Metal–Oxide Nanowire Molecular Sensors and Their Promises

chemosensors

Review Metal–Oxide Nanowire Molecular Sensors and Their Promises

Hao Zeng 1 , Guozhu Zhang 1,*, Kazuki Nagashima 1,2, Tsunaki Takahashi 1,2 , Takuro Hosomi 1,2 and Takeshi Yanagida 1,3,*

1 Department of Applied Chemistry, Graduate School of Engineering, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8654, Japan; [email protected] (H.Z.); [email protected] (K.N.); [email protected] (T.T.); [email protected] (T.H.) 2 JST-PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan 3 Institute for Materials Chemistry and Engineering, Kyushu University, 6-1 Kasuga-Koen, Kasuga, Fukuoka 816-8580, Japan * Correspondence: [email protected] (G.Z.); [email protected] (T.Y.)

Abstract: During the past two decades, one–dimensional (1D) metal–oxide nanowire (NW)-based molecular sensors have been witnessed as promising candidates to electrically detect volatile organic compounds (VOCs) due to their high surface to volume ratio, single crystallinity, and well-deﬁned crystal orientations. Furthermore, these unique physical/chemical features allow the integrated sensor electronics to work with a long-term stability, ultra-low power consumption, and miniature device size, which promote the fast development of “trillion sensor electronics” for Internet of things (IoT) applications. This review gives a comprehensive overview of the recent studies and achievements in 1D metal–oxide nanowire synthesis, sensor device fabrication, sensing material functionalization, and sensing mechanisms. In addition, some critical issues that impede the practical

application of the 1D metal–oxide nanowire-based sensor electronics, including selectivity, long-term stability, and low power consumption, will be highlighted. Finally, we give a prospective account of

Citation: Zeng, H.; Zhang, G.; the remaining issues toward the laboratory-to-market transformation of the 1D nanostructure-based Nagashima, K.; Takahashi, T.; sensor electronics. Hosomi, T.; Yanagida, T. Metal–Oxide Nanowire Molecular Sensors and Keywords: nanowire; oxide; gas sensor; device; 1D nanostructure; sensing mechanism Their Promises. Chemosensors 2021, 9, 41. https://doi.org/10.3390/ chemosensors9020041 1. Introduction Academic Editor: Simas Rackauskas For the upcoming “trillion sensor electronics” era, molecular sensor and electronic recognition devices, which collect the enormous chemical information as big data from Received: 15 January 2021 various volatile organic molecules (VOCs), are gaining increasing interests in health Accepted: 19 February 2021 Published: 22 February 2021 care [1–3], environment [1,4,5], security [6–9] and agriculture areas [3,4,10,11]. Among various molecular sensors, chemiresistive sensors integrated with metal–oxide semi-

Publisher’s Note: MDPI stays neutral conductor (MOS) nanostructures are of particular interest due to their high sensitivity with regard to jurisdictional claims in and fast response [12–16]. Especially with the advancement of nanomaterial fabrication published maps and institutional affil- technology, a large number of functional MOS nanostructures, such as nanodots [17,18], iations. nanowires [19–22], nanosheets [23–25], and hierarchical nanostructures [26–28], are synthesized as building blocks for the fabrication of sensor electronics. Among these nanostructured forms, 1D MOS nanowires offer an ideal platform for nanoscale sensor integration due to their high surface to volume ratio, comparable sized Debye length, high crystallinity, excellent surface chemical reaction, and low power Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. consumption [29–31]. Ever since the first report of using the 1D metal–oxide nanostructure This article is an open access article as the gas sensor by Comini, great progress has been achieved in the past two decades [32]. distributed under the terms and To date, hundreds of papers have been published on molecular sensors integration based conditions of the Creative Commons on 1D metal–oxide nanostructures (SnO2 [29], In2O3 [33], Fe2O3 [34], V2O5 [35], CeO2 [36], Attribution (CC BY) license (https:// ZnO [37], WO3 [38], NiO [39], CuO [40], NaNbO3 [41], Zn2SnO4 [42,43], CdIn2O4 [44], etc.), creativecommons.org/licenses/by/ and the numbers of publications related to nanowires and metal–oxide nanowire sensors 4.0/). can be seen in Figure1. Meanwhile, to fulfill multiple demands of molecule detection

Chemosensors 2021, 9, 41. https://doi.org/10.3390/chemosensors9020041 https://www.mdpi.com/journal/chemosensors Chemosensors 2021, 9, 41 2 of 41 Chemosensors 2021, 1, x FOR PEER REVIEW 2 of 40

sensors can be seen in Figure 1. Meanwhile, to fulfill multiple demands of molecule detection and monitoring,and monitoring, nanowire-based nanowire-based sensor sensor devices devices with withdifferent different structures, structures, such such as as ﬂat [45], flat [45], suspendedsuspended [46], [46 bridgi], bridgingng [47], [47 ],and and vertical vertical structure [48[48],], have have also also been been addressed. ad- Despite dressed. Despitemuch much effort effort being devotedbeing devot to advancinged to advancing the metal–oxide the metal–oxide nanowire-based nanowire- molecular sensor based molecularelectronics, sensor electronics, molecular sensors molecula basedr sensors on 1D based MOS on nanowires 1D MOS have nanowires not yet have been successfully not yet been commercializedsuccessfully commercialized compared to compared the MOS ﬁlm to the structures. MOS film structures.

Figure 1. NumbersFigure of 1.publicationsNumbers ofin the publications area of nanowires in the area and of nanowire nanowires sensors and nanowirefrom 1997 sensorsto 2021 from 1997 to (internet search2021 of the (internet Web of search Science of on the Jan. Web 15, of 2021). Science Keywords on 15 January for search: 2021). metal–oxide Keywords nanowire; for search: metal–oxide metal–oxide nanowirenanowire; + gas/molecular metal–oxide nanowire sensor. + gas/molecular sensor.

The inherent Thelimitations inherent for limitations the practical for a thepplications practical of applications the 1D MOS of nanowire-based the 1D MOS nanowire-based molecular sensorsmolecular can be sensors summarized can be summarizedas follows: as follows: (1) Lack of effective(1) Lack method of effective for the method large-scale for the synthesis large-scale of synthesisgeometrically of geometrically uniform uniform single-crystallinesingle-crystalline nanowires—as nanowires—as is known, the is known, electrical the [49], electrical optical [49 [50],], optical thermal [50], [51], thermal [51], and and chemicalchemical [52] properties [52] properties of nanomaterials of nanomaterials are strongly are stronglyaffected affectedby their bysize their and sizeshape and shape due due to the quantumto the quantum confinement confinement effect [53]. effect The [ 53electrical]. The electrical conduction conduction becomes becomes more sen- more sensitive sitive to the tofield-effect the field-effect as the asnanowire the nanowire diameter diameter decreases decreases [54]. Until [54]. Untilnow, now,although although highly highly crystallinecrystalline MOS nanowire MOS nanowire with various with various diameters diameters [55], compositions [55], compositions [28–40] [and28– 40] and het- heterostructureserostructures [56,57] can [56 be,57 grown] can be in grown vapor in phase vapor (physical phase (physical vapor vapordeposition deposition (PVD) (PVD) [58,59], [58,59], pulsedpulsed laser laser ablation ablation deposition deposition (PLD) (PLD) [60,61] [60,61 ]and and chemical vaporvapor deposition deposition (CVD) [62,63], (CVD) [62,63],etc.) etc.) and and solution solution phase phase (hydrothermal (hydrothermal [64 [64,65],65] and and solvothermal solvothermal [66 [66,67]),,67]), the large-scale the large-scalesynthesis synthesis of of geometry geometry uniform uniform (diameter) (diameter) nanowires nanowires is is still still a biga big challenge challenge [68 ]. (2) Poor reproducibility—it has been demonstrated that the nanowires present a [68]. fantastic performance as they are integrated into single nanowire devices [69]. For such (2) Poor reproducibility—it has been demonstrated that the nanowires present a fan- a kind of device, lithography and sputtering techniques are frequently utilized to design tastic performance as they are integrated into single nanowire devices [69]. For such a and deposit interdigitated electrodes on randomly distributed nanowires for the device kind of device, lithography and sputtering techniques are frequently utilized to design fabrication [70]. However, this process is only accessible for fundamental laboratory and deposit interdigitated electrodes on randomly distributed nanowires for the device research, and assembling scalable and controllable nanowires on arbitrary substrates fabrication [70]. However, this process is only accessible for fundamental laboratory re- remain a major challenge toward performance reproducible sensor device fabrication [71]. search, and assembling scalable and controllable nanowires on arbitrary substrates re- (3) Poor environmental/thermal stability—to deeply exploit the data science from the main a major challenge toward performance reproducible sensor device fabrication [71]. obtained sensor signal, the long-term stability of the sensor response is highly required (3) Poor environmental/thermal stability—to deeply exploit the data science from the for time-series data collection [72]. However, performance degradation usually occurs in obtained sensorthe nanowire-basedsignal, the long-term sensor stability electronics of the because sensor theresponse surrounding is highly oxygen, required water, and con- for time-seriestaminates data collection would react[72]. However, with the active performance nanowire degradation surface as well usually as the occurs nanowire–electrode in the nanowire-basedcontact whensensor the electronics sensors arebecaus exposede the tosurrounding ambient air oxygen, for molecular water, detection and con- [47,73,74]. taminates would react(4) Poor with sensor the active selectivity—although nanowire surface as the well sensitivity as the nanowire–electrode of the single nanowire sensor contact whendevices the sensors has been are exposed demonstrated to ambi toent possess air for an molecular exponential detection enhancement [47,73,74]. as compared with (4) Poorthose sensor thin-film selectivity—although devices [75,76 ],the it issensitivity still desired of the to significantlysingle nanowire improve sensor the selectivity devices has beenof nanowire-based demonstrated to sensor possess electronics. an exponential Therefore, enhancement further as efforts compared are still with encouraged to those thin-filmpromote devices MOS [75,76], nanowire-based it is still desired sensor to significantly electronics for improve the IoT the applications. selectivity of

Chemosensors 2021, 1, x FOR PEER REVIEW 3 of 40 Chemosensors 2021, 9, 41 3 of 41 nanowire-based sensor electronics. Therefore, further efforts are still encouraged to promote MOS nanowire-based sensor electronics for the IoT applications. Here, in this review, we first give a comprehensive overview of the recent studies and achievements in 1DHere, MOS in nanowire this review, synthesi we ﬁrsts, givesensor a comprehensivedevice fabrication, overview device of func- the recent studies and tionalization, and achievementssensing mechanisms. in 1D MOS Then, nanowire some critical synthesis, issues sensor that impede device fabrication,the practi- device function- cal application of alization,the 1D MOS and nanowire-based sensing mechanisms. sensor Then, electronics, some critical including issues selectivity, that impede the practical long-term stability,application and low power of the consumption, 1D MOS nanowire-based will be pointed sensor out. electronics, Finally, we including give an selectivity, long- outlook of the remainingterm stability, issues and towards low power the lab-to-market consumption, transfer will be pointedof the 1D out. nanostruc- Finally, we give an outlook ture-based sensor ofelectronics. the remaining issues towards the lab-to-market transfer of the 1D nanostructure-based sensor electronics. 2. Metal–oxide Nanowires Growth 2. Metal–oxide Nanowires Growth Nanowire-based electronics offers an excellent possibility for future systems beyond Nanowire-based electronics offers an excellent possibility for future systems beyond the limitation of Moore’s law and provides a promising platform to explore the physical the limitation of Moore’s law and provides a promising platform to explore the phys- origin and intrinsic properties. In recent decades, significant efforts and progress have ical origin and intrinsic properties. In recent decades, signiﬁcant efforts and progress been made, and many methods have been developed to synthesize 1D metal–oxide nan- have been made, and many methods have been developed to synthesize 1D metal– owires, including PVD, CVD, PLD, thermal oxidization and solution-based growth for oxide nanowires, including PVD, CVD, PLD, thermal oxidization and solution-based single-crystalline nanowires, and template growth, electro-spun for polycrystalline/amor- growth for single-crystalline nanowires, and template growth, electro-spun for polycrys- phous nanowires. Here, this section will mainly focus on the metal–oxide nanowires talline/amorphous nanowires. Here, this section will mainly focus on the metal–oxide growth mechanism. nanowires growth mechanism.

2.1. Vapor–Liquid–Solid2.1. Vapor–Liquid–Solid Growth (VLS Growth) Growth (VLS Growth) The vapor–liquid–solidThe vapor–liquid–solid (VLS) mechanism (VLS)is widely mechanism employed is widelyto grow employed high-quality to grow high-quality nanowires [77–89].nanowires In a typical [77 VLS–89 growth,]. In a typicalas shown VLS in growth,Figure 2, asa metallic shown incatalyst Figure drop-2, a metallic catalyst let is firstly formeddroplet via heating is ﬁrstly the formed pre-depo viasited heating metal the catalyst pre-deposited thin film. metal Then, catalyst by con- thin ﬁlm. Then, by tinuously feedingcontinuously the vapor species feeding to the the metall vaporic species catalyst to theat an metallic elevated catalyst temperature, at an elevated a temperature, liquid eutectic alloya liquid is formed. eutectic Finally, alloy once is formed. the alloy Finally, reaches once super-saturation, the alloy reaches nanowires super-saturation, nanowires can be anisotropicallycan be grown anisotropically from the grownnucleated from seeds the nucleatedat the liquid–solid seeds at the interface liquid–solid and interface and the the length of nanowireslength can of nanowires be controlled can by be the controlled growth by time. the growth time.

Figure 2. A typical vapor–liquid–solidFigure 2. A typicalvapor–liquid–solid (VLS) growth mechanism (VLS) growth in metal–oxide mechanism nanowire in metal–oxide fabrica- nanowire fabrication. tion. In 1998, the Lieber group firstly reported a laser ablation technique for gaining VLS In 1998, the Liebersilicon group nanowires firstly [ 81reported]. Following a laser this ablation interesting technique work, for various gaining methods VLS such as CVD, silicon nanowiresPVD, [81]. Following PLD, electron this beaminteresting evaporation, work, various and molecular methods beamsuch epitaxyas CVD,(MBE) have been PVD, PLD, electrondeveloped beam evaporation, to grow the and metal–oxide molecular beam nanowires epitaxy ITO (MBE) [90 –have92], NiObeen [de-93–95], ZnO [83,96], SnO [97,98], TiO [99,100], In O [101,102], MgO [103–105], etc. [106,107]. veloped to grow the metal–oxide2 nanowires2 ITO2 [90–92],3 NiO [93–95], ZnO [83,96], SnO2 In recent years, the Yanagida Lab (The University of Tokyo) has made great efforts in [97,98], TiO2 [99,100], In2O3 [101,102], MgO [103–105], etc. [106,107]. the VLS growth of metal–oxide nanowires [85,86,94,98,99,105,108–131]. They have pointed In recent years, the Yanagida Lab (The University of Tokyo) has made great efforts out that materials flux is an important variable which has been underestimated in VLS in the VLS growth of metal–oxide nanowires [85,86,94,98,99,105,108–131]. They have nanowires growth. For example, via precisely controlling the material flux within an order pointed out that materials flux is an important variable which has been underestimated of magnitude, the structure and composition of VLS growth ITO nanowires can be altered in VLS nanowires growth. For example, via precisely controlling the material flux within from Rutile structure (Sn 100%, In 0%) to Fluorite structure (Sn 28%, In 72%) and Bixbyite an order of magnitude, the structure and composition of VLS growth ITO nanowires can structure (Sn 21%, In 79%) [86]. Moreover, the proposed “material flux window” concept from their group has been successfully applied for the synthesis of SnO2, In2O3, ITO, ZnO,

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Chemosensors 2021, 9, 41 be altered from Rutile structure (Sn 100%, In 0%) to Fluorite structure (Sn 28%, In 72%) 4 of 41 and Bixbyite structure (Sn 21%, In 79%) [86]. Moreover, the proposed “material flux window” concept from their group has been successfully applied for the synthesis of SnO2, In2O3, ITO, ZnO, NiO, MnO, CaO, Sm2O3, Eu2O3 and MgO nanowires [85]. Nevertheless, NiO, MnO, CaO, Sm2O3, Eu2O3 and MgO nanowires [85]. Nevertheless, the VLS process of the VLS processmetal–oxides of metal–oxides growth generally growth requiresgenerally relatively requires high relatively temperatures high temperatures (600–1000 ◦ C), which (600–1000essentially ℃), which limitsessentially their practicallimits their applications. practical applications. To solve thisTo solveproblem, this Zhu problem, et al. Zhudemonstrated et al. demonstrated a rational amethod rational to method reduce the to reduce growth the growth temperaturetemperature via using the via concept using the of concept“materials of “materialsflux window” flux [83]. window” As shown [83]. in As Figure shown 3(a), in Figure3a , it was foundit was that found a reduction that a in reduction growth temp in growtherature temperaturecan be successfully can be achieved successfully via pre- achieved via cisely controllingprecisely the controlling vapor flux the which vapor originates flux which from originates the suppression from the of suppressionnucleation at ofthe nucleation vapor–solidat theinterface. vapor–solid By optimizing interface. an Byappr optimizingopriate vapor an appropriate flux for VLS vapor nanowire, flux forthis VLS con- nanowire, cept can bethis applied concept to reduce can be appliedthe growth to reducetemperature the growth for various temperature metal–oxides, for various including metal–oxides, ◦ ◦ ZnO (fromincluding 750 to 400 ZnO℃), SnO (from2 (from 750 to750 400 to 400C), ℃ SnO), and2 (from MgO 750(from to 800 400 to C),350 and℃), as MgO shown (from 800 to in Figure 3(b–d).350 ◦C), In as addition, shown init Figurehas shown3b–d. a Insuccessful addition, application it has shown of this a successful concept for application the of VLS growththis SnO concept2 nanowire for the on VLS tin-doped growth indium SnO2 nanowire oxide (ITO) on glass tin-doped at 500 indium ℃ and oxideZnO nan- (ITO) glass at owires on 500polyimide◦C and (PI) ZnO substrates nanowires at on350 polyimide ℃. (PI) substrates at 350 ◦C.

Figure 3. (aFigure) The concept 3. (a) The of concept “materials of “materials flux window” ﬂux window” for nanowire for nanowire growth growth (VLS, Vapor–Liquid– (VLS, Vapor–Liquid–Solid; Solid; VS, Vapor–Solid);VS, Vapor–Solid); SEM SEM image image of (b of) SnO (b) SnO2 nanowires;2 nanowires; (c) ZnO (c) ZnOnanowires; nanowires; and ( andd) MgO (d) MgO nan- nanowires owires fabricatedfabricated under under the guidance the guidance of ‘materials of ‘materials flux window’. ﬂux window’. Reprinted Reprinted from reference from reference[83] with [83] with permissionpermission from the Amer fromican the Chemical American Society. Chemical Society.

2.2. Vapor–Solid2.2. Vapor–Solid Growth (VS Growth Growth) (VS Growth) In the absenceIn the of absence a metal of catalyst, a metal 1D catalyst, material 1D structures material structures can also be can directly also be formed directly formed from the vaporfrom thephase vapor into phase the solid into phase, the solid that phase, is, the VS that growth is, the VS(Figure growth 4). In (Figure the view4). Inof the view the currentof studies, the current VS growth studies, mechanisms VS growth mechanismsare generally are concluded generally as concluded three types: as (1) three types: The anisotropic(1) The growth anisotropic mechanism: growth nanowires mechanism: can nanowires be synthesized can be via synthesized the preferential via the re- preferential reactivity and binding of gas reactants on a speciﬁc surface to minimize the total surface activity and binding of gas reactants on a specific surface to minimize the total surface energy [132–134]; (2) The defect-induced growth (or screw dislocations) mechanism: the energy [132–134]; (2) The defect-induced growth (or screw dislocations) mechanism: the growth of crystal proceeds by adding atoms at the kink sites of a surface step [135–139]; growth of crystal proceeds by adding atoms at the kink sites of a surface step [135–139]; (3) The self-catalytic growth mechanism: metal–oxide can be decomposed into metal and (3) The self-catalytic growth mechanism: metal–oxide can be decomposed into metal and oxygen by heating under vacuum condition, then the metal vapors are condensed and oxygen by heating under vacuum condition, then the metal vapors are condensed and forming liquid droplets on a lower temperature substrate, and such droplets are the ideal forming liquid droplets on a lower temperature substrate, and such droplets are the ideal catalysts for metal–oxide nanowire growth [140–151]. Similar to VLS growth, various catalysts for metal–oxide nanowire growth [140–151]. Similar to VLS growth, various metal–oxide nanowires, such as Ga2O3, ZnO, MgO, In2O3, and SnO2 [140–153], have been metal–oxide nanowires, such as Ga2O3, ZnO, MgO, In2O3, and SnO2 [140–153], have been successfully synthesized via VS growth. Furthermore, by using surface-roughness-assisted or seed-layer-assisted VS growth, nanowires can be also grown in a selective area [154–156].

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Chemosensors 2021, 1, x FOR PEER REVIEW 5 of 40 successfully synthesized via VS growth. Furthermore, by using surface-roughness-assisted Chemosensors 2021, 9, 41 5 of 41 or seed-layer-assistedsuccessfully synthesized VS growth, via VS nanowires growth. Furthermore, can be also by grown using surface-roughness-assisted in a selective area [154– 156]. or seed-layer-assisted VS growth, nanowires can be also grown in a selective area [154– 156].

Figure 4. A typical Vapor–Solid (VS) growth mechanism in nanowire fabrication. Figure 4. A typical Vapor–SolidFigure 4. A typical (VS) growth Vapor–Solid mechanism (VS) growth in nanowire mechanism fabrication. in nanowire fabrication.

Compared withCompared VLS growth, with VLS VS growth growth, nano VSwires growth are nanowires suffering from are sufferinginsufficient from abil- insufﬁcient Compared with VLS growth, VS growth nanowires are suffering from insufficient ability to controlability uniformity, to control which uniformity, usually result whichs usuallyin a tapered results morphology in a tapered of morphologynanowires. Re- of nanowires. ity to controlcently, uniformity, Anzai et al. whichreported usually a negative result effects in ofa taperedVS growth morphology on the tapered of nanowires. SnO2 nanowire Re- Recently, Anzai et al. reported a negative effect of VS growth on the tapered SnO2 nanowire cently, Anzaielectrical et al.performanceelectrical reported performance [79].a negative By using [ 79effect ].spatially By of using VS resolved spatiallygrowth single on resolved the nanowire tapered single electrical nanowire SnO2 nanowire measure- electrical measure- electricalments, performance plane-viewments, [79]. plane-viewelectron By using energy-loss electron spatially energy-lossspectr resolvedoscopy spectroscopy single and molecular nanowire and dynamics molecular electrical simulations, dynamics measure- simulations, ments, plane-viewit has been observedit electron has been that energy-loss observed the VS growth that spectr the willoscopy VS ge growthnerate and a willlargemolecular generate number dynamics of a largedefects number simulations,in the oxides, of defects in the it has beenthus observed makingoxides, anthat unintentional the thus VS making growth doping an will unintentional in ge thenerate nanowire a large doping structure. number in the Asof nanowire defectsshown inin structure. Figurethe oxides, 5(d), As shown in thus makingsuch an an unintentionally unintentionalFigure5d, such doping doping an unintentionally effect in the will na causnowire dopinge a seven structure. effect orders will increaseAs cause shown ain seven the in conductance,Figure orders 5(d), increase in the which entirely alters the intrinsic properties of nanowires. Furthermore, in light of their lat- such an unintentionallyconductance, doping which effect entirelywill caus alterse a seven the intrinsic orders propertiesincrease in of the nanowires. conductance, Furthermore, in est results, oxygenlight of deficiency their latest is results, the critical oxygen factor deﬁciency of the unintentionally is the critical doping factor effect of the in unintentionally the which entirely alters the intrinsic properties of nanowires. Furthermore, in light of their lat- VS growth doping[157]. Moreover, effect in the increasing VS growth oxygen [157 ].pa Moreover,rtial pressure increasing during oxygenthe VS growth partial pressureis an during est results,alternative oxygen theway deficiency VS to growthprevent is the isthis an critical drawback. alternative fact or way of tothe prevent unintentionally this drawback. doping effect in the VS growth [157]. Moreover, increasing oxygen partial pressure during the VS growth is an alternative way to prevent this drawback.

Figure 5. AnomalousFigure 5. electricalAnomalous properties electrical of properties VS growth of nanowire: VS growth (a nanowire:) a schematic (a) image a schematic of uninten- image of unintentional carriertional doping carrier occurs doping at occursthe vapor–solid at the vapor–solid interface interface during duringVS growth; VS growth; (b) single (b) single tapered tapered VS+VLS VS+VLS growthgrowth SnO SnO2 nanowirenanowire device; device; (c) single (c) single untapered untapered VLS VLS growth growth SnO SnO2 nanowirenanowire device; device; (d ()d ) IV curves 2 2 IV curves ofof VS VS growth growth (93.0 (93.0 kΩ kΩ) and) and VLS VLS growth growth nanowire nanowire (2.04 (2.04 TΩ T);Ω );Reprinted Reprinted from from reference reference [79] with Figure 5.[79] Anomalous with permissionpermission electrical fromfrom properties the the American American of VS Chemical growth Chemical Society.nanowire: Society. (a) a schematic image of unintentional carrier doping occurs at the vapor–solid interface during VS growth; (b) single tapered VS+VLS growth SnO2 nanowire device; (c) single untapered VLS growth SnO2 nanowire device; (d) IV curves of VS growth (93.0 kΩ) and VLS growth nanowire (2.04 TΩ); Reprinted from reference [79] with permission from the American Chemical Society.

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2.3. Solution Phase Growth The2.3. solution-phase Solution Phase Growthgrowth, for example, the hydrothermal growth, is extensively used for theThe synthesis solution-phase of metal–oxide growth, nanowi for example,res [64,65,158–160]. the hydrothermal Compared growth, with isthe extensively vapor phaseused growth for the (both synthesis VLS and of metal–oxideVS), a much lower nanowires temperature [64,65,158 is –required160]. Compared for the solu- with the tion growth,vapor which phase growthallows the (both nanowires VLS and to VS), be directly a much integrated lower temperature on the flexible is required organic for the substratessolution for the growth, modern which wearable allows device. the nanowiresIn addition, to the be morphology directly integrated and properties on the of flexible the solutionorganic phase substrates growth fornanowires the modern can be wearable conveniently device. manipulated In addition, by tuning the morphology the con- and centrationproperties of the composition, of the solution introducing phase growth additional nanowires composition, can be conveniently or adding additives manipulated in by the solvent.tuning the concentration of the composition, introducing additional composition, or adding Inadditives the case of in ZnO the solvent.nanowires, the concept of a “concentration window” in the control of ZnO nanowireIn the morphology case of ZnO has nanowires, been demonstrat the concepted by of He a “concentration et al., as shown window” in Figure in6(a) the con- [111]. Viatrol carefully of ZnO controlling nanowire morphology the concentratio hasn been of Zn demonstrated ionic species within by He a etspecific al., as con- shown in centrationFigure range,6a [ 111a selective]. Via carefully growth on controlling the (0001) theplane concentration can be realized. of Zn Meanwhile, ionic species under within a the guidancespecific of concentration a “concentration range, window”, a selective Sakai growth et al. on achieved the (0001) a plane significant can be improve- realized. Mean- ment inwhile, the growth under therate guidance of ZnO nanowires of a “concentration (up to 2000 window”, nm/h) with Sakai the et al.assistance achieved of a significantam- monia,improvement as shown in Figure in the growth6(b) [161]. rate Moreover, of ZnO nanowires Joo et al. (up pointed to 2000 out nm/h) that the with axial the and assistance the radialof ammonia, growth of as ZnO shown nanowire in Figure can6 alsob [ 161 be]. controlled Moreover, by Joo introducing et al. pointed and outtuning that the the axial and the radial growth of ZnO nanowire can also be controlled by introducing and tuning amount of Cd (Cu, Mg, Ca) ions or Al (In, Ga) ions in the growth solution, respectively the amount of Cd (Cu, Mg, Ca) ions or Al (In, Ga) ions in the growth solution, respec- [161]. Not only in the growth of ZnO nanowires, other anisotropic 1D nanowires can also tively [161]. Not only in the growth of ZnO nanowires, other anisotropic 1D nanowires be controlled via tuning the concentration of additives. For example, Choi et al. reported can also be controlled via tuning the concentration of additives. For example, Choi et al. a straightforward process for synthesizing tungsten oxide nanostructure with various reported a straightforward process for synthesizing tungsten oxide nanostructure with morphologies [162]. By changing the composition of the solvent, the crystalline phase of various morphologies [162]. By changing the composition of the solvent, the crystalline tungsten oxide, such as mono-clinic W18O49 nanowires (with ethanol), hexagonal WO3 phase of tungsten oxide, such as mono-clinic W18O49 nanowires (with ethanol), hexagonal platelets (with water/ethanol ratio (1:9)), and monoclinic WO3 nanosheets (with water) can WO3 platelets (with water/ethanol ratio (1:9)), and monoclinic WO3 nanosheets (with be alternatively controlled. Currently, many kinds of nanowires, such as SnO2, ZnO, TiO2, water) can be alternatively controlled. Currently, many kinds of nanowires, such as SnO2, In2O3, MnO2, and WO3, have been successfully synthesized via solution-phase growth ZnO, TiO2, In2O3, MnO2, and WO3, have been successfully synthesized via solution-phase with/without the assistance of the seed layer [160,163–173]. Owing to the low cost, less growth with/without the assistance of the seed layer [160,163–173]. Owing to the low cost, hazardous,less hazardous, no metal catalysts, no metal the catalysts, effectively the effectivelycontrollable controllable morphology morphology and properties, and properties, the solutionthe phase solution growth phase strategy growth has strategy been has succe beenssfully successfully integrated integrated into a well-developed into a well-developed micro-electro-mechanicalmicro-electro-mechanical system system (MEMS) (MEMS) [174–176]. [174 – 176].

Figure Figure6. Face-selective 6. Face-selective control control of hydr ofothermal hydrothermal zinc oxide zinc oxidenanowire: nanowire: (a) schematic (a) schematic of a concentra- of a concentration tion dependencedependence on onthe the crystal crystal growth growth of hydrot of hydrothermalhermal ZnO ZnO nanowires nanowires as a as crystal-plane a crystal-plane depend- dependence ence onon a critical a critical nucleation nucleation concentration. concentration. Reprin Reprintedted from fromreference reference [111] wi [111th] permission with permission from the from the AmericanAmerican Chemical Chemical Society; Society; (b) Zn concentration (b) Zn concentration dependence dependence of ZnO of nanowire ZnO nanowire morphology morphology with with HMTA HMTA15 mM 15with mM ammonia with ammonia addition addition 500 mM. 500 The mM. Zn Theconcentration Zn concentration range, where range, a where nanowire a nanowire can can be grown,be grown,is highlighted is highlighted by a red by color. a red Reprinte color. Reprintedd from reference from reference [161] wi [161th permission] with permission from the from the Nature Publishing Group. Nature Publishing Group.

2.4. Thermal2.4. Thermal Oxidation Oxidation Growth Growth The thermalThe thermal oxidation oxidation method method is an iseasy an easy way way to massively to massively grow grow metal–oxide metal–oxide nan- nanowires owiresin in situsitu [177]. [177]. For For example, example, Sn Sn and and Ga Gaoxide oxide nanowires nanowires can be can easily be easily obtained obtained as the as the environmentenvironment temperature temperature increases increases over their over melting their melting points pointsin the inair theor airwater or watervapor vapor surroundingssurroundings [31,178]. [31 ,178Interestingly,]. Interestingly, Zn, Zn,W, Fe, W, Fe,Mo, Mo, and and Cu Cu oxide oxide nanowires nanowires can bebe grown grown by the thermal oxidation method at an even lower temperature [179–187]. As

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shownby the in thermalFigure 7 oxidation(a)–(c), several method possible at an evenmechanisms lower temperature have been invoked [179–187 to]. account As shown for in theFigure thermal7a–c oxidation, several growth: possible (1) mechanisms Evaporation have and been condensation invoked to [179,188]; account (2) for Fast the thermalinter- naloxidation diffusion growth: along a (1)tunnel Evaporation centered andon the condensation core of a screw [179 ,dislocation188]; (2) Fast [180,189]; internal (3) diffusion Sur- facealong diffusion a tunnel along centered the sides on of the nanowires core of a screw[181,190]. dislocation For example, [180,189 α-Fe]; (3)2O3 Surface nanowires diffusion are spontaneously grown due to the fact Fe ions tend to leave from the compressed regions along the sides of nanowires [181,190]. For example, α-Fe2O3 nanowires are spontaneously at the Fe2O3/Fe3O4 interface and diffuse to the stress-free Fe2O3 surface layer, whereas CuO grown due to the fact Fe ions tend to leave from the compressed regions at the Fe2O3/Fe3O4 nanowires are basically grown due to the continuous diffusion of Cu ions from the sub- interface and diffuse to the stress-free Fe2O3 surface layer, whereas CuO nanowires are stratebasically to thegrown tip of nanowires due to the continuousthrough the diffusioninternal grain of Cu boundary, ions from theas depicted substrate in to Figure the tip 7 of (d)nanowires and (e) [181,189]. through the internal grain boundary, as depicted in Figure7d,e [181,189].

FigureFigure 7. 7.TheThe various various mechanisms mechanisms proposed proposed for for meta metal–oxidel–oxide nanowire nanowire (NW) (NW) growth growth during during the the oxidationoxidation of ofmetal: metal: (a) (evaporationa) evaporation and and condensation; condensation; (b) ( binternal) internal diffusion diffusion along along with with the the core core of ofa a screwscrew dislocation; dislocation; (c) (surfacec) surface diffusion diffusion along along the the sidewall sidewall of ofnanowires; nanowires; (d ()d Fe) Fe2O23O nanowire3 nanowire growth growth followingfollowing the the surface surface diffusion diffusion mechanism. mechanism. Reprin Reprintedted with with permission permission from from Reference Reference [181] [181 Copy-] Copy- rightright 2012 2012 Elsevier; Elsevier; (e) (CuOe) CuO nanowire nanowire growth growth follows follows the the inte internalrnal diffusion diffusion mechanism. mechanism. Reprinted Reprinted with permission from Reference [189] Copyright 2011 Elsevier; and (f) CuO nanowire device fabri- with permission from Reference [189] Copyright 2011 Elsevier; and (f) CuO nanowire device fab- cated by thermal oxidation method. Reprinted with permission from Reference [177] Copyright ricated by thermal oxidation method. Reprinted with permission from Reference [177] Copyright 2012 Elsevier. 2012 Elsevier.

OwingOwing to tothe the in insitu situ growth growth behavior behavior of ofnanowires, nanowires, a novel a novel and and simple simple technology technology hashas been been reported reported by bySteinhauer Steinhauer et etal. al.for for the the fabrication fabrication of ofCuO CuO na nanowirenowire sensors sensors [177]. [177 ]. ViaVia heating heating the the Cu Cu layer layer loaded loaded on on the the patterned patterned electrodes electrodes in inair, air, CuO CuO nanowires nanowires were were naturallynaturally grown grown from from the the electrodes. electrodes. By By increa increasingsing the the growth growth time, time, nanowires nanowires on on foreign foreign electrodeselectrodes were were mutually mutually attached, attached, forming forming the the nanowire–nanowire nanowire–nanowire conductive conductive junctions junctions as asa functional a functional sensing sensing channel. channel. Due Due to toits itssimplicity simplicity and and lack lack of ofpost-processing post-processing steps steps afterafter nanowire nanowire synthesis, synthesis, this this technique technique has has been been extensively extensively employed employed to tointegrate integrate CuO CuO nanowiresnanowires in ina sensing a sensing device device [191–193]. [191–193 ].

2.5.2.5. Template-Assisted Template-Assisted Growth Growth Metal–oxideMetal–oxide nanowires nanowires can can be achieved be achieved by me byrely merely depositing depositing oxide oxide materials materials into an into organican organic template template such suchas poly(methyl as poly(methyl methac methacrylate)rylate) (PMMA) (PMMA) or an or aninorganic inorganic template template suchsuch as asanodic anodic aluminum aluminum oxide oxide (AAO). TheThe diameter,diameter, density, density, and and length length of theof the nanowires nan- owirescan be can conveniently be conveniently controlled controlled based based on the on template the template design, design, as shown as shown in Figure in Figure8. Kol- 8. Kolmakovmakov et al. et hasal. has shown shown a typical a typical inorganic inorga template-assistednic template-assisted growth growth method method to synthesize to syn- thesizeSnO2 SnOnanowires2 nanowires [29]. [29]. By depositing By depositing Sn into Sn into nanoporous nanoporous alumina alumina and and further further etching etch- the template by NaOH solution (or cracking by sonication), highly crystalline Sn nanowires

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ing the template by NaOH solution (or cracking by sonication), highly crystalline Sn nanowires are harvested. Then, via processing an air annealing on the metallic Sn nanowires, SnO2 nanowires are obtained. ChemosensorsChemosensors 20212021, 1,, x9 ,FOR 41 PEER REVIEW 8 of8 40 of 41

ing the template by NaOH solution (or cracking by sonication), highly crystalline Sn nanowires are harvested. Then, via processing an air annealing on the metallic Sn nanowires, are harvested. Then, via processing an air annealing on the metallic Sn nanowires, SnO SnO2 nanowires are obtained. 2 nanowires are obtained.

Figure 8. Schematic image of (a) the highly regular AAO template; (b) the template cross-section with nanowires deposited inside the nanopores; and (c) nanowires after removing the AAO template.

Figure 8. Schematic image of (a) the highly regular AAO template; (b) the template cross-section Ra et al. reported the use of an organic template for obtaining ZnO nanowires [194]. Figurewith 8. nanowires Schematic deposited image of inside (a) the the highly nanopores; regular and AAO (c) nanowires template; after (b) the removing template the cross-section AAO template. As depicted in Figure 9, a-carbon was first deposited on the SiO2/Si wafer as an etch-stop with nanowires deposited inside the nanopores; and (c) nanowires after removing the AAO tem- layer. Then,plate. the wafer Ra et was al. reportedcoated by the a sacrif use oficial an organic layer via template plasma-enhanced for obtaining chemical ZnO nanowires va- [194]. por depositionAs (PECVD). depicted inThen, Figure the9, a-carbonPECVD waslayer first was deposited patterned on theusing SiO conventional2/Si wafer as anli- etch-stop thography andlayer. etchedRa et Then, al. by reported the the wafer CF 4the/Ar was use inductively coated of an byorganic a coupled sacrificial template plasma layer for via obtaining(ICP) plasma-enhanced process. ZnO nanowiresThen, chemical an [194]. vapor atomic layerAs depositiondeposition depicted in (ALD) (PECVD). Figure process 9, Then, a-carb was theon PECVDusedwas first tolayer create deposited was a 70 patterned nmon the ZnO SiO using layer2/Si conventional wafer over the as an pre- etch-stop lithography patterned sacrificiallayer.and Then, etched layer. the by Then,wafer the CF bywas4/Ar ICP coated inductively etching by a with sacrif coupled Clicial2/Ar plasmalayer discharges, via (ICP) plasma-enhanced process. the ZnO Then, layer chemical an was atomic va- layer removed exceptpordeposition deposition for the spacer (ALD) (PECVD). processpart. Then,Finally, was the used ZnO PECVD to nanowire create layer a 70 wasarrays nm patterned ZnO were layer obtained using over conventional the by pre-patterned re- li- moving thethography sacrificialsacrificial andoxide layer. etched layer. Then, by A by the similar ICP CF etching4/Ar fabrication inductively with Cl 2process/Ar coupled discharges, for plasma CuO, the NiO,(ICP) ZnO andprocess. layer Cr was2O Then,3 removed an nanowires hasatomicexcept also layer been for deposition the recently spacer reported (ALD) part. Finally, process [195]. ZnO Suchwas nanowire used patterning to create arrays methods a 70 were nm obtainedbyZnO using layer bylithog- over removing the pre- the raphy techniquespatternedsacrificial can sacrificial prevent oxide layer. layer. the Aproblems Then, similar by fabricationICP associated etching process withwith Cl the for2/Ar alignment CuO, discharges, NiO, andof the the CrZnO 2nan-O 3layernanowires was has also been recently reported [195]. Such patterning methods by using lithography owires. removed except for the spacer part. Finally, ZnO nanowire arrays were obtained by re- movingtechniques the sacrificial can prevent oxide the layer. problems A similar associated fabrication with theprocess alignment for CuO, of theNiO, nanowires. and Cr2O3 nanowires has also been recently reported [195]. Such patterning methods by using lithography techniques can prevent the problems associated with the alignment of the nanowires.

Figure 9. Schematic diagram illustrating the fabrication processes of the ZnO nanowire device based on nanoscale spacer Figure 9. Schematic diagram illustrating the fabrication processes of the ZnO nanowire device based lithography: (a) thermal SiO2 deposition; (b) a-carbon deposition (etch-stop layer); (c) Plasma-enhanced chemical vapor on nanoscale spacer lithography: (a) thermal SiO2 deposition; (b) a-carbon deposition (etch-stop deposition (PECVD) of SiO2 (sacrificial layer); (d) sacrificial layer patterning; (e) ZnO atomic layer deposition (ALD); (f) top layer); (c) Plasma-enhanced chemical vapor deposition (PECVD) of SiO2 (sacrificial layer); (d) sacri- view after ZnO plasma etching; (g) sacrificial layer removal; and (h) ZnO nanowire device after metal electrode deposition. ficial layer patterning;Figure 9. Schematic (e) ZnO atomicdiagram layer illustrating deposition the fabrication (ALD); (f )processes top view of after the ZnO ZnO nanowire plasma etch- device based Reprinted from reference [194] with permission from the Copyright 2008 Wiley. ing; (g) sacrificialon nanoscale layer removal; spacer lithography: and (h) ZnO (a ) nanowirethermal SiO device2 deposition; after metal (b) a-carbonelectrode deposition deposition. (etch-stop Reprinted fromlayer); reference (c) Plasma-enhanced [194] with pe rmissionchemical fromvapor the deposition Copyright (PECVD) 2008 Wiley of SiO.2 (sacrificial layer); (d) sacri- ficial2.6. layer Electro-Spun patterning; Growth (e) ZnO atomic layer deposition (ALD); (f) top view after ZnO plasma etch- 2.6. Electro-Spuning; (g Growth) Electro-spinningsacrificial layer removal; is one and of the (h) easiest,ZnO nanowire template-free, device after versatile, metal electrode and cost-effective deposition. ap- Reprintedproaches from to producereference 1D[194] nanostructures. with permission Almost from the all Copyright the oxides 2008 and Wiley mixed. oxides nanowires can be fabricated using this method [196–210]. Figure 10 shows a typical example of 2.6.nanowires/nanofibers Electro-Spun Growth fabrication by electro-spun growth [201]. As shown in Figure 10a, an appropriate ratio of dimethylformamide, poly(vinyl acetate) (PVA), titanium(IV) propox-

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Electro-spinning is one of the easiest, template-free, versatile, and cost-effective ap- Chemosensors 2021, 1, x FOR PEERproaches REVIEW to produce 1D nanostructures. Almost all the oxides and mixed oxides9 of nan- 40 owires can be fabricated using this method [196–210]. Figure 10 shows a typical example of nanowires/nanofibersElectro-spinning is fabricone ofation the easiest, by electro-spun template-free, growth versatile, [201]. and As cost-effective shown in Figure ap- 10(a),proaches an appropriate to produce ratio 1D ofnanostructures. dimethylformamide, Almost poly(vinylall the oxides acetate) and mixed(PVA), oxides titanium(IV) nanowires can be fabricated using this method [196–210]. Figure 10 shows a typical example Chemosensors 2021propoxide,, 9, 41 and acetic acid are mixed as the precursor solution. Then, the solution is 9 of 41 loadedof nanowires/nanofibers into a syringe, which fabric is connectedation by electro-spun to a high-voltage growth supply.[201]. As Afterward, shown in theFigure pre- 10(a), an appropriate ratio of dimethylformamide, poly(vinyl acetate) (PVA), titanium(IV) cursor solution was directly spun onto the Al2O3 substrate with pre-patterned Pt electrode arrays.propoxide, Finally, and nanofibers acetic acid are are pressed mixed under as the 120 precursor ℃ to prevent solution. sticking Then, theand solutionfurther cal-is loaded into a syringe,ide, and which acetic acidis connected are mixed to as a the high-voltage precursor solution. supply. Then, Afterward, the solution the pre- is loaded into a cined in air at 450 ℃, and thus TiO2 nanofibers sensors with the mat-like structure are cursor solution syringe,was directly which spun is connected onto the Al to2 aO high-voltage3 substrate with supply. pre-patterned Afterward, Pt the electrode precursor solution obtained. Due to the simple and inexpensive fabrication process, electrospinning has at- arrays. Finally, wasnanofibers directly are spun pressed onto the under Al2O 3120substrate ℃ to prevent with pre-patterned sticking and Pt electrodefurther cal- arrays. Finally, tracted increasingnanofibers attention arein the pressed commercial under 120 field.◦C to prevent sticking and further calcined in air at cined in air at 450 ℃, and thus TiO2 nanofibers sensors with the mat-like structure are 450 ◦C, and thus TiO nanofibers sensors with the mat-like structure are obtained. Due to obtained. Due to the simple and inexpensive2 fabrication process, electrospinning has at- the simple and inexpensive fabrication process, electrospinning has attracted increasing tracted increasing attention in the commercial field. attention in the commercial field.

Figure 10. (a) Schematic diagram of the processing steps used to fabricate TiO2 nanofiber mats on Al2O3 substrates with interdigitated Pt electrode arrays. PVA (Polyvinyl acetate); and (b) the micro- Figure 10. (a) Schematic diagram of the processing steps used to fabricate TiO2 nanoﬁber mats Figure 10. (a) Schematic diagram of the processing steps used to fabricate TiO2 nanofiber mats on graph of a calcined TiO2 nanofiber mat on top of the Al2O3 substrate (dark region) and Pt electrode on Al2O3 substrates with interdigitated Pt electrode arrays. PVA (Polyvinyl acetate); and (b) the Al2O3 substrates with interdigitated Pt electrode arrays. PVA (Polyvinyl acetate); and (b) the micro- (bright region). Reprintedmicrograph from of reference a calcined [201] TiO2 wi nanoﬁberth permission mat on from top the of the American Al O substrate Chemical (dark Soci- region) and graph of a calcined TiO2 nanofiber mat on top of the Al2O3 substrate (dark region)2 3 and Pt electrode ety. (bright region). ReprintedPt electrode from (bright reference region). [201] Reprinted with permission from reference from the [201 American] with permission Chemical from Soci- the American ety. Chemical Society. 3. Metal–oxide Nanowire Devices Fabrication 3. Metal–oxide Nanowire Devices Fabrication 3. Metal–oxideTo study the Nanowireintrinsic properties Devices Fabrication of nanowires and use their functionality as a sensing To study the intrinsic properties of nanowires and use their functionality as a sensing element, one necessary process is transferring nanowires from the growth substrate and To study theelement, intrinsic one properties necessary of process nanowi isres transferring and use their nanowires functionality from the as growtha sensing substrate and integratingelement, onethem necessary integratingwith contact process them electrodes withis transferri contact on ang electrodes platform. nanowires on As a from platform.shown the in growth As Figure shown substrate11, in the Figure current and 11, the current nanowire-basedintegrating them devicenanowire-based with cancontact be divided electrodes device caninto on be tw a divided oplatform. types: into (a) As two individual shown types: in (a) Figure nanowire individual 11, the device; nanowire current and device; and (b)nanowire-based multiple nanowire(b) device multiple devices. can nanowirebe divided devices. into two types: (a) individual nanowire device; and (b) multiple nanowire devices.

FigureFigure 11. Schematic 11. Schematic image of image (a) an individualof (a) an individual nanowire device nanowire with device different with structures different (ﬂat, structures suspended, (flat, and sus- vertical); Figure 11. Schematic image of (a) an individual nanowire device with different structures (flat, sus- and (b)pended, multiple and nanowire vertical); device and (ﬂat, (b) bridging, multiple and nanowire vertical). device (flat, bridging, and vertical). pended, and vertical); and (b) multiple nanowire device (flat, bridging, and vertical).

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3.1. Individual Metal–Oxide Nanowire Device 3.1. Individual Metal–Oxide Nanowire Device Due to the fast development of modern electronics, individual metal–oxide nanowire Due to the fast development of modern electronics, individual metal–oxide nanowire devices such as the field-effect transistor (FET) offer a great chance to explore the electri- devices such as the ﬁeld-effect transistor (FET) offer a great chance to explore the electrical, cal, chemical, and mechanical properties of the nanowire. chemical, and mechanical properties of the nanowire.

3.1.1.3.1.1. Device with FlatFlat StructureStructure AA “pick“pick andand place” place” is is a commona common technique technique in fabricatingin fabricating individual individual nanowire nanowire devices. de- First,vices. oneFirst, must one “pick”must “pick” metal–oxide metal–oxide nanowires nanowires from thefrom growth the growth substrate. substrate. The substrate The sub- withstrate nanowires with nanowires should should be immersed be immersed in a readilyin a readily volatile volatile solvent solvent such such as isopropanol,as isopropa- ethanol,nol, ethanol, methanol, methanol, and evenand even water. water. Then, Th theen, metal–oxide the metal–oxide nanowires nanowires are transferred are transferred from thefrom growth the growth substrate substrate and dispersed and dispersed into solvents into solvents under under the ultrasonic the ultrasonic wave. wave. One must One thenmust ‘place’ then ‘place’ the nanowires. the nanowire One drops. One of solventsdrop of containingsolvents co dispersedntaining dispersed nanowires nanowires should be placedshould on be theplaced platform on the with platform pre-deposited with pre-deposited electrode pads electrode and dried pads by and simple dried evaporation. by simple Withevaporation. the assistance With ofthe optical assistance scope of or optical SEM observation, scope or SEM the relativeobservation, positions the relative of nanowires posi- totions the of pre-deposited nanowires to markers the pre-deposited are recorded. mark Then,ers are the recorded. platform Then, should the be platform covered should with a layerbe covered of photoresist with a layer by spin of photoresist coating technique. by spin coating By combining technique. laser/electron-beam By combining laser/elec- lithog- raphytron-beam and sputteringlithography deposition and sputtering (or using depositi focusedon (or ion using beam focused technique), ion beam nanowires technique), can benanowires connected can with be theconnected electrode with pads. the Finally, electrod thee pads. additional Finally, metal the and additional photoresist metal can and be removedphotoresist by can a lift-off be removed process inby the a lift-off organic proc solventess in such the organic as N-methyl-2-pyrrolidone solvent such as N-methyl- (NMP), dimethylformamide2-pyrrolidone (NMP), (DMF), dimethylformamide and ethanol. The (DMF schematic), and imageethanol. of theThe individual schematic nanowire image of devicethe individual fabrication nanowire process device can be fabrication seen in Figure process 12. Othercan be novel seen fabricationin Figure 12. processes Other novel can befabrication seen in the processes latest review can be [211 seen]. in the latest review [211].

Figure 12. A schematic diagram illustrating the fabrication processes of individual nanowire devices: Figure 12. A schematic diagram illustrating the fabrication processes of individual nanowire de- (vices:a) substrate; (a) substrate; (b) transferring (b) transferring nanowires nanowires onto the onto substrate; the substrate; (c) photoresist (c) photoresist coating coating (sacrificial (sacrificial layer); (layer);d) sacrificial (d) sacrificial layer patterning; layer patterning; (e) contact (e) electrodecontact electrode deposition; deposition; (f) individual (f) individual nanowire nanowire device after de- lift-offvice after process. lift-off process. 3.1.2. Device with Suspended Structure 3.1.2. Device with Suspended Structure Due to the sensor heating, the hot air near the sensor surface will form a layer which Due to the sensor heating, the hot air near the sensor surface will form a layer which is likely to support convective processes leading to the concentration gradients of reactive is likely to support convective processes leading to the concentration gradients of reactive analyte gases, termed as stagnant layer [212]. Such a layer can measure up to several tens analyte gases, termed as stagnant layer [212]. Such a layer can measure up to several tens of nanometers, and it is comparable with the nanowire size, which will cause insufficient of nanometers, and it is comparable with the nanowire size, which will cause insufficient exposure to the target molecules [213,214]. To prevent this problem, a suspended structure exposure to the target molecules [213,214]. To prevent this problem, a suspended structure has been developed to improve sensor performance. The suspended structure is a highly has been developed to improve sensor performance. The suspended structure is a highly favorable configuration because the nanowires are surrounded by the gas atmosphere. As favorable configuration because the nanowires are surrounded by the gas atmosphere. As shown in Figure 13, the fabrication process of a suspended nanowire device is similar to shown in Figure 13, the fabrication process of a suspended nanowire device is similar to that of a flat nanowire device. However, it requires one more sacrificial layer (inorganic MgO/organicthat of a flat nanowire PMMA) device. before However, the dropof it nanowirerequires one solvent more (Figuresacrificial 13 b).layer The (inorganic overall MgO/organic PMMA) before the drop of nanowire solvent (Figure 13(b)). The overall en-

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hancement inenhancement the performance in the of performance the nanodevi ofce the is nanodevicebenefited from is beneﬁted the suspended from the struc- suspended structure. For example,ture. ForNima example, et al. fabricated Nima et al. a stable, fabricated reproducible a stable, reproducibleZnO nanowire ZnO humidity nanowire humidity sensor with asensor suspended with structure a suspended [215]. structure It is observed [215]. that It is the observed suspended that nanowire the suspended de- nanowire vice presenteddevice an exponential presented change an exponential (over five change orders) (over of resistance ﬁve orders) in response of resistance to rela- in response to tive humidityrelative from humidity0% (dry air) from to 0% 60% (dry at air)room to 60%temperature. at room temperature.This improvement This improvement is is strongly associatedstrongly with associated a subthreshold with a subthresholdcarrier modulation carrier in modulation the nanowire in the core, nanowire the high core, the high surface-to-volumesurface-to-volume ratio of the nanowire, ratio of the and nanowire, most importantly, and most importantly,complete exposure complete of the exposure of the nanowire surfacenanowire to air. surface Owing to to air. the Owing gigantic to enhancement the gigantic enhancement in gas sensing in response, gas sensing the response, the developmentdevelopment of metal–oxide of nanowire metal–oxide gas nanowire sensors with gas sensorsthe suspended with the structure suspended has structurebeen has been extensively studiedextensively [214,216–220]. studied [214 ,216–220].

Figure 13. A schematicFigure 13. diagramA schematic of the diagram individual of the suspended individual nanowire suspended device nanowire fabrication device process: fabrication (a) substrate; process: (b) photoresist coating(a) (sacrificialsubstrate; ( layer);b) photoresist (c) transferring coating (sacrificial nanowires layer); onto the (c) substrate;transferring (d )nanowires photoresist onto coating the substrate; (sacrificial layer); (e) sacrificial layer(d) photoresist patterning; coating (f) contact (sacrificial electrode layer); deposition; (e) sacrificial and (g layer) individual patterning; nanowire (f) contact device electrode with suspended deposi- structure after the lift-offtion; process. and (g) individual nanowire device with suspended structure after the lift-off process.

3.1.3. Device with Vertical Structure 3.1.3. Device with Vertical Structure Similar to the suspended nanowire device, sensors with a vertical structure have also Similar to the suspended nanowire device, sensors with a vertical structure have been considered a promising design for gas detection due to their 3D architecture. Unfor- also been considered a promising design for gas detection due to their 3D architecture. tunately, the conventional “pick and place” technique cannot obtain individual nanowire Unfortunately, the conventional “pick and place” technique cannot obtain individual devices with nanowirevertical structures devices with due verticalto the cu structuresrrent limitation due to in the nanotechnology. current limitation To indate, nanotechnology. a few papersTo reported date, a fewthe papersindividual reported nanowire the individual sensor devices nanowire with sensor a vertical devices structure with avertical struc- [222,223]. Offermansture [221 et,222 al.]. offered Offermans a potential et al. offered way to a achieve potential the way individual to achieve nanowire the individual gas nanowire sensor with verticalgas sensor structure with vertical[221,224]. structure As shown [223 in,224 Figure]. As 14, shown vertical in Figureindium 14 arsenide, vertical indium ar- (InAs) nanowiressenide are (InAs) firstly nanowiresgrown on an are In firstlyP(111)B grown wafer. on Then, an InP(111)B the nanowires wafer. were Then, pat- the nanowires terned into arrayswere ranging patterned in intosize arraysfrom 30 ranging μm × 30 in μ sizem to from 100 μ 30m µ×m 100× μ30mµ viam tocombining100 µm × 100 µm via the sacrificialcombining resist layer the and sacrificial etching process. resist layer Finally, and etchingusing the process. sacrificial Finally, resist using layer the with sacrificial resist sputtering deposition,layer with air sputtering bridges were deposition, formed air to bridgesmake electrical were formed connections. to make In electrical view of connections. their device structure,In view of it theiris possible device to structure, achieve a it device is possible with tofewer achieve nanowire a device connections with fewer nanowire or even a singleconnections nanowire or connection even a single by shri nanowirenking the connection size of the by patterned shrinking array. the size Mean- of the patterned while, it was array.shown Meanwhile, that this vertical it was structure shown that of the this sensor vertical device structure presents of the a sensorvery high device presents sensitivity to a100 very ppb high of NO sensitivity2 at room to temperature. 100 ppb of NOHowever,2 at room duetemperature. to the uncertainty However, and due to the complexity ofuncertainty this process, and it is complexity impossible of to this massively process, fabricate it is impossible such a device to massively with indi- fabricate such a vidual verticaldevice nanowires. with individual To the best vertical of our knowledge, nanowires. to To date, the bestthere of has our been knowledge, no report to date, there using individualhas beenmetal–oxide no report nanowires using individual with vertical metal–oxide structure nanowires as the gas with sensor. vertical structure as the gas sensor.

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Figure 14. TheFigure fabrication 14. The processfabrication of the process individual of the vertical individual nanowire vertical device: nanowire (a) as-growndevice: (a nanowires;) as-grown nan- (b) patterned nanowire arraysowires; with (b air) patterned bridging contact;nanowire (c )arrays a side wi viewth air of anbridging air bridge contact; contact; (c) a (d side) schematic view of image an air of bridge the individual nanowire withcontact; a vertical (d) schematic structure; image (e) the of gas the sensing individual performance nanowire of with vertical a vertical aligned structure; indium (e arsenide) the gas (InAs) sensing nanowires sensor. Reprintedperformance from reference of vertical [223 aligned] with permission indium arsenide from the(InAs) American nanowires Chemical sensor. Society. Reprinted from reference [221] with permission from the American Chemical Society. 3.2. Metal–Oxide Multiple Nanowire Device 3.2. Metal–Oxide Multiple Nanowire Device Due to the requirement of precise alignment between the single nanowire and pre- Due to the requirement of precise alignment between the single nanowire and pre- deposited patterned electrodes, the overall fabrication process of the individual nanowire deposited patterned electrodes, the overall fabrication process of the individual nanowire device is complicated, time-consuming, and expensive [225–229]. Furthermore, high device is complicated, time-consuming, and expensive [225–229]. Furthermore, high ac- accuracy and stable electrical measurement systems are needed to obtain the electrical curacy and stable electrical measurement systems are needed to obtain the electrical sens- sensing signals of the single nanowire sensor devices [75]. To simplify the fabrication ing signals of the single nanowire sensor devices [75]. To simplify the fabrication process process and electrical signal measurement, multiple nanowire devices become the most and electricalwidely signal acceptedmeasurement, device multiple design in nanowire practical devices sensor device become applications. the most widely accepted device design in practical sensor device applications. 3.2.1. Device with Flat Structure 3.2.1. Device with Flat Structure A mat-like nanowire structure is similar to the single nanowire device with a flat A mat-likestructure, nanowire while structure the fabrication is similar process to the single is much nanowire easier than device that with of the a singleflat nanowire structure, whiledevice. the fabrication In a typical process mat-like is much structure, easier the than device that canof the be single loaded nanowire easily via two steps. device. In a typicalAs shown mat-like in Figure structure, 15, nanowires the device can can be be firstly loaded transferred easily via fromtwo steps. the growth As substrate shown in Figureonto 15, a nanowires platform, and can thenbe firstly the contacttransferred electrodes from the are growth deposited substrate by utilizing onto a a mask with platform, andsputtering. then the contact Alternatively, electrodes the are electrodes deposited can by be utilizing firstly pre-deposited a mask with sputter- on the platform, and ing. Alternatively,finally the nanowires electrodes are can transferred be firstly pre-deposited on the surface on of the electrodes. platform, Both and offinally the two methods nanowires areare transferred simple, costless, on the surface and unnecessary of electrodes. pre/post-process. Both of the two Due methods to its convenienceare sim- in terms ple, costless, andof fabrication unnecessary process, pre/post-process. various metal–oxide Due to its nanowire convenience gas sensors,in terms includingof fabri- SnO2, ZnO, cation process,WO various3, TiO2 ,metal–oxide NiO, CuO [230 nanowire–235], are gas successfully sensors, including obtained SnO by this2, ZnO, method. WO Moreover,3, this TiO2, NiO, CuOtechnique [230–235], is in are favor successfully of flexible obtained sensor electronics by this method. integration. Moreover, this technique is in favor of flexible sensor electronics integration. 3.2.2. Device with Bridging Structure Due to the easy fabrication, in situ controllable nanowire growth position, and 3D architecture, bridging structure is another promising design for nanowire gas sensors [43,177,191–193,236–267]. In the typical fabrication process of the device with a bridging structure, bottom electrodes should be firstly deposited on the substrate, then a seed/catalyst layer will be deposited on the top of the bottom electrodes. Finally, nanowire will be grown in situ only on the seed/catalyst layer, as shown in Figure 16. With the increase in the growth time, nanowires will incline and contact other nanowires, forming nanowire–nanowire junctions, which can be utilized as a sensing channel for gas detection. Such on-chip growth nanowire sensors can prevent nanowire–electrode contact issues encountered by the flat nanowire sensor devices, and these kind of sensors show a more stable physical and electrical performance than the sensors fabricated by spinning, dropping, or spray coating technique. Park et al. firstly reported a comparison study of NO2 sensing between air-bridged nanowire sensors and conventional nanoparticle sensors [251].

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Figure 14. The fabrication process of the individual vertical nanowire device: (a) as-grown nanowires; (b) patterned nanowire arrays with air bridging contact; (c) a side view of an air bridge contact; (d) schematic image of the individual nanowire with a vertical structure; (e) the gas sensing performance of vertical aligned indium arsenide (InAs) nanowires sensor. Reprinted from reference [221] with permission from the American Chemical Society.

3.2. Metal–Oxide Multiple Nanowire Device Due to the requirement of precise alignment between the single nanowire and pre- deposited patterned electrodes, the overall fabrication process of the individual nanowire device is complicated, time-consuming, and expensive [225–229]. Furthermore, high accuracy and stable electrical measurement systems are needed to obtain the electrical sensing signals of the single nanowire sensor devices [75]. To simplify the fabrication process and electrical signal measurement, multiple nanowire devices become the most widely Chemosensors 2021, 1, x FOR PEER REVIEW accepted device design in practical sensor device applications. 13 of 40 Chemosensors 2021, 9, 41 13 of 41 Figure 15.3.2.1. Two Devicetypical withmethods Flat Structurefor making a mat-like metal–oxide nanowires device: (a) platform; (b) nanowires transferred onto the platform; (c) contact electrode deposition; (d) contact electrode A mat-like nanowire structure is similar to the single nanowire device with a flat deposition; and (e) nanowires transferred onto the platform. structure,Owing while to the a higher fabrication exposed process area tois themuch surroundings, easier than thethat bridging of the single structure nanowire shows a faster device. In a typical mat-like structure, the device can be loaded easily via two steps. As 3.2.2. Device withresponse Bridging and Structure recovery rates as compared with that of nanoparticle sensors. Meanwhile, shown inKim Figure et al., 15, Jung nanowires et al., and can Hungbe firstly et al. transferred systematically from studiedthe growth the substrate effect of nanowire onto a junc- Dueplatform, to the tioneasy and density fabrication,then the on contact the devicein situelectrodes sensingcontrollable are performance deposited nanowire by [245 utilizing ,growth253,260 ].a position,mask Through with theand sputter- controlling 3D of architecture,ing. Alternatively,bridgingnanowire structure growththe electrodes time,is another the can number be promising firstly of nanowire–nanowirepre-deposited design for on nanowirethe junctions platform, gas within and sensors finally the bridging [43,177,191–193,236–267].nanowiressensor are transferred structures In the can typicalon the be quantitativelysurface fabricat ofion electrodes. process manipulated. Bothof the of Furthermore, devicethe two withmethods it a was bridging are found sim- that the structure,ple, bottom costless,bridged electrodes and structures unnecessary should with bepre/post-process. a firstly high density deposited of Due junctions on to theits convenience substrate, show a high then in sensing terms a seed/cat- of performance fabri- to alyst layercation will process,beNO deposited2. However, various on metal–oxide thethe response top of th nanowire ande bottom recovery gas electrodes. timesensors, of the includingFinally, bridging nanowire SnO nanowire2, ZnO, will deviceWO be3 , with a grown inTiO situ2, NiO,onlyhigh CuOon density the [230–235], seed/catalyst of junctions are successfully are layer, slower as obtained thanshown that in by of Figure thethis low-densitymethod. 16. With Moreover, nanowirethe increase this device, tech- in because nique is ain long favor time of flexible is needed sensor for the electronics molecules’ integration. diffusion in the high density of nanowire network. the growth time, nanowires will incline and contact other nanowires, forming nanowire– nanowire junctions, which can be utilized as a sensing channel for gas detection. Such on- chip growth nanowire sensors can prevent nanowire–electrode contact issues encountered by the flat nanowire sensor devices, and these kind of sensors show a more stable physical and electrical performance than the sensors fabricated by spinning, dropping, or spray coating technique. Park et al. firstly reported a comparison study of NO2 sensing between air-bridged nanowire sensors and conventional nanoparticle sensors [251]. Ow- ing to a higher exposed area to the surroundings, the bridging structure shows a faster response and recovery rates as compared with that of nanoparticle sensors. Meanwhile, Kim et al., Jung et al., and Hung et al. systematically studied the effect of nanowire junction density on the device sensing performance [245,253,260]. Through the controlling of nanowire growth time, the number of nanowire–nanowire junctions within the bridging sensor structures can be quantitatively manipulated. Furthermore, it was found that the bridged structures with a high density of junctions show a high sensing performance to NO2. However, Figurethe response 15. Two typical and recovery methods for time making of the a mat-like bridging metal–oxide nanowire nanowires device device: with ( aa) platform; high density of junctions(b) nanowires are transferred slower than onto that the platform;of the low-density (c) contact electrode nanowire deposition; device, (d because) contact electrode a long time is neededdeposition; for the and molecules’ (e) nanowires diffus transferredion in onto the thehigh platform. density of nanowire network.

Figure 16.FigureA typical 16. A methodtypical formethod making for bridging making metal–oxidebridging metal–oxide nanowires device:nanowires (a) substrate; device: (a (b) )substrate; bottom electrode (b) deposition;bottom (c) seed/catalyst electrode deposition; layer deposition; (c) seed/catalyst and (d) in situ layer growth deposition; of metal–oxide and (d) nanowire in situ growth on electrodes. of metal–oxide nanowire on electrodes. 3.2.3. Device with Vertical Structure 3.2.3. Device with VerticalRecently, Structure the nanowire array sensors with the vertical structure are promising due to Recently, thetheir nanowire porosity array structure sensors and reliablewith the electrical vertical performance structure are [268 promising–270]. To obtaindue to a vertical nanowire sensor, three steps are typically needed, as shown in Figure 17. First, nanowires their porosity structure and reliable electrical performance [268–270]. To obtain a vertical are grown vertically on the substrate through solution–phase/vapor–phase methods. Then, nanowire sensor,the three nanowires steps are are typically covered byneed spined, coating as shown a layer in Figure of organic 17. First, photoresist. nanowires By precisely are grown verticallycontrolling on the the thicknesssubstrate of through photoresist solution–phase/vapor–phase on a nanowire array via etching methods. process, the tips Then, the nanowiresof nanowires are covered will be by exposed. spin coating Afterwards, a layer a of metal organic contact photoresist. layer is deposited By pre- onto the cisely controllingphotoresist the thickness and connectedof photoresist with on all thea nanowire tips of the array nanowires. via etching Finally, process, a vertical the structure tips of nanowiresof will the sensorbe exposed. device Afterwards can be achieved, a metal by removing contact the layer unnecessary is deposited photoresist. onto the Cao et al. photoresist and hasconnected demonstrated with all the the integration tips of the of a nanowires. vertical WO 3Finally,nanowire a sensorvertical with structure high sensitivity of the sensor device can be achieved by removing the unnecessary photoresist. Cao et al. has demonstrated the integration of a vertical WO3 nanowire sensor with high sensitivity to NO2 [268]. It is shown that the vertical device exhibited the capability to detect NO2 concentrations down to the 50 ppb level, which shows a promising future in the field of

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Chemosensors 2021, 1, x FOR PEER REVIEW 14 of 40 to NO2 [268]. It is shown that the vertical device exhibited the capability to detect NO2 concentrations down to the 50 ppb level, which shows a promising future in the ﬁeld of low low concentrationconcentration gas detection. gas detection.In addition, In addition, Chen et Chenal. has et proposed al. has proposed a sensor a sensor array array com- composed posed of verticalof device vertical structures device structures with diff witherent different noble metal noble decorations metal decorations (Pd, Pt, (Pd, and Pt, Au) and Au) for for selective gasesselective detection gases [48]. detection Several [48 ].diffe Severalrent different types of types sensors of sensors are fabricated are fabricated via al- via altering tering the selectionthe selectionof decorating of decorating materials. materials.

Figure 17.FigureA typical 17. A method typical for method making for a vertical making metal–oxide a verticalnanowires metal–oxide device: nanowires (a) the substrate device: with(a) the the substrate seed/catalyst layer; (bwith) nanowires the seed/catalyst are grown vertically;layer; (b) (cnanowires) photoresist are coating grown with vertically; the further (c) etchingphotoresist process; coating and ( dwith) top the contact electrodefurther deposition etching and process; the removal and of ( photoresist.d) top contact electrode deposition and the removal of photoresist.

4. Current Progress4. Current in Performance Progress in Tailor Performanceing of TailoringMetal–Oxide of Metal–Oxide Nanowire-Based Nanowire-Based Gas Gas Sensors Sensors Although metal–oxide nanowires have already shown great potential as gas sens- Although metal–oxideing materials, nanowires further efforts have are already still encouraged shown great to enhancepotential the as sensinggas sensing performance materials, furtherof efforts nanowire-based are still encouraged sensor electronics, to enhance such asthe gas sensing species performance discrimination of from nan- mixtures, owire-based sensorlow electronics, concentration, such and as room gas species temperature discrimination detection. Tofrom achieve mixtures, such goals,low con- introducing centration, and roomadditives temperature on the nanowire detection. surface To hasachieve been provedsuch goals, to be introducing an easy and effective additives approach. on the nanowire surface has been proved to be an easy and effective approach. 4.1. Nanoparticles Decoration (NPs Decoration) 4.1. Nanoparticles DecorationThe loading (NPs of Decoration) noble metal/oxide nanoparticles with a nanometer size onto a metal– oxide nanowire surface has been widely employed for functionalizing the nanowire-based The loadingsensors of noble due metal/oxide to their low nanoparticles cost and simplicity with ina nanometer the fabrication size process. onto a metal– Here, we intro- oxide nanowire ducesurface one has common been methodwidely (notemployed least) to for load functionalizing nanoparticles on the nanowires. nanowire- For noble based sensors duemetal to nanoparticletheir low cost decoration and simplicity as an example, in the afabrication noble metal process. (M) precursor Here, solutionwe was introduce one commonfirstly prepared method by (not dissolving least) to appropriated load nanoparticles HMClx· Hon2O nanowires. into H2O (ethanol, For noble etc.). Then, metal nanoparticleby simplydecoration immersing as an nanowiresexample, ina noble the above metal solution (M) precursor for a proper solution time and was calcined at firstly prepared highby dissolving temperature, appropriated M-loaded nanowires HMClx·H can2O into be obtained. H2O (ethanol, This method etc.). Then, can also by be used for metal–oxide nanoparticle decoration, despite using different precursors. Currently, simply immersing nanowires in the above solution for a proper time and calcined at high there are two coexisting mechanisms to understand the sensing performance of decorated temperature, M-loadednanowires: nanowires (1) chemical can sensitizationbe obtained. (spillover This method effect): can noble also metal be used nanoparticles for can metal–oxide nanoparticlepromote the decoration, dissociation despite of molecules using different and oxygen precursors. on nanowire Currently, surface, there leading to an are two coexistingefficient mechanisms chemical to reaction. understand Moreover, the sensing owing performance to the noble metal-induced of decorated nan- lower energy owires: (1) chemicalbarrier sensitization for gas adsorption (spillover and effect): desorption, noble the metal response nanoparticles and recovery can promote rates can be accel- the dissociation eratedof molecules [271]; (2) and electronic oxygen sensitization: on nanowire the interfacesurface, between leading oxidized to an efficient noble metals (or chemical reaction.oxide Moreover, nanoparticles) owing and to nanowiresthe noble metal-induced usually form a thicker lower electronenergy depletionbarrier for layer with gas adsorption anda narrow desorption, channel. the During response exposure and torecovery the target rates molecules, can be theaccelerated concentration [271]; of charge carriers can be efficiently modulated [272]. To date, researchers have made significant (2) electronic sensitization: the interface between oxidized noble metals (or oxide nano- progress in fabricating nanoparticles decorated nanowire sensors. Through optimizing particles) and nanowiresthe decorated usually material form selection/combination, a thicker electron depletion the size of layer nanoparticles, with a narrow and the surface channel. Duringcoverage exposure of to nanowires, the target it molecu is accessibleles, the to realize concentration the fabrication of charge of functionalized carriers can nanowire be efficiently modulatedsensors with [272]. high To sensitivity, date, resear highchers selectivity, have made and lowsignificant working progress temperature. in fab- A detailed ricating nanoparticlesoverview decorated of the sensing nanowire performance sensors. of nobleThrough metal-decorated optimizing metal–oxide the decorated nanowire gas material selection/combination,sensors is summarized the size in Tableof nanoparticles,1. and the surface coverage of nanowires, it is accessible to realize the fabrication of functionalized nanowire sensors with high sensitivity, high selectivity, and low working temperature. A detailed overview of the sensing performance of noble metal-decorated metal–oxide nanowire gas sensors is summarized in Table 1. As for sensitivity, Kolmakov et al. firstly reported the use of noble metal (Pd) functionalized with metal–oxide nanowire (SnO2) device for the sensitivity enhancement [273]. As shown in Figure 18 (d), due to (1) oxygen dissociation on Pd nanoparticles by spillover

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Chemosensors 2021, 1, x FOR PEER REVIEW 15 of 40 As for sensitivity, Kolmakov et al. ﬁrstly reported the use of noble metal (Pd) functionalized with metal–oxide nanowire (SnO2) device for the sensitivity enhancement [273]. As effect,shown inand Figure (2) 18the d, diffusion due to (1) oxygen of weakly dissociation adsorb on Pded nanoparticles oxygen on by nanowire spillover effect, surface to Pd nanoparticles,and (2) the diffusionit is found of weakly that adsorbedthe Pd oxygennanoparticles on nanowire decoration surface to Pd can nanoparticles, dramatically it enhance the deviceis found sensing that the Pd performance nanoparticles decorationto O2 and can H dramatically2. Motivated enhance by this the device encouraging sensing result, to date, tremendousperformance to efforts O2 and via H2. noble Motivated metal/oxide by this encouraging nanoparticles result, to decoration date, tremendous are being intensively efforts via noble metal/oxide nanoparticles decoration are being intensively investigated investigatedfor enhancing devicefor enhancing sensitivity [device87,273–277 sensitivity]. [87,273–277].

Figure 18. (a–c) Nanoparticles decorated metal–oxide nanowire with different coverage; (d) schematic

Figuredepiction 18. of the(a)– three(c) Nanoparticles major processes taking decorated place ata metal–oxide SnO2 nanowire surfacenanowire and their with band different diagram; coverage; (d) sche- maticand (e )depiction compared studies of the of three the sensing major performance processes of taking SnO2 and place SnO2 at/Pd a nanowireSnO2 nanowire sensor devices. surface and their band diagram;Reprinted fromand reference(e) compared [273] with studies permission of the from sensing the American performance Chemical Society.of SnO2 and SnO2/Pd nanowire sensor devices.Nanoparticle Reprinted decoration from is reference also an alternative [273] with way permission to dramatically from improve the Amer theican selec- Chemical Society. tivity of the nanowire-based sensors [174,272,274–286]. For example, Kim et al. reported a ZnONanoparticle nanowire sensor decoration with high sensitivity is also and an selectivityalternative toward way H to2 via dramatically Pd nanoparticle improve the selec- tivitydecoration of the [274 nanowire-based]. Due to the synergistic sensors effect [174,272 of chemical,274–286]. sensitization For of Pdexample, nanoparticles Kim et al. reported a and metallization effect of ZnO, the sensing response to H2,O2, NO2,C6H6, and C7H8 ZnOare increased nanowire by 3214.1, sensor 1.6, with 7.8, 17.2, high and sensitivity 166.9%, respectively. and selectivity Moreover, toward compared H with2 via Pd nanoparticle decorationpristine nanowire, [274]. the Due Pd-decorated to the synergistic ZnO nanowire effect shows of a chemical capability to sensitization detect 0.1 ppm Hof2 Pd nanoparticles ◦ andwith metallization a wide temperature effect range of from ZnO, 150 the to 350 sensingC. Byoun response et al. reported to H a2 TeO, O2, nanowiresNO2, C6H6, and C7H8 are increasedgas sensor byby surface 3214.1, decoration 1.6, 7.8, with 17.2, oxide and nanoparticles 166.9%, [respectively.280]. Due to the formationMoreover, of compared with p–n heterojunctions between p-type TeO nanowire and n-type ZnO nanoparticles, the pristine nanowire, the Pd-decorated2 ZnO nanowire shows a capability to detect 0.1 ppm TeO2 nanowires become more resistant and suitable for sensing oxidizing gas, especially Hsince2 with it shows a wide anexcellent temperature NO2 selectivity range from in comparison 150 to 350 with ℃ interfering. Byoun gases et al. such reported as a TeO2 nan- owiresSO2, CO, gas and sensor C2H5OH. by surface decoration with oxide nanoparticles [280]. Due to the for- Furthermore, surface decoration with noble metal/oxide nanoparticles has been re- 2 mationgarded as of an p–n effective heterojunctions way in lowering downbetween the sensor p-type operation TeOtemperature nanowire [280 and,287 n-type–295]. ZnO nanoparticles,For example, the TeO as reported2 nanowires by Liang become et al., due more to the spilloverresistant effect and and suitable the change for in sensing deple- oxidizing gas, especiallytion layer caused since by it Au shows nanoparticles, an excellent the Au NO decorated2 selectivity VO2 nanowires in comparison sensor exhibits with an interfering gases excellent sensing performance to NO at 0.5–5 ppm at room temperature [285]. Lupan et al. such as SO2, CO, and C2H5OH.2 have reported that Au nanoparticle decoration on ZnO nanowire surface can realize the H2 room Furthermore, temperature detection surface [289 decoration], Choi et al. foundwith thatnoble Pd nanoparticlesmetal/oxide decoration nanoparticles on has been re- garded as an effective way in lowering down the sensor operation temperature [280,287– 295]. For example, as reported by Liang et al., due to the spillover effect and the change in depletion layer caused by Au nanoparticles, the Au decorated VO2 nanowires sensor exhibits an excellent sensing performance to NO2 at 0.5–5 ppm at room temperature [285]. Lupan et al. have reported that Au nanoparticle decoration on ZnO nanowire surface can realize the H2 room temperature detection [289], Choi et al. found that Pd nanoparticles decoration on ZnO nanowires enables the CO sensing at room temperature [287] and Hong et al. have reported the Cu2O nanoparticles-decorated ZnO nanowire sensor could realize the room temperature sensing of NO [296]. In addition, the effects of decorated nanoparticle size and density have also been intensively investigated [192,271,282,296,297]. For example, Lee et al. have studied the size effect of Au nanoparticles on the CuO nanowire sensor [192]. As the diameter of Au nanoparticles decreases, the sensing responses of the nanowire sensor present an increase to NO2 and CO, and a maximum performance appears when 60 nm of Au nanoparticles are

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ZnO nanowires enables the CO sensing at room temperature [287] and Hong et al. have reported the Cu2O nanoparticles-decorated ZnO nanowire sensor could realize the room temperature sensing of NO [296]. In addition, the effects of decorated nanoparticle size and density have also been intensively investigated [192,271,282,296,297]. For example, Lee et al. have studied the size effect of Au nanoparticles on the CuO nanowire sensor [192]. As the diameter of Au nanoparticles decreases, the sensing responses of the nanowire sensor present an increase to NO2 and CO, and a maximum performance appears when 60 nm of Au nanoparticles are decorated on the CuO nanowire surface. Meanwhile, they have pointed out that the size of nanoparticles should be carefully optimized to prevent sufﬁcient surface coverage, since excessive surface coverage would cause the steric hindrance on the nanowire surface, which is harmful to gas sensing.

Table 1. A detailed overview of sensing performance of noble metal-decorated metal–oxide nanowire gas sensors.

T LOD Response NWs NPs Target Gas oper Response Ref. (◦C) (ppm) Time(s)

Ag NH3 450 0.05 300 @ 100 ppm 45 [298] Au NO2 200 0.1 2 @ 0.1 ppm N/A [299] Au/ZnO-branching NO2 300 2 13 @ 10 ppm 118 [300] Au/ZnO-shell CO 300 0.0026 26.6 @ 0.1 ppm 75 [301] Pd H2 300 1 55.72 @ 100 ppm 22 [302] Pd NO2 300 0.1 505 @ 0.1 ppm 20 [271] Pd H2 300 1 16.95 @ 1 ppm N/A [88] SnO2 Pd H2 150 10 4.5 @ 100 ppm N/A [303] Pt NO2 300 0.1 700 @ 0.1 ppm 10 [271] Pt H2 25 N/A 1.87 @ 1000 ppm 0.33 [288] Pt Ethanol 300 0.1 6.5 @ 100 ppm N/A [304] Pt Benzene 350 0.1 18 @ 100 ppm N/A [304] Pt Acetone 300 0.1 5.8 @ 100 ppm N/A [304] Pt H2 350 0.1 4 @ 100 ppm N/A [304] Pt Toluene 300 0.1 58 @ 100 ppm N/A [304] Pd Ethanol 260 - 5 @ 500 ppm 6 [305] Ag Ethanol 450 5 228.1 @ 100 ppm 40–80 [306] Au NO2 150 1 31.4 @ 1 ppm 29 [284] Au NO2 25(UV) 1 2.6 @ 1 ppm 39.5 [292] Au H2 25 <1 ppm 40 @ 100 ppm N/A [289] Au Acetone 172 15 50.5 @ 100 ppm 1 [307] Au H2 25 20 32.9 @ 1000 ppm N/A [282] ZnO Au/Fe2O3 NO2 400 150 247 @ 250 ppm N/A [308] Au/Pd NO2 100 1 94.2 @ 1 ppm 35 [309] Pd Benzene 25(UV) 0.0067 2.2 @ 50 ppm N/A [310] Pd H2 350 1 87 @ 100 ppm N/A [274] Pd H2S 300 10 20 @ 500 ppm 720 [311] Pd/BN H2 200 0.1 12.3 @ 50 ppm 240 [307] Pt H2S 260 0.0011 65 @ 0.3 ppm 40 [312] Pt Toluene 25(UV) 0.0003 2.86 @ 50 ppm N/A [310] Au n-butanol 250 5 147 @ 100 ppm 16.5 [313] Au Acetone 250 5 72 @ 200 ppm 17.5 [313] Pd/Au Acetone 300 200 152.4 @ 200 ppm 96 [314] WO 3 Pd/Au n-butanol 200 5 93 @ 50 ppm 12 [315] Rh Acetone 300 0.2 75 @ 5 ppm 11 [316] Ru Acetone 350 0.05 78 @ 5 ppm 11.7 [202]

W18O49 Ag/Pt Trimethylamine 240 0.071 22 @ 2 ppm 15 [277] Ag Ethanol 25 0.5 1900 @ 100 ppm N/A [317] In2O3 Au CO 25 0.5 2200 @ 100 ppm N/A [317] Pt H2 25 0.5 1400 @ 100 ppm N/A [317] Chemosensors 2021, 9, 41 17 of 41

Table 1. Cont.

T LOD Response NWs NPs Target Gas oper Response Ref. (◦C) (ppm) Time(s)

Au NO2 300 1 2.2 @ 50 ppm N/A [192] Au CO 350 1 1.25 @ 50 ppm N/A [192] CuO Pt Ethanol 200 1000 3.8 @ 1000 ppm 480 [318] Pd H2 200 1000 4.5 @ 1000 ppm 600 [318] Pd H2S 100 1.9 @ 100 ppm N/A [319]

PdO Pt H2 25 10 1.2 @ 100 ppm 166 [290]

VO2 Au NO2 25 0.5 3.22 @ 5 ppm N/A [285]

NWs: nanowires; NPs: nanoparticles; Toper: operation temperature of sensor; LOD: limit of detection); Response: Rg/Ra (Oxidizing gas) or Ra/Rg (Reducing gas).

4.2. Branched Nanowire In addition to surface decoration, branched nanowires (nanowire hierarchical nanostructures) offer a high specific surface area for gas diffusion and adsorption, which are beneficial for molecular sensing. Khoang et al. reported a controllable and scalable route for preparing n-type ZnO branched n-type SnO2 nanowire hierarchical nanostructures via a combination of thermal evaporation method (SnO2 nanowires) and hydrothermal method (ZnO nanowires). As shown in Figure 19, compared with pure SnO2 nanowires, the branched nanowires sensor shows three–five-fold sensitivity enhancement to 25–500 ppm ethanol. Kaur et al. demonstrated an n-type ZnO branched p-type NiO nanowire hierarchical structures for sensing performance enhancement [233]. In such a kind of sensor, the surface of NiO nanowires is fully covered by ZnO nanowires, leading to a response mode transformation from the p-type to n-type. In addition, the lowest detection limits of these sensors to ethanol and acetone can be down to 7 and 11 ppm, respectively. Moreover, to further improve the device sensing performance, Choi et al. attempted Chemosensors 2021, 1, x FOR PEER REVIEW to decorate Au nanoparticles on the ZnO branched SnO2 nanowire surface18 of 40 [300 ]. It was indicated that the sensing response of the ZnO-branched SnO2 nanowire sensor is dramatically enhanced after Au nanoparticle decoration due to the spillover effect of noble metal. Meanwhile, the sensor shows an excellent selectivity toward NO2. Similar results have Meanwhile, the sensor shows an excellent selectivity toward NO . Similar results have also been reported recently by Bang et al. [320]. They realized the low temperature2 and also been reported recently by Bang et al. [320]. They realized the low temperature and selective NO2 sensing via using synergistic effects of Pt decoration and Bi2O3 branching. selective NO2 sensing via using synergistic effects of Pt decoration and Bi2O3 branching.

Figure 19. Left ﬁgure: branches nanowire with different length on ZnO nanowires: (a) as fabricated ZnO nanowires;

(b) 1 hFigure growth 19. SnO Left2 branched figure: branches ZnO nanowire; nanowire (c) 2 hwith growth; different (d) 4 h length growth. on Right ZnO ﬁgure: nanowires: SnO2 nanowire (a) as fabricated and SnO 2–ZnO hierarchicalZnO nanowires; structure sensing (b) 1h performance; growth SnO (e2) branched as fabricated ZnO ZnO nanowire; nanowires; (c) (f )2h 1 h growth; growth SnO(d) 24hbranched growth. ZnO Right nanowire; (g) 2 hfigure: growth; SnO and2 nanowire (h) 4 h growth. and ReprintedSnO2–ZnO from hierarchical reference [321structure] with permissionsensing performance; from the American (e) as Chemicalfabricated Society. ZnO nanowires; (f) 1h growth SnO2 branched ZnO nanowire; (g) 2h growth; and (h) 4h growth. Reprinted from reference [321] with permission from the American Chemical Society.

4.3. Core–Shell Structure (C–S Structure) Core–shell (C–S) metal–oxide nanowire sensors have also been extensively studied due to their ability to detect an extremely low concentration of chemical species and mitigate the poor selectivity [322,323]. The C–S structure is similar to the nanoparticle-decorated structure but with a continuous layer and higher stability [271]. Such a shell layer can serve as a catalytic layer to promote the molecular oxidizing [324], narrow the conduction core part of nanowire [325], facilitate the adsorption of gas molecules to increase the sensing activity [326], and work as a main conductive channel for sensing [327]. Cur- rently, various C–S nanowires with a combination like n–n (n-type core/n-type shell) [323,327–330], n-p (n-type core/p-type shell) [256,331], and p-n (p-type core/n-type shell) [252,324,332] have been employed as gas sensors. According to recent C–S nanowire gas sensors, the shell layer selection and its deposition thickness are crucially important to tailor the performance of the nanowire-based gas sensors [332,333]. Park et al. firstly demonstrated an exciting model to understand the sensitivity enhancement in SnO2-ZnO C–S nanofibers [328]. As the deposition shell thickness is comparably shorter than or equivalent to the Debye length of the shell materials, the conducting channel can be fully depleted during the exposure to target molecules. As a result, the conductance variation is favorably considerable compared with that of the non-C–S structure. Katoch et al. also reported a similar result via systematically studying the impact of shell thickness on the sensing performance of core/shell structures [330]. By gradually changing the thickness of ZnO on SnO2 nanofibers, it was observed that the nanofiber sensor device has the highest sensing response towards CO when 20 nm of ZnO shell is loaded on the SnO2 nanofiber surface. Such thickness is identically close to the Debye length of ZnO. As shown in Figure 20(a)–(c), by controlling the ALD growth time of the shell layer, Choi et al. proposed a dual functional sensing mechanism in the SnO2/ZnO C–S nanowires [327]. First, as the shell layer is thinner than its Debye length, the shell layer will experi- ence a large resistance modulation during the sensor exposure to reducing gas. As a result, a substantial portion of electron transport occurs through the inner core nanowire, which would weaken its sensing performance. Second, as the shell layer is slightly larger than its Debye length, electrical transport will be mostly confined within the shell layer, leading to a high modulation/response in sensing performance.

Chemosensors 2021, 9, 41 18 of 41

4.3. Core–Shell Structure (C–S Structure) Core–shell (C–S) metal–oxide nanowire sensors have also been extensively studied due to their ability to detect an extremely low concentration of chemical species and mitigate the poor selectivity [322,323]. The C–S structure is similar to the nanoparticle-decorated structure but with a continuous layer and higher stability [271]. Such a shell layer can serve as a catalytic layer to promote the molecular oxidizing [324], narrow the conduction core part of nanowire [325], facilitate the adsorption of gas molecules to increase the sensing activity [326], and work as a main conductive channel for sensing [327]. Currently, various C–S nanowires with a combination like n–n (n-type core/n-type shell) [323,327–330], n-p (n-type core/p-type shell) [256,331], and p-n (p-type core/n-type shell) [252,324,332] have been employed as gas sensors. According to recent C–S nanowire gas sensors, the shell layer selection and its deposition thickness are crucially important to tailor the performance of the nanowire-based gas sensors [332,333]. Park et al. firstly demonstrated an exciting model to understand the sensitivity enhancement in SnO2-ZnO C–S nanofibers [328]. As the deposition shell thickness is Chemosensors 2021, 1, x FOR PEER REVIEW comparably shorter than or equivalent to the Debye length of the shell materials,19 of 40 the conducting channel can be fully depleted during the exposure to target molecules. As Singh et al. havea result, discussed the conductance carrier transport variation is based favorably on In considerable2O3 nanowires compared and with In2O that3/ZnO of the non-C–S structure. Katoch et al. also reported a similar result via systematically studying core–shell nanowiresthe impactnetwork of shell devices thickness [334]. on the Du sensinge to the performance large resistance of core/shell of structures the polycrys- [330]. By talline ZnO shell, carriersgradually mainly changing flow the through thickness ofthe ZnO single on SnO crystalline2 nanofibers, In2 itO was3 core. observed To achieve that the electron transportationnanofiber from sensor nanowire device has to thenanowire, highest sensing only responseone potential towards barrier CO when (core–core, 20 nm of ZnO shell is loaded on the SnO2 nanofiber surface. Such thickness is identically close to the C/C) has to be overcomeDebye lengthin the of In ZnO.2O3 nanowires device (Figure 20 (d)), while the electrons are essentially to overcomeAs shown three in potential Figure 20a–c, barriers by controlling (core to the shell ALD (C/S), growth shell time ofto theshell shell (S/S), layer, and shell to core (S/C))Choi etin al. the proposed core–shell a dual nanowire functional devices sensing (Figure mechanism 20(e)). in the Owing SnO2/ZnO to the C–S nanowires [327]. First, as the shell layer is thinner than its Debye length, the shell layer will modulation of potentialexperience barriers, a large resistancesignificant modulation enhancements during the in sensor sensor exposure response to reducing for the gas. re- As a ducing gases have beenresult, achieved. a substantial This portion mechan of electronism transport also can occurs be used through to explain the inner the core sensing nanowire, performance enhancementwhich would in weaken other itsp/n sensing and performance. n/p types Second,of core/shell as the shell nanowire layer is slightly sensors larger than its Debye length, electrical transport will be mostly confined within the shell layer, [256,325,329,331,335].leading to a high modulation/response in sensing performance.

Figure 20. (a) Nanowire with SnO core and ZnO shell structure; (b) EDS of core–shell nanowire; (c) shell thickness- Figure 20. (a) Nanowire with2 SnO2 core and ZnO shell structure; (b) EDS of core–shell nanowire; (c) dependent sensing performance towards CO, benzene, toluene, and NO2. Reprinted from reference [327] with permission shell thickness-dependent sensing performance towards CO, benzene, toluene, and NO2. Reprinted from the American Chemical Society. (d,e) Schematics of the hydrogen-sensing mechanism for In O nanowires and from reference [327] with permission from the American Chemical Society. (d)–(e)2 Schematics3 of the In2O3/ZnO C–S nanowires sensors. Reprinted from reference [334] Copyright 2011 Elsevier. hydrogen-sensing mechanism for In2O3 nanowires and In2O3/ZnO C–S nanowires sensors. Re- printed from reference [334] Copyright 2011 Elsevier.

4.4. Impurities Doping Introducing dopants have a significant impact on the electrical and chemical properties of semiconductor metal–oxides [336–338]. Experimental studies have proved that small amounts of impurity additives (< 0.1 at% or lower) can significantly alter the electronic and chemical properties of metal–oxide nanowires due to the modulation of the Fermi level and introducing additional adsorption sites [339]. For example, Zhang et al. have studied the oxygen vacancies doping effect in In2O3 nanowires for NH3 sensing [340]. According to their report, via varying the concentration of oxygen vacancies in In2O3 nanowires, the Fermi level (EF) of the In2O3 nanowires can be controlled close to the conduc-

tion band and above the energy level of NH3 (ENH3) in a heavily doped condition, or the

Fermi level (EF) can be below ENH3 in a relatively low doping concentration. As shown in

Figure 18 (a), when the EF of nanowires is higher than ENH3, electrons should transfer from the nanowire to the adsorbed NH3 and result in a reduction in the nanowire carrier con-

centration. When the EF of nanowires is lower than ENH3, electrons should migrate from adsorbed NH3 to the nanowire and result in an enhanced conductance, as shown in Figure 21(a) and 21(b). This work indicates that the density of dopants can determine the signal and amplitude of the nanowire sensor response. Moreover, Singh et al. have reported an approach to tune the Fermi level of In2O3 nanowires via Zn doping and achieving the room temperature detection of CO [341]. As shown in Figure 21(c) and 21(d), due to the small chemical potential gradient between the adsorbed CO molecule and Fermi level of undoped In2O3 nanowires, there is less electron transfer (to reach equilibrium) and the nanowire-based sensor shows a poor sensitivity to CO at room temperature. In contrast, as shown in Figure 21(e) and 21(f), a big difference of the chemical potential between the Fermi level of nanowire and CO molecule is presented after In2O3 nanowires are doped with Zn, as a result, the Zn doped In2O3 nanowire sensor device offers a fast (response

Chemosensors 2021, 9, 41 19 of 41

Singh et al. have discussed carrier transport based on In2O3 nanowires and In2O3/ZnO core–shell nanowires network devices [334]. Due to the large resistance of the polycrystalline ZnO shell, carriers mainly ﬂow through the single crystalline In2O3 core. To achieve electron transportation from nanowire to nanowire, only one potential barrier (core–core, C/C) has to be overcome in the In2O3 nanowires device (Figure 20d), while the electrons are essentially to overcome three potential barriers (core to shell (C/S), shell to shell (S/S), and shell to core (S/C)) in the core–shell nanowire devices (Figure 20e). Owing to the modulation of potential barriers, signiﬁcant enhancements in sensor response for the reducing gases have been achieved. This mechanism also can be used to explain the sensing performance enhancement in other p/n and n/p types of core/shell nanowire sensors [256,325,329,331,335].

4.4. Impurities Doping Introducing dopants have a significant impact on the electrical and chemical properties of semiconductor metal–oxides [336–338]. Experimental studies have proved that small amounts of impurity additives (< 0.1 at% or lower) can significantly alter the electronic and chemical properties of metal–oxide nanowires due to the modulation of the Fermi level and introducing additional adsorption sites [339]. For example, Zhang et al. have studied the oxygen vacancies doping effect in In2O3 nanowires for NH3 sensing [340]. According to their report, via varying the concentration of oxygen vacancies in In2O3 nanowires, the Fermi level (EF) of the In2O3 nanowires can be controlled close to the conduction band and above the energy level of NH3 (ENH3) in a heavily doped condition, or the Fermi level (EF) can be below ENH3 in a relatively low doping concentration. As shown in Figure 18a, when the EF of nanowires is higher than ENH3, electrons should transfer from the nanowire to the adsorbed NH3 and result in a reduction in the nanowire carrier concentration. When the EF of nanowires is lower than ENH3, electrons should migrate from adsorbed NH3 to the nanowire and result in an enhanced conductance, as shown in Figure 21a,b. This work indicates that the density of dopants can determine the signal and amplitude of the nanowire sensor response. Moreover, Singh et al. have reported an approach to tune the Fermi level of In2O3 nanowires via Zn doping and achieving the room temperature detection of CO [341]. As shown in Figure 21c,d, due to the small chemical potential gradient between the adsorbed CO molecule and Fermi level of undoped In2O3 nanowires, there is less electron transfer (to reach equilibrium) and the nanowire-based sensor shows a poor sensitivity to CO at room temperature. In contrast, as shown in Figure 21e,f, a big difference of the chemical potential between the Fermi level of nanowire and CO molecule is presented after In2O3 nanowires are doped with Zn, as a result, the Zn doped In2O3 nanowire sensor device offers a fast (response time 20s and recovery time 10s) and large transfer of electrons (1–5 ppm CO detection). However, due to the difficulties in introducing impurities into single-crystalline nanowires, impurities doping is less popular than nanoparticles decoration, branched structures, and core–shell structures for achieving high-performance device. Chemosensors 2021, 1, x FOR PEER REVIEW 20 of 40 time 20s and recovery time 10s) and large transfer of electrons (1–5 ppm CO detection). However, due to the difficulties in introducing impurities into single-crystalline nanowires, impurities doping is less popular than nanoparticles decoration, branched structures, andChemosensors core–shell2021, 9, 41 structures for achieving high-performance device. 20 of 41

Figure 21. (a) I−V curves of heavily doped In2O3 nanowires device before and after exposure to 1% NH3. Inset shows the energy band diagrams of heavily doped In O and NH molecules; (b) I−V curves of lightly doped In O nanowires device Figure 21. (a) I−V curves of heavily doped2 3 In32O3 nanowires device before 2and3 after exposure to 1% before and after exposure to 1% NH3. Inset shows the energy band diagrams of lightly doped In2O3 and NH3 molecules. NH3. Inset Reprintedshows fromthe reference energy [340 band] with permission diagrams from theof Americanheavily Institute doped of Physics In2O (AIP)3 and Publishing; NH3 (cmolecules;) sensing (b) I−V curves of lightlyresponse doped of Undoped In In22OO33nanowires nanowires (d) Energy device band diagrams before of undopedand after In2O3 exposureand CO molecules; to 1% (e) sensing NH the3. Inset shows response of Zn-doped In2O3 nanowires; (f) energy band diagrams of Zn-doped In2O3 and CO molecules. Reprinted from the energy bandreference diagrams [341] Copyright of 2010 lightly Elsevier. doped In2O3 and NH3 molecules. Reprinted from reference [340] with permission from the American Institute of Physics (AIP) Publishing; (c) sensing response of 5. Gas Sensing Mechanism Undoped In2O3 nanowires (d) Energy band diagrams of undoped In2O3 and CO molecules; (e) sens- Today, nanowire chemiresistors are being widely investigated due to their convenience ing the response of Zn-dopedin dataIn2O analysis,3 nanowires; high accuracy, (f) lowenergy cost, and band their diagrams ability to dynamically of Zn-doped monitor the In2O3 and CO molecules. Reprinted fromchannel refe resistance/conductancerence [341] Copyright variation 2010 during Elsevier. the exposure to the target molecules. Generally, the sensing mechanism of chemiresistors can be divided into three types. 5. Gas Sensing Mechanism5.1. Ohmic-Contacted Sensing In a traditional chemiresistor sensor, Ohmic contact is highly preferred to study the Today, nanowire chemiresistorsintrinsic properties are of nanowires. being Generally,widely the investigated classical D–L model due is responsible to their for conven- the sensitivity of nanowire gas sensors, where D represents nanowire diameter (D), and L ience in data analysis, highrepresents accuracy, the depth low of the cost, surface depletionand their layer (L)ability [342,343 to]. Considering dynamically the n-type monitor oxide case, when a sensor is exposed to the atmosphere, oxygen can be adsorbed on the channel resistance/conductancethe surface of metal–oxide variation nanowire during and becomesthe exposure negatively charged to the by target the following molecules. Generally, the sensing mechanismequations [22]: of chemiresistors can be divided into three types. O2(gas) ↔ O2(ads) (1) 5.1. Ohmic-Contacted Sensing In a traditional chemiresistor sensor, Ohmic contact is highly preferred to study the intrinsic properties of nanowires. Generally, the classical D–L model is responsible for the sensitivity of nanowire gas sensors, where D represents nanowire diameter (D), and L represents the depth of the surface depletion layer (L) [342,343]. Considering the n-type oxide case, when a sensor is exposed to the atmosphere, oxygen can be adsorbed on the surface of metal–oxide nanowire and becomes negatively charged by the following equations [22]:

O2(gas) ↔ O2(ads) (1)

- - O2(ads) + e ↔ O2 (ads) (<100 ℃) (2)

- - - (3) O2 (ads) + e ↔ 2O (ads) (100-300 ℃) O-(ads) + e- ↔ O2- (ads) (>300 ℃) (4)

Chemosensors 2021, 1, x FOR PEER REVIEW 21 of 40

Chemosensors 2021, 9, 41 21 of 41 Such oxygen-charged behavior would extract electrons and create a shell-like surface depletion layer on the nanowire surface (Figure 22(a)), which can effectively modulate channel conductivity. Moreover, due to the adsorbed oxygen ions (O- and O2-), highly ac- O (ads) + e− ↔ O − (ads) (<100 ◦C) (2) tive towards reducing gas, high temperatures2 (> 100 ℃) 2are usually required to generate − − − ◦ oxygen ions and enhance chemical reaction.O2 (ads) Fo +r e example,↔ 2O (ads)when (100–300 the sensorC) is exposed to (3) the gases (such as toluene, ethanol, CO,O −NH(ads)3, H +2 e,− methanol,↔ O2− (ads) shown (>300 in◦C) Equations (5)-(7) (4) [22]), the charged oxygen (O-) can react with these gases, and release electrons back to the Such oxygen-charged behavior would extract electrons and create a shell-like surface nanowire bulk. As a result, the thickness of the depletion layer is shrunk, leading to an depletion layer on the nanowire surface (Figure 22a), which can effectively modulate increase in conductancechannel conductivity. (Figure 22(b)). Moreover, In contrast, due towhen the adsorbed the sensor oxygen is exposed ions (O to− andthe Ooxi-2−), highly dizing gases (suchactive as towardsNO2, O3 reducing, Equations gas, high(8), (9) temperatures [22]), these (>100 gases◦C) can are extract usually requiredmore elec- to generate trons from the nanowireoxygen ions surface. and enhance Then, chemical the depletion reaction. layer For example,thickness when increases the sensor and isresults exposed to the in a reduction ingases nanowire (such asconductance toluene, ethanol, (Figure CO, 22(c)): NH3,H 2, methanol, shown in Equations (5)–(7) [22]), the charged oxygen (O−) can react with these gases, and release electrons back to the nanowire bulk. As a result, the thickness of the depletion layer is shrunk, leading to - - an increase in conductanceCO(ads) + O (Figure(ads) → 22COb).2(gas)+ In contrast, e when the sensor is exposed(5) to the oxidizing gases (such as NO- 2,O3, Equations (8) and (9) [22]), these- gases can extract more CH3CH2OH(ads) + 6O (ads) →2CO2(gas)+ 3H2O(gas)+ 6e (6) electrons from the nanowire surface. Then, the depletion layer thickness increases and - - results in a reductionH2(ads) in nanowire+ O (ads) conductance→H2O(gas)+ (Figuree 22c): (7)

- − - − e + NOCO(ads)2(ads)↔ + NO O 2(ads)(ads)→ CO2(gas)+ e (8) (5)

- - − - − CH2e3CH + O2OH(ads)3(ads)↔ +O 6O2 (ads)+(ads) O→(ads)2CO 2(gas)+ 3H2O(gas)+ 6e (9) (6) − − H2(ads) + O (ads) →H2O(gas)+ e (7) Due to its convenience in data analysis, tremendous− works have− used the Ohmic con- e + NO2(ads)↔ NO2 (ads) (8) tacted device as a platform for fundamental studies to improve the device sensing perfor- − − − mance. 2e + O3(ads)↔ O2 (ads)+ O (ads) (9)

Figure 22.FigureA schematic 22. A schematic diagram ofdiagram conventional of conventional chemiresistor chemiresistor sensors: (a) nanowire sensors: conducting(a) nanowire channel conducting exposed to ambient;channel (b) nanowire exposed conducting to ambient; channel (b) exposed nanowire to reducingconducting gases; channel and (c) ex nanowireposed to conducting reducing channelgases; and exposed (c) to oxidizingnanowire gases. conducting channel exposed to oxidizing gases.

Due to its convenience in data analysis, tremendous works have used the Ohmic con- 5.2. Schottky-Contacted Sensing tacted device as a platform for fundamental studies to improve the device sensing performance. Recently, the Schottky-contacted nanowire sensors have attracted considerable attention due to the higher5.2. Schottky-Contacted sensitivity, faster Sensing sensing response, shorter recovery time, and lower cost as compared withRecently, the conventional the Schottky-contacted Ohmic-contacted nanowire nanowire sensors have sensors. attracted In a considerabletypical at- Schottky-contactedtention sensor due (Figure to the higher 23(a)), sensitivity, the overall faster resistance/conductance sensing response, shorter of the recovery device time, and is dominated bylower the Schottky cost as compared barrier height with the (SBH), conventional and its value Ohmic-contacted is susceptible nanowire to external sensors. In a typical Schottky-contacted sensor (Figure 23a), the overall resistance/conductance of the stimulus. Wei et al. firstly reported a Schottky-contacted nanowire sensor for gas mole- device is dominated by the Schottky barrier height (SBH), and its value is susceptible to cules detection [344].external The stimulus. device Weicontains et al. ﬁrstlyone Pt/ZnO reported junction a Schottky-contacted (Schottky contact) nanowire and sensor one for gas Pt–Ga/ZnO junctionmolecules (Ohmic detection contact). [344]. By The comparing device contains the onesensing Pt/ZnO performance junction (Schottky of the contact) Schottky-contactedand device one Pt–Ga/ZnO and the conventional junction (Ohmic Ohmic-contacted contact). By comparing device, the it sensingis shown performance that of the Schottky-contactedthe Schottky-contacted sensor device device presents and 1085 the conventional times and 8776 Ohmic-contacted times sensitivity device, im- it is shown provement to oxidizingthat the Schottky-contacted gas (O2) and reducing sensor devicegas (CO), presents respectively. 1085 times Furthermore, and 8776 times the sensitivity Schottky contacted sensor device exhibited a faster response and shorter reset time, improved by a factor of 7. This new sensing concept has been extensively applied to other

Chemosensors 2021, 9, 41 22 of 41 Chemosensors 2021, 1, x FOR PEER REVIEW 22 of 40

metal–oxide nanowire systems such as the well-known SnO2 [345] and In2O3 [346] nanowire-based sensors.improvement Here, the to oxidizingauthor recommends gas (O2) and the reducing latest gascomprehensive (CO), respectively. review Furthermore, of Chemosensors 2021, 1, x FORSchottky-contacted PEER REVIEW the Schottkynanowire contacted sensors sensorpresented device by exhibited Meng et aal. faster [21]. response and shorter22 of reset40 time, improved by a factor of 7. This new sensing concept has been extensively applied to metal–oxideother nanowire metal–oxide systems nanowire such systemsas the well-known such as the SnO well-known2 [345] and SnO In22O[3453 [346]] and nan- In2O3 [346] owire-basednanowire-based sensors. Here, sensors. the author Here, therecommends author recommends the latest thecomprehensive latest comprehensive review of review of Schottky-contactedSchottky-contacted nanowire nanowire sensors presented sensors presented by Meng by et Mengal. [21]. et al. [21].

Figure 23. A schematic diagram of Schottky-contacted sensors: (a) structure of a Schottky-contacted sensor; and (b) change of Schottky barrier height and depletion region under the external stimulation. Figure 23.Figure A schematic 23. A schematic diagram of diagram Schottky-contacted of Schottky-contacted sensors: ( sensors:a) structure (a) structureof a Schottky-contacted of a Schottky-contacted sensor; andsensor; (b) change and (b )of change Schottky of Schottky barrier height barrier and height depletion and depletion region regionunder underthe external the external stimula- stimulation. 5.3. Nanowire–Nanowiretion. Junctions Sensing In the case of5.3. an Nanowire–Nanowire n-type nanowire–nanowire Junctions Sensing junction sensor, as shown in Figure 24(a), both the5.3. thickness Nanowire–Nanowire ofIn the the electron case Junctions of an depletion n-type Sensing nanowire–nanowire layer and the potential junction barrier sensor, asheight shown of in theFigure 24a, nanowire–nanowireIn theboth case theinterface of thickness an n-type dominates ofnanowire–nanowire the electron the overall depletion junction conductance layer sensor, and theof as potentialtheshown device. in barrierFigure Further- 24(a), height of the nanowire–nanowire interface dominates the overall conductance of the device. Further- more, if boththe electron the thickness depletion of the layerelectron is smaldepletionler than layer the and nanowire the potential diameter, barrier the height potential of the more, if the electron depletion layer is smaller than the nanowire diameter, the potential barrier heightsnanowire–nanowire of nanowire–nanowire interface dominates junctions the will overall play conductance a crucial role of thein the device. gas sensingFurther- more, if barrierthe electron heights depletion of nanowire–nanowire layer is smaller junctionsthan the nanowire will play adiameter, crucial role the inpotential the gas sensing performance of nanowire sensors. Similar to the Schottky-contacted nanowire devices, the barrier heightsperformance of nanowire–nanowire of nanowire sensors. junctions Similar will to theplay Schottky-contacted a crucial role in the nanowire gas sensing devices, the nanowire–nanowire barrier height can be decreased/increased when devices are exposed performancenanowire–nanowire of nanowire sensor barriers. Similar height to can the be Schottky-contacted decreased/increased nanowire when devicesdevices, arethe exposed to a reducing/oxidizingnanowire–nanowireto a reducing/oxidizing atmosphere. barrier height atmosphere. can be decreased/increased when devices are exposed to a reducing/oxidizing atmosphere.

Figure 24. A schematic diagram of conventional nanowire–nanowire junction sensors: (a) nanowire conducting channel Figure 24.Figure A schematic 24. A schematic diagram diagram of conventional of conventional nanowire–nanowire nanowire–nanowire junction junction sensors: sensors: (a ()a )nanowire nanowire exposedconducting to ambient;conducting (channelb) nanowire channelexposed conducting exposed to ambient; channel to ambient; (b exposed) nanowire (b) nanowire to oxidization conducting conducting gas; cha and channel (cnnel) nanowire exposed exposed conductingto to oxidization oxidization channel exposedgas; to reducing and (gas;c)gas. nanowire and (c) nanowire conducting conducting channe channel exposedl exposed to reducing to reducing gas. gas. Many works have been performed using the interfacial sensing concept, not only Many worksMany haveworks been have performed been performed using using the interfacialthe interfacial sensing sensing concept, concept, not not only only forfor for the simplicity in device fabrication but also for higher sensitivity. Most importantly, the simplicity in device fabrication but also for higher sensitivity. Most importantly, this the simplicity inthis device concept fabrication also shows but strong also fo integrationr higher sensitivity. capability onto Most a sensing importantly, platform this due to the concept also shows strong integration capability onto a sensing platform due to the devel- concept also showsdevelopment strong integration of nanowire capability devices with onto bridging a sensing structures platform [246 ].due Park to et the al. devel- ﬁrstly reported opment of nanowire devices with bridging structures [246]. Park et al. firstly reported a opment of nanowirea metal–oxide devices with nanowire bridging sensor stru withctures a bridging [246]. Park structure et al. for firstly NO2 detectionreported [a251 ]. As metal–oxide nanowire sensor with a bridging structure for NO2 detection [251]. As re- metal–oxide nanowirereported sensor in their wi work,th a SnO bridging2 nanowires structure are in situfor grownNO2 detection at the pre-deposited [251]. Aselectrode re- on- ported in their work, SnO2 nanowires are in situ grown at the pre-deposited electrode on- chip without using any arduous and individual lithography process. Due to self-connection ported inchip their without work, using SnO 2any nanowires arduous areand in individual situ grown lithography at the pre-deposited process. Due toelectrode self-connec- on- with other nanowires, multiple nanowire–nanowire junctions are naturally formed as chip withouttion with using other any nanowires, arduous andmultiple individual nanowire–nanowire lithography process.junctions Dueare naturally to self-connec- formed sensing channels. Consistent gas sensing characteristics can be seen due to the averaging tion withas other sensing nanowires, channels. Consistent multiple nanowire–nanowiregas sensing characteristics junctions can be areseen naturally due to the formed averag- effect of multi nanowire connections, and high sensitivity to NO2 is obtained (Rg/Ra = 230). as sensinging channels. effect of multi Consistent nanowire gas connections, sensing ch andaracteristics high sensitivity can be to seen NO 2due is obtained to the averag- (Rg/Ra = Inspired by their encouraging work, other metal–oxide nanowires (ZnO, TiO2, and CuO) ing effect230). of multi Inspired nanowire by their connections,encouraging work,and high other sensitivity metal–oxide to nanowiresNO2 is obtained (ZnO, TiO (Rg2/R, anda = 230). InspiredCuO) have by their also beenencouraging applied to work, fabricate other brid metal–oxideging structure nanowires sensors to combine (ZnO, TiO the 2merits, and CuO) haveof in also situ been growth applied and nanowire–nanowire to fabricate bridging junctions structure sensing sensors [177,191,250,263,347]. to combine the merits of in situ growth and nanowire–nanowire junctions sensing [177,191,250,263,347]. 6. Critical Issues for Metal–Oxide Nanowire-Based Gas Sensor Devices 6. Critical Issues for Metal–Oxide Nanowire-Based Gas Sensor Devices

Chemosensors 2021, 9, 41 23 of 41

have also been applied to fabricate bridging structure sensors to combine the merits of in situ growth and nanowire–nanowire junctions sensing [177,191,250,263,347].

6. Critical Issues for Metal–Oxide Nanowire-Based Gas Sensor Devices Despite all the extraordinary achievements of 1D nanowire-based sensors, some critical problems are still encouraged to be studied and investigated further to advance the nanowire sensor from the lab to the market. Here, we mainly discuss four critical issues: reproducibility, selectivity, stability, and energy consumption.

6.1. Reproducibility of Devices Integrating a nanowire with contact electrodes on a platform allows us to study the intrinsic properties of the nanowire. However, a tiny variation in the size or shape of nanostructures will powerfully alter their physical and chemical properties. Thus, the nanowire-based electronics would inevitably suffer from poor reproducibility. To prevent the reproducibility issue, one easy method is to use a multiple nanowire sensor to eliminate the individual differences among nanowires. Currently, a commercially available fabrication process for the thin ﬁlm sensor is used for the mat-like nanowire network sensor fabrication. Zhang et al. have compared the difference in electrical properties between single and multiple nanowire sensors [75]. It was found that all the multiple nanowire devices possess similar and consistent characteristics such as conductance, threshold gate voltage, and sensitivity to gas species. The fabrication of multiple nanowire gas sensors with the bridging structure is also an alternative approach to obtain molecular sensors with high uniformity [247]. The fabrication process in detail was described in Section 3.2.2. Unfortunately, those sensors did not really face/solve the reproducibility issues in the single nanowire devices. In fact, there is no difference between using multiple nanowire devices and conventional nanoparticle devices for gas sensing despite the porous structure.

6.2. Selectivity of Device Another critical issue is the selectivity of the sensor device. Metal–oxide nanowire sensors generally have high sensitivity to the gas analytes, but they are also suffering from poor selectivity. To enhance the selectivity of sensors, a widely accepted method is introducing recognition elements on the sensing materials, for example, impurity elements doping, nanoparticle surface decorating, core/shell heterostructure construction. In addition, operational temperature modulation and nanowire architecture altering are also served as effective methods to improve the selectivity of the metal–oxide nanowire-based molecular sensors toward the target gas molecules. However, the results are usually far from prospects due to the low operational stability. In addition, an alternative way to selectively detect the target gas molecules is to use the sensor array, because sensor arrays combined by a number of non-selective sensors which can acquire more information on a specific analyte than an individual sensor [348]. Subsequently, molecule information can be extracted from sensor arrays by appropriate pattern recognition techniques, such as principal component analysis (PCA) and back- propagation artificial neural network (BP-ANN) [349]. However, according to the research from Chen et al., multi-component sensing combined with PCA cannot work well if the reactions have no big difference between a specific target species and different metal–oxide surfaces [350]. Thus, enhancing the overall sensing performance of functional materials is the priority rather than data extraction and process. Unfortunately, this is usually not an easy task.

6.3. Long-Term Stability of the Device Stability, including chemical stability and physical stability, is the essential key to open the gate for sensor practical applications. Sysoev et al. ﬁrstly pointed out that using the nanowire network sensor can obtain long-term stable sensing performance compared with the conventional nanoparticle sensors because nanowires can prevent nanostructure Chemosensors 2021, 9, 41 24 of 41

aggregation [351]. However, although numerous efforts are devoted to improving the long-term stability of metal–oxide nanowire sensors, the inﬂuence of humidity, poisoning, and new contact issues still exist to prevent such a kind of sensors from the practical applications [47]. Steinhauer et al. reported that the sensing performance of CuO nanowire- based gas sensors is strongly weakened with the increase in surrounding H2O[47]. This is because the formation of the surface hydroxyl groups induced by H2O adsorption prevents the oxygen chemisorption from the surface active sites. Meanwhile, Nakamura et al. have noticed that a ppm-level CO2 in the air can react substantially with ZnO nanowire surfaces even at room temperature [74]. Such a reaction will form an electrically insulating zinc carbonate thin layer, which can critically determine the electrical stability of hydrothermally grown single-crystalline ZnO nanowires. In addition, Zeng et al. reported a contact issue in the conventional metal–oxide nanowire device [73]. Titanium has been most widely used to contact typical metal–oxide (e.g., SnO2, ZnO) by counting the energy level matching and good adhesion to the substrate, while the Ti contact can be oxidized easily, which causes an increase in contact resistance and further degradation in sensor performance after long-time operations under high temperature. Up to now, only a few papers have mentioned how to achieve long-term stability for metal–oxide nanowire sensors, and it is still remains a crucial challenge for commercialization.

6.4. Energy Consumption An external heater is usually needed to elevate the temperature of metal–oxide gas sensors for effective gas sensing, which inevitably increases the power consumption and sensor size. There are currently two methods to solve this problem: (1) developing room temperature sensing materials; (2) using the self-Joule-heat technique. As for low-temperature sensors, tremendous progress has been made to reduce the operating temperature of the metal–oxide nanowire sensors, such as surface modiﬁcation, additives doping, UV/visible- light irradiation. For example, Lupan et al. reported that the Pd-modiﬁed ZnO nanowires sensor could achieve room temperature sensing for H2. However, it appears to have other annoying troubles such as lower sensing response, longer response time, and slow recovery rate [352]. Self-Joule-heating is a nearly ideal strategy for operating nanowire gas sensors at ultralow power consumption, without additional heaters. Currently, it has been reported that conducting self-Joule-heat for nanowire sensors and remarkably decrease power consumption. Prades et al. have reported that the energy consumption of individual SnO2 nanowire sensor devices can be down to 20 µJ/s as self-Joule-heat is performed [218]. In addition, the power consumption of the nanowire sensor device could be further decreased via the pulsed self-heating technique [353]. However, recent studies on the self-Joule-heat technique only focused on controlling the spatial thermal properties of nanowires rather than tailoring the sensing performance of these sensors.

7. Prospective towards Metal–Oxide Nanowire Gas Sensor Electronics As for the reproducibility of nanowire devices, we should carefully consider how to synthesize nanowires with high uniformity. Template-assisted nanowire growth is considered an alternative approach to achieve a highly ordered nanowire array. However, the complexity of template fabrication, low density of nanowires, and the extensive distribution of nanowire diameter have limited the wide application of such method [354]. Recently, Zhao et al. reported a two-step method to fabricate ZnO nanowires with uni- formly shaped structures, as shown in Figure 25[ 355]. Firstly, ZnO nanowires of random + size are etched by NH4 as the seed layer, and then, a very similar diameter (average about 17 nm with σ 1.3 nm, shown in Figure 25c) of ZnO nanowires are grown in the second step, which signiﬁcantly increases the reproducibility of metal–oxide nanowire. This unique ﬁnding paves the way for the fabrication of nanowires-integrated nanodevices with reliable performance. Chemosensors 2021, 9, 41 25 of 41

ChemosensorsChemosensors 2021, 1, 2021x FOR, 1, PEER x FOR REVIEW PEER REVIEW 25 of 4025 of 40

Figure Figure25. (a) SEM25. 25. (a) image(a SEM) SEM ofimage two-step image of two-step of growth two-step growth ZnO growth nanowire; ZnO ZnO nanowire; (b) nanowire; SEM (b) image SEM (b) ofimage SEM conventional imageof conventional of single- conventional single- step growthstepsingle-step growth ZnO nanowire; growthZnO nanowire; ZnO (c) comparison nanowire; (c) comparison (ofc) ZnO comparison of nanowire ZnO nanowire of diamet ZnO nanowire diameter distributioner diameterdistribution grown distribution grownby single- by single- grown step andstepby two-step single-step and two-step method; and method; two-step (d) the (d)mechanism method; the mechanism (d )of the the mechanism synthesisof the synthesis of of monodispersed the of synthesis monodispersed of sized monodispersed ZnO sized nan- ZnO sizednan- owires owiresZnOfrom nanowiresrandomly from randomly sized from seeds. randomly sized Reprintedseeds. sized Reprinted seeds.from reference Reprintedfrom reference [355] from with [355] reference permission with [permission355] from with the permission from Amer- the Amer- from ican Chemicalicanthe American Chemical Society. ChemicalSociety. Society.

Poor selectivityPoor selectivity selectivity is a perpetually is is a a perpetually perpetually perplexing perplexing perplexing problem problem problem that limits that that thelimits limits wide the the application wide wide application application of of the MOStheof thegasMOS MOSsensor, gas gassensor, including sensor, including the including metal–oxide the metal–oxide the metal–oxide nanowire-based nanowire-based nanowire-based sensors. sensors. Inspired sensors. Inspired by the Inspired by the catalyticalcatalyticalby thechemistry, catalytical chemistry, it is chemistry,worth it is worthconsidering it isconsidering worth combining considering combining porous combining porous materi materials poroussuchals as such materialszeolites, as zeolites, such metal–organicmetal–organicas zeolites, frameworks, metal–organic frameworks, and mesostructured frameworks,and mesostructured andoxides mesostructured oxides with a withmetal–oxide a metal–oxide oxides nanowire with nanowire a metal–oxide [356– [356– 358]. Canlas358].nanowire Canlas et al. [ 356reported et– al.358 reported]. a Canlas novel a etmethodnovel al. reported method to fabricate ato novel fabricate molecules method molecules toimprinted fabricate imprinted oxide molecules catalyst,oxide imprinted catalyst, and theandoxide fabrication the catalyst, fabrication process and theprocess is fabricationshown is shown in Figure process in Figure 26 is [359]. shown 26 Using[359]. in Figure Usingthis structure, 26 this[ 359 structure,]. Usingthe nanocav- thisthe nanocav- structure, ities canitiesthe preferentially nanocavities can preferentially react can preferentially with react nitrobenzene with nitrobenzene react rather with nitrobenzene ratherthan nitroxylene than nitroxylene rather in thanthe photoreduc-in nitroxylene the photoreduc- in the tion modeltionphotoreduction model and react and with react model benzyl with and benzyl alcohol react alcohol with rath benzyler raththaner alcohol2,4,6-trimethylbenzyl than 2,4,6-trimethylbenzyl rather than 2,4,6-trimethylbenzyl alcohol alcohol in the in the photo-oxidationphoto-oxidationalcohol in model. the photo-oxidation model. This technique This technique model. can be canThisapplied be technique applied to the metal–oxideto can the be metal–oxide applied nanowire to nanowire the sensor metal–oxide sensor to giganticallytonanowire gigantically improve sensor improve toselectivity gigantically selectivity due improveto due thei tor preferential thei selectivityr preferential interactions due to interactions their with preferential specific with specific interactionsVOC VOC molecules,molecules,with even speciﬁc with even VOC a with chemically molecules, a chemically similar even similar with structure. a chemicallystructure. similar structure.

Figure 26. (a) The fabrication process of molecules imprinted oxide; and (b) the preferential reaction Figure Figure26. (a) The 26. (a)fabrication The fabrication process process of molecules of molecules imprinted imprinted oxide; andoxide; (b) and the (b)preferential the preferential reaction reaction with gas species. Reprinted from reference [359] with permission from the Nature publishing group. with gaswith species. gas species. Reprinted Reprinted from reference from reference [359] wi [359]th permission with permission from the from Na turethe Na publishingture publishing group. group.

Chemosensors 2021, 1, x FOR PEER REVIEW 26 of 40

Solving the degradation issues, including nanowire degradation, contact degradation, and electrode degradation, is the only way to achieve long-term stability in nanowire de- Chemosensors 2021, 9, 41 26 of 41 vices. As shown in Figure 27(a), Nakamura et al. demonstrated a strategy for achieving the atmospheric electrical stability of ZnO nanowires [74]. Via using a thermal annealing treat- ment in vacuum/air, the insulating layer induced by the unstable –OH layer on as fabricated Solving the degradation issues, including nanowire degradation, contact degradation, ZnO nanowireand electrode surface degradation,is efficiently is eliminated the only way. Such to achieve a simple long-term and low-cost stability inmethod nanowire can enhance nanowiredevices. atmospheric As shown in Figure stability 27a, Nakamurafor at leas ett 40 al. demonstrateddays with stable a strategy electrical for achieving properties. Meanwhile,the Zeng atmospheric et al. offered electrical a stability way to of overco ZnO nanowiresme the degradation [74]. Via using in a thermal conventional annealing sensor contact, astreatment shown in in Figure vacuum/air, 27(b) the[73]. insulating It is found layer that induced the nanowire by the unstable device –OH can layerobtain on good as fabricated ZnO nanowire surface is efficiently eliminated. Such a simple and low- stability forcost at methodleast over can enhance2000 hours nanowire by replacing atmospheric the stability easily oxidized for at least contact 40 days withmetal stable (Ti) with heavily dopedelectrical metal–oxide properties. Meanwhile,(antimony Zengdoped et al.tin offered oxide). a way Moreover, to overcome Yan the et degradation al. reported an unusual annealingin conventional process sensor on contact, oxide as thin shown film ins Figure for highly 27b [73 ].thermal It is found and that chemical the nanowire stability shown in Figuredevice can 27(c) obtain [360,361]. good stability It was for indica at leastted over that 2000 Al-doped hours by replacing ZnO (AZO) the easily nano-thin oxidized films could efficientlycontact metalsuppress (Ti) with the heavily inevitable doped crysta metal–oxidel defect (antimony formation doped in tin the oxide). as-fabricated Moreover, thin Yan et al. reported an unusual annealing process on oxide thin films for highly thermal film via sequentialand chemical annealing stability shown process in Figure under 27 cair [360 and,361 ].Zn It wasvapor indicated atmosphere, that Al-doped resulting ZnO in a stable electrical(AZO) resistivity nano-thin films (~10 could−4 Ω·cm) efficiently in air, suppress even at the high inevitable temperature crystal defect(up to formation 500 °C). This thermally instable the as-fabricated thin film can thin be film utilized via sequential as electrodes annealing for process gas sensors, under airwhich and Znobtain vapor stable −4 performanceatmosphere, over 250 resulting hours compared in a stable to electrical the conventional resistivity (~10 Ti/PtΩ contact·cm) in air,sensor. even Meanwhile, at high temperature (up to 500 ◦C). This thermally stable thin film can be utilized as electrodes for the sequentialgas sensors, annealed which AZO obtain nano-thin stable performance films al overso show 250 hours highly compared chemic toal the stability conventional in buffer solution (pH:Ti/Pt 3~11) contact compared sensor. Meanwhile, with non-anneal the sequentialed AZO annealed nano-thin AZO nano-thin films and films ZnO also thin show films. These proposedhighly chemicalstrategies stability can successfully in buffer solution suppress (pH: 3~11) the comparedelectrical withperformance non-annealed degradation AZO of the nanowirenano-thin devices films andand ZnO have thin a great films. potent These proposedial to be strategiesapplied canto various successfully oxide suppress nanostructures, whichthe electricalwould performancegive a foundation degradation for of th thee nanowiredesigning devices and andfabrication have a great of potentialoxide nano- to be applied to various oxide nanostructures, which would give a foundation for the material-baseddesigning IoT and sensors fabrication with of long-term oxide nanomaterial-based stability. IoT sensors with long-term stability.

Figure 27. Figure(a) Atmospherically 27. (a) Atmospherically stable stable ZnO ZnO nanowires nanowires via via using using thethe annealing annealing process. process. Reprinted Reprinted from referencefrom [74] reference with [74 permission] with permission from fromthe American the American Chemical Chemical Society; (b ()b gas) gas sensor sensor with with long- long- term stability via using heavily doped oxide as the contact. ATO, Sb-doped SnO2. Reprinted from term stability via using heavily doped oxide as the contact. ATO, Sb-doped SnO2. Reprinted from reference [73] with permission from the American Chemical Society; (c) Al-doped ZnO thin ﬁlm with reference [73] with permission from the American Chemical Society; (c) Al-doped ZnO thin film thermal stability via using sequential annealing. Reprinted from reference [360] with permission with thermal stability via using sequential annealing. Reprinted from reference [360] with permis- from the American Chemical Society. sion from the American Chemical Society.

With regard to the low power consumption, Meng et al. recently reported an excellent thermal management approach in metal–oxide nanowire sensors via a pulsed self- Joule-heating technique, as shown in Figure 28 [220]. It was found that the thermal conductivity of the device was reduced due to the prohibition of heat dissipation from nanowire to surroundings, and its thermal relaxation times can be decreased down to a microsecond range, while several tens of seconds are needed for conventional MEMS gas

Chemosensors 2021, 9, 41 27 of 41

With regard to the low power consumption, Meng et al. recently reported an excellent thermal management approach in metal–oxide nanowire sensors via a pulsed self-Joule- Chemosensors 2021, 1, x FOR PEER REVIEW heating technique, as shown in Figure 28[ 220]. It was found that the thermal27 conductivity of 40 of the device was reduced due to the prohibition of heat dissipation from nanowire to surroundings, and its thermal relaxation times can be decreased down to a microsecond ∼ 2 sensors. This methodrange, enables while the several reduction tens of in seconds energy are consumption needed for conventional down to MEMS10 pJ/s gas and sensors. the enhancement ofThis sensitivity method enablesfor electrical the reduction sensing in of energy NO2 consumption (100 ppb). This down proposed to ∼102 pJ/s ther- and the mal management conceptenhancement of nanowires of sensitivity in forboth electrical spatial sensing and time of NO domains2 (100 ppb). offers This proposeda strategy thermal for exploring novel managementfunctionalities concept of nanowi of nanowiresre-based in both sensors spatial andwith time high domains performance. offers a strategy for exploring novel functionalities of nanowire-based sensors with high performance.

Figure 28. (a) SuspendedFigure nanowire 28. (a) Suspended device; nanowire and (b) device;comparison and (b) between comparison the between conventional the conventional external external heating device and pulsedheating self-jou devicele-heating and pulsedself-joule-heating device. Reprinted device. from Reprinted reference from [220] reference with [220 permission] with permission from the American Chemicalfrom the AmericanSociety. Chemical Society. With the improvement of the metal–oxide nanowire synthesis and device fabrication With the improvementprocesses withof the high metal–oxide uniformity, the nanowire integrated synthesis nanowire sensorand device electronics fabrication would have a processes with highhigh uniformity, reproducibility, the integrated selectivity, and nanowire long-term sensor stability. electronics This would promotewould have the nanowire a high reproducibility,sensor selectivity, electronics and to be long-term widely used instability. the IoT applications, This would such promote as medicine, the food nan- industry, security, and environment protection. owire sensor electronics to be widely used in the IoT applications, such as medicine, food industry, security, andAuthor environment Contributions: protection.Writing—original draft preparation, H.Z.; writing—review and editing, G.Z., H.Z., K.N., T.T., T.H. and T.Y.; supervision, T.Y. All authors have read and agreed to the published Author Contributions:version writing—original of the manuscript. draft preparation, H.Z.; writing—review and editing, G.Z., H.Z., K.N., T.T., Funding:T.H. andThis T.Y.; research supervision, was funded T.Y. by KAKENHI, All authors grant have number read JP17H04927, and agreed JP18H01831, to the JP18H05243, published version of theand manuscript. JP18KK0112. T.T. was supported by JST PRESTO Grant Number JPMJPR19M6, K.N. was supported by JST PRESTO Grant Number JPMJPR19J7, T.H. was supported by JST PRESTO Grant Funding: This researchNumber was funded JPMJPR19T8, by KAKENH Japan. T.T.,I, G.Z., grant K.N., number and T.Y. JP17H04927, were supported JP18H01831, by JST CREST Grant Number JP18H05243, and JP18KK0112.JPMJCR19I2, T.T. Japan. was T.Y.supported and K.N. by were JST supported PRESTO by Grant CAS− NumberJSPS Joint JPMJPR19M6, Research Projects (Grant K.N. was supported byNumber JST PRESTO JPJSBP120187207) Grant Numb and byer Mirai JPMJPR19J7, R&D of JST. T.H. was supported by JST PRESTO Grant NumberInstitutional JPMJPR19T8, Review Japan. Board T.T., Statement: G.Z.,Not K.N., applicable. and T.Y. were supported by JST CREST Grant NumberInformed JPMJCR19I2, Consent Japan. Statement: T.Y. Notand applicable. K.N. were supported by CAS−JSPS Joint Re- search Projects (Grant Number JPJSBP120187207) and by Mirai R&D of JST. Data Availability Statement: Data sharing not applicable. Institutional Review Acknowledgments:Board Statement:This Not work applicable. was performed under the Cooperative Research Program of “Network Joint Research Center for Materials and Devices” and the MEXT Project of “Integrated Research Informed Consent Statement:Consortium onNot Chemical applicable. Sciences”. Data Availability Statement:Conﬂicts of Data Interest: sharingThe authors not applicable. declare no conﬂict of interest. Acknowledgments: This work was performed under the Cooperative Research Program of “Net- work Joint Research Center for Materials and Devices” and the MEXT Project of “Integrated Re- search Consortium on Chemical Sciences”. Conflicts of Interest: The authors declare no conflict of interest.

Chemosensors 2021, 9, 41 28 of 41

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