ZnO Nanosheets Decorated with Graphite-Like Carbon Nitride Quantum Dots as Photoanodes in Photoelectrochemical Water Splitting

Abstract

The efficient utilization of solar power is becoming an important strategy for its conversion into a storable, clean, and renewable energy source like H 2 . To generate H 2 as a chemical fuel from solar power, attempts are being made to establish photoelectrochemical (PEC) water splitting as an efficient, greener pathway. Here, the surfaces of ZnO 2D nanosheets are adorned by graphite-like carbon nitride (g-C 3 N 4 ) quantum dots (QDs) with the intention of developing efficient photoanodes. Sensitization of ZnO nanosheets with C 3 N 4 QDs leads to a more enhanced PEC performance than that of bare ZnO. The observed enhancement in PEC is due to the high light absorbance and photon-generated charge-carrier separation. The best-obtained ZnO/C 3 N 4 photoanode exhibits a nearly 2.29 times as high photocurrent density compared to bare ZnO. ZnO 2D sheets can generate a photocurrent density of 0.414 mA cm −2 at 0.5994 V versus reversible hydrogen electrode (RHE), whereas ZnO/C 3 N 4 can produce 0.952 mA cm −2 at 0.5994 V versus RHE under uninterrupted conditions of light illumination. Further, there is improvement in the observed PEC activity of the heterostructure because of enhancement in the carrier density. The carrier density enhances nearly 2.2 times in the heterostructure compared to the bare ZnO sheet. ZnO/C 3 N 4 shows a maximum photoconversion efficiency (η) of 0.70%. Both ZnO 2D sheets and the ZnO/C 3 N 4 heterostructure show efficient stability under chopped irradiation of light for 1000 s. The stability of ZnO/C 3 N 4 is also determined for 1 h under continuous illumination.

Custom Synthesis of ZnO Nanowires for Efficient Ambient Air-Processed Solar Cells

“…ZnO was chosen as the carrier material of the nanostructure. It is widely used as a functional material in several optoelectronic applications such as light-emitting diodes (LEDs), dye PV cells, − sensors, , thermoelectric devices, transistors, photoelectrodes, , and photo-assisted batteries because it presents a unique combination of functional properties, structural properties, chemical stability, and tunable micro/nanostructure. , In terms of functional properties, semiconducting ZnO has an optical gap of 3.37 eV at 300 K, a resistivity of 10 –4 Ω Cm, and an electronic mobility between 10 and 60 cm 2 s –1 V –1 depending on the microstructure . At the atomic scale, ZnO can crystallize in three different forms: cubic (rocksalt), face-centered cubic (blende), and compact hexagonal (wurtzite).…”

A Comprehensive Review of Graphitic Carbon Nitride (g-C3N4)–Metal Oxide-Based Nanocomposites: Potential for Photocatalysis and Sensing

“…In another work, the effect of boron addition and carbon nitride content did increase the H 2 evolution up to 85% compared to the bare TiO 2 , which is mainly due to charge carriers' generation and separation [109]. It was also shown that the photoconversion efficiency of the low charge-transfer resistance of ZnO decorated with g-C 3 N 4 is 2.3 times higher than that of pure ZnO [406]. As discussed previously, this composite had a high specific surface area, promising electronic conductivity, and excellent charge transfer interfaces and would be excellent candidates for the water splitting and H 2 evolution [407].…”

“…Other metal oxide-g-C 3 N 4 based composites such as WO 3 -g-C 3 N 4 and ZnO-g-C 3 N 4 are used for H 2 generation. Mahala et al demonstrated that the prepared ZnO nanosheets decorated with g-C 3 N 4 quantum dots composites on the fluorine-doped tin oxide (FTO) coated glass slide could be utilized as a photoanode for water splitting via PEC (Figure 17b) [406]. In another work, the effect of boron addition and carbon nitride content did increase the H 2 evolution up to 85% compared to the bare TiO 2 , which is mainly due to charge carriers' generation and separation [109].…”

Optimization and Modulation Strategies of Zinc Oxide-based Photoanodes for Highly Efficient Photoelectrochemical Water Splitting

“…It was reported that various ZnO nanostructures (such as wire, rod, belt, wafer, spheres, etc. ) can be prepared by adding surfactants or polymers in a precursor base on two different strategies: acting as templates or structure directing agents and controlled crystallization (including adjusting amount of nucleation sites, controlling growth rates, and adjusting the pH value of precursor solution). , The crystallization process of ZnO in aqueous solution is strongly reliant on the pH of the solution; in other words, the pH of the precursor solution is a key factor for modulating the ZnO surface structure . Moreover, there is modulation of the growth velocity of specific ZnO facets by passivating the other facets in consideration of its positive polar plane rich in Zn cations and negative polar plane rich in O anions. − …”