A review on plasmonic Au-ZnO heterojunction photocatalysts: Preparation, modifications and related charge carrier dynamics☆
Introduction
Compared to the pure phase semiconductors, hybrid nanomaterials have entailed intense research to amplify the photoresponse in the solar spectrum due to their photon absorption ability in different wavelength range and built-in electric field assisted spatial separation of charge carriers across the integrated components [1], [2], [3], [4]. As the electrons and holes accumulates in different components due to vectorial carrier transfer, simultaneous redox reactions proceeds with minimal carrier recombination. The distinct types of hybrid systems that are researched to surpass the limited efficiency of single component are semiconductor-semiconductor [1], [2], [3], [4] and metal-semiconductor heterojunctions [5], [6], [7]. The interdiffusion of cations during the fabrication (annealing step) in the former results in the formation of impure compounds possessing unfavorable interfacial structures which inevitably lower their efficiency [8], [9], [10], [11], [12]. Alternatively, metal-semiconductor composite is convenient as the metal is deposited over the semiconductor support at ambient conditions without distorting the pristine crystal structure. The metal loadings including the size and shape can be easily tailored by altering the reaction parameters, which provides an opportunity to modulate both carrier lifetime and opto-electronic properties [13], [14], [15], [16], [17], [18], [19]. In addition, noble metal can catalyze redox reactions and therefore integrating with semiconductor are likely to trigger the peculiar functionalities [20].
As a counterpart to TiO2 and other copious metal oxides, ZnO have become prevalent in photocatalysis owing to their easier crystallization and facile to large scale synthesis combined with significant electron affinity (4.2 eV), fast carrier mobility (∼ 205–1000 cm2 V−1 s−1), exciton binding energy (60 meV), photosensitivity and transparent conductivity [8], [9], [10], [11], [12]. The prominent ionicity of hexagonal wurtzite crystal structure of ZnO and their polar surfaces favor the dimensional anisotropy enabling the crystal growth in different directions forming the diverse morphologies and also promote the fine tuning of surface properties [8], [9], [10], [11], [12]. Despite these shining merits, large direct band gap (3.4 eV) of ZnO imparts restricted response for major fraction of solar spectrum, while low degree of charge carrier separation, acidic/basic medium induced corrosion and deactivation during the photocatalytic reactions hampers their extended utility [6], [8], [9], [10], [11], [12].
Among the various strategies reported to surmount these intrinsic drawbacks [8], [9], [10], [11], [12], noble metal deposition on the ZnO surface is under the spotlight of current research from the prospect of effective charge carrier separation, their easy preparation, improved photostability, and expanding the bandwidth spanning the entire solar spectrum [6], [7], [8], [9]. Compared to widely used Ag (4.26 eV), Au (~ 5.1 eV) has higher work function and band bend upwards for Au-ZnO promoting the electron transfer process from ZnO to Au and simultaneously inhibits the electron flow in opposite directions [21]. In addition, hydrolysis of Au precursor is not complicated and stable complexes associated with aqueous phase are not prominent. As a result, Au precursor (HAuCl4) is chemically stable, resistant to photocorrosion and can be conveniently deposited in the metallic state over a wider pH range [22], [23]. In contrast, formation of oxidized Pt species (PtO, PtO2, Pt(OH)2, [Pt(OH)4Cl2]2-) and Ag7O8NO3 phase are encountered during the deposition of Pt and Ag respectively under the specified reaction conditions [24], [25], [26]. This might be an obstacle to attain dependable optical signals and also to perform long-term operations. Few review articles emphasized the prolonged carrier lifetime in Au-ZnO compared to Ag/Pt-ZnO due to facile electron transfer and Fermi-level equilibration [8], [11], [12]. Furthermore, easy tuning of localized SPR band of Au NPs by varying the size-shape-morphology is an added advantage to tailor the optical response for the solar spectrum.
On the basis of these viewpoints, plethora of research activities towards the fabrication of Au-ZnO hybrid assemblies have pursued interests towards photocatalytic applications. This review article centers on understanding the correlation between the charge carrier dynamics, SPR effects, surface functionalization, defects, size of Au NPs, free radicals participation and morphological effects of Au-ZnO in relevance to the photocatalytic reactions. The secondary objective is to address the key challenges and issues allied to the preparation method, gap analysis of experimental data and our perspective to elicit the further research in designing the Au-ZnO for energy conversion fields.
Section snippets
Junction formation and interfacial structure
Plasmonic structures are conjugated with semiconductors to achieve concerted effects between the band gap and plasmonic excitation from semiconductor and plasmonic NPs respectively. When Au decorates over ZnO surface, an Ohmic or Schottky contact is established depending on the surface state defects, morphological features and the intrinsic defects of ZnO [27], [28], [29], [30], [31], [32], [33], [34]. The as formed junction directly impacts the optical properties, band bending, dynamics of
Preparative methods of Au-ZnO
The synthesis of Au-ZnO maintaining their crystal structure integrity, interfacial structure with minimized defects, intergranular electronic interactions, morphology and large surface area, uniform deposition of Au and well defined interfaces are the primitive requisite for efficient photocatalytic applications. The proper expose of active facets under the light illuminating conditions to maximize the photon absorption should be engineered during the preparative method itself. A great deal of
Hierarchical Au-ZnO
Despite the higher performance of Au-ZnO, their post recovery is quiet difficult and energy consumptive in the nanopowder form. Also, particles form usually suffers from reduced carrier mobility arising from electron trapping/scattering at the grain boundaries. Thus, integrating Au and ZnO ingeniously into an oriented architecture at the nanoscale regime holds promise for efficient photocatalytic systems. The hierarchical nanostructure overcome these aforementioned drawbacks and are
Functionalization of Au-ZnO surface for improving performance
Given the weaker interactions between the metal and semiconductor that often results in uncontrollable Au NPs size and uneven distribution, functional molecule directed controllable attachment through the aid of molecular linkers either to the surface of ZnO or Au NPs or both are indispensable towards the formation of self-assembled nanostructures [52], [73], [84], [101], [108], [117], [118], [119], [123], [124], [125]. The further improvement in the performance of Au-ZnO was achieved by
Influence of particle size and Au loadings
The literature survey unanimously substantiates that the charge carrier separation and transfer pathways are principally related to the metal content, electronic structures, magnitude of interfacial interactions and nature of interface between the Au and ZnO. The size of Au NPs together with the content of Au and ZnO in the composite plays a pivotal role in shifting the Fermi level, altering the reduction potential of transferred electrons, localized electric field magnifications, wavelength
Ternary composites associated with Au-ZnO
With the reports discussed above, it is evident that the enhancement in the activity of Au-ZnO is although significant, it however does not vindicates the Au-ZnO for large scale applications. The further development in this regard was attempted via heterostructuring the Au-ZnO with other metals, carbon materials and semiconductors to form ternary hybrids without compensating its beneficial properties and also to impart the unique functionality like magnetic, intense visible light response,
Conclusion and future prospects
The successful transfer of charge carrier to the surface prior to their recombination is vital to achieve the photocatalytic activity. As the photon flux is much larger in the visible region of solar spectrum, integrating the disparate crystal structures of plasmonic Au with ZnO in the nanoscale regime offers unique interfacial structure, ease of carrier transport, interphase boundaries and extended optical response to longer wavelength that is essential from the prospect of utilizing solar
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Dedicated to Prof. K. S. R. Koteswara Rao, Department of Physics, Indian Institute of Science, Bengaluru.