Optical and Photoconductive Response of CuO Nanostructures Grown

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C: Energy Conversion and Storage; Energy and Charge Transport

Optical and Photoconductive Response of CuO Nanostructures Grown by a Simple Hot Water Treatment Method Khalidah Al-Mayalee, Nawzat Saeed Saadi, Emad Badradeen, Fumiya Watanabe, and Tansel Karabacak J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06783 • Publication Date (Web): 25 Sep 2018 Downloaded from http://pubs.acs.org on September 26, 2018

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The Journal of Physical Chemistry

Optical and Photoconductive Response of CuO Nanostructures Grown by a Simple Hot Water Treatment Method Khalidah H. Al-Mayalee†, ‡, Nawzat Saadi†, Emad Badradeen†, Fumiya Watanabe§ and Tansel Karabacak†*

†

Department of Physics and Astronomy, University of Arkansas at Little Rock, Little Rock, AR 72204, USA. ‡

Physics Department, Faculty of Education for Women, University of Kufa, Najaf, Iraq.

§

Center for Integrative Nanotechnology Sciences, University of Arkansas at Little Rock, Little Rock, AR 72204, USA.

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ABSTRACT

In this work, we report the fabrication and characterization of copper (II) oxide (CuO) nanoleaf structures (NS) grown on Cu sheets by a facile hot water treatment (HWT) method without using catalyst materials. In addition, simple photoconductive devices based on as prepared CuO nanoleaves were fabricated to study the optical and photocurrent response of CuO NSs. SEM images revealed the formation of uniform and dense nanoleaves morphology of CuO on Cu sheets. XRD patterns indicated that synthesized nanostructures have a monoclinic CuO structure. Furthermore, XPS results demonstrate the formation of Cu–O chemical bond which confirmed the formation of CuO phase. For the fabrication of the photoconductive devices, the CuO/Cu samples were coated with aluminum doped ZnO (AZO) shell by sputter deposition at room temperature. CuO NSs show high-broadband ultraviolet-visible spectroscopy (UV/Vis) absorbance with marked enhancement after AZO coating. Current density-voltage (J-V) measurements show that AZO/CuO/Cu devices exhibit a photocurrent density response of 9.65±0.43µA/cm2 with a rise-time of 0.195 s and decay-time of 0.192 s. They also indicate a Schottky contact between p-type CuO NSs and Cu substrate. Photocurrent increases, and risetime and decay-time decrease with an applied forward bias (e.g. ~19.00 µA/cm2 at 1.0 V with rise-time of ~0.100 s and decay-time of 0.096 s). The value of the optical energy gap of CuO NSs was calculated to be 1.44±0.13eV, by the analysis of Tauc’s plot. These results indicate that our photoconductive devices based on CuO NSs prepared by HWT can achieve high light absorption and good photocurrent response for optoelectronic applications.


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Introduction Nanostructured semiconductor materials demonstrated notable performance in a wide range of technological applications including medicine, imaging, computing, and catalysis. These materials show exclusive physical and chemical properties with nanoscale size, shape and large surface-to-volume ratio, which can enhance their device functionalities

1-3

. For example, metal

oxides (MOs) showed improved physicochemical, magnetic, thermal, and optical properties with the variation in size and shape when their structural feature size is reduced down to the nanoscale 1, 4

. MOs such as TiO2, ZnO, NiO, CuO, and SnO2 have found many uses in electronics,

catalysis, energy storage and conversion, biomedicine, and sensors

1, 4-7

. Copper oxide

nanostructures are among the most extensively studied nanostructured MOs, because of their unique properties in comparison to other MOs

8-9

. Exclusive optical, electrical, thermal, and

magnetic properties of CuxO made them the materials with extraordinary potential for a broad range of technological applications such as in solar cells, gas sensors, magnetic phase transitions, catalysts, and superconductors. Several distinct phases of CuxO exist, mainly including copper (II) oxide or cupric oxide (CuO), copper (I) oxide or cuprous oxide (Cu2O), and paramelaconite (Cu4O3) 4, 8, 10. CuO is a widely studied form of copper oxide due to its remarkable characteristics such as superior thermal conductivity, photovoltaic property, stability, antimicrobial activity, and super-hydrophobic properties

1, 4, 6, 11-12

. CuO is a p-type semiconductor having a monoclinic

crystal structure with a band gap in the range ~1.2-2.0 eV depending on the synthesis conditions. It has a high optical absorption coefficient, suitable to be a semiconductor photo-absorber material

1, 4-5, 8-9, 13-14

. Because of its attractive properties, CuO is a promising MO for various

high-level technological applications such as in batteries, supercapacitors, solar cells, gas sensors, photodetectors, and biomedical devices 1, 4, 15. 3 ACS Paragon Plus Environment


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A wide range of physical and chemical techniques have been developed to produce CuO NSs of different sizes, shapes, and compositions. Some of these methods include thermal oxidation, sol-gel, hydrothermal, reactive magnetron sputtering, microwave irradiation, and more recently hot water treatment (HWT)

4, 6, 8, 16

. The surface treatment of metallic foils with hot de-ionized

water (70–100 °C) during HWT has been successfully used to produce rough metallic oxide nanostructures of CuO, ZnO, and Al2O3 17-22. HWT offers a simple and low-cost alternative in MO NS synthesis without using any chemical additives or catalyst materials. Recently, Khedir et al.

17

studied HWT copper oxide nanostructures, which were grown by

treatment of Cu sheets with hot de-ionized water (80 °C) for different time periods to fabricate nanoscale superamphiphobic materials. They found out that controlling the treatment time had a significant effect on the morphology and crystal structure of HWT nanostructures. Na et al.

19

reported the recrystallization of Cu2O to CuO in water at room temperature and presented the changes in morphology, crystallinity, oxidation states, and liquid-solid heterogeneous Cu2O oxidation reactions induced by visible light. Hassan et al.

23

utilized a combination of

sandblasting and HWT technique to introduce microroughness and nano-roughness, respectively, on copper sheets. The fundamental understanding of the growth process of metal oxide nanostructures formation by a facile HWT method has been discussed by Saadi et al.

24

.

Furthermore, HTW has been utilized for modifying wetting properties for several metals including Zn, Al, and Cu 4, 18, 21-22, 25. However, photoconductive properties of HWT metal oxide nanostructures have no been investigated in the literature before. In this study, we investigate the formation of CuO nanostructures during HWT and their application in a simple photodetector device. SEM, XRD, and XPS analyses were used to


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characterize the morphology and structure of the synthesized CuO NSs. Electrical and optical properties of the CuO samples and the fabricated devices will also be presented and discussed.

Experimental Synthesis of Copper Oxide Nanostructures 0.127 mm thick, annealed, 99.9% purity copper sheets (Alfa Aesar) were used as the substrate. The substrates were cut into 1.5 cm × 2 cm pieces. Native oxide and surface contaminants were removed by mechanical polishing using ultrafine aluminum oxide sandpaper of (3000 grit) until a shiny smooth surface was obtained. A schematic diagram of the HWT process and photographs of the Cu sheet samples before and after the treatment at 75°C for 24 hrs to synthesize CuO nanostructures are shown in Fig. 1. Cu foils were sequentially cleaned with acetone and ethanol for 20 min using an ultrasonic cleaner followed by rinsing with distilled water several times to remove any metal and organic contaminations. Finally, the cleaned Cu substrates were dried by blowing nitrogen gas. The resulting Cu foils were immersed in clean glass beaker hot de-ionized (DI) water which was placed on a hot plate to maintain the constant temperature of 75 °C for 24 hr. After this HWT process, samples were taken out of the hot water and dried at room temperature for further characterization. During HWT oxidation, apperance of the samples changed from reddish color of a pristine copper toward a dark black color as shown in photographs of Fig. 1.



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Figure 1: Schematic diagram of the HWT process and photographs of Cu sheet samples before and after the treatment at 75°C for 24 hrs to synthesize CuO nanostructures.

Device Fabrication Process For the fabrication of a simple photoconductive device utilizing HWT CuO NSs. First, CuO NSs as grown on Cu sheet substrates were coated with a layer of aluminum-doped zinc oxide (AZO) by sputter deposition at room temperature to serve as a transparent conductive oxide layer. Cross-sectional and top-view design of the AZO/CuO/Cu photoconductive device is illustrated in Fig. 2. AZO shell layer was sputtered on CuO surface within small circles of 0.08 cm2

by

using

a

shadow mask as shown in the photograph of Fig. 2.


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Figure 2: Cross-sectional and top-view illustrations of the AZO/CuO/Cu photoconductive device structure. AZO shell and top-contact layers were deposited by RF-magnetron sputtering using an aluminum oxide doped zinc oxide target (2 in. diameter, 99.99% purity, composed of 98 wt% ZnO and 2 wt% Al2O3) provided by Plasmaterial Inc. Before deposition, a base pressure of ~106

mbar was achieved inside the sputtering chamber by a turbomolecular pump backed up by a

mechanical pump. The argon gas flow rate was maintained at 10 SCCM and the RF power was set to 110 W during AZO deposition with 0 W reflected power at room temperature. For high pressure sputtering (HIPS), Ar gas pressure was set to ~1*10-2 mbar for 60 min to achieve a conformal AZO shell around the CuO NSs. This was followed by low pressure sputtering (LPS) at ~3*10-3 mbar for 15 min to obtain a dense top contact. AZO was deposited at normal incidence on substrates that were azimuthally rotated at 20 RPM. The total AZO layer thickness was estimated to be ⁓372 nm based on the deposition rate measured by a quartz crystal microbalance (QCM) and analysis of cross-sectional SEM images. Keithley 2400 source meter with two probes was used to investigate the electrical characterization of the AZO/CuO/Cu photoconductive devices. Devices were tested under simulated AM 1.5G sunlight with an incident light power of 100 mW.cm-2. During characterization, an individual device was positioned in the center uniform portion of the light beam source. Each nanostructured photoconductive device consisted of small circles of 0.08 cm2 AZO contacts on CuO surface achieved by using a shadow mask. Here, we should mention that I-V measurements were repeated several times on three independent samples with each sample consisting of six circular devices. The photoresponse measurements were carried out at zero (0), 0.1, 0.5 and 1.0 V bias to examine the dynamic response of the photocurrent. For this purpose, 7 ACS Paragon Plus Environment


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Cu electrode was connected to the ground terminal of the source meter, AZO contact was connected to the positive terminal, and the light source was turned on and off at regular intervals of time. Applied voltage corresponds to a forward bias along Schottky junction at the CuO/Cu interface as will be discussed below.

Results and Discussion Morphological Analysis The surface morphology of samples was analyzed using scanning electron microscopy (SEM; JEOL JSM-7000F). Figures 3 (a), (b), and (c) show the top-view SEM surface morphological images of the polished flat copper surface, CuO NSs as prepared by HWT, and AZO shell deposited on CuO NSs with different scales of magnification. As seen in Fig. 3b, leaf-like CuO NSs uniformly formed on the Cu sheets substrate similar to those reported by Khedir et al. 17 and Hassan et al.

23

. Some of these nanoleaves are also tilted with respect to the substrate normal,

which leads to a 3D geometry. CuO NSs growth rate and orientation can be affected by substrate polishing process, immersion time and diffusion rates

24

. The initial substrate surface chemistry

with high density defect sites, which act as nucleation regions, may lead to the formation of rough leaf-like nanostructures observed in this study. Further, Low surface diffusion rates of CuO molecules on the Cu surface (i.e., low HWT temperature) with long immersion times can lead to the formation of fractal-like rough nanostructures 24. Moreover, it is well known that the principle rule for crystal growth and the morphology progress depend on the surface energy reduction

19

. Therefore, the crystal planes with low surface energy will derive the re-deposited

CuO molecules to move towards the edges to reduce surface energy which extends the size of the facets and determine the final shape of the CuO NSs

24

. In addition, Fig. 3c demonstrates the 8

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conformal AZO coating around the CuO NSs. HIPS of AZO was used first to provide a relatively more conformal AZO shell around CuO NSs. It is observed that the CuO NSs is almost entirely covered by AZO. HIPS provides an incident flux of atoms with broader angular

distribution. Therefore, HIPS atoms have more efficient penetration through the gaps of the nanostructured surface 26-27. On the other hand, LPS at normal incidence results in a denser AZO contact layer that helps to provide the electrical conductivity among isolated NSs 27. Figure 3: Top-view SEM images in different magnification of (a) polished flat copper surface, (b) HWT CuO nanostructures grown on copper sheet (HWT: 75ᴼC, 24 hr), and (c) AZO shell deposited on CuO nanostructures. Microstructure of the CuO NSs is further investigated by transmission electron microscopy (TEM) as shown in Fig. 4. The morphological details obtained from the TEM images showed a good accord with the SEM results. The TEM images indicate that CuO leaf-like structures are 9 ACS Paragon Plus Environment


composed of interpenetrating 2D nanosheet subunits with lengths and widths ranging from 200 to 820 nm and 60 to 200 nm, respectively, as building blocks. The CuO nanoleaf architecture may be formed by accumulation of several of small CuO nanosheets which were self-assembled to form 3D nanoleaves structures as shown in the insets of Fig. 4 (a) and (b). The CuO NSs are found to have lengths 900-1300 nm with widths 500-630 nm.

Figure 4: TEM images of CuO nanostructures formed by HWT.

Growth Mechanism Recently, a growth mechanism of the formation of metal oxide nanostructures (MONSTRs) by HWT method has been proposed by Saadi et al.

24

. The mechanism is believed to involve

“plugging” and “surface diffusion” processes as the main contributors. While plugging process results in isolated NSs, shape of MONSTRs was mainly claimed to depend on the surface diffusion. In this mechanism, the process is initiated with the formation of a metal oxide molecule on the metal surface. Accordingly, as illustrated in Fig. 5, copper cations (Cu2+) are first formed during the HWT of a Cu plate, leaving electrons behind on the surface, then the electrons on the surface react with dissolved oxygen and water molecules to produce hydroxyl ions. Finally, copper cations from the copper sheet can react with hydroxyl ions on the surface to form copper oxide molecules along with the release of hydrogen gas. After the formation of 10 ACS Paragon Plus Environment

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metal oxide, the plugging process takes place that involves the release, migration through water, and re-deposition of a copper oxide molecule on the Cu substrate surface. Along with the plugging process, surface diffusion of the re-deposited copper oxide and their crystal structure will determine the shape of MONSTRs.

Figure 5: Schematic illustration of the growth mechanism during HWT of (a) CuO molecules, (b) CuO nanostructures.



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For copper-hot water reaction, Cu surface favors the growth of Cu2O with cubic nanostructures 24

. The formation of Cu2O continues until the surface completely gets covered by Cu2O. With the

progress of HWT time and the presence of hydroxyl ions, the transformation of Cu2O cubic nanostructures to CuO (more thermodynamically stable phase) nanoleaves take place and continue until the whole surface is transformed into CuO nanoleaves

17, 19, 23-24

. The chemical

process of CuO NSs formation during HWT can be summarized as 19, 24. The experimental parameters such as dissolved oxygen, treatment time, and water temperature can affect the oxidation rate as well as the final dimension, size, and crystal structure quality of the CuO NSs

17, 25

. The overall process is also believed to result in good adhesion of the CuO

NSs to the Cu substrate.

Crystallographic and Compositional Analysis

Cu Cu + 2e

O + 2H O + 4 2e 4OH

2Cu + 2OH 2Cu O + H

Cu O + 2OH 2 CuO + H


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The crystal structure and orientation of the as-grown CuO NSs were studied by a Rigaku MiniFlex X-ray diffractometer (XRD) with a Cu Kα X-ray source. From XRD spectrum in Fig. 6, we can observe three sharp Cu peaks located at 2θ angles of 43.22ᴼ, 50.37ᴼ, and 74.1°, which are associated with Cu(111), Cu(200) and Cu(220) orientations of the metallic substrate, respectively

17, 28

. In addition to the Cu peaks, CuO(002) and CuO(111) peaks can be seen at

35.44ᴼ and 38.54ᴼ respectively. This indicates the formation of monoclinic CuO phase the nanostructures

11, 13, 23

. A weak diffraction peak at 36.32° also points to the presence of Cu2O

oriented along the (111) direction 15, 17.

Figure 6: XRD spectrum for treated Cu sheet in hot de-ionized water at 75ᴼϹ for 24 h The average crystalline grain size D of CuO NSs was estimated analyzing the diffraction peaks from XRD data using to the Debye-Scherer’s formula

: K λ⁄β cos θ, where λ is

29-30

the X-ray wavelength (1.54060 Å), K is Scherrer constant (0.9), β is the full-width-at-halfmaximum of diffraction peak (in Rad), and θ is the Bragg diffraction angle. Average grain size



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values were calculated to be 21.6 nm and 17.5 nm for the CuO (002) and (111) crystal planes, respectively. In addition, Thermo Scientific K Alpha X-ray photoelectron spectroscopy (XPS) was utilized to investigate the surface chemical states of the CuO nanoleaves grown on Cu sheets 2. The Cu 2p core-level XPS spectra in Fig. 7 shows the range of energies associated with the Cu (2p3/2) and Cu (2p1/2) photoelectric lines. These peaks, which are located around 933.7ev and 953eV (with the splitting of 19.3 eV), are attributed to the presence of the Cu2+ chemical state as a

further indication of the formation of CuO. Figure 7: Core level XPS spectra of Cu 2p for a CuO/Cu sample after HWT, the inset shows the XPS depth profiles of the CuO/Cu interface.


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Moreover, the spectrum displays several satellite peaks, which are characteristic of CuO having a d9 configuration in the ground state. The shake-up satellite peaks of the Cu (2p3/2) and Cu (2p1/2) at 942.3 and 962.2 eV, respectively (~ 9 eV greater than the corresponding significant peaks), confirmed the formation of Cu2+ on the surface

28, 31-32

. Copper (II) exhibits one or two

satellite peaks at 5-10 eV higher binding energy from the main peak. Whereas, the XPS of copper (I) complexes exhibit a symmetrical main peak in either the 2p3/2 or 2pI/2 region with no evidence of a shake-up satellite at higher binding energy 4. The depth profiles of the CuO/Cu interface shows Cu2+ growth from the the Cu bulk into surface as shown in the inset of Fig. 7. All these results support the formation of CuO on the Cu substrate surface after HWT.

Optical Properties Optical properties of the as-synthesized CuO NSs and photoconductive devices have been studied by using Shimadzu UV-3600 UV/Vis/NIR Spectrometer. Fig. 8 shows the Absorptance (A) spectra of CuO NSs with and without AZO coating. Spectral absorptance was estimated from reflectance (R), and transmittance (T) as A = 1- (R + T), where R was measured using an integrating sphere and T was ~0% for the nanostructured CuO/Cu samples. The results demonstrate that the as-grown CuO NSs exhibit a broadband spectrum in the range of wavelengths from ultraviolet through the visible part to the near-infrared region. This behavior can be attributed to the broad size distribution in CuO NSs size and also surface defects that can induce new energy levels in the band gap 5, 11, 33-34. Furthermore, the 3D morphology of CuO NSs can provide enhanced light scattering and therefore improved light trapping 35-36.



110 100 90 80

Absorptance %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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AZO/CuO/Cu

70 60

CuO/Cu 50 40 30 20 10 200

400

600

800

1000

1200

1400

1600

1800

2000

Wavelength (nm) Figure 8: UV-Vis-NIR absorptance spectra of CuO/Cu samples with and without AZO. In addition, the absorptance spectrum of the AZO/CuO/Cu device demonstrated enhanced light absorption in the NIR compared to CuO/Cu. This can originate from multi-scattering effects introduced by the AZO layer and a more gradual change of the refractive index from air to the CuO NSs active layer compared to that of the CuO/Cu sample index within the range 1.74-2.13

40

35-39

. AZO can have a refractive

in visible region of the spectrum (about 360-760 nm). This

falls in between the refractive indexes of air (~1.00) and CuO (~2.63) 5, which can enhance the antireflection characteristics

41-42

. Therefore, AZO/CuO/Cu devices seem to have an

antireflection property over wide range of wavelengths. To estimate the optical band gap Eg of CuO NSs, we used diffuse reflectance results and Kubelka–Munk function ( . Reflectance was measured using UV-Vis absorptance spectroscopy with diffuse reflectance accessory. Reflection measurements with an integrating sphere included both the direct reflected light and the diffuse scattering light. The experimental 16 ACS Paragon Plus Environment

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UV/VIS/NIR diffuse reflectance results of CuO nanostructures are shown in the inset of Fig. 9. The Kubelka–Munk function (F(R∞)) is given by 43:

1 − … … … … … … . 1 2

where R is the diffuse reflectance, α absorption coefficient and s is the scattering factor. Absorption coefficient α of a direct bandgap semiconductor is related through the Tauc equation 44: +

αhυ Ahυ − E* … … … … … … . 2 Where A is a proportionality constant. Typically, is proportional to the absorption coefficient α when the incident light reflects off surfaces in a perfectly diffuse manner. In this case, we consider the scattering factor s as a constant independent of wavelength and weakly dependent on energy 45-46. Therefore, Eq. (2) can be transformed to Kubelka–Munk function by: … … … … … … . 3 +

. ℎυ Ahυ − E* … … … … … … . 4 The value of the direct optical band gap (E* ) is determined by plotting of (FR . hυ)2 versus the photon energy hυ. The extrapolation of this straight line in a certain region will intercept the (hυ)-axis where F corresponds to zero

37, 47-48

. Fig. 9 shows Tauc’s plot of CuO

nanostructures samples. The average band gap for CuO NSs samples has been calculated to be 1.44 ± 0.13 eV, where the coefficient of determination r2 has maximum value of 0.999. This results is within the range of previous reported band gap values for CuO 1, 5, 11, 44, 48.



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Figure 9: The [F (R∞) hν] 2 versus hυ Plot (using Kubelka–Munk method) to measure band gap; the inset shows the diffuse reflectance spectrum for synthesized CuO NSs.

Electrical Measurements The average of photocurrent density (Jph) versus time profiles of the AZO/CuO/Cu devices at 0 V are shown in Fig. 10a. Jph is the pure photocurrent density obtained by the photogenerated charge carriers, which is calculated by (|Ilight| − |Idark|)/(device area). AZO/CuO/Cu photoconductors demonstrated a positive Jph when Cu metal was connected to the ground terminal of the source meter. This indicates a dominant Schottky junction at the CuO/Cu interface. The mean value of Jph ± SD from all the devices was calculated to be about 9.64 ± 0.43µA/cm2 (Fig 10a).


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Figure 10: (a) Current density vs time curve of the AZO/CuO/Cu devices under zero bias; (b) An enlarged portion of the photocurrent response; (c) Photocurrent response of the AZO/CuO/Cu devices as the light is turned on off under different bias voltages: 0.1, 0.5 and 1.0 V and (d) Current density vs voltage (J-V) profiles measured under light and dark. The conventional photoelectric device performance based on the conversion of light energy into photocurrent is the outcome of several complicated processes such us electron-hole generation, recombination, and trapping

38-39,

49

. However, relatively low photocurrent

performance of the AZO/CuO/Cu device, despite the high absorption, might be attributed to the 19 ACS Paragon Plus Environment


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small active contact area between the CuO/Cu junction. Furthermore, low photocurrent performance may be linked to high recombination rate of the photogenerated electrons. Since the large surface area of the CuO NSs may introduce a large number of defects; they can act as recombination sites. Also, a large surface area is typically prone to rapid nonradiative recombination, which means reducing the collection of photo-generated carriers and carrier lifetimes 49. The AZO layer is believed to help to decrease this critical issue by capturing charge carriers, which overcome undesired surface recombination velocities and increase decaytime. Moreover, AZO shell enables photons generated by electron-hole recombination to be reabsorbed that can lead to further charge carrier generation 5, 36, 49. From Fig 10a, we can see that the magnitude of photocurrent in every on/off cycle is steady and repeatable. The rise-time(τon ; the time in which current increased to its saturation value) of AZO/CuO/Cu was about 195 ms as illustrated in Fig 9b. Meanwhile, the decay-time (τoff ; the time in which current decreased to its saturation value) was about 192 ms. These values of risetime and decay-time are shorter than the ones reported in the literature (e.g. τon =15 s and τoff =17 s for a CuO infrared photodetector

47

). In metal/p-type-semiconductor Schottky contacts, the

current conduction is driven by holes which are the majority carriers

50

. We believe that steady

photocurrent and fast response in our devices can be attributed to the rapid and large amounts of electron-hole pair photogeneration at the CuO/Cu Schottky junction. These pairs will be swiftly separate and driven by the strong built-in electric field in the interface depletion region into reverse directions, which is beneficial to photocurrent response speed

50-52

. Fast response speed

can also be attributed to the short transit time of photogenerated charge carriers due to the relatively high carrier mobility in CuO/Cu. Furthermore, the stability of photocurrent over time (Fig 10a) suggests that charge accumulation and surface trapping do not significantly impact the 20 ACS Paragon Plus Environment

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electric field within the CuO-Cu interface. Moreover, the fast decay-time when the illumination source is turned off can be due to rapid electron-hole recombination, which leads to a short recovery time of the photocurrent in CuO-Cu junction. In addition, as it can be noticed from Fig 10c, the photocurrent density of the device increases with increasing the forward bias voltage. Photocurrent density can change according to the equation 53 : 012 345∆712 89 + Δ;12 81

Optical and Photoconductive Response of CuO Nanostructures Grown

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