, Volume: 8( 1)
(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.
- Yun Liu Research School of Chemistry, The Australian National University, Canberra, Australian Capital Territory, 2601, Australia
Tel: 61261251124; E-mail: [email protected]
Received: April 07, 2017 Accepted: June 22, 2017 Published: June 30, 2017
Citation:Sun Q, McBride BR, Liu Y. (N3-, M5+) Co-Doping Strategies for the Development of TiO2-Based Visible Light Catalysts. Res Rev Electrochem. 2017;8(1):106.
(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.
Photocatalysis; Solar energy; Doping
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, N3- and M5+ (M=Nb, V, Ta) co-doping strategies were designed to develop TiO2-based VLCs. That is, N3- 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 N3- 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.
Figure 1: The development process, advantages (bottom) and disadvantages (top) of different TiO2-based photocatalysts.
This work briefly reviews the recent progress of (N3-, M5+) co-doping strategies for the development of TiO2-based VLCs. We first explain why N3- 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 (N3-, M5+) co-doped TiO2 materials according to the type of used M5+ ions. Finally, we point out the existing concerns from current investigations into (N3-, M5+) co-doped TiO2 materials and prospects for the future development of TiO2-based VLCs.
The Selection of M5+ Cations for Co-Doping with N3- Anions
In (N3-, M5+) co-doped TiO2 materials, N3- 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 41st 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, rion=0.078 nm in six-coordinated octahedral), Nb4+ (rion=0.082 nm) and Nb3+ (rion=0.086 nm) .Tantalum (Ta) is the 73rd 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., rion of Ta5+=0.078; Ta4+=0.082 nm; Ta3+=0.086 nm). Vanadium (V) is the 43rd 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+ (rion=0.0745 nm) and their elements show the similar electronegativity to Ti (1.54), they are normally co-doped together with N3- 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 N3- anions. The molar ratio of M5+ and N3- 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, (N3-, M5+) co-doping is actually accompanied by N3- 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 N3- and M5+ ions in TiO2 host materials are considered to play three important roles in the enhancement of photocatalytic effects:  (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 N3- anions) when N3- and M5+ ions are bound together.
Synthesis, Characterization, Photocatalytic Properties and Related Theoretical Calculations of (N3-, Nb5+) Co- Doped TiO2
Various experimental routes have been tried to date to synthesize (N3-, Nb5+) co-doped TiO2 materials. Table 1 lists the synthesis processes, characterized properties and VLC effects for (N3-, 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), Ti1-xNbx alloys (titaniumniobium alloys), C10H15O5Nb (niobium ethoxide) and (NH4)[(NbOF4)(NbF7)26 (ammonium uoroniobate salt). As for the Ti sources, TiCl4 (titanium tetrachloride), Ti metal, Ti1-xNbx alloys, C16H36O4Ti (titanium n-Butoxide), C12H28O4Ti (titanium tetraisopropoxide) are normally used.
|N source||Nb source||Ti source||Method and condition||N (at.%)||Nb (at.%)||Phase||Band gap (eV)||Shape||Light source||Dye (mg/L)||VLCs (g/L)||C/C0 (%)|
|HNO3||NbCl5||TiCl4||Solvothermal||5.3 (XPS, TGA, N-O determinator)||5.6 (XPS)||Anatase||2.2||NP (<10 nm)||Xe lamp (500 W,>400 nm, 10 cm)||RhB (20)||1||[email protected] min|
|(200°C, 12 h)|
|NH4OH/||NbCl5||Ti(SO4)2||Microwave-assisted hydrothermal||15 (nominal)||10 (XPS)||Anatase||3.1||NP (~9 nm)||halogen lamp (500 W,>400 nm)||H2O||0.4||100 umol/h|
|C6H12N4||NbCl5||TiCl4||Microwave-assisted hydrothermal||-||2 (EDX)||Anatase||2.8||NP||LED lamps||NO (2 ppm)||-||31%|
|(190°C, 0.5 h)|
|NH3||NbCl5||C16H36O4Ti||Sol-gel and Post-sintering||-||1-33.3 (nominal)||Anatase||2||NP||Xe lamp (150 W, AM 1.5 G filter)||MB (40)||-||[email protected] min|
|(500°C, 5 h, NH3)||(20 nm)|
|NH3||NbCl5||C16H36O4Ti||Sol-gel and Post-sintering||0.2 (XPS)||25 (XPS)||Anatase||2.2||NP||xenon lamp (150 W, AM 1.5 G filter)||H2O||1||7 umol/h|
|(500°C, 5 h, NH3)||(20 nm)|
|NH3||Ti1-xNbx||Ti1-xNbx||Anodization and Post-sintering||8.2 (XPS)||10 (nominal)||Anatase||2.8||NTA||halogen lamp (3.0 m W/cm2,>400nm)||MB (2)||-||[email protected] min|
|(450°C, 0.5 h,NH3)|
|NH3||(NH4) [(NbOF4)(NbF7)2]||Ti foil||Anodization and Post-sintering||6.9 (XPS)||4 (bulk)||Anatase||-||NTA||Oriel EmArc (200 W)||-||-||-|
|(550°C, 2 h, NH3)|
|NH4OH||NbCl5||C12H28O4Ti||Sol-gel and Post-sintering||-||0.5||Anatase||2.98||NP||xenon lamp (300 W,>420 nm)||4-CP||1||[email protected] min|
|and HNO3||(400°C, 3 h, air)|
|C4H11N||C10H15O5Nb||C12H28O4Ti||Aerosol assisted chemical vapour deposition||0.06-0.09 (XPS)||2-10 (XPS)||Anatase||2.4-3.5||film||-||-||-||-|
Table 1. The synthesis processes, characterized properties and VLC effects for (N3-, 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 (N3-, 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. . They designed a novel solvothermal reaction route to directly synthesize (N3-, 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 N3-/Nb5+ and efficiently increase the doping concentration of difficult-dopant N3- 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 N3- and Nb5+ co-dopants should locally form defect-pairs. Figure. 2 shows the TEM image of their synthesized (5.3 at% N3-, 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 N3-- 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).
Figure 2: (a) The TEM image of (5.3 at % N3-, 5.6 at % Nb5+) co-doped anatase TiO2 nanocrystals, (b) the resultant local defect-pair motif and (c) the decomposition curve of Rhodamine B under only visible light illumination using these defectpairs modified VLCs. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission from reference 3.
In addition, a one-step microwave-assisted hydrothermal method was also designed to simultaneously introduce N3- 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  or a gradient distribution of chemical compositions . Moreover, an excess of Nb5+ dopants over N3- 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 (N3-, Nb5+) co-doping. Additionally, Chadwick et al.  designed an aerosol assisted chemical vapor deposition method to directly prepare (N3-, M5+) co-doped TiO2 films by using n-butylamine and niobium ethoxide. The co-doping level of N3- 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 (N3-, 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 N3- 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 (N3-, 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 N3- 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 N3- doping concentration .
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 (N3-, 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 (N3-, Nb5+) co-doped TiO2 achieved by different researchers. However, one commonly accepted fact is that (N3-, Nb5+) co-doping is more efficient in the enhancement of VLC properties than N3- or Nb5+ mono-doping.
Synthesis, Characterization, Photocatalytic Properties and Related Theoretical Calculations of (N3-, Ta5+) Co- Doped TiO2
In the synthesis of (N3-, Ta5+) co-doped TiO2 materials, the synthesis methods, experimental processes and utilized Ti/N sources are very similar to that of (N3-, 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 (N3-, Ta5+) co-doped TiO2 [14-18]. It can be found that (N3-, 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 N3- ions into the as-prepared Ta monodoped rutile nanocrystals to form (N3-, Ta5+) co-doped rutile TiO2. Comparing with the N mono-doped rutile TiO2, the co-doping of N3- and Ta5+ ions can prohibit the formation of amorphous layers on the nanowire surfaces and thus enhance the incident photon to current conversion efficiency . However, the chemical composition, especially the doping levels of N3- ions, is not discussed at all. It is thus difficult to compare their results with that of co-doped anatase TiO2 nanomaterials.
|N source||Ta source||Ti source||Method and condition||N (at. %)||Ta (at. %)||Phase||Band gap (eV)||Shape||Light source||Dye (mg/L)||VLCs (g/L)||C/C0 (%)|
|CH4N4O||TaCl5||C16H36O4Ti||Sol-gel and Post-sintering
(500°C, 1 h, air)
|13.4 (XPS)||12.8 (XPS)||Anatase||2.68||NP||Xe lamp (500W, > 420nm)||MB (5)||1||[email protected]|
|NH4OH||Ta||C12H28O4Ti||Hydrothermal and Post-sintering
(300°C, 1 h, air)
|1.7 (XPS)||0.29 (XPS)||Anatase||2.85||NP (20 nm)||Hg-Xe lamp (500W, > 420nm)||MB (5)||3||[email protected] min|
|N2||(Ta2O5)0.01(TiO2)0.99||Ti and (Ta2O5)0.01(TiO2)0.99||Magnetron sputtering
|0.5-0.6 (XPS)||2.3-1.3 (XPS)||Anatase||3.07-3.16||Film||Xe lamp (420-500nm)||Oleic acid||-||[email protected] min|
|NH3||C15H35O5Ta||TiCl4||Solvothermal and Post-sintering
(500°C, 2 h, NH3)
|-||0.29 (XPS)||Rutile||-||NW||Visible light (>420 nm)||-||-||-|
|NH3||C10H25O5Ta||C12H28O4Ti||Microwave-assisted solvothermal and Post-sintering
(350°C, 1 h, NH3)
|-||-||rutile||~2.6||NR||Xe lamp (500W, > 420nm)||H2O||0.5||0.7 umol h-1|
Table 2. The synthesis processes, characterized properties and VLC effects for (N3-, Ta5+) co-doped TiO2 (NP: Nanoparticle; NW: Nanowire; NR: Nanorod; MB: Methylene Blue).
The measured bandgaps of (N3-, 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 N3- 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 (N3-, Ta5+) co-doped TiO2, the degradation of MB and oleic acid was characterized under visible light illumination. For example, Zhao et al.  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.  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 (N3-, Ta5+) co-doped TiO2 to disclose where N3- and Ta5+ ions are located in the TiO2 crystal structure, how the synergistic effects between N3- 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, N3- and Ta5+ co-dopants locally form similar defect-pairs to the N3- and Nb5+ co-doping system . 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 (N3-, Ta5+) co-doped rutile TiO2, there are no associated theoretical calculations to date.
Figure 3: (a) A 108-atom supercell containing substituted N and Ta co-dopants, (b) the calculated total DOS and (c) PDOS of un-doped, Ta mono-doped, N mono-doped and (N, Ta) co-doped anatase TiO2. Reprinted with permission from reference 19.
Synthesis, Characterization, Photocatalytic Properties and Related Theoretical Calculations of (N3-, V5+) Co- Doped TiO2
The synthesis methods used for the preparation of (N3-, V5+) co-doped TiO2 materials are the same as that of (N3-, Nb5+) and (N3-, V5+) co-doped TiO2. For example, the high-temperature nitradation in NH3 is also used to introduce N3- ions into as-prepared V mono-doped TiO2. Table 3 lists the synthesis processes, characterized properties and VLC effects for (N3-, V5+) co-doped TiO2 [20-25]. NH4VO3 is predominantly used as a V source. In contrast to the synthesis of (N3-, Nb5+) and (N3-, V5+) co-doped TiO2, the hydrothermal route is frequently used to synthesize (N3-, V5+) co-doped TiO2 [21-25]. It involves the incorporation of N3- 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.
|N source||V source||Ti source||N (at. %)||V (at. %)||Phase||Band gap (eV)||Shape||Light source||Dye (mg/L)||VLCs (g/L)||C/C0 (%)|
|C6H15N||NH4VO3||C16H36O4Ti||4 (nominal)||2 (nominal)||Anatase||2.3||NP (7 nm)||Xe lamp (150 W, 15 cm)||RhB (95.8)||0.29||[email protected]|
|C6H15N||NH4VO3||Ti(SO4)2||3.12 (XPS)||1.0 (ICP) and 0.5 (XPS)||Anatase||2.5||NP (13 nm)||Xe lamp (400 W>400 nm)||PCP-Na (20)||0.4||[email protected] min|
|C6H15N||V4+||C16H36O4Ti||-||-||Anatase||2.8||NP (5 nm)||Xe lamp (400 W>400 nm, 25 cm)||MB (1.6)||-||[email protected] min|
|NH4VO/NH3OH||NH4VO3||TiO2||2.97 (XPS)||20 (nominal)||Anatase||-||NP (11 nm)||Xe lamp (300 W>420 nm)||MO (3.3)||-||[email protected] min|
|NH4VO3||NH4VO3||N- TiO2||3.4 (XPS)||4.2 (XPS)||Anatase||2.3||NTA||Hg lamp (300 W>420 nm)||CO2||-||64.5 ppm h-1 cm-2|
|C6H15N||NH4VO3||C16H36O4Ti||0.62 (XPS)||2 (nominal)||Anatase||2.5||NP||Halide lamp (400 W>420 nm||CAP (25)||1||325 × 10-4 min-1|
Table 3. The synthesis processes, characterized properties and VLC effects for (N3-, 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 (N3-, V5+) co-doped TiO2. This means that N3- and V5+ co-doping can efficiently lower the bandgap and extend the light absorption to visible light regime. In almost all (N3-, 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 (N3-, 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.  used their synthesized (N3-, V5+) co-doped TiO2 to treat antibiotics/bacteria and found that (N3-, V5+) co-doping would strengthen VLC properties comparing with N3- or V5+ mono-doping. The key roles of (N3-, V5+) co-doping on the enhancement of VLC properties are also emphasized by other related researchers.
Theoretical calculations on (N3-, V5+) co-doped TiO2 demonstrate that (N3-, 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, N3- 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 . 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.
Figure 4: (a) The schematic illustration of (N, V) defect-pair configurations in anatase TiO2 and (b) the calculated total DOS. The orange and red balls represent the co-doped Ta and N, respectively. Reprinted with permission from reference 26.
Conclusions and Prospects
In (N3-, M5+) co-doped TiO2 materials, the introduced N3- 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 N3- 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 N3- 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 (N3-, 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.
- The ionic radius and electronegativity of Nb, Ta and V elements and ions can be found from the below website of https://www.webelements.com.
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