Soumen Maiti

Low temperature solution processed ZnO/CuO heterojunction photocatalyst for visible light induced photo-degradation of organic pollutants

The possibility of integrating manifold functionalities, coupled with various associated noble interface phenomena in the hierarchical nanoforms, either comprised of geometrical intricacies or achieved via the rational coupling of several components, has made them immensely pertinent from both research and technological aspects. Here, an oxide based nanostructure hybrid has been realized by integrating low bandgap copper oxide nanosheet with high bandgap one dimensional zinc oxide nanowires on a flexible carbon cloth as well as on a flat substrate. These bandgap modulated hybrid nanostructures are generated for the efficient absorption of visible light, targeting their possible use in waste water management. Our work presents a novel ambient condition protocol for morphological tuning in the nanoscale or their organization in a hierarchical structure. Environmental remediation through catalytic activity under the visible light irradiation of the synthesized samples was inspected using both anionic and cationic dyes (methyl orange and Rhodamine B, respectively) as the model contaminants, where the optimized heterostructure exhibits significantly better performance than the mono component oxides. Such enhanced performance could be explained by the formation of favorable staggered gap multiple p–n junctions at ZnO/CuO interface, which in turn retards the photogenerated electron–hole pair recombination within the heterostructure. The signature of successful p–n junction formation at ZnO nanorod/CuO nanosheet interface has been identified via current–voltage measurements with a conducting tip AFM in contact mode. The creative designing of novel heterojunctions adopting this protocol will pave the way for the utilization of the entire visible light range: thus, offering potential in solar energy conversion devices.

Recent advances in low temperature, solution processed morphology tailored ZnO nanoarchitectures for electron emission and photocatalysis applications

Having the benefit of several features such as low cost, straightforward processing, easy fabrication, large area deposition and physical flexibility, low temperature solution processed electronic devices have gained traction in the eyes of the research community as the difficulties associated with conventional higher temperature crystalline semiconductor devices are incrementally rendering them outdated. Over the last decade, amongst metal oxides, ZnO with its rich variety of nanoforms has been documented as the candidate with the highest economic impact by virtue of its diverse use in a plethora of electronic and optoelectronic applications relying on its associated unique functional features such as high mobility, excellent thermal stability and high transparency. To date, most review articles in the literature focused on ZnO nanostructures realized via the vapour phase method whereas a comprehensive study on solution processed nanoforms and their widespread use is still lacking. The current article mainly highlights an overview of recent developments in multi-dimensional ZnO nanoarchitectures processed via low temperature, rational approaches and their functional properties in field emission devices and environmental remediation. In addition to these descriptions of controlled morphology design and the usage perspective, significant issues pertinent to such geometrical evolution and device performance determination and the possible outlook for low temperature research on ZnO are also described.

Ultra-thin graphene edges at the nanowire tips: a cascade cold cathode with two-stage field amplification

A multistage field emitter based on graphene-linked ZnO nanowire array is realized by means of spin-coating a graphene dispersion (reduced graphene oxide) over a nanostructured platform followed by plasma modification. Spin-coating leads to interlinking of graphene sheets between the neighboring nanowires whereas plasma etching in the subsequent step generates numerous ultra-sharp graphene edges at the nanowire tips. The inherent tendency of graphene to lay flat over a plane substrate can easily be bypassed through the currently presented nanostructure platform based technique. The turn-on and threshold field significantly downshifted compared to the individual components in the cascade emitter. Through the facile electron transfer from nanowires to graphene due to band bending at the ZnO-graphene interface together with multistage geometrical field enhancement at both the nanowire and graphene edges remain behind this enriched field emission from the composite cold cathode. This strategy will open up a new direction to integrate the functionalities of both the graphene array and several other inorganic nanostructure array for practical electronic devices.

Room temperature deposition of ultra sharp ZnO nanospike arrays on metallic, non-metallic and flexible carbon fabrics: Efficient field emitters

Synthesis of large scale arrays of ZnO nanospikes with ultra sharp tips (<10 nm) is achieved through a newly developed chemical methodology. This is the first room temperature report of growing 1D ZnO nanostructures on heterogeneous substrates. The deposition was performed on various substrates including planar (silicon, conducting glass, copper foil), cylindrical (metallic wire) and flexible carbon cloth. The deposition technique is a universal one in the sense that it is compatible with complex shape substrates as well as independent of the nature of the substrate material. The arrangement of the nanospikes over the substrates may be altered from a dense array to a distributed flower like structure depending on the substrate pretreatment. The nanospikes exhibit excellent field emission performance caused by their emission beneficial geometry with turn-on and threshold fields showing significant dependence on the growth substrate. The turn-on field and the field enhancement factor are found to be the lowest and the highest for the ZnO deposited on carbon cloth substrates having values of 0.7 V μm−1 and 2.6 × 104 respectively. These are the best values compared to all reported chemically synthesized ZnO nanostructures and comparable to the best results of vapor phase deposited ZnO nanostructures. The synthesis method reported here will open-up a new door for future development of nano-based flexible electronics.

An ambient condition, one pot route for large scale production of ultrafine (<15 nm) ZnO nanowires from commercial zinc exhibiting excellent recyclable catalytic performance: Approach extendable to CuO nanostructures

A one pot synthesis strategy has been demonstrated for the production of ultraslim (<15 nm) zinc oxide nanowires from commercial zinc powder at completely ambient conditions. The nanowire yield in a single synthesis run remains in the gram scale. Dynamic release of zinc ions in a highly alkaline oxidative environment from zinc powder serves as the source for nanowire growth, where the intrinsic anisotropic crystal growth habit of zinc oxide plays a pivotal role during the growth process. Cathodoluminescence spectroscopic investigation reveals that the nanowires are rich with oxygen vacancies, having singly and doubly ionized charge states. Environmental remediation through photocatalytic activity of the as-synthesized nanowires was investigated taking two different commercial dyes (Rhodamine B and Orange II) as the model contaminants. The exhibited high photocatalytic performance by the nanowire sample is linked with their high surface area as well as large surface oxygen vacancy content. 3D flower-like hierarchical structures composed of copper oxide nano-leaves were also obtained employing the same growth plan, evidencing the versatility of the synthesis technique.

Flexible cold cathode with ultralow threshold field designed through wet chemical route.

A flexible cold cathode based on a uniform array of ZnO nanowires over carbon fabrics was designed via a simple wet chemical route. The structural parameters of the nanowires (i.e. length, diameter) as well as their arrangement over the carbon fibers were tailored by adjusting nutrient solution composition and growth duration. The optimized arrays of ZnO nanowires exhibit excellent electron emission performance with ultralow turn-on as well as threshold fields of 0.27 and 0.56 V µm(-1). This threshold field value is the lowest compared to any of the previous zinc-oxide-based cold cathodes realized through either chemical or vapor phase processes. In addition, the current density can reach an exceptionally high value of ∼ 11 mA cm(-2) at an applied electric field of only 0.8 V µm(-1). Flexible electronic devices based on a field emitter cold cathode may thus be realized through chemical processing at low budget but having high efficiency.

Ambient condition oxidation of zinc foil in supersaturated solution for shape tailored ZnO nanostructures: low cost candidates for efficient electron emitter and UV-detector

A well-ordered array of ZnO nanostructures is realized by a one pot ambient condition approach where the nanoforms are spontaneously formed over zinc foil in alkaline supersaturated solution containing zinc salt. Shape of nanostructure, a key issue for practical application, can be tailored by only manipulating the counter anion of the zinc salt used. The synthesized array nanostructures exhibit great promise for their application as electron field emitters as well as UV photo-detectors. A comparative study reveals that nanospikes having emission beneficial ultra-sharp tips with large inter-tip separation, shows the highest field emission current. Nanowire arrays with high surface defect constitution were observed to show the highest photocurrent gains as high as 1.1 × 103. Nanostructures organized over substrate for direct integration into nanodevices and their performance optimization through structure tailoring will be boosted by this chemical method.

Three dimensional ZnO nanostructures realized through a polymer mediated aqueous chemical route: candidate for transparent flexible electronics

A brand new three dimensional zinc oxide nanosheet array was developed through manipulating its crystal growth habits with the anionic polymer, polysodium 4-styrene sulfonate (PSS). Along with being low cost, the process separates itself from other nanosheet array production protocols not only in its unique hierarchical assembly but as it is also devoid of any post-growth high temperature pyrolysis thus offering the opportunity to integrate devices over temperature sensitive substrates. Efficient electron emissions with low operating fields (turn-on field 3.5 V μm−1) are registered from the transparent field emitter based on three dimensional nanosheet assemblies on polymeric substrates and it can preserve its high performance even in a bended state. UV photodetectors based on the same material exhibit photocurrent gains as high as 1.95 × 105 which is 42 times higher when compared to nanowires and this is attributable to the much higher surface area; even after 100 consecutive bending cycles the detector performance degradation is negligible. This study will motivate the development of new morphologies in chemical synthesis routes employing polymers as growth modifiers.

Controlling the sharpness of ZnO tetrapods by restricted zinc oxidation in the open air: a low turn-on field emitter stabilized by graphene

A simple, novel and economical approach was developed for the synthesis of ZnO tetrapods through the controlled open air oxidation of a commercial zinc foil. Unrestricted by stringent criteria such as the maintenance of a high vacuum level or the usage of catalysts, this protocol yields scalable amounts of high quality tetrapods, thus highlighting its convenience over other reported methods. A thin coating of gold on top of the zinc foil is found to restrict the free open air oxidation and thus plays a pivotal role in this alternative kind of tetrapod formation. Investigations using cathodoluminescence spectroscopy confirmed the high optical quality of the tetrapods. Additionally, these tetrapods are found to exhibit an excellent electron emission performance. Finally, with the expectation of gaining better field enhancement, thermally reduced graphene oxide is attached to the tetrapods. This hybrid system is observed to possess outstanding electron emission activity with very low turn-on and threshold field values, as well as much improved stability, which were not achieved for the separate components. This indicates the potential usability of these materials in field emission nanodevices.

Spontaneous hyper-branching in ZnO nanostructures: morphology dependent electron emission and light detection

Higher dimensional ZnO nanoforms offer unprecedented advantages over their low dimensional counterparts in emerging technologies. This motivated the researchers to design ZnO hierarchical architectures which are expected to offer performances improved to device benchmarks. Starting from 1D ZnO nanospike arrays, hierarchical cactus and tree-like ZnO arrays with increasing branching and complexities have been grown via a simple wet chemical approach in ambient conditions. The zero thermal budget, large area of fabrication and absence of any structure-directing agents are the novel highlights of the current synthesis protocol. Cathodoluminescence spectroscopic investigation reveals a gradual increment in defect constitution in these nanoforms with increasing structural complexity. The synthesized nanostructure arrays display promise in electron field emission owing to their structural uniqueness as indicated by field distribution calculations using ANSYS electromagnetic software. Furthermore, these higher order nanostructures are capable of detecting UV light with photocurrent gains as high as 2.21 × 104.