Hydrothermal Synthesis and in Situ Powder X-ray Diffraction Study of

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Hydrothermal synthesis and in situ powder X-ray diffraction study of bismuth substituted ceria nanoparticles Kasper Houlberg, Espen D. Bojesen, Christoffer Tyrsted, Aref Mamakhel, xueqin wang, Ren Su, Flemming Besenbacher, and Bo B. Iversen Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b00678 • Publication Date (Web): 16 Jun 2015 Downloaded from http://pubs.acs.org on June 24, 2015

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Crystal Growth & Design

Hydrothermal synthesis and in situ powder Xray diffraction study of bismuth substituted ceria nanoparticles

Kasper Houlberg,1‡ Espen D.Bøjesen,1‡ Christoffer Tyrsted,1† Aref Mamakhel,1 Xueqin Wang,2 Ren Su,2 Flemming Besenbacher,2 and Bo B. Iversen1* 1

Center for Materials Crystallography, Department of Chemistry and iNANO, Aarhus

University, Langelandsgade 140, 8000 Aarhus, Denmark 2

Interdisciplinary Nanoscience Center (iNANO), University of Aarhus, DK-8000

Aarhus C, Denmark *Corresponding author: [email protected]

KEYWORDS In situ PXRD, Hydrothermal synthesis, Flow synthesis, Bismuth substituted Ceria, Photocatalysis

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ABSTRACT

Ceria is a widely used material for multiple purposes e.g. as catalysts, sensors and solid electrolytes. Substitution of Ce with Bi in nanocrystalline ceria facilitates an even broader field of applications. Here we present an easy-reproducible autoclave based hydrothermal route for preparing phase-pure nanocrystalline bismuthsubstituted ceria as well as a continuous flow synthesis method that is suitable for large-scale production. The produced materials were characterized using a wide range of techniques to understand the synthesis dependent changes in crystallographic structure, crystallite size, optical properties, type and number of surface adsorbed groups and overall nanostructure. Furthermore, photodecomposition of a model dye compound (Rhodamine B) was performed to investigate the photocatalytic properties of the as-synthesized materials. To investigate the formation mechanisms in situ powder X-ray diffraction data were measured, revealing that miniscule crystallites of BixCe1-xO2-x/2 formed already during mixing of the precursor solutions via coprecipitation at room temperature. Pristine samples with tunable crystallite size and band gaps were obtained, thus demonstrating that the method is viable for tailoring of bismuth-doped ceria with controllable physical properties.

INTRODUCTION

Metal oxides exhibiting fluorite like structure have attracted great attention as a result of the remarkable properties they exhibit within the fields of solid electrolytes, oxygen sensors, oxygen-storage systems, UV blocking and (photo)-catalysis.1-9 CeO2 is part of this family of compounds and doped ceria is being investigated as an 2 ACS Paragon Plus Environment

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alternative to yttria-stabilized zirconia, which is one of the most used solid electrolytes in solid oxide fuel cells (SOFCs). It is well established that aliovalent metal substitution with ions of lower valence commonly leads to increased ionic conductivity in compounds such as yttria.10 A high ionic conductivity is the key aspect for high performing solid electrolyte materials in SOFCs. In the case of bismuth substituted ceria, the improvement of ionic conductivity compared with the pure ceria has been attributed to the unsymmetrical coordination of Bi3+ in the compound and the introduction of a higher degree of oxygen ion vacancies.11-15 A different field of applications in which doped, nanostructured or otherwise band gap engineered ceria is of interest is within the field of photocatalysis. Modifications of the band structure are necessary to facilitate optimal performance in the optical region.6, 16,

17

Additionally, engineering at the nanoscale is expected to further alter

and possibly enhance the photocatalytic performance compared with that of the bulk counterparts, by shortening the charge transfer path.18,

19

Thus, nanosized bismuth

substituted ceria possibly could be a candidate for photocatalysis applications. Various methods of producing a range of substituted ceria compounds have been used previously; including hydrothermal and precipitation based methods.11,

14, 16, 20, 21

Thorough mixing of individual elemental components is crucial for obtaining proper aliovalent substitutions in solid solutions and thus solution based methods commonly are observed to be very effective.1, 2, 22 Furthermore, in contrast to classic solid state syntheses methods, solution based methods require comparably lower temperatures and therefore less energy, while at the same time producing high quality nanocrystalline samples. For large scale purposes, continuous flow synthesis is considered as an attractive alternative to autoclave based batch type hydrothermal syntheses that are commonly 3 ACS Paragon Plus Environment


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used on laboratory scale. Large quantities can be produced reasonably fast by continuous flow synthesis while retaining control over morphology, size, and phase purity. The technology is, at the present, at a state where various start-up companies are trying to commercialize it, and thus upscale production volumes to 1000 tons per annum.23 Sub- and supercritical flow synthesis has been used to produce a wide variety of inorganic materials including CeO2, ZrO2, TiO2, α-Fe2O3, Fe3O4, AlOOH, Al5(Y+Yb)3O12, Co3O4, LiCoO2, ZnO, BaO6·Fe2O3 and BixZr1-xO2 etc.24-29 Conducting flow synthesis at supercritical conditions has proven as a viable method for producing inorganic materials with very narrow crystallite size distributions due to the instantaneous nucleation of nanoparticles upon the dramatic changes observed in the solvent properties of water at supercritical conditions.25 The number of adjustable parameters in the hydrothermal synthesis method is vast. The synthesis regulating parameters include temperature, pressure, precursor composition and concentration, pH, and reaction time providing a variety of “chemical knobs” that can be adjusted to alter the synthesis pathway and consequently the product. To unravel the mechanisms governing hydrothermal and supercritical synthesis, in situ X-ray powder diffraction, at times combined with other techniques such as small angle X-ray scattering and total scattering, has proven to be a most valuable tool.26,

30-40

Achieving an

understanding of the formation and growth of various synthesized materials is of great importance since the properties of nanomaterials are highly dependent on size, crystallinity and morphology. Here we present the first in situ powder X-ray diffraction (PXRD) study on the hydrothermal synthesis of the BixCe1-xO2-x/2 system. The study focuses on acquiring a greater understanding of the formation and growth of BixCe1-xO2-x/2. We extract the information based on a combination of ex situ and in situ studies and additionally a 4 ACS Paragon Plus Environment

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range of continuous flow syntheses are conducted to demonstrate the industrial feasibility. Finally, selected samples of the prepared nanomaterials have been tested for their photocatalytic activity using a model dye compound as probe molecule.

EXPERIMENTAL PROCEDURES

Autoclave synthesis Samples of BixCe1-xO2-x/2 (x = 0.0, 0.1, 0.2, 0.4, 0.6, 0.7, 0.8 and 1.0) were prepared by hydrothermal synthesis in Teflon-lined autoclaves. Metal salts of Bi(NO3)3·5H2O (Sigma Aldrich, purity > 98%) and (NH4)2Ce(NO3)6 (Sigma Aldrich, purity > 98%) were dissolved in deionized (DI) water and mixed by magnetic stirring before drop wise addition of 8 M aqueous solution of NaOH (Sigma Aldrich, purity > 98%) while stirring. The final metal concentration ([Ce4+ + Bi3+]) was 0.4 M and the final alkali concentration was 2 M. The addition of NaOH transformed the colorless solutions into yellow gelatinous suspension. 8 mL of each suspension was loaded into 16 mL Teflon-lined stainless steel autoclaves. The autoclaves were placed in a preheated furnace at 220 °C for 20 hours (without stirring). The products were spun down by centrifugation and washed twice with deionized (DI) water and once with ethanol. Subsequently the powders were dried in a vacuum oven at room temperature for 24 h.

Continuous flow synthesis A selected range of samples within the BixCe1-xO2-x/2 compositional range were prepared by continuous flow synthesis using a setup very similar to the one described 5 ACS Paragon Plus Environment


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by Adschiri et al.24,

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Near-supercritical water was used as solvent and no in-line

filters were used. The precursor solution was prepared similarly to the precursors used for the autoclave syntheses. Bi(NO3)3·5H2O was dissolved completely in a small amount of water by addition of nitric acid under agitation with a magnetic stirrer. Subsequently the solution was neutralized using an aqueous solution of NaOH. (NH4)2Ce(NO3)6 was added to the solution and the mixture was once more neutralized after dissolution of the metal salt. The final metal salt concentration ([Ce4+ + Bi3+]) was 0.1 M, the lower concentration was chosen to prevent clogging in the flow reactor. The internal reactor conditions were set at 350 °C and 375 °C respectively at 250 bars (only pure CeO2 was synthesized at 375 °C). The reactor residence time (treactor) was calculated to be on the order of 15 seconds using the formula: = /( / ) = 15 where Vreactor is the volume of the reactor (7.3 mL), ρT is the density of water at synthesis temperature and pressure (0.62545 g/cm3 at 350 °C and 25 MPa) and F is the flow rate (18 ml/min).27

Characterization

Ex situ Powder X-ray diffraction and Rietveld refinement The nanoparticles synthesized in autoclaves and by the continuous flow process were characterized by Powder X-ray diffraction (PXRD). The PXRD data were collected on a Rigaku SmartLab diffractometer equipped with a rotating Cu Kα rotating anode, parallel beam optics, Nickel filter, and a D/tex Ultra 1D detector. Powder patterns were analyzed by Rietveld refinements using the Fullprof software suite,41 for 6 ACS Paragon Plus Environment

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refinement details see Table SI1 in the supporting information. The PXRD pattern of a LaB6 (NIST SRM 660s) standard collected on the same diffractometer was Le Bail fitted to extract the instrumental contribution to the peak profiles. The volume weighted crystallite sizes were calculated using Scherrer’s formula:

(2) =

cos ()

where B is the integral breadth (IB) corrected for instrumental broadening, L is the crystallite size, λ is the X-ray wavelength used, θ the scattering angle of the given reflection and K is a constant close to unity (1 is used here).42, 43

In situ PXRD In situ PXRD experiments were performed at beamline I71144 at MAX-lab (Lund, Sweden) using a capillary based setup described by Becker et al.39 Precursors were prepared in the same way as for the ex situ autoclave syntheses. However, as an alternative to loading suspensions into autoclaves, the gels were loaded into a sapphire capillary having an inner diameter of 0.7 mm. The capillary was subsequently pressurized to 250 bar by a HPLC pump. Synthesis was initiated by directing a hot air jet onto the capillary thereby reaching the set temperature within 20 seconds (see supporting information Figure SI11 for heating profiles). The capillary was penetrated by a monochromatic X-ray beam (λ = 0.9912 (5) Å). The 2D PXRD data were recorded with 5 seconds intervals on an Oxford Diffraction Titan CCD detector at a sample to detector distance of 88.94 mm. The 2D data were integrated using Fit2D45 and analyzed similarly to the ex situ PXRD by Rietveld refinement using the Fullprof Software suite.



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FTIR Fourier transformed infrared (FTIR) spectra were recorded on a NICOLET 380 spectrophotometer equipped with a Smart Orbit, and data were measured in the 4000400 cm-1 range.

Optical diffuse reflectance Optical diffuse reflectance spectra (DRS) were measured at room temperature using a Shimadzu UV-3600 UV-vis-NIR spectrophotometer operated in the 220-2000 nm region. The diffuse reflectance spectra were used to estimate the band-gap of the materials by converting reflectance curves to absorption curves according to the Kubelka-Munk function α/S = (1-R)2(2R)-1, where R is the reflectance and α and S are the absorption and scattering coefficients, respectively.46, 47

Electron microscopy Transmission electron microscopy (TEM) images were obtained on a TALOS F200A with a TWIN lens system, X-FEG electron source, Ceta 16M Camera. Spatially resolved elemental analysis, with a spatial resolution better than 2 nm, was obtained using the same TALOS microscope, equipped with a Super-X EDS Detector, in STEM mode. Exposures times of five minutes where used to create elemental distribution maps with satisfying counting statistics, while minimizing problems due to beam damage and specimen drift. STEM pictures were obtained using a High angle 8 ACS Paragon Plus Environment

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annular dark field detector (HAADF). Particle sizes obtained from TEM images were estimated using the software FIJI (v.1.49q) .48

Photo-induced de-colorization of Rhodamine B The photo-induced de-colorization of Rhodamine B (RhB) was carried out under ambient conditions to evaluate the photocatalytic performance of the catalysts. All samples were UV irradiated for 2 hours prior to photocatalytic testing ensuring removal of surface adsorbed organic molecules prior to the measurements. Catalysts with a loading of 0.5 g/L were added to 50 mL RhB solutions with a concentration of 10 µM or 25 µM respectively. Throughout, DI water was used as solvent. The suspensions were kept in the dark for 1 hour to establish adsorption equilibrium. The photocatalytic reactions were initiated by continuously irradiating the nanoparticle – Rh B suspensions using a LED UV light source (365 nm diode, Optimax 365, Spectroline). Aliquots of 1.2 mL suspension were collected at given time intervals and were immediately centrifuged and analyzed using an UV–VIS spectrometer (UV1800, Shimadzu). The extinction coefﬁcient of RhB at 554 nm was measured to quantify the concentration of RhB using Lambert-Beer’s Law.

RESULTS AND DISCUSSION

Ex situ structural and microstructural study In Figure 1 PXRD patterns, unit cell parameters and crystallite sizes of the synthesized BixCe1-xO2-x/2 samples are shown. Pristine solid solutions of BixCe1-xO2x/2,

exhibiting the cubic fluorite-type phase (space group Fm-3m), were obtained via 9 ACS Paragon Plus Environment


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autoclave syntheses in the compositional range of x ≤ 0.6. A secondary monoclinic αBi2O3 phase (space group P21/ c) formed at compositional values greater than 0.7. A peak shift toward smaller angles was observed upon Bi3+ substitution of Ce4+, owing to an expansion of the unit cell caused by the larger size of Bi3+ ions compared with Ce4+ (radii of 1.31Å and 1.11Å respectively).49 The solid solution nature was indicated by a linear relationship between unit cell dimension and composition in correspondence with Vegard’s law as shown in Figure 1C.50,

51

The elemental

compositions of selected samples was investigated by elemental analysis (see supporting information) and found to concur well with the chosen fixed occupancies used in the Rietveld refinements (Figure 1E). The Rietveld refinements were based on a structural model derived from the structure presented in the International Crystal Structure Database (ICSD) of pure CeO2 (ICSD ID: 72155). The Bi3+ ion is assumed to occupy the same site as Ce4+. The crystallites were assumed to be spherical, and refined crystallite diameters were in the range of 3.5-5 nm. Refinement results are summarized in Table SI1 in the supporting information. The crystallite sizes of the single phase powders decreased with increasing Bi3+ content (Figure 1C), suggesting an increasing incompatibility of incorporating Bi3+ ions into the CeO2 structure at high Bi3+ levels. PXRD patterns of BixCe1-xO2-x/2 nanoparticles synthesized in the continuous flow reactor at sub-critical conditions displayed the same trends of decreasing crystallite size and increasing unit cell size with increasing amounts of substituted Bi3+ (Figure 1B, 1D, and 1F). With the exception of the pure CeO2 samples synthesized at 375 °C (supercritical conditions), the crystallite sizes of the products synthesized in the continuous flow reactor were in the range of 2.5-4.5 nm. The comparatively smaller crystallites produced via the flow syntheses were presumably due to the significantly shorter reaction time (seconds) in the flow process


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compared with the batch autoclave process (hours). Results of the Rietveld refinements are summarized in the supporting information. The unit cell dimensions are in correspondence with Vegard’s law, as demonstrated on Figure 1D, where they are plotted alongside the crystallite sizes. In contrast to the autoclave syntheses, it was possible to produce phase-pure BixCe1-xO2-x/2 with a compositional value of 0.7 by the continuous flow synthesis. At compositional values of 0.8 and above a secondary bismuth phase formed, which is different from the α-Bi2O3 phase formed in the autoclave based syntheses (Figure 1B). The secondary impurity phases consisted mainly of a tetragonal Bi2O3 phase (space group P-4b2). It thus appears that the higher temperatures, heating rates and pressures as well as lower concentrations in the flow syntheses compared with the autoclave process facilitates the incorporation of Bi3+ into the fluorite type structure of the solid solution BixCe1-xO2-x/2. Furthermore, the rapid heating achievable with the flow method allowed for nanocrystalline powders to be produced on a significantly shorter time scale (< 1 minute) compared with the autoclave syntheses (20 h).



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Figure 1. PXRD patterns for samples of (A) autoclave synthesized BixCe1-xO2-x/2 and (B) continuous flow synthesized BixCe1-xO2-x/2. Unit cell parameters and crystallite sizes for phase-pure cubic-stabilized samples of (C) autoclave synthesized BixCe1-xO2x/2

and (D) continuous flow synthesized BixCe1-xO2-x/2 as a function of composition, x.

Representative Rietveld refinements of (E) autoclave synthesized Bi0.4Ce0.6O1.8 and (F) continuous flow synthesized Bi0.4Ce0.6O1.8.


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TEM micrographs did not provide any indications of anisotropic morphologies (see Figure 2); therefore the assumption of spherical crystallites applied in the refinements of PXRD data was considered to be valid. Cloudy areas with low diffraction contrast are visible in the autoclave synthesized Bi0.4Ce0.6O1.8 sample, possibly indicating the existence of an amorphous phase. In the flow synthesized samples similar regions were not observed, this might be due to the higher temperatures employed in the flow process. The TEM images support the notion that crystallite sizes were indeed in the order of a few nanometers. The HR-TEM pictures show that both the autoclave and flow synthesized samples consists of crystalline isotropic shaped particles. The particles are however lacking definite sharp facets, supporting the notion of possible disorder at the crystallite surfaces. The non-phase-pure Bi0.8Ce0.2O1.6 sample showed large platelet like Bi rich precipitates, most likely Bi2O3 impurities, as indicated by the PXRD patterns. STEM-EDX maps on the impure sample, shown in Figure 2A, highlight the compositional difference between the small spherical particles and the large plate like aggregates. Similar STEM-EDX investigations of the flow synthesized Bi0.4Ce0.6O1.8 sample revealed a homogeneous distribution of Bi and Ce.



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Figure 2. The pictures in row A) show, at the left; a STEM picture of the impure “Bi0.8Ce0.2O1.6” sample, in the middle; a spatially resolved elemental distribution map based on EDX measurements showing the distribution of Bi in the sample, to the right; a similar map as the middle one showing the Ce distribution. B) The same three types of pictures as in A), but for the flow synthesized Bi0.4Ce0.6O1.8 sample. C) shows a TEM micrograph of the flow synthesized Bi0.4Ce0.6O1.8 sample and D) a HR-TEM of the same sample. E) Shows a TEM micrograph of the Autoclave synthesized Bi0.2Ce0.8O1.9 sample, F) shows a HR-TEM picture of the same sample.

In situ PXRD Incorporation of Bi3+ ions into the fluorite-like structure and general growth of BixCe1-xO2-x/2 nanoparticles was studied using in situ PXRD. In situ synthesis experiments of phase-pure samples revealed that the cubic structure is formed directly without going through other crystalline intermediates. Generally, it appeared that the phase-pure crystallites were formed immediately upon heating. However, comparison of the diffractograms obtained prior to heating (0 minutes) with the final diffractogram (15 minutes) revealed that a significant amount of the final product also forms prior to any application of heat (see Figure 3). It is, therefore, evident that the solid solution for both systems must form as a consequence of time dependent coprecipitation. The in situ PXRD measurements consequently give insight into the crystallite growth but not the initial reaction between the gelatinous Bi- and Cespecies. The formation of the solid solution compounds during aging was moreover indicated by visual inspection as the suspensions turned increasingly yellow during magnetic agitation (~½ hour) prior to heating. These observations open up for the,



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from an industrial up-scaling point of view, intriguing possibility of room-temperature synthesis of this system. We chose to investigate this prospect further; with positive results (see supporting information). It therefore appears that heating is not a key aspect in the formation of the stabilized crystalline structure, yet it influences other material characteristics such as crystallite size, most likely crystallinity and accordingly the physical properties of the product. Furthermore it seems that at high Bi concentrations high temperatures do actually facilitate the formation of the highly substituted product.


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Figure 3. (A) Time-resolved diffractograms of Bi0.4Ce0.6O1.8. Start and end diffractograms for the synthesis of (B) Bi0.4Ce0.6O1.8 and (C) “Bi0.8Ce0.2O1.6”.

Based on Rietveld refinements of the collected in situ powder X-ray diffraction patterns it was determined that the unit cell decreases slightly (less than 0.8 Å3) during the first minutes of heating followed by a practically constant period. This is similar to results from previous investigations of pristine ceria growth.31 The compositional dependence of the lattice parameters based on the values obtained in the last collected diffraction pattern follow Vegard’s law (see Figure 4). This once again demonstrates the solid solution nature of the phase-pure compounds. Furthermore, the experiments revealed a strong temperature dependence of the observed crystallite size evolution. Higher temperatures led to formation of bigger crystallites. No relation between the final crystallite sizes and composition could be established in the in situ experiments. This contradicts the result from the ex situ and continuous flow syntheses where a strong correlation was observed. Nonetheless, the relative change in crystallite size was found to decrease with increasing amounts of substituted Bi3+ (Figure 4A and 4B). Thus it appears that the heat induced growth is hindered either due to kinetic or thermodynamic reasons. The lack of difference in the final size may be related to the effect of different heating rates, concentrations, duration and reaction environment compared with the autoclave syntheses and continuous flow syntheses. Previous studies on the synthesis of various ferrites have shown that the details during initial mixing in synthesis were amorphous or semicrystalline precursor cluster-states are involved are of high significance to the evolution of both the average crystallite size and size distribution.52 The flow



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syntheses were conducted at significantly lower concentrations and thus the initial coprecipitation is most likely very different from the one observed for the in situ experiments. Similarly the autoclave experiments proceed for reactions times orders of magnitude longer and at low temperatures and heating rates, probably supporting more steady state-like conditions for the incorporation of Bi and crystallite growth. In the time-resolved refinements the scale factor stays constant after a short increase upon heating, whereas the average crystallite volume keeps increasing. The scale factor is directly proportional to the total volume of crystalline material, thus the observed trend implies that larger crystallites grow by feeding on smaller crystallites (Ostwald ripening). Additional results may be found in supporting information.

Figure 4: In situ experiments at 320 °C. (A) Crystallite growth curves for the synthesis of BixCe1-xO2-x/2. The initial crystallite sizes are subtracted from the absolute sizes.(B) Unit cell sizes and relative change in crystallite sizes obtained after 15 minutes in situ synthesis. (C) Change in unit cell volume compared to final unit cell 18 ACS Paragon Plus Environment

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volume. (D) Change in unit cell volume compared to final unit cell volume as function of crystallite size.

Optical and photocatalytic properties Both compositionally pure CeO2 and Bi2O3 powders exhibited a pale yellow colour, while powders with mixed compositions exhibited various colors ranging from brown to orange. It was therefore, expected that the materials would demonstrate difference in the magnitude of their band gaps. The DRS analysis revealed that indeed the optical band-gaps of pure CeO2 and Bi2O3 are on the order of 3.1 and 2.8 eV respectively; these values are in general accordance with literature.16, 53 Introduction of Bi3+ into the ceria structure leads to a drastic lowering of the band-gap to values of about 2.3 eV. DRS analysis and the calculated band-gaps are presented in Figure 5. No obvious relation between synthesis methods (flow vs. autoclave) and band-gap values could be established. However, a trend of higher absorbance in the UV region for the autoclave synthesized samples with increasing Bi content was observed.



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Figure 5. DRS of (A) autoclave synthesized and (B) continuous flow synthesized BixCe1-xO2-x/2 samples. For all analyses the Kubelka-Munk function was applied. (C) Calculated band gaps for all phase-pure samples of BixCe1-xO2-x/2. Estimated standard deviations are smaller than the individual marker sizes. The dotted line is intended as a guide to the eye.

FTIR spectra of representative samples were collected to examine possible surface adsorbed groups on the as-synthesized particles. The FTIR spectra revealed that the Bi0.1Ce0.9O1.95 and Bi0.4Ce0.6O1.8 samples presumably contained hydroxide, water,


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nitrate and other NOx groups adsorbed on the surface of the particles (Figure 6).

54

The density of surface groups appeared to be relatively high compared with commercial CeO2 sample. Calcination of Bi0.1Ce0.9O1.95 powder in air at various temperatures did minimize the amount of hydroxide and water surface groups (less absorption at 3200-3600 cm-1 and around 1630cm-1); however even calcination at 500

°C did not seem to change the amount of adsorbed NOx surface groups significantly. Removing NOx type surface groups thus might require prolonged calcination at higher temperatures in reducing atmosphere, which is known to induce oxygen surface defects.55, 56 A different, possibly more energy efficient approach would be avoiding the use of nitrates as precursors if clean particle surfaces were essential. The absorption maxima observed at < 800 cm-1 are mainly due to metal-oxygen vibration modes of the lattice.55,

56

The FTIR spectra of flow synthesized BixCe1-xO2-x/2

nanoparticles revealed the presence of similar amounts and identities of surface groups (Figure 6C). FTIR spectra of the pure CeO2, on the contrary did not suggest the presence of any significant amounts of attached surface groups. The calcination process was accompanied by a reduction in band gap for autoclave synthesized Bi0.2Ce0.8O1.9 to 2.20 eV (from c. 2.30 eV) and increased the crystallite size to c. 10.8 nm but did not remove all nitrate surface groups.



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Figure 6. FTIR spectra of BixCe1-xO2-x/2. (A) Samples of Bi0.1Ce0.9O1.95 and Bi0.4Ce0.6O1.8 compared with a commercial sample from SCF Technology. (B) assynthesized Bi0.1Ce0.9O2-δ , calcined Bi0.1Ce0.9O2-δ at 200°C for two hours, and further calcinated at 500°C for two hours. (C) FTIR spectra of representatives of continuous flow synthesized BixCe1-xO2-x/2.


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Successful photocatalytic degradation of RhB and methyl orange using bismuth substituted ceria has previously been reported.6,

16

Here, initial measurements of

photocatalytic degradation RhB under UV irradiation were unsuccessful (Figure 7A). The futile results may be explained by the surface inhibition of the NOx groups or a high concentration of surface defects leading to an increase in recombination kinetics of photogenerated electron-hole pairs. Initial calcination of the autoclave synthesized powders at low temperature (200 °C for 2h + 500 °C for 2h) did not improve the photocatalytic activities. However, prolonged calcination (4 h at 600 °C) led to an improvement of photocatalytic activity (Figure 7B and 7C). The improvement occurred despite the fact that contaminants still remained on the surface (see FTIR spectra Figure 6B). Nevertheless, as a consequence of prolonged annealing at high temperature, the crystallinity and crystallite size may have increased, which in turn will influence the photocatalytic performance of the materials. Such phenomena have been observed for other photocatalyst systems.6,

57

The calcinated phase-pure

Bi0.2Ce0.8O1.9 and mixed-phase “Bi0.8Ce0.2O1.6” were able to photocatalytically decolorize RhB. The decolorizaion efficiency was essentially identical for both samples. The need to calcine the products before gaining any photocatalytic activity is problematic if these particles are to be used as photocatlayst. It functions as a reminder of the fact that it is not only a question of producing a large quantity of nanocrystalline product having the right band gap; a clean particle surface is equally important. Furthermore, the results support the notion that problems with “dirty nano particle surfaces” are not only an issue when organic reagents and surfactants are involved; nitrates can similarly be a nuisance to get rid of and can cause a deterioration of the photo catalytic properties of functional oxide materials.



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Figure 7 UV-vis absorption spectra obtained at different irradiation intervals for photocatalytic decolorization of RhB by (A) Bi0.4Ce0.6O1.8 nanopowder and (B) calcined Bi0.2Ce0.8O1.9 (both synthesized in autoclaves). (C) Change in RhB concentration over irradiation time derived from the UV-vis absorption spectra for the two calcined samples that was synthesized in autoclaves.


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CONCLUSIONS

Phase-pure solid solutions of BixCe1-xO2-x/2 with the fluorite structure were obtained in the compositional range of 0.0 ≤ x ≤ 0.6 via hydrothermal autoclave synthesis and the product powders consisted of nanosized crystallites. By means of continuous hydrothermal flow synthesis it was possible to obtain phase-pure solid solutions in the somewhat broader compositional range of 0.0 ≤ x ≤ 0.7. The products demonstrated band-gaps between 2.3 eV and 3.1 eV, with the narrowest band-gap being in the range of 0.2 ≤ x ≤ 0.6. Via in situ PXRD experiments it was recognized that the cubic structure of BixCe1-xO2-x/2 was formed directly due to aging and that no other phases or intermediates were formed during heating. Temperature controls crystallite size but is not a pre-requisite for the actual formation of bismuth doped ceria. Additionally it was noted that as-prepared samples contained surface adsorbed NOx, water and hydroxide groups, most likely present at the particle surfaces. Annealing the samples for partial removal of the surface groups, renders the photocatlytical decomposition of Rh B possible. This demonstrates that it is not sufficient to produce nanocrystalline samples with an appropriate band-gap to ensure photocatalytic activity. Control of the surface adsorbed species and, possibly, also crystallinity is equally important in this respect.

ASSOCIATED CONTENT Supporting Information Supporting information contains Rietveld refinements details, elemental analysis, aging synthesis; changes caused by annealing, measurements of photodecomposition,



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additional TEM and additional in situ PXRD results. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected]

Present Addresses † Ren Su is currently employed at Syngaschem BV/SynfuelsChina, Christoffer Tyrsted is currently at Haldor Topsøe Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

Acknowledgements The research was supported by the Danish National Research Foundation (Center for Materials Crystallography, DNRF93), the Danish Ministry of Higher Education and Research (DanScatt) and the European Community's Seventh Framework Programme (FP7/2007-2013) CALIPSO under grant agreement nº 312284. Beamline I711, MAXlab synchrotron radiation source, Sweden, is gratefully acknowledged for beam time.


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For Table of Contents Only

Hydrothermal synthesis and in situ powder X-ray diffraction study of bismuth substituted ceria nanoparticles

Kasper Houlberg, Espen D.Bøjesen, Christoffer Tyrsted, Aref Mamakhel, Xueqin Wang, Ren Su, Flemming Besenbacher, and Bo B. Iversen

Phase pure bismuth-substituted ceria nanocrystals have been synthesized by an easyreproducible autoclave based hydrothermal route as well by continuous flow synthesis. Furthermore, the formation mechanisms was studied by in situ synchrotron powder X-ray diffraction.


Hydrothermal Synthesis and in Situ Powder X-ray Diffraction Study of

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