Review ArticleiMedPub Journalshttp://www.imedpub.comNano Research & ApplicationsISSN 2471-9838Role of Nanomaterials and their Applicationsas Photo-catalyst and Senors: A ReviewAbstractLack of environmental sustainability is a vital and growing problem due to theissues: such as climate change, pollution, and disturbances associated withbiodiversity. A major cause of these environmental threats is pollutants in theatmosphere. Semiconducting metal oxide nanostructures play an important role indeveloping smart materials that are well efficient for sensing and simultaneouslydestroy harmful chemical contaminants from our environment. This review articlehighlights some recent advances of nano-science in the area of environmentalhazardous contaminant detection by sensing followed by the remediation; focusedon water remediation especially through photo-catalysis. In addition, the state-ofthe-art research activities involved in importance of metal oxide and their variousapplications are also discussed. The compilation consists of three main sectionsvis a vis (a) semiconducting metal oxides and applications, (b) Photocatalysis:fundamentals, processes and mechanism and (c) sensors: processes, types andtheir respective applications to monitor levels of environmentally importantparameters. Also an inclusion of a section on future trends which discusses someof the new approaches being used to improve the selectivity and sensitivity ofmetal oxide semiconductors. This review concluded the perspectives and outlookon the future developments in the metal oxide nanostructure research area aswell as summarizes a comprehensive compilation of the work done in order toaddress the challenges followed by prevailing achievements till date.Keywords: Semiconducting metal-oxides; Photo-catalysis; Sensing2016Vol. 2 No. 1:10Preeti Singh1,Abdullah M. M2 andSaiqa Ikram11 Bio/Polymers ResearchLaboratory,Department ofChemisrty,Jamia Millia Islamia,New Delhi-25.2 Promising Centre for Sensors andElectronic Devices (PCSED), Departmentof Physics, College of Science andArts, Najran University, P.O. Box-1988,Najran–11001, Saudi ArabiaCorresponding author: Ikram Saiqa sikram@jmi.ac.inBio/Polymers Research Laboratory,Department of Chemistry, Jamia Millia Islamia,New Delhi-110025.Citation: Preeti P, Abdullah MM, Ikram S.Role of Nanomaterials and their Applicationsas Photo-catalyst and Senors: A Review. NanoRes Appl. 2016, 2:1.Received: October 15, 2015; Accepted: December 03, 2015; Published: December10, 2015IntroductionNanotechnology is an emerging field that covers a wide rangeof technologies which are presently under development atnanoscale. Behavior of materials at the nanoscale as compared tomacroscale often found to be highly desirable properties whichare created due to size confinement, the dominance of interfacialphenomena, and quantum effects. These new and uniqueproperties of nanostructured materials, nanoparticles, and otherrelated nanotechnologies lead to improved properties such ascatalysts, tunable photoactivity, increased strength, and manyother interesting characteristics [1]. It plays a major role in thedevelopment of innovative methods to produce new products, tosubstitute existing production equipment and to reformulate newmaterials and chemicals with improved performance resultingin less consumption of energy and materials and reduced harmto the environment as well as environmental remediation. Copyright iMedPub Environmental applications of nanotechnology address thedevelopment of solutions to the existing environmental problems,preventive measures for future problems resulting from theinteractions of energy and materials with the environment, andany possible risks that may be posed by nanotechnology itself.Use of pesticides, herbicides, dyes, solvents, etc., rapidly inagriculture and large scale industrial development activities arecausing much trouble and concern for the scientific communitiesand environmental regulatory authorities around the world.These organic pollutants adversely affect the environment andare a major source of aesthetic pollution, eutrophication andecological disturbance in aquatic life due to their toxicity andpersistence. To safeguard of our environment, it is very importantto detoxify these hazardous organic pollutants. Among severalproposed techniques for wastewater treatment: photocatalyticoxidation process provides a route for the detoxification ofvarious toxic and hazardous pollutants as well as remedies water.1This article is available in: http://nanotechnology.imedpub.com/archive.php
ARCHIVOSDE MEDICINANano Research& ApplicationsISSN1698-9465ISSN2471-9838In the present scenario all the researchers and scientist aregetting more and more attracted towards the metal oxide (MO)nanostructures due to their technology important applications inelectronic as well as optoelectronic devices, sensors, medicinesand renewable energy sources. Reducing the size of materials(metal oxide) to nano-level imparts properties which are differentfrom the bulk or crystalline form and these nanoparticlesdemonstrate behavior like an isolated atoms and molecules[2]. Therefore, semi-conducting metal oxides (SCMO) are morepotential due to their capability to generate charge carrierson stimulation with the required amount of energy and usedfor environmental remediation as well as electronics. Variousmethods of synthesis of SCMO are Chemical vapor depositiontechnique, hydrothermal method, Laser ablation technique,and electro-deposition method, etc. Out of these techniques,hydrothermal method is to friendliness use because of its costreducing and easy to handle as well as chemically reactive at lowtemperature.For healthy environment, it is very important to maintain theecological balance, in which controlling the environmentaltoxic pollutants is of great importance. MO semiconductorsnanostructures are inevitable materials due to their novelcharacteristics and potential applications such as catalysis, ionexchange, molecular adsorption etc. Their outstanding propertiesare due to huge exposed active surface area, high stability,quantum confinement consequence, and high porosity as wellas permeability (mesoporous in nature). This affects the varioustechnological advancements mainly the particular advantagesover traditional sensing methods, such as lower sensitivity, higherresponse time, and cost effective [3]. MOs are very strong decoloringagent used as photocatalytic degradation of organic pollutants. Itplays an effective role in the treatment of waste water effluents.These promising applications of MO characterized as charge transportin electronic structure stimulates its light absorption properties andmade it possible to be utilized as photocatalyst as well as sensor [4].Specially, the SCMO nanostructures based sensors demonstratedbetter efficiency for the detection of various hazardous chemicalssuch as hydrogen, acetone, ammonium hydroxide, formaldehyde,and chloroform etc. It is because of the fact that: the low dimensionof metal oxide nanostructures allows very sensitive transduction ofthe liquid/surface interactions into a change in the electrochemicalproperties [5].Accordingly, it’s a credible contribution of researchers towards thehealthier environment to organize a set-up for the photocatalyticand sensing activity for removal and detection of healthhazardous compound form environment as well remediation ofwater by using SCMO. This review article presents the importanceof nanotechnology mainly in terms of cleaning and monitoring ofthe environment. Here we discussed SCMO and their applicationin water remediation by using as photocatalyst and sensing in theform of chemical, gas and biosensor/s.Photo-catalysis: fundamentals, processesand mechanismCatalysts are utilized for speeding up the chemical reactionand similarly, a photocatalyst employ the catalyst for speeding22016Vol. 2 No. 1:10up chemical reactions in the presence of UV light. In this way,the absorption of light produces the electron–hole pairs thatenable chemical transformations of the reaction participantsand regenerate its chemical composition after each cycle of suchinteractions. There are two types of photocatalytic reactions, i.e.,homogeneous photocatalysis and heterogeneous photocatalysis.Significant features of the photocatalytic systems are: properband gap, morphology of the material, more exposed surfacearea, stability and its reusability. Photocatalysis is widelybeing practiced for the degradation and mineralization ofhazardous organic compounds to CO2 and H2O and thus leads tothe reduction of toxic metal ions into non-toxic states, deactivateand destruct all the water borne microorganisms, decomposesthe air pollutants such as NO2, CO and NH3, degraded the wasteplastics and green synthesis of industrially important chemicals.Thus, Photocatalysis refers to the oxidation and reductionreactions on the surfaces of photocatalyst material, mediatedby the valence band (VB) such as holes(h ) and conduction band(CB) such as electrons (e-) generated by the absorption of UV-VISlight radiation. Such photo-generated pairs of h and e- inducesthe formation of aggressive species such as hydroxyl (OH ) orsuperoxide radicals from the moisture and atmospheric oxygen.These species are strong enough to oxidize and decomposeorganic materials or smelling gas and kill bacteria. Photocatalysishas been established as an efficient process for the mineralizationof toxic organic compounds, hazardous inorganic materials andmicrobial disinfection as a result of the formation of the OH ions,which acts as a strong oxidizing agent [6,7]. Figure 1 shows theschematic representation of photo catalytic mechanism. Briefly,the SCMO on irradiation with an appropriate wavelength (i.e.,greater than or equal to band gap energy) lead to the excitationof an electron from the VB to the CB yielding an e-/h pair. Whenthe reaction is conducted in the presence of water and oxygen,the electron in the conduction band is picked by oxygen givingrise to superoxide radical anion and in the oxidation site water isoxidized to give OH radical. These are the two reactive species,which react with the organic pollutants leading to completemineralization. The reactions of this e-/h pair with a variety ofelectron acceptors and donors, as well as e-/h recombinationprocesses have been well-studied [8,9]. The formation of cationradicals of organic substrates, following electron transfer toexcited semiconductors have been studied in several cases, bothby product analysis [10] as well as by spectroscopic studies [11].Extensive research has shown SCMO can photo-oxidise a widerange of organic substrates including alkanes, alkenes, aromatics,surfactants and pesticides [12-14]. Several MO such as TiO2, ZnO,MoO3, ZrO2, WO3, α-Fe2O3, SnO2, SrTiO3 and some chalcogenidemetals (ZnS, CdS, CdSe, WS2, MoS2) can be used as photocatalystswhich is an area of intensive research. However, in terms ofthermodynamics, the valence band (VB) and conduction band(CB) in semiconductor which act as photocatalyst should bepositioned in such a way that, the oxidation potential of thehydroxyl radicals and the reduction potential of superoxideradical lie well within the band gap. In other word, the redoxpotential of the VB hole must be sufficiently positive to generateOH- radicals and that of the CB electron must be sufficientlynegative to generate superoxide radicals [15].This article is available in: http://nanotechnology.imedpub.com/archive.php
ARCHIVOSDE MEDICINANano Research& ApplicationsISSN1698-9465ISSN2471-98382016Vol. 2 No. 1:10efficient in imaging and monitoring of nanomaterial, biological,chemical, and pathological samples. In the present scientificera, the development of sensors using nanostructured metaloxides, conducting oxides, semi-conducting oxides, polymers,and composites are the subject of study for detection andquantification of various hazardous gasses, chemicals and biochemicals [19].Chemical SensorFigure 1 Schematic representation of photocatalytic mechanism (CBConduction band, VB-Valence band).Fundamental Chemistry Involved in PhotocatalysesThe standard reactions involved in the process of photocatalysis todegrade water contaminant or pollutant consist OH radical to bethe primary oxidant in the photocatalytic system and suggestedthe seven steps, all based on attack of OH-, lead to detoxificationof harmful compound [16,17]. They follow:(a)Photon energy greater than the band gap excites thecatalyst and generates electrons and holes.(b)Organic pollutants get adsorbed on the catalyst surfacevia lattice oxygen at the surface.hν e h (1)h H2O H OH(2) A chemical sensor is a device that transforms chemicalinformation (composition, the presence of a particular elementor ion, concentration, chemical activity, partial pressure.) into ananalytically useful signal. The chemical information, mentionedabove, may originate from a chemical reaction of analyte or froma physical property of the system investigated. They can haveapplications in different areas such as medicine, home safety,environmental pollution and many others. Chemical sensorsusually contain two basic components connected in series:a chemical (molecular) recognition system (receptor) and aphysicochemical transducer. Chemical sensing is quite commonlyused in industry for process control and for monitoring, includingmonitoring for safety, plays important role in environmentalprotection, in racking of hazardous materials, tracking natural andman-made occurrences such as pollution, waterways infestation,migration of species, weather prediction and tracking etc.Fabrication of chemical sensor is relatively very simple basedon silicon processes or other thin or thick film technologies.The basic principle is that when an oxide is held at elevatedtemperatures, the surrounding gases react with the oxygen inthe oxide causing changes in the resistivity of the material. Theessential components are the high temperature, the oxide andthe reaction in the oxide.h OH OH (3)e O2 O2 (4)2e O2 2H H2O2(5)The principle of this mechanism (as shown in Figure 2) is basedon redox reaction which takes place at the surface of MOnanostructures as shown in equation (8) and (9)e H2O2 OH OH (6)e (MO nanostructures) O2 O2 (8)e (MO nanostructures) O2 2O2(9)Organic pollutant OH O2 CO2 H2O other degradation productform(7)Sensor: processes, methods and mechanismMetal oxides are being extensively studied due to their uniquesurface activities imparted by huge surface areas, which can makethem ideal sensing elements as chemi-sensors. High surfaceto-volume ratios and high chemical and thermal stabilities ofnanostructures under the operating conditions are responsiblefor their good sensing applications [18]. There are different typesof sensors such as Chemical sensor, Bio sensor, gas sensor, opticalsensor, magnetic sensor, temperature sensor, and humiditysensor etc. In this article we will discuss about three types:Chemical, Bio and gas sensors and their applications.A sensor; in general is a technological device that detects orsenses a signal, physical condition and chemical compounds.New and novel nano-materials have been playing a major rolein the development of very sensitive, accurate, and reliablesensors. Nano-structured based devices have been found Under License of Creative Commons Attribution 3.0 License Thus, the sensitivity of analyte is accredited to the oxygendeficiency and increase in the oxygen adsorption on the surfaceof MO. The oxidation of analyte depends on the magnitude ofadsorbed oxygen. If the adsorbed oxygen on the sensor surfacebe high, the oxidizing potential will be higher and the oxidationof chemicals (such as ethanol) will be faster. Negatively charged(adsorbed oxygen) oxidizes the chemicals (such as ethanol) tocarbon dioxide and water and releases free electrons (6e ) to theconduction band of MO nanostructures as shown in equationbelow. These electrons increase the conductance of the film, andthus enhance the sensitivity.Analyte (such as: CH3CH2OH) (ads) 6O (ads) 2CO2 3H2O 6e (CB)(10)Gas SensorGas sensor measures the concentration of gas in its vicinity.Each gas has a unique breakdown voltage, i.e., the electricfield at which it is ionized. Sensor identifies gases by measuring3
ARCHIVOSDE MEDICINANano Research& ApplicationsISSN1698-9465ISSN2471-98382016Vol. 2 No. 1:10biochip device, leading to the design of high-density bioarrays.Several biosensors based on different detection for differentanalytes such as: hydrogen, ammonium and sodium ions as wellas urea, creatynine, triglycerides, acetylcholine, butyrylcholine,pesticides and heavy metal ions were presented [20].Biosensorshelps in detecting emerging contaminants like pharmaceuticals,personal care products (PPCPs), steroids, xenoestrogens, andother endocrine disrupting compounds (EDCs), algal toxins,giardia (and other pathogens), and a variety of miscellaneouschemicals such as caffeine, cholesterol [21], etc.Figure 2 Schematic representation of sensing mechanism.these voltages. The concentration of the gas can be determinedby measuring the current discharge in the device. Variousapplications of gas sensor are such as process control industries,environmental monitoring, boiler control, fire detection, alcoholbreath tests, detection of harmful gases in mines, home safety,grading of agro-products like coffee and spice. There are twooperating parameters of gas sensor (1) operating temperatureand (2) humidity. Also the disadvantages are: bulky, consumelots of power and require “risky” high voltage to operate. Thedetection principle of resistive sensors is based on change of theresistance of a thin film upon adsorption of the gas molecules onthe surface of a semiconductor. The gas-solid interactions affectthe resistance of the film because of the density of electronicspecies in the film.BiosensorNanostructured metal oxides (NMOs) have recently becomeimportant as materials and have been found to exhibit interestingnano-morphological, functional biocompatible, non-toxic andcatalytic properties, especially those of zinc, iron, cerium, tin,zirconium, titanium, metal and magnesium. These materialsalso exhibit enhanced electron-transfer kinetics and strongadsorption capability, providing suitable micro-environments forthe immobilization of biomolecules and resulting in the enhancedelectron transfer and improved biosensing characteristics asbiosensors. To investigate the effect of optical and electrochemicalproperties of NMOs, suspended media, solid–liquid interfacesand nano-bio-interfaces are being conducted for biosensorapplications.A biosensor is an integrated miniaturized device that employsa biological element (antibody, enzyme, receptor protein,nucleic acid, whole cell or tissue section) as a sensing elementcoupled to a transducer for signal detection. A biosensor utilizesthe selectivity of the biomolecule and the processing power ofmodern microelectronics and optoelectronics and is hence apowerful analytical tool with applications in medical diagnosticsand other areas. Due to their specificity, portability, rapid responsetime and low cost, biosensors are projected to play a criticalrole in both clinical and non-clinical applications. The prevailingminiaturization tools allow packing of numerous microscopicelectrodes along with transducers into a small footprint of a4To fabricate an efficient biosensor, it is crucial to select an NMOthat is suitable for immobilization of the desired biomolecule.The interface formed due to binding between an NMO and abiomolecule is known to affect the performance of a biosensor.The formation and properties of a nanobiointerface dependon the nature of the NMO; parameters like effective surfacearea, surface charge, energy, roughness and porosity, valence/conductance states, functional groups, physical states andhygroscopic nature all affect the formation of a biointerf
Arts, Najran University, P.O. Box-1988, Najran–11001, Saudi Arabia Corresponding author: Ikram Saiqa sikram@jmi.ac.in Bio/Polymers Research Laboratory, Department of Chemistry, Jamia Millia Islamia, New Delhi-110025. Citation: Preeti P, Abdullah MM, Ikram S. Role of Nanomaterials and their Applications as Photo-catalyst and Senors: A Review. Nano
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