Black TiO2 Nanoparticles by a Microwave-induced Plasma
over Titanium Complex Aqueous Solution
Yohei Ishida, Wataru Doshin, Hiroki Tsukamoto, and Tetsu Yonezawa*
Division of Material Science and Engineering, Faculty of Engineering, Hokkaido University,
Kita 13 Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628(E-mail: tetsu@eng.hokudai.ac.jp)

We report a novel synthesis of black TiO2 nanoparticles
by using a microwave-induced plasma over Ti complex
aqueous solution. Detailed HAADF-STEM analysis
revealed that this plasma method enables doping W atom
inside the TiO2 crystal structure, resulting in strong
absorption in visible light regions.

REPRINTED FROM

Vol.44 No.10

2015 p.1327–1329
CMLTAG
October 5, 2015

The Chemical Society of Japan

Received: June 1, 2015 | Accepted: June 27, 2015 | Web Released: July 4, 2015

CL-150531

Black TiO2 Nanoparticles by a Microwave-induced Plasma
over Titanium Complex Aqueous Solution
Yohei Ishida, Wataru Doshin, Hiroki Tsukamoto, and Tetsu Yonezawa*
Division of Material Science and Engineering, Faculty of Engineering, Hokkaido University,
Kita 13 Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628(E-mail: tetsu@eng.hokudai.ac.jp)
We herein report a novel synthesis of black TiO2 nanoparticles by using a microwave-induced plasma over Ti complex
aqueous solution. The resultant TiO2 nanoparticles showed verystrong reﬂectance over the whole visible light region. Detailed
HAADF-STEM analysis of the obtained particles revealed that
this plasma method enables doping of W ions inside the TiO2
crystal structure.
Titanium dioxide (TiO2) has been the focus of many researchﬁelds during the past several decades due to its scientiﬁc andapplication importance,1­5 especially photocatalysis. TiO2 is acommercially available photocatalyst that can decompose wateror organic pollutants upon its excitation by sunlight. Many
preparation methods of TiO2 materials have been investigated. As
for anatase TiO2, hydrothermal methods using amorphous TiO2,6TiCl4,7,8 or TiOCl2 aqueous solution,9 and sol­gel methods usingtitanium alkoxides10­12 are worth noting. However, these methods
generally require complicated large equipment and synthesis ofTiO2 nanocomposite by these methods is still limited.
Recently, a synthesis of metal nanocomposites by plasma inliquid has been extensively investigated. While the radio wavelength region (usually 13.56 MHz) has been used as the main
source in the last decade,13 very recently the microwave region(2.45 GHz) was newly used as a more easily available powersource.14 The microwave-induced plasma is easily produced byrelatively small equipment according to the high frequency ofmicrowave. During the plasma production, bubbles continuously
form in liquid and are necessary for generating the plasma because
plasma is apparently generated inside bubbles or around thebubble boundary in the liquid as reported in a previous paper.15
We and other groups have recently reported the synthesisof Au,16,17 Ag,18 Pt,19 and ZnO15,20 nanoparticles by using a
microwave-induced plasma in aqueous solution containing metalions. The mechanism for the metal nanoparticle formation isattributed to the reaction of metal ions with H• (hydrogen radicals)that come from the decomposed water by plasma. According tothese backgrounds, we herein report a novel synthesis of TiO2
nanoparticles by using a microwave-induced plasma over watersoluble Ti complex aqueous solution. The series of Ti complexes
have not been water-soluble and very recently Kakihana’s grouphas reported a novel water-soluble Ti complex.21,22
Water-soluble titanium complex was synthesized according toprevious reports (see Supporting Information for the method).21,22
The synthesis of TiO2 nanoparticles by microwave-induced
plasma in water was carried out by the method outlined in SI.
38.3 mg of black powder was obtained, and longer reaction timeor larger plasma energy will result in better synthetic yield. It iswell known that TiO2 changes their color gradually from white(pure TiO2) to light yellow, green, dark blue, and black according
Chem. Lett. 2015, 44, 1327–1329 | doi:10.1246/cl.150531

to the diﬀerent amount of oxygen defects in their structure.23
Judging from the black color of our TiO2 powder, it is estimatedthat our TiO2 partially consists of TiOx (x < 2).
First, the obtained black powder was characterized by XRF
as summarized in Table S1. In the as-prepared sample, Ti wasmainly detected (97.7 atom %), however a little tungsten (W,ca. 1.4 atom %) was also found. This tungsten component wasa contaminant from the tungsten electrode used in the plasma
equipment, as we reported previously.19 To obtain pure Ti
components and accurately investigate the characteristics, we
examined removal the W components by alkaline solution. It isknown that nanocomponents of W are easily oxidized and theresultant WO3 can be dissolved in concentrated aqueous NaOH.24
We introduced 30 cm3 of NaOH solution (ca. pH 14) into 20 mgof black TiO2 powder and stirred overnight. It was ﬁltered andrepeatedly washed with deionized water (>18 M³). The W
component in the sample after the alkaline treatment decreased
drastically (Table S1), thus we successfully puriﬁed the blackTiO2 powder.
The black TiO2 powder was characterized by XRD(Figure 1). Most importantly, the characteristic peaks of TiO2were observed at 24­30 and 52­58 degrees. Judging from theirbroad shapes, it can be considered that the resultant TiO2 powder
is not a monocomponent, and it consists of various kinds ofTiO2 including anatase, rutile, brookite, and TiOx (x < 2). Forexample, sharp peaks at 25.3, 53.9, and 55.1 degrees corresponded to anatase, and rutile was observed at 36.1 degree.
Rather small peaks at 25.3 and 25.7 degrees matched brookite.
Furthermore, small peaks at 20.8, 29.1, 31.7, and 40.9 degrees
did not match TiO2 crystals, and correspond to O-defective

Figure 1. XRD patterns before and after the alkaline treatment.

© 2015 The Chemical Society of Japan | 1327

Figure 2. TEM images and size-distribution histograms of black TiO2 powder after the alkaline treatment (a). Small (upper) and large (lower)
nanoparticles were counted separately. HAADF-STEM images and STEM-EDX mapping proﬁles of black TiO2 powder before (b) and after (c) thealkaline treatment.

titanium oxides (Ti9O17 and Ti4O7). Thus, the formation of TiO2
as well as O-defective TiOx was revealed by these XRD proﬁles.
As noted before, black TiO2 basically indicates a large amountof oxygen defects.23 Plausible reasons for the formation of Odefected black TiO2 are, (i) the H• generated by the decomposition of water by plasma reduces the nucleation stage of TiO2,and (ii) the extremely quick refrigeration from plasma temperature (several thousands °C) to liquid temperature (several tens°C) suppresses the bond-formation with O atoms and complete
crystallization. XRD peaks of O-defective TiOx (at 20.8, 29.1,31.7, and 40.9 degrees) disappeared and the color turned ashy
after sintering at 500 °C under air, indicating oxidation (seesample images, XRD patterns, detailed experimental condition inFigure S1). A previous report on liquid plasma has also shown aformation of O-defective TiOx by using Ti wire as a precursor.25
They have observed a XRD peak of O-defective TiOx at32 degree that was also detected in the current work. Synthesisof visible-light responsive black TiO2 nanoparticles is now
attracting much attention as a future photocatalysis utilizes wide
wavelength regions of sunlight. Our liquid plasma method thus
will widen the spectrum of the preparation/modiﬁcation strategy
for functionalized TiO2 nanoparticles. Characteristic peaks of
α-W (at 40.3 degree) and β-W (at 39.9 and 43.9 degrees) wereobserved in TiO2 powder before alkaline treatment, anddisappeared after. These XRD patterns also support the successful puriﬁcation by alkaline treatment as described in Table S1.
Figure 2a shows TEM images of our black TiO2 powder after
the alkaline treatment. The size was not uniform, and consists ofvery small particles (average diameter of 5.6 « 1.4 nm) as well aslarge ones (90 « 64 nm). Since we observed small nanoparticles
below 10 nm and larger ones over 30 nm, two size distribution
histograms were obtained. We consider that this large particle size
distribution is attributed to the variety of bubble sizes formed
during the plasma. In liquid plasma system, inside or around thebubble boundary is the “reaction ﬁeld” of plasma. We synthesized
TiO2 nanoparticles in various sizes of bubble ﬂasks at various
total amounts of precursors. The very high temperature of plasma
immediately decomposed the Ti complex and the surrounding
water quickly refrigerated formed Ti nanocomponent. Thus, weobtained the various sizes of TiO2 particles.
To quantitatively investigate the elemental composition of
the obtained nanoparticles, high-angle annular dark-ﬁeld scan-1328 | Chem. Lett. 2015, 44, 1327–1329 | doi:10.1246/cl.150531

ning transmission electron microscopy (HAADF-STEM) andenergy-dispersive X-ray spectroscopy (STEM-EDX) were carriedout. HAADF-STEM method enables observation of the chemical
contrast according to the atomic number (Z). In the present case,the atomic number of 22Ti is much smaller than that of 74W, thus
the contaminating W components just after the plasma synthesis
can be detected. STEM-EDX directly characterizes the elemental
compositions at an angstrom order.
A HAADF-STEM image in Figure 2b (before alkalinetreatment) shows the presence of large particles with low-contrast
as well as small particles with high-contrast. STEM-EDX
analysis for this image region revealed that the larger particle
mainly consists of Ti and O, and smaller ones consist of W.
Since the elemental mapping of O atoms did not overlap that ofW, it is estimated that the small particles are metal W, not WO3 orother tungsten oxides. On the other hand, the sample after thealkaline treatment (Figure 2c) did not show such high-contrast
small particles and W component from HAADF-STEM image
and EDX-mapping, respectively. Thus, we can conclude that theobtained black powder is certainly composed of titanium oxides,
and the alkaline treatment successfully removed the contaminated
W components. This result supports XRF (Table S1) and XRD(Figure 1). It should be noted that we still observed the weak, butenough strong W components even after the alkaline treatment(Figure 2c). The counts of W mapping were rather stronger thanthe backgrounds and were overlapped the whole region of theparticle. Therefore it is expected that W atoms were doped inside
the TiO2 crystal structure from the electrode of the plasma
equipment.
Figure 3 shows the high-magniﬁcation HAADF-STEM
image of the black TiO2 powder after the alkaline treatment.
Two kinds of lattice spacing of 3.52 and 3.45 ¡ were observed.
These values correspond to {120} and {111} lattices of brookitetype TiO2. Moreover, the angle between these two lattice spacing
was 82.9°. This value matches well to the theoretically calculated
angle between (120) and (111) lattices of brookite TiO2 (82.94°).
Thus, we can conclude that the particle shown in Figure 3 is
identiﬁed as the brookite-type TiO2. Other crystal structures suchas rutile were also observed in diﬀerence particles.
Very small bright spots were observed as indicated by red
arrows in Figure 3a. These spots should be composed of heavier
atoms than the backgrounds (Ti and O), thus they are estimated to© 2015 The Chemical Society of Japanstructural characterization platform as a program of Nanotechnology Platform of the Ministry of Education, Culture, Sports,Science and Technology (MEXT), Japan. Authors thank Prof. B.
Ohtani (Hokkaido Univ.) and Dr. T. Tokunaga (Nagoya Univ.)
for their fruitful discussions.
Supporting Information is available electronically on J-STAGE.

Figure 3. (a) High-magniﬁcation HAADF-STEM image of black
TiO2 nanoparticles after alkaline treatment. (b) Diﬀuse reﬂectancespectra of black TiO2 nanoparticles and commercially available whiteTiO2 nanoparticles (ISK, ST-01). Inset: Electrical photo of black TiO2powder.

be W atoms introduced from the electrode of the plasma
equipment. The atomic radii of Ti and W are 75 and 74 pm,
respectively,26 thus the W atom should be able to substitute for Ti
atom in the crystal structure. This HAADF-STEM image cannot
directly conclude the substituent type doping of W atom however,these ﬁndings indicate that the microwave-induced plasma canbe a promising candidate for novel preparation and modiﬁcation
method of functionalized TiO2 nanoparticles.
In order to investigate the photophysical characteristic of
obtained black TiO2 nanoparticles, diﬀuse reﬂectance spectrum
was recorded for 200­800 nm as shown in Figure 3b (also see
the spectra for 200­2000 nm region in Figure S2). Commerciallyavailable TiO2 nanoparticle (ISK, ST-01, anatase crystal forphotocatalyst with an average diameter of ca. 5 nm) was also
observed as a reference (black line). As can be seen from
Figure 3b, our black TiO2 nanoparticles showed signiﬁcant
reﬂectance over the whole visible light region. The spectralfeature was completely diﬀerent from reference white TiO2,
therefore we could not determine the band-gap for our black
TiO2 nanoparticle. To the best of our knowledge, such strong
absorption over all visible light regions has not been reported forvisible light responsive TiO2 materials.27,28 The doping of W
atoms as well as the formation of O-defects as described andrevealed in Figures 2 and 3a are the most plausible mechanisms
for this usual, and tremendous absorption. We believe thatdesired photocatalysts, not only TiO2 but also other oxides,
will be realized by controlling and adjusting the liquid plasma
conditions in the near future.
In conclusion, we presented a novel synthetic method ofblack TiO2 nanoparticles using a microwave-induced plasma
over water-soluble Ti complex aqueous solution. The crystal
structure of TiO2 was observed, moreover the doped W atoms
were revealed by STEM-EDX and high-magniﬁcation HAADFSTEM image, resulting in the black TiO2 nanoparticles with analmost complete reﬂectance in whole visible light regions.
Simple and easy procedure presented here therefore will widenthe strategy for preparation and modiﬁcation of functionalized
TiO2 photocatalyst.
This work was partly supported by a Research Fund from
Hokkaido University to YI and TY, and by Grant-in-Aid for
Scientiﬁc Research (A) from JSPS (No. 24241041). This work
was also partially supported by Hokkaido University, micro-Chem. Lett. 2015, 44, 1327–1329 | doi:10.1246/cl.150531

References
1 A. Fujishima, K. Honda, Nature 1972, 238, 37.
2 S. Baskaran, L. Song, J. Liu, Y. L. Chen, G. L. Graﬀ, J. Am.
Ceram. Soc. 1998, 81, 401.
3 C. Xue, T. Narushima, Y. Ishida, T. Tokunaga, T. Yonezawa,Chem. Lett. 2014, 43, 1514.
4 K. Koelsch, S. Cassaignon, J. F. Guillemoles, J. P. Jolivet, Thin
Solid Films 2002, 403­404, 312.
5 M. R. Hoﬀmann, S. T. Martin, W. Choi, D. W. Bahnemann,
Chem. Rev. 1995, 95, 69.
6 K. Yanagisawa, J. Ovenstone, J. Phys. Chem. B 1999, 103,7781.
7 H. Cheng, J. Ma, Z. Zhao, L. Qi, Chem. Mater. 1995, 7, 663.
8 J. Moser, M. Grätzel, J. Am. Chem. Soc. 1983, 105, 6547.
9 S.-J. Kim, S.-D. Park, Y. H. Jeong, S. Park, J. Am. Ceram. Soc.
1999, 82, 927.
10 I. Moriguchi, H. Maeda, Y. Teraoka, S. Kagawa, Chem. Mater.
1997, 9, 1050.
11 H. Uchida, S. Hirao, T. Torimoto, S. Kuwabata, T. Sakata, H.
Mori, H. Yoneyama, Langmuir 1995, 11, 3725.
12 X.-Z. Ding, Z.-Z. Qi, Y.-Z. He, J. Mater. Sci. Lett. 1995, 14, 21.
13 K. Köhler, J. W. Coburn, D. E. Horne, E. Kay, J. H. Keller,J. Appl. Phys. 1985, 57, 59.
14 K. Mori, S. Sato, M. Ishizashi, O. Ariyada, Proceedings of theGCMEA 2008 MAJIC 1st, 2008, 891.
15 T. Yonezawa, A. Hyono, S. Sato, O. Ariyada, Chem. Lett. 2010,39, 783.
16 J. Hieda, N. Saito, O. Takai, J. Vac. Sci. Technol., A 2008, 26,854.
17 A. A. Ashkarran, A. Iraji zad, S. M. Mahdavi, M. M. Ahadian,
M. R. H. Nezhad, Appl. Phys. A: Mater. Sci. Process. 2009, 96,423.
18 D.-C. Tien, K.-H. Tseng, C.-Y. Liao, T.-T. Tsung, J. Alloys
Compd. 2009, 473, 298.
19 S. Sato, K. Mori, O. Ariyada, H. Atsushi, T. Yonezawa, Surf.
Coat. Technol. 2011, 206, 955.
20 A. A. Ashkarran, A. Iraji zad, S. M. Mahdavi, M. M. Ahadian,Mater. Chem. Phys. 2009, 118, 6.
21 K. Tomita, V. Petrykin, M. Kobayashi, M. Shiro, M. Yoshimura,
M. Kakihana, Angew. Chem., Int. Ed. 2006, 45, 2378.
22 M. Kobayashi, H. Kato, M. Kakihana, Nanomater. Nanotechnol.
2013, 3, 1.
23 S. Tominaka, Y. Tsujimoto, Y. Matsushita, K. Yamaura, Angew.
Chem., Int. Ed. 2011, 50, 7418.
24 R. Jedamzik, A. Neubrand, J. Rödel, J. Mater. Sci. 2000, 35,477.
25 Y. Nakasugi, G. Saito, T. Yamashita, T. Akiyama, J. Appl. Phys.
2014, 115, 123303.
26 R. D. Shannon, Acta Crystallogr., Sect. A: Cryst. Phys., Diﬀr.,
Theor. Gen. Crystallogr. 1976, 32, 751.
27 Y. H. Hu, Angew. Chem., Int. Ed. 2012, 51, 12410.
28 J. Dong, J. Han, Y. Liu, A. Nakajima, S. Matsushita, S. Wei, W.
Gao, ACS Appl. Mater. Interfaces 2014, 6, 1385.

© 2015 The Chemical Society of Japan | 1329