Supplementary Information Three Dimensional-tio 2 Nanotube Array Photoanode Architectures Assembled on Thin Hollow Nanofibrous Backbone and Their Performance in Quantum Dot- Sensitized Solar Cells

A. Electrode and solar cell preparation: (a) Fabrication of TiO 2 nanotube arrays on the FTO glass The TiO 2 nanotube arrays (TiO 2-NTs) were synthesized on the FTO glass as per our previous report. 1,2 A thin ZnO seed layer was deposited on the FTO glass by radio frequency magnetron sputtering. Arrays of ZnO nanorods, which serve as sacrificial templates for the TiO 2 nanotubes, were grown on the seed layer by a hydrothermal method at 85 o C for 10 h. Zinc nitrate hexahydrate (0.025 M) and hexamethylenetetramine (0.025 M) were used as precursor chemicals. The synthesized ZnO nanorod arrays on FTO glass were immersed in an aqueous solution consisting of 0.075 M ammonium hexafluorotitanate and 0.2 M boric acid at room temperature for 0.5 h. In this solution, ammonium hexafluorotitanate hydrolyzed to TiO 2 on the individual ZnO nanorod while ZnO dissolved simultaneously in the solution with acids produced by ammonium hexafluorotitanate hydrolysis. Subsequently, the resulting TiO 2 nanotube arrays immersed in 0.5 M boric acid solution for 1 h to remove the residual ZnO inside the tubes. The arrays were finally rinsed with DI water and calcined in Ar at 500 o C for 0.5 h to increase crystallinity. Fig. S1a shows a SEM picture of directly grown TiO 2 NT on FTO substrates.


Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted
demonstrate their excellent candidature as a photoanode in QDs-sensitized solar cells, exhibiting ~ 3 fold higher energy sc =8.8 mAcm -2 ) than that of the directly grown nanotube arrays on transparent conducting oxide (TCO) sc =2.5 mAcm -2 ) .15   The mesoscopic sensitized-solar cell is the emerging candidate in electrical power production though direct conversion of solar energy to electrical energy without green house effect. 1 Recently, quantum dot (QDs) semiconductors have attracted a great deal of interest as sensitizers in mesoscopic sensitized solar cells. 2,3 cause of the outstanding abilities in multiple hot carrier generation, panchromatic solar harnessing and high extinction coefficient, the quantum dot-sensitized solar cells (QDSCs) are being the future solar energy conversion systems. 46][7] These sensitizers are decorated on a wide band gap metal oxide framework (TiO 2 , ZnO and SnO 2 ) that acts as a photoanode (selective electron contact).Though QDSCs demonstrate feasible performance utilizing a variety of QDs sensitizers, still it requires more improvement to compete with the commercial dye-sensitized solar cells.
Semiconductor QDs sensitizers are relatively larger in size than dye molecules; therefore it is difficult to penetrate deeper parts of TiO 2 electrode and thus limiting the sensitizer loadings.
Although, the higher extinction coefficient of semiconductor QDs, in comparison with molecular dyes, partially compensates the loss of the effective surface and subsequently the decrease in 40 the sensitizer loading, configuring the photoanode framework with large-pore network is necessary to further promote the QDs sensitizer loading. 8In Addition, such photoanodes could demonstrate high charge transport from sensitizer to a charge collector, ultimately, overwhelming the charge recombination at photoanode/electrolyte interface.Thus, to achieve high sensitizer loading, fast electron transport channel, and good electrolyte pore-filling, establishing vertically aligned nanostructures, in particular, directly synthesized on transparent conductive oxides (TCO) has been identified as the promising approach in dye or QDs-sensitized solar cells. 9Most importantly, vertically grown nanotube (NT) arrays have longer electron diffusion length and more benefits in pore-filling of solid state hole transport materials (HTM), compared to disordered TiO 2 mesoporous films. 10iverse methods were demonstrated for the fabrication of TiO 2 55 NT arrays, including electrochemical anodization, 11 hydrothermal treatment 12 and vapour-liquid-solid methods.Recently, Gao group developed directly assembled TiO 2 NT arrays on TCO using ZnO nanowire templates. 13Though direct assembly of NT arrays on TCO substrates, is more adventurous, 14 template-based 60 NT arrays have wide tube-tube voids which resulted in lesser distribution compared to anodization technique.Besides, such less density of NTs on a TCO substrate markedly lowers the internal surface area of the electrode as well as limits the QD loading.

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One simple way to promote the interface surface area of the NT array is to extend their length, 15 however there exist a tradeoff between the NT length and mechanical stability.][18] Scheme 1 illustrates the fabrication stages of hierarchical 3-D hollow TiO 2 nanofibers (H-TiO 2 -NF).Our proposed hierarchical 3-D hollow TiO 2 NFs would be the optimum nanostructure for achieving higher sensitizer loading and fast electron transport for 75 QDSCs.In this communication, we demonstrate the fabrication of TiO 2 nanotubes branched on TiO 2 hollow nanofiber photoanode, directly grown on TCO and elucidate their candidature as an excellent photo anode in QDSCs.Figure 1 shows the scanning electron microscopy (SEM) image (Figure 1a) of backbone TiO 2 NFs confirming the continuous 1D geometry.The distribution of the fiber diameter lies between 200 and 500 nm with the average wall thickness of 20 nm.The ZnO NR templates with an average diameter ~ 25 nm 5 and a length of ~500 nm were vertically grown on the outer surface of TiO 2 NF which completely covered the backbone (Fig. 1b).After the TiO 2 thin layer coating on ZnO NRs, the ZnO templates were finally removed by selective etching (Fig. 1c).Fig. 1d shows the QDs-sensitized 3-D TiO 2 nanotubes branched 10 on TiO 2 hollow nanofibers (H-TiO 2 NF).The high resolution TEM images and the selective area electron diffraction pattern (SAED) reveal that the TiO 2 hollow nanofiber possess anatase phase and polycrystalline nature (Fig. 1e).Fig. 1(f) reveals that the spatially decorated ZnO NT arrays on TiO 2 NF have good contact with the TiO 2 backbone.Furthermore, TEM image (Fig. 2g and 2h) suggests that the TiO 2 tubular branches have sufficiently large pore channels for electrolyte filling as well as good structural stability even after removing the ZnO templates and QDs sensitization, respectively.2b and the estimated PV parameters are summarized in Table 1.The directly grown TiO 2 -NTs on a FTO electrode resulted in a photoconversion efficiency (PCE) of photovoltage, V oc =0.62 V, photocurrent, J sc =2.5 mAcm -2 and fill factor, F.F=58.3%.As anticipated, the hierarchical TiO 2 nanotube 60 branches grown on hollow NF backbone shows unprecedentedly promoted PCE oc =0.61 V, J sc =8.8 mAcm -2 and F.F.=50.3%.It clearly evidences that the TiO 2 NTs spatially assembled on the hierarchical 3D-nanofibrous backbone promote the QDSCs performance by a factor of three than the directly 65 grown TiO 2 NTs on a TCO substrate.We can explain the enhancement of photocurrent generation with the H-TiO 2 NF photoanodes by several contributions: (a) higher effective surface area and consequently higher QD loading and light harvesting; (b) highly efficient charge collection throughout the photoanode 70 with less boundary layers and (c) multiple scattering effect of the comb-like hierarchical NT arrays, in particular, red photons harvesting.On the other hand, it is interesting to point out that V oc obtained for both devices are similar, in spite of the larger effective surface area of H-TiO 2 NF, expecting a higher recombination rate (and consequently lower V oc ).But this is not the case as observed in Fig. 2b, where similar V oc values are observed for both the samples.For further understanding of this behaviour, the QDSCs recombination has been analyzed using the electrochemical impedance spectroscopy (EIS).The stability of the samples during the impedance measurement was verified by comparing the cyclic voltammograms before and after EIS measurement (See supporting information S3).Fig. 3 shows the recombination resistance obtained for the samples analyzed in Fig. 2b.Similar recombination resistances are observed for both samples.Despites the larger effective surface area of H-TiO 2 NFs, the recombination resistance does not become significantly  In this sense, the recombination rate does not increase for the hierarchical sample; rather it decreases as shown in Fig. 3.This fact may be contributable significantly to the 3 fold enhancement in the solar cell efficiency observed for the H-TiO 2 NFs in comparison with the TiO 2 -NTs.The huge increase of photocurrent is not deleteriously compensated by a reduction in 30 V oc , giving place to a final efficiency improvement of 310%.In addition high collection efficiency can be deduced for H-TiO 2 NF QDSCs (See supporting information S4).
In summary, 3-D hierarchical TiO 2 nanotube branches were 35 successfully assembled onto the primary hollow TiO 2 nanofibrous backbone.The newly designed H-TiO 2 NF photoanode has offered large surface area for high QD loading with high light scattering property.In comparison with the directly grown NT arrays on a TCO substrate, the introduction of 40 NTs on the continuous hollow nanofibrous layer results in the effective charge collection.In addition, the hierarchical structure enhances effective surface area without altering the recombination rate, as it should be expected.The proposed H-TiO 2 NF architecture fabricated from the simple protocol can 45 allow wide applications in electrochemical energy conversion and storage devices including QDSCs, DSSCs, photocatalyst and batteries, where high catalytic/electroactive materials have to be loaded and fast charge transport characteristics is required.

15 higher
than the resistance observed for TiO 2 -NT.

Table 1 .
Photovoltaic parameters of QDSCs using different