Hybrid bismuth oxyhalides@gypsum as self-cleaning composites: novel aspects of a sustainable photocatalytic technology for solar environmental cleanup

Article abstract of DOI:10.1039/c5ra09993e

Herein we report the fabrication of a novel heterojunctioned BiOCl_xBr_1-x@gypsum composite and its application as a self-cleaning surface for the straightforward use in construction and building materials. The morphology, chemical composition and crystal structure of the hybrid material were characterized using X-ray diffraction and scanning electron microscopy. The photocatalytic activity of the new composite was investigated for the degradation of naphthalene and rhodamine B (RhB) under visible light (λ ≥ 420 nm) irradiation. Our results show rapid and complete destruction of the RhB dye after only 4 minutes under Xe visible light irradiation (422-740 nm) and 2.5 minutes under mild sunlight illumination. Total organic carbon measurements could verify the absolute mineralization of the recalcitrant naphthalene contamination, as a state of the art example, within 20 minutes of visible light illumination.

Full text of DOI:10.1039/c5ra09993e

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Hybrid bismuth oxyhalides@gypsum as self-
cleaning composites: novel aspects of a sustainable
photocatalytic technology for solar environmental
cleanup†
Cite this: RSC Adv., 2015, 5, 66650
a
b
a
a
*
*
Hani Gnayem, Vladimir Uvarov, Ofer Lahad and Yoel Sasson
Herein we report the fabrication of a novel heterojunctioned BiOClxBr1ꢀx@gypsum composite and its
application as a self-cleaning surface for the straightforward use in construction and building materials.
The morphology, chemical composition and crystal structure of the hybrid material were characterized
using X-ray diffraction and scanning electron microscopy. The photocatalytic activity of the new
composite was investigated for the degradation of naphthalene and rhodamine B (RhB) under visible
light (l $ 420 nm) irradiation. Our results show rapid and complete destruction of the RhB dye after only
4 minutes under Xe visible light irradiation (422–740 nm) and 2.5 minutes under mild sunlight
illumination. Total organic carbon measurements could verify the absolute mineralization of the
recalcitrant naphthalene contamination, as a state of the art example, within 20 minutes of visible light
illumination.
Received 27th May 2015
Accepted 29th July 2015
DOI: 10.1039/c5ra09993e
www.rsc.org/advances
Despite the intensity and diffusion of these studies,3–7 there
were no major practical breakthroughs in the eld.
1. Introduction
The oxidative properties of titanium dioxide were utilized to
achieve self-cleaning surfaces by means of solar light8 and, in
parallel, it was sought to use the capacity of the aforementioned
oxide combined with ultraviolet radiation, to transform
surfaces into highly hydrophilic textures, so rain contributes to
their cleaning.9 These applications had a reasonable commer-
cial success. In what seems to be the beginning of a new era of
photocatalysis, recent work oriented toward the use of visible
light to drive chemical processes. The approach is to modify the
catalyst properties by varying the chemical composition so it
can be activated by the visible broader spectrum of the sun. In
addition to diverse applications in the elds of air, water and
wastewater treatment, heterogeneous photocatalytic oxidation
is central for self-cleaning purposes.10,11
Scientists are nowadays facing major challenges in countering
air, water and soil pollution. Of particular interest is conceiving
ways to decompose organic materials and mould deposited on
building surfaces, which are responsible for the deterioration of
their appearance. Inorganic semi-conductor based photo-
catalysts applied as coatings on buildings and stone structures
can be a motivating solution in order to lower maintenance
costs by reducing surface soiling and the need for costly
cleaning operations.1
In the eld of photocatalytic construction and building
materials, titanium dioxide (TiO2) is the most widely used
substance which has been used as a white pigment in paints,
cosmetics and foodstuff since ancient times.2
The use of photocatalysts blended with building materials
startedfromtheearly1990s. Theversatilefunction ofTiO2, which
can serve as a photocatalyst and as a structural material, has
facilitated its application as an exterior construction component
and an interior furnishing matter, combined with e.g. cement
mortar, exterior tiles, paving blocks, glass and PVC fabric.
Consequently a smart and elegant application of solid-state
semiconductors photocatalysis is its application in construc-
tion and building materials. Given their large surface areas,
buildings can conveniently be used to promote the elimination
of organic pollutant gases from the surrounding air.
To protect the surfaces of cementitious materials and
minimize the negative inuence of environment conditions on
buildings, the advanced oxidation processes such as heteroge-
neous photocatalysis started to play a major role in building
industry.12–14
aCasali Center of Applied Chemistry, The Institute of Chemistry, The Hebrew University
of Jerusalem, Jerusalem 91904, Israel. E-mail: ysasson@huji.ac.il; hani.gnayem@
mail.huji.ac.il; Fax: +972 2652 9626; Tel: +972 6584530/+972 2658 6567
bThe Unit for Nanoscopic Characterization, The Center for Nanoscience and
Recently, bismuth oxyhalides have received considerable
attention and are of immense signicance due to their excellent
visible light photocatalytic activity.15–19
Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
† Electronic supplementary information (ESI) available: Table S1: solid TOC
measurements, Fig. S1: BET surface area isotherm of the investigated alloy. See
DOI: 10.1039/c5ra09993e
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As a ternary oxide semiconductor, BiOX (X ¼ F, Cl, Br, I) satisfactory manner by means of an addition of a water soluble
comprises a layered crystal structure consisted of tetragonal halide salt. Thus, the bismuth salt is fed to the ltrate together
[Bi2O2]2+ positive slabs interleaved by double negative slabs of with water-soluble halide salt (e.g., alkali halide such as sodium
halogen atoms along the c axis. Their outstanding optical, light chloride and potassium bromide) and the regenerated ltrate is
harvesting and electrical properties render them promising supplied to the next production batch for use as a reaction
substances for applications as pharmaceuticals,20 pigments21 medium. The products which are formed in a fresh or a recycled
and catalysts22 as well as gas sensing materials.23
reaction medium are equally excellent in respect of their pho-
Herewith, as a rst novel study discussing BiOX family of tocatalytic activity. The process of the invention is hence readily
materials for self-cleaning purposes, we report the facile and applicable for large scale production either in a batch or
straightforward integration of BiOClxBr1ꢀx (0 # x # 1) solid continuous mode of operation.
solutions19 in construction materials while focusing on gypsum
(Ca(SO4)$2H2O) as an archetypal candidate.
Rhodamine B (RhB) and naphthalene were selected for
probing and evaluating the photodestruction capacity of the
2.2 Characterization
X-ray powder diffraction measurements were performed on the
D8 Advance diffractometer (Bruker AXS, Karlsruhe, Germany)
with a goniometer radius 217.5 mm, secondary graphite
above alloys while being incorporated into gypsum surfaces.
The fabricated BiOClxBr1ꢀx@gypsum formulations were
monochromator, 2ꢁ Sollers slits and 0.2 mm receiving slit. XRD
exposed to visible light irradiation using Xe arc lamp, natural
patterns within the range 5ꢁ to 70ꢁ 2q were recorded at room
aernoon sunlight as well as simple battery-powered LED
lamps.
˚
temperature using CuKa radiation (l ¼ 1.5418 A) with following
measurement conditions: tube voltage of 40 kV, tube current of
40 mA, step-scan mode with a step size of 0.02ꢁ 2q and counting
2. Experimental section
time of 1 s per step. Gypsum plasters coated by BiOCl0.8Br0.2
2.1 Materials and methods: catalysts and gypsum integrated
composites-preparation and characterization
were located on sample stage that is regulated along the vertical
axis, and allows obtaining XRD patterns from as-manufactured
samples with various sizes. Uncoated material of plaster was
grinded in agate mortar to powder and placed into low-
background quartz sample holders.
Materials. All materials were purchased from Aldrich-Sigma
and were used without further purication.
Preparation of BiOCl0.8Br0.2 solid-solution. Deionized water
(75 mL), glacial acetic acid (35 mL) and bismuth nitrate (9.18 g)
are added to a ask and mixed at room temperature for een
minutes until a clear, transparent solution is formed. Cetyl-
trimethylammonium bromide-CTAB (1.378 g dissolved in
10 mL of water) and cetyltrimethylammonium chloride-CTAC
(4.85 g in the form of 25 wt% aqueous solution) are added to
the solution, for additional 30 minutes of mixing at room
temperature. The white precipitate thus formed is separated
from the liquid phase by ltration, washed ve times with
ethanol (20 mL) and ve times with water (50 mL), in order to
remove the non-reactive organic species. The solid is then dried
(in air). The weight of the solid collected is 7 g (yield ¼ 91%).
Gypsum composites. The BiOClxBr1ꢀx@gypsum composites
used in this study were prepared by mixing the commercially
available Tambour gypsum with stoichiometric amount of
distilled water followed by spaying the BiOCl0.8Br0.2 photo-
catalyst. Composites with 1, 2 and 4 wt% of the photocatalyst
were prepared and applied as material for coating of gypsum
plates using commercial spray coater. These plates were used
for both structural and photocatalytic studies.
Morphological observations and chemical analysis were
performed using Environmental Scanning Electron Microscope
(ESEM) Quanta 200 (FEI, Eindhoven, Netherlands) equipped
with EDS detector (EDAX-TSL, USA) and operated in low vacuum
mode (0.6 Torr pressure) at 20 kV accelerating voltage, and with
the High Resolution Scanning Electron Microscope (HR SEM)
Sirion (FEI, Eindhoven, Netherlands) in Ultra-High resolution
mode with Through the Lens Detector. Accelerating voltage and
working distance were set at 5 kV and 5 mm correspondingly.
Diffuse Reectance Analysis was carried out by Integrating
Sphere (JASCO V-650 Series ISV-722).
Surface area was determined by the N2 Brunauer–Emmett–
Teller (BET) method (NOVA-1200e).
Total Organic Carbon (TOC) measurements have been con-
ducted using SKALAR PRIMACSSLC Solid TOC ANALYZER.
Model no. 2CS22901.
3. Results and discussion
3.1 Material characterization
Sustainable aspects of the synthetic protocol: recycle of raw Fig. 1 shows the XRD patterns acquired from as-synthesized
materials. The ltrate formed in the above procedure contains BiO(ClxBr1ꢀx) sample, from coated gypsum plaster and from
some major ingredients of the process and can be easily recy- pure gypsum (for comparison). Fig. 1b shows XRD pattern
cled. These are water, acetic acid and particularly the quaternary obtained from coated gypsum plaster, and diffraction peaks of
ammonium salts (probably with nitrate counter anion). This both phases (BiOCl0.8Br0.2 and gypsum) are clearly visible.
ltrate can directly serve as a reaction medium for the succes- Analysis of the XRD pattern (Fig. 1b) revealed that one of the
sive batch. It is easily regenerated; fresh amounts of the reac- crystal phases is BiOClxBr1ꢀx (unit cell parameters a ¼ b ¼ 3.89
˚
˚
tants which were consumed in the reaction are added to the A, c ¼ 7.484 A, P4/nmm (no. 129) space group), and second phase
ltrate, except for the quaternary ammonium salt surfactant is gypsum Ca(SO4)$2H2O. The composition of photocatalyst was
whose activity as structure-directing agent can be restored in a calculated as described in our recent works19,24 using value of
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the c-parameter obtained from XRD data via x ¼ 10.966 ꢀ 1.354c
Fig. 2 shows images acquired from dry as-synthesized
(x is content of chlorine). We obtained x ¼ 0.81 that is very close BiOCl0.8Br0.2 and from the same material on gypsum surface.
to the claimed composition of the photocatalyst. Moreover, very Imaging in secondary electrons (Fig. 2a and b) revealed topo-
broad BiOCl0.8Br0.2 peaks indicate the nanosize of its crystal- graphical details of the observed morphology. As is seen in
lites. The crystallite sizes of BiOCl0.8Br0.2 photocatalyst calcu- Fig. 2, BiOCl0.8Br0.2 particles have a shape of microspheres with
lated using Scherrer equation were about 8 nm.
a size about 3 mm, which are built of thin plates having lateral
dimensions of hundreds of nanometers. Although the lateral
dimensions of plates were about hundreds of nanometers, their
thickness was about 10 nm only. On gypsum surface we see both
BiOCl0.8Br0.2 microspheres and their component plates (Fig. 2c
and d). Elongated crystals of gypsum are also perfectly visible.
The adduced EDS spectrum conrms the chemical composition
of the material.
To verify the location of BiOCl0.8Br0.2 we analyzed cross
sectional samples of the coated material with EDS in SEM. We
found that BiOCl0.8Br0.2 is absent in the bulk of the plaster while
it is present in the supercial layer. The SEM image obtained
from cross-sectional sample is shown in Fig. 3. As is clearly seen
from Fig. 3, BiOCl0.8Br0.2 photocatalyst is composed of micron
size particles exhibiting very strong contrast at back scattered
electron (BSE) imaging. The thickness of coating layer is about
50–80 mm. EDS spectra acquired from the marked areas are
shown in the in Fig. 3. The EDS analysis conrms that the
plaster material beneath the coating layer is gypsum. A weak
calcium peak appears in the EDS spectra acquired at coated
regions (brown line) and indicates presence of gypsum in the
coated layer, but in the latter the gypsum content is much less
than in the plaster body.
Fig.
1 XRD patterns acquired from as-synthesized BiOCl0.8Br0.2
nanoparticles (a), BiOCl0.8Br0.2 layer on gypsum (b) and pure gypsum
(c).
3.2 Photocatalytic activity
Once a pollutant has been deposited, its disappearance and
those of its degradation intermediate products can be followed
with various techniques. The cleaning efficacy is best evaluated
by optical methods, particularly when stains are studied.
Successive pictures can also be taken and analyzed by
graphical modeling. These optical measurements have obvi-
ously the interest of taking into account the effects on the visual
aspect of both the initially deposited compound(s) and the
degradation intermediate products. As was aforementioned, the
variations in color can be quantied when the deposited
compound is a dye.
For these reasons we have evaluated the photocatalytic
degradation of RhB (30 ppm) dye sprayed on the prepared
BiOClxBr1ꢀx@gypsum composites while exposing the later to
various irradiation sources, at different day times, including
natural sunlight and 300 W Xe arc lamp (Max-302, Asahi
spectra). Power consumption of Max-302 is 500 VA.
For visible light experiments a 422 nm cut-off lter was used.
The light intensity was xed at 70 mW cmꢀ2 and the samples
were placed 10 cm away from the light's source mirror.
The fabricated well-dried coated gypsum plates were
contaminated by an aqueous solution containing 30 ppm of
RhB, simply by trickling 10 drops of the dye solution on the
Fig. 2 SEM images acquired from dry powders of as-synthesized
BiOCl0.8Br0.2 (a and b) and from coated gypsum surface (c and d). EDS
spectrum (e) was acquired from coated surface at 30 kV accelerating
voltage.
center of the tested surfaces. These contaminated BiOCl0.8
-
Br0.2@gypsum composites were kept for 60 min on dark thus
the adsorption–desorption equilibrium of RhB stain will be
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4 minutes of irradiation under Xe lamp (422–740 nm). As it can
be effortlessly guaranteed, the photolysis of the dye is con-
dently negligible.
Fig. 5 demonstrates the gradual photocatalytic decomposi-
tion of a RhB stain over BiOCl0.8Br0.2@gypsum (2 wt%)
composite under Xe lamp visible light irradiation (l ¼ 422–740
nm). The clear-cut disappearance of the dye stain is undoubt-
edly observed and can be easily and visually monitored by
naked eye. Nine images have been taken for recording the
photodegradation processes with time interval of 0.5 min
between one image to the next one. As it is clearly shown, the
complete and swi destruction of the RhB dye could be
successfully achieved within only 4 minutes of irradiation.
Fig. 6 elucidates the swi photo-oxidation of the RhB dye
over BiOCl0.8Br0.2@gypsum (2 wt%) composite while exposing
the surface to natural aernoon solar light. Complete self-
cleaning is monitored aer 2.5 minutes of illumination.
It is of utmost importance to clarify that there was no
signicant difference (in terms of kinetic rates) in the photo-
catalytic efficacy between the samples containing 2 and 4 wt%
of photocatalyst, thus indicating that even a minor portion (>1
wt%) of well dispersed powerful photocatalyst is unequivocally
sufficient for fabricating a photostable light harvesting oxida-
tive material.
The ultimate visible light driven photodecomposition of
RhB contamination presented above was tested in numerous
number of cycles without any loss in activity. Additionally, the
prepared surfaces maintained their superb photocatalytic
efficiency even aer seven months since were rst fabricated.
Lastly, further experiments, conducted under the same
conditions, using simple 11 W table lamp and even 6 W LED
lamps demonstrated rapid photodestruction of RhB which
was accomplished within less than 60 minutes of
Fig. 3 SEM image and EDS spectra acquired from the cross-section
sample. The areas from which the EDS spectra were acquired are
marked with dashed squares. The EDS spectrum acquired from the top
region (the upper dashed square) is shown as a brown line, while that illumination.
of the plaster region (the lower dashed square) is shown as a red line.
Photodecomposition of naphthalene over BiOCl0.8Br0.2
@
gypsum composite. Following the molecular photodecomposi-
tion of RhB shown above we have continued towards investi-
gating the challenging photo-driven destruction of
naphthalene.
Shedding a light on the mineralization of this specic
colorless and persistent organic compound (photolysis under
visible light is condently negligible) underscores the practical
highlights of semi-conductors based photocatalysis.
For this goal we have fabricated very thin surfaces (ꢂ1 mm)
of gypsum coated with BiOCl0.8Br0.2 (2% W).
completely accomplished. Subsequently, the photolysis and
photocatalytic capacities were warily evaluated.
Fig. 4a & b refer respectively to the dark adsorption–
desorption equilibrium and photolysis processes of uncoated
gypsum plates. The later were used as blank and contrast
experiments. Fig. 4b shows the photolysis of the RhB stain aer
Simple mixing and molding of gypsum with stoichiometric
amount of water inside Petri dish followed by spraying the
photocatalyst has enabled a controlled deposition of our alloy
on the curing gypsum.
In fact, these plates were created in a thin form in order to
insure trustworthy solid TOC measurements as the complete
content of the aforementioned surfaces has been utilized for
carrying out the photodegradation analysis.
Two duplicates of hybrid BiOCl0.8Br0.2@gypsum were
analyzed. The rst one has been contaminated with naphtha-
lene (ꢂ50 mg dissolved in EtOH, which has been later evapo-
Fig. 4 (a) Dark adsorption–desorption equilibrium and (b) photolysis
of RhB over uncoated gypsum plate under Xe visible light illumination. rated) and kept in the dark while the second identical sample
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Fig. 5 Photodecomposition of RhB over BiOCl0.8Br0.2@gypsum under Xe visible light at 0.5 min time intervals while a ¼ 0 min, b ¼ 0.5 min, c ¼ 1
min, d ¼ 1.5 min, e ¼ 2 min, f ¼ 2.5 min, g ¼ 3 min, h ¼ 3.5 min, i ¼ 4 minutes.
Fig. 6 Photodecomposition of RhB over BiOCl0.8Br0.2@gypsum under natural sunlight at 0.5 min time intervals while a ¼ 0 min, b ¼ 0.5 min, c ¼
1 min, d ¼ 1.5 min, e ¼ 2 min, f ¼ 2.5 min.
has been illuminated with visible light (Xe, 422–740 nm) for 20
minutes prior to conducting the solid TOC analysis.
Fig. 7 depicts the solid total organic carbon measurements.
The le peak shows the total content of organic carbon in the
reference sample, namely before treatment. The right peak,
designated as aer treatment, indicates the same for the irra-
diated sample.
3.3 Photophysical properties
Additional physical variables which are of supreme importance
limiting the photoefficiency of semiconductors are the band
gap and the conduction and valence band energy edges.
The determination of CB and VB edge electrochemical
potentials of a semiconductor is essential in order to under-
stand the photodegradation mechanism of pollutants. The
bands edge positions of the photocatalyst were theoretically
predicted using atom's Mulliken electronegativity denition:
As it can be seen, over 90% decline in the TOC values was
observed (for more details please see Table S1 in the ESI†).
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Fig. 7 TOC measurements of naphthalene over BiOCl0.8Br0.2@gypsum surfaces. Left peak illustrates the solid TOC measurement before
treatment while the right peak belongs to the same after the controlled photodecomposition.
EVB ¼ c ꢀ Ee + 0.5Eg
where c is the electronegativity of the semiconductor, Ee is the
energy of free electrons on the hydrogen scale (z4.5 eV), and Eg
is the band gap energy of the semiconductor.
According to this empirical expression, the calculated CB
(ECB), VB (EVB) edge positions and the band gap of the sample
estimated from the onsets of the absorption edge as shown in
Fig. 8 (the diffuse reectance spectrum of BiOCl0.8Br0.2) are:
(ECB) ¼ 0.36 eV, (EVB) ¼ 2.96 eV, (Eg) ¼ 2.60 eV.
Fig. 9 illustrates the band alignment, position of CB and VB
of the studied semi-conductor.
Fig. 9 A model of energy band matching and flow of electrons for
BiOCl0.80Br0.20 system.
The resultant BET isotherm of the as synthesised BiOCl0.8
Br0.2 alloy is presented at the ESI (Fig. S1†). The recorded
surface area according to this method is 25.75 m2 gꢀ1
-
.
3.4 Mechanistic study
In order to detect the presence of OHc radical in BiOClxBr1ꢀx
photocatalyst we used terephthalic acid (TA) as a uorescent
probe. OHc radical is known to be trapped by TA to produce the
uorescent 2-hydroxyterephthalic acid.25 There were no
observable peaks when the BiOCl0.8Br0.2 suspensions were
illuminated.
In another experiment, it was found that the addition of
2-propanol, as a scavenger of OHc radicals,26 into the photore-
action system did not cause any noteworthy changes in the
degradation rates of RhB and naphthalene.
Running the above experiment in the presence of benzo-
quinone, as a scavenger for super oxide anion, also had no
Fig. 8 The diffuse reflectance spectrum of BiOCl0.8Br0.2
.
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apparent effect on the reaction rate of the photodecomposition
process.
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In a valence band of Bi3+, holes formed by photoexcitation27
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¨
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In the present study, we demonstrate a potent self-cleaning
featured composite based on photocatalytic BiOCl0.8Br0.2
semi-conductor and commercially available gypsum. Since this
form of Ca(SO4)$2H2O is utilized in the construction industry
where gypsum is applied as nish for walls or as a material for
interior walls in buildings, the composite described here can be
applied directly, akin to general plasters, on a large variety of
surfaces (plastic, aluminum, glass, and quartz). In particular,
we assume that our proposed system may nd use in public for
self-cleaning applications in construction and building arenas.
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Acknowledgements
This study was supported by the Israel Science Foundation
Grant No. 12/207. We would like to acknowledge Dr Nimer Taha
for his assistance with the TOC measurements.
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