Crystallography and Properties of Polyoxotitanate Nanoclusters

Review pubs.acs.org/CR

Crystallography and Properties of Polyoxotitanate Nanoclusters Philip Coppens,* Yang Chen, and Elzḃ ieta Trzop Chemistry Department, University at Buffalo, SUNY, Buffalo, New York 14260-3000, United States the oxidation of organic compounds in polluted air and wastewater.2 In photovoltaic cells, electron injection occurs from a photoactivated sensitizer molecule to a large band gap semiconductor, which is typically composed of titania nanoparticles. The electron injection depends on the energy levels of the excited sensitizer molecule and the Fermi level and band gap of the semiconductor, but also on the mode of attachment of the chromophores and the structure of the semiconductor to which they are attached. However, notwithstanding extensive theoretical and experimental studies on the mechanism of photoinduced electron injection, little precise structural CONTENTS information has been available on the binding modes of the 1. Introduction 9645 sensitizers to the semiconductor surfaces. Without such 2. Summary of Synthetic Methods 9645 information, assumptions must be made in theoretical 3. Geometrical Aspects of Polyoxotitanate Clusters 9648 calculations,3,4,42 or theoretical optimization is required. 3.1. Survey of Structures 9648 Following a recently developed approach, the synthesis of 3.2. Comparison with Bulk TiO Phases and homodisperse polyoxotitanate nanoparticles by methods Theoretical Calculations of Small Particles 9649 summarized briefly in the following section, and subsequent 4. How Do Chromophores Attach to the Nanocrystallization, produces a crystalline nanophase, the structure clusters? 9650 4.1. Modes of Attachment and Substitution 9650 of which can be determined by X-ray crystallographic methods. 4.2. Binding Modes of Carboxylate, AcetylacetoWe and others have used this method to reveal the structures of nates, and Phosphonates 9650 a large variety of nanoclusters with and without sensitizer 4.3. Charge Injection and Geometry 9652 molecules attached. In this Review, we restrict our discussion to 4.4. Dependence of Cluster Geometry on Funcclusters with nuclearities of 11 titanium atoms and larger. As tionalization 9652 band gap calculations of the smaller clusters show large 5. The Geometry of Doped Polyoxotitanate Nanovariations with cluster size not typical for larger particles,5 we clusters 9653 will only refer to smaller complexes, which are well covered in 5.1. Importance of Doping 9653 an earlier review article,6 as building blocks of larger clusters 5.2. Structural Relation between Doped and when appropriate. The large amount of information now Undoped Clusters 9654 available allows a systematic analysis of the structural feature of 5.3. Doped Clusters with Trapped Guest Atoms 9655 the clusters and their binding modes to chromophores. As 6. Band Structure and the Effect of Doping on the Band Gap of Polyoxotitanate Nanoclusters 9655 extensively studied in earlier work, doping of the bare clusters 6.1. Quantum Effects and Particle Size 9655 may lead to a reduction of the band gap,7−10 with a 6.2. Band Gap Measurements 9655 bathochromic shift of the absorption often into the visible 6.3. Band Gap Calculations 9657 range, which is clearly of importance for catalytic applications 7. Photoinduced Electron- and Hole-Transfer 9657 and solar energy capture. The structures and electronic 8. Frameworks of Polyoxotitanate Nanoparticles 9658 properties of a number of stoichiometrically doped homo9. Concluding Remarks 9659 disperse clusters have now been analyzed by X-ray diffraction, Author Information 9659 allowing theoretical calculations based on well-defined Corresponding Author 9659 structures. Structures, spectroscopy, and physical properties Notes 9659 based on both calculations and experiments are discussed in Biographies 9659 this Review. It follows a critical 2011 review on titanium oxo Acknowledgments 9660 References 9660 cluster by Rozes and Sanchez6 and an extensive earlier review on photocatalysis on TiO2 surfaces by Yates and co-workers.11 Special Issue: 2014 Titanium Dioxide Nanomaterials

1. INTRODUCTION Polyoxotitanate nanoparticles play a crucial role as anodes in photovoltaic cells,1 and as photocatalysts in processes such as © 2014 American Chemical Society

Received: December 20, 2013 Published: May 12, 2014 9645

dx.doi.org/10.1021/cr400724e | Chem. Rev. 2014, 114, 9645−9661

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Table 1. Polyoxotitanate Nanoclusters with TinOm Core Only: Formula, Coordination Number of Titanium (CN), Degree of Condensation m/n, Nature of Oxo Bridges, and Ligandsa nanosized polyoxotitanate clusters

space group

CN

O/Ti

O bridges

ligands

core type

ref

CCDC

Ti11O13(OiPr)18 Ti11O13(OEt)5(OiPr)13·EtOH Ti12O16(OiPr)16 Ti12O16(OiPr)16·1.4 MeCl2 Ti12O16(OEt)6(OiPr)10 Ti12O12(OAc)6(OnPr)18 [Ti12O16(OiPr)18]·[Mn3Br8(HOiPr)4] Ti12O16(OEt)4(OtBu)12·2 tBuOH Ti12O16(OCH2tBu)16 [Ti12O15(OiPr)17]·[Ti15(BrCo)6O24(OiPr)18(Br)]·iPrOH Ti14O20(OAc)4(OtBu)13(H)·MeCOOtBu Ti14O18(OiBu)8(HOiBu)2(OOCC Me2Et)12 Ti14O14(OEt)22(PhenylPO3)3

P21/c P21 P21/n P21/n P21/n P21/n P21/c P21/n P1̅ P21/c

5,6 5,6 5,6 5,6 5,6 6 5,6 5,6 5,6 5,6/5

1.18 1.18 1.33 1.33 1.33 1 1.33 1.33 1.33 1.25/1.60

μ3-O μ2-O; μ3-O μ2-O; μ3-O μ2-O; μ3-O μ2-O; μ3-O μ3-O; μ4-O μ3-O μ2-O; μ3-O μ2-O; μ3-O μ3-O/μ3-O

OiPr; μ2OiPr OEt; OiPr; μ2OiPr OiPr; μ2 OiPr OiPr; μ2OiPr OEt; OiPr; μ2OiPr OnPr; μ2OnPr; μ2,η2-OAc OiPr; μ2OiPr OtBu; μ2OEt η-OCHH2tBu; μ2OCH2tBu OiPr; μ2OiPr/OiPr

I I I

P1̅ P1̅

4,5,6 6

1.43 1.29

μ2-O; μ3-O μ2-O; μ3-O

III

P1̅

5,6

1

II

977158

Ti15O14(OEt)32

P21/c

6

0.93

μ4-

IV

977145

Ti16O16(OEt)32

C2/c

6

1

μ4-

OEt; μ2OEt

IV

33

SEGVOP11

Ti16O16(OEt)32

P42/n

6

1

μ4-

OEt; μ2OEt

IV

33

SEGVOP02

Ti16O16(OEt)28(OnPr)4·2 MeC6H5

Pbca

6

1

μ4-

OEt; μ2OEt; OnPr

IV

34

MAKDIM

Ti16O16(OEt)24(OnPr)8·2 MeC6H5

Pbca

6

1

μ4-

OEt; μ2OEt; OnPr

IV

34

MAKDOS

Ti16O16(OEt)26(OCH2CCl3)6

I41/acd

6

1

μ4-

OEt; μ2OEt; OCH2CCl3

IV

35

ONUTUN

Ti17O24(OiPr)20

P21/c

4,5,6

1.41

μ4-

OiPr; μ2OiPr

V

36

LIDJUD

Ti17O24(OiPr)20

C2/c

4,5,6

1.41

μ4-

OiPr; μ2OiPr

V

37

QAYCEA

Ti17O24(OiPr)20(Py)·C6H6

P1̅

4,5,6

1.41

μ4-

OiPr; μ2OiPr; η-N Py

V

Ti17O24(OiPr)16(INA)4

I41/a

4,6

1.41

μ4-

OiPr; μ2OiPr; μ2,η2-INA

V

38

Ti17O24(OiPr)16(cat)4·2C6H6

P1̅

4,6

1.41

μ4-

OiPr; μ2OiPr; η2-cat

V

37

LUVHOA/ LUVHOA01 LUVHIU

Ti17O24(OiPr)18(CA)2

C2/c

4,5,6

1.41

μ4-

OiPr; μ2OiPr; μ2,η2-CA

V

38

QAYDUR

Ti17O24(OiPr)16(BA)4

I41/a

4,6

1.41

μ4-

OiPr; μ2OiPr; μ2,η2-BA

V

Ti17O24(OiPr)16(DMACA)4 2CH3CN

P21/m

4,6

1.41

μ4-

OiPr; μ2OiPr; η2-DMACA

V

38

QAYDEB

Ti17O24(OiPr)16(DMABA)4 disordered solvent Ti17O24(OiPr)16(acac)4

I41/a

4,6

1.41

μ4-

OiPr; μ2OiPr; η2-DMABA

V

38

QAYDAX

Pnma

4,6

1.41

μ4-

OiPr; μ2OiPr; η2-acac

V

QAYCIE

Ti17O24(OiPr)16(NPA)4·7C6H6

C2/c

4,6

1.41

μ4-

OiPr; μ2OiPr; η2-NPA

V

QAYDOL

Ti17O24(OiPr)16(APA)4·MeCl2

P1̅

4,6

1.41

μ4-

OiPr; μ2OiPr; η2-APA

V

38

QAYCOK

Ti17O24(OiPr)18(343coumarin)2·3iPrOH Ti18O22(OnBu)26(acac)2

C2/c

4,5,6

1.41

μ4-

38

QAYCUQ

6

1.22

μ4-

OiPr; μ2OiPr; η2-343coumarin OnBu; μ2OnBu; η2-acac

V

P1̅

μ2-O; μ3-O; O μ2-O; μ3-O; O μ2-O; μ3-O; O μ2-O; μ3-O; O μ2-O; μ3-O; O μ2-O; μ3-O; O μ2-O; μ3-O; O μ2-O; μ3-O; O μ2-O; μ3-O; O μ2-O; μ3-O; O μ2-O; μ3-O; O μ2-O; μ3-O; O μ2-O; μ3-O; O μ2-O; μ3-O; O μ2-O; μ3-O; O μ2-O; μ3-O; O μ2-O; μ3-O; O μ2-O; μ3-O; O μ2-O; μ3-O; O μ2-O; μ3-O; O μ2-O; μ3-O; O; μ5-O μ2-O; μ3-O; O μ2-O; μ3-O; O μ2-O; μ3-O; O μ2-O; μ3-O; O

OtBu; μ2,η2-OAc; μ2OH OiBu; μ2OiBu; HOiBu; μ2,η2OOCCMe2Et OEt; μ2OEt; μ3,η3PhenylPO3 OEt; μ2OEt

39

JUJJIH

μ4-

O Bu; μ2OH

V

16

ZUCTIA

μ4-

OtBu; μ2OH

V

16

ZUCTIB

μ4-

OtBu; μ2OtBu; μ2,η2-OAc

VI

31

OPUQIA

μ4-

OtBu; μ2OtBu; μ2,η2C2H5COO

VI

38

QAYFAZ

OEt; μ2OEt; μ3,η3PhenylPO3

II

40

936876

t

t

Ti18O28(O Bu)17(H)· BuOH

P21/c

4,5,6

1.56

Ti18O28(OtBu)17(H)

P4/nmm

4,5,6

1.56

Ti18O25(OtBu)12(OAc)10·4tBuOH

P1̅

5,6

1.39

Ti18O25(OtBu)12(C2H5COO)10t BuOH·EtCOOtBu Ti25O26(OEt)36(PhenylPO3)6

P1̅

5,6

1.39

Pa3

6

1.04

μ4-

μ2-O; μ3-O; μ4O 9646

t

I I I I I/−

27 19 19 28 19 29

30

31 32

DIQYUX YAVYOJ YAVYEZ GEQLET YAVYID IZIHOP 977142 977143 AMADIC FEBVAL OPUQEW KAWYUE

977160

977159


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Table 1. continued nanosized polyoxotitanate clusters

space group

CN

O/Ti

[Ti26O26(OEt)39(PhenylPO3)6]·Br

I213

6

1.00

Ti28O40(OtBu)20(OAc)12·MeC6H5 Ti28O34(OEt)44

R3c̅ C2/c

5,6 6

1.43 1.21

Ti34O50(OiPr)36·MeC6H5

P1̅

4,5,6

1.47

Ti34O50(OiPr)30(DMABA)6·12C6H6

P1̅

4,6

1.47

O bridges μ2-O; O μ2-O; μ2-O; O μ2-O; O μ2-O; O

μ3-O; μ4-

ligands

core type

ref

CCDC

II

40

936877

μ3-O μ3-O; μ4-

OEt; μ2OEt; μ3,η3PhenylPO3 OtBu; μ2,η2-OAc OEt; μ2OEt

IV

31 41

OPUQOG 895467

μ3-O; μ4-

OiPr; μ2OiPr

V

μ3-O; μ4-

OiPr; μ2OiPr; η2-DMABA

V

977157 38

QAYFED

a

Symbols: acac = acetylacetonate; APA = 4-aminophenylacetonate; BA = benzoic acid; CA = trans-cinnamic acid; cat = catechol; DMABA = dimethylaminobenzoic acid; DMACA = dimethylamino trans-cinnamic acid; INA = isonicotinic acid; MeC(CH2OH)3 = 1,1,1-tris(hydroxymethyl)ethane; NPA = nitrophenyl acetylacetonate; PhenylPO3H2 = phenylphosphonic acid; Py = pyridine.

Figure 1. The seven basic types of polyoxotitanate nanoparticle cores. Atom color codes: O, red; 4-coordinated Ti, cyan; 5-coordinated Ti, light blue; 6-coordinated Ti, purple; 7-coordinated Ti, dark blue. The local point group symmetry is indicated.

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Figure 2. Nuclearity versus condensation ratio for polyoxotitanate clusters listed in Table 1. (a) No distinction between alkoxide groups. (b) For ethoxo- and isopropoxo-substituted polyoxotitanate clusters.

2. SUMMARY OF SYNTHETIC METHODS The synthesis of polyoxotitanates followed by gel formation has been discussed in a number of publications.12,13 They can be synthesized in a nucleophilic reaction of metal-alkoxides, specifically Ti(OR)4 (R = alkyl) in the case of polyoxotitanates, with water. On the basis of calorimetric measurements, Golubko and co-workers propose a two-step process, described by the reactions Ti(OR)4 + H2O → Ti(OR)3OH + ROH followed by nTi(OR)3(OH)+ → TinOm(OR)4n−2m, in which the degree of polymerization n depends crucially on the [H2O]/ [Ti(OR)4] ratio in the initial hydrolysis reaction. A large ratio is likely to lead to formation of the bulk TiO2 phases. In practice, H2O is supplied by traces of water, or generated in the reaction mixture by an esterification reaction of, for example, acetic acid and ROH.6 Solvothermal synthesis is a suitable method for the synthesis of pure and doped, as well as functionalized polyoxotitanate clusters under well-controlled conditions. Products obtained may vary depending on the maximum temperature selected, typically 120−150 °C, and the cooling period, which may be as long as 3 days. Crystals are frequently obtained during the solvothermal heating−cooling cycle or during a posteriori partial evaporation of the mother liquor remaining after the solvothermal synthesis. Once the clusters have been obtained, they can often by functionalized by postmodification via substitution reactions under ambient conditions. In a recent experiment, the alkoxo ligands (OR) were eliminated through esterification by performing the solvothermal synthesis with a large excess of carboxylic acids, leading to clusters reportedly much less sensitive to moisture.14

I−VII in Tables 1 and 3, and shown in Figure 1. When all clusters are grouped together, no clear trend emerges relating the number of Ti atoms, or nuclearity, n, to the number of oxygen atoms, m, defined as the condensation ratio n/m (Figure 2a). However, a regularity emerges when the clusters are sorted into groups of different alkoxide ligands, which show a condensation ratio dependence on the size of the ligand, as illustrated in Figure 2b. The condensation ratio is relatively low for the compact (OEt) terminated clusters. The bulkier isopropoxo ligand allows a large condensation ratio, likely due to the effect of steric repulsion in the coordination shell composed of larger alkoxide ligands. The few butoxide terminated clusters in our sample conform to the condensation ratios of the isopropoxides. The largest value of the ratio observed in the known nanoclusters so far, 1.56, occurs for Ti18O28H(OtBu)17. All ratios are smaller than the value of 2, found in the bulk TiO phases rutile, anatase, and brookite. With one exception, Ti14O14(OEt)22(PhenylPO3)3, the predominantly OEt substituted clusters do not contain reactive 5-coordinate titanium atoms, again indicating their compact structure. 4-Coordination of Ti is an exception for the clusters with larger alkoxide substituent up to a nuclearity n of 17. In a 1995 report, Day et al. concluded, based on a limited sample with variable alkoxides, that the stability of the TiO is correlated with the condensation ratio, a view repeated in subsequent reports.6,15,16 This conclusion is at variance with the current survey of a much larger number of structures. For example, the OEt substituted clusters have lower condensation ratios and are generally more stable when exposed to the atmosphere, a result that may be influenced by the absence of the reactive 5coordinate sites. The atoms in clusters of types V and VII are arranged around a central 4-coordinate titanium or dopant atom, and are Keggin-type structures.17 The prototype V structure is Ti17O24(OiPr)20; Ti28MnO38(OEt)40H2 is a typical type VII cluster.18 Structural relationships between different clusters are common. For Ti18, three structural types are observed. The core of Ti28O34(OEt)44 is composed of two overlapping prototype structures of Ti16O16(OEt)32 (type IV), as shown in Figure 3a. The cluster has therefore been classified as type IV

3. GEOMETRICAL ASPECTS OF POLYOXOTITANATE CLUSTERS 3.1. Survey of Structures

Information on structurally analyzed polyoxotitanate clusters with 11 or more Ti atoms, without and with attached chromophores, is listed in Table 1, while Ti/O clusters incorporating dopant atoms are tabulated in Table 3. Most of the observed clusters fall into seven structural types marked as 9648


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Figure 3. (a) Structural overlay of Ti28O34(OEt)44 (blue) and two Ti16O16(OEt)32 clusters (green and pink). (b) Structure overlay of Ti11O13(OEt)5(OiPr)13 (cyan)19 and Ti12O16(OiPr)16 (red).19 The ethyl and isopropyl groups are omitted for clarity.

Figure 4. Octahedral structures of rutile (b) and anatase (a). The μ3 oxygen atoms are shown in red.

substitution by photosensitizers. As indicated by STM measurements on the (110) surface of rutile, 5-coordinate Ti sites are reactive surface binding sites of bulk TiO2 also.23 Similar conclusions have been reached in theoretical studies on binding to anatase24 and rutile surfaces.25,26 The similarity indicates that the nanocluster phase offers the possibility of obtaining information on the geometry at the surfaces of bulk titanium oxide phases by accurate X-ray diffraction methods. A number of theoretical calculations of the geometry of smaller TiO clusters up to n = 15 have been reported. Hamad et al. conclude that for particles with n = 9−15 the structures are compact with a preference for a central octahedron, surrounded by a layer of 4- and 5-coordinate Ti atoms.20 Shevlin and Woodley conclude that in (TiO2)13 a titanium atom in its center has a coordination of seven,21 while Qu and Kroese find an n-odd-n-even fluctuation of the structural features in the n = 10−16 region.22 None of these features are observed in the experimental structures. However, it must be emphasized that the experimental nanoparticles are terminated by a shell of alkoxide groups, while the theoretical results concern bare clusters. The difference between the structures of the (OEt) and (OiPr) substituted clusters, summarized in section 3.1, shows that the effect of substitution cannot be neglected in the comparison of experiment and theory.

in Table 1. A second example relating Ti11 and Ti12 clusters, both type I, is shown in Figure 3b. The only significant difference is the cap at the top, which is missing in the Ti11 cluster. The third structural type of Ti28 (type VII) has only been observed when dopant atoms are present and is further described in section 5. 3.2. Comparison with Bulk TiO Phases and Theoretical Calculations of Small Particles

The bulk TiO phases rutile, anatase, and brookite all consist of 6-fold coordinated titanium atoms, linked by μ3 oxygen bridges. The linkage of the octahedra differs significantly in the three phases, as shown for two of the three phases in Figure 4. While the octahedral arrangement occurs in all of the nanoclusters, a striking difference is the occurrence of 5-coordinate Ti atoms near the surfaces of the clusters in types I, III, V, VI, and VII (light blue in Figure 1). In addition, the Keggin-types clusters (types V and VII) are distinguished by a central 4-coordinate titanium, while 7-coordination is observed in type VII clusters (dark blue atoms in Figure 1). The Ti11 and Ti12 clusters, such as Ti11O13(OiPr)18 and Ti12O16(OiPr)16 (type I), are hollow shells without a central titanium atom; similarly, Ti16O16(OEt)32 (type IV) lacks a central metal atom. A central 4-coordinate metal atom also occurs in the larger Ti28 clusters (type VII). The 5-coordinate sites in the observed structures are highly reactive as discussed in the next section. Some of the clusters such as Ti15O14(OEt)32, Ti16O16(OEt)32, and Ti28O34(EtO)44 lack 5-coordinate Ti atoms and are therefore inert to 9649


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4. HOW DO CHROMOPHORES ATTACH TO THE NANOCLUSTERS?

Scheme 2. Schematic Representation of the Bidentate Ligand Attachment Reaction Observed on the Larger Clustersa

4.1. Modes of Attachment and Substitution

Photoinduced injection of electrons from sensitizing chromophores to semiconductor substrates is dependent on the mode of attachment of chromophore. An extensive theoretical literature on the nature of such binding has appeared,3,4,42−45 but accurate experimental information is only now becoming available. Gigant et al. determined a wide variety of binding modes of catechol, salicylic acid, and 2.2′-biphenol with TiO clusters containing up to 10 Ti atoms and identified a number of binding modes shown in Scheme 1.46 Experimentally Scheme 1. Different Binding Modes for Ligands with Two Linking Oxygen Atomsa

a

Red arrows represent the alkoxide ligands, blue lines the attached chromophores. Reprinted with permission from ref 38. Copyright 2012 American Chemical Society.

larger clusters. They are labeled μ2-L (L = ligand) in the column titled “ligands” in Table 1 as opposed to η2-L for the chelate binding. The chelate arrangement is observed for the dimethylaminobenzoate and dimethylaminocinnamate (Figure 5a), perhaps related to the electron-donating effect of the dimethylamino group. Additional structures will have to be determined to further explore this interpretation. It is remarkable that the bridging cinnamate (Figure 5b), the chelate aminophenyl acac, and the chelate 343 coumarin (Figure 5d) chromophores are found to use only two of the four almost coplanar 5-coordinate sites of Ti17, which are arranged in an S4-symmetric arrangement. Attempts to increase the substitution in the case of the cinnamate by changing the ratio of reactant or adding another chromophore in a follow-up reaction of this complex have not been successful. It is noticeable that none of the available experiments reported so far have described different binding modes for the same chromophore−cluster combination, which would indicate that the binding mode is specific for each particular chromophore. Acetylacetonates (acacs), which are recognized as strong ligating species,47,48 do not show the bridging attachment, and always have been found to bind in the chelate configuration (Figure 5c). The 343-coumaric acid binds to Ti17 through one oxygen of its carboxylic group and an adjacent carboxyl oxygen, in an acac-like binding mode rather than through the carboxylate group (Figure 5d). A theoretical investigation of the energy of alternative binding modes to Ti17 has been reported by Sokolow et al.38 In the calculations, the TiO core was fixed at the crystallographic geometry, but the geometries of the alkoxide ligands and the chromophore were optimized. The 6-31G B3LYP calculations were performed both with isopropoxide and with the smaller methoxo ligands. Results for the bridging and chelate modes are shown in Table 2. Only small differences are found between isopropoxide and methoxide binding. In all but one of the cases examined, the observed binding mode is found to be the most stable one. A discrepancy occurs for the cinnamate versus dimethylaminocinnamate acid substitution. For both, the

a (a) Monodentate μ1-(O), (b) chelate bidentate μ1-(O,O′), (c) bridging bidentate μ2-(O,O′), (d) bridging chelate μ2-(O,O′,O′), and (e) doubly bridging chelate μ3-(O,O′,O,O′). Oxygen atoms, red; titanium, purple. Reprinted with permission from ref 38. Copyright 2012 American Chemical Society.

determined binding modes of carboxylic acids and acetylacetonates to larger clusters show less variation than observed in the smaller clusters. For these, only modes (b) and (c) have been observed, although mode (a) of Scheme 1 has been considered for neutral acids in theoretical treatments. The binding of the photosensitizers to the nanoclusters invariable involves a 5-coordinate (Ti 5) surface site, unless the chromophore is attached to a dopant atom. Unlike the (Ti6) sites, the (Ti5) sites are terminated by only one bond to an alkoxide ligand, as illustrated by the red arrows in Scheme 2a. On substitution in a chelate bidentate binding mode, the (Ti6) site is not involved, as only the ligand on the (Ti5) site is replaced (Scheme 2b). When substitution involves the bridging bidentate mode, one alkoxide on the (Ti6) site is replaced by a bond to the chromophore, while the (Ti5) site accepts the second bond and thus becomes (Ti6) also, as shown in Scheme 2c. In both cases, charge neutrality is conserved. 4.2. Binding Modes of Carboxylate, Acetylacetonates, and Phosphonates

Crystallographic and spectroscopic studies on small clusters (n < 11) show the carboxylate ligand to be generally bridging,6 but for larger clusters both chelate and bidentate carboxylate binding occurs (Figure 5a and b, the difference being dependent on the nature of the carboxylic acid). Several carboxylic acids are found to attach in the bridging mode to the 9650


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Figure 5. Structural diagrams showing examples of bridging (b) and chelate (a,c,d) modes. Hydrogen atoms are omitted for clarity.

bidentate bridging mode is calculated to be the most stable one, in disagreement with the binding mode observed for the latter. Theoretical geometries, not reproduced here, were found to be in reasonable agreement with the experimental values. A theoretical study of the binding of the doubly protonated dye N719, [cis-(dithiocyanato)-Ru-bis(2,2′-bipyridine-4,4′-dicarboxylate)], shows the complex to anchor to a TiO2 surface through three of its four carboxylic acid groups, two of which bind in a monodentate mode while the third is calculated to bind in a bridging mode,24 as observed for the majority of nanoclusters linked to fully dissociated carboxylic acids. For the phosphonate group attached to small oxotitanates, a number of binding modes have been observed as shown in Scheme 3. Yet its binding to larger polyoxotitanates clusters has only been examined experimentally recently.40 The clusters Ti 2 5 O 2 6 (OEt) 3 6 (PhenylPO 3 ) 6 and [Ti 2 6 O 2 6 (OEt) 3 9 (PhenylPO3)6]Br can be considered as composed of building blocks of the smaller cluster [Ti4O (OEt)12(PhenylPO3)]. In all three, the phenylphosphonate group has been found to be bonded in a μ3 tridentate configuration. In a theoretical study, Nilsing et al. concluded that a monodentate linkage, assisted by

Table 2. Final Total Energies (hartree) for the Optimized Isonicotinic Acid (INA), Nitophenylacetylacetonate (NPA), and Dimethylaminocinnamic Acid (DMACA) Binding Modes of the Ti17 Clustera calculations isopropoxide

methoxide

theory

(H)

Δ

(H)

Δ

INA_bridging INA_chelate NPA_bridging NPA_chelate CA_bridging CA_chelate DMACA_bridging DMACA_chelate

−21 093.705 −21 093.587 −22 471.191 −22 471.210 −20 731.555 −20 731.499 −21 874.915 −21 874.805

0.000 0.118 0.019 0.000 0.000 0.056 0.000 0.109

−19 835.787 −19 835.677 −21 213.303 −21 213.317 −19 316.401 −19 316.348 −20 616.995 −20 616.896

0.000 0.110 0.014 0.000 0.000 0.054 0.000 0.099

The columns labeled Δ show the difference from the lowest energy binding mode. The observed binding mode is highlighted in column 1. From ref 38.

a

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particles have received extensive attention, there are as yet few studies on systems of which the geometry has been accurately determined. As discussed by Galoppini, the dynamics of electron injection and recombination depends strongly on the distance of the dye to the nanoparticles and varies strongly with the length of the linker between the two, and their relative orientation.51 Meyer and Galoppini and co-workers showed that the, albeit low-yield, electron transfer is considerably slowed down for rigid star-shaped homoleptic mononuclear Ru2+ complexes with injection occurring on a nanosecond time scale, while charge recombination was not yet complete on a 90 ms time scale.52 A subsequent theoretical study shows that that use of rigid rod linkers forces confinement of the frontier orbitals away from the anchor groups, thus providing an explanation for slow interfacial transfer observed.53 A first study of electron injection based on a precisely defined experimental geometry has now been reported. A theoretical treatment of Ti17O24(OiPr)20 functionalized with four pnitrophenol acetylacetonate (NPA) adsorbates (see Table 1) is used to follow electron injection in the first 5 fs after initial excitation to the NPA LUMO+1 orbital.54 No such transfer is found to occur from the LUMO of NPA. On a 15 fs time scale, delocalization of the injected electron occurs over multiple chromophores. Parallel EPR studies of frozen solutions show radical formation with long-pass filters in the 295−400 nm range, while the bare Ti17 cluster only shows a signal at 295 nm. Extension of such studies to more general techniques, starting with the interaction of the ground state and incident photons, and applied to a variety of functionalized homodisperse particles, is called for.

Scheme 3. Binding Modes of the Phosphonate Group Observed in Small Oxotitanatesa

a

Reprinted with permission from ref 40. Copyright 2013 John Wiley and Sons.

two hydrogen bonds, is the most stable arrangement of nondissociated and singly dissociated phosphonic acid on a model anatase (101) surface.44 Thornton et al. showed by combined experimental and theoretical studies that deprotonated phosphonic acid binds to the (110) face in a bridging bidentate arrangement.49 Luschtinetz and co-workers found by using advanced theory that the geometry of binding of organic phosphonic acids on anatase (101) and rutile (110) surfaces strongly favors a bidentate configuration, but note the possibility of tridentate linkage,25 as indeed observed in the nanoclusters described above. Dimethyl methylphosphonate, considered by Bermudez, necessarily binds in a monodentate arrangement.50 The phosphonate binding is evidently versatile and a function of the underlying substrate and the degree of deprotonation. The phosphonate group is a strong linker, as the linking O−Ti bonds have an average length of 2.010(7) Å considerably shorter than the average length of 2.087(7) Å reported for the corresponding bonds in carboxylate-functionalized TiO clusters.

4.4. Dependence of Cluster Geometry on Functionalization

Because in a number of instances functionalization involves solvothermal synthesis at high temperatures, there could be a change in the cluster’s atomic connectivity on substitution. To shed light on this possibility, two structural overlays are depicted in Figure 6. The complex Ti17O28(OiPr)16[Fe-phen]2 was synthesized under pressure at 150 °C, possibly leading to changes in connectivity. The overlay with the Ti17 cluster shown in Figure 6a indicates that no such changes occur, although closer examination of the geometry shows that two of the oxygen atoms coordinated to each of the iron atoms in the functionalized complex have moved by about 0.2 Å on

4.3. Charge Injection and Geometry

Although electron and hole injection and subsequent recombination from chromophores to polyoxotitanate nano-

Figure 6. (a) Overlay of the structures of Ti17O28(OiPr)16[Fe-phen]2 (red and magenta) and Ti17O24(OiPr)20 (blue). (b) Overlay of Ti17O24(OiPr)16(INA)4 (green) and Ti17O24(OiPr)20 (blue). Alkoxide groups and hydrogen atoms are omitted for clarity. 9652


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Table 3. Polyoxotitanate Nanoclusters with TinOm Core and Dopant Atoms: Formula, Coordination Number of Titanium (CN), Degree of Condensation m/n without and with Guest Atoms, Nature of Oxo and Bridges, and Ligandsa nanosized polyoxotitanate clusters

space group

CN

O/Ti

O/M

O-bridges

ligands

core type

ref

CCDC

Ti11(CoBr)O14(OiPr)17iPrOH Ti11(CoI)O14(OiPr)17·0.5iPrOH Ti11(MnCl)O14(OiPr)17·iPrOH Ti11(MnBr)O14(OiPr)17·iPrOH Ti11(MnI)O14(OiPr)17·iPrOH Ti11(FeBr)O14(OiPr)17·iPrOH [N(CH3)4]10[Ti12Nb6O38(O2)6]·38H2O

Pn Pn Pn Pn Pn Pn P1̅

5,6 5,6 5,6 5,6 5,6 5,6 6

1.27 1.27 1.27 1.27 1.27 1.27 3.17

1.17 1.17 1.17 1.17 1.17 1.17 2.11

μ3-O μ3-O μ3-O μ3-O μ3-O μ3-O μ3-O; μ5-O

OiPr; μ2-OiPr OiPr; μ2-OiPr OiPr; μ2-OiPr; Cl OiPr; μ2-OiPr; Br OiPr; μ2-OiPr; I OiPr; μ2-OiPr; Br μ4,η4-O4Nb(O2)

([N(CH3)4]10[Ti12Nb6O44])2·73H2O

P21/c

6

3.17

2.11

μ3-O; μ5-O

μ4,η4-O5Nb

56

[N(CH3)4]10 [Ti12Nb6O44]·6.5(H2O) Ti13Ba4O18(OEtOMe)24

P21/c C2/c

6 6

3.17 1.385

2.11 1.06

μ3-O; μ5-O μ3-O; μ5-O

57 58

(Ti13Mn4O16[MeC(CH2O)3]4(OEt)12Br4)∞

I41md

4,6

1.23

0.94

59

913441

Ti14MnO16(OEt)28H2

I4̅

6

1.14

1.07

[Ti12O15(OiPr)17][Ti15Co6Br6O24(OiPr)18] (Br)·iPrOH [Ti15Mn2I2O22(OiPr)17](I) Ti17O28(OiPr)16[Co-phen]2

P21/c

5,6/5

1.25/1.60

μ2-O; μ3-O; μ4-O μ2-O; μ3-O; μ4-O μ3-O

μ4,η4-O5Nb OEtOMe; μ3,η2OEtOMe OEt; μ5,η3MeC(CH2O)3; Br OEt; μ2-OEt; μ3-OEt

992716 XICGAT 977205 977206 977207 977203 ICSD 419424 ICSD 380450 COKVUU TAMPEC

P21/n P21/n

5 4,5,6

1.53 1.65

1.35 1.56

Ti17O28(OiPr)18[Cd-phen]2

C2/c

4,5,6

1.65

1.56

Ti17O28(OiPr)16[Fe-phen]2

P21/n

4,5,6

1.65

1.56

Ti28+δ O38(OEt)39·NH4

P1̅

4,5,6,7

1.36

1.31

[Ti28LaO38(OEt)40(H)2]Cl

P1̅

4,5,6,7

1.36

1.31

[Ti28CeO38(OEt)40(H)2]Cl

P1̅

4,5,6,7

1.36

1.31

Ti28MnO38(OEt)40H2·EtOH

P1̅

4,5,6,7

1.32

1.28

Ti28+δLiO38(OEt)39

P1̅

4,5,6,7

1.36

1.31

Ti28+δNaO38(OEt)39

P1̅

4,5,6,7

1.36

1.31

Ti28+δSrO38(OEt)39·1.5H2O

P1̅

4,5,6,7

1.36

1.31

Ti28+δBaO38(OEt)39·1.5H2O

P1̅

4,5,6,7

1.36

1.31

Ti28+δPrBrO38(OEt)39

P1̅

4,5,6,7

1.36

1.31

a

−/1.14

μ2-O; μ3-O μ2-O; μ3-O; μ4-O μ2-O; μ3-O; μ4-O μ2-O; μ3-O; μ4-O μ2-O; μ3-O; μ4-O μ2-O; μ3-O; μ4-O μ2-O; μ3-O; μ4-O μ2-O; μ3-O; μ4-O μ2-O; μ3-O; μ4-O μ2-O; μ3-O; μ4-O μ2-O; μ3-O; μ4-O μ2-O; μ3-O; μ4-O μ2-O; μ3-O; μ4-O

I I I I I I

55

56

V V

930833

OiPr; μ2-OiPr/OiPr

I/−

60

FEBVAL

OiPr OEt; μ2-OEt; phen

V

61

977208 BIKDOQ

OEt; μ2-OEt; phen

V

61

BIKDEG

OEt; μ2-OEt; phen

V

62

977452

OEt; μ2-OEt; μ3-OEt

VII

OEt; μ2-OEt; μ3-OEt; μ2-OH OEt; μ2-OEt; μ3-OEt; μ2-OH OEt; μ2-OEt; μ3-OEt

VII

63

DEJQUG

VII

63

DEJRER

VII

18

970095


VII

41

913776


VII

41

895468


VII

977443


VII

977442


VII

977444

977441

Symbols: MeC(CH2OH)3 = 1,1,1-tris(hydroxymethyl)ethane; phen = 1,10-phenanthroline.

region. Anatase, for example, has a direct band gap of 4.03− 4.04 eV and an indirect band gap of 3.21 eV.65 Thus, dopants are required to exploit the use of solar energy. Preparation methods of doped titanium dioxide photocatalysts have been reviewed by Zaleska,7 whereas the mechanism of activity and results up to 1995 are covered in a detailed review by Yates and co-workers.11 It has been noted that the photoreactivity of doped TiO2 is a complex function of the dopant concentration, the energy levels of the dopants within the TiO2 lattice, and the distribution and the concentration of the dopant atoms.66 It is therefore of importance that synthetic and diagnostic methods have become available, making it possible to determine the structure of doped nanoclusters, thus allowing calculations based on the experimental structures. Available structures are discussed in the current section, while theoretical and experimental band gaps are treated in the following section.

complexation to the iron atom. A similar overlay for Ti17O24(OiPr)16(INA)4 is shown in Figure 6b. This cluster was synthesized by solution methods at room temperature. No significant changes in atomic connectivity on functionalization are observed. We note that the Ti17 core appears sufficiently stable to resist any rearrangement on being subjected to the reaction conditions.

5. THE GEOMETRY OF DOPED POLYOXOTITANATE NANOCLUSTERS 5.1. Importance of Doping

The photocatalytic activity of TiO2 was first described in 1972 when Fujishima and Honda reported the photoinduced decomposition of water on TiO2 surfaces.64 Since then, titanium dioxide has been used extensively for water and air purification, for cleaning of surfaces, and as an antibacterial agent. The pure TiO2 phases do not absorb in the visible-light 9653


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Figure 7. Relations between doped and undoped clusters. (a) Structural overlay of Ti17O24(OiPr)20 (type V) and Ti14MnO16(OEt)28H2 oxoclusters. Blue, Ti17; magenta, Ti14Mn. (b) The Ti28 type VII structural type of Ti28MnO38(OEt)40H2 and Ti28+δNaO38(OEt)39, the top of which is identical to the type V configuration of Ti17O24(OiPr)20 in blue.

Figure 8. (a) Atomic arrangement of Ti28+δNaO38(OEt)39. Ti, purple; O, red; C, gray.41 The Na atom (blue) is located on the surface at the bottom of the cluster. (b) Perspective of the Ti28NaO core of the cluster along its approximate C3v axis (ethyl groups omitted for clarity). Reprinted with permission from ref 41. Copyright 2013 American Chemical Society.

ways. In Ti28MnO38(OEt)40H2·EtOH, the dopant Mn atom replaces the central 4-coordinate titanium as in Ti14MnO16(OEt)28H2. The four Mn−O distances in the Ti28 cluster average to 2.0030 Å, which is shorter than the four equivalent distances of 2.057 Å in Ti14MnO16(OEt)28H2.67 Both are well in the range reported in the International Tables for Crystallography for 4-coordinate Mn2+ of 2.100 Å with a spread of 0.104 Å.68 In both cases, the 2+ valency of Mn has been confirmed by X-ray absorption spectroscopy.18,67 In other known doped Ti28 clusters with Li, Na,41 Sr, Ba,69 La, Ce,63 and Pr,69 the dopant atoms are located on the periphery close to the pseudo 3-fold axis of the TiO framework, in a configuration reminiscent of that occurring in crown ethers (Figure 8). Dopant atoms are similarly located near the surface of the clusters in the framework compound Ti13Mn4Br4 (Figure 9)59 and in the Ti11 complexes listed in Table 3. An additional atom or group is generally linked to the dopant and positioned away from the core. Several of the Ti28 crystal structures show

5.2. Structural Relation between Doped and Undoped Clusters

Doped clusters of which the structure has been determined are listed in Table 3. In several cases, the structural type of the undoped cluster is preserved; in others, the introduced guest atoms are structure-determining. For example, the first six entries of Table 3, Ti11(CoX)O14(OiPr)17, Ti11(MnX)O14(OiPr)17, and Ti11(FeX)O14(OiPr)17 with X = Cl, Br, I, in which Co, Mn, and Fe are linked to halogens, belong to clusterstructure type I as does Ti12O15(OiPr)16. The cluster Ti14MnO16(OEt)28H2 has the type V structure of which Ti17 is the prototype (Figure 7a), although the central 4-coordinate Ti atom of the Keggin structure, colored cyan in diagram V of Figure 1, is replaced by Mn2+, and the two purple-colored Ti atoms at the top and bottom of the diagram in Figure 1 are absent. Part of a third type of doped cluster (VII), typical for doped Ti28, consists of a type V configuration (Figure 7b). Substitution in the type VII clusters occurs in two different 9654


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Figure 9. (a) Perspective view of the Ti13Mn4O16[MeC(CH2O)3]4(OEt)12Br4 cluster unit.59 Atom color codes: Ti, dark purple; Mn, cyan; Br, light purple; O, red; C, gray; H, pale gray. (b) Projection of part of the three-dimensional framework of the clusters connected by bromine bridges. Reproduced with permission from ref 59, http://dx.doi.org/10.1039/C3DT52218K. Copyright 2013 The Royal Society of Chemistry.

almost perfect superpositions of two or three largely identical clusters with sometimes an additional or missing Ti atom and associated oxygen atoms and are therefore labeled Ti28+δ. The original literature should be consulted for details. As noted above, the latter case occurs for the majority of the known Ti28 clusters.

6. BAND STRUCTURE AND THE EFFECT OF DOPING ON THE BAND GAP OF POLYOXOTITANATE NANOCLUSTERS

5.3. Doped Clusters with Trapped Guest Atoms

6.1. Quantum Effects and Particle Size

Lv et al. reported an unusual ion-separated combination of clusters [Ti 1 2 O 1 5 (O i Pr) 1 7 ]·[Ti 1 5 Co 6 Br 6 O 2 4 (O i Pr) 1 8 ](Br)·iPrOH, the anion of which has a hollow core containing a “naked” Br− anion. A second such cluster [Ti15Mn2I2O22(OiPr)17](I) encapsulating a free I− has now been synthesized (Figure 10) (see Table 3 for CCDC reference). A second unusual feature of both is that they contain exclusively 5-coordinate Ti atoms and thus may be

The photochemical properties of the TiO phases and the TiO nanoclusters are crucially dependent on the HOMO−LUMO spacing between the occupied and unoccupied energy levels, typically referred to as the band gap in the case of semiconductors. An analysis by both theoretical and experimental methods of the size dependence of nanoparticles on composition and size has been reported by Yamamoto et al.70 It shows a small crystal-structure difference between anatase and rutile nanoparticles and quantum effects leading to widening of the band gap when particle sizes decrease below about 1.7 nm, corresponding to 30 Ti atoms, for which the band gap is quoted as being 3.34 eV. A similar conclusion was reached by Serpone et al., who concluded that quantum size effects were not significant in anatase particles down to about ∼2.1 nm diameter.65 Yamamoto and co-workers show that quantum effects become important already at larger particle sizes for other semiconductors examined such as ZnO, InSb, GaAs, and CdS, a result attributed to the smaller band gap of the latter group.71 Much larger quantum size effects have been observed for the radical anion [Ti17]•− and the excited cluster Ti17*.71 A number of studies examining the spectroscopic and electrochemical changes of doping of heterodisperse TiO2 nanoparticles have been reported.72−74

highly susceptible to substitution. However, no functionalized derivatives have been reported so far.

6.2. Band Gap Measurements

Two spectroscopic methods are commonly applied to measure band gaps of polyoxotitanates. The first is based on absorption of solutions, the second on diffuse reflectance spectroscopy of solids. A distinction is made between direct and indirect transitions. The former are vertical Franck−Condon type transitions, which occur without momentum transfer. Indirect transitions occur away from the equilibrium position of the

Figure 10. Structure of [Ti15Mn2I2O22(OiPr)17](I) (see Table 3 for CCDC reference). Hydrogen atoms are omitted for clarity. Atom color codes: Ti, pale gray; Mn, light purple; I, purple; O, red; C, gray. 9655


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Table 4. Experimental Band Gaps of Anatase and Nanoclusters (eV) compound

solution, direct/indirect

ref

anatase Ti17O24(OiPr)20 Ti28O40(OtBu)20(OAc)12 (i.e., Ti14 dimer) Ti14MnO16(OEt)28H2 (Ti13Mn4O16[MeC(CH2O)3]4(OEt)12-Br4)∞ Ti17O28(OiPr)16(Co-phen)2 Ti17O28(OiPr)18(Cd-phen)2 Ti28MnO38(OEt)40H2·EtOH [Ti28LaO36(OH)2(OEt)40]Cl [Ti28CeO36(OH)2(OEt)40]Cl

4.03−4.04/2.97−3.21 4.26 (n = 0.5)/ 3.36 (n = 3) 4.53(2)/3.43(1)

65 37 31

4.39 (n = 0.5)/ 3.56 (n = 3) 3.50 (n = 3)

solids, diffuse reflectance

ref

3.19 3.8

67 61

2.64 2.57 ∼2.50 3.3 2.74

67 59 61 61 18

63 63

Figure 11. Calculated partial density of states of Ti28O34(OEt)44 (marked as the Ti14 dimer in the top diagram), of Ti28O34(OEt)44 (second from above), Ti28LiO37(OEt)39 (second from below), and Ti28NaO37(OEt)39 (lowest). Reprinted with permission from ref 41. Copyright 2013 American Chemical Society.

complex, with momentum transfer. They typically correspond to a smaller band gap. The band gap is experimentally determined using the equation:75

α(hν) =

the indirect transitions, lower concentration on the stronger direct excitations. The second method employed is based on the inverse relation between the diffuse reflectance spectra of solids and their absorption coefficients. The diffuse reflectance spectrum can be measured on a spectrometer equipped with an integrating sphere. Strong absorption eliminates diffuse reflection, which, unlike specular reflection against a surface, originates inside the samples in layers close to the surface, leading to an inverse relation between reflectance and absorption. The inversion is commonly performed with an expression developed theoretically by Kubelka and Munk, also referred to as the remission function.76

B(hν − Eg )n hν

(1)

in which α is the measured absorption coefficient, B is the measured absorption constant for the transition, Eg (in eV) is the energy of the band gap, and hν (in eV) is the photon energy at each point of the scan. The exponent n depends on the nature of the transition. The recommended values are n = 0.5 and 2 for allowed direct and indirect transitions, respectively, and 1.5 and 3 for forbidden direct and indirect transition. The n-dependence of the shape of a plot of α versus the photon energy hν gives information on the nature of the transition. High concentrations typically give information on

α(hν)/S = 9656

[1 − R d(hν)]2 2R d(hν)

(2)


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In this expression, Rd is the fractional diffuse reflectance relative to a strongly scattering standard such as barium sulfate, magnesium oxide, or Teflon, and S is defined as the scattering factor, which becomes constant when the thickness of the sample is much larger than the size of the individual particles.77 Because the reflectance R is a fraction and therefore

Crystallography and Properties of Polyoxotitanate Nanoclusters

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