In the following, we focus on mesoporous MgTa2
films templated by the KLE block copolymer. PIB6000 generates films with very similar mesopore structure and crystallinity, with only the average lateral size of the mesopores (ca. 20 nm) being larger than in the case of KLE template films (ca. 15 nm). Since the mesopore structure (spherical mesopores) and also photocatalytical activity of these films are very similar, in the following, mainly KLE-templated films are discussed. Additional results for PIB6000-templated mesoporous films are presented in Supporting Information File 1
Both the mesoporous and the nonporous (nontemplated) films show distinct X-ray diffraction (XRD) reflections corresponding to a tri-rutile structure MgTa2O6 (JCPDS card 32-0631), only after calcination at temperatures beyond 760 °C, as illustrated in . The crystallinity of MgTa2O6 in the KLE-templated mesoporous film increased with increasing calcination temperature from 760 up to 1100 °C. The average grain size, which was estimated by applying the Rietveld refinement, is shown in as a function of the calcination temperature for both mesoporous and nonporous films. Owing to the lower overall crystallinity of the investigated material and a high background in some of the analyzed XRD patterns, the microstrain part in the size-microstrain procedure within the Rietveld refinement was not refined and was kept to the instrumental values determined by the crystalline standard. The standard was also used for the deconvolution of the instrumental-broadening contribution. Since the initial refinement gave rather high disagreement between the experimental and calculated curves, particularly in the  direction, an anisotropic broadening with a larger crystallite size along the  direction was assumed. Accordingly, the difference curves were then smooth and all refinements were satisfactory. In , vertical error bars represent the distribution interval of crystallite sizes for all reflections, and therefore the error interval is actually a measure of crystal anisotropy. It is obvious that the nonporous crystallites appear with a larger anisotropy (larger grains in  direction) compared to their mesoporous counterparts. When the calcination temperature was increased from 760 to 1100 °C the grain size of MgTa2O6 in the mesoporous film showed a gradual increase from 1.1 to 9.4 nm. When both films were heated to the same temperature of 760 and 790 °C, the crystallinity of MgTa2O6 in the nonporous film was more pronounced than in the mesoporous one, which can be attributed to the retarding effect of the template and the mesopores on the grain growth.
XRD patterns of (a) the KLE-templated mesoporous and (b) the nonporous MgTa2O6 thin film after thermal treatment at various temperatures.
Average grain size and its standard deviation as a measure of anisotropy of MgTa2O6 in the mesoporous KLE-templated and nonporous films, as a function of the calcination temperature.
Grain growth of MgTa2O6 in the nontemplated film with increasing calcination temperature from 760 up to 1100 °C was not so conspicuous. The grain size only increased from 5.3 to 8.9 nm.
shows the SEM surface morphology of the MgTa2O6 thin film subjected to various calcination temperatures. The EISA procedure achieved a crack-free surface with randomly distributed micrometer-sized round concaves, while the film as a whole consisted of highly ordered mesopores. The dual nanoarchitecture remained almost unchanged upon heating to 760 °C, with crystallized MgTa2O6 appearing within the walls. After being heated to the highest temperature available (1100 °C), conspicuous cracks appeared within the round concaves and the ordered mesopores decayed to a wormlike state. However, the microstructure was still preserved, and most thermally induced cracks were restricted to each concave, that is, they did not coalesce or propagate throughout the film upon heating to 1100 °C. At this temperature, well-crystallized MgTa2O6 walls were obtained, as seen by XRD.
SEM surface morphologies of a mesoporous KLE-templated MgTa2O6 film after thermal treatment at (a) 760 °C and (b) 1100 °C, and (c) of a mesoporous PIB6000-templated MgTa2O6 film after thermal treatment at 760 °C.
illustrates the AFM surface morphology of the MgTa2
film after calcination at 760 °C, which is in agreement with the SEM observation. The corresponding height profile obtained from the AFM image suggests that the depth of the randomly distributed round concaves was ca. 60 nm. The film thickness was estimated to be ca. 200 nm; therefore, the ordered mesoporous MgTa2
film thoroughly covered the Si substrate. The distance between two neighboring mesopores remained constant at ca. 22 nm after calcination at 760 °C. The ordered mesoporous structure was further confirmed by the TEM morphology of flakes scratched off from the MgTa2
film before and after heating at 760 °C, as indicated in . From such TEM images a spherical mesopore shape was inferred with an average mesopore diameter of ca. 15 nm. TEM images of PIB6000-templated films are shown in Supporting Information File 1
AFM (a, c) morphology of the KLE-templated mesoporous MgTa2O6 film after calcination at 760 °C. The corresponding profiles along the lines indicated in (a) and (c) are shown in (b) and (d), respectively.
TEM morphology of a fragment scratched off from a mesoporous KLE-templated MgTa2O6 thin film before (a) and after calcination at 760 °C (b).
shows the SAXS patterns (measured in symmetric reflection with a 1-D detector) of the mesoporous MgTa2
film after calcination at various temperatures. The diffuse SAXS pattern does not allow for an unambiguous determination of the pore morphology. Taking into account previous studies on KLE as a template and the TEM images, the SAXS data can be interpreted in terms of spherical mesopores, although fcc, bcc and hcp structures cannot be distinguished. The scattering maxima are characteristic of ordered mesopores, being assigned as the (110) reflection of a distorted bcc mesoporous structure, and became diffuse as the calcination temperature increased to 760 °C, i.e., the onset of crystallization temperature. However, the SAXS scattering maxima can still be discerned after calcination up to 1000 °C. Therefore, the present ordered mesoporous MgTa2
film possessed a relatively high thermal stability. This property was attributed to the special templating properties of KLE, which forms relatively large micelles during the EISA procedure and hence helps to stabilize the ordered mesopores even after the template has been removed [15
]. In addition, the randomly distributed round concaves also possibly aid the release of thermal stress, thus helping to maintain the ordered mesopores up to calcination temperatures as high as 1000 °C.
SAXS patterns (measured in symmetric reflection) of the mesoporous KLE-templated MgTa2O6 thin film after calcination at various temperatures.
2-D-SAXS patterns of the as-stabilized mesoporous MgTa2
film and of those heated to 550, 650 and 760 °C are shown in . Here, the material was synthesized on ultrathin Si wafers (thickness ca. 30 μm), allowing the penetration of the covered wafers by X-rays. The maxima attributable to a bcc mesostructure in  orientation relative to the substrate can be discerned in the 2-D patterns measured with an angle of incidence of β = 10°, even after the onset of crystallization [20
]. In addition, the 2-D-SAXS patterns became increasingly ellipsoidal, suggesting an anisotropic shrinkage of the mesostructure in the direction normal to the substrate, which has often been observed previously [20
]. For the SAXS patterns obtained in transmission geometry (β = 90°) the 1,−1,0 ring, corresponding to a random orientation of mesostructured domains of the bcc mesoporous structure within the plane parallel to the substrate, was observed for all the films. No in-plane contraction in the plane occurred even for a film heated to 760 °C, which is in agreement with the crack-free surface observed by SEM () and AFM (). The in-plane d
-spacing of 22 nm derived from the 2-D-SAXS patterns collected at β = 90° agreed well with the AFM measurement (). 2-D-SAXS data of F127- and PIB6000-templated films are shown in Supporting Information File 1
Figure 7 2-D-SAXS patterns of the KLE-templated mesoporous MgTa2O6 thin film after calcination at various temperatures. The values β = 10° and β = 90° indicate the angles of incidence between the surface of the film and the X-ray (more ...)
The EISA procedure has been widely utilized to synthesize thin films with ordered mesopores, with the help of certain surfactants; however, such round concaves containing ordered mesopores have not been noticed before. As a preliminary result, we ascribe the formation of such a nanoarchitecture to the Mg2+ ions in the precursor, which increased the polarity of the ethanol and enhanced the evaporation speed of the solvent. The rapid evaporation produced randomly distributed bubbles, and when these collapsed they left randomly distributed, open, round concaves.
The mesoporosity was additionally investigated by nitrogen physisorption at 77 K (see Supporting Information File 1
). Since the amount of material is by far too small for a single film, as an example, four films of PIB6000-templated MgTa2
were deposited on top of each other in order to provide a sufficiently large absolute surface area. These multilayer films were studied by SAXS and SEM to ensure that indeed the multilayer deposition did not disturb the mesostructure. For comparison, multilayers of nontemplated films were studied by nitrogen physisorption.
For such a PIB6000-templated mesoporous multilayer MgTa2O6 film with four single layers a BET surface area of ca. 260 m2 cm−3 was observed (after heat treatment at 790 °C), which is typical of mesoporous films with such large mesopores. While the quality of the isotherm is limited, owing to the low amount of material, an average mesopore size of ca. 12 nm can be formally deduced from the BJH approach applied to the adsorption branch. Such a value has to be regarded with care, but nevertheless is consistent with the suggestion of the spherical mesopores being substantially contracted perpendicular to the surface. Thus, nitrogen physisorption provides a well-defined accessible mesopore structure up to high temperatures. A nontemplated film did not exhibit substantial uptake of nitrogen, and no reasonable isotherm was obtained.
Photocatalytic activity evaluation
In the absence of the oxide film, the UV irradiation induced no remarkable degradation of rhodamine B (RhB) in water. When the MgTa2
film was exposed to radiation, RhB underwent gradual decomposition depending on the irradiation time. As an example, shows the absorption curve of the aqueous RhB solution as a function of the UV irradiation time in the presence of the KLE-templated MgTa2
film after calcination at 790 °C. The change in RhB concentration as a function of the irradiation time, in the presence of various oxide films, is illustrated in . Here, two oxide films, namely, a mesoporous anatase film and a F127-templated MgTa2
film, were included as references. The fabrication and characterization of the mesoporous anatase film is reported elsewhere [21
]. F127-templated MgTa2
films also exhibited an ordered mesoporous structure with concaves, even after calcination at 760 °C; however, the pore-to-pore distance was much smaller and the mesopores were less ordered compared to those templated by KLE. In addition, the round concaves were much smaller and distributed less homogeneously. After calcination, the F127-templated MgTa2
film exhibited XRD patterns corresponding to tri-rutile structure MgTa2
(see Figure S1 for the 2-D-SAXS patterns, Figure S2 for the SEM morphology, and Figure S3 for the XRD pattern in Supporting Information File 1
). From it can be seen that, of all the various oxide films, the KLE derived mesoporous MgTa2
film possessed the highest activity in assisting the photodegradation of RhB in water.
Figure 8 (a) Absorption spectra of RhB after photodegradation for given durations, assisted by the mesoporous KLE-templated MgTa2O6 thin film after calcination at 790 °C. (b) The RhB concentration as a function of the photodegradation time, assisted by (more ...)
Interestingly, PIB6000-templated mesoporous MgTa2O6 thin film showed a very similar dye-degradation activity as compared with the KLE-templated films.
Photocatalytic degradation of RhB in water roughly follows a pseudo-first-order reaction [22
is the normalized RhB concentration, t
is the illumination time, and k
is the apparent reaction rate in terms of min−1
. All of the data demonstrated good linearity for all the curves based on Eq. 1
(Figure not shown). The derived reaction rate constant is listed in . Also included in is the so-called turnover frequency (TOF), which defines the ratio of the reaction rate constant to the catalyst content and reflects the intrinsic activity per site of catalysis [25
]. The catalyst content in the mesoporous film was calculated assuming a condensed film with a porosity of ca. 30% and a thickness of 200 nm (the densities of MgTa2
] and anatase TiO2
] are 7.2 g cm−3
and 3.8 g cm−3
, respectively), which would overestimate the catalyst content and hence underestimate the TOF value for the mesoporous films. A detailed description of the result in will be presented later.
Reaction rate constant (k) and turnover frequency (TOF) of selected photocatalysts.
shows the reaction rate constants of the mesoporous and nonporous MgTa2O6 films as a function of the calcination temperature. The photocatalytic activity of the nonporous film reached an optimum value of ca. 0.0175 min−1 after being heated to 950 °C. However, the evolution in photocatalytic activity for the mesoporous MgTa2O6 film with increasing calcination temperatures was considerably more complicated. Between 550 °C and 760 °C the photocatalytic activity initially increased to a maximum of 0.0216 min−1 at 700 °C, but then decreased as the calcination temperature rose to 760 °C. Interestingly, the photocatalytic activity increased again to a maximum of 0.0260 min−1 when the calcination temperature rose from 760 °C to 790 °C. There was a dramatic drop in photocatalytic activity as the temperature increased to 850 °C followed by a gradual increase as the temperature rose to 1100 °C.
Figure 9 Reaction rate constants of the photodegradation reaction in the presence of the KLE-templated mesoporous and nonporous MgTa2O6 thin films after thermal treatment at various temperatures. The value for a mesoporous TiO2 thin film is represented by a star (more ...)
Since this peculiar increase and decrease in the activity is one of the most relevant findings of the present study, the measurements were repeated three times. As the results were similar to PIB6000-templated films, shows only one of the films for KLE-templated MgTa2O6.
Under UV illumination, MgTa2
adsorbs photons with a wavelength shorter than ca. 278 nm and generates electron–hole pairs corresponding to its bandgap of ca. 4.4 eV [10
] (see Figure S4, Supporting Information File 1
, for the UV–vis diffuse-reflectance spectrum). The photogenerated holes diffuse all the way to the surface where they oxidize the preadsorbed RhB molecules directly, or indirectly by forming hydroxyl radicals, which abstract H atoms from the organics on or near the semiconductor surface [26
]. During subsequent calcination, three factors readily affecting the photocatalytic activity changed dramatically in the current investigation, i.e., crystallinity, grain size, and ordered mesopores [27
]. Defects serve as traps for photogenerated electron–hole pairs [26
]; therefore, improved crystallinity prolongs the lifetime of the photogenerated pairs, thus increasing the number of holes that are able to diffuse all the way to the surface to be involved in the photodegradation reaction. Small grains are assumed to facilitate the photocatalytic reaction, as it takes less time for the photogenerated pairs of holes and electrons to migrate all the way to the surface to be involved in the reaction, which thus inhibits the recombination of the pairs [27
]. Ordered mesopores being accessible from the top favors the diffusion of organic molecules to the oxide surface and at the same time significantly increases the number of catalytically active sites [15
]. In a word, improved crystallinity and small grain size of oxides, together with well-ordered mesopores contribute to the photocatalytic activity of semiconductor oxide films with ordered mesoporous structures.
Increasing the calcination temperatures promotes crystallinity, but at the same time causes grain growth and deterioration of the ordered mesoporous structure; consequently, choosing the calcination temperature is always a balancing act. Considering that 760 °C is the onset of crystallization for the present MgTa2
(), the fact that the first maximum appeared at 700 °C in the temperature range of 550–760 °C for the mesoporous MgTa2
film can be attributed to the balancing act between the crystallinity and the ordered mesopores. When heated to a temperature tens of degrees below the onset of crystallization, e.g., 700 °C, the decay in the ordered mesopores was not so significant, as can be discerned from the SAXS results shown in and . Therefore, the enhanced crystallinity obtained by increasing calcination temperature produced a near linear improvement in the photocatalytic activity at 700 °C. As the calcination temperature approached that of the onset of crystallization, decay in the ordered mesoporous structure became significant due to the further mechanical stress accompanying the amorphous–crystalline phase transformation [29
]. There was a decline in photocatalytic activity at the point where the advantages of the increasing crystallinity were outweighed by the disadvantage introduced by the deterioration of the ordered mesopores.
After the onset of crystallization, the negative effect caused by the collapsed mesoporous structure stabilized. In this case, one should expect the gradually increasing crystallinity to produce a gradual increase in photocatalytic activity as calcination temperatures rise. However, the other factor readily affecting the photocatalytic activity of a crystallized oxide, i.e., the grain size, should not be neglected. After the onset of crystallization, the grain size of MgTa2O6 increased steadily with increasing calcination temperature from 760 °C to 950 °C, as indicated in . The negative effect caused by larger grains, in combination with the positive effect originating from the improved crystallinity, resulted in the second maximum of photocatalytic activity at 790 °C in the temperature range of 760–850 °C. The gradual increase in photocatalytic activity of the films after calcination beyond 850 °C can be ascribed to a consistently improved crystallinity and also to the fact that the changes in the grain size and mesopores were not overly significant.
The positive impact of the ordered mesoporous structure on the photocatalytic activity is reinforced by the significantly higher photocatalytic performance of the mesoporous MgTa2
film compared to the nonporous one (). In addition, the KLE-templated mesoporous MgTa2
film exhibited higher photocatalytic activity than the F127-templated counterpart at the optimized calcination temperature of 790 °C. In the current investigation, a MgTa2
film with an ordered mesoporous structure was also successfully synthesized by using the commercial F127 template even after calcination at 760 °C; however, the mesopores were much smaller and less ordered. It is not possible to synthesize ordered mesoporous MgTa2
films with crystallized walls by using the other widely used block polymer of P123, because of the even smaller mesopores formed during the EISA procedure [11
]. Therefore, the present KLE template contributes greatly to the MgTa2
film with microstructures favoring the photocatalytic property.
Besides the ordered mesoporous structure, the randomly distributed round concave areas with depths of ca. 60 nm might further contribute to the photocatalytic activity. Compared to the generally achieved ordered mesoporous structure with a smooth surface, such a 3-D nanoarchitecture enlarged the surface area of MgTa2O6 exposed to the RhB solution. Meanwhile, as evidenced from , cracks that initiated during the calcination procedure, due either to the thermal mismatch between the MgTa2O6 film and the substrate or to the mechanical stress originating from the amorphous–crystalline transformation, were effectively confined within the round concaves, which improved the interfacial strength of the thin film.
The present nanoarchitectured mesoporous MgTa2
film possessed an optimized photocatalytic activity significantly higher than that of the ordered mesoporous anatase film. When considering the TOF value, the nanoarchitectured MgTa2
film calcinated at 790 °C possessed a photocatalytic activity ca. four times that of the mesoporous anatase film (). Although MgTa2
powders have been reported to possess the ability to assist photoinduced water-splitting, their activities are not promising [10
]. The successful synthesis of the present novel nanoarchitectured MgTa2
film significantly promoted the potential property of MgTa2
as a photocatalyst, which we believe could find practical applications in heterogeneous photocatalysts and solar-induced water splitting.