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Facile growth of ZnO nanowire arrays and nanoneedle arrays with flower structure on ZnO-TiO2 seed layer for DSSC applications
Accepted Manuscript Facile growth of ZnO nanowire arrays and nanoneedle arrays with flower structure on ZnO-TiO2 seed layer for DSSC applications T. Marimuthu, N. Anandhan, R. Thangamuthu, S. Surya PII:
S0925-8388(16)33023-7
DOI:
10.1016/j.jallcom.2016.09.260
Reference:
JALCOM 39094
To appear in:
Journal of Alloys and Compounds
Received Date: 26 April 2016 Accepted Date: 23 September 2016
Please cite this article as: T. Marimuthu, N. Anandhan, R. Thangamuthu, S. Surya, Facile growth of ZnO nanowire arrays and nanoneedle arrays with flower structure on ZnO-TiO2 seed layer for DSSC applications, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.09.260. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical abstract
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NNAs
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NWAs
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Facile growth of ZnO nanowire arrays and nanoneedle arrays with flower structure on ZnO-TiO2 seed layer for DSSC applications
a
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T. Marimuthua, N. Anandhana*, R. Thangamuthub and S. Suryab Advanced Materials and Thin Film Physics Lab, Department of Physics, Alagappa
University, Karaikudi-630 003, India. b
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Electrochemical Materials Science Division, CSIR-Central Electrochemical Research
Institute, Karaikudi-630 003, India.
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E. mail: [email protected] Abstract
In this present investigation, zinc oxide-titanium oxide (ZnO-TiO2) thin films were prepared by sol-gel spin coating technique and used as a blocking as well as seed layer, and
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followed by, ZnO nanowire arrays (NWAs) and nanoneedle arrays (NNAs) were synthesized on ZnO-TiO2 seed layer by facile hydrothermal technique by varying some parameters such as solution concentration, growth time and growth temperature. They are used as a photoanode for
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dye sensitized solar cell (DSSC) and characterized by spectroscopic and microscopic technique to investigate about the crystal structure, morphology and optical properties. The presence of
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hexagonal wurtzite structure of the prepared nanostructures grown along with (002) plane was confirmed by using X-ray diffraction (XRD), Raman spectra and transmittance electron microscope (TEM). NWAs and NNAs were observed with flower like structure consisting of nanowires and nanoneedles, respectively by field emission scanning electron microscope (FESEM). UV-Vis spectra imply that NNAs have a better light absorbance with more dye loading than NWAs. It could be found from current density-voltage (J-V) curve that the photovoltaic 1
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conversion efficiency of the NWAs and NNAs based DSSC is 0.91% and 1.47%, respectively. Electrochemical impedance spectroscopy (EIS) shows that DSSC based on NNAs photoanode has a better electron lifetime with less electron recombination than DSSC based on NWAs
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photoanode.
Keywords: zinc oxide-titanium oxide, nanowire arrays, nanoneedle arrays, dye sensitized solar
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cell, flower like structure. 1. Introduction
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Dye sensitized solar cells (DSSCs) have been serving for last two decades as environmentally friendly [1]. They are also of more attention due to their easy fabrication, low cost and facile accessibility. DSSC is one of the third generation solar cells, which has a considerable higher indoor efficiency than silicon solar cell [2]. Generally, DSSC consists of
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mesoporous TiO2 nanoparticles sensitized with N719 dye used as a photoanode and platinum (Pt) coated fluorine doped tin oxide (FTO) glass substrate as a counter electrode, which are separated by (I-/I-3) electrolyte [2].
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Titanium oxide (TiO2) is most known material for photoanode, and TiO2 mesoporous based DSSC has exhibited efficiency of more than 12 %. However, it is difficult to further
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increase the current limit of the efficiency. Hence, Zinc oxide (ZnO) is the most attractive and alternative material to TiO2. It is an n-type semiconductor material and has an exciton binding energy of 60 meV with band gap energy of 3.37 eV. ZnO has high electron mobility with similar electronic band structure compared to TiO2 [3]. Most of the research groups have devoted more time to synthesize different novel structures such as nanoparticle (zero dimensional) [4], nanorod or nanowire [5], nanotube (one dimensional) [6], nanosheet (two dimensional) [7] and 2
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microsphere particle (three dimensional) [8]. The efficiency of the DSSC depends on morphology of the photoanode [9]. The photoanode should posse high electron mobility, high charge collection, good charge separation, large surface area and high light scattering ability. In
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this case, the nanoparticle based DSSC has large surface area to more dye loading. Although it has high surface area, the efficiency of the DSSC is low owing to presence of large number of grain boundary, which significantly affects the electron mobility. The poor electron mobility
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leads to recombination between back electrons transfer and oxidized dye molecules or oxidized species in electrolyte [10]. In order to retard these types of recombination, some researchers have
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synthesized one dimensional (1D) nanostructures such as nanowires and nanorods which offer direct pathway to injected electron to reach the FTO conductive substrate. The high electron mobility of the 1D nanostructure has inhibited the recombination between back electrons transfer and oxidized dye molecules or oxidized species in electrolyte. 1D nanowire based DSSC has
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achieved low efficiency compared to nanoparticles based DSSC. This is due to the limited surface area to dye loading, instability, poor coverage of nanowire on FTO substrate which causes recombination between injected electron from uncovered FTO substrate and oxidized
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electrolyte species or oxidized dye molecules [10]. To prevent this type of recombination, Wang et al introduced TiO2 blocking layer between FTO substrate and ZnO seed layer. Eventhough,
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they used the blocking as well as seed layer, the efficiency of the DSSC is low [11]. Further to enhance the conversion efficiency, some of the researchers have employed scattering layer to improve the light absorption through multi-scattering of light within photoanode, and they have significantly increased the efficiency of the DSSCs [12-14]. The various nanostructured ZnO thin films have been synthesized by chemical methods such as electrochemical deposition [15], spin coating [16], chemical bath deposition [17] and hydrothermal technique [18]. Among the 3
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various methods, spin coating and hydrothermal are the best methods to synthesize the blocking as well as seed layer and 1D nanostructure for light scattering on account of low cost and easy
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scalable. In the present work, we have simply prepared ZnO-TiO2 thin film by sol-gel spin coating technique. The prepared thin film is used as a blocking as well as seed layer. 1D nanostructure has further synthesized on seed layer coated FTO glass substrate in viscous solution. By
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changing solution concentration, growth time and growth temperature, the dense and upright
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nanowire arrays (NWAs) and nanoneedle arrays (NNAs) with flower like structure are synthesized to increase the light scattering ability for higher light absorption. The structural, morphological and optical properties of the nanostructures are investigated. The DSSCs are constructed using NWAs and NNAs respectively as a photoanode. The current density verse voltage and electrochemical characterization are carried out. NNAs show better conversion
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efficiency with less recombination between photoanode/electrolyte interface and higher electron lifetime than that of the NWAs.
2.1 Materials
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2. Experimental details
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Fluorine doped tin oxide (FTO) conductive substrate, zinc nitrate hexahydrate and
polyvinyl alcohol were purchased from Sigma Aldrich. Zinc acetate dehydrate and titanium oxide were purchased from Merck. Ruthenium dye (N719), surlyn spacer and electrolyte were purchased from Solaronix. Ethanol was purchased from Alfa Aesar. 2.2 Photoanodes preparation
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ZnO nanostructures were synthesized via two-step process. First step, ZnO-TiO2 seed layers were prepared onto FTO conductive substrates by spin coated technique. The 0.3 M of zinc acetate dehydrate (90%) and titanium oxide powder (10%) was dissolved and dispersed in
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isopropyl alcohol. Followed by, 0.3 M of monoethanolamine was added as a stabilizer. The solution was continuously stirred at 60˚C for 2 hours to get the sol. ZnO-TiO2 thin film was coated at 3000 rpm for 20 sec. The coated film was eventually annealed at 450˚C for one hour
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(The detailed results and discussion of ZnO-TiO2 seed layer are given in ESI†). Before seed layers deposition, FTO substrates were ultrasonically rinsed with acetone and deionized water
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for 10 minutes separately. Thereafter, the rinsed FTO substrates were baked at 100˚C in air for 10 minutes. Second step, the nanostructures were synthesized on ZnO-TiO2 seed layer by hydrothermal technique. An aqueous growth solution consisting of zinc nitrate hexahydrate (0.01 M), 0.1% of polyvinyl alcohol (0.01 M) and hexamethylenetetramine (0.01 M) were dissolved in
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deionized water. The ZnO-TiO2 seed layer was immersed into bottle contained solution at 45˚ angle to avoid any unwanted precipitation from top of the solution on growing ZnO nanostructures. The temperature of growth solution was maintained at 80˚C. In order to synthesis
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the well defined nanostructures, the concentrations, growth times and growth temperatures were varied about 0.005-0.05 M, 80-95˚C and 3-5 hours, respectively. The synthesized ZnO
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nanostructures were eventually rinsed with ethanol and deionized water. Afterward, they were baked at 100˚C for 30 minutes in air atmosphere. 2.3 DSSCs construction
The synthesized ZnO NWAs and NNAs films were soaked into 0.3 mM of N719 dye solution prepared in ethanol for 3 hours in a dark room. Thereafter, the dye loaded photoanode films were rinsed with ethanol to remove the unbounded dye molecules. The Pt electrodes were 5
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prepared on FTO by spin coating at 500 rpm for 30 sec using a 5 mM solution of chloroplatinic acid in isopropyl alcohol, followed by the coated films were baked at 100˚C for 10 minutes. Once again the procedure was repeated. Finally, the pre-heated Pt electrodes were annealed at
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400°C for one hour. The dye loaded ZnO nanostructured photoanodes and Pt electrodes were sandwiched between 60 µm thicknesses of surlyn spacer, and then they were holed by clamp. The active area of the cells was 0.20 cm2, Liquid electrolyte was filled into active area by
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pouring a few drops through the hole on Pt electrode, which contained a mixture of 0.6 M 1methyl-3-propylimidazolium iodide, 0.1 M lithium iodide, 0.05 M iodide and 0.5 M tert-
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butylpyridine. Finally, the hole was sealed with surlyn film. 2.4 Characterizations
X-ray diffraction (XRD) patterns were recorded to find out the crystal structure of the nanostructures by X’Pert PRO PAN analytical powder X-ray Diffraction meter. The
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morphologies of the nanostructures were observed using filed emission scanning electron microscopy (FE-SEM, FEG Quanta 250) and transmission electron microscope (TEM, Tecnai instruments). Raman spectra were performed to observe the vibrational modes and crystal phase
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by micro Raman spectroscopy at an excitation wavelength of 514.5 nm (Princeton instrument Acton sp 2500). Viscosity of the solution was measured by Oswald viscometer. Optical
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properties and dye loading of the optimized nanostructures were measured using UV-Vis spectroscopy (Perkin Elmer’s LAMBDA 35). Thickness of the films was measured by a Stylus profilometer. Current density-voltage (J-V) curves were recorded using Keithley 2400 for constructed DSSCs under the illumination of 100 mW light from Xenon lamp. Electrochemical impedance spectra (EIS) were performed under the dark condition by µ-autolab type III
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Potentiostat/Galvanostate in the frequency range of 500 kHz – 0.1 Hz at an open circuit voltage of 400 mV.
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3. Results and discussion 3.1 Growth Mechanism
ZnO nanostructures are hydrothermally synthesized based on following reactions [19,20].
4NH3 + 6HCHO NH4+ + OH-
NH3 + H2O
Zn2+ + 2OH-
Zn(OH)2 (s)
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(CH2)6N4 + 6H2O
(1)
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Zn2+ (aq) + 2NO3- (aq)
Zn(NO3)2 (s)
ZnO (s) + H2O
(2)
(3)
(4)
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Zinc nitrate (Zn(NO3)2) salt dissociates into Zn2+ (aq) and NO3- ions in deionized water. Hexamethylenetetramine (CH2)6N4) reacts with water (H2O) and gives formaldehyde (HCHO)
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and ammonia (NH3), respectively and it acts as a pH buffer to provide the controllable supply of OH- ions through the slow decomposing. Further, NH3 reacts with H2O and forms ammonium
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hydroxide (NH4OH) which also gives OH-. This OH- complex with Zn2+ forms zinc hydroxide (Zn(OH)2). Eventually, Zn(OH)2 dehydrates into ZnO. Polyvinyl alcohol (PVA) does not react with any compound in the reaction process. In this study, PVA is used to prepare the viscous solution for grow the nanostructures. Viscosity of the solution is found to be 1.31 Pa.s -----Insert Figure 1-----
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3.2 Effect of concentration 3.2.1 X-ray diffraction analysis
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-----Insert Figure 2----Crystal structure of the ZnO nanostructures grown at different concentrations at constant temperature of 80˚C was analyzed by X-ray diffraction (XRD) pattern. As shown in Fig. 2, the
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diffraction peaks are indexed by black dot symbol corresponding to F doped SnO2 substrates, all other peaks except peak indicated by star symbol can be indexed to hexagonal wurtzite structure
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and good concordance with International Center for Diffraction Data (ICDD) card no 36-1451. The prominent diffraction peaks 2θ angles at 31.68, 34.34 and 36.16 correspond to (100), (002) and (101) planes, respectively. The weak peak (indicated by star symbol) appeared at 25.7 is due to the presence of TiO2 anatase phase in seed layer for nanostructure synthesized at a
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concentration of 0.01 M, which is well coincided with ICDD card no.89-4921. When the concentration of growth aqueous precursor is increased from 0.01 M to 0.05 M, the intensity of the diffraction peaks is also increased, which means crystallinity of the nanostructure is good at
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higher concentration. In addition, the intensity of the (002) plane is increased with respect to concentration, which implies that the crystalline is grown along the C-axis to surface of the
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substrate [15]. The other diffraction peaks reveal that the crystalline is grown at random orientation or at their preferential orientation. 3.2.2 Scanning Electron Microscope -----Insert Figure 3-----
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The surface morphologies of the nanostructures prepared at different Zn2+ concentrations were observed by field emission scanning electron microscope (FE-SEM) and its images are shown in Fig. 3. Fig. 3(a, a1) shows nanowires prepared at a concentration of 0.005 M (Zn2+). It
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can be clearly seen that some nanowires are agglomerated and found less dense as shown in Fig. 3(a1). The diameter and length of the nanowires are varied in the range between 30-111 nm and 300-485 nm, respectively. Further the concentration of Zn2+ is raised to 0.01 M, nanowire arrays
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(NWAs) with hexagonal wurtzite structures are vertically and densely grown with diameter of 61-103 nm on ZnO-TiO2 seeded FTO plate (inset image of Fig. 3(b)), which is good concurrence
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with (002) plane in XRD result. The NWAs can favor for higher electron mobility. A few flowers like structures are appeared as shown in Fig. 3(b) and compose of large number of nanowires which are grown with different orientation as shown in Fig. 3 (b1). The random orientation of nanowires is confirmed by the presence of other satellite planes (100), (101),
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(102), (110), (103) and (112) in XRD pattern. Flower consisting of randomly oriented nanowire can be acted as a light scattering center to absorb more number of photons [12]. The diameter and length of the NWAs are changed in the range of 42-47 nm and 202-504 nm, respectively. As
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the concentration of Zn2+ is further increased to 0.05 M, SEM images of Fig. 3(c, c1) show similar structure, but the significant changes are occurred in diameter of the NWAs in the range
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between 123-382 nm as shown in Fig. 3 (c). The increased diameter of the NWAs decreases surface area of the film. In addition, a few nanotubs are randomly grown whose diameters are changed between 0.7-1.263 µm. 3.2.3 Raman spectroscopy -----Insert Figure 4-----
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Fig. 4 shows Raman spectra of 1D nanostructures prepared at different precursor concentrations. It is the best tool to study the crystal phase, crystallinity and defect of the materials. Raman peak at 636 cm-1 corresponds to Eg mode, which is ascribed to the tetrahedral
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anatase phase of TiO2. Raman peaks are appeared at 332, 437 and 580 cm-1 owing to E2L-E2H, E2 (high), and E1 (LO) mode, respectively. The small peak centered at 332 cm-1 is assigned to second order non-polar mode of Raman scattering. The peak at 580 cm-1 is found to be presence
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of oxygen vacancies (or) Zn interstitials. The high intense peak around at 437 cm-1 is attributed to hexagonal wurtzite structure and non-polar optical mode of ZnO, which is owing to the heavy
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vibration of an oxygen atom. The intensity of the E2 (high) mode is significantly enhanced as the concentration of the growth solution is increased from 0.005 to 0.05 M as shown in inset Fig. 4. This is an indication of good crystallinity of the prepared nanostructures which is good in accordance with XRD results and SEM images [21]. According to Raman selection rule, the
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absence of A1 (TO) and E1 (TO) modes represents that the synthesized NWAs are grown perpendicular to seed layer coated FTO substrate [22]. This can be confirmed from the presence of high intense (002) plane in XRD pattern. The Raman peak is appeared in the range of 1050-
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1200 cm-1due to the multi phonon process [23].
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3.3 Effect of growth time and temperature 3.3.1 X-ray diffraction analysis -----Insert Figure 5-----
Fig. 5G shows XRD patterns of the nanostructures prepared at constant temperature of 80˚C at different time. All the diffraction peaks (except peaks indicated by black dot and star symbol) are indexed to hexagonal wurtzite structure and also in good agreement with ICDD card no. 3610
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1451. The small peak observed at 25.7 is owing to presence of TiO2 anatase phase. As the growth time is increased from 3 to 5 hours, the intensity of the (002) plane is remarkably increased as shown in Fig. 5G(d). It confirms that crystallinity of the nanostructures is enhanced as the
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growth time is increased. From this XRD results, we could confirm that the crystalline nature is higher for nanostructure grown at 5 hours than 3 hours. Fig. 5H shows XRD patterns of microstructures grown at different growth temperatures for 3 hours in 0.01 M solution of zinc
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nitrate hexahydrate. When the growth temperature is raised from 80 to 95˚C, the intensity of the ZnO peaks are dramatically decreased and FTO peaks are increased as shown in Fig. 5H(e). It
3.3.2 Scanning Electron Microscope
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can be confirmed that film growth at higher temperature is less.
-----Insert Figure 6-----
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Fig. 6(d, d1) displays a nanoneedle arrays (NNAs) like structure grown for 5 hours at 80˚C. The flowers like structures are observed at a few places in prepared thin film as shown in Fig. 6(d). On other hand, the NNAs are vertically with densely grown along the C-axis to ZnO-TiO2 seeded
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substrate, which is well consent with high intense (002) plane in XRD pattern. The flower like structure is composed of large number of nanoneedles which are attached at one big grain and a
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few nanoneedles are combined to form a bigger one. The nanoneedles show length, diameter of the tip and base in the range of 600-830 nm, 40-70 nm and 60-160 nm, respectively. The flower like structures having nanoneedles act as the light scattering center for higher light absorption. From this observation, when the growth time of the nanostructure is increased from 3 to 5 hours, the morphology change is occurred from NWAs to NNAs and also increased the length and diameter. SEM image of the Fig. 6(e) presents less dense nanorods as the growth temperature is 11
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raised about 95˚C. The rolling comb like structures are appeared which compose the vertical nanorods attached on a big microrod as shown in Fig. 6(e1). The vertical nanorod is to be a
3.3.3 Raman spectroscopy -----Insert Figure 7-----
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length and diameter of 326 nm and 71 nm, respectively.
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Raman spectra of Fig. 7G display E2L-E2H, E2 (high) and E1 (LO) modes corresponding to hexagonal wurtzite structure of ZnO. Eg mode is observed due to the tetrahedral anatase phase of
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TiO2. As the growth time is extended from 3 to 5 hours, the intensity of the E2 (high) mode is only increased as shown in inset Fig.7G, it can be confirmed that crystallinity of the NNAs is dramatically enhanced compared to NWAs structures. This is good in accordance with XRD pattern and SEM images [21]. Fig. 7H shows additional peaks appeared at 395 and 513 cm-1
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corresponding to B1g and A1g mode which are originated to the tetrahedral anatase phase of TiO2 [24]. When the growth temperature is raised about 95˚C, the intensity of the E2 (high) mode is significantly elevated as shown in Fig. 7H(e), which is due to increasing crystallinity of the
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microstructure compared to nanostructures. Even, XRD result showed less crystalline nature.
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3.3.4 Transmission electron microscopy (TEM)
Transmission electron microscope (TEM) was analyzed to study the crystalline structure
and particle size of the NNAs. The NNAs were scraped from thin film and dispersed in isopropanol. Thereafter, the solution was sonicated for 10 minutes. Fig.
8(d)
displays particle size of 10-15 nm which is found that after sonication, the NNAs are broken into 12
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different particle size. Fig. 8(d1) corroborates that the nanoparticles have a well defined lattice space of about 2.6 Å which is well matched with lattice space of the (002) plane of the hexagonal wurtzite structure. This is an indication of NNAs grown along the c-axis to the seed layer.
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Selected area electron diffraction (SAED) pattern proves that the prepared NNAs are polycrystalline nature as shown in inset Fig. 8(d1) [20].
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3.3.5 UV-Visible spectroscopy -----Insert Figure 9-----
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The absorbance and transmittance spectra of the nanostructures prepared for 3 (NWAs) and 5 hours (NNAs) were carried out in the wavelength range of 300-1100 nm. The absorbance spectra are shown Fig. 9. The absorbance of the NNAs is higher than NWAs in the range of 4001100 nm, which could be more light scattering ability of the NNAs as shown in schematic
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diagram Fig. 1(d). The high light scattering ability increases more number of light traveling pathways in NNAs due to the multi scattering of incident light [10]. Transmittance spectra of the pure FTO, NWAs and NNAs are shown in Fig. S3. The transmittance spectra of the NWAs and
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NNAs are significantly lower than the pure FTO substrate. The NNAs show less transmittance compared to NWAs. The reduction of transmittance is attributed to the structural properties and
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film thickness [25]. It can be seen that NNAs have good light absorptions which can increase the photo to current conversion efficiency. 3.3.6 J-V Characterization
-----Insert Figure 10-----
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In order to study the light harvesting efficiency, the optimized ZnO NWAs and NNAs nanostructures are used as a photoanode for DSSCs fabrication. Fig. 10 presents current density versus voltage (J-V) curves of the NWAs and NNAs, respectively. The measured and calculated
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parameters such as open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF) and efficiency (η) are given in table 1. The Voc and Jsc of the NWAs are about 656 mV and 3.51 mA with FF of 38%. The photo to current conversion efficiency is about 0.91%. As compared to
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NWAs, NNAs obtain the highest Jsc and slight less Voc of 5.70 mA and 617 mV, respectively. The increased Jsc from 3.51 to 5.70 mA mainly depends on the increased internal surface area of
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the NNAs to more dye loading and superior light scattering effect to increase the light travelling path way for harvest more light, which also causes to be increased the film thickness from 3.68 to 4.47 µm for NWAs and NNAs, respectively. The determined η of the NNAs is about 1.47% which is higher than NWAs. The slightly decreased Voc of the NNAs from 686 to 617 mV
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depends on the energy different between quasi Fermi band edge and reduction potential of I-/I-3 redox couple in electrolyte. [20]. However, FF of the NNAs is significantly increased from 38% to 42%, which might be ascribed to reduction of electron recombination between photoanode and
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electrolyte interface [26]. This study reveals that the DSSC based on NNAs show the highest Jsc,
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FF and η. The obtained η is higher than the previously reported values. [27,28]. -----Insert Table 1-----
3.3.7 UV dye absorbance
-----Insert Figure 11-----
In order to find out the reason for higher Jsc of the NNAs, both NWAs and NNAs photoanodes were soaked into 5 mM of eosin yellow dye solution for 3 hours. Thereafter dye 14
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absorbed photoanodes were washed with ethanol to remove the abundant dye molecules. Dye was detached by dipping the photoanode in 10.5 pH of KOH solution for 24 hours. Fig. 11 displays UV-Vis absorbance spectra of the dye solution. The absorbance peak is observed at 515
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nm due to a specific interaction of the chromophores in eosin yellow dye [29]. No other impurity future is observed for KOH solution in the spectra. It is illustrated that dye detached from NNAs photoanode shows better absorbance than that of the NWAs photoanode. It obviously indicates
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that NNAs have large internal surface area for more dye loading. The dye absorbance is calculated about 279 nM and 300 nM for NWAs and NNAs, respectively. In general, more dye
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loaded photoanode harvests more number of photons from incident light, which also generates higher Jsc. These results are good concurrent with J-V results. 3.3.8 Electrochemical impedance spectroscopy (EIS)
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Electrochemical impedance spectroscopy (EIS) was carried out to investigate the kinetics of charge transfer process for DSSC based on NWAs and NNAs photoanode under the dark condition in the frequency range of 500 kHz to 0.1 Hz by applying AC signal of 400 mV open
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circuit voltage. Fig. 12 shows Nyquist plots of both experimental and fitting data of the DSSCs, which are fitted with an equivalent circuit using Z SimpWin software. The fitted equivalent
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circuit is shown in Fig. S4. The fitted and estimated parameters are summarized in table 2. -----Insert Figure 12-----
The two peaks are observed in Nyquist plots. First one, the peak is appeared at high
frequency region in the range of 500-1 kHz as shown in Fig. S5, which is attributed to the charge transfer resistance (Rct1) between Pt electrode and electrolyte interface. Second peak is observed at middle frequency region in between 1000-1 Hz, which is due to the charge transfer 15
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recombination resistance (Rct2) between injected electron and oxidized dye molecules or oxidized specie of (I-/ I-3) in electrolyte [30]. The charge transfer recombination resistance of the NWAs (633.3 Ω) is lower than NNAs (954.5 Ω), which is ascribed to more number of recombination
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that take place at dye loaded photoanode and electrolyte interface in NWAs based DSSC [31]. The increased Rct2 value can increase the FF of DSSC based on NNAs photoanode, which is good concurrent with J-V results.
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-----Insert Figure 13-----
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Fig. 13 shows Bode plots of the DSSCs. The peak frequency of DSSC based on NNAs photoanode at middle frequency region is shifted toward to low frequency region compared to DSSC based on NWAs photoanode, which implies that the frequency shifting not only increases Rct2 but also increases electron lifetime. The small peak appeared at high frequency region is
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owing to the charge transfer resistance between Pt and electrolyte interface. The electron lifetime of the DSSCs is calculated by following equation [3,32]. 1 2πf max
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τr =
where fmax is the peak frequency of the semicircle at middle frequency region (Fig. 13). The
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calculated electron lifetime is listed in table 2. The electron lifetime of DSSC based on NWAs and NNAs is 196 µs and 374 µs, respectively. The increased electron lifetime is not only increased FF but also increased Jsc and η. It could be found that more number of electrons are injected from large amount of dye loaded NNAs than that of NWAs, which are reached to FTO conducting substrate with less recombination and increase Jsc. Herein it is found that DSSC
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based on NNAs photoanode exhibited a better performance than DSSC based on NWAs photoanode.
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-----Insert Table----4. Conclusion
ZnO-TiO2 thin film was spin coated on FTO substrate as a blocking as well as seed
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layer, and NWAs and NNAs were successfully grown on ZnO-TiO2 seed layer by hydrothermal technique. The crystal structure, morphology and optical properties of the nanostructures were
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evaluated by means of XRD, FE-SEM, TEM and Raman spectroscopy, respectively. The results show that the prepared nanostructures were hexagonal wurtzite structure oriented along the perpendicular direction to ZnO-TiO2 seed layer with NWAs and NNAs like morphology. The optimized NWAs and NNAs were used as a photoanode to fabricate the DSSCs. NNAs structure
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had more surface area for dye loading with better light absorbance than that of the NWAs structure. As compared to two nanostructures from J-V results, NNAs based photoanode DSSC exhibited the best photovoltaic conversion efficiency of 1.47%. EIS results revealed that DSSC
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based on NNAs photoanode had higher recombination resistance and electron lifetimes than
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DSSC based on NWAs photoanode.
Acknowledgements
Author T.Marimuthu wishes to thank University Grants Commission (UGC), New Delhi,
India for the financial support provided through Basic Scientific Research (BSR) scheme and Department of Industrial Chemistry, Alagappa University, India for providing facility to study the morphology of nanostructures. 17
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6. A. B. F. Martinson, J. W. Elam, J. T. Hupp, M. J. Pellin, Nano Lett. 7 (2007) 2183. 7. E. Hosono, S. Fujihara, I. Honna, H. S. Zhou, Adv. Mater. 17 (2005) 2091. 8. C. Zhu, Y. Shi, C. Cheng, L. Wang, K. K. Fung, N. Wang, J. Nanomater. 4 (2012) 212653.
9. Z. H. Liu, X. J. Su, G. L. Hou, S. Bi, Z. Xiao, H. P. Jia, RSC Adv. 3 (2013) 8474.
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10. X. Kang, C. Jia, Z. Wan, J. Zhuang, J. Feng, RSC Adv. 5 (2015)16678. 11. Y. Q. Wang, Y. M. Sun, K. Li, Mater. Lett. 63 (2009) 1102. 12. T. P. Chou, Q. Zhang, G. E. Fryxell, G. Cao, Adv. Mater. 19 (2007) 2588.
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13. K. S. Kim, H. Song, S. H. Nam, S-M. Kim, H. Jeong, W. B. Kim, G. Y. Jung, Adv.
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Mater. 24 (2012) 792.
14. J. Qu, Y. Yang, Q, Wu, P. R. Coxon, Y. Liu, X. He, K. Xi, N. Yuan and J. Ding, RSC Adv. 4 (2014)11430.
15. T. Marimuthu, N. Anandhan, R. Thangamuthu, M. Mummoorthi, S. Rajendran, G. Ravi, Mater. Res. Express 2 (2015) 015502. 16. N. Huang, M. W. Zhu, L. J. Gao, J. Gong, C. Sun, X. Jiang, Appl. Surf. Sci. 257 (2011) 6026. 18
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17. E. Edri, E. Rabinovich, O. Niitsoo, H. Cohen, T. Bendikov, G. Hodes, J. Phys. Chem. C 114 (2010) 13092.
F. S. Senthil, Sol. Energy 106 (2014) 129.
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18. M. Raja, N. Muthukumarasamy, D. Velauthapillai, R. Balasundaraprabhu, S. Agilan and
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263501.
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29. T. Yoshida, K. Terada, D. Schlettwein, T. Oekermann, T. Sugiura, H. Minoura, Adv. Mater. 12 (2000) 1214. 19
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30. X. Guo, H. Dong, G. Niu, Y. Qiu, L. Wang, RSC Adv. 4 (2014) 21294. 31. J. J. Wu, G. R. Chen, H. H. Yang, C. H. Ku, J. Y. Lai, Appl. Phys. Lett. 110 (2007) 13872.
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32. C. Zhao, D. Child, Y. Hu, N. Robertson, D. Gibson, S. C. Wang, Y. Q. Fu, RSC Adv. 4
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(2014) 61153.
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Dye loading (nM cm-2)
B
279
D
300
Voc (mV)
Jsc (mA cm-2)
FF (%)
η (%)
686
3.51
38
0.91
617
5.70
42
1.47
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Samples
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Table 1. Dye loading and photovoltaic parameters of the DSSC based on NWAs and NNAs.
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Table 2. Rs, Rct2 and τr value of the DSSC based on NWAs and NNAs. Rs
b
27.83
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d
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Samples
33.5
Rct2
τr
633.3
196 X 10-6
954.5
374 X 10-6
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(b)
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(a)
(d)
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(c)
Fig. 1. Schematic diagram of the nanostructures and their light scattering ability (a) ZnO-TiO2 seed layer, (b) NWAs grown for 3 hours, (c) NNAs grown for 5 hours and (d) the flower like
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structure consisting of nanoneedles offer a good light scattering ability, and red and green arrow
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represent direct transmitted light and multi scattered light, respectively.
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FTO substrate
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Fig. 2. XRD patterns of ZnO nanostructures grown at different Zn(NO3)2 concentrations: (a)
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0.005 M, (b) 0.01 M and (c) 0.05 M.
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(a1)
(b)
(b1 )
(c1)
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(a)
Fig. 3. FE-SEM images of ZnO nanostructures grown at different concentration: (a, a1) 0.005 M, (b, b1) 0.01 M and (c, c1) 0.05 M.
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Fig. 4. Raman spectra of ZnO nanostructures grown at different concentration of (a) 0.005 M, (b)
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0.01 M and (c) 0.05 M.
(G)
(H)
FTO substrate
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FTO substrate
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Fig. 5G. XRD patterns of ZnO nanostructures grown at a concentration of 0.01 M at 80˚C for different growth time (b) 3 hours, (d) 5 hours, and Fig. 5H XRD patterns of ZnO microstructures
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grown for 3 hours at different growth temperatures (b) 80˚C (e) 95 ˚C.
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(d)
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(d1)
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(e)
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Fig. 6. FE-SEM images of ZnO nanostructures grown at a concentration of 0.01 M at 80˚C at different growth time of (d, d1) 5 hours and microstructure grown for 3 hours at different growth temperatures of (e, e1) 95 ˚C.
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(H)
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Fig. 7G. Raman spectra of ZnO nanostructures grown at a concentration of 0.01 M at 80˚C at different growth time (b) 3 hours and (d) 5 hours, and Fig. 7H Raman spectra of ZnO
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microstructures grown for 3 hours at different growth temperatures (b) 80˚C and (e) 95 ˚C.
(d)
(d1)
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d=2.60 Å
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hkl=(002)
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Fig. 8. TEM image of the of the ZnO nanoneedle arrays.
10 1/nm
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Fig. 9. UV-Vis absorbance spectra of the ZnO nanostructures grown at different growth time (b)
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3 hours and (d) 5 hours.
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Fig. 10. J-V curve of ZnO nanostructures (b) NWAs and (d) NNAs.
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Fig. 11. UV-Vis absorption spectra of dye detached from (b) NWAs and (d) NNAs.
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Fig. 12. Electrochemical impedance spectra of experimental (line) and fitting data (dot) of DSSC
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Fig. 13. Bode plots of DSSC constructed using (b) NWAs and (d) NNAs.
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Highlights
ZnO nanowire arrays (NWAs) and nanoneedles arrays (NNAs) were synthesized. TEM study confirmed the hexagonal wutrzite structure of ZnO NNAs.
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The vibration modes of NWAs and NNAs films were studied by Raman spectra. The efficiency of DSSC based on NWAs and NNAs was found to be 0.91 and 1.47 %.
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DSSC based on NNAs exhibited the highest charge transfer recombination resistance.
S0925-8388(16)33023-7
DOI:
10.1016/j.jallcom.2016.09.260
Reference:
JALCOM 39094
To appear in:
Journal of Alloys and Compounds
Received Date: 26 April 2016 Accepted Date: 23 September 2016
Please cite this article as: T. Marimuthu, N. Anandhan, R. Thangamuthu, S. Surya, Facile growth of ZnO nanowire arrays and nanoneedle arrays with flower structure on ZnO-TiO2 seed layer for DSSC applications, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.09.260. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical abstract
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NNAs
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NWAs
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Facile growth of ZnO nanowire arrays and nanoneedle arrays with flower structure on ZnO-TiO2 seed layer for DSSC applications
a
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T. Marimuthua, N. Anandhana*, R. Thangamuthub and S. Suryab Advanced Materials and Thin Film Physics Lab, Department of Physics, Alagappa
University, Karaikudi-630 003, India. b
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Electrochemical Materials Science Division, CSIR-Central Electrochemical Research
Institute, Karaikudi-630 003, India.
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E. mail: [email protected] Abstract
In this present investigation, zinc oxide-titanium oxide (ZnO-TiO2) thin films were prepared by sol-gel spin coating technique and used as a blocking as well as seed layer, and
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followed by, ZnO nanowire arrays (NWAs) and nanoneedle arrays (NNAs) were synthesized on ZnO-TiO2 seed layer by facile hydrothermal technique by varying some parameters such as solution concentration, growth time and growth temperature. They are used as a photoanode for
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dye sensitized solar cell (DSSC) and characterized by spectroscopic and microscopic technique to investigate about the crystal structure, morphology and optical properties. The presence of
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hexagonal wurtzite structure of the prepared nanostructures grown along with (002) plane was confirmed by using X-ray diffraction (XRD), Raman spectra and transmittance electron microscope (TEM). NWAs and NNAs were observed with flower like structure consisting of nanowires and nanoneedles, respectively by field emission scanning electron microscope (FESEM). UV-Vis spectra imply that NNAs have a better light absorbance with more dye loading than NWAs. It could be found from current density-voltage (J-V) curve that the photovoltaic 1
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conversion efficiency of the NWAs and NNAs based DSSC is 0.91% and 1.47%, respectively. Electrochemical impedance spectroscopy (EIS) shows that DSSC based on NNAs photoanode has a better electron lifetime with less electron recombination than DSSC based on NWAs
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photoanode.
Keywords: zinc oxide-titanium oxide, nanowire arrays, nanoneedle arrays, dye sensitized solar
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cell, flower like structure. 1. Introduction
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Dye sensitized solar cells (DSSCs) have been serving for last two decades as environmentally friendly [1]. They are also of more attention due to their easy fabrication, low cost and facile accessibility. DSSC is one of the third generation solar cells, which has a considerable higher indoor efficiency than silicon solar cell [2]. Generally, DSSC consists of
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mesoporous TiO2 nanoparticles sensitized with N719 dye used as a photoanode and platinum (Pt) coated fluorine doped tin oxide (FTO) glass substrate as a counter electrode, which are separated by (I-/I-3) electrolyte [2].
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Titanium oxide (TiO2) is most known material for photoanode, and TiO2 mesoporous based DSSC has exhibited efficiency of more than 12 %. However, it is difficult to further
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increase the current limit of the efficiency. Hence, Zinc oxide (ZnO) is the most attractive and alternative material to TiO2. It is an n-type semiconductor material and has an exciton binding energy of 60 meV with band gap energy of 3.37 eV. ZnO has high electron mobility with similar electronic band structure compared to TiO2 [3]. Most of the research groups have devoted more time to synthesize different novel structures such as nanoparticle (zero dimensional) [4], nanorod or nanowire [5], nanotube (one dimensional) [6], nanosheet (two dimensional) [7] and 2
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microsphere particle (three dimensional) [8]. The efficiency of the DSSC depends on morphology of the photoanode [9]. The photoanode should posse high electron mobility, high charge collection, good charge separation, large surface area and high light scattering ability. In
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this case, the nanoparticle based DSSC has large surface area to more dye loading. Although it has high surface area, the efficiency of the DSSC is low owing to presence of large number of grain boundary, which significantly affects the electron mobility. The poor electron mobility
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leads to recombination between back electrons transfer and oxidized dye molecules or oxidized species in electrolyte [10]. In order to retard these types of recombination, some researchers have
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synthesized one dimensional (1D) nanostructures such as nanowires and nanorods which offer direct pathway to injected electron to reach the FTO conductive substrate. The high electron mobility of the 1D nanostructure has inhibited the recombination between back electrons transfer and oxidized dye molecules or oxidized species in electrolyte. 1D nanowire based DSSC has
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achieved low efficiency compared to nanoparticles based DSSC. This is due to the limited surface area to dye loading, instability, poor coverage of nanowire on FTO substrate which causes recombination between injected electron from uncovered FTO substrate and oxidized
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electrolyte species or oxidized dye molecules [10]. To prevent this type of recombination, Wang et al introduced TiO2 blocking layer between FTO substrate and ZnO seed layer. Eventhough,
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they used the blocking as well as seed layer, the efficiency of the DSSC is low [11]. Further to enhance the conversion efficiency, some of the researchers have employed scattering layer to improve the light absorption through multi-scattering of light within photoanode, and they have significantly increased the efficiency of the DSSCs [12-14]. The various nanostructured ZnO thin films have been synthesized by chemical methods such as electrochemical deposition [15], spin coating [16], chemical bath deposition [17] and hydrothermal technique [18]. Among the 3
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various methods, spin coating and hydrothermal are the best methods to synthesize the blocking as well as seed layer and 1D nanostructure for light scattering on account of low cost and easy
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scalable. In the present work, we have simply prepared ZnO-TiO2 thin film by sol-gel spin coating technique. The prepared thin film is used as a blocking as well as seed layer. 1D nanostructure has further synthesized on seed layer coated FTO glass substrate in viscous solution. By
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changing solution concentration, growth time and growth temperature, the dense and upright
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nanowire arrays (NWAs) and nanoneedle arrays (NNAs) with flower like structure are synthesized to increase the light scattering ability for higher light absorption. The structural, morphological and optical properties of the nanostructures are investigated. The DSSCs are constructed using NWAs and NNAs respectively as a photoanode. The current density verse voltage and electrochemical characterization are carried out. NNAs show better conversion
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efficiency with less recombination between photoanode/electrolyte interface and higher electron lifetime than that of the NWAs.
2.1 Materials
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2. Experimental details
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Fluorine doped tin oxide (FTO) conductive substrate, zinc nitrate hexahydrate and
polyvinyl alcohol were purchased from Sigma Aldrich. Zinc acetate dehydrate and titanium oxide were purchased from Merck. Ruthenium dye (N719), surlyn spacer and electrolyte were purchased from Solaronix. Ethanol was purchased from Alfa Aesar. 2.2 Photoanodes preparation
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ZnO nanostructures were synthesized via two-step process. First step, ZnO-TiO2 seed layers were prepared onto FTO conductive substrates by spin coated technique. The 0.3 M of zinc acetate dehydrate (90%) and titanium oxide powder (10%) was dissolved and dispersed in
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isopropyl alcohol. Followed by, 0.3 M of monoethanolamine was added as a stabilizer. The solution was continuously stirred at 60˚C for 2 hours to get the sol. ZnO-TiO2 thin film was coated at 3000 rpm for 20 sec. The coated film was eventually annealed at 450˚C for one hour
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(The detailed results and discussion of ZnO-TiO2 seed layer are given in ESI†). Before seed layers deposition, FTO substrates were ultrasonically rinsed with acetone and deionized water
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for 10 minutes separately. Thereafter, the rinsed FTO substrates were baked at 100˚C in air for 10 minutes. Second step, the nanostructures were synthesized on ZnO-TiO2 seed layer by hydrothermal technique. An aqueous growth solution consisting of zinc nitrate hexahydrate (0.01 M), 0.1% of polyvinyl alcohol (0.01 M) and hexamethylenetetramine (0.01 M) were dissolved in
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deionized water. The ZnO-TiO2 seed layer was immersed into bottle contained solution at 45˚ angle to avoid any unwanted precipitation from top of the solution on growing ZnO nanostructures. The temperature of growth solution was maintained at 80˚C. In order to synthesis
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the well defined nanostructures, the concentrations, growth times and growth temperatures were varied about 0.005-0.05 M, 80-95˚C and 3-5 hours, respectively. The synthesized ZnO
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nanostructures were eventually rinsed with ethanol and deionized water. Afterward, they were baked at 100˚C for 30 minutes in air atmosphere. 2.3 DSSCs construction
The synthesized ZnO NWAs and NNAs films were soaked into 0.3 mM of N719 dye solution prepared in ethanol for 3 hours in a dark room. Thereafter, the dye loaded photoanode films were rinsed with ethanol to remove the unbounded dye molecules. The Pt electrodes were 5
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prepared on FTO by spin coating at 500 rpm for 30 sec using a 5 mM solution of chloroplatinic acid in isopropyl alcohol, followed by the coated films were baked at 100˚C for 10 minutes. Once again the procedure was repeated. Finally, the pre-heated Pt electrodes were annealed at
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400°C for one hour. The dye loaded ZnO nanostructured photoanodes and Pt electrodes were sandwiched between 60 µm thicknesses of surlyn spacer, and then they were holed by clamp. The active area of the cells was 0.20 cm2, Liquid electrolyte was filled into active area by
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pouring a few drops through the hole on Pt electrode, which contained a mixture of 0.6 M 1methyl-3-propylimidazolium iodide, 0.1 M lithium iodide, 0.05 M iodide and 0.5 M tert-
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butylpyridine. Finally, the hole was sealed with surlyn film. 2.4 Characterizations
X-ray diffraction (XRD) patterns were recorded to find out the crystal structure of the nanostructures by X’Pert PRO PAN analytical powder X-ray Diffraction meter. The
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morphologies of the nanostructures were observed using filed emission scanning electron microscopy (FE-SEM, FEG Quanta 250) and transmission electron microscope (TEM, Tecnai instruments). Raman spectra were performed to observe the vibrational modes and crystal phase
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by micro Raman spectroscopy at an excitation wavelength of 514.5 nm (Princeton instrument Acton sp 2500). Viscosity of the solution was measured by Oswald viscometer. Optical
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properties and dye loading of the optimized nanostructures were measured using UV-Vis spectroscopy (Perkin Elmer’s LAMBDA 35). Thickness of the films was measured by a Stylus profilometer. Current density-voltage (J-V) curves were recorded using Keithley 2400 for constructed DSSCs under the illumination of 100 mW light from Xenon lamp. Electrochemical impedance spectra (EIS) were performed under the dark condition by µ-autolab type III
6
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Potentiostat/Galvanostate in the frequency range of 500 kHz – 0.1 Hz at an open circuit voltage of 400 mV.
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3. Results and discussion 3.1 Growth Mechanism
ZnO nanostructures are hydrothermally synthesized based on following reactions [19,20].
4NH3 + 6HCHO NH4+ + OH-
NH3 + H2O
Zn2+ + 2OH-
Zn(OH)2 (s)
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(CH2)6N4 + 6H2O
(1)
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Zn2+ (aq) + 2NO3- (aq)
Zn(NO3)2 (s)
ZnO (s) + H2O
(2)
(3)
(4)
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Zinc nitrate (Zn(NO3)2) salt dissociates into Zn2+ (aq) and NO3- ions in deionized water. Hexamethylenetetramine (CH2)6N4) reacts with water (H2O) and gives formaldehyde (HCHO)
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and ammonia (NH3), respectively and it acts as a pH buffer to provide the controllable supply of OH- ions through the slow decomposing. Further, NH3 reacts with H2O and forms ammonium
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hydroxide (NH4OH) which also gives OH-. This OH- complex with Zn2+ forms zinc hydroxide (Zn(OH)2). Eventually, Zn(OH)2 dehydrates into ZnO. Polyvinyl alcohol (PVA) does not react with any compound in the reaction process. In this study, PVA is used to prepare the viscous solution for grow the nanostructures. Viscosity of the solution is found to be 1.31 Pa.s -----Insert Figure 1-----
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3.2 Effect of concentration 3.2.1 X-ray diffraction analysis
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-----Insert Figure 2----Crystal structure of the ZnO nanostructures grown at different concentrations at constant temperature of 80˚C was analyzed by X-ray diffraction (XRD) pattern. As shown in Fig. 2, the
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diffraction peaks are indexed by black dot symbol corresponding to F doped SnO2 substrates, all other peaks except peak indicated by star symbol can be indexed to hexagonal wurtzite structure
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and good concordance with International Center for Diffraction Data (ICDD) card no 36-1451. The prominent diffraction peaks 2θ angles at 31.68, 34.34 and 36.16 correspond to (100), (002) and (101) planes, respectively. The weak peak (indicated by star symbol) appeared at 25.7 is due to the presence of TiO2 anatase phase in seed layer for nanostructure synthesized at a
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concentration of 0.01 M, which is well coincided with ICDD card no.89-4921. When the concentration of growth aqueous precursor is increased from 0.01 M to 0.05 M, the intensity of the diffraction peaks is also increased, which means crystallinity of the nanostructure is good at
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higher concentration. In addition, the intensity of the (002) plane is increased with respect to concentration, which implies that the crystalline is grown along the C-axis to surface of the
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substrate [15]. The other diffraction peaks reveal that the crystalline is grown at random orientation or at their preferential orientation. 3.2.2 Scanning Electron Microscope -----Insert Figure 3-----
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The surface morphologies of the nanostructures prepared at different Zn2+ concentrations were observed by field emission scanning electron microscope (FE-SEM) and its images are shown in Fig. 3. Fig. 3(a, a1) shows nanowires prepared at a concentration of 0.005 M (Zn2+). It
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can be clearly seen that some nanowires are agglomerated and found less dense as shown in Fig. 3(a1). The diameter and length of the nanowires are varied in the range between 30-111 nm and 300-485 nm, respectively. Further the concentration of Zn2+ is raised to 0.01 M, nanowire arrays
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(NWAs) with hexagonal wurtzite structures are vertically and densely grown with diameter of 61-103 nm on ZnO-TiO2 seeded FTO plate (inset image of Fig. 3(b)), which is good concurrence
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with (002) plane in XRD result. The NWAs can favor for higher electron mobility. A few flowers like structures are appeared as shown in Fig. 3(b) and compose of large number of nanowires which are grown with different orientation as shown in Fig. 3 (b1). The random orientation of nanowires is confirmed by the presence of other satellite planes (100), (101),
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(102), (110), (103) and (112) in XRD pattern. Flower consisting of randomly oriented nanowire can be acted as a light scattering center to absorb more number of photons [12]. The diameter and length of the NWAs are changed in the range of 42-47 nm and 202-504 nm, respectively. As
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the concentration of Zn2+ is further increased to 0.05 M, SEM images of Fig. 3(c, c1) show similar structure, but the significant changes are occurred in diameter of the NWAs in the range
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between 123-382 nm as shown in Fig. 3 (c). The increased diameter of the NWAs decreases surface area of the film. In addition, a few nanotubs are randomly grown whose diameters are changed between 0.7-1.263 µm. 3.2.3 Raman spectroscopy -----Insert Figure 4-----
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Fig. 4 shows Raman spectra of 1D nanostructures prepared at different precursor concentrations. It is the best tool to study the crystal phase, crystallinity and defect of the materials. Raman peak at 636 cm-1 corresponds to Eg mode, which is ascribed to the tetrahedral
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anatase phase of TiO2. Raman peaks are appeared at 332, 437 and 580 cm-1 owing to E2L-E2H, E2 (high), and E1 (LO) mode, respectively. The small peak centered at 332 cm-1 is assigned to second order non-polar mode of Raman scattering. The peak at 580 cm-1 is found to be presence
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of oxygen vacancies (or) Zn interstitials. The high intense peak around at 437 cm-1 is attributed to hexagonal wurtzite structure and non-polar optical mode of ZnO, which is owing to the heavy
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vibration of an oxygen atom. The intensity of the E2 (high) mode is significantly enhanced as the concentration of the growth solution is increased from 0.005 to 0.05 M as shown in inset Fig. 4. This is an indication of good crystallinity of the prepared nanostructures which is good in accordance with XRD results and SEM images [21]. According to Raman selection rule, the
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absence of A1 (TO) and E1 (TO) modes represents that the synthesized NWAs are grown perpendicular to seed layer coated FTO substrate [22]. This can be confirmed from the presence of high intense (002) plane in XRD pattern. The Raman peak is appeared in the range of 1050-
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1200 cm-1due to the multi phonon process [23].
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3.3 Effect of growth time and temperature 3.3.1 X-ray diffraction analysis -----Insert Figure 5-----
Fig. 5G shows XRD patterns of the nanostructures prepared at constant temperature of 80˚C at different time. All the diffraction peaks (except peaks indicated by black dot and star symbol) are indexed to hexagonal wurtzite structure and also in good agreement with ICDD card no. 3610
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1451. The small peak observed at 25.7 is owing to presence of TiO2 anatase phase. As the growth time is increased from 3 to 5 hours, the intensity of the (002) plane is remarkably increased as shown in Fig. 5G(d). It confirms that crystallinity of the nanostructures is enhanced as the
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growth time is increased. From this XRD results, we could confirm that the crystalline nature is higher for nanostructure grown at 5 hours than 3 hours. Fig. 5H shows XRD patterns of microstructures grown at different growth temperatures for 3 hours in 0.01 M solution of zinc
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nitrate hexahydrate. When the growth temperature is raised from 80 to 95˚C, the intensity of the ZnO peaks are dramatically decreased and FTO peaks are increased as shown in Fig. 5H(e). It
3.3.2 Scanning Electron Microscope
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can be confirmed that film growth at higher temperature is less.
-----Insert Figure 6-----
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Fig. 6(d, d1) displays a nanoneedle arrays (NNAs) like structure grown for 5 hours at 80˚C. The flowers like structures are observed at a few places in prepared thin film as shown in Fig. 6(d). On other hand, the NNAs are vertically with densely grown along the C-axis to ZnO-TiO2 seeded
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substrate, which is well consent with high intense (002) plane in XRD pattern. The flower like structure is composed of large number of nanoneedles which are attached at one big grain and a
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few nanoneedles are combined to form a bigger one. The nanoneedles show length, diameter of the tip and base in the range of 600-830 nm, 40-70 nm and 60-160 nm, respectively. The flower like structures having nanoneedles act as the light scattering center for higher light absorption. From this observation, when the growth time of the nanostructure is increased from 3 to 5 hours, the morphology change is occurred from NWAs to NNAs and also increased the length and diameter. SEM image of the Fig. 6(e) presents less dense nanorods as the growth temperature is 11
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raised about 95˚C. The rolling comb like structures are appeared which compose the vertical nanorods attached on a big microrod as shown in Fig. 6(e1). The vertical nanorod is to be a
3.3.3 Raman spectroscopy -----Insert Figure 7-----
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length and diameter of 326 nm and 71 nm, respectively.
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Raman spectra of Fig. 7G display E2L-E2H, E2 (high) and E1 (LO) modes corresponding to hexagonal wurtzite structure of ZnO. Eg mode is observed due to the tetrahedral anatase phase of
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TiO2. As the growth time is extended from 3 to 5 hours, the intensity of the E2 (high) mode is only increased as shown in inset Fig.7G, it can be confirmed that crystallinity of the NNAs is dramatically enhanced compared to NWAs structures. This is good in accordance with XRD pattern and SEM images [21]. Fig. 7H shows additional peaks appeared at 395 and 513 cm-1
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corresponding to B1g and A1g mode which are originated to the tetrahedral anatase phase of TiO2 [24]. When the growth temperature is raised about 95˚C, the intensity of the E2 (high) mode is significantly elevated as shown in Fig. 7H(e), which is due to increasing crystallinity of the
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microstructure compared to nanostructures. Even, XRD result showed less crystalline nature.
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3.3.4 Transmission electron microscopy (TEM)
Transmission electron microscope (TEM) was analyzed to study the crystalline structure
and particle size of the NNAs. The NNAs were scraped from thin film and dispersed in isopropanol. Thereafter, the solution was sonicated for 10 minutes. Fig.
8(d)
displays particle size of 10-15 nm which is found that after sonication, the NNAs are broken into 12
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different particle size. Fig. 8(d1) corroborates that the nanoparticles have a well defined lattice space of about 2.6 Å which is well matched with lattice space of the (002) plane of the hexagonal wurtzite structure. This is an indication of NNAs grown along the c-axis to the seed layer.
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Selected area electron diffraction (SAED) pattern proves that the prepared NNAs are polycrystalline nature as shown in inset Fig. 8(d1) [20].
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3.3.5 UV-Visible spectroscopy -----Insert Figure 9-----
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The absorbance and transmittance spectra of the nanostructures prepared for 3 (NWAs) and 5 hours (NNAs) were carried out in the wavelength range of 300-1100 nm. The absorbance spectra are shown Fig. 9. The absorbance of the NNAs is higher than NWAs in the range of 4001100 nm, which could be more light scattering ability of the NNAs as shown in schematic
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diagram Fig. 1(d). The high light scattering ability increases more number of light traveling pathways in NNAs due to the multi scattering of incident light [10]. Transmittance spectra of the pure FTO, NWAs and NNAs are shown in Fig. S3. The transmittance spectra of the NWAs and
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NNAs are significantly lower than the pure FTO substrate. The NNAs show less transmittance compared to NWAs. The reduction of transmittance is attributed to the structural properties and
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film thickness [25]. It can be seen that NNAs have good light absorptions which can increase the photo to current conversion efficiency. 3.3.6 J-V Characterization
-----Insert Figure 10-----
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In order to study the light harvesting efficiency, the optimized ZnO NWAs and NNAs nanostructures are used as a photoanode for DSSCs fabrication. Fig. 10 presents current density versus voltage (J-V) curves of the NWAs and NNAs, respectively. The measured and calculated
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parameters such as open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF) and efficiency (η) are given in table 1. The Voc and Jsc of the NWAs are about 656 mV and 3.51 mA with FF of 38%. The photo to current conversion efficiency is about 0.91%. As compared to
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NWAs, NNAs obtain the highest Jsc and slight less Voc of 5.70 mA and 617 mV, respectively. The increased Jsc from 3.51 to 5.70 mA mainly depends on the increased internal surface area of
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the NNAs to more dye loading and superior light scattering effect to increase the light travelling path way for harvest more light, which also causes to be increased the film thickness from 3.68 to 4.47 µm for NWAs and NNAs, respectively. The determined η of the NNAs is about 1.47% which is higher than NWAs. The slightly decreased Voc of the NNAs from 686 to 617 mV
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depends on the energy different between quasi Fermi band edge and reduction potential of I-/I-3 redox couple in electrolyte. [20]. However, FF of the NNAs is significantly increased from 38% to 42%, which might be ascribed to reduction of electron recombination between photoanode and
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electrolyte interface [26]. This study reveals that the DSSC based on NNAs show the highest Jsc,
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FF and η. The obtained η is higher than the previously reported values. [27,28]. -----Insert Table 1-----
3.3.7 UV dye absorbance
-----Insert Figure 11-----
In order to find out the reason for higher Jsc of the NNAs, both NWAs and NNAs photoanodes were soaked into 5 mM of eosin yellow dye solution for 3 hours. Thereafter dye 14
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absorbed photoanodes were washed with ethanol to remove the abundant dye molecules. Dye was detached by dipping the photoanode in 10.5 pH of KOH solution for 24 hours. Fig. 11 displays UV-Vis absorbance spectra of the dye solution. The absorbance peak is observed at 515
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nm due to a specific interaction of the chromophores in eosin yellow dye [29]. No other impurity future is observed for KOH solution in the spectra. It is illustrated that dye detached from NNAs photoanode shows better absorbance than that of the NWAs photoanode. It obviously indicates
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that NNAs have large internal surface area for more dye loading. The dye absorbance is calculated about 279 nM and 300 nM for NWAs and NNAs, respectively. In general, more dye
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loaded photoanode harvests more number of photons from incident light, which also generates higher Jsc. These results are good concurrent with J-V results. 3.3.8 Electrochemical impedance spectroscopy (EIS)
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Electrochemical impedance spectroscopy (EIS) was carried out to investigate the kinetics of charge transfer process for DSSC based on NWAs and NNAs photoanode under the dark condition in the frequency range of 500 kHz to 0.1 Hz by applying AC signal of 400 mV open
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circuit voltage. Fig. 12 shows Nyquist plots of both experimental and fitting data of the DSSCs, which are fitted with an equivalent circuit using Z SimpWin software. The fitted equivalent
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circuit is shown in Fig. S4. The fitted and estimated parameters are summarized in table 2. -----Insert Figure 12-----
The two peaks are observed in Nyquist plots. First one, the peak is appeared at high
frequency region in the range of 500-1 kHz as shown in Fig. S5, which is attributed to the charge transfer resistance (Rct1) between Pt electrode and electrolyte interface. Second peak is observed at middle frequency region in between 1000-1 Hz, which is due to the charge transfer 15
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recombination resistance (Rct2) between injected electron and oxidized dye molecules or oxidized specie of (I-/ I-3) in electrolyte [30]. The charge transfer recombination resistance of the NWAs (633.3 Ω) is lower than NNAs (954.5 Ω), which is ascribed to more number of recombination
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that take place at dye loaded photoanode and electrolyte interface in NWAs based DSSC [31]. The increased Rct2 value can increase the FF of DSSC based on NNAs photoanode, which is good concurrent with J-V results.
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-----Insert Figure 13-----
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Fig. 13 shows Bode plots of the DSSCs. The peak frequency of DSSC based on NNAs photoanode at middle frequency region is shifted toward to low frequency region compared to DSSC based on NWAs photoanode, which implies that the frequency shifting not only increases Rct2 but also increases electron lifetime. The small peak appeared at high frequency region is
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owing to the charge transfer resistance between Pt and electrolyte interface. The electron lifetime of the DSSCs is calculated by following equation [3,32]. 1 2πf max
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τr =
where fmax is the peak frequency of the semicircle at middle frequency region (Fig. 13). The
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calculated electron lifetime is listed in table 2. The electron lifetime of DSSC based on NWAs and NNAs is 196 µs and 374 µs, respectively. The increased electron lifetime is not only increased FF but also increased Jsc and η. It could be found that more number of electrons are injected from large amount of dye loaded NNAs than that of NWAs, which are reached to FTO conducting substrate with less recombination and increase Jsc. Herein it is found that DSSC
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based on NNAs photoanode exhibited a better performance than DSSC based on NWAs photoanode.
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-----Insert Table----4. Conclusion
ZnO-TiO2 thin film was spin coated on FTO substrate as a blocking as well as seed
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layer, and NWAs and NNAs were successfully grown on ZnO-TiO2 seed layer by hydrothermal technique. The crystal structure, morphology and optical properties of the nanostructures were
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evaluated by means of XRD, FE-SEM, TEM and Raman spectroscopy, respectively. The results show that the prepared nanostructures were hexagonal wurtzite structure oriented along the perpendicular direction to ZnO-TiO2 seed layer with NWAs and NNAs like morphology. The optimized NWAs and NNAs were used as a photoanode to fabricate the DSSCs. NNAs structure
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had more surface area for dye loading with better light absorbance than that of the NWAs structure. As compared to two nanostructures from J-V results, NNAs based photoanode DSSC exhibited the best photovoltaic conversion efficiency of 1.47%. EIS results revealed that DSSC
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based on NNAs photoanode had higher recombination resistance and electron lifetimes than
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DSSC based on NWAs photoanode.
Acknowledgements
Author T.Marimuthu wishes to thank University Grants Commission (UGC), New Delhi,
India for the financial support provided through Basic Scientific Research (BSR) scheme and Department of Industrial Chemistry, Alagappa University, India for providing facility to study the morphology of nanostructures. 17
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17. E. Edri, E. Rabinovich, O. Niitsoo, H. Cohen, T. Bendikov, G. Hodes, J. Phys. Chem. C 114 (2010) 13092.
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30. X. Guo, H. Dong, G. Niu, Y. Qiu, L. Wang, RSC Adv. 4 (2014) 21294. 31. J. J. Wu, G. R. Chen, H. H. Yang, C. H. Ku, J. Y. Lai, Appl. Phys. Lett. 110 (2007) 13872.
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32. C. Zhao, D. Child, Y. Hu, N. Robertson, D. Gibson, S. C. Wang, Y. Q. Fu, RSC Adv. 4
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Dye loading (nM cm-2)
B
279
D
300
Voc (mV)
Jsc (mA cm-2)
FF (%)
η (%)
686
3.51
38
0.91
617
5.70
42
1.47
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Samples
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Table 1. Dye loading and photovoltaic parameters of the DSSC based on NWAs and NNAs.
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Table 2. Rs, Rct2 and τr value of the DSSC based on NWAs and NNAs. Rs
b
27.83
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d
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Samples
33.5
Rct2
τr
633.3
196 X 10-6
954.5
374 X 10-6
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(b)
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(a)
(d)
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(c)
Fig. 1. Schematic diagram of the nanostructures and their light scattering ability (a) ZnO-TiO2 seed layer, (b) NWAs grown for 3 hours, (c) NNAs grown for 5 hours and (d) the flower like
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structure consisting of nanoneedles offer a good light scattering ability, and red and green arrow
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represent direct transmitted light and multi scattered light, respectively.
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FTO substrate
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Fig. 2. XRD patterns of ZnO nanostructures grown at different Zn(NO3)2 concentrations: (a)
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0.005 M, (b) 0.01 M and (c) 0.05 M.
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(a1)
(b)
(b1 )
(c1)
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(c)
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(a)
Fig. 3. FE-SEM images of ZnO nanostructures grown at different concentration: (a, a1) 0.005 M, (b, b1) 0.01 M and (c, c1) 0.05 M.
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Fig. 4. Raman spectra of ZnO nanostructures grown at different concentration of (a) 0.005 M, (b)
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0.01 M and (c) 0.05 M.
(G)
(H)
FTO substrate
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FTO substrate
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Fig. 5G. XRD patterns of ZnO nanostructures grown at a concentration of 0.01 M at 80˚C for different growth time (b) 3 hours, (d) 5 hours, and Fig. 5H XRD patterns of ZnO microstructures
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grown for 3 hours at different growth temperatures (b) 80˚C (e) 95 ˚C.
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(d)
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(d1)
(e1)
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(e)
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Fig. 6. FE-SEM images of ZnO nanostructures grown at a concentration of 0.01 M at 80˚C at different growth time of (d, d1) 5 hours and microstructure grown for 3 hours at different growth temperatures of (e, e1) 95 ˚C.
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(H)
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(G)
Fig. 7G. Raman spectra of ZnO nanostructures grown at a concentration of 0.01 M at 80˚C at different growth time (b) 3 hours and (d) 5 hours, and Fig. 7H Raman spectra of ZnO
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microstructures grown for 3 hours at different growth temperatures (b) 80˚C and (e) 95 ˚C.
(d)
(d1)
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d=2.60 Å
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hkl=(002)
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Fig. 8. TEM image of the of the ZnO nanoneedle arrays.
10 1/nm
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Fig. 9. UV-Vis absorbance spectra of the ZnO nanostructures grown at different growth time (b)
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3 hours and (d) 5 hours.
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Fig. 10. J-V curve of ZnO nanostructures (b) NWAs and (d) NNAs.
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Fig. 11. UV-Vis absorption spectra of dye detached from (b) NWAs and (d) NNAs.
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Fig. 12. Electrochemical impedance spectra of experimental (line) and fitting data (dot) of DSSC
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based on (b) NWAs and (d) NNAs.
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Fig. 13. Bode plots of DSSC constructed using (b) NWAs and (d) NNAs.
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Highlights
ZnO nanowire arrays (NWAs) and nanoneedles arrays (NNAs) were synthesized. TEM study confirmed the hexagonal wutrzite structure of ZnO NNAs.
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The vibration modes of NWAs and NNAs films were studied by Raman spectra. The efficiency of DSSC based on NWAs and NNAs was found to be 0.91 and 1.47 %.
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DSSC based on NNAs exhibited the highest charge transfer recombination resistance.