Optical and Structural Property Tuning in Physical
Vapor Deposited Bismuth Halides Cs3Bi2(I1–xBrx)9 (0 ≤ x ≤ 1)
Crystalline films of lead-free all-inorganic Cs3Bi2X9 (X = Br, I) perovskites have been deposited by radio frequency (RF)-magnetron sputtering providing high-quality, single-phase films as confirmed by structural, morphological, and optical property characterization. Progressive tuning of crystal structure characteristics and optical absorbance has been achieved in mixed Br/I phases Cs3Bi2(I1–xBrx)9 (0 ≤ x ≤ 1), highlighting a shift of the band gap from about 2.0 eV for Cs3Bi2I9 to 2.64 eV for Cs3Bi2Br9. X-ray diffraction and Raman scattering allowed defining the range of alloyed compositions where single-phase compositions are found. Finally, preliminary photocatalytic activity tests on the degradation of methylene blue provided solid data indicating the future possible exploitation of Bi-based perovskite derivative materials as active photocatalysts.
Introduction<!>Cs3Bi2I9 and Cs3Bi2Br9 Perovskite Films<!><!>Cs3Bi2I9 and Cs3Bi2Br9 Perovskite Films<!><!>Cs3Bi2I9 and Cs3Bi2Br9 Perovskite Films<!><!>Cs3Bi2I9 and Cs3Bi2Br9 Perovskite Films<!><!>Cs3Bi2I9 and Cs3Bi2Br9 Perovskite Films<!>Mixed Cs3Bi2(I1–xBrx)9 (0 ≤ x ≤ 1) Films<!><!>Mixed Cs3Bi2(I1–xBrx)9 (0 ≤ x ≤ 1) Films<!><!>Mixed Cs3Bi2(I1–xBrx)9 (0 ≤ x ≤ 1) Films<!><!>Mixed Cs3Bi2(I1–xBrx)9 (0 ≤ x ≤ 1) Films<!><!>Mixed Cs3Bi2(I1–xBrx)9 (0 ≤ x ≤ 1) Films<!><!>Mixed Cs3Bi2(I1–xBrx)9 (0 ≤ x ≤ 1) Films<!>Conclusions<!>Film Deposition<!>XRD Diffraction<!>EDX and Scanning Electron Microscopy (SEM)<!>AFM<!>Absorption and Diffuse Reflectance Spectroscopy<!>Raman Spectroscopy<!>Methylene Blue Photodegradation<!>
<p>Bismuth-based perovskite
derivatives of general formula Cs3Bi2X9 (X = Br, I) are attracting huge
interest from several diverse communities due to their technological
potential spanning from photocatalysis to photodetectors, organic
synthesis, and photovoltaics.1−11</p><p>Triggered by the issue of overcoming the concerns related
to Pb-toxicity
in metal halide perovskites (MHPs) for photovoltaics, the field of
lead-free perovskites and perovskite derivatives has significantly
extended toward novel applications by exploiting the photoactivity
of lead-free materials which possess, in addition to relevant catalytic
performance, water stability which is still a key problem of lead-based
systems. Among Bi-containing perovskites, Cs3Bi2Br9 and Cs3Bi2I9 have
been the object of several recent publications highlighting, in particular,
their potential use in organic synthesis and photodetection.5,6,8,11 Cs3Bi2Br9 has a trigonal crystal structure
(space group. S.G., P3̅m)
characterized by a layered structure of alternating corner-sharing
octahedra, and shows strong optical absorbance at around 400–500
nm depending on the material state, i.e. powder,
single-crystals, or thin films.12−16 Its large excitonic binding energy prevents this material from being
suitable for photovoltaics, nevertheless it has offered good performance
as an active layer in photodetectors and, even more interestingly,
in photocatalytic applications.6,11,12 Cs3Bi2Br9 was found to be effective
in the direct selective photocatalyzed oxidation of hydrocarbons with
high conversion rates and excellent selectivity, and in the ring-opening
reaction of epoxides.11 In both cases,
Cs3Bi2Br9 exhibited good stability
and recyclability while its lead-based counterpart performed significantly
worse. The Cs3Bi2I9 perovskite, crystallizing
in a hexagonal symmetry (S.G., P63/mmc), on the other hand, has been deeply investigated for
its possible use as an active layer in perovskite solar cells (PSCs)
since 2015, due to its high absorption coefficient and a band gap
of about 2.10 eV.1 Strong effort has been
put in place for optimizing the film morphology for PSCs due to the
known dissolution problems of inorganic precursors when applying solution-based
methods.1 The potential applications of
Cs3Bi2I9 have been further extended
in the last few years to photodetection and photocatalysis, in analogy
with the bromide-containing phase.5,6,9 As an example, perovskite single-crystal thin films
of Cs3Bi2I9 have been grown by a
space-limited solvent evaporation crystallization method providing
highly efficient photodetectors with impressive stability without
any encapsulation for 1000 h in humid air (50% relative humidity (RH)).17 The same compound has been used in photocatalysis
for hydrogen evolution and organic pollutant degradation.18,19</p><p>Mixed Cs3Bi2I9–xBrx have also been the object of
some investigation for both photovoltaics and photodetection applications.2,5 An extensive and deep study of the I/Br solid solution is reported
in the work of Yu et al., where thin films of both end-members and
intermediate mixed compositions have been prepared by spin coating
and used in PSCs.2 While showing relatively
low power conversion efficiencies (PCEs), with Cs3Bi2I6Br3 delivering 1.15% as the best-performing
compound, the data confirm the significant stability of Bi-based phases.2 Liu et al., studied the photodetection performances
of Cs3Bi2Br9–xIx films by varying x, and achieved the best results for the Cs3Bi2I6Br3 composition with an excellent photosensitivity
of 4.1 × 104 at zero bias, as well as with the responsivity
and detectivity reaching 15 mA W–1 and 4.6 ×
1011 Jones.5 Moreover, in this
case, excellent stability in the ambient environment, maintaining
over 96% of the initial value after 100 days, was observed.5</p><p>All pieces of evidence reported so far
refer to works carried out
in the last couple of years suggesting that Bi-based layered perovskites
show promise for future exploration and possible applications in several
technologically relevant fields, as also recently demonstrated by
the interest in tunable Cs3Bi2(Cl1–xIx)9 halide
perovskites.20 In this respect, the possibility
of film deposition scale-up is an urgent issue. Recently, physical
vapor deposition methods have been triggering significant interest;
they seem to be a valuable path to take the required step from the
laboratory scale to the industrial scale in all of the fields where
good quality thin films are required, in particular when considering
all-inorganic perovskite materials where solution processing is more
complicated with respect to hybrid organic–inorganic phases.21</p><p>Based on the above considerations and
the strong appeal of Bi-based
layered perovskites, in this work we carried out the vapor phase deposition,
by means of radio frequency (RF)-magnetron sputtering, of the Cs3Bi2(I1–xBrx)9 system (0 ≤ x ≤ 1). To date, no vapor phase approaches have been
used to prepare Cs3Bi2(I1–xBrx)9 films.
In addition, RF-magnetron sputtering has scarcely been used for perovskite
films notwithstanding its huge potential for scale-up, providing stoichiometry
control, good morphology, and high crystallinity and quality of deposited
films, as we demonstrated recently.22,23 Together with
the structural and optical property characterization of the deposited
films we report a significant photocatalytic activity in organic pollutant
degradation combined with excellent water stability of the prepared
materials. In the following, we start discussing the two end-members
of the solid solution, namely Cs3Bi2Br9 and Cs3Bi2I9, and then moving to
the investigation of the mixed compositions.</p><!><p>RF-magnetron
sputtering has been adopted to deposit high-quality films of Bi-based
layered perovskites starting with the preparation of Cs3Bi2I9 and Cs3Bi2Br9.</p><p>Cs3Bi2I9 films, with
an average thickness of about 500–1000 nm (as determined by
profilometry, see the Experimental Section), have been deposited on glass substrates starting from a target
composed of stoichiometric amounts of CsI and BiI3 (see
the Supporting Information, SI, for more
details on the synthesis conditions) without substrate heating. Post-deposition
thermal treatment has been carried out at 200 °C for 2 h under
mild vacuum.</p><p>Characterization has been initially carried out
by X-ray diffraction
(XRD), revealing the crystal structure. Figure 1a displays the film pattern superimposed
to the calculated diffraction for hexagonal (P63/mmc—JCPDS card 01-070-0666) Cs3Bi2I9. There is a perfect match between
experimental and calculated data confirming the deposition of single-phase
films with good crystallinity as also evidenced by the narrow diffraction
peaks. The lattice parameters are a = b = 8.4081(5) Å and c = 21.1520(8) Å. The
inset of Figure 1a
shows a picture of the film deposited by sputtering, which looks dark-red
and highly reflective. A sketch of the hexagonal crystal structure
is also reported in the inset of Figure 1a. In addition, a slight preferential orientation
along the (00l) direction (Miller indexes in bold
in Figure 1a) is observed
by comparing the expected and experimental intensities. In addition,
nearly (h0l) fully oriented films
can be obtained by slightly changing the deposition conditions and,
in general, such an effect is observed for films with a thickness
of about 500–600 nm. A typical pattern of such an oriented
film of Cs3Bi2I9 is shown in Figure 1b.</p><!><p>(a) XRD pattern of a
Cs3Bi2I9 film
superimposed to the calculated pattern of the hexagonal structure
(vertical red lines), inset: right, photo of a Cs3Bi2I9 film deposited on a glass substrate; left: sketch
of the crystal structure of Cs3Bi2I9; and (b) XRD pattern of a (h0k) fully oriented film of Cs3Bi2I9.</p><!><p>Further insight into the film
crystal quality has been obtained
by Raman spectroscopy. Figure 2a shows the spectra of the films reported in Figure 1a,b. The spectrum of the unoriented
film shows broadened Raman features with respect to reference Raman
spectra of a single-crystalline sample of Cs3Bi2I9.24 Nevertheless, one can
appreciate different peaks: according to ref (23), bridge Bi–I asymmetric
stretching at around 90 cm–1, terminal Bi–I
asymmetric stretching at around 120 cm–1, and the
terminal Bi–I symmetric stretching at about 150 cm–1, are clearly visible, while other vibrational modes are less defined.
On the other hand, for the oriented film, an intrinsic mode selection
leads to a Raman spectrum dominated by the signal of asymmetric stretching
at around 120 cm–1.</p><!><p>(a) Raman spectra of the unoriented (red
line) and oriented (blue
line) Cs3Bi2I9 films and (b) optical
absorption edge of an oriented Cs3Bi2I9 film (direct gap red curve and indirect gap blue curve extrapolations).</p><!><p>Finally, the optical response of Cs3Bi2I9 sputtered films have been determined
by absorption and diffuse
reflectance spectroscopy. The absorption edge has been first argued
at about 670 nm, i.e. 1.85 eV on films of different
thicknesses (Figure S2, SI). The band gap
has been then better estimated from the extrapolation of the linear
part of [F(R)hν]2 where F(R) is the Kubelka–Munk
function F(R) = (1 – R)2/2R, as reported in Figure 2b. for an oriented
Cs3Bi2I9 film. A clear absorption
edge is observed corresponding to a direct band gap of about 2.0 eV.
Such a value is in agreement with the only paper reporting thin films
of Cs3Bi2I9 prepared by spin coating
where, however, the authors observed a more structured absorption
edge identifying a direct and an indirect band gap.25 The indirect gap edge is estimated at 1.96 eV (blue curve)
from the indirect gap extrapolation of the linear part of [F(R)hÎ½]1/2.</p><p>Cs3Bi2Br9 films have been
deposited
starting from stoichiometric amounts of CsBr and BiBr3 followed
by thermal treatment at 200 °C for 2 h under mild dynamic vacuum
(as for the Cs3Bi2I9 material). This
approach allowed us to prepare single-phase, highly crystalline, Cs3Bi2Br9 films, as shown in Figure 3, with the typical aspect of
the yellow film reported in the inset. Cs3Bi2Br9 films grow in the trigonal crystal structure (P3̅m—JCPDS card 01-070-0493)
as sketched in the inset of the figure, with a slight preferential
growth along the (00l) direction (Miller indexes
in bold in Figure 3) as can be inferred by comparing the experimental pattern with the
calculated one (vertical red lines).</p><!><p>XRD pattern of Cs3Bi2Br9 film
superimposed to the calculated pattern of the Cs3Bi2Br9 crystal structure (red lines), inset: from
the left: typical aspect of deposited film and sketch of the crystal
structure of Cs3Bi2Br9.</p><!><p>Raman spectroscopy has also been used in this case to further
probe
the film structure. Figure 4a shows the Raman spectrum of a single-phase Cs3Bi2Br9 film which is in excellent agreement
with the published data.24 Indeed, the
two expected A1g and Eg normal modes give strong
Raman features centered at ∼190 and 165 cm–1, respectively, as a result of Bi–Br vibrations inside the
corner-sharing [BiBr6]3– octahedra. Additional
weaker Raman features are correctly measured at 91, 76, and 66 cm–1. The high quality of the obtained structure is confirmed
by the full width at half-maximum (FWHM) values of A1g and
Eg bands (6.9 and 5.3 cm–1 respectively),
very close to the values obtained for single-crystals.26</p><!><p>(a) Raman scattering spectrum of the single-phase Cs3Bi2Br9 film and (b) optical absorption
edge
of the same film (direct gap red curve and indirect gap blue curve
extrapolations).</p><!><p>The optical response
is blue-shifted with respect to Cs3Bi2I9, as shown in Figure 4b, and is characterized by a sharp edge corresponding
to a band gap of about 2.64 eV to be compared to the value of 2.67
eV reported for the only available Cs3Bi2Br9 film study in the current literature.2 Indirect edge is at 2.6 eV (blue curve). The presence of a direct
and indirect band gap in Cs3Bi2Br9 has been described in detail by Zhang and co-workers.27</p><p>So far the first successful application
of RF-magnetron sputtering
to grow high-quality, single-phase Cs3Bi2I9 and Cs3Bi2Br9 films has
been demonstrated, also reporting the first vapor phase growth on
these Bi-based perovskite derivatives. The two compositions have distinct
crystal structures and optical properties, which, as shown in the
next section, can be tuned by halide alloying, also providing a demonstration
of the effectiveness of the sputtering method to grow mixed phases.</p><!><p>Mixed Cs3Bi2(I1–xBrx)9 (0 ≤ x ≤ 1) have
been grown by RF-magnetron sputtering according to the experimental
conditions.</p><p>Figure 5a reports the XRD patterns of a series of mixed films with
an average thickness of about 300 nm as a function of x (bromide content). energy-dispersive X-ray (EDX) analysis was used
to determine the effective Br/I content, which was very close to the
nominal one with deviations on the order of ±5%. EDS analysis
also confirmed the good atomic homogeneity of the prepared films.</p><!><p>(a) XRD
pattern of the Cs3Bi2(I1–xBrx)9 (0 ≤ x ≤ 1) series of films; (b) trend of lattice volume
of Cs3Bi2(I1–xBrx)9 (0 ≤ x ≤ 1) films as a function of the Br content.</p><!><p>For the range 0 ≤ x ≤
0.3 the crystal
structure of the mixed compositions is the same as that of Cs3Bi2I9 (P63/mmc), with the diffraction lines of the samples
showing a progressive shift toward higher angle (smaller lattice parameters)
on increasing the Br amount. At x around 0.4, the
XRD pattern shows the presence of the characteristic diffraction peaks
of Cs3Bi2I9 (with significant preferential
orientation along the 00l reflections) together with
the appearance of diffraction peaks related to the trigonal crystal
structure of Cs3Bi2Br9 (P3̅m, marked with an asterisk in Figure 5a) which, in addition, are
found at significantly lower angles with respect to the reference
structure (vertical bluelines of the middle part of Figure 5a), indicating a bigger unit
cell due to iodide presence. By further increasing the Br amount, x, to 0.5, single-phase samples in mixed system Cs3Bi2(I1–xBrx)9 are found. Now, the crystal structure
is compatible with that of Cs3Bi2Br9. As it can be observed in the top panel of Figure 5a, and with reference to the reflection of
Cs3Bi2Br9 (vertical blue bars), the
diffraction peaks of the mixed compositions in the 0.5 ≤ x ≤ 0.9 range are shifted to lower angles as a result
of the presence of bigger I which increases the cell volume. The trend
of lattice volume, determined by profile matching of the diffraction
patterns, for the series of films reported in Figure 5a is shown in Figure 5b. A roughly linear trend of V, with different slopes, in the two regions of phase stability is
observed as a function of the bromide content.</p><p>The present results
mark a transition from hexagonal to trigonal
symmetry at about x = 0.3, i.e.
Cs3Bi2I6.3Br2.7, a small
intermediate region of mixed-phase samples, and another region of
single-phase mixed compositions with trigonal symmetry from Cs3Bi2I4.5Br4.5 to Cs3Bi2Br9. The data reported on spin-coated films
by Yu et al. evidenced a similar phase transition at a composition
equal to that of Cs3Bi2I6Br3, indicating the existence of mixed samples around Cs3Bi2I7Br2, while alloying in the
analogous system in the form of nanocrystals occurs at Cs3Bi2I3.6Br5.4 in excellent agreement
with the present data.2</p><p>Morphological
characterization of some selected films of the Cs3Bi2(I1–xBrx)9 (0 ≤ x ≤ 1) series
has been assessed by atomic force microscopy
(AFM). In Figure 6 are
reported the images collected on 20 × 20 μm2 area for films at x = 0, 0.3, 0.5, 0.7, and 1.
Images of smaller and larger areas are reported in the SI. From the 90 × 90 μm2 area (Figure S1) images it is evident
that the RF-magnetron sputtering method provides a very good substrate
coverage. The root mean square (RMS) roughness spans from 15 to 60
nm, with no clear trend as a function of the Br content with a value
of around 30 nm for Cs3Bi2I9 and
around 60 nm for Cs3Bi2Br9. From
the images in Figure 6 (and the 4 × 4 μm2 in the SI) it is possible to note that the films are composed of
small particles of about 50–300 nm, depending on the composition
(e.g., average grain size of around 40–70
nm for Cs3Bi2I9 and around 200–300
nm for Cs3Bi2Br9). In general, there
are no significant differences in the film morphology, apart from
the slightly less compacted film obtained for Cs3Bi2Br9.</p><!><p>AFM images on 20 × 20 μm2 area for Cs3Bi2(I1–xBrx)9 samples for x = 0, 0.3, 0.5, 0.7, and 1.</p><!><p>The behavior derived from XRD results for mixed samples is further
confirmed by Raman data. Room temperature Raman spectra have been
registered for all of the investigated samples in the region of 50–1000
cm–1. In Figure 7a, for ease of viewing, a selection of mixed samples
is reported in the region 80–220 cm–1 where
Raman yield can be measured. The Raman spectra from the two end-members
are also reported for comparison. The spectra in Figure 7a result by averaging all spectra
obtained from linear scans as described in the SI. This protocol allowed to check the homogeneity of the
Raman behavior of the deposited films from different sample regions.
In particular, for x < 0.4 and >0.6, the films
exhibit a higher degree of homogeneity, as can be seen in Figure S3 where the result from a linear scan
of Raman mapping from a sample with x = 0.3 is reported
as a representative example.</p><!><p>(a) Raman spectra at room temperature of different
mixed samples
plotted with the two end-members and (b) peak position (open symbols)
and linewidth (fully symbols) for the Raman bands corresponding to
A′ mode in Cs3Bi2I9 (squares)
and to A1g mode in Cs3Bi2Br9 (circles).</p><!><p>The initial substitution of I
by Br leads to a Raman spectrum dominated
by the typical A′ mode due to terminal Bi–I symmetric
vibrations in Cs3Bi2I9, different
from what was already observed for the pure sample, whose spectrum
is mainly due to the mode at about 117.0 cm–1 attributed
to terminal Bi–I asymmetric vibrations. This fact confirms
that the addition of low amounts of Br leads to films with a crystal
structure equal to that of Cs3Bi2I9 but with different orientations, in accordance with the XRD data.
Increasing the Br alloying leads to increasing Raman activity at energies
higher than 160 cm–1. A very weak signal at around
162 cm–1 is already present in the spectrum for
the sample with x = 0.3 and becomes more and more
pronounced with a clear blue-shift with increasing Br content. In
the range of 0.3 < x < 0.6 it is possible to
observe Raman features pertinent to both crystal structures. Indeed, Figure S4 reports the Raman spectrum in the region
of 100–200 cm–1 obtained for the sample with x = 0.5 during a linear scan. The data are interpolated
by a superposition of six Lorentzian curves evidencing a mixing between
the two phases. It is important to underline that this kind of spectra
are in any case rarely observed and by averaging over 15 spectra,
the contribution from the minority phase tends to be smeared out.
At higher Br amounts, the Raman activity in the region of 80–150
cm–1 decreases indicating that the contribution
from Bi–I vibrations is gradually quenched, while that from
Bi–Br is strengthened. The transition between the two different
crystal structures can be further appreciated by monitoring Raman
band parameters as reported in Figure 7b.</p><p>The Raman modes we used as reference are the
already mentioned
A′ symmetric mode, involving terminal Bi–I vibrations,
for Cs3Bi2I9, and the A1g mode due to the Bi–Br vibrations inside the corner-sharing
[BiBr6]3– octahedra in Cs3Bi2Br9, peaked in pure samples at 144.0 and 191.0 cm–1, respectively. According to Yu et al., the insertion
of Br causes a small blue-shift of the A′ phonon energy with
the hardening of the mode of about 2 cm–1 for the x = 0.4 sample.2 On the other
hand, the linewidth of this Raman mode increases markedly as the result
of the disordering due to the Br insertion in the Bi–I vibrations
units. At higher Br amounts the Raman fingerprints of the trigonal
crystal structure (P3̅m) appear
at around 160 cm–1. The broadened and unresolved
feature denotes a higher disorder degree which is gradually quenched
leading to the fully ordered phase in the pure sample. In this case,
the blue-shift of the A1g mode is greater than that observed
for the A′ mode but this is consistent with the contraction
of cell volume and the involved anion masses. The higher disorder
in the mid-range of substitution and the presence of Raman features
ascribable to both crystal systems (see Figure S4) indicate the presence of a mixed-phase.</p><p>Figure 8a reports
the diffuse reflectance spectra of selected films (for ease of representation)
for the Cs3Bi2(I1–xBrx)9 system showing
a clear blue-shift from x = 0 to 1. The direct gap
energy for all of the films considered shows a progressive shift of
the fundamental absorption edge toward higher energies with increasing
Br content, with a good linear trend all through the visible range
(Figure 8b) and with
a slight deviation at x = 0.5 which could be possibly
correlated to the Raman results indicating, locally, a mixed-phase
nature of the sample. The monotonic band gap variation on increasing
the Br content, as shown in Figure 8b, well follows the literature trend, as experimentally
observed by Gu et al. on only three compositions (asterisks in the
Figure).25</p><!><p>(a) Diffuse reflectance spectra of different
mixed Cs3Bi2(I1–xBrx)9 samples plotted
with the two end-members;
(b) fundamental gap energy trend (dots) with linear trend interpolation
and a comparison with the literature data (asterisks).</p><!><p>Finally, we report in the present work some preliminary results
aiming at testing the photocatalytic properties of Cs3Bi2I9 and Cs3Bi2Br9 films, which have been assessed by determining their ability in
the photodegradation of an organic dye by selecting methylene blue
(MB) as a representative model compound of this class. As mentioned
in the Introduction section, Cs3Bi2I9 and Cs3Bi2Br9 have attracted significant recent interest due to their relevant
photoactivity, which, however, to date has been determined only on
particualte samples.18,19</p><p>Figure 9 shows the
variation of the percentage of MB removal (calculated as C0 – C/C0 where C0 is the initial concentration)
as a function of irradiation time, compared to that of direct photolysis
(see details in the SI).</p><!><p>Percent of MB removal
as a function of irradiation time for Cs3Bi2I9 (red dots) and Cs3Bi2Br9 (blue dots) films, compared to the photolysis
effect (empty dots); conditions: 0.5 g L–1 catalyst,
250 W m–2 simulated solar light. Relative standard
deviation (RSD) < 10% (n = 3).</p><!><p>It is clear from Figure 9 that the two films show relevant activity in the degradation
of MB which provides the first evidence of photoactivity by Cs3Bi2Br9 and Cs3Bi2I9 thin films. Slightly different behavior in the MB degradation
between the two compositions may be the result of morphology/grain
size effects (see AFM data above). The degradation of MB was also
investigated concerning the kinetics. For quantitative evaluation,
experimental data for MB photodegradation were fitted to different
kinetic equations to determine the suitable model to represent the
kinetics of degradation.28 Plotting the
natural logarithm of the ratio between the original concentration
and the concentration after photocatalytic degradation ln([MB]0/[MB]t) versus the irradiation time (min) provided a linear relationship, as shown
in Figure S5. The apparent first-order
rate constants (min–1) were 0.0295 and 0.0389 for
Cs3Bi2Br9 and Cs3Bi2I9 films, respectively. Concerning the mechanism
of MB degradation, previous data on bulk Cs3Bi2Br9 and Cs3Bi2I9 indicated
that MB degradation occurs via the formation of hydroxyl
radicals.29,30 These promising results open the way to
further explore the application of Bi-based perovskites in a series
of relevant photocatalytic reactions.</p><!><p>In
the present paper, we provide and demonstrate an efficient vapor
deposition route for the preparation of single-phase, high-quality
films of Cs3Bi2Br9 and Cs3Bi2I9 based on RF-magnetron sputtering, which
is a simple and effective method making use of a single target source.
The films have been characterized in terms of their crystal structure,
morphology, optical and Raman spectra, and by assessing preliminary
photocatalytic activity by investigating the solar-driven MB degradation.
RF-sputtering has also been successfully employed to deposit mixed
I/Br films Cs3Bi2(I1–xBrx)9 (0 ≤ x ≤ 1), confirming the suitability of this approach
in preparing alloyed compositions with a very good stoichiometry control,
as well. A structural phase transition has been observed at about x = 0.4, as confirmed by X-ray diffraction, with a scaling
of lattice parameters with the halide content. Raman spectroscopy
was revealed to be effective in following the structural evolution
of mixed compositions. Finally, a variation of the fundamental band
gap from 2.0 eV for Cs3Bi2I9 to about
2.64 eV for Cs3Bi2Br9 has been observed
with the tuning of Eg between the two
end-members achieved in the mixed compositions. To conclude, this
paper demonstrates the efficiency of sputtering in preparing thin
films of all-inorganic lead-free Bi-based perovskites which are of
huge interest for the wide community of photovoltaics, optoelectronics,
and more recently, photocatalysis. As we already demonstrated for
other metal halide perovskites, sputtering seems to be a method to
be extended to several systems to further boost the scale-up of perovskite-based
technology, in particular for all-inorganic systems which face relevant
problems in wet-chemistry depositions.21−23</p><!><p>All of the thin films were deposited
by means radio frequency (RF) magnetron sputtering on substrates made
by cutting microscopy slides (1 mm thick) in 25 × 25 mm2 pieces. The substrates were mechanically cleaned with 2-propanol
(Aldrich, ≥99.7%), sonicated in the same solvent for 15 min,
and heated at 200 °C on a hot plate just before deposition. The
targets (diameter 5.08 cm, thickness 1 mm) were made of pressed powders
of stoichiometric CsBr/CsI/BiI3/BiBr3 mixtures
(Aldrich, 99.9%). Overall, the starting mass of the target was about
10 g.</p><p>Deposition parameters were: (i) target-to-substrate distance,
10 cm, (ii) RF-power, 50 W, (iii) argon pressure, 2.3 × 10–2 mbar, and (iv) argon flux 20 sccm. The depositions
were carried out in power-control mode. Film thickness was determined
using a P-6 stylus profilometer KLA Tencor equipped with a silicon
tip (tip radius 2 Î¼m; applied force 2 mg). After the deposition,
the films have been heated and cooled in a vacuum using a BÜCHI
glass drying oven for 2 h at 200 °C.</p><!><p>The structural properties of the deposited
thin films were characterized by X-ray diffraction (XRD) using a Bruker
D8 Advance instrument (Cu radiation) in a Bragg–Brentano setup.</p><!><p>EDX analysis
and microstructural characterization of the samples were performed
using a high-resolution scanning electron microscope (SEM, TESCAN
Mira 3) operated at 25 kV.</p><!><p>Atomic force microscopy (AFM)
images were obtained
with an AutoProbe CP microscope (ThermoMicroscopes-Veeco), operating
in tapping mode, by means of sharpened silicon tip Nanosensors (resonance
frequency: 300 kHz; force constant: 40 N m–1). For
each analyzed film, scans from 90 μm × 90 μm to 4
μm × 4 μm have been carried out with the scan rate
ranging from 0.5 to 1 Hz. A standard second-order flatten processing
of the images has been performed to correct the scanner nonlinearity.</p><!><p>Ultraviolet–visible–near
infrared (UV–vis–NIR) optical measurements were performed
under ambient conditions using a Varian Cary 6000i spectrophotometer
equipped with a double monochromator, a deuterium lamp and a tungsten
filament lamp as light sources, and a photomultiplier (UV–vis)
and an InGaAs photodiode (NIR) as detectors. The spectral range was
200–1800 nm, in a step of 1 nm. Both near-normal absorption
spectra and diffuse reflectance spectra with a 110 mm diameter integrating
sphere were recorded.</p><!><p>Micro-Raman measurements
were carried
out at room temperature using a Labram Dilor spectrometer equipped
with an Olympus microscope HS BX40. The 632.8 nm light from a He–Ne
laser was employed as excitation radiation. The samples, mounted on
a motorized xy stage, were tested with a 50×
objective and with a laser spot of ∼1.5 μm diameter.
The spectral resolution was about 1 cm–1. A cooled
charge-coupled device (CCD) camera was used as a detector and the
typical integration times were about 2 min. The sample phase homogeneity
was verified by performing linear scanning over a length of about
30 Î¼m in three different sample regions. From these scans, an
average spectrum for each sample has been derived. These spectra were
processed by best-fitting procedures based on Lorentzian functions.</p><!><p>Methylene blue degradation
was conducted in a batch setup. Methylene blue solution (3 mL, 0.5
g L–1) was placed in a quartz cuvette together with
the catalyst film, sputtered onto a 0.8 cm × 4.0 cm glass substrate.
The progress of the reaction was monitored measuring the absorption
of the solution at 664 nm. The catalyst film was placed in a direction
perpendicular to the irradiation path. Irradiation was conducted in
a solar box equipped with a Xe lamp and the irradiance was set to
250 W m–2, to avoid excessive degradation of the
methylene blue during photolysis (without the catalyst). Before the
start of the irradiation, the filled cuvette was kept in the dark
for half an hour to enable the establishment of an adsorption–desorption
equilibrium between methylene blue and the thin film. The experiments
were repeated three times (RSD < 10%).</p><!><p>Additional AFM
and Raman and diffuse reflectance spectra
(PDF)</p><p>ic1c01545_si_001.pdf</p><p>The authors declare no
competing financial interest.</p>