Lu
Cao
a,
Dehong
Chen
*a,
Wu-Qiang
Wu
a,
Jeannie Z. Y.
Tan
ab and
Rachel A.
Caruso
*ab
aParticulate Fluids Processing Centre, School of Chemistry, The University of Melbourne, Melbourne, Victoria 3010, Australia. E-mail: dehongc@unimelb.edu.au; rcaruso@unimelb.edu.au
bCSIRO Manufacturing, Private Bag 10, Clayton South, Victoria 3169, Australia
First published on 27th January 2017
To fabricate an effective antibacterial coating on the surfaces of diverse sanitary ceramic utensils, efficient titania photocatalysts with integrated features including high temperature anatase phase stability (>800 °C), excellent particle mobility for the formation of uniform thin coatings, high crystallinity and narrow particle size distribution are desirable. In this study, monodisperse dopant-free titania microspheres were synthesized with large pore size (∼80 nm) that remain anatase even after calcination at 900 °C. These titania microspheres were fabricated via a facile solvothermal treatment of amorphous spheres in the presence of 4.5 wt% ammonia solution and consisted of well-crystallized and faceted anatase nanocrystals with a uniform size of 24 nm. The anatase nanocrystals with high crystallinity and narrow crystal size distribution are responsible for their high temperature stability. The resulting anatase titania microspheres exhibited enhanced photocatalytic performance even after calcination at high temperature due to the retention of the anatase phase and the enhanced crystallinity. Such monodisperse anatase microspheres have potential to be applied as smart coating materials for antibacterial and self-cleaning applications.
Among the three most common polymorphs of titania (anatase, brookite and rutile), anatase is considered to be the most photoactive phase for both antibacterial coatings and degradation of organic pollutants because of the low recombination rate of the photogenerated charge carriers and the high surface adsorptive infinity towards the organic compounds.36,37 However, anatase is a metastable phase and can be readily transformed to the thermodynamically stable rutile phase at temperatures from 500 to 700 °C,23,38–41 and this phase transformation is irreversible. This restricts the use of anatase photocatalysts for some important applications (e.g., self-cleaning and antibacterial coatings on sanitary ceramic ware) that need to be processed at high temperature (usually >800 °C).42 Therefore, the development of a high performance anatase photocatalyst that tolerates the high sintering temperature of diverse ceramic substrates has become a challenge in smart coating technology. In recent years, rational control of the onset temperature of the anatase-to-rutile transformation has been researched. The presence of certain cationic dopants, such as Zr, Al, La and Si, could effectively stabilize the anatase phase at elevated temperatures due to the formation of Ti interstitials.39,43–46 However, secondary impurity phases (e.g., Al2TiO5) may be produced at high temperatures,46 which will generate additional charge carrier recombination sites and thus reduce the photocatalytic activity.39,43–46 High temperature stabilization of the anatase phase through anion doping (S, F, N, etc.) has also been widely investigated.47–49 In this case, the concentration of oxygen vacancies may be increased, which could enhance recombination between photogenerated electrons and holes and therefore deteriorate the photocatalytic activity of the titania photocatalyst. Moreover, excess anion doping can inhibit interfacial charge transfer, which is detrimental to the photocatalytic activity as well.47 A sol–gel templating method has been reported to retard the phase transformation by creating hierarchically porous structures to reduce the interfaces between the titania nanocrystals in porous titania networks. The reduced interfaces can significantly decrease interface nucleation of the rutile phase and effectively retard the anatase to rutile phase transformation.23 The effect of the heating atmosphere on the anatase to rutile transformation was also investigated:50,51 the phase transformation rate was decreased under vacuum or oxygen and increased in reducing atmospheres, which could be due to the formation of Ti interstitials and oxygen vacancies, respectively. Although an increase in the onset temperature of phase transformation has been achieved, it is still lower than the sintering temperature for ceramic coatings. Another important issue that should be considered is the thermal stability of the porous structures. In most cases, the pores collapsed and most of the porosity was lost after being sintered at high temperature. Therefore, preparation of anatase titania photocatalysts with spherical morphology, large pore size and high temperature stability (>800 °C) without any chemical modification would be of considerable interest.
Herein, a facile route is reported for the synthesis of monodisperse anatase titania microspheres with a large pore size and high thermal stability by combining sol–gel chemistry and a solvothermal treatment process. In the absence of any doping strategy or surface modification, the anatase-to-rutile phase transformation was effectively inhibited at high temperatures giving rise to high thermal stability (up to 900 °C) and highly crystallized anatase titania microspheres. The anatase microspheres possess a uniform and large pore size, ∼80 nm. The possible reasons for the superior thermal stability were investigated and discussed. The photocatalytic activity was evaluated by degrading methylene blue under UV illumination.
WR = AR/A0 = AR/(AR + 0.884AA) |
A high resolution field emission environmental scanning electron microscope (SEM, Quanta 200 FEI) was used to observe the morphologies of the samples without metal sputter coating pretreatment. The transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns of the resulting samples were obtained on a FEI Tecnai F20 microscope (USA) operating at 200 kV. Nitrogen gas sorption isotherms at 77 K were collected on a Micromeritics Tristar 3000 surface area and porosity analyser (USA). All the samples were degassed under vacuum at 160 °C for at least 18 h prior to measurement. The Brunauer–Emmett–Teller (BET) equation was used to calculate the specific surface area. Pore size distributions were obtained using the Barrett–Joyner–Halenda (BJH) method from the adsorption branch of the isotherms. Diffuse reflectance spectroscopy (DRS) was measured on a UV-vis-NIR diffuse reflectance spectrophotometer (Perkin-Elmer lambda 1050) with an 150 mm integrating sphere attachment in the range of 200–800 nm. The band gap of the materials was estimated by extrapolating a linear part of the plots to (αhν)2 = 0. Raman spectroscopy was performed on a Renishaw inVia confocal microscope system. Specimens were illuminated with an argon laser (785 nm wavelength) through a 50× objective. The spot was estimated to be in the range of 1 μm. Spectra were collected over the wave number range of 100 to 700 cm−1 on the surface of the sample. The UV light illumination intensity was measured using a Tenmars TM-213 UV light meter. The pH values of the aqueous suspensions were measured using an Oakton pH/mV 11 meter. Prior to the measurement, the electrode was calibrated using fresh standard pH buffer solutions. The X-ray photoelectron spectrometer (XPS) data were recorded on a VG ESCALAB 220i-XL spectrometer (UK) equipped with a twin crystal monochromated Al Kα X-ray source, which emitted a photon energy of 1486.6 eV at 10 kV and 22 mA. Samples were secured onto Al holders and were measured in the analysis chamber at a typical operating pressure of ∼7 × 10−9 mbar. An electron flood gun was used to compensate the charging effect of non-conductive materials. Spectra were obtained at a step size of either 1.0 eV (survey scans) or 0.05 eV (regional scans). Quantification and curve fitting of XPS spectra were performed using CasaXPS software. The C1s peak at 285.0 eV was used as a reference for the calibration of the binding energy scale.
The solvothermally treated sample consists of individual spherical entities with an average diameter of 2.23 ± 0.12 μm, Fig. 1a and S4a.† Each solvothermally-treated MTS is composed of uniform titania nanocrystals of ∼22 nm in size (Fig. 1b). After calcination at temperatures of 800 °C or higher, an obvious shrinkage in the sphere diameter was observed (Fig. 1c and d and S2†). The sphere diameter decreased to 1.97 ± 0.09, 1.88 ± 0.10 and 1.58 ± 0.09 μm when the calcination temperature increased to 800, 900 and 1000 °C, respectively (Fig. S4†). As clearly illustrated by the higher magnification SEM images (Fig. 1c and d), a crystal coarsening process occurred when the solvothermally-treated MTS were calcined at 800 °C or higher, giving rise to enlarged crystal sizes. A slight increase in crystal size (50 to 60 nm) was observed when the calcination temperature increased from 800 to 900 °C, Fig. 1c and d, and then the crystal size increased further for the MTS calcined at 1000 °C in air (Fig. S2d†). Along with the increasing crystal size, the pore size/void between the nanocrystals became larger as a result of the increased calcination temperature (Fig. 1b–d).
The crystal phase and size of the MTS were studied by powder X-ray diffraction (XRD); the corresponding XRD patterns of the samples calcined at different temperatures are displayed in Fig. 2a. The anatase content (as a weight percentage) in each sample was calculated according to a previously reported method,52Fig. 2b. The XRD pattern of the solvothermally-treated MTS sample showed well-resolved diffraction peaks that can be ascribed to anatase titania (JCPDS card no. 21-1272). The anatase diffraction peaks for the MTS-800 °C and MTS-900 °C were sharper compared to the solvothermally-treated MTS sample. The crystal size of each sample can be estimated from the full width at half maximum of the (101) peak using the Scherrer equation, giving 22.0, 48.0 and 57.5 nm for MTS, MTS-800 °C and MTS-900 °C, respectively. This is in a good agreement with the crystal coarsening (increased crystal size) occurring with elevated calcination temperature observed from the SEM images.
The physical properties of the MTS materials are summarized in Table 1. It is worth noting that no rutile phase was detected in the MTS-900 °C sample within the X-ray detection limit. This result was further confirmed by the corresponding Raman spectra (Fig. S5†), in which five high-intensity peaks at 142, 196, 397, 514 and 640 cm−1 can be attributed to characteristic Eg, Eg, B1g, A1g(B1g) and Eg modes of the anatase titania, respectively.55,56 No other Raman peaks were observed, indicating the pure anatase phase for this MTS-900 °C sample. Thus the resulting anatase MTS materials possessed an excellent thermal stability (up to 900 °C), which is much higher than previous reports.23,35,38–40 Further increasing the calcination temperature to 1000 °C (for 2 h in air) resulted in a phase transformation from anatase to rutile, leading to the formation of faceted titania particles (Fig. S3†) containing 7.1 wt% anatase (Fig. 2b). In contrast, for the Degussa (Evonik) P25 nanoparticles, only 8.3 wt% anatase remained after calcination at 800 °C for 2 h in air (Fig. 2b and S6†). When the temperature was increased to 900 °C, P25 was completely converted to the rutile phase. Phase transformation of the amorphous precursor spheres (APS) derived from sol–gel synthesis was also investigated (Fig. 2b and S7†): the anatase-to-rutile phase transformation occurred above 600 °C and was almost complete at 800 °C (only 2.7 wt% anatase remained at this temperature). UV-vis diffuse reflectance spectra and the corresponding Kubelka–Munk function (relationship of [F(R)hν]1/2versus photon energy) plots of the MTS and Degussa (Evonik) P25 samples calcined at diverse temperatures were recorded to determine the band gaps of the resulting materials (shown in Fig. S8†). Solvothermally-treated MTS, and calcined MTS-800 °C and MTS-900 °C samples show a very similar band gap (3.20 ± 0.01 eV) because of their similar anatase phase. In contrast, the as-received P25 nanoparticles are a mixed phase titania with the crystallized material consisting of 84.6 wt% anatase and 15.4 wt% rutile, and such mixed-phase titania possesses a relatively low band gap of 3.10 eV, which is consistent with the band gap values reported previously.57 After calcination at 900 °C, the resulting P25-900 °C sample has been completely converted into the rutile phase, giving rise to a reduced band gap of 2.99 eV. These results clearly illustrate the excellent thermal stability of the MTS materials.
Sample name | W A [wt%] | S BET [m2 g−1] | PDc [nm] | V sp [cm3 g−1] | CSXRDe [nm] | Bandgapf [eV] |
---|---|---|---|---|---|---|
a W A = weight fraction of the anatase phase calculated using the integrated intensities of anatase (101) and rutile (110) peaks according to a previously reported method.52 b S BET = BET specific surface area obtained from adsorption data in the P/P0 range from 0.05 to 0.20. c PD = pore diameter determined by BJH model from the adsorption data. d V sp = single-point pore volume calculated from the adsorption branch at P/P0 = 0.98. e CSXRD = anatase crystallite size estimated from XRD data using the Scherrer equation. f Band gap estimated according to Kubelka–Munk equation based on diffuse reflectance data.53,54 ‘n/a’ indicates the corresponding methods are not applicable for the samples. | ||||||
MTS | 100 | 71.5 | 30.5 | 0.420 | 22.0 | 3.21 |
MTS-800 °C | 100 | 17.0 | 65.9 | 0.191 | 48.0 | 3.20 |
MTS-900 °C | 100 | 10.9 | 77.0 | 0.123 | 57.5 | 3.19 |
MTS-1000 °C | 7.10 | 1.10 | n/a | 0.003 | n/a | 2.96 |
P25 | 84.6 | 50.1 | n/a | 0.164 | n/a | 3.10 |
P25-800 °C | 8.30 | 14.1 | n/a | 0.042 | n/a | 3.00 |
P25-900 °C | 0 | 7.10 | n/a | 0.019 | n/a | 2.99 |
Nitrogen gas sorption isotherms and pore size distributions of the solvothermally-treated MTS and the MTS materials calcined at various temperatures are shown in Fig. 3. Except for the MTS-1000 °C sample, the samples showed characteristic type IV isotherms with a sharp capillary condensation step at relative high pressures (P/P0 = 0.90–0.98), reflecting the existence of large pores. Sharp H1 type hysteresis loops were observed for solvothermally-treated MTS, and calcined MTS-800 °C and MTS-900 °C samples, indicating the narrow pore size distributions (PSD) of these samples. For solvothermally-treated MTS, a PSD centered at ∼30.5 nm was calculated from the adsorption branch using the Barrett–Joyner–Halenda (BJH) method. This sample has a specific Brunauer–Emmett–Teller (BET) surface area of 71.5 m2 g−1 and a total pore volume of 0.42 cm3 g−1. Using this BET surface area, and assuming spherical crystals, the crystal size of this solvothermally-treated MTS is estimated to be 21.5 nm on the basis of the anatase titania density (3.9 cm3 g−1), which is consistent with the result derived from XRD calculation (∼22.0 nm). The hysteresis loops of the MTS-800 °C and MTS-900 °C samples shifted to higher relative pressure, illustrating an enlargement in pore size during the calcination process. As shown in Fig. 3b, sharp PSDs centered at 65.9 and 77.0 nm for the MTS-800 °C and MTS-900 °C samples were observed, respectively. It is worth noting that such uniform large pores in titania spheres have rarely been reported. After calcination at 1000 °C in air, as illustrated by nitrogen gas sorption and SEM images, the porosity of the resulting MTS-1000 °C materials disappeared, indicating a significant crystal growth and densification of the titania spheres (XRD results).
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Fig. 3 (a) Nitrogen gas sorption isotherms and (b) the corresponding pore size distributions of the MTS, MTS-800 °C, MTS-900 °C and MTS-1000 °C samples. |
Specific surface areas of the MTS and P25 samples as a function of the calcination temperature ranging from 500 to 1000 °C are presented in Fig. 4. For the MTS sample, the specific surface area showed a slight decrease of 8.9% up to 600 °C. This small drop in specific surface area could be attributed to the high crystallinity of the samples induced by solvothermal treatment at 220 °C. However, a significant drop in specific surface area from 64.0 to 17.0 m2 g−1 was observed when the calcination temperature was increased from 600 to 800 °C. This decrease is due to the coarsening of the anatase nanocrystals as revealed by the corresponding XRD results (Fig. 2). With a further increase in the calcination temperature to 900 °C, the specific surface area decreased to 10.9 m2 g−1. The specific surface area of the P25 sample calcined at varied temperatures was also investigated. As P25 is produced by flame hydrolysis of TiCl4 at elevated temperature (up to 1000 °C), it is reasonable that only a slight drop in the specific surface area was observed when the calcination temperature is raised to 700 °C. The surface area dropped dramatically from 43.5 to 13.8 m2 g−1 when the calcination temperature increased from 700 to 800 °C. Nevertheless, it is worth noting that the specific surface areas of the calcined MTS are higher than those of P25 calcined at 800 and 900 °C.
XPS measurements were used to detect any N-doping in the resulting MTS materials.6 As shown in Fig. S9,† the as-prepared sample contained a small quantity of N (0.9 at%) and this N peak disappeared after washing with ethanol three times, indicating that the N on the as-prepared sample was due to the ammonia adsorbed on the surfaces of the sample and can be readily removed by thorough washing with ethanol.
It is documented that the thermal stability of anatase depends on its crystal size,45,52,56,59,62 as the rate of surface nucleation is proportional to the total number of anatase crystals in a given sample volume.60 To investigate the crystal size effect on the transformation from anatase to rutile phase, monodisperse anatase titania microspheres (MTS-0 wt%) consisting of smaller nanocrystals (11.6 nm in diameter estimated by XRD) were prepared in the absence of ammonia during the solvothermal treatment. The XRD patterns, Fig. 5a, show an anatase to rutile transformation has begun by 900 °C. The rutile content in the MTS-0 wt% samples was determined as 19.6% and 100% after calcination at 900 °C and 1000 °C, respectively, indicating a phase transformation at lower temperatures compared to the microspheres with larger crystallite size (22.0 nm, Fig. 2; 0% and 92.9% rutile after calcination at 900 °C and 1000 °C, respectively). The spherical morphology was maintained with shrinkage in diameter and increasing crystal size as the calcination temperature increased (SEM images, Fig. 5b–d).
In conjunction with XRD characterization, the MTS-0 wt% and MTS samples were also ultramicrotomed to ∼90 nm thick sections and observed by transmission electron microscopy (TEM) to investigate the uniformity of the crystal size (Fig. 6a and b). As clearly illustrated in Fig. 6a1–a3 and b1–b3, the titania microspheres consisted of uniform crystals and the sample prepared in the absence of ammonia possessed a relatively small crystallite size (Fig. 6a2 and a3). The corresponding selected-area electron diffraction (SAED) patterns (Fig. 6a4 and b4) indicate that these microspheres are well crystallized anatase phase titania. To statistically determine the equivalent diameter of the anatase nanocrystals in the microspheres, these titania microspheres were crushed using an agate pestle and mortar and then dispersed in ethanol by sonication for 30 min. The resulting colloidal suspension was then dropped onto Cu grids and examined by TEM analysis, and the corresponding crystal size distributions were summarized in Fig. S11.† The crystal size is 12.0 ± 2.5 and 24.1 ± 5.7 nm for the titania microspheres prepared in the absence (0 wt%) and presence (4.5 wt%) of ammonia, respectively. This result is in agreement with the XRD result. The resulting faceted anatase nanocrystals had a truncated bipyramid crystal morphology, exposing the (101) side and (001) top facets (Fig. S12 in ESI†).
Purpose-built titania microspheres (MTS-17.4 wt%) featuring a bimodal crystal size distribution were prepared by increasing the ammonia concentration to 17.4 wt% to verify the crystal size effect on the phase transformation. As shown in Fig. 6c1–c3 and S13c1 and c2,† these unique titania microspheres were composed of two distinct anatase crystals (with relatively small anatase crystals (∼20 nm) in the core and larger anatase crystals (∼60 nm) decorating the outer surface of the spheres). As these small and large anatase crystals are located within the same microsphere and therefore are in close proximity (within a distance < 2.2 μm), these anatase nanocrystals have been exposed to the same temperature during the calcination. Thus, the variation in anatase and rutile phases can be correlated to the size of the anatase nanocrystals within the microspheres.
According to the corresponding XRD patterns (Fig. S14†), a small portion of rutile (2.8%) was observed when this MTS-17.4 wt% sample was calcined at 900 °C, which is believed to be derived from the relatively ‘unstable’ small anatase crystals in the core. When this MTS-17.4 wt% was calcined at 1000 °C, about 63.1 wt% anatase still remained in this sample, which can be attributed to the presence of relatively stable larger anatase crystals located on the surface of the spheres. In contrast, the titania spheres featuring monodisperse anatase crystals (12.0 or 24.1 nm shown in Fig. S11†) retained 0 wt% (Fig. 5a) or 7.1 wt% anatase (Fig. 2a) when calcined at 1000 °C. On the basis of the above results, it is clear that the crystal size is a key parameter in determining the onset temperature of the anatase-to-rutile phase transformation (thermal stability). This onset temperature increased with increasing initial crystal size as a result of the reduction in free surfaces and interfaces available for the rutile nucleation.56,60
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Fig. 7 Photocatalytic degradation kinetics of the methylene blue (MB) probe under UV light in the presence of MTS-800 °C, MTS-900 °C, P25-800 °C and P25-900 °C materials. |
Footnote |
† Electronic supplementary information (ESI) available: Photocatalytic set-up, SEM images, size distributions, Raman spectrum, XRD patterns, UV-vis diffuse reflectance, XPS spectra, Arrhenius plot, TEM images, MB absorbance and degradation plot. See DOI: 10.1039/c6ta08981j |
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