High energy photonic band gap in Sb2S3 inverse opals by sulfidation processing
One of the significant requirements to develop a full PBG is the existence of a high dielectric function contrast in
the crystal. In this way, Sözüer et al. proved the existence of a full PBG in
face-center cubic (FCC) arrays of air spheres immersed in a high dielectric
constant medium (εm>8.4)
For this reason, 3D macroporous silica or polystyrene opals, are
widely used as supports in order to fill the interconnected cavities with
materials possessing a high dielectric function. Then, silica or polystyrene
matrices can be finally removed by means of chemical etching or thermal
treatment. In this case, the remaining structures are known as inverse opals and
they consist of air spheres in a high dielectric constant medium showing the
negative replica of the initial FCC lattice. Porous silica matrices have been
successfully infiltrated by CBD (Chemical Bath Deposition) and CVD (Chemical
Vapor Deposition) methods. It is well known that synthesis methods
involving gas reactants lead to syntheses with a better materials quality:
Silicon and Germanium are
two very good examples. Due to the
limitation imposed by the required high dielectric constant, there are very few
materials capable of showing a full PBG. In
Silicon and Germanium inverse opals a full PBG is predicted in the NIR range of
However, the existence of a limited number of transparent
materials in the VIS possessing a high dielectric constant decreases the chance
to find materials exhibiting a full PBG in this spectra range. Sb2S3 fulfils the
optical requirements to obtain a PBG in the VIS or the NIR, depending on its
amorphous or crystalline nature. The electronic gap of Sb2S3
lies around 2.2 eV (564 nm) for amorphous and 1.78 eV (697 nm) for crystalline
Sb2S32S3inverse opals with a full PBG is a very
important target in photonic crystal research. thin films. The maximum values of the dielectric
function are 10.9 and 14.4 for the amorphous and crystalline forms respectively. Therefore, Sb
For the synthesis of these structures, a modified CBD method
has been employed showing promising results. Although the quality of the
material obtained by the wet method is very good, the control on the degree of
infiltration and the uniformity of the composite can only be improved by
substituting as many liquid phase precursors as possible for similar ones
entering the reaction in gas phase. Here, we report on a two-reaction procedure
in which a gas phase is the trigger of the reaction to grow crystalline Sb2S3in
opals. This newly developed method includes the hydrolysis and condensation of
an antimony alkoxide precursor within the opal matrix, sulfidation of the
hydrolyzed product (antimony oxide, Sb2O3) and finally,
chemical etching for silica removal. This method yields inverted samples of
remarkable quality and optical properties largely improved. The wet impregnation
of a pure liquid precursor benefits the complete opal infiltration (for its
liquid nature) and produces the maximum amount of solid oxide after the
hydrolysis reaction (for not being dissolved). During the second reaction, the
gas phase assures a better diffusion of the reactant for the chemical conversion.
We present the reflectance response of
crystalline Sb2S3 inverse opals synthesized by using this
method showing a broad and flat peak centered at ca. 760 nm with an 85%
reflectivity that can be assigned to the full PBG. The full width at half
maximum of this peak when recorded from the (111) direction is 100 nm which
corresponds to 14 % width-to-centre ratio.
The sulfidation of the hydrolyzed
precursor was performed in an upflow fixed-bed quart reactor at atmospheric
pressure and 300ºC.
Thermodynamic data allows to predict that the sulfidation of
Sb2O3 at the operating conditions used is highly favoured
-31 Kcal/mol) through an exhothermic reaction (DHº573=-32
Kcal/mol). Silica, however,
cannot be affected by this sulfidation process. A variation of the operating
conditions (temperature, flow rate of H2S as a reactant, etc) allows
different rates of sulfide growth leading to the formation from amorphous to well crystallized Sb2S3.
Incidentally, spherical morphology of the
beads and mechanical and sulfidation properties of Sb2O3-silica
composites make them potentially suitable for the development of sorbents of H2S.
Fig. 1. SEM images of a Sb2O3-silica
(a) and Sb2S3-silica composites (b).
The synthesis of crystalline Sb2S3
inverse opals involves several steps. In the first stage, it is necessary to
obtain a Sb2O3-Silica composite for further sulfidation.
Porous silica was impregnated with liquid antimony trietoxide, Sb(OEt)3,
under vacuum conditions. This opal was then exposed to a moist atmosphere at
room temperature, promoting the hydrolysis of antimony trietoxide to antimony
oxide Sb2O3 through the reaction:
Ethanol, as a
hydrolysis product, was removed by vaporization heating up the samples at 100ºC.
Three successive impregnations of Sb(OEt)3 followed by further
hydrolysis provided an infilling degree of nearly 90 % of Sb2O3.
Due to the different molar volume of Sb2O3 and Sb2S3,
an expansion is expected after the conversion. The Sb2O3-silica
composites were subsequently sulfidated in a fixed-bed reactor at atmospheric
pressure and 300 oC. The involved chemical reaction is:
images of the prepared Sb2O3-silica and Sb2S3-silica
composites are shown in Figs 1 a) and b), respectively. During impregnation, the
liquid alkoxide filled the interstitial cavities among silica spheres. After
hydrolysis and condensation, the deposited solid Sb2O3
fills these cavities as can be seen in Fig 1 a). The presence of holes is due to
applied cleavage for sample preparation but it is worth to mention that the
interstitial filling is shown to be complete and homogenous. Fig. 1 b) shows the
appearance of a sulfide-silica composite after sulfidation of the oxide.
Fig. 2. SEM image of a (111) facet of a Sb2S3
inverted sample. The inset shows a lower magnification image.
To obtain a full PBG photonic material, opal inversion is
subsequently required. This new structure is just the
negative replica of the original opal as silica spheres are removed by chemical
etching. In this inverted opal, interconnected air spheres embedded in a Sb2S3
matrix provide enough contrast in the dielectric function to open a full PBG.
The method used for silica removal is chemical treatment with a hydrofluoric
solution 1 wt. % in concentration for 7 h. Figs. 2 show the appearance of the
inverted opal after chemical etching. A high degree of connectivity between
voids and high fidelity to the original opal is evidenced in large areas
of the sample. It is worth mentioning the remarkable quality of both, composites
and inverted samples, as can be appreciated from the SEM images.
The composition of the inverted opals was determined in a
SEM coupled to an energy dispersive X-ray (EDX) spectrometer. The respective Sb
and S concentrations were found using a commercial Sb2S3
99.999% purity as standard. The obtained composition of the inverted opal was
very close to that expected for the stoichiometric antimony sulfide Sb2S3.
of the chemical nature and structure of the compounds formed at the different
stages of preparation has been achieved by powder X-Ray diffraction (XRD). To
minimize the detrimental effect derived from the presence of amorphous silica in
oxide-silica composites, bulk hydrolyzed alkoxide was obtained by using
identical operating conditions to those used in the preparation of hydrolized
opals. To study the structural changes involved in the sulfidation of Sb2O3
bulk hydrolyzed precursor was sulfidated in the reactor together with Sb2O3-silica
composite. Two different runs were performed by using reaction times of 6 and 10
h, respectively. Operating temperature, gas flow rate and pressure was kept
constant for both experiments. The respective XRD patterns were processed
using Rietveld methods, allowing the refinement of different structural
parameters from the starting structure, including lattice and peak shape
parameters. In addition, this method allows quantitative analysis of the
crystalline chemical species found. The application of this method also reveals that the
material lattice parameters underwent relevant changes (not shown). The
conversion of oxidic into sulfided phase implies an increase in the molar volume
that can account for crack development if a complete filling of voids in the
preceding stage of preparation is achieved and depending on the operating
conditions used during the sulfidation procedure.
For this reason, incomplete infilling was used in the preceding stage of
Fig. 3.XRD diffractograms from
hydrolyzed Sb(OEt)3: cubic senarmonite Sb2O3
(a) and from the latter after sulfidation: Sb2S3.
In the 10 h
sulfidation test, the XRD pattern reveals that a 4 wt% of the oxide was present.
However, the final inverted opal does not contain any traces of oxide since Sb2O3
remnants are removed together with silica during chemical etching with HF. Experiments to remove
silica spheres in Sb2O3-silica composites before
sulfidation processing failed. The 1 % wt. HF solution dissolved not only the
silica but also the Sb2O3
in the voids.
Fig 3 a)
shows XRD pattern with well defined reflections assigned to cubic Sb2O3
senarmonite. The presence of a broad peak centered at 2q =
28º suggests the partial presence of amorphous Sb2O3. The powder
XRD pattern for the sulfidated compound is shown in fig 3 b). Narrow reflections
show the crystalline nature of this sample composed of 96 wt% Sb2S3
and 4 wt% Sb2O3.
Fig. 4. Reflectance spectrum from a
Sb2O3-silica composite. The shift in the first stop-band
is used to assess the degree of infiltration.
Optical reflectance spectra were obtained
in several stages of sample preparation to track the filling degree of voids in
bare opal. Since the position of the Bragg peak is a function of the dielectric
constants of both the silica and the medium occupying the voids, material
infiltration entails a shift of the diffraction peak.
Plots in figure 4 show the reflectance
spectra for a bare opal before and after the first precursor condensation. Using
a dielectric constant of 4.4 for the oxide, an estimate in the degree of
infilling can be done. In this case, after the first condensation cycle, 65 % of
the pore volume was filled with Sb2O3.
Fig. 5. Reflectance spectrum form a 410 nm
diameter sphere loaded to a 100% of the pore volume with Sb2S3
and band structure calculations for a 100 % infilled opal.
Optical properties for sulfide composite and inverted opal
were also monitored with the support of band structure calculations. Figure 5
shows theoretical calculations and experimental reflectance spectrum for a 100%
Sb2S3 infilled opal. Sb2S3-silica
composite shows a broad and well-defined peak at low energy corresponding to the
first stop band opened around the L point of the reciprocal lattice, the Bragg
reflection peak. Again, from the position of this peak it is possible to assess
the degree of infilling because this peak shifts to higher wavelengths and
broadens as the Sb2O3 becomes sulfidated. This fact is a
consequence of the increase in the dielectric constants ratio as the oxide is
replaced by sulfide as sulfidation proceeds.
Two extra peaks appear at higher energies that are in good
agreement with theoretical calculations.
Fig. 6. Reflectance spectra from crystalline Sb2S3
inverse opal made of 410 nm sphere diameter and the band structure for a
crystalline Sb2S3 inverted opal.
Figure 6 shows the band structure and the
reflectance spectrum for the inverse structure made of crystalline Sb2S3.
In fig 6, clear correspondence can be
established between the experimental peaks and gaps in the calculated bands. The
marked region corresponds to the predicted full PBG in the inverted opal. This
broad and well-defined peak is centred at 760 nm. The shape and high absolute
reflectivity (85%) of this peak confirms the quality of the sample. To assure
the existence of the full PBG, a detailed study in other crystallographic
directions is required that will be the subject of future work.