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Short Communication

Titanium dioxide nanospheres with wide spectral absorption prepared by

low-voltage plasma electrolysis

Zhan Wu, Zhi-Kun Zhang, Deng-Zhu Guo ⇑

Key Laboratory for the Physics and Chemistry of Nanodevices, Department of Electronics, Peking University, Beijing 100871, People’s Republic of China

a r t i c l e i n f o

 Article history:

Received 28 February 2012

Accepted 9 October 2012

Available online 22 October 2012

Keywords:

TiO2

Nanosphere

Optical absorption

Plasma electrolysis

a b s t r a c t

TiO2   nanospheres with diameters mostly in the range of 20–200 nm are prepared by using cathodic

plasma electrolysis at low voltage of 70 V. It is found that the low voltage could efficiently depress the

particle sizes and their distribution, and result in more anatase phases. The nanospheres have an excel-

lent optical absorption from 240 nm to 2600 nm.

 2012 Elsevier Inc. All rights reserved.

TiO2   nanomaterials are important in solar energy harvesting

and pollutant photodegradation [1–5]. But the wide band gap con-

fines their optical absorption within ultraviolet (UV) region. In or-

der to expand its absorption into visible region, many efforts havebeen done by chemical doping [1–3] and surface sensitization [6–

8]. These strategies have been proven to be effective in enhancing

the visible absorption. Nevertheless, more effective and cost-

efficient strategies are still highly expected to expand the optical

absorption of TiO2   itself into the full solar spectrum.

Oxygen vacancies in metal oxides are known to induce elec-

tronic states within the band gap and result in absorption of low

energy photons [1,2,9,10]. The coexistence of multiphases of TiO2

could also expand the optical absorption [11]. We recently synthe-

sized TiO2 nano-/micro-spheres by using cathodic plasma electrol-

ysis, which generates abundant oxygen vacancies in TiO2   and

results in an excellent absorption from 240 nm to 2600 nm  [12].

However, the diametric distribution of the particles is too wide

from nanometers to tens of micrometers, limiting their applica-tions. Here we will focus on the preparation of TiO2  nanospheres

by using low-voltage plasma electrolysis.

The preparation of the TiO2 nanospheres was performed using a

plasma electrolysis technique as described in Ref.   [12]. Here we

added two steps to improve the diametric distribution of the par-

ticles. Firstly, the Ti wire was electropolished in a solution of per-

chloric acid, n-butyl alcohol, and ethanol (volume ratio 1:6:9)

under 20 V DC potential, and then used as the cathode. Secondly,

the electrolyte (3 M NH4NO3   aqueous solution) for the plasma

electrolysis was heated to   70 C, so that the voltage for plasma

ignition was lowered to   70 V (while room temperature ignition

requires 90 V). During the plasma electrolysis process, the solution

gradually became turbid, and the particles in the suspension werethen purified for characterization.

At the beginning, we found that there are a few big particles as

large as tens micrometers in the SEM observation. We supposed

that they were formed due to the locally enhanced Joule heating

at surface roughness. The tips of the surface roughness were

melted off and then oxidized to form big particles. It is found that

adding electropolishing step can effectively depress the big parti-

cles. It is also found that lowering voltage can dramatically im-

prove the diametric distribution of particles. Shown in   Fig. 1a

and b are typical SEM images of the particles prepared at 90 V

and 70 V, respectively. Spheres with diameter of several microme-

ters are obvious in Fig. 1a, but in Fig. 1b, the particle sizes and their

distribution are highly depressed (mostly in 20–200 nm).

Two XRD patterns are shown in Fig. 2, in which the upper andbottom spectra were obtained from the nanospheres (70 V) and

the nano-/micro-spheres (90 V), respectively. In both cases, three

phases of TiO2, i.e., rutile, anatase and oxygen-deficient phases,

can be identified. The diffraction peaks are labeled according to

their crystal indices. A close comparison can find some differences

between the two cases. Firstly, the anatase (101) peak in the upper

spectrum is stronger than the rutile (110) peak, implying more

anatase phase has been formed during low voltage plasma electrol-

ysis. On the contrary, in the bottom spectrum the rutile (110) peak

is stronger than the anatase (101) peak, meaning the existence of 

more rutile phase. Secondly, broadened peaks in the upper spec-

trum imply that most particles are very small, according to

0021-9797/$ - see front matter   2012 Elsevier Inc. All rights reserved.http://dx.doi.org/10.1016/j.jcis.2012.10.016

⇑ Corresponding author. Fax: +86 10 62765112.

E-mail address:  guodz@pku.edu.cn (D.-Z. Guo).

 Journal of Colloid and Interface Science 392 (2013) 463–464

Contents lists available at SciVerse ScienceDirect

 Journal of Colloid and Interface Science

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Scherrer equation. This is in good agreement with the SEM obser-

vation. The oxygen-deficient phases can be assigned to Ti10O19,

Ti5O9  or Ti3O5, etc., in which some oxygen atoms are absent com-

paring with stoichiometric TiO2. These phases were formed due to

the insufficient oxidization during the plasma electrolysis.

The optical absorption spectrum of the as-prepared TiO2  nano-

spheres was measured in the wavelength range between 240 nm

and 2600 nm (Fig. 3). For comparison, the absorption of Degussa

P25 was also measured in the same condition. One can see that

the P25 only absorbs photons within a wavelength below

400 nm, while the synthesized TiO2  nanospheres exhibit a strong

absorption in the full wavelength range, similar to the nano-/mi-

cro-spheres reported previously  [12]. The insets in   Fig. 3 are the

digital images showing the bright white color of P25 and black col-

or of TiO2   nanospheres, respectively, in good agreement with the

optical absorption measurement. The black color comes from the

excellent wide-spectral absorption, which originates from the oxy-

gen vacancies-mediated defect states within the band gap of TiO2,

as in our previous report  [12].

In summary, TiO2 nanospheres are synthesized via plasma elec-

trolysis at a low voltage of 70 V. The diametric distribution and

phase composition have been changed due to the improvement

of synthesis process, while the wide spectral absorption is retained.

The oxygen vacancies-mediated defect states within the band gap

of TiO2  are supposed to be the mechanism.

 Acknowledgment

This work was financially supported by the Natural Science

Foundation of China (Grant No. 60971002).

References

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Fig. 1.   SEM images of TiO2  (a) nano-/micro-spheres synthesized at 90 V and (b) nanospheres synthesized at 70 V.

Fig. 2.   XRD spectra of the TiO2   spheres, the upper and bottom curves are for

samples synthesized at 70 V and 90 V, respectively.

Fig. 3.   Optical absorption spectrum of TiO2   nanospheres compared with that of 

commercial P25.

464   Z. Wu et al. / Journal of Colloid and Interface Science 392 (2013) 463–464