Sensors and Actuators B 152 (2011) 73–81 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb Improving the ethanol sensing of ZnO nano-particle thin films—The correlation between the grain size and the sensing mechanism Thanh Thuy Trinh a,c , Ngoc Han Tu c , Huy Hoang Le c , Kyung Yul Ryu a , Khac Binh Le c , Krishnakumar Pillai a , Junsin Yi a,b,∗ a b c Information and Communication Device Laboratory, School of Information and Communication Engineering, Sungkyunkwan University, Republic of Korea Department of Energy Science, Sungkyunkwan University, Republic of Korea University of Sciences, Vietnam National University, Ho Chi Minh City, Vietnam a r t i c l e i n f o Article history: Received 15 January 2010 Received in revised form September 2010 Accepted 22 September 2010 Available online 27 October 2010 Keywords: ZnO:Sn thin films Neck-controlled sensitivity Ethanol sensing Sol–gel process a b s t r a c t ZnO and Sn doped ZnO (ZnO:Sn) thin films at various doping concentrations from to 10 at.% were prepared by the sol–gel method for an ethanol sensing application The Sn doping significantly influenced the film growth, grain size and response of the films The XRD patterns showed that the hexagonal wurtzite structure of the ZnO film was retained even after the Sn doping The crystallite grain sizes of the ZnO:Sn thin films at 0, and at.% were estimated by using the typical Scherrer’s equation The crystalline quality of the films at 6, and 10 at.% of Sn was degenerated Typical FESEM images demonstrated the different morphologies for the ZnO:Sn thin films at various Sn concentrations; many pores of various dimensions were observed depending on the doping level A TEM analysis of the ZnO:Sn thin films at 0, and at.% was performed to verify the grain size The optimum Sn doping level of ZnO:Sn thin film for ethanol sensing was estimated to be at.% The at.% sample obtained the highest response to ethanol vapor in the 10–400 ppm level range at a low operating temperature of 250 ◦ C The sensing mechanism was explained by a variation in the sensitivity model from a neck–grain-boundary controlled sensitivity to a neck-controlled sensitivity Our work demonstrates the ability to reduce the working temperature as well as to increase the response of ZnO thin film based gas sensors to detect ethanol, which would be of great merit for commercialized applications © 2010 Elsevier B.V All rights reserved Introduction Semiconductor oxides such as SnO2 , Fe2 O3 , Ga2 O3 , and Sb2 O3 are widely used for the detection of inflammable gases (CH4 , C3 H8 , H2 , etc.) and toxic gases (CO, H2 S, etc.) due to their distinct advantages, such as high response time and low cost [1,2] In particular, zinc oxide is a potential candidate for toxic and combustible gas sensing applications The gas sensing mechanism of ZnO type gas sensors involves the chemisorptions of oxygen onto the oxide surface at high temperatures, creating a space charge layer around the particles, followed by a charge transfer during the interaction between the chemisorbed oxygen and the target gas molecules, thus leading to a change in the surface resistance of the sensor element [3,4] The lower edge of the conduction band of ZnO is ∼4.3 eV below the vacuum energy, which is higher than the chemical potential of oxygen (∼5.7 eV below the vacuum energy) [5] So when ZnO is exposed to air, O2 will be adsorbed; it acts as electron acceptors ∗ Corresponding author at: Tel.: +82 31 290 7139; fax: +82 31 290 7159 E-mail address: yi@yurim.skku.ac.kr (J Yi) 0925-4005/$ – see front matter © 2010 Elsevier B.V All rights reserved doi:10.1016/j.snb.2010.09.045 that generate a space charge layer around the crystallite particles in the material [6] The gas sensing properties of zinc oxides, which are difficult to explain, depend naturally on their catalytic or surface chemical properties as well as on their physical or morphological properties Because of the surface reaction, this type of gas sensor shows a lack of selectivity resulting in an unspecific gas detection mechanism and so many types of reducing gases can be detected simultaneously Furthermore, most of the reactions involved in the detection of gases rely on the reaction between the adsorbed oxygen and the test gas, so that the working temperature is usually quite high [7] Zinc oxide thin films doped with various materials such as Fe [8], Cu [9] and Al [10] have been widely explored for the development of selective gas sensors It has been shown that a lower operating temperature may be achieved by the doping effect and a significant resistance change can be obtained in doped ZnO rather than in undoped ZnO, which results in a higher sensor response [11] Consequently one of the techniques used to improve the performance of ZnO thin film type sensor is by doping the films with suitable elements which vary the surface morphology of the films The surface morphology 74 T.T Trinh et al / Sensors and Actuators B 152 (2011) 73–81 and therefore the effective specific surface area of the sensor film depend on the concentration of the dopants That is, dopants effectively vary the contact and adsorption area between the gas sensing element and the target gas [12] which would change the response of the film Thus the sensing mechanism of a sensor film will be altered depending on the concentration of dopants in the film It has been reported that the surface area as well as the ratio between the crystallite size (D) and the space charge layer thickness (L) play an important role in enhancing the gas response characteristics and in the gas selectivity of the SnO2 gas sensor [13] However, to date, there are hardly any reports regarding the correlation between the grain size of the particles and the sensitivity of the ZnO thin film In addition, the exact role of the dopants in the gas sensing process is not well understood, though there are various reports discussing the role of dopants based on the catalytic effect and/or the oxygen transfer effect on the surface of the ZnO particles [14] Controlling and monitoring the ethanol concentration is vital in fields such as the testing of the alcohol levels of drivers and the monitoring of chemical synthesis [7] Both undoped [4,15,16] and doped ZnO [3,8] have been investigated widely due to their high sensitivity to ethanol However, the working temperatures of zinc oxide sensors are quite high, in the range of 400–500 ◦ C [8,15] The sensitivity of zinc oxide to ethanol vapor can still be improved upon, and there is need to further explore new ethanol-sensitive materials as well Gas sensing films can be deposited by several methods, such as thermal evaporation, successive ionic layer adsorption and reaction (SILAR) [17], pulsed laser deposition (PLD) [18], and sol-gel process The solution-based sol–gel process offers a simple, low cost and large area thin film coating method as an alternative to vacuum deposition techniques [19] Moreover, it has the advantage of fabricating thin films with a small grain size, porous microstructure, and a large surface area, useful for gas sensing applications In this study, ZnO and Sn doped ZnO (ZnO:Sn) thin films at various Sn concentrations were prepared using the sol-gel process for an ethanol sensing application The structural and morphological properties of the ZnO thin films as well as the effect of Sn doping on the ethanol sensing behavior were investigated Sn was doped into the ZnO film in order to reduce the average grain size and vary the surface morphology, which was expected to increase the response When the average grain size was close to the space charge layer thickness (L), the sensitivity mechanism of ZnO:Sn thin films changed from a neck–grain-boundary controlled sensitivity to a neck-controlled sensitivity model This property was found to significantly increase the film’s response strates were dried in a flow of N2 gas The ZnO:Sn gel films were coated onto the glass substrates (25 mm × 25 mm × mm) using the dip coating method at a speed of 15 cm/min These as-coated films were pre-heated at 250 ◦ C for 20 immediately after coating After repeating the coating procedure times the films were annealed in air at 500 ◦ C for h The thickness of the films was estimated to be around 100–150 nm using the alpha-step method The surface morphology of the films was studied using a Hitachi S-4800 model ultra high resolution field emission scanning electron microscope (FESEM) X-ray diffraction (XRD) patterns were obtained by employing a Siemens Kristalloflex using CuK␣ radi˚ The measuring range was 20–60◦ , at room ations ( = 1.54059 A) temperature The crystalline grain size of the ZnO and the ZnO:Sn thin films at and at.% was measured by transmission electron microscopy (TEM) (Model JEM-3101-JEOL) In order to prepare a specimen for ZnO (Fig 4(a)), the film deposited on the glass substrates was scraped and dispersed in a solvent followed by placing a drop of the solution on a carbon coated copper grid The specimens for the TEM observation of the ZnO:Sn thin films with and at.% Sn concentration (Fig 4(d) and (e)) were prepared by a carbon and metal coating method A thin layer of platinum (∼10 nm) above which a thick layer of carbon was coated by the ion beam method (Model Gatan 682 PECS) on top of the deposited ZnO:Sn film in order to provide conductivity and beam damage protection 2.2 The sensing measurements The characteristics of the sensors were studied using a homemade heated gas flow chamber (6.7 dm3 in volume) The sensor film with the CrNi electrode was placed on a heater which was kept inside the gas flow chamber; the temperature of the heater was controlled from room temperature to 500 ◦ C by a heat controller The current to the heater was controlled with a variable voltage transformer The temperature of the sensor was detected using a copper-constantan thermocouple The temperature inside the chamber was always maintained at over 80 ◦ C, which is over the boiling point of the ethanol solution The response S, of the film was defined as Ra /Rg where Ra and Rg are the electrical resistances in the air and in the ethanol–air mixed gas, respectively [3,13] The ethanol was injected into the hot-chamber by a micro pipette in the range of 0.2–2 l The ethanol is then converted to its vapor phase in approximately The sensing characteristics were studied at a temperature range of 200–300 ◦ C The volume of the ethanol injected into the chamber was estimated by the following equation [20,21]: C (ppm) = Experimental 2.1 The sensing films fabrication and characterization Fig depicts the details regarding the preparation of the films Zinc acetate dehydrate (Zn(CH3 COO)2 ·2H2 O, 99.99% pure) was dissolved in a mixture of 2-methoxyethanol (99.99% pure) and monoethanolamine ((MEA) HOCH2 CH2 NH2 , 99% pure) solution and tin tetrachloride (SnCl4 ·5H2 O, 99% pure) was dissolved in 2-methoxyethanol to prepare two different types of solutions, solutions A and B MEA acted as a solution stabilizer The concentration of the Zn ions in all of the ZnO:Sn sols was controlled to 0.75 M; the Sn/Zn ratio was varied from to 10 at.% in steps of at.% Both the A and B solutions were stirred for h and then aged at 60 ◦ C for 44 h until a transparent and homogenous sol was obtained Prior to dipping, the glass substrates were cleaned ultrasonically by a freshly prepared dilute hydrochloric acid, detergent solution, a sodium hydroxide solution, acetone, and distilled water Finally, the sub- ı × Vr × R × T × 106 M × Pb × Vb (1) where ı is the ethanol density, Vr is the volume of the ethanol injected, R is the universal gas constant, T is the absolute temperature, M is the molecular weight, Pb is the pressure after the ethanol vaporization inside the chamber, and Vb is the volume of the chamber Results and discussions 3.1 The structural characteristics Fig depicts the XRD patterns of the ZnO and ZnO:Sn films at various Sn concentrations As shown in the figure, the undoped and low percentage Sn doped films have (0 2) as the preferred orientation This (0 2) preferred orientation is due to the minimal surface energy in which the hexagonal structure, c-plane to the ZnO crystallites, corresponds to the densest packed plane No phases corresponding to tin or the related tin compounds were detected in the XRD pattern due to the low doping concentration, implying T.T Trinh et al / Sensors and Actuators B 152 (2011) 73–81 SnCl4.5H2O Additive MEA 2ME solvent 75 2ME solvent Magnetic stirring, 30 minutes Magnetic stirring, hr Solution 2ME + MEA Zn acetate Magnetic stirring, 30 minutes Solution B (ZnO source) Solution A (Sn source) Magnetic stirring, hrs, 60 °C Solution C Aging at 60 °C, 44 hrs Transparent sol Dip coated times Preheat at 250 °C, 20 minutes Wet films Annealing at 500 °C, hrs XRD, FESEM, TEM, ethanol sensing test Thin films Fig Experimental process used to fabricate the sensing films that the Sn dopant did not alter the typical ZnO hexagonal wurtzite structure The figure shows that the (0 2) peak was slightly shifted to higher diffraction angles, especially for the at.% Sn doped sample when compared to the undoped one For the undoped sample, the (0 2) peak was at 2 around 33.7◦ whereas for the at.% doped sample the peak was at around 34.1◦ This is in agreement with other report [22] When the ZnO film was doped with Sn, Sn4+ substituted into the Zn2+ site in the crystal structure The difference in the ion radius between Sn4+ (0.069 nm) [23] and Zn2+ (0.074 nm) [24] might have resulted in a small lattice distortion and so therefore reduced the XRD Bragg peak intensity as well as the grain size [22] Additional peaks corresponding to the (1 1) and (1 0) planes of the ZnO were also observed in Fig 2, but with low relative intensities As the Sn concentration increased, the intensity of the Bragg peaks decreased and the full width at half maximum (FWHM) increased The average crystallite size D, of the films at various Sn doping was estimated by Scherrer’s formula using the equation: D= 0.9 ˇ cos  (2) where and ˇ are the wavelength of CuK␣ radiation and the FWHM of the strongest peak, respectively Table presents the average grain sizes of the films which show that the grain size decreases with an increase in the Sn concentration The grain sizes of the ZnO:Sn thin films with 0, and at.% Sn concentration were estimated to be ∼23, 16.2 and 11.5 nm, respectively As the grain size in the film decreases, the total surface area of the grains in the film is expected to increase, which could enhance the response of the film when used as a sensor The crystalline quality of the films with 6, and 10 at.% of Sn doping is degenerated The disappearance of the (0 2), (1 0) and (1 1) peaks in the samples with the at.% and 10 at.% Sn doping imply a deterioration of the crystalline quality with an increase in the Sn doping level The degeneration of the crystallinity with an increase of the Sn concentration was also observed earlier [22] 3.2 The surface morphology Fig depicts the representative FESEM images of the undoped ZnO thin films and the ZnO:Sn thin films at 2, 4, and at.% Sn doping levels As demonstrated in the figure, variations in the surface morphology of the ZnO:Sn films with an increase in the Sn doping concentration were observed The average particle size on the surface of the at.% film decreased significantly when compared to the undoped ZnO and at.% ZnO:Sn thin film Further Sn doping to Table The average grain sizes of the ZnO:Sn thin films with 0, and at.% Sn concentrations Fig XRD patterns of the undoped and Sn-doped ZnO thin films at various doping levels %Sn Crystalline size (nm) 23 16.2 11.5 – – 10 – 76 T.T Trinh et al / Sensors and Actuators B 152 (2011) 73–81 Fig Representative FESEM images of (a) undoped ZnO, (b) at.%, (c) at.%, (d) at.%, and (e) at.%, Sn doped ZnO thin films at.% appears to slightly increase the overall grain size, as shown in Fig 3(d) Many pores of various dimensions can be observed in the films The figure shows that the number of pores increases when the Sn concentration increases from to at.% (Fig 3(a)–(c)) The increase of the Sn concentration to and at.% (Fig 3(d) and (e)) reduced the number of pores; and crystallite grains in the films seem to be agglomerated The pores on the surface of the films are likely to improve the surface area, in the sense that the real specific surface that the sensing gas can enter into and contact with effectively increases This could play a significant role in the surface reactions resulting in an increase in the sensitivity of the films Thus, from the above observations, ZnO:Sn thin films with a at.% Sn concentration are expected to show the highest response when compared to the other films The HRTEM method was used to reaffirm the grain sizes in the ZnO:Sn thin films The results of the undoped, and at.% Sn doped films are shown in Fig Fig 4(a) shows that the crystallite size of the ZnO film is in the ∼20–25 nm range (marked by white lines) Fig 4(b) and (c) shows that the crystallite sizes of the ZnO:Sn thin films with the and at.% Sn concentrations are ∼10 to 20 nm and ∼8 to 10 nm, respectively These size variations of the crystallites to the Sn concentrations are almost in accordance with the sizes calculated by Scherrer’s equation using XRD results Fig 4(d) and (e) shows cross-sectional views of the TEM images of the ZnO:Sn thin films with the and at.% Sn concentrations on the glass substrates The white background indicates the pores in the film The figures demonstrate that the film with the at.% Sn concentration has more of pores when compared to the at.% Sn concentration film This is in accordance with our earlier FESEM observations In accordance to the structure and morphology observations above, the sample with the at.% Sn doping level was expected to have the highest response to ethanol vapor compared to the other samples Section 3.3 will present and discuss the sensing characteristics of the undoped and the Sn doped ZnO films 3.3 The sensing characteristics 3.3.1 The influence of doping concentration on the ethanol sensitivity of films Fig shows the response of the films with the different doping concentrations as a function of the ethanol concentration at a working temperature of 300 ◦ C As demonstrated in the figure, the undoped sample showed a poor response with a value of around 7.7 In contrast, the response of the ZnO:Sn films increased with an increase in the Sn doping concentration up to at.% Above the at.% doping concentration, the response of the films tended to T.T Trinh et al / Sensors and Actuators B 152 (2011) 73–81 Fig Representative TEM images of (a) undoped ZnO, (b) at.%, and (c) at.%, Sn doped ZnO thin films 77 78 T.T Trinh et al / Sensors and Actuators B 152 (2011) 73–81 the ZnO:Sn films estimated from the XRD pattern decreased when the Sn concentration in the film increased from to at.% The ZnO:Sn film with the at.% Sn concentration showed the lowest crystalline size of ∼11.5 nm As the grain size in the film decreases, the total surface area of the film is expected to increase, which enhances the response of the film In fact, oxygen adsorption plays an important role in the electrical properties of the Sn-doped ZnO nano material with multi microstructures and depends strongly on temperature At low temperatures, O2 − is chemisorbed, while at high temperatures O2− and O− are chemisorbed, and the O2− disappears rapidly The complete process of the oxygen adsorption can be described by the following equations [12]: Undoped 1at% 2at% 3at% 4at% 5at% 6at% 60 50 Response (Ra/Rg) 40 O2(gas) ↔ O2(adsorbed) 30 − O2(adsorbed) + e ↔ O2(adsorbed) − − (3) − O2(adsorbed) + e ↔ 2O(adsorbed) 20 (4) − O− + e− ↔ O(lattice) 2− (5) (6) When exposed to a reduction gas, such as ethanol vapor, the reaction between the ethanol vapor and the oxygen that is adsorbed onto the surface of the film can be expressed by: 10 CH3 CH2 OH(adsorbed) + 6O(adsorbed) − → 2CO2 + 3H2 O + 6e− 0 100 200 300 400 Gas concentration (ppm) Fig Response of the undoped and Sn doped ZnO thin films at various doping levels as a function of the ethanol concentration at 300 ◦ C decrease, as we can see for the at.% and at.% samples from the figure The response of the at.% and 10 at.% doping samples (not shown here) decrease further, when compared to the at.% sample The highest response, at over 50 (S = Ra /Rg ), was obtained from the at.% doping sample in ethanol vapor concentrations ranging from 100 to 400 ppm Repeated experiments were also performed in order to determine the reliability of the films The sensing mechanism of the pure and the ZnO:Sn sensor films can be explained as follows: at an elevated temperature, the reactive oxygen species, such as O2 − , O2− and O− , are adsorbed on the ZnO:Sn film surface and the concentration of these oxygen species is changed by the chemisorptions due to surface reactions [4] The doping of the ZnO by Sn creates electronic defects in the same way that Al doped ZnO does [25,26], and also changes the surface morphology of the films (as noted from the FESEM image in Fig 3), which causes the variations in the adsorbed oxygen This develops a potential barrier which enhances the resistance of the material [4] When exposed to ethanol vapor, the chemisorbed oxygen will react with the ethanol vapor due to the sensing reaction and re-inject the free carriers, thereby reducing the resistance of the ZnO and the ZnO:Sn The observed variations in the response of the ZnO:Sn films at various Sn doping concentrations can be attributed to the variations in the electronic defects created due to the Sn doping, the surface morphology, and to the variations in the adsorbed oxygen quantity As shown in the FESEM images from Fig 3, there are many pores in the films that allow the gas to quickly diffuse into the films from the outside This means that the oxygen as well as the ethanol vapors can diffuse into the film and come into contact with the inner surface thereby increasing the effective surface area during sensing [12] It should be noted from Fig that the number of pores increases when the Sn concentration increases from to at.% (supported by the TEM images in Fig 4), so that the response also increases Above the at.% Sn concentration, the response of the film decreases due to the reduced number of pores in the film Furthermore, it should be noted that the average crystalline size of (7) Due to the reaction, a number of free electrons are re-injected into the film, so that the resistance of the films decreases as the ethanol gas flows into the test chamber and is subsequently adsorbed onto the surface of the ZnO:Sn thin film 3.3.2 The effect of working temperature on the ethanol sensitivity of films In order to estimate the efficacy of the films in reduced working temperature for sensing ethanol, experiments were performed using the ZnO:Sn thin films at various doping concentrations ranging from to at.% at the temperatures of 200, 250 and 300 ◦ C The results are shown in Fig Fig 6(a) presents the response of the films as a function of the various doping concentrations for 300 ppm of ethanol at different temperatures The figure shows that at 200 ◦ C the response of the ZnO:Sn film improved dramatically when compared to the pure ZnO film The response of the undoped film was around 3, whereas for the 1, 2, 3, and at.% ZnO:Sn films, the response was around 30, 64, 72, 55 and 42, respectively When the working temperature was increased to 250 ◦ C, all the doped-films exhibited an extremely high response A higher response of around 150 was observed for the at.% sample Above at.% doping value, the response of the film tended to decrease, however this value was still higher than the undoped films The experiments were repeated to verify their reproducibility; the trend was observed to be same We also estimated the film’s ability to detect ethanol vapor at a low concentration of 50 ppm The results are shown in Fig 6(b) The figure demonstrates that the at.% sample could detect the ethanol vapor even at the very low concentration of 50 ppm with a high response when compared to all the other doped samples This result suggests that ZnO:Sn thin films with an appropriate doping concentration will be able to detect ethanol at low concentrations From Fig 6(a) and (b), it is obvious that the working temperature plays a crucial role in determining the response of the ZnO:Sn film Typically, the optimum working temperature of a sensor depends on the target gas, specifically the dissociation mechanism and the reactions between the gas and the oxygen chemisorption on the sensor surface [27] Furthermore, the chemisorptions of atmospheric oxygen depend on the sensor surface morphology In our case, the optimum operating temperature was found to be ∼250 ◦ C for the at.% sample As described earlier, the sensing mechanism is controlled by surface reactions, due to which a potential barrier T.T Trinh et al / Sensors and Actuators B 152 (2011) 73–81 (a) 160 C2 H5 OH in 200 o C 250 o C 140 300 o C 79 C2 H5 OH in C2 H5 OH in 10 120 Resistance (MΩ) Response (Ra/Rg) 100 80 60 10 40 C2 H5 OH out 10 20 C2 H5 OH out 200 250 C o 300 C Response (Ra/Rg) 30 20 10 0 1500 C o 40 1000 Fig Response and recovery time of the undoped and the at.% Sn doped ZnO thin films at 250 ◦ C 50 o 500 Time (sec) Doping concentration (at%) (b) 4at% Sn doped Undoped C2 H5 OH out Doping concentration (at%) Fig Block diagram representing the response of undoped and Sn doped ZnO films at different operating temperatures as a function of the doping concentration: (a) 300 ppm ethanol concentration and (b) 50 ppm ethanol concentration is created to further charge transfer When doping with Sn, due to the low concentration, Sn may substitute the Zn site in the lattice of the ZnO, forming more trap states on the surface of the films These surface trap states make the oxygen chemisorption process occur more easily, even at low temperatures (lower than 250 ◦ C), and enhances the O− concentration on the surface On a further increase of the working temperature over 250 ◦ C, the adsorbed oxygen species available on the film surface may not be enough to react with the ethanol vapors, which in turn may decrease the response of the film More studies are needed to arrive at a specific conclusion The response and the recovery times are vital parameters in the design of sensors for desired applications The response time is usually defined as the time taken to achieve 90% of the final change in the current value following a step change in the gas concentration at the sensor The recovery time can be defined as the time needed to return to 90% of the initial current value after recovering to a dry air flow as opposed to the operating temperature [28] Fig depicts the response and recovery time of the undoped and the at.% Sn doped samples at 250 ◦ C for 300 ppm of ethanol As shown in the figure, a high response (∼150), a short response time (∼40 s) and recovery time (∼60 s) to 300 ppm ethanol were observed for the at.% sample at 250 ◦ C Repeated experiments showed the same trend The short response and recovery times will be a merit for the ZnO:Sn thin films for use in sensor applications Overall, our studies illustrate that the ZnO:Sn thin films with the at.% doping have a high response to ethanol vapor This high response characteristic can be explained by the surface reaction mechanism of the crystallite grains or nano particles associated with the thin films As explained earlier, at elevated temperatures, the oxygen adsorption from the gas phase results in the formation of an acceptor surface state in n-type semiconductors [29] The oxygen vacancies govern the position of the Fermi level in the ZnO films A near-surface portion of the vacancies can capture oxygen from the surrounding atmosphere As a result, the concentration of free vacancies in the subsurface layer and, hence, the concentration of free charge carriers tend to decrease Thus, under oxygen adsorption, the crystal surface of n-type semiconductors has an electron depleted layer in which the concentration of electrons is lower than that in the bulk [30] The size of this layer, which is created due to the oxygen chemisorptions onto the film surface, strongly depends on the ratio of the crystallite size (D) to the space charge layer thickness (L) on the crystallite grains of the film [13] The space charge layer thickness can be calculated by: L= εKB T q2 No 1/2 (8) where ε is the static dielectric constant, KB is Boltzmann’s constant, T is the absolute temperature, q is the electrical charge of the carrier, and No is the carrier concentration The estimated value of L for ZnO was around 7.5 nm, obtained by substituting the values of the physical parameters as T = 573 K, ε = 7.9 × 8.85 × 10−12 F m−1 , and No = 4.0 × 1017 cm−3 [31] It is apparent from the above discussions that as the quantity of adsorbed oxygen increases, the region where the movement of the 80 T.T Trinh et al / Sensors and Actuators B 152 (2011) 73–81 electrons is disturbed increases When ethanol is introduced, the adsorbed oxygen ions are removed (by Eq (7)) and therefore the potential barrier decreases so that the mobility of the charge carriers increases Typically, a semiconductor sensor film like ZnO:Sn consists of crystallites which are connected to each other by necks, forming aggregates of large particles which in turn are connected to the neighboring particles by the grain boundary contacts The response of an oxide semiconductor such as ZnO:Sn thin film is independent of the crystallite size if the crystallite size is significantly large compared to the space charge layer (D 2L) At this condition, the resistance of the sensor film is mostly determined by the resistance offered by the potential barrier at the grain boundary contacts, which is independent of the crystallite size If the crystallite size approaches the space charge layer thickness (D ≥ 2L), the space charge layer penetrates deeper into each of the crystallites and forms channels at each neck within a particle or crystallite grain [13,32] At this stage the conduction electrons must move through these channels and so experience an extra potential barrier in addition to that found at the grain boundaries Due to the large number of crystallite grains and hence necks in the film; the resistance of the sensor film is determined predominantly by the neck resistance Since the neck size is observed to be proportional to the crystallite size, the response of the gas sensor is dependent on the crystallite size [13,32] Thus, the actual crystal size (D) relative to the space charge depth is one of the most important factors affecting the sensing properties of the ZnO-like semiconductor oxide gas sensor Compared to the L value (7.5 nm) of ZnO, typically, a high response can be expected for the ZnO:Sn based gas sensor if the crystallite size is below ∼15 nm In our case, the highest response was obtained for the ZnO:Sn film with the at.% doping, which showed a surface morphology having small crystallite grains (∼11.5 nm) and many pores, as shown in Fig This makes the sensitivity model vary from the neck–grain-boundary controlled sensitivity to a neck-controlled sensitivity In addition, the at.% sample exhibited the best surface area due to the association of the small grain size and the many surface pores The number of pores increases when the Sn concentration increases to at.% At the and at.% Sn doping, the number of pores reduced, perhaps due to the agglomeration of the crystallite grains (Fig 3) As discussed earlier, the gas sensing mechanism of undoped and ZnO:Sn thin film is based on the surface reactions Therefore, the samples with higher surface area are expected to show a better response Conclusions ZnO and ZnO:Sn thin films were prepared using a simple sol–gel method The XRD patterns of the as-prepared samples showed that the hexagonal wurtzite structure of the ZnO thin film was retained even after Sn doping No traces of tin or related tin compounds were detected A TEM analysis of the ZnO:Sn thin films at 0, and at.% was performed to verify the grain size The FESEM images showed that the ZnO:Sn thin film consists of nano particles associated with small grains and many pores The ethanol sensing mechanisms of the ZnO:Sn thin films at various doping levels were analyzed using a custom built home-made device It was found that the proper Sndoping of the ZnO film greatly improved the response of the gas sensor to ethanol The best response (∼150), the shortest response time (∼40 s) and recovery time (∼60 s) to 300 ppm of ethanol was observed for the sample with the at.% Sn concentration at a temperature of 250 ◦ C Our work demonstrates the ability to reduce the working temperature and to 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semiconductor sensor materials on oxygen chemisorption on their surface, Russ J Gen Chem 78 (12) (2008) 2556–2565 [31] C.C Li, Z.F Du, H.C Yu, T.H Wang, Low-temperature sensing and high sensitivity of ZnO nanoneedles due to small size effect, Thin Solid Films 517 (2009) 5931–5934 [32] H Ogawa, M Nishikawa, A Abe, Hall measurement studies and electrical conduction model of tin oxide ultrafine particle films, J Appl Phys 53 (1982) 4448–4455 Biographies Thanh Thuy Trinh completed her undergraduate studies at the University of Science, Ho Chi Minh City, Vietnam and now studying M.Sc course in Sungkyunkwan University, South Korea in 2009 81 Huy Hoang Le finished his undergraduate studies at the University of Science, Ho Chi Minh City, Vietnam in 2008 Ngoc Han Tu finished her undergraduate studies at the University of Science, Ho Chi Minh City, Vietnam in 2006 and now is a M.Sc student in College of Technology, Hanoi, Vietnam Khac Binh Le currently he is a Professor of Physics at the Faculty of Material Science, University of Science, Ho Chi Minh City, Vietnam Junsin Yi is a Professor at the School of Information and Communication Engineering, Sungkyunkwan University, Korea ... reducing the resistance of the ZnO and the ZnO: Sn The observed variations in the response of the ZnO: Sn films at various Sn doping concentrations can be attributed to the variations in the electronic... of the films with 6, and 10 at. % of Sn doping is degenerated The disappearance of the (0 2), (1 0) and (1 1) peaks in the samples with the at. % and 10 at. % Sn doping imply a deterioration of the. .. representative FESEM images of the undoped ZnO thin films and the ZnO: Sn thin films at 2, 4, and at. % Sn doping levels As demonstrated in the figure, variations in the surface morphology of the ZnO: Sn films with