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Nanoscale calcium bismuth mixed oxide with enhanced photocatalytic performance under visible light
Applied Catalysis A: General 382 (2010) 190–196
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Nanoscale calcium bismuth mixed oxide with enhanced photocatalytic performance under visible light Renata Solarska a,∗, Andre Heel a, Joanna Ropka b, Artur Braun a, Lorenz Holzer a, Jinhua Ye c, Thomas Graule a a b c
Laboratory for High Performance Ceramics, EMPA – Swiss Federal Laboratories for Materials Testing & Research, Uberlandstrasse 129, CH-8600 Dübendorf, Switzerland Laboratory of Crystallography, University of Geneva, 1211 Geneva, Switzerland Photocatalytic Materials Center, National Institute for Materials Sciences (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
a r t i c l e
i n f o
Article history: Received 19 February 2010 Received in revised form 21 April 2010 Accepted 22 April 2010 Available online 29 April 2010 Keywords: Flame spray synthesis Mixed oxide Nanoparticles Oxygen vacancies Photocatalysis
a b s t r a c t The objective of materials research is the development of economical, safe and efficient synthesis routes that lead to the formation of a photocatalyst which is able to overcome performance problems related to particle size, crystallinity, or low surface area. Here, we report high-quality functional nanoparticles of calcium bismuth mixed oxide with 15 nm nominal size corresponding to a specific surface area of 41 m2 /g which were produced by single-step flame spray synthesis (FSS). The high temperature of the flame afforded creation of oxygen vacancies which were quantified by near edge X-ray absorption fine structure (NEXAFS) spectra. These two parameters, developed active surface area and created in the flame oxygen vacancies, allowed to enhance the photocatalytic activity of calcium bismuth oxide by a factor of 6, in comparison to previously reported calcium bismuth mixed oxide produced by conventional methods which required additional temperature treatment steps. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Toxic organic pollutants from both domestic and industrial activities pose environmental problems. Therefore, an environmental remediation becomes a pressing concern which can be solved with photocatalytic materials. However the choice of the suitable system is restricted to the semiconducting oxides which are photochemically stable and which are able to absorb efficiently visible light. An additional critical factor influencing the photocatalytic efficiency of the involved system is the suitable position of energy bands. For example, although the oxidation of carbon in aromatic hydrocarbons into CO2 is relatively facile on different photocatalysts, total dearomatization leading to complete mineralization of organic pollutants is generally too slow, if not impossible. In this regard, the pioneering work of Zou et al. [1] on In1−x Nix TaO4 (x = 0–0.2) which is able to split efficiently water under visible light, gave an important impetus to explore other
Abbreviations: FSS, flame spray synthesis; RDOC, relative degree of crystallinity; SSA, specific surface area; FWHM, full width at half maximum; NEXAFS, near edge X-ray absorption fine structure; MB, methylene blue; MeOH, methanol. ∗ Corresponding author. Present address: Department of Chemistry, Warsaw University, Pasteura 1, PL-02-093 Warsaw, Poland. Tel.: +48 22 822 0211; fax: +48 22 822 5996. E-mail addresses: [email protected], [email protected] (R. Solarska). 0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.04.043
binary or ternary oxides for photocatalytic purposes [2]. Much of the interest in mixed semiconducting oxide materials arises because of their upper shift of the valence band which is assigned to the hybridizations between the O 2p orbital and transition metal orbitals, such as Bi(6s), Ag(4d), Ta(5d), and V(3d). In consequence, the hybridized valence band permits reduction of the band gap of the resultant photocatalyst. Based on this empirical rule, many promising materials have been carefully designed in order to eliminate potential routes of losses in performance and then promptly implemented for environmental remediation [3–9]. At this point it is worth mentioning that the recently reported Ag based solid solution perovskite (Ag0.75 Sr0.25 )(Nb0.75 Ti0.25 )O3 [10] exhibits excellent photocatalytic efficiency for acetaldehyde decomposition. However, with demands for nanoscale materials it is essential that as-synthesized photocatalysts do not require any additional heat treatment that may lead to particle growth and coarsening. Currently, this is an essential drawback of the aforementioned materials implemented for environmental remediation. In this regard, our approach is to decrease the recombination problem related to the particle size, crystallinity, and low surface area by one-step flame spray synthesis. This bottom-up synthesis method allows the production of a wide variety of materials including metal oxides, mixed metal oxides, sulfides, nitrides, and metal alloys and it has several advantages over solution powder preparation or top-down methods like solid-state reaction [11–13]. A specific advantage of our setup is use of high combustion enthalpy acety-
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Switzerland) and calcium nitrate tetrahydrate (Ca(NO3 )2 ·4H2 O, ≥99%, Riedel-de-Haen, Switzerland) in stoichiometric ratio dissolved in pure acetylacetone (C8 H5 O2 , ≥99.3%, Fluka). The total cation concentration (Ca and Bi) was fixed at 0.3 M. A controlled amount of this precursor solution was fed to the nozzle through a mass controller (Bronkhorst HI-TEC, The Netherlands), with different flow rates: 10, 15, 18 and 20 ml/min and dispersed by oxygen (35 l/min). The resulting spray was then burned in a premixed acetylene–oxygen flame under constant flow rate of 13 and 17 l/min, in order to form crystalline calcium bismuth oxide. The nanoparticles were collected on glass fiber filters (Type GF/A 150, Whatman, UK) placed inside a stainless steel filter holder connected to a vacuum pump. The powder samples were then labeled F10, F15, F18 and F20 in accordance to the precursor flow rate. The aim of the precursor flow rate variation was to influence the specific surface area and the crystallinity of the final product. During the entire flame synthesis run, the oxygen need () was close to 1 in order to ensure the complete oxidation of acetylene and for effective mixing of oxygen and fuel. =
O2,real O2,stoichiometric
(1)
2.2. Photocatalyst characterization
Fig. 1. Schematic presentation of the flame spray plant and the particles formation path.
lene flame instead of a methane flame, which is most often used in conventional plants. Acetylene based flame spray synthesis providing high temperature (>3000 ◦ C) and high cooling rates allows the successful transformation of aqueous precursor solution of calcium and bismuth nitrates into highly crystalline functional nanoparticles. Given that, among many materials, very good photocatalytic efficiency for organic waste decomposition has been reported for bismuth based mixed oxides but their photocatalytic performance was limited due to a very low specific surface area in range of 0.6 m2 /g [5–7], we saw potential for improvement by producing calcium bismuth oxide with developed specific surface area by one-step FSS. Herein we report a progress made in synthesis and in photocatalytic performance of calcium bismuth mixed oxide. A decrease in particle size to 15 nm, corresponding to a specific surface area (SSA) of 41 m2 /g and created within the flame oxygen vacancies affected the microstructure of the photocatalyst and subsequently, allowed to improve remarkably its photocatalytic performance under visible light irradiation. 2. Experimental 2.1. Photocatalyst synthesis Calcium bismuth mixed oxide (CaBi2 O4−x ) has been prepared in a one-step flame spray synthesis involving the passage of the liquid precursor to an aerosol generator and next to the burning flame. Fig. 1 shows a schematic sketch of the experimental setup consisting of a liquid precursor supply line, gases delivery system, an aerosol flame reactor and the particles collection unit. The detailed operating procedure has been described elsewhere [14–16]. Acetylene (C2 H2 , purity ≥ 99.96%, Carbagas, Switzerland) was employed as a fuel, while the oxygen served as a burning and atomizing gas (O2 , purity ≥ 99.95%, Carbagas, Switzerland). The high power density acetylene flame premixed with oxygen allows combustion of an inorganic metal precursor mixture, containing bismuth nitrate pentahydrate (Bi(NO3 )3 ·5H2 O, Riedel-de-Haen,
2.2.1. XRD, BET, TEM and SEM measurements The crystal structure of the as-synthesized calcium bismuth oxide powders was determined by X-ray diffraction (XRD) method with a PANalytical X’Pert PRO diffractometer under Ni-filtered CuK␣ radiation. The space group, crystal lattice, cell parameters and stoichiometry of the photocatalyst were evaluated from the corresponding XRD patterns, fitted and refined with FullProf software. To assess the crystallite size of flame-made calcium bismuth oxide we applied Scherrer’s formula which can be used to estimate the diameter of the nanocrystallites smaller than 100 nm [17]: dXRD =
K ˇ cos
(2)
where K was assumed as 0.89, is the copper XRD wavelength: CuK␣ 1.5418 Å and ˇ is the full width at half maximum (FWHM) of calcium bismuth oxide peak measured at 2 = 28◦ . Additionally, the relative degree of crystallinity (RDOC) was estimated from the ratio of intensity and FWHM of the most pronounced peak in the pattern to the background offset for all four samples. Taking into account that the photocatalytic performance scales with accessible surface area, the specific surface area of all samples was determined by Brunauer–Emmett–Teller (BET) method from a 6-point nitrogen adsorption isotherm at 77 K (Gemini 2360, Shimadzu). The BET-equivalent particle diameter of the primary particles was calculated from formula (3) presented below which assumes spherical geometry: is the theoretical density of our cubic calcium bismuth oxide which was estimated from the Rietveld refinement (9.59 g/cm3 ) and S the specific surface area of as-synthesized calcium bismuth mixed oxide. dBET =
6 CaBi2 O4 S
(3)
Subsequently, the morphology of as-synthesized powders was investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) in order to detect, characterize and monitor changes in surface morphology, homogeneity, size of primary particles, crystallinity and state of agglomeration. TEM was performed with a Philips CM30, operating at 300 kV and equipped with an EDX system from EDAX. The SEM investigations were performed with a Hitachi S4800 (with cold FEG). The microscope was equipped with in-lense SE- and ExB-detectors and with a bright
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Fig. 2. Relative degree of crystallinity (RDOC) for powders F10, F15 and F18 vs. samples (A) F20 and (B) F20T which was additionally treated in 500 ◦ C during 1 h.
field STEM-detector. For both, TEM and SEM observations, the nanoparticles were dispersed in isopropanol (purity ≥ 99.5%, Fluka) and deposited on the formvar/carbon coated copper grids (Plano GmbH, Germany). 2.2.2. X-ray absorption spectroscopy Soft X-ray absorption spectra at the oxygen K-shell edge were recorded at beamline 9.3.2 at the Advanced Light Source in Berkeley, California in the energy range from 500 to 580 eV in steps of 0.1 eV and recording time of 1 s/step at an energy resolution of E/E ∼ 1/5000. The powder samples were roll-pressed into thin indium metal foil and then placed on a sample holder which was transferred into the ultra high vacuum recipient with a base pressure of 10E−10 Torr. The spectra were normalized to unity at 550 eV and deconvoluted using the WinXAS program. 2.3. Photocatalytic activity Evaluation of photocatalytic performance of calcium bismuth oxide was based on methylene blue (MB) degradation in aqueous medium. MB degradation was carried out with 0.15 g of the catalyst powder (F10, F15, F18, and F20) suspended in a solution of MB (5 × 10−5 M, Vtot = 100 ml). Prior to the irradiation, the reaction was conducted in the darkness for 30 min to establish equilibrium of MB adsorption/photolysis. The optical system for the photocatalytic reaction included a 300 W Xe arc lamp equipped with a cutoff filter providing the visible light (>420 nm) and a glass water filter to prevent IR irradiation. The degradation process of MB was investigated by comparing the UV–Vis absorbance spectra (UV-2500 spectrometer, Shimadzu) of residual MB with initial MB concentration. The concentration of residual MB was checked every 10 min. 3. Results and discussion 3.1. Structure characterization The calcium bismuth oxide powders were produced in acetylene flame, with precursor flow rates varied from 10 to 20 ml/min. Powders produced with lower flow rates were milky and fluffy (F10 and F15) while those produced with higher flow rates became more yellowish and compact (F18 and F20). The color and physical appearance of as-synthesized nanopowders reflected their degree of crystallinity and an order of magnitude of the specific surface area. The relative degree of crystallinity (RDOC) was estimated from XRD patterns based on the ratio between peak intensity, peak width at half-maximum and normalized background [18]. Not surprisingly, the milky and fluffy powders showed lower degree of crystallinity (RDOC), suggesting that these materials are
less ordered than samples produced with higher flow rates. As it is shown in Fig. 2, their RDOC are of the order of 75% vs. sample F20, whereas vs. sample F20T (additionally annealed in 500 ◦ C during 2 h) reaches only 30%. This drop in RDOC is related to lower combustion enthalpy by reduced solvent content in the flame. Going further, lower energy density in the flame leads to the formation of surface crystalline defects which is also associated with high specific surface area (SSA). In case of nanoparticles with sizes of 10–20 nm, the proportion of surface atoms (surface defects) is higher than in the bulk and it may lead to the disturbance in 3D structural order and affects the crystallinity. This is the case of samples F10 and F15 which have lower RDOC but exhibit larger specific surface areas, in range of 47 m2 /g. For samples F18 and F20 the SSA slightly decreases to 41 m2 /g as a direct consequence of higher precursor feed rate into the flame. It has been reported that an increase in precursor concentration in the flame enhances the growth of particles by their surface growth, thus, larger particles are formed [19]. On the other hand, the surface growth reduces the defect concentration which is concomitant with increase in crystallinity. The relation between precursor flow rate and SSA or particle diameters calculated from BET are summarized in Table 1. At this point, it should be emphasized that although precursor flows of 18 or 20 ml/min lead to the formation of slightly bigger primary particles than flow of 10 ml/min, the F18 and F20 powders represent an optimal balance between specific surface area and crystallinity. This optimum manifests in the enhanced photocatalytic activity which will be discussed in details further. The particle size of 15 nm, calculated from Brunauer–Emmett–Teller (BET) isotherms, showed good agreement with the crystallite size calculated from the peak width at half-maximum of the calcium bismuth oxide diffraction peaks which revealed the presence of crystallites in the range of 10–12 nm (Table 1). The BET-equivalent of the particle diameter is always slightly larger than the calculated from Scherrer’s formula crystallite size, since the surface defects, amorphous content or porosity are not accounted for in X-ray. The convergence in particles and crystallites size, suggests that the nanoparticles are virtually single-crystalline. The average particle diameters derived from BET and XRD of as-synthesized F10, F15, F18, and F20 Table 1 Particle sizes based on BET and XRD measurements. Sample
F10 F15 F18 F20
Precursor flow rate (ml/min)
(g/h)
10 15 18 20
72 108 130 145
SSA (m2 /g)
dBET (nm)
dXRD (nm)
= 9.59 g/cm3 47.6 47.2 43.7 41.5
13.1 13.2 14.3 15.1
9.5 10.0 11.2 11.7
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Fig. 3. SEM (A) and TEM (B and C) images of calcium bismuth oxide nanoparticles produced with flow rate of 20 ml/min.
powders are shown in Table 1. It should be noted that samples F10 and F15 exhibit almost similar structural features, thus we will focus our further investigations on two external samples: F10 and F20. In addition to the primary particle size, the particle size distribution and morphology are important measures of the quality of a powder and often determine the usefulness of the powder in subsequent steps and applications. Therefore, scanning and transmission electron microscopies (SEM and TEM) were used to size particles, to investigate their microstructure, to quantify phase composition and distribution, and together with energydispersion spectroscopy (EDX), to identify the elements present in a sample. Computer supported analysis of SEM and TEM images revealed small particles (less than 20 nm) showing a strong tendency to form soft agglomerates, however with visible grain boundaries and crystalline lattices, which is typically indicative of good quantum efficiency (Fig. 3). TEM images confirmed also the single-crystalline nature of samples showing the crystalline patterns in range of 15 nm (Fig. 3C). It is worth noting that the particle size distribution, which is based on a set of 500 counted particle diameters from TEM measurements, is relatively narrow and characterized by a small geometric standard deviation of g = 1.27 (Fig. 4). At this point, it should be mentioned that powders produced by salt-based flame techniques are typically characterized by rather broad ( g ∼ 1.5) and skewed particle size distributions with a long right tail indicating non-negligible mass fraction of larger particles [20]. It is equally important to mention that coupled with TEM, EDX measurements confirmed the expected stoichiometry of CaBi2 O4−x
Fig. 5. Experimental ( ) and calculated (—) pattern of freshly synthesized nanoparticles of calcium bismuth mixed oxide (F20).
showing Ca and Bi contents in the measured F20 sample of 17.3 and 40.5%, respectively. Calcium bismuth mixed oxide was reported in the monoclinic crystalline system with cell parameters: a = 1.4002 nm, b = 1.1596 nm, c = 1.2198 nm, and ˇ = 101.541◦ [5]. However, calcium bismuth mixed oxide produced by FSS, crystallizes in cubic structure (space group Fm − 3m, lattice parameter a = 0.5530 nm, reliability factors: 2 = 1.91, RB = 2.96, and RF = 1.986) (Fig. 5), which is probably related to the strong quenching effect in the flame. In the present case, the precursor conversion takes place in the early stage flame with temperature of approx. 3000 ◦ C. In this part of the
Fig. 4. Particles size distribution of F20 sample based on set of 500 particle sizes from TEM analysis.
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Fig. 6. MB degradation on calcium bismuth powders (䊉 F10 and F20) and in the absence of photocatalyst ().
Fig. 7. Diffuse reflection spectra of F10 and F20 calcium bismuth mixed oxide samples as-recorded and transformed to the absorption spectra by KM theory.
flame, the particles have an initial speed in the range of 150 m/s and then they are rapidly cooled down to approx. 700 ◦ C, which gives the quenching rate of about 500,000 K/s [14–16]. In this regard, the cation diffusion time is very short and therefore the material formed has no time to relax and the final structure is frozen in cubic crystal system. However, when additional slow heat treatment is applied the structure relaxes to the thermodynamically preferred, monoclinic structure and additional Bragg reflections appear in the XRD pattern.
F10 (Fig. 6 and Table 2). Sample F20 has slightly lower surface area than F10 but significantly higher crystallinity. In consequence, the optimal ratio between these two factors (SSA and RDOC) allows an enhancement in the MB degradation rate. In Table 2, the MB degradation rates are related to the SSA and RDOC. The degradation of MB is not a simple one-hole oxidation process but its degradation kinetics is limited by an adsorption of the dye itself on the photocatalyst, then by a formation of intermediates which re-adsord/desorb from photocatalyst surface. The MB degradation pathway is a multi-step process requiring diffusion of the photogenerated charges to the surface, which then have to traverse the surface of the photocatalyst without any recombination in order to perform proper photocatalytic action. Therefore, the prerequisite for improved photocatalytic activity is not only high SSA or excellent crystallinity, separately but a combination of both these factors [18,23]. We managed to achieve the optimum with sample F20 which represents an excellent compromise between still high surface area of 41.5 m2 /g and acceptable degree of crystallinity (70%). Such an optimization of synthesis conditions allowed us to improve photocatalytic performance of calcium bismuth mixed oxide by a factor of 6 in comparison to calcium bismuth oxide with low surface area, reported elsewhere [5]. Photocatalytic performance is also determined by the stability of the catalyst within the time. At this point we should mention that we tried to perform the hydrogen evolution in the presence of sacrificial reagents such as Pt and MeOH. This test took more than 30 h and although no evolution of H2 was observed under visible light, the catalyst was stable and we could generate the H2 under UV irradiation, immediately after the previous test. This experiment showed that the CB is too low to allow the hydrogen generation under visible illumination, but the calcium bismuth mixed oxide neither dissolved nor photo-corroded under visible or UV light.
3.2. Photocatalytic performance Photocatalytic decomposition of organic species is a surface driven reaction. Therefore, our powders possess a surface area that is well suited for environmental applications. Although the developed surface area is also associated with surface defects and hence, lower crystallinity, FSS offers a possibility to optimize these two conflicting parameters which influence directly photocatalytic activity. On the other hand, it is known that defects produced by high temperature can extend the optical absorbance and hence, the activity range of a photocatalyst. These defects are the oxygen vacancies and can be easily created in the hot part of a flame. As it was reported previously, in case of TiO2 such defects play a crucial role in the improvement of its photocalatytic performance [21]. Despite the fact that the photocatalytic activity due to defects is much smaller than the activity coming from intrinsic absorption, it is definitely an important issue in view of environmental remediation. MB is a textile dye, which can be easily found in wastewater discharges, and although MB can self-photolyse (Fig. 6) [22], its degradation rate is low under solar irradiation: only 20% decomposed within the first hour of the photolysis under irradiation of > 420 nm. On the contrary, almost 100% is decomposed within the same time in the presence of calcium bismuth oxide, revealing its high photocatalytic performance under visible light, despite the fact that its absorption spectrum shows an effective intrinsic absorption only to 450 nm (Fig. 7). In this regard, we claim that the extension of its photocatalytic activity is due to developed surface and oxygen vacancies which are formed in the flame. MB is oxidized on CaBi2 O4−x surface mainly via direct photocatalytic effect enhanced by the created substoichiometry in oxygen but possibly also by MB–dye sensitization [22]. It means that this decomposition is driven by light and the surface properties of a photocatalyst. Indeed, in the present case there is a clear difference in the photocatalytic activity for MB decomposition between powder F20 and
3.3. Spectroscopic characterization In order to understand the difference in photocatalytic behavior of samples F10 and F20 we probe the valence band by soft X-ray absorption spectroscopy at the anion absorption edge. This technique is particularly sensitive to orbital and spin effects. Fig. 8 shows the near edge X-ray absorption fine structure (NEXAFS) spectra of samples F10 and F20. Of particular relevance are the pre-edge peaks between 520 and 530 eV right before the upper Hubbard band. The first peak at 523 eV is next to the Fermi energy
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Table 2 Comparison of photocatalytic activity of calcium bismuth mixed oxides produced by FSS and soft chemistry methods. MB degradation rate (mol/(h × g))a
Photocatalyst
Synthesis
SSA (m2 /g)
CaBi2 O4−x F20 F10
FSS – 20 ml/min FSS – 10 ml/min
41.5 47.6
70 30
63 40
CaBi2 O4 [5]
Soft chemistry
0.6
100
10
a
RDOC (%)
The MB degradation rate is based on the reaction rate at the initial 30 min with 420-nm cutoff filter.
Fig. 8. Oxygen (1s) NEXAFS spectra of F10 and F20 samples.
at about 520 eV and has eg symmetry. This peak corresponds to doped holes and accounts for electric conductivity in the samples. The following peaks at 528 and 530 eV have t2g and eg symmetries and may block conductivity, depending on the molecular environment. These three peaks derive from hybridization between the O(2p) and the Bi(5d) bands and are sensitive to the superexchange between oxygen and metal ions. Comparison of both spectra shows that sample F10 has negligible intensity at 523 eV, i.e. virtually no existing holes. The situation is entirely different for F20, which has a pronounced intensity in this energy range, indicative to a high concentration of doped holes. The observed difference in concentration of doped holes between samples F10 and F20 emerges from difference in oxygen substoichiometry which is related to precursor flow rate. Hence, based on the precursor flow rate we can estimate not only the particle size, SSA or crystallinity but also the conductivity and extent of the oxygen deficiency. All these parameters play a crucial role in photocatalytic performance.
exploring of materials in view of effective environmental remediation. Finally, it should be emphasized that FSS allows control of the particles size very precisely. Such a precise control of particles size subsequently, allows to influence not only many physicals properties of a photocatalyst but also to reduce the extent of charge recombination effect. In this regard, FSS appears as a powerful tool to design and synthesize efficient photocatalytic systems.
Acknowledgments We gratefully acknowledge Dr. Andri Vital and Dr. Kranthi Akurati who initiated this project, part of which was supported by the Empa Board of Directors’ Fund. Financial support for the exchange visit of Dr. Renata Solarska at NIMS by the Swiss National Science Foundation (SNF Project IZAJZO-123331/1) and the Japan Society for Promotion of Science (JSPS/RCI-2/08035, ID no. RC 20830001) is also acknowledged.
4. Conclusions Photocatalytic performance of calcium bismuth oxide for environmental remediation was improved by a new preparation route going through evaporation and decomposition of precursor in a flame, followed by nucleation, condensation, coagulation and agglomeration steps, which lead to the formation of fine photocatalyst particles with nominal size of 15 nm. Decrease in particle size and flame-formed oxygen deficiencies enhanced the surface properties and extend the absorption spectrum towards the visible range. As a consequence, the photocatalytic activity of MB degradation of our flame-made calcium bismuth oxide was enhanced by a factor of 6. The soft X-ray absorption spectroscopy measurements showed a significant difference in concentration of doped holes between samples produced with different precursor flow rates which allows the synthesis conditions to be related to the conductivity, thus to the oxygen substoichiometry. These observations are very important for further
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Nanoscale calcium bismuth mixed oxide with enhanced photocatalytic performance under visible light Renata Solarska a,∗, Andre Heel a, Joanna Ropka b, Artur Braun a, Lorenz Holzer a, Jinhua Ye c, Thomas Graule a a b c
Laboratory for High Performance Ceramics, EMPA – Swiss Federal Laboratories for Materials Testing & Research, Uberlandstrasse 129, CH-8600 Dübendorf, Switzerland Laboratory of Crystallography, University of Geneva, 1211 Geneva, Switzerland Photocatalytic Materials Center, National Institute for Materials Sciences (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
a r t i c l e
i n f o
Article history: Received 19 February 2010 Received in revised form 21 April 2010 Accepted 22 April 2010 Available online 29 April 2010 Keywords: Flame spray synthesis Mixed oxide Nanoparticles Oxygen vacancies Photocatalysis
a b s t r a c t The objective of materials research is the development of economical, safe and efficient synthesis routes that lead to the formation of a photocatalyst which is able to overcome performance problems related to particle size, crystallinity, or low surface area. Here, we report high-quality functional nanoparticles of calcium bismuth mixed oxide with 15 nm nominal size corresponding to a specific surface area of 41 m2 /g which were produced by single-step flame spray synthesis (FSS). The high temperature of the flame afforded creation of oxygen vacancies which were quantified by near edge X-ray absorption fine structure (NEXAFS) spectra. These two parameters, developed active surface area and created in the flame oxygen vacancies, allowed to enhance the photocatalytic activity of calcium bismuth oxide by a factor of 6, in comparison to previously reported calcium bismuth mixed oxide produced by conventional methods which required additional temperature treatment steps. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Toxic organic pollutants from both domestic and industrial activities pose environmental problems. Therefore, an environmental remediation becomes a pressing concern which can be solved with photocatalytic materials. However the choice of the suitable system is restricted to the semiconducting oxides which are photochemically stable and which are able to absorb efficiently visible light. An additional critical factor influencing the photocatalytic efficiency of the involved system is the suitable position of energy bands. For example, although the oxidation of carbon in aromatic hydrocarbons into CO2 is relatively facile on different photocatalysts, total dearomatization leading to complete mineralization of organic pollutants is generally too slow, if not impossible. In this regard, the pioneering work of Zou et al. [1] on In1−x Nix TaO4 (x = 0–0.2) which is able to split efficiently water under visible light, gave an important impetus to explore other
Abbreviations: FSS, flame spray synthesis; RDOC, relative degree of crystallinity; SSA, specific surface area; FWHM, full width at half maximum; NEXAFS, near edge X-ray absorption fine structure; MB, methylene blue; MeOH, methanol. ∗ Corresponding author. Present address: Department of Chemistry, Warsaw University, Pasteura 1, PL-02-093 Warsaw, Poland. Tel.: +48 22 822 0211; fax: +48 22 822 5996. E-mail addresses: [email protected], [email protected] (R. Solarska). 0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.04.043
binary or ternary oxides for photocatalytic purposes [2]. Much of the interest in mixed semiconducting oxide materials arises because of their upper shift of the valence band which is assigned to the hybridizations between the O 2p orbital and transition metal orbitals, such as Bi(6s), Ag(4d), Ta(5d), and V(3d). In consequence, the hybridized valence band permits reduction of the band gap of the resultant photocatalyst. Based on this empirical rule, many promising materials have been carefully designed in order to eliminate potential routes of losses in performance and then promptly implemented for environmental remediation [3–9]. At this point it is worth mentioning that the recently reported Ag based solid solution perovskite (Ag0.75 Sr0.25 )(Nb0.75 Ti0.25 )O3 [10] exhibits excellent photocatalytic efficiency for acetaldehyde decomposition. However, with demands for nanoscale materials it is essential that as-synthesized photocatalysts do not require any additional heat treatment that may lead to particle growth and coarsening. Currently, this is an essential drawback of the aforementioned materials implemented for environmental remediation. In this regard, our approach is to decrease the recombination problem related to the particle size, crystallinity, and low surface area by one-step flame spray synthesis. This bottom-up synthesis method allows the production of a wide variety of materials including metal oxides, mixed metal oxides, sulfides, nitrides, and metal alloys and it has several advantages over solution powder preparation or top-down methods like solid-state reaction [11–13]. A specific advantage of our setup is use of high combustion enthalpy acety-
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Switzerland) and calcium nitrate tetrahydrate (Ca(NO3 )2 ·4H2 O, ≥99%, Riedel-de-Haen, Switzerland) in stoichiometric ratio dissolved in pure acetylacetone (C8 H5 O2 , ≥99.3%, Fluka). The total cation concentration (Ca and Bi) was fixed at 0.3 M. A controlled amount of this precursor solution was fed to the nozzle through a mass controller (Bronkhorst HI-TEC, The Netherlands), with different flow rates: 10, 15, 18 and 20 ml/min and dispersed by oxygen (35 l/min). The resulting spray was then burned in a premixed acetylene–oxygen flame under constant flow rate of 13 and 17 l/min, in order to form crystalline calcium bismuth oxide. The nanoparticles were collected on glass fiber filters (Type GF/A 150, Whatman, UK) placed inside a stainless steel filter holder connected to a vacuum pump. The powder samples were then labeled F10, F15, F18 and F20 in accordance to the precursor flow rate. The aim of the precursor flow rate variation was to influence the specific surface area and the crystallinity of the final product. During the entire flame synthesis run, the oxygen need () was close to 1 in order to ensure the complete oxidation of acetylene and for effective mixing of oxygen and fuel. =
O2,real O2,stoichiometric
(1)
2.2. Photocatalyst characterization
Fig. 1. Schematic presentation of the flame spray plant and the particles formation path.
lene flame instead of a methane flame, which is most often used in conventional plants. Acetylene based flame spray synthesis providing high temperature (>3000 ◦ C) and high cooling rates allows the successful transformation of aqueous precursor solution of calcium and bismuth nitrates into highly crystalline functional nanoparticles. Given that, among many materials, very good photocatalytic efficiency for organic waste decomposition has been reported for bismuth based mixed oxides but their photocatalytic performance was limited due to a very low specific surface area in range of 0.6 m2 /g [5–7], we saw potential for improvement by producing calcium bismuth oxide with developed specific surface area by one-step FSS. Herein we report a progress made in synthesis and in photocatalytic performance of calcium bismuth mixed oxide. A decrease in particle size to 15 nm, corresponding to a specific surface area (SSA) of 41 m2 /g and created within the flame oxygen vacancies affected the microstructure of the photocatalyst and subsequently, allowed to improve remarkably its photocatalytic performance under visible light irradiation. 2. Experimental 2.1. Photocatalyst synthesis Calcium bismuth mixed oxide (CaBi2 O4−x ) has been prepared in a one-step flame spray synthesis involving the passage of the liquid precursor to an aerosol generator and next to the burning flame. Fig. 1 shows a schematic sketch of the experimental setup consisting of a liquid precursor supply line, gases delivery system, an aerosol flame reactor and the particles collection unit. The detailed operating procedure has been described elsewhere [14–16]. Acetylene (C2 H2 , purity ≥ 99.96%, Carbagas, Switzerland) was employed as a fuel, while the oxygen served as a burning and atomizing gas (O2 , purity ≥ 99.95%, Carbagas, Switzerland). The high power density acetylene flame premixed with oxygen allows combustion of an inorganic metal precursor mixture, containing bismuth nitrate pentahydrate (Bi(NO3 )3 ·5H2 O, Riedel-de-Haen,
2.2.1. XRD, BET, TEM and SEM measurements The crystal structure of the as-synthesized calcium bismuth oxide powders was determined by X-ray diffraction (XRD) method with a PANalytical X’Pert PRO diffractometer under Ni-filtered CuK␣ radiation. The space group, crystal lattice, cell parameters and stoichiometry of the photocatalyst were evaluated from the corresponding XRD patterns, fitted and refined with FullProf software. To assess the crystallite size of flame-made calcium bismuth oxide we applied Scherrer’s formula which can be used to estimate the diameter of the nanocrystallites smaller than 100 nm [17]: dXRD =
K ˇ cos
(2)
where K was assumed as 0.89, is the copper XRD wavelength: CuK␣ 1.5418 Å and ˇ is the full width at half maximum (FWHM) of calcium bismuth oxide peak measured at 2 = 28◦ . Additionally, the relative degree of crystallinity (RDOC) was estimated from the ratio of intensity and FWHM of the most pronounced peak in the pattern to the background offset for all four samples. Taking into account that the photocatalytic performance scales with accessible surface area, the specific surface area of all samples was determined by Brunauer–Emmett–Teller (BET) method from a 6-point nitrogen adsorption isotherm at 77 K (Gemini 2360, Shimadzu). The BET-equivalent particle diameter of the primary particles was calculated from formula (3) presented below which assumes spherical geometry: is the theoretical density of our cubic calcium bismuth oxide which was estimated from the Rietveld refinement (9.59 g/cm3 ) and S the specific surface area of as-synthesized calcium bismuth mixed oxide. dBET =
6 CaBi2 O4 S
(3)
Subsequently, the morphology of as-synthesized powders was investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) in order to detect, characterize and monitor changes in surface morphology, homogeneity, size of primary particles, crystallinity and state of agglomeration. TEM was performed with a Philips CM30, operating at 300 kV and equipped with an EDX system from EDAX. The SEM investigations were performed with a Hitachi S4800 (with cold FEG). The microscope was equipped with in-lense SE- and ExB-detectors and with a bright
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Fig. 2. Relative degree of crystallinity (RDOC) for powders F10, F15 and F18 vs. samples (A) F20 and (B) F20T which was additionally treated in 500 ◦ C during 1 h.
field STEM-detector. For both, TEM and SEM observations, the nanoparticles were dispersed in isopropanol (purity ≥ 99.5%, Fluka) and deposited on the formvar/carbon coated copper grids (Plano GmbH, Germany). 2.2.2. X-ray absorption spectroscopy Soft X-ray absorption spectra at the oxygen K-shell edge were recorded at beamline 9.3.2 at the Advanced Light Source in Berkeley, California in the energy range from 500 to 580 eV in steps of 0.1 eV and recording time of 1 s/step at an energy resolution of E/E ∼ 1/5000. The powder samples were roll-pressed into thin indium metal foil and then placed on a sample holder which was transferred into the ultra high vacuum recipient with a base pressure of 10E−10 Torr. The spectra were normalized to unity at 550 eV and deconvoluted using the WinXAS program. 2.3. Photocatalytic activity Evaluation of photocatalytic performance of calcium bismuth oxide was based on methylene blue (MB) degradation in aqueous medium. MB degradation was carried out with 0.15 g of the catalyst powder (F10, F15, F18, and F20) suspended in a solution of MB (5 × 10−5 M, Vtot = 100 ml). Prior to the irradiation, the reaction was conducted in the darkness for 30 min to establish equilibrium of MB adsorption/photolysis. The optical system for the photocatalytic reaction included a 300 W Xe arc lamp equipped with a cutoff filter providing the visible light (>420 nm) and a glass water filter to prevent IR irradiation. The degradation process of MB was investigated by comparing the UV–Vis absorbance spectra (UV-2500 spectrometer, Shimadzu) of residual MB with initial MB concentration. The concentration of residual MB was checked every 10 min. 3. Results and discussion 3.1. Structure characterization The calcium bismuth oxide powders were produced in acetylene flame, with precursor flow rates varied from 10 to 20 ml/min. Powders produced with lower flow rates were milky and fluffy (F10 and F15) while those produced with higher flow rates became more yellowish and compact (F18 and F20). The color and physical appearance of as-synthesized nanopowders reflected their degree of crystallinity and an order of magnitude of the specific surface area. The relative degree of crystallinity (RDOC) was estimated from XRD patterns based on the ratio between peak intensity, peak width at half-maximum and normalized background [18]. Not surprisingly, the milky and fluffy powders showed lower degree of crystallinity (RDOC), suggesting that these materials are
less ordered than samples produced with higher flow rates. As it is shown in Fig. 2, their RDOC are of the order of 75% vs. sample F20, whereas vs. sample F20T (additionally annealed in 500 ◦ C during 2 h) reaches only 30%. This drop in RDOC is related to lower combustion enthalpy by reduced solvent content in the flame. Going further, lower energy density in the flame leads to the formation of surface crystalline defects which is also associated with high specific surface area (SSA). In case of nanoparticles with sizes of 10–20 nm, the proportion of surface atoms (surface defects) is higher than in the bulk and it may lead to the disturbance in 3D structural order and affects the crystallinity. This is the case of samples F10 and F15 which have lower RDOC but exhibit larger specific surface areas, in range of 47 m2 /g. For samples F18 and F20 the SSA slightly decreases to 41 m2 /g as a direct consequence of higher precursor feed rate into the flame. It has been reported that an increase in precursor concentration in the flame enhances the growth of particles by their surface growth, thus, larger particles are formed [19]. On the other hand, the surface growth reduces the defect concentration which is concomitant with increase in crystallinity. The relation between precursor flow rate and SSA or particle diameters calculated from BET are summarized in Table 1. At this point, it should be emphasized that although precursor flows of 18 or 20 ml/min lead to the formation of slightly bigger primary particles than flow of 10 ml/min, the F18 and F20 powders represent an optimal balance between specific surface area and crystallinity. This optimum manifests in the enhanced photocatalytic activity which will be discussed in details further. The particle size of 15 nm, calculated from Brunauer–Emmett–Teller (BET) isotherms, showed good agreement with the crystallite size calculated from the peak width at half-maximum of the calcium bismuth oxide diffraction peaks which revealed the presence of crystallites in the range of 10–12 nm (Table 1). The BET-equivalent of the particle diameter is always slightly larger than the calculated from Scherrer’s formula crystallite size, since the surface defects, amorphous content or porosity are not accounted for in X-ray. The convergence in particles and crystallites size, suggests that the nanoparticles are virtually single-crystalline. The average particle diameters derived from BET and XRD of as-synthesized F10, F15, F18, and F20 Table 1 Particle sizes based on BET and XRD measurements. Sample
F10 F15 F18 F20
Precursor flow rate (ml/min)
(g/h)
10 15 18 20
72 108 130 145
SSA (m2 /g)
dBET (nm)
dXRD (nm)
= 9.59 g/cm3 47.6 47.2 43.7 41.5
13.1 13.2 14.3 15.1
9.5 10.0 11.2 11.7
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Fig. 3. SEM (A) and TEM (B and C) images of calcium bismuth oxide nanoparticles produced with flow rate of 20 ml/min.
powders are shown in Table 1. It should be noted that samples F10 and F15 exhibit almost similar structural features, thus we will focus our further investigations on two external samples: F10 and F20. In addition to the primary particle size, the particle size distribution and morphology are important measures of the quality of a powder and often determine the usefulness of the powder in subsequent steps and applications. Therefore, scanning and transmission electron microscopies (SEM and TEM) were used to size particles, to investigate their microstructure, to quantify phase composition and distribution, and together with energydispersion spectroscopy (EDX), to identify the elements present in a sample. Computer supported analysis of SEM and TEM images revealed small particles (less than 20 nm) showing a strong tendency to form soft agglomerates, however with visible grain boundaries and crystalline lattices, which is typically indicative of good quantum efficiency (Fig. 3). TEM images confirmed also the single-crystalline nature of samples showing the crystalline patterns in range of 15 nm (Fig. 3C). It is worth noting that the particle size distribution, which is based on a set of 500 counted particle diameters from TEM measurements, is relatively narrow and characterized by a small geometric standard deviation of g = 1.27 (Fig. 4). At this point, it should be mentioned that powders produced by salt-based flame techniques are typically characterized by rather broad ( g ∼ 1.5) and skewed particle size distributions with a long right tail indicating non-negligible mass fraction of larger particles [20]. It is equally important to mention that coupled with TEM, EDX measurements confirmed the expected stoichiometry of CaBi2 O4−x
Fig. 5. Experimental ( ) and calculated (—) pattern of freshly synthesized nanoparticles of calcium bismuth mixed oxide (F20).
showing Ca and Bi contents in the measured F20 sample of 17.3 and 40.5%, respectively. Calcium bismuth mixed oxide was reported in the monoclinic crystalline system with cell parameters: a = 1.4002 nm, b = 1.1596 nm, c = 1.2198 nm, and ˇ = 101.541◦ [5]. However, calcium bismuth mixed oxide produced by FSS, crystallizes in cubic structure (space group Fm − 3m, lattice parameter a = 0.5530 nm, reliability factors: 2 = 1.91, RB = 2.96, and RF = 1.986) (Fig. 5), which is probably related to the strong quenching effect in the flame. In the present case, the precursor conversion takes place in the early stage flame with temperature of approx. 3000 ◦ C. In this part of the
Fig. 4. Particles size distribution of F20 sample based on set of 500 particle sizes from TEM analysis.
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Fig. 6. MB degradation on calcium bismuth powders (䊉 F10 and F20) and in the absence of photocatalyst ().
Fig. 7. Diffuse reflection spectra of F10 and F20 calcium bismuth mixed oxide samples as-recorded and transformed to the absorption spectra by KM theory.
flame, the particles have an initial speed in the range of 150 m/s and then they are rapidly cooled down to approx. 700 ◦ C, which gives the quenching rate of about 500,000 K/s [14–16]. In this regard, the cation diffusion time is very short and therefore the material formed has no time to relax and the final structure is frozen in cubic crystal system. However, when additional slow heat treatment is applied the structure relaxes to the thermodynamically preferred, monoclinic structure and additional Bragg reflections appear in the XRD pattern.
F10 (Fig. 6 and Table 2). Sample F20 has slightly lower surface area than F10 but significantly higher crystallinity. In consequence, the optimal ratio between these two factors (SSA and RDOC) allows an enhancement in the MB degradation rate. In Table 2, the MB degradation rates are related to the SSA and RDOC. The degradation of MB is not a simple one-hole oxidation process but its degradation kinetics is limited by an adsorption of the dye itself on the photocatalyst, then by a formation of intermediates which re-adsord/desorb from photocatalyst surface. The MB degradation pathway is a multi-step process requiring diffusion of the photogenerated charges to the surface, which then have to traverse the surface of the photocatalyst without any recombination in order to perform proper photocatalytic action. Therefore, the prerequisite for improved photocatalytic activity is not only high SSA or excellent crystallinity, separately but a combination of both these factors [18,23]. We managed to achieve the optimum with sample F20 which represents an excellent compromise between still high surface area of 41.5 m2 /g and acceptable degree of crystallinity (70%). Such an optimization of synthesis conditions allowed us to improve photocatalytic performance of calcium bismuth mixed oxide by a factor of 6 in comparison to calcium bismuth oxide with low surface area, reported elsewhere [5]. Photocatalytic performance is also determined by the stability of the catalyst within the time. At this point we should mention that we tried to perform the hydrogen evolution in the presence of sacrificial reagents such as Pt and MeOH. This test took more than 30 h and although no evolution of H2 was observed under visible light, the catalyst was stable and we could generate the H2 under UV irradiation, immediately after the previous test. This experiment showed that the CB is too low to allow the hydrogen generation under visible illumination, but the calcium bismuth mixed oxide neither dissolved nor photo-corroded under visible or UV light.
3.2. Photocatalytic performance Photocatalytic decomposition of organic species is a surface driven reaction. Therefore, our powders possess a surface area that is well suited for environmental applications. Although the developed surface area is also associated with surface defects and hence, lower crystallinity, FSS offers a possibility to optimize these two conflicting parameters which influence directly photocatalytic activity. On the other hand, it is known that defects produced by high temperature can extend the optical absorbance and hence, the activity range of a photocatalyst. These defects are the oxygen vacancies and can be easily created in the hot part of a flame. As it was reported previously, in case of TiO2 such defects play a crucial role in the improvement of its photocalatytic performance [21]. Despite the fact that the photocatalytic activity due to defects is much smaller than the activity coming from intrinsic absorption, it is definitely an important issue in view of environmental remediation. MB is a textile dye, which can be easily found in wastewater discharges, and although MB can self-photolyse (Fig. 6) [22], its degradation rate is low under solar irradiation: only 20% decomposed within the first hour of the photolysis under irradiation of > 420 nm. On the contrary, almost 100% is decomposed within the same time in the presence of calcium bismuth oxide, revealing its high photocatalytic performance under visible light, despite the fact that its absorption spectrum shows an effective intrinsic absorption only to 450 nm (Fig. 7). In this regard, we claim that the extension of its photocatalytic activity is due to developed surface and oxygen vacancies which are formed in the flame. MB is oxidized on CaBi2 O4−x surface mainly via direct photocatalytic effect enhanced by the created substoichiometry in oxygen but possibly also by MB–dye sensitization [22]. It means that this decomposition is driven by light and the surface properties of a photocatalyst. Indeed, in the present case there is a clear difference in the photocatalytic activity for MB decomposition between powder F20 and
3.3. Spectroscopic characterization In order to understand the difference in photocatalytic behavior of samples F10 and F20 we probe the valence band by soft X-ray absorption spectroscopy at the anion absorption edge. This technique is particularly sensitive to orbital and spin effects. Fig. 8 shows the near edge X-ray absorption fine structure (NEXAFS) spectra of samples F10 and F20. Of particular relevance are the pre-edge peaks between 520 and 530 eV right before the upper Hubbard band. The first peak at 523 eV is next to the Fermi energy
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Table 2 Comparison of photocatalytic activity of calcium bismuth mixed oxides produced by FSS and soft chemistry methods. MB degradation rate (mol/(h × g))a
Photocatalyst
Synthesis
SSA (m2 /g)
CaBi2 O4−x F20 F10
FSS – 20 ml/min FSS – 10 ml/min
41.5 47.6
70 30
63 40
CaBi2 O4 [5]
Soft chemistry
0.6
100
10
a
RDOC (%)
The MB degradation rate is based on the reaction rate at the initial 30 min with 420-nm cutoff filter.
Fig. 8. Oxygen (1s) NEXAFS spectra of F10 and F20 samples.
at about 520 eV and has eg symmetry. This peak corresponds to doped holes and accounts for electric conductivity in the samples. The following peaks at 528 and 530 eV have t2g and eg symmetries and may block conductivity, depending on the molecular environment. These three peaks derive from hybridization between the O(2p) and the Bi(5d) bands and are sensitive to the superexchange between oxygen and metal ions. Comparison of both spectra shows that sample F10 has negligible intensity at 523 eV, i.e. virtually no existing holes. The situation is entirely different for F20, which has a pronounced intensity in this energy range, indicative to a high concentration of doped holes. The observed difference in concentration of doped holes between samples F10 and F20 emerges from difference in oxygen substoichiometry which is related to precursor flow rate. Hence, based on the precursor flow rate we can estimate not only the particle size, SSA or crystallinity but also the conductivity and extent of the oxygen deficiency. All these parameters play a crucial role in photocatalytic performance.
exploring of materials in view of effective environmental remediation. Finally, it should be emphasized that FSS allows control of the particles size very precisely. Such a precise control of particles size subsequently, allows to influence not only many physicals properties of a photocatalyst but also to reduce the extent of charge recombination effect. In this regard, FSS appears as a powerful tool to design and synthesize efficient photocatalytic systems.
Acknowledgments We gratefully acknowledge Dr. Andri Vital and Dr. Kranthi Akurati who initiated this project, part of which was supported by the Empa Board of Directors’ Fund. Financial support for the exchange visit of Dr. Renata Solarska at NIMS by the Swiss National Science Foundation (SNF Project IZAJZO-123331/1) and the Japan Society for Promotion of Science (JSPS/RCI-2/08035, ID no. RC 20830001) is also acknowledged.
4. Conclusions Photocatalytic performance of calcium bismuth oxide for environmental remediation was improved by a new preparation route going through evaporation and decomposition of precursor in a flame, followed by nucleation, condensation, coagulation and agglomeration steps, which lead to the formation of fine photocatalyst particles with nominal size of 15 nm. Decrease in particle size and flame-formed oxygen deficiencies enhanced the surface properties and extend the absorption spectrum towards the visible range. As a consequence, the photocatalytic activity of MB degradation of our flame-made calcium bismuth oxide was enhanced by a factor of 6. The soft X-ray absorption spectroscopy measurements showed a significant difference in concentration of doped holes between samples produced with different precursor flow rates which allows the synthesis conditions to be related to the conductivity, thus to the oxygen substoichiometry. These observations are very important for further
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