Efficient photocatalysis triggered by thin carbon layers coating on photocatalysts: recent progress and future perspectives

TiO2 nanotube arrays-based photoelectrocatalyst: Tri-Doping engineering and carbon coating engineering boosting visible activity, and stable hydrogen evolution

The integration of non-metallic doping and carbon coating for TiO2-based photoelectrocatalysts can be recognized as a promising strategy to enhance their hydrogen production performance. To this end, this study explored the carbon coating engineering to induce stable multi-element doping with an aim to develop high-performance TiO2 nanotube array-based photoelectrocatalysts. The resulting structures consisted of carbon-nitrogen-sulfur-tri-doped TiO2 nanotube arrays with a nitrogen-sulfur-codoped carbon coating (CNS-TNTA/NSC). The fabrication process involved a one-step, low-cost strategy of the carbon-coated tridoped reaction confined in vacuum space, utilizing polymer thiourea sealed in a controlled environment. Compared the photocurrent density of CNS-TNTA/NSC with pristine TNTA, the photocurrent enhancement of approximately 18.3-fold under simulated sunlight and a remarkable increase of 32.8-fold under simulated visible light conditions. The enhanced photocatalytic activity under visible light was ascribed to two factors: First, C, N, and S tri-doping and Ti3+ created a diverse array of impurity energy levels within the band gap, which synergistically narrowed the band gap and further enhanced response to the visible light range. Second, the presence of a carbon coating shell doped with N and S can greatly promote electron transfer and efficient electron-hole pair separation. This study could provide significant insights concerning the design of sophisticated photoanodes.

Electronic Interactions on Platinum/(Metal-Oxide)-Based Photocatalysts Boost Selective Photoreduction of CO2 to CH4

By supporting platinum (Pt) and cadmium sulfide (CdS) nanoparticles on indium oxide (In2 O3 ), we fabricated a CdS/Pt/In2 O3 photocatalyst. Selective photoreduction of carbon dioxide (CO2 ) to methane (CH4 ) was achieved on CdS/Pt/In2 O3 with electronic Pt-In2 O3 interactions, with CH4 selectivity reaching to 100 %, which is higher than that on CdS/Pt/In2 O3 without electronic Pt-In2 O3 interactions (71.7 %). Moreover, the enhancement effect of electronic Pt-(metal-oxide) interactions on selective photoreduction of CO2 to CH4 also occurs by using other common metal oxides, such as photocatalyst supports, including titanium oxide, gallium oxide, zinc oxide, and tungsten oxide. The electronic Pt-(metal-oxide) interactions separate photogenerated electron-hole pairs and convert CO2 into CO2 δ- , which can be easily hydrogenated into CH4 via a CO2 δ- →HCOO*→HCO*→CH*→CH4 path, thus boosting selective photoreduction of CO2 to CH4 . This offers a new way to achieve selective photoreduction of CO2 .

Carbon coated In2O3 hollow tubes embedded with ultra-low content ZnO quantum dots as catalysts for CO2 hydrogenation to methanol

CO₂ hydrogenation coupled with renewable energy to produce methanol is of great interest. Carbon coated In₂O₃ hollow tube catalysts embedded with ultra-low content ZnO quantum dots (QDs) were synthesized for CO₂ hydrogenation to methanol. ZnO-In₂O₃-II catalyst had the highest CO₂ and H₂ adsorption capacity, which demonstrated the highest methanol formation rate. When CO₂ conversion was 8.9%, methanol selectivity still exceeded 86% at 3.0 MPa and 320 °C, and STY of methanol reached 0.98 g_MeOHh^-1g_cat^-1 at 350 °C. The ZnO/In₂O₃ QDs heterojunctions were formed at the interface between ZnO and In₂O₃(222). The ZnO/In₂O₃ heterojunctions, as a key structure to promote the CO₂ hydrogenation to methanol, not only enhanced the interaction between ZnO and In₂O₃ as well as CO₂ adsorption capacity, but also accelerated the electron transfer from In³⁺ to Zn²⁺. ZnO QDs boosted the dissociation and activation of H₂. The carbon layer coated on In₂O₃ surface played a role of hydrogen spillover medium, and the dissociated H atoms were transferred to the CO₂ adsorption sites on the In₂O₃ surface through the carbon layer, promoting the reaction of H atoms with CO₂ more effectively. In addition, the conductivity of carbon enhanced the electron transfer from In³⁺ to Zn²⁺. The combination of the ZnO/In₂O₃ QDs heterojunctions and carbon layer greatly improved the methanol generation activity.

Photocatalytic aerobic oxidative functionalization (PAOF) reaction of benzyl alcohols by GO-MIL-100(Fe) composite in glycerol/K2CO3 deep eutectic solvent

An MIL-100 (Fe)/graphene oxide (GO) hybrid, a fairly-known composite, was made through a simple one-step procedure and played a highlighted role in the photo-induced oxidative functionalization of the benzylic C–H bond. To identify the given binary composite, various techniques were applied: FT-IR, P-XRD, SEM, nitrogen absorption–desorption analysis, TGA, TEM, and UV–Visible DRS spectra. Proportions of GO used within the structure of the prepared composite differently ranged from low to high amount, and the most optimized ratio met at 38.5% of GO as the most efficient catalyst. Additionally, the reaction ran in Glycerol/K₂CO₃ (2:1) as the optimal solvent. The elemental roles of O₂^·− and OH⁻ were supposed to be the major ones for running a tandem oxidation-Knoevenagel reaction. The heterogeneity and reusability of the catalyst were also examined and confirmed after five successive runs.

Fast and efficient electrocatalytic oxidation of glucose triggered by Cu2O-CuO nanoparticles supported on carbon nanotubes

Electrocatalytic glucose oxidation reaction (GOR) is the key to construct sophisticated devices for fast and accurately detecting trace glucose in blood and food. Herein, a noble-metal-free Cu/C-60 catalyst is fabricated by supporting Cu₂O-CuO nanoparticles on carbon nanotubes through a novel discharge process. For GOR, Cu/C-60 shows a sensitivity as high as 532 μA mM⁻¹ cm⁻², a detection limit as low as 1 μM and a steady-state response time of only 5.5 s. Moreover, Cu/C-60 has outstanding stability and anti-interference ability to impurities. The synergistic effect of Cu₂O-CuO could improve the adsorption and conversion of glucose, thus enhancing GOR performance. By using Cu/C-60, we fabricate a three-electrode chip. A portable and compact electrochemical system is constructed by connecting the three-electrode chip with Cu/C-60 to an integrated circuit board and a mobile phone for recording and displaying data. The portable and compact electrochemical system results in a GOR sensitivity of 501 μA mM⁻¹ cm⁻², which is close to the data measured on the bloated electrochemical workstation. The detection limit of the portable and compact electrochemical system in GOR is 50 μM. This is higher than those obtained on the bloated electrochemical workstation, but is much lower than the common blood glucose concentration of human body (>3 mM). This demonstrates the accuracy, reasonability and applicability of the portable and compact electrochemical system. The results of the present work are helpful for fabricating fast, efficient and portable devices for detecting trace amount of glucose in blood and food.

Carbon-doped WO3 electrochemical aptasensor based on Box-Behnken strategy for highly-sensitive detection of tetracycline

Aptamer has been proved to be an important probe for antibiotic detection. Here, the electrical signal was doubly amplified by the synergistic effect of C-WO₃ and AuNPs. The probe structure has a specific recognition effect on tetracycline, which improves the selectivity and anti-interference of the sensor. With the assistance of BBD strategy, the experimental errors of the C-WO₃@AuNPs aptasensor were reduced and the best conditions for its preparation were obtained. This was conducive to obtain the best electrical signal transmission capacity of the electrode, greatly improved the sensor sensitivity. Under this mechanism, the antibiotic sensor achieved a low detection range (0.1 nM-100 nM) and a low detection limit (4.8 × 10^-2 nM). The sensor showed excellent selectivity even in the presence of coexisting pollutants. This work explored the mechanism of charge change and demonstrated the role of probes in antibiotic sensing, providing important prospects of future applications in electrochemical sensors.

Enhanced light-driven hydrogen generation on carbon quantum dots with TiO2 nanoparticles

Solar to hydrogen (H2) conversion systems based on carbon nanomaterials have shown great potentials in the clean energy field recently. However, for most systems, energy level alignments and light-induced redox processes are still unclear, which hinder artificial designing for higher efficiency of solar energy conversion and further applications. Here we report 77% enhancement in the light-driven H2 generation efficiency of N,S co-doped carbon quantum dot (N,S-CQD) aqueous system by adding TiO2 nanoparticles. Using steady-state and transient spectroscopy, four specific energy levels of CQDs are confirmed with the band gaps of 3.55 eV (X4), 2.99 eV (X3), 2.76 eV (X2) and 1.75 eV (X1), respectively. The X2 energy band is highly active for H+ reduction with a longer lifetime of 13.38 ns. Moreover, the observed low efficiency of intrinsic transition from X3 to X2 band of N,S-CQDs accounts for the poor performance of solar to H2 conversion for pure N,S-CQDs based on H2 generation and detailed time-resolved spectroscopic results. The mechanism of H2 generation enhancement can be explained by multiple electron transfer processes between N,S-CQDs and TiO2 NPs where TiO2 NPs act as electron intermediates that efficiently transfer electrons from the inert band (X3) to the active band (X2) for H2 generation. This study enriches the fundamental understanding of N,S-CQDs and provides a new pathway toward high-performance N,S-CQD-based solar to H2 conversion systems.

Carbon-Coated Tungsten Oxide Nanospheres Triggering Flexible Electron Transfer for Efficient Electrocatalytic Oxidation of Water and Glucose

Electrocatalytic oxidation of water (i.e., oxygen evolution reaction, OER) plays crucial roles in energy, environment, and biomedicine. It is a key factor affecting the efficiencies of electrocatalytic reactions conducted in aqueous solution, e.g., electrocatalytic water splitting and glucose oxidation reaction (GOR). However, electrocatalytic OER still suffers from problems like high overpotential, sluggish kinetics, and over-reliance on expensive noble-metal-based catalysts. Herein, 15 nm thick carbon-based shell coated tungsten oxide (CTO) nanospheres are loaded on nickel foam to form CTO/NF. An enhanced electrocatalytic OER is triggered on CTO/NF, with the overpotential at 50 mA cm^-2 (317 mV) and the Tafel slope (70 mV dec^-1) on CTO/NF lower than those on pure tungsten oxide (360 mV, 117 mV dec^-1) and noble-metal-based IrO₂ catalysts (328 mV, 96 mV dec^-1). A promoted electrocatalytic GOR is also achieved on CTO/NF, with efficiency as high as 189 μA mM^-1 cm^-2. The carbon-based shell on CTO is flexible for electron transfer between catalyst and reactants and provides catalytically active sites. This improves reactant adsorption and O-H bond dissociation on the catalyst, which are key steps in OER and GOR. The carbon-based shell on CTO retains the catalyst as nanospheres with a higher surface area, which facilitates OER and GOR. It is the multiple roles of the carbon-based shell that increases the electrocatalytic efficiency. These results are helpful for fabricating more efficient noble-metal-free electrocatalysts.