Synthesis of Hydroxyl-Rich Anatase Nanocrystallites, Their Characterization and Performance as Photoanode in Dye-Sensitized Solar Cells

Abstract

In this work an environmentally-friendly aqueous solution process was elaborated to produce hydroxyl-rich anatase nanocrystallites of very large surface area. The aqueous-synthesized nanocrystalline anatase material was shown to have favorable physical properties (e.g. 160-280 m2/g specific surface area, 5~8 nm crystallite size, high concentration of OH surface groups) for fabricating photoanodes for dye-sensitized solar cells (DSSCs). The fabrication process involved paste preparation, "double-layer" screen printing, annealing and sensitization with the N719 dye [cis - (2,2'- bipyridyl - 4,4'- dicarboxylate)2 (NCS)2 ruthenium(II)]. Photocurrent -voltage measurements of DSSCs based on the aqueous-synthesized titania showed higher photocurrent and conversion efficiency when compared to cells built from commercial benchmark nanotitania materials. The enhanced performance of the aqueous titania-based DSSC is discussed on the basis of extensive characterization work that included XRD, TEM, UV-Vis and Raman/ATR-FTIR spectroscopy.

Further Understanding of the Adsorption Mechanism of N719 Sensitizer on Anatase TiO2 Films for DSSC Applications Using Vibrational Spectroscopy and Confocal Raman Imaging

“…The spectra of the pure Dyesol and Aqueous TiO 2 films were published recently and reproduced in Supporting Information Figure S3 . The Dyesol TiO 2 film spectrum shows a weak ν(OH) infrared vibration at 3400 cm −1 , while our Aqueous TiO 2 shows a much stronger broad hydroxyl vibration occurring at 3365 cm −1 .…”

“…The spectra of the pure Dyesol and Aqueous TiO 2 films were published recently and reproduced in Supporting Information Figure S3 . The Dyesol TiO 2 film spectrum shows a weak ν(OH) infrared vibration at 3400 cm −1 , while our Aqueous TiO 2 shows a much stronger broad hydroxyl vibration occurring at 3365 cm −1 . These bands may be attributed to Ti-OH/Ti-OH 2 groups as also done recently by Minella et al In addition, a small band is seen at 3697 cm −1 typical of linear hydroxy groups. , From the observation of these vibrations, it is evident that the TiO 2 surface groups do not consist only of Ti-O sites, as considered in previous dye-TiO 2 binding studies, − but more importantly of Ti-OH/Ti-OH 2 groups.…”

“…In previous studies on the binding of the N719 onto TiO 2 , the high wavenumber spectra of the pure TiO 2 films was not presented or discussed in relation to the adsorbed samples. ,, The presence of Ti-OH/Ti-OH 2 units, as those identified in the commercial (Dyesol) and our synthetic (Aqueous) samples, need to be considered in understanding the binding mechanism between the anchoring groups (COOH vs COO − ) and the TiO 2 surface. This is especially relevant and important to elucidate in the case of our in-house-made TiO 2 from aqueous media …”

“…The Ru complex ((bis(tetrabutylammonium)[cis-di(thiocyanato)-bis(2,2′-bipyridyl-4-carboxylate-4′-carboxylic acid)-ruthenium(II)]): N719) used in this work was obtained from Dyesol. The study of the binding of the Ru complex involved two nanostructured TiO 2 (both anatase: one commercial, Dyesol; and our aqueous produced variety, Aqueous) films prepared by paste deposition, annealing, and sensitization . The TiO 2 paste of the Aqueous product was prepared with ethyl cellulose and α-terpineol as per procedure for screen printing pastes described elsewhere .…”

“…Recently, Hirose et al , in a departure from previous studies proposed that the N719 adsorption on the TiO 2 surface is facilitated by the presence of surface OH sites that leads to the formation of bidentate chelating linkage (mode II, Figure ) with one of its four carboxylate groups (Supporting Information Figure S1). Wang and Lin reported that surface hydroxyl groups induced on the TiO 2 semiconductor oxide surface by O 2 plasma treatment lead to increased dye adsorption, although no surface characterization or discussion of the binding mechanism was offered. , On the basis of the literature reviewed above and our own interest in hydroxyl-rich TiO 2 as photoanode in DSSCs, a greater understanding of the role of the N719 anchoring (COO − and COOH) groups and TiO 2 ’s surface groups (Ti-O, Ti-OH, Ti-OH 2 ) is needed. , To this end, we have undertaken this spectroscopic study with the aim of (a) resolving and unifying the binding mechanism ideas that have been proposed to operate between the N719 complex and nanocrystalline anatase TiO 2 films and (b) in particular shedding light as to the way in which molecular groups involved in the binding mechanism are distributed. In order to elucidate the latter aspect, TiO 2 films with two different anatase powders are used: a commercial one (Dyesol) and our house variety produced via an aqueous synthesis route .…”