Nanorod Arrays Decorated with CdSe Using an Upconversion TiO

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Enhanced Photocatalytic Activity of TiO2 Nanorod Arrays Decorated with CdSe Using Upconversion TiO2:Yb3+, Er3+ Thin Film Jinzhao Huang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie504204z • Publication Date (Web): 24 Dec 2014 Downloaded from http://pubs.acs.org on January 1, 2015

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Enhanced Photocatalytic Activity of TiO2 Nanorod Arrays Decorated with CdSe Using Upconversion TiO2:Yb3+, Er3+ Thin Film

Journal: Manuscript ID: Manuscript Type: Date Submitted by the Author: Complete List of Authors:

Industrial & Engineering Chemistry Research ie-2014-04204z.R2 Article 23-Dec-2014 Fu, Ke; School of Physics and Technology, School of Physics and Technology Huang, Jinzhao; School of Physics and Technology, Yao, Nannan; School of Physics and Technology, School of Physics and Technology Xu, Xijin; University of Jinan, School of Physics and Technology Wei, Mingzhi; School of Material Scien Qilu Uce and Engineering, School of Material Science and Engineering

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Enhanced Photocatalytic Activity of TiO2 Nanorod Arrays Decorated with CdSe Using Upconversion TiO2:Yb3+, Er3+ Thin Film Ke Fu1, Jinzhao Huang1*, Nannan Yao1, Xijin Xu1*, and Mingzhi Wei2 1School of Physics and Technology, University of Jinan, Jinan 250022, Shandong Province, P R China 2School of Material Science and Engineering, Qilu University of Technology, Jinan 250353, Shandong Province, P R China E-mail: [email protected] ABSTRACT: A new composite photocatalyst CdSe-sensitized TiO2 nanorod arrays (CdSe/TiO2 NRAs) grown on fluorine-doped tin oxide (FTO) coated with TiO2:Yb3+, Er3+ thin film was constructed. The TiO2:Yb3+, Er3+ thin film acted as a medium for converting near-infrared light to visible light via upconversion process and reduced electron−hole recombination. TiO2 nanorod arrays with high specific surface areas, well-aligned nanostructures can provide a fast transfer pathway for photogenerated electrons. The CdSe/TiO2 NRAs extended optical response from ultraviolet to visible region, which can generate more electron-hole pairs. The composite photocatalyst TiO2 NRAs/TiO2:Yb3+, Er3+ exhibited excellent photocatalytic activity toward the degradation of Rhodamine B. The composite structure can not only extend the absorption of TiO2 but also consequently reduce electron−hole recombination, which improve the photocatalytic efficiency of the composite photocatalyst. Moreover, the photocatalyst grown on FTO substrates directly makes it possible to collect and



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recycle.

1. INTRODUCTION Humanity is facing tremendous pressure from environmental pollution. It is important for us to control destruction of chemical pollutants to the environment effectively. Although conventional catalytic technology plays a certain role in cleaning pollutants, some disadvantages also exist, such as the decomposition of pollutants is not complete, and the secondary pollution can be produced easily. Fujishima and Honda discovered the hydrogen production phenomenon on the TiO2 electrode in 1972, this marked the beginning of the era of semiconductor photocatalysis.1 By photocatalytic technology, a variety of organic pollutants can be completely mineralized to carbon dioxide, water and other inorganic small molecules or ions.2-6 However, many challenges need to be overcome for improving the efficiency of the photocatalytic reaction.9,10 TiO2 semiconductor has attracted much attention because of its physical and chemical stability, environmental friendliness, and its strong oxidizing power under ultraviolet (UV) light, low cost and so on.12-15 However, the band gap of the semiconductor material of TiO2 is 3.2 eV, which determines that the TiO2 semiconductor material only be irradiated by UV to stimulate electron-hole pairs.11 As we all know, the UV light only occupies 5% of the solar spectrum, and the rest of the solar spectrum in the visible is about 48% and near-infrared (NIR) light is about 47%, which is not used sufficiently for photocatalysis.16,17


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To solve these problems, some innovative work has been done. To reach the goal of extending the limited optical absorption edge of TiO2 from UV light to the visible light for better utilization of solar energy, many studies have been made, which include the introducing of nonmetal or metal impurities,18-20 depositing of noble metals,21,22 coupling with semiconductor,23,24 and modifying with carbon-based materials to adjust the band gap toward visible light energies.26 Among these methods, sensitizing with narrow band-gap semiconductors has received widespread attention in recent years. The CdSe with a band gap of around 1.7 eV is often used to sensitize wide band gap semiconductors such as TiO2.27-29 The band gap of CdSe is narrow relatively and the conduction band of CdSe is slightly higher than TiO2. Therefore, CdSe can easily transfer electron to the TiO2. The light absorption edge can be extended to the visible region with the help of these strategies mentioned above. However, it is still a challenge to find an appropriate way to extend the absorption of TiO2 to the NIR regions.18 To overcome this limitation, upconversion has been considered as one of the most promising solutions and have already been applied in photocatalysis. Rare-earth ions act as a medium for converting NIR to visible light via two-photon or multiphoton upconversion processes.30,32 In recent years, although there have been reported on efficient upconversion material such as β‑NaYF4 as the host material to enhance photocatalytic activity of TiO2.16,18 However, the β‑NaYF4 is a glass ceramic material with poor electrical conductivity, and there is still some distance away from practical application. It is important to find new matrix material to further improve the



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efficiency. Excellent properties of TiO2 make itself have great potential in upconversion materials. One dimensionally (1D) aligned single-crystalline semiconductors made on the conductive substrates such as TiO2 nanorod arrays (NRAs), have been receiving more and more attention from researchers owing to its a variety of potential application in the photocatalysis and dye-sensitized solar cells.7,33,34 Ordered TiO2 NRAs with high specific surface areas, well-aligned nanostructures can provide a fast transfer pathway for photogenerated electrons, which facilitates electron transfer and subsequently improves the electron transfer efficiency.4,18,35 In this paper, a new attempt has been made to construct CdSe-sensitized TiO2 NRAs (CdSe /TiO2 NRAs) on fluorine-doped tin oxide (FTO) coated with a layer of TiO2 thin film doping with Yb3+, Er3+( TiO2:Yb3+, Er3+). To the best of our knowledge, the composite structure of CdSe/TiO2 NRAs/TiO2:Yb3+, Er3+ used in the photocatalytic degradation is rare. The photocatalyst grown on FTO will not fall off in the photocatalytic process, which will not produce new pollution. The structural, optical and photocatalytic performance of the composite structure CdSe / TiO2 NRAs / TiO2: Yb3+, Er3+ were investigated.

2. EXPERIMENTAL SECTION 2.1. Photocatalyst Preparation For the fabrication of TiO2 sol, 10 mL (29 mmol) tetrabutyl titanate (Ti(OBu)4) was dissolved in 30 mL ethanol with stirring at 25 °C, then the glacial acetic acid was


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added in the solution. After stirring for 15 min, the transparent faint yellow solution was obtained. Under the strong stirring, nitric acid-ethanol solution (0.5 mL nitric acid, 10 mL ethanol and 1 mL DI water, (1~2) D·S-1) was added in the above solution. After stirring for 1 h, the TiO2 sol was obtained. For the Yb3+ and Er3+ doping experiment, measured quantity of the ytterbium nitrate and erbium nitrate were added into the nitric acid-ethanol solution. The Ti:RE mole ratio 1:0.01 and the mole ratio of Yb3+ to Er3+ is 1:1 were used in this doping experiment. To facilitate the nucleation of TiO2 NRAs , the TiO2 seed layer was deposited on the FTO by spin-coating method, and the optimum coating condition of a 2 cm × 2 cm substrate was 3000 RPM for 30 S (with acceleration time of 1 S) using the TiO2 sol. And then the seed layer was calcined at 400 °C for 1 h. The FTO glass substrates used for the growth of TiO2 NRAs were cleaned in an ultrasonic bath with acetone, isopropanol, and DI water. For the synthesis of TiO2 NRAs, 15 mL of DI water was mixed with 15 mL of HCl (38 wt. %). The mixture was stirred for 5 min in a Teflon-lined stainless steel autoclave before the addition of 0.5 mL of titanium butoxide. After stirring for another 5 min, the FTO substrates (coated with TiO2:Yb3+, Er3+) were placed against the wall of the Teflon-liner with the conductive side facing down. The hydrothermal synthesis was conducted at 150 °C for 6 h in an electric oven. Then the autoclave was cooled down to room temperature under flowing water. The CdSe were synthesized by a hot-injection method. In a three-neck flask, a 70-mL octadecene (ODE) solution containing 5.54 g Oleic acid and 0.51 g CdO was



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first heated to 180°C to form a clear solution under N2 atmosphere. After that, it was heated up to 270°C trioctylphosphine (TOP)-Se solution (1.3 mmol Se powder and 0.5 g TOP dissolved in 10 mL ODE, stirred more than 1 h) was quickly injected into the flask. Finally, methanol and acetone were added to precipitate CdSe, which were then dissolved in toluene for storage. The CdSe was electrodeposited on the TiO2 NRAs via an electrophoresis adsorption technique in the CdSe solution for 30s using a regulated dc power supply. The reaction was under +23 V for 30s. The final architecture is shown in Scheme 1.

Scheme 1. Schematic Illustration for the Preparation of CdSe/TiO2 NRAs/TiO2:Yb3+, Er3+ Upconversion Photocatalyst

2.2. Materials Characterizations The structure of TiO2 was investigated by X-ray diffraction (XRD) using a D8 ADVANCE with Cu Kα at λ=0.15406 nm. The absorption spectra of samples were


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examined with a UV-vis-NIR scanning spectrophotometer (UV-3101PC). The surface morphology and energy dispersive spectrometer (EDS) of the samples were investigated using a field emission scanning electron microscopy (FE-SEM, Quanta FEG250). The photoluminescence (PL) spectra were obtained by using the FLS920 fluorescent spectrometer made by Edinburgh instruments using a Xe lamp as the excitation source. 2.3. Photoelectrochemical Measurements The photocurrent by irradiating the photoanode with sunlight was recorded with an electrochemical workstation. The photoelectrochemical cell was a three-electrode system: The prepared samples of the composite structure saturated calomel electrode (SCE), and Pt electrode acted as the working, reference, and counter electrodes, respectively. The variations of the photoinduced current with time (I−t curve) were measured during the light on and off. The photoinduced current−voltage (I−V) curves were measured from −0.5 to 2.0 V. The gap between the switching on and turning off of the light was 50s. a Xe lamp was used as the light source. The measurements were performed in 1 M KOH solution and in a light tight environment at ambient temperature. The photocatalytic behavior was investigated by evaluating the degradation of RhB in an aqueous solution. The prepared composite structure photocatalysts were placed into 10 ml RhB solution (5 mg/L). After irradiation for a designated time (20min), 3 mL of the RhB aqueous solution was taken out to measure the absorption spectrum and then put back into the quartz bottle for further measurement. A 500 W Xe lamp was used as the radiation source. All of these measurements were carried out at room temperature.



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3. RESULTS AND DISCUSSION The SEM images of TiO2 NRAs/TiO2:Yb3+,Er3+ and TiO2 NRAs/TiO2 are shown in Figure 1(a) and 1(b). It can be seen that TiO2 NRAs were successfully synthesized. It is clear that the TiO2 NRAs are evenly distributed on the FTO. The TiO2 NRAs are vertical growth on the FTO. The diameter of nanorods is relatively uniform. These results illustrate that morphology has no impact on the photocatalytic performance of the catalyst. Figure 1(c) shows SEM image of CdSe/TiO2 NRA/TiO2:Yb3+, Er3+, it can be seen that the CdSe decorated the TiO2 nanorod uniformly. The chemical compositions of the CdSe/TiO2NRA/TiO2:Yb3+, Er3+ is shown in Figure1(d) using EDS. The EDS elemental analysis confirms the existence of Cd and Se.

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Figure 1. SEM images of (a) TiO2 NRA/TiO2 ,(b) TiO2 NRA/TiO2:Yb3+, Er3+,(c) CdSe/TiO2NRA/ TiO2:Yb3+, Er3+ and (d) EDS spectrum of CdSe/TiO2NRA/ TiO2:Yb3+, Er3+ Figure 2 shows XRD patterns of TiO2:Yb3+, Er3+. The peaks are readily indexed to anatase TiO2, which illustrates crystal mainly based on the anatase TiO2. 31 However, we don’t find the rare earth elements or rare earth oxides in the XRD pattern of TiO2: Yb3+, Er3+, which may be attributed to the amount of doping is extremely low or a small amount of rare earth ions into the TiO2 crystal lattice.



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Figure 2. XRD patterns of TiO2:Yb3+, Er3+ The photocatalytic performance of the samples toward to degradation of RhB dyes are shown in Figure 3. From Figure 3, the RhB dye solution was observed to be very stable under Xe lamp irradiation in the absence of catalyst. TiO2 NRAs/TiO2:Yb3+, Er3+ exhibit better photocatalytic activities than TiO2 NRAs/TiO2, which could degrade 60% and 50% of RhB in 80 min respectively. These results demonstrated that the introduction of Yb3+, Er3+ makes the electron binding energy of various elements especially Ti element on the surface of the photocatalyst to be changed and significantly increased the content of the Ti3+ on the surface of photocatalyst, which make surface states to capture photo-generated electrons of the conduction band more efficiently and photogenerated electron- hole pairs recombination rate was suppressed. In addition, the similar procedures were followed to analyze CdSe/TiO2 NRAs/ TiO2:Yb3+, Er3+. The photodegradation efficiency of RhB was about 98% after 80 min irradiation obviously. However, in the reference experiment, only 73% of RhB


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decomposition was observed within 80 min irradiation with CdSe/TiO2 NRAs/TiO2. The results show that Yb3+, Er3+ play a significant role in converting NIR to visible light. The converted visible light was absorbed by CdSe, which can generate more electron-hole pairs, thus improving the photocatalytic efficiency.

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Figure 3. Photocatalytic degradation of RhB in the presence of various catalysts. To further study the separation efficiency of photogenerated electrons and holes in the photocatalytic process, the PL spectra of the prepared composite structure TiO2 NRAs/TiO2 and TiO2 NRAs/TiO2:Yb3+, Er3+ were studied. It is well known that the decrease of the PL intensity can be accounted for the higher electron–hole separation that gives long-lived photogenerated charge carriers. The PL spectra excited by 320 nm for different samples are shown in Figure 4. According to the PL analysis, the



tendency of PL spectra of the TiO2 NRAs/TiO2 and TiO2 NRAs/TiO2:Yb3+, Er3+ are quite similar. In our study, the PL intensity of TiO 2 NRAs/TiO2 is significantly higher than the PL intensity of TiO2 NRAs/TiO2:Yb3+, Er3+, which indicated the TiO2 NRAs/TiO2:Yb3+, Er3+ might suppress the recombination process of photogenerated electrons and holes. Therefore, it is concluded that presence of Yb3+, Er3+ in the composite samples could be effective to enhance the separation efficiency of electron-hole pairs during the photocatalysis.

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Figure 4. PL of TiO2 NRAs/TiO2 and TiO2 NRAs/TiO2:Yb3+, Er3+.

To distinguish the mechanisms for the RhB degradation under a Xe lamp irradiation with the different samples, the photoelectrochemical performance of TiO2 NRAs/TiO2 , TiO2 NRAs/TiO2:Yb3+, Er3+, CdSe/TiO2 NRAs/TiO2 and CdSe/TiO2 NRAs/ TiO2:Yb3+, Er3+ were studied using photocurrent I−t and I−V curves. The photocurrent responses


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of the samples are shown in Figure 5(a). In the case of Xe lamp illumination, an apparently boosted photocurrent response appeared. The photocurrent of TiO2 NRAs/TiO2, TiO2 NRAs/TiO2:Yb3+, Er3+, CdSe/TiO2 NRAs/TiO2 and CdSe/TiO2 NRAs/TiO2:Yb3+, Er3+ electrodes were 0.001, 0.0064, 0.0081 and 0.012mA, respectively. Much more free carriers could be generated and more separation of photogeneratad electrons and holes were thought as the key cause of photocurrent rise. Figure 5(b) shows the I−V curves of the samples. When the bias potential is larger than 0.7 V, The slope of photocurrent is CdSe/TiO2 NRAs/TiO2:Yb3+, Er3+>CdSe/TiO2 NRAs/TiO2>TiO2 NRAs/TiO2:Yb3+, Er3+>TiO2 NRAs/TiO2 electrodes. This result indicates that the introduction of Yb3+, Er3+ effectively enhance the separation efficiency of photogenerated electron−hole pairs. The result is consistent with the results of photocatalytic tests.

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Figure 5. Photoinduced I-t curves (a) and I-V curves (b) of TiO2 NRAs/TiO2 and TiO2 NRAs/TiO2:Yb3+, Er3+ The solar radiation spectrum is shown in the Figure 6(a). The excitation spectrum of TiO2:Yb3+, Er3+ is shown in the Figure 6(b). The emission spectrum of TiO2:Yb3+, Er3+ is presented in Figure 6(c). A 980 nm laster is used as the source the PL emission. The emission peaks at 526, 547 and 659 nm of Er3+ are observed. The emission peaks of Yb3+, Er3+ almost locate in the visible region and match well with the absorption region of the CdSe. CdSe with a band gap of around 1.7 eV responds well in the visible range. The absorption spectrum of CdSe examined by UV−visible spectroscopy is shown in Figure 6(d), and there is strong absorption in visble light. Especially a strong absorption peak at 556 nm, which can better overlap with the PL of TiO2: Yb3+, Er3+. Therefore, the addition of TiO2:Yb3+, Er3+ coated on FTO can extend the absorption spectrum of CdSe/TiO2 NRAs/TiO2:Yb3+, Er3+. The catalyst can


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absorb more of the sun's energy to generate more carriers. Consequently, the photocatalytic performance of the composite structure is improved. 1.0

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Figure 6. (a) Spectrum of solar radiation (b) The excitation light of TiO2:Yb3+, Er3+ (c) PL of TiO2:Yb3+, Er3+ (d) Absorption of CdSe To further demonstrate upconversion process of TiO2: Yb3+, Er3+, UV-vis-NIR absorption spectra of CdSe/TiO2 NRAs/TiO2 and CdSe/TiO2 NRAs/TiO2:Yb3+, Er3+ are shown in Figure 7. The spectrum shows significant absorption in the visble light region. Compared to the spectrum of CdSe/TiO2 NRAs/TiO2 , it is obvious that the absorption of visible light of CdSe/TiO2 NRAs/TiO2:Yb3+, Er3+ significantly higher than the CdSe/TiO2 NRAs/TiO2, indicating that Yb3+, Er3+ acts as a medium for converting NIR to visible light, which means the absorption spectrum of TiO2 is expanded. The result agreed well with the results of photocatalytic tests.



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Figure 7. UV-vis-NIR absorption spectra of CdSe/TiO2 NRAs/TiO2 and CdSe/TiO2 NRAs/TiO2:Yb3+, Er3+ Photocatalytic mechanism of CdSe/TiO2 NRAs/TiO2:Yb3+, Er3+ upconversion photocatalyst under the full spectrum light irradiation were shown in Scheme 2. Upon the light irradiation, the high penetrability of the NIR allows it to go further into the inner structure to excite the upconversion materials TiO2:Yb3+, Er3+, then Yb3+ and Er3+ act as a medium for converting NIR to visible light, CdSe/TiO2 NRAs can quickly absorb the UV and visible light for photocatalysis. These emission peaks are totally in the CdSe/TiO2 NRAs absorption range. The photo-generated electrons react with the absorbed O2 on the edge of FTO to form OH·. These hydroxyl radicals are known to be very reactive oxidative species which react with the organic or water pollutants that can be degraded to CO2 and H2O. Additionally, the remaining holes in CdSe sensitized TiO2 NRAs can transfer to the


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surface of composite structure through the nanorod structure, and take part in the redox reaction to generate OH·, which is further used to remove RhB.

Scheme 2. Photocatalytic Mechanisms of CdSe/TiO2 NRAs/TiO2:Yb3+, Er3+ Photocatalyst Under the Full Spectrum Light Irradiation.

4. CONCLUSIONS The composite structure photocatalyst CdSe/TiO2 NRAs/TiO2:Yb3+, Er3+ has been successfully prepared. The characterization results show that the introduction of Yb3+, Er3+ makes a contribution to suppress the photogenerated electron−hole pair recombination. More importantly, the optical absorption edge of photocatalyst is expanded. The CdSe/TiO2 NRAs/TiO2:Yb3+, Er3+ has better photocatalytic activity compared with CdSe/TiO2 NRAs/TiO2 under the Xe lamp irradiation. The result



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shows an important insight on enhancing the photocatalytic activity to us by converting NIR to visible light via the photon upconversion. Furthermore, the photocatalysts grown on FTO makes the collection and recycle of the photocatalysts much easier.

ACKNOWLEDGMENTS Project supported by the National Natural Science Foundation of China (Grant No. 61106059, 11304120), the Encouragement Foundation for Excellent Middle-aged and Young Scientist of Shandong Province (Grant No. BS2011NJ003, BS2012CL005, BS2013CL020), the Science-Technology Program of Higher Education Institutions of Shandong Province (Grant No. J11LA10, J14LA01)

REFERENCES (1) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature. 1972, 238, 37. (2) Tanabe, I.; Ryoki, T.; Ozaki, Y. Significant Enhancement of Photocatalytic Activity of Rutile TiO2 Compared with Anatase TiO2 upon Pt Nanoparticle Deposition Studied by Far-Ultraviolet Spectroscopy. Phys. Chem. Chem. Phys. 2014, 16, 7749. (3) Yan, W.; He, F.; Gai, S.; Gao, P.; Chen, Y.; Yang, P. A Novel 3D Structured Reduced Graphene Oxide/TiO2 Composite: Synthesis and Photocatalytic Performance. J. Mater. Chem. A 2014, 2, 3605. (4) Natarajan, T, S.; Natarajan, K.; Bajaj, H, C.; Tayade, R, J. Energy Efficient


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Scheme 1. Schematic Illustration for the Preparation of CdSe / TiO2 NRAs / TiO2:Yb3+, Er3+ Upconversion Photocatalyst Scheme 2. Photocatalytic Mechanisms of CdSe / TiO2 NRAs / TiO2:Yb3+, Er3+ Photocatalyst Under the Full Spectrum Light Irradiation.


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figure captions Figure 1. SEM images of (a) TiO2 NRA/TiO2 ,(b) TiO2 NRA/TiO2:Yb3+, Er3+,(c) CdSe/TiO2NRA/ TiO2:Yb3+, Er3+ and (d) EDS spectrum of CdSe/TiO2NRA/ TiO2:Yb3+, Er3+ Figure 2. XRD patterns of TiO2:Yb3+, Er3+ Figure 3. Photocatalytic degradation of RhB in the presence of various catalysts. Figure 4. PL of TiO2 NRAs/TiO2 and TiO2 NRAs/TiO2:Yb3+, Er3+. Figure 5. Photoinduced I-t curves (a) and I-V curves (b) of TiO2 NRAs / TiO2 and TiO2 NRAs / TiO2:Yb3+, Er3+ Figure 6. (a) Spectrum of solar radiation (b) The excitation light of TiO2:Yb3+, Er3+ (c) PL of TiO2:Yb3+, Er3+ (d) Absorption of CdSe Figure 7. UV-vis-NIR absorption spectra of CdSe / TiO2 NRAs / TiO2 and CdSe / TiO2 NRAs / TiO2:Yb3+, Er3+



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Nanorod Arrays Decorated with CdSe Using an Upconversion TiO

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