**3. Metal oxide nanowires: Influence of the preparation method type (wet or dry) on their structural, morphological and optical properties**

Arrays of metal oxides (ZnO and CuO) nanowires were prepared involving two simple cost-effective wet and dry approaches: chemical synthesis in aqueous solution and thermal oxidation in air.

#### **3.1 CuO nanowires**

CuO nanowire arrays obtained by a wet method were chemically synthesized in aqueous solution based on the procedure described in Ref.s [43]. Thus, 0.0045 mol NH4OH in 30 ml aqueous solution and 0.007 mol NaOH in 6 ml aqueous solution were added, under vigorous stirring, in a glass beaker with 100 ml aqueous solution containing 0.004 mol CuSO4. The beaker was covered and stored for 7 days without stirring at ambient temperature. The precipitate was collected through centrifugation, washed several times with water and dried at room temperature.

**Figure 1(a)** illustrates a SEM image of the CuO nanowires chemically synthesized in aqueous solution, indicating that these nanowires have a cylindrical shape, lengths up to 2 μm and diameters of about 40 nm. The XRD pattern of the prepared CuO nanowires (**Figure 1(b)**) evidences peaks corresponding to the Miller indexes of the reflecting planes for CuO in a monoclinic phase (JCPDS reference code 00–048-1548). Based on the reflectance spectrum of the obtained CuO nanowires (**Figure 1(c)**), the band gap value was estimated as being around 1.6 eV, in agreement with previously reported data for CuO nanowires [44].

Arrays of CuO nanowire were prepared also by a dry method, using thermal oxidation in air according to the procedure given in Ref.s [11, 30]. Briefly, metallic substrates consisting in 2 cm2 copper foils were cleaned in ultrasonic bath with acetone and isopropyl alcohol and then annealed in air for 24 h at 400°C, 500°C and 600°C in a convection oven.

The SEM images in cross-sectional view of the annealed Cu foils (**Figure 2(a), (c), (e)**) revealed that there are three distinct regions with different morphologies from the bottom up: the Cu foil, a Cu2O thin film and the CuO nanowire arrays. Moreover, the SEM images in plan-view of the CuO nanowire arrays (**Figure 2(b), (d), (f )**) prepared by thermal oxidation in air at different temperatures disclose that the increase of the annealing temperature favors a higher density and a larger diameter of the CuO nanowires. Thus, the diameters and lengths of the CuO nanowires can be tuned as a function of the applied annealing temperatures. At 400°C, there is a low density of nanowires with diameters of

#### **Figure 1.**

*(a) SEM image, (b) XRD pattern and (c) reflectance spectrum of the CuO nanowire arrays obtained by chemical synthesis in aqueous solution.*

**29**

to 30 μm.

**Figure 2.**

*(c), (d) 500°C and (e), (f) 600°C.*

*Metal Oxide Nanowires as Building Blocks for Optoelectronic Devices*

about 40 nm and lengths up to 1 μm. At 500°C, there is a high density of nanowires with diameters of about 60 nm and lengths up to 30 μm. Also, at 600°C, there is a much higher density of nanowires with diameters of about 100 nm and lengths up

*SEM images of the CuO nanowire arrays prepared by thermal oxidation in air at (a), (b) 400°C,* 

The structural properties of the Cu foils thermally oxidized in air at different temperatures consisting in the XRD patterns (**Figure 3(a), (c), (e)**) evidence the presence of diffraction peaks assigned to the Miller indexes of the reflecting planes for three crystalline phases: Cu in face-centered-cubic phase (JCPDS reference code 00–004-0836), Cu2O in cubic phase (JCPDS reference code 01–071-3645) and CuO in monoclinic phase (JCPDS reference code 00–048-1548). These results are in accordance with the data obtained for the CuO nanowire arrays in the crosssectional SEM images (**Figure 2(a), (c), (e)**) in which there were clearly observed

The band gap values for the CuO nanowire arrays obtained by thermal oxidation in air at various temperatures were assessed based on the reflectance spectra (**Figure 3(b), (d), (f )**) as being around 1.6 eV, in agreement with data previously

three distinct areas with different morphologies.

reported in the literature for CuO nanowires [44].

*DOI: http://dx.doi.org/10.5772/intechopen.94011*

*Metal Oxide Nanowires as Building Blocks for Optoelectronic Devices DOI: http://dx.doi.org/10.5772/intechopen.94011*

**Figure 2.**

*Nanowires - Recent Progress*

and thermal oxidation in air.

substrates consisting in 2 cm2

600°C in a convection oven.

**3.1 CuO nanowires**

**3. Metal oxide nanowires: Influence of the preparation method type (wet or dry) on their structural, morphological and optical properties**

Arrays of metal oxides (ZnO and CuO) nanowires were prepared involving two simple cost-effective wet and dry approaches: chemical synthesis in aqueous solution

CuO nanowire arrays obtained by a wet method were chemically synthesized in aqueous solution based on the procedure described in Ref.s [43]. Thus, 0.0045 mol NH4OH in 30 ml aqueous solution and 0.007 mol NaOH in 6 ml aqueous solution were added, under vigorous stirring, in a glass beaker with 100 ml aqueous solution containing 0.004 mol CuSO4. The beaker was covered and stored for 7 days without stirring at ambient temperature. The precipitate was collected through centrifuga-

**Figure 1(a)** illustrates a SEM image of the CuO nanowires chemically synthesized in aqueous solution, indicating that these nanowires have a cylindrical shape, lengths up to 2 μm and diameters of about 40 nm. The XRD pattern of the prepared CuO nanowires (**Figure 1(b)**) evidences peaks corresponding to the Miller indexes of the reflecting planes for CuO in a monoclinic phase (JCPDS reference code 00–048-1548). Based on the reflectance spectrum of the obtained CuO nanowires (**Figure 1(c)**), the band gap value was estimated as being around 1.6 eV, in agree-

Arrays of CuO nanowire were prepared also by a dry method, using thermal oxidation in air according to the procedure given in Ref.s [11, 30]. Briefly, metallic

acetone and isopropyl alcohol and then annealed in air for 24 h at 400°C, 500°C and

The SEM images in cross-sectional view of the annealed Cu foils (**Figure 2(a), (c), (e)**) revealed that there are three distinct regions with different morphologies from the bottom up: the Cu foil, a Cu2O thin film and the CuO nanowire arrays. Moreover, the SEM images in plan-view of the CuO nanowire arrays (**Figure 2(b), (d), (f )**) prepared by thermal oxidation in air at different temperatures disclose that the increase of the annealing temperature favors a higher density and a larger diameter of the CuO nanowires. Thus, the diameters and lengths of the CuO nanowires can be tuned as a function of the applied annealing temperatures. At 400°C, there is a low density of nanowires with diameters of

*(a) SEM image, (b) XRD pattern and (c) reflectance spectrum of the CuO nanowire arrays obtained by* 

copper foils were cleaned in ultrasonic bath with

tion, washed several times with water and dried at room temperature.

ment with previously reported data for CuO nanowires [44].

**28**

**Figure 1.**

*chemical synthesis in aqueous solution.*

*SEM images of the CuO nanowire arrays prepared by thermal oxidation in air at (a), (b) 400°C, (c), (d) 500°C and (e), (f) 600°C.*

about 40 nm and lengths up to 1 μm. At 500°C, there is a high density of nanowires with diameters of about 60 nm and lengths up to 30 μm. Also, at 600°C, there is a much higher density of nanowires with diameters of about 100 nm and lengths up to 30 μm.

The structural properties of the Cu foils thermally oxidized in air at different temperatures consisting in the XRD patterns (**Figure 3(a), (c), (e)**) evidence the presence of diffraction peaks assigned to the Miller indexes of the reflecting planes for three crystalline phases: Cu in face-centered-cubic phase (JCPDS reference code 00–004-0836), Cu2O in cubic phase (JCPDS reference code 01–071-3645) and CuO in monoclinic phase (JCPDS reference code 00–048-1548). These results are in accordance with the data obtained for the CuO nanowire arrays in the crosssectional SEM images (**Figure 2(a), (c), (e)**) in which there were clearly observed three distinct areas with different morphologies.

The band gap values for the CuO nanowire arrays obtained by thermal oxidation in air at various temperatures were assessed based on the reflectance spectra (**Figure 3(b), (d), (f )**) as being around 1.6 eV, in agreement with data previously reported in the literature for CuO nanowires [44].

**Figure 3.**

*(a), (c), (e) XRD patterns and (b), (d), (f) reflectance spectra of the CuO nanowire arrays prepared by thermal oxidation in air at (a), (b) 400°C, (c), (d) 500°C and (e), (f) 600°C.*

#### **3.2 ZnO nanowires**

ZnO nanowire arrays were chemically synthesized in aqueous solution based on the procedures described in Ref.s [13, 28, 29]. Thus, a glass beaker with 300 ml aqueous solution containing 0.1 mmol Zn(NO3)2 and 0.1 mmol (CH2)6N4 was covered and placed in a hot air oven, preheated at 90°C. After 5 h, the substrates, Si/SiO2 pieces coated with a thin Ti/Au layer, were dipped and kept in the aqueous

**31**

**Figure 4.**

*Metal Oxide Nanowires as Building Blocks for Optoelectronic Devices*

solution for 2 days. The Ti layer behaves as an adhesion promoter for the Au layer,

The morphological properties of the ZnO nanowire chemically synthesized in aqueous solution are presented in **Figure 4(a)**, the SEM image revealing that the nanowires have a cylindrical shape with lengths up to 10 μm and very thin diameters of about 20 nm. The XRD pattern of the ZnO nanowires obtained by a wet method (**Figure 4(b)**) evidences peaks corresponding to the Miller indexes of the reflecting planes for ZnO crystalized in a hexagonal wurtzite phase (JCPDS reference code

The optical properties of the ZnO nanowire arrays were analyzed by reflectance and photoluminescence measurements (**Figure 4(c)** and **(d)**). From the reflectance spectrum, a band gap value was estimated of about 3.3 eV, similar with the values reported in the literature for ZnO nanowires [13]. The photoluminescence spectrum of the obtained ZnO nanowires (**Figure 4(d)**) disclose only the presence of a broad, intense emission band, centered at approximately 2.2 eV. Usually, for the ZnO nanowires synthesized in water, this broad emission band from the visible region is linked to the higher concentrations of point defects like: zinc vacancy, interstitial

Arrays of ZnO nanowire obtained by a dry route were prepared according to the

sonic bath with acetone and isopropyl alcohol and thermally oxidized in air for 24 h

*(a) SEM image, (b) XRD pattern, (c) reflectance spectrum and (d) photoluminescence spectrum of the ZnO* 

*nanowire arrays obtained by chemical synthesis in aqueous solution.*

zinc foils were cleaned in an ultra-

which acts as a nucleation layer assisting the growth of ZnO nanowires.

zinc, oxygen vacancy, interstitial oxygen, hydroxyl group, etc. [13].

method from references [13, 37]. Thus, 2 cm<sup>2</sup>

at 400°C, 500°C and 600°C in a convection oven.

*DOI: http://dx.doi.org/10.5772/intechopen.94011*

00–036-1451).

#### *Metal Oxide Nanowires as Building Blocks for Optoelectronic Devices DOI: http://dx.doi.org/10.5772/intechopen.94011*

*Nanowires - Recent Progress*

**30**

**3.2 ZnO nanowires**

**Figure 3.**

ZnO nanowire arrays were chemically synthesized in aqueous solution based on the procedures described in Ref.s [13, 28, 29]. Thus, a glass beaker with 300 ml aqueous solution containing 0.1 mmol Zn(NO3)2 and 0.1 mmol (CH2)6N4 was covered and placed in a hot air oven, preheated at 90°C. After 5 h, the substrates, Si/SiO2 pieces coated with a thin Ti/Au layer, were dipped and kept in the aqueous

*(a), (c), (e) XRD patterns and (b), (d), (f) reflectance spectra of the CuO nanowire arrays prepared by* 

*thermal oxidation in air at (a), (b) 400°C, (c), (d) 500°C and (e), (f) 600°C.*

solution for 2 days. The Ti layer behaves as an adhesion promoter for the Au layer, which acts as a nucleation layer assisting the growth of ZnO nanowires.

The morphological properties of the ZnO nanowire chemically synthesized in aqueous solution are presented in **Figure 4(a)**, the SEM image revealing that the nanowires have a cylindrical shape with lengths up to 10 μm and very thin diameters of about 20 nm. The XRD pattern of the ZnO nanowires obtained by a wet method (**Figure 4(b)**) evidences peaks corresponding to the Miller indexes of the reflecting planes for ZnO crystalized in a hexagonal wurtzite phase (JCPDS reference code 00–036-1451).

The optical properties of the ZnO nanowire arrays were analyzed by reflectance and photoluminescence measurements (**Figure 4(c)** and **(d)**). From the reflectance spectrum, a band gap value was estimated of about 3.3 eV, similar with the values reported in the literature for ZnO nanowires [13]. The photoluminescence spectrum of the obtained ZnO nanowires (**Figure 4(d)**) disclose only the presence of a broad, intense emission band, centered at approximately 2.2 eV. Usually, for the ZnO nanowires synthesized in water, this broad emission band from the visible region is linked to the higher concentrations of point defects like: zinc vacancy, interstitial zinc, oxygen vacancy, interstitial oxygen, hydroxyl group, etc. [13].

Arrays of ZnO nanowire obtained by a dry route were prepared according to the method from references [13, 37]. Thus, 2 cm<sup>2</sup> zinc foils were cleaned in an ultrasonic bath with acetone and isopropyl alcohol and thermally oxidized in air for 24 h at 400°C, 500°C and 600°C in a convection oven.

**Figure 4.**

*(a) SEM image, (b) XRD pattern, (c) reflectance spectrum and (d) photoluminescence spectrum of the ZnO nanowire arrays obtained by chemical synthesis in aqueous solution.*

The SEM images in cross-sectional view of the thermally oxidized Zn foils (**Figure 5(a), (c), (e)**) show that there are two distinct regions with different morphologies: one as a film, attributed to the metallic Zn and the second one, as nanowire arrays, associated to ZnO.

Additionally, similar with the CuO nanowire arrays obtained by a dry technique, the SEM images in plan-view of the ZnO nanowire arrays prepared by thermal oxidation in air at different temperatures (**Figure 5(b), (d), (f )**) evidence that the increase of the annealing temperature favors a higher density and a larger diameter of the ZnO nanowires. Hence, at 400°C there is a low density of nanowires with diameters of about 20 nm and lengths up to 1 μm, while at 500°C and 600°C there is a much higher density of nanowires with diameters of about 30 nm (500°C) and 60 nm (600°C) and lengths up to 30 μm.

The XRD patterns of the Zn foils thermally oxidized in air at different temperatures (**Figure 6(a), (d), (g)**) disclose the presence of diffraction peaks assigned to the Miller indexes of the reflecting planes for two crystalline phases: Zn in hexagonal phase (JCPDS reference code 00–004-0831) and ZnO crystalized in a hexagonal wurtzite phase (JCPDS reference code 00–036-1451). The structural properties are in agreement with the two distinct regions with different morphologies observed in

**33**

arrays.

**Figure 6.**

*(g), (h), (i) 600°C.*

reports for ZnO nanowires [13].

various type of point defects [13].

**based on single metal oxide nanowires**

*Metal Oxide Nanowires as Building Blocks for Optoelectronic Devices*

the cross-sectional SEM images (**Figure 5(a), (c), (e)**) for the ZnO nanowire

*(a), (d), (g) XRD patterns, (b), (e), (h) reflectance spectra and (c), (f), (i) photoluminescence spectra of the ZnO nanowire arrays prepared by thermal oxidation in air at (a), (b), (c) 400°C, (d), (e), (f) 500°C and* 

oxidation in air at different temperatures (**Figure 6(b), (e), (h)**), the band gap values were estimated as being at about 3.3 eV, in accordance with data previously

The photoluminescence spectra of the ZnO nanowires obtained by a dry method at different annealing temperatures (**Figure 6(c), (f ), (i)**) reveal the presence of two emission bands: one intense, sharp and centered at approximately 3.3 eV in the UV region and another one, weak and broad, centered at approximately 2.3 eV in the visible region. The sharp emission band in the UV region is related to the band-edge emission and the one in the visible region is linked to the

**4. Electronic devices (diodes and field effect transistors: FETs)** 

In order to evaluate the electrical properties and to integrate single CuO or ZnO nanowires obtained by wet (chemical synthesis in aqueous solution) and dry (thermal oxidation in air) methods into electronic devices like diodes or field effect

Based on the reflectance spectra of the ZnO nanowire arrays obtained by thermal

*DOI: http://dx.doi.org/10.5772/intechopen.94011*

**Figure 5.** *SEM images of the ZnO nanowire arrays prepared by thermal oxidation in air at (a), (b) 400°C, (c), (d) 500°C and (e), (f) 600°C.*

*Metal Oxide Nanowires as Building Blocks for Optoelectronic Devices DOI: http://dx.doi.org/10.5772/intechopen.94011*

#### **Figure 6.**

*Nanowires - Recent Progress*

nanowire arrays, associated to ZnO.

60 nm (600°C) and lengths up to 30 μm.

The SEM images in cross-sectional view of the thermally oxidized Zn foils (**Figure 5(a), (c), (e)**) show that there are two distinct regions with different morphologies: one as a film, attributed to the metallic Zn and the second one, as

the SEM images in plan-view of the ZnO nanowire arrays prepared by thermal oxidation in air at different temperatures (**Figure 5(b), (d), (f )**) evidence that the increase of the annealing temperature favors a higher density and a larger diameter of the ZnO nanowires. Hence, at 400°C there is a low density of nanowires with diameters of about 20 nm and lengths up to 1 μm, while at 500°C and 600°C there is a much higher density of nanowires with diameters of about 30 nm (500°C) and

Additionally, similar with the CuO nanowire arrays obtained by a dry technique,

The XRD patterns of the Zn foils thermally oxidized in air at different temperatures (**Figure 6(a), (d), (g)**) disclose the presence of diffraction peaks assigned to the Miller indexes of the reflecting planes for two crystalline phases: Zn in hexagonal phase (JCPDS reference code 00–004-0831) and ZnO crystalized in a hexagonal wurtzite phase (JCPDS reference code 00–036-1451). The structural properties are in agreement with the two distinct regions with different morphologies observed in

*SEM images of the ZnO nanowire arrays prepared by thermal oxidation in air at (a), (b) 400°C,* 

**32**

**Figure 5.**

*(c), (d) 500°C and (e), (f) 600°C.*

*(a), (d), (g) XRD patterns, (b), (e), (h) reflectance spectra and (c), (f), (i) photoluminescence spectra of the ZnO nanowire arrays prepared by thermal oxidation in air at (a), (b), (c) 400°C, (d), (e), (f) 500°C and (g), (h), (i) 600°C.*

the cross-sectional SEM images (**Figure 5(a), (c), (e)**) for the ZnO nanowire arrays.

Based on the reflectance spectra of the ZnO nanowire arrays obtained by thermal oxidation in air at different temperatures (**Figure 6(b), (e), (h)**), the band gap values were estimated as being at about 3.3 eV, in accordance with data previously reports for ZnO nanowires [13].

The photoluminescence spectra of the ZnO nanowires obtained by a dry method at different annealing temperatures (**Figure 6(c), (f ), (i)**) reveal the presence of two emission bands: one intense, sharp and centered at approximately 3.3 eV in the UV region and another one, weak and broad, centered at approximately 2.3 eV in the visible region. The sharp emission band in the UV region is related to the band-edge emission and the one in the visible region is linked to the various type of point defects [13].
