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The effect of nature and location of defects onbandgap narrowing in black TiO2 nanoparticles.

Alberto Naldoni, Mattia Allieta, Saveria Santangelo, Marcello Marelli, Filippo Fabbri,Serena Cappelli, Claudia Letizia Bianchi, Rinaldo Psaro, and Vladimiro Dal Santo

J. Am. Chem. Soc., Just Accepted Manuscript DOI: 10.1021/ja3012676 Publication Date (Web): 21 Apr 2012

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The effect of nature and location of defects on bandgap narrowing in black TiO2 nanoparticles

Alberto Naldoni,*, Mattia Allieta,§ Saveria Santangelo, Marcello Marelli, Filippo Fabbri,# Serena Cappelli,§ Claudia L. Bianchi,§ Rinaldo Psaro, and Vladimiro Dal Santo*,

CNR–Istituto di Scienze e Tecnologie Molecolari, Via C. Golgi 19, Milano 20133, Italy; § Dipartimento di Chimica Fisica ed Elettrochimica, Università degli Studi di Milano, Via Golgi 19, Milano 20133, Italy; Dipartimento di Meccanica e Materiali, Università ‘‘Mediterranea’’ di Reggio Calabria, Loc. Feo di Vito, 89122 Reggio Calabria, Italy; # IMEM-CNR Institute, Parco Area delle Scienze 37/A , 43100 Parma, Italy.

KEYWORDS. bandgap engineering, defects, electronic structure, core-shell nanoparticles, titanium dioxide Supporting Information Placeholder

ABSTRACT: The increasing need for new materials capable of solar–fuel generation is central in the development of a green energy economy. In this contribution, we demonstrate that black TiO2 nanoparticles obtained through a one-step reduction/crystallization process exhibit a bandgap of only 1.85 eV, which well matches with visible light absorption. The electronic structure of black TiO2 nanoparticles is determined by the unique crystalline and defective core/disordered shell morphology. We introduce new insights that will be useful for the design of nanostructured photocatalysts for energy applications.

A high surface area (BET surface area ≥ 500 m2/g) amor-phous TiO2 was chosen as a precursor for facilitating gas dif-fusion into TiO2 and interaction with its structure.14

White TiO2 was obtained by heating of the precursor under O2 flow (Figure S1) at 500 °C for 1 h. Under the same condi-tions, black TiO2 powder (Figure 1a), stable in air over ten months, was obtained by precursor heating under H2 stream, followed by fast cooling in inert environment until room tem-perature (RT). The use of very slow cooling rate or instanta-neous exposure to air resulted in a gray coloration, hinting at a (heat- or oxygen-induced) rearrangement of the TiO2 lattice into a more stoichiometric form. Therefore, it is argued that freezing of a metastable defective phase is the mechanism leading to the formation of black TiO2 NPs. In fact, reducing crystalline samples (white TiO2 and P25 Degussa) generated only pale blue colored powders with unmodified absorption spectra (Figure S2).

Diffusive reflectance UV–vis spectroscopy (Figure S2) re-vealed that the optical bandgap of white TiO2 was 3.15 eV, whereas the UV onset of black TiO2 absorption occurred at ～2.75 eV (Table S1). In the latter, a broad absorption, starting at ～400 nm and extending in the near-infrared (NIR) region of the spectrum, was also noted, which gave it the black colora-tion.

Aiming to clarify the physical origin of the visible-NIR light absorption of black TiO2, its structural, luminescence and electronic properties were investigated by using a combination of advanced characterization tools.

Figures S1 and 1a show synchrotron x-ray powder diffrac-tion (SXRPD) patterns of white and black TiO2 samples, re-spectively. TiO2 produced in O-rich conditions was composed by 100% in anatase (A), whereas black TiO2 NPs presented 81 % A and 19 % rutile (R) phases (Table S2–S3). Both the sam-ples were highly crystalline with average particle size of 15 nm (white TiO2) and 23 nm (black TiO2) for the A phase.

Understanding the electronic properties of reduced titanium dioxides (TiO2) is the focus of intense current interest. Com-pared to bare TiO2, defective TiO2 is more attractive for pho-tovoltaics,1 photocatalysis,2 and fuel cells owing to its narrow-er bandgap (< than typical 3 eV value) enabling absorption of visible light, and relatively high electrical conductivity.3 In-deed, bandgap engineering is a crucial requirement for opti-mizing TiO2 solar light harvesting capability. Several attempts consisted in adding metal4,5 or nonmetal6–8 impurities. Howev-er, the introduction of dopants, acting as charge carrier recom-bination centers, is a major issue affecting the photocatalytic efficiency.9 Recently, appealing approaches based on dopant-free, pure TiO2 phase were proposed in order to overcome this limitation.2,10–12 Tao et al. synthesized a new phase, with bandgap of only 2.1 eV, forming at the surface of rutile TiO2 (011) by oxidation of bulk Ti interstitials.11 Chen et al. ob-tained black TiO2 nanoparticles (NPs) with ～1.0 eV bandgap through high-pressure hydrogenation of crystalline TiO2.2 Self-doping with Ti3+ bulk species was also demonstrated.12 Cited examples show the decisive role of surface disorder2 and point defects, such as oxygen vacancies (VOs)12 and Ti intersti-tial,13 in dictating the bandgap narrowing in TiO2.

Herein, we present the structural and electronic analysis of novel core-shell black TiO2 NPs with a reduced bandgap ob-tained via a one-step crystallization-reduction process. Provid-ed experimental evidences clearly identify the nature and loca-

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Figure 1. (a) SXRPD pattern and picture of black TiO2 sam-ples. (b) Micro-Raman spectra of P25 Degussa and black TiO2. Reference spectra of pure anatase and rutile phases (con-tinuous and dashed blue lines, respectively) are also shown. Inset: the most intense Eg peak of anatase TiO2 for both sam-ples, along with corresponding peak center positions and widths. (c) CL spectra of P25 and black TiO2.

Interestingly, reduction of TiO2 at 400°C and 450°C gener-ated samples composed by 100% A with unaltered absorption features (Figure S2–S3, Table S1).

In the O–rich regime, the temperature of A to R phase transi-tion exceeds 700 °C, while upon our reduction conditions it drops down to 500°C. As known, annealing in reducing envi-

ronment creates point defects in the TiO2 crystal structure: the interaction between TiO2 host matrix and hot H2 molecule gives rise to VOs that overcome the activation energy of TiO2 lattice rearrangement and accelerate it.15

From the Rietveld-refinement of the black sample SXRPD data, the factional occupancy of Ti and O sites in the A phase were obtained (Table S3). The absence of structural anomaly in R phase suggested a weak contribution of this phase to the overall sample properties. For the A phase, the occupation fac-tor of O was ～0.95, which corresponds to a VO concentration of 5% in black sample.

Structural properties of black TiO2 were further examined by measuring Raman scattering. Analysis was carried out also on P25 Degussa that has the same phase composition. The six (3Eg+2B1g+A1g) Raman active modes of A phase were detected in both the investigated samples (Figure 1b).16 Since black TiO2 spectral features were superimposed to a VO related pho-toluminescence band (Figure S5),17 the light emission proper-ties were studied by cathodoluminescence (CL) spectroscopy. CL spectra of both P25 and black TiO2 (Figure 1c) show a broad structured band arising from three different radiative transitions (Figure S6), two (at 2.63 eV and 2.36 eV) related to VO intra-gap states,17 and one (at 2.77 eV) attributed to the self-trap exciton emission.17 The higher intensity of VO related emissions in the black TiO2 spectrum is an evidence of the higher concentration of VOs in this sample.

Finite size of the grains (< 10 nm) or shortening of the corre-lation length because of the presence of defects result in pho-non confinement effects.18,19 Theoretical calculations show that phonon confinement leads to the blue-shift and broaden-ing of the most intense Eg peak with respect to bulk A.19 The smallest between correlation length of the phonons and parti-cle size ultimately determines the Raman spectral change.18 Here, the size of crystallites resulting from SXRPD data (23 nm) ruled out the occurrence of finite-size effects, indicating that structural disorder2,19 (localized defects associated with VOs) was rather responsible for the large blue shift and broad-ening of the peak observed in black TiO2 (see position and FWHM in inset of Figure 1b).

HRTEM images and related Fast Fourier Transform (FFT), provided detailed information on the structure of TiO2 sam-ples. FFT reported in Figure 2b-2c provided evidences that in some region of the sample, A and R phases have grown sepa-rately in different NPs of black TiO2. From the FFT (inset c2) related to a well-formed A nanocrystal reported in Figure 2c, a set of interplanar distances were calculated. These distances obtained at the local scale were compared with the average ones (from SXRPD) and a contraction of about 1–4% was found. This result is consistent with a lattice contraction in-duced by the presence of VO in the A black TiO2 structure. Remarkably, only black TiO2 NPs presented a unique core-shell morphology characterized by a ～1.5 nm thick disordered surface layer (Figure 2b and SI).2

The reduction of TiO2 results in samples with VOs (TiO2–x) or Ti interstitials (Ti1+xO2). In both cases, (i) Ti is in excess with respect to O, and (ii) reduction is accompanied by the ap-pearance of Ti3+ species.20,21

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Figure 2. HRTEM micrographs of white (a) and black (b,c) TiO2 along with their FFT shown in the insets. (c) EELS spec-tra of the Ti–L2,3 edge for white (black line) and black (red line) TiO2.

Electron energy loss near edge structure (ELNES) appears above the absorption edge in the electron energy loss spectrum (EELS) and allows a qualitative interpretation of electronic states. O–K (Figure S11) and Ti–L edges are the main features present in the TiO2 spectrum. The Ti L2;3 edge shown in Figure 2d mainly reflects Ti 3d unoccupied states split into the t2g and eg sub–bands because the octahedral coordination of Ti atoms with O.22 Comparing white and black TiO2, the t2g–eg splitting is 1.69 eV vs. 1.61 eV in L3 and 1.78 eV vs. 1.45 eV in L2, re-spectively. The noteworthy 20% decrease observed for the L2 peak splitting in black TiO2 can be related to the presence of Ti3+ in its lattice. In fact, t2g–eg states are extremely sensitive to the presence of point defects.23 With the increase of d elec-trons, which corresponds to the decrease of the formal charge of the Ti ion (i.e., formation of Ti3+), the occupation number of the antibonding orbital increases. As a matter of fact, the t2g–eg splitting becomes smaller. Other titanium oxides, such as Ti2O3 or TiO, with their rather featureless ELNES,22 are repre-sentative examples for this situation.

The presence of Ti3+ centers in the black TiO2 structure was confirmed by electron paramagnetic resonance (EPR) spec-troscopy (Figure S12).24–27 Interestingly, the absence of the superoxide (O2–) radical signal would indicate that no Ti3+ are present at the surface. Thus, Ti3+ are present exclusively in the bulk, which is a key factor for the observed stability of black TiO2 NPs.

X-ray photoelectron spectroscopy (XPS) analysis of Ti 2p and O 1s regions evidenced no significant differences in the NP surface of P25 and black TiO2 (Figure S13, Table S4). The Ti region was regular and the binding energy (Ti_2p3/2 = 458.3±0.2 eV) compared well with data for TiIV in TiO2.28 The O 1s peaks of P25 and black TiO2 showed, as often reported in the case of oxides, the presence of two components, attributed to lattice oxygen (OL) in TiO2 (529.5±0.2 eV) and to surface OH (OOH) species (531.0±0.2 eV).28 The shell of black TiO2 might have origin during the fast cooling step, which freezes, as highly reactive and disordered phase, the outer surface lay-ers (～1.5 nm). Upon exposure to air at RT, the metastable shell can subse

Figure 3. Valence band XPS spectra of P25 Degussa (black

line) and black TiO2 (red line). Thin black lines show the line-ar extrapolation of the curves used for deriving the band edge position of TiO2 samples.

Figure 4. Schematic of nanoparticle’s structure and DOS for (a) TiO2 P25 Degussa and (b) black TiO2. Both representations were built using experimental data from SXRPD, CL, UV–vis spectroscopy and XPS analysis. The energy position of VO lo-calized states were calculated subtracting components at 2.36 eV and 2.63 eV of black TiO2 CL band to the valence band maximum in reference TiO2.

quently become near stoichiometric and preserves the nano-crystal core from further oxidation.

In our case, SXRPD and CL analyses revealed that Ti3+ spe-cies in black TiO2 NPs obtained by mild thermal treatment are due to the presence of VOs. The presence of interstitial Ti ions, most probably formed under more severe conditions (i.e., high

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12345678910111213141516171819202122232425262728293031323334353637383940414243444546474849505152535455565758temperature vacuum annealing),11,13,20,21 was ruled out since in the O–poor regime the formation of VOs should be energetical-ly favored compared to them.29,30

Figure 3 reports VB XPS of reference (P25) and defective (black) TiO2. P25 Degussa displayed the characteristic VB density of states (DOS) of TiO2, with the band edge at ～1.2 eV below the Fermi energy. Since the optical bandgap of P25 is 3.25 eV (Table S1), the conduction band (CB) minimum would occur at –2.05 eV.2 On the other hand, VB XPS of black TiO2 showed notable differences: the main absorption onset was located at 0.6 eV, whereas the maximum energy as-sociated with the band tail blue-shifted further toward the vac-uum level at about –0.3 eV.

Chen et al. reported that surface disorder induces a substan-tial shift (2.18 eV) of the VB position for hydrogenated black TiO2 NPs,2 while Wang et al. recently showed very similar VB spectra for both TiO2 and TiO2 hydrogenated nanowires.1 Hy-drogenation conditions and crystallinity of TiO2 are key pa-rameters in the reduction process. In our case, starting from an amorphous precursor addresses the formation of TiO2 NPs, having crystalline and defective core and disordered shell, with a peculiar DOS structure (Figure 4). If for black TiO2 the same CB energy shape is assumed as for P25, the surface dis-order induces a remarkable bandgap narrowing (1.85 eV). This value further reduces considering an already predicted slight CB tailing.2 Furthermore, VOs introduce localized states at 0.7–1.0 eV below the CB minimum of black TiO2.12,21 There-fore, electronic transitions from both tailed VB and VO local-ized states to CB, and from tailed VB to VO localized states are responsible for the black TiO2 vis–NIR absorption.

In summary, we have shown that our black TiO2 NPs exhibit unique crystalline core/disordered shell morphology. VOs are present in the bulk anatase crystalline phase, while the disor-dered NP surface appears to be nearly stoichiometric. The bandgap narrowing is dictated by the synergistic presence of VOs and surface disorder. The findings of this work provide new insights for developing nanostructures tailored for solar–fuel generation device and other applications via controlled bandgap engineering.

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ASSOCIATED CONTENT

Supporting Information. Detailed synthesis, experimental meth-ods, and additional materials characterization. This material is available free of charge via the Internet at http://wendang.chazidian.com.

AUTHOR INFORMATION

Corresponding Author

a.naldoni@http://wendang.chazidian.comr.it; v.dalsanto@http://wendang.chazidian.comr.it

ACKNOWLEDGMENT

Financial support from Regione Lombardia through the project “ACCORDO QUADRO Regione Lombardia e CNR per l’attuazione di programmi di ricerca e sviluppo” and from the Italian Ministry of Education, University and Research through the FIRB Projects “ItalNanoNet” (RBPR05JH2P) and “Oxides at the nanoscale: multifunctionality and applications” (RBAP115AYN) is gratefully acknowledged.

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