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Photoelectrochemical cell performance Cu doped ZnO photoanode sensitized by xanthene dyes
Deepak Kumbhar1'4, Sarita Kumbhar2, Sagar Delekar3, Rekha Nalawade3, Avinash Nalawade4*
department of Chemistry, Raje Ramrao Mahavidyalaya, Jath, 416 404, affiliated to Shivaji University Kolhapur 416004, MS, India 2Department of Physics, Rajashri Chhatrapati Shahu College, Kolhapur, 416 005, affiliated to Shivaji University Kolhapur 416004, MS, India 3Department of Chemistry, Shivaji University, Kolhapur, 416 004, MS, India 4Department of Chemistry, Lal Bahadur Shastri College, Satara, 415 002, affiliated to Shivaji University Kolhapur 416004, MS, India *avinashnalawaderes@gmail.com
DOI 10.17586/2220-8054-2019-10-4-466-474
In this study transition metal Cu is doped into ZnO framework at 1, 3 and 5 mol% concentrations by sol-gel method and Photoelectrochemical performance under sensitization is recorded. Structural analysis XRD and Raman gives information of wurtzite structure formation without any mislaid peaks. It also informs decrement in crystalline size and lattice parameters as doping level increases. SEM and EDAX provide nono structure formation with appropriate compositions. Optical analysis by FTIR and PL gives peaks at expected positions while DRS UV-visible peaks show humps with red shift due to effect of Cu doping. After structural and morphological study NPs are deposited on conducting glass surface of FTO substrate by doctor blade method and sensitized with mixed xanthene dyes (Eosine Y., Rhodamine B., and Rose Bengal) for 12 h and photoelectrochemical cell performance are recorded under solar simulator under standard AM 1.5 one sun illumination in that 1 % Cu/ZnO photoanode shows good performance as compared to other.
Keywords: Sol-gel, Cu doped ZnO NPs, xanthene dyes, sensitization, photoelectrochemical cell. Received: 2 August 2019 Revised: 10 August 2019
1. Introduction
The role of energy is crucial in the human development in all areas of human activity, such as economic, ecosystem, employment, prosperity and equity. For many years, human have utilized several resources to generate energy and fulfil all needs for better life. But, currently, because of increased population the use of traditional resources for energy are not adequate and this creates an issue of global warming [1,2]. Thus, humanity is continuously in the search of alternative energy sources with environmental friendliness. Many research groups already worked on solar, wind, hydro, bio and geothermal energies and catch beneficial outcomes [3-5]. Still this era is under increased pressure to obtain satisfactory outcomes. Solar energy is considered to be more beneficial as compared to other renewable energy sources due to its key benefits of clean, non-polluting, noise less, low maintenance, long life etc. [6,7]. To acquire this energy role of PV devices are originated. In that solar cell play role to convert solar radiation into electricity. The desire of human fabricated first to fourth generation solar cells with diverse materials and technologies. In the market silicon based solar cells existing but it has limitation to use in wide range due to environmental and cost consequence. O'Regan and Gratzel in 1991 invented low cost and simple construction based dye sensitized solar cell (DSSC). In comparison to silicon cells, it is still in a state of growth to get high efficiency and stability. Researchers are currently working on this and have formulated a number of nanomaterials [8].
In DSSC type of photoanode, electrolyte and sensitizer are crucial to bring beneficial outcome. TiO2 is typically used photoanode material however it has limitations of carcinogenic nature, lower electron lifetime and transport rate [9-11]. So, research groups worked on several alternative materials. ZnO is another choice with parallel properties of TiO2 having wide band-gap of 3.37 eV at room temperature, 60 meV of electron binding energy [12,13], higher electron mobility, anti-oxidation and chemically stability. However, considerable research has revealed lower performance of ZnO based DSSC due to some limitations of ZnO. To circumvent these limitations, different strategies have been developed such as modified fabrication process, utilization of different morphological materials, mixing of other nanomaterial, utilization of new dyes, co-sensitization method, material increased porosity and use of varied electrolyte in DSSC. Doping is one more strategy that offers an effective means to enhance and control the structural, optical and electrical properties of ZnO NPs. Already different dopants (Al, Mn, Cd, Mg, Ni, Ga, Ag, Cr etc.) in the lattice structure of ZnO utilized to boost its properties [14-18]. Doping is one of the methods to modify the band
gap of ZnO and shift to the large visible spectrum of light by creating energy levels inside the band gap [19-21]. Our study focused on doping of Cu due to its large solubility in ZnO matrix, rich electronic states and close ionic radius. Until now, various researchers studied and reported effect of Cu doping on ZnO with respect to morphological and optical properties [22-27]. Assimilation of Cu dopants in ZnO is carried out at low level i.e. 1, 3 and 5 mol% for decrement of band gap and to improve electron shifting. This was successfully carried out by simple sol-gel approach. After structural, morphological and optical analyses, photoanodes were prepared by reported doctor blade technique. To improve DSSC performance and reduce Zn2+-dye aggregation photoanodes were sensitized in metal free sensitization in xanthene dyes and the photoelectrochemical cell performance measured under solar simulator standard AM 1.5 one sun illumination visible illumination of 100 mW/cm2. Among employed photoanodes 1 % Cu/ZnO shows notable efficiency of n = 0.1165 % and proves fruitful effect of doping and Co-sensitization.
2. Experimental section
2.1. Materials
All the chemicals i.e. zinc acetate dihydrate (Sigma-Aldrich), copper acetate monohydrate (Sigma-Aldrich), sodium dodecyl sulfate (SDS of SD Fine), ammonium hydroxide (SD Fine) used in the experiment were of analytical grade without any prior treatment.
2.2. Synthesis of Cu doped ZnO NPs by sol-gel method
For this 0.2 M zinc acetate dihydrate (for doping with stoichiometric doping amount of copper acetate monohydrate) solution was prepared and 0.02 M of SDS mixed into it with continuous stirring. To this solution ammonium hydroxide was added drop wise till pH of solution becomes near to 8.0 ± 0.1. Here formation of zinc hydroxide sol takes place, which was further dried at 80 °C for 9 h to get gel. The obtained gel was calcinated at 350 °C for 3 h in a muffle furnace, results in Cu doped ZnO NPs.
3. Results and discussion
3.1. X-ray diffraction analysis
The XRD spectra (Fig. 1) indicate pure and Cu doped (1,3 and 5 mol%) ZnO NPs. XRD patterns of both undoped and Cu doped samples were found to be exactly same. This data was found to be of hexagonal wurtzite phase and JCPDS card No. 36-1451 [25,28,29]. The peak intensities of (100), (002), (101), (102), (110), (103), (200), (112), (201), (004) and (202) in the XRD spectrum are slightly decreased as doping concentration increases this can be also attributed to the replacement of Zn+2 by Cu+2. The crystalline size of Cu doped ZnO NPs were obtained by fitting the XRD data to the Debye Scherrer formula and full-width at half-maximum (FWHM) of the XRD lines [30]:
D = ^ <■>
p cos 0
where A is the wavelength of X-ray (1.5406 A), p is the full-width at half-maximum in radian, and 0 is the angle of diffraction. It is seen that average crystalline size (D) decreased from 60 to 31 nm as Cu doping. The variation in
_Ja | S'i. Cu/ZnO
1 li [i II 3% Cu.'ZnO L. ! Jl / .A,
1% Cu'ZnO J Jl » A.
ZnO 1 1 ¡1 I . A.
20 30 40 5 0 6 0 70 80 90
20 (degree)
FIG. 1. XRD patterns of pure ZnO and 1,3,5 mol% Cu doped ZnO Nanoparticles
the grain size by doping ZnO with Cu2+ up to 5 mol% is mainly due to the alteration in the host ZnO lattice, which decreases the nucleation and subsequent growth rate by the addition of Cu concentrations up to 5 mol% as same in some reference [26, 27]. By increasing the Cu content, the carrier concentration and mobility in the conduction band of the semiconductor also increases [31]. The lattice parameters (a, c), volume (V), c/a ratio and crystalline size obtained are given in Table 1. It conclude that values of lattice parameters and volume slightly decreases as doping contartion increases from 1 to 5 mol%. This reduction is due to successful replacement of Zn2+ with ionic redii 0.74 A by Cu2+ with ionic redii 0.73 A [32-35].
Table 1. Lattice constant a, c volume of unit cell (V), ratio (c/a) and crystallite size (D) of ZnO
and Cu/ZnO sample
Samples
Lattice Parameter (A)
Volume (A3)
c/a ratio
Crystalline size (D) nm
ZnO 1 % Cu/ZnO 3 % Cu/ZnO 5 % Cu/ZnO
2.9495 2.9441 2.9440 2.9336
5.1065 5.0993 5.0961 5.0811
44.4244
44.2023 44.1696 43.7300
1.732 1.732 1.732 1.732
60 40 37 31
a
c
3.2. Raman spectroscopy
Raman spectra in the range of 250 - 600 cm-1 of doped and undoped ZnO are showed in Fig. 2. In this range, there are five main bands at 326, 400, 433, 572, 589 cm-1, corresponding to Ai(TO), Ei(TO), E2(high), Ei(low) and Ai (low) for ZnO. The strong and sharp band observed at 433 cm-1 corresponds to the nonpolar optical phonons E2 (high) mode of ZnO attribute that incorporation of Cu in ZnO leads to a decrease in crystal quality, but there is no change in wurtzite crystal structure. The features located at 326 and 400 cm-1 correspond to the multi-phonon scattering process E2 (high)-E2 (low) and A1 (phonons of ZnO crystal, respectively). The signal located at 572 cm-1 could be attributed to the E1 , longitudinal optical phonon (LO) feature, associated with the formation of defects such as oxygen vacancy. According to the literature, as the Cu doping concentration was increased, intensities of spectra decreased and the A1 transverse optical phonon (TO) mode vanished [36]. It could be successfully explained in terms of resonant anharmonic interaction of the high E2 mode with a band of combined transverse and longitudinal acoustic modes, as the steep variation of the two-phonon density of states around the high E2 frequency leads to a distorted phonon line shape [37,38].
Fig. 2. Raman spectra of the ZnO and Cu doped ZnO NPs
3.3. Scanning electron microscope (SEM) and Energy Dispersive X-Ray Analysis (EDX)
Figure 3 showed that the ZnO and Cu doped ZnO NPs are in regular or almost in granular shape, though some agglomerations have been observed. It was observed that doping of Cu in ZnO slightly affect the size of ZnO NPs, as also suggested by XRD study. It is due to less ionic radii (0.87 A) of Cu than Zn (0.88 A) [31]. Fig. 3(b) shows the EDAX spectra of ZnO and Cu doped ZnO nanoparticles. The elements of Zn, O and Cu in doped samples clearly observed. The Compositions are given below (Table 2).
Fig. 3. SEM images of 1, 3 and 5 % Cu/ZnO doped ZnO NPs (a); EDAX images of 1, 3 and 5 % Cu/ZnO doped ZnO NPs (b)
Table 2. Composition of ZnO and Cu doped ZnO nanoparticles
Sample O (Wt %) Zn (Wt %) Cu (Wt %)
ZnO 16.84 83.16 —
1 % Cu/ZnO 17.12 82.10 0.87
3 % Cu/ZnO 16.25 81.15 2.60
5 % Cu/ZnO 16.55 79.05 4.40
3.4. Fourier transform infrared spectroscopy (FTIR) studies
The broad absorption peaks in Fig. 4 around 3500 cm-1 attributed to normal polymeric O-H stretching vibration of H2O in Cu-Zn-O lattice and the weak bands from 500 to 881 cm-1 are assigned due to the change in the microstructural features Cu doping into Zn-O lattice that shifts frequencies. Copper atom is slightly lighter than Zn atom so, according to the well-established theories of vibrational modes in mixed crystals the substitution should result in an upward shift of the fundamental transverse optical phonon mode. The frequency shift towards the lower side reveals the substitution of Cu2+ ion into the Zn-O lattice [39,40].
3.5. Photoluminescence spectroscopy
ZnO and Cu doped ZnO PL spectra in Fig. 5 displays peaks at ^300 nm excitation wavelength, in the UV range, which is associated with exciton emission i.e. band edge emission (NBE), another in the visible range at around ^600 nm, which originates from electron-hole (e-h) recombination at the deep level (DLE) caused by oxygen vacancy or zinc interstitial defect [29,41]. The lower intensity of UV emission in ZnO than Cu doped ZnO indicated suppression in recombination of photogenerated charge carriers that enhances optical properties [42,43]. Here 1 % Cu/ZnO samples showed lower recombination rate as compared to other samples.
Fig. 4. ATR-FTIR spectra of ZnO and Cu doped ZnO NPs
Fig. 5. Photoluminescence spectra of ZnO and Co doped ZnO NPs
3.6. UV-Visible diffuse reflectance spectroscopy studies (DRS UV-Visible) and Tauc plots
The UV-Vis absorption spectra of pure and Cu doped ZnO samples were recorded and presented in Fig. 6(a) The spectra shows increase in absorption ability and red shift with humps as incorporation of Cu content from 1 to 5 mol%. It can be interpreted that there is the formation of new energy levels in the band gap, thereby leading to spectral red shift [23,44]. Therefore the red shift of Cu doped ZnO helps to electron transfer from Zn3d to O2p. It is observed that the doping effect narrows or shrinks the band gap of ZnO as increasing Cu doping densities.
Fig. 6. UV-Visible diffuse reflectance spectrum of ZnO and Cu doped ZnO NPs (a); Tauc plots for band gap determination (b,c)
The Tauc plots Fig. 6(b, c) are drawn using Tauc equation (ahv)2 = A(hv — Eg), where, v is the frequency of light, A is a constant, h is the Planck's constant and Eg is energy band gap of the prepared samples. By extrapolating the line we measure the band gap of prepared ZnO NPs. The computed band gap values are 3.16 for ZnO and 2.84, 2.83, 2.78 eV for 1, 3 and 5 mol% Cu/ZnO respectively i.e. the band gap decreases from 1 to 5 mol%. The absorption edge of samples were associated to the presence of the acceptor level produced by Cu, over the valance band of ZnO [31,45].
4. Preparationand investigation of dye solution
It is reported that ZnO DSSC are almost unstable in acidic and metal based dyes. Here, we applied sensitization with organic xanthene dyes contain Rose Bengal, Eosin Y. and Rhodamine B. dye. 1:1 proportion of acetonitrile and tert-butanol used to prepare dye solution. UV-Vis absorption investigation Fig. 7 show absorption in visible region at 518, 530, and 535 nm for Eosin Y., Rose Bengal, and Rhodamine-B. Individual dyes have limited absorption so we used mixed dye concept that gives better absorption in the range of 500 - 600 nm which is confirmed by its absorption study [46]. So such sensitization solution is prepared by equal mixing of dyes and sensitization of photoanodes are done.
Fig. 7. Optical absorbtion spectra study of single Eosine Y., Rose Bangal, Rhodamine B. dyes and mixed dye in tert-butyl alcohol/acetonitrile
5. Fabrication of photoelectrochemical cell and I-V measurements
5.1. Construction of photoelectrode
Photoelectrodes are prepared by simple doctor blade method [47]. Here NPs were sonicated in alcohol and ethyl cellulose for 30 min. Later a terpineol was added (Sigma-Aldrich) to obtain a better suspension. This suspension deposited on FTO glass surface at conducting side and annealed it at 350 K for 2 h.
5.2. Construction of cell and I-V measurement
The prepared photoanodes are sensitized in dye solution for 12 h and washed with alcohol to remove excess dye. The photoanode of average area 1.0 cm2 act as working electrode and Pt-coated FTO of similar area were used as the counter electrodes. I-V measurements are taken under under solar simulator standard AM 1.5 one sun illumination visible illumination of 100 mW/cm2 using a Keithley source meter (model 2460).
First ZnO photoanode without sensitization directly employed in electrolyte solution of 0.1 M lithium iodide and 0.05 M iodine in propylene carbonate then results recorded in dark as well as in light. Later sensitization of ZnO and Cu doped ZnO samples for 12 h carried out and alter I-V measurements are taken Fig. 8. All outcomes i.e. open circuit voltage (Voc), short circuit current density (Isc) and fill factor (FF), efficiency in percentage (n %) are recorded in tabular form in Table 3. It displays worthy efficiency after sensitization i.e. the highest efficiency obtained for 1 % Cu/ZnO of n = 0.1156 % and other with notable efficiencies 0.0438 and 0.0471 % for 3, 5 % Cu/ZnO respectively.
Fig. 8. Photocurrent-voltage curves in I /I3 redox electrolyte solution under dark and visible light for: (a) ZnO/dye; (b) 1% Cu/ZnO; (c) 3% Cu/ZnO; (d) 5%Cu/ ZnO with dye
Table 3. Photoelectrochemical parameters of the cell
Sample Jsc (mA/cm2) Voc (V) FF n %
ZnO without dye 0.065 0.262 0.27 0.0045
ZnO/dye 0.211 0.278 0.62 0.0363
1 % Cu/ZnO 0.98 0.319 0.37 0.1156
3 % Cu/ZnO 0.277 0.352 0.45 0.0438
5 % Cu/ZnO 0.236 0.256 0.78 0.0471
6. Conclusion
We effectively employed the effective sol gel technique for the synthesis of Cu/ZnO NPs. The XRD and Raman analysis clearly indicated that Cu/ZnO samples possess a hexagonal wurtzite crystal structure. It also revealed the slight decrement of particles and lattice parameters as Cu doping increses in low concentration. Here, nanograin morphologies are formed with respective elemental compositions. FT-IR studies substantiate the presence of the ZnO peaks into the samples with upward shifting due to Cu doping. Photoluminescence (PL) analysis of all samples of ZnO displays peaks at ^300 nm in the UV range i.e. band edge emission (NBE), and at ^600 nm in the visible region i.e. deep level emission (DLE). The UV-Vis absorption spectra depicts an increase in absorption ability and red shift with humps as incorporation of Cu content from 1 to 5 mol% while Tauc equation (ahv)2 = A(hv — Eg), computed band gap values indicating band gap decrease from 1 to 5 mol% doping i.e 3.16 for ZnO and 2.84, 2.83, 2.78 eV for 1, 3 and 5 mol% Cu/ZnO respectively. I-V measurements of PEC cell is studied that gives notable results with
highest efficiency of n = 0.1156 % for 1 % Cu/ZnO photoanode with Jsc = 0.98 mA/cm2 and Voc = 0.319 V. ZnO photoanode without dye sensitization show low n % value, but when it is sensitized, high increment seen for ZnO and 1 % Cu/ZnO but later for 3 and 5 % values found to be decreased. It is may be due intense color of films for high doping that may affect absorption of photon even after sensitization. Photoluminescence studies also indicate higher electron hole recombination for higher doping These outcomes still very low but doping and co-sensitization such working strategy may give another way to enhance DSSCs outcome in the fast growing solar energy sector.
Acknowledgements
This research was made possible by support given by DST-FIST Analytical Instrumentation Laboratory of Lal Bahadur Shastri College, Satara and Jaysingpur College, Jaysingpur, Maharashtra, India.
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