Tuning the Surface Chemistry of Nanoporous Carbons for Enhanced Nanoconfined Photochemical Activity

Article abstract of DOI:10.1002/cctc.201500355

We showed the effect of surface oxidation on the conversion of light into a chemical reaction in the confined pore space of nanoporous carbons. The photoactivity of carbons is caused by the combination of high porosity and the presence of photoreactive sites that favor the splitting of the exciton inside the pores, which boosts its efficient use in chemical reactions. The incorporation of O-containing groups in the carbon matrix decreased the photoconversion inside the pores, although values were higher than those attained in solution. This is attributed to the lower stabilization of the exciton through the delocalization within the conjugated sp² network of the basal planes because of the electron-withdrawing effect of the O-containing groups. The photochemical conversion of light inside the pores is very sensitive to the acidic/basic nature of the O-containing groups of the carbon matrix, and can be enhanced by balancing the surface composition, porosity, and electronic mobility. Reactions in small spaces: The effect of surface oxidation on the conversion of light into chemical reactions in the confined pore space of nanoporous carbons is explored. The density and the acidic/basic strength of the surface functionalities are of paramount importance to the photochemical response of nanoporous carbons.

Full text of DOI:10.1002/cctc.201500355

DOI: 10.1002/cctc.201500355
Full Papers
Tuning the Surface Chemistry of Nanoporous Carbons for
Enhanced Nanoconfined Photochemical Activity
[a]
[a]
[b]
[a]
Leticia F. Velasco, Alicia Gomis-Berenguer, Joao C. Lima, and Conchi O. Ania*
We showed the effect of surface oxidation on the conversion
of light into a chemical reaction in the confined pore space of
nanoporous carbons. The photoactivity of carbons is caused
by the combination of high porosity and the presence of pho-
toreactive sites that favor the splitting of the exciton inside the
pores, which boosts its efficient use in chemical reactions. The
incorporation of O-containing groups in the carbon matrix de-
creased the photoconversion inside the pores, although values
were higher than those attained in solution. This is attributed
to the lower stabilization of the exciton through the delocaliza-
2
tion within the conjugated sp network of the basal planes be-
cause of the electron-withdrawing effect of the O-containing
groups. The photochemical conversion of light inside the
pores is very sensitive to the acidic/basic nature of the O-con-
taining groups of the carbon matrix, and can be enhanced by
balancing the surface composition, porosity, and electronic
mobility.
Introduction
The conversion of light energy has been long explored in envi-
ronmental chemistry for the degradation of pollutants as the
excitation of electronic molecular states may induce chemical
bond breaking. Indeed, after the studies in 1977 that reported
the performance of zinc and titanium oxides to decompose cy-
TiO , and the enhanced performance of carbon/titania photo-
2
catalysts was attributed to i) the porosity of the inert porous
carbon support or ii) strong interfacial electronic effects in the
case of carbon additives with a high electronic mobility (i.e.,
[
5]
carbon nanotubes, graphenes).
[
1]
anides in solution, the interest in the development of ad-
vanced oxidation processes based on semiconductor photo-
catalysis for the degradation of pollutants in air and water has
Our recent research has demonstrated the photochemical
activity of semiconductor-free nanoporous carbons under dif-
[
6]
ferent irradiation conditions, showing their ability to photo-
generate radical oxygen species (ROS) in aqueous environ-
[
2]
increased.
[
7]
Triggered by the low photonic efficiency of most semicon-
ductors, the optimization of the optical features of photoactive
ments. This has opened new perspectives in the field of ap-
plied photochemistry based on carbon materials that covers
environmental remediation, water splitting, enhanced adsorp-
[3]
materials remains a widely investigated topic. Aside from
transition metal oxides and sulfides as photocatalysts, hybrid
materials prepared by their immobilization on appropriate sub-
[
8]
tion/oxidation, and photoluminescence.
In spite of increasing interest in this field, there is still a multi-
tude of fundamental questions that are worth investigating to
understand the underlying mechanisms that govern the con-
version of light into a chemical reaction to exploit the poten-
tial applications of light-responsive carbons in different fields.
With the aim to shed light on this topic, we herein provide
an overview on the effect of the surface composition of nano-
porous carbons on the photochemical reactions hosted inside
the nanopores. By combining catalytic, spectroscopic, and pho-
toelectrochemical tools, we show the dependence of the pho-
tochemical response in the confined pore space with the sur-
face functionalization of the carbon matrix, and we chose the
photo-oxidation of phenol as a model reaction.
[4]
strates have been investigated extensively. Although carbons
are strong light-absorbing materials, their incorporation in
hybrid carbon/semiconductor composites is an interesting
strategy to attain high photoconversion efficiencies in the deg-
[5]
radation of a variety of pollutants. The first investigations in
this field focused on the use of carbons as inert additives to
[
a] Dr. L. F. Velasco, A. Gomis-Berenguer, Dr. C. O. Ania
Instituto Nacional del Carbon (INCAR, CSIC)
C/Francisco Pintado Fe 26, 33011 Oviedo (Spain)
Fax: (+34)985297662
E-mail: conchi.ania@incar.csic.es
[
b] Dr. J. C. Lima
Department of Chemistry, REQUIMTE/CQFB
Faculdade de CiÞncias e Tecnologia
Universidade Nova de Lisboa
Results and Discussion
2
829-516 Lisboa (Portugal)
The elucidation of the mechanism of the photochemical reac-
tions that occur in the constrained pore space of nanoporous
solids is a complex task because of the simultaneous coexis-
tence of various processes inherent to high-energy irradiation
sources and porous materials: direct and indirect photo-oxida-
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500355.
This publication is part of a Special Issue on Carbon in the Catalysis
Community. Once the full issue has been assembled, a link to its Table of
Contents will appear here.
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tion/reduction, reactions on the catalyst surface and adsorp-
tion and diffusion phenomena that lead to changes in the re-
action rate and reactant concentration. To disregard these con-
tributions, we have developed a strategy that allowed us to
isolate and evaluate the efficiency of the photochemical reac-
Table 1. Main physicochemical and textural properties of the nanoporous
carbons studied.
Sample
B
BOH
BO
2
À1
SBET [m g
]
1033
0.52
0.32
0.09
8.9
2.1
491
167
1045
0.52
0.32
0.08
5.7
6.4
1903
238
989
0.50
0.31
0.07
3.3
7.5
1945
647
[6]
[a]
3
À1
V
V
V
total [cm g
]
]
]
tion inside the pore voids of a catalyst. The approach consist-
ed of the introduction of a target compound inside the pore
structure (adsorbed) before illumination. After irradiation, the
compounds still retained inside the pores are extracted into an
appropriate solvent and analyzed to determine the yield of the
photochemical reaction that takes place inside the pores of
the material (Figure 1a). As no desorption occurs during the ir-
radiation of the preadsorbed catalysts, the extent of the photo-
chemical reaction provides direct evidence on the existence
and fate of the host–light interactions in the confined pore
space. Our previous studies have validated this experimental
approach of monitoring the reaction from inside the pores for
the photo-oxidation of phenol in aqueous solution using nano-
[
b]
3
À1
micro [cm g
[b]
3
À1
meso [cm g
pHPZC
[
c]
O [wt%]
CO [mmolg
CO [mmolg ]
2
À1 [d]
]
À1 [d]
[a] Evaluated at a relative pressure of 0.99. [b] Evaluated from DFT meth-
ods. [c] On a dry basis. [d] From TPD-MS measurements.
a series of oxidized materials by mild wet oxidation. It is impor-
tant to highlight that the surface modification did not change
the textural parameters of the pristine carbon (Table 1, see fur-
ther characterization details in Figures S1–S5 in the Supporting
Information) in terms of pore volume or surface area. This is
critical as the photochemical experiments were designed to
control the amount adsorbed and the confinement state inside
the pores of the carbons.
[
6]
porous carbons.
With the aim to explore the effect of the functionalization of
the carbon matrix on the photochemical activity, we prepared
The incorporation of O-containing groups on nanoporous
carbons decreases the uptake of phenol, although it does not
alter the adsorption mechanism, which is governed by disper-
sive interactions between the pore walls and the phenol mole-
[
9]
cules. To overcome these differences in the adsorption ca-
pacity of the carbons upon oxidation, the amount of phenol
preadsorbed before irradiation was fixed and kept below the
saturation limit. This allows the formation of a single adsorp-
tion layer in the pores (phenol molecules are adsorbed pre-
dominantly in an edge-wide orientation in which the aromatic
ring is parallel to the pore walls and the hydroxyl moiety proj-
[
9]
ects toward the solution), and we assume that the confine-
ment of phenol is the same for all the samples. Hence, the dif-
ferences in the photochemical performance should be caused
by the nature and fate of the carbon–light interactions in the
pore space.
The evolution of phenol conversion and the intermediates
detected in the extracts upon irradiation of the preadsorbed
carbons by using a low-pressure Hg lamp is shown in Figure 1.
Data that correspond to direct photolysis (absence of catalyst)
are included for comparison. The three tested materials led to
very similar phenol conversions. Furthermore, the superior
photo-oxidation performance of the carbons over the photo-
lytic reaction confirmed that the light conversion is boosted in
the nanoconfined pore space. The effect is already very pro-
nounced at short irradiation times, and for instance, after
1
50 min of irradiation the conversion of phenol is approxi-
mately eight times higher inside the porous network of sam-
ple B than in the absence of catalyst. This is remarkable if we
consider that the carbon matrix absorbs a large fraction of the
incident irradiation and the actual fraction of light responsible
for the photochemical conversion of phenol inside the pores is
expected to be much lower than that in solution.
Figure 1. a) Scheme of the experimental procedure designed to evaluate the
photochemical yield inside the nanoporous structure of the carbon materi-
als; b) phenol conversion and c) intermediates formed during the photocata-
lytic reaction in the presence of activated carbons B, BOH, and BO. Data that
correspond to direct photolysis are included for comparison.
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The amount of intermediates detected inside the nanopores
is also larger than that in solution (Figure 1c), which seems
reasonable if we consider that the phenol conversion is higher.
Interestingly, although the amount of intermediates decreases
with time for the carbons, there is a clear accumulation of sub-
products of the photolytic reaction. An important difference
can be seen in the speciation of the intermediates (Figure 1b);
catechol is the predominant intermediate in all cases, although
its concentration is higher if phenol is decomposed inside the
nanopore space. This finding is significant as the regioselective
degradation of phenol in the ortho position is considered
termined by actinometry and the integrated absorption frac-
tion over the wavelength range used in the experiments (see
details in the Supporting Information).
If the photochemical reaction occurs inside the nanopores, it
becomes very complex to evaluate the actual photon flux that
reaches the molecules adsorbed in the pores because of the
contributions of light scattering by the particles suspended in
solution and the strong light absorption by the carbon matrix
itself. For comparison, we calculated an apparent or pseudo-
photochemical quantum yield (f ) for the carbons by assum-
ps
ing that all emitted photons eventually reach the fraction ad-
sorbed inside the pores. Although this is a simplistic approxi-
mation, it allows a straightforward comparison of this family of
nanoporous carbons as all the experiments were recorded
under similar conditions (in terms of carbon matrix, catalyst
loading, and particle size). Besides, the actual photon flux
would be smaller because of the above-mentioned side reac-
[10]
more advantageous than conversion via quinones
as the
mechanism involves a lower number of intermediates. This
confirms that the uncatalyzed photodegradation is less effi-
cient than photo-oxidation inside the pores of the carbons.
As for the effect of surface chemistry, the better per-
formance of the pristine carbon than that of the oxidized
counterparts indicates the negative impact of the oxidation on
the photochemical conversion of phenol. The abundance of
degradation subproducts followed the trend: BO>BOH>B.
This means that the incorporation of O-containing functionali-
ties does not only reduce the conversion of phenol itself, but
also the subsequent oxidation of the intermediates. Hence, the
overall phenol photo-oxidation is more efficient on the parent
carbon.
tions that consume photons, hence even if f cannot be strict-
ps
ly considered as a quantum yield, it accounts for the minimum
limit of the actual quantum yield value. Importantly, it provides
a more accurate viewpoint of the overall photochemical reac-
tion that occurs inside the pores of the carbons by considering
the intermediates formed in the course of the reaction.
The dependence of the number of moles reacted per
photon flux with the irradiation time is shown in Figure 2. The
profile of direct photolysis in solution followed the expected
linear pattern with the irradiation time, with a quantum yield
To gain further insight into the effect of the surface chemis-
try of the carbons in the photochemical conversion of light in
the confined pore space, we calculated the photochemical
quantum yield (f), defined by IUPAC as the ratio between the
amount reacted DN [mol], per mole of photons absorbed
[
12]
of 12 mmol/Einstein, in agreement with reported values.
Interesting features are revealed if the photochemical reac-
tion is performed inside the pores of the carbons. As opposed
to the uncatalyzed reaction, the dependence of fps with time
shows two different regimes. Below 30 min of irradiation, the
number of moles reacted is very high for the nanoporous car-
bons with fps values close to unity, whereas at longer times,
[
11]
(
IADt), from the slope of the correlation between the moles
of degraded compounds per incident photon flux versus the ir-
radiation time (Figure 2) with Equation (1):
a second less steep linear region is observed and the fps
DN ¼ ꢀI Dt
ð1Þ
A
values decrease by an order of magnitude (but they are still
larger than that in solution). This indicates that the photo-
chemical reaction inside the nanopores is very fast at short ir-
radiation times and leads gradually to steady conversion
values over time.
in which I is the photon flux absorbed by the sample evaluat-
A
ed from the product of the incident photon flux I , which is de-
o
With regard to surface chemistry, fps values followed the
trend B>BOH>BO. Given the similar textural properties of the
carbons (Table 1), differences in the photochemical response
must be discussed in terms of the surface functionalization of
the carbon matrix. The dependence of phenol conversion and
fps values with selected parameters of the nanoporous carbons
characteristic of the degree of surface functionalization
(
oxygen content, surface pH, nature of O-containing groups) is
shown in Figure 3. Data show that the incorporation of oxygen
groups of an acidic nature is the most determinant parameter
in the decrease of the photochemical activity. This is supported
by correlations with the amounts of gases (CO and CO ) quan-
2
tified from the thermodesorbed species by temperature-pro-
grammed desorption (TPD) followed by MS.
Figure 2. Correlation of the amount [mol] reacted per incident photon flux
with the irradiation time for the nanoporous carbons and the photolytic re-
action. The points represent experimental values, and the lines are guides
for the eye.
As seen, the amount of CO -evolving groups (attributed to
2
carboxylic acids and anhydrides of an acidic nature) is approxi-
mately three times higher in BO than in BOH, whereas both
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hole oxidation and/or radical
mediated mechanisms) with
electron donors present in the
reaction medium if splitting is fa-
[
14,15]
vored.
If we take this into consider-
ation, the decrease in the photo-
chemical conversion of the func-
tionalized carbons can be associ-
ated with several factors that
would affect either their optical
features (because of the creation
of structural defects or changes
2
3
in the sp /sp hybridization state
of carbon atoms derived from
the incorporation of the O-con-
[
6d,15]
taining groups)
or the stabi-
lization of the photogenerated
carriers through delocalization
2
within the conjugated sp net-
work of the basal planes.
Several scenarios are possible;
the first is that the holes might
react directly with the adsorbed
phenol molecules. This seems
plausible as the reaction occurs
inside the pores and the split-
ting of the carriers would be fa-
vored by the immediate reaction
with the adsorbed molecules.
This explains the higher conver-
sion values in the tight nano-
pore confinement compared to
that in solution. However, as the
carbons display similar textural
features, this cannot account for
the differences between the car-
Figure 3. Effect of surface functionalization on phenol conversion (plots a and b) and fps values (plots c–f) for the
nanoporous carbons calculated for the first (plots c and d) and second (plots e and f) regime of the correlation of
the moles reacted per incident photon flux with the irradiation time (see Figure 2).
carbons displayed similar amounts of CO-releasing groups
bons themselves, which must be attributed to the fate of the
charge carriers in the pores.
[13]
(
mainly phenolic and quinone-type groups). Thus, the higher
performance of BOH than BO is because of its increased sur-
face hydrophobicity (Table 1, Figure 3). If we analyze data by
considering the number of moles reacted (Figure 3c–f) and
not just phenol conversion (Figure 3a–b), a similar trend is ob-
served. In this case, the dependence of the chemical features
of the carbons follows a polynomial pattern, which indicates
a more sensitive response of the light conversion yield to small
changes in the surface acidity of the nanoporous carbons.
According to literature reports, the light absorption features
of amorphous nanoporous carbons depend on the density of
Another possibility is the stabilization of the holes through
the oxidation of water molecules coconfined in the nanopore
space and the effect of the surface functionalization on the ex-
citon splitting. This was supported by EPR spectroscopy, which
allowed the detection of radical species upon the illumination
of aqueous suspensions of the carbons using a nitrone spin-
trapping
Figure 4).
agent,
5,5-dimethylpyrroline-N-oxide
(DMPO;
Similar EPR spectra were obtained for all the carbons, in
which the characteristic quartet peak profile with a 1:2:2:1 in-
tensity (g=2.006, a =ab =14.8 G hyperfine splitting con-
2
3
electronic states (DOS; mainly sp /sp hybridization of the
carbon atoms), and in the UV range are dominated by p–p*
and s–p* transitions that involve free zig-zag sites and car-
N
H
stants) of the DMPO-OH adduct was attributed to the presence
[
16]
of hydroxyl and superoxide radicals. The quantification of
the relative abundance of the ROS showed lower concentra-
tions in the functionalized carbons. Interestingly, similar
amounts of radicals were detected for BO and BOH, which in-
dicates the importance of the acidic/basic nature of the O-con-
taining groups that decorate the carbon surface (Table 1) in
[
14]
bine-like sites. Under sunlight irradiation, some other transi-
tions that involve the activation of chromophoric groups on
[15]
the carbon surface have been proposed.
These electronic
transitions generate carriers (holes and electrons) that can par-
ticipate in charge transfer reactions (which either involve direct
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Figure 4. a) Typical EPR spectrum of the DMPO adducts obtained after
0 min of irradiation of aqueous suspensions of the nanoporous carbons.
b) Quantification of the radical species that correspond to DMPO-OH ad-
ducts determined from the intensity of the second line (marked by a star) in
the profiles.
Figure 5. a) Transient photocurrent densities vs. bias potential of the nano-
porous carbons. b) Example of the chronoamperometric response of photo-
electrodes built on carbon B that show a square-shaped profile and the
cathodic/anodic shoots in the transient photocurrent response.
2
for efficient exciton separation. Furthermore, the potential
onset of the photocurrent for the carbon anodes is approxi-
mately +0.8 V vs. Ag/AgCl, which is clearly lower than the
value for the Ti collector.
the formation of radicals. Indeed, the amount of ROS correlates
well with the CO-evolving groups of a basic nature (Figure S6),
whereas the strongly acidic groups (CO -evolving) seem to in-
2
hibit the formation of radicals. The strong electron withdrawal
effect of carboxylic acid and anhydride groups on the p-elec-
tron density of the carbon basal planes would increase the sur-
face recombination of the carriers, which would result in the
formation of fewer radicals. Notably, low-intensity EPR signals
should not be considered as an indication of low photochemi-
cal activity; EPR spectroscopy only provides information about
the formation of radicals, whereas the reaction may proceed
The transient photocurrent response presents a square-
shaped profile if the light is switched on (Figure 5b) with
a prompt initial increase followed by a smooth decrease until
a steady-state regime is achieved; the photocurrent reverts to
the original values if the illumination is turned off. In the aque-
ous electrolyte in which water molecules are the only hole
scavengers, the anodic photocurrent corresponds to water oxi-
dation, which is corroborated by measuring the O concentra-
2
[9]
by other mechanisms (e.g., direct hole transfer). Thus, the
lower-intensity EPR signals for the oxidized carbons indicate
that the radical-mediated mechanism might not be the domi-
nant mechanism.
tion in the electrolyte, whereas the photogenerated electrons
migrate to the substrate through the electrode film. The pho-
tocurrent response was stable and reproducible during repeat-
ed on/off cycles of illumination.
Further insights into the fate of the photogenerated carriers
were obtained by exploring the photo-electrochemical re-
sponse of the carbons. The transient photocurrent response of
the photoanodes after on/off illumination at various potentials
is shown in Figure 5. Dark currents at the applied potentials
were allowed to equilibrate before irradiation to stabilize the
large capacitive contributions of the nanoporous carbon elec-
trodes. The background photocurrent generated by the illumi-
nation of the bare Ti foil used as the current collector is shown
for comparison.
In some electrodes the initial anodic increase upon illumina-
tion was preceded by a cathodic undershoot (Figure 5b),
whereas the steady-state regime was followed by an anodic
overshoot if the illumination was turned off. Anodic and catho-
dic current shoots are frequently reported for semiconductor
materials and attributed to surface recombination processes of
[
17]
the photogenerated charges or photocorrosion phenomena.
In our materials the cathodic undershoot is most likely attribut-
ed to either the reduction of trace amounts of dissolved O2
that remains in the pores of the carbons that could not be re-
moved by N2 bubbling (photocurrent disappears in subse-
quent cycles) and/or the reduction of photogenerated COH rad-
A remarkable photocurrent was generated if the electrodes
were illuminated and the bias potential was positive enough
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À
À
icals (according to the reaction COH+1e $OH ) as evidenced
by the EPR spectra (Figure 4).
affects the stabilization/splitting of the exciton by charge prop-
agation through the carbon matrix, which results in higher sur-
face recombination (i.e., lower photocurrents). Further support
for the superior electron mobility in the pristine carbon is pro-
vided by its high DC conductivity.
The presence of undershoots can be used as an indicator for
insufficient mass transport and poorly efficient reaction. This is
expected if we consider the low electron mobility of the stud-
ied nanoporous carbons (BO, BOH, and BO have a DC conduc-
The stability of the carbon photoanodes after the on/off illu-
mination cycles was also explored; the materials proved to be
quite resistant to illumination (Figures S7 and S8) but became
oxidized if the bias potential was higher than 0.8 V vs. Ag/AgCl
because of the effect of the oxygen released during water oxi-
dation. The TPD-MS analysis of the used photoanodes evi-
denced the oxidation and decrease in surface hydrophobicity,
whereas the appearance of new humps in the cyclic voltam-
mograms showed the loss of conductivity caused by these
changes in the surface chemistry.
À1
tivity of ꢀ0.5, 0.2, and 0.02 mScm , respectively).
At 1 V and above, the photocurrent density increased con-
siderably for all the carbons because of the more efficient pho-
toassisted oxygen evolution reaction if higher potential values
are applied (extensive bubbling was seen on the electrode sur-
face, not observed in the dark or during the irradiation of the
bare collector).
All the carbons showed the same potential onset for the
photocurrent detection, although current densities decreased
for the functionalized materials. This contrasts with our previ-
ous work in which we studied the photo-electrochemical oxi-
dation of water using visible light and highly functionalized
Finally it should be highlighted that current densities up to
À2
0.70 mAcm were recorded for the highest potentials on sam-
ple B. Although these are low values compared to data in the
[15]
[20]
carbon photoanodes. The ability to oxidize water was linked
to the presence of S- and N-containing groups that work as
chromophores and leave reactive vacancies that are able to
accept electrons from oxygen in water molecules upon light
excitation. The role of the sulfur and nitrogen species was sup-
ported by the lack of activity detected on a commercial nano-
porous carbon free of these heteroatoms.
literature for the photo-electrochemical splitting of water,
they are remarkable if we consider the nature of the carbon
photoanodes (metal-free and amorphous nanoporous carbons)
and the low overpotential for the photo-electrochemical oxida-
tion of water.
Conclusions
The fact that the nanoporous carbons studied herein do not
have S- and N-containing groups but display photochemical
activity gives a new perspective to the origin of this behavior
and on the role of the surface chemistry. Firstly, the porosity of
these nanoporous carbons is more developed than that stud-
ied previously (Table 1, Figures S3 and S4), with larger pore vol-
umes adapted for the adsorption of water and phenol (by dis-
Our results show that the conversion of light inside the porous
network of nanoporous carbons depends on the porosity, sur-
face functionalization, and presence of photoreactive sites that
lead to the photogeneration of charge carriers that can be
used effectively in chemical reactions. The presence of a well-
developed porosity is essential to obtain a high conversion in
the constrained pore space, which distinguishes low-cost
nanoporous carbons from graphene, carbon nanotubes, and
other nanostructured carbon materials.
[
9,18]
persive forces).
Besides porosity, hydrogen bonding with
the surface groups that exist in the pores could be expected
to promote the surface wetting and hence increase the photo-
electrochemical oxidation of water inside the pores. Converse-
ly, oxidation caused a reduced hydrophobicity of the carbons
With regard to functionalization, the decoration of the
carbon surface with O-containing groups caused a decrease in
the photo-oxidation of phenol inside the pores; however, con-
versions were still higher in the confined pore space of the
functionalized carbons than in solution.
(
Table 1). It seems that beyond surface wetting, the carbon ma-
terials must have photoactive sites inside the pores at which
carbon–light–water and/or carbon–light–phenol interactions
can take place.
Besides the density of surface groups, the yield of the light
conversion is very sensitive to small changes in the acidic/basic
strength of the oxygen groups, particularly to the presence of
The photoactivity of these carbons is certainly not linked to
the presence of chromophoric surface groups, our previous
[
7]
studies also disregarded the contribution of metal impurities,
acidic groups (CO -releasing groups). The number of moles re-
2
but to the generation of carriers in the reactive sites. Such re-
active sites in carbons are located at the edges of the basal
planes, either associated with surface functionalities or to free
edges sites linked to various configurations (carbyne-like and
acted increased with the surface hydrophobicity, as inferred by
the superior performance of the carbons that show either low
functionalization or mainly phenolic and quinone-type groups.
This behavior is linked to the lower ability of the acidic carbons
to stabilize/promote the splitting of the photogenerated exci-
[
19]
carbene-type). The free sites are also responsible for the re-
activity of carbons to incorporate heteroatoms to give rise to
2
ton through the delocalization within the conjugated sp net-
[19]
stable surface functionalities. If we consider this, the reduced
photoactivity of the oxidized carbons would be linked to
a lower density of free reactive sites (at which the O-containing
groups are most likely incorporated).
work of the basal planes because of the high electron-with-
drawing effect of O-donating groups. This was corroborated
by the lower amount of oxygen radical species and lower pho-
tocurrent densities measured in the functionalized carbons.
As a result of the versatility of nanoporous carbons and
abundance of precursors (which allow simple and cost-effec-
tive synthetic methods), it is highly feasible to champion nano-
Additionally, because of the presence of O-containing
groups and their high electron-withdrawing effect on the p
electron density of the basal planes, oxidation of the carbons
ChemCatChem 2015, 7, 3012 – 3019
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porous carbons as sustainable metal-free photoanode/cathode
materials for different applications (e.g., photo-electrochemical
water splitting for hydrogen and oxygen evolution, pollutant
photo-oxidation). The future challenge is in the further en-
hancement of the photochemical activity by balancing the sur-
face composition (by the incorporation of adequate photosen-
sitizer groups), porosity, reactivity, and change-carrier mobility
HPLC. Extraction yields were determined for each pure compound
on each carbon material. The corresponding blank of direct phenol
photolysis was performed under similar conditions for comparison.
All the measurements were performed at least in duplicate (the
standard deviation was lower than 5%), and average values are
presented.
(
increasing conductive graphene-like units). Work focused on
Photo-electrochemical measurements
the optimization of the nanoporous carbon layout in a con-
trolled way to achieve stable carbon photoelectrodes with
higher efficiency is ongoing.
A standard three-electrode cell equipped with an optically flat
quartz window on the side, the nanoporous carbon photoanode
as the working electrode, a graphite rod as the counter electrode,
and a saturated Ag/AgCl reference electrode was used. For the
preparation of the electrodes, a slurry of the nanoporous carbon
and polyvinylidene fluoride (90:10) in N-methyl-2-pyrrolidone was
Experimental Section
2
coated onto a 1 cm Ti foil collector and dried at 1208C before
usage. A Xe lamp, which emitted between 200 and 600 nm
Materials
(
150 W), was used as the irradiation source to amplify the signal of
A nanoporous carbon obtained from the CO activation of bitumi-
2
the carbon electrodes. A potentiostat (Biologic) was used to evalu-
ate the electrochemical behavior. The transient photocurrents were
obtained under a constant bias potential between 0 and +1.5 V vs.
Ag/AgCl under on/off illumination. Dark current equilibrium at the
applied potential was allowed before irradiation. The photoanodes
were suspended in an aqueous solution of 0.1m Na SO (20 mL)
nous coal was selected for this study (sample B). The pristine
carbon was submitted to several treatments to modify its composi-
tion to explore the effect of the surface chemistry on the photo-
chemical response. The incorporation of oxygen was performed by
mild wet oxidation. Sample B (ꢀ1 g) was put in contact with a sa-
turated solution of ammonium persulfate in H SO (10 mL; 4n) at
RT and left to stir overnight. After oxidation, the carbon was col-
lected by filtration, washed with distilled water until a constant pH
was obtained, and dried at 110 8C overnight (sample BO). Subse-
2
4
2
4
and purged with N before illumination (the dissolved O concen-
2
2
tration in the electrolyte was measured by using a sensor). The
electrochemical behavior of the electrodes was also explored by
À1
cyclic voltammetry at a potential sweep of 20 mVs before and
quently, a portion of this carbon was submitted to a thermal treat-
À1
after light exposure.
ment at 4008C for 30 min under a flow of N (50 mLmin ) to par-
2
tially remove the incorporated functionalities (sample BOH).
Spin-trapping EPR measurements
Characterization
The formation of paramagnetic species in solution during the irra-
diation of the carbon suspensions was detected by a nitrone spin
trapping agent, DMPO. This compound is able to form spin ad-
ducts with hydroxyl and superoxide radicals, which creates more
stable nitrone radicals that are detected easily by EPR spectroscopy
The textural properties of the samples were determined by N ad-
2
sorption at À1968C (Micromeritics, ASAP 2010). Before the experi-
ments, samples were outgassed at 1208C overnight to constant
À4
vacuum (10 Torr). The main textural parameters such as specific
À1
in aqueous solution. The carbon sample (ꢀ0.5 gL ) was suspend-
surface area, SBET, pore volume, and distribution of pore sizes were
evaluated from the N adsorption isotherms. The carbon materials
were further characterized by elemental analysis (LECO CHNS-932
and LECO VTF-900 automatic analyzers). A custom-made device for
TPD-MS was used to evaluate the surface chemistry of the activat-
ed carbons. The samples were heated in a fused-silica reactor up
to 9008C at a heating rate of 108Cmin . The analysis was per-
formed under high-vacuum conditions (below 10 mbar) and the
gas phase was monitored continuously by MS. The acidic/basic
character of the nanoporous carbons was determined by the mea-
surement of the pH of point of zero charge (pHPZC) by a modifica-
ed in HClO4 buffer (pH 3, 5 mL), and the appropriate volume of
DMPO was added to the suspension to reach a final concentration
of 18 mm. Samples were introduced into quartz capillary tubes and
irradiated for 5, 10, 20, and 60 min (Philips TL K40W/05 lamp, with
a broad emission peak centered at 365 nm). EPR spectra were re-
corded immediately from the solution (after the solids were re-
moved by filtration) at RT by using a Bruker ESP 300E X band spec-
2
À1
À5
5
trometer with the following spectral parameters: receiver gain 10 ;
modulation amplitude 0.52 G; modulation frequency 100 kHz, mi-
crowave frequency 9.69 GHz; microwave power 5.024 mW; conver-
sion time 40.96 ms; center field 3450 G; sweep width 120 G. The
intensity of the second line in the spectrum was used for the quan-
tification of the signals.
[21]
tion of the mass-titration method by Noh and Schwarz.
Phenol photo-oxidation
Acknowledgements
A low-pressure mercury lamp (6 W, which emitted at 250, 401, 433,
and 542 nm) was used as the irradiation source. The incident
À9
À1
The financial support of the Spanish MINECO (grant CTM2014-
photon flux of the lamp (ꢀ3.3”10 Einsteins ) was measured by
[11]
56770-R) is acknowledged.
ferrioxalate actinometry and used to normalize the photochemi-
cal conversions. Suspensions of the carbons in a phenol solution
were allowed to equilibrate until all the phenol was completely re-
moved and the suspensions were irradiated for different times. The
solution was filtered and analyzed by reversed-phase HPLC (C18
column, water/methanol 95:5). The carbons were further extracted
with ethanol, and the alcoholic solution was also analyzed by
Keywords: carbon
· electrochemistry · photochemistry ·
radical reactions · surface chemistry
ChemCatChem 2015, 7, 3012 – 3019
www.chemcatchem.org
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
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