Effects of Substitutional Dopants on the Photoresponse of a

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Effects of Substitutional Dopants on the Photoresponse of a Polyoxotitanate Cluster Junyi Hu,† Lijie Zhan,† Guanyun Zhang,† Qun Zhang,‡ Lin Du,‡ Chen-Ho Tung,† and Yifeng Wang*,† †

Key Lab for Colloid and Interface Science of Ministry of Education, School of Chemistry and Chemical Engineering and Environment Research Institute, Shandong University, Ji’Nan 250100, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: In this paper, using a simple method, 17 isostructural polyoxotitanates (POTs) were synthesized, including the pristine [Ti12O16(OiPr)16], the monodefected [Ti11O13(OiPr)18], and the heterometal-doped [Ti11O14(OiPr)17(ML)] (M = Mg, Ca, Zn, Cd, Co, or Ni; L = Cl, Br, I, or NO3). The electronic structures of these POTs were determined by UV−vis spectroscopy and DFT calculations. Upon UV irradiation of the POTs, electron spin resonance showed the formation of TiIII under anaerobic conditions and superoxide (O2•−) in the presence of O2. The photoactivities of the POTs were then probed with TiIII production and short-circuit photocurrent experiments. The photophysical processes were studied using steady-state and transient photoluminescence. The results show that within the very similar structures, the deexcitation processes of the photoexcited POTs can be greatly affected by the dopants, which result in enhanced or decreased photoactivities. Co and Ni doping enhances the absorption of the visible light accompanied by serious loss of UV photoactivities. On the other hand, a Ti vacancy (in [Ti11O13(OiPr)18]) does not reduce the band gap of a POT but improves the UV photoactivities by serving as surface reaction site. The POTs were then used as molecular models of titanium oxide nanoparticles to understand some important issues relevant to doped titanate, i.e., coordination environment of the dopant metal, electronic structure, photoactivities, and photophysical processes. Our present findings suggest that for solar energy harvesting applications of titanium oxides like photocatalysis and solar cells substitution of titanium atoms by transition metal ions (like Co and Ni) to extend the absorption edges may not be an efficient way, while loading of Ti vacancies is very effective.

1. INTRODUCTION Titanium oxides (e.g., TiO2) have been the most important photocatalysts in solar energy conversion and in environmental applications because of their earth abundance, low toxicity, and high chemical stability. However, due to the large band gap, only ultraviolet light of the solar spectrum can be absorbed. In order to improve the photoabsorption properties, especially to extend the absorption edge to the visible light region of the solar spectrum (which accounts for 45−50% of solar flux on Earth), numerous studies have been reported in the past decades about the effects of the metal or nonmetal dopants on the optical properties and the photocatalytic activities of TiO2.1 On the other hand, many approaches have also been made to introduce intrinsic defects to improve the photocatalytic activities of TiO2, such as oxygen defects2 and titanium vacancies.3 Although the doping of TiO2 with titanium vacancies does not reduce the band gap, it does bring about a significant impact on the physicochemical properties of TiO2.4 It is well known that the photoactivity of metal-doped or defected TiO2 depends substantially on both the dopant type and its concentration5 and seems to be closely related to the coordination environment of the dopant within the titanate. © 2016 American Chemical Society

However, due to the complexity of the preparation method, the complicated structures of the nanomaterials, the low concentration of the dopants (typically 400 nm).

higher than the blank experiment. The current intensity of {Ti12} is larger than that of Co-doped compounds but is smaller than Mg- or Ca-doped POTs. This conclusion holds true for bias < 0.7 V, while for larger bias, decomposition of POTs is suspected to occur at the electrode surface. On the other hand, under visible light irradiation, no detectable photocurrent was observed for any of the POTs. In a recent report by Chen et al.,16 transition-metal doping (e.g., ML = FeIIBr) was found to generate a weak photocurrent by visible light irradiation of a {Ti11FeBr}-modified FTO anode. The discrepancy in the results might be due to the low light intensity or the different experimental setup of the present study. 3.5. Photoproduction of TiIII. Next, quantitative comparison of the photoactivity of various POTs is performed by

a

gx, gy, and gz are anisotropic g values; meanwhile, Wx, Wy, and Wz (units Gauss) are the corresponding line widths.

rare cases of directly detecting superoxide formed at photoirradiated titanium oxide surfaces at room temperature without using a spin trap.23 For the lacunary POT, {Ti11}, this signal is stronger than that of {Ti12}. By contrast, for the Co- or Nidoped POTs, very weak signals of superoxide radical were produced by 355 nm irradiation. 3.4. Photocurrent. An important aspect of solar energy harvesting is converting photon energy to electrical energy. To compare the activities of the POTs as photoanodes of the 8497

DOI: 10.1021/acs.inorgchem.6b01071 Inorg. Chem. 2016, 55, 8493−8501

Article

Inorganic Chemistry measurement of production of TiIII upon UV irradiation of their isopropanol solutions of equal concentrations under identical conditions. Herein, the isopropanol molecules or the isopropoxide ligands serve as the hVB+ scavenger. Upon UV irradiation, the solutions become blue due to the production of TiIII, which has been revealed by the above ESR measurements. The extinction coefficient at 740 nm of every compound is measured by UV−vis titration (see Figure S8) of the solutions using [FeCp2]BF4 as a pure electron acceptor;25 values are 870 M−1 cm−1 for {Ti12}, 1265 M−1 cm−1 for {Ti11}, 1490 M−1 cm−1 for {Ti11MgCl}, 1229 M−1 cm−1 for {Ti11CaCl}, and 2043 M−1 cm−1 for {Ti11ZnCl}. Concentrations of TiIII formed as a function of irradiation time is then quantitatively determined (Figures 6 and S8).

while the lifetime of the photogenerated charge carriers should be as long as possible.26 To understand the photophysical processes of the POTs after photoexcitation, steady-state and transient PL experiments are carried out. The POT samples were excited by a 300 nm laser in order to study both the high-energy and the low-energy PL bands (Figure 7). However, it is worth noting that the emission

Figure 7. PL spectra of (a) {Ti12}, (b) {Ti11}, (c) {Ti11CaCl}, and (d) {Ti11CoCl}, collected by 300 nm excitation at room temperature in solid form. (Inset) PL spectra obtained by 350 nm excitation. Due to the weakness of the signals and the sensitivity of the signal intensity to measurement conditions, comparison of the signal intensity among the samples is not attempted.

Figure 6. Production of TiIII upon light irradiation of isopropanol solutions of the POTs. After a reaction time for 90 min, less than 65% of the POT molecules were charged with one electron. Reaction conditions: POT concentration = 2.3 mM; light source 300 W xenon lamp (200−700 nm); photon intensity = 176 mW cm−2.

spectrum is likely independent from the excitation energy, since the 350 nm laser produces very similar spectra as the 300 nm laser (inset of Figure 7). The PL measurements for TiO2 nanoparticles are often carried out at low temperatures, like 77 K or lower,27 because the PL intensity of an indirect semiconductor like TiO2 is typically very weak at room temperature. Meanwhile, literature studies on PL of TiO2 at room temperature mainly use TiO2 samples, which likely contain alkoxide ligands on the surfaces, and it has been believed that the alkoxide can affect the emission intensity (by blocking surface traps by OR groups).27a,28 This situation is just analogous to the present isopropoxide-protected POTs. From Figure 7 it can be seen that when excited by 300 nm (4.14 eV), the samples exhibit a few emissions in the range of 350−550 nm. For TiO2 nanoparticles, the broad high-energy PL bands are attributed to direct transitions (e.g., X1b → X2b, 3.59 eV; X1b → X1a, 3.45 eV) and indirect transitions (e.g., X1b → Γ3, 3.19 eV; Γ1b → X2b, 3.05 eV).27a Analogously, herein the emission of the POTs in the range of 350−420 nm can be assigned to direct transitions and indirect transitions as well. Again, in TiO2, two dominant mechanisms29 were used to explain the red-shifted (or upconverted27a) emission, i.e., recombination of the electrons in the surface traps with the valence band holes (designated as type I herein) and recombination of the conduction band electrons with the holes captured by the surface traps (type II). Our DFT calculation results (Figure S6) show that the O atoms of the isopropoxide ligands have a large contribution to the VBM of an alkoxide-protected POT, and hence, it is reasonable to believe these O atoms can serve as surface traps for photogenerated holes; therefore, the red PL emissions (i.e., the several peaks between 430 and 500 nm in Figure 7) are assigned as type II mechanism herein. No surface

To simulate the spectrum of solar energy, a high-pressure xenon lamp was used to irradiate the solutions. It was found out that the rate of TiIII production follows the order {Ti11} > {Ti 11 CaCl} > {Ti 11 MgCl} > {Ti 11 ZnCl} > {Ti12} ≫ {Ti11NiCl} and {Ti11CoCl}. A negligible amount of TiIII is produced in the {Ti11NiCl} or {Ti11CoCl} solutions. Using a monochromatic irradiation of 365 nm, a similar trend was observed. However, under visible light irradiation (450 nm), no color change was measured for any of the cluster compounds. A little black precipitate, likely to be Co0, was formed after 24 h irradiation of {Ti11CoCl} by the 300 W xenon lamp (200−700 nm) but was not observed by vis-light irradiation. These information together with the ESR measurements clearly demonstrate that (1) for photocatalytic oxidation of isopropanol (or the isopropoxide ligand) by hVB+ the activity of the titanium−oxo clusters with respect to solar spectrum and to UV follows the order {Ti11} > {Ti11CaCl} > {Ti11MgCl} > {Ti11ZnCl} > {Ti12} ≫ {Ti11NiCl} and {Ti11CoCl} and (2) the visible light activity of Ni- or Co-doped titanium−oxo clusters should be very low. This trend is similar to that in Figure 5 for photocurrent measurements, except that the photocurrents of {Ti12} and {Ti11ZnCl} are close. 3.6. Photoluminescence. Photoexcitation of a semiconductor occurs on a time scale of 10 fs,17a while the subsequent charge carrier trapping occurs in nanoseconds.19 Because the electron transfer processes at the surface of a titanium oxide particle are typically slower (e.g., on a time scale of 100 ns19), an efficient photocatalyst usually needs to exhibit a relatively low electron−hole recombination rate, and mean8498

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Inorganic Chemistry

Co, Ni) have the shortest lifetime (τavg < 1.5 ns). The typical PL lifetime of pristine TiO2 is 2−4 ns,30 shorter than that of {Ti12}. Comparing {Ti12}, {Ti11}, and {Ti11MCl} (M = Mg, Ca, Zn, or Cd), it can be concluded that the lacunary site and the closed-shell dopants cannot prolong or even shorten the lifetime of photogenerated carriers, but they indeed enhance the photoactivities (Figures 5 and 6). Meanwhile, it appears that Co and Ni dopants greatly shorten the PL lifetime as well as reduce the photoactivity of a POT. Discussions on the photophysical processes and the observed photoactivities are included in the Discussion.

trap states near the CBM can be assigned from the DFT results of {Ti12} (Figure S6). The very similar emission peaks in the range of 430−500 nm in the emission spectra of all samples in Figure 7 indicate that these samples all exhibit the same red PL emission mechanism/ pathways, i.e., type II mechanism. This is understandable because their titanium-oxo frameworks are also similar. However, Co and Ni doping causes the high-energy emission (direct and indirect transition) to significantly decrease, which can only be explained by the existence of nonemissive pathways for quenching of photogenerated charge carriers. Herein, this is correlated with the many new states between the band gaps produced by the Co or Ni dopants (Figure S6), which work as nonemissive recombination centers for the photogenerated charge carriers and dramatically compete with the direct/ indirect radiative electron−hole recombination paths (an illustration for this is provided in the Discussion). The transient PL decay technique measures the dynamics of charge carriers. A longer PL lifetime can indicate higher efficiency of charge separation, while a shorter PL lifetime is also correlated with quenching of the charge carriers by nonradiative processes. The PL decay spectra were recorded at 468 nm (note that this peak originates from the same mechanism for all the compounds) with 378 nm excitation under otherwise identical conditions to Figure 7. It is seen from Figure 8 and Table 3 that the PL intensity of all samples decays

4. DISCUSSION 4.1. Coordination Environment of the Dopants. A series of nanosized {Ti11ML} clusters was synthesized herein by substituting one TiIV ion of {Ti12} to mimic the latticesubstituted titanium oxide. This situation may suggest that lattice doping of TiO2 by a heterometal is thermodynamically attainable. The discrepancy in the charge of Ti(IV) and the dopant metal ion (2+ herein) is compensated by the ligands, e.g., two of the μ2-O2− in [Ti12O16(OiPr)16] are replaced with two μ2-OiPr− in [Ti11O14CoCl(OiPr)17], and meanwhile one of the μ1-OiPr− of the former is substituted by Cl− in the latter. These ligands are considered analogs to the chemically adsorbed species27a,28 on a titanium oxide nanoparticle surface. On the other hand, the {Ti11} structure may be viewed as a Tivacancy-doped POT: a molecule analogous to the Ti-vacancydoped TiO2 which has been documented to be a p-type semiconductor and a highly active photocatalyst.3 As the orbitals of the closed-shell-metal ions (e.g., Ca2+, Mg2+, Zn2+, and Cd2+) do not overlap with those of the titanium−oxo framework, these dopants do not modify the electronic structure of the POTs. Comparing to Co and Ni, the closedshell-metal ions at the lacunary sites are more like counter cations, related to the alkali metal ions bonded at the lacunary sites of polyoxotungstates like [SiW11O39]8−.31 Hence, the POTs containing closed-shell metals are also considered as the Ti-vacancy-doped POTs. Numerous theoretical studies have addressed the electronic structures of metal-doped TiO2 nanoparticles. Importantly, these DFT calculations have applied the hypothetical (TiO2)n nanoclusters which contain the heteroatom dopants as models, and the geometries of the models are obtained by truncating from the bulk TiO2 structure (i.e., units isolated from anatase, rutile, or brookite structures) and then by optimization according to energy minimization. It is indicated by DFT calculations that metal doping can introduce new states above the VBM, which contributes to narrowing the band gap of TiO2 and results in the visible light response. This is common for both doped TiO2 in the literature and the doped POTs in the present study.1,5a,8d,e,16,32 4.2. Two-Sided Effects of Co and Ni Dopants. For the activities of the POTs, two aspects of the most important applications of POTs have been examined, namely, the oxidation power of the valence band holes and the power of electron injection for electricity output. The results indicate that Co and Ni doping extends the absorption band to the visible light region but reduces the UV photoactivity as well as the total utilization efficiency of the solar spectrum. Such two-sided effects of transition-metal dopants on the light absorption and on the photoactivity of TiO2 have indeed been frequently observed previously. For example, a systematic study by Choi et al. demonstrated that doping with CoIII or AlIII

Figure 8. PL decay spectra at 468 nm: (a) {Ti12}; (b) {Ti11MgCl}; (c) {Ti11}; (d) {Ti11NiCl}; (e) {Ti11CoCl}. Red smooth curves are fitted curves. Excitation wavelength = 378 nm. More spectra are included in Figure S11.

Table 3. Lifetime of {Ti12}, {Ti11}, and {Ti11ML} compound

τavg (ns)

{Ti12} {Ti11} {Ti11MgCl} {Ti11CaCl} {Ti11ZnCl} {Ti11CdCl} {Ti11CoCl} {Ti11NiCl}

10.7 2.47 11.4 10.4 6.76 7.58 1.33 1.49

on the nanosecond time scale, comparable to that of a titanium oxide particle.19 The average PL lifetimes of {Ti12} and {Ti11MCl} (M = Mg, Ca) are similar (τavg > 10 ns) and longer than those of {Ti11MCl} (M = Zn and Cd; τavg ≈ 7 ns). The average PL lifetime of {Ti11} is shorter, while {Ti11MCl} (M = 8499

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vacancy serves as the surface active site for charge transfer, thereby efficiently quenching the trapped holes at O atoms on the POT surfaces, and this greatly enhances the accumulation of conduction band electrons, which is beneficial for the production of TiIII and the photocurrent. A closed-shell-metal ion at the lacunary site is beneficial for the stability of the whole structure, while it slightly decreases the photoactivity by spatially shielding the lacunary site. This mechanism is consistent with the remarkably higher photocatalytic activity of titanium-vacancy-doped TiO2 with respect to pristine TiO2, which was attributed to the more efficient charge transfer at the semiconductor/electrolyte interface.3

decreases the photocatalytic activity of TiO2, consistent with the present observation, while a lot of other dopant metals like FeIII, MoV, RuIII, and VIV are beneficial for the photocatalytic reductive degradation of CHCl3.33 The doped metal ions in TiO2 could improve the photocatalytic performance by forming surface defects or by causing the anatase-lattice distortion, and the defects can act as electron or hole traps to enhance the efficiency for the spatial separation of the photogenerated charge carriers,33 and this has been supported by the intensive theoretical studies.1 At the same time, decreased photocatalytic efficiency has commonly been observed once the concentration of a dopant exceeds ca. 0.5 wt %.33 Considering the effective quenching of the high-energy PL (direct and indirect band transition) due to the CoII or NiII dopants and the new states they bring about to the {Ti11MCl} (M = Co or Ni) POTs, it is therefore proposed that CoII or NiII can develop new paths for quenching the photogenerated charge carriers (Scheme 1B, path e). In contrast, path b

5. CONCLUSION Systematic examination of the photoactivities and the photophysical properties of a series of isostructural POTs was performed. Although Co and Ni doping enhances the absorption of the solar light, it causes serious loss of UV photoactivity. The new states brought about by Co or Ni dopants serve as electron traps for the nonemissive recombination of the trapped electrons and the trapped holes. Meanwhile, although loading of Ti vacancy does not enhance the light-absorption properties of a POT, it indeed improves the utilization efficiency of the solar spectrum. The POT structures provide information for better understanding the performance of TiO2 doped with transition metals and with Ti vacancies. Our present findings suggest that lattice doping with transition metal ions may not be an efficient way, while loading of surface active sites is very effective for both photocatalysis and solar cells applications of titanium oxides.

Scheme 1. Schematic Illustration of the Deexcitation Processes of the Photogenerated Carriers in (A) {Ti12}, (B) {Ti11MCl} (M = Co or Ni), and (C) {Ti11}a

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a

Processes: (a) photoproduction of electrons in the CB and holes in the VB, (b) recombination by direct or indirect band transition to give blue PL, (c) capture of holes by shallow traps, (d) recombination of conduction band electrons and trapped holes (type II emission in section 3.6), (e) capture of electrons by doped CoII or NiII states, (f) nonemissive quenching of the charge carriers, and (g) electron transfer from isopropyl group to the trapped holes. Straight and wavy lines indicate radiative and nonradiative transitions, respectively.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01071. Crystallographic data, detailed syntheses and characterization, spectra, and ESR simulation (PDF) Crystallographic information (CIF)

(Scheme 1A) is attributed to the high-energy PL, while path d is the recombination of conduction band electrons with trapped holes (type II mechanism). Due to the nonemissive recombination of the trapped electrons with the trapped holes (path f), path e−f−c leads to quenching of the photogenerated charge carriers and hence effectively competes with path b and shortens the apparent lifetime of paths b and d (e.g., the 468 nm emission peak in Figure 7). 4.3. Effect of the Defect Site. While a defect site of {Ti11} does not enhance the absorption efficiency of the titanium−oxo compound, it indeed enhances the photoactivity (recall Figures 5 and 6; relative to the pristine {Ti12}). Substitution of one Ti atom of {Ti12} with a closed-shell-metal ion like MgII, CaII, or ZnII can also increase the photoactivity. These closed-shellmetal ions have relatively weak coordination to the oxo ligands at the lacunary site and may become more labile upon solvation in the isopropanol solution to expose the active sites during the photocatalytic reaction. However, in contrast to the enhanced photoactivity, the PL lifetime of the defected or the closedshell-metal-doped POTs was not prolonged (for {Ti11MgCl} and {Ti11CaCl}) and even shortened (in the cases of {Ti11}, {Ti11ZnCl}, and {Ti11CdCl}). This paradox can be solved only if one assumes that the lacunary site of the POT is highly oxidative, which efficiently abstracts electrons from isopropyl ligand or isopropanol (path g in Scheme 1), namely, the Ti(IV)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone/Fax: +86-531-88363632. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Y.W. gratefully acknowledges the financial support by the National Natural Science Foundation of China (21473104& 21401117) and Natural Science Foundation of Shandong Province (ZR2014BQ003). C.-H.T. acknowledges Shandong University for a startup fund (104.205.2.5).

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.6b01071 Inorg. Chem. 2016, 55, 8493−8501