Oxygen-Deficient Titania with Adjustable Band Positions and Defects

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Oxygen-Deficient Titania with Adjustable Band Positions and Defects; Molecular Layer Deposition of Hybrid OrganicInorganic Thin Films as Precursors for Enhanced Photocatalysis Debabrata Sarkar, Sergey Ishchuk, Dereje Hailu Taffa, Niv Kaynan, Binyamin Adler Berke, Tatyana Bendikov, and Roie Yerushalmi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11795 • Publication Date (Web): 02 Feb 2016 Downloaded from http://pubs.acs.org on February 6, 2016

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The Journal of Physical Chemistry

Oxygen-Deficient Titania with Adjustable Band Positions and Defects; Molecular Layer Deposition of Hybrid Organic-Inorganic Thin Films as Precursors for Enhanced Photocatalysis Debabrata Sarkar,† Sergey Ishchuk,† Dereje Hailu Taffa,† Niv Kaynan,† Binyamin Adler Berke,† Tatyana Bendikov,‡ and Roie Yerushalmi*† †

Institute of Chemistry and The Center for Nanoscience and Nanotechnology, The Hebrew

University of Jerusalem, Edmond J. Safra Campus, Givat Ram, Jerusalem, 91904 Israel ‡

Department of Chemical Research Support, Weizmann Institute of Science, Rehovot, 76100,

Israel

AUTHOR INFORMATION Email: [email protected] Tel: +972-2-658-5608

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ABSTRACT: Molecular Layer Deposition (MLD) of TiCl4 and ethylene-glycol (Ti-EG) was recently demonstrated as a vapor phase synthetic route for preparation of photocatalytic thin films via hybrid organic-inorganic thin films. The organic moieties of the hybrid material function as a sacrificial components that undergo controlled decomposition during thermal annealing. Anneal temperature was shown to be an important factor determining the overall photocatalytic performances of the treated films with 650 °C optimal for photodegradation of dye molecules and anneal at 520 °C showing optimal performance for the direct photocatalytic production of H2O2. Both systems exhibit activities that are not typically attainable by Titania, yet fundamental understanding of the underlying details leading to these improved reactivity for the specific cases is still lacking. Here we demonstrate that thermal anneal of hybrid organicinorganic thin films prepared by MLD yield oxygen-deficient Titania with controllable levels of oxygen vacancies (OVs) and defect states that are adjusted by the temperature of the anneal process, performed under air. The anneal process result in non-stoichiometric oxide films with unique electronic properties including the tuning of band positions, accessibility to significantly deeper valence band position and controlled formation of electronic defect states that assist in charge separation for Au-Titania catalyst. We correlate the oxygen deficiency and electronic structure of the annealed film with the photocatalytic activity for shedding light on the details that lead to the improved reactivity. These results extend the scope of using MLD in the context of photocatalysis with new routes for obtaining non stoichiometric oxides which are key for enhancing and tailoring the reactivity of metal oxide (photo)catalysis.


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1. INTRODUCTION Metal Oxides (MOs) are central in a wide range of research fields and used in numerous applications including catalysis, sensing, photonics, optoelectronic devices, renewable energy, electrochemistry and more.1–3 MOs exhibit a unique myriad of properties including their high chemical and physical stability, durability, rich surface reactivity, catalytic and photocatalytic functions.4 Numerous approaches were developed for preparing MOs with optimized chemical and electronic properties for different applications. Recently, there is an increasing interest in photocatalytic thin film coatings based on MOs with emerging applications including selfcleaning surfaces, light absorbing layers, removal of organic pollutants, H2O2 production, H2 production and more.5–7 Utilization of MOs thin films for the various applications is closely related to the capacity of the photocatalytic material in harnessing light energy and converting it to chemical potential that is used for chemical transformations including oxidation and reduction processes that take place at the thin film surface. MO photocatalytic properties and performance are typically optimized by controlling the crystalline phases, grain size, incorporation of impurities in the MO lattice, also commonly termed doping, formation of noble metal-MOs hybrids, and more.8–12 Photocatalytic TiO2 thin films, including films prepared by atomic layer deposition (ALD) were reported for the decomposition of dyes such as methylene blue, thin solid film of stearic acid, volatile organic compounds such as toluene, and more.13–17 Recent studies highlight the importance of non-stoichiometric oxides and the introduction of defects for optimizing the reactivity and photocatalytic performances of MO catalysts.18,19 Specifically for MOs, oxygen vacancies (OVs) affect the photocatalytic performances.20–23 OV alter the reactivity of MOs by introduction of new electronic states in the material’s band gap (BG) and alteration of the electronic structure. In addition, OVs often function as adsorption sites for Lewis acids and 3 ACS Paragon Plus Environment


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bases making them surface active sites for heterogeneous catalysis.24 OVs design offers additional valuable handles for optimizing MO electronic structure. One of the most widely studied MOs in the context of OV is non-stoichiometric, oxygen-deficient titania, TiO2-δ, where δ denotes the deviation from ideal oxide stoichiometry. The electronic structure, charge transport, and surface properties of TiO2-δ are closely related to the details of the defects and OV.25,26 Previous studies demonstrated that non-stoichiometric titania films exhibit improved properties and reactivity compared with stoichiometric TiO2 for a wide range of applications with a number of routes reported for introduction OV, often by incorporating impurity atoms into the lattice.8,27–30 For example, small concentrations of nitrogen, or nitrogen-carbon doping may yield OV by introducing interstitial defects under mild conditions.28,31 Others demonstrated metal cation doping as a route for creating oxygen vacancies.29,32 Recently, several groups, including our group demonstrated the synthesis and characterization of organic-inorganic hybrid films using TiCl4 and ethylene-glycol (Ti-EG) by molecular layer deposition (MLD).33,34 Furthermore, our group demonstrated the use of Ti-EG MLD for the formation of highly photoactive thin films by applying thermal anneal to the organic-inorganic Ti-EG hybrid films and transformation to the corresponding oxide.33 The organic moiety, ethylene glycol (EG) of the Ti-EG film function as a sacrificial component that undergo controlled decomposition during thermal annealing, resulting in carbon rich TiO2 films with molecular voids and defects which are key for the enhanced photocatalytic activity,33 however the details that lead to the enhanced photocatalytic activity remain unclear. Here we study in detail the stoichiometry, band positions, and defect states for annealed TiEG films demonstrating that annealing Ti-EG films in air at different temperatures results in oxides with adjustable deviation from ideal stoichiometry (TiO2-δ) and shifting of band positions. 4 ACS Paragon Plus Environment

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We correlate the photocatalytic performance and electronic structure of the annealed films showing that the controlled introduction of OVs and shifting of band positions contribute to enhanced reactivity with distinct optimal anneal conditions required for driving specific photocatalytic reactions. The present work provides a unified understanding of the evolution of the electronic structure and defects that takes place when annealing Ti-EG films at different temperatures capturing the electronic details associated with the specific reactivity for each case. We further demonstrate the generality of our findings by studying the reactivity of annealed TiEG films for the photodegradation of Terephthalic acid (TPA) in water. We show that Ti-EG films annealed at 650 oC exhibit downshift of the VB, making this material optimal for photodegradation of organic compounds, including highly stable compounds such as TPA, while Ti-EG films annealed at 520 oC yield the direct photocatalytic production of hydrogen peroxide (H2O2) as previously reported by us.7 Specifically, for the photocatalytic production of H2O2 our results suggest that the unique reactivity demonstrated by the gold decorated catalyst based on annealed Ti-EG is owing to the introduction of defect states that mediate charge separation and injection of the excited electrons from the oxide to the gold nanoclusters of the hybrid catalyst.7 Both the photodegradation of TPA and direct production of H2O2 are not typically attainable by titania underscoring the importance of understanding the fine details of the annealed Ti-EG films.

2. METHODS The as-prepared films were annealed under air at the specified temperature for 30 minutes, cooled to room temperature and used for further analysis employing various surface analysis and spectroscopic methods as detailed below. The oven was pre-heated and equilibrated to the 5 ACS Paragon Plus Environment


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specified anneal temperature prior to sample loading. All analyses were performed at room temperature. 2.1 Ti-EG MLD Process. Ti-EG films were prepared using TiCl4 (Acros, 99.9%) and ethylene glycol (Aldrich, > 99%). Ultrahigh purity Ar gas was used as the carrier gas in a hot wall reactor and for purge between reactant exposures. MLD films were prepared by dosing the reactant precursors into Ar carrier gas. The duration of precursor dosing was controlled using computer controlled pneumatic valves (EG dose 70 sec, Ar purge 30 sec, TiCl4 dose 0.3 sec, Ar purge 9 sec). A steady state pressure of 2.1 x 10-1 mBar was maintained during the process. Ultra pure water (>18 MΩ, ELGA purification system) was used for ALD of TiO2. The water and EG precursor chambers were set to 40 and 80 °C, respectively, and the sample reactor temperature was set to 110 °C. Thin films were prepared on quartz slides that were cleaned by O2 plasma prior to film deposition. Ti-EG samples (40 MLD cycles for BA degradation and 120 cycles for XPS analysis) were prepared and thermally annealed at different temperatures for 30 minutes. For thermal anneal of the films, the oven was equilibrated to the specified temperatures prior to sample loading. 2.2. Photocatalytic Degradation of Terephthalic Acid in Aqueous Solutions. Terephthalic acid (TPA, λmax = 240 nm), was obtained from Sigma-Aldrich and used as received. 0.1 mM aqueous solution was prepared in 2 mM NaOH solution. Photocatalytic decomposition of the TPA was performed in a quartz cuvette photoreactor with 1.5 mL volume. Ti-EG and TiO2 coated quartz slides were immersed for 30 min in the dark to attain adsorption/desorption equilibrium prior to UV illumination. Similarly, TiO2 NPs (P25, surface area 50 m2/g, Degussa) were suspended in TPA solutions and magnetically stirred for 30 min in the dark. The amount of P25 NPs used in each experiment was of equivalent mass of the thin film catalyst. All


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photocatalytic experiments were performed using 40 MLD cycles for Ti-EG and 75 ALD cycles for TiO2 films both with thickness of ~6 nm after thermal annealing. The light source used was a 9 W, 365 nm low pressure mercury lamp (Philips, PL-S9W/10). Aliquot of the reaction solution (50 µL) was collected at different time intervals and analyzed using UV-Vis spectrophotometer (Lambda 1050, Perkin-Elmer). Fluorescence measurements were performed using Perkin-Elmer LS55 fluorimeter. TPA solutions gave fluorescence intensity maxima at 430 nm for excitation wavelength of 315 nm. 2.3. FTIR Measurements. ATR-IR measurements were performed using Bruker Vertex 70V vacuum FTIR spectrophotometer equipped with a multi-reflection ATR accessory. Ti-EG films were prepared on a Si(100) polished

prism (52.2X20.0 mm), annealed at the specified

temperatures and measured under vacuum.

13

C labelled Ti-EG films were prepared using 99%

enriched EG obtained from Sigma-Aldrich. 2.4. XPS and VB-XPS Measurements. X-ray photoelectron spectroscopy (XPS) data was collected with a Kratos Axis Ultra X-ray photoelectron spectrometer. Spectra were acquired with monochromatic Al(kα) radiation. The valence band spectra of Ti-EG films were performed for sample area of 300 x 700 microns and pass energy of 20 eV. 2.5. PL Measurements. The photoluminescence (PL) spectra were recorded using a FLSP920 spectrophotometer (Edinburgh Instruments Ltd, Edinburgh, U.K.) with a 450 W xenon arc lamp (Xe900) as the light source. Photoluminescence lifetime measurements were performed on FLSP-920 using 270 nm pulsed light emitting diode (EPLED-270) as an excitation source with a pulse period 500 nsec. 2.6. UPS Measurements. UPS measurements were carried out with Kratos AXIS ULTRA system using a concentric hemispherical analyzer for photo-excited electron detection. UPS was 7 ACS Paragon Plus Environment


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measured with helium discharge lamp, using He I (21.22 eV) and He II (40.8 eV) radiation lines.35 Energy scale was referenced to the Fermi level measured on bare Au substrate.36 The vacuum level was obtained from the secondary-electron cutoff (photoemission onset) measured in the low kinetic energy region of the He(I) spectra.36,37 Total energy resolution was less than 100 meV, as determined from the Fermi edge of Au reference sample. 2.7. Raman Measurements. The Raman spectra were collected on a Renishaw inVia (Glousestershire, UK) Raman spectrometer, operated as follows: Ar+ laser (514.5 nm), 50× magnification.

3. RESULTS AND DISCUSSION Ti-EG films were prepared by MLD as detailed in the methods section. The as-prepared films were annealed under air at the specified temperatures, cooled to room temperature and used for further analysis and photocatalytic studies employing various surface analysis and spectroscopic methods as detailed below (Figure 1a). All analyses were performed at room temperature. The photocatalytic reactivity of annealed Ti-EG films greatly depends on the anneal temperature with distinct reactivity peaks obtained at different temperatures for different types of photocatalytic reactions (Figure 1b). The fine details of the oxides obtained at each anneal temperature and the contribution to the photocatalytic reactivity of each case is studied in the following sections.


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Figure 1. (a) Film synthesis and anneal schematics. (b) Photocatalytic reactivity of annealed TiEG films as a function of anneal temperatures in air. Normalized steady state values of H2O2 (▲-), and normalized first order rate constants for photocatalytic degradation of Terephthalic acid, (-●-) are presented. H2O2 data adapted from Ref. 7.

3.1. Characterization of Annealed Ti-EG Films by ATR-IR and XPS Spectroscopy. Attenuated total reflection Fourier transform infrared spectroscopy (ATR-IR) and X-ray Photoelectron Spectroscopy (XPS) were employed for studying the anneal process of Ti-EG. ATR-IR spectroscopy was measured using a Bruker V70v FTIR spectrophotometer operating under low vacuum (all measurements were performed at approx. 1 mbar, at room temperature) affording high sensitivity and elimination of background IR signals from atmosphere gases. The ATR-IR spectra showed the expected gradual decomposition of the organic EG component above 350 °C, with the loss of C-H vibrations and concomitant evolution of new peak that was 9 ACS Paragon Plus Environment


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deconvoluted to two Lorentzian components centered at 2339 and 2351 cm-1, suggesting CO2 absorption (ν3 vibration mode) (Figure 2a). The absorption evolved with increasing anneal temperature to a maximal peak area at around 500 °C followed by a sharp decrease for higher temperatures. Notably, the ATR-IR measurements were carried out in vacuum, exhibiting absorption peaks that did not change with time over several days in vacuum. This result suggests that the IR signal at the CO2 vibration region is due to strongly adsorbed or encapsulated molecules within the annealed oxide structure. In order to assign the evolving IR peaks we prepared Ti-EG films using 13C labeled EG (99%). Anneal of 13C labeled Ti-EG in air resulted in two peaks at 2339 cm-1, the same as for un-labeled Ti-EG, and a new peak at 2273 cm-1 with the expected 66 cm-1 isotope shift for CO2 corresponding to

12

CO2 and

13

CO2, (Figure 2b).38 These

experiments suggest the presence of CO2 molecules encapsulated in the annealed film structure and possibly adsorbed at cationic, or at -OH surface sites, (Figure S1a).39 In addition, no peaks were observed at 1300-1700 cm-1 excluding the presence of adsorbed carbonate species. 13

CO2/12CO2 peak area ratios decreased from ~2/1 to ~1/1.3 with increasing anneal temperature

(Figure S1b). Thus, at low anneal temperatures most of the CO2 signal originate from the decomposition and combustion of the EG moiety of the film while for higher anneal temperatures about half of the adsorbed CO2 originate from the film and the rest derive from adsorption of CO2 from the ambient air at which the anneal process was performed. This result is in-line with previous studies demonstrating that defect sites at oxide surfaces such as those found for oxygen deficient titania, TiO2-δ, exhibit high affinity towards Lewis acids and bases such as CO2 molecules. Namely, the CO2 molecules may serve as marker molecules for studying oxide defect sites.40 XPS analysis showed the expected binding energy for Ti2p peak with predominantly Ti4+


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Figure 2. ATR-IR spectra of Ti-EG films prepared on Si prism and annealed at the specified temperatures in air. (a) IR spectra for non-labelled EG. (b) CO2 region (ν3 vibration) for Ti-EG films prepared with

13

C labelled EG annealed at the specified temperatures in air. Films were

annealed under air at the specified temperature, cooled to room temperature and analysed. (c) Evolution of deviation from ideal oxide stoichiometry, δ, determined by XPS and quantification of

13

CO2 absorption peak area determined by ATR-IR for different anneal temperatures. (d)

Correlation of δ and 13CO2 peak area at 2339 cm-1. Deviation from the linear correlation for 520 °C is shown (▲), see text for discussion. 11 ACS Paragon Plus Environment


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species typical of TiO2 (Ti2p3/2 458.8 eV and Ti2p1/2 464.6 eV) as well as an additional peak at lower binding energy (Ti2p3/2 457.5 eV and Ti2p1/2 462.9 eV) attributed to TiO2-δ phase (Figure S2, S3 and Table S1).41 XPS in the O1s region showed two components for all annealed films at 530.0 eV assigned to O2− anions in the lattice of TiO242 and the additional species with higher binding energy attributed to C=O species. This assignment is consistent with the C1s XPS spectra peaks at 285.8, 288.9 and 290.9-291.2 eV attributed to C-O, C=O and COOH species, respectively.43,44 The lower binding energy of 285.8 eV for C-O species compared to the typical C-O species usually found at 286.2-286.5 eV may be attributed to C-O-Ti species present for the annealed Ti-EG films.45 In addition, adventitious carbon peak was observed at 285.0 eV for all samples.46 The overall film stoichiometry at each anneal temperature was derived by O/Ti atomic ratios obtained by XPS. The stoichiometry of Ti-EG films annealed at 450 °C was close to TiO2 (δ≈0), reaching a maximum deviation from ideal stoichiometry for 550 °C (δ≈0.25), then showing a sharp drop in oxygen deficiency reaching a stoichiometric oxide again for 650 °C (δ≈0) (Figure 2c). Comparing the XPS data and ATR-IR analysis yielded a good linear correlation for 13CO2 adsorption measured by ATR-IR with the oxygen deficiency quantified by XPS (R2~ 0.99, Figure 2d) with the exception of 520 °C where we obtained a significant deviation from the linear correlation (Figure 2d, ▲). The linear correlation suggests that the incorporation of CO2 molecules is related to the presence of defect sites in the oxide which is known to play an important role in the adsorption of CO2 molecules and preferential adsorption of Lewis acids and bases in surface defect sites.47 The outlying results obtained for 520 °C were reproducible for multiple experiments and therefore reflect specific property of the films annealed at this temperature. The deviation from the linear correlation found for 520 °C and


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additional distinctive electronic structure features that were obtained repeatedly for Ti-EG films annealed at this temperature are discussed further below. 3.2. Evolution of the Electronic Structure of Ti-EG Films with Thermal Treatment (I); Mid Band Gap States. Photoluminescence (PL) spectroscopy provides information on the electronic states and defects lying within the band gap of the oxide. Deconvolution of the PL spectra to individual components and bands yield information of the electronic states present within the BG which, in turn, can be related to a specific defect states. For TiO2, OV states are typically located at ~0.75 to 1.20 eV below the CB, commonly referred to as “green emission band”.48 For Ti-EG films annealed at 650 °C and higher the main PL band is obtained at 2.41 eV, corresponding to the expected green emission band. In addition, all samples showed the expected emission at 3.5-3.64 eV attributed to direct band to band transition (conduction to valence band), typically referred to as the ‘UV band’ for TiO2 (Figure 3 and S4). PL intensity varied significantly with anneal temperatures with lowest intensities measured for Ti-EG films annealed at 450 °C and 650 °C, both showed a stoichiometric oxide (δ≈0) by XPS analysis (Figure 2c). Ti-EG films annealed at 520 °C showed significantly higher PL intensity with the main bands at 2.86 eV and 3.0 eV, in addition to the expected green and UV bands. These additional PL bands are attributed to shallow defect states.49 The mechanisms of introducing shallow defect states in titania is still a subject of ongoing research and may involve several routes, including OV,50 Ti interstitials,51 both OV and Ti interstitials52,53 or other donor states.54 Notably, our PL data show that Ti-EG films annealed at 520 °C exhibit the highest PL intensity for these shallow defect states (Figure 3).



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Figure 3. Photoluminescence (PL) spectra of Ti-EG films annealed at the specified temperatures. All spectra were obtained at room temperature. Overall, the room temperature PL spectra obtained for films annealed at 520 °C was deconvoluted to five bands centered at 2.41 eV, 2.86 eV, 3.00 eV, 3.36 eV and 3.64 eV (Figure S4). Meticulous study of the PL spectra for films annealed at the temperature range of 500-550 °C showed that the intense bands at 2.86 eV and 3.00 eV were unique to anneal at 520 °C. The occurrence of the additional PL bands at 2.86 eV and 3.0 eV as the primary PL emission for films annealed at 520 °C coincide with the deviation from the linear correlation between adsorbed CO2 and oxygen deficiency presented in Figure 2d for this anneal temperature. Furthermore, we previously reported the unique reactivity of Ti-EG films annealed at 520 °C for the direct photocatalytic production of H2O2, using annealed Ti-EG films decorated with gold clusters.7 The absorption spectra and BG analysis gave almost constant BG of 3.24 eV regardless 14 ACS Paragon Plus Environment

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of the anneal temperature, with no evidence for visible absorption that would imply carbon doped titania.55 Nevertheless, carbon impurities may be an important factor in obtaining the unique electronic characteristics of the annealed films by promoting the introduction of structural defects, and OVs, in the annealed oxide, and possibly, by acting as mild redox agent during the thermal processes. These findings are currently under further study. PL lifetime measurements were carried to study the recombination dynamics of photogenerated electrons and holes by excitation at 270 nm and probing the PL emission decay at 360, 425, and 515 nm. Figures S5-S7 present the PL decay for Ti-EG films annealed at the specified temperatures and probe wavelengths. The PL decay curves were fitted using single-, or bi- exponential model using equation (1):56 = ∑ −

(1)

Where I(t) is the intensity as a function of time (t), Ai is a pre-exponential factor representing fractional contribution to the time-resolved decay of the component with a lifetime τi. The PL lifetime results are summarized in Table 1 and Figure S5-S7. The faster decay component (~0.51.61 ns) is ascribed to radiative recombination of photogenerated charge carriers.57 For Ti-EG films annealed at 520 °C and 650 °C, a bi-exponential decay was obtained with a longer lifetime of 2.72-4.42 ns indicating that some of the photogenerated charge carriers are trapped in defect states. While τ1 showed typical lifetimes comparable with other reported TiO2 systems, τ2 was significantly longer, comparable to the highest reported lifetimes for TiO2 systems (see Table S2 for comparison). The prolonged lifetime exhibited for Ti-EG films annealed at 520 and 650 °C



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Table 1. PL Lifetimes for Ti-EG Films Annealed at the Specified Temperatures with Excitation at 270 nm and Probed at the Emission Wavelengths Indicated All measurements were performed at room temperature. Emission Ti-EG film Lifetime Lifetime Wavelength annealed τ1 (ns) τ2 (ns) (nm) temp. (°C) 360

425

515

450 520 650 710

0.65 0.56 0.70 0.77

̲ ̲ 2.72 ̲

450 520 650 710

1.61 0.99 0.65 0.51

̲ 4.42 ̲ ̲

450 520 650 710

0.66 0.93 0.59 0.65

̲ ̲ 2.82 ̲

may be an important factor contributing to the enhanced photocatalytic activity of these films by enhancing the steady-state population of photogenerated holes and therefore the overall probability of participating in surface reactions. In addition, the longer lifetime obtained for the shallow defect sates for Ti-EG films annealed at 520 °C may further contribute to the improved photocatalytic production of H2O2 by allowing more efficient injection of excited electrons into the gold clusters.7 3.3. Evolution of the Electronic Structure of Ti-EG Films with Thermal Treatment (II); Band Positions. The photocatalytic activity of semi-conductors (SCs) and MOs is largely determined by the band structure, including the conduction band minimum energy (CBM) and valence band maxima (VBM) positions, band-gap energy, and mid band gap states. Valenceband XPS (VB-XPS) and Ultraviolet Photoemission Spectroscopy (UPS) were employed for determining the VBM with respect to the vacuum energy level for Ti-EG films annealed at 16 ACS Paragon Plus Environment

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various temperatures. Optical absorption spectroscopy was used to measure the optical band gap (BG), and together with the VBM values the conduction band (CB) position of thermally annealed Ti-EG films relative to the vacuum level were determined.36,58 Figure 4a-d shows the VB-XPS spectra at the VBM region for Ti-EG films annealed at different temperatures. The VBM of each film was determined by linearly fitting the leading edge of the valence band and the flat energy distribution, and finding the intersection of these two lines. The VBM as well as ionization energy (IE) for the Ti-EG film annealed at 450 °C were determined by UPS measurements (Figure S8). The conduction band minimum position was calculated by subtracting optical band gap value from ionization energy. Our results are summarized in Figure 4e showing the evolution of the electronic structure and band positions with respect to the vacuum energy level for various anneal temperatures combining the PL, UPS, VB-XPS and optical spectroscopy data. While the BG remains almost unchanged for all anneal temperatures studied (Figure S9 and Table S3), the CB and VB positions varied significantly. In addition, the PL characteristics were indicative of defect states present for specific anneal conditions, as discussed in the previous section (for optical absorption data, BG values, UPS and VB-XPS spectra and analysis see the SI, Figures S8, S9 and Tables S3, S4). The VB and CB positions obtained for Ti-EG film annealed at 450 °C correspond to typical literature values of TiO2 anatase, (Figure 4e).59–61 In contrast, the band positions were shifted to lower energies for Ti-EG films annealed at higher temperatures (more negative w.r.t. vacuum level), with the most negative VB position



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Figure 4. Evolution of the electronic structure of Ti-EG films with thermal treatment; VB-XPS spectra of Ti-EG films annealed at (a) 450 °C, (b) 520 °C, (c) 650 °C and (d) 700 °C. (e) Band position and defect level. Band positions and defect states for Ti-EG films annealed at different temperature determined by VB-XPS and PL spectroscopy, respectively. determined for Ti-EG annealed at 650 °C (Figure 4e and Table S4). This result is in agreement with the highest photocatalytic activity for degradation of dyes previously reported by us for Ti-EG films annealed at 650 °C exhibiting approximately a 5-fold increased activities 18 ACS Paragon Plus Environment

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compared to TiO2 films prepared by ALD.33 The VB position of samples annealed at temperatures above 650 °C were again typical of TiO2 literature values and the photoctalytic activity was significantly reduced compared to Ti-EG annealed at 650 °C.33 The deeper VB position obtained for 650 °C imply a higher driving force for oxidative processes involving the photogenerated holes and reactive oxygen species. Namely, the deep VB position determines the overall capacity of the photo-generated holes to take part in oxidative processes that lead to the decomposition of organic molecules at the oxide film surface, and to promote the generation of reactive oxygen species that act as aggressive oxidizers.62,63 We further demonstrate here that the deeper VB position obtained for annealed Ti-EG films at 650 °C not only improves the decomposition kinetics of adsorbed molecules as we previously reported, but that the higher oxidation potential associated with the deeper holes generated at the down-shifted VB enables the degradation of aromatic compounds that are typically not attainable using TiO2 (vide infra discussion regarding photodegradation of Terephthalic acid (TPA)). 3.4. Evolution of the Crystal Phases of Ti-EG Films with Thermal Treatment. Raman spectroscopy is a sensitive tool to follow the structural evolution of titania thin films providing detailed information regarding the oxide phases and deviation from ideal structure. TiO2 anatase (tetragonal, space group D194h: I41/amd), has six active Raman modes (A1g+2B1g+3Eg) and three IR active modes.64,65 Raman spectra measured for the annealed Ti-EG films showed three bands centered around 146, 197, and 639 cm−1 assigned to anatase Eg modes, a band at 399 cm-1 assigned to the B1g mode of anatase and a band at 516 cm-1 assigned to combination of A1g and B1g modes (Figure 5 and Table S5). The Eg mode is associated with planar O–O interactions making it a sensitive marker to oxygen defects while A1g and B1g modes originate from Ti–O stretching modes.66,67 Specifically, Janotti et al.50 showed that lattice defects such as the removal



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Figure 5. Evolution of the crystal phases of annealed Ti-EG films characterized by Raman spectroscopy. (a) Raman spectra showing anatase phase and the formation of rutile (446 cm-1 peak for anneal above 650 °C), (b) Eg Raman band obtained for Ti-EG films annealed at specified temperatures, and (c) Eg Raman band position-linewidth correlation (R2~ 0.92).


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of oxygen atom from its lattice position results in dangling bonds and shift of neighboring titanium atoms with respect to their mean position. Our results show that the Eg Raman band located around 146 cm-1 shifted with anneal temperatures between 144 cm-1 (for 800 °C) to 149 cm-1 (for 520 °C). Furthermore, we find a linear correlation between the peak position and the full width at halfmaximum (FWHM) for this band (Figure 5c). This result is in-line with the expected sensitivity of the Eg Raman band to the fine details of O–O modes. Specifically, the band broadening and shift to higher wavenumbers is associated with lattice contraction as a result of introducing OVs.68 The most blue-shifted and highest FWHM value is obtained for samples annealed at 520 °C (see Figure 5c), in agreement with our XPS results showing high oxygen deficiency for this anneal temperature. For anneal temperatures above 650 °C two additional Raman peaks were present at 446 cm-1 and 610 cm-1 associated with the rutile Eg and A1g modes, respectively (Figure 5a and Table S5). Absence of D and G band Raman peaks ruled out the presence of a graphitic phase for the annealed films, as expected for anneal under oxidative conditions (air) studied here. 3.5. Photocatalytic Performance (I); Tuning the Reactivity of Annealed Ti-EG Films for Photodegradation of Terephthalic acid. Aromatic compounds such as benzoic acids (BAs) are relatively stable compounds, soluble in water and involve toxic by-products generated during their degradation processes that persist in the environment for long periods of time. Solar assisted decomposition of these compounds on semiconductor surfaces is considered a promising green technology for their effective removal. Ti-EG layers annealed at 650 °C exhibit approximately 5-fold higher activity compared to TiO2 obtained by other preparation methods, such as atomic layer deposition (ALD) when comparing the photodegradation of porphyrin dye 21 ACS Paragon Plus Environment


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molecules.33 We previously showed by Temperature-programmed desorption (TPD) and porphyrin adsorption experiments that the surface area of our films do not change significantly with anneal temperature,1 therefore the higher activity of Ti-EG films annealed at 650 °C should be ascribed to changes in the inherent reactivity of the films. Here we demonstrate that the enhanced activity of annealed Ti-EG films owing to the deep VB position obtained for 650 °C can be utilized for photodegradation of aromatic carboxylic acids, BAs, which require high oxidative capacity. In fact, having simply higher surface area of the photocatalyst would have not yielded photodegradation of BAs since the typical oxidative capacity of the photogenerated holes of TiO2 is not sufficient for such processes. Specifically, Terephthalic acid (TPA) is a BA derivative that is commonly used as fluorescence probe for quantifying photogeneration of hydroxyl radical (OH )̇ involved in TiO2 photoreactivity.69,70 TPA reacts with OH ̇ to form 2hydroxyterephthalic acid (TPAOH) yielding photoemission at approximately 430 nm. TPAOH is commonly employed as a probe molecule for the photogenerated species for TiO2 as it is quite stable towards further photodegradation by TiO2.69,70 Annealed Ti-EG films were prepared on quartz slides, annealed at the specified temperatures, immersed in 0.1 mM solution of TPA and illuminated with UV light (365 nm, see experimental section for additional details). Both TPA absorbance (λmax = 240 nm) and fluorescence intensity due to formation of TPAOH were measured (Figure 6). Experiments were carried out in basic conditions (pH~10) to ensure complete de-protonation and solubility. Additionally, alkaline condition facilitates the rate of hydroxyl radical formation (OH·) for TiO2–based photocatalysis.71,72 Photodegradation of TPA was measured for annealed Ti-EG films (MLD), TiO2 (ALD) and Degussa P25 NPs. Ti-EG films annealed at 650 °C exhibited the highest degree of degradation


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(lowest C360/C0 values in Figure 6a), followed by TiO2 films prepared by ALD, and Degussa P25 NPs. TPA decomposition over annealed Ti-EG thin films showed first order kinetics (Figure 6b).

Figure 6. Photocatalytic degradation of Terephthalic acid. (a) Comparison of performance for TiO2 (ALD), Ti-EG (MLD), and Degussa P25 NPs quantified by C360/Co, the concentration of BA remaining after UV irradiation for 360 minutes. (b) Degradation of TPA vs. time for annealed Ti-EG and TiO2 films prepared by MLD and ALD, respectively, (c) time evolution of the formation of TPA-hydroxyl radical adduct, TPAOH, measured by fluorescence spectroscopy, (d) first order rate constants obtained for TPA degradation over annealed Ti-EG films at specified temperature (∆), and VBM values (−) obtained by VB-XPS for the respective Ti-EG films. All solutions contained initial BA concentration of 0.1 mM in 2 mM NaOH. 23 ACS Paragon Plus Environment


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TPA was almost fully decomposed after 360 minutes as indicated by the absorbance measurements and total organic content measurements (TOC, Figure S10). In contrast, for TiO2 thin films, ~60% of the initial TPA is still present after the same illumination time (Figure 6a). Furthermore, the fluorescence data showed significantly different kinetics for TPAOH formation over annealed Ti-EG and TiO2 films. For Ti-EG a sharp increase in the fluorescence intensity is obtained until ~100 minutes longer times (Figure 6c, ∆) indicative of decomposition of the TPAOH species over Ti-EG films. In contrast, for TiO2 films prepared by ALD the fluorescence intensity increases monotonically, indicating the transformation of TPA to TPAOH, without further degradation, as expected (Figure 6c, ▲).69 To better understand the enhanced and different reactivity of Ti-EG towards TPA and correlate the reactivity with the electronic structure details of the annealed Ti-EG films presented here the photocatalytic degradation of TPA was studied for Ti-EG films annealed at various temperatures (Figure 6d). We find a sharp dependence of the first order rate constants for TPA degradation with Ti-EG film anneal temperatures (Figure 6d). Ti-EG films annealed within the temperature window of ~450-700 °C showed photodegradation of TPA that is associated with the VBM position with the highest activity corresponding to the deepest VBM position at 650 °C and negligible activity for samples annealed at temperatures outside this window where VBM values are typical for TiO2 literature values (Figure 6d). Thus, the high reactivity of annealed Ti-EG films exhibiting the photocatalytic degradation of TPA is in close relation with the deep VBM position. Our results indicate the deep VB position obtained for Ti-EG films annealed at 650 °C is a dominant factor in this unique reactivity. Notably, the photodecomposition of TPA involves multiple steps and intermediates. This includes generation of reactive oxygen species, hydroxyl radicals, and


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some recent studies also indicate possibility of direct hole oxidation.73,74 Further study of reaction intermediates is currently in progress. 3.6. Photocatalytic Performance (II); Tuning the Reactivity of Annealed Ti-EG Films for Direct Production of H2O2. Recently we reported the direct production of H2O2 from O2 and H2O by a heterogeneous catalyst based on annealed Ti-EG films deposited on Si nanowire scaffolds and decorated with gold clusters, SiNW–(Ti-EG)anneal–Au structures.7 The photocatalytic production of H2O2 was demonstrated using UV light energy (a 9W, 365 nm low pressure mercury lamp) without the need for additional chemical energy (sacrificial compounds) or externally applied potential. We demonstrated that fine-tuning of the catalyst architecture and film structural details based on annealed Ti-EG films were key for attaining H2O2 by direct photo-induced redox reactions. Additionally, the photocatalytic production of H2O2 varied with the anneal temperature of the Ti-EG coated Si NW scaffolds with optimal performance obtained for 520 °C.7 Further details and characterization of the SiNW–(Ti-EG)anneal–Au structures involving the direct photocatalytic production of H2O2 are discussed elsewhere.7 Steady state (SS) values of H2O2 obtained for SiNW–(Ti-EG)anneal–Au catalyst prepared with various anneal temperatures are plotted against δ, the deviation from ideal stoichiometry for TiO2-δ, as determined by XPS (Figure 7). The correlation obtained (R2~ 0.92) indicates that OV plays an important role in the photocatalytic production of H2O2, with a deviation from the linear correlation for the optimal anneal temperature at 520 °C where significantly higher SS values were obtained (Figure 7, ▲) The enhanced production of H2O2 by catalyst annealed at 520 °C match the PL data showing two shallow defect levels at 0.25 eV and 0.39 eV below the oxide CB at this anneal temperature (Figures 3 and 4e). We suggest that the enhanced photocatalytic production of H2O2 measured for catalyst prepared by anneal at 520 °C may be ascribed to the 25 ACS Paragon Plus Environment


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shallow defect states (see Figure 4e, Ds states present for 520 °C) which are positioned between the CB and the gold Fermi level thereby improving the overall kinetics of electron transfer by mediating charge separation and improving the injection of the excited electrons from the oxide CB to the gold clusters where molecular oxygen is adsorbed and catalytically reduced (see Figure 7, ∆ DS [ H 2 O 2 ] SSC ).

Figure 7. Correlation of H2O2 steady state values obtained for SiNW–(Ti-EG)anneal–Au catalyst7 and δ parameter denoting the deviation from ideal oxide stoichiometry for nonstoichiometric titania, TiO2-δ, (R2~ 0.92). Deviation from the linear correlation for 520 °C sample is shown (▲), see text for discussion. This result is in-line with our previously reported data indicating that the main function of gold clusters in this system is for improving the charge separation.7 In this case the anneal process applied to the Ti-EG inorganic-organic hybrid films present a versatile handle for 26 ACS Paragon Plus Environment

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introducing specific defect states that mediate electron transfer between the oxide and gold clusters. This type of control is central for attaining the direct production of H2O2 from water and molecular oxygen without the use of sacrificial compounds which is typically not accomplished with TiO2 based catalyst. Thus, the combination of OVs and shallow defects with finely tuned energy positioning facilitate charge transfer and contributes to the overall photocatalytic production of H2O2 by this system.

4. CONCLUSIONS The present work provides a comprehensive study of the evolution of the electronic structure of Ti-EG films with anneal temperature. The organic components of the Ti-EG organic-inorganic hybrid films decompose during the thermal treatment yielding gradual and manageable transformation of the meta-stable hybrid films to the respective TiO2-δ oxide thin films. We show that varying the anneal temperature is an effective means for adjusting the resulting thin film oxide stoichiometry, generating oxygen deficient titania and tuning of the electronic structure of the films. We demonstrate significant tuning of the electronic properties including the introduction of defect states and shifting the band positions studied by photoluminescence (PL), valence-band XPS (VB-XPS), and UPS spectroscopy. Specifically, films annealed at 520 oC exhibit shallow defect states which are positioned between the CB and the gold Fermi level thereby improving the overall kinetics of electron transfer by mediating charge separation and improving the injection of the excited electrons from the oxide CB to the gold clusters where molecular oxygen is adsorbed and reduced, thereby enhancing the photocatalytic production of H2O2. The high reactivity of Ti-EG films annealed at 650 °C for photodegradation of organic molecules, including Benzoic acids that requires high oxidation capacity is in close relation with 27 ACS Paragon Plus Environment


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the deep VB position determined for this anneal temperature. The use of MLD and anneal processes results in versatile method for attaining highly optimized oxide materials for photocatalysis as demonstrated in detail for Ti-EG. The concept of utilizing a meta-stable organic-inorganic hybrid film for controllable introduction of OV defects in the evolving MO demonstrated here for TiO2-δ is expected to extend the utility of titania and possibly other MO materials by applying similar approaches to other types of oxides.

ASSOCIATED CONTENT Supporting Information. Electronic Supplementary Information (ESI) available: XPS, UPS spectra, quantification of IR components,

13

CO2/12CO2 peak area evolution, PL spectra

deconvolution, PL decay time profile, Optical absorbance and band gap analysis, Raman analysis summary, and TPA degradation and total organic content. XPS binding energies, Optical band gap values, VB position shift, Raman modes are in tables. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT This work was partially funded by a starting grant from the European Research Council (ERC) under the European Community’s Seventh Framework Programme Grant agreement no. 259312, and the Israeli National Nanotechnology Initiative (INNI, FTA project).


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TOC IMAGE


Oxygen-Deficient Titania with Adjustable Band Positions and Defects

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