Engineered Electronic States of Transition Metal Doped TiO₂ Nanocrystals for Low Overpotential Oxygen Evolution Reaction

Abstract

Electrochemical oxygen evolution reaction (OER) involves high overpotential at oxygen evolving electrode and thereby suffers significant energy loss in the proton exchange membrane water electrolyzer. In order to reduce the OER overpotential, precious ruthenium and iridium oxides are most commonly used as anode electrocatalyst. Here we report marked reduction in overpotential for the OER using transition metal (TM) doped TiO 2 nanocrystals (NCs). This reduction in overpotential is attributed to d-orbitals splitting of the doped TMs in the TM-doped TiO 2 NCs and their interactions with the oxyradicals (intermediates of OER) facilitating the OER. The d-orbital spitting of TMs in TM-doped TiO 2 NCs is evident from the change in original pearl white color of undoped TiO 2 NCs and UV-vis absorption spectra. Hu et al. reported improved OER activity of IrO 2 /Nb−TiO 2 catalyst as compared to unsupported IrO 2 catalyst. 11 This suggests potential of TiO 2 as electrocatalyst in addition to its advantageous properties of high thermal and chemical stability, lost cost, and commercial availability. Recently, Cai et al. reported enhanced OER with Co-doped TiO 2 nanowires synthesized by sol-flame process. 20 A few other recent reports also confirm the reduction of OER overpotential with TM-doped TiO 2 . 21,22 However, the exact rationality behind such improved performance remains unclear. Here we report reduced overpotential of OER using TM-doped TiO 2 nanocrystals (NCs) synthesized by a facile low temperature hydrothermal technique. This reduced overpotential is attributed to d-orbital splitting of TMs in TiO 2 NCs producing mid gap energy states as confirmed from the UV-vis absorption spectroscopy. The d-orbital splitting of three TMs (Fe, Co, and Cu) and their role in OER overpotential is discussed in the present report. 2. EXPERIMENTAL DETAILS 2.1 Materials. All the chemicals were analytical grade and used as received without further purification. Titanium tetraisopropoxide (TTIP) [99.999%], Cu(NO 3 ) 2 .3H 2 O, Fe(SO 4 ).7H 2 O, Co(NO 3 ) 2 .6H 2 O, tetrabutyl ammonium hydroxide (TBAH) [(C 4 H 9 ) 4 NOH in 0.1 N aqueous], and diethanolamine (DEA) were purchased from Merck. Synthesis of TM doped TiO 2 NCs.In a typical synthesis, 3 mmol of TTIP was added in a mixture of TBAH (40 mmol) and DEA (160 mmol), and stirred for 5 min at room temperature. Then the viscous solution was transferred to a Teflon-lined stainless steel autoclave and heated at 225 °C for 24 h. After the heat treatment, the autoclave was allowed to cool down to room temperature naturally and the product was collected by centrifuge and washed with water and ethanol several times. The final product was dried in air at 60 °C for 24 h. The overall yield of the product was 90−95 %. TM-doped TiO 2 NCs was synthesized by

The electrocatalytic properties of doped TiO2

“…Several studies have been performed regarding doped RuO 2 [48,49,27] or IrO 2 [50], or doped anatase TiO 2 [51,52]. The favorable agreement between the results of our screening and experimental knowledge regarding Ru-doped TiO 2 was discussed in detail in our previous work [19], and will not be discussed further.…”

A facile and general synthesis strategy to doped TiO2 nanoaggregates with a mesoporous structure and comparable property

“…Figure 2a shows the XRD patterns of pure and doped TiO 2 products. 28,33 There are also two symmetric peaks in Figure 2g. 71-1187) of pure tetragonal anatase TiO 2 , which indicates that the doping of metal does not change the anatase crystal structure of TiO 2 .…”

“…Figure 1a and b show the typical SEM and TEM images of titanium glycolate precursor synthesized according to the previous work. 26,28 The corresponding UV-vis diffuse reflectance spectra of the pure TiO 2 and doped TiO 2 samples are shown in Figure 2d. Once putting those precursor powders into the autoclave and performing the hydrothermal treatment at 180 °C for 6h, as shown in Figure 1c and d, aggregates consisting of many small TiO 2 nanocrystals were obtained.…”

Bandgap Engineering and Signature of Ferromagnetism in Ti1−xMnxO2­ Diluted Magnetic Semiconductor Nanoparticles: A Valence Band Study

“…Ti‐O σ, Ti‐O π, and O π molecular orbitals forms the valence band whereas there are two conduction bands (upper and lower). Ti‐O σ* molecular orbitals (MOs) comprise of the upper level of the conduction band while Ti‐Ti σ, Ti‐Ti σ*; Ti‐Ti π, Ti‐Ti π*; and Ti‐O π* MOs form the lower level of the conduction band of TiO 2 NCs . Possible interactions of dopant (here Mn) with various molecular orbitals may change the optical properties of TiO 2 .…”