Design of nanostructured cadmium tantalate and niobate and their photocatalytic properties

Article abstract of DOI:10.1039/c3ra42667j

In the present study a template-free hydrothermal method under optimal alkaline conditions and suitable temperature leads to growth of nanocubes (～15 nm edge) of cadmium tantalate and cadmium niobate which show significantly higher (nearly seven times) photocatalytic activity compared to that obtained by the same oxides by the solid state method. The above synthesis is carried out at much lower temperature (600 °C) than the solid state method (1000 °C). The evolution of the nanostructures as a function of reaction time (hydrothermal synthesis) was followed by microscopy and the optimal time of 48 h leads to oxides with high photocatalytic efficiency. Nanostructures with cuboidal morphology show high surface area (57 m² g^-1) and highly negative surface charge and this leads to enhanced adsorption of the cationic dye (Rhodamine B). These nanoparticles are highly stable under UV light irradiation for a long period of time and retain the photocatalytic efficiency. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra42667j

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Table of Contents:
Cuboidal shaped Cd2Ta2O7 and Cd2Nb2O7 nanoparticles via template free hydrothermal route
crystallizing in pyrochlore structure. High surface area and optimal surface charge leads to
efficient degradation of Rhodamine B dye.
1

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Design of nanostructured cadmium tantalate and niobate and their photocatalytic
properties
Debashree Das a and Ashok K Ganguli a,b*
a
Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi 110016,
INDIA
b Institute of Nano Science & Technology, Phase – X, Mohali, Punjab, 160062, INDIA
*Author for Correspondance
eꢀmail: ashok@chemistry.iitd.ernet.in
Tel no:91ꢀ11ꢀ26591511
Fax: 91ꢀ11ꢀ26854715
1

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Abstract
In the present study a template – free hydrothermal method under optimal alkaline conditions
and suitable temperature leads to growth of nanocubes (~ 15 nm edge) of cadmium tantalate and
cadmium niobate which show significantly higher (nearly seven times) photocatalytic activity
compared to that obtained by the same oxides by the solid state method. The above synthesis is
carried out at much lower temperature (600ºC) than the solid state method (1000ºC). The
evolution of the nanostructures as a function of reaction time (hydrothermal synthesis) was
followed by microscopy and the optimal time of 48h leads to oxides with high photocatalytic
efficiency. Nanostructures with cuboidal morphology show high surface area (57m2/g) and
highly negative surface charge and this leads to enhanced adsorption of the cationic dye
(Rhodamine B). These nanoparticles are highly stable under UV light irradiation for a long
period of time and retain the photocatalytic efficiency.
Keywords: hydrothermal, nanocubes, band gap, adsorption, Photocatalysis.
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Introduction
Oxide semiconductor based photocatalysis has attracted extensive attention due to its wide
application of photon energy conversion from the viewpoint of solar energy utilization in
environmental processing such as air purification, water disinfection, hazardous waste
remediation and degradation of toxic compounds.1ꢀ5 Among the semiconducting oxides, TiO2
(UV ꢀ photocatalyst) is most widely studied due to its chemical stability, nontoxicity, resistance
to corrosion and high photocatalytic activity. Although extensive work has been carried out
towards the improvement of the photocatalytic performance of TiO2, studies have shown that
TiO2 suffers from deactivation during the photocatalytic process after few cycles due to
adsoption of intermediates on the TiO2 surface and thereby blocking the active sites.6,7 To
overcome this, continuous efforts have been devoted in the search for other photocatalytic
materials. One of the indispensable properties required for materials to drive the photocatalytic
reaction is that the valence and conduction band of semiconductor oxides should have
appropriate potentials alongwith efficient generation of photoexcited electrons (eꢀ) and holes
(h+).8 Alongwith the above the ease of migration of photoexcited electrons and holes to the
surface of the catalyst material leading to fast reduction and oxidation of surfaceꢀadsorbed dye /
organic moiety for their decomposition is also important. In order to meet all these requirements
new materials are being explored and examined for the purpose of photocatalytic degradation of
organic waste and for facilitating water splitting.9ꢀ12
The conduction band of tantalates consisting of 5d orbitals is located at a more negative potential
than that of titanates (3d) and niobates (4d). The lower potential of the conduction band is
advantageous towards the photocatalysis reactions. There are some Ta5+ containing compounds
like Sn2Ta2O7, BaTa2O6, K3Ta3B2O12, and NiTa2O6 which have been studied for photocatalytic
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water splitting under UV light irradiation.13ꢀ16 The structure of the photocatalyst plays an
important role for the smooth migration of electronꢀhole pairs. Tantalates with corner ꢀ shared
TaO6 octahedra possess TaꢀOꢀTa bond angles close to 180˚ and this accounts for the easy
migration of photogenerated electronꢀhole pairs in the corner – shared framework of TaO6 units,
which is beneficial for the photocatalytic reactions.10,17 Several tantalates and niobates crystallize
in the ABO3(perovskite) or A2B2O7(pyrochlore) structure where the BꢀOꢀB bond angle is close
to 180˚. Pyrochlores include a large number of compounds with the general formula A2B2X6X’,
in which A and B are metal atoms and X and X’ are anions at different crystallographic sites.18, 19
In pyrochlore – based oxides however both X and X’ are ‘O’ atoms. Tantalates and niobates
have been synthesized by conventional solid state route or the polymer precursor method at high
temperature (typically 1000ꢀ1300˚C).20,21 The major problems with high temperature methods
are larger particle size, segregation of components and possible loss of stoichiometry due to
volatilization of more volatile constituents. These factors lead to decrease in the photocatalytic
activity. In addition, for synthesis of catalysts, low temperatures are desirable to avoid particle
growth which leads to a low surface area. As a result, low – temperature, wet chemical routes
have been employed to prepare nanosized crystallites with higher purity and uniform sizes to
enhance photocatalytic activity.22ꢀ24 Hydrothermal method is a low temperature route and
generates highly crystalline products with narrow size distribution and less aggregation. The
morphology and size of the products can be controlled by adjusting the hydrothermal reaction
conditions.
In this work, we describe a hydrothermal route for the synthesis of Cd2Ta2 O7 and Cd2Nb2O7
nanoparticles, and also their photocatalytic activity. There is one patent reported on the synthesis
of nanoscale cadmium tantalate photocatalyst by solꢀgel method at 900˚C.25 In photocatalysis,
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Wang et al. and Guo et al. have studied the photocatalytic efficieny of Cd2Ta2O7.7,26 However,
little work has been done to investigate in detail the photocatalytic activity of Cd2Nb2O7 and
Cd2Ta2O7 nanoparticles for the organic contaminants degradation. This is the first time we are
reporting synthesis of cadmium tantalate and cadmium niobate by a template free hydrothermal
method, which was optimized to yield nanocubes of 15 nm size. The aim was to control the
surface morphology and surface charge and to enhance the photocatalytic degradation of
Rhodamine B. The above studies show the enhanced efficiency of these nanostructured material
compared with niobate and tantalate particles obtained by the solid state method and also better
than TiO2 in term of stability.
Experimental Section
For synthesis of Cd2Ta2O7 and Cd2Nb2O7, cadmium nitrate (SigmaꢀAldrich, 99.0%), tantalum
pentachloride (Alfa Aesar, 99%) and niobium pentachloride (Alfa Aesar, 99.8%) were used.
In a typical synthesis of Cd2Ta2O7 nanoparticles, tantalum pentachloride solution (0.2 mol) in 25
mL ethanol and 25 mL aqueous solution of cadmium nitrate (0.35mol) were first stirred for 15
min. The white slurry was loaded into a Teflonꢀlined stainlessꢀsteel container containing NaOH
solution with continuous stirring. The container was sealed and heated at 180˚C for 48h under
autogenous pressure. The vessel was cooled to room temperature and the resulting particles were
collected by centrifugation. The precipitate was washed thoroughly with ethanol and deionized
water. The product was dried at 60˚C for 6h in an oven and further heated at 600˚C for 8h to
yield the pure Cd2Ta2O7 nanoparticles. Similar procedure was followed for the synthesis of
Cd2Nb2O7 nanoparticles taking niobium pentachloride and cadmium nitrate as the reactants.
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Cd2Ta2O7 and Cd2Nb2O7 (bulk, micron sized) were also synthesized by solid state route by taking
CdO (SigmaꢀAldrich, 99.99%), Nb2O5 (SigmaꢀAldrich, 99.99%) and Ta2O5 (SigmaꢀAldrich,
99.99%) in stoichiometric ratio and heated at 1000˚C for 12h.
Characterization
The structure of the synthesized materials were confirmed by powder XꢀRay diffraction studies
on a Bruker D8 Advance Diffractometer using Niꢀfiltered CuꢀKα radiation. The crystallite size
was deduced using the Scherrer formula: d = 0.9λ/ (BsꢀBr) cosθ, where λ is the Xꢀray
wavelength, Bs indicates the full width at half maxima of the sample, while Br is that of a
standard(Quartz). PXRD data for Rietveld refinement was collected using a step size of 0.02⁰
and a step time of 8 s per step were used. Structural refinement was carried out using the TOPAS
software 27 in the space group (Fd
3
m). The background was modeled by a shifted Chebyschev
polynomial and the peak profile was simulated with a pseudoꢀVoigt function. The refinement
involved the cell parameter, scaling factor, positional parameters, and isotropic thermal
parameters. Diffuseꢀreflectance spectra (DR) spectra were recorded on Shimadzu UVꢀ2450
spectrophotometer where the baseline was fixed using a barium sulfate reference. The
fluorescence intensities were measured using HORIBA fluromaxꢀ4 spectrofluorometer. Nitrogen
adsorptionꢀdesorption isotherms were recorded at liquid nitrogen temperature (77K) on a Nova
2000e (Quantachrome corp.) and the surface area was determined by the BrunauerꢀEmmettꢀ
Teller (BET) method. The asꢀprepared Cd2Ta2O7 and Cd2Nb2O7 samples were degassed at 150˚C
for 12h prior to the surface area measurements.
The nanoparticles were studied by transmission electron microscopy (TEM) on an FEI Technai
G2 20 electron microscope operated at 200KV. Samples were dispersed in ethanol in an
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ultrasound bath for few minutes and a few drops of suspension were deposited on carbon coated
microgrid for the TEM studies.
Photocatalytic Tests
The photocatalytic efficiency of Cd2Ta2O7 and Cd2Nb2O7 nanoparticles under UV light was
estimated by measuring the decomposition rate of Rhodamine B in an aqueous solution. The
photocatalytic reactions were carried out in a quartz reactor with a water circulating jacket under
UV light irradiation with a 125W high pressure Hg lamp equipped with a liquid filter to remove
IR radiation. Experiments were also performed in the absence of photocatalyst and
illumination(blank tests). Rhodamine B was selected as a model pollutant because it is a
common contaminant present in industrial waste water.28 100ml of Rhodamine B solution at
concentration of 15 ꢁM containing suspended photocatalyst (80 mg) was stirred continuously
using a magnetic stirrer during the photocatalytic processs. After 30 min of stirring in dark to
attain the adsorption equilibrium, the UV lamp was positioned above the reactor. Aliquots of
1cm3 of the aqueous suspension were collected at regular time periods during irradiation and
absence of light. These aliquots were centrifuged to remove photocatalyst particles. Rhodamine
B concentration was estimated using UVꢀvis spectroscopy at its maximum absorption. The
photocatalytic decomposition experiments were done for Degussa P25 TiO2 (80 mg with 55m2/g
surface area). The whole catalytic process was carried out under O2 bubbling.
Analysis of hydroxyl radicals (•OH)
Hydroxyl radicals (•OH) produced by the photocatalysts under UV light were measured by
fluorescence method using terephthalic acid (TA) as probe molecule. 80 mg of photocatalyst was
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dispersed in 100 mL of aqueous solution containing 3 X 10ꢀ3 M TA aqueous solution in a diluted
NaOH aqueous solution (10 X 10ꢀ3 M ). The resulting suspension was then exposed to UV light.
At regular intervals, 5 mL of the suspension was collected and centrifuged to measure the
maximum fluorescence emission intensity with an excitation wavelength of 315 nm. This
method relies on the fluorescence signal at 426 nm of the hydroxylation of terephathic acid with
generated •OH.29
Results and Discussion
Fig 1a and 1b shows the XRD patterns of Cd2Ta2O7 and Cd2Nb2O7 obtained by heating the
product (obtained hydrothermally at 180˚C for 48h and 0.5M alkali concentration) at 600˚C for
8h. In the hydrothermal synthesis of Cd2Ta2O7 and Cd2Nb2O7 nanoparticles, the concentration of
alkali is a key factor that affects the purity of the final product.30 Therefore, detailed
investigations have been carried out with varying NaOH concentration, reaction time and
temperature in the preparation of the above nanoparticles. Hydrothermal reaction was carried out
over a temperature range between 40˚C and 180˚C for varying time period of 6h to 48h. The
white product (precursor, Fig S1) obtained after hydrothermal treatment was heated at 600˚C for
8h. XRD patterns of precursor of Cd2Ta2O7 and Cd2Nb2O7 indicates not well crystalline powders
and shows presence of Ta2O5 and Nb2O5. In the case of Cd2Ta2O7 when NaOH concentration was
kept at 0.5 mol.dmꢀ3 the products were found to be monophasic and all the reflections could be
indexed on the cubic pyrochlore structure (Fd
3
m). The average crystallite size of the particles
31,32
obtained from the Xꢀray line broadening studies (using Scherrer’s equation)
of the (222)
reflection was found to be 20 nm and 25 nm for Cd2Ta2O7 and Cd2Nb2O7 respectively which is
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in accordance with TEM studies. Pure phases of Cd2Ta2O7 and Cd2Nb2O7 were stabilized in the
cubic phase at high alkali content.
The structure of the ternary oxides (Cd2Ta2O7 and Cd2Nb2O7 nanoparticles) obtained by
hydrothermal treatment at 180˚C for 48h followed by calcination at 600˚C for 8h could be
refined satisfactorily (Rietveld method) with a refined composition of Cd2Ta2O7 and Cd2Nb2O7
respectively (Fig 2 a and b ). The Rꢀfactor and χ2 value indicate a good quality fit. The relevant
crystallographic parameters are included in Table 1a and b.
Role of alkali concentration: When the NaOH content was between 0.1 and 0.3M a mixture of
Cd2Ta2O7 and CdO was observed. At low alkali content CdO is observed as the secondary phase.
Similar results were observed in the case of Cd2Nb2O7 (see Figure S2 and S3 in the Supporting
Information). Thus, monophasic cadmium tantalate and cadmium niobate phases could be
obtained only at higher alkali content. The presence of alkaline hydroxylated TaꢀCl ionic groups
formed under strongly alkaline condition is the key to the stabilization of the Cd2Ta2O7 and
Cd2Nb2O7 phase.33,34 With the addition and increase in NaOH concentration, the amount of the
hydroxylated TaꢀCl ionic groups increase, resulting in the increase in amount of Cd2Ta2O7 and
Cd2Nb2O7 in the final product and pure phase was obtained at 0.5 M of NaOH under
hydrothermal condition. The phase purity also depends on reaction time and temperature.
Role of reaction time and temperature: The temperature and reaction time for obtaining pure
Cd2Ta2O7 and Cd2Nb2O7, was optimized by the hydrothermal process at various temperatures
from 40˚C to 180˚C and varying time period (6h to 48h) with a fixed alkali concentration of
0.5M NaOH solution. As shown in the Supporting Information, (Figures S4 and S5), it is
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observed that at 80˚C the XRD pattern shows CdO implying incomplete reaction.While, pure
Cd2Ta2O7 and Cd2Nb2O7 were obtained when the precursor was synthesized at a hydrothermal
temperature of 180˚C. The requirement of higher temperature (180˚C) and pressure
(hydrothermal condition) for the synthesis of pure phase of Cd2Ta2O7 and Cd2Nb2O7 is guided by
thermodynamics. Under normal conditions TaꢀCl bond is difficult to hydrolyze but under high
temperature and pressure provided by the hydrothermal conditions, TaCl5 can be dissolved and
hydroxylated leading to the formation of the oxide. Thus, the precursor obtained by
hydrothermal treatment at 180˚C followed by calcination at 600˚C for 8 h leads to the formation
of Cd2Ta2O7 and Cd2Nb2O7.
In addition to temperature, the reaction time also has a strong effect on the formation of pure
crystalline Cd2Ta2O7 and Cd2Nb2O7 nanoparticles. Figure S6 in the Supporting Information
shows the XRD pattern of Cd2Ta2O7 and Cd2Nb2O7 heated at 180˚C for 24h and 48h. It is
observed that Cd2Ta2O7 and Cd2Nb2O7 starts forming when the precursor is further calcined at
600˚C for 8 h after nearly 24 h of hydrothermal treatment at 180˚C. The intensity of xꢀray
reflections become stronger as the hydrothermal reaction time is increased to 48h and well
crystalline Cd2Ta2O7 and Cd2Nb2O7 phases are obtained after 48h of hydrothermal reaction
followed by calcination at 600˚C for 8 h. In the TEM studies of the product obtained (after the
hydrothermal treatment of the precursor for 24h followed by calcination at 600˚C for 8 h), small
spherical particles of Cd2Ta2O7 and Cd2Nb2O7 were observed alongwith nanocubes (see Figure
S8 in the Supporting Information). As the reaction time is increased, more nanocubes (average
edge length of 15 nm) are formed and these grew at the cost of smaller particles (fig 3). This is
due to difference in solubility between the large and small particles according to Gibbsꢀ
Thomson Law and is a typical hydrothermal ripening process where a supersaturated solution
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acts as the precursor for the formation of crystalline nanoparticles.35 In the early stages,
examination of the precursor showed the formation of irregular nanoparticles which on
prolonged heating led to the growth of nanocubes at the cost of the smaller particles.
We have also carried out the reaction at higher temperature as well as with longer duration of
hydrothermal conditions i.e. at 200˚C and 72 h and on calcination of the precursor at 600˚C for
8h we observed formation of pure phase of Cd2Ta2O7 and Cd2Nb2O7. Our aim was to lower the
synthesis temperature to obtain smaller sized particles as we are studying the catalytic properties
of Cd2Ta2O7 and Cd2Nb2O7 nanoparticles, would be dependent on the particle size (surface area).
Since with increase in the hydrothermal temperature and reaction duration, the particle size
increases (thereby decreasing the surface area) the photocatalytic performance will be lowered.
Therefore, to increase the surface area for better adsorption and thereby to enhance the
photocatalytic efficiency, the best hydrothermal reaction conditions for the synthesis of
Cd2Ta2O7 and Cd2Nb2O7 nanoparticles is heated at 180˚C for 48h, followed by calcination at
600˚C for 8h. In addition, when the precursor was calcined at 400℃ and 500℃, we observed
presence of CdO as secondary phase alongwith Cd2Ta2O7 indicating incomplete reaction (fig S7a
and S7b). Thus, we have chosen 600˚C as the optimal temperature.
From TEM micrographs it is observed that nanocubes of Cd2Ta2O7 and Cd2Nb2O7 of size 15ꢀ30
nm were obtained by the hydrothermal method at 180˚C for 48h and heating at 600˚C for 8h (Fig
3). On heating at 800˚C nanocubes with larger grain size were obtained (Fig S 9a and 9b). The
lattice fringes spacing of 0.299 nm and 0.298 nm assigned to (222) planes of Cd2Ta2O7 and
Cd2Nb2O7 (fig3b and 3d) could be seen in the high – resolution TEM (HRTEM). TEMꢀEDX
shows the presence of constituent elements and further confirms the purity of the synthesized
nanoparticles (fig S9 c and d). We have also performed FESEMꢀEDX on the Cd2Ta2O7 and
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Cd2Nb2O7 nanoparticles (fig S9 e and f) and did not observe any presence of sodium in the
synthesized materials indicating absence of sodium incorporation into these oxides.
We have also synthesized Cd2Ta2O7 and Cd2Nb2O7 by the solid state route to compare the size of
particles which was found to be in the range of 100 ꢀ130 nm (Fig S10).
The optical properties of Cd2Ta2O7 and Cd2Nb2O7 were probed by diffuse reflectance studies.
The band gap estimated from the absorption edge and was found to be 3.5 eV and 3.4 eV for
Cd2Ta2O7 and Cd2Nb2O7 nanocubes respectively (Fig 4). In general, the wavelength at the
absorption edge for a semiconductor is determined as the intercept on the photon energy axis,
Energy band gap of the synthesized materials were calculated using the formula (Eg = 1240/ λ).36
The specific surface area for Cd2Ta2O7 and Cd2Nb2O7 nanocubes synthesized by the
hydrothermal method was found to be 57.6 m2/g and 53.6 m2/g. whereas, for solid state
synthesized Cd2Ta2O7 and Cd2Nb2O7 the surface area was much lower and was found to be 7.3
m2/g and 3.6 m2/g. The surface area plays a crucial role in determining the photocatalytic activity
of the Cd2Ta2O7 and Cd2Nb2O7 nanocubes, which accounts for increased adsorption of dye on
the surfaces of Cd2Ta2O7 and Cd2Nb2O7 nanocubes.
Therefore, we have successfully optimized the reaction parameters, where the calcination
parameters were reduced from 1000ºC 12h (solid state method) to 600 ºC 8h with hydrothermal
method. Hydrothermal route incorporated with various reaction parameters(concentration of
alkali, hydrothermal reaction time and temperature) aided in the formation of smaller sized
particle and inhibited their agglomeration, resulting in large specific surface area.
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Photocatalytic properties of Cd2Ta2O7 and Cd2Nb2O7 nanocubes
The photocatalytic efficiency of Cd2Ta2O7 and Cd2Nb2O7 nanocubes was estimated by
photodegradation reactions of Rhodamine B under UV irradiation. The UV absorption maxima
of Rhodamine B (λ ~546nm) was recorded at regular intervals. Before the photocatalytic reaction
a blank study was carried out in which the dye solution containing the dye (Rhodamine B) was
illuminated in the absence of the photocatalyst to examine its stability. The dye did not
decompose even after long time (2h) of illumination. In addition the concentration of dye nearly
remained same (absorption intensity remain constant) under dark conditions and Cd2Ta2O7 and
Cd2Nb2O7 nanocubes and dye solution after certain time reached the adsorption – desorption
equilibrium. Fig 5 shows the degradation plot for Rhodamine B in the presence of Cd2Ta2O7 and
Cd2Nb2O7 nanocubes synthesized by hydrothermal and solid state method at pH 8.5 under O2
purging. It is observed that photocatalytic efficiency of Cd2Ta2O7 and Cd2Nb2O7 nanocubes
synthesized by hydrothermal method is much more efficient than that synthesized via the solid
state route. The smaller particle size and high surface area increases the probability of the surface
reaction of electrons and holes with adsorbed dye during the course of photocatalytic reaction
comparison with recombination reactions in the bulk. The adsorption of the dye on the catalyst
surface is regarded as a critical step for efficient photocatalysis and this is highly influenced by
the crystallinity, size, surface charge and surface area. The photocatalytic performance of
Cd2Ta2O7 and Cd2Nb2O7 nanocubes synthesized by hydrothermal route is nearly 7 times more
efficient than samples synthesized by solid state route. In addition, Cd2Ta2O7 nanocubes
synthesized by hydrothermal route showed almost comparable degradation rate of photocatalysis
with commercial TiO2 (Fig6).
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The enhanced photocatalytic efficiency of the hydrothermally synthesized Cd2Ta2O7 and
Cd2Nb2O7 nanocubes compared to those obtained by the solid state route is mainly due to the
small size of the particles and high surface area. We investigated the role of pH on the rate of
photocatalytic degradation shown by Cd2Ta2O7 and Cd2Nb2O7 nanocubes (see Figure S11 in the
Supplementary Information). The rate of photocatalytic activity of Cd2Ta2O7 and Cd2Nb2O7
nanocubes was significantly higher under basic pH (8.5) with continuous purging of oxygen
during the irradiation. Absence of oxygen or lower pH (4.5) decreased the photocatalytic
efficiency. The reason for this difference is the extent of adsorption of dye on the surface of
catalyst. From the zeta potential measurements, the surface charge on Cd2Ta2O7 and Cd2Nb2O7
nanocubes at pH 8.5 was found to be ꢀ28.7mV and ꢀ26.8 mV. Whereas, at pH 4.5 the surface
charge on Cd2Ta2O7 and Cd2Nb2O7 nanocubes was observed to be 16.2 mV and 17.8mV. Under
basic conditions, since the surface of the catalyst is negatively charged, the dye can be easily
adsorbed on the catalyst surface (Rhodamine B being a cationic dye) leading to increase in the
rate of photocatalysis. In acidic media, both Rhodamine B molecules and catalyst surface are
positively charged. Because of the strong electrostatic repulsion due to the similarly charged
surfaces, Rhodamine B molecules cannot adsorb easily on the Cd2Ta2O7 and Cd2Nb2O7
nanocubes surface and hence the cationic dye is electrostatically destabilized resulting in lesser
adsorption and lower degradation rate of dye.37 In addition the samples obtained by the solid
state route were comprised of aggregated particles. It has been reported that large value of the
surfaceꢀtoꢀvolume ratio increases the number of surface active sites where the photogenerated
electronꢀhole pairs are able to induce more hydroxyl and superoxide radicals to react with
adsorbed molecules.38,39
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Photocatalytic mechanism
In photocatalysis, it has been widely accepted that hydroxyl radicals (•OH) generated by the
illumination of photocatalyst are considered to be the main oxidation agents for the degradation
of organic pollutants. The formation of •OH on the surface of photocatalysts was detected by the
fluorescence technique using TA as a probe molecule, which readily reacts with •OH to produce
the highly fluorescent product, 2ꢀhydroxyterephthalic acid (TAOH). The fluorescence intensity
of TAOH is proportional to the amount of •OH produced on the surface of photocatalysts. The 2ꢀ
hydroxyterephthalic acid on excitation at 315 nm shows emission around 426 nm. Using
terephthalic acid as the probe molecule, the •OH produced by photocatalyst nanostructures under
UV light was measured by fluorescence method Figure 7. The increase in the intensity of
photoluminescent intensity of 2ꢀhydroxyterephthalic acid with increasing irradiation time is
directly proportional to the •OH generated during the photocatalytic process under UV light. The
above result clearly suggested that the photocatalysts (Cd2Ta2O7 and Cd2Nb2O7) were
photocatlytically active under UV light and the extent of degradation ability was consistent with
the rate of formation of the active •OH under UV irradiation. The mechanism for the
photocatalytic degaradtion of RhB can be proposed as follows:
Photocatalyst + hν
Photocatalyst ( eꢀCB… + h+
●OH
)
VB
Photoatalyst (h+VB) + H2O
ꢀ
eꢀ + O2
●O2
ꢀ
●
●O2 + H2O
HO2 + OHꢀ
●
HO2 + H2O
●OH + H2O2
H2O2
2●OH
CO2 + H2O
●OH + RhB
15

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Where, eꢀCB and h+VB stand for the electron in the conduction band and hole in the valence band,
respectively.
Further, it is observed that when the sample contained homogenously distributed nanocubes of
Cd2Ta2O7 and Cd2Nb2O7, the photoactivity is higher compared to the sample with spherical
particles alongwith nanocubes of Cd2Ta2O7 and Cd2Nb2O7 (see the Supplementary Information ꢀ
in the inset of Figure S11). Highly dispersed and uniformly sized catalyst particles provide
higher reaction rates.40 The photocatalytic activity of cadmium tantalate is found to be better than
cadmium niobate which can be attributed to the Taꢀ5dꢀconduction band being located at a more
negative potential compared to Nbꢀ5d band. Another advantage of these cadmium based
photocatalyst materials is the presence of Cd2+, which acts as a poor recombination centre due to
the stable +2 oxidation state of Cd, thereby facilitating the reaction efficiently towards the
degradation of dye. Using the equation, ln(Co/C) = kt, where Co and C are the concentration of
Rhodamine B solution, respectively, at time 0 and t, we have employed the pseudo first order
model to calculate the rate constant (k) of the photodegradation process. Our observations
revealed that the cadmium tantalate nanocubes showed nearly twice the decomposition rate
under UV irradiation with a pseudo first order rate constant of 0.859 hꢀ1 compared to niobate
nanocubes with a rate constant of 0.414 hꢀ1 and nearly seven times photocatalytic activity
compared to that obtained by Solid state method 0.177 hꢀ1(Cd2Ta2O7) and 0.119 hꢀ1 (Cd2Nb2O7).
Further, to check the reusability of the Cd2Ta2O7 and Cd2Nb2O7 nanocubes, reactions were
carried out on the same catalyst for five cycles (see Figure S12 in the Supporting Iinformation).
The synthesized Cd2Ta2 O7 and Cd2Nb2O7 nanoparticles demonstrated very stable activity under
UV light irradiation for a longer period of time compared to commercial TiO2. The degradation
rate was almost same for nearly four consecutive cycles for Cd2Ta2O7 and Cd2Nb2O7 (Fig 8)
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nanoparticles indicating stable performance by these photocatalyst materials and not deactivated
during the photodegradation of the organic dye; thus it has potential applications in treating
contaminated water. Figure S13 in the Supplementary Information shows XRD of Cd2Ta2O7 and
Cd2Nb2O7 before and after photocatalytic reaction. We observe that Cd2Ta2O7 and Cd2Nb2O7
quite stable since no modification was observed in the XRD peaks. The observations indicate
that rate of photocatalytic degradation of Rhodamine B by Cd2Ta2O7nanocubes is higher than the
rate for Cd2Nb2O7 nanocubes and the synthesized Cd2Ta2O7 and Cd2Nb2O7 nanoparticles can
perform as a novel photocatalyst material towards degradation of organic pollutants.
Conclusions
Cd2Ta2O7 and Cd2Nb2O7 nanoparticles of cubic morphology have been successfully synthesized
by a template free hydrothermal route. The optimal condition for the formation of pure phase of
Cd2Ta2O7 and Cd2Nb2O7 nanoparticles was obtained by hydrothermal treatment at 180˚C for 48h
followed by calcination at 600˚C for 8 h. The various synthetic parameters such as concentration
of alkali, reaction temperature and reaction time play important role in the formation of
monophasic Cd2Ta2O7 and Cd2Nb2O7 nanoparticles. Photocatalytic activity towards Rhodamine
B indicated that nanosized Cd2Ta2O7 and Cd2Nb2O7 obtained by hydrothermal method show high
photocatalytic activity under UV radiation due to small size, large surface area and crystalline
nature. Cd2Ta2O7 nanocubes are better photocatalysts than the Cd2Nb2O7 analogues.
Acknowledgments
AKG thanks Department of Electronics and Information Technology (DeitY), Govt. of India and
DST (Solar initiative) project for financial support. DD thanks CSIR for a senior research
fellowship.
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Table 1: Refined positional parameters for (a) cadmium tantalate (Cd2Ta2O7) and (b) cadmium
niobate(Cd2Nb2O7) treated hydrothermally at 180˚C for 48h and calcined at 600˚C for 8h. The
numbers in parentheses are the estimated standard deviations in the last digit.
1 (a)
Spaceꢀgroup: Fd
3
m, a = 10.38425(2) Å Rꢀexp: 4.77 Rwp: 8.17 Rp: 6.60 χ 2 : 1.71
2
Atoms
Position
X
UISO (Ǻ )
Fractional
occupancy
Y
Z
Cd
Ta
O
0
0
0
0.500(5)
0.100(5)
2.8(3)
1
1
1
1
0.5
0.5
0.5
0.125
0.125
0.125
0.4104(5)
0.125
0.125
O
1.32(4)
1 (b)
Spaceꢀgroup: Fd
3
m, a = 10.3796(1) Å Rexp: 4.65 Rwp: 7.36 Rp: 5.64 χ 2 : 1.58
2
Atoms
Position
X
UISO (Ǻ )
Fractional
occupancy
Y
Z
Cd
Nb
O
0
0
0
0.62(4)
0.100(5)
5.0(1)
1
1
1
1
0.5
0.5
0.5
0.125
0.125
0.125
0.3867(6)
0.125
0.125
O
7.65(8)
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Figure captions:
Figure 1. XRD patterns of (a) cadmium tantalate and (b) cadmium niobate obtained by
hydrothermal treatment at 180˚C for 48h followed by calcination at 600˚C for 8h.
Figure 2. Rietveld plot of (a) cadmium tantalate and (b) cadmium niobate obtained by
hydrothermal treatment at 180˚C for 48h followed by calcination at 600˚C for 8h.
Figure 3. (a) TEM image of cadmium tantalate after calcining at 600˚C (b) HRTEM image of
Cd2Ta2O7 nanocubes (c) TEM image of cadmium niobate after calcining at 600˚C (d) HRTEM
image of Cd2Nb2O7 nanocubes.
Figure 4: Diffuse reflectance studies of (a) Cd2Ta2O7 and (b) Cd2Nb2O7 nanocubes.
Figure 5. Photocatalytic degradation of Rhodamine B using Cd2Ta2O7 and Cd2Nb2O7 nanocubes
at pH 8.5 with O2 purging. SS (solid state route); HM(hydrothermal method).
Figure 6. (a)Degradation activity of commercial TiO2, Cd2Ta2O7 and Cd2Nb2O7 synthesized by
hydrothermal(HM) and solid state route(SS)
Figure 7. Fluorescence spectra of the UVꢀlight irradiated (a)Cd2Ta2O7 (b) Cd2Nb2O7
suspensions in 3mM terephthalic acid at different irradiation time; Time dependences of
fluorescence intensity at 426 nm with photocatalysts (c) Cd2Ta2O7 and (d) Cd2Nb2O7
Figure 8. Cycling studies of photodecomposition of (b) Cd2Ta2O7 nanocubes (c) Cd2Nb2O7
nanocubes
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(222)
a
(440)
(400)
(622)
(311)
(111)
(444)
(511)
(731)
10
20
30
40
50
60
70
2-Theta(degree
)
(222)
b
(440)
(400)
(622)
(731)
(444)
(711)
(311)
(111)
(511)
(531)
10
20
30
40
50
60
70
2-Theta(Degree)
Figure 1. XRD patterns of (a) cadmium tantalate and (b) cadmium niobate obtained by
hydrothermal treatment at 180˚C for 48h followed by calcination at 600˚C for 8h.
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Structure
0.00 %
8,000
7,000
6,000
5,000
4,000
3,000
2,000
1,000
0
Cd2Ta2O7 obtained at 600˚C for 8h
a
10
20
30
40
50
60
70
80
90
100
110
120
13
2Th Degrees
Structure
hkl_Phase
100.00
0.00
%
10,000
8,000
6,000
4,000
2,000
0
%
Cd2Nb2O7 obtained at 600˚C for 8h
b
-2,000
10
20
30
40
50
60
70
80
90
100
110
120
13
2Th Degrees
Figure 2. Rietveld plot of (a) cadmium tantalate and (b) cadmium niobate obtained by
hydrothermal treatment at 180˚C for 48h followed by calcination at 600˚C for 8h.
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b
a
(d = 0.299 nm)
20 nm
5 nm
d
c
(d = 0.298 nm)
20 nm
5 nm
Figure 3. (a) TEM image of cadmium tantalate after calcining at 600˚C (b) HRTEM image of
Cd2Ta2O7 nanocubes (c) TEM image of cadmium niobate after calcining at 600˚C (d) HRTEM
image of Cd2Nb2O7 nanocubes.
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0.5
0.4
0.3
0.2
0.1
0.0
a
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Photon energy (eV)
0.8
b
0.6
0.4
0.2
0.0
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Photon energy (eV)
Figure 4. Diffuse reflectance studies of (a) Cd2Ta2O7 and (b) Cd2Nb2O7 nanocubes.
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Cd2Ta2O7(SS)
Cd2Nb2O7(SS)
Cd2Ta2O7(HM)
Cd2Nb2O7(HM)
Light ON
1.0
0.8
0.6
0.4
0.2
0.0
TiO2(commercial)
0
25
50
75
100
125
time (min)
Figure 5. Photocatalytic degradation of Rhodamine B using Cd2Ta2O7 and Cd2Nb2O7
nanocubes at pH 8.5 with O2 purging. SS (solid state route); HM(hydrothermal method).
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100
80
60
40
20
0
92%
85%
68%
30%
20%
TiO2
commercial
(HM) (SS)
Cd2Ta2O7
(HM) (SS)
Cd2Nb2O7
Figure 6. (a)Degradation activity of commercial TiO2, Cd2Ta2O7 and Cd2Nb2O7 synthesized by
hydrothermal(HM) and solid state route(SS)
28