Influence of solvent on the morphology and photocatalytic properties of ZnS decorated CeO2 nanoparticles Cristiane W. Raubach, Lisânias Polastro, Mateus M. Ferrer, Andre Perrin, Christiane Perrin, Anderson R. Albuquerque, Prescila G. C. Buzolin, Julio R. Sambrano, Yuri B. V. de Santana, José A. Varela, and Elson Longo Citation: Journal of Applied Physics 115, 213514 (2014); doi: 10.1063/1.4880795 View online: http://dx.doi.org/10.1063/1.4880795 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/115/21?ver=pdfcov Published by the AIP Publishing Articles you may be interested in MoS2@ZnO nano-heterojunctions with enhanced photocatalysis and field emission properties J. Appl. Phys. 116, 064305 (2014); 10.1063/1.4893020 Synthesis of ZnO decorated graphene nanocomposite for enhanced photocatalytic properties J. Appl. 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Raubach,1,a) Lisânias Polastro,1 Mateus M. Ferrer,1 Andre Perrin,1,b) Christiane Perrin,1,b) Anderson R. Albuquerque,2 Prescila G. C. Buzolin,2 Julio R. Sambrano,2 Yuri B. V. de Santana,3 Jos�e A. Varela,3 and Elson Longo3 1INCTMN-UFSCar, Universidade Federal de S~ao Carlos, Rod.Washington Lu�ıs Km 235, S~ao Carlos 13565-905, SP, Brazil 2Grupo de Modelagem e Simulaç~ao Molecular, INCTMN-UNESP, S~ao Paulo State University, P.O. Box 47 3, Bauru 17033-360, SP, Brazil 3INCTMN-UNESP, Universidade Estadual Paulista, P.O. Box 355, Araraquara 14801-907, SP, Brazil (Received 9 November 2013; accepted 15 April 2014; published online 4 June 2014) Herein, we report a theoretical and experimental study on the photocatalytic activity of CeO2 ZnS, and ZnS decorated CeO2 nanoparticles prepared by a microwave-assisted solvothermal method. Theoretical models were established to analyze electron transitions primarily at the interface between CeO2 and ZnS. As observed, the particle morphology strongly influenced the photocatalytic degradation of organic dye Rhodamine B. A model was proposed to rationalize the photocatalytic behavior of the prepared decorated systems taking into account different extrinsic and intrinsic defect distributions, including order-disorder effects at interfacial and intra-facial regions, and vacancy concentration. VC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4880795] INTRODUCTION The removal of organic pollutants from waste waters generated by the industry sector is an important challenge because these compounds are usually non-biodegradable. Unlike methods involving the transfer of organic pollutants to different phases, the advanced oxidation process (AOP)1 which is a recently adopted approach affords complete destruction of the organic pollutants. In the AOP, a broad range of organic compounds are nonselectively oxidized by reactive species such as hydroxyl radicals.2,3 Several approaches can be used to produce these active radicals, including the use of photocatalysts. Among the wide range of available photocatalysts, TiO2 is the most widely used. Additionally, CeO2 has been considered as a potential candi- date, as detailed in previous reports.4,5 Many methods have been developed for the preparation of small particles, high surface area oxides, and mixed oxides suitable for catalytic applications. More specifically, several processing routes have been investigated to synthe- size CeO2 powders, including spray pyrolysis,6 electrosyn- thesis,7 gas condensation,8 flux method,9 sonochemical and microwave-assisted thermal decomposition,10 hydrothermal method,11 as well as precipitation from oxalate,12 carbon- ate,13,14 peroxide,15 hydroxide,16 and polymeric precur- sors,17,18 including complexation with citric acid,17 organometallic decomposition.19 Previously, we reported the preparation of CeO2 by a simple microwave-assisted hydro- thermal (MAH) method.20 We showed that the microstruc- ture of the final cerium oxide crystals was very sensitive to the experimental conditions studied that included the cerium source, solvent, solution additives (such as surfactants), tem- perature, and reaction time. As recently reviewed, a wide va- riety of particle morphologies have been reported from spheres and cube to anisotropic polyhedra and rods.20,21 ZnS coverage of nanoparticles is well established method employed to enhance the specific physical properties of the base nanoparticles. An example of such a system is CdSe-ZnS core-shell particles prepared from zinc diethyldithiocarbamate Zn(S2CNEt2) in an organic medium, such as tri-n-octylphos- phine,22 in the presence of oleylamine as a surfactant addi- tive.23 Earlier synthesis methods involved two different (and hazardous) precursors: diethyl zinc and bis(trimethylsilyl)sul- fide,24,25 zinc inorganic salts (chlorate and acetate) precipitated by sodium and ammonium sulfide,26,27 have also been employed. Simpler inorganic precursors (metal oxide and ele- mental sulfur) have been proposed, however, these required high temperatures (230 �C) for dissolution.28 In contrast, a low temperature (60 �C) synthesis can be achieved using zinc ace- tate and thiourea in 1-octadecene.29 However, because of the low synthesis temperature, an additive is required to increase the solubility of the precursors at the low temperature. In this paper, we report the synthesis of CeO2, ZnS, and ZnS decorated CeO2 nanoparticles using the microwave- assisted solvothermal (MAS) method that involves simple and nonhazardous chemicals as precursors and solvents, and affords the synthesis of different particle morphologies. The prepared particles were characterized X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDS, elemental analy- sis), transmission electron microscopy (TEM), Fourier trans- form infrared (FT-IR) spectroscopy, ultraviolet-visible (UV-vis) spectroscopy, and photoluminescence (PL) spectros- copy. Subsequently, the photocatalytic activity of the a)cristiane@liec.ufscar.br b)On leave from Institut des Sciences Chimiques de Rennes, UMRCNRS 6226, Universit�e de Rennes, 1, Rennes Cedex 35042, France. 0021-8979/2014/115(21)/213514/10/$30.00 VC 2014 AIP Publishing LLC115, 213514-1 JOURNAL OF APPLIED PHYSICS 115, 213514 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 186.217.234.103 On: Fri, 10 Oct 2014 16:50:31 http://dx.doi.org/10.1063/1.4880795 http://dx.doi.org/10.1063/1.4880795 http://dx.doi.org/10.1063/1.4880795 http://dx.doi.org/10.1063/1.4880795 mailto:cristiane@liec.ufscar.br http://crossmark.crossref.org/dialog/?doi=10.1063/1.4880795&domain=pdf&date_stamp=2014-06-04 synthesized particles was examined towards the photodegrada- tion of organic dye Rhodamine B (RhB). The effect of particle morphology on the photocatalytic activity of the prepared par- ticles is demonstrated. EXPERIMENTAL DETAILS Preparation of CeO2 particles First, 5� 10�3 mol of Ce(NO3)3.6H2O (99% Vetec Qu�ımica Fina) and 0.01 mol of cetyltrimethylammonium bromide (CTAB, 99.9% Acros Organics) were dissolved in 100 ml of a urea (CO(NH2)2) 0.5 M in either water or ethyl- ene glycol (EG), under constant stirring at room temperature. The solution was transferred to a sealed Teflon autoclave (120 ml) and placed in a microwave oven (2.45 GHz, maxi- mum power of 800 W). The reaction system was heated at 150 �C for 30 min at a heating rate of 10 �C/min. The pres- sure in the sealed autoclave was maintained at 3.3 atm. Following reaction, the autoclave was allowed to naturally cool to room temperature. The resulting white precipitate was collected, washed with water, and dried in air at room temperature. Then, the powder was calcined at 500 �C for 1 h, producing a straw yellow powder. Preparation of ZnS decorated CeO2 nanoparticles The ZnS decorated CeO2 nanoparticles were synthesized using 0.017 mol of the previously prepared CeO2 powder dis- persed in 25 ml of either water or EG (solution 1). Then, 0.017 (or 0.0017) mol of zinc chloride and 0.03 mol of thiou- rea were dissolved in 75 ml of either water or EG (solution 2) no surfactant was added at this stage. Solutions 1 and 2 were then mixed in a 120 ml Teflon autoclave and placed in the microwave system at 180 �C for 32 min. The resulting precipitates were washed several times with water until a neutral (pH� 7) solution was obtained, and the powders were collected and air dried at 100 �C for 5 h. For compari- son, ZnS nanoparticles were also prepared from solution 2 under similar conditions. Table I list the various samples pre- pared in this study. CHARACTERIZATION Powder XRD data were collected in a 2h range of 10�–105� using a step scanning mode of 0.02� step size and a 1 s/step, a 0.5� divergence slit, a 0.3 mm receiving slit, and Cu Ka1 radiation on a (Rigaku-DMax/2500PC). Microstructural characterization was performed on a field- emission scanning electron microscope Carl Zeiss Supra 35-VP and high-resolution transmission electron microscope FEI Tecnai G2TF20, operating at 200 kV. EDS (for elemen- tal analysis) was conducted on the above mentioned scan- ning and transmission electron microscopes. FT-IR spectra were recorded in the range of 400–4000 cm�1 on a Bruker Equinox-55 (Germany) in transmittance mode using the KBr pellet technique. UV-vis spectra were collected on a Varian Cary 5G spectrophotometer in diffuse reflectance mode. PL spectra were collected using a Thermal Jarrel-Ash Monospec monochromator and a Hamamatsu R446 photomultiplier. Krypton ion laser (Coherent Innova) with an exciting wavelength of 350.7 nm (2.57 eV) was used, and the output of the laser was maintained at 200 mW. All measurements were taken at room temperature. PHOTOCATALYTIC ACTIVITY The photocatalytic activity of the samples towards the photo oxidation of RhB dye [C28H31ClN2O3], (99.5%, Mallinckrodt) in aqueous solution was tested under UV-light illumination. First, 50 mg of catalyst powder was dispersed 50 ml of RhB solution (0.1 mM), adjusted to a pH of. The suspension was sonicated for 10 min in a Branson 1510 ultrasonic cleaner at a frequency of 42 kHz. Before illumination, the resulting suspension was stored in the dark for 10 min to allow equilibrium adsorption of RhB onto the catalyst. The solution containing beakers were then placed in the photo reactor at 20 �C and illuminated by six UV lamps (TUV Philips 15W) with maximum intensity at 254 nm. The light flux was measured by a Coherent Power Max PM10; the optical energy density was 20 mW cm�2. At 30 min intervals, aliquots (2 ml) were sample and centri- fuged at (9000 rpm) for 5 min to separate the powder from the solution. Changes in the absorbance at kmax (520 nm) of the supernatant solutions were measured on a UV-vis dual beam spectrophotometer using a double monochromator and a photomultiplier tube detector (JASCO V-660). Reference photoactivity experiments containing different added radical scavengers (0.1 mmol) in the reaction system (ammonium oxalate as a scavenger for photogenerated holes,30 AgNO3 as a scavenger for photogenerated elec- trons,31,32 tert-butyl alcohol as a scavenger for hydroxyl rad- icals,30,33 and benzoquinone as a scavenger for superoxide radical species34) were performed under similar conditions to those employed for the above photocatalytic oxidation of samples.35 THEORETICAL METHODS AND MODELS Periodic DFT calculations with the B3LYP hybrid func- tion36 were performed using a CRYSTAL09 computer code37,38 that employ a Gaussian type basis set to represent Khon-Sham orbitals in periodic systems within the linear combination of atomic orbitals (LCAO) approximation. We have successfully used this method to study bulk and surfa- ces of oxides and sulfides.39–42 The CeO2 fluorite structure (Fm-3m) is defined by one lattice parameter (a¼ 5.41 Å) (Ref. 43) where the cerium TABLE I. Concentrations of zinc ions, solvent used and identification of the samples synthesized by MAS method. Sample Identification Solvent Concentration (mol) zinc ion CeO2 pure (CeO2-EG) EG … CeO2@ZnS (CZ1-EG) EG 0.0017 CeO2@ZnS (CZ2-EG) EG 0.017 CeO2 pure (CeO2-H2O) H2O … CeO2@ZnS (CZ1-H2O) H2O 0.0017 CeO2@ZnS (CZ2-H2O) H2O 0.017 ZnS pure (ZnS-EG) EG 0.017 213514-2 Raubach et al. J. Appl. Phys. 115, 213514 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 186.217.234.103 On: Fri, 10 Oct 2014 16:50:31 center is surrounded by eight oxygen atoms, which occupy tetrahedral sites [OCe4] with a Td local symmetry (see Fig. 1(a)). In contrast, the ZnS structure crystallizes either as rock salt (NaCl), cubic zinc blende, or hexagonal wurtzite structures.44 Under normal conditions, the thermodynami- cally most stable phase is wurtzite. In the (P63mc structure), each Zn atom is surrounded by four S atoms [ZnS4] at the corners of a tetrahedron (see Fig. 1(c)) with lattice parame- ters a¼ 3.82 Å and c¼ 6.26 Å.44 In CeO2, oxygen centers are described by the all elec- tron basis set 8-411d11G.45 For cerium, a modified version of the small core ECP28MWB pseudopotential46 (for core electrons) with valence shells was adopted as proposed by D�esaunay et al.47 within the contraction scheme: (16s 13p 10d 7f)/[4s 3p 3d 2f]. In the ZnS hexagonal crystal (P63mc), the atomic centers are described by the modified version41 of the all electron basis set Zn_86-411d31G (Ref. 48) for Zn and S_86-311G for sulfur.49 The accuracy of the Coulombic and exchange integral calculations was controlled by five parameters were, set at (8 8 8 8 18). The shrinking (Monkhorst-Pack) factor was set at 6 which corresponds to 16k-points for CeO2 and 34k-points for ZnS in the irreducible part of the Brillouin zone integra- tion in primitive unit cells. From the full optimized primitive cells, four periodic (2� 2� 2) supercells were built to simulate the effects of a slight structural deformation on the electronic structure owing to the decorated architecture: (i) ordered structure CeO2-o and ZnS-o (see Figs. 1(a) and 1(c)), and (ii) disor- dered structures CeO2-d and ZnS-d (see Figs. 1(b) and 1(d)), where the O and S atoms are displaced by 0.3 Å in the [001 direction]. Fig. 1 shows ordered and disordered supercell models. These models are not intended to represent the exact reality of a disordered structure, but they offer a simple scheme to understand the concurrent effects in the resulting electronic structures. Band structures were obtained for 100k-points along appropriate high-symmetry paths of the first Brillouin zone. The XCrySDen program50 was used for the unit cell and den- sity charge map designs. RESULTS AND DISCUSSION Structural identification Fig. 2 illustrates the XRD patterns of CeO2 and ZnS decorated CeO2 nanoparticles synthesized at two different zinc solution concentrations prepared in either EG or water using the MAS method at 180 �C. XRD patterns of CeO2 prepared in either EG or water, and calcined at 500 �C were in full agreement with the data as obtained in the Inorganic Crystal Structure Database (ICSD) for this compound. Peak broadening, as assessed by the full width half maximum (FWHM) method, was observed when comparing samples synthesized in water with those prepared in EG.51 This suggests that the particles pre- pared in EG were smaller. However, no significant changes in the patterns were apparent following decoration of the CeO2 nanocrystals by ZnS. The blende phase is stable at low temperature. However, in the solution chemistry approach, both wurtzite52 and blende53–56 allotropes were formed. For comparison, a pure ZnS sample (prepared in EG) was syn- thesized under similar conditions and clearly appeared to be poorly crystallized wurtzite. However, because CeO2 and blende ZnS are both face centered cubic structures with FIG. 1. Supercell (2� 2� 2) from primitive unit cell of (a) CeO2–o, (b) CeO2–d, (c) ZnS–o, and (d) ZnS–d (labels o and d refer to ordered and disordered structures, respectively). FIG. 2. XRD powder patterns of the CeO2 and CeO2/ZnS synthesized in (a) EG and (b) H2O. Insets show the zoomed XRD patterns from 24� to 40�. The vertical lines indicate the position and relative intensity of the ICSD cards No. 72155 (CeO2) and 41985 (ZnS blende). 213514-3 Raubach et al. J. Appl. Phys. 115, 213514 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 186.217.234.103 On: Fri, 10 Oct 2014 16:50:31 comparable unit cell constants, we believe that the blende ZnS allotrope can be stabilized by epitaxial relationships. As a consequence, owing to the similarity of their patterns (illustrated by the bulk powder data reported in the ICSD files), it is not possible to experimentally prove the actual growth of a ZnS shell on the CeO2 core solely from XRD data. To overcome this limitation, EDS analyses were per- formed. As shown in Figs. 3(c) and 3(f), Zn and S peaks were observed in addition to Ce and O peaks. For layered samples of small dimensions, it is difficult to achieve an accurate quantitative analysis. However, the standard semi- quantitative results, obtained herein, were consistent with the expected Zn/S ratio (i.e., �1). A complete optimization procedure was adopted for determining lattice parameters a for CeO2-o and a, c, and u for ZnS-o. The calculated and experimental values are shown in Table S2 and are in good agreement with results reported in the literature.51 The prepared particles were further examined by FT-IR spectroscopy.51 The bands at 1610 and 3500 cm�1 confirmed that all samples, including CeO2 particles, had water adsorbed on their surface (although the samples were previ- ously fired at 500 �C (however, they were not stored under specific atmospheric moisture restrictions because they were subsequently used in a water environment). Owing to its fluorite structure, bulk CeO2 displayed only one IR active Ce-O vibrational mode, slightly below 500 cm�1. This band appeared at the lower limit of the detection range used in this study. However, several additional bands were clearly observed within the 700–1600 cm�1 range. These bands were ascribed to vibrational bond in molecules either coordinated or adsorbed on the surface. It should be noted that both CeO2-EG and CeO2-H2O exhibited the same spectra, despite the different synthetic media employed. Because the samples were fired at 500 �C, the presence of urea and surfactant is improbable. Thus, these bands were probably associated with surface hydroxyls20 and carboxyl groups (namely the bands near 1340 and 1540 cm�1).57–59 Comparison of CeO2-EG and CZ1-EG samples confirmed that the extent of the splitting of the C-O vibration was lower in the latter samples. This is in- dicative, of a change from bidentate to monodentate coordi- nation,57 suggesting that at the very early stages of the ZnS growth, some of the Ce-O-C bonds were broken. Conversely, most of the bands observed within the 500–1600 cm�1 range for pure ZnS particles (denoted as ZnS-EG in Fig. S151 agreed reasonably well with thiourea vibrations,60 especially when comparing with Zn[CS(NH2)2]3 2� complexes.61 Additionally, bands near 2915 and 3310 cm�1 that overlapped the hydroxyl broad absorption band were a close fit to bands corresponding to coordinated thiourea.62 However, the most prominent feature FIG. 3. FE-SEM images and EDS analysis of crystals synthesized by MAS method (a) CeO2-EG, (b) CZ1-EG, (c) CZ2-EG, (d) CeO2-H2O, (e) CZ1-H2O, and (f) CZ2-H2O and EDS (20 kV and 5 kV) analysis. 213514-4 Raubach et al. J. Appl. Phys. 115, 213514 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 186.217.234.103 On: Fri, 10 Oct 2014 16:50:31 was the strong absorption band at �2140 cm�1: the intensity of the band was stronger for the ZnS decorated CeO2 and was maximum for the ZnS-EG particles. Thus, this band is clearly related to zinc sulfide; however, it is unlikely to cor- respond to Zn-O, Zn-S, and S-O vibrations owing to the strong intensity of the peak. In fact, only a few vibrators are possible in this spectral range, e.g., S-H stretching vibration occurs at �2500–2600 cm�1 in H2S and thiols.63,64 Therefore, this vibration was tentatively assigned to weak S-H(OH) bond stretching present in species such as ZnS-H2O that is consistent with the significant reduction in frequency observed. The other band at �1250 cm�1 that dis- played similar behaviors would correspond to the same vibrator. The ZnS-EG spectrum data highlights an important point that bands characteristic of bound sulfur appear in the samples. In the case of the decorated compounds synthesized in water, a similar conclusion could not be made because of the unsuccessful preparation of pure ZnS-H2O, possibly because of the compound hydrolysis in water.51 Microstructure analysis Figs. 3(a)–3(f) show FE-SEM images of the samples synthesized by MAS method. The CeO2 nanocrystals synthe- sized in water and EG exhibit different shapes and aspect ratios. A higher magnification analysis reveals that CeO2-H2O crystals are truncated parallelepipeds with flat surfaces, whereas the CeO2-EG nanocrystals consist of open structures formed by sheets that tend to orientate parallel to each other, resulting in oval shaped particles. However, these sheets do not correspond to a specific orientation because no specific broadening (or narrowing) was observed in the XRD patterns, as confirmed by HR-TEM (see Fig. 4(c)). Different observations were made for the CeO2 sample obtained by precipitation at room temperature in the presence of EG. A strong peak broadening was observed for the 111 peak while all other peaks remained narrow.16 This was indicative of strongly anisotropic crystal growth with thin sheets develop- ing parallel to the (111) plane. Thus, simply changing the solvent (EG or water) enabled control over the shape of the resulting CeO2 nanocrystals. Mesoporous flower-like CeO2 nanocrystals with an open structure were prepared by a hydrothermal method (simulta- neous polymerization of a precipitation reaction, metamor- phic reconstruction, and mineralization under hydrothermal conditions), followed by calcinations.65,66 Unlike the CeO2-EG samples prepared in this study that featured paral- lel sheets, those obtained nanospheres were composed of sheet-like crystallites oriented in flower petal-like fashions. Other flower-like CeO2 architectures that consist of nanorods arranged in a spherical morphology, reminiscent of urchin structures, with no sheet-like components, have also been reported.67 Finally, when compared with the above CeO2 samples, the MAS technique used herein for the synthesis of CeO2 samples afforded a unique open structure when EG solution was used as solvent. In contrast, synthesis in water generated material morphologies as commonly reported. After decoration of the CeO2-H2O and CeO2-EG sam- ples with ZnS, spherical particles were observed on both samples surfaces, indicative of the onset of ZnS nucleation during the coverage process. Some crystals in the CeO2-H2O samples were not covered, and resulting samples appeared quite porous. Under higher magnification analysis (see Fig. 3(e)), flat and elongated crystals were visible on the sur- face owing to the presence of a ZnS shell, as confirmed by EDS analyses. Comparison analysis with core-shell nanopar- ticles reported in the literature reveals that the shell is not uniformly developed around the core and the occurrence of nucleation of individual shell particles instead of the forma- tion of a regular shell around the nanoparticles is common.68,69 Fig. 4 displays TEM images of the CeO2-EG particles before and after zinc sulfide decoration. Fig. 4(a) confirms the microstructure of CeO2-EG. Fig. 4(b) is a HR-TEM image of the thin sheet, as enlarged in Fig. 4(c). The images show that the cerium oxide nanocrystals (�5–10 nm) are ran- domly oriented, as consistent with the relatively constant XRD FWHM values. Figs. 4(d) and 4(e) (in comparison with Fig. 4(b)) shows that ZnS (identified in the EDS analysis in Fig. 4(f)) grow onto the CeO2 sheets as quasi spherical par- ticles with a typical diameter of �50–60 nm. TABLE II. Fermi level, top of valence band, bottom of conduction band, electronic band gap of ordered and disordered CeO2 and ZnS by periodic B3LYP cal- culations and experimental values. Top of VB (eV) Bottom of CB (eV) Band gap (eV) Experimental CeO2-o �3.26 (O 2p) �0.83 (Ce 4f) 2.43 (O 2p – Ce 4f) … 4.70 (Ce 5d) 7.96 (O 2p – Ce 5d) CeO2-EG … … … 3.10 CeO2-H2O … … … 3.05 CeO2-d �3.17 (O 2p) �0.83 (Ce 4f) 2.34 (O 2p – Ce4f) … 4.53 (Ce 5d) 7.70 (O 2p – Ce 5d) CZ1-EG … … … 3.15 CZ2-EG … … … 3.05 CZ1-H2O … … … 2.91 CZ2-H2O … … … 2.78 ZnS-o �5.84 (S 3p) �1.96 (Zn 3d) 3.88 (S 3p – Zn 3d) 4.20 ZnS-d �5.82 (S 3p) �1.97(Zn 3d) 3.85 (S 3p – Zn 3d) 213514-5 Raubach et al. J. Appl. Phys. 115, 213514 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 186.217.234.103 On: Fri, 10 Oct 2014 16:50:31 Absorption spectroscopy analysis UV-vis spectral data of the samples are given in Table II; the band gap energies were calculated using the fol- lowing equation: ðahtÞ2 ¼ f ðhtÞ.70 Single crystal data con- firm that ZnS exhibits a direct band gap of 3.66 eV for the blende allotrope and 3.74–3.78 eV for the wurtzite allo- trope.71 Additionally, a wurtzite band gap of (3.723 eV) was determined for wurtzite ZnS epilayers.72 ZnS nanoparticles exhibited a blue shift that was related to a pronounced quan- tum confinement effect.44,73 The band gap value (4.20 eV) for the wurtzite allotrope also indicated a blue shift. For CeO2, the bulk optical band gap was reported as 3.14 eV, indicating the onset of non direct transitions in sputtered films.74 Based on the optical studies of nanoparticles (2.6–4.1 nm), an indirect transition (Ei¼ 2.87 and 2.73 eV), and a direct transition (Ed¼ 3.44 and 3.38 eV) were observed. Despite the small increase in the band gap energy observed as the size decreased, no quantum size effects were evident in these nanoparticles.75 Inoue reported optical band gaps of 2.90 and 3.52 eV for very small particles (2 nm), and concluded the presence of quantum size effects in these par- ticles.76 The optical band gaps of the prepared CeO2 and ZnS decorated CeO2 particles were within 2.78–3.10 eV. The decorated samples featured reduced band gaps (i.e., red shift) that suggested the presence of interfacial interactions. Importantly, the prepared CeO2 and ZnS decorated CeO2 samples featured absorption properties at the high energy region of the solar spectrum when compared with those of pure ZnS and TiO2, thereby suggesting possible use under natural light. For the disordered models, all lattice parameters remained constant because the small atomic dislocations and amount in bulk did not significantly modify these structural parameters. However, perturbation owing to anion shift induced symmetry breakage and splitting in the electronic band structure with a reduction in the band gap for the oxide and sulfide.51 Quantum mechanical calculations of the dislocated CeO8 and ZnO4 complex clusters indicated that localized states generated in the band gap reduced the gap energies (Table II). Levels above the valence band (VB) and below the conduction band (CB) of CeO2-o are mainly composed of O-2p and Ce-5d orbitals, respectively, with unfilled Ce-4f orbital in the intermediate gap level.51 Thus, two band gaps can be identified: 2.43 eV O-2p – Ce-4f (C-X) and 7.96 eV O-2p – Ce 5d (C-X) with a Fermi level of �3.26 eV. These results agree with theoretical results reported by Sanz and co-workers77 who evaluated the use of hybrid functions to assess the properties of ceria oxide. Experimental studies have demonstrated that those two gaps in ceria are ��2.6–3.9 eV (O2p-Ce4f) and �6–8 eV (O2p-Ce5d). FIG. 4. The TEM images at different magnifications of the (a)-(c) CeO2-EG and (d)-(e) CZ2-EG crystals; on the right: EDS analysis using 6 keV for energy, providing ratios Ce/O/Zn/S. 213514-6 Raubach et al. J. Appl. Phys. 115, 213514 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 186.217.234.103 On: Fri, 10 Oct 2014 16:50:31 For the CeO2-d model, a decrease in the two band gaps values was observed that was mainly attributed to the split of the electronic levels.51 However, a Fermi level increase was observed, owing to a cluster disorder, thereby clarifying that this disordered cluster was more reactive than the ordered cluster. The band gaps of CeO2-d were 2.34 eV (O2p-Ce4f) (C-X), and 7.70 eV (O2p – Ce5d) (C-X). The same band structure properties were observed for ZnS-o and ZnS-d. ZnS-o displayed direct band gap energy of 3.88 eV S-3p Zn 3d) with a Fermi level of �5.84 eV.51 The ruptured local symmetry of the ½ZnS4�x cluster instigated splitting in the bands, a small increase (of�0.02 eV) in the Fermi level, and a decrease in the gap to 3.85 eV.77 The ½CeO8� and ZnS4½ � cluster polarization, owing to a local disorder, can be visualized according to the charge den- sity maps in Figs. 5(a) and 5(b), respectively. The defect is indicated by the region within the dotted circle showing a reduced cation-anion overlap region. Clusters outside the circle featured small electronic perturbation. The present model though simple is useful to explain the disorder owing to the crystallite oxide sulfide interfaces in the core-shell representation. In this case, the band gap reductions were proportional to the short range disorder, and the increase in the Fermi level was related to the photolumi- nescence and photocatalytic effect of the polarized clusters, as discussed further. Photoluminescence and photocatalytic activity PL spectra confirmed the ZnS coverage of the CeO2 dec- orated material. As observed in Fig. 6, the fluorescence emis- sion of pure CeO2 was not significant, where CZ1 and CZ2 exhibited PL spectra comparable that of ZnS-EG; the inten- sity increased with increasing Zn contents. The decorated samples prepared in water and EG displayed different fluo- rescence profiles. Similar behaviors have been reported in previous studies.78,79 A detailed study on the mechanisms involved underway. The photocatalytic activity of the samples towards the photodegradation of RhB is shown in Figs. 7(a) and 7(b). The two CeO2 samples prepared in EG and water showed distinct photocatalytic activities. Nearly complete photode- gradation of the dye solution was achieved over CeO2-EG within 120 min, where only a small portion of the dye solu- tion was degraded over CeO2-H2O within the same irradia- tion time. This behavior was obviously related to the different morphologies of the particles, thereby confirming that the photocatalytic properties of CeO2 nanocrystals are strongly dependent on the shape of the particles.80 As FIG. 5. Charge density maps of (a) CeO2–d and (b) ZnS–d. The arrow inside the dotted circle region points the decrease of charge density due to the anion dislocation. The arrow outside the circle points the normal bond charge density between cation-anion. FIG. 6. Photoluminescence spectra of the CeO2 and CeO2/ZnS nanoparticles in EG and H2O. Inset is a zoom of the PL spectra of CeO2 in EG and H2O where the intensities are enlarged by a factor 1000�. FIG. 7. Normalized absorbance of RhB solution after various times of under UV irradiation in presence of CeO2, ZnS, and ZnS decorated CeO2 synthesized in (a) EG and (b) H2O. 213514-7 Raubach et al. J. Appl. Phys. 115, 213514 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 186.217.234.103 On: Fri, 10 Oct 2014 16:50:31 exemplified further, lamellar CeO2 crystals displayed an excellent photocatalytic yield towards the photodegradation of methylene blue.4 The current mesoporous nanocrystals are of special interest because of their high surface to volume ratios and open structures that are suited for application in catalysis. The ZnS-EG system shows intriguing photocatalytic results. Noticeable dye discoloration was observed prior to solution exposure to UV radiation. Under light exposure, degradation continues sharply before quickly reaching equi- librium. It is believed that the initial discoloration was related physical adsorption of the dye onto the surface, espe- cially in the system involving ZnS.81 However, this initial discoloration seldom occurred in the presence of the CeO2-EG sample despite its very high surface area. Another possible explanation is that the as grown ZnS crystals pos- sess activate sites that deactivate under illumination. Studies on the degradation and regeneration of ZnS catalysts52 have shown strong changes following reaction cycles. In the latter regeneration study, the platelets that formed the microspheri- cal structure of the catalyst disappeared following irradia- tion, yielding smooth ZnS spheres. Moreover, the resulting XRD pattern suggested the onset of severe amorphization. The initial characteristics of the ZnS catalyst were recovered by a new sulfidation treatment. In our system, we believe that ZnS could be oxidized by activated hydroxyls to pro- duce Zn(OH)2, xH2O for instance. The decorated sample exhibited different behaviors. For samples prepared in H2O, a distinct improvement in the pho- tocatalytic activity was observed for CZ1-H2O relative to the (poorly efficient) CeO2-H2O sample. This could be ascribed to the decoration effect that promoted electronic transfer at the interface. However, when the thickness of the ZnS layer was increased, CZ2-H2O started to behave like pure ZnS. For samples grown in EG, ZnS coverage reduced the poros- ity of the resulting decorated samples. Consequently, CZ1-EG displayed a low yield. Moreover, similar behaviors to those displayed by ZnS in the absence of light irradiation, as discussed earlier, were observed for CZ1-EG. Furthermore, when the ZnS layer thickness was increased, the CZ2-EG sample behaved as pure ZnS particles. The cluster-like elucidation of the photocatalytic per- formance was supported and strengthened by different ex- trinsic (surface) and intrinsic (bulk) defect distributions including structural order-disorder effects (interfacial region, intra-facial region, and vacancy concentration). The applica- tion of stress or interfacial strain may induce significant modifications of the decorated sample band gap, thus pro- moting the development of a heterogenous structure. Thus, we consider that the bulk (CeO2) and decorating (ZnS) com- ponents are neutral and have the same relevance in terms of the electronic structure. This effect only applies when there is an intermediate level of order-disorder between the inter- faces of the decorated samples. The defect structure and density variation in the interfa- cial and/or intra-facial regions may be responsible for the different photocatalytic properties of CeO2, ZnS, and ZnS decorated CeO2. Effective charge separation (electron/hole) requires the presence of a cluster to cluster charge transfer (CCCT) of electrons or holes from ½ZnS4�xo–½ZnS4�xd, ½CeO8�xo–½CeO8�xd, or ½CeO8�xB–½ZnS4�xS (decorated) (where o¼ order, d¼ disorder, B¼ bulk, and S¼ surface). A way to enhance the photocatalytic efficiency is to convert ordered complex clusters into disordered complex clusters. Consequently, the effect of surface properties on photocata- lytic performance should be considered in terms of ½CeO8�xo, ½ZnS4�xo, or ½ZnS4�xS clusters and ½CeO8�xd , ½ZnS4�xd, or ½CeO8�xB clusters. The first effect is intrinsic to CeO2, ZnS, or ZnS decorated CeO2 and is derived from the bulk/surface material composed of an asymmetric ½CeO8�d or ½ZnS4�d and ordered ½CeO8�o or ½ZnS4�o. The ordered complex clusters often behave as elec- trons sinks, thereby improving charge separation within the semiconductor photocatalytic system. These electrons can then react with acceptors (O2) at the interface, with a relatively lower overpotential of reduction. Consequently, the effect of surface properties on photocatalytic activity should be consid- ered in terms of the following reactions: ½CeO8�xo þ ½CeO8�xd �!h� ½CeO8�0o þ ½CeO8��d; (1) ½ZnS4�xo þ ½ZnS4�xd �!h� ½ZnS4�0o þ ½ZnS4��d; (2) ½CeO8�xB þ ½ZnS4�xS�! h� ½CeO8�0B þ ½ZnS4��S; (3) ½CeO8��d þ H2O! ½CeO8�xd … OH þ H�; (4) ½ZnS4��d þ H2O! ½ZnS4�xd … OH þ H�: (5) Upon absorption of a photon with energy equal to or greater than the band gap of the semiconductor, an electron/ hole pair is generated in the bulk/surface. These charge car- riers migrate towards the catalytic surface where charge transfer between the defect-free or defective surface and adsorbed oxygen molecules produces several types of charged species including O2 0 superoxide ions. Molecular oxygen reactivity with ½CeO8� or ½ZnS4� results in further the species and subsequent oxygen incorporation into the lattice, ½CeO8��d þ O2 þ ½CeO8�0o ! ½CeO8��d … O02 adsð Þ þ ½CeO8�xo; (6) ½ZnS4��d þ O2 þ ½ZnS4�0o ! ½ZnS4��d … O02 adsð Þ þ ½ZnS4�xo (7) ½ZnS4��SþO2þ ½CeO8�0B! ½ZnS4��S …O02 adsð Þþ ½CeO8�xB: (8) The CeO2, ZnS, ZnS decorated CeO2 complex clusters react with water to produce hydroxyl radicals and hydrogen ions according to the following reactions: The primary products formed from the partial oxidation reaction between water and complex cluster ½CeO8��d or ½ZnS4��d are hydroxyl radicals, OH*. These radicals exhibit high oxidation power that enables mineralization of organic compounds in water (anodic reaction). The primary reaction 213514-8 Raubach et al. J. Appl. Phys. 115, 213514 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 186.217.234.103 On: Fri, 10 Oct 2014 16:50:31 (cathodic) involves the formation of superoxide species ½CeO8��d … O02 or ½ZnS4��d … O02. These species then react with hydrogen H� to form a hydrogen peroxide radical (O2H*) according to the following reactions: ½CeO8��d � � �O02 adsð Þ þ H� ! ½CeO8��d … O2H ; (9) ½ZnS4��d � � �O02 adsð Þ þ H� ! ½ZnS4��d … O2H ; (10) ½ZnS4��S � � �O02 adsð Þ þ H� ! ½ZnS4��d … O2H : (11) The radicals OH* and O2H* then react with the organic compound instigating the oxidation of the organic compound. The nature of the superoxide and hydroxyl radicals can be described using a complex cluster model where the elec- tron/hole transfers from the disordered structure to ordered structure, followed by absorption of molecular oxygen and water. The studies of the samples afforded the characteriza- tion of the defect-free and defective complex cluster struc- tures and determination of surfaces with large numbers of defective complex clusters associated with morphological and structural properties. Fig. 8 illustrates electronic models of CeO2 and ZnS ordered and disordered structure before and after O2 and H2O interaction. Further, understand the nature of primary active species, semiconductor (Eqs. (1)–(3)) and degradation agent (Eqs. (4)–(11)), involved for visible light degradation of RhB in an aqueous phase over the CeO2/ZnS nanoparticles, we have carried out the control experiments with adding scavenger for electrons for electrons (e0), holes (h�), hydroxyl radicals (OH*), and superoxide radicals (O2H*). Fig. 8 shows the results of adding different radical scavengers over the CeO2/ZnS photocatalyst reaction system under UV irra- diation. When the radical scavenger, tert-butyl alcohol (TBA) for OH* and benzoquinone (BQ) for O2H*, are added into the reaction system, the degradation of RhB is signifi- cantly inhibited (Figs. 9(a) and 9(b)), these results suggests that, under UV irradiation, the OH* and O2H* (Eqs. (7)–(11)) play an important role toward the degradation of RhB over the CeO2/ZnS nanoparticles. Indeed, the addition of electron scavenger, AgNO3 (Ag) and hole scavenger ammonium oxalate (AO) to the reaction system had intermediate effects on the photodegradation of RhB (Figs. 9(c) and 9(d)). Thus, OH* and O2H* radicals cannot be formed before polarization of the semiconductor and reaction with water and oxygen, Eqs. (1)–(8). The reac- tion over ZnS decorated CeO2 is primarily driven by photo- generated electrons and holes in the semiconductor that subsequently, reacts with water, Eqs. (1)–(5). CONCLUSIONS CeO2, ZnS, and ZnS decorated CeO2 nanoparticles were synthesized by the efficient MAS method and their properties were investigated in detail. FE-SEM and TEM analyses confirmed a clear relationship between the nature of the solvent used in the synthesis and resulting CeO2 crys- tals morphologies. Truncated parallelepipeds with flat surfaces were obtained using water as a solvent where oval particles with an open structure built from sheets were obtained using EG as a solvent. The presence of ZnS on the surface of CeO2 was evidenced by EDS analyses and IR spectroscopy. The decorated nanoparticles exhibit charac- teristic absorption bands in the IR spectra that were also visible in the pure ZnS sample. More specifically band that was attributed to weak S-H bonding in Zn-S-H-OH surface species was observed. The intensities of the PL spectra were directly related to the amount of ZnS used in the syn- thesis. The photocatalytic studies of these nanoparticles support a clear relationship between the particle morphol- ogy and RhB degradation rate. Finally, a model was pro- posed to explain the photocatalytic behavior of these decorated systems taking into account different extrinsic and intrinsic defect distributions, including order-disorder effects at interfacial and intra-facial regions and vacancy concentration. FIG. 8. Mechanism models of ZnS decorated CeO2 system interface in the photodegradation process. FIG. 9. Controlled experiments using different radical scavengers for the photocatalytic selective degradation of RhB over CeO2/ZnS in an aque- ous solvent; (a) reaction with tert-butyl alcohol (TBA), as a scavenger for hydroxyl radicals (OH*), (b) reaction with benzoquinone (BQ) as a scavenger for superoxide radicals (O2H*), (c) reaction with ammonium oxalate (AO) as a scavenger for photogenerated holes (h�), (d) reaction with AgNO3 as a scavenger for photogenerated electrons (e0), and (e) the reaction in the absence of radical scavengers under UV irradiation for 120 min. 213514-9 Raubach et al. J. Appl. Phys. 115, 213514 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. 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