Zwitterionic buffer-induced visible light excitation of TiO2 for efficient pollutant photodegradation

Article abstract of DOI:10.1039/c6ra02979e

Excitation of TiO₂ for visible light absorption by surface complexation with antenna organic molecules is an effective strategy to improve its solar utilization efficiency for photocatalytic application, but the existing antenna molecules are mostly toxic and environmentally-aggressive, severely limiting their practical application. In this study, we tested the potential of zwitterionic buffers (Good's buffers) as an environmentally-benign alternative. The addition of Good's buffers significantly enhanced methyl orange (MO) photodegradation by TiO₂ under visible light, but the enhancement degree varied with the different buffer types, buffer concentration and solution pH. The presence of 4-(2-hydroxyerhyl) piperazine-1-erhanesulfonic acid (HEPES) as a typical Good's buffer led to over 90% MO removal within 180 min, whereas only slight MO removal was observed in the TiO₂ alone system during the same period. Such an induced visible light photocatalytic activity was attributed to a complexation between the conjugate acid structured buffer molecule and TiO₂, which favors a ligand-to-metal charge transfer (LMCT). The LMCT activity was strongly dependent on the molecule structure, especially the states of hydroxyl and amino groups of Good's buffers. The pH buffering ability of the buffers also contributed to the efficient MO photodegradation. This study suggests a great potential of Good's buffers as both green antenna molecules and pH buffer for strengthening TiO₂-based photocatalytic remediation processes.

Full text of DOI:10.1039/c6ra02979e

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Zwitterionic buffer-induced visible light excitation of TiO2 for
efficient pollutant photodegradation
Xiang Xiaoa,b, Wen-Wen Zhua, Yu-Bin Leia, Qiu-Yue Liua, Qian Lia, Wen-Wei Lib,*
Received 00th January 20xx,
Accepted 00th January 20xx
Excitation of TiO2 for visible light absorption by surface complexation with antenna organic molecules is an effective
strategy to improve its solar utilization efficiency for photocatalytic application, but the existing antenna molecules are
mostly toxic and environmentally-aggressive, severely limiting their practical application. In this study, we tested the
potential of zwitterionic buffers (Good's buffers) as an environmentally-benign alternative. The addition of Good's buffers
was found to significantly enhance methyl orange (MO) photodegradation by TiO2 under visible light, but the
enhancement degree varied with the different buffer types, buffer concentration and solution pH. The presence of 4-(2-
hydroxyerhyl) piperazine-1-erhanesulfonic acid (HEPES) as a typical Good’s buffer led to over 90% MO removal within 180
min, whereas only slight MO removal was observed in the TiO2 alone system during the same period. Such an induced
visible light photocatalytic activity was attributed to a complexation between the conjugate acid structured buffer
molecule and TiO2, which favors a ligand-to-metal charge transfer (LMCT). The LMCT activity was strongly dependent on
the molecule structure, especially the states of hydroxyl and amino groups of Good's buffers. The pH buffering ability of
the buffers also contributed to the efficient MO photodegradation. This study suggests a great potential of Good's buffers
as both “green” antenna molecules and pH buffer for strengthening TiO2-based photocatalytic remediation processes.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Zwitterionic N-substituted aminosulfonic acids, first introduced
by Good and co-workers,14 are widely used as biological buffers in
various reactions due to their nontoxicity, biocompatibility, weak
Introduction
TiO2 has become the most widely investigated photocatalyst since
the first discovery of its ability for photo-catalyzing water splitting in
1972.1 It has many advantages such as low toxicity and cost, good
water insolubility, hydrophilicity, chemical stability and resistance
to photocorrosion. However, due to the large band gap (3.2 eV for
anatase and brookite, 3.0 eV for rutile), the excitation wavelength
of TiO2 falls in the UV region,2-4 which accounts for only 4-5% of the
solar spectrum. To extend its photo response to the visible region,
various strategies, such as metal ion/nonmetal ion doping,
sensitizing by organic compounds or metal complexes, and noble
metal deposition,5-7 were explored. Surface modification by directly
adding chemicals is considered as a promising methods to boost the
photoreaction.8 Especially, the ligand-to-metal charge transfer
(LMCT) mediated pathway of TiO2 under visible light has received
intensive attentions.9-13 Surface complexation of TiO2 by some
antenna organic molecules, such as aromatic organics, organic acids
and amines that are commonly seen in the environment, confer it
the ability of visible light absorption,9-11 but most of these
complexation agents are toxic and environmentally-aggressive.
complexation properties, and conveniently-controllable pKa values.
The hydroxyl or amino functional groups of such zwitterionic
buffers (or called Good's buffers) could induce LMCT excitation.15-17
Thus, we hypothesize they may be used to enhance photocatalytic
reactions by playing two roles: 1) serving as green antenna
molecules to strengthen visible light adsorption by TiO2; 2) to
provide a favourable pH condition for photoexcitation of TiO2.18
Here, we aim to validate the above hypothesis and explore the
potential of Good’s buffers as an enhancer of visible light
photocatalytic processes. Five kinds of Good's buffers were tested
as antenna molecules for TiO2 nanoparticles, and the visible light
activities of the corresponding complexes for methyl orange (MO)
photodegradation were evaluated. In addition, the mechanisms of
Good's buffer-induced visible light excitation of TiO2 were
investigated by using HEPES as a model buffer and evaluating the
MO photodegradation performances under different buffer
concentration and pH conditions. This work may have valuable
implications for developing more efficient and sustainable
photocatalytic processes for broad-range applications such as
environmental remediation and renewable energy production.
a.School of The Environment and Safety Engineering, Jiangsu University, Zhenjiang,
212013, China
CAS Key Laboratory of Urban Pollutant Conversion, Department of Chemistry,
University of Science & Technology of China, Hefei 230026, China
† Electronic Supplementary Informaꢀon (ESI) available: This secꢀon contains four
figures. See DOI: 10.1039/x0xx00000x
b
Experimental
Chemicals
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Anatase TiO2 nanoparticle used in this study were purchased from
Wanjing New Material Co., China. The nanoparticles were found to
be uniform with 25-nm average size (Fig. S1). The biological buffers
(listed in Table 1) were purchased from Sangon Biotech Co., Ltd.
(Shanghai, China).
Results and discussion
MO photodegradation with different Good’s buffers
Photocatalytic efficiencies of TiO2 for MO degradation in the
absence or presence of different Good's buffers as antenna
molecules were evaluated (Fig. 1). The buffer-free control showed
only 10% MO removal after irradiation for 4 h, confirming a low
photocatalytic activity of TiO2 under visible light. This result also
indicates that photosensitization was not a main cause of the MO
photodegradation. In contrast, the presence of Good's buffers
significantly accelerated MO removal, but the removal rate varied
for different buffers. After irradiation for 90 min, the MO was
depleted in the treatment with TRIS, but only 32% MO removal was
achieved in PIPES treatment.
Previous studies have shown that hydroxyl group, especially
when present as multidentate ligand, could promote LMCT.15, 16
This was confirmed by our results where the buffers with hydroxyl
group (i.e., TAPS, TRIS and HEPES) all exhibited higher MO removal
than those without hydroxyl groups (i.e., PIPES and MOPS). In
addition, addition of methanol, which contains hydroxyl groups but
without pH buffering capability, significantly enhanced MO
degradation, further confirming that antenna molecules with
hydroxyl group are conducive to LMCT excitation (Fig. S2).
Photodegradation Tests
The photocatalytic efficiency of TiO2 was evaluated by using MO as
a model pollutant. The reaction solution contained 10 mg L-1 MO,
500 mg L-1 TiO2, and 50-mM Good's buffer. The solution pH was
adjusted to 7.0 unless otherwise specified. The serum vials, each
contained 20-ml mixed solution, were bubbled with N2 for 20 min
to remove the O2, sealed with butyl rubber stoppers and then
sonicated. All the serum vials were continuously stirred in the dark
for 20 min on a shaker (200 rpm) to achieve adsorption-desorption
equilibrium. Then, illumination with two incandescent lamps (60 W)
was applied. Samples were collected during the reaction period and
the MO concentration was monitored using a UV-vis spectrometer
at 464 nm. The photocatalytic activity was quantitatively evaluated
according to the equation:
C
Residualrate(%)= ×100%
C0
where C0 refers to the initial MO concentration and C represents
the concentration at a given time during the photodegradation.
The influences of MO concentration (with gradients of 10, 20, 50
and 100 mM) and solution pH (6.5, 7.0, 7.5 and 8.0, adjusted with
1M NaOH) on the LMCT excitation of TiO2-HEPES was evaluated. To
elucidate the MO degradation pathway, ammonium oxalate (AO)
and Cr(VI) were used as scavengers of holes (h+)19 and electrons (e-
),20, 21 respectively.
Chemical analysis
The interaction between HEPES and TiO2 was examined by UV-vis
diffuse reflectance spectroscopy (DRS). TiO2 suspensions (1.0 g L-1)
were buffered with 50 mM HEPES at pH 7.0, and stirred for
overnight in the dark. After centrifugation at 6000 rpm for 10 min,
the solids was collected and dried at room temperature. The DRS
spectra were measured on a UV-2450 spectrometer (Shimadzu)
equipped with an integrating sphere attachment, using BaSO4 as
background.
Fig. 1 Photodegradation of MO by LMCT excitation of TiO2 with
different Good's buffers under visible light irradiation.
Agilent (Waldbronn, Germany) 6890N Series gas chromatograph
equipped with a 7683B injector and a 5975B mass spectrometry
detector was used to analyze the MO degradation products. The GC
parameters were as follows: split ratio, 2:1; column, HP-5-MS
capillary (30 m×0.25 mm I.D., 0.25 μm film thickness, Agilent,
Waldbronn, Germany); injection port temperature, 220oC; carrier
gas, helium; flow rate, 1.5 mL/min; oven temperature, initially 80oC
for 3 min, ramped to 150oC at 10oC/min, 150oC for 5 min, and
ramped to 250oC at 20oC/min, 250oC for 4 min. Before sample
injection the sample solution was extracted by CH2Cl2. One
microliter of extract was filtered through 0.22-μm membrane filter
and then 1 µL of filtrate was injected into GC/MS for measurement.
Pathway of MO Photodegradation by TiO2-HEPES complex
To better understand the roles of Good’s buffers in LMCT excitation
of TiO2 for MO degradation, HEPES (with pH buffering range
of 7.3~8.7) was chosen as a representative antenna molecular for
in-depth investigation. Our results showed no obvious MO removal
in the dark and in the treatments with HEPES alone (Fig. 2A),
indicating that MO was removed by photodegradation but
photosensitization was not the main route. The TiO2 alone system
showed only slight MO degradation, because TiO2 with wide band
gap could not be excited by visible light. In contrast, the co-
presence of HEPES and TiO2 led to rapid degradation of MO. The
adsorption spectra (Fig. 2B) shows that the characteristic
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Table 1 List of biological buffers used in this study.
pKa
Name
Abbreviation
Structure
(25oC)
Piperazine-1,4-
PIPES
6.96
7.18
7.56
bisethanesulfonic acid
3-Morpholinopropanesulfonic acid
MOPS
HEPES
4-(2-Hydroxyethyl)-1-
piperazineethanesulfonic acid
Tris(hydroxymethyl)
aminomethane
TRIS
8.07
8.44
[(2-Hydroxy-1,1-bis(hydroxymethyl)ethyl)amino]-1-
propanesulfonic acid
TAPS
Fig. 2 Visible light photodegradation of MO by TiO2 (0.5 g L-1) in the presence of 50 mM HEPES at pH =7.0. (A), Photodegradation process of
different treatments for 240 min; (B), Absorption spectra changes of the MO solution during photodegradation.
absorption peak of MO at 464 nm rapidly decreased over time, MO was broken to form amino group. The other degradation
coupled with an arising new peak at 247 nm, indicting the cleavage product of MO, 4-aminobenzenesulfonic acid, was not detected in
of the azo bond22-24 and formation of sulfanilic acid.25
this condition, because its intense hydrophilicity makes it difficult to
The degradation products were further confirmed by GC-MS be extracted by methylene chloride. Thus, the above results
analysis. As shown in Fig. 3A, degradation products displayed a indicate that HEPES played an essential role in the process of TiO2-
distinct peak at 9.78 min, which was identified as N,N- catalyzed MO photodegradation under visible light, and the
dimethylbenzene-1,4-diamine (Fig. 3B). Apparently, the azo bond of degradation occurred through a reduction pathway.
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Since TiO2 alone could not be excited by visible light, the visible
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light adsorption of TiO2-HEPES complex should be attributed to the
HEPES molecules that are adsorbed on the surface of TiO2. This was
validated by the DRS spectra results. As shown in Fig. 4, neither
HEPES nor TiO2 alone showed visible light response, but the HEPES-
TiO2 complex exhibited obvious visible light absorbance at 400~650
nm. This result indicates the formation of charge-transfer-complex
(CTC) between TiO2 and HEPES,17 which contributed to the
enhanced degradation of MO in our study.
Fig. 4 UV-vis diffuse reflectance spectra of HEPES, TiO2 and TiO2-
HEPES complex.
Based on the above results, we propose the following pathways
of enhanced MO photodegradation by TiO2 in presence of HEPES.
HEPES as an antenna molecule is first absorb on the surface of TiO2
to form TiO2-ligand complex; Under visible light irradiation,
photoelectrons are excited on the TiO2-HEPES surface complexes
and then transferred to the conduction band (CB) of TiO2;12, 26 MO
molecules are then reduced by the photoelectrons on the surface of
TiO2-HEPES complex,27, 28 forming 4-aminobenzenesulfonic acid and
N,N-dimethylbenzene-1,4-diamine (Fig. S3).
Fig. 3 GC chromatogram of the degradation products of MO in
HEPES solution (A) and the MS fragmentation pattern at the time
9.78 min (B).
To validate that MO photodegradation under visible light was
indeed driven by LMCT and to elucidate the degradation pathway,
Cr(VI) and AO were used as the scavengers of e- and h+,
respectively. As shown in Fig. 5, addition of AO inhibited the
degradation of MO slightly, indicating that h+ had no remarkable
contribution to the MO degradation. A competition between AO
and HEPES for the adsorption sites on TiO2 surface might also
contributed to the suppression.
Fig. 5 Effect of 1 mM scavengers of h+ and e- on the MO
photodegradation (CHEPES=50 mM).
Mechanisms of LMCT excitation of TiO2 by HEPES
Our results showed that the hydroxyl group of HEPES played an
important role in LMCT excitation of TiO2 for enhanced
photodegradation (Fig. 1 and Fig. 2A). Thus, it can be expected that
a higher concentration of HEPES would accelerate the degradation
of MO because more hydroxyl groups become available. This was
validated by our tests with different HEPES concentrations at a
In contrast, the MO degradation was completely inhibited in the
presence of Cr(VI), indicating that the excited e- played a key role in
the azo-bond cleavage. These results confirm that the degradation
of MO in TiO2-HEPES was mediated by LMCT process via a reductive
pathway.
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constant pH of 7.0 (Fig. 6A). Thus, raising the HEPES concentration pKa value of the buffer in the order of PIPES, MOPS, HEPES and
led to formation of more TiO2-ligand complex and a higher visible TAPS, the MO photodegradation rate increased accordingly due to
light photodegradation activity. The excess of HEPES may also alter the formation of conjugate acid structure that is more conducive to
the zero point potential of TiO2 and enhance its binding with MO,17 electronic excitation by LMCT than conjugate base structure.
thereby further improving the MO degradation.
Interestingly, the TiO2-TRIS complex showed the highest LMCT
The molecule structure of HEPES may also affect its binding with activity and the fastest MO photodegradation despite of its slightly
TiO2 and thus the photodegradation activity. Since the solution pH lower pKa value than TAPS. This extraordinary performance of TRIS
poses influences on the molecular structure of HEPES, it is also might be attributed to its highest number of hydroxyl groups and
supposed to affect MO photodegradation by TiO2-HEPES complex. the influence of amine functional group. Like the hydroxyl group,
As expected, our results showed an obviously negative correlation the amine groups can also complex with TiO2 and contribute to
between solution pH value and the MO photodegradation efficiency LMCT, but is less effective than hydroxyl group. Thus, as the amine
(Fig. 6B). With decreasing pH, the MO removal was improved functional groups become protonated (at pH lower than pKa), the
gradually and the maximum degradation rate was achieved at pH nitrogen-metal interaction would be diminished, which causes less
6.5. This increased photodegradation activity at acidic pH may be competition for hydroxyl group to access the TiO2 and hence
associated with the pronation of HEPES molecules, forming an acid improves the formation of more effective complex for LMCT (Fig.
structure with more hydroxyl groups that can readily form charge S4). The superior degradation performance of TAPS over TRIS might
transfer complex with TiO2 and induce visible light excitation of the be due to its different amino group structure that favours a more
LMCT process.29
efficient LMCT process.
A consistent result can be seen from the comparison of
different buffers. In general, there was an good positive correlation
between the MO removal efficiencies at pH 7.0 and the pKa values
of different Good's buffers except for TRIS (Fig. 1). With increasing
Implications
Given the significant influences of pH on the LMCT activity, our
results also suggest that the good pH buffer capability of Good’s
buffer could bring extra benefits by maintaining a favourable pH
condition for TiO2-based photocatalytic reactions.
In addition, the findings in this study imply that the potential
influences of various soluble organic matters (DOMs) in natural
aquatic environment (many of them possess hydroxyl functional
groups) should be paid attention. A better understanding and
utilization of the LMCT roles of such DOMs may lead to
development of more efficient photocatalytic remediation process.
Conclusions
This study demonstrates that Good's buffers could be used as
effective antenna molecules to induce visible light photocatalytic
activity of TiO2 through LMCT excitation. HEPES, as a typical Good's
buffer, was found to complex with TiO2. Under visible light
irradiation, electrons were excited from the TiO2-HEPES complex
through LMCT process and transferred to the conduction band of
TiO2, leading to reductive degradation of MO. The LMCT activity of
Good's buffers was strongly dependent on their molecule structure,
especially the states of hydroxyl and amino groups. These
functional groups varied with the concentration and type of
adopted buffer and was significantly affected by solution pH. LMCT
excitation was favoured by a conjugate acid structure of the buffers
with the presence of more hydroxyl function and was affected by
the amino group. This study broadens our knowledge of LMCT and
has valuable implications for development of more efficient
photocatalytic remediation process.
Acknowledgements
Fig. 6 MO photodegradation by TiO2-HEPES at different buffer
This study was partially supported by National Natural Science
Foundation of China (51478208, 51278479, 51522812, 51538012)
concentration (A) and pH (B).
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and the Collaborative Innovation Center of Suzhou Nano Science
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and Technology of the Ministry of Education of China
.
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