Notable Effects of Aliovalent Anion Substitution on the Electronic

Jul 30, 2015 - This is particularly true of materials cosubstituted by two anions (such as N3– and F– in place of O2– or P3– and Cl– in plac...
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Notable Effects of Aliovalent Anion Substitution on the Electronic Structure and Properties of Metal Oxides and Sulfides C. N. R. Rao* International Centre for Materials Science, New Chemistry Unit, Sheikh Saqr Laboratory and CSIR Centre of Excellence in Chemistry, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560 064, India S Supporting Information *

ABSTRACT: Although it is customary to substitute cations in metal oxides, sulfides, and other materials to modify their structure and properties, effects of anion substitution have not been investigated sufficiently. This is particularly true of materials cosubstituted by two anions (such as N3− and F− in place of O2− or P3− and Cl− in place of S2−). Substitution of a trivalent anion along with a monovalent anion helps to eliminate defects, the three anions being isoelectronic and of nearly the same size. Furthermore, such aliovalent anion substitution gives rise to marked changes in the electronic structure and properties. Isovalent anion substitution (e.g., S2− in place of O2− or Se2− in place of S2−) does not bring about such changes. In this Perspective, we examine the electronic structures and properties of several oxides involving cosubstitution of N and F for oxygen. The oxides discussed are TiO2, ZnO, Cr2O3, and BaTiO3. Aliovalent anion substitution decreases the band gaps of the oxides and affect the magnetic and ferroelectric transitions. Sulfides such as CdS and ZnS where sulfur is substituted by P and Cl also show a large decrease in band gaps. Unlike in cation substitution where the conduction band is mainly affected, in the aliovalent anionsubstituted materials the p-states of the trivalent anions (N3− and P3−) dominate the top of the valence band, with the metaltrivalent anion (N, P) bond being shorter and the metal−halogen bond longer. Such materials with high visible absorption extending to long wavelengths may indeed find uses.

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TiO212−15 and improved photoactivity of N,F-codoped samples has been observed due to greater absorption of visible radiation. In most of the studies, the doping level of N or F has been generally moderate. It has been found recently that N and F can be cosubstituted for oxygen to fairly high concentrations in oxides such as ZnO and TiO2, thereby significantly changing their electronic structure and properties.16,17 ZnO with orangeyellow color has been obtained by N, F substitution. The N,Fcosubstituted oxides are essentially defect free. In this article, we present the important results obtained by the cosubstitution of oxygen in a few oxides by N and F. The oxides discussed are ZnO, TiO2, Cr2O3, and BaTiO3, the last two to examine the effect of anion substitution on magnetic and ferroelectric transitions. Anion substitution in metal chalcogenides would be of great interest in view of the interesting properties of semiconducting materials such as CdS and ZnS. There has been success recently in substituting sulfur by phosphorus and chlorine in these sulfides.18 Such substitution does indeed bring about marked changes in the electronic structure of the sulfides. We shall examine the changes in the electronic structure and properties of CdS and ZnS by cosubstitution of sulfur by P and Cl as well as N and F. Here again, P3−, Cl−, and S2− are isoelectronic and

n order to alter the structure and properties of metal oxides, sulfides and other inorganic materials, it is common practice to substitute the cation with other metals ions. However, the changes in the electronic structure brought about by such substitution are often marginal. It seemed possible that anionic substitution may bring about more significant changes in the electronic structure and properties of materials because they may affect the valence bands more markedly. It is known that ZnO substituted by nitrogen brings about marked changes in the Raman spectrum and modifies the photocatalytic activity.1−4 Fluorine substitution, on the other hand, changes the electronic properties of ZnO marginally because the halogen atoms give rise to deep-lying levels.5,6 There has been some effort to extend the absorption of TiO2 to the visible region and doping by nitrogen is reported to create a band above the valence band thereby narrowing the gap.7 Photocatalytic activity of N-doped TiO2 has been examined by a few workers. Fluorine doping in TiO2 does not appear to change the band gap, and photocatalytic properties of F-doped TiO2 have been studied with ultraviolet irradiation.8−10 It must be noted the N-doping in oxides creates oxygen vacancies. The oxygen vacancies in N-doped oxides gives rise to weak roomtemperature magnetism in nanoparticles.11 This can be avoided by codoping with N and F because one N3− and one F− are equivalent to two oxide ions. Furthermore, N3−, O2−, and F− are isoelectric and have comparable sizes. Thus, O2− has a radius which is the average of the radii of N3− and F−. There has been some effort to study catalytic properties of N, F doped © XXXX American Chemical Society

Received: May 15, 2015 Accepted: July 30, 2015

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bonds (1.96 Å) in TiO2. The changes in bond lengths are less dramatic in the ab-plane. The behavior of N and F is like negative and positive charges respectively and the local distortions are more pronounced in N,F-codoped TiO2 than in F or N doped oxide. Energetically, cosubstitution is preferred over individual substitution, and electrostatic interaction between N and F favors pairwise clustering. In bulk TiO2, the valence band is primarily of O 2p orbitals with little mixing with the Ti 3d states. In the band structure of N,F-codoped TiO2, an isolated band appears at the top of the valence band (Figure 2). Analysis of the density of electronic states shows that this uppermost band is derived primarily from the N 2p states. The F 2p states are deep-lying in energy and the lowest energy conduction bands essentially have Ti 3d character. Formation of the sub-band of 2p states of nitrogen is enhanced by fluorine cosubstitution.14 This is also evidenced in the density of states. Supporting Information Figure S2 helps to visualize the uppermost valence states of N,F-TiO2. The charge is localized largely on N atoms and spreads weakly into the p orbitals of the nearest oxygen atoms. There is weak covalent π interaction between px of oxygen and dxz of Ti. Methyl orange gets degraded by visible light at a much faster rate by N,Fdoped TiO2 than pure TiO2. More interestingly, N,F-TiO2 shows H2 evolution from water under visible light irradiation without the presence of any noble metal (Supporting Information Figure S3). ZnO. Substitution of oxygen in ZnO with nitrogen alone can be done by heating the oxide in NH3 or with urea. Cosubstitution of oxygen by N and F has been accomplished by heating ZnO nanoparticles with NH4F at 600 °C in an atmosphere of NH3.16 Cosubstitution of N and F can also be carried out by heating the NH4ZnF3complex at 600 °C in an atmosphere of NH3.16 By these means, N and F have been substituted up to 10−20 at. %. ZnO1−x−yNxFy has the wurtzite structure and exhibits characteristic N(1s) and F (1s) signals(Supporting Information Figure S4) and show a decrease in the hexagonal lattice parameters (a = 3.2355 Å, c = 5.1879 Å with 10−15 at. % N,F substitution compared to a = 3.2486 Å and c = 5.2031 Å for pure ZnO).16 Raman spectra show the emergence of many of the forbidden bands of ZnO. More importantly, the optical absorption spectra show broad absorption extending up to 750 nm (Figure 3). The decrease in band gap is substantial (∼1.3 eV). Photoluminescence spectra show the disappearance of the defect band of ZnO around 700 nm after N, F substitution.

have nearly the same size. Aliovalent anion substitution has a profound effect on the valence band unlike isovalent anion substitution (e.g., S2− in place of O2− or Se2− in place of S2−). It is hoped that the discussion of aliovalent anion substitution in metal oxides and sulfides presented in this Perspective will arouse interest in this strategy for modifying the properties of inorganic materials to render them more useful.

It is common to substitute metal ions in oxides and other inorganic materials. Isovalent anion substitution is also common, but aliovalent anion substitution is not sufficiently investigated. TiO2. TiO2 can be codoped with N and F by heating a mixture of TiO2 nanoparticles with urea and NH4F at 600 °C in a stream of nitrogen.17 This can also be accomplished by heating (NH4)2TiF6 in air at 550 °C.17 Only N doping can be accomplished by several ways, heating TiN in air at 450 °C being one of them. N,F-doped TiO2 prepared by the above methods has the anatase structure, the lattice parameters being slightly larger than those of pure TiO2. The N(1s) and F(1s) core-level spectra of TiO2 codoped with N and F clearly show the expected signals (Supporting InformationFigure S1) and help to ascertain the composition, the sample obtained by the urea route having ∼15 at. % of both N and F. The contents of N and F achieved generally vary from 5 to 15 at. %, UV−visible spectra of the N,F-codoped samples show absorption extending to the visible region (up to ∼550 nm) with the band gap decreasing by ∼1 eV (Figure 1). The band edges of N,F-doped TiO2 prepared by the urea and the complex routes are around 2.2 and 2.5 eV, respectively. The sample is yellow in color. Photoluminescence spectra show that the band around 600 nm due to defects in N-doped TiO2 disappears on N,F-codoping (Figure 1). Note that N atoms alone cause oxygen vacancies due to charge difference with oxygens. First-principles calculations17 show that the lattice parameters of TiO2 vary only slightly on substitution of N and F, each at 6.25 at. %. Local changes in bond lengths on N, F substitution are significant. The Ti−N bond length along the caxis is 1.82 Å while the Ti−F bond is 2.40 Å which are 7.5% shorter and 22.3% longer respectively relative to the Ti−O

Figure 1. (a) UV−visible and (b) PL spectra of undoped TiO2, N-doped and N,F-TiO2 (prepared by complex and urea routes). Insets in (a) show the Tauc plots (reprinted from ref 17). Note that the N,F-codoped TiO2 prepared by the urea route with a higher percentage of N and F has the band edge at a lower energy. 3304

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Figure 2. (a) Electronic band structure of undoped TiO2 and (b) comparison of the band structures of pure TiO2 (in red) and N,F-TiO2 (in black). The N 2p derived sub-band is visible above the valence band of the N,F-TiO2. This is not present in (a) (reprinted with permission from ref 17).

Figure 3. UV−visible spectra of undoped and doped ZnO prepared (a) by the nanoparticle route and (b) by complex decomposition. Inset in (a) shows the colors of the samples (reprinted with permission from ref 16).

magnetic ordering from first-principles calculations. The highest energy valence band in pure Cr2O3 primarily consists of Cr 3d states weakly hybridized with O 2p. In contrast, the topmost valence band of N,F-codoped Cr2O3 largely consists of N 2p states. The band gap decreases by 0.7 eV due to the presence of the N 2p sub-band. Cosubstitution by N, F leaves the cell size nearly unchanged and the lowest energy configuration has both the N and F atoms bonded to the same Cr atom. The Cr−N bond is shorter, whereas the Cr−F bond is longer with respect to the Cr−O bond (Figure 4). The local spins in N, F- substituted Cr2O3 are predicated to be canted due to the spin−orbit coupling. Experimentally it is found that on N, F substitution (at 10 at. % each), there is evidence for spin-canting (Figure 5), with the lattice parameters of the N,F-doped sample being slightly smaller than those of Cr2O3. BaTiO3. BaTiO3 cosubstituted with 10 at. % each of N and F has been prepared by heating a mixture of BaCO3, TiO2, and BaF2 in NH3 at 950 °C.18 On N,F cosubstitution (∼10 at % of each), BaTiO3 becomes more cubic and the material turns green in color. The visible absorption band, therefore, extends to 500 nm, the band gap decreasing to 2.5 eV (from 3.15 eV in pure BaTiO3). The most interesting aspect is that the sharp ferroelectric transition at 120 °C changes to a diffuse transition upon N, F substitution as shown in Figure 6.21 This is because there are strong local electric dipole moments due to the presence of N−Ti−F clusters which induce relaxor or diffuse ferroelectric behavior. Disorder associated with heterovalent cation substitution and short-range chemical ordering are known to result in relaxor properties. N, F substitution in BaTiO3 and the associated disparity in Born charges of N, F, and O results in the relaxor behavior. As in other oxides, the topmost valence band in N,F-BaTiO3 has a major contribution

Metal ion substitution only brings about changes in the conduction bands of oxides and sulfides, but aliovalent anion substitution markedly affects the valence bands. First-principles calculations predict the change in the lattice parameters of ZnO upon cosubstitution of N and F (up to 12.5 at. % each) to be within 1% and that cosubstitution is favored over N substitution alone. The energetics of different chemically ordered configurations of N,F-ZnO reveal that N and F preferentially occupy the nearest neighbor sites bonded through the Zn cation. The tendency of N and F to be together seems evident. In Supporting Information Figure S5, the electronic band structures of ZnO and N,F-ZnO are shown. The uppermost valence bands are most affected on N, F substitution. Density of states calculations show that the uppermost band in N,F-ZnO is primarily derived from N 2p states appearing as a sub-band at the top of the valence band with only minor contributions from Zn 3d and O 2p derived states. The 2p states of the F atoms are deep down in energy below the VB. The reduction in the band gap is the effect of cosubstitution of N and F, the latter enhancing the effect of N substitution.16 Semiconductor heterostructures of the type ZnO/Pt/Cd1−xZnxS are found to exhibit improved hydrogen evolution on visible-light irradiation if ZnO is substituted with N and F as shown in Supporting Information Figure S6.19 Cr2O3. The effect N,F cosubstitution in antiferromagnetic Cr2O3 has been studied.20 In Supporting Information Figure S7, we show the electron density of states of pure Cr2O3 and N,F-codopedCr2O3 (15 at. % each) obtained with collinear 3305

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Figure 6. Dielectric properties of BaTiO3 and N,F-BaTiO3 (reprinted with permission from ref 21).

Figure 4. Structural changes due to N, F substitution in Cr2O3 (reprinted with permission from ref 20.).

from the N 2p states. The band gap in N,F-BaTiO3 is direct in contrast to pure BaTiO3 where it is indirect.

On substitution of N and F in place of oxygen in ZnO and TiO2, the band gaps decrease markedly (∼1 eV). CdS. First-principles calculations on CdS doped, with ∼6 or 12 at. % each of P and Cl, show that the lattice parameters are not affected much by anion substitution.18 Cosubstitution of P and Cl is more favored than substitution of either P or Cl alone. The Cd−P bond length is ∼4% shorter, whereas the Cd−Cl bond is ∼12% longer that the average bond lengths. In the electronic structure, the uppermost valence band is most affected by the phosphorus 3p states emerging as a sub-band with a bandwidth of 0.6 eV (Figure 7). The 3p states of Cl

Figure 7. (a) Electronic structure and (b) projected density of states of hexagonal CdS and P,Cl-CdS. P 3p orbital contribution is at the top of the valence band (reprinted with permission from ref 18).

atoms are deep-lying in energy. The decrease in the band gap in hexagonal and cubic CdS due to anion substitution is similar

Figure 5. Comparison of the magnetization data of (a) pure and (b) N,F-doped Cr2O3 (reprinted with permission from ref 20). 3306

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crystal structures. N, F substitution appears to have a greater effect on the electronic structure of the cubic phase. ZnS substituted with P and Cl (10 at. % each) has been prepared by heating ZnS with Zn3P2 and NH4Cl at 600 °C. N, F-substituted ZnS could be prepared by heating a mixture of ZnS and NH4F at 600 °C in an atmosphere of NH3. The lattice parameters of P,Cl-ZnS (at 10 at % of each dopant) are a = 3.82 Å and c = 6.25 Å. In Figure 8, we show X-ray photoelectron spectra of P,Cl and N,F-substituted ZnS samples. Optical absorption spectra of these materials (Figure 10) show a decrease in the band gap of ZnS by 0.7 eV on P,Cl substitution. N,F substitution brings about a similar decrease in the band gap.18

(0.5−1.0 eV). The decrease in band gap is predicted to be comparable for N, F substitution as well.18 Experimentally, CdS cosubstituted with P and Cl has been prepared by reaction of CdS with Cd3P2 and NH4Cl at 400 °C in a nitrogen atmosphere or by heating CdS with red phosphorus and NH4Cl in two steps.15 X-ray photoelectron spectra show the P(2p) and Cl(2p) signals (Figure 8), giving

Figure 10. Electronic absorption spectra of ZnS along with that of (a) P,Cl-ZnS and (b) N,F-ZnS (reprinted with permission from ref 18).

On substitution of P and Cl in place of sulfur in CdS and ZnS, the band gap is reduced by 0.5−1 eV

Figure 8. X-ray photoelectron spectra (a) P 2p and (b) Cl 2p of P ClCdS, (c) P 2p and (d) Cl 2p of P, Cl-ZnS and (e) N 1s and (f) F 1s of N,F-ZnS. Residual plots are shown in gray color (reprinted with permission from ref 18).

Outlook. The above discussion should suffice to demonstrate how anionic substitution in oxides and sulfides has a major effect on the electronic structure and properties. Cosubstitution of N,F or P,Cl in these materials results in a large shift of the optical absorption band to longer wavelength, thereby affording discovery of new materials as well as new uses or applications requiring only visible light excitation. It is to be noted that the p band of the trivalent substituent (N3− or P3−) dominates the top of the valence band in all the cosubstituted samples. This is significant because cation substitution generally affects only the unoccupied states (conduction band). Besides lowering the band gap, cosubstitution of N, F and P, Cl in oxides and sulfides eliminates anion vacancies, which give rise to roomtemperature ferromagnetism.19 Preliminary studies indicate that the photoluminescence spectra and associated dynamics of anion codoped samples are significantly different and this aspect has to be examined further. It is indeed possible that anion-substituted materials such as P,Cl−CdS may find uses in sensor technology and elsewhere, considering that P, Cl or N, F cosubstituted materials are reasonably stable up to moderate temperatures and in aqueous medium. It would be rewarding to explore several of the anion-substituted chalcogenides including selenides, topological insulators as well as other materials, including quantum dots, heterostructural superlattices, and other nanostructures of N,F- or P,Cl-substituted materials. Preliminary studies have shown that the well-known metal− insulator transition of V2O3 is shifted to a considerably lower temperature on nitrogen substitution up to 10 at. % or less.22,23

the atomic percent of the two as ∼15%. The unit cell parameters of undoped and doped CdS (hexagonal) are a = 4.09 Å, c = 6.67 Å and a = 4.12 Å, c = 6.69 Å, respectively. Optical absorption spectra reveal the band gap of P, Cl-CdS to be lower by 0.5 eV compared to pristine CdS (Figure 9). Preliminary photocatalytic studies of hydrogen generation by CdS1−x−yPxCly/Pt relative to CdS/Pt shows a significant enhancement H2 yield by the former.18 ZnS. First-principles calculations predict that P,Cl cosubstituted ZnS (up to 12.5 at. % of each dopant) should exhibit a decrease in band gap of 0.7 and 0.2 eV in the hexagonal and cubic phases, respectively. The decrease in band gap on cosubstitution of N and F (∼12.5 at %) is around 0.5 eV in both the

Figure 9. Electronic absorption spectra of CdS and P, Cl-CdS prepared by two methods (reprinted with permission from ref 18). 3307

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(10) Yu, J.; Wang, W.; Cheng, B.; Su, B. L. Enhancement of Photocatalytic Activity of Mesporous TiO2 Powders by Hydrothermal Surface Fluorination Treatment. J. Phys. Chem. C 2009, 113, 6743− 6750. (11) Sundaresan, A.; Rao, C. N. R. Ferromagnetism as a Universal Feature of Inorganic Nanoparticles. Nano Today 2009, 4, 96−100. (12) Wang, Q.; Chen, C.; Ma, W.; Zhu, H.; Zhao, J. Pivotal Role of Fluorine in Tuning Band Structure and Visible-Light Photocatalytic Activity of Nitrogen-Doped TiO2. Chem. - Eur. J. 2009, 15, 4765− 4769. (13) Liu, G.; Yang, H. G.; Wang, X.; Cheng, L.; Pan, J.; Lu, G. Q.; Cheng, H. M. Visible Light Responsive Nitrogen Doped Anatase TiO2 Sheets with Dominant {001} Facets Derived from TiN. J. Am. Chem. Soc. 2009, 131, 12868−12869. (14) Hojamberdiev, M.; Zhu, G.; Sujaridworakun, P.; Jinawath, S.; Liu, P.; Zhou, J.-P. Visible-Light-Driven N−F-Codoped TiO2 Powders Derived from Different Ammonium Oxofluorotitanate Precursors. Powder Technol. 2012, 218, 140−148. (15) Li, D.; Haneda; Hishita, S.; Ohashi, N. Visible-Light-Driven N− F−Codoped TiO2 Photocatalysts. 1. Synthesis by Spray Pyrolysis and Surface Characterization. Chem. Mater. 2005, 17, 2588−2595. (16) Saha, R.; Revoju, S.; Hegde, V. I.; Waghmare, U. V.; Sundaresan, A.; Rao, C. N. R. Remarkable Properties of ZnO Heavily Substituted with Nitrogen and Fluorine, ZnO1‑x(N,F)x. ChemPhysChem 2013, 14, 2672−2677. (17) Kumar, N.; Maitra, U.; Hegde, V. I.; Waghmare, U. V.; Sundaresan, A.; Rao, C. N. R. Synthesis, Characterization, Photocatalysis, and Varied Properties of TiO2 Cosubstituted with Nitrogen and Fluorine. Inorg. Chem. 2013, 52, 10512−10519. (18) Kouser, S.; Lingampalli, S. R.; Chithaiah, P.; Roy, A.; Saha, S.; Waghmare, U. V.; Rao, C. N. R. Extraordinary Changes in the Electronic Structure and Properties of CdS and ZnS by Anionic Substitution: Cosubstitution of P andCl in Place of S. Angew. Chem., Int. Ed. 2015, 54, 8149−8153. (19) Lingampalli, S. R.; Rao, C. N. R. Remarkable Improvement in Visible-Light Induced Hydrogen Generation by ZnO/Pt/Cd1−yZnyS Heterostructures through Substitution of N and F in ZnO. J. Mater. Chem. A 2014, 3, 7702−7705. (20) Pan, J.; Waghmare, U. V.; Kumar, N.; Ehi-Eromosele, C. O.; Rao, C. N. R. Effect of Nitrogen and Fluorine Cosubstitution on the Structure and Magnetic Properties of Cr2O3. ChemPhysChem 2015, 16, 1502−1509. (21) Kumar, N.; Pan, J.; Aysha, N.; Waghmare, U. V.; Sundaresan, A.; Rao, C. N. R. Effect of Cosubstitution of Nitrogen and Fluorine in BaTiO3 on Ferroelectricity and Other Properties. J. Phys.: Condens. Matter 2013, 25, 345901. (22) Mott, N. F. Metal-Insulator Transitions; Taylor and Francis: London, 1990. (23) Edwards, P. P.; Ramakrishnan, T. V.; Rao, C. N. R. The MetalNonmetal Transition: A Global Perspective. J. Phys. Chem. 1995, 99, 5228−5239.

Many more such dramatic changes in properties of materials are likely to be driven by anion substitution.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.5b01006. Figures of core-level X-ray photoelectron spectra, visualization of the uppermost valence states, hydrogen evolution with time under visible light irradiation, X-ray photoelectron spectra, electronic band structures, effect of codoping, electronic density of states. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biography C. N. R. Rao obtained his Ph.D. degree from Purdue University (1958) and D.Sc. degree from Mysore University (1960). His main interests are in the chemistry of materials; in this area, he has published over 1200 papers and 40 books. He is a member of the Royal Society, U.S. National Academy of Sciences and several other academies. He is the recipient of the Royal Medal of the Royal Society, August Wilhelm Hoffmann medal of the German Chemical Society and Dan David Prize for Materials research.

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ACKNOWLEDGMENTS The author has immensely benefitted from collaboration with Prof. Umesh Waghmare on theoretical investigations of anion substituted materials.

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REFERENCES

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