Rapid Synthesis of Porous, Mixed Phase Titania Films with Tailored

Dec 3, 2013 - We report on a new, one-step electrochemical oxidation method for the rapid synthesis of mixed phase, polycrystalline TiO2 porous films ...
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Rapid Synthesis of Porous, Mixed Phase Titania Films with Tailored Orientation of Rutile for Enhanced Photocatalytic Performance Ren Su,† Mogens Christensen,‡ Yanbin Shen,‡ Jakob Kibsgaard,† Björn Elgh,§ Ronnie T. Vang,† Ralf Bechstein,† Stefan Wendt,† Anders Palmqvist,§ Bo B. Iversen,‡ and Flemming Besenbacher*,† †

Interdisciplinary Nanoscience Center (iNANO) and Department of Physics and Astronomy, Aarhus University, Gustav Wieds Vej 14, DK-8000 Aarhus C, Denmark ‡ Center for Materials Crystallography, Department of Chemistry and iNANO, Aarhus University, Langelandsgade 140, DK-8000 Aarhus C, Denmark § Department of Chemical and Biological Engineering, Chalmers University of Technology, SE-41296 Göteborg, Sweden S Supporting Information *

ABSTRACT: We report on a new, one-step electrochemical oxidation method for the rapid synthesis of mixed phase, polycrystalline TiO2 porous films with oriented rutile within a few minutes. The orientation as well as the surface chemical composition of rutile nanocrystallites can readily be tuned by adjusting the additive concentrations of HCl or HF in the electrolyte during synthesis. All TiO2 films show similar large specific surface area, which is ideal for the application of photocatalysis and photoelectrocatalysis. Compared to the random-oriented TiO2 film, films with an increasing portion of exposed rutile (101) facets were found to be characterized by enhanced photocatalytic oxidation and photoelectrochemical performances. We also observed a synergistic promotion effect of the orientation and surface F impurity. Most interesting, our tailor-oriented porous TiO2 films prepared using HF as additive show an impressive photocurrent generation at zero bias, which is ∼50 times higher compared to that of the random-oriented TiO2 film.

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INTRODUCTION Titanium dioxide (TiO2)-based materials have been addressed in numerous studies featuring applications in biomedical devices,1 energy production and storage,2 and heterogeneous (photo)catalysis.3,4 The performance of TiO2 in photocatalytic applications is governed by a complex interplay of its physical properties.5,6 It has been shown that engineering of the crystallinity,7 crystallite size,8,9 the nature and concentration of impurities,10,11 the surface decoration,12 and the polymorph composition13 can affect the recombination kinetics, the charge separation, and charge trapping efficiency and thus alter the photocatalytic performance of the material significantly.14−16 Furthermore, modifying these parameters allows adjustment of the band structure of TiO2 and may enable visible-light photocatalytic activity.17,18 Since Hotsenpiller et al.19,20 observed that the photoreactivity of polycrystalline rutile depends on the exposed facets, engineering the shape or orientation of titania has been considered as a promising route to enhance the photocatalytic reactivity of pristine TiO2.21−37 Although the underlying mechanism of the shape effect on the photocatalytic reactivity is still under investigation, a recent surface science study indicated that the enhanced activity may arise from a higher density of the undercoordinated Ti and O atoms on the surface.38 It is often observed that anatase (001) and rutile © 2013 American Chemical Society

(101) are more efficient in photo-oxidation reactions, whereas the reduction sites tend to be located at anatase (011) and rutile (110) facets, respectively.19,23,33 However, theoretical calculations revealed that these facets with high densities of undercoordinated Ti and O atoms are thermally metastable,39 explaining the difficulties in synthesizing TiO2 photocatalysts with large portions of highly reactive facets. In order to overcome the thermal equilibrium limitations, special precursors, shape-control agents, and deliberate preparation conditions have been applied.20,28,29,33 The strategy of using specialized adsorbates (i.e., H, B, F, and Cl) to synthesize microsized shape controlled anatase single crystals with large portions of (001) facets has proven to be very successful.28 However, such elaborate methods are timeconsuming and costly, and the prepared photocatalyst materials are characterized by relatively small specific surface areas because of the large particle size. Moreover, the impurities and surface adsorbed ligands introduced during the synthesis can be detrimental for the photocatalytic performance.40 Although heat treatments have been reported to be sufficient for the removal of such impurities,40 such processes bear the risk of Received: October 29, 2013 Revised: November 25, 2013 Published: December 3, 2013 27039

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particle sintering, causing the surface area to shrink, which may lead to a decrease of the photocatalytic activity. Among the three most stable polymorphs of TiO2 (anatase, rutile, and brookite), shape-controlled anatase particles with a large portion of (001) have been intensively investigated.28,29,33 While anatase is often considered to be the most active polymorph owing to higher adsorption affinity for organic molecules41 and lower recombination rate,14 some studies have suggested that nanosized rutile exhibits a comparable or even higher photocatalytic activity.8,37 However, due to the difficulties of their preparation, shape-controlled rutile nanoparticles or oriented rutile films have rarely been studied.34,36 Here, we present a new, one-step plasma electrolytic oxidation (PEO) method for synthesizing polycrystalline (essentially rutile) TiO2 porous films with tunable orientation of the crystallites.6,42 The films could be rapidly synthesized within a few minutes, and the orientation of rutile could be easily adjusted by tuning the concentration of HCl or HF in the electrolyte. In contrast to conventional hydrothermal synthesis, our method employs transient high temperature that was generated by the plasma, which prevents the sintering of TiO2 nanocrystals. More importantly, the surface chemical composition of the as-synthesized films can be well controlled by tuning the electrolyte composition, which allows us to address the effect of orientation and surface impurity on the photocatalytic performance of the TiO2 films.

Figure 1. (a) Schematic drawing of the home-designed PEO setup. The Ti sheet (1) is pressed into a Teflon seal (2) by a copper block (3) that serves as heat sink and provides electric connection to the power supply. The heat is removed by a Peltier element (4) and a second cooling block with circulated cooling water (5). The sample is pressed against a round opening of the stainless steel tank with cooling jacket (6) that contains the electrolyte. The temperature of the copper cone is measured by a Pt100 sensor, as indicated by the arrow. (b) representative V−t and I−t curves for the preparation of the porous Ran-TiO2 film.

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MATERIALS AND METHODS Sample Preparation. The porous films were synthesized by the PEO method using pure Ti sheets as anode. Pure titanium disks (0.5 mm thick, 99.6%, Advent, UK) with a diameter of 12 mm were used for film growth. Both sides of the specimens were mechanically and chemically polished. The polished specimens were cleaned by acetone under sonication and dried out in air. Titanium foil (25 μm thick, 99.6%) was utilized to prepare TiO2 films for cross-section investigations. A 0.15 M H2SO4 solution was chosen as electrolyte for the preparation of randomly oriented TiO2 film (Figure S1 and Table S1).6,42 For the synthesis of oriented TiO2 films, 0.05, 0.10, and 0.20 M additives (HCl or HF) with 0.15 M H2SO4 were used as electrolyte. The randomly oriented and the oriented TiO2 porous films prepared using different electrolytes are labeled as Ran-TiO2, 0.05HCl-TiO2, 0.10HCl-TiO2, 0.20HCl-TiO2, 0.05HF-TiO2, 0.10HF-TiO2, and 0.20HF-TiO2. A home-designed plasma electrolytic oxidation (PEO) setup with additional cooling sample holder was applied for the synthesis of porous TiO2 films, as shown in Figure 1. This configuration was developed to absorb extra heats that are generated during the PEO process. As shown in the inset figure, a Peltier element (UltraTEC, Laird) was pressed against a copper cone, which serves as both electrical connector and heat absorber of the metal piece. A Pt 100 with ceramic coating was inserted in the copper cone to measure the temperature of the sample at reverse side. A power supply (SM400-AR-4, Delta) controlled by a computer was employed for the PEO process. The high voltage was applied in a square-wave fashion for 0.5 s per cycle with a repetition rate of 0.8 Hz (duty cycle ∼40%). Representative V−t and I−t curves for the preparation of a RanTiO2 film are shown in Figure 1b. The three regions shown in the figure corresponding to the formation of thin, dense titania insulating layer (I), the formation of porous, polycrystalline TiO2 films (II), and completion of porous film growth (III). Therefore, it is clear that the rapid film synthesis only takes ∼4

min. The electric parameters and the electrolyte composition for the synthesis of porous films with variable orientation are shown in Table S1. Characterization of Physical Properties. The stoichiometric composition and the chemical states of elements within the films were analyzed using an X-ray photoelectron spectrometer (XPS, AXIS Ultra, Kratos, UK). A monochromatic aluminum Kα source equipped with a 500 mm Rowland circle silicon single crystal monochromator was operated using a 15 mA emission current at an accelerating voltage of 15 kV. Survey scans were collected using the following instrument parameters: energy scan range of 1100 to −10 eV; pass energy of 160 eV; step size of 1 eV; sweep time of 111 s; and an X-ray spot size of 700 × 300 μm. High-resolution spectra were acquired in the region of interest using the following experimental parameters: 20−50 eV energy window; pass energy of 40 eV; step size of 0.1 eV; and the scan rate is fixed at 0.2 eV/s. One sweep was used to acquire all the regions. The absolute energy scale is calibrated to the C 1s peak binding energy of 285 eV using the adventitious carbon. Depth profiles of the porous films were executed by argon (Ar+) sputtering with an emission current and an accelerating voltage of 5 mA and 5 kV, respectively. A scanning electron microscope (SEM, NOVA600, FEI) equipped with an energy-dispersive X-ray (EDX) spectroscope was employed to characterize the microscopic features of the films. A Micromeritics ASAP2010 (Micromeritics, US) adsorption analyzer was used to determine the surface area of the films by the Brunauer−Emmett−Teller (BET) method. Krypton gas was used as adsorbate due to its relative low vapor pressure that allows measuring small surface areas. The samples were degassed at 400 K for at least 8 h in vacuum prior to the measurement. The Kr adsorption isotherm was measured at 77 27040

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K, and multipoint measurements were used to calculate the specific surface area. The vapor pressure (P0) of Kr was measured prior to each measurement. The crystallite structure of the films was analyzed by grazing incidence X-ray diffraction (GIXRD) using an X-ray diffractometer (XRD, SmartLab, Rigaku) with Cu Kα radiation. Depth profiling the orientation and polymorph composition was realized by varying the incidence angle. The scan rate and integration time for grazing angles ≥2° were 0.04°/s and 10.0 s, while 0.04°/s and 20.0 s were used for grazing angle of 1°, respectively. Rietveld refinement was applied to extract the unit cell parameters, phase compositions, and orientations.43 The crystallographic information files (CIF) of anatase, rutile, and metallic Ti used for the refinement were adopted from the International Union of Crystallography (IUCr).44 For the case of preferred orientation, a correction factor G is introduced to refine the diffraction pattern.

Icorr = Icorr exp( −Gα 2)

Figure 2. (a, b) Representative XPS survey spectra and Ti 2p spectra of the Ran-TiO2, 0.10HCl-TiO2, and 0.10HF-TiO2 films. (c) F 1s spectra of the 0.05HF-TiO2, 0.10HF-TiO2, and 0.20HF-TiO2 films. (d) Depth profile of the F 1s spectra of the as-prepared 0.10HF-TiO2 films. Solid lines represent 3-point averages of the raw data (dots).

(1)

where Icorr and Iobs are the corrected and observed peak intensities, respectively. The acute angle between the scattering vector and the normal of the crystallites is denoted as α. Photocatalytic Performance Analysis. The photocatalytic decomposition of methylene blue (MB) was used to evaluate the photocatalytic oxidation performance of the films. A UV LED (OPTMAX-365, Spectroline) was used as the light source. The emission peak is located at 365 nm with a full width at half-maximum (fwhm) of 10 nm. The photon flux is 4 × 1017 photons/s.42 The initial concentration of MB was adjusted to ∼4 μM to ensure that the dissolved oxygen (∼250 μM) is sufficient for the complete oxidation of MB. A continuous analyzing setup has been designed to evaluate the photocatalytic performance of the porous TiO2 films based on the temporal decrease of MB concentration.6 Prior to the activity test, each film was cleaned by UV irradiation for 2 h in deionized water and then transferred into MB solution and kept in the dark for 2 h to establish adsorption equilibrium. The experiment was performed under conditions that favor the oxidation of MB; i.e., the pH of the solution was adjusted to 7 using NaOH and HCl, and the whole degradation process was executed under strong stirring. The photoelectrochemical performance of the films was measured in a Teflon compression cell using 0.1 M NaOH as electrolyte. The same LED light source was used. Transient photocurrent and open circuit photopotential measurements were performed using a standard three-electrode configuration. The sample, a platinum wire, and a Ag|AgCl (Metrohm, 3 M KCl) electrode were served as working electrode (WE), counter electrode (CE), and reference electrode (RE), respectively.11 The potential between WE and RE was set to zero for the photocurrent analysis. For comparison, the photocurrent generation of selected samples at zero bias was also measured using a two-electrode configuration, and there was very little difference using the two different configurations.42

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Table 1. Chemical Composition of the Films As Determined by XPS Analysis catalyst

Ti (at. %)

Ran-TiO2 0.05HCl-TiO2 0.10HCl-TiO2 0.20HCl-TiO2 0.05HF-TiO2 0.10HF-TiO2 0.20HF-TiO2

32.9 32.5 32.6 32.8 31.2 33.4 33.3

O (at. %) S (at. %) 65.8 66.1 66.0 66.3 66.8 64.1 63.6

1.3 1.4 1.4 0.9 1.0 0.7 0.8

Cl (at. %)

F (at. %)

− − − − − − −

− − − − 1.0 1.8 2.3

all samples (Figure 2b), indicating fully oxidized TiO2. The presence of S (0.95 in the bulk, indicating that the structures become increasingly more random. The films prepared with either HF or HCl additives exhibited G factors that were significantly smaller than 1, and the G factors decreased with increasing concentrations of the additives. This indicates that a higher portion of exposed rutile (101) facets can be achieved by increasing the additive concentrations. It is also noticed that the G factor of the HCl-TiO2 films increased gradually as the incidence angle increased, whereas the G factor observed for the HF-TiO2 films decreased slightly. These observations suggest that the HF-TiO2 films show a more homogeneous orientation of rutile than the HCl-TiO2 films. Further, we evaluated the photocatalytic oxidation and the photoelectrochemical performances of the porous TiO2 films. Figures 7a and 7b depict the photocatalytic MB degradation using the HCl-TiO2 and HF-TiO2 series films, respectively. Throughout the degradation processes, a pseudo-first-order

Figure 6. (a, b) GIXRD patterns measured at various incidence angles of the Ran-TiO2 and 0.10HF-TiO2 films; the main anatase (A) and rutile (R) peaks are indicated. (c, d) Depth profiles of the polymorph compositions of all the HCl-TiO2 and HF-TiO2 films. (e, f) Depth profiles of the G factor of all the HCl-TiO2 and HF-TiO2 films. The G factor denotes the amount of lattice planes oriented in [001] direction.

films (Figure 6b) were characterized by polycrystalline structures, containing exclusively rutile and anatase crystallites. No sign of amorphous TiO2 was seen, and no hint was found for the presence of brookite. The ratios of the anatase (101) and rutile (110) peak intensities were similar in both films at all incidence angles, indicating rather homogeneous polymorph compositions in the films. Metallic Ti peaks of the substrate were only observed at higher incidence angles. However, the intensity ratios of rutile (110):(101):(111) displayed very different characteristics for the two films considered. Whereas the Ran-TiO2 film shows similarities to the standard randomly oriented rutile particles with an intensity ratio of ∼6:3:1 for the (110):(101):(111) peaks,49 the 0.10HFTiO2 film was characterized by more intense rutile (101) and (111) peaks. In fact, all the films prepared using either HCl or HF as additives show more intense rutile (101) and (111) peaks.42 Powder X-ray diffraction (PXRD) analysis of selected samples extracted from the films (Figure S3) confirmed that the differences in the peak intensity ratios originate from the crystallite orientation.42 The GIXRD patterns of the 0.10HFTiO2 film revealed stronger peaks for lattice planes containing c-axis components (Miller index l > 0) that stems from a preferred orientation along the [001] direction. Rietveld refinement was applied to quantify the polymorph compositions and the preferred orientation (Ph) of the rutile

Figure 7. Comparison of the photocatalytic MB decomposition using the (a) HCl-TiO2 series and (b) HF-TiO2 series. 27043

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reaction kinetic was observed for all cases. The slope increases following an increase of the additive concentrations, suggesting a gradual enhancement of the photocatalytic performance. Note the improvement was more obvious for the HF-TiO2 series. Consistently, a similar trend was observed in the photocurrent measurement, as demonstrated in Figures 8a,b. A

indicates that the fluorine as surface contamination in the HFTiO2 film does not influence the recombination kinetics. Since previous studies show that the surface adsorbed F can mediate the charge transfer between semiconductor and charge acceptors,52−54 the significant increase of the photocatalytic and photoelectrocatalytic performance for the HF-TiO2 films may be contributed to a synergetic effect of the orientation and surface F contamination. Since all HCl-TiO2 and HF-TiO2 series films show similar physical properties (i.e., chemical composition, polymorph composition, and surface area), it was possible to correlate their photocatalytic performances with the G factors and thus the influence of the crystallite orientation. Clearly, as shown in Figures 9a,b, both photoreactivity and photoelectrocatalytic

Figure 8. Photoelectrochemical performances of TiO2 films prepared using HCl and HF as additives. (a, b) Transient photocurrent measurements in 0.1 M NaOH. The potential between WE and RE was set to zero. (c) Open circuit photopotential measurement of the Ran-TiO2, 0.20HCl-TiO2, 0.10HF-TiO2, and 0.20HF-TiO2.

Figure 9. (a) MB photocatalytic decomposition rate constants and (b) transient photocurrent as a function of the G factor.

performances of the TiO2 films increased following a decrease of the G factor; i.e., the photoreactivity is the higher the larger the area of the exposed rutile (101) surfaces is. In addition, F as surface impurity also played a key role in further boosting the catalytic performance.52,53 Such synergistic effect has been demonstrated in Scheme 1. The surface of rutile nanocrystallites within Ran-TiO2 film is dominated by (110) facets, which is characterized by relative low density of undercoordinated Ti and O. Therefore, the reactivity is lower in comparison with the HCl-TiO2 series that exposed more rutile (101) facet, which has a higher density of undercoordinated Ti and O.38 Meanwhile, the HF-TiO2 series films have the additional surface F impurity on rutile (101) facets, which facilitate the charge transportation and the adsorption of electron acceptors; thus, the optimum photocatalytic performance is achieved.

remarkable 10-fold enhancement of the photocurrent was observed for the 0.05HCl-TiO2 in comparison with the RanTiO2. While an increase of the HCl concentration from 0.05 to 0.10 M during the synthesis resulted in a further improvement of the photocurrent, doubling the HCl concentration to 0.20 M showed little improvement. The enhancement of the photoelectrocatalytic performance was more significant for the HFTiO2 films (Figure 8b), where an impressive photocurrent of ∼3 mA cm−2 was achieved for the 0.20HF-TiO2 film, which is ∼50 times larger than that of the Ran-TiO2. Since a welldefined light source was applied in this study, we estimate that the 0.20HF-TiO2 film shows an incident photon-to-current efficiency of ∼11% at 365 nm.42 Thus, the oriented porous TiO2 films show great potential in photoelectrochemical applications. While the improved catalytic performance of the HCl-TiO2 films can be solely correlated to the orientation effect as there is no impurity in these films, the enhanced performance of the HF-TiO2 films may arise from a combination effect of orientation and surface impurity. In order to explore how the orientation and surface impurity affect the reactivity of the films, we performed open circuit photopotential analysis on selected samples, as shown in Figure 8c. Here, all films created a similar potential upon irradiation, which suggests that they can be characterized by similar recombination kinetics regardless of their morphology or surface area.11 It also

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CONCLUSIONS In conclusion, we have synthesized high-quality porous TiO2 films with tunable orientation of rutile via a rapid, one-step electrochemical route. The fraction of exposed rutile (101) facets can be adjusted by modifying the orientation of rutile using HCl or HF additives without altering other parameters, e.g., polymorph composition and surface area. Furthermore, the surface chemical composition, which may also influence the photocatalytic performance of the films, can be well controlled. The HCl-TiO2 and HF-TiO2 series are found to be chemically pure and F-contaminated oriented films, respectively. We have shown that a strongly oriented rutile film with a large portion of 27044

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Scheme 1. Proposed Promotion Mechanism of Photocatalytic Performance by Orientation and Surface F Impuritya

a A and A− represent electron acceptors and reduced electron acceptors.

exposed rutile (101) facets can enhance the photocatalytic performance of pristine TiO2. Additionally, the presence of F as surface contamination can further improve the photocatalytic performance. Our approach provides an easy and rapid route for the synthesizing of highly reactive TiO2 films with tailored properties.

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ASSOCIATED CONTENT

S Supporting Information *

Details on sample preparation and characterization (TEM and XRD). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (F.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Danish Strategic Research Council (Center for Energy Materials), the Danish National Research Foundation (Center for Materials Crystallography, DNRF93), Haldor Topsøe A/S, and the Carlsberg Foundation for financial support.

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REFERENCES

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dx.doi.org/10.1021/jp4106713 | J. Phys. Chem. C 2013, 117, 27039−27046