J. Cent. South Univ. Technol. (2010) 17: 218-222

DOI: 10.1007/s11771-010-0033-3                                                                                                                

Photogenerated carrier transfer mechanism and photocatalysis properties of TiO₂ sensitized by Zn(¢ò) phthalocyanine

LI Li(ÀîÀö)^{1, 2}, XIN Bai-fu(ÐÁ°Ø¸£)^{1, 2}

1. Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education,

Heilongjiang University, Harbin 150080, China

2. School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China? Central South University Press and Springer-Verlag Berlin Heidelberg 2010

Abstract: The Zn(¢ò) phthalocyanine sensitized TiO₂ (ZnPc-TiO₂) nanoparticles were prepared by hydrothermal method via impregnation with ZnPc. The as-prepared photocatalysts were characterized by X-ray diffractometry (XRD) and diffuse reflectance spectroscopy (DRS), and the surface photovoltage spectroscopy (SPS) and photocatalytic degradation of rhodamine B (RhB) were studied under illuminating. The experimental results indicate that TiO₂ sensitized by ZnPc extends its absorption band into the visible region effectively, and the sensitized TiO₂ has higher activity than TiO₂ (Degussa P-25) under the simulated solar light and the visible light. Based on the DRS and SPS results, the mechanism about the photogenerated carrier transfer between TiO₂ and ZnPc is proposed. At a lower ZnPc content (¡Ü0.20 ?mol/g), ZnPc monomer acts as the electron donor, which provides the photoinduced electrons to the conduction band of TiO₂. These photoinduced electrons can transfer to molecular oxygen (O₂), leading to the formation of active species, such as superoxide/hydroxide radicals and singlet oxygen, which is beneficial to the photocatalytic reaction. While at a higher ZnPc content (£¾0.20 ?mol/g), the formation of ZnPc dimer results in the decrease of photocatalytic activities of ZnPc-TiO₂ photocatalyst.

Key words: Zn(¢ò) phthalocyanine (ZnPc); TiO₂; nanoparticles; photocatalyst; sensitization; photodegradation; mechanism

1 Introduction

The utilization of TiO₂ as a catalyst for photodegradation of organic pollutants in water is a relevant topic in view of a possible application in economically advantageous and environmental friendly process [1-2]. However, these photocatalysts are generally materials with a wide band gap, thus they can only absorb a small fraction of visible light. Coupled semiconductor systems, such as organic functional dye/TiO₂, which can extend its absorption band into the visible region, are used to overcome this defect [3-5]. Phthalocyanine was found to have many applications. This group of compounds have a strong absorption in the visible/near-IR region [6], a good photostability and a low probability of desorption from the semiconductor surface once adsorbed, which make them interesting candidates [7-8].

Some photocatalysts were reported, such as iron, copper, cobalt and metal-free phthalocyanine with carboxylic acid or sulfonic acid groups and without any functional group, which were efficient in photocatalytic reactions used for the degradation of organic pollutants in water [9-11]. In these research reports, the mechanisms of TiO₂ photocatalysis reactions are almost the same, ILIEV and TOMOVA [11] thought that the high photocatalytic activity of the sample is explained by the realization of an electron transfer from the conduction band of the excited phthalocyanine particle to the conduction band of TiO₂ support. The increase of the quantum yield of the redox process results from the additional formation of superoxide radical on the TiO₂ conduction band [11].

These reports, however, mainly focus on the investigation of reaction mechanism via detection of intermediate active species, which lack the sufficient evidences in the photogenerated charge carrier transfer. In this work, the mechanism about the photogenerated carrier transfer between TiO₂ and ZnPc was studied using diffuse reflectance spectroscopy (DRS) and surface photovoltage spectroscopy (SPS).

2 Experimental

2.1 Reagents

Zinc (¢ò) phthalocyanine was purchased from Acros Organics (USA) with a purity of 98%. Titanium dioxide (TiO₂, Degussa P-25) was used as a comparison photocatalyst. Degussa P-25 consisted of 75% anatase and 25% rutile with a specific BET-surface area of 50 m²/g and a primary particle size of 20 nm. Other reagents were all analytical grade compounds and were used without any further purification.

2.2 Preparation of ZnPc-TiO₂ photocatalyst

The synthesis of TiO₂ particles followed the procedure reported by HAO et al [12] and was slightly modified. A mixture of 40 mL tetrabutyl titanate and   10 mL isopropanol was slowly added into 250 mL deionized H₂O containing 2 mL nitric acid solution at a rate of 1-2 drop/s under cooling and stirring. After refluxing at 80 ¡æ for 4 h, the reactor was cooled to room temperature, and the pH value of the suspension was adjusted to 10 using NH₃¡ÁH₂O. The suspension was then transferred into a stainless steel Teflon-lined autoclave of 50 mL capacity and heated at 180 ¡æ for 8 h without shaking or stirring during the heating. After the autoclave was naturally cooled to room temperature, the obtained sample was sequentially washed with deionized water and absolute ethanol several times. Then, anatase phase TiO₂ nanoparticles with white color were obtained.

As-prepared TiO₂ nanoparticles were impregnated with various amounts of ZnPc (0, 0.05, 0.15, 0.20, 0.30, and 0.40 ?mol/g TiO₂) dissolved in N-methyl-2- pyrrolidone for 24 h in the dark, dried at 100 ¡æ, and ground to obtain ZnPc-TiO₂ nanoparticles.

2.3 Characterization of samples

The surface photovoltage spectroscopy (SPS) instrument was assembled at Jilin University, China, and monochromatic light was obtained by passing light from a 500 W xenon lamp (CHF-XQ500W, China) through a double prism monochromator (SBP300, China). The slit widths of entrance and exit were 2 and 1 mm, respectively. A lock-in amplifier (SR 830, USA), synchronized with a light chopper (SR 540, USA), was employed to amplify the photovoltage signal. The powder sample was sandwiched between two ITO glass electrodes. The DRS patterns were obtained in the wavelength range of 300-700 nm using a Shimadzu UV-2550 UV-Vis spectrophotometer equipped with the integrating sphere accessory for diffuse reflectance spectra. BaSO₄ was used as a reference.

2.4 Evaluation of photocatalytic activity of ZnPc-TiO₂

The photocatalytic degradation of rhodamine B (RhB) over ZnPc-TiO₂ was carried out in a home-built reactor. A 160 W high-pressure mercury lamp with and without a 410 nm cutoff filter was used as the visible light source and the simulated solar light source, respectively. In each run, 0.1 g ZnPc-TiO₂ catalyst was added into 20 mL RhB solution of 10 mg/L. After the mixture was premixed in the dark for 20 min, the light was turned on to initiate the reaction for 30 min. A Shimadzu UV-2550 UV-Vis spectrophotometer was used to determine the concentration of RhB solution before and after photocatalytic degradation.

3 Results and discussion

3.1 DRS analysis

Fig.1 shows DRS patterns of pure TiO₂ and ZnPc-TiO₂ particles adsorbing different ZnPc contents. The absorption edge can be calculated by inflection point of UV spectra. Table 1 lists the shift values of UV spectra absorption edge. From Fig.1 and Table 1, a novel blue shift of TiO₂ absorption edge can be found, which gradually increases with the increase in the content of ZnPc from 0 up to 0.20 ?mol/g (vs TiO₂), and then drops down.

Fig.1 DRS patterns of ZnPc-TiO₂ photocatalysts

Table 1 Shift values of UV spectrum absorption edge of ZnPc-TiO₂ with different contents of sensitizer

3.2 SPS measurement

The surface photovoltage (SPV) method is a well- established contactless technique for the characterization of semiconductors. It can offer important information about semiconductor surface, interface, and bulk properties, mainly reflecting the carrier separation and transfer behavior with the aid of light [13].

The SPS principle can be explained by the top left corner of Fig.2. The absorbed photons induce the formation of free carriers by creating electron-hole pairs via band-to-band transitions. The photoinduced electrons may be transferred from the surface to the bulk, and the photogenerated holes can be moved to the surface under the built-in electric field. Thus, the surface potential is changed. The variational value of surface potential (¦¤V) is the signal of SPS.

Fig.2 Schematic diagram of photogenerated carrier transfer mechanism between TiO₂ and ZnPc (CB and VB are energy levels of conduction band and valence band, respectively; G is energy level of ground state; E_s is singlet energy level; E_T is triplet energy level; E¡ä and E¡å are singlet band split energy levels, respectively)

Fig.3 shows the SPS spectra of pure TiO₂ and ZnPc-TiO₂ with various ZnPc contents. It can be found that the SPS spectrum of pure TiO₂ is different from that of sensitized TiO₂. The SPS intensity decreases with the increase in sensitizer ZnPc content from 0 up to 0.20 ?mol/g, and then increases.

Fig.3 SPS spectra of pure TiO₂ and ZnPc-TiO₂

3.3 Mechanism of photoinduced carrier transfer

The above experimental results indicate that the shift value of UV spectrum absorption edge and the intensity of SPS of ZnPc-TiO₂ vary with the increase in the amounts of ZnPc adsorbed on the surface of TiO₂ nanoparticles. The substrates are the same TiO₂ nanoparticles. So, the fact derives from the variety of the amounts of ZnPc adsorbed on the surface of TiO₂ nano- particles. ZnPc contains a planar configuration of ¦Ð-conjugated Pc ring structure and tends to aggregate into dimer, so-called H-aggregate, which is commonly seen in many phthalocyanine systems [14]. In this work, the mechanism about the photogenerated carrier transfer between TiO₂ and ZnPc is proposed based on the UV-Vis and SPS experimental results as well as the nature of ZnPc (shown in Fig.2). At a rather lower ZnPc content (¡Ü0.20 ?mol/g), ZnPc molecules adsorbed on the surface of TiO₂ may exist as the monomer form. Because E_s, the energy level of the singlet of ZnPc, is located above E_CB of TiO₂, as shown in Fig.2. While the surface of ZnPc-TiO₂ is irradiated by the light (h¦Ô¡ÝE_g), the photoexcited electrons of ZnPc can be easily transferred into the conduction band of TiO₂ semiconductor, resulting in the increase of E_g of TiO₂ and variational value of surface potential of TiO₂ before and after the decrease of illumination, so the blue shift of absorption edge and descend of SPS intensity (i.e., ¦¤V¡ä£¼¦¤V) can be observed. At a higher ZnPc content (£¾0.20 ?mol/g), a fraction of ZnPc molecules adsorbed on the surface of TiO₂ may aggregate to form ZnPc dimer [14]. The overlap of the intermolecular- conjugated electron orbital within ZnPc dimer leads to a band split. Because transitions from the ground state to the excited state E¡ä are forbidden while transitions from the ground state to the excited state E¡å are allowed [14], electrons of the ground state can be directly excited to the excited state E¡å. Due to rapid internal conversion between singlet states, the excited electron in the excition state E¡å transfers to the excition state E¡ä quickly. E¡ä is located under E_cb of TiO₂ semiconductor, resulting in the fact that ZnPc dimer cannot provide photoinduced electrons to TiO₂. Hence, the photoinduced electrons transferring from the excited state E_s of ZnPc to the conductor band of TiO₂ reduce, the blue shift may decrease and the intensity of SPS may increase with increasing ZnPc content.

3.4 Photocatalytic reactions

Fig.4 shows the photocatalytic degradation ratio curves of RhB vs content of ZnPc under simulated solar light. It can be found that the degradation ratio of RhB increases with the increase of the ZnPc content from 0 up to 0.20 ?mol/g, and then drops down. This fact can be explained by the mechanism of photoinduced carrier transfer between TiO₂ and ZnPc mentioned above.

Fig.4 Photocatalytic degradation ratio of RhB vs content of ZnPc

At a lower ZnPc content (¡Ü0.20 ?mol/g TiO₂), ZnPc monomer acts as the electron donor, which provides the photoinduced electrons to the conduction band of TiO₂. These photoinduced electrons can transfer to molecular oxygen (O₂), leading to the formation of active specials, such as superoxide/hydroxide radicals and singlet oxygen in the system. These active elements can degrade the pollution [15]. The number of photoinduced electrons increases with the increase in ZnPc content, so the photocatalytic activity of photocatalysts increases with the increase of ZnPc content.

While at a higher ZnPc content (£¾0.20 ?mol/g), the formation of ZnPc dimer on the surface of TiO₂ leads to the decrease of photoinduced electrons transferring from the excited state E_s of ZnPc to the conductor band of TiO₂, so photocatalytic activity is reduced. The photocatalysis experimental results are consistent with the conclusion of SPS and DRS test.

To examine the stability of as-prepared photocatalysts, the repetition experiments were carried out at the 0.2 ¦Ìmol/g ZnPc-TiO₂ under the same condition mentioned above. Fig.5 shows that the degradation ratio of RhB is stable during circulating photodegradation test.

Fig.5 Stability of ZnPc-TiO₂ photocatalyst

Fig.6 shows the photodegradation ratio of RhB to P-25 and 0.2 ¦Ìmol/g ZnPc-TiO₂ powders under different light sources. From Fig.6, it can be found that the sensitized TiO₂ has a higher activity than P-25 under the simulated solar light and the visible light. Herein, ZnPc plays an important role in the system. The experimental results indicate that TiO₂ sensitized by ZnPc extends its absorption band into the visible region effectively and has an excellent activity.

Fig.6 Comparison of photocatalytic activity of P-25 and    0.2 ¦Ìmol/g ZnPc-TiO₂

4 Conclusions

(1) ZnPc-TiO₂ nanoparticles are prepared by the impregnation method. The absorption band of TiO₂ extends into the visible region effectively. The photocatalytic activity is the highest when the ZnPc content on the surface of TiO₂ is 0.20 ?mol/g, and the photocatalytic activity is higher than that of P-25 under the simulated solar light and the visible light.

(2) A novel blue shift of absorption edge and the SPS intensity varieties of TiO₂ sensitized by ZnPc are observed. The mechanism about the photogenerated carrier transfer between TiO₂ and ZnPc is proposed.

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Foundation item: Project(20431030) supported by the National Natural Science Foundation of China; Project(2006RFQXS096) supported by the Foundation for Science and Technology Innovation Talents of Harbin, China; Project(1152Z002) supported by the Key Projects of Educational Department of Heilongjiang Province, China; Project(LBH-Q07111) supported by Heilongjiang Postdoctoral Funds for Scientific Research Initiation

Received date: 2009-05-16; Accepted date: 2009-12-20

Corresponding author: XIN Bai-fu, PhD, Professor; Tel: +86-451-86608781; E-mail: xinbf@263.net

(Edited by CHEN Wei-ping)