Hierarchically Grown CaMn3O6 Nanorods by RF Magnetron

Sep 18, 2014 - Hierarchically Grown CaMn3O6 Nanorods by RF Magnetron Sputtering for Enhanced ... substrates by the radio frequency (RF) magnetron sput...
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Hierarchically Grown CaMn3O6 Nanorods by RF Magnetron Sputtering for Enhanced Visible-Light-Driven Photocatalysis B. Barrocas,† S. Sério,*,§ and M. E. Melo Jorge‡ †

Departamento de Química e Bioquímica and ‡Centro de Química e Bioquímica, Faculdade de Ciências da Universidade de Lisboa, Campo Grande C8, 1749-016 Lisboa, Portugal § CEFITEC, Departamento de Física, Faculdade de Ciências e Tecnologia da Universidade Nova de Lisboa, 2829-516 Caparica, Portugal ABSTRACT: CaMn3O6 films with hierarchical nanostructures were deposited for the first time on unheated quartz glass substrates by the radio frequency (RF) magnetron sputtering technique using a polycrystalline CaMnO3 sample as sputtering target, and their photocatalytic activity was evaluated on the decolorization of Rhodamine 6G (Rh6G) aqueous solutions. The films scanning electron microscope (SEM) images evidence a growth of nanorods (NRs) arrays with planar endings and a narrow size distribution centered at about 30 nm in diameter. The CaMn3O6 films surface is uniform and presents a high density of nanorods (116 nanorods per μm2). The high surface area combined with the tunnels crystallographic structure, evidenced by X-ray diffraction (XRD), results in an effective photocatalyst for Rh6G degradation under visible light irradiation. Based on the photodegradation experiments, it is suggested that a process of dye self-sensitization can be one of the key factors of the superior photocatalytic performance of CaMn3O6 NRs. The kinetics of photocatalytic degradation of Rh6G follows a first-order reaction. Furthermore, XRD of the used films did not reveal additional phases indicating high photochemical stability, and the diffuse reflection infrared Fourier transform spectrum (DRIFT) does not show adsorbed organic species on the CaMn3O6 NRs surface. This work provides a potential route to develop high-performance immobilized nanostructures, and the achievements open up many possibilities to tailor visible light active materials for environmental applications.

1. INTRODUCTION

Over the past 20 years the scientific and engineering interest in the application of semiconductor photocatalysis has grown exponentially. Though a wide variety of materials have been tested for photocatalysis,6 TiO2 has generally been the most extensively used and has the ability to detoxificate water from a large number of organic pollutants.7−9 However, TiO2 has a major drawback in processes associated with solar photocatalysis due to its wide band gap (∼3.2 eV), performing rather poorly, thus making it difficult to implement an overall technological process based on TiO2. As the largest proportion of the solar energy spectrum lies in the visible range, more efficient photocatalytic conversion of solar energy could be achieved using a suitable material that is active under visible light. To effectively use the energy of sunlight, several approaches have been used to develop suitable photocatalysts, and basically two broad strategies have been proposed. One is to modify the wide band gap of the photocatalysts (such as TiO2 and ZnS) by cation or anion doping or by producing heterojunctions between them and other materials.10−17 The other approach involves the exploration of novel semiconductor materials capable of

Nowadays, textile dyes are a major source of water contamination, and they create severe environmental pollution problems by releasing toxic and potential carcinogenic substances into the aqueous phase. Even a very low concentration of dyes in the effluents is highly visible and undesirable. In order to address these problems, considerable efforts have been devoted to develop techniques more effective than the conventional processes to eliminate these pollutants. Adsorption and coagulation are common practices applied to treat dyes1 but always result in secondary pollution. In line with these objectives, the application of alternative methods like the advanced oxidation processes (AOP), for example photocatalysis, which involve generation and subsequent reaction of hydroxyl radicals, are often used to treat wastewater and have received great attention in the past years.2 These processes include systems such as TiO2/UV, H2O2/UV, photo-Fenton, and ozone and have been discussed in several reviews.2,3 Hydroxyl radicals can also be generated when a solid semiconductor absorbs radiation based on its band gap. In aqueous solution, the reactive OH radicals produced on the catalyst surface can promote the oxidation of a broad range of organic pollutants quickly and nonselectively and eventually their mineralization.4,5 © 2014 American Chemical Society

Received: July 15, 2014 Revised: September 3, 2014 Published: September 18, 2014 24127

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light irradiation using the Rh6G as a target contaminant. Rh6G dye is a complex molecule and is extensively used for coloring leather, paper, silk, and wool, which should be treated before being extruded into the environment. Because of its high stability, it is interesting to find photocatalytic systems that are able to decompose Rh6G to smaller biodegradable species or eventually allow it total mineralization. The obtained catalytic performance is compared with the one exhibited by TiO2 and Ca0.6Ho0.4MnO3 perovskite films, previously prepared and described in detail elsewhere9,29 for similar experimental photodegradation conditions. It was found that the resulting immobilized CaMn3O6 NRs by this new approach exhibit much higher photocatalytic activity than the TiO2 films on the degradation of Rh6G under visible light irradiation. Furthermore, it is reported here the importance of this new nanostructured material in obtaining active visible-light photocatalysts. We believe this work can provide a directional guide for exploratory study of other member’s materials of interest.

absorbing visible light, namely compounds as La2Ti2O7:Cr/ Fe,18 Bi2WO6,19 PbBi2Nb2O9,20 BiFeO3,21,22 LaFeO3,23 LaMO3 (M = Cr, Mn, Fe, Co, Ni),24 and Bi4Ti3O12.25 Other studies include the modification of semiconductor oxides in order to extend the absorption of light to the visible range by dye sensitization.26−28 The enhancement in the photocatalytic activity is attributed to the increase of light absorption and retarding of the photogenerated electron−hole recombination. Despite many of these photocatalysts being effective for the degradation of organic pollutants, presently, the design of materials showing a high activity under visible light is still a main challenge in the field. Moreover, the stability and efficiency of these catalysts are still low and need improvement. In search of photocatalysts with high photocatalytic activity under visible light irradiation, we have recently discovered that a novel perovskite immobilized Ca0.6Ho0.4MnO3 phase is an efficient photocatalyst for Rh6G aqueous solutions under visible light.29 It is much more active than recently disclosed TiO2 for the same photocatalytic reaction.29 The study revealed that band gap value and the redox ability of the Mn4+/Mn3+ couple are determining factors in the catalytic activity of the Mn-based perovskite oxides. Furthermore, the preparation of the catalysts as supported films is an important approach for the implementation of the photocatalysis process at industrial scale at relatively low cost. Several techniques can be used to prepare oxides thin films; among them sputtering is one of the most versatile techniques used for the deposition of thin films, even when device quality films are required. This technique presents several advantages30−32 such as high deposition rates, films with high purity, high adherence, accurate control of the film thickness, and a wide industrial applicability. In the present study, it is reported for the first time the growth of CaMn3O6 NRs by RF magnetron sputtering onto quartz glass substrates using a nanosized powder target of CaMnO3, previously synthesized by the citrate sol−gel method.33−36 Recently, some works focusing on the synthesis, characterization, and application of CaMn3O6 have been published.37−43 In most of these works37−40 the molten salt synthesis method was chosen to prepare CaMn3O6 nanoribbons, using CaMnO3 powders as the precursor. Others studies revealed the formation of porous microspheres with agglomerated nanorods of CaMn3O6 by heating the solid−solution carbonate precursors in recrystallized alumina boats in air at 800 °C41 or by solid state reactions through the mixture of solid reactants.42,43 The CaMn3O6 crystal structure is based on a framework built of double chains of edge-sharing MnO6 octahedra propagating along Cm. The chains are linked by common corners, producing six-sided tunnels, wherein the Ca2+ cations are located.42 These 1D manganites nanostructures have revealed interesting electronic and magnetic phase transitions due to tunnel and open structures exhibited by these phases.38−40 Moreover, these morphologies enhance their catalytic activity for oxygen reduction reaction in electrocatalysis processes41 and also improve the water oxidation activity.43 The synthesis of new nanostructures is still a huge goal to materials scientists. To the best of our knowledge, comparatively little work has been performed on the fabrication of nanostructures of manganites, particularly in the immobilized form. In this paper is described the preparation of mixed valence manganites CaMn3 O 6 NRs by RF magnetron sputtering and the photocatalytic properties under visible

2. EXPERIMENTAL METHODS All chemicals used in this work were of analytical grade or chemical grade (Aldrich and Fluka) and were used as received. The solutions were prepared with Millipore Milli-Q ultrapure water (18 MΩ cm). 2.1. Preparation of CaMn3O6 NRs Films. CaMn3O6 NRs films were grown by RF magnetron sputtering on unheated quartz glass substrates using CaMnO3 solid solutions as sputtering target. The polycrystalline CaMnO3 samples were previously prepared by the self-combustion method using citric acid, as described in detail in previous reports.33−36 After the decomposition at 600 °C, for 6 h, the resulting amorphous powder was grounded and heated in air at 800 °C, for 18 h, in alumina crucibles (Alsint 99.7). The sputtering target (25 mm diameter and 1 mm of thickness) was prepared by mixing the synthesized polycrystalline CaMnO3 sample with acetone. The mixture was placed and compacted until the acetone evaporation on the top of a cylindrical body planar magnetron cathode, a prototype that was developed in our laboratory to support small powder targets, previously reported and described in detail elsewhere.29 The sputtering of the target was performed in 99.999% pure argon at fixed gas pressure of 0.23 Pa and for 80 min. It was used a RF power supply: Plasmaloc 2HF, a sputtering power of 15 W and a frequency of 100 kHz. Prior to the depositions, the quartz glass substrates (25 mm × 40 mm and 1 mm thick) were cleaned successively in acetone, isopropanol, and deionized water for 5 min each step and dried with nitrogen gas to remove any organic contamination. A turbomolecular pump was used to achieve a base pressure of 10−4 Pa (before introducing the sputtering gas). Details of the sputtering conditions are summarized in Table 1. Since the as-deposited films were amorphous, they were thermal annealed in air at 800 °C for 6 h, in a tubular furnace. 2.2. Decolorization Studies with CaMn 3O6 NRs Catalysts. The photocatalytic activity of CaMn3O6 NRs films was evaluated by the photocatalytic decolorization of Rh6G aqueous solution at ambient temperature under visible light irradiation. The photodegradation experiments were carried out in a photoreactor refrigerated by water circulation (Figure 1). The reaction vessel with a capacity of 250 mL, manufactured from borosilicate glass, accommodates a cooled central quartz tube, in which a 450 W Hanovia medium-pressure mercuryvapor lamp with 0.37 W/cm2 of watt density is placed to 24128

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vis spectroscopy at 526 nm. The blank experiment (Rh6G photolysis) was conducted in the presence of irradiation without any photocatalyst. The structural stability and surface morphology of the photocatalysts were analyzed by XRD and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) before and after the photodegradation experiments. 2.3. Catalysts Characterization. The films structural characterization was performed by XRD using a Panalytical X’pert Pro diffractometer (θ/2θ) equipped with an X’Celerator detector. The data were recorded in the 2θ range 10°−90° with a 2θ step size of 0.017° and a step time of 20 s, employing Cu Kα radiation accelerated at a voltage of 40 kV and a current of 30 mA. A field emission scanning electron microscope (FEG-SEM JEOL 7001F) operating at 15 keV and a transmission electron microscope (TEM, JEOL 200CX) operating at 300 kV equipped with an energy-dispersive spectrometer (EDS) were used for morphology observation and chemical composition analysis. The films thickness was evaluated from the SEM crosssectional images. In order to prevent charge buildup, a thin chromium film was coated on the films surface before SEM analysis. For the size distribution of the CaMn3O6 NRs the SEM images were analyzed using the software ImageJ version 1.46, which allows the estimative of the coverage area, nanorods diameter, and number per unit area. The surface morphology of the films was also characterized by atomic force microscopy (AFM) using a Topometrix TMX 2000 (Veeco Instruments), in contact mode. Silicon cantilevers were employed. All images were taken with 400 × 400 pixels resolution and with an area of 1 × 1 μm2. The surface morphology of the films was characterized by the root-mean-square roughness (RRMS), which was calculated by Gwyddion software. UV−vis diffuse reflectance spectra (DRS) were measured on an UV−vis spectrometer Shimadzu UV-2600PC equipped with an integrating sphere ISR 2600plus over the spectral range 200−900 nm, using BaSO4 as reference. The absorption of the Rh6G solutions was monitored by an UV−vis spectrophotometer Shimadzu UV-2600, in the range 200−650 nm at a scanning speed of 400 nm min−1. The diffuse reflection infrared Fourier transform spectra (DRIFTs) were collected on a Nicolet 6700 FT-IR attached to smart diffuse reflectance accessory, in the range 400−4000 cm−1. The data acquisition was carried out with the program Omnic, and the background was performed with a gold plate. A commercial water contact angle goniometer (NRL contact angle goniometer, model 100-00) was used to evaluate the wettability of the films, by measuring the water contact angle (CA) of a water droplet on the film surface at room temperature. Assuming that the geometry of the drop was a spherical section, the contact angle can be estimated directly from the diameter of the contact circle measured by an optical microscope. Four droplets were deposited in different places of the film surface, and the average value of the contact angles was determined with a typical variance of 90°) when the surface does not tend to adsorb water or be wetted by water. The average CA value estimated for CaMn3O6 films was 84°, evidencing that these photocatalysts are hydrophilic which may contribute positively to the photodegradation ability of these materials. 3.2. Photocatalytic Activity for the Degradation of Rh6G Dye. The photocatalytic activity of the CaMn3O6 NRs was investigated under visible light irradiation on the decolorization of 5 ppm Rh6G solution for a period of 4 h (Figure 6). The Rh6G degradation was monitored by

Figure 4. TEM image for CaMn3O6 NRs (a) and EDS results of a representative CaMn3O6 NR structure (b).

molar ratio of Mn/Ca is about 2.95, which corresponds well to the expected atomic ratio of Mn/Ca = 3.00 for CaMn3O6 phase and therefore confirms the presence of this phase in the films. To determine the band gap energy (Eg) of the studied films, the Kubelka−Munk (KM) method based on the diffuse reflectance spectra was employed. The Eg of the CaMn3O6 NRs films was estimated from the plot of (F(R)hν)1/2 versus photon energy (hν) (shown in Figure 5) being FKM(R) = (1 −

Figure 6. UV−vis absorption data for photocatalytic degradation of Rh6G solution upon irradiation with visible light, using CaMn3O6 NRs films as catalyst. The inset shows a photograph of the corresponding color change of the samples of Rh6G solution taken at t = 0, 5, 15, and 30 min and after that at 30 min of interval up to 4 h during the photodegradation assay.

measuring the absorbance of the samples taken at regular intervals during a period of 4 h. The blank experiment without catalyst (photolysis) was also investigated. From Figure 6, it is observed that the characteristic absorption peaks of Rh6G decrease with the increase of irradiation time. The Rh6G presents three characteristic absorption peaks: the peak at 247 nm is attributed to the benzene ring structure, the peak around 275 nm is related to the naphthalene ring structure, and the third peak at 526 nm is the characteristic peak of Rh6G.9,29 The intensity of the peak at 526 nm dropped with the increase of irradiation time, which means that the chromophoric unsaturated conjugated bond in the dye molecule was destroyed gradually. The peak of benzene ring and naphthalene ring also decreased progressively with exposure time. These results indicate that the photocatalytic degradation not only destroys the conjugate system but also partly decomposes the benzene and naphthalene rings in Rh6G molecule. Figure 7a shows the photodegradation percentage evolution of Rh6G using CaMn3O6 NRs for 4 h of irradiation exposure. For better comparison it was also included in Figure 7, the data previously published by us,29 namely the photodegradation

Figure 5. Kubelka−Munk (KM) plot (after conversion of diffusereflectance to KM) for the CaMn3O6 NRs films. Inset: (FKMhν)2 versus hν for determination of the band gap energy.

R)2R = K/S, where K is the absorption and S the scattering coefficient), after conversion of diffuse-reflectance to KM plot.29 The estimated band gap value for CaMn3O6 NRs was 5.3 eV. Contact angle (CA) measurements were performed to determine the wettability of CaMn3O6 NRs, which is an important surface property for photocatalytic applications. The CA is a measure of the wetting behavior of a particular liquid on 24131

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degradation, respectively), while for TiO2 was merely 63.7%. These results point out that the manganites are better dyedecolorizing agents as compared to the immobilized TiO2. The photocatalytic mechanism of many dyes usually follows Langmiur−Hinshelwood kinetics type kinetic model (L−H model),46 which is described by eq 2: r = −dC /dt = k rK aC /(1 + KC)

(2)

where r is the reaction rate, t the irradiation time, C the dye concentration, kr the reaction rate constant, and Ka the adsorption equilibrium constant. When the initial dye concentration is very low (KaC ≪ 1), this equation can be approximated as a first-order process, whose rate is given as r = kC, where k is the pseudo-first-order reaction rate constant.46 In this case, the variation of the dye concentration with time can be described by eq 3 ln(C0/C) = kt

(3)

where C0 and C are the dye initial concentration and its concentration after visible light irradiation for a given reaction time t, respectively. The relation between ln(C0/C) and t is linear. The ln(C0/C) of Rh6G is equal to its ln(A0/A) according to the linear eq 3, where A0 is the initial absorbance of aqueous Rh6G and A is the absorbance of aqueous Rh6G at the reaction time of t. Figure 7b shows the fitted plots of ln(A0/ A) as a function of visible irradiation time t. For comparative purposes, the results for Rh6G photolysis (in the absence of catalyst), Ca0.6Ho0.4MnO3 films and TiO2 films are also depicted.29 As can be seen by Figure 7b, there is a linear dependence between ln(C0/C) and t for the photocatalytic degradation of Rh6G by the different catalysts and the kinetics of the photodegradation reactions follow a first-order reaction. The apparent reaction rate constants calculated by the slope of the line are listed in Table 2 and plotted in Figure 7c. It can be Table 2. Photocatalytic Degradation Percentage, Reaction Rate Constants, and Correlation Constants on the Decolorization of Rh6G Solution Figure 7. Temporal course of the photodegradation of Rh6G aqueous solution (a); relationship between ln(A0/A) and treatment time (b); photocatalytic degradation and reaction rate constants of Rh6G degradation (c) without catalyst (photolysis) and with CaMn3O6 NRs, TiO2, and Ca0.6Ho0.4MnO3 films as catalysts under visible irradiation.

photolysis CaMn3O6 NRs TiO2a Ca0.6Ho0.4MnO3a a

percentage evolution of Rh6G in the presence of TiO2 films (catalyst most commonly used) and another one with perovskite structure, Ca0.6Ho0.4MnO3 films. For these catalysts the degradation assays were also carried out using the same experimental conditions. All the systems exhibited significant degradation for Rh6G under visible light irradiation. Nevertheless, the photocatalytic activity of the CaMn3O6 NRs was found to result in a higher conversion of Rh6G dye compared to TiO2 and photolysis, but it is somewhat lower than the one exhibited by the immobilized Ca0.6Ho0.4MnO3 phase. It can be seen from Figure 7a that Rh6G dye shows some decolorization without any catalyst (48.1% reduction). In the presence of Ca0.6Ho0.4MnO3, CaMn3O6 NRs, and TiO2 catalysts, only 70, 60, and 150 min, respectively, were required to degrade the same amount of Rh6G dye. It is further observed in Figure 7a, after 4 h of visible light irradiation, that the Rh6G dye was almost completely degraded when the CaMn3O6 NRs and Ca0.6Ho0.4MnO3 films were used (83.1% and 91.5% of dye

Rh6G degradation (%)

K (h−1)

R2

48.1 83.1 63.7 91.5

0.1541 0.3926 0.2465 0.5542

0.9978 0.9999 0.9949 0.9984

Data from ref 29.

seen that the apparent reaction rate constant k increases for the reaction involving catalysts and CaMn3O6 NRs (k = 0.3926 h−1) show a significant higher value as compared with TiO2 (k = 0.245 h−1) followed by the Manganite Ca0.6Ho0.4MnO3 films (k = 0.5542 h−1). Globally, this work reveals that the manganite films used as catalysts (CaMn3O6 NRs and Ca0.6Ho0.4MnO3) in the photodegradation of Rh6G dye are more efficient, when compared to the immobilized TiO2 films, exhibiting a faster reaction rate (were 1−2 times higher than the TiO2 catalyst reaction rate). It is well-known that the materials catalytic performance results from the balance of several factors such as the surface area, crystal structure, adsorption activity, band gap, and so on. In these mixed valence manganites (CaMn3O6 NRs and Ca0.6Ho0.4MnO3 films) the high activities can be attributed in part to the presence of the Mn4+/Mn3+ redox couple, which 24132

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higher catalytic activity was attributed to the strong adsorption capacity of CeO2.48 In that work was proposed that the dye selfsensitization is the governing mechanism for the degradation process. These results allow to conclude that the CaMn3O6 NRs photocatalysts can be manipulated to obtain the desirable microstructures, which are proved to be key factors in the photocatalytic activity. With this in mind, it is important to stress that controllable design of CaMn3O6 photocatalysts with various textures and hierarchical morphologies is the focus of the future research. Additionally, the structural flexibility of this manganite, CaMn3O6, and its tunnels structure, making it possible to incorporate another metal or element to form a new oxide structure, offers an opportunity to tailor the chemical− physical properties and consequently the catalytic activity of these manganese oxides. 3.3. Stability of the Photocatalysts. After photoirradiation, the CaMn3O6 NRs films were monitored by XRD and DRIFTS analysis. The XRD pattern of CaMn3O6 NRs films after the photodegradation assay (not shown) is in good agreement with the pristine CaMn3O6 films, implying that the catalysts maintain their structural integrity after the photocatalysis reaction, and do not suffer photodissolution in the irradiation conditions. Moreover, from DRIFTS spectrum adsorbed organic species were not detected on the CaMn3O6 surfaces (Figure 8).

plays an essential role in the catalytic action and has a positive influence on the catalytic activity.29 For Mn-based oxides, the most active catalyst, usually has an average surface Mn valence between +3 and +4, which affords moderate bond strength between the catalyst surface and the dye. According to Song et al.,47 the coexistence of aliovalent cations (Mn2+, Mn3+, Mn4+) may facilitate the formation of more ionic defects (e.g., vacancies and misplaced ions) and electronic defects (electrons and holes), thus changing the ionic, electronic, and catalytic properties of the manganese oxides. These defects may accelerate the kinetics of the surface redox reactions, in particular the oxygen vacancies could cause an increase in the adsorbed hydroxyl group density and lead to the formation of hydrophilic regions. Moreover, the nominal distribution of Mn valence is CaMn23+Mn4+O6 (average oxidation state of Mn is +3.33) and Ca0.6Ho0.4Mn0.43+Mn0.64+O3 (average oxidation state of Mn is +3.6) for CaMn3O6 NRs and Ca0.6Ho0.4MnO3, respectively. Taking into account the high Eg value obtained for the CaMn3O6 NRs (Eg = 5.3 eV), it is surprising the photocatalytic performance achieved by this catalyst. In fact, comparing with the TiO2 (Eg = 3.27 eV) and Ca0.6Ho0.4MnO3 (Eg = 1.8 eV),9,29 the activity is dramatically improved, and a degree of decomposition of 83.1% was achieved, 30% higher than TiO2 and 10% lower than Ca0.6Ho0.4MnO3. The excellent photocatalytic degradation of Rh6G exhibited by CaMn3O6 NRs under visible light irradiation can be essentially attributed to the high specific area caused by the presence of a large number of nanorods with an orientation oblique and parallel to the substrate. This particular morphology leads to the exposure of more active sites for the adsorption of the reactant molecules, and therefore the photocatalytic activity is favored. Besides the surface area, the small diameters of CaMn3O6 NRs are probably the other key factor for the high activity observed for the nanorods photocatalyst. From the microstructural data shown by SEM/TEM images (Figures 3 and 4), CaMn3O6 films present nanorods, and this rod-like structures seem to favor a higher photocatalytic activity, which can be attributed to the different crystal surfaces exposed on the surface and their preferential photocatalytic responses, contributing to the enhancement of the photocatalytic property. On the other hand, the CaMn3O6 crystallographic structure is also a very important parameter in the Rh6G photodegradation. Using transmission electron microscopy, X-ray, and neutron diffraction techniques, Hadermann et al.42 concluded that the CaMn3O6 crystal structure is based on a framework built of double chains of edge-sharing MnO6 octahedra with a geometry that involves differences in the Mn−O distances. The chains are linked by common corners, which produce six-sided tunnels for Ca2+ ions to reside in. Therefore, this tunnels crystallographic structure can also explain the higher photocatalytic activity detected for CaMn3O6 NRs. Moreover, considering the strong adsorption of Rh6G on CaMn3O6 NRs catalyst by the analysis of Figure 7 and the estimated band gap, it can be inferred that the employed irradiation source with a wavelength well above 234 nm (which corresponds to the CaMn3O6 onset of absorption) is not absorbed by the photocatalyst though is able to excite the Rh6G dye. Therefore, a process of dye self-sensitization can be another ruling factor of the enhanced photocatalytic performance of CaMn3O6 NRs. This type of behavior was reported for CeO2 (with a band gap in the UV region) in the degradation of azodye acid orange 7 under visible light irradiation, and the

Figure 8. DRIFT spectra for CaMn3O6 films before and after the photodegradation assay. The inset shows the DRIFT spectrum of Rh6G.

Considering this advantageous property (the photochemical stability), it should be possible their reuse. Only kinetic experiments carried out with reused samples are adequate to demonstrate the stability of the photocatalysts. Experiments in this sense are in progress.

4. CONCLUSIONS This work reports the study of CaMn3O6 NRs grown in quartz glass substrates by RF magnetron sputtering method using CaMnO3 policrystalline compacted target previously synthesized via the citrate route method at low synthesis temperature. The structure, morphology, and photocatalytic properties on the photocatalytic decolorization of the Rh6G dye with new CaMn3O6 nanostructures have been investigated. The data showed a clear enhancement of the catalytic performance in the degradation of Rh6G dye under visible irradiation as compared 24133

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to TiO2 films, and the kinetics of photocatalytic degradation follows a first-order kinetics equation. This study further revealed that the high catalytic efficiency of CaMn3O6 NRs probably arises as a result of the complex interaction between the double chains of edge-shared MnO6 octahedra, the mixture between Mn3+ and Mn4+, and/or the higher surface-to-volume ratio (surface morphology) afforded by the nanorods geometry, together with a process of dye selfsensitization. XRD and DRIFTS evidenced high photochemical stability of the CaMn3O6 films after photodegradation experiment. These findings are meaningful and encouraging for future studies on the improvement of the CaMn3O6 photocatalytic efficiency and promote its practical application in environmental remediation using solar irradiation.

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AUTHOR INFORMATION

Corresponding Author

*Tel +351-21-294 8576; Fax +351-21-294 8549; e-mail susana. [email protected] (S.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Doctor Yuri Nunes for the AFM image acquisition (CEFITEC/FCT/UNL), the financial support from FEDER, through Programa Operacional Factores de Competitividade − COMPETE and Fundaçaõ para a Ciência e a Tecnologia − FCT, for the project PTDC/AACAMB/103112/2008. This work was also supported by FCT, through the projects PEst-OE/QUI/UI0612/2013 and PEstOE/FIS/UI0068/2011. The authors thank the use of UV−vis spectrometer Shimadzu UV-2600PC equipped with an integrating sphere ISR 2600plus, financed by the project PTDC/CTM-NAN/113021/2009. S. Sério thanks FCT for the Programme Ciência 2007.

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