An Insight into the Role of Oxygen Vacancy in Hydrogenated TiO₂ Nanocrystals in the Performance of Dye-Sensitized Solar Cells

Abstract

Hydrogenated titanium dioxide (H-TiO2) nanocrystals were successfully prepared via annealing TiO2 in H2/N2 mixed gas flow at elevated temperatures ranging from 300 to 600 °C. Electron paramagnetic resonance (EPR) spectra were used to determine the produced oxygen vacancy in H-TiO2. Variations in temperature were studied to investigate the concentration change of oxygen vacancy in H-TiO2. The H-TiO2 nanocrystals prepared at different temperatures were employed into photoanodes sensitized by N719 dye and found to have exceptional effect on the solar-to-electric energy conversion efficiency (η). Photoanodes with H-TiO2 nanocrystals hydrogenated at 300 °C show the highest short-circuit current density (Jsc) of 18.92 mA cm(-2) and photoelectrical conversion efficiency of 7.76% under standard AM 1.5 global solar irradiation, indicating a 27 and 28% enhancement in Jsc and η, respectively, in comparison to those with TiO2. The enhancement is attributed to high donor density, narrow band gap and positive shift of flat band energy (Vfb) of H-TiO2 that promote the driving force for electron injection. Intensity-modulated photocurrent spectroscopy (IMPS) accompanied by intensity-modulated photovoltage spectroscopy (IMVS) and other analyses were applied to shed more light on the fundamental mechanisms inside the charge transfer and transport in these systems.

Fast and Large-Scale Anodizing Synthesis of Pine-Cone TiO2 for Solar-Driven Photocatalysis

“…As illustrated in Figure 3A, at the beginning, a compact oxide layer with small holes was formed. Mg 2+ and NO 3 − ions in the solution moved to the cathode and anode under the electrical field, respectively. The NO 3 − ions reacted with the oxide layer and soluble species such as [TiO 2−x (NO 3 ) x ] m−n (m > n, 0 < x < 2) were formed, leading to the local dissolution/thinning of the oxide layer and forming many small pores rapidly.…”

“…The NO 3 − ions reacted with the oxide layer and soluble species such as [TiO 2−x (NO 3 ) x ] m−n (m > n, 0 < x < 2) were formed, leading to the local dissolution/thinning of the oxide layer and forming many small pores rapidly. Meanwhile, as shown in Figure 3B, as NO 3 − and OH − reacted with the oxide layer/Ti interface to form soluble [Ti(NO 3 ) n ] m−n (m = 3, 4; n > 4) and Ti(OH) 4 species [29] the pores continued to grow. However, the formation of oxide layers led to lattice expansion and generated stress at the metal/oxide interface.…”

“…The concentration of N calculated from the N 1s XPS spectrum of the PCT film was~2.93 at %, which included interstitial N (Ti-O-N; 399.6 eV; 81.3%) [36], substitutional N (Ti-N; 397.1 eV; 8.7%) [37], and molecularly chemisorbed γ-N 2 (401.3 eV; 10%) [38], as shown in Figure S12. N atoms were introduced by the anodizing technique, likely from the nitric irons attacking the oxide layer and some adsorbed complexes, such as [TiO 2−x (NO 3 ) x ] m−n (m > n, 0 < x < 2) and [Ti(NO 3 ) n ] m−n (m = 3, 4; n > 4), during the formation of TiO 2 [29,39]. These complex anions [Ti(NO 3 ) n ] m−n were thermodynamically unstable and decomposed into thermodynamically stable TiO 2 with N doping [29].…”

“…N atoms were introduced by the anodizing technique, likely from the nitric irons attacking the oxide layer and some adsorbed complexes, such as [TiO 2−x (NO 3 ) x ] m−n (m > n, 0 < x < 2) and [Ti(NO 3 ) n ] m−n (m = 3, 4; n > 4), during the formation of TiO 2 [29,39]. These complex anions [Ti(NO 3 ) n ] m−n were thermodynamically unstable and decomposed into thermodynamically stable TiO 2 with N doping [29]. In addition, the NO species from the decomposition of the nitrate radical might be chemisorbed on the surface or incorporated into the defects of the resultant TiO 2 [40].…”

“…Nano-structured photoactive TiO 2 materials are believed to have a great promise for many photocatalytic applications such as pollution degradation [1], watersplitting [2], and dye-sensitized solar cells [3]. Many methods have been created to fabricate TiO 2 , such as sol-gel [4,5], hydrothermal treatment [6,7], assisted-template method [8][9][10], laser ablation [11][12][13][14], and electrochemical anodic oxidation [15,16].…”