the glycine concentration (G/N = 0.7) and reducing the particle surface area increased the yield of metallic Cu. The effects of varying reaction parameters, such as catalyst activity, catalyst dosage, and H2O2 concentration on nonylphenol-9-polyethoxylate (NP9EO) degradation, were assessed. With a copper-based catalyst in a heterogeneous system, the NP9EO and total organic carbon removal efficiencies were 83.1% and 70.6%, respectively, under optimum operating conditions (pH, 6.0; catalyst dosage, 0.3 g/L; H2O2 concentration, 0.05 mM). The results suggest that the removal efficiency increased with an increase in H2O2 concentration but decreased when the H2O2 concentration exceeded 0.05 mM. Furthermore, the trend of photocatalytic activity was as follows: G/N = 0.5 > G/N = 0.7 > G/N = 0.3. The G/N = 0.5 catalysts showed the highest photocatalytic activity and resulted in 94.6% NP9EO degradation in 600 min.

Importance of ZnTiO3 Phase in ZnTi-Mixed Metal Oxide Photocatalysts Derived from Layered Double Hydroxide

In this study, ZnTi-mixed metal oxides (ZTM), such as ZnTiO₃, were synthesized from ZnTi layered double hydroxides by varying the molar ratio of Zn/Ti, calcination temperatures, and synthesis methods (hydrothermal or reflux). The surface electronic characteristics of ZTM were investigated by the energy-resolved distribution of electron traps (ERDTs) using reversed double-beam photoacoustic spectroscopy. The ZTM samples obtained by conducting hydrothermal synthesis at 500 °C showed similar ERDT patterns independent of the molar ratio of Zn/Ti, although ZnTiO₃ phase was not observed in the X-ray diffraction pattern, when the Zn/Ti ratio was high. When the ERDT patterns demonstrated a high electron accumulation level near the conduction band bottom in hydrothermal products at 500 °C, a higher photocatalytic phenol degradation efficiency was observed due to the formation of ZnTiO₃ phase. This suggested that the product with the high Zn/Ti molar ratio (Zn/Ti = 6) constituted amorphous ZnTiO_3.The enhanced photocatalytic performance of ZTM could be attributed to the heterojunction of electrons among ZnO, TiO₂, and ZnTiO₃, which enabled electron transfer in the composites, prevented charge recombination, and promoted a wider visible light adsorption by ZnTiO₃ phase irrespective of its crystallinity.

Improved charge separation through H2O2 assisted copper tungstate for enhanced photocatalytic efficiency for the degradation of organic dyes under simulated sun light

In the recent years, copper tungstate (CuWO₄) has been widely researched for its photocatalytic properties as it responds in the visible light range to augment the utilization of solar energy. In the present report, CuWO₄ was synthesized through a facile and cost-effective solvothermal method, followed by annealing process at 700^∘C. The structural, morphological, compositional and optical property of the synthesized powders were examined by X-ray diffraction, scanning electron microscope, UV-visible, Raman and photoluminescence studies. The photocatalytic activity of the nanostructured CuWO₄ was evaluated by the degradation of methylene blue (MB) and methyl orange (MO) in aqueous solution under one sun light irradiation. The degradation efficiency of MB was found to be about 70% while that of MO was only 57% at 240 min in the same irradiation time. Surprisingly, the degradation process was accelerated by the addition of electron capturing agent H₂O₂ and thus MB dye was completely degraded within the time interval of 30 min while MO degraded in 75 min. These results prove that CuWO₄ nanoparticles possess significant photocatalytic activity towards MO and MB dyes, thus indicating the feasibility of using CuWO₄ for the active treatment of organic contaminants in the industrial effluents.

A new synthesis methodology for SiO2 gel-based nanostructures and their application for elimination of dye pollutants

A new procedure for preparation of a high specific surface area silica-based nanostructure and its copper-containing active photocatalyst is described.

Controllable preparation of CuO/Cu2O composite particles with enhanced photocatalytic performance

Spherical CuO/Cu₂O nanocomposites synthesized in PEG-400 have excellent catalytic activity because of transferring photogenerated electrons from Cu₂O to CuO.

Highly efficient visible-light photoactivity of Z-scheme MoS2/Ag2CO3 photocatalysts for organic pollutants degradation and bacterial inactivation

Here, a novel Z-scheme MoS₂/Ag₂CO₃ heterojunction photocatalyst was assembled from two-dimensional MoS₂ nanosheets and Ag₂CO₃ nanoparticles through facile hydrothermal and in-situ precipitation method. The MoS₂/Ag₂CO₃ heterojunction exhibited much enhanced visible-light photocatalytic performance in probe experiment for organic pollutants degradation and Escherichia coli (E. coli) inactivation compared to pristine Ag₂CO₃ and MoS₂. The degradation rates of Lanasol Red 5B, rhodamine B, ciprofloxacin, and metronidazole reached 95%, 90%, 80%, and 72%, respectively. On the other hand, E. coli was completely inactivated in 80 min in the presence of 5%-MoS₂/Ag₂CO₃. The improved photocatalytic performance was ascribed to the enhanced photogenerated charge separation efficiency and increased lifetime of the charge carriers, proved by photoluminescence spectra, time-resolved fluorescence emission decay spectra, and electrochemical measures. In addition, the active species trapping and ESR experiments all indicated that holes (h⁺) exhibited a significant contribution and superoxide radicals (O₂^-) acted as assistants. Based on experiment results, the photocatalytic enhancement mechanism for organic pollutants degradation and E. coli inactivation was discussed. The effect of representative environmental factors on the degradation of Lanasol Red 5B was investigated. The experiment results indicated that the degradation efficiency was partially influenced in the presence of inorganic salt. Furthermore, the appearance of a small amount of Ag nanoparticles not only enhanced the charge transfer, but also improved the stability of photocatalyst. Overall, MoS₂/Ag₂CO₃ heterojunction has a great application potential for future water purification.

Green Synthesis of a Cu/SiO2 Catalyst for Efficient H2-SCR of NO

In this work, the synthesis of Cu/SiO2 catalysts starting from pre-formed copper nanoparticle (CuNP) colloidal suspensions was carried out. Two different protocols for the CuNP synthesis were tested: (i) a green approach using water as solvent and ascorbic acid as reducer and stabilizing agent, and (ii) a second solvothermal method involving the use of diethylene glycol as solvent, sodium hypophosphite (NaH2PO2) as reducer, and polyvinylpyrrolidone (PVP) and cetyltrimethylammonium bromide (CTAB) as stabilizing agents. In addition, and for the sake of comparison, a third catalyst was prepared by solid state conventional grinding of CuO with SiO2. The catalysts were tested in the environmentally relevant catalytic reduction of NOX with H2, in a temperature range from 300 to 500 °C. The catalysts were characterized by X-ray diffraction (XRD), temperature programmed reduction (TPR) cycles, Raman spectroscopy, and N2 adsorption for specific surface BET measurements. From these techniques CuO and Cu(0) species were detected depending on the synthesis protocol. CuNP size and size distribution in the colloid suspensions were determined by transmission electronic microscopy (TEM). The catalyst prepared from the aqueous suspension (CuAsc/SiO2) exhibited higher NO conversion (100%) and selectivity (85%) toward N2 at the lower reaction evaluated temperature (300 °C). The CuCTAB/SiO2 catalyst obtained by the solvothermal approach showed activity at high reaction temperature (400 °C) preferentially. The metal–support mechanical mixture exhibited a negligible response at low temperature and low conversion (68%) and selectivity (88%) at 500 °C. Nanoparticle size and distribution on the support, together with the metal–support interaction, were postulated as the most plausible parameters governing the catalytic performance of the different Cu/SiO2 materials.

Solar light active silver/iron oxide/zinc oxide heterostructure for photodegradation of ciprofloxacin, transformation products and antibacterial activity

This paper reports on the multitasking potential of a silver/iron oxide/zinc oxide (Ag/Fe₂O₃/ZnO) heterostructure, which was used for the photocatalytic decomposition of ciprofloxacin (CPX) and bacterial disinfection. The Ag/Fe₂O₃/ZnO heterostructure was successfully prepared using a facile precipitation method, and characterization results showed interesting structural, morphological, compositional and luminescent properties. The morphological results of the prepared heterostructure confirmed the deposition of Ag nanoparticles onto the surface of ZnO nanoplates and Fe₂O₃ nanorods. Treatment studies showed that the Ag/Fe₂O₃/ZnO heterostructure had superior solar light driven photocatalytic activity towards CPX degradation (76.4%) compared to bare Fe₂O₃ nanorods (43.2%) and ZnO nanoplates (63.1%), Ag/Fe₂O₃ (28.2%) and Ag/ZnO (64.5%) under optimized conditions (initial CPX concentration: 10 mg/L; pH 4; catalyst loading: 0.3 g/L). Reactive species study confirmed the roles of e^-, h⁺, OH and O₂^- in the photocatalytic degradation process. This photocatalytic behaviour of the Ag/Fe₂O₃/ZnO heterostructure could be attributed to the improved full solar spectrum harvesting capacity, separation of charge carriers and migration of e^-/h⁺ across the heterostructure interface. In addition, the Ag/Fe₂O₃/ZnO heterostructure also showed good antibacterial activity against Escherichia coli (E. coli) under both dark and visible light conditions. This might be due to generation of reactive oxygen species during the reaction. To the best of our knowledge, this is the first study till date on the utilization of Ag/Fe₂O₃/ZnO heterostructure for the photocatalytic degradation of CPX and E. coli bacteria disinfection. Therefore, this work offers an attractive path to design ZnO-based ternary heterostructures for solar-driven applications in wastewater remediation.