The synthesis via chemical solutions (aqueous) (SCS) wet route is a low-temperature and cost-effective growth technique of high crystalline quality oxide semiconductors films. Here we report on morphology, chemical composition, structure and ethanol sensing performances of a device prototype based on zincdoped copper oxide nanocrystallite layer. By thermal annealing in electrical furnace for 30 min at temperatures higher than 550 ˚C, as-deposited zinc doped Cu2O samples are converted to tenorite, ZnxCu1-xOy, (x=1.3wt%) that demonstrate higher ethanol response than sensor structures based on samples treated at 450 ˚C. In case of the specimens after post-growth treatment at 650 ˚C was found an ethanol gas response
of about 79 % and 91 % to concentrations of 100 ppm and 500 ppm, respectively, at operating temperature of 400 ˚C in air

Ababii, N.

Adelung, R.

Cretu, V.

Hoppe, M.

Lupan, O.

Postica, V.

Schütt, F.

Smazna, D.

Sontea, V.

Trofim, V.

SocioEconomic Challenges Journal (Sumy State University)

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE  
NANOMATERIALS: APPLICATIONS AND PROPERTIES 
Vol. 4 No 2, 02NAESP02(4pp) (2015) 
 
 
2304-1862/2015/4(2)02NAESP02(4) 02NAESP02-1  2015 Sumy State University 
Ethanol Sensing Performances of Zinc-doped Copper Oxide Nano-crystallite Layers 
 
V. Cretu1,V. Postica1, N. Ababii1, F. Schütt2, M. Hoppe2, D. Smazna2, V. Trofim1,  
V. Sontea1, R. Adelung2, O. Lupan1,2 
 
1 Technical University of Moldova, 168 Stefan cel Mare Blvd, MD-2004, Chisinau, Republic of Moldova  
2 Functional Nanomaterials, Institute for Materials Science, University of Kiel, D/24143, Germany  
 
(Received 01 March 2015; published online 22 August 2015) 
 
The synthesis via chemical solutions (aqueous) (SCS) wet route is a low-temperature and cost-effective 
growth technique of high crystalline quality oxide semiconductors films. Here we report on morphology, 
chemical composition, structure and ethanol sensing performances of a device prototype based on zinc-
doped copper oxide nanocrystallite layer. By thermal annealing in electrical furnace for 30 min at tempera-
tures higher than 550 ˚C, as-deposited zinc doped Cu2O samples are converted to tenorite, ZnxCu1-xOy, 
(x=1.3wt%) that demonstrate higher ethanol response than sensor structures based on samples treated at 
450 ˚C. In case of the specimens after post-growth treatment at 650 ˚C was found an ethanol gas response 
of about 79 % and 91 % to concentrations of 100 ppm and 500 ppm, respectively, at operating temperature 
of 400 ˚C in air. 
 
Keywords: Chemical synthesis, nanocrystalline, ethanol, film, copper oxide, Cu2O. 
 
 PACS numbers: 07.07.Df, 68.47.Gh 
 
 
1. INTRODUCTION 
 
Ethanol (EtOH), that is also known as ethyl alcohol, 
is a colorless, flammable, volatile liquid and is exten-
sively utilized in pharmacology, cosmetic production, 
chemical and food industry, especially in breath analy-
sis and wine-quality monitoring. Ethanol can be used 
as a fuel for motor vehicles, thus it is an important 
alternative fuel that produces less greenhouse emission 
compared to other conventional fuels like methane, 
propane, etc. Moreover, ethanol consumption is one of 
the major causes of traffic accidents [1]. In this context 
appear necessities for monitoring of ethanol vapors to 
offer safety in homes, vehicles, industry and for detec-
tion of the blood alcohol level (BAC) in the body of the 
drivers [1]. Conventional ethanol sensors are based on 
oxide semiconductors and are widely used in this do-
main due to their exceptional properties at nanoscale 
range, that offer high response, long life cycle, good 
selectivity, robustness and low cost. Many research 
activities have been done in the field of ethanol sensors 
based on oxide semiconductors. One of the most popu-
lar among them are n-type oxide semiconductors like 
ZnO [2], SnO2 [3], WO3 [4], etc. On the other hand, p-
type oxide semiconductors like CuO, Cu2O, NiO, Cr2O3, 
Co3O4, etc. have received relatively less attention.  
Cuprous oxide (Cu2O) has been widely studied as a 
promising material for conversion of the solar energy 
due to its nontoxicity, low-cost and relatively simple 
synthesis process [5]. But there are several reports on 
the ethanol sensing of Cu2O based sensors with differ-
ent structures such as combo-like nanorods [6], uni-
faceted crystals [7], etc. On the other side, cupric oxide 
(CuO) is known to be an important material in fabrica-
tion of high-temperature superconductors [8]. Many 
researchers have reported that CuO nanostructured 
films have higher gas response than those of Cu2O [9]. 
Thus, it is of practical interest to investigate influence 
on ethanol gas response of such parameters, like tem-
perature of thermal treatment and operating tempera-
ture of device in case of doping of copper oxide films. 
Generally, there are only few reports on doped copper 
oxide based sensors, on Zn-doped CuO in particular are 
less. A motivation of our research were previous prom-
ising results proving that by doping CuO nanorods 
with Cr it was possible to obtain good selectivity to NO2 
gas [10]. Hübner et. al. has reported that gas response 
of sensors based on the p-type oxide semiconductors is 
lower than for n-type based ones and is equal to the 
square root of gas response for sensor based on n-type 
oxide semiconductors [11]. Thus, according to previous 
papers it is more difficult to obtain a p-type oxide semi-
conductor based sensor with a high response.  
In current report, we present experimental data on 
as-grown Zn-doped Cu2O nanocrystalline films, which 
were subject of post-growth thermal annealing (TA) in 
electrical furnace and changed from cuprite phase to 
cuprite/tenorite phase, then to tenorite crystal phase. 
Ethanol response of the specimens after different TA 
treatments and operating temperatures (OPT) was 
investigated and reported below. 
 
2. EXPERIMENTAL 
 
2.1 Growth of zinc-doped copper oxide 
nanostructured films 
 
Glass substrates were used for growth of zinc-doped 
copper oxide nanostructured films by SCS method. The 
cleaning and synthesis process were reported in our 
previous work [12]. Thickness of the films is in range of 
0.85-0.90 µm as measured in cross-sectional view in 
scanning electron microscope. 
 
2.2 Characterization  
 
The samples of zinc-doped copper oxide nanostruc-
tured films were analyzed using scanning electron mi-
croscopy (SEM) instrument Carl Zeiss (7 kV, 10 µA). The 
compositional analysis of the specimens was carried out 
by EDX analysis in combination with SEM. Crystallo-
graphic information was derived from X-ray powder 
 V. CRETU, V. POSTICA, N. ABABII, ET AL. PROC. NAP 4, 02NAESP02 (2015) 
 
 
02NAESP02-2 
diffraction (XRD) data obtained using a Seifert 3000 TT 
unit operating at 40 kV and 40 mA, with graphite mono-
chromatized CuKα1 radiation (=1.541 Å). Different 
characterization techniques confirmed that nanostruc-
tured films are crystalline. Gas sensing experiments 
were performed as reported before [13]. Ethanol gas 
response was calculated according to relation S = 
(ΔR/Rair) ∙ 100 %, where ΔR = REtOH - Rair, REtOH is re-
sistance of the sensor under exposure to ethanol vapors 
and Rair is the resistance of sensor in the air. 
 
  
 
Fig. 1 – SEM images of zinc-doped copper oxide nanostructured films thermally annealed at: (a) 450 ˚C; (b) 550 ˚C; (c) 650 ˚C.  
 
 
 
 
Fig. 2 – Compositional images taken by EDX elemental mapping at the microstructural level of Zn-doped CuO nanostructured 
films: (a) Cu; (b) O; (c) Zn; and (d) EDX spectrum of Zn-doped copper oxide nanostructured films treated TA at 650 ˚C.  
 
3. RESULTS AND DISCUSSION  
 
In this work we report on morphological, chemical 
and structural properties of zinc-doped copper oxide 
nanostructured films TA treated in electrical furnace 
for 30 min at different temperatures. Also, ethanol 
vapor response is compared for different values of op-
erating temperatures OPT (300 ˚C, 400 ˚C and 450 ˚C). 
 
3.1 Morphological and Chemical Characteriza-
tion 
 
In Fig. 1 are presented SEM images of zinc-doped 
copper oxide nanostructured films grown by SCS and 
post-growth treated TA at (a) 450 ˚C, (b) 550 ˚C and (c) 
650 ˚C. Films are relatively uniform on substrate and 
are composed of interconnected nanocrystallites with 
diameter in range of 50 – 200 nm, depending on re-
gime. It can be observed that crystallinity and diameter 
of nanocrystallites increased with the temperature of 
TA. Thus, samples treated at 650 ˚C (see Fig. 1c) have 
larger nanocrystallites than specimens annealed at 450 
˚C or 550 ˚C (see Fig. 1a-b). In result, surface-to-volume 
ratio was increased and it affects sensing properties.  
Fig. 2a-c presents compositional images taken by 
EDX elemental mapping at the microstructural level of 
Zn-doped CuO. The uniform distribution of elements 
Cu, O and Zn-dopant is clearly seen in the film of the 
nanostructures used in our sensor studies. Fig. 2d re-
veals an EDX spectrum of Zn-doped copper oxide 
nanostructured films treated TA at 650 ˚C for 30 min. 
Cu and O can be distinguished from copper oxide film.  
Incorporation of zinc dopant is confirmed by Zn peaks 
in EDX spectrum. Zinc content in copper oxide 
nanostructured films treated TA at 650 ˚C was found to 
be as low as 1.32 wt% in different regions. 
 
3.2 Structural Characterization 
 
Structural properties of the thermal annealed TA 
samples were investigated by X-ray diffraction technique 
(XRD). In previous work [14] is demonstrated that as 
deposited Zn-doped CuO nanostructured films possess 
the cuprite phase. Fig. 3 shows the XRD patterns of the 
zinc-doped copper oxide nanostructured films treated 
TA at 450 ˚C, 550 ˚C and 650 ˚C for 30 min, recorded in 
the two theta range of 20-100˚. The peaks were ob-
served that correspond to simple cubic Cu2O (cuprite 
JCPDS # 00-050-0667, see curve 1) and monoclinic CuO 
(tenorite JCPDS # 00-045-0937, see curves 1-2-3). 
 
 
 
Fig. 3 – XRD patterns of the Zn-doped copper oxide nanostruc-
tured films treated TA at 450 ˚C, 550 ˚C and 650 ˚C for 30 min. 
 ETHANOL SENSING PERFORMANCES OF ZINC-DOPED COPPER OXIDE… PROC. NAP 4, 02NAESP02 (2015) 
 
 
02NAESP02-3 
 
 
 
Fig. 4 – Gas response to ethanol vapors of zinc-doped copper oxide nanostructured films based sensor versus: (a) Operating tem-
perature for 100 ppm and 500 ppm EtOH, treatment regime TA was at 650 ˚C for 30 min; (b) concentration of EtOH for different 
temperature of treatment TA in electrical furnace. Dynamic response to ethanol of zinc-doped copper oxide nanostructured films 
based sensor versus operating temperature and concentration of EtOH: (c) 100 ppm; and (d) 500 ppm. 
 
Peaks that correspond to the Zn impurities are not 
observed in XRD patterns, that indicate incorporation of 
Zn ions into the lattice of copper oxide without changes 
in structural properties [15], see Fig. 3. Also peaks that 
correspond to Cu(OH)2 or Cu were not observed in dif-
fractograms. In the case of sample treated TA at 450 ˚C 
(Fig. 3, curve 1) mixed phase is observed and the domi-
nant phase is cuprite. The characteristic peaks at 36.3˚, 
42.4  ˚and 61.6˚ correspond to reflections from (111), (200) 
and (220) planes, respectively. In the case of samples 
treated in air at 550 ˚C and 650 ˚C the peaks have been 
observed that corresponds only to tenorite phase. Nair et 
al. [16] demonstrate that as-deposited Cu2O films by 
chemical method converts to CuO phase at temperatures 
higher than 350 ˚C. Improved crystallinity of the Zn-
doped copper oxide films with increasing TA tempera-
ture, observed in the SEM images (see Fig. 1) has been 
confirmed by XRD measurements by appearance of 
stronger peaks for samples treated at 650 ˚C. 
 
3.3 Ethanol Vapor Response 
 
Fig. 4a unveils gas response of sensors based on 1.3 
wt% zinc-doped copper oxide nanostructured films to 
ethanol versus OPT and concentration of ethanol vapors. 
The inset schematically represents the structure of sen-
sor investigated in this research. Aluminum (Al) contacts 
are deposited by evaporation in vacuum using an Al 
mask with meander configuration. Width of meander was 
1 mm. Type of formed contacts are one of the most im-
portant factors that control the sensing properties of 
oxide semiconductor sensors. The current-voltage charac-
teristics of sensor structures based on zinc-doped copper 
oxide nanocrystalline films treated TA at 650 ˚C is pre-
sented in inset of Fig. 4b, and it demonstrates the linear 
behavior indicating Ohmic contacts. Theoretically Al 
contacts form a Schottky barrier with p-type copper ox-
ide, due to lower work function of Al, which was experi-
mentally demonstrated by Ushio et al. [17]. In our case, 
formation of Ohmic contacts is probably due to defects at 
the Al/CuO:Zn interface or due to the doping effect. The 
Ohmic behavior ensures that sensors signal is not influ-
enced by the contacts between Al layer and zinc–doped 
CuO nanostructured film, but is originating only from 
sensing layer [18]. Optimal operating temperature OPT 
was found to be at about 400 ˚C for both concentration of 
ethanol vapors (see Fig. 4a). Also was investigated de-
pendence of ethanol gas response on temperature of post-
growth treatment TA (see Fig. 4b). In the presence of 
mixed phase of cuprite/tenorite gas response was lower ~ 
40 % for 100 ppm and ~ 52 % for 500 ppm (sample set TA 
450 ˚C). For sensors based only on the tenorite phase gas 
response was found to be higher ~ 79 % for 100 ppm and 
about 91 % for 500 ppm (sample set TA 650 ˚C), that is 
higher than for the sample treated at 550 ˚C, probably 
due to lower crystallinity of Zn-doped CuO films treated 
at 550 °C and a higher surface-to-volume ratio for sample 
Zn-doped CuO films treated at 650 °C. Barreca et al. [19] 
and Zoolfakar et al. [9] demonstrate that ethanol re-
sponse of sensors based on CuO film is higher than for 
sensors based on Cu2O. One of the reasons can be differ-
ent type of reaction at the surface of nanocrystallites that 
include a different type of sensing mechanism. Further 
possible sensing mechanisms of ethanol sensors based on 
copper oxide nanostructured films will be discussed. 
In Fig. 4c and 4d are presented dynamic responses 
to ethanol 100 ppm and 500 ppm, respectively, of sen-
sor structures based on zinc-doped copper oxide 
nanostructured films. The response time of sensors at 
OPT 400 ˚C and 450 ˚C was ~ 15 s, and at 300 ˚C of 30 
– 40 s. The recovery time was much higher, in range of 
35 – 50 s for OPT 400 ˚C and 450 ˚C, while for OPT 300 
˚C it was higher than 60 s. 
It is well known, that at high temperatures oxygen in-
teracting with surface of oxide semiconductors by the 
following equations [20]: 
 V. CRETU, V. POSTICA, N. ABABII, ET AL. PROC. NAP 4, 02NAESP02 (2015) 
 
 
02NAESP02-4 
 2( ) ( )
1
2
g adO O  (3.1) 
  '( ) ( )ad adO O h  (3.2) 
 
 
Thus, oxygen atom, after adsorption on copper oxide 
surface, has left a hole (h+) in the lattice, forming the 
hole accumulation layer (HAL) on a surface of nano-
crystalline film. Resistance of this layer is much lower 
than the one of the insulating core. After exposure to 
reducing gas, in our case to ethanol vapors, the concen-
tration of electrons in HAL will increase, that result in 
recombination of electron-holes and in a narrowing of 
HAL [21]. Thus, the resistance of sensor structure will 
increase after exposure to ethanol vapors due to reduc-
tion of charge carriers’ concentration. Interaction of 
adsorbed oxygen species on surface of copper oxide 
semiconductor with ethanol molecules can be described 
by the following equation [22]: 
 
    2 5 ( ) ( ) 2( ) 2 ( )C H OH 6O 6 2CO 3H Og ad g gh  (3.3) 
 
In results of equation (3.3), via interaction of one 
molecule of ethanol with six ions of oxygen on the sur-
face, recombination of six holes from HAL with six elec-
trons released by desorbed species on surface occurs. 
Another important factor that need to be taken into 
account is surface band bending VS between nanocrystal-
lites. The conductance of ethanol gas sensor structure 
based on zinc-doped copper oxide nanocrystallite films is 
dependent of VS value by the following equation [11]: 
 
   exp 2p SG qV kT   (3.4) 
 
In results of equation 3.3, value of VS will be modi-
fied and gas response will be determined by ΔVS [11], 
where ΔVS = VS air –VS EtOH, VS air – is value of band 
bending in air and VS EtOH – is value of band bending 
under exposure to ethanol vapors in air. General de-
pendence of the response is represented by [11]: 
 
    exp 2SS q V kT  (3.5) 
 
In our case, a higher ethanol response of sensors 
based on the zinc-doped copper oxide nanostructured 
films could be due to nanocrystallites shapes that re-
sult in higher volume-to-surface ratio, and in more 
barriers which are formed between them. In such a 
case, larger amount of gas molecules is adsorbed on 
surface of nanostructured zinc doped copper oxide, 
which produces a higher gas response. 
 
4. CONCLUSIONS 
 
We have demonstrated doping of copper oxide nano-
crystalline films with zinc by SCS method at a relatively 
low temperature. High crystallinity of samples was de-
tected by the XRD studies. Sensors based on these films 
shows good response to ethanol vapor in concentrations 
of 100 ppm and 500 ppm at 300 – 450 ˚C operating tem-
peratures. The highest gas response of around 80 % to 
100 ppm ethanol and of approximately 95 % to 500 ppm 
ethanol was obtained for films treated TA at 650 ˚C for 
30 min, which demonstrates presence of only tenorite 
phase. Values of response time and recovery time were 
found to be dependent on operating OPT value due to 
temperature dependency of adsorption/desorption coeffi-
cient. Also, the value of ethanol response was decreased 
with lowering temperature of annealing TA down to 450 
˚C. In results, according to these experimental data, the 
tenorite phase of copper oxides is more sensitive to etha-
nol vapors than cuprite or cuprite/tenorite mixed phases. 
 
AKNOWLEDGEMENTS 
 
The authors gratefully acknowledge financial 
support of the STCU in Ukraine through Grant 5989. 
 
 
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Ethanol Sensing Performances of Zinc-doped Copper Oxide Nano-crystallite Layers

Electronic Sumy State University Institutional Repository

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE  NANOMATERIALS: APPLICATIONS AND PROPERTIES Vol. 4 No 2, 02NAESP02(4pp) (2015)   2304-1862/2015/4(2)02NAESP02(4) 02NAESP02-1  2015 Sumy State University Ethanol Sensing Performances of Zinc-doped Copper Oxide Nano-crystallite Layers  V. Cretu1,V. Postica1, N. Ababii1, F. Schütt2, M. Hoppe2, D. Smazna2, V. Trofim1,  V. Sontea1, R. Adelung2, O. Lupan1,2  1 Technical University of Moldova, 168 Stefan cel Mare Blvd, MD-2004, Chisinau, Republic of Moldova  2 Functional Nanomaterials, Institute for Materials Science, University of Kiel, D/24143, Germany   (Received 01 March 2015; published online 22 August 2015)  The synthesis via chemical solutions (aqueous) (SCS) wet route is a low-temperature and cost-effective growth technique of high crystalline quality oxide semiconductors films. Here we report on morphology, chemical composition, structure and ethanol sensing performances of a device prototype based on zinc-doped copper oxide nanocrystallite layer. By thermal annealing in electrical furnace for 30 min at tempera-tures higher than 550 ˚C, as-deposited zinc doped Cu2O samples are converted to tenorite, ZnxCu1-xOy, (x=1.3wt%) that demonstrate higher ethanol response than sensor structures based on samples treated at 450 ˚C. In case of the specimens after post-growth treatment at 650 ˚C was found an ethanol gas response of about 79 % and 91 % to concentrations of 100 ppm and 500 ppm, respectively, at operating temperature of 400 ˚C in air.  Keywords: Chemical synthesis, nanocrystalline, ethanol, film, copper oxide, Cu2O.   PACS numbers: 07.07.Df, 68.47.Gh   1. INTRODUCTION  Ethanol (EtOH), that is also known as ethyl alcohol, is a colorless, flammable, volatile liquid and is exten-sively utilized in pharmacology, cosmetic production, chemical and food industry, especially in breath analy-sis and wine-quality monitoring. Ethanol can be used as a fuel for motor vehicles, thus it is an important alternative fuel that produces less greenhouse emission compared to other conventional fuels like methane, propane, etc. Moreover, ethanol consumption is one of the major causes of traffic accidents [1]. In this context appear necessities for monitoring of ethanol vapors to offer safety in homes, vehicles, industry and for detec-tion of the blood alcohol level (BAC) in the body of the drivers [1]. Conventional ethanol sensors are based on oxide semiconductors and are widely used in this do-main due to their exceptional properties at nanoscale range, that offer high response, long life cycle, good selectivity, robustness and low cost. Many research activities have been done in the field of ethanol sensors based on oxide semiconductors. One of the most popu-lar among them are n-type oxide semiconductors like ZnO [2], SnO2 [3], WO3 [4], etc. On the other hand, p-type oxide semiconductors like CuO, Cu2O, NiO, Cr2O3, Co3O4, etc. have received relatively less attention.  Cuprous oxide (Cu2O) has been widely studied as a promising material for conversion of the solar energy due to its nontoxicity, low-cost and relatively simple synthesis process [5]. But there are several reports on the ethanol sensing of Cu2O based sensors with differ-ent structures such as combo-like nanorods [6], uni-faceted crystals [7], etc. On the other side, cupric oxide (CuO) is known to be an important material in fabrica-tion of high-temperature superconductors [8]. Many researchers have reported that CuO nanostructured films have higher gas response than those of Cu2O [9]. Thus, it is of practical interest to investigate influence on ethanol gas response of such parameters, like tem-perature of thermal treatment and operating tempera-ture of device in case of doping of copper oxide films. Generally, there are only few reports on doped copper oxide based sensors, on Zn-doped CuO in particular are less. A motivation of our research were previous prom-ising results proving that by doping CuO nanorods with Cr it was possible to obtain good selectivity to NO2 gas [10]. Hübner et. al. has reported that gas response of sensors based on the p-type oxide semiconductors is lower than for n-type based ones and is equal to the square root of gas response for sensor based on n-type oxide semiconductors [11]. Thus, according to previous papers it is more difficult to obtain a p-type oxide semi-conductor based sensor with a high response.  In current report, we present experimental data on as-grown Zn-doped Cu2O nanocrystalline films, which were subject of post-growth thermal annealing (TA) in electrical furnace and changed from cuprite phase to cuprite/tenorite phase, then to tenorite crystal phase. Ethanol response of the specimens after different TA treatments and operating temperatures (OPT) was investigated and reported below.  2. EXPERIMENTAL  2.1 Growth of zinc-doped copper oxide nanostructured films  Glass substrates were used for growth of zinc-doped copper oxide nanostructured films by SCS method. The cleaning and synthesis process were reported in our previous work [12]. Thickness of the films is in range of 0.85-0.90 µm as measured in cross-sectional view in scanning electron microscope.  2.2 Characterization   The samples of zinc-doped copper oxide nanostruc-tured films were analyzed using scanning electron mi-croscopy (SEM) instrument Carl Zeiss (7 kV, 10 µA). The compositional analysis of the specimens was carried out by EDX analysis in combination with SEM. Crystallo-graphic information was derived from X-ray powder  V. CRETU, V. POSTICA, N. ABABII, ET AL. PROC. NAP 4, 02NAESP02 (2015)   02NAESP02-2 diffraction (XRD) data obtained using a Seifert 3000 TT unit operating at 40 kV and 40 mA, with graphite mono-chromatized CuKα1 radiation (=1.541 Å). Different characterization techniques confirmed that nanostruc-tured films are crystalline. Gas sensing experiments were performed as reported before [13]. Ethanol gas response was calculated according to relation S = (ΔR/Rair) ∙ 100 %, where ΔR = REtOH - Rair, REtOH is re-sistance of the sensor under exposure to ethanol vapors and Rair is the resistance of sensor in the air.     Fig. 1 – SEM images of zinc-doped copper oxide nanostructured films thermally annealed at: (a) 450 ˚C; (b) 550 ˚C; (c) 650 ˚C.      Fig. 2 – Compositional images taken by EDX elemental mapping at the microstructural level of Zn-doped CuO nanostructured films: (a) Cu; (b) O; (c) Zn; and (d) EDX spectrum of Zn-doped copper oxide nanostructured films treated TA at 650 ˚C.   3. RESULTS AND DISCUSSION   In this work we report on morphological, chemical and structural properties of zinc-doped copper oxide nanostructured films TA treated in electrical furnace for 30 min at different temperatures. Also, ethanol vapor response is compared for different values of op-erating temperatures OPT (300 ˚C, 400 ˚C and 450 ˚C).  3.1 Morphological and Chemical Characteriza-tion  In Fig. 1 are presented SEM images of zinc-doped copper oxide nanostructured films grown by SCS and post-growth treated TA at (a) 450 ˚C, (b) 550 ˚C and (c) 650 ˚C. Films are relatively uniform on substrate and are composed of interconnected nanocrystallites with diameter in range of 50 – 200 nm, depending on re-gime. It can be observed that crystallinity and diameter of nanocrystallites increased with the temperature of TA. Thus, samples treated at 650 ˚C (see Fig. 1c) have larger nanocrystallites than specimens annealed at 450 ˚C or 550 ˚C (see Fig. 1a-b). In result, surface-to-volume ratio was increased and it affects sensing properties.  Fig. 2a-c presents compositional images taken by EDX elemental mapping at the microstructural level of Zn-doped CuO. The uniform distribution of elements Cu, O and Zn-dopant is clearly seen in the film of the nanostructures used in our sensor studies. Fig. 2d re-veals an EDX spectrum of Zn-doped copper oxide nanostructured films treated TA at 650 ˚C for 30 min. Cu and O can be distinguished from copper oxide film.  Incorporation of zinc dopant is confirmed by Zn peaks in EDX spectrum. Zinc content in copper oxide nanostructured films treated TA at 650 ˚C was found to be as low as 1.32 wt% in different regions.  3.2 Structural Characterization  Structural properties of the thermal annealed TA samples were investigated by X-ray diffraction technique (XRD). In previous work [14] is demonstrated that as deposited Zn-doped CuO nanostructured films possess the cuprite phase. Fig. 3 shows the XRD patterns of the zinc-doped copper oxide nanostructured films treated TA at 450 ˚C, 550 ˚C and 650 ˚C for 30 min, recorded in the two theta range of 20-100˚. The peaks were ob-served that correspond to simple cubic Cu2O (cuprite JCPDS # 00-050-0667, see curve 1) and monoclinic CuO (tenorite JCPDS # 00-045-0937, see curves 1-2-3).    Fig. 3 – XRD patterns of the Zn-doped copper oxide nanostruc-tured films treated TA at 450 ˚C, 550 ˚C and 650 ˚C for 30 min.  ETHANOL SENSING PERFORMANCES OF ZINC-DOPED COPPER OXIDE… PROC. NAP 4, 02NAESP02 (2015)   02NAESP02-3    Fig. 4 – Gas response to ethanol vapors of zinc-doped copper oxide nanostructured films based sensor versus: (a) Operating tem-perature for 100 ppm and 500 ppm EtOH, treatment regime TA was at 650 ˚C for 30 min; (b) concentration of EtOH for different temperature of treatment TA in electrical furnace. Dynamic response to ethanol of zinc-doped copper oxide nanostructured films based sensor versus operating temperature and concentration of EtOH: (c) 100 ppm; and (d) 500 ppm.  Peaks that correspond to the Zn impurities are not observed in XRD patterns, that indicate incorporation of Zn ions into the lattice of copper oxide without changes in structural properties [15], see Fig. 3. Also peaks that correspond to Cu(OH)2 or Cu were not observed in dif-fractograms. In the case of sample treated TA at 450 ˚C (Fig. 3, curve 1) mixed phase is observed and the domi-nant phase is cuprite. The characteristic peaks at 36.3˚, 42.4  ˚and 61.6˚ correspond to reflections from (111), (200) and (220) planes, respectively. In the case of samples treated in air at 550 ˚C and 650 ˚C the peaks have been observed that corresponds only to tenorite phase. Nair et al. [16] demonstrate that as-deposited Cu2O films by chemical method converts to CuO phase at temperatures higher than 350 ˚C. Improved crystallinity of the Zn-doped copper oxide films with increasing TA tempera-ture, observed in the SEM images (see Fig. 1) has been confirmed by XRD measurements by appearance of stronger peaks for samples treated at 650 ˚C.  3.3 Ethanol Vapor Response  Fig. 4a unveils gas response of sensors based on 1.3 wt% zinc-doped copper oxide nanostructured films to ethanol versus OPT and concentration of ethanol vapors. The inset schematically represents the structure of sen-sor investigated in this research. Aluminum (Al) contacts are deposited by evaporation in vacuum using an Al mask with meander configuration. Width of meander was 1 mm. Type of formed contacts are one of the most im-portant factors that control the sensing properties of oxide semiconductor sensors. The current-voltage charac-teristics of sensor structures based on zinc-doped copper oxide nanocrystalline films treated TA at 650 ˚C is pre-sented in inset of Fig. 4b, and it demonstrates the linear behavior indicating Ohmic contacts. Theoretically Al contacts form a Schottky barrier with p-type copper ox-ide, due to lower work function of Al, which was experi-mentally demonstrated by Ushio et al. [17]. In our case, formation of Ohmic contacts is probably due to defects at the Al/CuO:Zn interface or due to the doping effect. The Ohmic behavior ensures that sensors signal is not influ-enced by the contacts between Al layer and zinc–doped CuO nanostructured film, but is originating only from sensing layer [18]. Optimal operating temperature OPT was found to be at about 400 ˚C for both concentration of ethanol vapors (see Fig. 4a). Also was investigated de-pendence of ethanol gas response on temperature of post-growth treatment TA (see Fig. 4b). In the presence of mixed phase of cuprite/tenorite gas response was lower ~ 40 % for 100 ppm and ~ 52 % for 500 ppm (sample set TA 450 ˚C). For sensors based only on the tenorite phase gas response was found to be higher ~ 79 % for 100 ppm and about 91 % for 500 ppm (sample set TA 650 ˚C), that is higher than for the sample treated at 550 ˚C, probably due to lower crystallinity of Zn-doped CuO films treated at 550 °C and a higher surface-to-volume ratio for sample Zn-doped CuO films treated at 650 °C. Barreca et al. [19] and Zoolfakar et al. [9] demonstrate that ethanol re-sponse of sensors based on CuO film is higher than for sensors based on Cu2O. One of the reasons can be differ-ent type of reaction at the surface of nanocrystallites that include a different type of sensing mechanism. Further possible sensing mechanisms of ethanol sensors based on copper oxide nanostructured films will be discussed. In Fig. 4c and 4d are presented dynamic responses to ethanol 100 ppm and 500 ppm, respectively, of sen-sor structures based on zinc-doped copper oxide nanostructured films. The response time of sensors at OPT 400 ˚C and 450 ˚C was ~ 15 s, and at 300 ˚C of 30 – 40 s. The recovery time was much higher, in range of 35 – 50 s for OPT 400 ˚C and 450 ˚C, while for OPT 300 ˚C it was higher than 60 s. It is well known, that at high temperatures oxygen in-teracting with surface of oxide semiconductors by the following equations [20]:  V. CRETU, V. POSTICA, N. ABABII, ET AL. PROC. NAP 4, 02NAESP02 (2015)   02NAESP02-4  2( ) ( )12g adO O  (3.1)   '( ) ( )ad adO O h  (3.2)   Thus, oxygen atom, after adsorption on copper oxide surface, has left a hole (h+) in the lattice, forming the hole accumulation layer (HAL) on a surface of nano-crystalline film. Resistance of this layer is much lower than the one of the insulating core. After exposure to reducing gas, in our case to ethanol vapors, the concen-tration of electrons in HAL will increase, that result in recombination of electron-holes and in a narrowing of HAL [21]. Thus, the resistance of sensor structure will increase after exposure to ethanol vapors due to reduc-tion of charge carriers’ concentration. Interaction of adsorbed oxygen species on surface of copper oxide semiconductor with ethanol molecules can be described by the following equation [22]:      2 5 ( ) ( ) 2( ) 2 ( )C H OH 6O 6 2CO 3H Og ad g gh  (3.3)  In results of equation (3.3), via interaction of one molecule of ethanol with six ions of oxygen on the sur-face, recombination of six holes from HAL with six elec-trons released by desorbed species on surface occurs. Another important factor that need to be taken into account is surface band bending VS between nanocrystal-lites. The conductance of ethanol gas sensor structure based on zinc-doped copper oxide nanocrystallite films is dependent of VS value by the following equation [11]:     exp 2p SG qV kT   (3.4)  In results of equation 3.3, value of VS will be modi-fied and gas response will be determined by ΔVS [11], where ΔVS = VS air –VS EtOH, VS air – is value of band bending in air and VS EtOH – is value of band bending under exposure to ethanol vapors in air. General de-pendence of the response is represented by [11]:      exp 2SS q V kT  (3.5)  In our case, a higher ethanol response of sensors based on the zinc-doped copper oxide nanostructured films could be due to nanocrystallites shapes that re-sult in higher volume-to-surface ratio, and in more barriers which are formed between them. In such a case, larger amount of gas molecules is adsorbed on surface of nanostructured zinc doped copper oxide, which produces a higher gas response.  4. CONCLUSIONS  We have demonstrated doping of copper oxide nano-crystalline films with zinc by SCS method at a relatively low temperature. High crystallinity of samples was de-tected by the XRD studies. Sensors based on these films shows good response to ethanol vapor in concentrations of 100 ppm and 500 ppm at 300 – 450 ˚C operating tem-peratures. The highest gas response of around 80 % to 100 ppm ethanol and of approximately 95 % to 500 ppm ethanol was obtained for films treated TA at 650 ˚C for 30 min, which demonstrates presence of only tenorite phase. Values of response time and recovery time were found to be dependent on operating OPT value due to temperature dependency of adsorption/desorption coeffi-cient. Also, the value of ethanol response was decreased with lowering temperature of annealing TA down to 450 ˚C. In results, according to these experimental data, the tenorite phase of copper oxides is more sensitive to etha-nol vapors than cuprite or cuprite/tenorite mixed phases.  AKNOWLEDGEMENTS  The authors gratefully acknowledge financial support of the STCU in Ukraine through Grant 5989.   REFERENCES  1. Sensors for Chemical and Biological Applications (Ed. M. K. Ram, V. R. Bhethanabotla) (Taylor & Francis Group, LLC: 2010). 2. Q. Wan, Q.H. Li, Y.J. Chen and T.H. Wang, Appl. Phys. Lett. 84, 3654 (2004). 3. Y.J. Chen, X.Y. Xue, Y.G. Wang, T.H. Wang, Appl. Phys. Lett. 87, 233503 (2005). 4. D. Chen, X. Hou, H. Wen, Y. Wang, H. Wang, X. Li,  R. Zhang, H. Lu, H. Xu, S. Guan, J. Sun, L. Gao, Nano-technology 21 035501 (2010). 5. A.O. Musa, T. Akomolafe, M. J. Carter, Solar Energ. Ma-ter. Sol. Cells 51 305 (1998). 6. L. Guan, H. Pang, J. Wang, Q. Lu, J. Yin, F. Gao, Chem. Commun. 46, 7022 (2010). 7. K.X. Yao, X.M. Yin, T. H. Wang, H. C. Zeng, J. Am. Chem. Soc. 132, 6131 (2010). 8. P. Raksa, A. 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Ethanol Sensing Performances of Zinc-doped Copper Oxide Nano-crystallite Layers

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