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Abstract—This paper reviewed the films used in formaldehyde gas sensing recently. They can be divided into three groups: 1) metal oxide semiconductor films; ...

IEEE SENSORS JOURNAL, VOL. 15, NO. 12, DECEMBER 2015

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Thin-Film Sensors for Detection of Formaldehyde: A Review Dazhi Chen and Yong J. Yuan, Senior Member, IEEE

Abstract— This paper reviewed the films used in formaldehyde gas sensing recently. They can be divided into three groups: 1) metal oxide semiconductor films; 2) polymer films; and 3) carbon nanotubes (CNTs) films. Detection limits of these three groups were 1 ppb, 1 ppm, and 20 ppb, respectively. In metal–oxide–semiconductor films, the sensitivity of ZnO conductimetric type sensor was down to 1 ppb with the detectable response of 7.4. Polyethyleneimine/TiO2 was the most sensitive film with quartz crystal microbalance sensor in polymer films, which could detect 1-ppm formaldehyde with the response ( f ) of 0.8 Hz. Either multi-wall CNTs (MWCNTs) or single-walled CNTs, forming a CNTs film, had the higher sensitivity so far, and the MWCNTs-NH2 interdigital electrodes sensor exhibited 1.73% relative resistance change to 20 ppb of formaldehyde. The further research will be, however, needed to deal with the situation of real scenario influences, such as temperature, humidity, and interferents. Index Terms— Film, selectivity.

formaldehyde,

sensor,

sensitivity,

I. I NTRODUCTION

F

ORMALDEHYDE is a volatile organic compound (VOC), colorless and strong smelling [1], which is found in many products used in daily life [2]–[4], such as furniture, carpeting, fiberglass, permanent press fabrics, paper products, antiseptics, medicines, cosmetics, cleaners, plastics, glues, and adhesives etc. Moreover, off gassing from new mobile homes, automobile engines, burning biofuels, smoke from cigarettes, and burning of wood products are also the environmental sources [3], [5], [6] of formaldehyde. The annual production of formaldehyde is 46 billion [7] approximately in the world. Only in the USA, more than 2 million of workers are occupationally exposed to it [3]. China is the largest producer of formaldehyde with production of 12,000 kt in 2007 [8]. Though formaldehyde is widely used as a base chemical material, it is a hazardous air pollutant. Also, prolonged exposure to formaldehyde can cause serious health effects

Manuscript received April 25, 2015; revised June 29, 2015; accepted July 14, 2015. Date of publication July 17, 2015; date of current version October 2, 2015. The work of Y. J. Yuan was supported by the National Natural Science Foundation of China through the General Program Fund under Grant 31170954. The associate editor coordinating the review of this paper and approving it for publication was Prof. Chang-Soo Kim. (Corresponding author: Yong J. Yuan.) The authors are with the Laboratory of Biosensing and Micromechatronics, School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu 610031, China (e-mail: [email protected]; [email protected] swjtu.edu.cn). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSEN.2015.2457931

such as central nervous system damage, immune system disorders, and respiratory system disease [9], [10]. Furthermore, formaldehyde has been identified as a major cause of sick building syndrome (SBS), the sufferers exhibit a range of symptoms which appear to be related to the time spent in a particular building [11]. International Agency for Research on Cancer (IARC) determines that formaldehyde is carcinogenic to humans [12]. Consequently, the World Health Organization (WHO) limits exposure to 0.08 ppm (80 ppb) over 30 min [13], while the Chinese Environmental Protection Agency has set a 30 min exposure limit of 0.06 ppm [14]. In recent years, a number of analytical methods to detect formaldehyde have been reported, such as spectrophotometry [1], [9], [15], gas chromatography [16], high-performance liquid chromatography [17], ion chromatography [18], polarography [19], as well as fluorescence [20], [21]. However, these approaches required rigorous conditions and bulky instrumentation, and are unable to detect formaldehyde exposure information on a real-time basis. To overcome these defects, sensing techniques are brought into action. As well known, the sensitive films are important parts for sensors to analyzing characteristics. In this paper, a thorough review of the films used in formaldehyde gas sensors has been summing up, after classifying them according to materials. The sensing characteristics of sensor-device coatings have been decrypting in this review. II. M ETAL OXIDE S EMICONDUCTOR F ILMS Metal oxide semiconductor films can determine a formaldehyde molecule by adsorption and desorption of oxygen on sensing surface, which changes resistance of sensors. When the metal oxide semiconductor films are exposed to air, oxygen molecules will capture electrons from the metal oxide semiconductor surface states in the condition of high temperature due to the strong electro negatively of the oxygen atom, and oxygen molecules adsorb on the surface [22]. Chemisorbed oxygen molecules undergo a delocalized charge transfer, which causes great band bending and a shift in the electrostatic potential toward the surface [23]–[25]. With the temperature increasing gradually, the chemical processes as follows [26]: O2(g) ←→ O2(ads) O2(ads) + e



←→

− + e− ←→ O2(ads)

− O2(ads) − 2O(ads)

− 2− O(ads) + e− ←→ O(ads)

1530-437X © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

(1) (2) (3) (4)

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TABLE I S ENSING P ROPERTIES OF SnO 2 F ILMS TO F ORMALDEHYDE

Fig. 1.

Schematic diagram of the conductimetric gas sensor.

When formaldehyde contains in the air, processes of disporting oxygen can be summarized as follows [27]: − ⇒ C O2 + H2 O + e − HCHOads + Oads

(5)

⇒ C O2 + H2 O + 2e



(6)

⇒ C O2 + H2 O + 2e



(7)

HCHOads + O2− (ads) 2− HCHOads + Oads A. SnO2

SnO2 is a typical n-type wide band gap semiconductor with high conductivity and good stability, which has been widely used as a gas sensing material [28]–[30]. The sensing properties of SnO2 to formaldehyde were shown in Table I. Morphology plays an important role in the formaldehyde gas-sensing properties of SnO2 nanostructure films, and SnO2 films with different morphologies have different sensing characteristics. In 2013, Wu et al. [31] compared the formaldehyde gas-sensing properties of three SnO2 morphologies films, such as nanocorals, nanofragments, and nanograss. When reaction temperature was up to 300 °C, the responses (for gas sensors, the response value was defined as S = Ra/Rg, where Ra and Rg were the resistance in air and test gas, respectively) to 50 ppm formaldehyde were respective 43.6, 39.4, and 55.6 for nanocoral, nanofragment and nanograss conductimetric gas sensors, which consisted of an alumina ceramic tube with a pair of Au electrodes and four Pt wires [11] (Fig. 1).

To 100 ppm formaldehyde the response and recovery times (we define the response time as the time to reach 90% of the maximum response and the recovery time as the time to decrease to 10% of the maximum [32]) were 6 s and 7 s for the nanograss, 7 s and 8 s for the nanocoral, and 9 s and 12 s for nanofragment the sensors, respectively. In the work presented by Gu et al. [33], porous flower-like SnO2 nanostructure film was prepared by annealing of the flower-like copper Cu3 SnS4 nanostructures. The porous flower-like SnO2 nanostructure film was directly coated on the surface of a conductimetric gas sensor with a small Ni-Cr alloy coil as heating wire by a pipette [34]. For 100 ppm formaldehyde, the response was 9.5, which was higher than benzene, methanol, ethanol, and acetone at the optimum operating temperature of 260 °C, and the response and recovery times were 2 s and 12 s, respectively. At a low concentration of 1ppm formaldehyde, the response was about 2.3. The detection limit of formaldehyde was approximately 0.33 ppm. However, the strongest interference was from toluene, the responses to 100 ppm and 1 ppm toluene were 9.7 and 2.7 [33], respectively. While, Iizuka et al. [35] had reported a porous SnO2 film developed by plasma spray physical vapor deposition (PS-PVD) on a conductimetric gas sensor. This sensor coated with an 8μm porous SnO2 film exhibited the noticeable response of formaldehyde at a concentration as low as 40 ppb at a fixed temperature of 540 K. The physical and chemical properties of initial materials can change by doping other materials. There is a great influence on gas sensing properties of SnO2 film via noble metal or another metal oxide doping. In 2012, Liu et al. [36] synthesized 2% Ce-doped SnO2 film by hydrothermal method using SnCl4 · 5H2 O and Ce(NO3 )3 · 6H2 O. The experimental results showed that the response of this film to 10 ppm formaldehyde was 3.6 at 320 °C. Iizuka et al. [37] synthesized a 0.1 mol% Pt-added porous SnO2 film by PS-PVD. This film successfully detected formaldehyde at a concentration as low as 20 ppb in the temperature of 220 °C. This result was more sensitive than Lee et al., in their work [38], 0.3 wt% Pt-SnO2 nanopowders

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TABLE II S ENSING P ROPERTIES OF TiO 2 F ILMS TO F ORMALDEHYDE

formaldehyde gas detected by this sensor was 0.03 ppm by active carbon enrichment method. B. TiO2

Fig. 2.

Sketch of electrodes in the micro-gas sensor.

films were prepared by a doping method which controls the face charge by controlling the pH, and the lowest detection limit of this film to formaldehyde was 0.45 ppm at 400 °C. Wang et al. [39] developed a micro-gas sensor by fabricating Pt electrodes on a SiO2 /Si substrate. The heating electrode and measuring electrodes in the micro-gas sensor were shown in Fig. 2. 1 mol% Pb-doped SnO2 thin films were synthesized on the surface of the electrodes by sol-gel method using SnCl4 · 5H2 O and PbCl2 . The lowest concentration detected by this film was 0.03 ppm at 250 °C with the response about 1.02, and the response and recover times to 0.05 ppm formaldehyde were about 50 s. However, the response of this film to ethanol was higher than formaldehyde. A more selective Pb-SnO2 film was synthesized by Tian et al. [40]. In their work, the Pb-SnO2 film was synthesized by loading Pb nanodots onto SnO2 fibers through a facile dipping-annealing process. The response of this film to 500 ppb formaldehyde (S = 4.63) was four times as high as that of 500 ppb ethanol, 500 ppb acetone, and 10 ppm benzene at 190 °C. The lowest detection concentration reached 50 ppb. The response and recovery times to 100 ppb formaldehyde were 53 s and 103 s, respectively. In 2008, Wang et al. [14] fabricated a conductimetric gas sensor of SnO2 doped with hydroxyl functionalized multiwall carbon nanotubes (MWCNTs), in which, SnO2 power and MWCNTs power were mixed at a suitable weight ratio and group with ion-free water to form a paste which was coated onto a clean ceramic tube with electrodes and wires, and then sintered at 500 °C for 2 h. The lowest concentration of formaldehyde detected by the 5 wt% MWCNTs-doped SnO2 sensor was 0.05 ppm with the response of 1.0 at 250 °C, and the response and recover times were about 100 s and 90 s. The response of this sensor to 50 ppm formaldehyde was about 3.8, which was 2-4 times as high as acetone, methanol, toluene, benzene, as well as ammonia. The lowest concentration of the

TiO2 is an n-type semiconductor which has been demonstrated to be a promising material for application in gas sensors [41], [42]. The sensing properties of TiO2 to formaldehyde were shown in Table II. In 2010, Zeng and Liu [42] synthesized 2-3% Ag-doped TiO2 solid powder with the sol-gel method using TiCl4 and AgNO3 . To 200 ppm formaldehyde, the response of this film was estimated to be 3.7 at the temperature of 360 °C, and the response and recovery times were evaluated to be about 30 s and 45 s, respectively. Compared to pure TiO2 , Ag-TiO2 had better sensitivity and higher selectivity to formaldehyde [42], [43]. However, as early as 2008, Yu et al. [44] had synthesized better doped TiO2 film. They prepared Cr-doped TiO2 powder and Cr2 O3 /TiO2 powder using sol-gel route by conventional solid state reaction of TiO2 and Cr2 O3 . The detection limit of the conductimetric sensor [45] coated with 32% Cr-doped TiO2 film for formaldehyde concentration was as low as 21 ppm at the temperature of 773 K, as well as the same sensor coated with 8% Cr2 O3 -doped TiO2 film at 393 K. However, Cr doped TiO2 based sensor was less sensitive than Cr2 O3 /TiO2 sensor due to lower porosity which was positive correlation to the amount of adsorbed oxygen [46]. C. NiO NiO is one of the relatively few metal oxides with p-type semiconductivity which has been extensively investigated because of the stable band gap [47]. The sensing properties of NiO to formaldehyde were shown in Table III. In 2001, Dirksen et al. [48] fabricated 0.5 μm NiO thin film on an alumina substrate, silver pads, and lead wires using a dipping process. The conductivity of this device was found to change as the formaldehyde concentration varied, and a linear formaldehyde sensitivity of 0.825 mV ppm−1 was attained at 600 °C. However, its detection limit was as high as 50 ppm. In 2006, Lee et al. [49] deposited a NiO thin film (2 μm) which was prepared by radio frequency (RF) magnetron sputtering system with a NiO target of 99.98% purity on Microelectromechanical Systems (MEMS)-based gas sensor [50] comprising micro-hotplates with Pt resistance heaters, a sputtered NiO layer, and Au interdigital electrodes (IDE) (Fig. 3). At the optimal temperature of 280 °C, a high degree of sensitivity of 0.14 ppm−1 and a detection

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TABLE III S ENSING P ROPERTIES OF NiO F ILMS TO F ORMALDEHYDE

TABLE IV S ENSING P ROPERTIES OF In2 O 3 F ILMS TO F ORMALDEHYDE

D. In2 O3

Fig. 3.

Schematic representation of the MEMS-based gas sensor.

capability of less than 1.2 ppm were found. The response time was determined to be 13 s in the formaldehyde concentration range of 1.0-8.0 ppm. Later, a MEMS-based NiO thin film (0.4 μm) sensor was developed. At the optimal temperature of 280 °C, this sensor had a higher degree of sensitivity of 0.33 ppm−1, a lower detection limit of less than 0.8 ppm, and a rapid response time of 13.2 s in the formaldehyde concentration range of 5.0-10.0 ppm [51]. In 2008, a SnO2 -doped NiO sensitive film (Sn:Ni = 22:1) was synthesized by a chemical co-precipitation method using SnCl2 · 2H2 O and NiSO4 · 6H2 O, and coated onto a MEMS-based sensor comprising micro-hotplate with Ti/Pt thin film (50 nm/200 nm) as electrodes to detect formaldehyde [52]. The experimental results indicated that this sensor had a lowest detection limit of 0.06 ppm, rapid response and recovery times (for 22.5 ppm formaldehyde, 11 s and 60 s, respectively), and high selectivity (response to 0.06 ppm formaldehyde was about five times as high as that of 0.1 ppm alcohol, toluene, acetone, and 0.5 ppm α-pinene) at 300 °C.

In2 O3 , an important n-type semiconductor with a band gap of approximately 3.55-3.75 eV, has been widely used in gas sensors for the detection of various toxic gases [53]–[55]. The sensing properties of In2 O3 to formaldehyde were shown in Table IV. In 2010, Li et al. [56] prepared In2 O3 nanofibers and nanoribbons by electrospinning combined with a poly (vinyl pyrrolidone)-assisted sol-gel technique, and compared their formaldehyde gas-sensing properties. The In2 O3 nanoribbons films exhibited a higher and faster response (the response, response and recovery times to 100 ppm formaldehyde were 4.2, 16 s and 15 s, respectively) than the nanofibers films at 300 °C. Zhang and Zhang [57] fabricated porous nanosheetbased corundum In2 O3 microflowers by one-pot hydrothermal treatment of D-fructose/In(NO3)3 mixtures using urea as a precipitating agent at 160 °C for 16 h. This film could detect formaldehyde in the range of 5-750 ppm with prompt response and recovery times (for 750 ppm formaldehyde, the response and recovery times were less than 10 s and 60 s, respectively) at the operating temperature of 215 °C [57], [58]. Moreover, this film showed high anti-interference ability. To 50 ppm formaldehyde and other gases or vapors such as ammonia, ethanol, acetone, CO, H2 , NO2 , and methane, this film showed the highest response of 18 to formaldehyde, and the highest response to other gases or vapors was 7.3. Chen et al. [59] investigated gas-sensing characteristics of CdO-mixed In2 O3 to formaldehyde. CdO-In2O3 powders (Cd:In = 1:2.5) were coated on a conductimetric gas sensor as sensing materials. The responses of this sensor to 10 ppm formaldehyde was more than 80 with the 2 min response time and 4 min recovery time at 95 °C, but to alcohol and

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TABLE V S ENSING P ROPERTIES OF ZnO F ILMS TO F ORMALDEHYDE

gasoline were very small. In another study [60], the CdO-mixed In2 O3 materials with a molar ratio of 1:1 calcining at different temperatures were prepared using the solid-state synthesis technologies. This film exhibited the lowest detection limit of 2 ppm formaldehyde after calcining at 650 °C when operating at 100 °C. In 2012, Chen et al. [61] synthesized grape-like porous microstructure In2 O3 /CdO composite by calcining mixture of In2 O3 and CdO (molar ratio 1:1) at 600 °C. The lowest detection limit of this film was 50 ppb with the response of 1.45 at 410 °C. The response of this film to 10 ppm formaldehyde was about 8.5, which was 6-8 times as high as that of 10 ppm ethanol, toluene, acetone, methanol, and ammonia. The response time and recovery time of this sensor to 10 ppm formaldehyde were about 70 s and 110 s, respectively. Wang et al. [62] prepared 8 wt% Ag-doped In2 O3 nanocrystalline powers by sol-gel method using In(NO3)3 · 5H2 O and AgNO3 as base-materials. This film could detect formaldehyde down to 2 ppm with a short response time of 10-15 s and an acceptable recovery time within 60-80 s at 100 °C. To 100 ppm formaldehyde, the response of this sensor was 175, which was 6-10 times as high as that of methanol, acetone, benzene, and carbon dioxide. E. ZnO As one of the key wide bandgap (∼3.4 eV at 1.2 K) semiconductors, ZnO has been proved to be an excellent gassensing material for measuring both oxidative and reductive target gases at ppm level and above [63], [64]. The sensing properties of ZnO to formaldehyde were shown in Table V. In 2009, Chu et al. [65] investigated formaldehyde conductimetric gas sensor with ZnO thick film which was prepared through microwave heating method using Zn(NO3)2 · 6H2 O. The response to 1 ppb formaldehyde of this sensor

attained 7.4 when operating at 210 °C. To 1000 ppm formaldehyde, benzene, and toluene, the responses were 997, 20, and 13, respectively. The response time and recovery time for 1000 ppm formaldehyde were about 10 s and 20 s, respectively. However, this sensor was not selective to formaldehyde when operating at other different temperatures. Peng et al. [66] compared formaldehyde gas sensing properties of nanoparticles and nanorods, they were prepared using Zn(Ac)2 · 2H2 O [67], [68]. The results showed that ZnO nanoparticles and nanorods with the UV light irradiation were more sensitive to formaldehyde gas than without the UV light irradiation [69], [70]. It was attributed to the large surfaceto-volume ratio and high photo-generated charge efficiency. However, photo-generated charge efficiency of gas sensing element decreases as the surface-to-volume ratio increasing. Therefore, the particles with highest sensitivity were of a certain size and the ZnO nanorods with diameter of ∼40 nm owned the highest response to formaldehyde with the UV light illumination. In the work reported by Han et al. [71], ZnO nanorod with the length of 500 nm showed higher gas sensing property than 200 nm and 2000 nm length ones. They also investigated the intrinsic defects by photoluminescence (PL). The results showed that high ratio of visible to ultra-violet luminescence could not account for high gas response, and the content of donor-related (DL) was high and that of acceptor-related (AL) was low, the gas response was high. In the work reported by Zhang et al. [72], nanoparticleassembled ZnO mico-octahedrons were synthesized by a facile homogeneous precipitation method using Zn(CH3 COO)2 · H2 O. This film represented excellent formaldehyde sensing properties, and when exposed to 1 ppm formaldehyde, the response was as high as 22.7 at 400 °C.

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Furthermore, the lowest detection limit is calculated to be 2.8 ppb. For 200 ppm formaldehyde, the response and recovery times were 46 s and 13 s, respectively, and the response was about 315 which was 6-10 times as high as that of methane, ethanol, ammonia, and benzene. The reason of high formaldehyde response was considered to be the synergistic effect of larger amount of electron donor defects and higher active oxygen species. They also synthesized Shuttlelike ZnO nano/microrods via a low temperature (80 °C) and simple hydrothermal process [73] in the solution of zinc chloride and ammonia water. This film exhibited high sensitivity to formaldehyde in the range of 10 (S = 24.8) and 1000 (S = 564.7) ppm at an optimum operating temperature of 400 °C. When exposed to 200 ppm formaldehyde, the response and recovery times were 41 s and 15 s, respectively. The response was also tested by exposing this film to 200 ppm potential interfering gases at 400 °C, including, CH4 , liquefied petroleum gas (LPG), C5 H12 , C6 H14 , C6 H12 , C7 H16 , CH2 Cl2 , CHCl3 , CCl4 , C6 H6 , C6 H5 CH3 , H2 , CO, NO, and NO2 . This film showed the highest response to formaldehyde (S = 225), which is 5-10 times as high as other gases. Tian et al. [74] processed conductimetric gas sensor based on ZnO nanotetrapods by sintering at different temperatures from 350 to 850 °C. The best gassensing performance toward formaldehyde was obtained after sintering at 450 °C, and the responses to 5 ppm and 50 ppm formaldehyde were about 8 and 68 at 320 °C, respectively. In 2010, Han et al. [75] prepared pure ZnO, Sn-, Ni-, Fe-, and Al-doped ZnO by coprecipitation of analytical purity ZnSO4 ·7H2 O, NH4 HCO3 , SnCl2 , Ni(NO3 )2 , FeCl2 and AlCl3 . Sn-doped ZnO could effectively increase the formaldehyde response by 2 folds (S = ∼130 @ 205 ppm formaldehyde) [75], [76], while Ni increased a little [75], [77] and Fe [75], [76] and Al [75] decreased the response. Further, CdO could effectively activate ZnO based formaldehyde gas sensing material, and 10 mol% CdO activated 2.2 mol% Sn-doped ZnO showed the highest formaldehyde sensitibity of 10/ppm at 200 °C. In the work presented by Xie et al. [78], nanostructured flat-type coplanar gas sensor arrays including Au IDE and RuO2 heater of ZnO with different MnO2 additive concentration were fabricated by a combination of screen-printing technology and solution growth process, and the nanostructures were composed of nanowalls and nanosheets with the thickness of about 50-200 nm and length of about 1-2 μm. The effect of MnO2 additive was positive when the concentrations of MnO2 were not larger than 1.0 wt%, and the negative function of MnO2 additive presented when the concentrations of MnO2 reached 5.0 wt% operating at 320 °C. However, the response and recovery times were delayed with increasing of MnO2 additive. Y-doped ZnO power [79] was synthesized by adding yttrium nitrate hexahydrate to the obtained ZnO power. The results showed that 4% Y-doped ZnO power exhibited the highest sensitivity, and the response to 50 ppm formaldehyde was 65.7 at 300 °C, and response time and recovery time were 4 s and 6 s, respectively. The responses to other VOCs including CH4 , NH3 , CH3 OH, CO, and (CH3 )2 CO were no larger

IEEE SENSORS JOURNAL, VOL. 15, NO. 12, DECEMBER 2015

than 16. Han et al. [80] prepared Ga-doped ZnO powders (∼8 μm in thickness) by coprecipitation [81] of analytical purity 1.5 mol/l ZnSO4 · 7H2 O, 1.26 mol/l NH4 HCO3 , and 2.2 mol% CaCl3 . The Ga-doped ZnO sensor with Ag paste and Pt wires could test the lowest formaldehyde concentration of 2.3 ppm at 400 °C. Additionally, there was no significant difference in the formaldehyde response during the relative humidity (RH) increasing from 0 to the maximum of 70+10% (which is also the upper limit of the comfortable RH for human beings [82]). Xu et al. [83] prepared mixed oxides of ZnO/ZnSnO3 doped with Au element by hydrothermal process using ZnAc2 ·2H2 O, SnCl4 ·5H2 O, and HAuCl4 , and the mole ratio of Zn and Sn was 2:1. Responses of the 1.0% Au-doped ZnO/ZnSnO3 conductimetric gas sensor to 14 kinds of gases were tested, such as ammonia (50 ppm), formaldehyde (50 ppm), benzene (50 ppm), ethanol (500 ppm), 90# petrol (500 ppm), isobutene (500 ppm), hydrogen (500 ppm), sulfurated hydrogen (50 ppm), etc. The response to formaldehyde was about 34.5, while other gases were no greater than 12 under the optimum heating voltage of 5.0 V. This sensor showed a favorable response to formaldehyde at a low concentration of 2 ppm, and the response and recovery times were less than 15 s and 12 s, respectively. In 2010, Zhai et al. [84] synthesized four type heterostructures of CdS nanoparticles (NPs)/ZnO (mass ratio (CdS:ZnO): 0.063:1, 0.079:1, 0.100:1, 0.330:1)) microcrystals by a sonochemical method using ZnO, CdCl2 · 2.5H2 O, and H2 NCSNH2 . The visible-light (λ > 450 nm) was used as the irradiation light for gas sensing measurement. The conductimetric sensor consisted of ITO electrode and glass substrate. As the qutantity of CdS NPs increasing, the photocurrent intensity increase. The number of photo-generated charge carriers had an important influence on the response to formaldehyde. But for CdS NPs/ZnO (0.330:1), the surface area of ZnO exposed to air drastically decreased due to postcoated, the number of oxygen ions chemisorbing also become a dominant factor to tune the sensitivity, so the response of CdS NPs/ZnO (0.330:1) to formaldehyde decreases. F. ZnSnO3 As a multifunctional n-type semiconductor material, ZnSnO3 [83] has been investigated as a new type of good gas sensing material and combustible gases. The sensing properties of ZnSnO3 to formaldehyde were shown in Table VI. In 2010, Wang et al. prepared monodisperse ZnSnO3 cubic crystallites via a solution process involving the reaction of ZnSnO3 · 7H2 O and Na2 SnO3 · 3H2 O with the molar ration of 1:1 at a reaction temperature of 0 °C for 5 h [85]. As the particle-size of ZnSnO3 cubic crystallites decreasing, the sensitivity of this film increased and the recovery time shorten rapidly. The detection limit of the 40 nm ZnSnO3 film could reach as little as 1 ppm at the working voltage of 5.25 V. Zeng et al. synthesized hollow ZnSnO3 cubic nanocages using a hydrothermal process, in which, Zn(CH3 COO)2 · 2H2 O, SnCl4 · 5H2 O, NaOH, and HMT((CH2 )6 N4 ) were used [86], [87]. The response of

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TABLE VI S ENSING P ROPERTIES OF ZnSnO 3 F ILMS TO F ORMALDEHYDE

TABLE VII S ENSING P ROPERTIES OF P3HT F ILMS TO F ORMALDEHYDE

this film to 50 ppm formaldehyde was 5.4 at 210 °C, and the response time and recovery time were about within 15 s and 60 s, respectively. In the same year, Guo et al. also synthesized hollow ZnSnO3 nanocages though the same process using Zn(CH3 COO)2 , SnCl4 · 5H2 O, NaOH, and monoethanolamine (MEA) as raw materials [27]. The response of this film to 50 ppm formaldehyde reached the maximum value of 57.6 at 350 °C, which was higher than ethanol (S = 34.5), acetone (S = 23.6), methanol (S = 12.8), and ammonia (S = 8.3) at their corresponding optimal temperatures, and the response and recovery times to formaldehyde (3 s, 5 s) were shorter than ethanol (7 s, 6 s), acetone (10 s, 13 s), methanol (11 s, 15 s), and ammonia (11 s, 14 s). This film could detect the formaldehyde as low as 1 ppm with the response of 9.3. III. P OLYMER F ILMS A. Poly-3-Hexylithiophene (P3HT) P3HT is one of the widely used polymer materials in organic thin film transistor (OTFT), and had been mainly reported on the detection of harmful gases [88]–[90]. The sensing properties of P3HT to formaldehyde were shown in Table VII. In 2013, Tai et al. [91] developed a novel formaldehyde OTFT sensor based on P3HT/Fe2 O3 nanocomposite thin film. The P3HT/Fe2 O3 mixed solution was ultrasonicated for 10 min, and then was sprayed onto the OTFT chips, of which the channel length and width was 25 and 4000 μm, the insulating layers SiO2 is 195 nm in thickness, 20 nm Titanium (Ti) and 50 nm gold (Au) bilayer was sputtered on top of the gate dielectric layer as the source and drain electrodes (Fig. 4). For OTFT, the gas response (R) is defined as R = (Igas - Iair )/Iair , where Iair is the value of Ids (drain current) before OTFT was exposed to formaldehyde and Igas is the steady value of Ids during the exposure to formaldehyde [92]. The P3HT/Fe2 O3 composite OTFT sensor had a better performance (R = 0.16) to 100 ppm formaldehyde than the P3HT OTFT (R = 0.08) at

Fig. 4.

Schematic illustration of the fabricated OTFT sensor.

room temperature. In the same year, Dan et al. [93] developed formaldehyde OTFT sensors based on airbrushed different ratios of P3HT/ZnO films (3:1, 1:1, and 1:3). The response of OTFT with P3HT/ZnO ratios of 1:1 exposed to 100 ppm formaldehyde was higher than the others with R = 20.12 at room temperature. The similar result was also found in the work reported by Li et al. [94]. B. Polyethyleneimine (PEI) PEI can be regarded as an ideal sensing material for the detection of formaldehyde due to the reversible interaction between formaldehyde molecules and amine groups of PEI [95]. The sensing properties of PEI to formaldehyde were shown in Table VIII. In 2009, Wang et al. [10] fabricated a formaldehyde sensor by electrospinning deposition of nanofibrous PEI/poly (vinyl alcohol) (PVA) membranes as sensitive coatings on QCM as illustrated in Fig. 5. The change in resonant frequency ( f ) can be related to the change in mass (m) due to the adsorption of gas molecules on the surface of the sensing material, and the change of 1 Hz corresponds to the mass of 16.67 ng of materials adsorbed onto the crystal surface of a 5 MHz QCM [96]. When exposed to 255 ppm formaldehyde, this sensor sensitivity of fibrous membrane (PEI/PVA weight ratio is 1.6/1) with higher specific

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TABLE VIII S ENSING P ROPERTIES OF PEI F ILMS TO F ORMALDEHYDE

TABLE IX S ENSING P ROPERTIES OF CNTs F ILMS TO F ORMALDEHYDE

surface area formed from the cosolvent ( f = 124 Hz) was three times higher than that of flat membrane. The sensors coated with fibrous membrane were tested against several conventional VOCs, including ethanol, acetone, benzene, toluene, chloroform, and dichloromethane, the tested results showed that the response to formaldehyde was 6-10 times higher than that of other VOCs. The effect of RH on sensor sensitivity was found. The sensors had no response to 255 ppm formaldehyde at the low RH of 10%, and the responses of these sensors were reinforced with the RH increasing. In the work reported by Zhang et al., the three-dimensional fibrous membranes comprising nanoporous polystyrene (PS) fibers were electrospun deposition on QCM, followed by the functionalization of the sensing PEI on the membranes [97]. The results showed that the PS concentration and PEI coating loads were positive correlation with sensibility, and the QCM-based PEI (0.83%)-PS (13 wt%) sensor with the PEI coating loads of 6000Hz had a slight response to the lowest limit of 3 ppm formaldehyde with the frequency shift of 15 Hz. In 2012, Wang et al. [98] developed a nanostructured complex of PEI functionalized TiO2 nanofiber (PEI-TiO2 ) as sensing materials coating on QCM for formaldehyde detection. TiO2 nanofibers were prepared by the combination of electrospinning and sol-gel process [99], and PEI solution was drop casted onto TiO2 fibers by micropipettor, through which PEI-TiO2 fibers were prepared. When exposed to 100 ppm of formaldehyde, the  f of this sensor was 13.7 Hz (PEI coating loads is 6600 Hz) at room temperature. The QCM sensor could detect the lowest limit of 1 ppm formaldehyde ( f = 0.8 Hz) and the response time was 120 s. The sensor also showed the highest response to 20 ppm formaldehyde with a  f up to 4 Hz, while for other vapors (20 ppm ethanol, acetone, N,N-dimethylformamide, dichloromethane, and tetrahydrofuran) the responses were less than 0.3 Hz. C. Conducting Polymers The possibility of the chemical and physical attributes of conducting polymers for chemical analysis was exploited by

incorporating their electronic and electrochemical properties to enhance the analytical figures of merit. Polypyrrole (PPy) and polyaniline (PANi) were used to identify formaldehyde. Itoh et al. [100] found that the PPy/MoO3 and PANi/MoO3 exhibited 4-8% change in conductivity to 50 ppm of formaldehyde. Hosono et al. [101] presented PPy/ 4-ethylbenzenesulfonic acid 40% resistance change upon 500 ppm. The result from Khan et al. [102] demonstrated PPy/zirconium selenoiodate nanocomposite good reversible response toward formaldehyde vapors ranging from 5 to 7%. Antwi-Boampong and BelBruno [103] found that the PANi/PEI had significant increases in the resistance by greater than 10 k at a concentration of 38 ppm formaldehyde exposed for 1 min. However, the sensing sensitivity, selectivity, range of response and detection limits of these conducting polymers were not clear compared with PEI. The structures of applicable sensors were also ambiguous, which lead to measurement standard of response incongruent. D. Nylon 6 Nano-Fiber/Nets (NFN) In 2012, Wang et al. [104] developed a novel label-free colorimetric sensor strip for real-time formaldehyde detection based on Methyl Yellow-impregnated electro-spinning/netting NFN. When the sensor strip was exposed to formaldehyde, the Methyl Yellow on the tape reacted with sulfuric acid produced by the reaction of hydroxylamine sulfate with formaldehyde to produce a yellow-to-red color change [105]. The limit of detection for this formaldehyde sensor was 50 ppb by naked eye. IV. C ARBON NANOTUBES (CNTs) F ILMS CNTs are considered as potentially applied materials for gas sensing because of their high specific surface areas for adsorption, nanoscale structure with numerous sites for chemical reactions, and special metal or semiconductor properties with electrons storage and transport [106]. The sensing properties of CNTs to formaldehyde were shown in Table IX.

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concentration significantly decrease the response properties by donating electrons into the nanotubes or introduction of charge trapping sites on the nanotubes [109], [110]. In 2010, Lu et al. [111] fabricated a sensor array consisting of 32 sensor elements with pristine SWCNTs, Rh-loaded SWCNTs, polyethyleneimine functionalized SWCNTs as sensing materials. The sensor elements consist of IDE with varying finger widths and gaps. The lowest detection limit of Rh-loaded SWCNTs to formaldehyde was down to 10 ppb at room temperature. Fig. 5. Schematic diagram illustrating the deposition of fibrous membranes onto the electrode of QCM via electrospinning.

In 2012, Xie et al. [107] fabricated gas sensors with MWCNTs modified with amino-groups on IDE to detect low concentration of formaldehyde at room temperature. The amine functionalized MWCNTs (MWCNTs-NH2 ) were the reaction products of chlorinated MWCNTs and NH2 (CH2 )2 NH2 in N,N-dimethylformamide at 160 °C for 12 h. MWCNTs with amino-group modification could absorb formaldehyde physically and chemically, and the chemical absorption was the main way [108]. The relative resistance changes of the sensors to formaldehyde increased with the percentage of amino groups increasing. Especially for detection of 20 ppb formaldehyde gas, the relative resistance change ((R − Ro )/Ro , where R and Ro are the resistances in target gas and in pure nitrogen, respectively.) of MWCNTs-NH2 sensor with 18% amino was 1.73%. To 200 ppb formaldehyde, the relative resistance change was 5.45%, while to the same concentration of methanol, ethanol, ammonia and carbon dioxide were 0.71%, 0.41%, 0.71%, 0.89%, respectively. However, with the content of amino-group more than 18.19%, the resistance baseline of the sensor was too large to be used as resistance sensors. In 2012, Shi et al. [109] fabricated Single-walled carbon nanotubes (SWCNTs) chemiresistive sensor. This sensor was fabricated by depositing IDE making by sputtering 50 nm of Ti and 180 nm of Au onto SiO2 /Si mode. The functionalization of the SWCNT sensors was performed by dropping one drop of tetrafluorohydroquinone (TFQ) solution (0.3% in acetone) onto the sensor and then vaporizing acetone at room temperature. The acid hydroxyl groups of TFQ could interact with formaldehyde to form week and reversible charged intermediate complexes. These charged intermediate complexes were close to the surface of the nanotube sensing elements, and it could significantly increase the conductance of the nanotubes via enriching hole carries in the semiconducting SWNTs. The lowest limit of 150 ppb formaldehyde was found with the relative resistance change of 0.28, and the response time was about 39 s. The sensitivity and selectivity of the sensors to some interfering organic vapors including water, mechanol, toluene, acetone, dichloromethane, hexane, and chloroform were also investigated. The results showed that all the interfering organic vapors caused a decrease in conductibity, whereas formaldehyde binged about a pronounced increase in the conductance. Specifically, increase in the water

V. C ONCLUSION This work reviews and compares the sensing characteristics to formaldehyde of different films coating on sensors. It is shown that metal oxide semiconductor films had lots of different materials and widely detection range, and SnO2 , In2 O3 and ZnO were within the materials with higher sensitivity to formaldehyde. Especially, the sensitivities of 1 mol% Pb-doped SnO2 [39], grape-like porous In2 O3 /CdO [61] and ZnO [65] were down to ppb level with detectable response value (>1) at their optimum operating temperatures. This suggested that the major factor to influence sensitivity was the capacity of the material to adsorb oxygen species and react with formaldehyde molecular but not the morphology of the sensing material. This capacity depended on the electronic properties of the metal oxide semiconductor such as the number of donors [7]. In polymer films, the sensitivity of PEI was higher than P3HT, and the detection limit of PEI/TiO2 was down to 1 ppm with the response ( f ) of 0.8 Hz [98]. However, the lowest detection limit come from NFN, and this film could detect 50 ppb formaldehyde by naked eye [104]. Regardless of MWCNTs or SWNTs, the CNTs films had higher sensitivity, which was due to high specific surface area for adsorption, numerous sites for chemical reactions, and special metal or semiconductor properties with electrons storage and transport [106], [112]. Although all the films coating on sensors were used to detect the same gas of formaldehyde, detection mechanisms and sensors were different. The sensors were divided into conductimetric type, piezoelectric type, and colorimetric type. The metal oxide semiconductor films and CNTs coating on conductimetric type sensors detected formaldehyde by resistance variation. The polymer films coating on piezoelectric type sensors detected formaldehyde by frequency and current variations, and coating on colorimetric type sensors detected formaldehyde by color variation. The operating temperatures for oxide semiconductor films were far higher than polymer and CNTs films, because metal oxide semiconductor films identified formaldehyde molecule by the adsorption and desorption of oxygen on the surface, and oxygen molecules captured electrons from metal oxide semiconductor in the condition of high temperature was essential to adopt oxygen on the surface [22]. It should be highlighted that in most cases experimental tests had not been carried out under real conditions. For example, humidity influence had only been tested by Han et al. [80] and Wang et al. [10]. On the other hand, formaldehyde and other indoor air pollutions

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always exist together, such as benzene, ammonia, acetone, ethanol, and methanol, the selectivity of the sensing materials against these toxic gases is very important. However, only a part of reports showed this characteristic. In this view, PEI films were coated on QCM to detect formaldehyde. However, results from Bruckemstein and Shay [113] and Grate and Klusty [114] showed that surface acoustic wave (SAW) sensors were able to operate at higher frequencies than the traditional QCM. Higher frequencies induce higher sensitivity. In our group [115], the SAW sensors with substrate using 128 °C YX LiNbO3 and guiding layer using polymethylmethacrylate (PMMA) could detect 10−15g mass. But, there have no reports about detecting formaldehyde using SAW sensors so far. In general room temperature and humidity conditions, identifying formaldehyde accurately in the presence of other harmful gases condition has a long way to go, and needing to develop more sensitive films and more accurate sensors. This view will provide theoretical basis for the future research. R EFERENCES [1] K. Kawamura, K. Kerman, M. Fujihara, N. Nagatani, T. Hashiba, and E. Tamiya, “Development of a novel hand-held formaldehyde gas sensor for the rapid detection of sick building syndrome,” Sens. Actuators B, Chem., vol. 105, no. 2, pp. 495–501, Mar. 2005. [2] Formaldehyde: A Brief History and Its Contributions to Society and the U.S. and Canadian Economies, Formaldehyde Council Inc., Arlington, VA, USA, 2005. [3] L. Zhang, C. Steinmaus, D. A. Eastmond, X. K. Xin, and M. T. Smith, “Formaldehyde exposure and leukemia: A new meta-analysis and potential mechanisms,” Mutation Res., vol. 681, nos. 2–3, pp. 150–168, Mar. 2009. [4] J. Flueckiger, F. K. Ko, and K. C. Cheung, “Microfabricated formaldehyde gas sensors,” Sensors, vol. 9, no. 11, pp. 9196–9215, 2009. [5] L. Turrio-Baldassarri et al., “Emission comparison of urban bus engine fueled with diesel oil and ‘biodiesel’ blend,” Sci. Total Environ., vol. 327, nos. 1–3, pp. 147–162, Jul. 2004. [6] R. R. Baker, “The generation of formaldehyde in cigarettes—Overview and recent experiments,” Food Chem. Toxicol., vol. 44, no. 11, pp. 1799–1822, Nov. 2006. [7] I. Castro-Hurtado, G. G. Mandayo, and E. Castano, “Conductometric formaldehyde gas sensors. A review: From conventional films to nanostructured materials,” Thin Solid Films, vol. 548, pp. 665–676, Dec. 2013. [8] X. Tang, Y. Bai, A. Duong, M. T. Smith, L. Li, and L. Zhang, “Formaldehyde in China: Production, consumption, exposure levels, and health effects,” Environ. Int., vol. 35, no. 8, pp. 1210–1224, Nov. 2009. [9] O. Bunkoed, F. Davis, P. Kanatharana, P. Thavarungkul, and S. P. Higson, “Sol-gel based sensor for selective formaldehyde determination,” Anal Chim Acta, vol. 659, nos. 1–2, pp. 251–257, Feb. 2010. [10] X. Wang, B. Ding, M. Sun, J. Yu, and G. Sun, “Nanofibrous polyethyleneimine membranes as sensitive coatings for quartz crystal microbalance-based formaldehyde sensors,” Sens. Actuators B, Chem., vol. 144, no. 1, pp. 11–17, Jan. 2010. [11] P.-R. Chung, C.-T. Tzeng, M.-T. Ke, and C.-Y. Lee, “Formaldehyde gas sensors: A review,” Sensors, vol. 13, no. 4, pp. 4468–4484, Apr. 2013. [12] A. Allouch, M. Guglielmino, P. Bernhardt, C. A. Serra, and S. L. Calve, “Transportable, fast and high sensitive near real-time analyzers: Formaldehyde detection,” Sens. Actuators B, Chem., vol. 181, pp. 551–558, May 2013. [13] Air Quality Guidelines for Europe, 2nd ed., World Health Organization, Regional Office for Europe, Copenhagen, Denmark, 2000. [14] J. Wang, L. Liu, S.-Y. Cong, J.-Q. Qi, and B.-K. Xu, “An enrichment method to detect low concentration formaldehyde,” Sens. Actuators B, Chem., vol. 134, no. 2, pp. 1010–1015, Sep. 2008. [15] Y.-L. Li, J. Liu, and W.-S. Guan, “Determination of trace formaldehyde in alcoholic beverage by chromotropic acid spectrophotometry,” in Proc. 3rd Int. Conf. Bioinformat. Biomed. Eng., Jun. 2009, pp. 1–4.

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Dazhi Chen received the master’s degree from the School of Life Science and Engineering College, Southwest Jiaotong University, Chengdu, China, in 2011, where he is currently pursuing the Ph.D. degree in things of sensor with the College of School of Materials Science and Engineering. His research interests include acoustic surface wave sensor and formaldehyde gas detection.

Yong J. Yuan (SM’08) received the Ph.D. degree in biosensors from the University of Western Sydney, Australia, in 1997. He was a Research Chemist with CSIRO (1997–1999) to work on oxygen scavenging materials, a Guest Researcher of the Agency of Industrial Science and Technology in Japan (1999–2000) to conduct research on the metabolic oscillation of yeast, and a Research Program Leader with Industrial Research Ltd., New Zealand (2000–2008) to develop biochips by the integration of novel materials, structures, devices, and processes. He joined the Southwest Jiaotong University (SWJTU), China, in 2008. He is currently a Professor of BioMEMS with the School of Materials Science and Engineering, SWJTU. His current research interests include biomedical sensors, microelectromechanical systems, nano/microdevices, and fabrication.

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