Bilayer stabilized Ln 3 +-doped CaMoO 4 nanocrystals with high luminescence quantum e ffi ciency and photocatalytic properties †

In this article, we discuss the microwave synthesis of sodium dodecyl sulphate (SDS) stabilized Ln-doped CaMoO4 nanocrystals (Ln = Eu, Er/Yb). The nanocrystals are quite monodispersed with an average size close to 100 nm. FTIR and TGA analyses suggest strong binding of the SDS molecules to the CaMoO4 nanocrystals surface. The high dispersibility of the nanocrystals in water implies that SDS stabilizes the nanocrystals as a bilayer structure. The SDS coating also assists in the easy dispersion of the nanocrystals in toluene without any additional surface chemistry. The Eu ions doped in the CaMoO4 nanocrystals display very strong red luminescence with a quantum yield close to 40%. Under 980 nm excitation, Er/Yb-doped CaMoO4 nanocrystals display Er emissions at 550 and 650 nm. In addition, interestingly, a NIR peak at around 833 nm is observed, which occurred via a three photon process. Furthermore, the CaMoO4 nanocrystals exhibit photocatalytic activity which is studied through the degradation of Rhodamine B (RhB) dye in neutral conditions. The RhB dye is significantly degraded by ∼80% under UV illumination within 4 h and the rate of degradation is comparable to that observed for well known ZnO nanoparticles. The high luminescence quantum efficiency and strong photocatalytic activity of the Ln-doped CaMoO4 nanocrystals make them a potential material for dual applications such as bio-imaging and photocatalysis.


Introduction
Lanthanide (Ln 3+ )-doped luminescent nanomaterials continuously draw much attention due to their characteristic sharp transitions arising from inner 4f orbitals.The sharp transition is due to shielding of the 4f orbitals by the 5s and 5p orbitals. 1hese transitions have low absorption coefficients as they are partially forbidden, resulting in the long luminescence lifetime from the µs to ms region. 2 Intra 4f transitions are narrow and occur over a wide spectral region (from ultraviolet to near infrared) and give potential advantages for use in colour displays due to their high colour purity, phosphors, and biomarking applications. 3In addition to the Stokes emission, these Ln 3+ ions possess several energy levels which are close enough to convert the lower energy radiation to higher energy visible emission, a process known as upconversion luminescence. 4ecently there has been an increasing demand for water dispersible Ln 3+ -doped nanocrystals due to their potential use as bioprobes. 57][8] However, studies on water dispersible oxide-based materials, such as oxides, vanadates, phosphates, sulphates, and molybdates, are quite limited. 9Oxide-based nanomaterials are interesting because of their robustness and lower toxicity. 10Among the various Ln 3+doped oxide nanomaterials, water dispersible molybdates are less explored.The molybdate (MoO 4 2− ) crystal has tetrahedral symmetry (Td) with the central Mo 2+ metal ion coordinated by four O 2− ions.3][24][25][26][27] Not only Ln-based molybdates but alkali metal molybdates [28][29][30][31] are interesting due to the ionic size of the alkali metals being close to Ln 3+ ions.Among the MMoO 4 (M = Ca 2+ , Ba 2+ & Sr 2+ ) materials, CaMoO 4 is attractive for the following reasons.First, CaMoO 4 is a highly transparent material which allows a wide range of light to pass through without much attenuation in luminescence.Second, CaMoO 4 is a robust phosphor due to its high density (4.25 g cm −3 ) and possess better physical and chemical properties compared to other oxide materials.3][34][35][36] Ju et al. have reported the synthesis of a CaMoO 4 :Eu 3+ phosphor via the sol-gel process.The quantum efficiency of this material is 20% higher than the one from a solid state reaction. 37Sharma et al. have achieved a series of CaMoO 4 phosphors doped with Dy 3+ and co-doped with K + by a hydrothermal method.They have performed photometric characterization of these nanocrystals which indicates the suitability of these phosphors in commercial white LEDs and other display devices. 38Apart from Stokes luminescence, there are some reports available on the upconversion luminescence of Ln 3+ -doped CaMoO 4 nanomaterials. 39Ryu et al. have reported controllable white upconversion luminescence in Ho 3+ /Yb 3+ /Tm 3+ -doped CaMoO 4 nanocrystals synthesized using a complex citrate-gel method. 40he same group has shown green upconversion luminescence from polycrystalline Er 3+ /Yb 3+ -co-doped CaMoO 4 nanocrystals and also the effect of Li + doping on the enhancement of the green UC emission. 41However, most of these reports discuss solid state materials.To fully explore the potential application of molybdates, it would be highly desirable to downscale their sizes to the nanoregime to expand their potential for various applications.
In this article, we report the synthesis of water dispersible Eu 3+ -doped CaMoO 4 nanocrystals under microwave conditions.The water dispersibility is achieved with a thin coating of sodium dodecyl sulphate (SDS) over the Ln 3+ -doped CaMoO 4 surface.The optimum concentration of Eu 3+ is 5 mol%, leading to a very intense luminescence signal in water.This led to an enhanced Eu 3+ luminescence quantum efficiency close to 40% in the CaMoO 4 nanocrystals.Under 980 excitation, Er 3+ /Yb 3+ -doped CaMoO 4 nanocrystals show a strong peak close to 833 nm in the NIR region, which, to our knowledge, is rarely reported. 42Furthermore, the nanocrystals display strong photocatalytic activity which is verified through the degradation of Rhodamine B (RhB) dye.We observed an 80% degradation of RhB under UV illumination, which is close to that reported for ZnO.The high luminescence quantum efficiency and strong photocatalytic properties of the Ln 3+ -doped CaMoO 4 nanocrystals make them suitable materials for dual applications.

Synthesis
Water dispersible Eu 3+ -doped CaMoO 4 nanocrystals were synthesized using a microwave (MW) procedure.Briefly, stoichiometric amounts of Eu 2 O 3 were converted into their corresponding nitrates by dissolving in 1 M HNO 3 whereas CaCl 2 , (NH 4 ) 6 Mo 7 O 24 •4H 2 O and SDS were used as received.In a typical procedure, a (NH 4 ) 6 Mo 7 O 24 •4H 2 O-H 2 O stock solution was prepared by dissolving 1 mmol (NH 4 ) 6 Mo 7 O 24 •4H 2 O in 25 ml H 2 O at room temperature.A mixture of CaCl 2 (0.95 mmol, 105.44 mg), Eu(NO 3 ) 3 (0.05 mmol, 16.9 mg), SDS (1 mmol, 288.38 mg) and 15 ml H 2 O was heated to about 60 °C under vigorous stirring, forming a transparent solution.2 ml from the (NH 4 ) 6 Mo 7 O 24 •4H 2 O-H 2 O stock solution was slowly added into the mixture and the stirring was continued for another 10 min.Subsequently, the transparent solution was transferred to a 10 ml vial used for microwave synthesis and placed in the microwave reactor (Anton Paar Monowave 300).The vial was tightly sealed with a Teflon cap and then microwave heated at 90 °C for 10 minutes.Finally, the white coloured product appeared as a precipitate and was collected by centrifugation and washed three times with ethanol followed by deionized water to remove any unreacted reactants.For Er 3+ /Yb 3+ -doped CaMoO 4 , the same protocol was followed, except Er(NO 3 ) 3 and Yb(NO 3 ) 3 were used instead of Eu(NO 3 ) 3 .

Characterization
The XRD patterns were collected using a Rigaku-SmartLab diffractometer attached with a D/tex ultra detector and Cu K α source operating at 35 mA and 70 kV.The scan range was set from 10-70°2θ with a step size of 0.02°and a count time of 2 s.The samples were well powdered and spread evenly on a quartz slide.Field emission scanning electron microscopy (FESEM) images were taken on a SUPRA 55-VP instrument with patented GEMINI column technology.Prior to loading the samples into the chamber, they were coated with a thin film of gold-palladium in order to avoid charging effects.TEM images were taken on a UHR-FEG-TEM, JEOL; JEM 2100 F model using a 200 kV electron source.The samples were prepared by placing a drop of an aqueous dispersion of the nanocrystals on a carbon coated copper grid and the grid was dried in air.Thermogravimetric analysis was performed using a Mettler Toledo TGA 851 instrument under a N 2 atmosphere at a heating rate of 10°min −1 .Room temperature optical absorption spectra of all the samples were recorded on a Hitachi U4100 spectrophotometer.The samples were taken in a 3 ml quartz cuvette ( path length, 1 cm).The PL spectra were measured on a Horiba Jobin Yvon spectrometer equipped with a 450 W Xe lamp.The excitation and emission light was dispersed using a Czerny-Turner monochromator with an optical resolution of 1 nm.The emitted photons were detected using a Hamamatsu R928 detector.The output signal was recorded using a computer.For the upconversion measurements, the nanocrystal dispersion was excited with a 980 nm diode laser from a RgBLase LLC, which was coupled with a fibre with a core diameter of 100 μm.The output signal was measured using a Jobin Yvon spectrometer.The PL lifetime measurements were performed with a Horiba Jobin Yvon Fluorolog CP machine equipped with a pulsed Xe source operating at 25 W.

Results and discussion
The microwave method used for the synthesis of SDS-capped Ln 3+ -doped CaMoO 4 [Ln 3+ = Eu 3+ , Er 3+ /Yb 3+ ] nanocrystals led to the formation of water dispersible nanocrystals in the size range of 100 nm.The high water dispersibility suggests that the SDS coating is in the form of a bilayer.This is confirmed from the luminescence studies (vide infra).These nanocrystals are quite monodisperse and spherical in shape, as confirmed from the HR-TEM image shown in Fig. 1.The high monodispersity in the size is further substantiated by the narrow distribution obtained with DLS measurements (Fig. 1B).The hydrodynamic size of the nanocrystals obtained from the DLS measurements is about 120 nm.This is also supported by the SEM images shown in Fig. S1 in the ESI.† The formation of pure tetragonal phase CaMoO 4 nanocrystals is verified from the XRD patterns shown in Fig. 2. The observed diffraction peaks match very well with the standard pattern of tetragonal phase CaMoO 4 (ICSD PDF card no: 01-085-1267).
The strong diffraction peaks clearly indicate good crystallinity in the as-synthesized products.The absence of any additional peaks suggests that CaMoO 4 nanocrystals with high phase purity can be easily obtained through a microwave synthetic method.The binding of SDS molecules to the surface of the CaMoO 4 nanocrystals is confirmed by the FTIR spectra shown in Fig. 3. Briefly, a broad band centered at 3350 cm −1 due to O-H stretching vibrations is observed.The C-H stretching vibrations of the methylene (-CH 2 ) units are observed near 2850 cm −1 and 2922 cm −1 .The region between 600 to 900 cm −1 indicates C-H bending frequencies of -CH 2 units.For the pure SDS, two strong peaks are noted near 1090 cm −1 (symmetric stretching) and 1225 cm −1 (asymmetric stretching), which are assigned to the SvO groups of sulphate.These peaks shift to lower energy and merge in the case of the SDS coated nanocrystals, indicating the binding of the SvO groups of SDS to the nanocrystals.To further confirm the SDS coating on the surface of the CaMoO 4 nanocrystals, TGA analysis was conducted.The TGA curves for SDS coated CaMoO 4 and the free SDS are compared in Fig. S2 in the ESI.† For the free SDS, the major weight loss is from 168 to 350 °C due to the decomposition of the SDS molecules.For the SDS coated CaMoO 4 nanocrystals, the onset of decomposition is shifted to a higher temperature (220 °C).This substantiates the FTIR results that the SDS molecules are attached strongly to the CaMoO 4 surface.In addition, the narrow decomposition range indicates that all the nanocrystals are more or less evenly coated with the SDS.Optical studies of the SDS coated Eu 3+doped CaMoO 4 nanocrystals show some interesting characteristics.Upon UV (394 nm) excitation, the nanocrystals emit strong emission near 614 nm, as shown in Fig. 4, which is assigned to a 5 D 0 → 7 F 2 transition.In addition to the strong emission at 614 nm, there are less intense emission peaks observed near 590, 650 and 700 nm.These peaks are respectively ascribed to 5 D 0 → 7 F 1 , 5 D 0 → 7 F 3 and 5 D 0 → 7 F 4 transitions.The 5 D 0 → 7 F 2 transition, being quite intense, gives rise to bright red light from a 0.1 wt% aqueous dispersion of the Eu 3+ -doped CaMoO 4 nanocrystals (shown in the inset of Fig. 4).The 5 D 0 → 7 F 2 emission is hypersensitive in nature and its intensity depends on the local crystallite environment around the Ln 3+ ions.An intense 5 D 0 → 7 F 2 transition is generally   observed when Eu 3+ ions are present in a non-centrosymmetric environment.However, in CaMoO 4 , both Ca 2+ and Mo 2+ have S 4 point symmetry possessing an inversion centre.This suggests a weaker emission intensity for the 5 D 0 → 7 F 2 transition.We believe that the observed strong emission might be due to the nanocrystals possessing some defects or disorder, leading to distortion in the symmetry sites where the Eu 3+ ions occupy.This is verified by preparing bulk Eu 3+ -doped CaMoO 4 nanocrystals via a sol-gel process.The material is heated at 800 °C to achieve better crystallinity.For the bulk powder sample, the asymmetric ratio (614/591) is 1.75 whereas for the SDS coated nanocrystals, the corresponding value is 7.75, suggesting more distortions near the crystal sites of Eu 3+ in the case of the nanocrystals.The luminescence spectrum of the bulk material is shown in Fig. S3 in the ESI.† On the other hand, we believe there can be an alternative reason, such as a polarization effect, which alters the crystal field strength as well as affecting the transition probabilities.36a, 43 The luminescence lifetime of the 5 D 0 state of the Eu 3+ ions in the SDS coated CaMoO 4 :Eu 3+ -doped nanocrystals is about 2.16 ms, as calculated from the decay analysis (shown in Fig. S4, ESI †).The intense luminescence signal and longer lifetime of the Eu 3+ ions in the CaMoO 4 :Eu 3+ -doped nanocrystals is further supported by the quantum efficiency of the nanocrystals in an aqueous medium.The calculated quantum yield is about 40%, which is measured using quinine sulphate as the reference standard.The details of the measurement and the calculations are given in Fig. S5 in the ESI.† This value is higher than the calculated value of 15% for YVO 4 :Eu 3+ (5%) colloidal nanocrystals. 44Haase et al. reported a luminescence quantum yield equal to 38% for colloidal YVO 4 :Eu 3+ (15%) nanocrystals with an average size of 30 nm at room temperature. 45The above points clearly suggest that the SDS coated CaMoO 4 :Eu 3+ (5%) nanocrystals are a better matrix for Eu 3+ ions.To our knowledge, there are very few reports with a high quantum yield for Eu 3+ ion in water.To understand the reason for the increased quantum efficiency for Eu 3+ ions in CaMoO 4 nanocrystals, we prepared Eu 3+ -doped CaMoO 4 nanocrystals without the SDS coating under identical conditions.The observed lifetime for the Eu 3+ ions is 1.93 ms, which is lower compared to the SDS coated CaMoO 4 nanocrystals (Fig. S6,  ESI †).This implies that one of the reasons for the enhanced quantum efficiency for the Eu 3+ ions in the CaMoO 4 matrix is the strong SDS coating, which can probably reduce the number of defects near the surface of the nanocrystals.
The high water dispersibility of the nanocrystals after SDS coating suggests that SDS is mostly in the form of a bilayer, and thus SO 3 − is on the surface of the nanocrystals.The bilayer formation is further verified by the slow addition of toluene to the water dispersible nanocrystals, which upon slight sonication led to the nanocrystals moving to the toluene phase.This phase transfer is supported by the strong Eu 3+ emission from the nanocrystals dispersed in toluene (shown in Fig. S7, ESI †).It is interesting that the nanocrystals can be used both in organic and aqueous phases without additional surface chemistry.
The luminescence studies were extended to understand the upconversion ability of the CaMoO 4 nanocrystals by doping with Er 3+ /Yb 3+ ions.Due to their high absorption coefficient, Yb 3+ ions are used as a sensitizer in the upconversion process.Moreover, it has only one excited electronic state ( 2 F 5/2 ) which matches with the 980 excitation wavelength.The nanocrystals in H 2 O showed a weak emission whereas in toluene they exhibit an intense upconversion emission, which is shown in Fig. 5.The green emission peaks centered at 530 and 550 nm are assigned to the 2 H 11/2 → 4 I 15/2 and 4 S 3/2 → 4 I 15/2 transitions, respectively.
The other emission peak at 668 nm is assigned to the 4 F 9/2 → 4 I 15/2 transition.It is interesting to note that along with the characteristic green and red emission peaks, the nanocrystals exhibit a relatively strong peak close to 833 nm in the NIR region.This peak is assigned to the 4 I 9/2 → 4 I 15/2 transition.To the best of our knowledge, this NIR emission peak is less reported in the literature.To investigate the origin of this peak, we prepared CaMoO 4 nanocrystals doped with Er 3+ and Fig. 4 Emission spectrum of the SDS coated 0.1 wt% Eu 3+ -doped CaMoO 4 nanocrystals in water.The inset shows a digital image of the colloidal nanocrystals under 366 nm UV light.Fig. 5 Upconversion emission spectrum of the SDS coated Er 3+ /Yb 3+doped CaMoO 4 nanocrystals in toluene, measured by exciting the sample with a 980 nm diode laser at a 400 mW laser power.
Yb 3+ independently using a similar synthetic protocol.Hardly any emission was observed near 830 nm from both the samples upon exciting at 980 nm.These results suggest the absence of excited state absorption as well as scattering of light from the Yb 3+ ions in the nanocrystals.The results are shown in Fig. S8 in the ESI.† To understand the mechanism responsible for the UC emission observed above, we prepared CaMoO 4 :Yb 3+ /Er 3+ nanocrystals without the SDS coating.The UC emission spectrum of this sample, shown in Fig. 6, is quite different from that obtained with the SDS coated nanocrystals.A very strong green UC emission is observed for the nanocrystals prepared without the SDS coating.For the SDS-coated nanocrystals, the intensity ratio I (NIR) /I (red) is about 1.40 whereas the corresponding value is only 0.38 for the nanocrystals without the SDS coating.These results suggest that the NIR emission is probably associated with surface properties of the nanocrystals.The number of photons involved in the energy transfer process is understood by conducting power dependent studies as the laser power, P α I n , where I is the intensity of the upconversion emission and n is the number of photons involved in generating the upconversion emission.A logarithmic plot of the laser power against the emission intensity gives a slope of n.As shown in Fig. S9 and S10 in the ESI, † the slopes (n) of the plots are 1.89 and 2.46 for the red and NIR emissions, respectively.Based on the above results, we propose the following mechanism, which is schematically shown in Fig. 7. First, Er 3+ in the ground state is excited to 4 I 11/2 via energy transfer (ET) from the Yb 3+ ions.Because of the short lifetime of the 4 I 11/2 state, most of the ions will non-radiatively decay and populate the 4 I 13/2 state.The second energy transfer will excite the Er 3+ ion and populate the 4 F 9/2 state.The red emission occurs from this level.In addition, some of the excited Er 3+ ions are excited higher in the energy to the 2 H 9/2 state via the third energy transfer.Due to the presence of energy levels which are close enough, the excited Er 3+ ions reach 2 H 11/2 / 4 S 3/2 non-radiatively.We propose the following cross-relaxation ( 2 H 11/2 / 4 S 3/2 , 4 I 13/2 )→( 4 I 9/2 , 4 I 9/2 ) occurs at this stage, thus populating the 4 I 9/2 state.In the absence of such a cross-relaxation, intense green emission occurs from the 2 H 11/2 / 4 S 3/2 states.Though the reason for the cross-relaxation is not clear, we presume that the SDS ligands might have a role.
Finally, we have extended the study to investigate the photocatalytic activity of the CaMoO 4 nanocrystals.The photocatalytic activity of PbMoO 4 and BiVO 4 with a scheelite structure has been reported. 46,47To the best of our knowledge, there is no report on the photocatalytic activity of CaMoO    that over half the concentration of the dye is degraded in the first 120 minutes.The RhB dye degradation follows apparent first order kinetics, in agreement with a general Langmuir-Hinshelwood mechanism; where, r is the degradation rate of the dye (mg min −1 ), C is the concentration of the dye (mg l −1 ), t is the illumination time, k 2 is the adsorption coefficient of the dye (mg −1 ), and k 1 is the reaction rate constant (mg min −1 ).If C is very small then the above equation could be simplified to lnðC 0 =CÞ ¼ k 1 k 2 t ¼ kt where C 0 is the initial concentration of the dye, C is the concentration of the dye after a certain time t with UV illumination, k is the rate constant and t is the time.A plot of ln(C 0 /C) vs. t (shown in Fig. S12, ESI †) provides a slope value of 0.0274 min −1 , which is nothing but the rate constant (k) of the dye degradation by the CaMoO 4 nanocrystals in the photocatalytic process.It is interesting to note that the resulting value is comparable with the reported value for a ZnO photocatalyst. 48

Conclusions
In this article, we have reported the microwave synthesis of sodium dodecyl sulphate coated Ln 3+ -doped CaMoO 4 nanocrystals (Ln 3+ = Eu 3+ , Er 3+ /Yb 3+ ).SEM and TEM measurements suggest the average size of the nanocrystals to be 100 nm.FTIR and TGA analyses confirm that the SDS binds strongly to the nanocrystals.The high water dispersibility of the nanocrystals after the SDS coating suggests that the SDS coating is in the form of a bilayer.Upon UV excitation, the Eu 3+ -doped CaMoO 4 nanocrystals show strong red emission, characteristic of Eu 3+ ions, with a high quantum efficiency close to 40%.Under 980 nm excitation, the Er 3+ /Yb 3+ -doped CaMoO 4 nanocrystals exhibit upconversion luminescence in toluene.In addition, the nanocrystals show strong photocatalytic activity, which is verified from RhB dye degradation under UV illumination.The high quantum efficiency and water dispersibility of the Ln 3+ -doped CaMoO 4 nanocrystals mean they can be used as potential materials for dual applications.

Fig. 1
Fig. 1 TEM image (A) and particle size distribution from DLS (B) of the water dispersed CaMoO 4 :Eu 3+ nanocrystals, indicating the formation of monodispersed nanocrystals.

Fig. 3
Fig. 3 FTIR spectra of (A) the free SDS molecules, (B) the SDS capped nanocrystals in H 2 O.
4 nanocrystals.Because of the relatively high band gap energy of CaMoO 4 (E g = 3.41 eV), the molybdate was tested as a photocatalyst under UV irradiation.The conduction band of CaMoO 4 is located above the RhB redox potential (E 0 RhB •+ = 1.34 eV) versus the standard calomel electrode (SCE), allowing CaMoO4 to be catalytically active.We believe that reactive oxygen species (e.g., HO • , O 2 −• , etc.), especially HO • radicals, can oxidize the organic dye molecules and perform photocatalysis.The Rhodamine B (RhB) dye degradation was demonstrated over the surface of the CaMoO 4 nanocrystals under UV illumination to elucidate the photocatalytic activity.To ensure good adsorption of the dye onto the nanocrystals surface, the photocatalytic property was studied without the SDS coating.Fig. 8 shows that the RhB dye is significantly degraded by ∼80% under UV illumination within 4 hours.The pie chart diagram of RhB dye degradation (see Fig. S11, ESI †) reveals

Fig. 8
Fig. 8 UV-Vis spectra of RhB dye as a function of time over the CaMoO 4 nanocrystals.

Fig. 6
Fig.6Upconversion emission spectrum of the Er 3+ /Yb 3+ -doped CaMoO 4 nanocrystals (without the SDS coating), measured by exciting the sample with a 980 nm diode laser operating at a 400 mW laser power.