Stable Co‐Catalyst‐Free Photocatalytic H₂ Evolution From Oxidized Titanium Nitride Nanopowders

Abstract

Abstract:In the present work, a simple strategy is used to thermally oxidize TiN nanopowder (~20 nm) to an anatase phase of a TiO 2 :Ti 3+ :N compound. In contrast to the rutile phase of such a compound this photocatalyst provides photocatalytic activity for hydrogen evolution under AM1.5 conditions -this without the use of any noble metal co-catalyst. Moreover the photocatalyst is active and stable over extended periods of time (tested for 4 months). Importantly, to achieve successful conversion to the active anatase polymorph, sufficiently small starting particles of TiN are needed. The key factor for catalysis is the stabilization of the co-catalytically active Ti 3+ species against oxidation by nitrogen present in the starting material. 2In 1972, Fujishima and Honda have reported groundbreaking work on the photolytic cleavage of water to H 2 and O 2 .[1] Since then, the concept of using solar light and a suitable semiconductor to generate the fuel of the future, H 2 , has received tremendous scientific attention. In their experiments, Fujishima and Honda used a two electrode approach with a TiO 2 single crystal as an illuminated photoanode and a Pt sheet as a counter electrode for hydrogen evolution (i.e., a photoelectrochemical configuration).More straightforward than using a two-electrode approach is to use a suspension of nanoparticles (that is without external electrochemical bias), where the photogenerated holes and electrons from the same particle react with the surrounding water. However, under these conditions, the presence of a suitable co-catalyst on the TiO 2 particles is required in order to efficiently generate H 2 . Over the past decades in virtually any investigation on photocatalytic hydrogen evolution, noble metals, such as Pt, Au, Pd[2], have been used as they present most effective co-catalysts due to their abilities to act as electron transfer mediator and recombination center for H 2 . Efforts to replace these expensive noble metal co-catalysts are limited to only a few reports [2a, 3].A key factor to form noble-metal-free active material may be the formation of a specific configuration An alternative approach to form Ti 3+ -rich TiO 2 is, in principle, to perform a controlled partial oxidation of Ti(III) compounds instead of a partial reduction of TiO 2 . Nevertheless, from previous work [5,6] we know that the two key requirements to obtain a stable and active catalyst are: i) the stabilization of the remaining Ti(III) against further oxidation, and ii) the formation of an anatase type of polymorph TiO 2 by oxidation (rutile is reported not to be active). In order to address requirement i), one may consider that theoretical work combined with experimental evidence indicate that nitrogen species situated in interstitial and/or substitutional positions are capable of stabilizing Ti 3+ centers by charge transfer resonance -as for example suggested by Livraghi and Hoang et al. [7]. Therefore the question arises, if an optimized oxidation of TiN may lead to a nitrogen stabilized Ti 3+ config...

Photocatalysis with Reduced TiO2: From Black TiO2 to Cocatalyst-Free Hydrogen Production

“…89,147 Follow-up work demonstrated that similarly active cocatalytic centers can be introduced by alternative approaches. 36,39,40,51,150,151 For example, high-energy proton ion-implantation was used to modify TiO 2 nanotubes selectively at their tops, and in the proton-implanted region, the creation of such intrinsic cocatalytic sites was observed, which were active for photocatalytic H 2 evolution. Proton implantation can induce specific defects and a characteristic modification of the electronic properties not only in nanotubes but also in anatase single crystal (001) surfaces (reaching cocatalyst-free solar H 2 evolution rates of ∼15 and ∼0.2 μmol h −1 cm −2 , respectively).…”

“…The high-temperature, highpressure (500 °C, 20 bar) hydrogenation of anatase or mixed anatase/rutile TiO 2 produced unique catalytic sites that enabled cocatalyst-free hydrogen production rate that were 2 orders of magnitude higher than those observed for stoichiometric powders, reaching values of more than 200 μmol g −1 h −1 . Later on, other synthetic procedures, such as high energy ion implantation, 39 hydride ball milling, 38 and partial oxidation of TiN powders, 40 have been shown to produce similar reduced TiO 2 varieties producing hydrogen without any addition of noble metals.…”

Hydrogen-Location-Sensitive Modulation of the Redox Reactivity for Oxygen-Deficient TiO2

“…As an accurate tool to detect the unpaired electron spins, electron paramagnetic resonance (EPR) was used to further probe Ti 3+ species over the hydrogenation time (Figure 2b). The g-tensor of 1.996 assigned to the Ti 3+ species can be observed in hydrogenated samples, 19 while the EPR signal in the P25 sample is absent. Furthermore, the EPR signal of the H-120 sample is stronger than that of the H-15 sample, which confirms the generation of Ti 3+ species during hydrogenation.…”

Ammonolysis‐Driven Exsolution of Ru Nanoparticle Embedded in Conductive Metal Nitride Matrix to Boost Electrocatalyst Activity

“…As evident from Figure 2e , all the Ti 1−x Ru x O 2 precursors exhibit typical phonon lines of the layered titanate phase, [ 33 ] whereas the RTN materials show phonon lines characteristic of the TiN phase, indicating a nitridation‐induced phase transition to rocksalt‐type titanium nitride structure. [ 20 , 21 , 34 ] The increase in Ru content led to gradual displacement of the acoustic phonon line at ≈200 cm −1 toward higher wavenumbers. Because the energy of this Raman peak is proportional to the concentration of nitrogen vacancies, [ 35 ] the increase in the phonon energy with increasing Ru content can be interpreted as further evidence for the creation of nitrogen vacancies caused by the exsolution of Ru nanoparticles.…”