Interplay of Corrosion and Photocatalysis during Nonaqueous Benzylamine Oxidation on Cadmium Sulfide

Article abstract of DOI:10.1021/acs.chemmater.7b02899

The photo(electro)chemical properties of bulk, nanowire, and chemical bath deposits of cadmium sulfide (CdS) for benzylamine oxidation to N-benzylidenebenzylamine (N-BB) in acetonitrile have been evaluated as a model for the activity and stability of CdS toward selective organic oxidations. CdS photocatalysts selectively deliver N-BB at rates ranging from 5 to 26 mM h^-1. Although CdS is a capable photocatalyst, SEM imaging and XPS analysis reveal significant morphological and compositional changes to the particles upon photolysis in benzylamine. These surface changes and surface sulfide oxidation are accompanied by Cd²⁺ leaching and hydrogen sulfide evolution, highlighting both redox and acid-base pathways of nonaqueous CdS corrosion. All facets of corrosion have been linked directly with amine reactivity, as the CdS particles are unaffected by substrate-free photolysis. A series of experiments using N,N-dimethylbenzylamine, 4-N,N-trimethylaniline, and ferrocene show that nonaqueous CdS corrosion is facilitated by acidic reaction intermediates opposed to photogenerated holes. Additionally, water and oxygen are essential components to corrosion, as photoelectrochemistry under dry/air-free conditions displays higher and stable photocurrent density as well as material stability. Finally, CdS nanowires display improved corrosion resistance, suggesting that control of particle morphology and/or electronic structure is essential for developing novel chalcogenide photocatalysts.

Full text of DOI:10.1021/acs.chemmater.7b02899

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Article
The Interplay of Corrosion and Photocatalysis During Non-
Aqueous Benzylamine Oxidation on Cadmium Sulfide
John L DiMeglio, and Bart M Bartlett
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b02899 • Publication Date (Web): 17 Aug 2017
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Chemistry of Materials
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The Interplay of Corrosion and Photocatalysis During Non-Aqueous
Benzylamine Oxidation on Cadmium Sulfide
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John L. DiMeglio and Bart M. Bartlett*
Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, MI 48109, United States
ABSTRACT: The photo(electro)chemical properties of bulk, nanowire, and chemical bath deposits of cadmium sulfide (CdS) for
benzylamine oxidation to N-benzylidenebenzylamine (N-BB) in acetonitrile have been evaluated as a model for the activity and
stability of CdS toward selective organic oxidations. CdS photocatalysts selectively deliver N-BB at rates ranging from 5 – 26 mM h–
1. Although CdS is a capable photocatalyst, SEM imaging and XPS analysis reveal significant morphological and compositional
changes to the particles upon photolysis in benzylamine. These surface changes and surface sulfide oxidation are accompanied by
Cd2+ leaching and hydrogen sulfide evolution, highlighting both redox and acid-base pathways of non-aqueous CdS corrosion. All
facets of corrosion have been linked directly with amine reactivity, as the CdS particles are unaffected by substrate-free photolysis.
A series of experiments using N,N-dimethylbenzylamine, 4,N,N-trimethylaniline, and ferrocene show that non-aqueous CdS corro-
sion is facilitated by acidic reaction intermediates opposed to photogenerated holes. Additionally, water and oxygen are essential
components to corrosion, as photoelectrochemistry under dry/air-free conditions display higher and stable photocurrent density as
well as material stability. Finally, CdS nanowires display improved corrosion resistance, suggesting that control of particle morphol-
ogy and/or electronic structure is essential for developing novel chalcogenide photocatalysts.
INTRODUCTION
corrosion,8–11 there are no experimental details elucidating
non-aqueous corrosion processes.
Cadmium sulfide (CdS) represents a well-studied II-VI
semiconductor with a band gap of 2.4 eV, which overlaps well
with the solar spectrum.1 Due to its impressive light absorbing
properties, CdS powders and electrodes have been pursued for
solar fuel prodution2–4 and pollutant decomposition (organic
dye degradation).5–7 However, the main challenge for imple-
menting CdS-based technologies is corrosion in aerated aque-
ous solutions.8 Extensive studies have demonstrated that
aqueous corrosion process may be initiated by either photo-
generated holes9–12 or by reactive intermediates, such as hy-
droxyl radicals.13,14 Corrosion of CdS often results in the for-
mation of water soluble CdSO4, surface dissolution, and loss
of catalytic activity.
To probe the driving forces behind non-aqueous CdS cor-
rosion, we employ the photo(electro)chemical oxidative cou-
pling of benzylamine (BnNH2) to N-benzylidenebenzylamine
(N-BB) as a test reaction. We start here because the photo-
chemical oxidation of BnNH2 to N-BB has benchmarked a va-
riety of photocatalysts,16,18 including CdS,21,22 BiVO4,28–30
Nb2O5,31–33 and TiO2
owing to the electron-rich amine
34–36
functionality. Bulk, nanowire, and chemically deposited CdS
powders and electrodes have been characterized by scanning
electron microscopy and X-ray photoelectron spectroscopy
before and after photolysis. Leached Cd2+ ions into the solu-
tion have been determined using inductively coupled plasma-
optical emission spectroscopy (ICP-OES). Through these an-
alytical techniques we establish that the corrosion of CdS in
organic solvents is highly dependent upon the substrate iden-
tity. This insight into non-aqueous CdS decomposition path-
ways will aid in the design of new inorganic light absorbing
platforms and target reactions.
To disfavor aqueous passivation, efforts have been aimed
at CdS photochemical applications in dry organic solvents. In
this realm, one promising application is the selective photo-
chemical oxidation of organic substrates,15–17 in which aque-
ous conditions generate reactive hydroxyl radicals, leading to
non-specific substrate oxidation to CO2.18–20 In order to pro-
mote selective oxidations as well as to disfavor catalyst pas-
sivation, the photochemical activity of CdS for the oxidation
of electron rich benzylic amine21,22 and alcohol22–27 substrates
in acetonitrile and trifluorotoluene solvents have been inves-
tigated. While high selectivity is observed, catalyst pas-
sivation is still apparent even in non-aqueous solvents.23–26
Catalyst corrosion is identified through a loss of photocurrent,
a morphological comparison by scanning electron micros-
copy, or detecting leached Cd2+ ions by atomic absorption
spectroscopy. Although past studies have not speculated on
the nature of CdS corrosion, the assumption is that aqueous
and non-aqueous passivation operate similarly. Compared to
the extensive work on the driving forces behind aqueous CdS
EXPERIMENTAL SECTION
Materials and Methods. Cadmium sulfide bulk particles
(99.995 %, trace metals basis), all solvents, and trimethylsilyl
cyanide were purchased from Sigma Aldrich. All amines were
purchased from Acros Organics; benzylamine and 4,N,N-tri-
methylaniline were further purified by vacuum distillation
over zinc dust and potassium hydroxide, respectively, then
subsequently stored over molecular sieves. Tetrabutylammo-
nium hexafluorophosphate (> 98 %) was purchased from TCI
America and purified by recrystallization from ethanol. Fer-
rocene was purchased from Sigma Aldrich and was recrystal-
lized from hexanes prior to use: all other reagents were used
without further purification.
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Preparation of CdS Nanowires. CdS Nanowires (NWs)
and was equipped with a Bragg-BrentanoHD X-ray optic deliv-
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were prepared by adopting a previously reported hydrother-
mal synthesis.37 Cadmium(III)nitrate tetrahydrate (751 mg,
2.4 mmol), thiourea (556 mg, 7.3 mol), and ethylenediamine
(13 mL) were mixed in a 18 mL PTFE lined autoclave. The
vessel was sealed and was warmed to 160 ˚C at a ramp rate of
10 ˚C/min, and was then held at 160 ˚C for two days. After
cooling naturally to room temperature, the mixture was cen-
trifuged at 4000 rpm for 5 minutes to isolate a bright yellow
powder. This powder was washed three times with Millipore
(MQ) water (3 × 15 mL), and then dried overnight in a vac-
uum oven.
ering only K radiation. Patterns were collected with a sam-
pling step of 0.017 ° and a scan rate of 6 °/min while spinning
at a rate of 0.25 Hz. X-ray photoelectron spectroscopy (XPS)
was conducted using a Kratos Axis Ultra X-ray Photoelectron
Spectrometer. The X-rays used were monochromatic Al K
X-rays (1486.7 eV). All data collected while the analysis
chamber pressure was ~ 1 × 10–9 torr. The charging effects of
the semiconducting substrates were compensated using an
electron flood gun. High-resolution spectra were collected at
a pass energy of 20 and a step size of 0.1 eV. XPS spectra
interpretation was performed using Casa XPS with a Shirley-
type baseline to calculate peak areas. Binding energies were
calibrated by positioning the adventitious carbon signal at
248.8 eV.
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Chemical Bath Deposition of CdS. CdS was deposited
onto fluorine-doped tin oxide (FTO) substrates by adopting a
previously reported procedure.38 Slides of FTO (1 × 4 cm)
were rinsed and sonicated sequentially with acetone, MQ wa-
ter, and acetone. These cleaned slides were fastened to the side
of a beaker (FTO side facing the center, and the bottom of the
FTO nearly touching the beaker bottom) containing a MQ wa-
ter (25 mL) solution of cadmium sulfate (522 mg, 2.5 mmol),
and warmed to 70 ˚C. While stirring the cadmium sulfate so-
lution, ammonium hydroxide (28-30%, 10 mL) was added
drop-wise, resulting in the evolution of a white vapor. In a
separate beaker, thiourea (190 mg, 2.5 mmol) was dissolved
in MQ water (25 mL). The thiourea solution was slowly added
to the stirring cadmium sulfate, ammonium hydroxide mix-
ture. The submerged FTO plates were left in the stirring solu-
tion for 15 minutes, which became a cloudy yellow suspen-
sion by the end of the deposition. The CdS-functionalized
FTO electrodes were then removed from the beaker to be
washed and sonicated with MQ water. Any deposits that
formed on the glass side of the FTO were removed by gently
rubbing that side with a Kim wipe wet with 1 M hydrochloric
acid followed by copious MQ water rinsing. The CdS-func-
tionalized FTO slides were annealed in a tube furnace with
flowing nitrogen at 300 ˚C for 30 minutes with a 10 ˚C/min
ramp rate. After annealing, the CdS films were clear and
bright yellow. Photoelectrodes were prepared from these de-
posits by attaching an exposed portion of the FTO slide to a
copper wire using silver print (Jameco Electronics). After dry-
ing for 12 hours, the exposed wire was inserted into a Pyrex
tube (length 10 cm, 3 mm ID, 4 mm OD). The exposed wire
and silver paint were masked using epoxy (Loctite EA 1C,
“Epoxi-patch Hysol”, VWR) to give electrodes with a surface
area of 1 cm2. The epoxy was allowed to cure at room temper-
ature for a minimum of 24 hours prior to use in any experi-
ments.
Photochemistry. CdS (bulk or nanowire, 10 mg) was sus-
pended in acetonitrile (2 mL) in a borosilicate test tube (13 ×
100 mm) equipped with a stir bar. The concentration of any
substrate employed was 250 mM. The test tube was capped
with a septum and vented using a needle. The reaction mixture
was placed in a blue LED photolysis reactor composed of
LED strips (Creative Lighting Solutions, flex LED strip 5050
High Density 12vDC, CL-FRS5050-12WP-12V) wrapped
around a water jacketed beaker (Figure S1) to deliver 5-10
mW/cm2 of 470 nm blue light. Substrate concentrations were
measured by periodically removing an aliquot (10 L) from
the reaction mixture and combining with a chlorobenzene
standard (50 L of a 21.6 mM standard) and acetonitrile (940
L). The samples were analyzed on either a Shimadzu QP-
2010 GC/MS or Thermo Fisher Trace 13-10 gas chromato-
graph equipped with a capillary column and a FID detector.
Quantification was enabled by the integration ratio of the an-
alyte and chlorobenzene signals in relation to an obtained cal-
ibration curve. For dry photochemistry experiments, the sam-
ples were prepared inside of a nitrogen glove box using ace-
tonitrile that was distilled over calcium hydride and stored
over molecular sieves. The sealed samples were brought out-
side of the glove box and saturated with dry air.
Leached Cadmium Detection. The post photolysis solu-
tions were analyzed for leached cadmium via inductively cou-
pled plasma optical emission spectrometry (ICP-OES) using
a Perkin-Elmer Optima 2000DV. Samples were prepared by
removal of the volatile organics via rotary evaporation, diges-
tion in a 5 % nitric acid solution, followed by filtration
through a syringe filter (VWR). Samples were referenced to a
yttrium internal standard (1 ppm) and concentrations of cad-
mium were determined via a series of standard cadmium so-
lution prepared from a 1000 ppm Cd reference solution
(Sigma Aldrich).
Material Characterization. Brunauer–Emmett–Teller
(BET) surface area measurements were obtained using a
NOVA4200e from Quantachrome surface area analyzer.
Scanning electron microscopy (SEM) images were obtained
from either a JEOL-7800FLV FE SEM or a Zeiss LEO
1455VP tungsten filament SEM at an accelerating voltage of
3 kV and working distance of 5 mm. UV-Vis spectra were
recorded using a Cary 5000 spectrophotometer (Agilent)
equipped with an external diffuse reflectance accessory. Spec-
tra were recorded in reflectance mode and transformed math-
ematically into the Kubelka–Munk function. Powder X-ray
diffraction (PXRD) data was collected on a Panalytical Em-
pyrean diffractometer at a power of 1.8 kW (45kV, 40 mA)
with Cu K ( = 1.5418 nm) radiation. The diffractometer
was a X’Celerator Scientific, a position-sensitive 1D detector
Hydrogen Sulfide Detection. A bismuth hydroxide hydro-
gen sulfide (H2S) chemical sensor was employed to verify the
production of H2S during photochemical amine oxidation.39
Bismuth(III) nitrate pentahydrate (100 mg) was suspended in
acetone (10 mL) and sonicated for 10 minutes. The resulting
white suspension was filtered through celite to give a clear
colorless solution that was syringed (100 L) onto the center
of a Kim wipe twice, with a minute delay between each dose
to allow for acetone evaporation. Once dry, a sodium hydrox-
ide solution (1 M, 100 L) was dropped onto the same spot,
generating the bismuth(III) hydroxide sensor. While wet, the
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Chemistry of Materials
Table 1. Metrics for Photochemical Catalysis and Corrosion of CdS Particles Under the Specified Conditions
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Conditions
1. CdS, BnNH2, LED
2. BnNH2, LED
Conversion / %
Rate / mM h–1
Solvated [Cd2+] / ppm
Surface SO42– / %a
63 ± 14
5 ± 1
2.0 ± 0.03
95
N/A
14
5
0
N/A
0
0
N/A
0
N/A
3. CdS, LED
0
4. CdS, BnNH2
0
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9
5. CdS, BnNH2, LED, N2
6. CdS, BnNH2, LED, Dry
7. CdS, BnNH2, tBuOH, LED
8. CdS, BnNMe2, LED
9. CdS, TMA, LED
10. CdS, Fc, LED
0
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< 0.1
60 ± 11
0
N/A
3.0 ± 0.1
0
0
1.6 ± 0.07
4.7 ± 0.6
0
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0
0
N/A
N/A
0
11. CdS NWs, BnNH2, LED
100
26 ± 5
5 ± 0.9
a Calculated as the percentage of total surface sulfur that exists as sulfate; all HR S2p XPS spectra are provided in Figure S11.
Kimwipe was affixed to the top of the test tubes and photo-
chemistry was performed as described above. Intermittently,
MQ water was used to gently wet the Kim wipe to improve
the gas sorption. The response of this sensor was verified by
affixing a Bi-loaded Kim wipe onto a vial containing bulk
CdS and 6 M hydrochloric acid; upon exposure to the H2S va-
pors, a yellow spot formed on the Kim wipe, indicative of bis-
muth(III) sulfide formation (Figure S2).
RESULTS AND DISCUSSION
The photochemical activity and stability of bulk cadmium
sulfide was assessed in 250 mM solutions of benzylamine
(BnNH2) in acetonitrile illumined by 5 – 10 mW/cm2 470 nm
blue LEDs for two days. The concentrations of BnNH2 start-
ing material and the coupled product, N-benzylidenebenzyla-
mine (N-BB) were quantified periodically using gas chroma-
tography (GC). Analysis of the products by GC reveals N-BB
to be the only product formed under these conditions (Figure
S5) at a rate of 5 ± 1 mM h–1 (Table 1, entry 1; Figure S6).
Repeating the experiment in the absence of any photocatalyst
material yields no conversion, highlighting the heterogeneous
catalytic activity of CdS (Table 1, entry 2). By measuring the
photochemical rate as a function of concentration, we have
confirmed that the catalysis is first order in both CdS and
BnNH2 (Figure S7). During the extended illumination of bulk
CdS particles necessary for full BnNH2 conversion, the first
order constant changes from 1.6 × 102 h–1 to 6.9 × 10–3 h–1 (Fig-
ure S8). We attribute the decreased activity of bulk particles
throughout long photolysis experiments to photocatalyst cor-
rosion.
Photoelectrochemistry. Chemical bath deposits of CdS on
FTO substrates were used as the working electrode, a polished
silver wire (CH instruments) as the reference, and a platinum
mesh (CH instruments) as the counter electrode in a custom
build two-compartment photoelectrochemical cell (Figure
S3). All electrochemical solutions contained 0.1 M Bu4NPF6
supporting electrolyte. A Nafion (NRE-212, Fuel Cell Earth)
membrane was used to separate the anode and cathode com-
partments. The photoelectrodes were back illumined using the
same LEDs described above for the photochemical experi-
ments (5 mW/cm2 of 470 nm blue light). Linear sweep volt-
ammetry (LSV) and bulk electrolysis in the dark and under
illumination were conducted using a CH instruments electro-
chemical workstation 1000A. LSVs were performed on qui-
escent solutions while the electrolyte was vigorously stirred
during bulk electrolysis All potentials are reported versus the
ferrocenium/ ferrocene (Fc+/0) couple by acquiring cyclic volt-
ammograms of ferrocene in the electrolytes used (Figure S4).
The solution-phase products were analyzed by gas chroma-
tography, by first concentrating the catholyte and precipitat-
ing the electrolyte using diethyl ether. The electrolyte was fil-
tered through celite and washed with diethyl ether. The or-
ganic filtrates were combined and concentrated in vacuo to a
yellow oil. This oil was combined with a chlorobenzene stand-
ard (50 L of a 21.6 mM standard) and diluted to 1 mL. For
the dry and air-free photoelectrochemistry, the cell was as-
sembled inside of a nitrogen glove box using dry solvents and
reagents. Activated molecular sieves (4A, Alfa Aesar) were
added to the anode compartment prior to brining the sealed
cell outside of the glove box. To maintain an air-free environ-
ment during photoelectrolysis a gentle stream of nitrogen was
introduced into the anode compartment.
Passivation of CdS catalysts by forming CdSO4 that leaches
Cd2+ into solution has been well established for aqueous pho-
tochemistry.9–12 Two mechanisms of CdS corrosion are often
evoked: acidic dissolution and sulfide oxidation. Under acidic
conditions, as predicted by the cadmium sulfide Pourbaix di-
agram,40 sulfide is thermodynamically unstable relative to hy-
drogen sulfide (H2S); upon gaseous H2S evolution, Cd2+
leaches out into the aqueous electrolyte.41,42 Alternatively, sul-
fide may be oxidized to sulfate either by photogenerated
holes9–12 or by hydroxyl radicals formed during water oxida-
tion.13,43 Non-aqueous examples of CdS corrosion have also
been reported during benzyl alcohol oxidations, but with no
discussion of how the non-aqueous solvent affects the corro-
sion mechanism.23–25 In order to determine the extent of CdS
corrosion and its role in non-aqueous photochemical transfor-
mations, we quantified Cd2+ dissolution as well as the surface
composition and morphology under a variety of photolysis
conditions. ICP-OES analysis detects 2.0 ± 0.3 ppm dissolved
Cd2+ in the solution after photolysis. SEM imaging revealed
the bulk CdS to contain large, smooth particles that are 20 –
50 m in size (Figure 1a). XPS of this
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iments in which we react BnNH2 substrate on CdS in the ab-
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sence of either light, oxygen, or water. Under conditions
where any one of the listed variables was removed, no con-
version of BnNH2 was detected, nor was any evidence for cor-
rosion detected by ICP-OES or XPS (Table 1, entries 4 – 6,
Figure S11). The lack of catalysis under either dark, nitrogen,
or dry conditions highlights: 1) the photochemical nature of
catalysis; 2) that oxygen serves as the sacrificial oxidant for
this system; 3) that, water is mechanistically involved through
a hydrolysis step as has been previously proposed, respec-
tively.35,44 Interestingly, under dry conditions, a small increase
in surface sulfate is observed from 6.3 to 17 %, similar to that
seen under substrate-free conditions, further demonstrating
the slow nature of sulfide to sulfate oxidation, by photogener-
ated holes.
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The control experiments described above begin to link oxi-
dative amine-coupling catalysis with both Cd2+ leaching and
sulfide oxidation. To understand how reaction intermediates
may be playing a part in these corrosion events, we propose
the amine oxidation mechanism suggested by others and
shown in Scheme 1.35,44 Principally, one-electron amine oxi-
dation generates a nitrogen-centered radical cation (Scheme
1, reaction 1). This oxidized intermediate contains acidic -
hydrogens in comparison to native BnNH2.44,45 Deprotonation
of the amine cation radical yields a neutral carbon-centered
radical intermediate (Scheme 1, reaction 2), which following
additional oxidation and deprotonation steps, produces an
imine (Scheme 1, reaction 3). In the presence of water, this
imine hydrolyzes to benzaldehyde (Scheme 1, reaction 4),
which undergoes a facile condensation reaction with BnNH2,
yielding N-BB (Scheme 1, reaction 5).
Figure 1. SEM images and HR-S2p XPS spectra of CdS (a,b),
post BnNH2 photolysis (c,d), and a substrate-free post-photolysis
control experiment (e,f). The blue components of the XPS fit rep-
resent sulfide and the red components represent sulfate.
Scheme 1. Proposed photochemical oxidation mechanism
material displayed one major sulfur signal in the S2p high-reso-
2
lution (HR) region at P3/2 = 161.8 eV, with a small (6 % of all
surface sulfur) amount of sulfate at 2P3/2 = 168.8 eV (Figure 1b).
The smooth pre-photolysis surface predominately comprised of
sulfide, is contrasted with the highly textured surface post photol-
ysis that is 95 % sulfate (Figure 1c,d). Sulfate assignment was
based on cadmium sulfate standard spectra (Figure S9). Although
Cd2+ reduction to metallic Cd by photogenerated electrons has
been reported on other CdS materials,12,14 energy dispersive X-
ray mapping of the post photolysis particles shows no Cd locali-
zation and HR Cd3d XPS shows no shift in the binding energy of
the 2D components or any deviations from a Cd:S atomic ratio of
one (Figure S10).
We suspect that oxidized BnNH2 intermediates, such as the
amine radical cation may serve as an oxidizing agent to cor-
rode sulfide to sulfate, as the hydroxyl radical does in aqueous
solutions.13,43 The BnNH2-mediated corrosion of CdS is ther-
modynamically allowed as the BnNH2 standard reduction po-
tential (0.9 V vs. NHE)46 is sufficiently positive to oxidize
CdS (0.3 V vs. NHE).47 Similar amine redox mediators have
been used in oxidative organic electrosynthesis, where an
electrochemically generated amine radical cation performs
homogenous oxidation chemistry.48,49 We propose that the
amine-radical cation serves as a similar mediator for non-
aqueous sulfide oxidation as shown in Figure 2. CdS photo-
generated hole corrosion is slow, demonstrated by the control
experiments discussed above. In addition to serving as an ox-
idizing agent, the acidic benzylic hydrogens formed upon
photolysis may serve as a source of protons for H2S evolution,
resulting in the observed Cd2+ leaching. We have confirmed
the production of H2S during amine oxidation using a bismuth
hydroxide chemical sensor (Figure S2).39 To ensure that hy-
droxyl radicals (•OH) are not involved in either the observed
catalysis or corrosion, we illuminated BnNH2
To understand the nature of catalyst corrosion during
BnNH2 photolysis, we carried out a control experiment in
which we illuminated a substrate-free CdS suspension in ace-
tonitrile. To our surprise, the pre- and post-photolysis materi-
als were morphologically indistinguishable; only a small par-
ticle size decrease due to the mechanical force of the stir bar
is observed (Figure 1e). In addition to the lack of morpholog-
ical change, XPS revealed only a minor increase in surface
sulfate from 6 % to 14 % (Figure 1f), contrasted with the 95
% sulfate present after BnNH2 photolysis (Figure 1d). Finally,
ICP-OES detected no leached Cd2+ in the post photolysis so-
lution (Table 1, entry 3). The lack of corrosion upon amine-
free photolysis suggests that non-aqueous CdS passivation oc-
curs though a substrate-mediated pathway, rather than by pho-
togenerated holes.
To probe the interplay between amine oxidation and CdS
corrosion, we performed a variety of additional control exper-
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Upon establishing a link between amine radical reactivity
and sulfide oxidation, we next sought to confirm that Cd2+
leaching is due to sulfide protonation by acidic hydrogens.
4,N,N-Trimethylaniline (TMA) was chosen as an appropriate
substrate, as it does not possess -hydrogens, and single elec-
tron oxidation yields a resonance-stabilized benzylic radical
with oxidizing power similar to that of BnNMe2 (Figure S12).
In agreement with the formation of stabilized oxidation inter-
mediates without -hydrogens, the CdS morphology and
composition are unaffected by TMA photolysis and no Cd2+
is leached into solution (Table 1, entry 9; Figure S11). We
have further demonstrated that sulfide oxidation and Cd2+
leaching is substrate dependent by performing photolysis ex-
periments in ferrocene (Fc) solutions. After two days of illu-
mination, the Fc solution was found to be nearly completely
bleached, indicating metal centered Fc/Fc+ oxidation followed
by irreversible cyclopentadienyl oxidation (Figure S14).57
Analogous experiments performed in the absence of CdS con-
firm that the oxidative decomposition of Fc is a heterogeneous
photochemical process. Although CdS oxidation has occurred
in these solutions, no Cd2+ leaching or any significant increase
in surface sulfate concentration was detected (Table 1, entry
10; Figure S11). Based on the lack of Cd2+ leaching and sul-
fide oxidation detected upon illuminating CdS in substrate-
free acetonitrile solution or in ferrocene solutions, we con-
clude that non-aqueous CdS stability is dictated by the acidity
and oxidation potential of the organic radical intermediates.
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Figure 2. Benzylamine-mediated oxidation of sulfide to sulfate.
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solutions with the •OH scavenger tBuOH.13,50 No change in N-
BB production, Cd2+ leaching, or sulfate formationdemon-
•
strates that trace water oxidation to OH is not a significant
contributor to the observed chemistries (Table 1, entry 7; Fig-
ure S11).
To elucidate the role of substrate in sulfide oxidation and
Cd2+ leaching, we performed photolysis experiments on N,N-
dimethylbenzylamine (BnNMe2). This tertiary amine is inca-
pable of producing a coupled product, yet reversible amine
oxidation may still occur to generate reactive intermediates
akin to those of BnNH2. Illuminating BnNMe2 in the presence
of CdS consumes a small amount of amine (14 ± 6 %), but
yields no detectable products. While ICP-OES suggests Cd2+
leaching (4.7 ± 0.6 ppm), the surface sulfur composition ap-
pears unchanged (Table 1, entry 8; Figure S11). Given that
tertiary radicals are stabilized compared to primary ones, we
propose that BnNMe2 radical cations are not sufficiently reac-
tive to corrode sulfide. Through a series of LSVs, we have
confirmed that the tertiary amine/radical cation couple is ~300
mV more negative than that of BnNH2 (Figure S12). Instead,
the stabilized radical provides protons from its -hydrogens,
leading to the observed Cd2+ leaching with concomitant H2S
evolution (Figure S2). The resulting deprotonated species is
likely prone to decomposition, accounting for the reduced
post photolysis BnNMe2 concentration. Homogeneous photo-
catalysts (e.g. [Ru(bpy)3]Cl2) have successfully oxidized sim-
ilar tertiary amines, to give unique reactions involving oxi-
dized intermediates trapped by an appropriate nucleophile or
electrophile.51–54 In light of these studies, we propose that
iminium cation formation (tertiary analogue to Scheme 1, re-
action 3) occurs upon tertiary amine oxidation and may be re-
sponsible for BnNMe2-enabled Cd2+ leaching. To verify imin-
ium cation formation, we carried out photochemical trapping
experiments using trimethylsilyl cyanide (Equation 1). Cya-
nide nucleophiles have been used to trap similar iminium cat-
ion intermediates from tertiary amine oxidations.55,56 We were
pleased to find that illuminating BnNMe2 with Me3SiCN pro-
duced the expected -cyanated species (68 ± 7 % conversion,
Figure S13), confirming the formation of reactive intermedi-
ates upon BnNMe2 photolysis.
To highlight how particle morphology affects radical-based
corrosion processes, we investigated the activity and stability
of CdS nanowires (NWs). Both the bulk particles and NWs
were found to contain crystalline wurtzite-phase CdS, with
comparable UV-Vis absorption spectra, and although the bulk
CdS absorption displays an Urbach tail characteristic of de-
fect-based absorption (Figure S15), Tauc analysis reveals both
materials to have similar direct bandgap transitions (Figure
S16).58 The CdS NWs oxidize BnNH2 to N-BB (Figure S6) at
a faster rate (26 ± 5 mM h–1, Table 1, entry 11) than bulk par-
ticles (5 ± 1 mM h–1, Table 1, entry 1), as expected for the
higher surface area (Figure S17). In accord with the faster rate
(as the faster reaction generates more of the acidic intermedi-
ate), the CdS NW post-photolysis solutions contain a higher
concentration of leached Cd2+ (5.0 ± 0.9 ppm) compared to
solutions of bulk CdS (2.0 ± 0.3 ppm). However, while the
NW data in Figure 3 show morphological and compositional
evidence for corrosion, post photolysis sulfate formation is
suppressed: 55 % surface sulfate for NWs, compared to 95 %
for bulk particles. In addition, the first order rate constant re-
mains unchanged through the course of NW photocatalysis
(Figure S8). We surmise that for a second, slower first-order
rate constant to be observed, a majority of the surface must
corrode to sulfate. Indeed, by illumining bulk CdS in BnNH2
for 6 – 12 hours (times within the first rate constant region)
sulfate concentrations between 20 – 60 % were detected (Fig-
ure S18). Future iterations of sulfide-based photocatalysts
may make use of the correlation between a stable first-order
rate constant and the lack of significant corrosion as a first
approximation of material stability. The decreased surface ox-
idation and stable rate constant suggest that the nanowire mor-
phology disfavors amine-based corrosion. Further experi-
ments are required to elucidate fully how the NW morphology
affects corrosion; one possible explanation is that the NW de-
fects
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Figure 3. SEM images and HR S2p XPS spectra of CdS NWs
pre- (a,b) and post- BnNH2 photolysis (c,d).
and band positions may be different from commercial CdS,
minimizing orbital overlap between sulfide and amine radi-
cals.
To assess the photoelectrochemical activity and stability of
CdS during non-aqueous amine oxidation, CdS photoelec-
trodes were prepared from a chemical bath deposition proce-
dure.38 With these chemically deposited photoelectrodes, a se-
ries of linear sweep voltammograms (LSVs) was obtained in
0.1 M tetrabutylammonium hexafluorophosphate (Bu4N PF6)
in acetonitrile solutions. As shown in Figure 4a LSVs ob-
tained under illumination display an anodic current onset at –
0.4 V vs. Fc+/0 (all potentials referenced to Fc+/0) that we at-
tribute to the photoelectrochemical corrosion of sulfide. This
current is not reproduced in the dark (Figure 4a), confirming
that this oxidation is due to photogenerated holes. In agree-
ment with the photochemical stability of CdS powders in the
absence of substrate (vide supra) there is a ~100 mV separa-
tion between the measured open circuit potential (–0.54 V)
and the anodic current onset; an additional potential is neces-
sary for acetonitrile-based CdS corrosion to occur.
Figure 4. LSVs obtained on CdS photoelectrodes in acetonitrile
solutions of 0.1 M Bu4NPF6 with and without 0.25 M BnNH2 at
a scan rate of 10 mV s–1, (a). Bulk photoelectrolysis performed at
the specified potential with blue LED illumination of 0.25 M
BnNH2 (b), the dashed data is performed under ambient condi-
tions, while the solid data was collected in the absence of air and
moisture (inert).
To determine the extent of corrosion during amine oxida-
tion, a series of LSVs were recorded BnNH2 solution. LSVs
recorded under blue LED illumination display anodic current
at –0.8 V, while the analogous dark LSVs display no current,
confirming the photoelectrochemical nature of BnNH2 oxida-
tion on CdS (Figure 4a, red traces). Amine oxidation appears
to be resolved from the photocorrosion of sulfide by ~600 mV,
however upon bulk photoelectrolysis either at –540 or –240
mV (Figure 4b; dashed blue and black lines, respectively) sig-
nificant current loss is observed within the first five minutes.
In addition to a loss in current, Figure 5 shows that the surface
morphology changes significantly from flat and featureless to
highly textured, while the film composition changes from
largely sulfide material to one that is nearly all sulfate (80 %).
These corrosion results suggest that even under photoelectro-
chemical conditions, amine oxidation engenders corrosion at
potentials where the material would otherwise be stable.
minimize corrosion. Dry and air free photoelectrochemistry
performed at either –0.54 or –0.24 V (Figure 4b solid lines)
resulted in photocurrents with significantly improved stabil-
ity. Analysis of the surface morphology and composition re-
vealed the photoelectrodes to be largely unchanged (Figure 5
e,f). Although stable photocurrents are obtained under dry and
inert conditions, the Faradaic efficiency for N-BB generated
is only ~20%. The suppressed formation of N-BB under these
conditions may be linked to the requisite dry reaction condi-
tions. The mechanism for BnNH2 oxidation outlined in
Scheme 1 requires water to hydrolyze the intermediate imine
bond; under dry conditions an alternative mechanism
(Scheme S1) may occur where BnNH2 acts as a nucleophile
towards the intermediate imine. This work is ongoing in the
lab.
Since both water and oxygen are involved in the oxidation
of sulfide to sulfate, removing these species is expected to
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ASSOCIATED CONTENT
1
Supporting Information. A photograph of the LED photolysis
setup, images of the H2S sensor results, rate data, total ion chro-
matographs, SEM images, EDX maps, additional XPS data, LSV
traces, UV-Vis data, XRD data, and a potential mechanism for
dry BnNH2 photoelectrochemical oxidation. This material is
available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
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E-mail: bartmb@umich.edu
Author Contributions
The manuscript was written through contributions of all authors.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This research was supported by a grant from the U.S. Department
of Energy, Basic Energy Sciences, Catalysis Science Program,
under Award # DE-SC0006587. We thank the Michigan Center
for Materials Characterization for use of the X-ray photoelectron
spectrometer and Dr. Kai Sun, Dr. Haiping Sun, and Dr. Bobby
Kerns for assistance, as well as the University of Michigan Col-
lege of Engineering and NSF grant # DMR-0420785. We would
also like to thank Mr. Roy Wentz for constructing our photo-elec-
trochemical cell and Dr. Antek Wong-Foy for assistance with
BET surface-area analysis.
Figure 5. SEM images and HR S2p XPS spectra of CdS photoe-
lectrodes pre- (a,b), post-ambient (c,d), and post- dry/free free
(e,f) BnNH2 photoelectrolysis.
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