Sensing of Ethanol with Nanosize Fe-ZnO Thin Films

1 Department of Environmental Engineering, National Cheng Kung University, Tainan City 70101, Taiwan 2 Sustainable Environment Research Center, National Cheng Kung University, Tainan City 70101, Taiwan 3 Department of Bioengineering, University of Washington, Seattle, WA 98105, USA 4 Department of Safety, Health and Environmental Engineering, National Kaohsiung First University of Science and Technology, Kaohsiung City 81184, Taiwan 5 Department of Chemistry, National Cheng Kung University, Tainan City 70101, Taiwan


Introduction
Real-time detection of toxic gases by analytical equipments such as FTIR, GC, or mass spectrometers is expensive and not practical mainly due to their bulky sizes.Metal oxide semiconductor sensors have advantages of small size, low mass, good sensitivity and low cost [1][2][3].ZnO which is an n-type semiconductor (band-gap = 3.2 eV) has been widely used in sensing of gases such as H 2 , H 2 O, and CH 4 [1][2][3][4][5][6][7][8].Methods of preparing ZnO thin films include spray pyrolysis, chemical vapor deposition, sputtering, electron beam evaporation, screen printing, and sol-gel methods [9][10][11][12][13][14][15][16].However, ZnO thin films have drawbacks, for instance, the working temperature is too high (>673 K) and their sensitivities are frequently not very desired [17].To improve sensitivity, transition metals can be doped onto ZnO thin films [18].Generally sensors doped with transition metals may also lead to a better stability [19].Thus the main objective of the present work was to investigate sensitivities of the ZnO thin films doped with iron (5-50%) in the presence of ethanol vapor.Nature of the sensing active species in the iron-doped ZnO (Fe-ZnO) thin films was also studied by in situ extended X-ray absorption fine structure (EXAFS) spectroscopy.

Experimental
The Fe-ZnO thin films were prepared by dissolving Zn(CH 3 COO) 2 • 2H 2 O (OSAKA) and Fe(NO 3 ) 3 • 9H 2 O (MERCK, 99%) in 2-methoxyethanol (MERCK), to which a surfactant, monoethanolamine (MEA) (WAKO); was subsequently added.The molar ratio of MEA to Zn(CH 3 COO) 2 was 0.02.The mole fractions of Fe(NO 3 ) 3 in the ZnO thin films were 0, 5, 10, 20, and 50%.The thin films were prepared by dip-coating on a home-made slow pulling device at a speed of 1.2 cm/d.The thin film samples were preheated at 523 K for 10 minutes to decompose organic compounds and annealed at 773 K for 60 minutes.
Chemical structure of the Fe-ZnO thin films was determined by X-ray powder diffraction (XRD) spectroscopy (D8 advance, Bruker) with a Cu Kα radiation (λ = 1.542Å).The surface morphology of the thin films was also measured by scanning electron microscope (SEM) (S-3000N, Phillips).In situ EXAFS spectra of zinc and iron in the Fe-ZnO thin

Results and Discussion
The X-ray diffraction (XRD) patterns of the Fe-ZnO thin films are illustrated in Figure 1.For the low-iron (0-10%) doped ZnO thin films, the main compound in the thin films is ZnO.As the fractions of the doped iron are greater than 20%, in addition to ZnO, ZnFe 2 O 4 is also observed.By SEM, it is found that the thin film is consisted of nanosize ZnO which is packed closely and well distributed (Figure 2).The mean particle size of ZnO in the thin film is in the range of 50-90 nm.
Figure 3 shows the sensitivities (R air /R ethanol ) of the ZnO and Fe-ZnO-Fe thin films in the presence of 1000 ppm of ethanol at 300 K.The sensitivity and response time of the ZnO thin film to ethanol are about 50 and 2.5 minutes, respectively.Interestingly, it is found that the 5% Fe-ZnO thin film has a relatively higher sensitivity (>70) and shorter response time (one minutes).Increasing the amount (>10%) of iron doped on the ZnO thin films leads to a rapid decrease of the sensitivity to less than 15.
It seems that a small amount (e.g., 5%) of iron can promote the sensitivity of the ZnO thin film to ethanol.The thin film doped with greater amount (20-50%) of iron has, on the contrary, a much less sensitivity to ethanol (R air /R ethanol < 15).To obtain a better understanding of interactions involved during sensing, the thin films have also been studied by in situ EXAFS spectroscopy.During sensing of ethanol, the chemical interactions between oxygen of ethanol and zinc on the Fe-ZnO thin film can be observed by EXAFS.An over 99% reliability of the EXAFS data fitting for zinc and iron species in the Fe-ZnO thin films is shown in Table 1.Their Debye-Waller factors (Δσ 2 ) were less than

Conclusions
at the ambient temperature (e.g., 300 K), it is found experimentally that iron can promote the sensivity of the ZnO thin

Call for Papers
Polymers reinforced with nanoparticles, such as carbon nanotubes, are of great interest due to their remarkable mechanical, thermal, chemical properties as well as optical, electronic, and magnetic applications.In the general research area of polymer nanocomposites, a number of critical issues need to be addressed before the full potential of polymer nanocomposites can actually be realized.While a number of advances have recently been made in the area of polymer nanocomposites, the studies on understanding of the effects of processing parameters on the structure, morphology, and functional properties of polymer nanocomposites are deficient.There is a need for characterization techniques to quantify the concentration and distributions of nanoparticles as well as to assess the strength at the interface between the polymer and nanoparticles.Also, there is a need for the development of better models able to predict the mechanical properties of the polymer nanocomposites as functions of myriad factors including nanoparticle orientation, the type of functional groups, and the molecular weight of polymer chain.The relationships between the structural distributions and the ultimate properties of the polymer nanocomposites also need to be elucidated.This special issue of the Journal of Nanomaterials will be devoted to emerging polymer nanocomposite processing techniques and call for new contributions in the field of characterization and applications of multifunctional nanocomposites.It intends to cover the entire range of basic and applied materials research focusing on rheological characterization, nanoparticle dispersion, and functional properties of polymer nanocomposites for sensors, actuators, and other applications.Fundamental understanding of the effects of processing and nanoparticles on the polymer structure and morphology, their optical, electrical, and mechanical properties as well as novel functions and applications of nanocomposite materials will be the highlights of this special issue.
Papers are solicited in, but not limited to, the following areas: • Solution and melt processing of polymer nanocomposites • Rheological and thermal characterization of nanocomposites • Generation of nanofibers using extrusion and electrospinning of nanocomposites

Call for Papers
The idea of magnetoelectricity has been around for decades, and several materials and concepts have been studied; however, recent discoveries of compounds in which the magnetoelectric (ME) coupling is strong have sparked a renaissance.ME symmetry allows magnetization (resp., polarization) to be induced by the application of an electric (resp., magnetic) field.The initial impetus driving ME materials was the prospect of employing a single field excitation, for example, voltage gradient, to tune both polarization and magnetization characteristics.Significant progress in ME nanostructures has been made in recent years, mainly driven by materialist use in new class of multifunctional device applications, for example, electric field-controlled magnetic data storage, antenna miniaturization using passive substrates, and field control for spintronics applications.Manipulation of electric polarizations by external magnetic fields (and vice versa) has been demonstrated in some of single-phase multiferroic materials, for example, BiFeO 3 .In this case, the intrinsic ME effect arises from the coupling between electric and magnetic order parameters.Intrinsic ME coupling occurs in compounds in which time-reversal and space-inversion symmetries are absent.A significant ME effect is also observed in the case of two-phase (thin films) composite multiferroics made by combining ferroelectric and ferromagnetic substances together, for example, BaTiO 3 /CoFe 2 O 4 , and lead-zirconate-titanate (PZT)/ferrite (or Terfenol-D).In these previous studies, it is commonly believed that elastic coupling amongst the ferroelectric and magnetic order parameters is at the origin of the ME effect, that is, an electric field E induces strain in the ferroelectric; this strain is passed on to the ferromagnet, where it causes a net magnetization M in the sample.A basic understanding of the distributions of electric, magnetic, and internal stress fields within the material allows one to manipulate the relative stability of domain structures.One would expect that the ME effect is large if the coupling at the interface is large; thus, composites with large surface area (such as multilayer systems and nanogranular mixtures) and strongly ferroelastic components are especially effective.The theoretical understanding of the microscopic mechanism that causes the strong coupling between the electric and magnetic degrees of freedom is quite limited.We therefore anticipate that ME nanostructures may open up a whole new dimension for a wide range of materials having cost effective, small size planar, and broadband multifunctional device applications.Papers are solicited in, but not limited to, the following areas: • Physical mechanisms that control the ME property in layered and granular nanostructures

Table 1 :
In situ EXAFS data of zinc and iron in the 5% Fe-ZnO thin film during sensing of ethanol at 300 K. , for instance, the thin film doped with 5% of iron has a relatively higher sensitivity (>70) and shorter response time (one minutes) when sensing of 1000 ppm of ethanol vapor.On the contrary, the thin films doped with 20-50% of iron have a very low sensitivity to ethanol (R air /R ethanol < 15).By in situ EXAFS, it is found that sensing of ethanol on the Fe-ZnO has caused changes of bond distances of Zn-O and Fe-O in the thin films to 1.90 and 1.98 Å and restored to 1.91 and 1.97 Å, respectively in the absence of ethanol.It is clear that the thin films doped with excess (>20%) iron have less amount of sensing active ZnO species which are consumed in formation of ZnFe 2 O 4 . film • Processing-induced orientation of nanoparticles • Quantification of nanoparticle dispersion • Effect of nanoparticle incorporation on polymerization • In situ nanoparticle formation in polymer matrix • Noncovalent functionalization techniques and characterization of properties at polymer-nanoparticle interface • Novel applications of polymer nanocompositesBefore submission authors should carefully read over the journal's Author Guidelines, which are located at http://www .hindawi.com/journals/jnm/guidelines.html.Prospective authors should submit an electronic copy of their complete manuscript through the journal Manuscript Tracking System at http://mts.hindawi.com/according to the following timetable: • Multiferroic composites • Multifunctional applications of nanostructures • Giant ME effects • Magnetoelectric interactions at electromechanical resonance • Magnetization and polarization tuning by external fields • Coexistence of magnetic and ferroelectric orders • Static and dynamic magnetoelasticity • Crystallographic engineering by assembling ME building blocks • ME effect in superconductors Before submission authors should carefully read over the journal's Author Guidelines, which are located at http://www .hindawi.com/journals/jnm/guidelines.html.Prospective authors should submit an electronic copy of their complete manuscript through the journal Manuscript Tracking System at http://mts.hindawi.com/according to the following timetable: