Oxygen Vacancy-Mediated Z-Scheme Charge Transfer in a 2D/1D B-Doped g-C₃N₄/rGO/TiO₂ Heterojunction Visible Light-Driven Photocatalyst for Simultaneous/Efficient Oxygen Reduction Reaction and Alcohol Oxidation

Abstract

Hydrogen peroxide (H2O2) is a powerful oxidant that directly or indirectly oxidizes many organic and inorganic contaminants. The photocatalytic generation of H2O2 is achieved by using a semiconductor photocatalyst in the presence of alcohol as a proton source. Herein, we have synthesized oxygen vacancy (Ov)-mediated TiO2/B-doped g-C3N4/rGO (TBCN@rGO) ternary heterostructures by a simple hydrothermal technique. Several characterization techniques were employed to explore the existence of oxygen vacancies in the crystal structure and investigate their impact on the optoelectronic properties of the catalyst. Oxygen vacancies offered additional sites for adsorbing molecular oxygen, activating alcohols, and facilitating electron migration from TBCN@rGO to the surface-adsorbed O2. The defect creation (oxygen vacancy) and Z-scheme mechanistic pathways create a suitable platform for generating H2O2 by two-electron reduction processes. The optimized catalyst showed the highest photocatalytic H2O2 evolution rate of 172 μmol/h, which is 1.9 and 2.5 times greater than that of TBCN and BCN, respectively. The photocatalytic oxidation of various lignocellulose-derived alcohols (such as furfural alcohol and vanillyl alcohol) and benzyl alcohol was also achieved. Photocatalytic activity data, physicochemical and optoelectronic features, and trapping experiments were conducted to elucidate the structure–activity relationships. The TBCN@rGO acts as a multifunctional Z-scheme photocatalyst having an oxygen vacancy, modulates surface acidity–basicity required for the adsorption and activation of the reactant molecules, and displays excellent photocatalytic performance due to the formation of a large number of active surface sites, increased electrical conductivity, improved charge transfer properties, outstanding photostability, and reusability. The present study establishes a unique strategy for improving H2O2 generation and alcohol oxidation activity and also provides insights into the significance of a surface vacancy in the semiconductor photocatalyst.

0D–2D Fe2O3/Boron-Doped g-C3N4 S-Scheme Exciton Engineering for Photocatalytic H2O2 Production and Photo-Fenton Recalcitrant-Pollutant Detoxification: Kinetics, Influencing Factors, and Mechanism

“…Additionally, the XPS band positioned at 717.3 eV represents the satellite peak, while the peak at 724.5 eV is ascribed to the Fe 3+ satellite peak of Fe 2 O 3 . These outcomes clearly confirm that Fe is present in both Fe 2+ and Fe 3+ valence states in the Fe 2 O 3 sample, which may be due to conversion of a small portion of Fe 3+ to Fe 2+ species during the synthesis procedure. , Furthermore, the XPS of O 1s shown in Figure b can be deconvoluted into two peaks at 529.2 and 530.9 eV attributed to Fe–O–Fe and −OH bonding, respectively. , The core XPS spectra of N 1s show peaks at 398.6 eV assigned to the sp 2 -hybridized pyridinic-bridged nitrogen atom, which has been bonded to a carbon atom (CN–C), while the peak centered at 399.8 eV represents the tertiary N–(C) 3 group in aromatic rings and that seen at 406.1 eV corresponds to the π–π* satellite peak (Figure c). − Moreover, the XPS spectrum of C 1s comprises two distinct peaks with binding energies of 284.9 and 288.5 eV ascribed to intended (C–C/CC) carbon species and sp 2 hybridization with N–CN coordination, respectively (Figure d). , In addition, the binding energy (BE) of B 1s is located at 191.8 eV, which can be attributed to B–N coordination and further suggests that the boron atom is surrounded by three nitrogen atoms as shown in the inset image in Figure e. From B–N coordination, it can be concluded that boron atoms have substituted carbon atoms in the aromatic ring (Figure e).…”

“…For the energy sector, H 2 O 2 is a promising and green alternative to H 2 fuel due to its high energy density (1.72 MJ/kg in 50 wt % H 2 O 2 ), water solubility, facile storage, safe conveyance, and having H 2 O as a byproduct. − However, the traditional anthraquinone method addresses nearly 95% of the global H 2 O 2 demand, which operates under elevated temperatures, produces intensive toxic byproducts, and uses organic solvents. , Moreover, direct H 2 O 2 preparation using H 2 , O 2 , and Pd or Pd/Au alloy is also explored extensively but the use of expensive noble metals and explosion characteristics of involved gases require extreme attention. , Hence, development of a green and budget-friendly preparation technique is of utmost necessity. As a sustainable and clean alternative, semiconductor-based artificial photocatalytic H 2 O 2 production employing renewable feedstock via (i) a direct two-electron single-step reaction, (ii) indirect one-electron two-step reaction, or (iii) a combination of two •OH species has received immense interest from the global research community. − Over the years, diverse catalytic systems have been experimented on toward improved H 2 O 2 production, including metal sulfide/oxide/phosphide, MOFs, carbonaceous species, and polymeric organic materials; however, low visible photon absorption ability, faster carrier recombination, limited charge-transfer efficiency, and poor O 2 adsorption-activation capacity reduce the overall quantum efficiency. , Out of the abundant semiconductor photocatalysts tested so far, two-dimensional (2D) metal-free graphitic carbon nitride (g-C 3 N 4 ) has attracted tremendous attention toward photocatalytic H 2 O 2 generation and pollutant degradation due to its superb intrinsic characteristics. ,− Moreover, the conduction and valence band position of g-C 3 N 4 encourages O 2 reduction and restricts oxidative decomposition of H 2 O 2 compared to oxide-based systems. For the first time ever, Shiraishi et al reported the photocatalytic generation of H 2 O 2 from a g-C 3 N 4 framework in a water/alcohol medium, but in the absence of a sacrificial agent, the activity is too low because of faster charge carrier recombination and low adsorption of O 2 on the catalyst surface .…”

“…Moreover, research minds across the planet applied various tailoring techniques for reducing the carrier recombination rate, broadening the light absorption window, and inducing defect states on g-C 3 N 4 , which involves non-metal doping (like boron) for achieving excellent reduction competencies of the aforesaid material. The insertion of boron results in the enhancement of photon absorption and transformation of band potentials for a specific redox reaction. , The boron atom is easily incorporated into g-C 3 N 4 lattices owing to its electron-deficient properties. However, the photo-activity of single boron-doped g-C 3 N 4 (BCN) is not satisfactory because of certain associated drawbacks that restrict its catalytic efficacy.…”

“…Hence, modification of single bulk g-C 3 N 4 is of first priority, and in this regard, developing a heterojunction with a suitable material is an ideal option for boosting the performance of photocatalysts. Consequently, a number of materials favoring double charge and a Z-scheme band arrangement have been adopted to fabricate BCN-based heterojunctions . Even though e – /h + are well-separated in these mentioned conventional systems, they show certain downsides from a kinetics point of view .…”

“…In the search toward finding a suitable candidate for designing robust g-C 3 N 4 -based S-scheme heterojunctions, Fe 2 O 3 quantum dots (FQDs) are found to be an ideal candidate because of their fascinating intrinsic characteristics . Adding more to the text, several g-C 3 N 4 -based heterojunctions have been reported toward enhanced H 2 O 2 production by photocatalytic O 2 reduction under ambient conditions. , Further, being a magnetic material with a suitable Fe­(II)–Fe­(III) oxidation state shuttling ability, FQDs can be utilized to carry out the photo-Fenton reaction, which is part of advanced oxidation processes (AOPs) toward pollutant degradation. In the photo-Fenton reaction, the production of hydroxyl radicals (•OH) occurs via a redox reaction, taking place between Fe 2+ and H 2 O 2 under visible light, which will further participate in AMX degradation .…”