full papers Thin Films
An All Low-Temperature Fabrication of Macroporous, Electrochemically Addressable Anatase Thin Films Michael Schröder, Sébastien Sallard, Matthias Böhm, Marcus Einert, Christian Suchomski, Bernd M. Smarsly,* Stephen Mutisya, and Massimo F. Bertino*
Macroporous TiO2
(anatase) thin films are fabricated by an all low-temperature process in which substrates are dip-coated in suspensions of mixed anatase nanoparticles and polystyrene beads, and the templating agents are removed by ultraviolet (UV) irradiation at a temperature below 50 °C. Scanning electron microscopy (SEM) and Raman spectroscopy show that the templating polymer beads are removed by UV irradiation combined with the photocatalytic activity of TiO2. X-Ray diffraction reveals that nanoparticle growth is negligible in UV irradiated films, while nanoparticle size increases by almost 10 times in calcined films that are prepared for comparison. The macroporous films are prepared on FTO-(fluorinedoped tin oxide) coated glass and ITO (indium tin oxide) coated flexible plastics and thereby used as working electrodes. In both cases, the films are electrochemically addressable, and cyclic voltammetry is consistent with the response of bulk TiO2 for calcined films and of nanoscale-TiO2 for UV-irradiated films.
1. Introduction Porous films of crystalline metal oxides are widely used and studied as electrodes or components of electrochromic and photovoltaic devices, Li-ion batteries and water
M. Schröder,[+] Dr. S. Sallard,[+] M. Böhm, M. Einert, Dr. C. Suchomski, Prof. B. M. Smarsly Justus-Liebig-Universität Institute of Physical-Chemistry Heinrich-Buff-Ring 58, D-35392, Giessen, Germany E-mail: [email protected] Dr. C. Suchomski Institute of Nanotechnology Karlsruhe Institute of Technology Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany Prof. M. F. Bertino, S. Mutisya Virginia Commonwealth University Department of Physics 701 West Grace Street, Richmond, VA, 23284–2000 E-mail: [email protected] [+]These authors contributed equally to this work and manuscript. DOI: 10.1002/smll.201300970
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decontamination systems.[1,2] Through-connected macroporosity (pore size > 50 nm) is beneficial with regard to mass transport and achieving a large pore volume, and it can be obtained by using templates such as polymer beads, typically resulting in pore sizes of several hundred nanometers.[3] In particular, macroporosity in the range of approx. 100–200 nm, i.e., significantly below 1000 nm, is regarded as favorable for electrochemical applications, e.g., Li insertion.[2c,3c] In most cases macroporous metal oxides are prepared via sol-gel routes such as alkoxide hydrolyzation and condensation, in combination with a templating strategy. In the vast majority of previous studies, macroporous metal oxides were generated in the form of powders or monolithic structures, e.g., using polymer colloids as template.[2c,3a,b] The polymer colloids and dispersions thereof are available by emulsion polymerization strategies.[4] Also, macroporous oxide films can be obtained by dip- and spin-coating of substrates with dispersions of polymer colloids, using molecular precursors.[3b] This strategy even allows for the generation of films with meso/macroporosity possessing interesting physicochemical properties, as demonstrated for SiO2 films endowed with hierarchical meso/macroporosity.[5] Calcination is typically employed, for meso- and macroporous metal oxides, to remove the templates[5] and crystallize
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An All Low-Temperature Fabrication of Macroporous, Electrochemically Addressable Anatase Thin Films
Scheme 1. Fabrication steps used to produce macroporous crystalline TiO2 anatase thin films.
the initially amorphous oxide,[2,3] but this treatment possesses certain inherent shortcomings. Calcination can induce changes in morphology (e.g., anisotropic shrinkage)[1,2a,d,f] and in the crystal structure (phase transformations, crystal growth), and evidently is incompatible with flexible organic substrates used in electronics. As a conceptually alternative strategy, dispersions of crystalline TiO2 nanoparticles can be utilized for generating thin films and other morphologies. Using metal oxide nanoparticles for the generation of templated porous structures has been already described as a versatile strategy for ordered mesoporous metal oxides, especially TiO2,[6] but meanwhile also for other metal oxides, for both powders and films.[7] In addition, dispersions of crystalline metal oxide nanoparticles in suitable solvents can serve for generating porous fibrous structures by electrospinning.[8] Preformed crystalline nanoparticles are advantageous in that the transformation from an amorphous to a crystalline matrix, involving rearrangement on the atomic scale, can be avoided. However, all these concepts usually requires treatment at elevated temperatures of several hundred degrees Celsius, in order to achieve densification and sufficient connectivity between the nanocrystals, especially for electrochemical applications. Hence, alternative low-temperature processes for the densification of the oxidic matrix would constitute a welcome strategy in the structural design of nanostructured metal oxides. Irradiation with UV light of colloidal films composed of pre-formed nanoparticles is a promising room temperature alternative to calcination, which was pioneered by the Itoh group and later applied by Tebby et al. to the fabrication of nanoporous electrodes, though not using well-defined templates.[9] Up until now, in the synthesis of nanoprous materials UV irradiation had thus been used to remove surfactant molecules, i.e., generating mesopores. Recently, Bertino et al. reported the densification of films prepared from dispersions of crystalline TiO2 nanoparticles by irradiation with UV light.[10] It was shown that benzyl alcohol molecules, being attached to the nanoparticles’ surface as a consequence of the special synthesis (Niederberger route)[7], are oxidized by UV light which in turn results in small 2014, 10, No. 8, 1566–1574
creating linkages between TiO2 nanoparticles and the densification of the matrix. These findings suggest that the densification and linkage between metal oxide nanoparticles by exposure to UV light can be an alternative to treatment at elevated temperature. Based on this concept, in the present study we use UV irradiation to fabricate mechanically stable TiO2 films with pore sizes between 50 and 100 nm, using polymer beads (poly(styrene), PS) and anatase TiO2 nanoparticles. Dispersions of TiO2 nanoparticles were synthesized by recently described procedures.[11] As one of the main features, the study was intended to reduce the temperature experienced by the films throughout all processing steps to room temperature, i.e., below 50 °C (Scheme 1). Irradiation by UV light was utilized for both, degrading the PS template and densification of the TiO2 matrix. This strategy is also motivated by the fact that TiO2 anatase is known to photocatalyze oxidation reactions, i.e. oxidizing toluene[12] and benzyl alcohol upon UV irradiation.[10] Thus, template removal at low temperatures represented another important challenge in this study. Thus, the degradation of PS was systematically studied by Raman spectroscopy. The targeted pore size is larger than for typical mesoporous metal oxides and concomittantly leads to smaller surface areas. Aside from electrochemical applications,[2c,3c] pore sizes of 50 to 200 nm are particularly attractive for diverse applications involving the interaction with larger molecules, for instance the filling with semiconducting polymers in hybrid organic/inorganic solar cells based on TiO2. Hole-conducting polymers can be combined with different oxidic nanomaterials, e.g., TiO2 and ZnO, providing a nanostructured donor/acceptor material.[13] One of the major issues of our approach was the colloidal stability of dispersions containing TiO2 nanoparticles and colloidal PS particles with average sizes between approx. 50 and 150 nm. Thus, our study was dedicated to develop suitable compositions and stabilization conditions allowing for the generation of stable colloidal dispersions, which then are useable for the reproducible deposition of films by dipcoating, targeting thicknesses of several hundred nanometers.
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“Colloidal stability” here is referred to as stability against agglomeration. Electrochemical characterization (cyclic voltammetry and galvanostatic measurements) was applied to study the electrochemical addressibility and also to compare UV-treated films with their calcined counterparts. Plastic substrates covered with a transparent, conductive ITO-(indium-tin oxide) layer were used to test the feasibility of the process for flexible and temperature-sensitive substrates.
2. Results and Discussion 2.1. Synthesis and Characterization of Macroporous TiO2 Films The fabrication of the films involved several synthetic challenges and required a thorough investigation of the influence of relevant processing parameters. In our strategy crystalline, TiO2 anatase nanoparticles (NP) were templated by PS beads, the latter being synthesized following a previously described procedure.[4b] We employed the synthetic methodology of Niederberger, which allows the generation of stable dispersions of crystalline TiO2 anatase NPs.[11b] Only such dispersions of crystalline metal oxides possess the properties required for the strategy presented. As determined by dynamic light scattering the polycrystalline TiO2 nanoparticles possessed a hydrodynamic diameter of about 10 nm (see Figure S1 in the Supporting Information), while the primary particle size was as approx. 3 nm, as determined from X-ray diffraction (XRD) using the Scherrer equation. The original formula approach was modified with respect to a variation of the primary crystallite size of the TiO2 anatase nanoparticles (see the Experimental Section). PS beads were synthesized by established formulae and their diameter could be varied between 80 and 120 nm. Two different fabrication schemes were investigated. In a first strategy, films of PS beads were deposited,[5] followed by infiltration by the anatase nanoparticles dispersed in an alcohol–water mixture. The films were then exposed to UV to remove the PS beads. The PS beads were removed by irradiation, but the films exhibited a high density of cracks and did not show a well-defined macroporosity (see Figure S2). It appeared that the NPs did not penetrate into the voids between the PS beads, but remained on the surface of the PS beads and formed a dense film. Probably, the penetration is impaired by the interaction of the NPs with the PS or by the surfactant (sodium dodecylsulfate, SDS) used to stabilize the polymer beads. By contrast, a one-step templating process proved to be successful (Scheme 1). Templated films were prepared by dipcoating a dispersion of TiO2 NPs mixed with PS beads in an alcohol/water mixture. Thin films formed free of cracks and became macroporous after irradiation by UVC, as shown in Figure 1. A temperature control setup kept the sample at controlled temperatures below 50 °C during UV exposure, thus excluding that any of the observed changes were caused by thermal activation or thermal degradation.
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Figure 1. SEM images of a film prepared from a dispersion of PS beads and anatase NPs (on Si wafer) before any treatment (A), after calcination at 550 °C (B) without UV treatment and after treatment by UVC irradiation solely (i.e., without thermal treatment) (C). Cross-section SEM image of UVC irradiated anatase film (D). Macroporous film on ITO-coated PET (E). TEM image of UVC irradiated anatase film (F). The films are issued from the same batch.
The quality of the films depended on several processing parameters. The most suitable solvent composition was found to be a combination of methanol and water in a weight ratio of about 1:5. Use of ethanol or of methanol-water weight ratios significantly different from 1:5 yielded films which were prone to micron-sized cracks and to delaminate. The need to use a methanol/water mixture is due to the antipodal solubility of PS beads and TiO2 nanoparticles. Dispersions of TiO2 nanoparticles, synthesized by the benzyl alcohol approach introduced by Niederberger,[6,11] are stable in solvents such as tetrahydrofuran and ethanol, while water-based emulsions of PS are incompatible with solvents for dispersions of TiO2 NPs. Thus, mixtures of methanol and water represent a balance with respect to colloidal stability, avoiding agglomeration. The optimal concentration of TiO2 was found to be between 3.5 and 4% of the weight of the solvents and the PS concentration was between 3.8 and 4.5% by weight. When higher concentrations of NPs and/or PS were used, aggregates formed in the suspension and the films were inhomogeneous. The weight ratio between TiO2 and PS was a critical parameter, too. Films prepared with excess of TiO2 NPs presented cracks, while excess PS increased the viscosity of the dipping solution and yielded films with cracks on the order of several micrometers (see Table 1 and Figure S3). Larger water concentrations lead to aggregation of TiO2 NPs, which is understandable, since the NPs are capped by a hydrophobic molecule (benzyl alcohol). The maximum allowable water concentration was 21% of the weight of methanol.
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An All Low-Temperature Fabrication of Macroporous, Electrochemically Addressable Anatase Thin Films
Table 1. Composition of dipping suspensions and resulting films characteristics. TiO2 [g]
PS [g]
TiO2/PS [weight]
MeOH [g]
H2O [g]
Humidity [%]
Cracks
SEM
0.234
0.268
0.87
4.7
1
90−95
No
S.2A
0.234
0.268
0.87
4.7
1
90−95
Yes
S.2B
0.211
0.251
0.84
5.6
1
95
No
S.2C
0.211
0.251
0.84
5.6
1
72
Yes
S.2D
Methanol concentrations in excess of about 5:1 of the weight of water lead to PS aggregation. Humidity in the chamber during withdrawal had to be quite high, typically 90–95%, probably needed to maintain the required water concentration in solution and to release stress within the colloid crystal of PS. Lower values of the relative humidity resulted in inhomogeneous films (see Figure S3). The precise composition of the suspensions used for dip coating is reported in Table 1, and the corresponding SEM images are shown in Figure S3. Figure 1 shows representative SEM images before and after irradiation. The PS beads were completely removed by irradiation, leaving pores with a diameter close to 100 nm, corresponding to the diameter of the PS beads. Cross-sectional measurements (Figure 1D) showed that the PS beads were removed for films up to 200 nm in thickness. To elucidate the degree and mechanism of template removal by UV, thin films of PS beads (i.e. without the presence of TiO2 nanoparticles) were irradiated under identical conditions. After exposure for 12 h the films were brownish and cracked, and the bead morphology was degraded (Figure S4), but these effects were not comparable to the complete elimination of PS observed in the presence of TiO2. Kr Physisorption, performed at 77 K, was applied on material which was scratched off the substrate (Si wafers), in order to determine the surface area of the films. Nitrogen physisorption (77 K) was not applicable because of the too small amount of material. Typically, films treated by UV irradiation for 40 h possessed a surface area of approx. 100 m2/g. However, these values have to be regarded with care, since it was difficult to determine the exact mass, taking into account
the small amount of material per area of these films. However, this value indicates that the surface area is mainly determined by the interparticular space between the TiO2 nanoparticles. Raman spectra of TiO2/PS and of pure PS films before and after irradiation are shown in Figure 2a, and, respectively, 2b. The spectra of unirradiated films (with and without TiO2) exhibited peaks that could be reconciled with the vibrational modes of polystyrene, the most intense of which was observed at 1602 cm−1 and corresponded to C-C stretches within the aromatic ring. Upon irradiation, all vibrational modes due to the organics disappear in films containing TiO2 NPs, and the spectra coincided within error with the spectra of calcined films (550 °C) of TiO2 nanoparticles, i.e. corresponding to complete removal (>99%) of carbon residues. The signal at 1450 cm−1 probably corresponds to residual aliphatic carbon moities. In films of pure PS NPs, instead, the vibrational modes of the organics were identifiable even after irradiation. Raman spectroscopy data ruled out temperature degradation and ozonolysis, which would have degraded the PS beads with and without TiO2. PS beads were removed by the photocatalytic action of anatase TiO2. We found that a UV C treatment for 5 h was sufficient to achieve removal of the PS template. Such exposure time does not affect significantly PET as flexible substrate for an exposure up to 20 h, as shown by reference experiments (Figure S5, Figure S8). For UV C exposure times > approx. 40 h a marked brownish color was observed owing to degradation of PET. In addition, Raman spectroscopy measurements were also performed on the TiO2 nanoparticles themselves, including the 100–600 cm−1 range, both in the as-prepared and the calcined state (550 °C), see supporting information (Figure S9). It is seen that the patterns agree with anatase TiO2, confirming a well-developed crystallinity after the synthesis based on benzyl alcohole. Furthermore, these measurements prove that the major signals observed for the macroporous films originate from TiO2. The broad shoulder observed at 1300 cm−1 in the UV-treated film can be attributed to residual carbon, while its small intensity speaks for a content of a few atom per cent at maximum. However, the quantification and determination of carbon is generally difficult by various
Figure 2. Raman spectra of a) TiO2 films templated with PS beads, and of b) PS beads without TiO2, after the indicated processing. The films were prepared on Si wafers, and the excitation wavelength was 532 nm. small 2014, 10, No. 8, 1566–1574
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analytical techniques (e.g., XPS), especially taking into account the small mass of the specimen. Furthermore, it can be excluded that residual benzyl alcohole contributes to the spectra (see Figure S9). For comparison, films were calcined at 550 °C without prior UV treatment. Calcined films possessed a morphology very similar to that of UV-irradiated films (Figure 1B), but they were mechanically quite weak and peeled off the substrate very easily. The brittleness of calcined films might have been a consequence of nanoparticle growth. The primary crystallite size of TiO2 nanoparticles was investigated by X-Ray Diffraction (XRD) on UV-irradiated and calcined films (Figure 3). The average size was determined by Scherrer’s equation and calculated to be nearly the same (diameter ≈ 3 nm) for as-grown and UV-irradiated particles. The XRD data were in good agreement with JCPDS card No.21-1272 (see Figure 3), suggesting the absence of anisotropic particle shape and preferred orientation. Note that the hydrodynamic diameter as determined from DLS was approx. 10 nm, indicating moderate aggregation of the NPs. Calcination at 550 °C yielded crystallites with a size of about 21 nm diameter, a nearly tenfold growth, which likely contributed to the mechanical weakness of calcined films. For several applications, e.g., for dye-sensitized solar cells, it is desirable to prepare macroporous TiO2 films on flexible substrates (Figure 4). That procedure was therefore used to generate films on a substrate of PET (poly(ethylene terephthalate)) covered with a coating of ITO (indium doped tin oxide). Crack-free macroporous films were obtained after UV treatment, using the same processing steps as described
Figure 4. Transparent macroporous anatase TiO2 film on flexible ITOcoated PET.
above. The films featured a reasonable transmittance (85% at λ = 550 nm), while they possessed a slightly brownish color, being attributable mainly to the decomposition of the PET polymer and certainly small amounts of residual carbonacous material originating from the decomposition of PS (Figure 1E, Figure S5). It is well known that even slightest amounts of carbon (below 1 wt.%) can result in such coloration. Since an exposure time of 5 h does not affect the PET substrate (see Figure S5) and since it is sufficient to remove the template, the films can be prepared without significant decomposition of the substrate. The high transparency suggests that indeed only minor carbonaceous leftovers of PS were present causing the color.
2.2. Electrochemical Characterization
Figure 3. XRD patterns of anatase nanoparticle films (A) as prepared, (B) UV C irradiated and (C) annealed at 500 °C. To determine the average grain size, the Scherrer equation was applied to the full width at half maximum (FWHM) of the (101) and (200) peaks. The average grain size of (A), (B) and (C) is 3.2 nm, 3.2 nm and 20.6 nm, respectively. The stick pattern shows JCPDS (Joint Committee on Powder Diffraction Standards) reference card no. 21–1272 for (single phase) anatase.
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The macroporous TiO2 films were studied with respect to their Li storage properties as electrochemical model system, using standard electrochemical characterization (CV, galvanostatic measurements). For both calcined and UV-treated films, FTO-coated glass was used as substrate. In addition, for the UV-treated films electrochemical characterization was also performed on plastics covered with ITO (indium tin oxide). SEM investigation after bending of the coated PET substrates did not show any pronounced scratches or damage in the macroporous TiO2 layer (see Figure S10, supporting information). CV of calcined and UV-irradiated films served to compare their electrochemical addressability (Figure 5). It is seen that irradiated and calcined films were both electrochemically active. Calcined films possessed an electrochemical response comparable to that of anatase obtained by calcination of molecular precursors.[2d] Sharp cathodic (1.74 V) and anodic (1.94 V) peaks were observed with a difference in the peak potential (ΔE) of 200 mV. UV-irradiated films (Figure 5) exhibited broad peaks at 1.76 V (cathodic) and 1.85 V (anodic) with ΔE = 90 mV. Thus, ΔE of UV-treated films was substantially smaller than the corresponding value for films treated at 550 °C. The dependence on processing was confirmed by galvanostatic cycles (Figure S6). The calcined materials featured a clearly defined plateau at 1.75 V
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Figure 5. Cyclic voltammetry of calcined and UV-irradiated macroporous anatase films, prepared on FTO-coated glass.
typical of anatase.[2d,6a] For the UV-irradiated material the plateau was less pronounced (Figure S6). The differences in the electrochemical responses can be explained based on the different size of the anatase nanoparticles in the two systems.[6a,14] For the comparatively large crystals of the calcined samples the cathodic and anodic processes occur mainly by insertion and extraction of Li ions.[6] In case of the smaller crystallites in the irradiated samples the adsorption of cations at the surface (pseudo-capacitance) for the anatase reduction-oxidation processes becomes relevant leading to increased charge/discharge kinetics, which results in a smaller ΔE and higher charge capacity.[6a] In order to evaluate the electrochemical performance of the PS/TiO2 thin films in further detail, CV measurements were carried out as a function of UV exposition time, i.e. for non-, 1 h and 40 h UV-illuminated dip-coated PS/TiO2 thin films (ITO/PET substrates) in a potential window between 1.55 V and 2.65 V as shown in Figure 6. It is seen that the overall shape of the CV curves is comparable to those obtained from films deposited on FTO-coated glass. All three types of samples demonstrate an anodic and cathodic peak at around 1.88 V and 1.77 V, respectively, corresponding to a difference in peak potential of approx. 110 mV in case of the film being exposed to UV light for 40 h. Thus, for all these UV exposition times ΔE was substantially smaller than for the calcined films, confirming the results obtained for films deposited on FTO-coated glass. The macroporous TiO2 film being illuminated for 40 h by UV light exhibited the most pronounced reduction/oxidation peaks, which is attributed to the entire removal of PS beads during the UV exposition making more anatase surface area accessible for the electrolyte. Interestingly, the electrochemical stability of macroporous TiO2 was not significantly influenced by the duration of UV treatment verified by constant cycling curves for all types of samples. This electrochemical stability is at least partially attributable to the good adhesion of the UV-treated films, compared to calcined films. small 2014, 10, No. 8, 1566–1574
The significant decrease of the cathodic-current after the first cycle can be assigned to the nature of the PET substrate (Figure 6D) and saturation of irreversible trapping sites on the anatase crystallite surface during the first reduction step. The electrochemical lithium insertion/extraction capacity of macroporous TiO2 films was further investigated by galvanostatic experiments (Figure S7). In agreement with the CV data, the macroporous TiO2 illuminated with UV light for 40 h depicts the highest specific charge capacities of 1.42 and 1.33 µAh·cm−2 for the 1st and 18th cycle (Figure S7 A), respectively. By comparison, the 1 h UV-exposed and nonirradiated PS/TiO2 thin films reveal capacities of 1.31 and 0.94 µAh·cm−2 (Figure S7 B and C), respectively, for the first cycle, being 8% and 34% lower than the film being exposed to UV C for 40 h, respectively. The higher capacity is most likely a consequence of the complete removal of PS template. As illustrated by the trend of discharge capacities (Figure S7 D), the thin-film TiO2 electrodes are able to maintain stable cycling behavior confirmed by loss of discharge capacity of only approx. 6, 8 and 11% from the 1st to the 18th cycle, for the 40 h, 1 h and non UVirradiated TiO2 sample, respectively. It is noteworthy that the UV-treated TiO2 thin-film electrodes show only moderate capacity fading, which further emphasizes their stability in spite of the absence of treatment at elevated temperature. The bare PET substrate demonstrates relatively low specific discharge capacities of 0.15 and 0.1 µAh·cm−2 for the 1st and 18th cycle, and sustains a stable cycling performance for theses cycles. This proves the electrochemical stability of ITO-coated PET as working electrode for electrochemical analysis.
3. Discussion and Conclusion In the present study, we have developed a methodology to produce macroporous, crystalline and electrochemically addressable TiO2 films, using dispersions of poly(stryene) and TiO2 nanoparticles. The main issue and challenge in the synthesis is the colloidal stability of the dispersion containing two types of colloidal particles, namely poly(styrene) and TiO2 nanoparticles, possessing diameters on the order of approx. 100 nm and a few nanometers, respectively. Since colloidal stability is identical to the prevention of agglomeration, it is worth while discussing the colloidal dispersions in terms of the principles of colloid chemistry, in particular with respect to the interaction between two significantly different types of particles in a solvent medium. In general, stability against aggregation is based on the delicate interplay between attracting van-der-Waals interaction and repulsive electrostatic forces. In the present system, the interaction is complex, because formally a completely different type of nanoparticles (metal oxide) is added to the poly(styrene) particle dispersion. Two aspects are of particular relevance regarding colloidal stability: 1. The colloidal particles are substantially different with respect to their Hamaker constants. While TiO2 possesses a
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Figure 6. Cyclic voltammograms of an as-prepared (a), 1 h UV-exposed (b), 40 h UV-exposed PS/TiO2 thin film (c) and a bare PET substrate (d) serving as working electrode. The TiO2 films (a-c) were prepared on ITO-PET substrates.
value of 19.5 × 10−20 J, the Hamaker constant of PS amounts to 7.8 × 10−20 J, i.e. is markedly smaller.[15] The Hamaker constant of the solvent mixture is certainly smaller (approx. on the order of 4 × 10−20 J).[15] Thus, the small TiO2 NPs enhance the average Hamaker constant of the solvent mixture. Hence, the interaction between PS particles is reduced by replacing the solvent by a mixture of solvents with TiO2 NPs. Also, the fact that the Hamaker constants of PS and TiO2 are significantly larger than that of the solvent mixutre, also formally leads to an attraction of PS particles and TiO2 particles.[15] Such attractive interaction is beneficial with respect to the desired positioning of TiO2 NP between the PS particles. 2. The TiO2 nanoparticles are more than one magnitude smaller than PS particles and thus occupy solvent volume between the PS particles. Also, the adsorption of TiO2 NPs on the surface of PS particles can be regarded as a similar situation as in Pickering emulsions. Both effects can be regarded as a formal increase in the volume fraction of the PS particles with respect to the solvent, possibly destabilizing the PS dispersion.
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Such discussion illustrates that the successsful preparation of stable dispersions based on mixing dispersions of PS with those of TiO2 NP is in accordance with the prediction of the classical concepts of colloidal science. The fabrication of the films is carried out at room temperature which is compatible with flexible electronics, as demonstrated by preparing films on flexible ITO-coated PET substrates. Our study determined optimal synthetic parameters, yielding porous films without macroscopic cracking or delamination (Figure S8). XRD and electrochemical measurements indicate that the crystallite size is not affected by UV irradiation, which opens the way for the fabrication of macroporous thin films of quantum-confined nanoparticles with tunable photochemical and electrochemical properties. Films with increased thickness can be obtained by repeated deposition, in the state prior to UV treatment. However, crack-free films can only be generated up to the fourth layer, i.e., larger numbers of deposition steps result in brittle films. This restriction may aggravate the usability of these films for certain applications (e.g., the “classical” Grätzel-type
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dye-sensitized solar cell); however, our macroporous TiO2 films, possessing thicknesses of approx. 150–300 nm, are suitable for polymer-based hybrid solar cells, which usually do not require thicknesses of several micrometers. The films will thus be studied with respect to the infiltration by suitable hole-conducting polymers and their photoelectrochemical properties. In order to check for the general validity of our synthetic approach, it is currently applied to other metal oxide nanoparticles. Evidently, one of the prerequisites regarding the metal oxide is a certain photocatalytic activity which degrades the polymer beads under UV irradiation. Within a first series of experiments, we succeeded in synthesizing ATO (antimony tin oxide) nanoparticles (Sb:Sn = 1:10, at. ratio) with suitable colloidal properties and in obtaining macroporous films thereof, applying the strategy developed for TiO2 nanoparticles (see Figure 7); the XRD of such ATO nanoparticles can be seen in the Supporting Information (Figure S11). While ATO is not reported as active photocatalyst, it might contribute to an oxidative degradation of the PS beads under the conditions used. Further studies are planned to study this decomposition process for different oxides in more detail, e.g., using gas chromatography coupled with mass spectometry. We consider this low-temperature process to be generally relevant in the field of porous metal oxides, since the
Figure 7. (A) SEM image of macroporous ATO film. (B) Cross-sectional SEM image of macroporous ATO film. small 2014, 10, No. 8, 1566–1574
successful synthesis of macroporous ATO films indicate that it might be applicable for other metal oxides and as the methodology opens new perspectives for the fabrication of flexible electronics devices.
4. Experimental Section Chemicals: Benzyl alcohol, 99.9% (Grüssing GmbH); Ethanol, 99.9% (VWR); Hexadecane, 98% (Fluka); Methanol 99.9% (Roth); Potassium peroxodisulfate, 99.6% (Fluka); 1,3-Propanediol, 99.6% (Aldrich); Sodium dodecyl sulfate, 98% (Sigma-Aldrich); Styrene, 99% (Sigma-Aldrich); Titanium(IV) chloride, 99.9% (Sigma-Aldrich); Lithium perchlorate, 99.99% (Sigma-Aldrich); Propylene carbonate, 99.7% (Aldrich) Synthesis of Polystyrene Spheres: Monodisperse polystyrene spheres were synthesized following the procedure reported in Ref. [4b]. 6 g styrene and 250 mg hexadecane were mixed and added to a solution of 0.033 g potassium peroxodisulfate, 0.1 g sodium dodecylsulfate (SDS) in 24 g H2O. The reaction was initiated by sonication and stirred 20 h by 77 °C under N2 atmosphere. The diameter of the beads could be varied between about 80 and 120 nm by varying the concentration of SDS or potassium peroxodisulfate. Decreasing the concentration of SDS or increasing the concentration potassium peroxodisulfate leads to bigger beadsizes. Nanoparticle Preparation: TiO2 nanoparticles (NPs) were synthesized by adapting the procedure originally reported by Niederberger et al.[11] In brief, 1 mL (1.74 g) of TiCl4 was added drop-wise to 5 mL (3.96 g) of absolute EtOH under stirring. After 5 min, 20 g of benzyl alcohol (BenOH) were added, and after another approx. 5 min 250 mg of 1,3-propanediol. The solution was then stirred for about 1 h at 110 °C. The solution was then immediately cooled to room temperature. To precipitate nanoparticles, 200 mL of diethyl ether were added to the parent solution, followed by centrifugation. The precipitate was washed again with diethyl ether, dried in air and redispersed under sonication in a methanol:water solution (90:10 by weight respectively) to get a clear dispersion. The solid concentration in the redispersed suspension was around 10% by weight. Film Preparation: Films were deposited at room temperature (20–22 °C) by dip-coating silicon wafers. The parent solution had the concentration indicated in Table 1. The weight ratio between TiO2 and PS was critical and had to be around 0.85 by weight. The withdrawal rate was kept constant (5 mm•s−1) and the relative humidity inside the dip-coating chamber was between 90 and 95%. The samples were dried for 10 min inside the dip-coating chamber, and for 1 h under ambient conditions. Calcination was done in oven from room temperature with a ramp of 2 °C/min and left 4 h at 500 °C. Exposure: A UV irradiator (model UV-P 250C) was employed (Hartmann). This irradiator is provided with a series of high pressure mercury and halogen lamps and with filters that allow ozonefree exposure of samples to visible light, UVA (320–400 nm), UVB (290–320 nm) and UVC (200–290 nm). The illumination power was measured at the sample position with a power meter to be 343 mW•cm−2. During irradiation, samples were kept at a constant temperature of 30 °C by cooling the supporting tray with water from a recirculating chiller.
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Characterization: Electrochemical measurements were performed in an Ar-filled glove-box (O2 and H2O below 1 ppm) in 1.0 M LiClO4 solution in propylene carbonate using a Verstat 3 potentiostat/galvanostat (Princeton Applied Research). Films deposited on FTO-(fluorine-doped tin oxide) coated glass and ITOcoated PET were used as working electrodes. The FTO coated glass was purchased vom Glaswerke Haller (Germany); Li foils were used as reference electrode and as counter electrode. Cyclic Voltammetry (CV) was performed between 1.45 V and 2.65 V vs. reference electrode at a fixed scan rate of 0.1 mV•s−1. Galvanostatic measurements were carried out in the same electrolytic solution, at a current density of 4 µA•cm−2 both for charge and discharge and potential boundaries at 1.55 V and 2.8 V, respectively. Scanning electron microscopy (SEM) was performed using a HSEM 982 Gemini from LEO. TEM measurements were performed using a CM30-ST microscope from Philips. X-ray Diffraction (XRD) measurements were performed with a PANalytical Xpert instrument (Cu-Kα wavelength); Raman spectra were measured by a Bruker Senterra with an excitation wavelength of 532 nm.
Supporting Information
[3]
[4]
[5] [6]
[7]
[8]
Supporting Information is available from the Wiley Online Library or from the author. [9]
Acknowledgements [10]
We thank D. Neher and C. Fahrenson from University of Potsdam (Germany) for providing cross-section SEM pictures; Bundesministerium für Bildung und Forschung (BMBF) is gratefully acknowledged for financial support (Förderkennzeichen 03 × 3525E, “SOHyb” project). This project was also supported by the Laboratory of Materials Research (LaMa) of JLU. Pascal Vöpel is thanked for substantial help with generating schemes for illustration.
[11]
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T. Yamato, I. Moriguchi, T. Kudo, Solid State Ionics 2004, 175, 195; d) D. Fattakhova-Rohlfing, M. Wark, T. Brezesinski, B. M. Smarsly, J. Rathouský, Adv. Funct. Mater. 2007, 17, 123; e) W. Wang, Y. X. Pang, S. N. B. Hodgson, J. Sol-Gel Sci. Technol. 2010, 54, 19; f) J. Zhao, S. Sallard, B. M. Smarsly, S. Gross, M. Bertino, C. Boissière, H. Chen, J. Shi, J. Mater. Chem. 2010, 20, 2831. a) B. T. Holland, C. F. Blanford, T. Do, A. Stein, Chem. Mater. 1999, 11, 195; b) W. C. Zhang, B.Q. Yin, R. Q. Shen, J. H. Ye, J. A. Thomas, Y. M. Chao, Appl. Mater. Interfaces 2013, 5, 239; c) B. Jache, C. Neumann, J. Becker, B. M. Smarsly, P. Adelhelm, J. Mater. Chem. 2012, 22(21), 10787. a) K. Landfester, N. Bechthold, F. Tiarks, M. Antonietti, Macromolecules 1999, 32, 5222; b) K. Landfester, M. Willert, M. Antonietti, Macromolecules 2000, 33, 2370; c) S. H. Im, G. E. Khalil, J. Callis, B. H. Ahn, M. Gouterman, Y. Xia, Talanta 2005, 67, 492. M. Etienne, S. Sallard, M. Schröder, Y. Guillemin, S. Mascotto, B. M. Smarsly, A. Walcarius, Chem. Mater. 2010, 22, 3426. a) J. Wang, J. Polleux, J. Lim, B. Dunn, J. Phys. Chem. C 2007, 111, 14925; b) T. Brezesinski, J. Wang, J. Polleux, B. Dunn, S. H. Tolbert, J. Am. Chem. Soc. 2009, 131, 1802. a) A. S. Desphande, N. Pinna, B. M. Smarsly, M. Antonietti, M. Niederberger, Small 2005, 1, 313; b) J. H. Ba, J. Polleux, M. Antonietti, M. Niederberger, Adv. Mater. 2005, 17, 2509; c) V. Müller, M. Rasp, J. Rathouský, B. Schütz, M. Niederberger, D. Fattakhova-Rohlfing, Small 2010, 6, 633; d) C. Weidmann, K. Brezesinski, C. Suchomski, K. Tropp, N. Grosser, J. Haetge, B. M. Smarsly, T. Brezesinski, Chem. Mater. 2012, 24, 486. a) C. Wessel, L. Zhao, S. Urban, R. Ostermann, I. Djerdj, B. M. Smarsly, L. Chen, Y.-S. Hu, S. Sallard, Chem. Eur. J. 2011, 17, 775; b) C. Wessel, R. Ostermann, R. Dersch, B. M. Smarsly, J. Phys. Chem. 2011, 115(2), 362. a) Z. Tebby, O. Babot, T. Toupance, D.-H. Park, G. Campet, M.-H. Delville, Chem. Mater. 2008, 20, 7260; b) Z. Tebby, O. Babot, D. Michau, L. Hirsch, L. Carlos, T. Toupance, J. Photochem. Photobiol. A 2009, 205, 70. M. F. Bertino, B. Smarsly, A. Stocco, A. Stark, Adv. Funct. Mater. 2009, 19, 1235. a) M. Niederberger, M. H. Bartl, G. D. Stucky, Chem. Mater. 2002, 14, 4364; b) M. Niederberger, G. Garnweitner, Chem. Eur. J. 2006, 12, 7282. G. Marci, M. Addamo, V. Augugliaro, S. Coluccia, E. Garcia-Lopez, V. Loddo, G. Martra, L. Palmisano, M. Schiavello, J. Photochem. Photobiol. A 2003, 160, 105. a) W. J. E. Beek, M. M. Wienk, R. A. J. Janssen, Adv. Funct. Mater. 2006, 16, 1112; b) L. E. Greene, M. Law, B. D. Yuhas, P. D. Yang, J. Phys. Chem. C 2007, 111, 18451. M. Wagemaker, W. J. H. Borghols, F. M. Mulder, J. Am. Chem. Soc. 2007, 129, 4323. Colloid Science, Ed. T. Cosgrove, Wiley, 2nd edition, Chippenham, UK 2010.
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Received: March 27, 2013 Revised: August 26, 2013 Published online: March 18, 2014
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An All Low-Temperature Fabrication of Macroporous, Electrochemically Addressable Anatase Thin Films Michael Schröder, Sébastien Sallard, Matthias Böhm, Marcus Einert, Christian Suchomski, Bernd M. Smarsly,* Stephen Mutisya, and Massimo F. Bertino*
Macroporous TiO2
(anatase) thin films are fabricated by an all low-temperature process in which substrates are dip-coated in suspensions of mixed anatase nanoparticles and polystyrene beads, and the templating agents are removed by ultraviolet (UV) irradiation at a temperature below 50 °C. Scanning electron microscopy (SEM) and Raman spectroscopy show that the templating polymer beads are removed by UV irradiation combined with the photocatalytic activity of TiO2. X-Ray diffraction reveals that nanoparticle growth is negligible in UV irradiated films, while nanoparticle size increases by almost 10 times in calcined films that are prepared for comparison. The macroporous films are prepared on FTO-(fluorinedoped tin oxide) coated glass and ITO (indium tin oxide) coated flexible plastics and thereby used as working electrodes. In both cases, the films are electrochemically addressable, and cyclic voltammetry is consistent with the response of bulk TiO2 for calcined films and of nanoscale-TiO2 for UV-irradiated films.
1. Introduction Porous films of crystalline metal oxides are widely used and studied as electrodes or components of electrochromic and photovoltaic devices, Li-ion batteries and water
M. Schröder,[+] Dr. S. Sallard,[+] M. Böhm, M. Einert, Dr. C. Suchomski, Prof. B. M. Smarsly Justus-Liebig-Universität Institute of Physical-Chemistry Heinrich-Buff-Ring 58, D-35392, Giessen, Germany E-mail: [email protected] Dr. C. Suchomski Institute of Nanotechnology Karlsruhe Institute of Technology Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany Prof. M. F. Bertino, S. Mutisya Virginia Commonwealth University Department of Physics 701 West Grace Street, Richmond, VA, 23284–2000 E-mail: [email protected] [+]These authors contributed equally to this work and manuscript. DOI: 10.1002/smll.201300970
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decontamination systems.[1,2] Through-connected macroporosity (pore size > 50 nm) is beneficial with regard to mass transport and achieving a large pore volume, and it can be obtained by using templates such as polymer beads, typically resulting in pore sizes of several hundred nanometers.[3] In particular, macroporosity in the range of approx. 100–200 nm, i.e., significantly below 1000 nm, is regarded as favorable for electrochemical applications, e.g., Li insertion.[2c,3c] In most cases macroporous metal oxides are prepared via sol-gel routes such as alkoxide hydrolyzation and condensation, in combination with a templating strategy. In the vast majority of previous studies, macroporous metal oxides were generated in the form of powders or monolithic structures, e.g., using polymer colloids as template.[2c,3a,b] The polymer colloids and dispersions thereof are available by emulsion polymerization strategies.[4] Also, macroporous oxide films can be obtained by dip- and spin-coating of substrates with dispersions of polymer colloids, using molecular precursors.[3b] This strategy even allows for the generation of films with meso/macroporosity possessing interesting physicochemical properties, as demonstrated for SiO2 films endowed with hierarchical meso/macroporosity.[5] Calcination is typically employed, for meso- and macroporous metal oxides, to remove the templates[5] and crystallize
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An All Low-Temperature Fabrication of Macroporous, Electrochemically Addressable Anatase Thin Films
Scheme 1. Fabrication steps used to produce macroporous crystalline TiO2 anatase thin films.
the initially amorphous oxide,[2,3] but this treatment possesses certain inherent shortcomings. Calcination can induce changes in morphology (e.g., anisotropic shrinkage)[1,2a,d,f] and in the crystal structure (phase transformations, crystal growth), and evidently is incompatible with flexible organic substrates used in electronics. As a conceptually alternative strategy, dispersions of crystalline TiO2 nanoparticles can be utilized for generating thin films and other morphologies. Using metal oxide nanoparticles for the generation of templated porous structures has been already described as a versatile strategy for ordered mesoporous metal oxides, especially TiO2,[6] but meanwhile also for other metal oxides, for both powders and films.[7] In addition, dispersions of crystalline metal oxide nanoparticles in suitable solvents can serve for generating porous fibrous structures by electrospinning.[8] Preformed crystalline nanoparticles are advantageous in that the transformation from an amorphous to a crystalline matrix, involving rearrangement on the atomic scale, can be avoided. However, all these concepts usually requires treatment at elevated temperatures of several hundred degrees Celsius, in order to achieve densification and sufficient connectivity between the nanocrystals, especially for electrochemical applications. Hence, alternative low-temperature processes for the densification of the oxidic matrix would constitute a welcome strategy in the structural design of nanostructured metal oxides. Irradiation with UV light of colloidal films composed of pre-formed nanoparticles is a promising room temperature alternative to calcination, which was pioneered by the Itoh group and later applied by Tebby et al. to the fabrication of nanoporous electrodes, though not using well-defined templates.[9] Up until now, in the synthesis of nanoprous materials UV irradiation had thus been used to remove surfactant molecules, i.e., generating mesopores. Recently, Bertino et al. reported the densification of films prepared from dispersions of crystalline TiO2 nanoparticles by irradiation with UV light.[10] It was shown that benzyl alcohol molecules, being attached to the nanoparticles’ surface as a consequence of the special synthesis (Niederberger route)[7], are oxidized by UV light which in turn results in small 2014, 10, No. 8, 1566–1574
creating linkages between TiO2 nanoparticles and the densification of the matrix. These findings suggest that the densification and linkage between metal oxide nanoparticles by exposure to UV light can be an alternative to treatment at elevated temperature. Based on this concept, in the present study we use UV irradiation to fabricate mechanically stable TiO2 films with pore sizes between 50 and 100 nm, using polymer beads (poly(styrene), PS) and anatase TiO2 nanoparticles. Dispersions of TiO2 nanoparticles were synthesized by recently described procedures.[11] As one of the main features, the study was intended to reduce the temperature experienced by the films throughout all processing steps to room temperature, i.e., below 50 °C (Scheme 1). Irradiation by UV light was utilized for both, degrading the PS template and densification of the TiO2 matrix. This strategy is also motivated by the fact that TiO2 anatase is known to photocatalyze oxidation reactions, i.e. oxidizing toluene[12] and benzyl alcohol upon UV irradiation.[10] Thus, template removal at low temperatures represented another important challenge in this study. Thus, the degradation of PS was systematically studied by Raman spectroscopy. The targeted pore size is larger than for typical mesoporous metal oxides and concomittantly leads to smaller surface areas. Aside from electrochemical applications,[2c,3c] pore sizes of 50 to 200 nm are particularly attractive for diverse applications involving the interaction with larger molecules, for instance the filling with semiconducting polymers in hybrid organic/inorganic solar cells based on TiO2. Hole-conducting polymers can be combined with different oxidic nanomaterials, e.g., TiO2 and ZnO, providing a nanostructured donor/acceptor material.[13] One of the major issues of our approach was the colloidal stability of dispersions containing TiO2 nanoparticles and colloidal PS particles with average sizes between approx. 50 and 150 nm. Thus, our study was dedicated to develop suitable compositions and stabilization conditions allowing for the generation of stable colloidal dispersions, which then are useable for the reproducible deposition of films by dipcoating, targeting thicknesses of several hundred nanometers.
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“Colloidal stability” here is referred to as stability against agglomeration. Electrochemical characterization (cyclic voltammetry and galvanostatic measurements) was applied to study the electrochemical addressibility and also to compare UV-treated films with their calcined counterparts. Plastic substrates covered with a transparent, conductive ITO-(indium-tin oxide) layer were used to test the feasibility of the process for flexible and temperature-sensitive substrates.
2. Results and Discussion 2.1. Synthesis and Characterization of Macroporous TiO2 Films The fabrication of the films involved several synthetic challenges and required a thorough investigation of the influence of relevant processing parameters. In our strategy crystalline, TiO2 anatase nanoparticles (NP) were templated by PS beads, the latter being synthesized following a previously described procedure.[4b] We employed the synthetic methodology of Niederberger, which allows the generation of stable dispersions of crystalline TiO2 anatase NPs.[11b] Only such dispersions of crystalline metal oxides possess the properties required for the strategy presented. As determined by dynamic light scattering the polycrystalline TiO2 nanoparticles possessed a hydrodynamic diameter of about 10 nm (see Figure S1 in the Supporting Information), while the primary particle size was as approx. 3 nm, as determined from X-ray diffraction (XRD) using the Scherrer equation. The original formula approach was modified with respect to a variation of the primary crystallite size of the TiO2 anatase nanoparticles (see the Experimental Section). PS beads were synthesized by established formulae and their diameter could be varied between 80 and 120 nm. Two different fabrication schemes were investigated. In a first strategy, films of PS beads were deposited,[5] followed by infiltration by the anatase nanoparticles dispersed in an alcohol–water mixture. The films were then exposed to UV to remove the PS beads. The PS beads were removed by irradiation, but the films exhibited a high density of cracks and did not show a well-defined macroporosity (see Figure S2). It appeared that the NPs did not penetrate into the voids between the PS beads, but remained on the surface of the PS beads and formed a dense film. Probably, the penetration is impaired by the interaction of the NPs with the PS or by the surfactant (sodium dodecylsulfate, SDS) used to stabilize the polymer beads. By contrast, a one-step templating process proved to be successful (Scheme 1). Templated films were prepared by dipcoating a dispersion of TiO2 NPs mixed with PS beads in an alcohol/water mixture. Thin films formed free of cracks and became macroporous after irradiation by UVC, as shown in Figure 1. A temperature control setup kept the sample at controlled temperatures below 50 °C during UV exposure, thus excluding that any of the observed changes were caused by thermal activation or thermal degradation.
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Figure 1. SEM images of a film prepared from a dispersion of PS beads and anatase NPs (on Si wafer) before any treatment (A), after calcination at 550 °C (B) without UV treatment and after treatment by UVC irradiation solely (i.e., without thermal treatment) (C). Cross-section SEM image of UVC irradiated anatase film (D). Macroporous film on ITO-coated PET (E). TEM image of UVC irradiated anatase film (F). The films are issued from the same batch.
The quality of the films depended on several processing parameters. The most suitable solvent composition was found to be a combination of methanol and water in a weight ratio of about 1:5. Use of ethanol or of methanol-water weight ratios significantly different from 1:5 yielded films which were prone to micron-sized cracks and to delaminate. The need to use a methanol/water mixture is due to the antipodal solubility of PS beads and TiO2 nanoparticles. Dispersions of TiO2 nanoparticles, synthesized by the benzyl alcohol approach introduced by Niederberger,[6,11] are stable in solvents such as tetrahydrofuran and ethanol, while water-based emulsions of PS are incompatible with solvents for dispersions of TiO2 NPs. Thus, mixtures of methanol and water represent a balance with respect to colloidal stability, avoiding agglomeration. The optimal concentration of TiO2 was found to be between 3.5 and 4% of the weight of the solvents and the PS concentration was between 3.8 and 4.5% by weight. When higher concentrations of NPs and/or PS were used, aggregates formed in the suspension and the films were inhomogeneous. The weight ratio between TiO2 and PS was a critical parameter, too. Films prepared with excess of TiO2 NPs presented cracks, while excess PS increased the viscosity of the dipping solution and yielded films with cracks on the order of several micrometers (see Table 1 and Figure S3). Larger water concentrations lead to aggregation of TiO2 NPs, which is understandable, since the NPs are capped by a hydrophobic molecule (benzyl alcohol). The maximum allowable water concentration was 21% of the weight of methanol.
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Table 1. Composition of dipping suspensions and resulting films characteristics. TiO2 [g]
PS [g]
TiO2/PS [weight]
MeOH [g]
H2O [g]
Humidity [%]
Cracks
SEM
0.234
0.268
0.87
4.7
1
90−95
No
S.2A
0.234
0.268
0.87
4.7
1
90−95
Yes
S.2B
0.211
0.251
0.84
5.6
1
95
No
S.2C
0.211
0.251
0.84
5.6
1
72
Yes
S.2D
Methanol concentrations in excess of about 5:1 of the weight of water lead to PS aggregation. Humidity in the chamber during withdrawal had to be quite high, typically 90–95%, probably needed to maintain the required water concentration in solution and to release stress within the colloid crystal of PS. Lower values of the relative humidity resulted in inhomogeneous films (see Figure S3). The precise composition of the suspensions used for dip coating is reported in Table 1, and the corresponding SEM images are shown in Figure S3. Figure 1 shows representative SEM images before and after irradiation. The PS beads were completely removed by irradiation, leaving pores with a diameter close to 100 nm, corresponding to the diameter of the PS beads. Cross-sectional measurements (Figure 1D) showed that the PS beads were removed for films up to 200 nm in thickness. To elucidate the degree and mechanism of template removal by UV, thin films of PS beads (i.e. without the presence of TiO2 nanoparticles) were irradiated under identical conditions. After exposure for 12 h the films were brownish and cracked, and the bead morphology was degraded (Figure S4), but these effects were not comparable to the complete elimination of PS observed in the presence of TiO2. Kr Physisorption, performed at 77 K, was applied on material which was scratched off the substrate (Si wafers), in order to determine the surface area of the films. Nitrogen physisorption (77 K) was not applicable because of the too small amount of material. Typically, films treated by UV irradiation for 40 h possessed a surface area of approx. 100 m2/g. However, these values have to be regarded with care, since it was difficult to determine the exact mass, taking into account
the small amount of material per area of these films. However, this value indicates that the surface area is mainly determined by the interparticular space between the TiO2 nanoparticles. Raman spectra of TiO2/PS and of pure PS films before and after irradiation are shown in Figure 2a, and, respectively, 2b. The spectra of unirradiated films (with and without TiO2) exhibited peaks that could be reconciled with the vibrational modes of polystyrene, the most intense of which was observed at 1602 cm−1 and corresponded to C-C stretches within the aromatic ring. Upon irradiation, all vibrational modes due to the organics disappear in films containing TiO2 NPs, and the spectra coincided within error with the spectra of calcined films (550 °C) of TiO2 nanoparticles, i.e. corresponding to complete removal (>99%) of carbon residues. The signal at 1450 cm−1 probably corresponds to residual aliphatic carbon moities. In films of pure PS NPs, instead, the vibrational modes of the organics were identifiable even after irradiation. Raman spectroscopy data ruled out temperature degradation and ozonolysis, which would have degraded the PS beads with and without TiO2. PS beads were removed by the photocatalytic action of anatase TiO2. We found that a UV C treatment for 5 h was sufficient to achieve removal of the PS template. Such exposure time does not affect significantly PET as flexible substrate for an exposure up to 20 h, as shown by reference experiments (Figure S5, Figure S8). For UV C exposure times > approx. 40 h a marked brownish color was observed owing to degradation of PET. In addition, Raman spectroscopy measurements were also performed on the TiO2 nanoparticles themselves, including the 100–600 cm−1 range, both in the as-prepared and the calcined state (550 °C), see supporting information (Figure S9). It is seen that the patterns agree with anatase TiO2, confirming a well-developed crystallinity after the synthesis based on benzyl alcohole. Furthermore, these measurements prove that the major signals observed for the macroporous films originate from TiO2. The broad shoulder observed at 1300 cm−1 in the UV-treated film can be attributed to residual carbon, while its small intensity speaks for a content of a few atom per cent at maximum. However, the quantification and determination of carbon is generally difficult by various
Figure 2. Raman spectra of a) TiO2 films templated with PS beads, and of b) PS beads without TiO2, after the indicated processing. The films were prepared on Si wafers, and the excitation wavelength was 532 nm. small 2014, 10, No. 8, 1566–1574
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analytical techniques (e.g., XPS), especially taking into account the small mass of the specimen. Furthermore, it can be excluded that residual benzyl alcohole contributes to the spectra (see Figure S9). For comparison, films were calcined at 550 °C without prior UV treatment. Calcined films possessed a morphology very similar to that of UV-irradiated films (Figure 1B), but they were mechanically quite weak and peeled off the substrate very easily. The brittleness of calcined films might have been a consequence of nanoparticle growth. The primary crystallite size of TiO2 nanoparticles was investigated by X-Ray Diffraction (XRD) on UV-irradiated and calcined films (Figure 3). The average size was determined by Scherrer’s equation and calculated to be nearly the same (diameter ≈ 3 nm) for as-grown and UV-irradiated particles. The XRD data were in good agreement with JCPDS card No.21-1272 (see Figure 3), suggesting the absence of anisotropic particle shape and preferred orientation. Note that the hydrodynamic diameter as determined from DLS was approx. 10 nm, indicating moderate aggregation of the NPs. Calcination at 550 °C yielded crystallites with a size of about 21 nm diameter, a nearly tenfold growth, which likely contributed to the mechanical weakness of calcined films. For several applications, e.g., for dye-sensitized solar cells, it is desirable to prepare macroporous TiO2 films on flexible substrates (Figure 4). That procedure was therefore used to generate films on a substrate of PET (poly(ethylene terephthalate)) covered with a coating of ITO (indium doped tin oxide). Crack-free macroporous films were obtained after UV treatment, using the same processing steps as described
Figure 4. Transparent macroporous anatase TiO2 film on flexible ITOcoated PET.
above. The films featured a reasonable transmittance (85% at λ = 550 nm), while they possessed a slightly brownish color, being attributable mainly to the decomposition of the PET polymer and certainly small amounts of residual carbonacous material originating from the decomposition of PS (Figure 1E, Figure S5). It is well known that even slightest amounts of carbon (below 1 wt.%) can result in such coloration. Since an exposure time of 5 h does not affect the PET substrate (see Figure S5) and since it is sufficient to remove the template, the films can be prepared without significant decomposition of the substrate. The high transparency suggests that indeed only minor carbonaceous leftovers of PS were present causing the color.
2.2. Electrochemical Characterization
Figure 3. XRD patterns of anatase nanoparticle films (A) as prepared, (B) UV C irradiated and (C) annealed at 500 °C. To determine the average grain size, the Scherrer equation was applied to the full width at half maximum (FWHM) of the (101) and (200) peaks. The average grain size of (A), (B) and (C) is 3.2 nm, 3.2 nm and 20.6 nm, respectively. The stick pattern shows JCPDS (Joint Committee on Powder Diffraction Standards) reference card no. 21–1272 for (single phase) anatase.
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The macroporous TiO2 films were studied with respect to their Li storage properties as electrochemical model system, using standard electrochemical characterization (CV, galvanostatic measurements). For both calcined and UV-treated films, FTO-coated glass was used as substrate. In addition, for the UV-treated films electrochemical characterization was also performed on plastics covered with ITO (indium tin oxide). SEM investigation after bending of the coated PET substrates did not show any pronounced scratches or damage in the macroporous TiO2 layer (see Figure S10, supporting information). CV of calcined and UV-irradiated films served to compare their electrochemical addressability (Figure 5). It is seen that irradiated and calcined films were both electrochemically active. Calcined films possessed an electrochemical response comparable to that of anatase obtained by calcination of molecular precursors.[2d] Sharp cathodic (1.74 V) and anodic (1.94 V) peaks were observed with a difference in the peak potential (ΔE) of 200 mV. UV-irradiated films (Figure 5) exhibited broad peaks at 1.76 V (cathodic) and 1.85 V (anodic) with ΔE = 90 mV. Thus, ΔE of UV-treated films was substantially smaller than the corresponding value for films treated at 550 °C. The dependence on processing was confirmed by galvanostatic cycles (Figure S6). The calcined materials featured a clearly defined plateau at 1.75 V
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An All Low-Temperature Fabrication of Macroporous, Electrochemically Addressable Anatase Thin Films
Figure 5. Cyclic voltammetry of calcined and UV-irradiated macroporous anatase films, prepared on FTO-coated glass.
typical of anatase.[2d,6a] For the UV-irradiated material the plateau was less pronounced (Figure S6). The differences in the electrochemical responses can be explained based on the different size of the anatase nanoparticles in the two systems.[6a,14] For the comparatively large crystals of the calcined samples the cathodic and anodic processes occur mainly by insertion and extraction of Li ions.[6] In case of the smaller crystallites in the irradiated samples the adsorption of cations at the surface (pseudo-capacitance) for the anatase reduction-oxidation processes becomes relevant leading to increased charge/discharge kinetics, which results in a smaller ΔE and higher charge capacity.[6a] In order to evaluate the electrochemical performance of the PS/TiO2 thin films in further detail, CV measurements were carried out as a function of UV exposition time, i.e. for non-, 1 h and 40 h UV-illuminated dip-coated PS/TiO2 thin films (ITO/PET substrates) in a potential window between 1.55 V and 2.65 V as shown in Figure 6. It is seen that the overall shape of the CV curves is comparable to those obtained from films deposited on FTO-coated glass. All three types of samples demonstrate an anodic and cathodic peak at around 1.88 V and 1.77 V, respectively, corresponding to a difference in peak potential of approx. 110 mV in case of the film being exposed to UV light for 40 h. Thus, for all these UV exposition times ΔE was substantially smaller than for the calcined films, confirming the results obtained for films deposited on FTO-coated glass. The macroporous TiO2 film being illuminated for 40 h by UV light exhibited the most pronounced reduction/oxidation peaks, which is attributed to the entire removal of PS beads during the UV exposition making more anatase surface area accessible for the electrolyte. Interestingly, the electrochemical stability of macroporous TiO2 was not significantly influenced by the duration of UV treatment verified by constant cycling curves for all types of samples. This electrochemical stability is at least partially attributable to the good adhesion of the UV-treated films, compared to calcined films. small 2014, 10, No. 8, 1566–1574
The significant decrease of the cathodic-current after the first cycle can be assigned to the nature of the PET substrate (Figure 6D) and saturation of irreversible trapping sites on the anatase crystallite surface during the first reduction step. The electrochemical lithium insertion/extraction capacity of macroporous TiO2 films was further investigated by galvanostatic experiments (Figure S7). In agreement with the CV data, the macroporous TiO2 illuminated with UV light for 40 h depicts the highest specific charge capacities of 1.42 and 1.33 µAh·cm−2 for the 1st and 18th cycle (Figure S7 A), respectively. By comparison, the 1 h UV-exposed and nonirradiated PS/TiO2 thin films reveal capacities of 1.31 and 0.94 µAh·cm−2 (Figure S7 B and C), respectively, for the first cycle, being 8% and 34% lower than the film being exposed to UV C for 40 h, respectively. The higher capacity is most likely a consequence of the complete removal of PS template. As illustrated by the trend of discharge capacities (Figure S7 D), the thin-film TiO2 electrodes are able to maintain stable cycling behavior confirmed by loss of discharge capacity of only approx. 6, 8 and 11% from the 1st to the 18th cycle, for the 40 h, 1 h and non UVirradiated TiO2 sample, respectively. It is noteworthy that the UV-treated TiO2 thin-film electrodes show only moderate capacity fading, which further emphasizes their stability in spite of the absence of treatment at elevated temperature. The bare PET substrate demonstrates relatively low specific discharge capacities of 0.15 and 0.1 µAh·cm−2 for the 1st and 18th cycle, and sustains a stable cycling performance for theses cycles. This proves the electrochemical stability of ITO-coated PET as working electrode for electrochemical analysis.
3. Discussion and Conclusion In the present study, we have developed a methodology to produce macroporous, crystalline and electrochemically addressable TiO2 films, using dispersions of poly(stryene) and TiO2 nanoparticles. The main issue and challenge in the synthesis is the colloidal stability of the dispersion containing two types of colloidal particles, namely poly(styrene) and TiO2 nanoparticles, possessing diameters on the order of approx. 100 nm and a few nanometers, respectively. Since colloidal stability is identical to the prevention of agglomeration, it is worth while discussing the colloidal dispersions in terms of the principles of colloid chemistry, in particular with respect to the interaction between two significantly different types of particles in a solvent medium. In general, stability against aggregation is based on the delicate interplay between attracting van-der-Waals interaction and repulsive electrostatic forces. In the present system, the interaction is complex, because formally a completely different type of nanoparticles (metal oxide) is added to the poly(styrene) particle dispersion. Two aspects are of particular relevance regarding colloidal stability: 1. The colloidal particles are substantially different with respect to their Hamaker constants. While TiO2 possesses a
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Figure 6. Cyclic voltammograms of an as-prepared (a), 1 h UV-exposed (b), 40 h UV-exposed PS/TiO2 thin film (c) and a bare PET substrate (d) serving as working electrode. The TiO2 films (a-c) were prepared on ITO-PET substrates.
value of 19.5 × 10−20 J, the Hamaker constant of PS amounts to 7.8 × 10−20 J, i.e. is markedly smaller.[15] The Hamaker constant of the solvent mixture is certainly smaller (approx. on the order of 4 × 10−20 J).[15] Thus, the small TiO2 NPs enhance the average Hamaker constant of the solvent mixture. Hence, the interaction between PS particles is reduced by replacing the solvent by a mixture of solvents with TiO2 NPs. Also, the fact that the Hamaker constants of PS and TiO2 are significantly larger than that of the solvent mixutre, also formally leads to an attraction of PS particles and TiO2 particles.[15] Such attractive interaction is beneficial with respect to the desired positioning of TiO2 NP between the PS particles. 2. The TiO2 nanoparticles are more than one magnitude smaller than PS particles and thus occupy solvent volume between the PS particles. Also, the adsorption of TiO2 NPs on the surface of PS particles can be regarded as a similar situation as in Pickering emulsions. Both effects can be regarded as a formal increase in the volume fraction of the PS particles with respect to the solvent, possibly destabilizing the PS dispersion.
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Such discussion illustrates that the successsful preparation of stable dispersions based on mixing dispersions of PS with those of TiO2 NP is in accordance with the prediction of the classical concepts of colloidal science. The fabrication of the films is carried out at room temperature which is compatible with flexible electronics, as demonstrated by preparing films on flexible ITO-coated PET substrates. Our study determined optimal synthetic parameters, yielding porous films without macroscopic cracking or delamination (Figure S8). XRD and electrochemical measurements indicate that the crystallite size is not affected by UV irradiation, which opens the way for the fabrication of macroporous thin films of quantum-confined nanoparticles with tunable photochemical and electrochemical properties. Films with increased thickness can be obtained by repeated deposition, in the state prior to UV treatment. However, crack-free films can only be generated up to the fourth layer, i.e., larger numbers of deposition steps result in brittle films. This restriction may aggravate the usability of these films for certain applications (e.g., the “classical” Grätzel-type
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An All Low-Temperature Fabrication of Macroporous, Electrochemically Addressable Anatase Thin Films
dye-sensitized solar cell); however, our macroporous TiO2 films, possessing thicknesses of approx. 150–300 nm, are suitable for polymer-based hybrid solar cells, which usually do not require thicknesses of several micrometers. The films will thus be studied with respect to the infiltration by suitable hole-conducting polymers and their photoelectrochemical properties. In order to check for the general validity of our synthetic approach, it is currently applied to other metal oxide nanoparticles. Evidently, one of the prerequisites regarding the metal oxide is a certain photocatalytic activity which degrades the polymer beads under UV irradiation. Within a first series of experiments, we succeeded in synthesizing ATO (antimony tin oxide) nanoparticles (Sb:Sn = 1:10, at. ratio) with suitable colloidal properties and in obtaining macroporous films thereof, applying the strategy developed for TiO2 nanoparticles (see Figure 7); the XRD of such ATO nanoparticles can be seen in the Supporting Information (Figure S11). While ATO is not reported as active photocatalyst, it might contribute to an oxidative degradation of the PS beads under the conditions used. Further studies are planned to study this decomposition process for different oxides in more detail, e.g., using gas chromatography coupled with mass spectometry. We consider this low-temperature process to be generally relevant in the field of porous metal oxides, since the
Figure 7. (A) SEM image of macroporous ATO film. (B) Cross-sectional SEM image of macroporous ATO film. small 2014, 10, No. 8, 1566–1574
successful synthesis of macroporous ATO films indicate that it might be applicable for other metal oxides and as the methodology opens new perspectives for the fabrication of flexible electronics devices.
4. Experimental Section Chemicals: Benzyl alcohol, 99.9% (Grüssing GmbH); Ethanol, 99.9% (VWR); Hexadecane, 98% (Fluka); Methanol 99.9% (Roth); Potassium peroxodisulfate, 99.6% (Fluka); 1,3-Propanediol, 99.6% (Aldrich); Sodium dodecyl sulfate, 98% (Sigma-Aldrich); Styrene, 99% (Sigma-Aldrich); Titanium(IV) chloride, 99.9% (Sigma-Aldrich); Lithium perchlorate, 99.99% (Sigma-Aldrich); Propylene carbonate, 99.7% (Aldrich) Synthesis of Polystyrene Spheres: Monodisperse polystyrene spheres were synthesized following the procedure reported in Ref. [4b]. 6 g styrene and 250 mg hexadecane were mixed and added to a solution of 0.033 g potassium peroxodisulfate, 0.1 g sodium dodecylsulfate (SDS) in 24 g H2O. The reaction was initiated by sonication and stirred 20 h by 77 °C under N2 atmosphere. The diameter of the beads could be varied between about 80 and 120 nm by varying the concentration of SDS or potassium peroxodisulfate. Decreasing the concentration of SDS or increasing the concentration potassium peroxodisulfate leads to bigger beadsizes. Nanoparticle Preparation: TiO2 nanoparticles (NPs) were synthesized by adapting the procedure originally reported by Niederberger et al.[11] In brief, 1 mL (1.74 g) of TiCl4 was added drop-wise to 5 mL (3.96 g) of absolute EtOH under stirring. After 5 min, 20 g of benzyl alcohol (BenOH) were added, and after another approx. 5 min 250 mg of 1,3-propanediol. The solution was then stirred for about 1 h at 110 °C. The solution was then immediately cooled to room temperature. To precipitate nanoparticles, 200 mL of diethyl ether were added to the parent solution, followed by centrifugation. The precipitate was washed again with diethyl ether, dried in air and redispersed under sonication in a methanol:water solution (90:10 by weight respectively) to get a clear dispersion. The solid concentration in the redispersed suspension was around 10% by weight. Film Preparation: Films were deposited at room temperature (20–22 °C) by dip-coating silicon wafers. The parent solution had the concentration indicated in Table 1. The weight ratio between TiO2 and PS was critical and had to be around 0.85 by weight. The withdrawal rate was kept constant (5 mm•s−1) and the relative humidity inside the dip-coating chamber was between 90 and 95%. The samples were dried for 10 min inside the dip-coating chamber, and for 1 h under ambient conditions. Calcination was done in oven from room temperature with a ramp of 2 °C/min and left 4 h at 500 °C. Exposure: A UV irradiator (model UV-P 250C) was employed (Hartmann). This irradiator is provided with a series of high pressure mercury and halogen lamps and with filters that allow ozonefree exposure of samples to visible light, UVA (320–400 nm), UVB (290–320 nm) and UVC (200–290 nm). The illumination power was measured at the sample position with a power meter to be 343 mW•cm−2. During irradiation, samples were kept at a constant temperature of 30 °C by cooling the supporting tray with water from a recirculating chiller.
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Characterization: Electrochemical measurements were performed in an Ar-filled glove-box (O2 and H2O below 1 ppm) in 1.0 M LiClO4 solution in propylene carbonate using a Verstat 3 potentiostat/galvanostat (Princeton Applied Research). Films deposited on FTO-(fluorine-doped tin oxide) coated glass and ITOcoated PET were used as working electrodes. The FTO coated glass was purchased vom Glaswerke Haller (Germany); Li foils were used as reference electrode and as counter electrode. Cyclic Voltammetry (CV) was performed between 1.45 V and 2.65 V vs. reference electrode at a fixed scan rate of 0.1 mV•s−1. Galvanostatic measurements were carried out in the same electrolytic solution, at a current density of 4 µA•cm−2 both for charge and discharge and potential boundaries at 1.55 V and 2.8 V, respectively. Scanning electron microscopy (SEM) was performed using a HSEM 982 Gemini from LEO. TEM measurements were performed using a CM30-ST microscope from Philips. X-ray Diffraction (XRD) measurements were performed with a PANalytical Xpert instrument (Cu-Kα wavelength); Raman spectra were measured by a Bruker Senterra with an excitation wavelength of 532 nm.
Supporting Information
[3]
[4]
[5] [6]
[7]
[8]
Supporting Information is available from the Wiley Online Library or from the author. [9]
Acknowledgements [10]
We thank D. Neher and C. Fahrenson from University of Potsdam (Germany) for providing cross-section SEM pictures; Bundesministerium für Bildung und Forschung (BMBF) is gratefully acknowledged for financial support (Förderkennzeichen 03 × 3525E, “SOHyb” project). This project was also supported by the Laboratory of Materials Research (LaMa) of JLU. Pascal Vöpel is thanked for substantial help with generating schemes for illustration.
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Received: March 27, 2013 Revised: August 26, 2013 Published online: March 18, 2014
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