Influence of doping on semiconductor nanocrystals mediated charge transfer and photocatalytic organic reaction

Article abstract of DOI:10.1039/c3cc41599f

Doped and undoped ZnS semiconductor nanocrystals having different recombination pathways are explored to study the charge transfer reaction between the nanocrystals and the 4-nitrophenol/sodium borohydride redox couple.

Full text of DOI:10.1039/c3cc41599f

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Volume 49 | Number 54 | 11 July 2013 | Pages 5999–6106
ISSN 1359-7345
COMMUNICATION
Narayan Pradhan et al.
Influence of doping on semiconductor nanocrystals mediated charge transfer and
photocatalytic organic reaction

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COMMUNICATION
Influence of doping on semiconductor nanocrystals
mediated charge transfer and photocatalytic organic
reaction†
Cite this: Chem. Commun., 2013,
4
9, 6018
Received 2nd March 2013,
Accepted 5th April 2013
Suresh Sarkar, Amit K. Guria and Narayan Pradhan*
DOI: 10.1039/c3cc41599f
www.rsc.org/chemcomm
Doped and undoped ZnS semiconductor nanocrystals having dif- exciton relaxation occurs through the conduction band to that of
ferent recombination pathways are explored to study the charge the Cu-state and the emission tunes with tuning of the host
transfer reaction between the nanocrystals and the 4-nitrophenol/ bandgap. In this communication, we explore these nanocrystals,
sodium borohydride redox couple.
both doped and undoped, having different characteristics of the
carrier relaxation mode as well as the excited state lifetime to
study the charge transfer induced catalytic reduction of 4-nitro-
phenol (p-NP) in basic medium.
A high bandgap, greener ZnS semiconductor nanocrystal and a
thermodynamically feasible but kinetically restricted redox
Charge transfer from semiconductor nanocrystals to organic
molecules can catalytically trigger varieties of solution mediated
1
chemical reactions. The phenomenon includes the photo-
generation of charge carriers in semiconductor nanocrystals
and their subsequent transfer to the surface adsorbed organic
species in the presence of a sacrificial donor. The importance of
using the quantum confined nanocrystals is their tunable band
gap which can help in charge transfer to a wide variety of organic
molecules having different HOMO–LUMO positions.
8
0
couple, 4-nitrophenol (E for 4-nitrophenol/4-aminophenol =
2
0
À
À0.76 V, NHE)–sodium borohydride (E for H BO /BH
=
3
3
4
À1.33 V, NHE), have been chosen to study the possible photo-
induced electron transfer reaction. In spite of being a strong
1a,3
À
This
reducing agent, BH
4
cannot reduce p-NP in aqueous dispersion
also draws attention towards understanding the photo-physics
of the associated system at the interface of the inorganic
nanomaterials and organic molecules.
and this redox pair has been used previously in metal nano-
particle catalyzed reactions but not with semiconductor mediated
9
1a,3a,4
photo-induced charge carrier transformation. The nanocrystals
7d
Recently, this photo-induced catalysis by semiconductor
nanocrystals has become an emerging area of research because
of its extensive contribution to the demand for energy, environ-
mental detoxification and synthesis of industrially valuable
are synthesized following a literature procedure in an organic
medium and transferred to water after being surface functiona-
lized with 3-mercaptopropanoic acid (MPA) to carry out the charge
transfer reaction in aqueous medium (for details see ESI†).
Fig. 1a shows the possible reaction conditions for ZnS
nanocrystals with the chosen redox couple. It is observed that
the reaction is feasible only in the presence of sodium boro-
1
a,b,d,2c,5
organic compounds.
crystal assisted photo-induced chemical reactions have been
Several semiconductor nano-
1
–4,6
already reported in the literature.
Usually, semiconductor
nanocrystals undergo exciton recombination directly through
their band edge or via surface defect states. Apart from this,
hydride (NaBH ) and ZnS nanocrystals under its band edge
4
6
excitation. Even when the excitation source is turned off or the
nanocrystals are excited at a lower energy than the band edge, no
reduction occurs. Fig. 1b shows a typical change in visible color
before and after the reduction. Fig. 1c presents the in situ
spectrophotometric observation of the reaction progress using
ZnS nanocrystals (B4 nm diameter, TEM and XRD in Fig. S1,
ESI†). It shows that the peak at 401 nm which corresponds to
p-NP (in basic medium) slowly decreases and the yellow colour of
the solution due to p-NP gradually turns colourless (Fig. 1b). In
several transition metal ion (Cu and Mn) doped nanocrystals are
developed where additional dopant states are introduced in
between the valence and conduction band, and ultimate recom-
7
bination occurs through these dopant states. In the case of the
2
+
Mn dopant, the exciton moves to Mn d-states, resulting in an
4
emission at 585 nm due to carrier recombination through T to
1
6
2+
A1 energy states of the Mn ion and the emission is indepen-
dent of the host band gap. But, in the case of Cu dopant, the
this process, an additional peak at B295 nm corresponding to
Centre for Advanced Materials and Department of Materials Science, Indian
Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India.
E-mail: camnp@iacs.res.in; Fax: +91-33-24732805; Tel: +91-33-24734971
9
4-aminophenolate appears concomitantly (Fig 1c).
Surprisingly, when a similar reaction has been carried out
with Mn doped ZnS, it does not proceed at all even with an hour
of irradiation whereas with Cu doped ZnS, the rate of reaction
†
Electronic supplementary information (ESI) available: Experimental procedure
and figures. See DOI: 10.1039/c3cc41599f
6
018 Chem. Commun., 2013, 49, 6018--6020
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Fig. 2 (a) Cyclic voltammogram of p-NP in sodium bicarbonate buffer (pH = 10) with
À1
a scan rate of 100 mV s . (b), (c) and (d) Changes in lifetime of ZnS, Mn:ZnS and
4
Cu:ZnS with the successive addition of p-NP in the presence of NaBH respectively.
sodium bicarbonate buffer of pH 10. Fig. 2a shows that the
formal potential of p-NP is À0.89 V (NHE) which is much higher
than that of the reduction potential of the photo-excited
Fig. 1 (a) Schematic presentation of different reaction conditions for possible
reduction of p-NP. (b) Digital images of the cuvettes before and after reduction of
À
10
p-NP. (c) In situ observations of the changes in optical absorption spectra during conduction electron of bulk ZnS (e CB = À2.24 V, NHE). This
4
the reduction of p-NP to p-aminophenol in the presence of NaBH using ZnS
implies that the photo-induced electron transfer from ZnS to
p-NP (in basic medium) is thermodynamically feasible. The
possible electron transfer has further been supported by Time
Correlated Single Photon Counting (TCSPC) measurements. We
have measured the lifetime of ZnS quantum dots (surface state
emission at 440 nm) in the absence and presence of p-NP and
under its band edge irradiation. Each spectrum has been collected with an
interval of 3 min. (d) and (e) Absorption spectra for the same reaction under
identical reaction conditions with Mn doped and Cu doped ZnS nanocrystals.
Insets show the PL spectra of the respective nanocrystals.
is drastically reduced. The absorption spectra during the pro- found a decrease in lifetime (53 ns to 31 ns) with successive
gress of the reaction for Mn and Cu doped ZnS nanocrystals are addition of p-NP in presence of NaBH (Fig. 2b, UV-vis pre-
sented in Fig. S4a, ESI†). This decrease in the excited state
4
shown in Fig. 1d and e respectively.
3
a,11
As the nanoparticle assisted catalytic reactions mostly occur lifetime
and the appearance of the 4-aminophenolate
9
through surface adsorption, here, we have also investigated absorption peak with subsequent reduction of the p-NP
whether this reaction is surface assisted or electron transfer attribute the electron injection from the conduction band of
mediated or both. Instead of MPA (3 carbon chain), when the ZnS nanocrystals to the LUMO of p-NP. Hence, it is confirmed
ZnS nanocrystals are capped with 11-mercaptoundecanoic acid that the feasibility of the reduction process needs both the
(
MUA, C11 chain), the reaction is observed to be delayed by exposed surface of nanocrystals for p-NP adsorption as well as
many folds (Fig. S2, ESI†). The decrease in the rate with the the band edge excitation of ZnS for exciton generation.
increasing chain length of the surface capping ligand implies On the other hand, when the lifetime of dopant emission has
that the approach of p-NP to the surface of nanocrystals is been measured in the presence of p-NP and NaBH no signifi-
4
severely hindered as the long chain ligands (MUA) render an cant change in either case is observed. For Mn:ZnS, the lifetime
obstacle compared to the short chain MPA for the penetration/ is found to be 0.74 ms which remains unchanged upon addition
adsorption at the surface of nanocrystals. Further, we have of p-NP in the presence of NaBH (Fig. 2c, UV-vis presented in
4
varied the concentration of the MPA capped ZnS nanocrystals Fig. S4b, ESI†) whereas for Cu:ZnS the lifetime decreases to
under all identical reaction conditions and found that with some extent from 0.35 ms to 0.32 ms (Fig. 2d, UV-vis presented in
increasing ZnS particles the rate of disappearance of p-NP is Fig. S4c, ESI†). The TCSPC measurement data are consistent with
also enhanced (Fig. S3, ESI†). These observations suggest that the spectroscopic data. While in the case of Mn:ZnS, it is
the process occurs through surface adsorption.
absolutely concluded that it does not allow the reduction pro-
However, as band edge excitation of ZnS is the key of this cess, whereas for Cu:ZnS, the reduction occurs to a little extent at
process, it indicates that the reaction is induced by photon an extremely slow rate in comparison to that of undoped ZnS
absorption and subsequent charge transfer. But to understand nanocrystals. However, the observed phenomenon is inconsis-
this it is further essential to know whether the reduction tent with an earlier report where the dopant enhances the photo-
1
2
potential of ZnS and the formal reduction potential of p-NP catalytic activity of semiconductor nanocrystals.
thermodynamically allow charge transfer. To confirm this we We have further analyzed the rate of the reaction with undoped
have estimated the formal reduction potential of p-NP in ZnS nanocrystals by varying the concentration of NaBH , p-NP and
4
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Chem. Commun., 2013, 49, 6018--6020 6019

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In addition, we have also carried out the same process with
doped and undoped ZnSe which is also a high bandgap semi-
conductor. The disappearance of the color of p-NP with ZnSe has
been observed similar to the ZnS system. Details of the observa-
tions using ZnSe have been provided in Fig. S6 (ESI†). However,
using lower bandgap ZnTe, CdSe and CdS such redox reaction has
not been observed (Fig. S7, ESI†). Hence, we can conclude here
that the alignment of the energy band positions of the donors and
acceptors is important for the charge transfer reaction.
In conclusion, we report here the photo-assisted charge
transfer from ZnS nanocrystals to p-NP and subsequently
studied the ZnS mediated catalytic reduction of p-NP in
Fig. 3 (a) and (b) Plausible mechanistic pathways of the reaction with undoped
and Mn doped ZnS respectively.
presence of NaBH as a sacrificial electron donor. This study
4
includes the photo-physics of ZnS quantum dots and their
concentration of nanocrystals. But, we have observed that the rate application in organic catalysis reaction producing a commercially
is retarded with an insufficient amount of NaBH and enhanced valuable compound, aminophenol. Here, we have observed that
4
with higher concentration of ZnS nanocrystals. We believe that the transition metal ion doping can seriously have an impact on
the rate of the reaction will not vary much with the change in the the basic fundamental photo-physical effects, protecting its
size of the ZnS nanocrystals as one ZnS nanocrystal can produce carriers rather than allowing them to be transferred to the
only one exciton under its band edge excitation and can transfer surface adsorbed molecules.
to a single organic molecule and that makes the difference
The authors thank DST and CSIR of India for funding. The
between the conventional metal nanoparticle mediated reduction authors also thank Subrata Das of department of spectroscopy
of p-NP and that of the semiconductor nanocrystals. However, of IACS for TCSPC analysis. NP acknowledges DST Swarnajayanti
with a fixed concentration of nanocrystals, the rate of the reaction for fellowship.
has been observed to be pseudo-first order with respect to p-NP as
the log(C /C ) vs. time plot is found to be linear (Fig. S5, ESI†). C is
t 0 t
Notes and references
the concentration at time t and C is the initial concentration of p-
NP. The rate constant (k) is estimated to be 0.0016 s from the
log(C /C ) vs. time (t) curve from the 1st cycle of a particular set of
t 0
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6
020 Chem. Commun., 2013, 49, 6018--6020
This journal is c The Royal Society of Chemistry 2013