w-ZnO nanostructures with distinct morphologies: Properties and integration into dye sensitized solar cells

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w-ZnO nanostructures with distinct morphologies: Properties and integration into dye sensitized solar cells Aneesiya K. Rajan, L. Cindrella∗ Fuel Cell, Energy Materials and Physical Chemistry Laboratory, Department of Chemistry, National Institute of Technology, Tiruchirappalli, 620015, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Zinc oxide Growth mechanism Band gap narrowing Dye sensitized solar cells

Herein, we report the cost-effective and single-step reaction strategies for the synthesis of distinct morphology of ZnO namely, nanoparticles, nanotubes and nanowires involving co-precipitation and hydrothermal methods. Powder X-Ray diffraction analysis indexed nine distinct peaks which correspond to the w-ZnO for all the synthesized nanostructures. The formation mechanism of nanostructures are proposed, in which the role of surfactants (polyvinylpyrrolidone and sodium dodecyl sulfate) and the oriented self-assembly are taken into account. The Raman spectral analysis of the nanostructures confirms the presence of highly intense E2low and E2high phonon vibrational modes corresponding to the movement of O-atoms and Zn sub-lattices of w-ZnO respectively. The prepared yellowish ZnO nanowires reveal intensification in optical absorption and shrinkage in band gap due to increased oxygen vacancies in the sample. The quenching of green luminescence in the photoluminescence spectrum of the nanowire further confirms the oxygen vacancies. All the nanostructures were integrated into dye sensitized solar cells as photoanode materials. The photovoltaic parameters and electrochemical charge transfer properties of the fabricated dye sensitized solar cells (DSSCs) were evaluated by using photocurrent density-photovoltage curves and Nyquist plots of electrochemical impedance spectroscopic (EIS) studies. Due to the presence of large pore size in nanowires and nanotubes, the dye infiltration and electrolyte diffusion rates are high and cause the highest photocurrent densities of 1.714 mA cm−2 and 1.813 mA cm−2 respectively. The specific hollow tube like nanostructures is channelizing the photo-injected electrons directly to the collector electrode and results in high photovoltaic conversion efficiency of 1.109%. High value of charge transfer resistance across the ZnO/dye/electrolyte interfaces in nanocapsule and nanoparticle based DSSCs enforce low conversion efficiency to the devices.

1. Introduction The extensive reliance on the limited prevailing fossil-fuel based power sources and the concern about global warming have captivated the attention on the production of renewable, efficient and economically viable alternatives. Sunlight is the clean and never-ending source of energy available on earth's surface. Light energy can be utilized in various ways like artificial photosynthesis and photovoltaic conversion [1]. Dye sensitized solar cell (DSSC), the most prominent third generation solar cell, is expected to provide solutions to the rising energy demand along with its lower production cost, flexibility in architecture, transparency, multicolor options and short energy pay-back time even though this field is dominated by solid-state junction devices. DSSCs are photo-electrochemical cells which contain adjacent layers of dye adsorbed semiconducting metal oxide, electrolyte with redox couple and conducting counter electrode materials like platinum or carbon [2].

∗

Due to the morphology dependent properties of zinc oxide (ZnO), a low-cost intrinsic semiconductor of direct band gap with a value of ~3.37 eV, the DSSCs fabricated with ZnO nanomaterial have received promising attention in recent research. ZnO nanomaterials are explored as alternatives to TiO2 (a widely studied semiconducting oxide in DSSC) because of its excellent bulk electron mobility (115–155 cm2 V−1 s−1) which is seven order of magnitude larger than that of TiO2 (~10−5 cm2 V−1 s−1) [3]. ZnO has a high exciton binding energy of 60 meV which assures effectual emissions and thermal separation of excitons [4]. Indeed, ZnO is manifested as the first studied metal-oxide semiconductor for irreversible electron injection from excited organic molecules to the energy levels of conduction band [5]. ZnO is a versatile functional material which includes a vast family of nanostructures in metal oxides known today; therefore several design opportunities are possible with ZnO for fabricating the photo-anodes. The crystalline structure of ZnO is conducive to its anisotropic growth, unlike TiO2,

Corresponding author. E-mail address: [email protected] (L. Cindrella).

https://doi.org/10.1016/j.ceramint.2019.12.045 Received 28 September 2019; Received in revised form 25 November 2019; Accepted 3 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Aneesiya K. Rajan and L. Cindrella, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.045

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followed by water and then dried at 80 °C for 6 h.

enabling it a favourable candidate for DSSCs. The isoelectric point of ZnO is ~9. In acidic dye environments, due to the basic nature of ZnO, the dissolution followed by the creation of Zn2+-dye complexes results. Thereby an insulating layer of aggregates of Zn2+-dye complexes will be formed which eventually blocks the injection of photo-excited electrons to the conduction band of ZnO from the dye [6]. In order to improve the efficiency of ZnO-based DSSCs, strategies like development of ZnO photo-anodes with efficient architectures, limiting the formation of Zn2+-dye complexes, limiting the rate of recombination reactions at various interfaces of the device by elemental doping, and the implementation of sensitizer dyes that are compatible with ZnO electrodes. ZnO is superior to TiO2 in terms of its electron mobility within the bulk single-crystal phases that are not influenced by defects, grain boundaries and electron trap centers which are common in randomly packed nano-particle based photo-electrodes. Solbrand et al. reported that, the electron diffusion coefficients of ZnO based devices vary between 10−4 to10−6 cm2 s−1 which are 1.5 × 10−5 cm2 s−1 in TiO2 based devices [7]. Tiwana et al. reported a “device diffusion coefficient” of value, 1.1 × 10−4 cm2 s−1 under short circuit condition, which is ascribed to bulk transport [8]. The photovoltage of the device is strongly influenced by the exponential scattering of trap energies in ZnO nanostructures [9]. Variety of ZnO nanostructures can be prepared by different synthetic strategies such as sol gel synthesis [10], chemical bath deposition including solvothermal growth [11], solid-state synthesis [12], magnetron sputtering [13], polyol hydrolysis [14], spary pyrolysis [15], cathodic electro-deposition [16], ultrasonic assisted precipitation [17], laser assisted flow deposition [18] etc. Of the various synthesis processes, co-precipitation and hydrothermal methods are simple and ecofriendly for the mass production of nano-structures with high aspect ratio. Also the addition of organic polymers and surfactants which can be used to regulate the size and shape of crystals are possible in hydrothermal methods. In the present study, various morphologies of ZnO such as nanoparticles, nanotubes and nanowires were synthesized through simple and cost-effective methods, and implemented in DSSCs as photo-anode materials. The influence of morphological variations of the photo-anodes on the photo-electrochemical properties of the DSSCs in using acidic dye N3 has been studied.

2.2.2. Preparation of ZnO nano-rods (ZNT) Hydrothermal method was used for preparing ZNT. 0.1 M solution of Zn(NO3)2·6H2O and HMTA were prepared separately and each solution was stirred for 30 min. A solution of the surfactant PVA (50 mg) was prepared in 40 ml of water and added to the solution containing precursor salt and stirred well for 15 min. HMTA was added dropwise to the precursor salt solution and stirred for 1 h at ambient temperature (30 ± 1 °C). The resultant cloudy solution was transferred to a Teflonlined autoclave and maintained at 120 °C for 16 h. The precipitate was collected and washed well with ethanol followed by water. The final product was dried at 60 °C for 6 h. 2.2.3. Preparation of ZnO nano-wires (ZNW) ZnO nanowires have been grown hydrothermally using simple colloidal growth technique. For this, ZnCl2 (0.035 M), SDS (0.01 M) and supersaturated solution of Na2CO3 (4.20 M) were prepared using Millipore water. Addition of Na2CO3 solution was made in drops to the precursor salt solution and the solution was stirred for 30 min, SDS was added and the cloudy solution obtained was stirred for 1 h. It was transferred to a Teflon-lined autoclave, sealed and kept for 24 h at 150 °C. The obtained product was rinsed using ethanol and then with water, and dried at 60 °C for 6 h. 2.3. Fabrication of DSSC About 0.5 g of ZNC, ZNP, ZNR and ZNW were mixed separately with a mixture containing 0.125 g of ethyl cellulose, 1.25 g of α-terpineol and 4 ml of ethanol. The resultant mixtures were stirred at room temperature for 48 h. The final colloidal form was mildly heated at 40 °C to get a paste of the corresponding nano-materials with suitable viscosity for coating. Doctor blade procedure was used to paint the as prepared pastes on the respective FTO plates to ~5 μm thickness (fixed by spacer). The coated plates were dried for 1 h in air and then calcined at 450 °C for 30 min. The plates were then cooled naturally to 80 °C and immersed in 3 × 10−4 M ethanolic solution of N3 dye. The immersion in dye was carried out for 24 h in dark atmosphere at ambient temperature (30 ± 1 °C). The electrodes were rinsed with ethanol to get rid of the un-adsorbed dye molecules, air-dried and used as the working electrodes. Counter electrodes were prepared by using Pt paste which was sintered at 450 °C for 15 min and cooled. The DSSCs were assembled by means of dye adsorbed working electrodes and counter electrodes in a sandwiched manner. Electrolyte used is composition of lithium iodide (0.5 M), iodine (I2) (0.05 M) and 4-tert-butyl pyridine (TBP) (0.5 M) in 3-methoxypropionitrile. A cyanoacrylate sealant was applied to prevent the leakage of electrolyte.

2. Experimental section 2.1. Materials Zinc acetate dihydrate (Zn(CH3COO)2·2H2O), zinc nitrate hexahydrate (Zn(NO3)2·6H2O), anhydrous zinc chloride (ZnCl2), sodium hydroxide (NaOH), hexamethylenetetramine (HMTA), sodium dodecyl sulfate (CH3(CH2)11OSO3Na)(SDS), sodium carbonate (Na2CO3), platinum paste, 4-tert-butyl pyridine, polyvinylpyrrolidone (PVP) and fluorine doped tin oxide coated (FTO) glass plate (~13 Ω cm−2) (Sigma Aldrich). Ethylcellulose, α-terpineol, 3-methoxypropionitrile and lithium iodide (LiI) (TCI chemicals), N3 dye (Aura chemicals) were used in the present study. Ethanol was procured from Merck and distilled prior to use. Millipore water was used for solution preparation. For the comparison of experimental results, ZnO nanostructures in the form of nanocapsules (Merck) were used and named as ZNC for convenience.

2.4. Characterization techniques X-ray diffraction studies were performed in Rigaku Ultima III X-ray diffractometer in which CuKα radiation (Wavelength = 1.5406 Å) was used with a scanning speed of 2°.min−1. JASCO-V-640 UV–Vis NIR spectrometer was used for recording the diffuse reflectance (DRS) in the range of 300–800 nm and photoluminescence (PL) was recorded by JASCO spectrofluorometer (FP8500). The surface morphology was analyzed by Carl Zeiss FESEM and the elemental analysis of the nanosturctures was done by Energy dispersive X-ray analysis (EDAX) while the average particle size and the morphological difference of the nanostructures were analyzed through JEOL JEM- 2100 TEM instrument. Raman scattering analysis was done using Horiba Xplora Raman microscope with laser emissions (532.15 nm) from Nd-YAG laser. The porosity, surface area and pore size distribution of various ZnO nanostructures were analyzed by Gemini VII series surface area analyzer. The photovoltaic parameters and the charge transfer properties at the interfaces of the fabricated DSSCs were evaluated by CHI608B

2.2. Synthesis of ZnO nanomaterials 2.2.1. Preparation of ZnO nano-particles (ZNP) Co-precipitation method was adopted for the synthesis of ZNP. For this, 0.25 M of (Zn(CH3COO)2·2H2O) (precursor salt) and 0.625 M of NaOH (precipitating agent) were dissolved separately in water and stirred for 30 min at ambient temperature conditions (30 ± 1 °C). Then NaOH was added dropwise to the precursor solution to form the precipitate. The resultant solution was stirred continuously at 60 °C for 5 h. The obtained white color precipitate was rinsed few times with ethanol 2

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The capsules are grown with hexagonal and cubic facets (Fig. 2(a and e)). The average particle size for ZNC is 80–90 nm. In ZNP, the nanoparticle formation is observed clearly and the formed particles are aggregated in the form of coral reefs. The surface morphology of the ZNP is shown in Fig. 2(b and f). In the case of ZNT (Fig. 2c and g), hollow tubular structures are formed with hexagonal facets. In Fig. 2(d and h), the nanowires with hexagonal facets and cross-section are observed for ZNW. The average particle size distribution in terms of diameter of ZnO nanocapsules, ZNP, ZNT and ZNW are shown in Fig. 2(i–l) respectively. The average length of the nanocapsules is ~376.29 nm. The average length of the nanotubes in ZNT is ~1565 nm and the wall thickness of the hollow tubes is ~47.28 nm with inner diameter of ~168.96 nm. TEM images of the nanostructures are shown in Fig. 3a-d. From the TEM images, the nanocapules, spherical-nanoparticles, hexagonal nanotubes and nanowires could be distinctly identified. The aspect ratio for the ZNT and ZNW are around 8 and 98 respectively. EDAX spectra of all nanostructures are shown in Figs. S1(a–d), which indicate that the nanostructures are completely free of contaminants. The atomic percentage ratios of Zn to O (Zn:O) are 1:0.75, 1:0.95, 1:1.02 and 1:1.01 for ZNC, ZNP, ZNT and ZNW respectively. Fig. 3e-h represents the highresolution TEM (HRTEM) images of the nanostructures. It can be seen that the distance between ZnO fringes that are perpendicular to the rod axis is 0.20 nm, 0.23 nm and 0.22 nm respectively for ZNC, ZNT and ZNW, and the distance is 0.18 nm in ZNP. The corresponding selected area diffraction patterns (SAED) show highly single crystalline nature of the nanostructures (Fig. 3i-l). As far as hexagonal wurtzite structure of ZnO is concerned, it has a Zn-terminated (0001) and an O-terminated (0001 ‾ ) polar surfaces along with six non-polar facets namely (101 ‾ 0) , (1 ‾ 010) , (01 ‾ 10) , (011 ‾ 0) , (11 ‾ 00) and (1 ‾ 100) planes [21]. As per theoretical studies, the surface energy of polar facets is twice as that of non-polar facets. The propensity for the formation of flat crystal faces of ZnO during vertical growth results in the simultaneous occurrence of stretching and contraction of Zn–O bonds along c-axis and perpendicular to the c-axis respectively, in preferential growth of 1D nanostructure along the c-axis (Fig. S2) [22]. During the crystal growth of hollow nanotubes and nanowires with hexagonal facets, the surfactants play significant role. Based on the influence of surfactants and its absence, a growth mechanism for ZNP, ZNT and ZNW has been proposed. In the case of ZNP, ZnO nanoparticles formed, aggregate into larger particles in the form of coral-reefs to minimize the surface energy and subsequently surface tension through Ostwald ripening without the presence of surfactants [23]. The chemical reactions responsible for the formation of coral-reef like nano aggregates are shown in Fig. S3. For obtaining ZNT, the surfactant PVP has been added. PVP has an ability to act as an anti-agglomerating agent due to the steric effect arising from the long polyvinyl chain. PVPbelonging to the non-ionic surfactant has an ability to aggregate and associate dynamically and spontaneously into cylindrical micelles in aqueous medium, above its critical micelle concentration (CMC) [24]. A “soft-template mediated growth mechanism” can be described for this structure creation [25]. The as grown PVP strands can act as “softtemplate” for nanostructure growth. The primary role of HMTA in aqueous solution is to release OH− ions constantly and slowly into the reaction medium. In aqueous medium, Zn2+ ions released from Zn (NO3)2·6H2O reacts with OH− ions to form insoluble Zn(OH)2, a quasiprecursor which gives a cloudy milky appearance to the reaction medium. Simultaneously OH− ions react with Zn(OH)2 quasi-precursor and forms Zn(OH)42− growth units. This reacts with cylindrical micelles of PVP through electrostatic interaction by amide groups of the pyrrolidone rings to form [PVP- Zn(OH)42−] complexes, which play a crucial role in controlling the size and shape of the nanotubes [26]. Under hydrothermal conditions, the complexes dehydrate to form ZnO nuclei, which nucleate and grow along (0001) direction. Due to the polar crystal growth, the as grown nuclei undergo oriented attachment along the c-axis to form hollow hexagonal tube like pattern [27]. The corresponding reactions are shown in Fig. S4.

Fig. 1. Powder X-ray diffractograms of ZnO nanocapsules (ZNC), nanoparticles (ZNP), nanotubes (ZNT) and nanowires (ZNW).

electrochemical analyzer equipped with Oriel LCS-100 solar simulator characterized (AM 1.5G and 100 mWcm−2). 3. Results and discussion 3.1. Structural studies The XRD patterns shown in Fig. 1 were used to deduce the specific crystal structure, crystallite size and phase composition of the synthesized ZnO nanostructures. Nine distinct diffraction peaks are identified for ZNW (JCPDS: 75-0576), ZNT (JCPDS: 89-0510), ZNP (JCPDS: 792205) and ZNC (JCPDS: 75-0576) and all the peaks are indexed which matched with the hexagonal wurtzite structure of ZnO. The lattice constants, a and c were calculated with respect to the (100) and (002) planes. No additional phase formations and impurity peaks are observed in the XRD patterns of each sample. The results reflect the high purity of the crystalline-phase of ZnO structures. The strong and tapered peaks at (002) of ZNW, ZNT, ZNP and ZNC show the preferential growth directions along the c-axis. The Scherrer equation has been used to calculate the average crystallite size of the nanostructures [19]. The crystallite size calculated for ZNC, ZNP, ZNT and ZNW corresponding to their (101) planes are 51.949 nm, 32.072 nm, 58.674 nm and 28.459 nm respectively. The strong peaks observed for (100) and (101) peaks indicate a high aspect ratio of the nano-structures, ZNT and ZNW. The increase in crystallinity in terms of the crystallite size is attributed to the relaxation of the in-built planar compressive strain developed in the ZnO structures. The unit cell volume (v), interplanar distance (dhkl), bond length (L) and Young's modulus of the nanostructures can be evaluated from the XRD data in accordance with the maximum intensity plane (101) by using the lattice geometry equations that have been described in detail elsewhere [20]. The c/a ratio is found to be constant for all the ZnO samples in such a way that the four tetragonal distances remain almost constant through the tetragonal distortion angle of the unit cell (Table 1). These values are also close to the ideal hcp of the hexagonal wurtzite ZnO reflecting the long range polar interactions. The obtained bond length values are also in good agreement with the bond length, Zn–O of the unit cell. 3.2. Morphological studies FESEM images of the synthesized nanostructures and the commercially obtained nanocapsules are shown in Fig. 2a-h at different magnifications. In ZNC, the nanostructures are in the form of nanocapsules. 3

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Table 1 Lattice constants (a and c), Atomic packing factor (c/a), interplanar distance (dhkl), unit cell volume (v), bond length (L) and the Young's modulus (Yhkl) of the nanostructures. Nanostructure

ZNC ZNP ZNT ZNW

Lattice constants a

c

3.248 3.253 3.251 3.249

5.199 5.206 5.202 5.194

c/a ratio

dhkl (Å)

ν (Å)3

L (Å)

Yhkl (GPa)

1.600 1.600 1.600 1.598

2.476 2.479 2.477 2.475

47.442 47.652 47.557 47.426

1.976 1.978 1.977 1.975

118.863 118.862 118.860 118.858

probing chemical bond vibrations. The space group of wurtzite structure of ZnO is P63mc. As per the Group theory, the optical phonon symmetry at the Γ point of the Brillouin zone can be represented by the irreducible representation, Γopt = A1+2B1+E1+2E2 [31]. Phonon of A1 and E1 symmetry is polar phonons and split up to transverse-optical (TO) and longitudinal-optical (LO) components. Both these modes are Raman and IR active. The Raman active non-polar E2 modes have two frequencies; E2low and E2high modes corresponding to the movement of oxygen atoms and zinc sub-lattice respectively [32]. B1 modes are both Raman and IR silent. Raman spectra observed for ZNC, ZNP, ZNT and ZNW fitted with Lorentzian functions are shown in Fig. 5(a–i). All the Raman spectra were split up to three for fitting. For describing Raman spectral modes, the sample ZNT has been considered here. A very narrow intense peak at ~97 cm−1 is associated with E2low mode of zinc sub lattice vibrations [33]. Acoustic phonon overtones 2 TA or 2E2low modes are observed at ~202 cm−1 [34]. The intense peak at ~329.63 cm−1 represents the second order multi-phonon Raman scattering arising from zone boundary phonons E2high - E2low for hexagonal wurtzite ZnO single crystals [35]. The vibrational modes at ~384.59 cm−1 and ~419.98 cm−1 are ascribed to A1(TO) and E1(TO) respectively [36]. Another very intense Raman mode is observed at ~436.49 cm−1 corresponding to E2high phonon vibrations of oxygen lattice [37]. A1(LO) mode and E1(LO) mode are observed at ~545.14 cm−1 and ~579.57 cm−1 as broad peaks due to the superposition of A1(LO) and E1(LO) modes which originated from second order Raman scattering. A highly intense broad peak corresponding to E1(LO) modes of ZNW shows that it is highly oxygen deficient. A

During the formation of ZnO nanowires, SDS, the anionic surfactant plays a key role in controlling the growth and morphology. Anionic surfactants in higher concentration lead to the formation of cylindrical reverse micelles in aqueous solution when the concentration reaches above its CMC [28]. These micelles can act as micro reactors during the hydrothermal process without any change in cylindrical nature [29]. A supersaturated Zn(OH)42− precursor solution is obtained at higher concentrations of Na2CO3. The space needed for nucleation and crystal growth is created in the interior of the reverse micelle, where the hydrophilic group of SDS ions and the Zn(OH)42− complex electrostatically interact. When the reaction proceeds, the micro reactors formed do not change their shape and act as nucleation center for space confined crystal growth of ZnO. The growth units of Zn(OH)42− are subsequently incorporated into the active sites formed by the micelle. To minimize the free energy of the entire system, preferential growth along [0001] direction would result in the formation of ultra-long nanowires with small diameter or prolongation of reaction time, due to surface energy anisotropy and oriented attachment. The corresponding chemical reactions are shown in Fig. S5. Fig. 4 represents the schematic depiction of the proposed mechanisms for ZNP (a), ZNT (b) and ZNW (c) growth [30]. 3.3. Raman spectral studies Raman spectral analysis is an effective tool to evaluate the nanosturcture quality, the phase and purity which further discuss the transport properties and phonon interactions with the free carriers by

Fig. 2. FESEM images of ZnO nanocapsules (a and e), nanoparticles (b and f), nanotubes (c and g) and nanowires (d and h). The scale bar for first row images is 1 μm and for the second row is 200 nm. The average particle size distribution in ZnO nanocapsules (i), nanoparticles (j), nanotubes (k) and nanowires (l). 4

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Fig. 3. TEM images of ZnO (a) nanocapsules, (b) nanoparticles, (c) nanotubes and (d) nanowires. HRTEM images of (e) nanocapsules, (f) nanoparticles, (g) nanotubes and (h) nanowires as lattice fringes (The interplanar spacing is marked). SAED pattern of (i) nanocapsules, (j) nanoparticles, (k) nanotubes and (l) nanowires.

shrinkage [41]. All the ZnO samples exhibit high reflectivity over the wavelength region (400–800 nm) except ZNW which shows a decreased reflectivity in 400–650 nm range and an increase above 650 nm. ZNT and ZNP show superior reflectivity than ZMP due to high light scattering ability in the visible light.

combination of the acoustic and optical modes of vibration is observed at ~663.96 cm−1 and ~1050.67 cm−1 [38]. The broad peaks observed at ~984.28 cm−1 and ~1107.05 cm−1 are due to the overlapping of 2TO and 2LO modes respectively. The broad peak at ~1153.70 cm−1 corresponds to 2A1(LO)/E1(LO)/2LO modes of vibrations [39]. The wavenumbers of Raman active wurtzite ZnO Γ-point phonon vibrational frequencies for ZNC, ZNP, ZNT and ZNW are listed in Table 2.

3.5. Photoluminescence studies Photoluminescence spectra (PL) of ZnO samples were recorded with an excitation wavelength of 350 nm at normal temperature (30 °C) and are shown in Fig. 7a. Presence of extrinsic and intrinsic deep level defects in ZnO result in emissions of different colors in visible region. PL spectrum of ZnO exhibits two major peaks: an intense and sharp near band emission peak at ~380 nm and a broad visible emission from 400 to 700 nm (mainly due to green-yellow emissions or deep level emission (DLE)) [42]. For representing the PL emission peaks, the spectrum for ZNT was Gaussian fitted and marked in Fig. 7a. In ZNT, UV-emission is observed at 402 nm, violet emissions at 420 nm and 432 nm, blue emission at 455 nm and 470 nm, green emission at 490 nm, and yellow emission at 562 nm. The yellow and orange-red emissions are mainly due to the presence of excess oxygen in the samples or due to the presence of oxygen interstitial defects [43,44]. The origin of green luminance (GL) is associated with the surface states formed by zinc vacancies (Vzn) localized at non-polar (101 0) surface especially in na‾ ZNW, a quenching in green norods and nanowires [45]. In the ZNT and emission (GL) is observed. This is mainly due to the lack of Vzn created at the grain boundaries, which modify the charge state of the neighboring oxygen atoms by creating local electron deficiencies at the surface and imparting charge neutrality especially in self-catalyzed homogeneous growth. ZNW is highly oxygen deficient and enriched with Zn. So it reflects least intense GL peak in comparison with other structures. GL peak of ZNT exhibits intermediate intensity. Also the near band emission is lowered while moving from base to the tip of the

3.4. Optical properties The optical properties of ZnO nanosturctures; ZNC, ZNP, ZNT and ZNW, in terms of the diffuse reflectance spectra (DRS) are shown in Fig. 6a. The UV–visible absorption spectra of the nanostructures are shown in Fig. S6. The absorption maxima of all the structures were shifted to lower wavelengths (356 nm, 353 nm, 358 nm and 360 nm for ZNC, ZNP, ZNT and ZNW respectively) from the ZnO bulk wavelength of 380 nm due to quantum confinement effect of excitons. At nanoscale dimensions, the energy levels are confined to the surface and a decreased overlapping and increased band gap energy are observed which indicate electronic coupling between nanocrystals in the aggregate [40]. All the samples exhibit fundamental absorption in the UV-range and low absorption in the visible range. Band gap energy of the samples were calculated from the Tauc plot (Fig. 6b) of the modified Kuelka–Munk function [F(R)hν]2. The band gap energy values are 3.27 eV, 3.32 eV, 3.23 eV and 3.22 eV for ZNC, ZNP, ZNT and ZNW respectively. The reduced band gap energy is a sign of high crystalline quality and grain size of the samples. Among all the structures, ZNW shows a narrower band gap of 3.22 eV and exceptional absorption pattern in comparison with other samples. Compared with the white color of the other samples, ZNW exhibits a pale yellow color and shows a red shifted absorption. The highest oxygen vacancy concentrations effectively extend the light absorption edge, since oxygen acts as trap center and creates impurity levels near the valence band, and induce band gap 5

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Fig. 4. The schematic depiction of the proposed growth mechanisms for the formation of (a) ZnO nanoparticles, (b) nanotubes and (c) nanowires.

3.6. N2 adsorption-desorption analysis

nanowires and nanotubes. Highly intense DLE peak observed in ZNC and in ZNP may be due to the absence of Zn ions. As a result, empty energy states are created at Fermi level in the close proximity of the valence band which leads to the formation of low density empty states for electron acceptance during emission [46]. The CIE 1931 color space chromaticity diagram in the (x, y) co-ordinates system for ZNC, ZNP, ZNT and ZNW are shown in Fig. 7b. The color purity for the nanostructures; ZNC, ZNP, ZNT and ZNW can be visualized as red, green and blue vertex regions with the chromaticity co-ordinates (0.25, 0.32), (0.32, 0.32), (0.28, 0.28) and (0.27, 0.23) respectively. The co-ordinates for ZNC are located at blue-green region and co-ordinates for ZNP is located at near white light edge whereas co-ordinates for ZNT and ZNW are shifted towards purple-blue region due to the decrement of emission peaks. The samples shine in accordance with the color coordinates under PL emission.

The N2 adsorption-desorption isotherms measured by BET analysis are shown in Fig. 8. Type-IV isotherms are observed for all the nanostructures with a type-H3 hysteresis loop at the P/P0 ranging from 0.7 to 1.0 due to the existence of mesoporous structure [47]. The appearance of small hysteresis is due to capillary condensation within the pores. The BET surface area, pore area, pore size and pore volumes for ZNW, ZNT, ZNP and ZNC are listed in Table 3. The highest surface area of 10.4052 m2 g−1 is observed for ZNP. High adsorption capacities of the samples were displayed at high relative pressure of P/P0 > 0.8 which reflects the co-existence of mesopores and macropores within the nanostructures. Besides surface area analysis, BJH pore size distribution analysis was performed to measure the diameter and distribution of mesopores in the nanostructures. The pore size distribution curves are represented in Fig. S7. It can be 6

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Fig. 5. Raman spectra of ZnO nanoparticles (a–c), nanotubes (d–f) and nanowires (g–i).

respectively within the thin films during sensitization. The increase in pore size improves the electrolyte diffusion and also prevents the formation of Zn2+/dye complex in the case of ZNW and ZNT [48].

Table 2 The wavenumbers of Raman active wurtzite ZnO Γ-point phonon vibrational frequencies for ZNP, ZNT and ZNW. Process

low

E2 2 TA,2E2low E2high – E2low A1(TO) E1(TO) E2high A1(LO) E1(LO) TA + LO 2TO TO + LO 2LO 2A1(LO), E1(LO), 2(LO)

Wavenumber (cm−1) ZNP

ZNT

ZNW

~97.17 ~202.91 ~328.83 ~378.96 ~406.90 ~435.84 ~538.21 ~583.75 ~651.14 ~982.03 ………… ~1112.74 ~1156.48

~97.38 ~185.43 ~329.63 ~384.59 ~419.98 ~436.49 ~545.14 ~579.57 ~663.96 ~984.28 ~1050.67 ~1107.01 ~1153.70

~99.32 ~200.53 ~330.60 ~381.12 ~417.43 ~437.82 ~543.46 ~578.65 ~663.10 ~973.92 ~1072.50 ~1110.63 ~1147.25

3.7. Photovoltaic studies of ZnO DSSCs The characteristic properties of the mentioned nanosturctures are highlighting the various ZnO architecture applications in DSSCs as photo-anode materials. The fabricated DSSCs with ZNC, ZNP, ZNT and ZNW were evaluated in terms of their J-V characteristics under test conditions. J-V characteristic curves of the fabricated DSSCs are shown in Fig. 9 and the photovoltaic performance parameters of the DSSCs along with that of the best reported in literature are listed in Table 4. The DSSC constructed with ZNT based photo-anode shows remarkably improved short-circuit photocurrent density (Jsc) of 1.812 mA cm−2, open-circuit potential (Voc) of 895 mV and hence the highest photovoltaic conversion efficiency of 1.109% among the other ZnO based DSSCs. All the ZnO films exhibit high reflectance in 390–800 nm wavelength range. The % reflectance decreased down to ~20% above 400 nm for all the nanostructures, but the decrease followed the order, ZNT > ZNP > ZNC > ZNW. Even though ZNT exhibits large particle size distribution of 250–300 nm, it possesses superior light scattering ability and comparatively low band gap. The second highest efficiency of 0.999% was observed for DSSC fabricated with ZNW. The highest efficiencies observed with one dimensional structure like nanotubes and nanowires are due to the unhindered direct pathways for fast electron transport to the collector electrode and further, the decrease in the probability of electron recombination reactions [49]. These unhindered electron transfer is also responsible for the highest open-

seen that there is an increase in pore diameter and in pore volume for the synthesized samples; ZNP, ZNT and ZNW in comparison with the relatively decreased pore diameter and pore volume of commercially available nanocapsules (ZNC). In ZNW, the macropore formation is observed with diameter of 150–250 nm. Formation of two peaks can be observed in the pore size distribution curve of ZNW. The first one centered at 24.2 nm represents supermicropores and the broad peak centered at 115.4 nm represents the wide range of mesoporosity in the nanowires. High surface area and pore volumes are remarkable parameters to host the dye molecules and dye infiltration process

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Fig. 6. Diffuse reflectance spectra (a) and Kubelka-Munk plot (b) of ZnO nanostructures.

circuit voltage observed with these 1D structures. The narrower band gap observed in ZNT and ZNW in turn produced highest photo-current among all the photoanodes. A decrease in efficiency of the cell made up of ZNW is due to relatively very low reflectivity of ZNW especially in the 400–600 nm range compared to other structures. In the case of ZNP and ZNC, the nanoparticles are randomly distributed and undergo a sequence of inter-particle hopping steps while moving towards the collector electrode. The tangled network constituted by dead-ends and innumerable nanoparticle boundaries in the sintered nanoparticle film of ZNP and ZNC cause electron bouncing which limits the fast electron transportation and unfavorable electron recombination reactions at the interface [50]. An increase in efficiency of ZNP based DSSC in comparison with ZNC based one is mainly because of the high surface area and porosity. The dye infiltration during sensitization can be improved by an increase in pore volume and the electrolyte diffusion under test conditions [48]. The irregularities in morphology observed in ZNC results in unfavorable electron recombination which lowers the photocurrent generation and the overall efficiency of the device. The photovoltaic conversion efficiency of 1.109% (present study) is the highest reported efficiency for DSSCs using ZnO semiconductor and N3 dye.

Fig. 8. N2 adsorption-desorption isotherms of ZnO nanocapsules (ZNC), nanoparticles (ZNP), nanotubes (ZNT) and nanowires (ZNW).

3.8. Electrochemical interface charge transfer studies of ZnO based DSSCs (EIS). The Nyquist plots for the DSSCs are shown in Fig. 10. A typical EIS spectrum is composed of three semicircles. One at frequency

Electrochemical interface charge transfer properties of the fabricated DSSCs are evaluated by electrochemical impedance spectroscopy

Fig. 7. (a) Photoluminescence (PL) spectra and (b) CIE 1931 color space chromaticity diagram of ZnO nanostructures. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 8

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Table 3 BET surface area characteristics and BJH pore-size distribution analysis of ZnO nanocapsules (ZNC), nanoparticles (ZNP), nanotubes (ZNT) and nanowires (ZNW). Nanostructure

BET surface area (m2.g−1)

Pore area (m2.g−1)

Pore diameter (nm)

Pore volume (cm3.g−1)

ZNW ZNT ZNP ZNC

2.6800 4.5545 10.4052 3.6040

1.6857 1.7792 7.9794 1.4208

53.6445 20.7009 18.2934 12.6836

0.021005 0.012074 0.039758 0.008389

Fig. 10. Nyquist plots of the electrochemical impedance spectroscopic analysis of DSSCs fabricated using ZnO nanocapsules (ZNC), nanoparticles (ZNP), nanotubes (ZNT) and nanowires (ZNW) constituting photoanodes.

frequency intercept on the real axis). CPE1 is the chemical capacitance in terms of constant phase element of the ZnO film [55]. R2 is the charge transfer resistance at the ZnO/N3-dye/electrolyte interface and the corresponding constant phase element is CPE2. R3 and CPE3 are the charge transfer resistance and double layer capacitance in terms of constant phase element at the counter electrode/Pt interface (platinized FTO) [56]. In the equivalent circuit of ZNP, presence of inductance at counter electrode/Pt interface was also observed. The R2 value is found to be high in ZNC (390.5 Ω) and low in ZNT (105.2 Ω). Therefore huge charge transfer resistance is observed in ZNC which limits electron transport across ZnO/N3-dye/electrolyte interface, resulting in low efficiency of the device.

Fig. 9. J–V curves of DSSCs fabricated using ZnO nanocapsules (ZNC), nanoparticles (ZNP), nanotubes (ZNT) and nanowires (ZNW) as photoanode materials.

interception is ascribed to a series resistance (R1). The largest semicircle at the mid frequency region is reflecting the charge transfer resistance (R2) at the ZnO/N3-dye/electrolyte interfaces [54]. The smallest third semicircle at the low frequency region either corresponds to the Warburg diffusion of I−/I3− redox couple within the electrolyte or the charge transfer resistance at the platinized FTO. In the observed spectra, the first and third semicircles are merged with the middle one. R2 values for the DSSCs follow the order, ZNT < ZNW < ZNP < ZNC. So the devices fabricated with ZNT and ZNW exhibit least interface charge transfer resistance and accordingly efficient charge transportation is possible through the ZnO/N3-dye/electrolyte interfaces. By using Randomize-Levenberg-Marquardt method, an equivalent circuit is employed to fit the experimental data of impedance to extract parameters of DSSC related to electron transport and recombination. The obtained parameters corresponding to the equivalent circuits represented in Fig. 11 of EIS are listed in Table 5. Here, R1 represents the series resistance, including the sheet resistance of the FTO glass and the contact resistance of the cell (the high

4. Conclusions In summary, ZnO nanostructures with distinct morphologies like nanoparticles, nanotubes and nanowires were synthesized through simple co-precipitation and hydrothermal methods. The formation of yellowish nanowires through simple sodium dodecyl sulfate (SDS) capped hydrothermal reaction was demonstrated. ZnO nanotubes with high crystallinity were synthesized by hydrothermal reaction at 120 °C using polyvinylpyrrolidone (PVP) as capping agent. Raman spectral characteristics of the synthesized nanostructures were correlated with the hexagonal wurtzite structure of ZnO. Comparatively low reflectance and PL emissions of ZnO nanowires confirm its oxygen deficiencies in the crystal lattice. All the synthesized nanostructures are integrated into solar cells as photoanode materials with N3-dye sensitizer. The material properties and dye solar cell performances of the nanostructures as photoanode materials were compared with the commercially available ZnO nanostructure. The maximum photovoltaic conversion efficiency of 1.109% was obtained by using nanotubes as photoanode materials

Table 4 Photovoltaic performance parameters of DSSCs fabricated using ZnO nanocapsules (ZNC), nanoparticles (ZNP), nanotubes (ZNT) and nanowires (ZNW) as photoanode materials along with that of the best reported in literature. DSSC

Dye

Jsc (mA.cm−2)

Voc (mV)

ff

η (%)

Ref.

ZNT ZNW ZNP ZNC ZnO nanosheets ZnO nanoparticle aggregation spheres ZnO bottle brush

N3 N3 N3 N3 D149 N719 N3

1.813 1.714 1.573 1.448 18.01 12.05 4.02

895 882 875 843 530 750 600

0.684 0.661 0.690 0.685 0.634 0.667 ……

1.109 0.999 0.950 0.835 6.060 6.010 1.000

Present Present Present Present [51] [52] [53]

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Fig. 11. Equivalent circuit models used to fit EIS data of the DSSCs fabricated with ZnO nanostructures; ZNC, ZNP, ZNT and ZNW.

Appendix A. Supplementary data

Table 5 The electrochemical interface parameters for DSSCs using equivalent circuits of EIS measurements. Parameter

R1 (Ω) R2 (Ω) R3 (Ω) CPE1 (F.s(a−1)) CPE2 (F.s(a−1)) CPE3 (F.s(a−1)) L3 (H) a1 a2 a3

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.12.045.

DSSC ZNW

ZNT

ZNP

ZNC

12.4 238.9 10.95 0.72 × 10−6 …… 4.5 × 10−6 ……. 0.137 …… 0.911

14.1 105.2 ……. ……. 19 × 10−6 ……. ……. ……. 0.775 …….

19.95 338.7 12.01 17.07 × 10−6 ……. ……. 9.373 0.7949 ……. …….

11.83 390.5 ……. ……. 5.17 × 10−6 ……. ……. ……. ……. …….

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due to its unhindered direct electron transport and comparatively high surface area and light scattering abilities. Electrochemical interface charge transfer resistances of all the fabricated devices were analyzed by EIS measurements and found that the nanotube based device experienced the least charge transfer resistance across the ZnO/dye/ electrolyte interface.

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements The authors thank the Ministry of Human Resource Development, India for the financial assistance for lab facilities and research fellowship.

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