Hierarchical rutile TiO2 aggregates: A high photonic strength material for optical and optoelectronic devices

Acta Materialia 119 (2016) 92e103

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Hierarchical rutile TiO2 aggregates: A high photonic strength material for optical and optoelectronic devices Ramireddy Boppella a, b, **, Arash Mohammadpour c, Sivaram Illa a, b, Samira Farsinezhad c, Pratyay Basak a, b, *, Karthik Shankar c, d, Sunkara V. Manorama a, b, *** a Nanomaterials Laboratory, Inorganic and Physical Chemistry Division, Council of Scientific & Industrial Research-Indian Institute of Chemical Technology (CSIR-IICT), Hyderabad 500 007, India b Nanomaterials Laboratory, Inorganic and Physical Chemistry Division, CSIR-Network Institutes for Solar Energy (CSIR-NISE), Hyderabad 500 007, India c Department of Electrical and Computer Engineering, University of Alberta, 9107e116 St, Edmonton, Alberta T6G 2V4, Canada d National Institute for Nanotechnology, National Research Council of Canada, 11421 Saskatchewan Drive Northwest, Edmonton, Alberta T6G 2M9, Canada

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 March 2016 Received in revised form 1 August 2016 Accepted 1 August 2016

Hierarchical monodisperse and poly-disperse rutile TiO2 aggregates featuring excellent light scattering properties were synthesized by a simple hydrolysis route at low temperature using sucrose as the structure directing agent. Various characterization techniques revealed that sucrose played a key role in controlling the shape and size distribution of hierarchical aggregates. The aggregates are formed by nano-sized crystallites and, therefore, are able to offer both a large specific surface area and strong light scattering behaviour. Films composed of monodisperse 465 nm rutile spheres coupled to a layer of commercial P25 anatase nanoparticles manifested a large photonic strength through the combination of a high haze ratio and a large back scattering-to-forward scattering ratio. The hierarchical rutile TiO2 aggregates when employed as the sole working electrode in DSSCs, achieved a 5.16% power conversion efficiency which is higher than that of commercial P25 (5.04%) with same thickness. In addition, the overall conversion efficiency of 7.26% was achieved when hierarchical aggregates were used as the light scattering overlayer of photoanode. The significant enhancement in overall conversion efficiency was attributed to the large photon absorption by increasing path length of photons due to the multiple scattering by hierarchical aggregates. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Rutile Hierarchical aggregates Mesoporosity Dye sensitized solar cells Scattering layer Power conversion efficiency

1. Introduction Dielectric spheres with diameters comparable to optical wavelengths are increasingly important in the science and technology of metamaterials due to their very low losses (especially when

* Corresponding author. Nanomaterials Laboratory, Inorganic and Physical Chemistry Division, Council of Scientific & Industrial Research-Indian Institute of Chemical Technology (CSIR-IICT), Hyderabad 500 007, India. ** Corresponding author. Nanomaterials Laboratory, Inorganic and Physical Chemistry Division, Council of Scientific & Industrial Research-Indian Institute of Chemical Technology (CSIR-IICT), Hyderabad 500 007, India. *** Corresponding author. Nanomaterials Laboratory, Inorganic and Physical Chemistry Division, Council of Scientific & Industrial Research-Indian Institute of Chemical Technology (CSIR-IICT), Hyderabad 500 007, India. E-mail addresses: [email protected] (R. Boppella), [email protected]. in (P. Basak), [email protected] (S.V. Manorama). http://dx.doi.org/10.1016/j.actamat.2016.08.004 1359-6454/© 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

compared to surface plasmons in metals), strong Mie resonances and their induction of both electric and magnetic dipoles in the farfield [1e3]. Films composed of dielectric spheres exhibit a range of interesting optical properties including but not limited to optical confinement, whispering gallery modes, directional light scattering, and slow and even frozen light [4e9]. Both photons and electrons take a random walk while propagating through photonic glasses consisting of disordered films made of monodisperse submicron semiconductor spheres. For this reason, high refractive index dielectric spheres and films thereof have been proposed as multifunctional elements for optical devices [1]. The main application areas for films composed of submicron dielectric spheres currently appear to be in photon management in solar cells, photon out-coupling in light emitting devices, random lasing, surface enhanced Raman scattering and high Q-factor sensing [10e14]. For photovoltaic devices, the principal feature of

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interest is the ability to vary the ratio of forward and reverse scattering, and potentially approach the first Kerker condition [15]. However, the desired angular distribution of scattering also depends on the particular solar cell configuration. In thin film solar cells based on silicon or CdS/CdTe active layers, dominant forward scattering with minimal back-scattering is desired to minimize reflection losses and a film of dielectric spheres would be a purely passive electronic element. However in dye-sensitized solar cells and other types of excitonic photovoltaics, the scattering layer is placed at the tail-end of the device (light passes through this layer last); dominant back-scattering with minimal forward scattering is desired in order to improve the harvesting of near-infrared photons poorly absorbed by the absorbing layer. In such cells, the scattering layer often also participates in light absorption and charge transport as an active photon management layer [16e20]. Ever since a power conversion efficiency of >11% for TiO2 based DSSCs was reported, in efforts to push the limits further, extensive work is focused to reproducibly design photoanode architectures that can promote higher efficiencies [21e23]. Several impressive attempts have been made to improve efficiency by designing new three-dimensional hierarchical nanostructures which can offer both high surface area for dye adsorption along with enhanced light scattering ability in one morphology [17e21]. Till date, anatase TiO2 particles with larger size (~in the 200e400 nm range) have been preferentially used to improve the light harvesting efficiency of DSSCs [16e20]. Even though rutile TiO2 possesses a higher refractive index, scatters white light more efficiently than anatase and is chemically more stable [24,25], it has not gained much ground as photoanodes. Only a few examples of employing rutile TiO2 phase having comparable particle size are available in the literature, primarily as comparative references to demonstrate the advantage of using anatase TiO2 [16,26,27]. The poor efficiencies recorded for rutile TiO2 were particularly ascribed to a low internal surface area that resulted in a lower amount of dye loading [28] and slow electron transport leading to lower VOC [29]. Consequently, little attention has been paid to improve upon the rutile TiO2 based photoanodes for DSSCs [29e33]. A recent study however, strongly indicates that the performance of DSSCs can be remarkably improved using rutile TiO2 microspheres constituting radially assembled nanorods when employed as a scattering over layer [29]. Evidently, the availability of a high surface area coupled with strong scattering can positively influence the overall conversion efficiency [30]. The confluence of these properties in one morphology with apparent performance gain merits wider investigation and poses a synthetic challenge for researchers to design exotic new architectures of rutile titania. In the present investigation, we establish a one-pot low temperature strategy to synthesize phase-pure hierarchical rutile titania mesoporous aggregates and demonstrate their superior performance when used as the scattering layer to fabricate DSSC photoelectrodes. The new synthetic approach is versatile, requires mild conditions, uses a classic solute (sucrose) as the structure directing agent, is relatively simple, reproducible and can be easily adopted for scale-up. Spherical aggregates of phase-pure rutile obtained are submicron sized, formed of numerous primary nanoparticles (~20 nm) which predominantly are monodomain crystallites as evidenced from electron microscopy and diffraction studies. The hierarchical architecture realized is in the targeted size ranges, and provides significant mesoporosity along with considerably high surface area and roughness as desired. Post synthesis and preliminary characterizations the products were formulated into a paste of suitable viscosity, used for fabrication of DSSC test cells and evaluated for their performance. Favorably, the morphology is also adequately polydispersed that aids in generating a tightly packed, co-continuous layer while retaining

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sufficient porosity in the photoanode film on FTO/glass substrate. Used as a scattering layer for the working electrode configuration, a clear enhancement in efficiency was observed when compared to photoanodes prepared with commercial Degussa P25 of similar thickness. An impressive overall power conversion efficiency (h) of 7.26% with an open-circuit voltage (VOC) of 0.78 V, short-circuit current density (JSC) of 13.9 mA cm
2 and a fill factor (FF) of 68% was achieved using these hierarchical mesoporous rutile TiO2 aggregates as a scattering layer. Notably, the efficiency gained in test cells fabricated is without the use of any co-sensitizers/surface modifiers and automation supported in-line assembly. We strongly believe, the possibility to considerably improve the performance levels further remains open employing such sophisticated state-of-the-art fabrication techniques. Besides, it is anticipated that these hierarchical mesoporous rutile architectures can also display remarkable properties as materials for photocatalytic application or anodes for storage/delivery devices. 2. Experimental 2.1. Synthesis of hierarchical rutile TiO2 aggregates Hierarchical rutile TiO2 aggregates were synthesized by a simple hydrolysis method at low temperature (55e60  C). In a typical synthesis, required amount of titanium tetrachloride was added to 5 mL of hydrochloric acid at room temperature in a fume hood followed by addition of 75 mL of water. During the addition, white fumes of HCl were evolved. Calculated amount of sucrose (TiCl4 to sucrose molar concentration ratio was 1:0.25, 1:0.5 and 1:1) was then added to the above resulting clear solution that was then hydrolyzed at 55e60  C under magnetic stirring for 4e5 h. The obtained TiO2 was washed with water several times and dried at 60  C in air overnight to produce the rutile titania spheres for characterization. The samples were calcined at 500  C for 3 h to get rid of the sucrose. Hereafter, the as synthesized hierarchical polydispersed and monodispersed spheres are referred to as RTS-1 (TiCl4: Sucrose 1:0.25) and RTS-2 (TiCl4: Sucrose 1:0.5) and calcined samples are denoted as CRTS-1 and CRTS-2, respectively. 2.2. Fabrication of photoelectrode The viscous paste of rutile TiO2 aggregates and nanocrystalline P25 TiO2 was obtained by the addition of ethyl cellulose and terpineol into the ethanol solution of respective TiO2 and ground for several hours. To prepare the DSSCs working electrode, nanocrystalline TiO2 film was deposited on fluorine doped tin oxide coated (FTO) glass (Sigma Aldrich, 7 U/sq, 2.3 mm thick) pre-treated with TiCl4 solution by doctor-blade technique, which was then heated to 500  C for 30 min. The over layer of hierarchical rutile TiO2 aggregates was deposited on the nanocrystalline film and heated by the same heating profile as above. The resulting TiO2 films were treated with a 40 mM TiCl4 (aq. solution) at 70  C for 30 min, then rinsed with ethanol and annealed again at 500  C for 30 min. The photoanodes prepared were immersed in acetonitrile/ tert-butanol solution (1:1 v/v) containing 5  10
4 M of cisdiisothio- cyanato-bis(2,20-bipyridyl-4,40-dicarboxylato) ruthenium(II) bis(tetrabutylammonium) (known as N719, Sigma Aldrich), and kept for 24 h at room temperature. The TiO2 film electrodes were further rinsed with anhydrous ethanol to remove physisorbed dye and dried at room temperature. Pt counter electrodes were prepared on the FTO glass using 0.5 mM H2PtCl6 solution, followed by heating at 400  C for 20 min in air. The electrolyte solution composed of 1-butyl-3-methyl-imidazolium iodide (BMII), 0.03 M I2, 0.1 M guanidinium thiocyanate (GSCN) and 0.5 M 4-tert-butylpyridine (TBP) in acetonitrile and valeronitrile

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(85:15 v/v). The resulting photoanode were assembled with the Ptcoated FTO counter electrode using a spacer (Surlyn 25 mm thick, Dyesol) to form a hole into which the electrolyte was filled to obtain the sandwiched DSSC cell. The active area of the working electrode film was representatively about 0.13 cm2. 3. Results and discussion Designing the photoanode structure in DSSCs is vital to realizing enhanced power conversion efficiencies (PCE). The material studied for the DSSCs in this work was fabricated by a simple one-step approach. Fig. 1 shows the SEM images of product obtained after hydrolysis and subsequent calcination at 500  C for 3 h to remove residual organic content. Fig. 1(a) and (b) are the images for the assynthesized polydisperse and monodisperse spheres, respectively. The size of the polydisperse spheres (RTS-1) ranged from ~150 to 650 nm (multimodal distribution, Fig. 1(e)) and the average size of the monodisperse spheres (RTS-2) was about y 466 ± 38 nm (Fig. 1(f)). After calcination, the hierarchical spheres produce relatively spherical aggregates and they preserve their spherical morphology

(Fig. 1(c)). It can be clearly seen that the hierarchical spheres possess a comparatively rough surface, which is favorable for high dye loading in DSSCs. The corresponding X-ray diffraction results showed (Fig. 1(d)) that the synthesized samples were well crystallized. The crystallite size and phase purity of synthesized samples were confirmed by X-ray diffraction analysis. The broad diffraction peaks ascribed to tetragonal rutile TiO2 (JCPDS No: 17e1272) and the maximum intensity (110) peak was used to estimate the crystallite size using the Scherrer's equation. The crystallite size was calculated to be 16.2 ± 0.5 and 14.6 ± 0.5 nm for RTS1 and RTS-2, whereas 19.1 ± 0.5 and 16.1 ± 0.5 for CRTS-1 and CRTS2 respectively, indicating that these hierarchical spheres are composed of nanocrystal subunits. High resolution transmission electron microscopy (HRTEM) reveals that the TiO2 hierarchical architecture consists of interconnected small nanoparticles with an average particle size of about 30e40 nm (Fig. 2(c)). A lattice spacing of about 3.2 Å corresponding to rutile (110) plane in the high magnification HRTEM images provides further evidence for the highly crystalline nature and phase purity of the samples (Fig. 2(d)). Closer observation also reveals the presence of multiple crystallites ~15e20 nm domain

Fig. 1. SEM images of as synthesized (a) polydisperse hierarchical spheres (RTS-1), (b) monodisperse hierarchical spheres (RTS-2) and (c) calcined hierarchical polydisperse aggregates (CRTS-1) (d) XRD pattern of as synthesized hierarchical polydispere (bottom) and monodisperse (top) spheres. Histograms representing the particle size distribution of (e) RTS-1 and (f) RTS-2 hierarchical rutile titania aggregates. The average particle size and standard deviation (s) was calculated by Gaussian fit to the data.

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Fig. 2. HRTEM images of hierarchical titania aggregates post calcination at 500  C: (a) CRTS-1 and (b) CRTS-2. Scale bar is 500 nm in both (a) and (b) micrographs. (c)e(d) HRTEM images of CRTS-1. Lattice fringes clearly visible in (d) could be indexed to the rutile (110) plane. The dotted regions depict the individual crystallites of the polycrystalline particles and are only guide to the eyes.

sizes in agreement with the XRD result that evidently form each individual particle (Fig. 2(d) and Figure SI-1). Interestingly, the crystallite boundary in each particle is almost indiscernible except for an evidence of slight mismatch in the lattice fringes. The observation strongly suggests a formation mechanism dominated by oriented attachment of crystallites during the stages of particle growth. The HRTEM results (Figure (a)-(b)) reveal the presence of evenly distributed nanopores inside the particle, which is beneficial for electrolyte diffusion and dye adsorption. This porous texture was confirmed by the corresponding nitrogen adsorption/desorption measurement, which will be discussed in the later section. To reveal the formation mechanism of hierarchical rutile TiO2 aggregates, multiple experiments were performed and the corresponding morphological evaluation was carried out by electron microscopy. The use of TiCl4 precursor as a function of changing sucrose concentration and without using sucrose, was studied and corresponding morphologies are presented in Figs. 1 and 2. When the hydrolysis of TiCl4 was performed in the absence of sucrose, needle shaped aggregates were observed with an average diameter of 400 nm (Figure SI-2(a)). The formation of needle shaped aggregates is similar to previous reports [34,35]. Addition of sucrose molecules to the TiCl4 solution delivers the diverse hierarchical spheres. As shown in Fig. 1(a), polydisperse hierarchical spheres were observed with a wide size distribution of 100e500 nm when the TiCl4 to sucrose molar concentration ratio was 1:0.25. When the sucrose concentration was increased to 1:0.5, well defined hierarchical monodisperse spheres were obtained with an average size of about 500 nm (Fig. 1(b)). Further increase in the sucrose concentration (1:1) leads to the formation of irregular shaped hierarchical

TiO2 structures (Figure SI-2(b)). The above SEM observations could imply that the sucrose concentration plays a decisive role in the formation of hierarchical spheres. On the basis of the above information, a possible growth mechanism has been proposed for the hierarchical rutile TiO2 spheres. Upon thermal hydrolysis of TiCl4, the crystalline structure of titania particles is strongly influenced by the acidity of the medium and the nature of additives [36,37]. Under such acidic conditions, formation of rutile structure is thermodynamically more favorable [37,38]. During the hydrolysis with continuous thermal treatment, rutile formation is favored by the corner shared bonding along the [001] direction yielding a needle like structure [38]. Our experimental observation along with morphological evolution confirms the formation of rutile TiO2 needles in absence of sucrose molecules with rapid and uncontrolled aggregation along the c-axis (Figure SI-2(a)). The SEM and TEM images reveal that the hierarchical structures appeared to be composed of several aggregated nanocrystals yielding larger spherical morphologies in sucrose solution. The mechanism of formation for this exotic hierarchical rutile titania structures can be rationalized on the following lines. Capping agents containing multiple hydroxyl groups (diols/triols/polyols) and/or carboxy groups chelates the metal atom at the surface and finds usage in stabilizing metal oxides nanostructures similar to the conventional surfactants and stabilizers [39]. Glucose, sucrose and other similar sugar moieties have shown interesting redox activities and stabilization through adsorption/chelation on certain crystal planes [39e41]. Sucrose, a non-reducing sugar in acidic medium (Hþ-ions) undergoes hydrolyzation to form glucose, a reducing sugar (see schematic in supporting information, Scheme

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SI-1). Concurrently, the initial hydrolysis of TiCl4 forming Ti(OH)4 undergoes a redox reaction with glucose under relatively mild temperature conditions to yield titania (TiO2) and gluconic acid. The byproduct gluconic acid and the excess glucose both stabilize the primary titania nanoparticles formed in-situ. The corner shared [TiO6] octahedral units of rutile phase post-nucleation (i.e. growth units) predominantly offer numerous exposed (001) planes. Hence, the chelation/adsorption of glucose and gluconic acid is primarily expected to occur on these planes thereby restricting the growth in the [001] direction and eventually leads to a spherical morphology of the primary particles. Post-stabilization of the primary nanoparticles, the slow self-aggregation is driven by the propensity of Hbond formation amongst the glucose/gluconic acid molecules adsorbed on the surface, coupled with the spontaneous tendency towards surface energy minimization [42]. This attribute was adequately evident in the present study, where we observed that the sucrose concentration also played a key role in controlling the morphology and size of the rutile titania hierarchical architectures. Driven by hydrogen bonding between glucose/gluconic acid molecules, the primary nanocrystallites form nanoaggregates via selfassembly that has been described as imperfect oriented attachment mechanism in literature [42,43]. The primary nanocrystallites possess high surface energies and have a strong tendency to aggregate into nanospheres by lattice fusion of the adjacent crystallographic planes. However, since there is no specific crystal plane orientation, anisotropic unidirectional crystal growth is not quite favored. Thus, the secondary structure also grows in a spherical geometry and the process continues until the nanospheres are selfstabilized by lowering their surface energy. The almost indiscernible crystallite boundary in the primary particles (HRTEM image, Fig. 2(d)) with matching lattice fringes strongly suggests a formation mechanism dominated by this imperfect oriented attachment of crystallites during the stages of particle growth. It is interesting to note that when the sucrose concentration is increased (1:0.5), the non-uniformly distributed hierarchical spheres transformed into uniformly distributed hierarchical spheres with significant reduction in overall size of the spheres. The formation of hierarchical aggregates relies on a synergetic effect of the mutual interactions between the sucrose/glucose and inorganic species. This could imply that the particle characteristics, such as, particle size, shape, distribution and degree of agglomeration are fairly dependent on the initial sucrose concentration. From experimental observations, the hydrolysis of TiCl4 and yield of the product notably decreases with increasing sucrose concentration. This might be due to the strong interaction of the organic molecules with the TiO2 nuclei limiting the random growth, resulting in smaller size of the particles with low polydispersity. TGA measurement obtained for the synthesized samples at different concentrations of sucrose are presented in Figure SI-4. As seen in Figure SI-4, the weight loss at 100  C can be attributed to the dehydration of the samples, and the weight loss at higher temperature corresponds to desorption and subsequent loss of the sucrose molecules. A higher percentage of weight loss is seen for RTS-2 samples, which correspond to the smaller particle size and indicates that higher sucrose concentration favors the formation of smaller nanoparticles. In addition to this, the results observed from XRD-crystallite measurements also revealed that the size of the nanocrystal subunits are relatively reduced in monodisperse aggregates compared to the polydisperse aggregates [44,45]. Apart from the concentration of the reactants, the reaction aging time and reaction temperature also show significant impact on the shape and phase of the resulting TiO2. At higher temperatures (65  C), a substantial amount of anatase phase was found which increases with increasing temperature resulting in irregular morphologies (Figure SI-3 and SI-5). These results imply that the sucrose

concentration plays an important role in the controlling the size and size distribution of hierarchical aggregates. The specific surface area and pore size distribution of the assynthesized and calcined samples were determined from nitrogen gas adsorption and desorption isotherms and are shown in Fig. 3. The BrunauereEmmetteTeller (BET) method was used to calculate the specific surface areas and the pore size distribution curves were calculated by the BarretteJoynereHalenda (BJH) model from desorption and adsorption branches of the isotherm [46]. The parameters estimated from these studies are summarized in Table SI-1. For the synthesized RTS-1 and RTS-2 samples, the specific surface area was 178.7 and 67.5 m2 g-1 respectively. The corresponding pore size distribution curves suggest that the samples exhibit type IV isotherms with H3 type hysteresis loop according to IUPAC classifications, indicating that the synthesized samples possess large porous structures [47]. After calcination treatment, the samples exhibited similar hysteresis loop that occurred at high relative pressure but with significantly lower adsorption/desorption values. As indicated in Table SI-1, the BET surface area decreases considerably for the calcined samples (CRTS1 and CRTS-2), with a concomitant decrease in pore size. The decrease in specific surface area for calcined samples along with decrease in pore size indicates densification and enlargement in crystal size [48]. It is also noticed that porosity (P) and surface roughness factors (R) are two important parameters which boost the efficiency of DSSCs by enabling high dye loading. Therefore P and R were estimated from the adsorption-desorption isotherms results [49,50]. The porosities (P) and roughness factor (R) are calculated using the following equations. P ¼ Vp/(q
1 þ Vp)

(1)

R ¼ q(1
P)S

(2)

where, Vp is the specific cumulative pore volume (cm3 g
1) and q is the density of rutile TiO2 (4.2743 cm3 g
1). The calculated data are enlisted in Table SI-1. The estimated porosity and roughness factors are 58%, 36%, 10.7%, 8.95% and 320.7, 184.6, 54.3, 45.8 for RTS-1, RTS2, CRTS-1 and CRTS-2 respectively, showing that the samples have high dye adsorption capacity. From these results, it can be anticipated that these rutile TiO2 hierarchical aggregates would be potential candidates for application in DSCCs. 3.1. Effect of rutile TiO2 hierarchical aggregates as a photoelectrode in DSSCs Dye sensitized solar cells (DSSCs) are being envisaged as a convenient and cost effective alternative to conventional silicon solar cells, particularly under diffused light conditions [51,52]. The cornerstone of any DSSC system is the photoelectrode (working electrode), a nanostructured and mesoporous layer of semiconducting oxide material/s, which apart from enabling dye adsorption and electrolyte diffusion, acts as a medium for photogenerated electron transport [53,54]. Nanostructured TiO2 has been perceived as an attractive electrode material primarily because of its commercial viability, chemical stability and unique electrical properties [55e60]. Attempts to fabricate photoelectrodes with TiO2 nanocrystals alone results in poor lightharvesting as a large portion of visible light is transmitted through the cell owing to low scattering [61]. Optical confinement of incident light within the photo-electrode can significantly increase the light absorption by back scattering [62e66]. According to Mie theory, particle dimensions comparable to the wavelength of light are best suited to do so [62]. Sizes apart, the refractive index of materials chosen is also a determining factor [67,68], and it has

0.0 40 0

20

40

3

Pore Radius (nm)

20 RTS-1 RTS-2 0 0.0

0.2 0.4 0.6 0.8 1.0 Relative Pressure (P/P0)

18 15

97

0.030 0.015

Pore Volume 3 -1 -1 (cm .g .nm )

0.2

21

(a)

-1

0.4

V (cm .g ) STP

0.6

60

3

-1

V (cm .g ) STP

80

Pore Volume -1 -1 3 (cm .g .nm )

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0.000

5 10 60 80 12 Pore Radius (nm) 9 6 3 0.0

(b) CRST1 CRST2

0.2 0.4 0.6 0.8 1.0 Relative Pressure (P/P0)

Fig. 3. Nitrogen sorption isotherms of the synthesized: (a) hierarchical rutile TiO2 spheres, and (b) calcined TiO2 aggregates. Inset shows the corresponding pore diameter distribution.

been established that a high refractive index material scatters light better [69,70]. Larger TiO2 particulates can promote scattering many-fold, extend the optical path-length of photons and excite more dye-molecules, thereby, increasing electron-hole pair generation within the electrode improving the photocurrent density [48]. However, large particles tender significantly reduced surfaceto-volume ratio that limits the dye-loading capacity of the photoelectrode. Over the years, hence, a multi-layered configuration has been adopted where use of ultrafine particles ensures higher dye loading while a layer of bigger grains enhances light scattering within and increases effective excitation. In general, most high efficiency DSSC photo-electrodes are patterned in a double layer structure that combines the benefits of high surface area nanocrystalline TiO2 (~20 nm) and a top layer comprising of considerably larger TiO2 particles (>200 nm) that improves scattering and light harvesting efficiency by optical confinement phenomena [48,71]. Here we investigate the large size (~400 nm) rutile TiO2 hierarchical aggregates employed directly as photoelectrode in DSSCs. The key factor motivating our research into submicron rutile spheres is to highlight their unique optical properties and present their potential for usage in optoelectronics and photonic devices. The influence of the size and size distribution of hierarchical aggregates was investigated to make preliminary evaluation on light harvesting efficiency. Considering the high surface area the hierarchical aggregates were directly employed as an active layer in DSSCs, and P25 was used for comparative purpose with same thickness (10 mm). P25 is low cost and widely reported in this field and also known as a mixed phase material (78% of anatase and 21% of rutile). The electrodes (Figure SI-6) were sensitized with N719 dye for 18 h before measuring the current-voltage (IeV) characteristics under standard AM 1.5 simulated sunlight with a power density of 100 mW cm
2. The photocurrent density-voltage (J-V) curves are presented in Figure SI-7(a) and the corresponding photovoltaic parameters are listed in Table SI-2. As shown in Figure SI-7(a), the DSSC photoanodes made with CRTS-1 exhibited the highest overall conversion efficiency of 5.16% with a shortcircuit photocurrent density (JSC) of 9.93 mA cm
2, an open circuit voltage (VOC) of 0.788 and a fill factor (FF) of 0.66. A shortcircuit current density of 9.56 mA cm
2, fill factor of 66% and open-circuit voltage of 0.801 V with overall conversion efficiency of 5.05% was attained for the P25 electrode, which is slightly lower than that of CRTS-1. In comparison with the CRTS-1 and P25, CRTS2 shows lower conversion efficiency (4.21%) due to the lower shortcircuit photocurrent density of 8.33 mA cm
2. The short-circuit

photocurrent density is the highest in device with CRTS-1, then P25 and lowest in the device with CRTS-2. This order is directly correlated with the dye adsorption data collected (Figure SI-7(b)), which can be attributed to their corresponding BET surface area results. It is observed that devices made with CRTS samples exhibits lower VOC than P25, which is ~12 mV less than that of mixed phase P25. This can be understood by the lower flat band potential (200 mV) of rutile TiO2 compared to its counterpart, anatase TiO2 [29,72,73]. Conversely, we observe almost similar VOC and fill factor for both CRTS-1 and CRTS-2 implying that size distribution does not significantly affect the difference between Fermi level and the redox potential of the electrolyte as well as charge transport [18]. It appears that only JSC is affected by the size distribution of rutile aggregates and directly relates to the different power conversion efficiencies. This is probably due to the better light harvesting efficiency (LHE) of CRTS sample and can be attributed to the improved dye loading capacity as well as light scattering properties of the films. The LHE of photoelectrode is significantly dependent on the extinction coefficient of the dye and the transport behaviour of light within the photoelectrode film. Generally, LHE is significantly enriched by the light scattering efficiency empowered by the optical absorption of scattered photons caused by the multiple reflections of photons within the photoelectrode. To investigate the light scattering efficiency of photoelectrode, diffused reflectance of each photoelectrode was recorded and is presented in Figure SI-8. As shown in Figure SI-8, CRTS photoelectrodes showed much more effective light scattering efficiency especially in 500e800 nm region than P25 photoelectrode. The reflectivity of the P25 decreased rapidly over 500 nm while the CRTS photoelectrode still maintained quite high reflectivity in the same range, indicating the CRTS photoelectrodes have better light-scattering ability than the P25. This can be attributed to the intrinsic scattering ability of rutile TiO2, which shows stronger reflectance than its counterpart of P25 and the comparable size of CRTS samples to the wavelength of visible light shows a resonant scattering. This is reasonable considering that the light scattering effect increases the light trapping phenomena when the particle size is comparable with incident light. It has been proved that the difference in the size and size distribution of spherical aggregates shows significant impact on the transport of light within the films so as to affect the optical properties of photoelectrode. Several investigations both theoretically and experimentally studied infer that the impregnation of larger particles into photoelectrode can improve the light harvesting efficiency by extending the light path. In addition to this,

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sample CRTS-1 showed higher light reflectivity over CRTS-2 in the region 500e800 nm. Thus, the polydispersed nature of CRTS-1 samples are expected to form disordered structure, which result in random multiple scattering when the light travels through the aggregate film [18,74]. Evidently, CRTS-1 sample showed significant absorption in visible region (shown in Fig. 4(c)), whereas almost no absorption was observed in the visible region for the film fabricated with CRTS-2 sample (This observation is explained in greater detail in the following section). Hence, the light scattering mechanism enhances the light harvesting efficiency of CRTS-1 photoelectrode as reflected by noticeable improvement in JSC, which subsequently leads to an overall higher conversion efficiency (h). These results imply that hierarchical aggregates have better optical and physical properties than P25 that significantly improves the power conversion efficiency. The effect of film thickness of CRTS-1 photoelectrode on photovoltaic performance was further examined in order to fully exploit its superior optical and physical properties. Considering the high specific surface area and superior optical properties, three different thicknesses of CRTS-1 films were fabricated and their photovoltaic properties were assessed. Fig. 4(a) and (b) shows the diffuse reflectance and optical absorption at different thickness of CRTS-1 film. As showed in Fig. 4(a), all three films showed excellent light scattering properties in the visible region of 400e800 nm and the scattering efficiency considerably increases with increase in film thickness. It should be noted that the scattering efficiency obtained is mainly due to the multidirectional scattering of the full visible and infrared spectra induced by the irregularity, nonperiodicity and broad size distribution of CRTS-1 sample. Therefore, the light scattering increases the photon life-time inside the film thereby improving light absorption of the film.

Correspondingly, the optical absorption (shown in Fig. 4(a)) obtained for different thickness of CRTS-1 films exhibits obvious difference in the absorption throughout the visible region (400e800 nm). The absorption below 420 nm represents the intrinsic optical absorption of rutile TiO2 caused by electron transitions from the valence band to the conduction band. The absorption above 420 nm i.e. in the visible region must be triggered by the size and size distribution of CRTS-1 samples [18]. The most significant increase in the absorption is observed for the films consisting polydispersed aggregates with thickness ~7 mm, and the absorption significantly reduces with increasing film thickness. Figure SI-7(b) and Fig. 4(c) shows the dye loading capacity and JeV characteristics respectively, for the DSSCs fabricated with different thickness and the detailed photovoltaic parameters are listed in Table SI-2. As showed in Figure SI-9, the dye loading capacity increases with increasing film thickness. Although the dye loading capacity is seen to greatly improve from 7 mm to 10 mm films, whereas from 10 mm to 15 mm film the dye loading capacity was not improved much (Figure SI-9). This may be due to the large size polydispersed aggregates that create more trap sites by forming a disordered structure with increasing film thickness. Consequently, it reduces the number of particles on film, correspondingly reducing the amount of adsorbed dye molecules. It can be seen from Fig. 4(a), when the film thickness increases from 7 to 10 mm, an obvious increase in JSC from 8.7 to 9.93 mA cm
2 was observed, resulting in a concomitant improvement of power conversion efficiency from 4.59% to 5.16%. When the thickness was further increased to 15 mm, the corresponding JSC increases to 11.1 mA cm
2, however the overall power conversion efficiency was decreased to 4.91%. This is mainly because of decrease in VOC and fill factor, which reduces the power conversion efficiency. The

Fig. 4. (a) Diffuse reflectance spectra obtained for CRTS-1 films with three different thickness, (b) Optical absorption spectra of CRTS-1 and CRTS-2 samples with different thickness, (c) Current density-voltage characteristics (JeV) for CRTS-1 photoelectrodes with different thickness measured under 1 sun illumination (100 mW cm
2 with a spectrum approximately AM 1.5G and an active area of 0.13 cm2), (d) Incident photon to current conversion efficiency (IPCE) curves for CRTS-1 based photoelectrodes with different thickness.

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reduction in VOC is due to the increase in the surface trap states with increasing film thickness resulting in a decrease in the Fermi level of TiO2 [75]. In order to investigate the charge transport and recombination in these photoelectrodes, the test cells fabricated were characterized using electrochemical impedance spectroscopy (EIS). Figure SI10(a) presents the Nyquist plots of the impedance spectra obtained at VOC under light illumination for different thickness of CRTS-1 photoelectrodes and the fitting data results are summarized in Table SI-3. Figure SI-11(c) depicts the Randle's equivalent circuit comprising several distributed elements appropriately connected in series and parallel that has been used to simulate the data generated in the EIS studies. All samples show two distinct regions in the complex-plane plots. The smaller semicircular contribution at higher frequencies with an evident asymmetry indicates presence of two contributions with significant overlap possibly because of similar relaxation time scales (t). The larger semicircle at lower frequency observed is primarily attributed to the dominant chargetransfer resistance at the TiO2/electrolyte interface coupled with the diffusionereaction impedance of electrons in TiO2 and the electrolyte redox species (I
/I
3 ) [76]. The smaller semicircle in the high frequency range is fitted to a charge-transfer resistance process at the redox electrolyte/Pt counter electrode interface. The larger semicircle in lower frequency range is ascribed to the recombination resistance and chemical capacitance across the TiO2/dye/electrolyte interface [77]. As can be seen from Table SI-3, the film with ~7 mm thickness showed high charge transfer and recombination resistance (Rct) than the film with higher thickness, implies that increasing film thickness will increase the chargerecombination between photoelectrode and electrolyte. This is because trap-sites including defects, surface states, and grain boundaries increases with film thickness and act as electron loss €tzel's research shows a centers. Several groups including Prof. Gra similar result and they demonstrated the optimum thickness of the nanocrystalline TiO2 film to produce highly efficient DSSCs is 12e14 mm [75,78]. Though, large number of electrons was generated in thick film as a result of higher dye loading; most of charge species are however captured by surface traps and do not effectively transfer to the external circuit. This could lower the electron density in conduction-band which would further lead to the lower VOC and fill factor. As a result, decrease in power conversion efficiency is observed for the thick film even though it exhibits high photocurrent density. The incident photon-to-current conversion efficiency (IPCE) spectra (Fig. 4(d)) could provide further evidence on the lightharvesting efficiency of the photoelectrodes [79,80]. The IPCE is defined as the ratio of the number of electrons extracted in the external circuit to the number of photons incident to the solar cell. As such, it is a function of LHE as well as charge injection (finj) and charge transport efficiencies (fcc) [29]. The IPCE of the ~15 mm film is higher than that of the other films between 400 and 650 nm, whereas 7 mm and 10 mm films showed the significant improvement in the long wavelength region (650e750 nm). The dyeloading capacity of each photoelectrode is reflected on the corresponding IPCE in the shorter wavelength region, while in the long wavelength region it was attributed to the light scattering efficiency of photoelectrodes. This implies that the lower IPCE values recorded in longer wavelength can be attributed to both lower fcc and light being reflected out of the cell before it can be absorbed by the dye molecule [29]. With the above considerations, hierarchical rutile TiO2 aggregates with higher light scattering properties are limited on the power conversion efficiency, however, photoelectrodes with 10 mm film can be considered as optimized framework for efficient dye adsorption, electron transport and light harvesting properties.

99

3.2. Effect of rutile TiO2 hierarchical aggregates as light scattering material in DSSCs In an additional series of experiments, the light scattering properties of the hierarchical aggregates were studied. In order to better scrutinize the light scattering properties, we fabricated four different photoelectrodes with the same thickness. An overlayer of ~5 mm thick of polydisperse and monodisperse aggregates were coated on ~10 mm standard P25 film, and the resulting optical and photovoltaic properties were compared with a 15 mm thick film composed of only standard Degussa P25 and with a film composed of polydisperse rutile TiO2 aggregates alone. Typical current density-voltage profiles of the test cells fabricated using the photoelectrodes are presented in Fig. 5(a) and the corresponding photovoltaic parameters are listed in Table SI-2. The results show that the DSSCs made with nanocrystalline TiO2 have a short-circuit current density (JSC) of 11.7 mA cm
2, fill factor (FF) of 66% and open-circuit voltage of 0.78 V with a power conversion efficiency (h) of ~6.02%. As discussed above, a short-circuit current density of 11.1 mA cm
2, fill factor of 59% and open-circuit voltage of 0.75 V with overall conversion efficiency of 4.92% was attained for the CRTS-1 electrode fabricated with only ~15 mm polydispersed rutile TiO2 aggregates. However, in both cases the power conversion efficiency was not much improved when compared to the previous case (Figure SI-7(a)). On the other hand, when the polydisperse and monodisperse TiO2 aggregates were employed as a scattering material on the nanocrystalline film, both these materials revealed higher photocurrent density of 13.9 mA cm
2 and 12.2 mA cm
2, respectively. The corresponding conversion efficiencies are 7.26% and 6.52% respectively, which are also remarkably higher than those of P25 and CRTS-1 working electrodes with same thickness. The most marked improvement was seen when CRTS-1 was employed as a scattering material. The superior conversion efficiency is mainly due to the improved JSC and FF obtained for P25þCRTS-1 and P25þCRTS-2, which are significantly higher than that obtained for only P25 photoelectrode. It is believed that the photocurrent density is strongly related to the dye adsorption capacity of the photoelectrode, however, the photoelectrode made with P25 has slightly higher dye adsorption values than the photoelectrodes made with P25þCRTS-1 and P25þCRTS-2 samples. The photoelectrodes derived from only CRTS-1 attain less dye adsorption than the other photoelectrodes. Irrespective of the dye adsorption, P25þCRTS-1 and P25þCRTS-2 photo-electrodes still exhibit higher efficiencies which are 17% and 9% respectively, higher than that of P25 electrode. This implies that the dye adsorption capacity of photo-electrode is not the only crucial factor responsible in improving the DSSCs efficiency [49]. The improvement in JSC may be due to the large surface area of first layer and the high photon absorption in the second layer owing to the enhanced scattering effect of the hierarchical aggregates [53,81]. In view of different dye adsorption capacities of these films, the variation in the solar cell conversion efficiency could hence be attributed to different light harvesting efficiencies (LHE) of the photoelectrodes. The LHE significantly depends on light scattering efficiency of scattering layer, which extends the travelling distance of light within the photo-electrode film and, thus, enhances the probability of interaction between the photons and dye molecules [74]. Therefore, the optical properties of the resulting photoelectrodes were studied and are shown in Fig. 6. It is important to note that the UVeVis transmittance spectra in Fig. 6(a) display the total transmittance, which consists of light that was scattered in the forward direction (diffuse transmittance) as well as regular transmittance. This data was collected from each sample by illuminating it at the entrance slit of an integrating sphere-based

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Fig. 5. (a) J-V characteristics of P25, P25þCRTS-1 and P25þCRTS-2 based DSSCs. Cells were illuminated at an intensity of 100 mW cm
2 with a spectrum approximately AM 1.5G, (b) Incident photon to current conversion efficiency (IPCE) curves of P25, P25þCRTS-1 and P25þCRTS-2 based DSSCs.

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Fig. 6. Optical properties of films composed of submicron rutile spheres employed as scattering layer compared to nanocrystalline TiO2 films. (a) Total integrated transmission (diffuse %T þ direct %T), (b) diffuse reflectance of corresponding films, (c) Ratio of light intensity scattered in the reverse (backward) direction to the intensity in the forward (propagation) direction and (d) Haze ratio.

spectrophotometer (Figure SI-12c). The 15 mm layer film of P25 nanoparticles has the highest transmission due to very low Mie scattering. The low levels of Mie scattering in the P25 film result from the diameter of the constituent nanoparticles (10e20 nm) being more than order of magnitude smaller than the wavelength of photons incident on the film. As the diameter of the nanoparticles becomes comparable to the wavelengths of incident photons, Mie scattering is strongly increased [22]. This behaviour is clearly seen for nc-TiO2 films coupled to a layer of polydisperse (P25þCRTS-1) and monodisperse (P25þCRTS-2) submicron rutile spheres, which respectively exhibit the lowest and second highest levels of optical transmission for much of the visible spectral range (500e750 nm). It is interesting to note that a single layer of polydisperse rutile spheres has the lowest transmittance of all samples for

wavelengths larger than 750 nm. The diffuse reflectance measurements obtained for corresponding photoelectrodes showed a good agreement with transmittance results (Fig. 6(b)). The direction of propagation of light through single- and double layer films has significant effect on the reflectance magnitude due to the different refractive index profiles experienced by light. When diffuse reflectance measurements are performed with the light incident first on the scattering layer, as is typical in most studies, the reflectance curves are generic and similar and do not provide much information on the specific optical properties of the scattering layers. Such curves are characterized by a sharp peak in the reflectance at ~400 nm
450 nm (Fig. 6(b)), followed by a downward slope in the reflectance for longer wavelengths and are also exhibited by P25 nanoparticles, ZnO nanotetrapods, 500 nm diameter anatase spheres, 150 nm long SnO2 nanorods and many

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Fig. 8. Schematic representation of structure of photoelectrodes and photon path analog.

transparency make it an attractive constituent material for dielectric spheres in every type of scattering layer. In addition, recent work from the Saenz group has shown that the optimal value of the refractive index to simultaneously achieve a large extinction coefficient and zero backscattering is 2.47 ± 0.1, a condition satisfied by only three materials: TiO2, SrTiO3 and diamond, for 75e150 nm diameter particles [2,85]. In this report however, back-scattering dominates and over a broad range because of two reasons: the much larger size of the rutile spheres studied here compared to the size required to achieve the first Kerker condition at visible wavelengths and the hierarchical nature of the rutile spheres which present void-like inclusions that produce larger bandwidth resonances [86]. The backscattered light excites more dye molecules in the transparent layer thereby increasing the light harvesting capacity of the photo-electrode manifested as a high overall conversion efficiency (see Fig. 8).

4. Conclusion In conclusion, here we described the synthesis of hierarchical rutile TiO2 structure consisting of both polydisperse and monodisperse aggregates of nanocrystallites and studied as scattering layer for dye-sensitized solar-cell electrodes. The as-prepared hierarchical aggregates with high surface areas demonstrate multifunctional properties, including remarkable dye loading and excellent scattering properties in visible region. The results reveal that the overall energy-conversion efficiency of the cells could be significantly affected when hierarchical aggregates were used as scattering layer. The high photonic strength of the hierarchical rutile aggregates results in unique and hitherto unreported forward-scattering and back-scattering behaviour when coupled to

Ratio of backscattered to forward scattered intensity

other types of scattering layers [4,82e84]. The generically similar behaviour originates in the refractive index maximum occurring close to the band edge of the wide band gap semiconductor (TiO2, ZnO, SnO2) leading to maximum index contrast at the air-scattering layer interface and concomitant maximum reflection at wavelengths in the 400e450 nm spectral range. On the other hand, we always illuminated the samples for UVeVis measurements such that the light was incident on the glass substrate first, as is the case in DSSCs. In this configuration, light passes through the glass first, then through the transparent conductive oxide (TCO) layer, subsequently through the active layer and finally through the scattering layer. The refractive index differences at every interface between dissimilar materials produce reflections. In addition to situating the samples at the entrance slit of the integrating sphere, we performed optical measurements on each sample by placing them at the center of the integrating sphere, well before the integrating sphere, and at the exit slit of the sphere with and without a Spectralon (calibrated diffuse reflectance standard) chuck behind the sample. These measurements allowed us to estimate the direct transmittance, diffuse reflectance, total absorbance and amount of waveguiding loss in the samples in order to obtain deeper insights into the distinctive optical behaviour of films containing the rutile spheres. Fig. 6(c) shows that films containing the hierarchical rutile aggregates vastly outperform the standalone nc-TiO2 films in terms of the ratio of backward-toforward scattering over nearly the entire visible and near-infrared spectra range. This is significant because it is the desired behaviour in DSSCs, where photons reaching the low surface area scattering layer at the tail end of the active layer, are sought to be backscattered to the high surface area dye-coated nc-TiO2 layer where most of the charge generation and charge separation takes place. The haze ratio, which is defined as the ratio of diffuse transmission to total transmission, is a measure of the forward scattering efficiency of a film and is shown in Fig. 6(d) for the various samples studied in this report. Here films containing a layer of polydispersed rutile aggregates (CRTS-1) are shown to have the highest backward-to-forward scattering ratio as well as the highest haze ratio pointing to their high photonic strength [2]. Consequently, these films exhibit the strongest optical absorption (estimated from combined measurements of transmission and reflection) amongst all the samples subsequent to sensitization by N719 dye molecules (Fig. 7(a)). The highest optical absorption is exhibited by P25 layers coupled to monodispersed rutile spheres (CRTS-1). Films exclusively composed of the hierarchical rutile spheres exhibit a much smaller absorption despite their high photonic strength due to their low surface area available for dye adsorption in comparison to ncTiO2 films. The high refractive index of TiO2 and its visible light

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Fig. 7. (a) Optical absorption of TiO2 particle films after sensitization by N719 dye and (b) Backward-to-forward scattering ratio of TiO2 particle films after N719 sensitization. The coloring scheme for the plots is consistent with the corresponding unsensitized samples in Fig. 6.

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P25 films. The highest overall energy-conversion efficiency of 7.26% was achieved with the film formed by polydisperse rutile TiO2 aggregates with a broad size distribution from 150 to 650 nm in diameter. High dye adsorption ability, improved generation of electron-hole pair and light scattering by the submicrometer-sized TiO2 aggregates were employed to explain the improved solar-cell performance through extending the distance travelled by the incident light so as to increase the light harvesting efficiency of the photoelectrode film. Acknowledgements BR and SI acknowledge the Council of Scientific and Industrial Research (CSIR), India, and University Grants Commission (UGC), India for the Senior Research Fellowship (SRF). PB and SVM duly acknowledges the strong support of MNRE-CSIR TAPSUN Project on Dye Sensitized Solar Cells (DyeCell: GAP-0366) and the CSIR XII-FYP Project M2D (CSC-0134) for the grants received. AM, SF and KS acknowledge funding and equipment support from NSERC, NRC and CFI. SF thanks Alberta Innovates Technology Futures for scholarship funding. Appendix A. Supplementary data

[19] [20]

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[24]

[25] [26]

[27] [28]

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Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.actamat.2016.08.004.

[30]

References

[31]

[1] A.I. Kuznetsov, A.E. Miroshnichenko, Y.H. Fu, J. Zhang, B. Luk’yanchuk, Magnetic light, Sci. Rep. 2 (2012) 492. [2] W.L. Vos, A. Lagendijk, A.P. Mosk, Light Propagation and Emission in Complex Photonic Media, 2015 arXiv preprint arXiv:1504.06808. [3] Q. Zhao, J. Zhou, F. Zhang, D. Lippens, Mie resonance-based dielectric metamaterials, Mater. Today 12 (2009) 60e69. pez-García, J.F. Galisteo-Lo pez, C. Lo pez, A. García-Martín, Light [4] M. Lo confinement by two-dimensional arrays of dielectric spheres, Phys. Rev. B 85 (2012) 235145. [5] M.J. Mendes, I. Tobías, A. Martí, A. Luque, Light concentration in the near-field of dielectric spheroidal particles with mesoscopic sizes, Opt. Express 19 (2011) 16207e16222. [6] J. Grandidier, D.M. Callahan, J.N. Munday, H.A. Atwater, Light absorption enhancement in thin-film solar cells using whispering gallery modes in dielectric nanospheres, Adv. Mater 23 (2011) 1272e1276. [7] Y. Yao, J. Yao, V.K. Narasimhan, Z. Ruan, C. Xie, S. Fan, Y. Cui, Broadband light management using low-Q whispering gallery modes in spherical nanoshells, Nat. Commun. 3 (2012) 664. [8] Y.H. Fu, A.I. Kuznetsov, A.E. Miroshnichenko, Y.F. Yu, B. Luk'yanchuk, Directional visible light scattering by silicon nanoparticles, Nat. Commun. 4 (2013) 1527. [9] D.S. Wiersma, Disordered photonics, Nat. Photo 7 (2013) 188e196. [10] A. Mihi, M. Bernechea, D. Kufer, G. Konstantatos, Coupling resonant modes of embedded dielectric microspheres in solution-processed solar cells, Adv. Opt. Mater 1 (2013) 139e143. pez, M. Ibisate, C. Lo pez, FRET-tuned resonant random lasing, [11] J.F. Galisteo-Lo J. Phys. Chem. C 118 (2014) 9665e9669. [12] Z. Zhu, B. Liu, C. Cheng, Y. Yi, H. Chen, M. Gu, Improved light extraction efficiency of cerium-doped lutetium-yttrium oxyorthosilicate scintillator by monolayers of periodic arrays of polystyrene spheres, Appl. Phys. Lett. 102 (2013) 071909. [13] V.V. Yakovlev, Ultrasensitive Raman sensor based on a highly scattering porous structure, in: Proceedings of SPIE, Frontiers in Pathogen Detection: From Nanosensors to Systems, 2010, 75530H-75530H-8. [14] G.N. Plass, Mie scattering and absorption cross sections for absorbing particles, Appl. Opt. 5 (1966) 279e285. [15] B. García-C amara, F. Moreno, F. Gonzalez, J.J. Saenz, M. Nieto-Vesperinas, R. Gomez-Medina, On the Optical Response of Nanoparticles: Directionality Effects and Optical Forces, INTECH Open Access Publisher, 2012. [16] K. Hyung-Jun, K. Yong Joo, L. Yoon Hee, I.L. Wan, K. Yungkon, P. Nam-Gyu, Nano-embossed hollow spherical TiO2 as bifunctional material for highefficiency dye-sensitized solar cells, Adv. Mater 20 (2008) 195e199. [17] H. Fuzhi, C. Dehong, L.Z. Xiao, A.C. Rachel, C. Yi-Bing, Dual-function scattering layer of submicrometer-sized mesoporous TiO2 beads for high-efficiency dyesensitized solar cells, Adv. Funct. Mater 20 (2010) 1301e1305. [18] Z. Qifeng, P.C. Tammy, R. Bryan, A.J. Samson, C. Guozhong, Polydisperse aggregates of ZnO nanocrystallites: a method for energy-conversion-efficiency

[32] [33]

[34]

[35]

[36]

[37]

[38]

[39]

[40]

[41]

[42]

[43] [44]

[45]

[46]

enhancement in dye-sensitized solar cells, Adv. Funct. Mater 18 (2008) 1654e1660. P. Wenqin, Y. Masatoshi, C. Han, H. Liyuan, Ellipsoidal TiO2 hierarchitectures with enhanced photovoltaic performance, Chem. Eur. J. 18 (2012) 5269e5274. H. Se-Hoon, L. Sangwook, S. Hyunjung, S.J. Hyun, A quasi-inverse opal layer based on highly crystalline TiO2 nanoparticles: a new light-scattering layer in dye-sensitized solar cells, Adv. Energy. Mater 1 (2011) 546e550. T.S. Kang, A.P. Smith, B.E. Taylor, M.F. Durstock, Fabrication of highly-ordered TiO2 nanotube arrays and their use in dye-sensitized solar cells, Nano Lett. 9 (2009) 601e606. K. Shankar, J.I. Basham, N.K. Allam, O.K. Varghese, G.K. Mor, X. Feng, M. Paulose, J.A. Seabold, K.S. Choi, C.A. Grimes, Recent advances in the use of TiO2 nanotube and nanowire arrays for oxidative photoelectrochemistry, J. Phys. Chem. C 113 (2009) 6327e6359. C. Hong-Yan, K. Dai-Bin, S. Cheng-Yong, Hierarchically micro/nanostructured photoanode materials for dye-sensitized solar cells, J. Mater. Chem. 22 (2012) 15475e15489. E. Hosono, S. Fujihara, K. Kakiuchi, H. Imai, Growth of submicrometer-scale rectangular parallelepiped rutile TiO2 films in aqueous TiCl3 solutions under hydrothermal conditions, J. Am. Chem. Soc. 126 (2004) 7790e7791. N.G. Park, J. van de Lagemaat, A.J. Frank, Comparison of dye-sensitized rutileand anatase-based TiO2 solar cells, J. Phys. Chem. B 104 (2000) 8989e8994. H. Sarmimala, V. Carmen, K. Rainer, S. Herman, H. Andreas, Influence of scattering layers on efficiency of dye-sensitized solar cells, Sol. Energy Mater. Sol. Cells 90 (2006) 1176e1188. P. Wenqin, Y. Masatoshi, C. Han, H. Liyuan, Ellipsoidal TiO2 hierarchitectures with enhanced photovoltaic performance, Chem. Eur. J. 18 (2012) 5269e5274. S. Hore, P. Nitz, C. Vetter, C. Prahl, M. Niggemann, R. Kern, Scattering spherical voids in nanocrystalline TiO2 e enhancement of efficiency in dye-sensitized solar cells, Chem. Commun. 15 (2005) 2011e2013. J. Lin, Y.U. Heo, A. Nattestad, Z. Sun, L. Wang, J.H. Kim, S.X. Dou, 3D hierarchical rutile TiO2 and metal-free organic sensitizer producing dye-sensitized solar cells 8.6% conversion efficiency, Sci. Rep. 4 (2014) 5769e5777. L. Yuan, M. Yu Tao, Y. Lei, X. Xu Rui, Z. Xiao Wen, L. Xue Ping, Computer simulations of light scattering and mass transport of dye-sensitized nanocrystalline solar cells, J. Electroanal. Chem. 588 (2006) 51e58. E. Hosono, S. Fujihara, K. Kakiuchi, H. Imai, Growth of submicrometer-scale rectangular parallelepiped rutile TiO2 films in aqueous TiCl3 solutions under hydrothermal conditions, J. Am. Chem. Soc. 126 (2004) 7790e7791. N.G. Park, J. van de Lagemaat, A.J. Frank, Comparison of dye-sensitized rutileand anatase-based TiO2 solar cells, J. Phys. Chem. B 104 (2000) 8989e8994. R. Yichuan, L. Yaogang, Z. Qinghong, W. Hongzhi, Size-tunable TiO2 nanorod microspheres synthesised via a one-pot solvothermal method and used as the scattering layer for dye-sensitized solar cells, Nanoscale 5 (2013) 12574e12581. L. Yuanzhi, F. Yining, C. Yi, A novel method for preparation of nanocrystalline rutile TiO2 powders by liquid hydrolysis of TiCl4, J. Mater. Chem. 12 (2002) 1387e1390. Y.Q. Zheng, E.W. Shi, R.L. Yuan, Hydrothermal preparation and formation mechanism of titanium dioxide micro crystallites, Sci. China. Ser. E Tech. Sci. 29 (1999) 206e213. H. Yin, Y. Wada, T. Kitamura, T. Sumida, Y. Hasegawa, S. Yanagida, Novel synthesis of phase-pure nano-particulate anatase and rutile TiO2 using TiCl4 aqueous solutions, J. Mater. Chem. 12 (2002) 378e383. ac, E. Tronc, L. Mazerolles, J.P. Jolivet, Synthesis of brookite A. Pottier, C. Chane TiO2 nano particles by thermolysis of TiCl4 in strongly acidic aqueous media, J. Mater. Chem. 11 (2001) 1116e1121. B. Ramireddy, P. Basak, S.V. Manorama, Viable method for the synthesis of biphasic TiO2 nanocrystals with tunable phase composition and enabled visible-light photocatalytic performance, ACS Appl. Mater. Interfaces 4 (2012) 1239e1246. B.L. Cushing, V.L. Kolesnichenko, C.J. O'Connor, Recent advances in the liquidphase syntheses of inorganic nanoparticles, Chem. Rev. 104 (2004) 3893e3946. X. Sun, C. Zheng, F. Zhang, Y. Yang, G. Wu, A. Yu, N. Guan, Size-controlled synthesis of magnetite (Fe3O4) nanoparticles coated with glucose and gluconic acid from a single Fe(III) precursor by a sucrose bifunctional hydrothermal method, J. Phys. Chem. C 113 (2009) 16002e16008. P. Manjula, B. Ramireddy, V. Sunkara, A. Manorama, Facile and green approach for the controlled synthesis of porous SnO2 nanospheres: application as an efficient photocatalyst and an excellent gas sensing material, ACS Appl. Mater. Interfaces 4 (2012) 6252e6260. S.Y. Ho, A.S.W. Wong, G.W. Ho, Controllable porosity of monodispersed tin oxide nanospheres via an additive-free chemical route, Cryst. Growth Des. 9 (2009) 732e736. R.L. Penn, J.F. Banfield, Imperfect oriented attachment: dislocation generation in defect-free nanocrystals, Science 281 (1998) 969e971. A.E. Saunders, M.B. Sigman, B.A. Korgel, Growth kinetics and metastability of monodisperse tetraoctylammonium bromide capped gold nanocrystals, J. Phys. Chem. B 108 (2004) 193e199. P.L. Alec, I. Bridget, F.T. Michael, D.T. Richard, Effect of surfactant concentration and aggregation on the growth kinetics of nickel nanoparticles, J. Phys. Chem. C 117 (2013) 16709e16718. C. Pengfei, D. Sisi, C. Yaxin, L. Fengmin, S. Peng, Z. Jie, L. Geyu, Tripartite layered photoanode from hierarchical anatase TiO2 urchin-like spheres and

R. Boppella et al. / Acta Materialia 119 (2016) 92e103

[47]

[48]

[49]

[50]

[51] [52]

[53]

[54] [55] [56] [57]

[58] [59]

[60]

[61] [62] [63] [64]

[65]

[66]

[67]

P25: a candidate for enhanced efficiency dye sensitized solar cells, J. Phys. Chem. C 117 (2013) 24150e24156. B. Yang, X. Zheng, Y. Hua, L. Zhen, A. Rose, W. Lianzhou, Porous titania nanosheet/nanoparticle hybrids as photoanodes for dye-sensitized solar cells, Appl. Mater. Interfaces 5 (2013) 12058e12065. C. Dehong, H. Fuzhi, C. Yi-Bing, A.C. Rachel, Mesoporous anatase TiO2 beads with high surface areas and controllable pore sizes: a superior candidate for high-performance dye-sensitized solar cells, Adv. Mater 21 (2009) 2206e2210. M. Xiaohuan, P. Kai, L. Yongping, Z. Wei, P. Qingjiang, T. Guohui, W. Guofeng, Controlled synthesis of mesoporous anatase TiO2 microspheres as a scattering layer to enhance the photoelectrical conversion efficiency, J. Mater. Chem. A 1 (2013) 9853e9861. K. Soon Hyung, C. Sang-Hyun, K. Moon-Sung, K. Jae-Yup, S.K. Hyun, H. Taeghwan, S. Yung-Eun, Nanorod-based dye-sensitized solar cells with improved charge collection efficiency, Adv. Mater 20 (2008) 54e58. B. O'Regan, M. Gratzel, A low-cost, high-efficiency solar cell based on dyesensitized colloidal TiO2 films, Nature 353 (1991) 737e740. M.K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Müller, P. Liska, N. Vlachopoulos, M. Gratzel, Conversion of light to electricity by cisX2bis(2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium(II) charge-transfer sensitizers (X ¼ Cl-, Br-, I-, CN-, and SCN-) on nanocrystalline titanium dioxide electrodes, J. Am. Chem. Soc. 115 (1993) 6382e6390. , F. Arendse, P. Comte, M. Jirousek, F. Lenzmann, V. Shklover, C.J. Barbe €tzel, Nanocrystalline titanium oxide electrodes for photovoltaic appliM. Gra cations, J. Am. Ceram. Soc. 80 (1997) 3157e3171. €tzel, Photoelectrochemical cells, Nature 414 (2001) 338e344. M. Gra P. Wenqin, Y. Masatoshi, C. Han, H. Liyuan, Ellipsoidal TiO2 hierarchitectures with enhanced photovoltaic performance, Chem. Eur. J. 18 (2012) 5269e5274. A.J. Nozik, R. Memming, Physical chemistry of semiconductor
liquid interfaces, J. Phys. Chem. 100 (1996) 13061e13078. Y. Chiba, A. Islam, Y. Watanabe, R. Komiya, N. Koide, L.Y. Han, Dye-sensitized solar cells with conversion efficiency of 11.1%, Jpn. J. Appl. Phys. 45 (2006) L638. M. Gr€ atzel, Conversion of sunlight to electric power by nanocrystalline dyesensitized solar cells, J. Photochem. Photobiol. A 164 (2004) 3e14. P. Tiwana, P. Docampo, M.B. Johnston, H.J. Snaith, L.M. Herz, Electron mobility and injection dynamics in mesoporous ZnO, SnO2, and TiO2 films used in dyesensitized solar cells, ACS Nano 5 (2011) 5158e5166. B. Enright, D. Fitzmaurice, Spectroscopic determination of electron and hole effective masses in a nanocrystalline semiconductor film, J. Phys. Chem. 100 (1996) 1027e1035. €tzel, Solar energy conversion by dye-sensitized photovoltaic cells, M. Gra Inorg. Chem. 44 (2005) 6841e6851. H.C. Van de Hulst, Light Scattering by Small Particles, Wiley, New York, USA, 1957. F. Qiang, S. Wenbo, Mie theory for light scattering by a spherical particle in an absorbing medium, Appl. Opt. 40 (2001) 1354e1361. S.H.A. Lee, N.M. Abrams, P.G. Hoertz, G.D. Barber, L.I. Halaoui, T.E. Mallouk, Coupling of titania inverse opals to nanocrystalline titania layers in dyesensitized solar cells, J. Phys. Chem. B 112 (2008) 14415e14421. T.Y. Cho, H. Haitao, Z. Limin, X. Keyu, W. Yu, F. Tianhua, Li Jensen, Y.T. Wing, Direct and seamless coupling of TiO2 nanotube photonic crystal to dyesensitized solar cell: a single-step approach, Adv. Mater 23 (2011) 5624e5628. Z. Xiaojiang, H. Fei, S. Bo, F. Samira, P.D. Greg, S. Karthik, Photocatalytic conversion of diluted CO2 into light hydrocarbons using periodically modulated multiwalled nanotube arrays, Angew. Chem. Int. Ed. 124 (2012) 12904e12907. H. Sarmimala, V. Carmen, K. Rainer, S. Herman, H. Andreas, Influence of scattering layers on efficiency of dye-sensitized solar cells, Sol. Energy Mater.

103

Sol. Cells 90 (2006) 1176e1188. [68] S. Hore, P. Nitz, C. Vetter, C. Prahl, M. Niggemann, R. Kern, Scattering spherical voids in nanocrystalline TiO2 e enhancement of efficiency in dye-sensitized solar cells, Chem. Commun. 15 (2005) 2011e2013. [69] L. Yuan, M. Yu Tao, Y. Lei, X. Xu Rui, Z. Xiao Wen, L. Xue Ping, Computer simulations of light scattering and mass transport of dye-sensitized nanocrystalline solar cells, J. Electroanal. Chem. 588 (2006) 51e58. [70] N.M. Lawandy, R.M. Balachandran, A.S.L. Gomes, E. Sauvain, Laser action in strongly scattering media, Nature 368 (1994) 436e438. [71] Q. Yongcai, C. Wei, Y. Shihe, Double-layered photoanodes from variable-size anatase TiO2 nanospindles: a candidate for high-efficiency dye-sensitized solar cells, Angew. Chem. Int. Ed. 49 (2010) 3675e3679. [72] L. Kavan, M. Gr€ atzel, S.E. Gilbert, C. Klemenz, H.J. Scheel, Electrochemical and photoelectrochemical investigation of single crystal anatase, J. Am. Chem. Soc. 118 (1996) 6716e6723. [73] D.O. Scanlon, C.W. Dunnill, J. Buckeridge, S.A. Shevlin, A.J. Logsdail, S.M. Woodley, C.R.A. Catlow, M.J. Powell, R.G. Palgrave, I.P. Parkin, G.W. Watson, T.W. Keal, P. Sherwood, A. Walsh, A.A. Sokol, Band alignment of rutile and anatase TiO2, Nat. Mater 12 (2013) 798e801. [74] Z. Qifeng, S.D. Christopher, P. Kwangsuk, L. Dawei, Z. Xiaoyuan, J. Yoon-Ha, C. Guozhong, Light scattering with oxide nanocrystallite aggregates for dyesensitized solar cell application, J. Nanophot. 4 (2010) 041540. [75] W.Y. Zhao, H. Bala, J.K. Chen, Y.J. Zhao, G. Sun, J.L. Cao, Z.Y. Zhang, Thicknessdependent electron transport performance of mesoporous TiO2 thin film for dye-sensitized solar cells, Electrochim. Acta 114 (2013) 318e324. [76] J. Bisquert, F. de Fabregat-Santiago, Impedance spectroscopy: a general introduction and application to dye-sensitized solar cells, in: Dye-sensitized Solar Cells, EPFL Press, 2010. ISBN 9781439808665. [77] Y. Hua, P. Jian, B. Yang, Z. Xu, L. Xinyong, W. Lianzhou, Hydrothermal synthesis of a crystalline rutile TiO2 nanorod based network for efficient dye-sensitized solar cells, Chem. Eur. J. 19 (2013) 13569e13574. €tzel, M.K. Nazeeruddin, [78] S. Ito, T.N. Murakami, P. Comte, P. Liska, C. Gra M. Gr€ atzel, Fabrication of thin film dye sensitized solar cells with solar to electric power conversion efficiency over 10%, Thin Solid Films 516 (2008) 4613e4619. [79] X. Wu, G.Q. Lu, L.Z. Wang, Shell-in-shell TiO2 hollow spheres synthesized by one-pot hydrothermal method for dye-sensitized solar cell application, Energy Environ. Sci. 4 (2011) 3565e3572. [80] A.C. Fisher, L.M. Peter, E.A. Ponomarev, A.B. Walker, K.G.U. Wijayantha, Intensity dependence of the back reaction and transport of electrons in dyesensitized nanocrystalline TiO2 solar cells, J. Phys. Chem. B 104 (2000) 949e958. [81] S. Kambe, K. Murakoshi, T. Kitamura, Y. Wada, S. Yanagida, H. Kominami, Y. Kera, Mesoporous electrodes having tight agglomeration of single-phase anatase TiO2 nanocrystallites: application to dye-sensitized solar cells, Sol. Energy Mater. Sol. Cells 61 (2000) 427e441. [82] W. Chen, S. Yang, Dye-sensitized solar cells based on ZnO nanotetrapods, Front. Optoelectron 4 (2011) 24e44. [83] J.D. Peng, C.P. Lee, D. Velayutham, V. Suryanarayanan, K.C. Ho, Dye-sensitized solar cells containing mesoporous TiO2 spheres as photoanodes and methyl sulfate anion based biionic liquid electrolytes, J. Mater. Chem. A 3 (2015) 6383e6391. [84] J. Huo, Y. Hu, H. Jiang, W. Huang, C. Li, SnO2 Nanorod@TiO2 hybrid material for dye-sensitized solar cells, J. Mater. Chem. A 2 (2014) 8266e8272. [85] Y. Zhang, M. Nieto-Vesperinas, J.J. S aenz, Dielectric spheres with maximum forward scattering and zero backscattering: a search for their material composition, J. Opt. 17 (2015) 105612. [86] S.A. Mann, R.R. Grote, R.M. Osgood, J.A. Schuller, Dielectric particle and void resonators for thin film solar cell textures, Opt. Express 19 (2011) 25729e25740.