A simple and facile one-pot synthesis of fluorescent carbon dots from orange waste peels was performed using the hydrothermal carbonization method at a mild temperature (180 °C). The chemical composition and morphological feature of the obtained carbon dots (C-dots) were characterized using various spectroscopies and a transmission electron microscopy. The prepared hydrothermal carbons were amorphous in nature, and clusters of polyaromatic hydrocarbons included a large quantity of oxygen functional groups. A composite of C-dot with ZnO was used as a photocatalyst for degradation of naphthol blue-black azo dye under UV irradiation, and the superior photocatalytic activity was demonstrated. Overall, the present preparation method of C-dots takes on meaning in the area of green synthesis in aqueous solutions, and the product has great potential as a component material in the development of a remarkably efficient catalytic system.

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Granular activated carbon (GAC) materials were prepared via simple gas activation of silkworm cocoons and were coated on ZnO nanorods (ZNRs) by the facile hydrothermal method. The present combination of GAC and ZNRs shows a core-shell structure (where the GAC is coated on the surface of ZNRs) and is exposed by systematic material analysis. The as-prepared samples were then fabricated as dual-functional sensors and, most fascinatingly, the as-fabricated core-shell structure exhibits better UV and H sensing properties than those of as-fabricated ZNRs and GAC. Thus, the present core-shell structure-based H sensor exhibits fast responses of 11% (10 ppm) and 23.2% (200 ppm) with ultrafast response and recovery. However, the UV sensor offers an ultrahigh photoresponsivity of 57.9 A W, which is superior to that of as-grown ZNRs (0.6 A W). Besides this, switching photoresponse of GAC/ZNR core-shell structures exhibits a higher switching ratio (between dark and photocurrent) of 1585, with ultrafast response and recovery, than that of as-grown ZNRs (40). Because of the fast adsorption ability of GAC, it was observed that the finest distribution of GAC on ZNRs results in rapid electron transportation between the conduction bands of GAC and ZNRs while sensing H and UV. Furthermore, the present core-shell structure-based UV and H sensors also well-retained excellent sensitivity, repeatability, and long-term stability. Thus, the salient feature of this combination is that it provides a dual-functional sensor with biowaste cocoon and ZnO, which is ecological and inexpensive.