the role of morphology and crystallographic structure of metal oxides

The effects of parameters such as film thickness, grain size, agglomeration, porosity, faceting, grain network, surface geometry, and film texture on the main analytical characteristics

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The role of morphology and crystallographic structure of metal oxides

in response of conductometric-type gas sensors

G Korotcenkova,b,*

a Korea Institute of Energy Research, Daejeon, Republic of Korea

b Technical University of Moldova, Chisinau, Republic of Moldova

Available online 18 March 2008

Abstract

This review paper discusses the influence of morphology and crystallographic structure on gas-sensing characteristics of metal oxide conductometric-type sensors The effects of parameters such as film thickness, grain size, agglomeration, porosity, faceting, grain network, surface geometry, and film texture on the main analytical characteristics (absolute magnitude and selectivity of sensor response (S), response time (tres), recovery time (trec), and temporal stability) of the gas sensor have been analyzed A comparison of standard polycrystalline sensors and sensors based on one-dimension structures was conducted It was concluded that the structural parameters of metal oxides are important factors for controlling response parameters of resistive type gas sensors For example, it was shown that the decrease of thickness, grain size and degree of texture is the best way to decrease time constants of metal oxide sensors However, it was concluded that there is not universal decision for simultaneous optimization all gas-sensing characteristics We have to search for a compromise between various engineering approaches because adjusting one design feature may improve one performance metric but considerably degrade another

Keywords: Metal oxides; Polycrystalline; One-dimensional; Gas sensor; Sensor response; Morphology and crystallographic structure influence

Contents

1 Introduction 2

2 Structural parameters of metal oxides controlling gas-sensing characteristics 3

2.1 The role of sensor geometry and contacts 3

2.2 The role of dimension factors in gas-sensing effects 7

2.2.1 The influence of thickness 7

2.2.2 Grain size influence 11

2.3 The role of crystallographic structure of metal oxides 16

2.3.1 Crystal shape 16

2.3.2 Surface geometry 20

2.3.3 Film texturing 22

2.3.4 Surface stoichiometry (disordering) 23

2.4 The role of morphology and porosity of metal oxides 24

2.4.1 Grain networks, porosity, and the area of inter-grain contacts 24

2.4.2 Agglomeration 28

2.5 Peculiarities of one-dimensional structure characterization 31

3 Concluding remarks 31

Acknowledgements 35

References 35

www.elsevier.com/locate/mser

Available online at www.sciencedirect.com

Materials Science and Engineering R 61 (2008) 1–39

* Correspondence address: Korea Institute of Energy Research, Daejeon, Republic of Korea.

E-mail address: ghkoro@yahoo.com

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1 Introduction

Conductometric (resistive) metal oxide sensors comprise a

significant part of the gas sensor component market While

many different approaches to gas detection are available[1–

23], metal oxide sensors remain a widely used choice for a

range of gas species[1–5,15,24–34] These devices offer low

cost, high sensitivity, fast response and relative simplicity,

advantages that should work in their favor as new applications

emerge, especially in the field of portable devices The working

principle of a typical resistive metal oxide gas sensor is based

on a shift of the state of equilibrium of the surface oxygen

reaction due to the presence of the target analyte The resulting

change in concentration of chemisorbed oxygen is recorded as a

change in resistance of gas-sensing material As an example,

reducing gases (CO, H2, CH4, etc.) lead to an increase of the

conductivity for n-type semiconductors and a decrease for

p-type material, respectively, whereas the effect of oxidizing

gases (O3, etc.) is vice versa The sensor response (sensitivity)

of such devices, that is, the ability of a sensor to detect a given

concentration of a test gas (analyte), is usually estimated as the

ratio of the metal oxide electrical resistance (conductivity)

(S = Rgas/Rair, or Rair/Rgas) measured in air and in an atmosphere

containing the target gas The rate of sensor response is

described in such parameters as the response or recovery times,

which characterize the time taken for the sensor output to reach

90% of its saturation value after applying or switching off the

respective gas in a step function

Numerous materials have been reported to be usable for

metal oxide sensors design including both single and

multi-component oxides[15,31–33] At that it has been established

that materials in different structural states can be used in those

resistive type gas sensors These states include amorphous-like

state, glass-state, nanocrystalline state, polycrystalline state,

and single crystalline state Each state has its own unique

properties and characteristics that can affect sensor

perfor-mance However, in practice, nanocrystalline and

polycrystal-line materials have found the greatest application in solid-state

gas sensors[4,15,24,27,32,35–37] Nanocrystalline and

poly-crystalline materials have the optimal combination of critical

properties for sensor applications including high surface area

due to small crystallite size, cheap design technology, and

stability of both structural and electro-physical properties

Typically amorphous-like and glassy materials are not stable

enough for gas-sensing applications, especially at high

temperature[32,38] Single crystalline and epitaxial materials

have maximum stability and therefore the use of materials in

these states for gas sensors may improve the temporal stability

of the sensor Unlike polycrystalline material, devices based on

epitaxial and single crystalline materials will not be plagued

with the problem of instability of grain size However, the high

cost and technological challenges associated with their

deposition limit their general use in gas sensors

One-dimensional structures, which are single crystalline

materials, can be synthesized using inexpensive, simple

technology[24,39,40] Wide use of one-dimensional structures

is however impeded by the great difficulties required for their

separation and manipulation [41,42] During the synthesisprocess of one-dimension structures one may observe aconsiderable diversity in their geometric parameters Inpolycrystalline and even nanocrystalline material we workwith averaged grain size, while using one-dimension structures,each sensor is characteristic by the specific geometry of theone-dimensional crystal Therefore, reproducibility of perfor-mance parameters for sensors based on one-dimensionstructures would depend on the uniformity of those structures.Unfortunately, the problem of separation, sizing, and manip-ulations of one-dimensional structures is not resolved yet Toachieve uniform sizing and orientation, new advancedtechnologies will need to be implemented, and these would

be expensive and not accessible for wide use There are a fewinteresting proposals for controlling one-dimensional structures

[43], but they require further improvement for practicalimplementation Thus, gas sensors based on individual one-dimension structures are not yet readily available commer-cially Further, the manufacturing cost of sensors based on one-dimensional structures would far exceed that of polycrystallinedevices Based on what was said above, it becomes clear that innear future, polycrystalline materials would remain thedominant platform for solid-state gas sensors

Nano- and polycrystalline materials are very complicatedobjects for study, because the electro-conductivity of thosematerials depends on great number of factors [34,44–55].Therefore, to specify optimal technologies for gas sensormanufacturing on the basis of such materials, it is necessary toexpand our understanding of gas sensor mechanism in nano-and polycrystalline oxides For example, it is necessary toestablish the role of morphology and crystallographic structure

in gas-sensing effects, because there is a lack of real, detailed,and integrated research establishing a connection betweenstructural parameters of oxides and parameters of sensorresponse One cannot find a large number of good works in thisfield There are the works of Yamazoe and coworkers in thefield of ceramic type sensors[26,44,56–62], in which a directcorrelation between grain size of metal oxides and gassensitivity of conductometric sensors was established; Ega-shira’s group[63–67]conducted a qualitative study of materialporosity influence on sensor response; Morante’s groupestablished a correlation between structural and gas-sensingproperties of metal oxides[68–73], and papers of Korotcen-kov’s group conducted research in the field of thin film gassensors[34,74–84] Korotcenkov’s works emphasized the needfor a broader approach for structural engineering of metal oxidefilms for solid-state gas sensors

While a lot of reviews and book chapters describe theworking principle of metal oxide gas sensors in detail [1–5,15,24–30,37,50], the aim of this review is to summarize theresults highlighting the correlation between material structureand gas-sensing properties, and formulating some generalconclusions typical for metal oxides Earlier assessments ofmodeling morphological effects were made in Refs

[34,47,79,85] The results used in the present review wereobtained mainly with SnO2and In2O3-based gas sensors Thesematerials are the most studied metal oxides for gas sensor

G Korotcenkov / Materials Science and Engineering R 61 (2008) 1–39 2

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applications [24,27,32,33,36,55,79–81,86–89] as well as the

most commercially available For example tin oxide is indeed

the most popular material for gas-sensing due to its relatively

low cost, its high sensitivity, and stability in different

environments

The main focus in this review is on the analysis of undoped

material Consideration of electro-physical and catalytic

proper-ties of a device with a second component would make the

analysis too complicated The introduction of the second

component changes both the catalytic activity of base material

and the chemical composition New compounds or solid

solutions with specific properties significantly different from

the undoped material can be formed during metal oxide doping

Additives could also influence grain size, the shape of

crystallites, bulk and surface stoichiometry, properties of

intercrystalline barriers, and bulk electro-physical properties

[34,74,87,90] Additional possible effects of metal oxide doping

include formation of p–n junctions, the appearance of transitional

areas and layers acting as catalytic filters, the changes in the

valency of metal state, and others[91–95] The analysis of those

interrelated processes requires individual consideration Some

important conclusions regarding the influence of the second

phase on structural, electro-physical and gas-sensing properties

of metal oxides can be found in Refs.[25,33,34,57,71,74,93,95–

99,] More detailed information about the effect of additives in

metal oxide sensors can be obtained also from earlier reviews

[24,25,35,46,49,50,52,57,101]

2 Structural parameters of metal oxides controlling

gas-sensing characteristics

As it was indicated earlier, the fundamentals of resistive type

sensor operation are based on the changes in resistance (or

conductance) of the gas-sensing material as induced by the

surrounding gas The changes are caused by various processes,

which can take place both at the surface and in the bulk of

gas-sensing material [24,34,35,48,51,52,100–105] Possible cesses, which can control gas-sensing properties, are presented

pro-inFig 1.The possible consequences of these processes for surfaceand electro-physical properties of metal oxides are shown in

Fig 2.Research has confirmed that all processes indicated inFig 1,including adsorption/desorption, catalysis, reduction/reoxida-tion, and diffusion are relevant in gas sensors and influenced bystructural parameters of the sensor material This affirms thatgas-sensing effects are structurally sensitive as well Takinginto account the complexity of the gas-sensing mechanism andits dependence on numerous factors, it becomes clear that wehave to consider the influence of a great number of variousstructural parameters of metal oxide matrix on gas sensors’parameters (seeFig 3)

It has been shown in Refs [25,46,47,50,85] that theinfluence of the above-mentioned parameters on gas-sensingcharacteristics takes place through the changes in the effectivearea of inter-grain and inter-agglomerate contacts, energeticparameters of adsorption/desorption, number of surface sites,concentration of charge carriers, initial surface band bending,coordination number of metal atoms on the surface, etc.2.1 The role of sensor geometry and contacts

Fig 4shows some reported gas sensor electrode geometries

To make measurements on a semiconductor gas-sensingmaterial it is possible to use a compressed pellet (see

Fig 4a), which may or may not be sintered, with metalelectrodes on each face This construction was used in Refs

[106,107]to obtain fundamental information on the ture dependence of conductance of tin dioxide In a study of thecompetition between water and oxygen adsorption in tindioxide [108], the electrode assembly consisted of twoconcentric tantalum cylinders with powdered tin dioxide

tempera-Fig 1 Diagram illustrating the processes, controlling the rate of sensor response.

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between them (Fig 4b) However, real devices usually have the

sensing material presented as a thin (e.g sputtered,

vacuum-evaporated, or deposited as a result of chemical reactions) or

thick (e.g screen-printed) film on a substrate (Fig 4c–f)[109–

114] Both electrodes can be fabricated together on the

substrate before (Fig 4g) or after (Fig 4h) the sensing film is

deposited This provides great flexibility in the fabrication

process, as it need not be compatible with the sensing material

At first approximation the sensor geometric parameters of

length (L) and width (W) do not influence sensor response The

L/W ratio influences only sheet conductivity of the gas sensor

(GS) As a rule, one needs to use inter-digital geometry of

contacts (Fig 4e) with small distance between contacts (L) in

order to get small sheet conductivity Appropriate adjustment of

these design parameters can achieve acceptable value of gas

sensor resistance suitable for further electronic processing

However, in reality the situation might be significantlydifferent First of all, the purely geometric effect arises becausethe film conductance does not change instantly or uniformlywhen the gas ambient changes: the gas must diffuse through thefilm, reacting with the particle surfaces as it does so This leads

to variations in local film conductance A numerical simulationindicated, for example, that where a sensor is highly sensitive tothe test gas, the sensitivity increased with electrode spacingwhen the electrodes were underneath the film, but decreasedwith spacing when the electrodes were deposited on top of thefilm[111] If electrode spacing was decreased to less than thefilm thickness, it was possible to detect a less-reactive gas in thepresence of a more reactive one [111] The possibility ofexploiting these effects to produce self-diagnostic sensors hasbeen considered in Ref.[110]: If two or more pairs of contactswith different separations are made on the sensor, then the

Fig 2 Diagram illustrating processes taking place in metal oxides during gas detection and their consequences for polycrystalline metal oxides properties (Reprinted with permission from Ref [105] Copyright 2007: Elsevier).

Fig 3 Diagram showing structural parameters of metal oxides, which control gas-sensing properties.

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conductance measured between any two pairs under a given set

of conditions will be related by a known function, even though

the individual values will, of course, change with test gas

concentration Thus, if the relationship observed deviates from

this function, the sensor must be malfunctioning

For thin films discussed above-mentioned effect does not

work However, even in this case the inter electrode distance

may be a strong influencing factor For example, in Ref.[109]it

was shown that the decrease of the distance between

inter-digitated electrodes from 400 mm to 200 mm may enhance the

CO response in ceramic type SnO2-based sensors Even greater

differences in sensor parameters could appear when the

distance between measurement electrodes in a sensor becomes

less than some critical value Such influences could be

connected with the following factors:

Electrode materials used (Pt, Pd, Au) are active catalysts

with specific catalytic properties As a result, in the area close to

contact (spillover zones), electrode materials act as catalysts

able to increase activity of gas-sensing metal oxides

[97,98,115,116] (see Fig 5) Spillover is a very important

term in catalysis[117] It is used as a shorthand description of

the diffusion of adsorbed species from an active adsorbent to an

otherwise inactive support For instance, this could be the

diffusion of atoms from active metal nanoparticles, where the

dissociation is non-activated, to a support, where the

dissociation directly from the gas phase is activated

Experimentally, this process was determined for hydrogen

and oxygen[117] Therefore, if the distance between contacts is

comparable with the width of spillover zone (seeFig 6b), the

influence of geometric parameters of sensors on their

gas-sensing characteristics would become noticeable The width of

spillover zone depends on the material and the nature of the

detected gas

The influence of the contacts on sensor response withdecreased length of the sensitive layer could become strongerbecause of another reason as well At some distance thecontact’s resistance could be comparable in magnitude or morethan the resistance of the gas sensitive layer, especially in theatmosphere of reducing gases (for n-type semiconductors) Insome cases, the potential barrier between the metal of theelectrode and the gas-sensing oxide could be comparable to thepotential barriers between the metal oxide grains Under thesecircumstances, the chemical reactions between gas and metal–metal oxide interface could affect the total conductance of thesensor, even without the influence of the spillover effect[119].Experimental data confirm both these effects[24,109,119–123] For example, Laluze et al [120] found very largedifferences in the operation of sintered SnO2sensors fabricatedusing different electrodes Fig 7 illustrates how strong thisinfluence could be One can see that the change of electrode

Fig 5 Schematic illustration of spillover effect at the SnO 2 surface at

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metal affects both the magnitude of the sensor response and the

temperature position of the maximum sensitivity

Other studies also established that different electrode

materials can affect sensor behavior [72,109,119,125] For

example, in Ref.[125]the characteristics of SnO2based sensors

with Pt, Au, and Pt–Au contacts were compared It was shown

that at approximately 550 8C, the conductance was about the

same and independent on the electrode material However,

below 150 8C the conductance of the sensors with a Pt electrode

was about three orders of magnitude higher than for those with

Au electrodes In Ref.[119]it was reported that for SnO2thick

film sensors the conductance changes induced by H2and CO

were very different for Pt and Au electrodes The sensor with Pt

electrodes was more sensitive to H2, whereas Au electrodes

seemed to provide a better response to CO In these

experiments, an inter-digital electrode design with a 5 mm

gap was used The same effect was observed in Refs.[124,126]

In Ref.[126]it was shown that a chlorine detector, made from

WO3and aluminum electrodes, had sensor response of about

400 for 1 ppm Cl2in air The sensor response dropped to 1 with

Pt electrodes

Response to humidity was also affected by the electrodematerial In Refs [109,125], it was established that theinfluence of water on the CO response of SnO2-based sensors isgreater in the case of Au contacts, and lower in the case of Ptcontacts

That the metal–semiconductor junction may be the maingas-sensing element responsible for the observed sensorresponse was confirmed in Ref.[127] In this study, the effect

of gap size on the sensor response to dilute NO2 wasinvestigated (seeFig 8) Gap sizes in WO3microsensors werevaried from 0.1 to 1.5 mm It was found that the response todilute NO2 was unchanged for gap sizes larger than 0.8 mm,whereas below 0.8 mm the sensor response tended to increasewith decreasing gap size The sensitivity to 0.5 ppm NO2was ashigh as 57 at a gap size of 0.11 mm For an explanation of theobserved effect it was assumed that the contribution of

Fig 6 Diagram illustrating the role of spillover zones in thin film gas sensors.

Fig 7 Influence of electrode material on gas-sensing characteristics of SnO 2

sensors, fabricated on the base of thin films deposited by electrostatic spray

Else-vier).

Fig 8 Sensitivities to dilute NO 2 of WO 3 microsensors as a function of gap size WO 3 microsensors with micro-gap electrodes were fabricated by means of MEMS techniques (photolithography and FIB) and suspension dropping method WO 3 powders were prepared by wet process Powders were calcined

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resistance at the electrode–grain interface to the total sensor

resistance becomes larger when the gap size is decreased It was

concluded that the resistance change at the electrode–grain

interface is much larger than that at the inter-grain boundary

when the microsensor is exposed to NO2 Thus, the sensitivity

is increased with the decreasing gap size

It means that at small distances between contacts, the role of

contact material is essential and should be considered in the

design of gas sensors Moreover, it is possible to control the

sensing properties of semiconductor gas sensors simply by

using different electrode materials Optimum electrode

material and electrode geometry could also be used to enhance

the gas-sensing properties[25,111,119]

Using the electrode geometry as a design parameter, one could

probe the variation of sensor signal with electrode position within

the porous sensor body (seeFig 4g and h) If the electrodes are

closely spaced, the current for configuration shown inFig 4g

probes only the base of the sensor layer, while the current probes

the whole sensor layer for electrodes that are spaced sufficiently

widely For configuration shown in Fig 4h we have other

situation The current can be pushed out into the layer by using

narrow inter-digitated electrodes, and pulled down towards the

base of the layer by using wide electrodes Such a measurement

could, for example, lead to a determination of the rate constant

for the surface-catalyzed decomposition, which should be a

characteristic parameter of the gas, the surface composition, and

the temperature To some degree, such measurements can be used

to identify the gas [25] Hoefer et al [128] used an array of

electrodes of differing width and separation to examine contact

resistance effects in tin dioxide sensors In its original form, they

used the transmission line method for measuring the total

resistance of a semiconductor sample as a function of electrode

separation The linear relation obtained allowed a determination

of sheet resistance and contact resistance, while an additional

‘‘end resistance’’ measurement allowed an estimation of a further

parameter: The ‘‘modified sheet resistance’’ of the film in the

vicinity of the electrode[129] It was shown that the modified

sheet resistance displayed greater sensitivity to CO and NO2than

either the sheet resistance itself or the contact resistance[128] In

this case, wide electrodes with narrow spacing would produce the

most sensitive detection An array of electrodes varying in width

and spacing (seeFig 4f), but all using the same sensing material,

could be used to resolve a mixture of CO, CH4, NO2and water

vapor into separate measurements of each component by first

determining the relative sensitivity of the total resistance of each

electrode pair to the individual gases[128] Further, simulations

have shown that a poorly reactive gas can be detected in the

presence of a highly reactive gas if electrode placement and film

thickness are chosen well[111] In Ref.[107]it was found out

that the lower detection limit can be improved by reducing the

number of grains between the electrodes

The electrode material also affects the stability of the gas

sensor It was shown in Refs.[109,130]that Au electrodes are

less stable as compared to Pt electrodes Scanning electron

microscopy (SEM) and resistance measurements, carried out in

Ref.[130], have shown that platinum on an adhesion layer of

titanium was stable up to 500 8C, while the changes in gold

films with various adhesion layers were observed at noticeablylower temperatures For example, gold on chromium startsdegrading at as low a temperature as 250 8C An increaseddiffusion coefficient and an inclination to form alloys areprobably the reasons for such behavior of Au electrodes Ifaluminum was to be used as an interconnect metallization, itwas found that the maximum stability had contacts with anadditional layer of platinum as the metal for making contactwith the sensor material (metal oxide) and a barrier layer oftitanium–tungsten between the aluminum and platinum Thiscombination was also usable up to 500 8C Other layerstructures show less thermal stability

Other important aspect of length’s influence appears whenthe distance between electrodes becomes less than thecrystallite’s size (see Fig 9c and d) In this case we couldobserve a situation, when the intercrystallite barriers stopaffecting the gas-sensing effects, which could induce sig-nificant changes in sensor performance parameters or even loss

of sensitivity This implies that as the distance is decreased, thegas sensor mechanism could change At small distances onlybulk grain effects would be present

It seems that a realization of this condition is impossible innear future for finely dispersed metal oxides This principle can

be realized only with single crystals, epitaxial films, and dimensional structures, where the grains and inter-grainsboundaries do not exist However, successful development ofnew advanced technologies [131] may make it feasible toproduce sensors based on one individual grain As it was shown

one-in Ref [112], state-of-the-art electron-beam techniques canproduce extremely narrow and closely spaced metallized lineswith features of less than 10 nm in size

2.2 The role of dimension factors in gas-sensing effects2.2.1 The influence of thickness

At present there are three main gas sensor designapproaches: (1) – ceramics; (2) – thick film; and (3) – thinfilm[35] Therefore, while analyzing the influence of thickness

on sensor parameters, it is necessary to remember that forceramics and thick film sensors the grain size does not depend

on the thickness; rather it is determined by the conditions ofsynthesis and the thermal treatment parameters The situationfor thin film sensors is fundamentally different The grain size isdetermined directly by the thickness of the deposited film Thestrength of that influence is shown in Fig 10 The mainregularities of film thickness influence on structural properties

of SnO2and In2O3deposited by spray pyrolysis were discussed

in Refs.[77–79,132].The influence of film thickness (d) on sensor response toozone and reducing gases for In2O3-based sensors fabricatedusing thin film technology is shown inFigs 11–13 In2O3films

in these experiments were deposited by spray pyrolysis It isseen that the change of film thickness can lead to a change inboth the magnitude and temperature position of the sensorresponse’s maximum At that, the effect of thickness on gas-sensing characteristics was most pronounced for oxidizinggases When the In O film thickness increases from 20 to

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Fig 10 The influence of the thickness of In 2 O 3 film deposited by spray

pyrolysis on the grain sizes measured by (1) XRD; (2) AFM; and (3) TEM

Fig 11 Thickness influence on sensor response to (1 and 2) ozone and (3) H 2 of

In 2 O 3 thin films deposited by (1 and 3) spray pyrolysis and (2) sputtering Results (2) were obtained immediately after In 2 O 3 photoreduction For this purpose the samples were directly irradiated in vacuum by mercury pencil lamp for 20 min (Adapted with permission from Refs [79,81,133] Copyright 2001 and 2004: Elsevier).

Fig 9 Diagram illustrating the influence of grain size on potential distribution along sensor.

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400 nm, the gas response to ozone drops by more than a factor

of 100 (Fig 11) This drop in sensitivity can be rationalized by

an increase in grain size [77,79] and a decrease of

gas-permeability within the film Due to high activity, ozone

decomposition occurs on the top layer of the metal oxide film

Thus, thin films designs should be used for effective detection

of oxidizing gases

Regarding the detection of reducing gases, especially

hydrogen, the opposite effect occurs on the films In general,

thick films work better for hydrogen and other reducing gases

For example, thin In2O3films, deposited from diluted solutions,

had lower sensitivity to hydrogen than thick films (Fig 11, curve

3) The same effect was observed earlier for SnO2films[134],

prepared by the spin-coating method An explanation of this

effect was presented in Refs.[59,64,110], where the

diffusion-reactive model of gas sensitivity was developed According to

Ref.[64], the increased sensitivity to H2in thick films arises

because H2has a much higher diffusion coefficient than oxygen

It is necessary to note that the sensitivity of sensorsfabricated by thick film technology is dependent on filmthickness as well However, different authors have observedsignificantly different dependencies with thickness (see

Fig 12) Some reports have observed an increase in sensitivity

to reducing gases with increased film thickness [137], whileothers have observed a loss in sensitivity[135], and one studyobserved that the sensor response would reach a maximum atsome thickness [138] Such disagreement demonstrates onceagain that gas sensitivity of metal oxides is dependent on manyfactors, which are hard to control

Because the depth of penetration of various gases into theoxide matrix depends on their diffusion coefficient and activity,the disposition of contacts (on top or below gas-sensing layer)starts playing an important role for ceramics or thick filmsensors This effect was studied in detail in Refs.[110,123], andwas used for the determination of gas diffusion parameters intotin oxide Research has shown that sensor characteristics, inparticularly the gas nature influence on the temperaturedependence of sensor response, are strongly dependent onthe position of electrodes (see Fig 14) Such a strong effectmight be used for definition of the nature of detecting gas Atsufficient thickness the top layer of the sensing material couldact as a filter for certain gas molecules[139] This effect couldalso explain the conclusion made in Ref.[140]regarding the H2response of the SnO2-based sensors with two types of noblemetal (Au, Pt, and Pd) electrodes covering the surface of the tinoxide nanohole arrays It was found that the temperaturedependence of the sensor response differed between the sensorsequipped with a pair of electrodes on both surfaces and thesensors equipped with a couple of inter-digital electrodes onone side At that, the H2response of the sensors equipped with apair of electrodes on both surfaces was much higher than that ofthe sensors equipped with inter-digital electrodes on one side.With increased film thickness, problems arise in usingphysical methods for the deposition of noble metals catalystsonto metal oxides[70,118] In Refs.[62,72,125], this problemwas studied for SnO film doping by Pd and Pt Some results are

Fig 12 Sensor response of SnO 2 -based sensors to reducing gases vs film thickness, determined in various laboratories: (a) devices were fabricated by dropping and spinning of sol suspension over an alumina substrate attached with comb-type Au electrodes SnO 2 powders were prepared by hydrothermal method; (b) SnO 2 films were deposited by (1) MOCVD method on alumina substrates with two Au electrodes, with following annealing at 600 8C for 15 h, and (2) by reactive DC sputtering with following annealing at 600 8C for 10 h (Adapted with permission from Refs [70,134,135] Copyright 1994, 2001, and 2003: Elsevier).

Fig 13 SnO 2 film thickness influence on normalized S(T oper ) dependencies of

sensor response to reducing gas Sensors were fabricated on the base of thin

films deposited by spray pyrolysis from SnCl 4 –water solution (Reprinted with

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shown inFig 15 It was concluded that the optimal technical

solution varied with the type of noble metal used as an additive

[72]

Other important aspect of the influence of film thickness on

gas sensor performance pertains to response and recovery

times The effect of film thickness on the time constants of

sensor response to ozone and hydrogen are shown inFigs 15

and 16 One can see that the time constants of sensor response

increase as the film thickness increases At that, the effect is

more pronounced with oxidizing gas than for reducing gases

Response and recovery times for reducing gas exposure on

In2O3-based thin film gas sensors increased nearly 10-fold as

the film thickness changed from 20 to 400 nm (Fig 16)

Response times during ozone detection in a dry atmosphere

changed almost two orders of magnitude (Fig 17), although inhumid atmosphere, this effect was weaker (seeFig 17).Thus, gas sensor designers should decrease thickness toimprove the sensing characteristics of metal oxide-based gassensors The use of thin films assures fast response andrecovery As shown empirically, there is not any diffusionlimitation in response kinetics in thin-film devices[81].The same conclusion was made by the authors of Ref.[143],studying gas-sensing properties of ZnO sensors fabricated bymagnetron sputtering It was found that sensors with minimalfilm thickness in the range 65–390 nm had the maximumresponse to CO and minimum response time These resultsindicate that thin film gas sensors (d < 100 nm) will always befaster than thick (d > 100 nm) film gas sensors[63] Another

Fig 14 Influence of electrode position (a) on gas sensitivity of SnO 2 -based thick film sensors (d 250–300 mm) loaded with 1.0 wt% of Pt or Pd to (b) H 2 and (c)

CH 4 Porous thick film sensors having (2) interior and (1) surface electrodes were fabricated on a porous mullite tube of 2 mm enter diameter and 1.7 mm inner diameter SnO 2 powders had a surface area (S surf ) of 75 m2/g) (Adapted with permission from Ref [64] Copyright 1998: Elsevier).

Fig 15 Scheme of the three doping methods (a), and influence of doping methods on the gas response of SnO 2 -based sensors (d 200 nm) to 100 ppm CO (T oper = 400 8C, RH = 40%) SnO 2 films and catalytic additives, (b) Pt and (c) Pd were deposited by reactive DC sputtering 1, 2, and 3 correspond to methods of doping shown in figure (Adapted with permission from Ref [72] Copyright 2003: Elsevier).

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confirmation of this statement can be found in Ref.[142], where

thick (d 1.0 mm) film In2O3-based ozone sensors were

described Because of optimization of the film structure

(increased film porosity), gas sensor sensitivity to ozone was

improved However, the authors failed to decrease both tresand

trec Even at operating temperatures of approximately 390 8C

the tresexceeded 6 min

Another important advantage of thin film sensors, especially

those designed in epitaxial and 1D structure versions, is better

temporal stability (seeFig 18) at high operating temperatures

The change of grain size in thin films during thermal treatment

is not as strong as for thick films

However, the diminution of film thickness has limitations,

controlled by the properties of material itself and the technical

difficulty in forming a continuous layer For example a

thickness of 10–15 nm is the limit for standard thin-filmtechnologies usually used for gas sensor fabrication For thickfilm technology, a minimal thickness of about 50–150 nm isrequired to form a continuous film [113]

Besides slow response and recovery processes in thick and ceramics-type sensors, it is necessary to take into accountthat above a certain thickness, ceramic and thick films have aninclination to cracking[87] An example of such a process isshown in Fig 19 As a result of cracking, the film electro-conductivity and gas-permeability could change fundamen-tally For example, both response and recovery times in suchsensors could decrease due to improved gas-permeability.However, the process of cracking is hard to control, because themagnitude of the sensor parameters change is dependent on thedepth and length of the cracks Probably therefore the authors of

film-[113], considering the influence of film thickness on sensorresponse, did not take into account the results, obtained forfilms with thickness more than 200 nm

2.2.2 Grain size influence

At present either the ‘‘grains’’ model, or ‘‘necks’’ models (see

Figs 9 and 20) are applied to rationalize the electro-physicalproperties of polycrystalline materials, which are dependentstrongly on their microstructure [24,46,47,50,144,145] It hasbeen established that the grain size and the width of the necks arethe main parameters that control gas-sensing properties in metaloxide films as well Moreover in the frame of modern gas sensormodels the influence of grains size and necks size on sensorresponse may be attributed to the fundamentals of gas sensoroperation [24,46,47,57,93,144–146] Usually it is displayedthrough the so-called ‘‘dimension effect’’ e.g., a comparison of

Fig 16 SnO 2 film thickness influence on response time during detection (1)

CO (T oper = 340 8C) and (2) CH 4 (T oper = 430 8C) SnO 2 films were deposited by

spray pyrolysis method (Reprinted with permission from Ref [281] Copyright

1999: Elsevier).

Fig 17 Dependencies of time constants of In 2 O 3 response to ozone on

thickness of porous films (T pyr = 475 8C): (1) recovery time; (2) response time

in dry atmosphere; (3) response time in wet atmosphere Dry air corresponds to

1–5% RH, wet air corresponds to 40–50% RH (Reprinted with permission from

Fig 18 Influence of annealing temperature on average grain sizes in In 2 O 3 films estimated on the base of AFM measurements In 2 O 3 films were deposited

by spray pyrolysis method: (1) T pyr = 520 8C; d 200 nm; (2) T pyr = 520 8C;

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the grains size (d) or necks width (X) with the Debye length (LD)

where k is the Boltzmann constant, T is the absolute

tempera-ture, e is the dielectric constant of the material, and N is the

concentration of charge carries

The distribution of the potential along polycrystalline oxides

for different d, X and L is presented inFigs 9 and 20 It is clear

that the width of the necks determines the height of the potential

barrier for current carriers, while the length of the necks

determines the depletion-layer width of the potential barrier It

is necessary to note that the increase of the necks lengthincreases the role of necks in the limitation of metal oxideconductivity, and correspondingly in gas-sensing effects Thegrain size would determine the depth of valley on the potentialdistribution within grains Corresponding potential diagramsfor one-dimension structures are given in theFig 9c and d Thedistribution of the potential looks similar to those observed inindividual grains except that at the boundary of a one-dimensional structure with Ohmic contact, the potential barrierwould be considerably lower than the potential barrier betweengrains

In brief, for explanation of the ‘‘dimension effect’’ on thegas-sensing effect it is possible to provide the following

Fig 20 Diagram illustrating the role of necks in the conductivity of polycrystalline metal oxide matrix and the potential distribution across the neck (Reprinted with permission from Ref [32] Copyright 2007: Elsevier).

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argument (seeFigs 9 and 20)[50,146] For large crystallites

with grain size diameter d 2Ls (Fig 9b), where Ls is the

width of surface space chargeðLS¼ LD

and ceramics usually is limited by Schottky barriers (VS) at the

grain boundary In this case the gas sensitivity is practically

independent of d

In the case of d 2Ls every conductive channel in necks

between grains is overlapped (see Fig 20) If the number of

long necks is much larger the inter-grain contacts, they control

the conductivity of the gas-sensing material and define the size

dependence of the gas sensitivity

If d < 2Ls, every grain is fully involved in the space-charge

layer (Fig 9a) and the electron transport is affected by the

charge at the adsorbed species In Ref [147], it was

demonstrated that, when the grain size becomes comparable

to twice the Debye length, a space-charge region can develop in

the whole crystallite The latter case is the most desirable, since

it allows achievement of maximum sensor response More

detailed descriptions of models used for explanation of both

grain size and necks influence on gas-sensing effects may be

found in Refs.[26,50,56,57,157]

It is necessary to note that the applicability of ‘‘grains’’ or

‘‘necks’’ models depends strongly on the technological routes

used for metal oxides synthesis or deposition and sintering

conditions[57] Usually the appearance of necks is a result of

high temperature annealing (Tan> 700–800 8C) Taking into

account processes which take place at inter-grain interfaces

during high temperature annealing, one can assume that the

forming of long necks in inter-grain space is a consequence of

mass transport from one grain to another According to Ref

[144]for metal oxide samples after high temperature annealing,

the neck size (X) is proportional to grain size (d) with a

proportionality constant (X/d) of 0.8 0.1 However, it is

necessary to note that this constant depends on the sintering

parameters and may be different For thin metal oxide film and

ceramics, which were not subject to high temperature

treatments, the gas-sensing matrix is formed from separately

grown grains Therefore, in such metal oxides, the necks

between grains are very short or are absent It means that for

description of their gas-sensing properties we can use the

‘‘grains’’ model According to this model, between grains there

are Schottky type contacts with the height of potential barrier

depending on the surrounding atmosphere In the frame of such

approach the grain boundary space charge or band bending on

inter-grain interfaces are the main parameters controlling the

conductivity of nanocrystalline metal oxides

The adequacy of above-mentioned model was estimated on

the base of results obtained during the impedance spectroscopy

of metal oxides At present, impedance spectroscopy is the

most effective method for experimental determination of

factors that limit the conductivity of metal oxides For example,

in Ref.[148], impedance spectroscopic measurements of SnO2

films with grain size equaling 5–14 nm showed that at low

operating temperatures (25–300 8C), both the grain and grain

boundary contribute to the conductivity At higher operating

temperatures (above 300 8C), the grain boundary contribution

for the conductivity is dominant The same results was obtained

in Ref [149] for SnO2 films (d 0.6–1.0 mm) duringinteraction with H2S and NH3 at 250 8C The impedance ofthese films was mainly contributed by the potential barriers atgrain boundaries Labeau et al.[150]also have found that themain contribution to the sensor impedance of polycrystallineSnO2sensors during interaction with CO and C2H5OH arisesfrom grain boundaries, although a small contribution from bulkwas also seen at high frequencies As was shown in Ref.[151],the grain boundaries are limiting elements in conductivity of

WO3thin films (d 50 nm) under dry air or/and ozone in thetemperature range from 150 to 375 8C as well

In Ref [152] using the same impedance method it wasshown that the contribution of the grain boundary in the totalconductivity of a metal oxide depends also on the grain size.The impedance spectra of SnO2 ultra dispersed ceramics(d 3–43 nm) have been investigated in the temperature range

25 8C < T < 300 8C under a dry oxygen atmosphere It wasestablished that the grain boundaries give the major contribu-tion to the electric transport for the samples with d less than

25 nm For the samples with larger grain size, the contributions

of the grain volume and grain boundaries to the conductivity are

of the same order The authors of Ref.[152]assumed that grainswith size smaller 24 nm are completely depleted by chargecarriers

Regarding experimental confirmation of grain size influence

on sensors response one can say that the dramatic increase insensitivity for metal oxides with grain size smaller than a Debyelength has been demonstrated many times for various materials,such as SnO2[26,34,56,60,85,153], WO3[154,155]and In2O3

[79,144,156] This effect for In2O3and SnO2-based sensors isillustrated inFigs 21 and 22 For example, in Ref.[30]it wasestablished that the sensitivity of sensors based on tin oxide

Fig 21 (1–5) Theoretical and (6–8) experimental dependencies of SnO 2 sensor response to reducing gas on grain size for (7) undoped SnO 2 ceramics and ceramics doped by (6) Sb and (8) Al: (1) N d = 1017cm3; 2–3 10 17

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nanoparticles dramatically increased when the particle size was

reduced down to 6 nm Below this critical grain size, the sensor

sensitivity rapidly decreased As the calculated Debye length of

SnO2is LD= 3 nm at 250 8C[147], the highest sensitivity was

actually reached when the particle diameter corresponded to

2 nm

However, it is necessary to note that the threshold value of

grain sizes determined in Ref.[30]is not an invariable constant

of SnO2 Wang et al [157] considered grain size effects in

particulate semiconductor gas sensors, which contained

mixtures of necks and grain boundary contacts, and concluded

that the gas sensitivity would increase sharply for particle

diameters below about 35 nm Studies on tin dioxide-based

hydrogen sensors carried out in Ref [159] indicated that

devices produced using 20 nm particles were around 10 times

more sensitive than devices made from 25 to 45 nm particles

Lu et al.[158]found that the SnO2-based sensor response to

500 ppm CO increases drastically if the particle diameter

becomes smaller than 10 nm

Because the Debye length depends on the concentration of

free charge carriers (see Eq (1)), it becomes clear that the

observed variation of threshold value of crystallite’s size is

valid Moreover, this parameter must be dependent on the

material’s properties and the doping For example, the Debye

length in SnO2(3 nm) estimated in Ref.[147]corresponds to

a donor concentration equal to nd= 3.6 1024m3 The

decrease of free charge carriers’ concentration, for example,

through metal oxide stoichiometry improvement induced by

annealing or doping, can considerably increase this threshold

value[144] Thus, Al-doped SnO2(seeFig 21, curve 8) shows

high sensitivity with increasing grain size even at d above

20 nm, while Sb-doped SnO2(seeFig 21curve 6) is insensitive

in the whole d region (see Figs 19 and 20) If d LD, the

increase of sensor response with grain size decrease is not so

significant[160]

The effect of grain size on the sensitivity of chemoresistivenanocrystalline metal-oxide gas sensors was evaluated also inRef [145] by calculating the effective charge carrierconcentration as a function of the surface state density for atypical sensing material, SnO2, with different grain sizesbetween 5 and 80 nm These calculations demonstrated a sharpdecrease in the charge carrier concentration when the surfacestate density reached a critical value that corresponds to acondition of fully depleted grains, namely, when nearly all theelectrons are trapped at the surface Assuming that thevariations in the surface state density are induced by surfaceinteractions with ambient gas molecules, authors of Ref.[145]

simulated the response curves of nanocrystalline gas sensors.The simulations showed that the conductivity increases linearlywith decreasing trapped charge densities, and that thesensitivity to the gas-induced variations in the trapped chargedensity is proportional to 1/d, where d is the average grain size.However, experimental results presented in Ref [79] shownthat this dependence can be stronger As it is seen inFig 22thedependence of sensor signal on grain size for In2O3-basedsensors obeys the correlation S t3for ozone detection in dryatmosphere It means that the authors of Ref.[145]probably didnot take into account all factors influencing the gas sensitivity

of solid-state sensors

It is necessary to note that relationship to grain size isdependent on the type of metal oxide, detection mechanism,and the analyzed gas For example, in In2O3-based gas sensors,the role of crystallites’ size is appreciably lower for thedetection of reducing gases in comparison with oxidizing gases

[79–81] In Ref [81], it was found that for the certaindeposition conditions, the sensor signal to the reducing gasescould either increase or decrease with increasing grain size Inother words, for reducing gases, the In2O3grain size is lessimportant than in the case of SnO2-based gas sensor[81] Thesensitivity of In2O3-based gas sensors to reducing gases mayhave an acceptable value even when the crystallite size isbigger than 80–100 nm With such crystallite sizes theresponse of In2O3sensors to oxidizing gases will be minimal

In the case of SnO2films, the increase in grain size leads todecrease of the sensor response to both oxidizing and reducinggases[45,56] For explanation of the observed effect in Ref

[100] it was assumed that in In2O3-based sensors duringreducing gases detection due to high surface unstoichiometry,the resistance of the films was not controlled by inter-grainbarriers X-Ray photoelectron spectroscopy (XPS) data and theabsence of dependences of film resistance normalized to filmthickness (Rd) on the grain size characteristic for conductivitylimited by the resistance on inter-grain barriers were the basisfor such a conclusion [81,104] For comparison in ozoneatmosphere such strong dependence of Rd on grain size hasappeared

For In2O3it was found also that sensors fabricated on thebase of thin films with minimal crystallites’ size had minimalresponse time (see Fig 23) in addition to maximum sensorresponse (seeFig 22) For example, the decrease in grain sizefrom 60 to 80 nm to 10–15 nm decreased tres during ozonedetection in dry air by a factor of 50–100 times In a humidified

Fig 22 The influence of In 2 O 3 film grain size on the sensor response to ozone

(T pyr = 475 8C; T oper = 270 8C): (1) films deposited by spray pyrolysis from

0.2 M InCl 3 –water solution; (2) 1.0 M InCl 3 solution; (3) extrapolated curve

according S t3dependence (Reprinted with permission from Ref [79]

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atmosphere, the established correlation was significantly

weakened[79,80]

Besides, the surface reactivity of particles is known to

rapidly increase with the increase of the surface-to-bulk ratio

because the strong curvature of the particle surface generates a

larger density of defects, which are the most reactive surface

sites [161] This high reactivity has largely been taken

advantage in catalysis, where ultrafine particles have been

used for decades When properly processed during the

fabrication of chemical semiconductors sensors, these

nano-particles are sufficiently reactive to make the use of catalytic

additives (such as Pt or Pd) unnecessary and to decrease the

working temperature of the sensors without any loss of

sensitivity[162,163]

However, the decrease in grain size cannot be unlimited At

some critical dimension, the number of free electrons in the

grain could become zero even at VS= 0 This leads to a grain

resistance that would not be dependant on the changes in the

surrounding atmosphere For charge carrier concentration in

metal oxides of 1021cm3, the critical crystallite size is 1 nm

The use of finely dispersed small crystallites can also have a

deleterious effect on the temporal stability of the sensor[34,76]

It was shown that an excessive decrease of grain size leads to a

loss of structural stability [34,77], and, as a consequence, to

change both surface and catalytic properties of the material

[164] For example, it was shown that for SnO2with a grain size

of about 1–4 nm, the grain-growth process begins already at

temperatures equal to200–400 8C (seeFig 24)[77,165] In

contrast, SnO2crystallites with average sizes ranging from 1.7

to 4.0 mm were stable up to 1050 8C

According to Refs.[169–171], there are two main reasons

for grain growth during thermal treatments This instability

may arise from a known defect in the bulk state, such as faulty

stoichiometry, or due to the finite size of the grains Recent

theoretical simulations have confirmed this statement[170] A

quantum mechanical study of the stability of SnO2talline grains has shown that the increase of both the grain size

nanocrys-in the range 0.3–4.0 nm, and the oxygen content nanocrys-in SnO2increased the stability of SnO2grains The fact that the size has

a strong influence on the melting temperature of thin metallicfilms is another direct confirmation of this statement It wasestablished that the decrease of Ag, Bi, Sn, and Pb films’thickness to 5 nm was accompanied by temperature decrease

of crystallization up to 0.6–0.7 of the melting temperature ofbulk samples[172] As it was established in Refs.[166–168],the decrease of grain size is accompanied by lowering of themelting temperature of the semiconductor nanocrystals aswell

However, the principle of surface and interface energyminimization explains only the appearance of a driving forcefor the recrystallization of polycrystalline films during theirannealing This principle cannot account for the thresholdnature of the grain size changes observed[77], i.e the presence

of a threshold temperature (Tst) below which the crystalliteswith fixed size remain stable, and the absence of t1/2type grainsize dependencies on time during thermal annealing

Because the process of coalescence starts through thebreaking of Me-atoms bonds with the lattice of metal oxides, inRef.[77]it was assumed that the formation energy of surfaceand bulk vacancies of Me-atoms (VIn, VSn) could be such acritical energy, characterizing the temperature threshold ofstructural stability The more the energy of VIn and VSnformation is, the more stable is the lattice, i.e the temperature

of possible grain coalescence is higher In spite of the fact thatthe observed process of grain growth in both In2O3and SnO2films takes place through diffusion of Me-atoms (In, Sn)following their incorporation in the lattice of a biggercrystallite, this process is not controlled by the coefficient ofself- or surface Me-atom diffusion From our point of view, thisprocess is controlled by certain energy parameters, for example

Fig 23 Dependencies of response time of In 2 O 3 sensors during gas detection

in dry atmosphere on the grain size of In 2 O 3 films deposited from 0.2 M InCl 3 –

water solution: (1) ozone (1 ppm) detection (T oper = 270 8C); (2) CO

(1000 ppm) detection (T oper = 270 8C) (Reprinted with permission from

Fig 24 Influence of annealing temperature on grain size in SnO 2 thin films, thick films and ceramics, fabricated using different manufacturing methods (Reprinted with permission from Ref [77] Copyright 2005: Elsevier).

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the energy of vacancy formation, characterizing the

thermo-dynamic stability of the crystallite

Theoretical calculations conducted for NiO[173]and Cu2O

[174]have shown that the energies of point defect formation in

polycrystalline ceramics are really smaller when compared

with the bulk Such expectations, for example, have been

experimentally confirmed for Cu2O[175] The obtained results

have shown clearly that the point defect concentration in

fine-grained metal oxides significantly exceeds the concentration of

point defects in large-grained materials

It thus becomes clear, that the presence of a finely dispersed

fraction with a grain size smaller than 2–5 nm will lead to some

structure instability of the metal oxide matrix at moderate

operating temperatures (T < 600 8C) This is true even for films

with average grain sizes greater than 100 nm Therefore, future

design methods of nano-scale devices, which assure grain size

stabilization during long-term operation at high temperature,

will gain priority over the design of methods producing

nano-scaled materials with minimal grain size The production of

both metal oxide powders and deposition of films with a small

dispersion of grain sizes is an effective method for

improve-ment of the temporal stability of solid-state gas sensors as well

Utilizing thin films in gas sensors also leads to an improved

stability of the gas-sensing matrix Research has shown that the

grain size in thin films during the annealing process changes

considerably less than in thick films (seeFig 18) Therefore,

unless there is a pressing need to improve detection limits

(sensitivity), it is not advisable to design sensors with

excessively reduced grain sizes This problem is especially

serious for operation in atmospheres of reducing gases

As it was discussed earlier, some studies [34,162] have

considered the possibility of introducing micro additives to

limit the mobility of adatoms and stabilize grain size This is an

interesting area of research It is well known in metallurgy that

trace impurities or second phase particles are very effective in

altering and inhibiting grain growth In accordance with results

of research carried out in this field, additives such as K, Ca, Al,

or Si salts may be such grain-growth inhibitors[27,98]

Both experimental research and theoretical simulations have

established that in the presence of a second phase, grain growth

is inhibited when the average domain size is comparable to the

average inter-particle distance Therefore, the controlled

addition of selected impurities may be one way to achieve a

particular grain size, morphology, and structural stability of

metal oxides necessary for practical applications However,

even small quantities of additives used for grain size

stabilization can affect catalytic and adsorption surface

properties of the gas-sensing material As a result, though

grain size may be stabilized, there may be a strong change in

both the electro-physical and the gas-sensing properties of the

metal oxide [34] This research is still in the early stages;

therefore detailed studies conformably to metal oxides are

needed to develop real technology for the stabilization of

nano-scaled materials

As the grain size decreased, one more interesting effect was

established With decreased grain size, a significantly increased

sensitivity to humidity was observed[34] (see Fig 25) The

same effect was observed experimentally in Ref [176].Williams and Coles found that alumina-based humidity sensorsalso increased their sensitivity by an order of magnitude whenprepared from 13 nm particles as compared to 300 nm particles

It means that the humidity influence of sensor parameters isstructure dependent and may be controlled through structuralengineering of the metal oxides used In Refs.[75,177–179], itwas confirmed that water adsorption on the surface of metaloxides depends on its crystallographic structure At that, thedifference of water attachment on different crystallographicSnO2planes appeared not only in concentration, but also in thetype of bonds between hydroxyl groups and the SnO2surface.The OH-groups participate in gas detection reactions, and wateradsorption/desorption processes can control the kinetics of gasresponse[24,87,141,180,181] Thus, if minimum sensitivity tohumidity is required, the size of crystallites in sensor materialshould be the maximum permissible

2.3 The role of crystallographic structure of metal oxides2.3.1 Crystal shape

At present, the affect that grain shape and faceting ofcrystallites has on gas-sensing is not being analyzed However,there are reasons to conclude that the role of these parameters isundeservedly understated The following statements can be madebased on the results given in Refs [34,36,75,85,132,177,182–186]:

(i) The external planes of nanocrystals participate in gas-solidinteraction, and therefore these very planes determine thegas-sensing properties of nanostructured materials Evenspherulities have micro-planes and facets The nanocrystalshape may determine such parameters as crystallographicplanes, inter-grain contacts, area of inter-grain contacts,gas-permeability, and so on

(ii) Every crystallographic crystal form has its own tion of crystallographic planes, framing the nanocrystal

combina-Fig 25 Influence of pyrolysis temperature on (2) grain size and (1) sensitivity

to air humidity during H 2 (1000 ppm) detection by In 2 O 3 -based sensors (d 60–80 nm) Sensitivity to air humidity was estimated as the ratio of sensor response at T oper = 370 8C to H 2 measured in dry and wet atmospheres In 2 O 3 films were deposited from 0.2 M InCl 3 solution.

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Every crystallographic plane has its own combination of

surface electron parameters, which include surface state

density, energetic position of the levels, induced by

adsorbed species, adsorption/desorption energies of

interacted gas molecules, concentration of adsorption

surface states, the energetic position of surface Fermi

level, activation energy of native point defects, and so on

This implies that the chemisorption characteristics

change noticeably from the crystal surface orientation

to another Thus, there is a large surface dependence at

the atomic level for chemical bonding of the adsorbed

particles

(iii) As chemical processes, the adsorption/desorption haveactivation energies The parameters controlling theseprocesses are orientation and grain size dependent Thedecrease in crystal size in the nm range notably strengthensthe crystallite shape influence on the adsorption properties.Both the shape and the size of nanocrystals have aprofound influence on the concentration of adsorbedspecies and on the type of bonding to the surface that takesplace It is known that depending on the type of bonding,some chemical species may have preferred adsorption oneither the edge/corner sites or on the plane facet Forexample in Ref [182], it was shown that monodentate

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adsorption on MgO and CaO is preferred on the edge/

corner sites, whereas the bidentate adsorption is favored by

smooth planes

Thus, depending on external form of the nanocrystallites, a

nanostructured gas-sensing matrix will have a unique

combination of structural, electronic, and

adsorption/deso-rption process parameters

Detailed studies [34,78,132] conducted on SnO2 films

deposited by a spray pyrolysis method confirmed the

appropriateness of this approach for modification of

gas-sensing properties For example, it was established that the

growth of grains, especially in the range from nm to mm, during

which the transition from spherulites to nanocrystallites and

from nanocrystallites to nanocrystals and crystals takes place, is

accompanied by the changes in both the size and the external

shape of crystallites[78,132] Possible morphologies of SnO2

films deposited by spray pyrolysis are shown inFig 26

In Ref.[78]it was shown that for SnO2films deposited by

spray pyrolysis, the shape of crystallites with different

crystallographic planes such as (1 1 0), (1 1 1), (2 0 0),

(0 1 1), đố1 ố1 2ỡ, and (2 1 0) is dependent on the deposition

parameters and film thickness Changing the facet of

nanocrystalls as their size and growth conditions are modified

has been observed in other metal oxides as well[187]

The predominance of different crystallographic planes

causes corresponding changes in atomic and electronic

properties along with surface energy parameters Examples

of some crystallographic planes observed in SnO2 films

deposited by spray pyrolysis are shown inFig 27 Consistent

with the results reported in Refs.[78,132,188Ờ190], the (1 1 0)

and (1 0 1) planes of the SnO2crystal are F (plane) faces, while

the (1 1 1) plane is K (kinked) This means that the (1 1 1) plane

has a much rougher surface than both (1 1 0) and (1 0 1) planes

According to the periodic bond chains (PBC) theory[189],

surface atoms of F-faces are strongly bound to each other in

directions parallel to the face Accordingly, these faces have a

small tendency to react with the arriving atoms In reality this

statement is not entirely correct, because besides ỔỔin-planeỖỖ

oxygen at the (1 1 0) SnO2surface, there are also ỔỔbridgingỖỖ

oxygen ions located at equal distance above and below the

surface plane However, the statement that the (1 1 0) surface

plane is the most stable facet, is correct Indeed, the facesparallel to only one PBC (stepped S-faces) or to none (kinkedK-faces) are highly reactive because there are many unsaturatedbonds cutting their surfaces

The catalytic activity of atomic planes to a large extent isdetermined by the surface concentration of non-saturatedcations and weakly bounded bridging oxygen The followingrank of catalytic activity (CA) of ideal atomic planes can beproposed: CA(110)< CA(001), CA(100)< CA(101) Simple esti-mations show that dissociative chemisorption on the surface ofSnO2is orientation dependent as well Various crystallographicplanes have different distances between Sn atoms, which formthe following series d(110) d(100)< d(101)< d(001)[132] Tinatoms are centers of oxygen chemisorption, and therefore thechange of indicated distance must influence the rate ofdissociative oxygen chemisorption, which in many cases is acontrolling factor of gas-sensing phenomena[4,24,28,29,46].The nature of the bonding of an adsorbed gas molecule withthe metal oxide is another important factor[37] For example,the authors of Ref.[182]observed that monodentate adsorption

on MgO and CaO is preferred on the edge/corner sites This isunique to small nanocrystallites (spherulites) In contrastbidentate adsorption is favored by flat planes, which are moreprevalent on the larger nanocrystals This feature of surfacespecies adsorption indicates that the control of surfaceroughness and grain shape can be exploited for improvement

of gas-sensing characteristics such as absolute magnitude andselectivity Such control may be as effective as the use ofcatalytic additives This last factor is an important one for thestability improvement of solid-state gas sensors[34]

Results of experimental research presented in Refs

[132,177,191] and theoretical simulations discussed in Refs

[75,177] gave the same conclusion For example, researchcarried out in Ref.[177]has shown that the (1 1 0), (1 1 0) and(1 0 1) SnO2planes have different surface energies, differentphase transition conditions, and different energy spectra ofsurface states generated during their reduction Differentcrystallographic planes also have different peculiarities ofinteraction with water[75,186] In Ref.[191], it was establishedthat SnO2films with different texturing have different catalyticactivity for selective oxidation of CH4 In these investigations,one type of SnO2 film had predominant orientation in the

Fig 27 The models of unrelaxed rutile SnO 2 surfaces: (a) (1 1 0); (b) (1 0 0); (c) (0 0 1); (d) (1 0 1) The light large balls represent tin atoms and the dark small balls oxygen atoms (Reprinted with permission from Ref [76] Copyright 2005: IOP Publishing Ltd.).

G Korotcenkov / Materials Science and Engineering R 61 (2008) 1Ờ39 18

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{1 1 0} crystallographic direction and another in the {2 1 1}

and {3 0 1} directions

So, the determination of crystallographic planes with

optimal combinations of adsorption/desorption and catalytic

parameters, and the development of methods for grains

deposition with indicated faces can be considered as a main

contemporary goal for thin film technology as applied to metal

oxide gas sensors Research, presented in Ref.[177]could be

considered as the first step in this direction

A proper choice for crystallite deposition technology or

synthesis with a necessary grain facet can be also one method to

decrease humidity effects in gas sensors For example, in Ref

[75], using a Mulliken population analysis it was shown that the

chemisorption of OH-groups on the (1 1 0) face is accompanied

by the localization of negative charge to a greater extend than

the chemisorption of OH-groups at the (0 1 1) surface of SnO2

This means that adsorption/desorption processes and surface

reactions with water vary with different SnO2crystallographic

planes

This consideration rationalizes the large dispersion of both

the temperature position of sensor response maximum and the

half-width of the temperature dependence of conductivity

response observed for SnO2 films deposited using various

technological parameters (see Fig 28) As indicated earlier,

every structure of a polycrystalline film has its own

combination of crystallographic planes that can participate

in the gas-sensing reaction Since a large variety of crystal

structures can be obtained during metal oxide deposition, a

variety of shapes for the S(T) dependencies, reflecting the

specific of adsorption/desorption (A/D) processes on these

planes will inevitably be observed Experimental S(T)

dependencies are a superposition of individual chemical

reactions taking place on the individual surface planes

Understandably, the preparation of polycrystalline metal

oxides with necessary grain faceting is difficult to control But,

it is achievable for one-dimensional sensors and should be a

high-priority area of research One-dimensional structures are

crystallographically perfect and have clear faceting with a fixed

set of planes Research has shown that the planes and faceting inone-dimensional structures depend on the parameters ofsynthesis For example, in Ref [192] it was reported thatusing solid–vapor phase deposition, it was a possibility tosynthesize In2O3nano-belts with {1 0 0} and {1 2 0} growthdirections Both types of nano-belts had the top and bottomsurfaces being (0 0 1), while the {1 0 0} nano-belts had sidesurface of (0 1 0) and a rectangular cross-section The {1 2 0}nano-belts had a parallelogram cross-section Furthermore, itwas reported in Refs [193,194]that In2O3nano-belts mighthave growth directions of {1 1 1} and {1 1 0} with side and topsurfaces being (1 0 0) In Ref [195] it was found that In2O3nano-belts were enclosed by (1 0 0) and (0 1 0) planes and thattheir growth direction was parallel to {0 0 1} As discussedearlier in this review, the possibility to control faceting planes

by growth conditions gives additional opportunities forcontrolling the sensor’s performance parameters

The crystallographic geometry of other metal oxide dimensional nanostructures is presented in Table 1 It isimportant to note that, for example, the SnO2nanostructures arenot enclosed by (1 1 0) planes, which are the most stablecrystallographic planes in the SnO2 lattice It means thatcrystallographic planes, which have never been analyzed bymethods of theoretical simulation, participate in gas-sensingeffects

one-It is necessary to note that semiconducting one-dimensionmetal oxide structures with well defined geometry and perfectcrystallinity could represent a perfect model-material familyfor systematic experimental study and theoretical under-standing of the fundamentals of gas-sensing mechanisms inmetal oxides

In analyzing the opportunities of one-dimensional structures

of various types for their practical application in gas sensors, it

is necessary to note that nano-belts (nano-ribbons) probablycould be the most demanding one-dimensional structure toexploit for gas-sensing applications Nano-belts are thin andplain belt-type structures with rectangular cross-section (see

Fig 29) At present, nano-belts have been obtained for nearlyall oxides used in gas sensors There is considerable datapertaining to the synthesis of nano-belts for SnO2, In2O3, ZnO,

Ga2O3, TiO2, etc.[39,192,195,199–205]) Typical nano-beltshave widths of 20–300 nm, and lengths from several mm tohundreds, or even some thousands of mm[195,200,206]) Thetypical width-to-thickness ratio for nano-belts ranges from 5 to

10 For comparison, for nanowires (or nanorods) this ratioequals 2–5[195] Synthesis of nano-belts could be done withvarious methods [195,199,207–210]which provide consider-able opportunities for research on such nano-size materials.Nano-belts do not have the mechanical strength ofnanotubes However, they have structural homogeneity andcrystallographic perfection It is well known that crystal-lographic defects may destroy quantum-size effects Because ofthe zero-defects of nano-belts, structural defects will not be aproblem as observed for nanowire type structures It isnecessary to emphasize that that suitable geometry (see

Fig 30), high homogeneity of the structure, and long lengthare important advantages of nano-belts for mass manufacturing

Fig 28 Temperature dependencies of sensor response to H 2 for undoped SnO 2

films deposited at different parameters of spray pyrolysis and corresponding

deconvoluted peaks (Reprinted with permission from Ref [85] Copyright

2001: Elsevier).

Tiêu đề	The Role of Morphology and Crystallographic Structure of Metal Oxides
Tác giả	G. Korotcenkov
Trường học	Korea Institute of Energy Research, Daejeon, Republic of Korea; Technical University of Moldova, Chisinau, Republic of Moldova
Chuyên ngành	Materials Science
Thể loại	Review Paper
Năm xuất bản	2008
Thành phố	Daejeon

Định dạng
Số trang	39
Dung lượng	4,69 MB