Coherent CuO-ZnO nanobullets maneuvered for photocatalytic hydrogen generation and degradation of a persistent water pollutant under visible-light illumination
Graphical Abstract
Introduction
Visible-light-driven photocatalysis is a promising alternative for sustainable H2 fuel production as well as environmental detoxification [1]. As a renewable, pollution-free, and high-yield-upon-combustion (122 kJ/mol) energy source, H2 is anticipated to solve the global energy crisis in the future [1], [2]. In addition, the photocatalytic degradation of persistent water pollutants is expected to resolve a likely potable water crisis through water purification for reuse [3], [4], [5], [6]. Hence, intensive research is in-full-swing to develop a highly efficient photocatalyst for H2 fuel production and wastewater decontamination. Accordingly, semiconductors are among the most explored materials for photocatalytic applications [1], [7].
Semiconductor photocatalysis is a preferred approach because semiconductors possess desirable chemical, physical, and catalytic characteristics and the potential to exploit UV and visible light (VL) [8]. Additional qualities associated with most semiconductors include complete purification or catalytic action, a long lifespan, and an environmentally benign nature [9]. Among the multitude of semiconductors, copper(II) oxide (CuO) and zinc oxide (ZnO) have attracted intensive interest because of their advantages of nontoxicity, low cost, rapid adsorption ability, high stability, high catalytic performance, and multifunctionality [3].
ZnO is a versatile n-type semiconductor comprising a wide bandgap (3.37 eV), substantial exciton binding energy (~60 meV), excellent thermal stability, and the ability to form heterojunction composites with other semiconductors. ZnO nanomaterials have been employed in a wide-ranging applications and devices, including photocatalysis, photochemical water-splitting catalysis, photodetectors, photonic crystals, supercapacitors, biosensors, transparent conducting oxides, and solar cells [10]. As a photocatalyst, ZnO outperforms TiO2 because it absorbs a comparatively greater portion of the solar spectrum [11]. The preparation method, which influences structural, morphological, and optical properties of the synthesized product, can also improve the photocatalytic and other capabilities of ZnO [12]. Nevertheless, the practical utility of ZnO alone as a photocatalyst is limited. The most pressing problems are the obstruction of catalytically active sites because of a low specific surface area, poor VL harvesting, serious agglomeration, poor carrier separation, instant recombination of photon-induced electron–hole pairs, and photo-corrosion of the ZnO catalyst itself [3], [8], [13].
CuO is a p-type semiconductor encompassed with a low bandgap energy (1.2–2.6 eV, depending on synthesis variables), remarkable electrical and thermal conductivity, and high stability [14]. It is nontoxic and naturally abundant and is appropriate for use in numerous applications and devices such as photocatalysis, lithium-ion batteries, solar energy devices, optoelectronic devices, and gas sensors [7], [11]. Moreover, particularly prepared CuO micro/nanostructures have revealed notable performance in photocatalytic dye degradation [15]. Although CuO promotes the absorption of VL, it suffers from comparatively low photocatalytic efficiency owing to the instant recombination of emanated charge carriers [16]. However, it is an excellent supporting photocatalyst to enhance the photocatalytic performance of heterojunction systems [8]. Liu et al., for instance, fabricated CuO–ZnO with a corn-like architecture and reported remarkable photocatalytic H2 generation, where the catalytic performance was enhanced by the formation of a heterojunction and by the special morphology of the product [13]. Taraka et al. synthesized hierarchical CuO–ZnO (p–n) heterojunctions and achieved high photo-induced reduction of CO2 into methanol [17]. Saravanan et al. constructed a CuO–ZnO heterojunction nanocomposite and achieved enhanced photodegradation of a textile dye under VL illumination [18]. Similar enhancements in the photocatalytic performance of various photocatalysts in consequence of the generation of a heterojunction between CuO and ZnO have been reported in numerous accounts [19], [20], [21], [22], [23].
We here substantiate a versatile approach to transform a wide-bandgap semiconductor into a VL-active photocatalyst via straightforward co-crystallization followed by high pressure annealing. The coupling of two semiconductors with different bandgap energies generated a permanent CuO–ZnO heterojunction and substantially upgraded the photocatalytic efficiency of both constituents. Our approach resolves shortcoming associated with the use of both ZnO and CuO individually, i.e., poor VL harvesting, agglomeration of particles, inadequate separation between photoinduced charge carriers and their instant recombination in ZnO, and the low photocatalytic efficiency of CuO (when used alone) because of immediate recombination of light-induced electrons and holes. As-constructed hybrid p–n-type heterojunction between ZnO and CuO shifts the optical absorption toward the VL region, dramatically promotes the separation between photon-induced charge carriers, and inhibits their recombination. The charge carriers are consequently available to accelerate photocatalytic reactions. Moreover, our approach is effective to inhibit the photo-corrosion of ZnO due to chemical bonding with CuO, which is critical for ensuring the recycling performance of a profitable photocatalyst.
The novelty of the present work is to address the difficulties prevailing in the CuO-ZnO nanocomposites. Previous catalyst synthesis involved the use of multiple steps or capping reagents. In addition, the optical absorbance range and bandgap energy were unsatisfactory, limiting to the single application of the photocatalyst. We achieved a unique structure with a significant redshift of the optical absorbance, a significant decrease in the bandgap energy, a maximum contraction of the PL intensity, and the formation of permanent heterojunctions in nano-sized composite particles. All improvements were critical for the remarkable catalytic performance of the products, which successfully constructed a photocatalyst to address the limitations of previous works. Also, our product is multi-functional; it is highly efficient for H2 fuel production as well as water pollutant degradation.
Section snippets
Chemicals
Zinc acetate dihydrate (99% purity, Sigma-Aldrich, MO, USA), copper(II) acetate hydrate (98% purity, Sigma-Aldrich, MO, USA), ethanol (99.5% purity, Sigma-Aldrich), methylene blue (MB) (Alfa Aesar, high purity, Lancashire, UK), sodium sulfite (98% purity, Sigma-Aldrich), and sodium sulfide nonahydrate (98% purity, Sigma-Aldrich) were used with no further purification. Distilled water was used in all solution preparations.
Synthesis of CuO–ZnO nanobullets
A series of CuO–ZnO nanobullet photocatalysts were prepared via an
Morphological characterizations
The morphological features of the as-fabricated CuO–ZnO nanobullets were scrutinized by FE-SEM; the corresponding micrographs are presented in Fig. 1. The CuO–ZnO composite particles (Fig. 1(b–d)) exhibit a peculiar small-bullet-like appearance and are therefore referred to as “nanobullets.”
The pristine ZnO nanoparticles (Fig. 1(a)) are more agglomerated and exhibit a less distinct shape than the pristine CuO nanoparticles (inset of Fig. 1(a)), which exhibit discrete monoclinic crystalline
Conclusion
We successfully synthesized a series of p–n CuO–ZnO heterojunction nano- composites through a straightforward co-crystallization reaction followed by calcination. The morphology of the nanocomposites and the formation of stable chemical bonds between CuO and ZnO were examined via standard techniques such as FE-SEM, HR-TEM, XRD, XPS, FTIR spectroscopy, PL spectroscopy, and UV–vis spectroscopy. We explored the effect of calcination time (while keeping other parameters constant) on the
CRediT authorship contribution statement
Kamal Prasad Sapkota: Conceptualization, Methodology, Investigation, Data curation, Software, Formal analysis, Writing – original draft. Insup Lee: Data curation, Formal analysis Visualization, Writing – review & editing. Santu Shrestha: Formal analysis. Md. Akherul Islam: Formal analysis. Md. Abu Hanif: Formal analysis. Jeasmin Akter: Formal analysis. Jae Ryang Hahn: Writing – review & editing, Validation, Data curation, Resources, Project administration, Investigation, Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research was supported by the Korean National Research Foundation (NRF-2021R1I1A3045310 and NRF-2018R1A2B6006155).
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