Nanoporous Nitrogen‐Doped Titanium Dioxide with Excellent Photocatalytic Activity under Visible Light Irradiation Produced by Molecular Layer Deposition

Abstract

Molecular layer deposition (MLD) is a processing technology for depositing ultrathin polymer or inorganic-organic hybrid films. It is a subset of atomic layer deposition (ALD) and has attracted tremendous attention because of its self-limiting nature, excellent conformality, chemical selectivity and low reaction temperature. [1][2][3][4][5][6] However, because of factors like low vapor pressure and thermal sensitivity of many organic molecules and poor understanding of vapor-phase organic coupling reactions, [7] the variety of combinations of organic and inorganic reactants that have been used for successful MLD is sparse. Up to now, only a few organic-inorganic polymer films, including alucone, [8][9][10][11] zincone, [12] and titanicone, [13,14] have been grown through MLD and those were mainly based on reactions between metal alkyl/chloride and glycol/glycerol. Given the great promise of MLD, it is highly demanded to develop new processing strategies for MLD.One important aspect for producing a larger range of organic-inorganic mixtures is the variability of compositions of the constituting organic and inorganic fractions of the resulting films. A limitation to rather short organic spacers, such as glycol, between the metal ions is a bottleneck for the synthesis of films with a broader range of organic-inorganic volume ratios. By designing new processing strategies for the organic component, the properties of the resulting films can be more easily tuned by varying their composition. These organic-inorganic hybrid films can be further used to produce porous metal oxide films by removing the organic component and the porosity of the resulting inorganic films will depend on the volume occupied by the organic constituting part, making such films attractive for many applications such as catalysis, separation, sensing, and energy conversion and storage.As an important semiconductor, TiO 2 has been intensely investigated using various ALD processes for its evident application potential in solar-energy devices, environmental remediation, and as catalyst support. [15][16][17][18][19][20] However, due to the ALD growth characteristics these TiO 2 films are intrinsically pinhole-free with low specific surface areas, resulting in low photocatalytic activity. [19,20] Recently, the research groups of George, Yerushalmi, and our group reported nearly in parallel the successful preparation of porous TiO 2 by annealing or UV exposure of Ti-containing hybrid organic-inorganic films (titanicones) produced by MLD using TiCl 4 and glycol or glycerol as reactants. [13,14,21] However, their activity is only efficient with ultraviolet light irradiation because of the wide band gap of TiO 2 (3.2 eV for anatase). Therefore, developing new MLD processes to grow porous TiO 2 films with visible light response and larger surface area is very welcome for such applications.Herein, we demonstrate a novel MLD process for preparing porous N-doped TiO 2 films. The process is based on a new strategy, a four-step ABCB reaction sequence using titan...

N-doped Na2Ti6O13@TiO2 core–shell nanobelts with exposed {1 0 1} anatase facets and enhanced visible light photocatalytic performance

“…After hybridizing titanate with TiO 2 , the resulted TTO-6 shows an enhanced photocatalytic activity than the pure Na 2 Ti 3 O 7 due to the formation of heterojunction and thus, the effective spatial separation of photogenerated electron-hole pairs [60]. By comparison, N-TTO-6-2 shows a further significantly enhanced photoactivity than TTO-6, indicating that N doping essentially tunes the electric structure of TTO-6 and thus, further effectively enhances the photocatalytic performance [61,62]. Combined with the results from UV-vis spectra, the observed photobleaching over N-TTO-6-2 under visible light irradiation should be mainly due to the oxidative photodegradation of dye molecules rather than self-photosensitized oxidation or other reasons.…”

Organic and inorganic–organic thin film structures by molecular layer deposition: A review

“…The various difficulties encountered when using organic precursors are discussed in detail in a review by George et al [ 24 ]. Several strategies have been employed to improve the controllability of the growth process, such as using organic precursors with stiff backbones [ 13 – 14 32 – 37 ] or with two different functional groups [ 36 – 39 ], using reactions requiring surface activation [ 10 , 39 – 46 ], using precursors in which ring-opening reactions occur [ 47 ], or using three different precursors instead of two [ 48 – 49 ].…”

“…Later, Chen et al [ 49 ] also utilized three different precursors to fabricate hybrid thin films, i.e., TiCl 4 , 2-aminoethanol and propanedioyl dichloride. Unlike with the TMA+2-aminoethanol+furan-2,5-dione system, the growth sequence of these films did not consist of repeated introduction of the precursors in three stages, but in four: One full deposition cycle consisted of supplying TiCl 4 , followed by 2-aminoethanol, then propanedioyl dichloride and finally a repeated pulsing of 2-aminoethanol.…”

Titanium dioxide thin films by atomic layer deposition: a review

“…After this, attention regarding the organic precursors has shifted more and more on aliphatic and aromatic organic alcohols, amines and carboxylic acids [9]. In most of the works TiCl4 has been used as the Ti precursor; it has been combined with ethylene glycol [847,848], glycerol [847,849], 8-hydroxyquinoline [842,850], ethanolamine [811], malonyl chloride [811] and 4,4'-oxydianiline [851,852]. Ideally, the growth of a hybrid thin film takes place via self-saturating ligand-exchange reactions between e.g.…”