Organohalide Precursors for the Continuous Production of Photocatalytic Bismuth Oxyhalide Nanoplates

Article abstract of DOI:10.1021/acs.inorgchem.0c03231

Metal heteroanionic materials, such as oxyhalides, are promising photocatalysts in which band positions can be engineered for visible-light absorption by changing the halide identity. Advancing the synthesis of these materials, bismuth oxyhalides of the form BiOX (X = Cl, Br) have been prepared using rapid and scalable ultrasonic spray synthesis (USS). Central to this advance was the identification of small organohalide molecules as halide sources. When these precursors are spatially and temporally confined in the aerosol phase with molten salt fluxes, powders composed of single-crystalline BiOX nanoplates can be produced continuously. A mechanism highlighting the in situ generation of halide ions is proposed. These materials can be used as photocatalysts and provide proof-of-concept toward USS as a route to more complex bismuth oxyhalide materials.

Full text of DOI:10.1021/acs.inorgchem.0c03231

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Organohalide Precursors for the Continuous Production of
Photocatalytic Bismuth Oxyhalide Nanoplates
Matthew N. Gordon, Kaustav Chatterjee, Alison L. Lambright, Sandra L. A. Bueno,
and Sara E. Skrabalak*
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ABSTRACT: Metal heteroanionic materials, such as oxyhalides,
are promising photocatalysts in which band positions can be
engineered for visible-light absorption by changing the halide
identity. Advancing the synthesis of these materials, bismuth
oxyhalides of the form BiOX (X = Cl, Br) have been prepared
using rapid and scalable ultrasonic spray synthesis (USS). Central
to this advance was the identification of small organohalide
molecules as halide sources. When these precursors are spatially
and temporally confined in the aerosol phase with molten salt
fluxes, powders composed of single-crystalline BiOX nanoplates
can be produced continuously. A mechanism highlighting the in
situ generation of halide ions is proposed. These materials can be
used as photocatalysts and provide proof-of-concept toward USS as a route to more complex bismuth oxyhalide materials.
BiOX (P4/nmm, No. 129) is also advantageous to photo-
INTRODUCTION
■
́
catalysis because of its layered Sillen structure of alternating
Every hour, enough solar energy reaches the surface of the
Earth to meet the energy needs of the entire planet for a year.1
This fact drives the motivation to harvest a fraction of this
energy and convert it to a solar fuel. One approach is through
solar water splitting wherein sunlight is used to split water into
hydrogen and oxygen, a process usually mediated with a light-
absorbing semiconductor.2 This reaction can then be reversed
upon energy demand in a hydrogen fuel cell, releasing the
stored chemical energy without producing carbon-based
byproducts.3 Fujishima and Honda first demonstrated water
photolysis with a titanium dioxide semiconductor in 1972.4
Since this work, tremendous efforts have been directed toward
the identification of new photocatalytic materials with
improved properties and their subsequent synthesis as
structurally well-defined materials.
Metal oxides typically meet the band-position requirements
for water splitting but absorb only ultraviolet light because of
their deep O 2p orbitals, which compose the valence band of
these materials.5 Secondary anions that are less electronegative
than oxygen can be incorporated to raise the valence-band
maximum, thus reducing the overall band gap.6−8 Mixed-anion
materials such as oxynitrides, oxysulfides, and oxyhalides
(where the halide is chloride, bromide, or iodide) have been
explored to combat this problem.9−13 For oxyhalides, the
identity and composition of the halide can tune the valence-
band position and band-gap energies.14
[Bi2O2]2+ layers and X− double layers. These alternating
charged layers create a static internal electric field perpendic-
ular to the layers, which spatially separates charge carriers and
reduces their recombination.16 BiOX materials are commonly
prepared via precipitation between Bi3+ and halide salts. These
facile syntheses are limited by the ability to control crystal
growth, often resulting in materials with poorly controlled
crystallinity (i.e., multiple crystalline domains in a particle). An
additional postsynthetic hydrothermal treatment is often used
to improve the resulting crystallinity.17−20 Here, both BiOCl
and BiOBr powders composed of single-crystalline nanoplates
with controlled faceting are achieved in a single step by
ultrasonic spray synthesis (USS), a continuous-flow means of
materials production.
USS is used for the commercial production of materials as
fine powders; however, typically the powders consist of
polycrystalline microspheres, not single-crystalline particles.
This particle morphology arises from the aerosol process
parameters and underlying chemistry that is typically used to
produce the materials. As shown in Figure 1A, a precursor
Special Issue: Inorganic Chemistry of Nanoparticles
Received: October 31, 2020
Published: December 27, 2020
Specifically, BiOX (X = Cl, Br, and I) has a tunable band gap
that decreases from BiOCl to BiOI.15 The crystal structure of
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Figure 1. (A) Schematic of a USS reactor consisting of a nebulization chamber, hot wall reactor, and collection vessel. Schemes highlighting the
processes that occur during USS within the droplet during heating (B) without a salt flux and (C) with a salt flux, where blue, red, green, and yellow
represent water, precursors, salt flux, and BiOX product, respectively.
solution is nebulized into aerosol droplets, which travel via a
carrier gas into a hot wall reactor where product formation
occurs.21 Solvent evaporation, precursor solidification and
reaction, and finally densification occur within the individual
droplets, with the solid particles being collected outside of the
hot wall reactor (in our setup, particles are collected in water;
Figure 1B). Typically, water-soluble precursors are selected
and decompose or react in a manner that produces only the
desired product and volatile byproducts that are easily flushed
from the droplets. Because of these chemical and physical
processes, multiple crystals form within a droplet and give rise
to the polycrystalline nature of product particles, where the
droplet sizes are governed by the Lang equation.22
Even so, we have shown that the temporal and spatial
confinement offered by the aerosol droplets can be exploited to
produce single-crystalline nanoscale crystals with shape control
by incorporating an inorganic salt into precursor solutions,
which becomes a molten flux upon heating in the reactor.23 In
such aerosol-assisted molten salt syntheses, the inorganic salt
serves as a high-temperature solvent, which facilitates faster
diffusion rates compared to solid-state reactions, and the flux
itself can inhibit particle−particle fusion within individual
droplets (Figure 1C).9,21,23−27 The latter point is central to
achieving discrete, single-crystalline particles as reported here.
Ideal reagents for USS are those that are water-soluble,
which then react at high temperature in the droplet to form an
insoluble product. Considering the synthesis of BiOX, water-
soluble bismuth precursors are rare, with common bismuth
salts hydrolyzing readily in water to produce insoluble
products.28 Polydentate hydroxycarboxylic or aminocarboxylic
acids can complex to Bi3+ to produce water-soluble
compounds.29,30 Simple halide salts would be the obvious
choice as halide precursors; however, these reagents immedi-
ately replace the ligands complexing to bismuth, resulting in
the uncontrolled precipitation of BiOX. Because BiOX
formation should occur in the droplet phase within the hot
wall reactor, a novel halide source is required.
Herein, small organohalides are shown to be effective water-
soluble halide sources that are compatible with a bismuth
lactate complex [denoted as Bi(lac)3], facilitating the
formation of single-crystalline BiOCl and BiOBr nanoplates
when used with the aerosol-assisted molten salt synthesis
approach. The mechanism of BiOX formation in the USS
system is found to be nucleophilic substitution of the
organohalide with water, releasing the halide to immediately
precipitate with Bi(lac)3. The reaction is temperature-
controlled such that product formation only occurs in the
heated droplets. USS-derived BiOCl and BiOBr were also
found to be capable photocatalysts, with this work opening up
a new synthetic path toward more complex bismuth-containing
oxyhalides given the versatility of USS.
EXPERIMENTAL SECTION
■
Materials. Bismuth nitrate pentahydrate (98%), bromoacetic acid
(98+%), 2-chloroethanol (99%), and 1,10-phenanthroline (anhy-
drous, 99%) were purchased from Alfa Aesar. Silver nitrate (≥99.0%),
iron(III) nitrate nonahydrate (≥99.95% metals basis), iron(III)
chloride hexahydrate (≥98%), potassium bromide (≥99%), and D2O
(99.9 atom % D) were obtained from Sigma-Aldrich. L-(+)-Lactic acid
(90% solution in water) was purchased from ACROS Organics.
Potassium nitrate (≥99.0%) was obtained from EMD. Potassium
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chloride (ACS grade) was purchased from BDH. Methanol (AR/ACS
grade), sulfuric acid (95.0−98.0%), and ethylene glycol (≥99.0%)
were obtained from Macron Fine Chemicals. Potassium oxalate
monohydrate (ACS grade) and sodium acetate (anhydrous, ACS
grade) were purchased from J.T. Baker. Glycolic acid (98%) was
obtained from Oakwood Chemicals. Ethanol (200 proof, ACS/USP
grade) was purchased from Pharmco. Tetraammineplatinum(II)
chloride monohydrate (99%, 99.99% platinum) was obtained from
Strem Chemicals. All chemicals were used as received. Milli-Q (18.2
MΩ·cm at 25 °C) purified water was used for all experiments, except
where D2O is noted.
Synthesis of Bi(lac)3. The bismuth lactate precursor Bi(lac)3 was
inspired by a literature synthesis.31 Bismuth nitrate pentahydrate
(8.73 g, 18.0 mmol, 1.00 equiv) and water (32 mL) were sonicated
and stirred, forming a white precipitate. L-(+)-Lactic acid (4.43 mL,
54.0 mmol, 3.00 equiv) was added, and the mixture was stirred
overnight, forming a clear Bi(lac)3 solution. The concentration of the
solution was calculated using the mass of Bi(NO3)3·5H2O used and
the final volume of the solution (typically ∼0.45 M). Note that the
exact structure of the Bi(lac)3 complex is not known in solution, but
because of the prevention of bismuth hydroxide precipitation, it is
likely that lactate stabilizes the Bi3+ ion because of its bidentate
ligation.31
mmol, 1.00 equiv) was dissolved in water (20 mL). The KBr solution
was added dropwise to the bismuth mixture with stirring. The mixture
was stirred for 5 h, and then the solid BiOX was collected via
filtration. Last, the product was washed three times with ethanol and
water and dried under vacuum. Bulk BiOCl was made similarly using
KCl (0.75 g, 10 mmol, 1.0 equiv) in place of KBr. These bulk samples
were compared to the USS BiOX samples in photocatalytic
experiments.
Electron Microscopy. Scanning electron microscopy (SEM) was
performed using a FEI Quanta 600 FEG microscope, operating at 30
kV with a spot size of 3, that interfaces with an Oxford INCA detector
for energy-dispersive X-ray spectroscopy (EDS) analysis. Samples for
SEM imaging were prepared by dispersing the particles in water with
sonication, followed by drop-casting 2 μL of the solution onto a
silicon wafer that was allowed to dry while loosely covered. High-
resolution transmission electron microscopy (TEM) images, scanning
transmission electron microscopy (STEM) images, and selected-area
electron diffraction (SAED) patterns were collected on a JEOL JEM
3200FS microscope operating at 300 kV using a Gatan 4K × 4K
Ultrascan 4000 camera. The instrument interfaces with an Oxford
INCA detector for EDS mapping. Samples for TEM/STEM analysis
were prepared by drop-casting 3 μL of a dispersed aqueous particle
solution onto a 300-mesh carbon-coated copper grid and allowed to
dry while loosely covered.
USS of BiOBr Nanoplates. Figure 1A highlights the major
components of the USS system, while full details of the apparatus can
be found in a previous publication.21 In a typical BiOBr synthesis,
Bi(lac)3 (2.00 mmol, solution from above, 1.00 equiv), bromoacetic
acid (360 μL, 5.00 mmol, 2.50 equiv), and KNO3 (2.02 g, 20.0 mmol,
10.0 equiv) are added to water, mixed, and diluted to a final volume of
20 mL. This precursor solution is transferred to the USS nebulization
chamber. Air is sparged into the solution at a flow rate of ∼175 sccm,
and the furnace is set to 584 °C (250 °C above the melting point of
KNO3). After the furnace is equilibrated and the solution is sparged
for more than 10 min, the nebulizer is turned on to full power (1.7
MHz, ∼5 W/cm2). At this frequency, the number-median droplet size
is estimated to be ∼3 μm in diameter, according to the Lang equation,
and ∼90% of the droplets are smaller than twice this value.22
Nebulized droplets are carried with the air flow through the hot wall
reactor. The flow is left undisturbed overnight or until a sufficient
amount of product is produced. The product is collected by bubbling
the flow output from the furnace into water contained in a gas
washing bottle. The solution containing product is transferred to a
centrifuge tube, where it is centrifuged to separate the powder from
the supernatant. The supernatant is discarded and the powder
dispersed in water and then collected again by centrifugation; this
washing step is conducted three times before the powder is dried
under vacuum. We note that product is lost to the reactor walls and
through the collection apparatus, and reactions are stopped before all
of the precursor solution is nebulized. Even with these losses, our
yields are ∼20% at a nebulization rate of ∼2 mL/h, which provides
100−200 mg/day. Commercial reactors are quite different in design
and can produce on the kilogram scale with minimal losses.23
USS of BiOCl Nanoplates. BiOCl is produced in a manner
similar to that above for BiOBr using 2-chloroethanol (335 μL, 5.00
mmol, 2.50 equiv) instead of bromoacetic acid and a furnace
temperature of 484 °C (150 °C above the melting point of KNO3).
Ex Situ Mechanism Studies. Experiments to probe the
mechanism by NMR were conducted using a solution analogous to
the nebulization mixture. A round-bottomed flask equipped with a stir
bar was charged with an organohalide (2.50 mmol, 1.00 equiv,
bromoacetic acid or 2-chloroethanol), silver nitrate (0.42 g, 2.5 mmol,
1.0 equiv), and D2O (10 mL). The flask was sparged with air and
equipped with a water condenser. The reaction was heated in an oil
bath at 100 °C. Aliquots (1 mL) were removed at specific time points
Powder X-ray Diffraction (pXRD). pXRD data was collected
with a PANalytical Empyrean instrument equipped with Cu Kα
radiation (1.54178 Å) and an X’Celerator linear strip detector. An
accelerating voltage of 45 kV and 40 mA current was used for all
measurements. Scanning was from 5 to 80° 2-theta, with a step size of
0.0167° 2-theta. Powders were measured on a zero-background
rotating silicon holder.
1
NMR Spectroscopy. H NMR spectra were recorded at room
temperature on a Varian I400 (400 MHz) or Varian VXR400 (400
MHz) spectrometer. Chemical shifts are reported in parts per million
from tetramethylsilane, with the residual solvent resonance as the
internal standard (D2O: δ = 4.79 ppm).
Optical Properties. Diffuse-reflectance spectroscopy (DRS) was
conducted on a Cary 100 UV−visible spectrophotometer equipped
with a Cary 301 diffuse-reflectance accessory using BaSO4 powder as a
reference.
X-ray Photoelectron Spectroscopy (XPS). XPS measurements
were collected using a PHI 5000 Versa Probe II scanning X-ray
microprobe under ultrahigh-vacuum conditions with a monochro-
matic Al Kα X-ray source. Spectra were shifted based on the C 1s
peak (284.8 eV) of adventitious carbon.
Photocatalytic Testing. Photocatalytic testing was performed in
a 25 mL quartz reactor connected to a gas closed-circulation system
with side irradiation by a xenon lamp (Newport 66902, 300 W)
attached with a water filter to avoid near-IR radiation above 850 nm
(Figure S1). The power density on the reactor was found to be 743
mW/cm2 (full arc) using a power meter (Newport 1917-R, 818P-040-
25 sensor). The rate of incident photons was calculated as 7.42 × 1017
photons/s (corresponding to 36.8 mW/mL) by ferrioxalate actino-
metry for λ < 500 nm (details can be found in the Supporting
Information). For oxygen evolution reaction (OER) tests, BiOX
powder (10 mg) was dispersed in the quartz reactor in aqueous
Fe(NO3)3 [5 mM, 10 mL, previously adjusted to pH 2.4 with HNO3
to prevent Fe(OH)3 precipitation]. For hydrogen evolution reaction
(HER) experiments, BiOX was first loaded with 1 wt % platinum
based on a literature method.26 BiOX powder was impregnated in a
[Pt(NH3)4]Cl2 solution, followed by annealing at 300 °C for 1 h
under forming gas (5% H2 in N2). Pt/BiOX powder (10 mg) was
dispersed in an aqueous solution containing methanol (10 vol %, 10
mL). For both the OER and HER experiments, the chamber was
evacuated under vacuum followed by backfilling of the system to
atmospheric pressure with helium (for OER experiments) or argon
(for HER experiments) three times. The headspace gas concentration
was monitored over the course of 4 h of irradiation with stirring. A
total of 0.5 mL of headspace gas was manually withdrawn using a
gastight syringe and injected into a gas chromatograph interfaced with
1
and analyzed by H NMR.
Synthesis of Bulk BiOX. For a comparison to USS samples, bulk
BiOBr and BiOCl samples were also synthesized by standard
precipitation routes from the literature.20 Bismuth nitrate pentahy-
drate (4.85 g, 10.0 mmol, 1.00 equiv) was dissolved in ethanol (50
mL), which forms a white suspension. Separately, KBr (1.19 g, 10.0
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Figure 2. (A) SEM image of BiOBr nanoplates. (B) TEM image of a BiOBr nanoplate. (C) STEM−EDS elemental mapping of part B showing the
presence of (Ci) bismuth (yellow), (Cii) oxygen (magenta), (Ciii) bromine (cyan), and (Civ) combined bismuth, oxygen, and bromine signals. (D)
SAED pattern of part B with reflections indexed. (E) Depiction of the BiOBr crystal structure (P4/nmm, No. 129) viewed along the [001]
direction. (F) pXRD pattern of BiOBr with a reference pattern (JCPDS 73-2061).
a thermal conductivity detector (Agilent Technologies 6890N gas
chromatograph, molecular sieve, MS6E column). Nitrogen was used
as an internal standard to approximate the introduction of dioxygen
from air during manual injection of the sample to the gas
chromatograph for OER measurements.
pH. The pH was measured with a VWR sympHony pH meter
calibrated before use using standard buffer solutions.
structure (Figure 2E). pXRD of the product shows BiOBr as
the only crystalline product (Figure 2F). Additionally, the
pXRD pattern shows a much higher relative signal for the
(001) reflection compared to the reference, further high-
lighting the exposed {001} facets and preferred orientation of
the nanoplates.
Similar results were achieved for the synthesis of BiOCl
nanoplates, but now with 2-chloroethanol as a Cl− source
(Figure S3). Again, optimized results were obtained with
KNO3 as the droplet flux but at a lower furnace temperature of
484 °C (150 °C above the melting point of KNO3). SEM and
TEM images show nanoplates that are confirmed to be single-
crystalline by SAED (Figure S3A,B,D,E). They have a
homogeneous elemental distribution by STEM−EDS (Figure
S3Ci−iv). The pXRD pattern confirms BiOCl as the only
crystalline phase (Figure S3F).
Other organohalide molecules were screened, including 2-
bromoethanol and chloroacetic acid as well as 2-bromoethyl
methyl ether, 2-bromopropionic acid, 2,2-dichloroethanol,
epichlorohydrin, and ethyl chloroacetate. All produced the
respective BiOX phase, although some had impurity phases
(such as Bi2O3) and less ideal particle morphologies.
Ultimately, bromoacetic acid and 2-chloroethanol were chosen
for further study because of their high water solubility and the
quality of the resulting BiOX samples in terms of phase purity
and plate morphology.
RESULTS AND DISCUSSION
■
Both BiOBr and BiOCl have been achieved as powders
composed of discrete, single-crystalline nanoplates through the
judicious selection of precursors for aerosol-assisted molten
salt synthesis. Shown in Figure 2 is the characterization of
BiOBr nanoplates produced in this manner. They were
achieved by nebulizing an aqueous solution containing
KNO3 as a flux, bromoacetic acid as a Br− source, and
Bi(lac)3 as a Bi3+ source. Although a number of fluxes and
reaction temperatures were surveyed, as discussed later, the
highest quality samples in terms of phase purity and
nanocrystal form were obtained with KNO3 at a reactor
temperature of 584 °C [250 °C above the melting point of
KNO3]. SEM (Figures 2A and S2) and TEM (Figure 2B)
show nanoplates that are roughly 300−1000 nm in length and
width, with thicknesses of about 30−80 nm. STEM-EDS
elemental mapping shows spatially homogeneous signals from
bismuth, oxygen, and bromine, consistent with a uniform
composition throughout individual particles (Figure 2Ci−iv).
The bulk elemental ratio of Bi/Br is 1.07 by SEM−EDS,
indicating a slightly halogen-deficient sample. A single set of
diffraction spots in the SAED pattern (Figure 2D) indicates
that individual nanoplates are single-crystalline. The SAED
pattern corresponds to the nanoplate being viewed down the
[001] zone axis, oriented as shown in the included crystal
Two synthetic parameters were found to be most critical to
the formation of single-crystalline nanoplates: precursor
selection and integration of a suitable flux into USS. Although
the high reaction temperatures and short droplet residence
times can limit analyses of the chemistry occurring during USS,
we undertook a detailed mechanistic study that accounts for
BiOX nanoplate formation. This work establishes organo-
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halides as a general source of halide ions that should be
applicable to other oxyhalide materials, even beyond USS
syntheses (e.g., hydrothermal routes). This is in complement
to work by Rabuffetti and co-workers that demonstrated metal
trifluoroacetates as precursor complexes toward layered
perovskite systems where the fluorinated ligand provides the
necessary F− for the reactions.32−34 In aqueous solution, Bi3+
and X− react quickly to form BiOX as a precipitate except at
extremely low pH values. With this in mind, we hypothesized
that Br− and Cl− were being generated from the organohalides
at elevated temperatures within the droplets via nucleophilic
substitution, with water acting as a nucleophile.35−39
°C for 90 min (Figure 3D), highlighting the need for a reagent
to remove halides from the reaction as they form. Applying this
mechanistic insight to the USS system, the Bi(lac)3 reagent
serves the same role as AgNO3 to precipitate any autogenous
halide ions as BiOX, our desired product (Figure 3E).
Analogous experiments were performed to verify the Cl−
release from 2-chloroethanol. The ex situ reaction scheme
shows a similar substitution reaction from heating 2-
chloroethanol with AgNO3 in D2O to produce ethylene glycol
and AgCl as major products and 2-nitrooxyethanol as a minor
product (Figure S6). Before heating, this solution shows two
triplet peaks centered at 3.55 and 3.7 ppm from 2-
chloroethanol, indicating that no reaction occurs at room
temperature (Figure S6A). Upon heating at 100 °C for 90 min,
a new major singlet peak appears at 3.5 ppm, which
corresponds to ethylene glycol, the product of a substitution
reaction of water displacing chloride (Figure S6B; note that the
peak is a singlet due to the symmetry of ethylene glycol). Two
minor triplet peaks centered at 3.75 and 4.5 ppm were just
visible as well, which correspond to 2-nitrooxyethanol, the
substitution product of chloride being replaced with a nitrate
group. Upon further heating at 100 °C for 6 h total, the peak
from the 2-chloroethanol starting material is gone, while the
peaks from both the major and minor products increase,
showing completion of the reaction (Figure S6C). The identity
of the minor product, 2-nitrooxyethanol, was confirmed
through a known synthesis and by comparison of the spectra
(Figure S6D).41 Similar to the mechanism for BiOBr
production, a nucleophilic substitution mechanism is proposed
to form the BiOCl products in the USS system (Figure S6E).
These ex situ results highlight the importance of each
component of the synthesis. Bi(lac)3 serves as a water-soluble
source of bismuth for BiOX but also as a necessary tool to
capture the released halide ions as they form. The organohalide
reagents (bromoacetic acid and 2-chloroethanol) serve as
halide sources but in a unique manner, such that they react
with water only at elevated temperatures. This property makes
the solution stable at room temperature and capable of
nebulization into aerosol droplets for transport into the USS
reactor, where the reactions occur to facilitate product
formation. While the ex situ experiments could only be
conducted at 100 °C, these reactions would proceed much
more quickly at the elevated temperatures of the droplets.
While the ex situ mechanistic experiments give insight into
the chemistry occurring within the aerosol droplets to form
BiOX, they do not account for the morphology of the BiOX
particles. The aerosol approach coupled with KNO3 as a flux is
central to achieving single-crystalline nanoplates of BiOBr and
BiOCl with facet control. Specifically, when an otherwise
identical USS experiment is conducted without the flux,
powders composed of polycrystalline, hollow spheres in the
case of BiOCl and hard aggregates of nanoplates in the case of
BiOBr are produced (Figure S7). The pXRD patterns of the
products show BiOX without other crystalline phases.
However, the product morphologies are undesired for
photocatalytic applications because polycrystalline materials
suffer from a high recombination of charge carriers and large
aggregates have limited active sites.42,43 These morphologies
are characteristic of those prepared by traditional USS because
multiple crystallites form per droplet and aggregate together as
the liquid evaporates.44 These results also highlight the
importance of flux addition in achieving single-crystalline
particles with facet control.
While in situ experiments are not possible, ex situ
experiments were performed to mimic the reaction and test
this hypothesis. In the case of Br− generation, a solution of
bromoacetic acid, silver nitrate, and D2O was heated and
1
monitored using H NMR (Figure 3). AgNO3 was selected as
1
Figure 3. H NMR of bromoacetic acid and AgNO3 in D2O (A)
without heating, (B) after heating at 100 °C for 45 min, (C) after
heating at 100 °C for 90 min, and (D) after heating at 100 °C for 90
min while omitting AgNO3. (E) Proposed BiOBr production scheme.
a scavenger of Br− instead of Bi(lac)3 in order to simplify the
NMR spectra, with AgBr(s) readily precipitating out with any
free Br−, just as BiOBr(s) forms when Bi3+ is exposed to free
1
Br−. Before heating, H NMR of this solution shows one
singlet peak at 3.85 ppm from the bromoacetic acid starting
material, indicating that no reaction occurs at room temper-
ature (Figure 3A). Additionally, no solid precipitate is
produced. Upon heating at 100 °C for 45 min, a new major
singlet peak appears at 4.0 ppm, corresponding to glycolic acid,
the product of a substitution reaction replacing the bromide
group with a hydroxyl group from water (Figure 3B). A minor
singlet peak at 4.9 ppm is visible as well, identified as
nitrooxyacetic acid, the substitution product of nitrate
replacing bromide (Figure S4).40 A solid precipitate is
observed as well, identified as AgBr by pXRD (Figure S5).
Upon further heating at 100 °C for 90 min total, the relative
size of the peak from the bromoacetic acid starting material
further decreases, while the peaks from both major and minor
products increase, showing further progression in the reaction
(Figure 3C). A control experiment without AgNO3 shows no
glycolic acid production or precipitation despite heating at 100
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Figure 4. (A) Time courses of photocatalytic OER on the BiOX USS samples compared to BiOX synthesized by bulk precipitation. (B) Oxygen
evolution rate for these samples after 1 h. The graph shows the average of a minimum of three experiments with standard deviations. The graph in
part A has its standard deviations removed for clarity but can be found in Figure S13C. Reaction conditions: OER, 10 mg sample in 10 mL of 5 mM
Fe(NO3)3 (previously adjusted to pH 2.4 with HNO3 to prevent Fe(OH)3 precipitation), 300 W xenon lamp (full arc).
Even though flux addition is important in achieving single-
crystalline particles, not all fluxes studied facilitated the
production of single-crystalline BiOX nanoplates or were
compatible with the precursors. For example, Bi(lac)3 reacts to
produce a precipitate in the presence of halide salts and other
strongly π-donating anions. Nitrate salts were soluble with
Bi(lac)3 and have low melting points, which makes them ideal
for USS. Among the alkali and alkali-earth nitrates, KNO3 did
not give rise to impurities/competing phases. The use of
KNO3 also led to the best product morphology. A KNO3/
Bi(lac)3 ratio of 10:1 was found to be optimal, although slight
changes were tolerable, giving rise to discrete and well-
developed nanoplates.
The particle morphology was also influenced by the reactor
temperature. For the case of BiOCl, the powder produced at
484 °C consisted of single-crystalline nanoplates, while higher
temperatures gave a powder of microspheres comprised of
many nanoplates (Figure S8). These differences in morphology
can be rationalized in terms of droplet supersaturation.
Specifically, fewer nucleation events occur per droplet at
lower temperatures (i.e., slower reaction rates keep super-
saturation low), resulting in single-crystalline plates separated
by the flux.23 In contrast, higher temperatures give a faster
reaction rate and higher supersaturation; this condition
provides more nucleation events and growing nanoplates per
droplet.44 Thus, the flux cannot adequately inhibit aggregation
of the many particles produced, and polycrystalline micro-
spheres are produced. This important role of KNO3 is further
supported by analysis of the BiOBr product collected prior to
the gas washing bottle. Together, SEM and pXRD reveal
multiple BiOBr nanoplates encased in each solidified KNO3
microsphere (Figure S9A,C); washing the sample with water
(as would occur as the droplets enter the gas washing bottle)
dissolves the KNO3 and releases the individual BiOBr
nanoplates (Figure S9B,C).
resolution scans of the bismuth, oxygen, chlorine, and bromine
regions show characteristic peaks that match literature values
(Figure S11B−E).46 While elemental analysis by SEM−EDS
(bulk measurement) only revealed slight halide deficiencies,
XPS (surface-sensitive measurement) found much greater
halide deficiencies (Table S1). This finding may be significant
to the use of these materials as photocatalysts because the band
structure may be different at the surface where reactions occur.
Both of the BiOBr and BiOCl nanoplates were evaluated as
photocatalysts for the OER and HER half-reactions that
compose the overall water splitting and compared to the
BiOBr and BiOCl references prepared by bulk precipitation.20
Characterization of the reference materials by SEM and pXRD
can be seen in Figure S12. Water oxidation (OER)
experiments show comparable activity between USS-synthe-
sized BiOX samples and their respective bulk counterparts
(Figure 4). BiOBr samples show higher evolution of oxygen
than BiOCl because of their smaller band gaps and thus greater
light absorbance.47 Oxygen production continued throughout
the 4 h experiments, with some decrease in the evolution rate
over time (Figure 4A and S13C). The deviation from linearity
is likely a result of some photocorrosion of BiOX, a known
limitation of these materials.46 Future work aims to develop
more complex bismuth oxyhalide materials without this
limitation. The oxygen evolution rate after 1 h is shown in
Figure 4B, and USS samples show slight improvements over
the bulk counterparts (although within the standard deviation
of the measurements). Such an improvement could be a result
of the improved crystallinity (single crystallinity vs poly-
crystallinity), which would lead to fewer defect sites and thus
less recombination.15
In analogous water reduction (HER) experiments of BiOX
USS and bulk samples, hydrogen production was not detected
by our analytical system without a cocatalyst. However,
platinum has been shown to be an ideal surface for HER
reactions, so 1 wt % was loaded on the materials.48 In this case,
excited electrons in BiOX migrate to the platinum surfaces,
where HER occurs. Figures S14 and S15 show characterization
after platinum loading of the USS and bulk BiOX materials,
respectively. The Pt/BiOBr sample shows higher evolution of
hydrogen than Pt/BiOCl because of its smaller band gap and
thus greater light absorbance (Figure S13). Hydrogen
production increased over the full 4 h of measurement, with
some increase in the rate of gas evolution over time (Figure
S13A). The increase in the rate is thought to be a result of
With the mechanisms accounting for nanoplate formation
better understood, the highest-quality BiOBr and BiOCl
nanoplates (Figures 2 and S3, respectively) were characterized
further, including for their photocatalytic capabilities. DRS
spectra and Tauc plots of the BiOBr and BiOCl nanoplates
indicate indirect band gaps of 2.83 and 3.38 eV, respectively
(Figure S10), which are comparable to the reported values.45
XPS was used to probe the surface chemical states of the
nanoplate samples. The survey scans did not detect any of the
unique elements from the KNO3 flux (Figure S11A). High-
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Inorg. Chem. 2021, 60, 4218−4225

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pubs.acs.org/IC
Forum Article
some reduction of Bi3+ to Bi(s), which may lead to enhanced
light absorbance due to the surface plasmon resonance and
cocatalytic properties of bismuth.46,49 The hydrogen evolution
rate after 1 h is shown in Figure S13B, and Pt/BiOBr by USS
clearly outperforms the bulk Pt/BiOBr sample. For Pt/BiOCl,
the comparison between the USS and bulk samples is much
closer; however, the USS sample has somewhat higher activity
over the bulk counterpart. Through these studies, we show that
single-crystalline BiOX nanoplates synthesized by USS match
or slightly outperform their bulk counterparts, demonstrating
their efficacy as photocatalysts for water splitting. These
materials have other applications in CO2 reduction and
nitrogen fixation and as films for photoelectrochemical water
splitting, which may be explored further.46,50,51
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c03231
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Funding
Funding for this project was made available from Indiana
University. S.L.A.B. was supported by National Science
Foundation (NSF) Grant DGE-1342962 (Graduate Research
Fellowship Program). Access to the powder diffractometer was
provided by NSF Grant CRIF CHE-1048613 and to the XPS
by NSF Grant DMR MRI 1126394.
In summary, USS was shown as a continuous route toward
BiOX materials. Central to this demonstration was the
identification of organohalides as a new halide source, being
water-soluble and forming stable solutions with bismuth-
containing complexes. As we found, such solutions could be
nebulized and transported into the heated USS reactor, where
nucleophilic substitution of the organohalides with water
releases the halide to immediately precipitate with the bismuth
complexes. Alkali nitrate salts can also be added to these
solutions to establish a molten flux within the aerosol droplets
that facilitates BiOX nanoplate formation in contrast to BiOX
of the ill-defined morphology. We anticipate that more
complex oxyhalide materials as well as perovskite halide
materials can be prepared with these organohalides as a source
of halide. An attractive feature of these organohalides is that
their reactivity is temperature-controlled, making them well
suited for USS but also by hydrothermal strategies toward new
materials.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the IU Electron Microscopy Center, Molecular
Structure Center, Nanoscale Characterization Facility, and
Nuclear Magnetic Resonance Facility for access to the
necessary instrumentation. We also thank Prof. Kevin Brown
for his feedback.
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AUTHOR INFORMATION
Corresponding Author
■
Sara E. Skrabalak − Department of Chemistry, Indiana
University - Bloomington, Bloomington, Indiana 47405,
United States; orcid.org/0000-0002-1873-100X;
Email: sskrabal@indiana.edu
Authors
Matthew N. Gordon − Department of Chemistry, Indiana
University - Bloomington, Bloomington, Indiana 47405,
United States; orcid.org/0000-0001-7212-9047
Kaustav Chatterjee − Department of Chemistry, Indiana
University - Bloomington, Bloomington, Indiana 47405,
United States; orcid.org/0000-0003-3527-9796
Alison L. Lambright − Department of Chemistry, Indiana
University - Bloomington, Bloomington, Indiana 47405,
United States
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