Review

, Volume: 8( 1)

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(N3-, M5+) Co-Doping Strategies for the Development of TiO2-Based Visible Light Catalysts

Qingbo Sun, Bethany R McBride and Yun Liu^*

Research School of Chemistry, The Australian National University, Canberra, Australian Capital Territory, 2601, Australia.

*Correspondence:

Yun Liu Research School of Chemistry, The Australian National University, Canberra, Australian Capital Territory, 2601, Australia
Tel: 61261251124; E-mail: yun.liu@anu.edu.au

Abstract

(N3-, M5+) co-doping is an efficient strategy to activate the visible light catalytic behavior of TiO2 for broad use in wastewater purification, air cleaning, hydrogen generation, and sterilization. Here, we briefly review the recent progress of (N3-, M5+) co-doping strategies for the development of TiO2-based visible light catalysts. The designed synthesis methods, the characterized material properties, the measured photocatalytic activity and the introduced local defect structures are summarized. It is expected that this mini review can build up a general framework for future/current research into (N3-, M5+) co-doped TiO2 materials and provide a direction for the further development of TiO2-based visible light catalysts.

Keywords

Photocatalysis; Solar energy; Doping

Introduction

The development of TiO₂-based visible light catalysts (VLCs) is significant to enhance the utilization efficiency of clean/renewable solar energy, and to remedy the current state of environmental pollution by directly harnessing sunlight to drive a range of chemical reactions. These chemical reactions include generating hydrogen from water, removing organic/toxic compounds in wastewater or air, automatically decomposing plastic rubbish, and even sterilization. In principle, the light energy absorbed by TiO₂-based VLC materials is used to generate electrons and holes. These photo-excited carriers subsequently migrate to the surfaces of VLC materials and chemically react with various targets. This simple photocatalytic process depends on three critical steps: (1) the light absorption ability of TiO₂-based VLCs. It determines how much photo-energy can be efficiently utilized by VLCs; (2) the efficient separation of electron-hole pairs. This dictates the numbers of active reductants/oxidants available for final chemical reactions; and (3) the migration distance and the lifetime of photo-excited carriers. Longer distances and shorter lifetimes are obviously undesirable for achieving excellent photocatalytic efficiency.

To meet the above criteria, N^3- and M⁵⁺ (M=Nb, V, Ta) co-doping strategies were designed to develop TiO₂-based VLCs. That is, N^3- and M⁵⁺ ions are simultaneously incorporated into TiO₂ crystal structures by substituting a proportion of the Ti⁴⁺ and O2- host ions. The atomic orbitals of these extrinsic co-dopant ions will hybridize with that of Ti⁴⁺, O2- or each other to extend the light absorption of TiO₂ towards the visible light regime, to reduce the recombination of electron-hole pairs, and finally to enhance VLC efficiencies.

The cation-anion co-doping, in this case N^3- and M⁵⁺ co-doping, leads to the appearance of the third generation of TiO₂-based photocatalysts. Prior to this, the first generation was designed based on intrinsic TiO₂. Through tuning the exposed crystal plane, particle size distribution, crystal structure, phase compositions, and surface chemistry, the photocatalytic properties of intrinsic TiO₂ could be controlled technologically. However, since this generation of TiO₂ only absorbs ultraviolet light, most of the solar energy spectrum is wasted and their applications are restricted. To increase the light absorption range, the second generation of TiO₂-based VLCs was subsequently developed based on cationic or anionic mono-doping routes. Although most of the elements in the atomic periodic Table have been tried, the photocatalytic effects of resultant products are still not good. On one hand, mono-doping cations is difficult to enhance visible light absorption and thus often results in unimproved or even worse visible light catalytic properties. On the other hand, mono-doping anions suffers a technical difficulty in their high doping levels. Furthermore, the generation of “trapping or recombination centers” would also worsen photocatalytic efficiency since they “kill” photo-excited electron and hole carriers. Figure. 1 summarizes the development process, advantages and disadvantages of different TiO₂-based photocatalysts.

This work briefly reviews the recent progress of (N^3-, M⁵⁺) co-doping strategies for the development of TiO₂-based VLCs. We first explain why N^3- and M⁵⁺ are chosen as co-dopants for TiO₂. Then, we summarize the synthesis methods, material properties, VLC performances and local defect structures of prepared (N^3-, M⁵⁺) co-doped TiO₂ materials according to the type of used M⁵⁺ ions. Finally, we point out the existing concerns from current investigations into (N^3-, M⁵⁺) co-doped TiO₂ materials and prospects for the future development of TiO₂-based VLCs.

The Selection of M⁵⁺ Cations for Co-Doping with N^3- Anions

In (N^3-, M⁵⁺) co-doped TiO₂ materials, N^3- anions are used to substitute O²- ions while M⁵⁺ cations are used to replace Ti⁴⁺ ions. The selected M⁵⁺ ions mainly include Nb⁵⁺, Ta⁵⁺ and V⁵⁺. In the atomic periodic Table, niobium (Nb) is the 41^st element with an electronegativity of 1.6 Pauling units and has the electronic configuration of 1s²2s²2p⁶3s²3p⁶3d¹⁰4s²4p⁶4d⁴5s¹. Nb ions normally have three chemical valences depending on the number(s) of electrons in 4d and 5s orbitals, i.e., Nb⁵⁺ (the ionic radius, r_ion=0.078 nm in six-coordinated octahedral), Nb⁴⁺ (r_ion=0.082 nm) and Nb³⁺ (r_ion=0.086 nm) [1].Tantalum (Ta) is the 73^rd element with an electronegativity of 1.5 Pauling units and has the electronic configuration of 1s²2s²2p⁶3s²3p⁶3d¹⁰4s²4p⁶4d¹⁰4f¹⁴5s²5p⁶5d³6s². Ta⁵⁺, Ta⁴⁺ and Ta³⁺ are their three sTable ions at normal conditions. The ionic radii of Ta ions are the same as that of Nb ions (i.e., r_ion of Ta⁵⁺=0.078; Ta⁴⁺=0.082 nm; Ta³⁺=0.086 nm). Vanadium (V) is the 43^rd element with an electronegativity of 1.63 Pauling units and has the electronic configuration of 1s²2s²2p⁶3s²3p⁶3d²4s². The ionic radii of V ions are smaller than that of Ta or Nb ions, and are 0.068 (V⁵⁺); 0.072 (V⁴⁺); and 0.078 nm (V³⁺), respectively. Since Ta⁵⁺ and Nb⁵⁺ ions have almost the similar ionic radius as Ti⁴⁺ (r_ion=0.0745 nm) and their elements show the similar electronegativity to Ti (1.54), they are normally co-doped together with N^3- ions into host TiO₂ without generating a large distortion in the local/average crystal structure. The smaller V⁵⁺ ions are also sometimes chosen as co-dopants due to their easy substitution of Ti⁴⁺ ions. It is more important that M⁵⁺ dopant ions lose one additional electron in contrast to the Ti⁴⁺ host ions. This electron is well compensated by the co-doping of N^3- anions. The molar ratio of M⁵⁺ and N^3- codopants is thus expected to be 1:1 for the charge balance of the whole co-doped system. Any deviation of the stoichiometric ratio will generate extra Ti³⁺ ions or additional oxygen vacancies. In these two scenarios, (N^3-, M⁵⁺) co-doping is actually accompanied by N^3- or M⁵⁺ mono-doping.

Two TiO₂ crystal structures, anatase with space group symmetry I4₁/amd and rutile with space group symmetry P4₂/mnm, are normally chosen as the host matrices since their syntheses are easier in comparison with other polymorphs. The coupling of N^3- and M⁵⁺ ions in TiO₂ host materials are considered to play three important roles in the enhancement of photocatalytic effects: [2] (1) activating the absorption of lower photon energies; (2) mutually compensating for the additional charges or defects generated by the introduction of dopants; and (3) facilitating a larger total dopant concentration (especially for N^3- anions) when N^3- and M⁵⁺ ions are bound together.

Synthesis, Characterization, Photocatalytic Properties and Related Theoretical Calculations of (N^3-, Nb⁵⁺) Co- Doped TiO₂

Various experimental routes have been tried to date to synthesize (N^3-, Nb⁵⁺) co-doped TiO₂ materials. Table 1 lists the synthesis processes, characterized properties and VLC effects for (N^3-, Nb⁵⁺) co-doped TiO₂ [3-10,12,13]. The sources of used N are categorized into three types: (1) the colorless liquid HNO₃ (nitric acid), C₄H₁₁N (n-Butylamine), NH₄OH (ammonia solution); (2) solid C₆H₁₂N₄ (hexamethylenetetramine), CH₄N₂O (urea) or (NH₄)[NbO(C₂O₄)2(H₂O)]•nH₂O (ammonium niobium oxalate); and (3) NH₃ (ammonia) gas. Meanwhile, the sources of Nb are mainly focused on NbCl₅ (niobium pentachloride), Ti_1-xNb_x alloys (titaniumniobium alloys), C₁₀H₁₅O₅Nb (niobium ethoxide) and (NH₄)[(NbOF₄)(NbF₇)₂₆ (ammonium uoroniobate salt). As for the Ti sources, TiCl4 (titanium tetrachloride), Ti metal, Ti_1-xNb_x alloys, C₁₆H₃₆O₄Ti (titanium n-Butoxide), C₁₂H₂₈O₄Ti (titanium tetraisopropoxide) are normally used.

Table 1. The synthesis processes, characterized properties and VLC effects for (N^3-, Nb⁵⁺) co-doped TiO₂ (NP: Nanoparticle; NTA: Nanotube Array; RhB: Rhodamine B; MB: Methylene Blue; 4-CP: 4-Chlorophenol).

Different synthesis methods have been reported for the preparation of (N^3-, Nb⁵⁺) co-doped TiO₂ materials depending on the selection of raw materials containing N, Nb and Ti elements. A simple approach has been recently demonstrated by Sun et al. [3]. They designed a novel solvothermal reaction route to directly synthesize (N^3-, Nb⁵⁺) co-doped anatase TiO₂ nanocrystals without any post-sintering treatment by using concentrated HNO₃, NbCl₅, TiCl₄ and ethanol. Through this reaction route, it is easier to control the doping ratio of N^3-/Nb⁵⁺ and efficiently increase the doping concentration of difficult-dopant N^3- ions. This chemical reaction at the atomic level is also one of the most promising ways to guarantee the homogeneous distribution of co-dopants in TiO₂ crystal structures. Experimental and theoretical investigations confirmed that N^3- and Nb⁵⁺ co-dopants should locally form defect-pairs. Figure. 2 shows the TEM image of their synthesized (5.3 at% N^3-, 5.6 at% Nb⁵⁺) co-doped anatase TiO₂ nanocrystals, the resultant local defect-pair motif, and the decomposition curve of Rhodamine B under only visible light illumination using the defect-pairs modified TiO₂-based VLCs. The formation of local N^3-- Nb⁵⁺ defect-pairs is critical to narrow the band gap to 2.2 eV from ~ 3.1 eV and to significantly enhance VLC efficiency (20 mg/L Rhodamine B solution is almost completely decomposed by loading 1 g/L defectpair modified TiO₂-based VLCs under visible light illumination).

In addition, a one-step microwave-assisted hydrothermal method was also designed to simultaneously introduce N^3- and Nb⁵⁺ codopants into anatase TiO₂ nanoparticles [4,5]. Their bandgaps, however, are too broad (3.1 and 2.8 eV, respectively) for practical applications as VLCs. Another normal synthetic procedure is to firstly prepare Nb mono-doped TiO₂ by sol-gel or anodization treatment, and then to incorporate N dopants through high-temperature nitridation in NH₃ gas [6-10]. The high-temperature nitridation process depends on the diffusion of N ions, leading to only surface co-doping [10] or a gradient distribution of chemical compositions [11]. Moreover, an excess of Nb⁵⁺ dopants over N^3- would introduce a large number of Ti³⁺ ions to balance the charges of the whole material. In fact, these additional dopant ions and associated defects may play a detrimental role on VLC properties when compared with uniform (N^3-, Nb⁵⁺) co-doping. Additionally, Chadwick et al. [13] designed an aerosol assisted chemical vapor deposition method to directly prepare (N^3-, M⁵⁺) co-doped TiO₂ films by using n-butylamine and niobium ethoxide. The co-doping level of N^3- ions are proven to be too low (only 0.09 at. %) to strengthen the light absorption behaviors and visible light catalytic properties.

It can be further seen from Table 1 that nearly all of the synthesized (N^3-, Nb⁵⁺) co-doped TiO₂ materials have an anatase crystal structure. The reported morphologies refer to 0-D nanoparticles with different average particle sizes, 1-D nanotubes and 2-D films. The doping concentrations of N^3- and Nb⁵⁺ co-dopants are usually analyzed by XPS (X-ray photoelectron spectroscopy). Confusion has arisen on the correct XPS core levels of interstitial, substitutional or contaminated N in (N^3-, Nb⁵⁺) co-doped TiO₂ materials. Moreover, XPS can only provide chemical information on sample surfaces. It is thus hard to quantitatively describe the doping levels of N^3- ions and further distinguish the “surface co-doping” or “bulk co-doping” by just relying on the XPS analysis. The combination of XPS with TGA-DSC (thermogravimetry and differential scanning calorimetry analysis) and N-O determinator measurements should give a more reasonable and more accepTable conclusion on the real N^3- doping concentration [3].

The decomposition of dyes like Rhodamine B (RhB), methylene blue (MB) and 4-chlorophenol (4-CP) under visible light was measured to present the VLC efficiency of (N^3-, Nb⁵⁺) co-doped TiO₂. Water splitting experiments were also conducted by some researchers. Due to the different experimental setup and operations such as the light sources, the dye concentrations and types, or the VLCs loading amounts, it is difficult to compare the VLC efficiency of (N^3-, Nb⁵⁺) co-doped TiO₂ achieved by different researchers. However, one commonly accepted fact is that (N^3-, Nb⁵⁺) co-doping is more efficient in the enhancement of VLC properties than N^3- or Nb⁵⁺ mono-doping.

Synthesis, Characterization, Photocatalytic Properties and Related Theoretical Calculations of (N^3-, Ta⁵⁺) Co- Doped TiO₂

In the synthesis of (N^3-, Ta⁵⁺) co-doped TiO₂ materials, the synthesis methods, experimental processes and utilized Ti/N sources are very similar to that of (N^3-, Nb⁵⁺) co-doped TiO₂, just with a replacement of the Nb sources with Ta sources. The selected Ta sources include TaCl₅ (tantalum pentachloride), Ta metal, (Ta₂O₅)0.01(TiO₂)_0.99 ceramic pellets, C₁₅H₃₅O₅Ta (tantalum isopropoxide) and C₁₀H₂₅O₅Ta (tantalum ethoxide). Table 2 lists the synthesis processes, characterized properties and VLC effects for (N^3-, Ta⁵⁺) co-doped TiO₂ [14-18]. It can be found that (N^3-, Ta⁵⁺) co-doped rutile TiO₂ was synthesized by a combination of solvothermal and post-sintering methods [17,18]. During the solvothermal reaction process, Ta mono-doped rutile TiO₂ nanowires or nanorods were first prepared. The subsequent high-temperature nitridation treatment was used to introduce N^3- ions into the as-prepared Ta monodoped rutile nanocrystals to form (N^3-, Ta⁵⁺) co-doped rutile TiO₂. Comparing with the N mono-doped rutile TiO₂, the co-doping of N^3- and Ta⁵⁺ ions can prohibit the formation of amorphous layers on the nanowire surfaces and thus enhance the incident photon to current conversion efficiency [17]. However, the chemical composition, especially the doping levels of N^3- ions, is not discussed at all. It is thus difficult to compare their results with that of co-doped anatase TiO₂ nanomaterials.

Table 2. The synthesis processes, characterized properties and VLC effects for (N^3-, Ta⁵⁺) co-doped TiO₂ (NP: Nanoparticle; NW: Nanowire; NR: Nanorod; MB: Methylene Blue).

The measured bandgaps of (N^3-, Ta⁵⁺) co-doped TiO₂ materials range from 2.6 to 3.1 eV. It seems that the narrowed band gap can only be achieved at a higher N^3- and Ta⁵⁺ co-doping concentration [14-16]. This conclusion is consistent with the claims of Sun et al.3 They point out that the higher and nearly equal doping concentrations of cation and anion co-dopants are key to tuning the light absorption behavior and are critical for significantly enhancing VLC properties. Using these synthesized (N^3-, Ta⁵⁺) co-doped TiO₂, the degradation of MB and oleic acid was characterized under visible light illumination. For example, Zhao et al. [14] investigated the visible light degradation of MB (5 mg/L) under 1 g/L VLCs solution. They found that the C/C₀ (C is the dye concentration at different illumination time and C₀ represents the initial dye concentration) was 31.6% at the reaction period of 240 min. At the same time, Le et al. [15] also investigated the visible light degradation of MB with the same concentration (5 mg/L). The C/C₀ was 7% at the reaction time of 180 min by increasing the loading amount of VLCs to 3 g/L. Due to their different light sources and different loading amounts of VLCs, it is not easy to judge whose VLCs are better for the visible light catalytic decomposition of MB.

Theoretical calculations were performed on (N^3-, Ta⁵⁺) co-doped TiO₂ to disclose where N^3- and Ta⁵⁺ ions are located in the TiO₂ crystal structure, how the synergistic effects between N^3- and Ta⁵⁺ co-dopants tune the bandgap and affect photocatalytic properties [14,16,19]. Figure. 3 shows a 108-atom super cell containing one substituted N and one replaced Ta. Among various co-doped configurations, N and Ta co-dopants prefer to directly bind together in one octahedron. The extension in the N-Ta distances will lead to higher total formation energy. Actually, N^3- and Ta⁵⁺ co-dopants locally form similar defect-pairs to the N^3- and Nb⁵⁺ co-doping system [3]. The hybridization of N2p and Ta5d states in N-Ta defect-pairs reduces recombination centers caused by impurity levels (Figures. 3b and 3c), narrows the bandgap, increases carrier mobility, and finally enhances the VLC properties. The calculated band gap of 2.7 eV is also consistent with the experimental results [14,18]. For (N^3-, Ta⁵⁺) co-doped rutile TiO₂, there are no associated theoretical calculations to date.

Synthesis, Characterization, Photocatalytic Properties and Related Theoretical Calculations of (N^3-, V⁵⁺) Co- Doped TiO₂

The synthesis methods used for the preparation of (N^3-, V⁵⁺) co-doped TiO₂ materials are the same as that of (N^3-, Nb⁵⁺) and (N^3-, V⁵⁺) co-doped TiO₂. For example, the high-temperature nitradation in NH₃ is also used to introduce N^3- ions into as-prepared V mono-doped TiO₂. Table 3 lists the synthesis processes, characterized properties and VLC effects for (N^3-, V⁵⁺) co-doped TiO₂ [20-25]. NH₄VO₃ is predominantly used as a V source. In contrast to the synthesis of (N^3-, Nb⁵⁺) and (N^3-, V⁵⁺) co-doped TiO₂, the hydrothermal route is frequently used to synthesize (N^3-, V⁵⁺) co-doped TiO₂ [21-25]. It involves the incorporation of N^3- or V⁵⁺ codopant ions into the as-prepared V/N mono-doped TiO₂ precursor in a hydrothermal reaction autoclave. This wet chemical reaction route avoids the traditional high-temperature nitridation treatment and reduces the agglomeration of nanoparticles. However, it is debaTable whether the dopants can be efficiently diffused into TiO₂ crystal structures at such mild reaction conditions and whether the “surface co-doping” dominates the photocatalytic properties.

Table 3. The synthesis processes, characterized properties and VLC effects for (N^3-, V⁵⁺) co-doped TiO₂ (NP: Nanoparticle; NTA: Nanotube Array; RhB: Rhodamine B; MB: Methylene Blue; MO: Methylene Orange; PCP-Na: Sodium Pentachlorophenate; CAP: Chloramphenicol).

Most of the measured bandgaps are around 2.3 and 2.5 eV for (N^3-, V⁵⁺) co-doped TiO₂. This means that N^3- and V⁵⁺ co-doping can efficiently lower the bandgap and extend the light absorption to visible light regime. In almost all (N^3-, V⁵⁺) co-doped samples prepared using NH₄VO₃ as raw materials, V⁵⁺ and V⁴⁺ ions are found to co-exist. If NH₄VO₃ is replaced by V⁴⁺-containing raw material, V⁴⁺ and V³⁺ will co-exist in the samples. The reasons for the easy reduction of V ions remain unclear to date. The VLC properties of (N^3-, V⁵⁺) co-doped TiO₂ were characterized through decomposing MB, MO (methylene orange), RhB and 4-chlorophenol; or reducing CO₂ into CH₄. In addition, Eswar et al. [25] used their synthesized (N^3-, V⁵⁺) co-doped TiO₂ to treat antibiotics/bacteria and found that (N^3-, V⁵⁺) co-doping would strengthen VLC properties comparing with N^3- or V⁵⁺ mono-doping. The key roles of (N^3-, V⁵⁺) co-doping on the enhancement of VLC properties are also emphasized by other related researchers.

Theoretical calculations on (N^3-, V⁵⁺) co-doped TiO₂ demonstrate that (N^3-, V⁵⁺) co-doping can efficiently enhance VLC properties. Figure. 4 presents the co-doping positions of N and V in anatase TiO₂ and the calculated total DOS (density of states). Here, N^3- and V⁵⁺ chemically bind together to form defect-pairs, again. The formation of defect-pairs narrows the band gap by about 0.45 eV through providing an acceptor level of about 0.33 eV above the valence band and a donor level of about 0.12 eV below the conduction band [26]. Furthermore, the N-V defect-pairs have a large binding energy of about 0.77 eV, making them rather sTable against separation. However, it is a technological challenge to experimentally control the chemical valences of V⁵⁺ dopants.

Conclusions and Prospects

In (N^3-, M⁵⁺) co-doped TiO₂ materials, the introduced N^3- and M⁵⁺ ions would chemically bind together to form local defect-pairs. These defect-pairs are critical to narrow the bandgap of host TiO₂, reduce the “trapping or recombination centers” of photo-generated carriers, increase the doping levels of difficult-dopant N^3- ions, and thus significantly enhance visible light catalytic properties. Since Nb⁵⁺ and Ta⁵⁺ ions are sTable in contrast to V⁵⁺, it is better to select them as the co-dopants of N^3- ions. Given the difficulty in comparing and analyzing the photocatalytic effects reported by different research groups,

(1) A standard photocatalytic reaction setup and conditions should be developed and followed. It would include the used light source, the light illumination intensity, a fixed dye type and concentration, identical loading amounts of the catalysts, and identical reaction times;

(2) At the very least, commercial Degussa P25 should be used as a reference and all experimental results should be quantitatively compared with it;

(3) The chemical compositions of the synthesized samples should be carefully analyzed to easily unveil the intrinsic origin of observed photocatalytic activities.

Based on local defect structure design, it is expected that co-doping TiO₂ with (N^3-, M⁵⁺) will significantly enhance their VLC properties. The development of defect-pair modified TiO₂-based VLCs is thus beneficial for the highly efficient utilization of clean and renewable solar energy.

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