Influence of Mn-doping on the photocatalytic and ...

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Inorganic Chemistry Communications 76 (2017) 71–76

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Influence of Mn-doping on the photocatalytic and solar cell efficiency of CuO nanowires Muzaffar Iqbal a, Akbar Ali Thebo b, Aamir Hassan Shah b, Azhar Iqbal a,⁎, Khalid Hussain Thebo c,⁎, Shahnawaz Phulpoto d, Muhammad Ali Mohsin e,⁎ a

Department of Chemistry, Quaid-I-Azam University, Islamabad 45320, Pakistan National Center for Nanoscience and Technology, Chinese Academy of Sciences, Beijing, China Institute of Metal research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang, Liaoning 110016, China d State Key Laboratory of Chemical resource Engineering, Beijing University of Chemical Technology, Beijing, China e Department of Chemistry, Institute of Chemical and Biological Sciences, University of Gujrat, Gujrat, Pakistan b c

a r t i c l e

i n f o

Article history: Received 12 September 2016 Received in revised form 23 November 2016 Accepted 27 November 2016 Available online 30 December 2016 Keywords: CuO Wet chemical method Nanowire Rhodamine B Photocatalyst

a b s t r a c t Pristine copper oxide (CuO) and manganese (Mn) doped CuO nanostructures with different ratios were synthesized via wet chemical method. As-prepared materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-rays (EDX), UV–Visible and emission spectroscopy. Further, the doped nanowires were employed in the hybrid solar cells in combination with P3HT and gave better current densities than their corresponding undoped counterparts. The photoactivity of synthesized materials was evaluated by the photocatalytic oxidation of Rhodamine B (RhB). The results showed that the 2% Mn doped CuO photocatalyst is highly photoactive than other corresponding undoped and doped CuO nanowires. The increase in solar cell efficiency and photocatalytic activity of the doped CuO nanowires is solely due to the improvement in the charge separation efficiency in the 2% Mn doped CuO nanowires. © 2017 Elsevier B.V. All rights reserved.

Transition metal oxides at the nanoscale have got considerable attention due to their unique properties and wide use in several applications [1–5]. Among such materials, cupric oxide (CuO) has fascinated the scientific community in recent decades [6–8] due to its exciting properties in electrical and photonic applications such as gas sensors and biosensors [9,10], solar cells [11], superconductors [12], energy storage material [13], and photocatalysts [14–17]. Several attempts have been made to synthesize variety of CuO nanostructured material in form of nanoparticles, nano-plates, nanorods, nanotubes and nanowires to enhance its performance [18–23]. Now it is well known fact that any controlled morphology variation can have high effect on the properties of the nanomaterials. In this regard, number of transition metals have been used for doping to enhance their physical and chemical properties. Even though much of the work is done on doping of nanostructured thin films, but still it is a challenge to produce high quality crystalline thin films with interesting properties. Recent investigations show that the photon property of CuO plays a significant role due to the variable oxidation states of copper i.e., Cu+, Cu2 +, and Cu3 +, because of which both hole and electron doping are possible [20]. It has been reported that there are many changes associated with ⁎ Corresponding authors. E-mail addresses: [email protected] (A. Iqbal), [email protected] (K.H. Thebo), [email protected] (M.A. Mohsin).

http://dx.doi.org/10.1016/j.inoche.2016.11.023 1387-7003/© 2017 Elsevier B.V. All rights reserved.

doping of transition metal into CuO lattice such as Ni-doped, Fedoped, Ti-doped, Cd-doped and Zn-doped [24–26]. Several attempts have also been made on mixed oxide catalysts especially containing MnO2 or CuO. Up to date, a less work has been done on Mn-doped bulk and/or nanostructured CuO [7,20,27]. Mn-doped CuO is an interesting material to enhance the visible light absorption because these transition metal ions can generate energy states within the band gap. Furthermore, these are used as intermediate steps for electrons during their transitions between the valence and conduction bands. In the present work, an environment friendly and simple method has been used to synthesize CuO nanowires with different doping concentrations of Mn. The structural, morphological and optical properties of the synthesized materials were investigated by the SEM, EDX, XRD, and UV–Vis analysis. Electrochemical measurements were performed by using Metrohm Autolab B.V., PGSTAT 302N, ADC164, External, DIO48 operated with NOVA 1.11.2 software. All electrochemical experiments were performed by using three electrode system, in which CuO, 2%Mn doped CuO and 10% Mn doped CuO were used as working, saturated calomel electrode as a reference and platinum wire was used as an auxiliary electrode. The synthesized nanostructures were studied for the photodegradation of Rhodamine B dye and also as the bulk heterostructure solar cells. Copper acetate and manganese chloride, thio-glyceriol, sodium hydroxide, rhodamine B, methanol, hydrochloric acid, ethanol and other

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Fig. 1. (a) SEM image of CuO nanowires; (b and c) SEM image of the different % Mn-doped CuO nanowires.

chemicals were received from Merck and Sigma Aldrich and used further without purification. Transition metal doped CuO nanowires such as (MnxCu1-xO) were synthesized by modified Ethirajet et al., method [28]. The dopant concentration ‘x’ ranged from 0.02 to 0.1, where x = 0.02 represents (2%), 0.06 (6%) and 0.1 (10%) doped samples, respectively. Copper acetate (4.15 g) and manganese salt at a desired concentration were dissolved in deionized water by heating up to 35 °C. 1 μL of thio-glyceriol (TG) was mixed to the solution, as a capping agent and stirred for few minutes. Sodium hydroxide solution (1 M) was added drop-wise followed by deionized water and stirred for few hours. The products were collected by centrifugation, washed with deionized water and were dried overnight at 35 °C. As-synthesized CuO nanowires and different % Mn-doped CuO nanowires were further characterized with help of aforementioned techniques. As-prepared doped semiconductor nanowires were dissolved in methanol and termed as solution A. Another solution of P3HT (20 mg/mL) was prepared in methanol under inert atmosphere and

termed as solution B and these two solutions were mixed thoroughly. A glass sheet of 3 × 2 cm2 dimensions covered with ITO (80 nm) was used as a substrate and etched a portion of it with hydrochloric acid (32%). Further it was thoroughly washed with ethanol to remove traces of HCl. Then the substrate was coated with a thin layer of PEDOT-PSS (poly (3, 4-ethylenedioxythiophene)-poly (styrene sulfonate)) by spin coating. In the subsequent step, the photo-functional material which is a blend of doped CuO nanowires and P3HT was spin coated over PEDOT-PSS at a speed of 1500 rpm and acceleration of 2000 rpm/min for 20 s, and then at a rate of 500 rpm and 100 rpm/min for 20 s in order to remove the solvent completely. Finally, Al is deposited by thermal evaporation in the metal evaporator under vacuum. 80 nm thick cathodic strips were made to extract and guide electrons to the external circuit. The device was annealed at 90 °C for 15 min under inert atmosphere (argon flow) to gain homogenous morphological distribution of the active blend. The photocatalytic degradation study was carried out in a cylindrical Pyrex glass reactor containing 0.01 g of the synthesized catalyst and

Fig. 2. (a) EDX spectrum of CuO nanowires; (b) EDX spectrum of Mn-doped CuO nanowires.

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Fig. 5. (a) UV–Vis spectrum of un-doped and Mn-doped CuO nanowires; (b) Band gap estimation of undoped and Mn-doped CuO nanowires. Fig. 3. XRD spectrum of un-doped CuO nanowires and with different amounts of Mndoped CuO nanowires.

100 mL aqueous solutions of RhB. The suspension was kept in the dark up to 90 min and stirred continuously to maintain equilibrium between adsorption-desorption processes. Further mixture was exposed to visible-light halogen lamp immersed within the photoreactor and surrounded by a circulating water jacket (Pyrex) to cool the reaction solution. An aliquot of the suspension was taken timely from the reactor and centrifuged. UV–Visible spectrophotometer (Shimadzu 3150) was used to measure the absorbance of the obtained solution at 527 nm to comprehend the effect of irradiation time as a function of concentration of the RhB dye. The low magnification SEM images of both CuO and Mn-doped CuO are presented in Fig. 1a–c. Pristine CuO nanowires showed an irregular and very thin configuration, due to agglomeration while the metal doped nanowires have shorter size in length but increase in diameter and also the density of wires is disturbed (Fig. 1). After doping of metal, the morphology was completely changed, indicating the density

Fig. 4. Emission spectra of un-doped and Mn-doped CuO nanowires.

of nanowires slightly increased by doping of Mn+2 ions. This is due to molecular disorder and lattice strain which results difference in ionic radii. EDX spectrum of undoped CuO showed the presence of Cu and O elements in the sample, from which we can easily see that atomic percent of Cu and O is 43.55% and 56.45% respectively, shown in Fig. 2(a). The EDX spectrum of 2% doped CuO nanowires is presented in Fig. 2(b), which clearly shows the dopant Mn peak signal. The characteristic XRD patterns of pure and Mn-doped CuO with varying Mn content were recorded in the range of 2θ between 20 and 80 at the scan rate of 0.011/s shown in Fig. 3. The XRD spectrum

Table 1 Band gap of Mn-doped and un-doped CuO nanowires. Material composition

λmax (nm)

Band gap (eV)

CuO nanowires 2% doped Mn CuO nanowires 6% doped Mn CuO nanowires 10% doped Mn CuO nanowires

476 539 516 496

2.6 2.3 2.4 2.5

Fig. 6. J–V plots of Mn doped CuO nanowires at different dopant concentrations.

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Table 2 Performance of Mn doped CuO–P3HT solar cells. Device composition

Jsc (mA/cm2)

Voc (V)

MPP (mW/cm2)

Fill factor (FF)

Efficiency (ɳ%)

P3HT-CuO light P3HT–CuO–Mn 2% P3HT–CuO–Mn 6% P3HT–CuO–Mn 10%

−2.1 −2.9 −2.7 −2.2

0.39 0.67 0.43 0.41

0.46 0.55 0.50 0.48

0.54 0.27 0.49 0.53

0.47 0.55 0.52 0.48

AM 1.5 conditions. Jsc = short circuit current density, Voc = Open circuit voltage, FF = fill factor and ɳ = cell conversion efficiency.

shows that the diffraction peaks located at 2θ 1/4 32.521, 35.551, 38.731, 48.761, 53.411, 58.311, 61.571, 66.221, 68.141, 72.411, and 75.021 which correspond to [110], [− 111], [111], [202], [020], [202], [113], [311], [220], [311] and [004] planes respectively and the peak positions were matched with JCPDS [card: 01-080-0076] of CuO. XRD results showed that no additional peak was found due to Mn or MnO2 which means that the substitution of Cu atoms by Mn has not changed the monoclinic structure of CuO. Further it is found that the intensities of peaks at [ 1 11] and [111], are stronger than those of other peaks as demonstrated in XRD results, which indicates that they are preferential crystal planes of the nanostructures. Doping of Mn even in small percentages caused a large decrease in the peak intensities at first. But as the doping concentration was increased, the [111] and [111] peaks intensity was also increased considerably. Broadening of the diffraction peak after Mn doping shows a slight crystallinity degeneration of the CuO films. However, it is worthy to mention that the observed XRD data shows no extra peaks due to manganese metal, other oxides or any copper manganese phase, indicating that the as-prepared samples are in a single phase. And it is safe to assume that the Mn ions have successfully substituted the Cu site without changing the monoclinic structure during the synthesis progression. The emission spectra of CuO nanowires, showed three main broad emission bands at 424, 460, and 488 nm (Fig. 4). The strong emission peak at 424 nm is related to the band-edge emission of CuO nanostructures. The other two weak emission peaks found at 460 nm and 488 nm are due to the band edge emission from the new sublevels at 273 K or maybe due to the defects present in the CuO nanostructures [29–32]. The emission spectra of Mn-doped CuO nanowires, show that main strong emission peak at 424 nm in undoped CuO nanowires is red shifted to 434 nm by Mn+2 ion doping CuO nanowires. The emission spectrum of pure CuO nanowires shows the presence of intense defects states but as CuO is doped, remarkable changes in the intensity of this

peak are observed i.e., the defect peak becomes less intense. Doping plays a significant role in eliminating these surface defects. UV–Visible absorption spectra of undoped CuO nanowires and different % Mn-doped CuO nanowires in deionized water were measured in the wavelength range of 200–800 nm shown in Fig. 5. The band gap calculated for CuO nanowires was [Eg = 2.6 eV], which is higher than the reported value for bulk CuO (Eg = 1.85 eV) [27]. This increase in the band gap of nanowire is attributed to the well-known quantum confinement effects for semiconductors [33,34]. The band gap calculated for 2, 6 and 10% Mn-doped CuO nanowires were 2.3, 2.4 and 2.5 eV as compare to undoped CuO nanowires which was 2.6 eV shown in Table 1. Doping of CuO nanowires with different percentage of Mn metal ions produce significant red shift in band gap as compared to undoped CuO nanowires, which is due to the incorporation of Mn+2 ion at position of Cu+ 2 ion into CuO nanowires, which have greater influence on the recombination time of charge carriers, because Mn+ 2 ion introduce new Fermi level above the valence band, which allows to absorb photon with less energy in the visible region. Therefore, 2% Mn doped CuO nanowires shows higher absorption in the visible region with the suitable band gap of 2.3 eV. With an increase the doping concentration the band gap also increases, as some defects from doping can also cause intra-gap defect states [35]. The band gaps were calculated from the first derivative of absorbance with energy with respect to energy (eV) shown in Fig. 5. In order to check the doping effect on the photovoltaic performance, solar cells were fabricated using Mn-doped CuO nanowires as semiconducting nanoparticles. These cells were subjected to the current voltage measurement by using an artificial solar simulator which irradiates white light of intensity 100 mW cm− 2 (AM 1.5 solar spectrums). When the photoactive material consists of only P3HT, the device did not perform well however using the blend of P3HT and CuO nanowires had a significant performance in the fabricated device. I–V curves of the reference device and devices containing Mn as dopant are shown in Fig. 6. The reference cell shows excellent diode behavior in the dark and it is exhibited by the I–V plot in black. When the solar cells were exposed to light the current density was increased. For 2% doped sample the current density and efficiency of the device was found better than the reference sample Table 2. Whereas for 6% and 10% doped samples the current density as well as the efficiency of the device was much lower. This could be explained as with the increase in concentration of metal, the structural crystallinity deteriorates or probably new phases appear as a result current decreases. Whereas, the slight increase in efficiency of the fabricated device could be related to the elimination of defect emission from the nanowires, however, unfortunately doping leads to growth in oriented attachment which makes the nanowires

Table 3 Catalytic efficiency and rate constant of doped and un-doped CuO nanowires with different dopant concentration. Material

Fig. 7. Nyquist plots of the catalysts in (1:1) 0.5 mM [Fe(CN)6]3−/4− in the frequency range of 10 Hz to 100 kHz at open circuit voltage.

Pure dye with catalyst CuO nanowires 2% Mn doped CuO nanowires 6% Mn doped CuO nanowires 10% Mn doped CuO nanowires

Weight (gm)

Photocatalytic efficiency

Rate constant k (min−1)

0.01 0.01 0.01 0.01

0% 79% 87% 83% 80%

2.5 × 10−20 2.66 × 10−3 5.6 × 10−3 4.59 × 10−3 4.21 × 10−3

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Fig. 8. Photocatalytic decomposition of RhB over different photocatalyst such as, un-doped CuO nanowires and [2, 6, 10%] doped CuO nanowires under visible light irradiation. Reaction conditions: C0 = 1.03 × 10−3 M; Catalyst loading: 0.01 g/L; (b). Determination of the apparent rate constants for RhB degradation reaction over the prepared un-doped CuO nanowires and 2, 6, and 10% Mn doped CuO nanowires.

rough. Such rough surfaced nanowires can develop resistance to the flow of current [36]. Electrochemical impedance spectroscopy (EIS) is an effective technique which allows finding out the interfacial properties of the material. It can be seen from Nyquist plot of CuO, 2% Mn doped CuO and 10% Mn doped CuO using 0.5 mM (1:1) [Fe(CN)6]3−/4− as the redox probe. It can be seen that the Nyquist plots for all these electrodes displayed a typical semicircle in the high frequency region with a linear trend observed in the low frequency region. The intercept of real axis in high frequency region is because of solution resistance. The diameter of semicircle in Nyquist plot is a measure of the charge-transfer resistance (Rct), which controls the electron-transfer kinetics of the redox probe at the electrode surface. This Rct value can be used as a direct and sensitive parameter to depict the interfacial properties of electrode/electrolyte interface [37]. From Fig. 7, it can be seen that the CuO electrode exhibited poor charge transfer kinetics as compared to 10% and 2% Mn doped CuO. While 2% Mn doped CuO exhibits lowest charge transfer resistance among all catalysts. The EIS results ensures the efficient charge transfer in 2% Mn doped CuO as compared to 10% Mn doped and undoped CuO (Table 3). Photocatalytic studies have been carried out for CuO nanowires and different % Mn-doped CuO nanowires for the photo degradation of Rhodamine (RhB) under UV–Visible light irradiation. The UV–Vis spectra of RhB solution after degradation over un-doped Mn-doped CuO nanowires at 25 °C as a photocatalyst under UV–Vis light irradiation for different hours are displayed in Fig. 8. 2% Mn-doped CuO nanowires completely mineralized the RhB solution and no intermediates were observed. The possible reason for the doped Mn+2 in CuO up to an optimal concentrations acts as trap for photo-excited electrons that extended the life time of charge carriers which results in enhanced photocatalytic activity of CuO nanowires. Under UV–Vis exposure, the valence electrons in the Mn2 +/CuO band are excited and jump into conduction band, leaving some holes in the valence band. Subsequently, hydroxide ions generated from water may react with these photo-generated holes to produce hydroxyl free radicals (•OH). On the other hand, the photogenerated electrons also combine with dissolved oxygen molecules to form oxygen radicals and hydrogen peroxide, after which hydroxyl radicals are formed. Further these highly reactive hydroxyl radicals play an important role in photo degradation of organic molecules [38]. The kinetics of degradation was studied for the RhB dye. The degradation reactions of undoped CuO nanowires and Mn doped CuO nanowires with RhB dye exhibited pseudo-first-order kinetics model with respect to the degradation time. The results were nearly consistent with the liner equation [39].  ln

C C0

where C is the concentration at time t, C0 is the initial concentration of dye molecule and k is the reaction constant of the first-order reaction. Linear correlation (R), suggesting that the degradation reaction follows the pseudo-first-order kinetics with respect to RhB shown in the Fig. 8. Initially, photocatalytic degradation rate constant (k) increased with increasing concentration of dopant up to 2% and then decreased by increasing the dopant concentration. Optimal concentrations of doped Mn2+ in CuO act as trap for photo-excited electrons and resulted in enhanced photocatalytic activity of CuO due to increase in the life time of the charge carriers. However, at higher concentration of dopant agent (Mn2+), the charge carriers may recombine through quantum tunneling leading to further decreased photocatalytic activity [40]. The stability test of the photocatalysts was performed by cycling experiments. Almost no change was observed in the dye degradation even after several hours under the light illumination as shown in Fig. 9. Thus, the obtained photocatalysts exhibit good stability toward efficient photocatalytic reduction of Rhodamine B dye. In summary, we have successfully synthesized novel Mn-doped CuO photocatalyst by the wet chemical method. The new nanostructure materials were characterized by different techniques such as UV–Vis and PL spectroscopy, XRD, SEM and EDX. The new developed photocatalyst have higher surface areas and their light absorption extends to the visible region, which can promote their photocatalytic activity. It was found that 2% Mn doped CuO nanowires photocatalyst is much highly

 ¼ −kt

ð1Þ Fig. 9. Various run for the dye degradation over 2% Mn doped CuO.

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