SnO2 Quantum Dots Synthesized with a Carrier Solvent Assisted

Jun 19, 2014 - SnO2 Quantum Dots Synthesized with a Carrier Solvent Assisted Interfacial Reaction for Band-Structure Engineering of TiO2 Photocatalyst...
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SnO2 Quantum Dots Synthesized with a Carrier Solvent Assisted Interfacial Reaction for Band-Structure Engineering of TiO2 Photocatalysts Kuan-Ting Lee, Cheng-Hsien Lin, and Shih-Yuan Lu* Department of Chemical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan S Supporting Information *

ABSTRACT: A simple, fast, room temperature, surfactant-free, carrier solvent assisted interfacial reaction method is developed for size-controllable syntheses of SnO2 quantum dots (QD). Narrowly size-ranged SnO2 QDs with average diameters of 2−2.7 nm, corresponding to energy band gaps of 5.7−4.39 eV, are produced at room temperature within 30 min through precursor concentration adjustment. These SnO2 QDs, when decorated onto the surfaces of TiO2 nanoparticles, enable precise engineering of the band structure of the TiO2 nanoparticles. The band structure of the SnO2 QD decorated TiO2 nanoparticles can be controlled to achieve not only enhanced charge separation but also generation of extra oxidizing species, peroxy radicals, both beneficial for boosting the photocatalytic performances of the TiO2 nanoparticles. The photocatalytic degradation efficiency of TiO2 toward Rhodamine B, in terms of the apparent reaction rate constant, is significantly improved from 0.025 to 0.055 min−1 with the band structure engineering achieved through SnO2 QD decoration.

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INTRODUCTION Semiconductor quantum dots (QD) continue to draw extensive research attention because of their unique size-tunable properties originating from the quantum confinement effects.1 Their applications in biomedical imaging,2 solar cells,3 lightemitting diodes,4 etc., have received great success. Consequently, there have been developed a wide variety of synthetic methods for QDs production, including laser ablation,5 microwave-assisted,6 hydrothermal,7 wet chemistry,8 thermal evaporation,9 sonochemical precipitation,10 and sol− gel processes.11 Most of the methods however require either high temperatures and long reaction times or high power energy input. Furthermore, many of the processes involve the use of structure-protecting or structure-deriving reagents. These reagents often cause problems for subsequent applications of the QDs. In this work, we develop a simple, fast, room temperature, surfactant-free, carrier solvent assisted interfacial reaction method for size-controllable syntheses of semiconductor quantum dots. Narrowly size-ranged QDs can be produced at room temperature within 30 min, and their sizes can be easily controlled through precursor concentration adjustment. We © 2014 American Chemical Society

demonstrate the use of the method with production of sizecontrollable SnO2 QDs. The majority of QD applications is based on the pronounced light excitation and emission properties of the QDs. Here, we explore the applications of these SnO2 QDs in photocatalysis through band-structure engineering a host semiconductor photocatalyst. Semiconductor photocatalysts receive continuous and intensive research attention with recent applications focusing on organic pollutant degradation and hydrogen production from water splitting. For semiconductor photocatalysts to function efficiently, the photon-induced charges, electrons and holes, need to be well separated so that they can perform desired functionality without recombination. It is thus a key issue to enhance charge separation for improving the photocatalytic efficiency of the semiconductor photocatalyst.12−15 Among the many approaches developed, formation of a heterostructure on the surface of the photocatalyst proves to be effective and has received a great deal of research Received: May 9, 2014 Revised: June 13, 2014 Published: June 19, 2014 14457

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engineering via SnO2 QD decoration, the photocatalytic degradation efficiency of TiO2 toward Rhodamine B, in terms of the apparent reaction rate constant, is significantly improved from 0.025 to 0.055 min−1. This approach can be readily extended to other heterojunction systems for creation of optimal band structure for relevant applications.

attention. At the interface of two semiconductors, which exhibit a staggered band structure, the photon-induced electrons tend to migrate to one semiconductor domain while the photoninduced holes tend to migrate to the other semiconductor domain, thus achieving charge separation.12,13,16,17 Once the charges are separated, either the photon-induced holes oxidize and remove the organic pollutants as in the case of photocatalytic degradation or the photon-induced electrons reduce protons to generate hydrogen as in the case of photocatalytic water splitting. Titanium dioxide is probably the most well-studied semiconductor photocatalyst because of its excellent photocatalytic efficiency, low cost, nontoxicity, photochemical stability, and earth abundance. It has been successfully applied in photocatalytic pollutant degradation,15 photocatalytic water splitting,18 and dye-sensitized solar cells.19 To further boost the photocatalytic efficiency of TiO2, particularly in the area of photocatalytic pollutant degradation, creation of heterostructure on its surface has been a promising approach. With a second semiconductor forming an interface with TiO2 to create a staggered band structure, the photon-induced electrons tend to migrate to the conduction band of the second semiconductor while the photon-induced holes tend to migrate to the valence band of TiO2, achieving effective charge separation. The photon-induced holes, which concentrate in the TiO2 domain and possess a strong oxidizing power (+2.8 eV vs NHE at pH 0),20 then proceed with the necessary oxidative degradation of the organic pollutant without recombination with photon-induced electrons. It however has been proved that not only photon-induced holes but also photon-induced electrons can lead to oxidative removal of organic pollutants. The photon-induced electrons, when energetically more negative than the reduction potential of O2/HO2• (−0.05 eV vs NHE at pH 0), can reduce O2 to produce peroxy radicals, HO2•, which are a strong oxidant and can proceed with oxidative removal of organic pollutants.21 There however remains a difficulty to be overcome. The energy level of the conduction band of the second semiconductor needs to be situated between the conduction band of TiO2 (−0.4 eV vs NHE at pH 0)20 and the reduction potential of O2/HO2•, to enable receiving of photoinduced electrons from the conduction band of TiO2 and performing oxidative removal of the pollutant through creation of the peroxy radicals. The workable energy gap for the conduction band position of the second semiconductor is thus only 0.35 eV, which is difficult to manage. Tin dioxide is a popular semiconductor to go with TiO2 for the heterostructure formation.12−15 The energy level of its conduction band however is more positive than the reduction potential of O2/HO2•. Consequently, decoration of SnO2 on TiO2 offers only enhanced charge separation, but not the extra oxidizing species, HO2•. In this work, we propose to resolve this issue by using SnO2 QDs as the second semiconductor. The band gap of SnO2 QD gets enlarged with decreasing QD size because of the quantum confinement effect.5−8,10 With the enlargement of the band gap, the energy level of the corresponding conduction band shifts to the more negative region and can be adjusted to fall within the workable energy gap through control of the QD size. For the above idea to work, one needs to be able to control the size of the SnO2 QD precisely. We develop a simple, fast, low cost, surfactant-free, carrier solvent assisted interfacial reaction method to prepare size-controllable SnO2 QDs. With suitable band structure

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EXPERIMENTAL METHODS Preparation of SnO2 QDs. A one-step carrier solventassisted interfacial reaction method is developed to synthesize size-controllable SnO2 QDs at room temperature with a reaction time of 30 min. The method is described below. First, a desired amount of SnCl4·5H2O (Baker, 99%) is dissolved in ethanol (Sigma-Aldrich, 99%). The solution is added into chloroform of equal volume (Sigma-Aldrich, 99%). Water, lighter than and immiscible with chloroform, is then added to the above solution to generate an interface between water and chloroform. Ethanol, miscible with both water and chloroform, serves as the carrier solvent, carrying the precursor from the chloroform domain to water domain to react there for formation of SnO2 QDs. It is to be noted that the precursor does not dissolve in chloroform, but only in ethanol, and ethanol has a higher affinity toward water than chloroform. Because of the above, once the water is added into the system and the water−chloroform interface is formed, ethanol will carry the precursor to move into the water domain for the sol− gel reaction to produce SnO2 QD. The moving rate of the ethanolic precursor solution plays an important role for controlling the size of the product QDs and can be adjusted by the precursor concentration. Preparation of SnO2 QD@TiO2 for Photodegradation of RhB. An amount of 0.05 g of P25 TiO2 is added into 10 mL of ethanol (95 vol %) under stirring for 15 min, followed by addition of 300 μL of ethanolic suspension of SnO2 QD (containing 2 mg of SnO2 QD). The final mixture is stirred for 30 min at room temperature. The product SnO2 QD@TiO2 is then collected with centrifuge and rinsed with ethanol several times before being dried in an oven set at 80 °C overnight to afford the final product. The photocatalytic performance of the SnO2 QD@TiO2 is measured by the photodegradation of RhB under UV illumination. The photocatalysis experiment is conducted at room temperature. In a typical run, 10 mg of photocatalyst is added into 20 mL of RhB solution (1.5 × 10−5 M). Prior to UV irradiation, the suspension is stirred in the dark for 60 min to reach the adsorption equilibrium between RhB and the photocatalyst. At certain time intervals of irradiation, samples are analyzed with a UV−vis spectrophotometer to determine the concentration evolution of RhB through recording the corresponding absorbance of the characteristic peak of 554 nm. Cyclic Voltammetry. The working electrode is prepared by drop-casting ethanolic suspension of SnO2 QD onto a graphite electrode followed by drying at 60 °C. The counter electrode is Pt coil, and Ag/AgCl serves as the reference electrode. The cyclic voltammograms are recorded in an electrolyte of 0.1 M Na2SO4(aq) with a negative scan starting from 0.5 to −1.5 V and then back to 0.5 V at a scan rate of 30 mV/s. Characterizations. The crystalline structure of the samples is determined by an X-ray diffractometer (XRD, MAC Science, MXP18, Cu Kα) and a field emission transmission electron microscope (FETEM, Philips, 200 kV). The quantum confinement effect and quantum yields of the sample are investigated with UV−vis spectroscopy (Hitachi, U-2800) and a fluo14458

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chloroform is 2:1:1. With a 30 min reaction time, almost all ethanol, carrying the precursor, moves into the water domain. Consequently, the height of the water−chloroform interface drops significantly. All cases, except case 6, show almost the same interface height. As for case 6, the precursor concentration is high enough to retard the movement of the ethanolic precursor solution, and no appreciable upward movement occurs within 30 min. The end result is that the water−chloroform interface remains almost unchanged. When the upward movement is fast, the precursor goes into the water domain fast and reacts fast to form more but smaller product QDs. These small size QDs are more energetic and aggregate more easily to form larger QD clusters. This is why the water domain appears cloudier with lower precursor concentrations. When the precursor concentration increases, the product QDs are with larger sizes and less intensive QD aggregation occurs. The water domain thus appears more transparent. In fact, for low precursor concentration cases, the aggregation of these QDs continues, and finally the cluster size becomes large enough to enable sedimentation of the clusters out of the water domain. The time needed for this sedimentation to occur increases from around 1−2 h for case 1 to 1−2 days for case 5. As for case 6, there is only very minor extent of reaction, and the water domain remains transparent within a week of storage. The crystalline structure of the SnO2 QDs is characterized by XRD and HRTEM. Figure 3a displays the XRD patterns of samples TQD-1 to TQD-5. These patterns match very well with that of SnO2 of the tetragonal phase (JCPDS 77-0447). No extra diffraction peaks can be identified from these patterns, and the broad diffraction peaks imply small grain sizes of the samples. The left and right panels of Figure 3b show the HRTEM images of samples TQD-1 and TQD-6, respectively. Both QD size and interlayer distances in the (110) and (101) directions are observed. The QD sizes are around 1.9 and 2.7 nm for samples TQD-1 and TQD-6, respectively. For both samples, the interlayer distances are estimated to be 0.33 and 0.26 nm in the (110) and (101) directions, respectively, in good agreement with the corresponding d-spacing of tetragonal SnO2. Figure 3c shows that the average crystal size of the SnO2 QDs increases with increasing precursor concentration. Evidently, the data obtained from the HRTEM images and from the Scherrer equation estimation agree with each other quite well, indicating the single crystallinity of the products. The reaction yield of the SnO2 QDs, taking sample TQD-5 as an example, is estimated to be around 64% from the precursor concentration and corresponding product weight measurement. Note that there are losses of the product SnO2 QDs during the centrifugal collection of rinsed products. Therefore, the true reaction yield should be higher than 64%. The exciton Bohr radius of SnO2 is around 2.7 nm so that SnO2 nanocrystals with a size of less than 5.4 nm would exhibit significant quantum confinement effects.10 According to this loose criterion, all products obtained from the present process are SnO2 QDs. The UV−vis absorption spectra of all QD samples are shown in Figure 4a. The blue-shifts in onset wavelength, thus enlargements in band gap energy, are evident and increase from that of sample TQD-6 to that of sample TQD-1, corresponding to decrease in precursor concentration and thus QD size. This trend confirms that smaller QDs give stronger quantum confinement effects. The band gap energy, Eg, of the SnO2 QDs can be determined by the equation

rescence spectrophotometer (Hitachi, F-4500). The band-edge positions of the samples are measured by an electrochemical analysis instrument (CHI-6275D).

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RESULTS AND DISCUSSION The carrier solvent assisted interfacial reaction method is illustrated in Figure 1. The whole process is operated at room

Figure 1. Schematic illustration of carrier solvent assisted interfacial reaction.

temperature, and the size of the SnO2 QD can be accurately controlled with the precursor concentration. Briefly, the precursor SnCl4·5H2O is first dissolved in ethanol, followed by dissolution of the resulting solution in chloroform. Water is then added on top of the chloroform domain, forming an interface between water and chloroform. As shown in Figure 1, the much greater affinity of ethanol toward water drives ethanol, carrying the precursor, to move upward from the chloroform into water domain. Once in the water domain, the precursor is converted to SnO2 QDs through the hydrolysis, condensation, and polymerization reactions with water within 30 min.22−24 The upward moving rate of the ethanolic solution depends on the precursor concentration. Increasing the precursor concentration enhances the molecular interactions between the ethanolic solution and chloroform, thus retarding the upward movement of the ethanolic solution. The lower the precursor concentration, the higher the moving rate of the ethanolic solution. Furthermore, the higher the moving rate of the ethanolic solution, the smaller the resulting SnO2 QDs. The higher moving rate of the ethanolic solution leads to higher precursor concentrations in the water domain, giving higher nucleation rates and thus more but smaller QDs. Consequently, the size of the SnO2 QD can be controlled by the precursor concentration. SnO2 QDs with a size range of 2−2.72 nm, corresponding to a band gap range of 5.7−4.39 eV, can be produced with the precursor concentration ranging from 0.005 to 0.15 M. We prepare SnO2 QDs of six different sizes by using six precursor concentrations, 0.005, 0.015, 0.025, 0.035, 0.05, and 0.15 M, with the products termed TQD-1 to TQD-6, respectively. Figure 2 shows photos for the reaction systems after 30 min of reaction. There are several points to be noted from Figure 2. The starting volume ratio of water:ethanol:-

Figure 2. Photos for cases 1−6. 14459

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(αhν)1/n = B(hν − Eg), where α is the absorption coefficient, hν the photon energy, n a value that depends on the nature of the transition (1/2 for direct band gap materials such as SnO2), and B a constant.5 The band gap energies of the SnO2 QDs are estimated from the inset plot of (αhν)2 versus photon energy of Figure 4a. The results are plotted against precursor concentration in Figure 3c. Figure 3c shows that the size and thus the corresponding band gap energy of the SnO2 QD can be adjusted with the precursor concentration. Photoluminescence (PL) emission spectra of the SnO2 QDs at room temperature are shown in Figure 4b. The results are consistent with those observed from the corresponding UV−vis absorption spectra. The emission peak is increasingly blueshifted from that of sample TQD-6 of a larger QD size to that of sample TQD-1 of a smaller QD size, exhibiting strong quantum confinement effects. The quantum yield of the SnO2 QDs, determined based on data collected from the UV−vis absorption and PL emission spectra and using Rhodamine B (RhB) as the standard reference, is 1.1% for sample TQD-5 as an example. The SnO2 QDs are decorated onto the surfaces of commercial P25 TiO2 nanoparticles for possible improvement of P25 TiO2 on photocatalytic degradation of RhB. Figure 5 shows the photodegradation scheme toward RhB and band structure of the SnO2 QD decorated P25 TiO2. Electron−hole pairs are produced upon the UV light excitation. The model pollutant, RhB, may be degraded by both the excited electrons and created holes through oxidation by oxidizing radicals (HO2• and HO•) and holes, h+, as shown in Figure 5. The decoration of SnO2 QDs of suitable band gaps (and thus suitable sizes) onto the surface of P25 TiO2 can achieve not only charge separation but also generation of HO2• radicals for degradation of RhB. From the band structure presented in Figure 5, a critical requirement should be met by the SnO2 QDs to achieve this goal. The conduction band of the SnO2 QDs should be less negative than that of the TiO2 to enable electron transfer from the TiO2 to SnO2 domain and should be more negative than that of the reduction potential of O2/HO2• so that the electron transferred from the TiO2 domain can generate HO2• through reduction of O2. Here, we determine the conduction band position of the SnO2 QDs with cyclic voltammetry analyses.25 The LUMO energy (ELUMO) of electroactive materials can be estimated from the onset reduction potential (Ered), according to the equation26

Figure 3. (a) XRD patterns of SnO2 QDs. (b) HRTEM images of samples TQD-1 (left panel) and TQD-6 (right panel). (c) Energy band gap and crystal size of SnO2 QDs as functions of precursor concentration.

E LUMO = −(Ered + 4.71) eV

(1)

Figure 4. (a) UV−vis absorption and (b) photoluminescence spectra of samples TQD-1 to TQD-6. 14460

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Figure 5. Photodegradation scheme toward RhB and band structure of the SnO2 QD decorated P25 TiO2.

distribution of SnO2 QDs (from sample TQD-5) on the surface of the P25 TiO2 nanoparticle. The inset of Figure 6b shows an EDX elemental mapping of SnO2 QD@TiO2 in Ti and Sn. The mapping confirms the coverage of SnO2 QD on the surface of the TiO2 nanoparticle. The interlayer distances of the SnO2 QDs along the (110), (101), and (211) directions with values of 0.33, 0.26, and 0.17 nm, can be determined from Figures 6c, 6d, and 6e, respectively, in good agreement with the corresponding d-spacing of tetragonal SnO2. We further investigate the photocatalytic performance of the SnO2 QD decorated P25 TiO2 nanoparticles. Figure 7a shows the evolution curves of the RhB concentration for RhB solutions containing P25, TQD-5 decorated, and TQD-6 decorated TiO2 nanoparticles. Data for plain RhB solution are also included as a control. Evidently, no appreciable decay in RhB concentration is observed for the control, while significant degradation of RhB is achieved for the photocatalyst containing samples, with the TQD-5 decorated TiO2 sample performing the best. An apparent first-order kinetic equation is adopted to fit the experimental data. Here, C0 is the initial concentration of RhB before light irradiation, C is the concentration of RhB remaining in the solution at time t, and Kapp is the apparent first-order reaction rate constant.29

Here, the onset potential is referenced to the Ag/AgCl electrode. The value of 4.71 represents the difference between the vacuum level potential of the normal hydrogen electrode (NHE) and the potential of the Ag/AgCl electrode versus NHE.27,28 We start from 0.5 V and proceed with a negative potential scan from 0.5 to −1.5 V and then back to 0.5 V. The onset reduction potentials of samples TQD-5 and TQD-6 are thus determined to be −0.40 and −0.33 V, respectively. These results together with relevant band structure data are summarized in Table 1 and plotted in Figure 5. Evidently, Table 1. Band Structure Parameters of SnO2 QDs sample

Ered (V)

LUMOa (eV)

HOMOb (eV)

λabsc (nm)

Egd (eV)

TQD-5 TQD-6

−0.40 −0.33

−4.31 −4.38

−8.86 −8.77

272 282

4.55 4.39

a

Determined by eq 1. bDetermined from LUMO and band gap energy. cMeasured by UV−vis absorption spectra. dEstimated from UV−vis absorption spectra.

the SnO2 QDs from samples TQD-5 and TQD-6 are both with a suitable conduction band to achieve not only charge separation enhancement but also generation of extra oxidizing radicals for photodegradation of RhB. Figure 6a shows the HRTEM images of SnO2 QD@TiO2. Evidently, the larger size P25 TiO2 nanoparticles are covered with much smaller size SnO2 QDs. Figure 6b shows the enlarged image of the dashlined region of Figure 6a, from which one can identify both the anatase and rutile phases of P25 TiO2 and the uniform

ln(C0/C) = K appt

(2)

Figure 7b shows the ln(C0/C) versus t plot, from which the apparent reaction rate constant can be determined as the slope of the line. The apparent reaction rate constant of the TQD-5 decorated TiO2 sample improves significantly over the

Figure 6. (a) HRTEM and (b) locally enlarged HRTEM images of SnO2 QD@TiO2. Inset of (b) shows an EDX elemental mapping of SnO2 QD@ TiO2 in Ti and Sn. HRTEM images of SnO2 QD decorated on TiO2 for interlayer distance in (c) (110), (d) (101), and (e) (211) directions. 14461

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Figure 7. (a) C/C0 and (b) ln(C0/C) versus time curves for RhB solutions containing P25, TQD-5 decorated, and TQD-6 decorated TiO2 nanoparticles.

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undecorated P25 TiO2 sample. For the TQD-6 decorated TiO2 sample, the conduction band of TQD-6 may be a bit too close to that of the reduction potential of O2/HO2• so that the effectiveness of HO2• generation decreases a little.

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CONCLUSIONS In conclusion, a simple, fast, room temperature, surfactant-free, carrier solvent-assisted interfacial reaction method for preparation of size-controllable SnO2 QDs is developed. The unique design of the present interfacial reaction method is the concept of using ethanol as a carrier solvent that carries precursor through the interface from the bottom to top domain for syntheses of SnO2 QDs. The application of these SnO2 QDs in photodegradation of RhB is demonstrated. Through decoration of SnO2 QDs of suitable sizes, the charge separation of the host photocatalyst, P25 TiO2, is much improved while maintaining a suitable electronic potential to generate extra oxidizing species, HO2•, for RhB degradation. The photocatalytic degradation efficiency of TiO2 toward Rhodamine B, in terms of the apparent reaction rate constant, is significantly improved from 0.025 to 0.055 min−1 with the band structure engineering achieved through SnO2 QD decoration.

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ASSOCIATED CONTENT

S Supporting Information *

Evolution of the interface position and aggregation of product QDs, plot for total ethanol−chloroform mixture volume and interface height versus precursor concentration, Stokes shift for sample TQD-5, CV analyses of SnO2 QDs, and UV−vis spectra recorded at increasing illumination times for photocatalytic degradation. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel +886-3-5714364; e-mail [email protected] (S.-Y.L.). Notes

The authors declare no competing financial interest.

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REFERENCES

ACKNOWLEDGMENTS

This work was financially supported by the National Science Council of Taiwan under grant NSC-101-2221-E-007-111-MY3 and by the Low Carbon Energy Research Centre of the National Tsing-Hua University. 14462

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