A novel Fe(OH)3/TiO2 nanoparticles and its high

photocatalytic activity

Nan Wang, Li Hua Zhu *, Jing Li, He Qing Tang *

Department of Chemistry and Chemical Engineering, Hubei Key Lab of Bioinorganic Chemistry and Materia Medica,

Huazhong University of Science and Technology, Wuhan 430074, China

Received 7 June 2007

Abstract

A very simple and controllable approach was proposed to synthesize novel Fe(OH)3/TiO2 nanoparticles. Compared with neat

TiO2, the Fe(OH)3/TiO2 increased the rate of the photocatalytic degradation of methyl orange at pH 6.0 by more than five times,

showing photocatalytic activity as excellent as P25. This enhancing effect is mainly attributed to the ferric hydroxide deposits as the

electron scavenger and the enriched surface hydroxyl groups.

# 2007 Published by Elsevier B.V. on behalf of Chinese Chemical Society.

Keywords: Titanium dioxide; Surface modification; Ferric hydroxide; Photocatalysis

TiO2 photocatalysis has been intensively investigated for its application to the destruction of environmental toxic

pollutants [1–6]. Numerous works are devoted to the elaboration of highly active photocatalysts, such as surface

deposition of noble metal clusters on TiO2 [1], coupling TiO2 with other semiconductors [5] and doping with transition

metal ions. Among these techniques, doping with iron seems mostly interesting, because only cheap and harmless

reagents are used. The most prevalent explanation for the improved photocatalytic performance of FeIII-doped TiO2 is

ascribed to the formation of shallow charge trapping sites within the TiO2 matrix and on the surface of the particles

through the replacement of TiIV by FeIII ions, which can partially prevent the undesirable recombination of photo-

induced e�–h+ pairs [7]. Several approaches of synthesizing Fe-doped TiO2 photocatalysts were reported, including

co-sputtering [8], co-combustion [9], plasma evaporating [10], and wet impregnation and coprecipitation process [11].

All these methods require a calcination treatment, and are time consuming. The present work will report a simple and

tunable approach to synthesize a novel Fe(OH)3/TiO2 photocatalyst, and demonstrate its increased photocatalytic

activity through the photocatalytic degradation of a non-biodegradable azo dye methyl orange (MO).

1. Experimental

To synthesize the Fe(OH)3/TiO2 nanoparticles, 2.0 g of commercial TiO2 powder (Zhoushan Nano Company,

China) and Fe(III) salt were dispersed in an acidic aqueous solution (pH � 2) by sonicating for 5 min. An aqueous

ammonia solution was dropwise added into the mixture suspension to a final pH of 8 � 9, leading to a coating of ferric

www.elsevier.com/locate/cclet

Chinese Chemical Letters 18 (2007) 1261–1264

* Corresponding authors.

E-mail addresses: lhzhu63@yahoo.com.cn (L.H. Zhu), hqtang62@yahoo.com.cn (H.Q. Tang).

1001-8417/$ – see front matter # 2007 Published by Elsevier B.V. on behalf of Chinese Chemical Society.

doi:10.1016/j.cclet.2007.08.020

hydroxide on the surface of TiO2 nanoparticles due to the precipitation of ferric ions. After evaporated to dryness, the

obtained powders finally were vacuum dried at 50 8C for 1 h. By controlling the fraction of iron in the mixture, the

nominal iron content of the particles was varied as desired. The Fe/Ti atomic ratio in the mixture suspension was

optimized at 0.05%, and the obtained Fe(OH)3/TiO2 in such a condition is referred to as 0.05-Fe(OH)3/TiO2 which

showed the best photocatalytic activity.

The crystalline structure of the photocatalyst was investigated by using an X’Pert PRO X-ray diffractometer

(PANalytical) with a Cu Ka radiation source. FT-IR spectra were recorded on a VERTEX 70 spectrometer (Bruker) in

the transmission mode in spectroscopic grade KBr pellets for all the powders. Thermogravimetric analysis (TGA) was

carried out on a TGS-2 instrument (Perkin-Elmer) at a heating rate of 10 8C min�1 in air.

The photocatalysis was performed in a 400-mL quartz reactor, being filled with 250 mL of aqueous suspension

containing catalyst (1 g L�1) and MO (10 mg L�1). The mixture suspension was stirred continuously for 20 min

before being illuminated, to favor the organic adsorption onto the catalyst surface. A 250W high-pressure mercury

lamp (Philips) was employed as a UV light source. At given time intervals, 2 mL aliquots were sampled, immediately

centrifuged at 14,000 rpm for 15 min to remove the TiO2 nanoparticles, and then analyzed on a Cary 50 UV–vis

spectrophotometer (Varian).

2. Results and discussion

In comparison with a conventional wet impregnation and coprecipitation process [10], our method does not require

a calcination treatment. This makes our modified method be much simpler for operation, and time and energy saving.

XRD patterns indicates that both the neat TiO2 and the Fe(OH)3/TiO2 nanoparticles possessed a crystalline anatase

lattice structure. We could not observed any peaks arisen from iron species, due to the quite low content of iron in the

Fe(OH)3/TiO2 particles. By using Scherrer equation, the average diameter of the 0.05-Fe(OH)3/TiO2 particles was

calculated as 21 nm, being larger than 13 nm of neat TiO2.

Fig. 1a illustrates the FT-IR spectra of the photocatalysts. For neat TiO2, a broad band at 3500–3000 cm�1 and a

band at 1622 cm�1 correspond to fundamental stretching vibration of O–H hydroxyl groups (free or bonded) and the

bending vibration of coordinated H2O and Ti–OH, respectively [12]. The absorption at 1405 cm�1, being similar to

that at 1409 cm�1 observed for freshly prepared TiO2 by a wet process, may be related to the coordinated H2O or

surface hydroxyls. Compared with the spectrum of neat TiO2, the bands at 1405 cm�1 and 3150 cm�1 are markedly

increased in strength in the Fe(OH)3/TiO2 nanoparticles, with a shifting to lower wavenumbers by about 20 cm�1. This

suggests that there are more coordinated H2O or surface hydroxyls bounded on the surface of TiO2 with stronger

chemical bonding. When the Fe(OH)3/TiO2 nanoparticles is heated at 250 8C for 120 min, the Fe(OH)3 coating will be

converted to a Fe2O3 coating, and consequently these two absorption bands are weaken significantly in strength.

The enriched surface hydroxyls or coordinated H2O on Fe(OH)3/TiO2 were confirmed by TGA. Fig. 1b depicts the

DTG curves of the neat TiO2 and the Fe(OH)3/TiO2 nanoparticles. In the case of neat TiO2 (curve 1), only one peak

appears at about 50 8C over the tested temperature range, which is attributed to the loss of loosely bound water. In

N. Wang et al. / Chinese Chemical Letters 18 (2007) 1261–12641262

Fig. 1. (a) FT-IR spectra and (b) DTG curves of: (1) neat TiO2; (2) 0.05-Fe(OH)3/TiO2; (3) 0.05-Fe(OH)3/TiO2 heated at 250 8C for 120 min; and (4)

0.2-Fe(OH)3/TiO2.

contrast, the Fe(OH)3/TiO2 nanoparticles yield a strong peak over the range from 150 to 240 8C (curve 2), which

originates from the loss of coordinated H2O or surface hydroxyls bounded on the surface of the particles with stronger

chemical bonding. When the nominal Fe content is increased to 0.2% (curve 4), this peak increases in height,

accompanied by a slight shifting to higher temperatures, indicative of hindered losing of the coordinated H2O or

surface hydroxyls due to the increased thickness of iron(III) hydroxide layer. When the composite is annealed at

250 8C for 2 h, the peak over the range from 150 to 240 8C disappears completely (curve 3), behaving like the neat

TiO2 without the Fe(OH)3 deposition.

Photocatalytic activity of Fe(OH)3/TiO2 was determined by using MO as a model pollutant. By monitoring UV–vis

absorption spectra of the MO solution, it was found that MO was discolored over each of the used catalysts under UV

irradiation. It is easily seen from Fig. 2 that the photocatalytic degradation over the neat TiO2 was slow, proceeding

with a kinetic behavior of pseudo-first-order reaction as ln(c/c0) = �0.00209t � 0.0418 (regression coefficient

R = 0.997). In contrast, the as-prepared 0.05-Fe(OH)3/TiO2 catalyst significantly increases the MO photodegradation,

following a pseudo-zero-order reaction as c/c0 = �0.00782t + 0.992 (R = 0.999). In consideration of different

dimensions of the rate constants, we will use the half-time (t1/2) of MO (the time required for one-half of MO to be

degraded) to compare the photocatalytic ability. For example, in 10 mg L�1 MO solutions at pH of 6.0, t1/2 was

decreased from 331.6 min over the neat TiO2 to 63.9 min over 0.05-Fe(OH)3/TiO2, even smaller than 72.6 min over

P25. This implies that Fe(OH)3 deposition increases the rate of MO degradation by more than five times, showing

photocatalytic ability as excellent as Degussa P25, a well-studied and highly efficient photocatalyst as a control.

Since heterogeneous photocatalytic reactions take place on the surface, the surface properties of TiO2 such as

surface hydroxyl groups, particle size, crystalline phase, surface metal deposits and the surface adsorbed ions play a

critical role in determining the efficiencies of the photocatalytic reaction [1–3,13]. As discussed in the XRD study,

these is no change in the crystalline phase of TiO2 after 0.05% Fe(OH)3 deposition and 0.05-Fe(OH)3/TiO2 exhibits

larger particle size, resulting in reducing the BET specific surface area from 180.9 m2 g�1 for neat TiO2 to

121.6 m2 g�1, being unfavorable to the increasing photocatalytic activity. With the aid of FT-IR and TGA

measurements, it is worthy noting that the Fe(OH)3 deposition on the TiO2 surface can increase significantly the

amount of surface hydroxyls and coordinated H2O, which will be lost by heating the Fe(OH)3/TiO2 at 250 8C for 2 h.

After the 0.05-Fe(OH)3/TiO2 nanoparticles was heated, as expected, the enhancing effect of the Fe(OH)3-deposited

catalyst was lost mostly (curve 3 in Fig. 2). This indicates that the increased surface hydroxyl groups or coordinated

H2O take an important role in the enhanced photocatalytic activity. However, the MO degradation over the heated

0.05-Fe(OH)3/TiO2 is faster than over the neat TiO2, due to the photo-induced electron trapping by the Fe3+ deposition

(Fe3+ + e� ! Fe2+), which reduces the electron-hole recombination and the produced Fe2+ initiates the Fenton

reaction, resulting in acceleration of �OH radicals generation.

Furthermore, the Fe(OH)3/TiO2 was found to have a good life time during its service. We carried out the

photocatalytic degradation of 10 mg L�1 MO at pH 6.0 in successive batches. For each run, the added MO was almost

completely degraded, then 10 mg L�1 of MO was freshly added and the next photodegradation cycle started. The t1/2

N. Wang et al. / Chinese Chemical Letters 18 (2007) 1261–1264 1263

Fig. 2. Kinetics of MO degradation over: (1) neat TiO2; (2) 0.05-Fe(OH)3/TiO2; (3) 0.05-Fe(OH)3/TiO2 heated at 250 8C for 120 min; and (4) P25 in

10 mg L�1 MO solutions at pH 6.0.

values were obtained as 63.9, 64.2, 66.2, 65.6, 69.5 and 69.2 min for the six cycles. The almost constant t1/2 values

demonstrates that the Fe(OH)3/TiO2 composite catalyst have fairly good photochemical stability.

In conclusion, we have prepared novel Fe(OH)3/TiO2 nanoparticles, whose important merits include outstanding

photocatalytic activity, easiness of preparation, and excellent life time. It has been confirmed that the enhanced effect

of Fe(OH)3 deposition is mainly attributed to the enriched surface hydroxyl groups and the Fe3+ function as an electron

scavenger. We have also used a similar route to prepare Cu(OH)2/TiO2 as efficient photocatalysts. The novel M(OH)X/

TiO2 composites may favor the actual applications in the rapid photocatalytic treatment of the biorefractory

compounds.

Acknowledgments

Financial supports from the National Natural Science Foundation of China (Nos. 30571536 and 20677019) are

gratefully acknowledged. The Center of Analysis and Testing of Huazhong University of Science and Technology is

thanked for characterization of Fe(OH)3/TiO2 catalysts.

References

[1] V. Subramanian, E. Wolf, P.V. Kamat, J. Phys. Chem. B 105 (2001) 11439.

[2] N. Wang, Z. Chen, L. Zhu, X. Jiang, B. Lv, H. Tang, J. Photochem. Photobiol. A: Chem. 191 (2007) 193.

[3] L. Wang, N. Wang, L. Zhu, H. Yu, H. Tang, J. Hazard. Mater., 2007, doi:10.1016/j.jhazmat.2007.06.063.

[4] X. Shen, L. Zhu, J. Li, H. Tang, Chem. Commun. (2007) 1163.

[5] H. Tada, Y. Konishi, A. Kokubu, S. Ito, Langmuir 20 (2004) 3816.

[6] J. Li, L. Zhu, Y. Wu, Y. Harima, A. Zhang, H. Tang, Polymer 47 (2006) 7361.

[7] J.A. Wang, R. Limas-Ballesteros, T. Lopez, A. Moreno, R. Gomez, O. Novaro, X. Bokhimi, J. Phys. Chem. B 105 (2001) 9692.

[8] J.O. Carneiro, V. Teixeira, A. Portinha, L. Dupak, A. Magalhaes, P. Coutinho, Vacuum 78 (2005) 37.

[9] B. Neppolian, H.S. Jie, J.-P. Ahn, J.-K. Park, M. Anpo, Chem. Lett. 33 (2004) 1562.

[10] X.H. Wang, J.-G. Li, H. Kamiyama, M. Katada, N. Ohashi, Y. Moriyoshi, T. Ishigaki, J. Am. Chem. Soc. 127 (2005) 10982.

[11] J.A. Navıo, G. Colon, M. Trillas, J. Peral, X. Domenech, J.J. Testa, J. Padron, D. Rodrıguez, M.I. Litter, Appl. Catal. B: Environ. 16 (1998) 187.

[12] Y. Gao, Y. Masude, K. Koumoto, Chem. Mater. 16 (2004) 1062.

[13] K. Nagaveni, G. Sivalingam, M.S. Hegde, G. Madras, Environ. Sci. Technol. 38 (2004) 1600.

N. Wang et al. / Chinese Chemical Letters 18 (2007) 1261–12641264