Review on hydrogen production photocatalytically using carbon quantum dots: Future fuel

Highlights

• CQDs/GQDs-based water splitting studies from 2015 to march 2021.

• Methods of synthesis of CQDs with their merits and demerits.

• Focus on optical features of CQDs.

• Critical review on CQD and GQD for H₂ gas production (future fuel).

• Research gaps and commercial-scale development possibilities are also enumerated.

Abstract

They are sometimes identified as zero-dimensional (0D) nanoparticles. These particles have gained much attention in water splitting into hydrogen and oxygen through photocatalytic conversion. CQDs act as semiconductor few nm sizes, due to very small size; their optical and electronic properties differ from larger particles. CQDs particle has high stability, mild toxicity besides conductivity. These particles are environmentally friendly due to low toxicity and also have excellent luminescence. Therefore they can be utilized as a potential source for the splitting of water photocatalytically. The parting of water into H₂ and O₂ will enable us to produce or collect hydrogen to be used as a future fuel. The review summarizes the efforts made by various researchers in the field of utilizing carbon quantum dabs for water splitting which may be further followed by future researchers for commercial-scale hydrogen production. Thus, the study concludes the methods for the production of CQDs and their utilization under sunlight by catalytically hydrogen gas production from water.

Introduction

Carbon quantum dots (CQDs) or carbon dots (CDs) [1], are small fluorescent carbon nanoparticles having a diameter below 10 nm [[2], [3], [4]]. They have attracted considerable interest over the decades since possessing excellent conductance higher chemical strength along with luminescence, and broadband optical absorption [5,6]. Graphene quantum dots (GQDs) [7], are a subgroup of CQDs having chemical and physical characters as graphene [8]. GQDs are referred to as tiny graphene sheets having cross-sections lower to 10 nm usually derived from graphene oxide [[9], [10], [11], [12]]. The supremacy of edge theory in graphene and quantum confinement effects of QDs creates the difference in properties of graphene and QDs [13,14]. CQDs and GQDs manifest superior photo-stability against decolorization, lesser malignant nature, and environmentally benign materials [[15], [16], [17]] when contrast with heavy toxic metal quantum dots such as CdSe, WO₃ [18], and CdS.

Accidental discovered CQDs by Xu et al., in 2004 while preparing carbon nanotubes [19], a great deal of effort has been devoted to exploring greener ways of formation of CDs and GQDs [20]. Size, morphology, and doping type or amount can alter the features of GQDs and CDs [21]. As their huge and tunable attributes namely photoluminescence (PL) property, biocompatibility, electrochemiluminescence, exceptional multi-photon excitation (up-conversion) property, they are broadly employed for numerous practices. These dots can be improved with bio-molecules since are chemically inert environmental benign hence used as efficient carriers for drug delivery [22], biological imaging [23], catalyst [24], sensing [25], and optronics applications [26] and energy also [27,28].

To date, excellent utilizations of CDs and GQDs have been reviewed by scientists [4,5,8,22,[29], [30], [31], [32]].

However, the rapidly expanding literature recently on the consumption of GQDs and CDs in future fuel [33,34] has yet to be reviewed. The review summarizes the greener approaches for the preparation of Carbon and graphene quantum dabs. Then we primarily focused on their typical applications in fuel production in the latest five years (2015 to March 2020) that utilized CQDs and GQDs as a key component in different techniques and principles. Future advances and perspectives for the applications of CQDs- and GQD-based nanomaterials are discussed. We hope this review article can help in guiding the future utilization of carbon for fuel production.

Three elementary necessities for human survival and sustainable development are water, environment, and energy [35]. Yet fast growth of industrialization and urbanization leads to hazardous effects on human welfare and the environment [36]. Numerous modern industrial signs of progress run on fossil energy including petroleum, coal, and fossil gas, thus causing air and water pollution as considered by Busaidi Baawain et al. [37]. Drastic increase in consumption of fossil fuels and finite availability of fossil energy may drive energy crises shortly. To solve the problem there is an urgent need for greener and sustainable sources and fabrication to create an alternative source of energy.

Nanotechnology has proved its importance in the domain of green and clean chemistry. It is an evolving field that is capable of generating smarter nanomaterials and utilized in the sustainable generation of energy. Renewable origin of energy such as solar energy drew the attention of many researchers and scientists since the most abundant, inexpensive, inexhaustible source of energy. Therefore the renovation of solar to fuel energy is one of the most encouraging techniques to resolve energy and environmental crises [[38], [39], [40]]. Mainly future fuel manufactured from water fissuring offers a potentially sustainable tactic to fulfill the future fuel needs without affecting the environment because of the usage of green and clean energy sources (Water, hydrogen, and solar energy) [[41], [42], [43], [44]].