Article pubs.acs.org/JPCC
Iodinated SnO2 Quantum Dots: A Facile and Efficient Approach To Increase Solar Absorption for Visible-Light Photocatalysis Pu Li,†,‡ Yong Lan,‡ Qian Zhang,‡ Ziyan Zhao,‡ Tonu Pullerits,§ Kaibo Zheng,*,§ and Ying Zhou*,†,‡ †
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, and ‡The Center of New Energy Materials and Technology, School of Materials Science and Engineering, Southwest Petroleum University, Chengdu 610500, China § Department of Chemical Physics, Lund University, Box 124, 22100 Lund, Sweden S Supporting Information *
ABSTRACT: Efficient visible-light-driven photocatalysis can be achieved over conventional wide band gap semiconductor, in which iodinated SnO2 quantum dots (QDs) are synthesized via hydrolysis of crystalline SnI4 in the absence of any additives or templates. The formation of SnO2 QDs reveals a wide preparation window and a very fast growth rate in minute scale. The iodine species may only exist on the surface of SnO2 QDs, which can be completely removed through heat treatment. SnO2 QDs reveal the light absorption in visible range which increases the limited optical absorption of bulk SnO2. Notably, the iodination can further enhance the visible light absorption due to the formation of band tail states. Therefore, iodinated SnO2 QDs exhibit significantly enhanced visible-light-driven photocatalytic activity toward degradation of rhodamine B and oxidation of NO in ppb level. Time-resolved spectroscopic studies reveal that the iodine species in QDs can not only server to passivate the surface traps to prolong the lifetime of the excited states when excited above the band gap, but they can also effectively absorb visible light and generate enhancement for photocatalytic reactions. The present study highlights the surface modification of wide band gap semiconductor for exploring efficient visible-light-active photocatalysts. visible-light photocatalytic activity.21 However, the origin of the visible-light activity over wide band gap semiconductor (SnO2) is still under debate. In addition, the preparation of SnO2 nanoparticles smaller than 10 nm is not straightforward.22 For example, Wu et al. reported a biomolecule-assisted hydrothermal method to generate SnO2 with diameters less than 10 nm.23 SnO2 QDs were synthesized via a microwave-assisted reaction of a SnO2 precursor in solution.20 SnO2 nanoparticles with a size of 3−4 nm are obtained through microwave irradiation using ionic liquid (BMIm)BF4 as capping agent.24 Yet, the majority of hitherto reported methods to SnO2 QDs require either evaluated temperature or pressure or the use of expensive instrument and capping agents. Therefore, it is highly desirable to develop facile approach for SnO2 QDs which can facilitate mass production in a short time with low energy consumption. Herein, iodinated SnO2 QDs (particle size ∼ 2.4 nm) are successfully fabricated via low temperature hydrolysis of crystalline SnI4 without any additives or templates. X-ray photoelectron spectroscopy (XPS) in combination with extended X-ray absorption fine structure (EXAFS) spectroscopy reveal that the iodine species from the SnI4 precursor may exist on the surface of SnO2 QDs. Such iodine species can be completely removed through heat treatment. Unlike bulk SnO2,
1. INTRODUCTION In the past decades, the quest for efficient visible-light-driven photocatalysts has attracted worldwide intense research interest as one of the potential solutions to harvest solar light and convert it to chemical energy.1−3 Semiconductors such as TiO2,4 SnO2,5 ZnO,6 Bi2WO6,7 and C3N48 have been widely used as heterogeneous photocatalysts in which the photoinduced electrons and holes from semiconductor can be utilized for various reactions. Among them, SnO2 is a n-type direct band gap semiconductor which possesses outstanding optical, electrical, and electrochemical properties.9,10 Unfortunately, due to its wide band gap (∼3.9 eV), SnO2 can only be excited by UV light. Therefore, in spite of its wide applications in gas sensors, lithium-ion batteries, solar cells, and so on,11−13 the application of SnO2 in visible-light-driven photocatalysis is still comparatively unexplored. On the other hand, semiconductor quantum dots (QDs) such as CdS,14 CdSe,15 PbS,16 and CdTe17 can effectively harvest visible light. Hence, these QDs have been widely used as photosensitizer attached to photocatalysts in order to improve their visible-light photocatalytic activity. The photogenerated electrons are injected from QDs into the conduction band (CB) of the photocatalyst to form reactive species.18,19 It is worth noting that recent studies indicated that pristine SnO2 QDs even without loading any other photocatalysts showed admirable visible-light photocatalytic degradation of methylene blue (MB).20 Moreover, Lee et al. found that V-shaped SnO2 bipods with small diameters ( S1.5h-70−450 > S1.5h-70. Previous work has demonstrated that the Sn K-edge EXAFS data can be used to estimate particle size.37 In this case, the reduction in magnitude of the peaks can be accounted for the decreasing of particle size. To get insight into the local structure information, curve fitting in R space was carried using a simple two shell model (for the details of the fitting results, cf. Figure S8 and Table S1). A good fit was obtained for both S1.5h-70−450 and S1.5h-70. The striking feature is that the Debye−Waller factor of S1.5h-70 is larger than that of S1.5h-70−450, confirming that the heat treatment can reduce the dynamic and static disorder in SnO2 QDs.36 However, no Sn−I or I−O bonds were detected in S1.5h-70. This is an indication that iodine species from SnI4 could be only on the surface and do not incorporate into the bulk structure of SnO2 QDs. 3.2. Optical Properties and Photocatalytic Performance. UV−vis optical absorption spectra of the S1.5h-70, S1.5h70−450, and SnO2-ref are shown in Figure 7a. From the tauc plots of absorption spectra (inset of the Figure 7a), we can estimate the optical bandgap of S1.5h-70−450 and SnO2-ref to
Figure 5. Raman spectra of S1.5h-70, S1.5h-70−450, and SnO2-ref. E
DOI: 10.1021/acs.jpcc.6b01530 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
Figure 7. (a) UV−vis absorption spectra of S1.5h-70, S1.5h-70−450, and SnO2-ref, the inset shows the tauc plots of the absorption spectra, and (b) steady state PL spectra of S1.5h-70, S1.5h-70−450, and SnO2-ref excited at 266 and 410 nm.
be about 3.75 and 3.97 eV. Although S1.5h-70−450 still possesses wide band gap, it shows additional obvious absorbance in the visible light range (between 400 and 520 nm) compared to that of SnO2-ref. The absorption spectra of S1.5h-70, however, exhibit an extra absorption band toward visible light region. Such an extra absorption band is unlikely the result of the sub-bandgap states of the SnO2, which usually appear as tail of the absorption edge. The band is most likely due to the adsorbed I2 species on the surface which has been confirmed by the XPS studies (Figure 4a). However, as Sn2+ can form shallower VBM level than that of O 2p state to narrow the band gap and the XPS spectra of SnO2 and Sn3O4 are very similar,38 we cannot completely exclude this possibility. On the other hand, the bandgap of S1.5h-70 in this case can be determined as 3.06 eV according to the tauc plots (inset of Figure 7a). Very recently, density functional theory (DFT) calculations have suggested that the strong orbital hybridization between O 2p and I 5s orbitals leads to the effect that doped I prefers to form a strong I−O bond in SnO2 and exists as an I5+ cation at Sn sites.39 Although I5+ has been identified in the Idoped TiO2,40 this is not the case in our current work as only I− and I2 were observed (Figure 4a). Therefore, we propose that the role of iodination on SnO2 QDs is similar to the introduction of disorder in the surface of TiO2 nanocrystals, which can yield midgap states and thus enhance the light absorption.41 We also measured the PL spectra of our samples as shown in Figure 7b. When excited above the band gap (266 nm), all samples exhibit strong UV emission with two peaks at 328 and 364 nm. The emission band at 328 nm represents the intrinsic band-edge emission, while the emission at longer wavelength can be attributed to the defect emission from the shallow trap states in the samples. We found that the relative amount of the band-edge emission compared to the defect emission is much higher in S1.5h-70 than the other two samples. This provides a strong indication that the trap states in S1.5h-70 have been partially passivated (or diminished), which would also be consistent with the PL kinetics results in the following part. Nevertheless, no pronounced featured emission can be detected when excited at 410 nm for all three samples (Figure 7b). The photocatalytic activities of the SnO2-ref, S1.5h-70, and S1.5h-70−450 were further evaluated through photodegradation of RhB (liquid phase) and photooxidation of NO (gas phase) at the indoor air level under visible light irradiation (λ > 420 nm). All suspensions of catalyst and RhB reach the adsorption− desorption equilibration after 80 min of adsorption in dark (Figure 8). Table 1 lists the summary of photocatalytic
Figure 8. Photodegradation of RhB over SnO2-ref, S1.5h-70, S1.5h-70− 450, and P25 under visible light irradiation (λ > 420 nm).
properties of these samples. As expected, SnO2-ref exhibited very low visible light activity toward to RhB degradation with the reaction rate constant kRhB of 0.002 min−1 due to its wide band gap. Both S1.5h-70 and S1.5h-70−450 can degrade RhB effectively under visible light. In particular, the photocatalytic activity of S1.5h-70 is much higher than that of S1.5h-70−450 (Figure 8). The corresponding kRhB of S1.5h-70 was calculated to be 0.073 min−1, which is more than 4× faster than that of S1.5h70−450 (0.016 min−1). The RhB degradation in the presence of S1.5h-70 is even higher than the values obtained for TiO2 nanoparticles (P25, 80% anatase phase). Furthermore, the removal ratio of NO over S1.5h-70−450 (8.4%) and P25 (12.0%) is quite low, whereas the photooxidation of NO over S1.5h-70 can reach as high level as 39.9% in 5 min under visible light (Figure 9a). Nevertheless, the NO concentration over this sample slightly increased with the irradiation time, which could be assigned to the accumulation of the oxidation products NO3− on the photocatalyst surface, restricting the diffusion of reaction intermediates over the catalysts. The reaction rate constant kNO of S1.5h-70 (0.098 min−1) is ∼3× faster than that of S1.5h-70−450 (0.032 min−1; cf. Table 1). It is worth noting that although the SBET of S1.5h-70 (138.9 m2/g) is significantly higher than that of S1.5h-70−450 (51.9 m2/g), the enhanced activity cannot be solely attributed to the specific surface area as the activity of S1.5h-70 is still prominently higher after normalizing the kRhB and kNO to specific surface area. In addition, S1.5h-70 possesses stable visible light photocatalytic activity reflected from the recycling experiments (Figure 9b). The activity of S1.5h-70 after 4 cycles is still as high as the fresh F
DOI: 10.1021/acs.jpcc.6b01530 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C Table 1. Summary of Photocatalytic Properties of the Obtained SnO2 QDs and Bulk SnO2
a
samples
crystallite sizea (nm)
particle sizeb (nm)
SBET (m2/g)
band gap (eV)
kRhB (min−1)
kNO (min−1)
S1.5h-70 S1.5h-70−450 SnO2-ref
8.5 9.6 46.8
2.4 7.0
138.9 51.9
3.06 3.75 3.97
0.073 0.016 0.002
0.098 0.032
Determined by the Scherrer equation from XRD patterns. bCalculated from the TEM images.
Figure 9. (a) Photocatalytic oxidation of NO over S1.5h-70, S1.5h-70−450, and P25; (b) cycling runs of photocatalytic activities of S1.5h-70 under visible light irradiation.
Figure 10. (a) Valence-band XPS of S1.5h-70 and S1.5h-70−450; (b) band structure diagram of S1.5h-70, S1.5h-70−450 and SnO2-ref.
the main reactive species in photocatalytic reactions. The calculated CB minima of S1.5h-70 and S1.5h-70−450 is 0.71 and 0.07 eV versus NHE, respectively, which are more positive than the standard redox potential of O2/•O2− (−0.28 eV vs NHE). Hence, the photogenerated electrons cannot react with the adsorbed O2 to form •O2− from thermodynamics point of view. On the other hand, the redox potential for S1.5h-70 (+2.06 eV) and S1.5h-70−450 (+3.82 eV) is large enough to oxidize OH− to • OH (+1.9 eV vs NHE). DMPO spin-trapping ESR spectra of S1.5h-70−450 confirmed that no signals of •O2− were observed no matter in the darkness or under light irradiation (Figure S11a). Moreover, no typical signals of DMPO-•OH with the characteristic intensity of 1:2:2:1 were observed (Figure S11b). Instead, the hyperfine splitting characteristics of EPR spectra indicated the direct oxidation of DMPO to DMPOX.43,44 On the basis of these results, the photocatalytic reaction over iodinated SnO2 and SnO2 QDs could be mainly governed by direct h+ oxidation. In order to further understand the photophysics of the prepared SnO2 QDs after excitation, the time-resolved PL measurements were performed. Two excitation wavelength 266 and 410 nm have been used to reveal the photocatalytic mechanism in both UV and visible region. Here we compared
sample. The cycling runs of photodegradation of RhB as well as the EDX spectra of the sample before and after photocatalytic reactions further confirm the stability of iodinated SnO2 QDs (Figures S9 and S10). 3.3. Band Structure and Time-Resolved Fluorescence Spectra. To understand the origin of the enhanced visible-light photocatalytic activity, the density of states (DOS) of the valence band (VB) of S1.5h-70 and S1.5h-70−450 were investigated through VB XPS (Figure 10a). The VB maximum of S1.5h-70−450 is at 3.18 eV. As the band gap of this sample is 3.75 eV, the CB minimum occurs at −0.57 eV. However, VB tails were observed for S1.5h-70, which could be attributed to the iodine species. Similar phenomenon has been previously observed through introducing disorder on the surface of TiO2 nanocrystals.41 Consequently, the VB maximum was shifted from 2.99 to 1.42 eV. Combining with the UV−vis absorption spectra (Figure 7), the CB minimum of S1.5h-70 is −0.07 eV. The band structures of these samples are shown in Figure 10b. In addition, the electronic potentials of S1.5h-70 and S1.5h-70− 450 can be determined at +2.06 and +3.82 eV, respectively, as the VB maximum of anatase TiO2 obtained by VB XPS is 0.64 eV lower than the value vs normal hydrogen electrode (NHE) at pH 7.42 In general, •O2−, •OH and photogenerated h+ are G
DOI: 10.1021/acs.jpcc.6b01530 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
Figure 11. PL decays of S1.5h-70, S1.5h-70−450, and SnO2-ref excited at (a) 266 and (b) 410 nm. (c) Schematics illustrated the photoinduced processes within these SnO2 samples after excitation with different wavelengths (see also Table 2).
Table 2. Triexponential Fitting Parameters of PL Decays of As-Obtained S1.5h-70, S1.5h-70-450, and SnO2-ref with 266 and 410 nm Excitation sample Exc. 266 nm S1.5h-70 S1.5h-70−450 SnO2-ref Exc. 410 nm S1.5h-70 S1.5h-70−450 SnO2-ref
A1
τ1 (ps)
A2
τ2 (ps)
A3
τ3 (ps)
0.55 0.82 0.69
30 ± 2 34 ± 2 25 ± 2
0.40 0.17 0.25
327 ± 4 312 ± 12 421 ± 24
0.05 0.01 0.06
2423 ± 90 5450 ± 700 3855 ± 70
0.61 0.69
35 ± 2 36 ± 2
0.37 0.27
245 ± 10 293 ± 26
0.02 0.04
1409 ± 372 2341 ± 1092
(Sn4+) on the QDs surface with a strong X-type bond. Actually, surface passivation via halide anions (Cl−, Br− and I−) has been reported in other colloidal QDs system (e.g. PbS QDs).46 After annealing at 450 °C for 3 h, I− has been removed from the QDs according to previous discussion, which again reactivates the surface traps (Figure 7b). Such a conclusion is consistent with the observation from steady-state PL spectra. Interestingly, we also observed PL kinetics of our QDs excited in visible region (410 nm) with the photon energy far lower than the band gap of bulk SnO2 (∼3.9 eV), as seen in Figure 11b. In absorption spectra (Figure 7a), narrower band gap can be found in both S1.5h-70 and S1.5h-70−450 samples compared with the reference bulk sample, while in VB XPS spectra (Figure 10a) band tail states can also be found blow the VB maximum. Based on those observations, we assume the presence of sub-bandgap states with optical transition strength in the QDs. Such states can be defect states (e.g., oxygen deficiencies), which have also been reported in organometal halide perovskite materials47 or I species related states. As the PL was observed both in iodinated and I free samples, we attribute such states to the oxygen deficiencies instead of I species. However, as the steady state emission can almost be negligible, as discussed above, we estimate a very low PL quantum yield of all samples when excited at 410 nm. In
the PL kinetics between as-obtained S1.5h-70 and S1.5h-70−450 together with bulk SnO2-ref as reference. When excited at 266 nm, all three samples show PL decays with lifetime
