## 1 Introduction
This further allows them to capture solar/visible light efficiently and smooth and long-term circulation of photogenerated electrons and holes over the surface, and ultimately increases the photocatalytic efficiency of materials [16; 17; 18]. To practically verify this subject and to increase the importance of the H\({}_{2}\) production method, this study was conducted, in which two bare semiconductors (CdWO\({}_{4}\) and ZrO\({}_{2}\)) with a large energy band gap were coupled to the developed heterojunction. Notably, monoclinic wolframite CdWO\({}_{4}\) is known for good photocatalytic performance due to its layered structure, self-trapped excitons, optical, chemical, and structural properties, and thermal stability. But modified (doped or coupled) CdWO\({}_{4}\) may have better photocatalytic performance than bare CdWO\({}_{4}\) due to a wider band gap in bare CdWO\({}_{4}\) (\(\sim\)3.5-4.0 eV) [16; 17; 18]. For instance, [PERSON] and others reported nanostructured CdS-sensitized CdWO\({}_{4}\) nanorods that exhibited highly efficient H\({}_{2}\) generation than bare CdWO\({}_{4}\)[20]. In addition to CdWO\({}_{4}\), ZrO\({}_{2}\), a n-type semiconductor, received much attention due to its substantial amount of oxygen vacancies, strong ion exchangeability, and physicochemical stability. However, it is a broad-band-gap semiconductor (5.5 eV) that requires UV-C light (\(<\)280 nm) for electron-hole generation [21]. One method to overcome this problem is by doping ZrO\({}_{2}\) with different transition metal ions or coupling with other metal oxides, creating composites, bringing about heterojunction formation, and incorporating co-catalysts. For instance, a binder-free WS\({}_{2}\)/ZrO\({}_{2}\) hybrid nanocomposite (NC)was reported as a photocatalyst for H\({}_{2}\) production via water splitting [22]. The coupling of ZrO\({}_{2}\) with other semiconductors like SiO\({}_{2}\), TiO\({}_{2}\), ZnO, WO\({}_{3}\), and NiO have also been reported [23; 24]. Considering the advantages of both these materials as bare photocatalysts, along with their composites, this study assessed the photocatalytic activity of the heterojunction material developed by coupling ZrO\({}_{2}\) and CdWO\({}_{4}\). Many methods like coprecipitation [25], hydrothermal [26], sol-gel [27], microwave [28], and bioprecipitation [16] have been used to develop heterojunction material. However, among all these methods, bioprecipitation is the cheapest, easiest, sustainable, and green [17; 21], hence the bioprecipitation method has been chosen as the synthesis method in the present study. In the bioprecipitation method, plant extracts were used, which act as reducing, capping, and stabilizing agents [16; 17; 18; 29; 30; 31]. The plant extracts are usually a rich source of phytochemicals that adsorb onto the material surface during preparation and enhance the properties of materials such as a stabilized particle, reduced particle size, enhanced surface functional sites, and large surface area [16; 17; 18; 21; 32]. The presence of functional groups on the surface of the material also enhances the photocatalytic activity of the material by easily absorbing sunlight [16; 17; 18]. Keeping all these features in mind, the green method was adopted for the present (ZrO\({}_{2}\)-CdWO\({}_{4}\)) preparation for which leaf extracts of _Brassica rapa_ plants were employed. _Brassica rapa_ leaf extracts contain abundant phytochemicals like kaempfer, isorhamtein, quercetin glycosides, phenylpropanol derivatives, indole alkaloids, sterol glucosides, and isorhammetin glycosylated acylated with different hydroxcinnamic [17; 18]. This plant extract has also been used for a number of nanoparticles (NPs)/(NCs) [16; 17; 18]. The objective of this study is to prepare a ZrO\({}_{2}\)-CdWO\({}_{4}\) heterojunction (type I) NC using a _Brassica rapa_ leaf extract. In this study, water-splitting photocatalytic activity has been explored by developing a heterojunction between ZrO\({}_{2}\) and CdWO\({}_{4}\).