A thick hierarchical rutile TiO2 nanomaterial with multilayered structure

Materials Research Bulletin 48 (2013) 1961–1966

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A thick hierarchical rutile TiO2 nanomaterial with multilayered structure Shengli Zhu a,b,*, Guoqiang Xie b, Xianjin Yang a,c, Zhenduo Cui a a

School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan c Tianjin Key Laboratory of Composite and Functional Materials, Tianjin 300072, China b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 10 September 2012 Received in revised form 27 November 2012 Accepted 26 January 2013 Available online 13 February 2013

In the present paper, we synthesized a new type of rutile TiO2 nanomaterial with a hierarchical nanostructure using a novel method, which combined dealloying process with chemical synthesis. The structure characters were examined using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The rutile TiO2 nanomaterial is thick in size (several 10 mm). The hierarchical structure of the rutile TiO2 nanomaterial consists of large quantities nanorods and nanoflower arrays. The nanoflowers consist of serveral nanopetals with diameter of 100–200 nm. The cross section of TiO2 nanomaterials presents a multilayer structure with the layer thickness of about 3– 5 mm. The rutile TiO2 nanomaterial has high specific surface area. The formation mechanism of the rutile TiO2 nanomaterial was discussed according to the experimental results. The rutile TiO2 nanomaterial has potential applications in catalysis, photocatalysis and solar cells. ß 2013 Elsevier Ltd. All rights reserved.

Keywords: A. Multilayers A. Nanostructures B. Chemical synthesis C. Electron microscopy C. X-ray diffraction

1. Introduction Titanium dioxide (TiO2) is among the most widely investigated materials for its unique properties and many promising applications [1–3]. In order to achieve high photocatalytic conversion rates, a high specific TiO2 surface area is typically desired. The high surface area is beneficial to many TiO2-based devices, as it facilitates reaction/interaction between the devices and the interacting media, which mainly occurs on the surface or at the interface and strongly depends on the surface area of the material. In order to achieve high specific surface area, lot of methods were applied to synthesis the TiO2 nanomaterials [1–6], such as nanoparticles, nanorods, nanowires, nanotubes and hierarchical nanoarrays. Recently, nanoporous materials have received increasing attention due to their special performance in many fields [7– 14]. A number of approaches have been developed to fabricate nanoporous materials. Among them, dealloying has the advantage of producing bicontinuous open nanoporosity extending in three dimensions [15]. Only limited crystalline alloy systems have been demonstrated to form uniform nanoporous structures [16–19]. In comparison with crystalline alloys, amorphous alloys are considered as good starting alloy for dealloying due to their monolithic phase with a homogeneous composition and structure down to

* Corresponding author at: School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China. Tel.: +86 22 27402494. E-mail address: [email protected] (S. Zhu). 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.01.049

subnanoscale [20,21]. There are several attempts to synthesize nanoporous metals using amorphous alloy as the starting alloy [15,22,23]. Our previous work reported the Ti oxides nanoporous surface structure with a thickness of about 500 nm prepared by electrochemical dealloying of Ti–Cu amorphous alloy [24]. More recently, TiO2 nanomaterials with hierarchical architectures have attracted much attention because of their unique shape-dependent properties [25,26]. Here we show the hierarchical rutile TiO2 nanomaterials with nanoporous structure and thickness of serveral 10 mm prepared by a simple chemical method, which combines dealloying process with chemical synthesis. 2. Materials and methods Ingot of Ti40Cu60 alloy was prepared by arc-melting the pure elements with purities above 99.9% in an argon atmosphere. The alloy composition is represented in nominal atomic percentage of the mixture. The amorphous alloy ribbons with width of 2 mm and thickness of 10–30 mm were prepared by melt spinning. Amorphous structure was confirmed by X-ray diffraction (XRD, Rigaku RINT-Ultima, monochromatic Cu Ka radiation). The amorphous alloy ribbons were cut into 3–4 cm in length, and then immersed in HNO3 solution with concentration of 13.14 mol/L. The reaction temperature was determined as 70 8C by a water bath. The structure, composition and phase structure of as-formed nanomaterial were characterized by using scanning electron microscope (SEM, Hitachi 4800S) coupled with X-ray energy dispersive spectrometer (EDS), XRD and high resolution transmission

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S. Zhu et al. / Materials Research Bulletin 48 (2013) 1961–1966

Fig. 1. Surface appearance and XRD patterns of the Ti40Cu60 amorphous alloy subjected to immersion in HNO3 solution for different durations. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)

electron microscope (HRTEM, JEOL JEM-2100F) coupled with selected area electron diffraction (SAED). The chemical state of the as-formed nanomaterials was characterized using X-ray photoelectron spectroscopy (XPS, PHLl600ESCA X). The specific surface areas were measured by a gas sorption analyzer (Autosorb-iQ, Quantachrome). 3. Results and discussion Fig. 1 shows the surface appearance and XRD patterns of the Ti40Cu60 amorphous alloy subjected to immersion in HNO3 solution for different durations. Original Ti40Cu60 amorphous ribbon exhibits shiny metallic luster. After several hours, some white substances covered in the samples were observed and the color of solution gradually turned green. Immersed for 24 h, the full surface of samples was almost covered by the white substance. The brown gas was found above the liquid level at the upper part of the bottle, which was suggested as NO2. The color of the solution became green, which should be attributed to the release of Cu ions from the Ti40Cu60 amorphous ribbon. The XRD pattern of the sample exhibits several diffraction peaks which superimpose over a broad halo peak from the amorphous phase. With increasing immersion time, the broad halo peak corresponding to amorphous phase becomes weak, and disappears at 48 h. The samples

subjected to 48 h immersion transformed to white ribbons completely. These results accord with the direct observation by the visual inspection. The crystal phase was indexed as a single rutile TiO2 phase which was the most stable phase in thermodynamics. The indices of lattice plane of rutile TiO2 were marked in the XRD patterns. No other phase was found according to the XRD results. Fig. 2(a) shows the surface morphology of TiO2 nanomateial after immersion for 96 h. The TiO2 nanomaterial exhibits nanoporous architecture structure which consists of large quantities nanorods and nanoflower arrays. The nanorods with diameter of 100–500 nm interlace and stack together (as shown in Fig. 2(a1)). The nanoflowers distribute homogenously on the surface of nanorods layer. The high magnification SEM images reveal that the nanoflowers consist of serveral nanopetals with diameter of 100–200 nm (as shown in Fig. 2(a2) and (a3)). The cross section of TiO2 nanomaterial presents a multilayer structure with the layer thickness of about 3–5 mm, as shown in Fig. 2(b) and (b1). The different layer exhibits different morphologies. The nanorods inside sample are thinner than those on the surface (Fig. 2(b2) and (b3)). Large quantity of nanorods interlace and joint together to form a nanoporous structure, as shown in Fig. 2(c) and (c1). Fig. 3(a) shows the TEM image and SAED pattern of the sample subjected to 96 h immersion. TEM image exhibits similar

S. Zhu et al. / Materials Research Bulletin 48 (2013) 1961–1966

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Fig. 3. TEM image and SAED pattern (a) and HRTEM image (b) of the hierarchical rutile TiO2 nanomaterial formed by immersing in HNO3 solution for 96 h.

Fig. 2. SEM images of the hierarchical rutile TiO2 nanomaterial formed by immersing in HNO3 solution for 96 h. (a, a1, a2 and a3) Surface morphologies; (b and b1) cross section morphologies; (b2 and b3) morphologies at different depth; (c) inside morphologies.

morphology with SEM image (Fig. 2(c)) and SAED pattern confirms the rutile phase structure. HRTEM image (Fig. 3(b)) shows two group of crystal planes with different orientation (plane angle: 70.78). The interplanar spacings of these crystal planes are 0.2491 and 0.3250 nm, corresponding to the (1 0 1) and (1 1 0) planes of rutile TiO2, respectively. Fig. 4 shows the EDS spectra of the samples subjected to 48 and 96 h immersion. Sites (I), (II), (III) and (IV) are along with the depth

from surface to inside. At any site only Ti and O elements are examined and no Cu is detected. For the sample immensed 48 h, the atom ratios of Ti/O at sites (I), (II) and (III) are almost 1/2, but 1/ 3 at site (IV). After 96 h immersion, the atom ratio of Ti/O is 1/2 in the full sample. The typical isotherms for nitrogen adsorption and desorption of the sample subjected to 96 h immersion is shown in Fig. 5. The Barrett–Joyner–Halenda (BJH) method was employed to analyze the pore-size distribution, and the results are shown in inset of Fig. 5. The TiO2 nanomateial has a Brunauer–Emmett–Teller (BET) specific surface area of 138.6 m2/g, which is much higher than that of commercial TiO2 nanoparticles: Degussa P25 (55 m2/g). The pore diameter is peaked at 16.5 nm with a narrow pore-size distribution. The total pore volume was determined to be 0.21 cm3/g. These results indicate that the multilayer hierarchical nanostructure would result in a higher specific surface area, which would facilitate reaction/interaction between the material and the interacting media. The formation of the hierarchical rutile TiO2 nanomaterials involves two simultaneous processes: dealloying and chemical synthesis. The Cu atoms in TiCu amorphous alloy were moved from the alloy by etching in HNO3 solution, as called ‘‘dealloying’’. It was reported that the surface of Cu-rich TiCu amorphous alloy consisted of oxides of both Ti and Cu, such as TiO, Ti2O3, TiO2 and Cu2O [27–29]. When the TiCu amorphous alloy was immersed in HNO3 solution, Cu2O in the surface oxides film would react with

S. Zhu et al. / Materials Research Bulletin 48 (2013) 1961–1966

3

Volume adsorbed / (cm /g STP)

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70

60

50

40

30 0.0

0.2

0.4

0.6

0.8

1.0

P/P0 Fig. 5. Nitrogen adsorption–desorption isotherm and pore size distribution (inset figure) of the hierarchical rutile TiO2 nanomaterial formed by immersing in HNO3 solution for 96 h.

dealloying process play an important role on the formation of hierarchical nanostructure. The second process (chemical synthesis) takes place through the nucleation and growth process. Due to the etching of HNO3, a rough and topological surface is formed on the TiCu amorphous alloy (not shown here). Ti oxides and Ti with rough structure would react with HNO3 following Eqs. (3)–(5). Furthermore, the H2TiO3 decomposed to TiO2, as shown in Eq. (6). The reactions (1)–(5) produced lots of NO2 gas that would be found at the upper part of the bottle. Fig. 4. EDS spectra of hierarchical rutile TiO2 nanomaterial formed by immersing in HNO3 solution for (a) 48 and (b) 96 h.

the HNO3 following Eq. (1). With the further etching, the Cu inside the alloy would be exposed to the HNO3 solution. Hence the reaction of Cu with HNO3 would occur following Eq. (2). þ

Cu2 O þ 6H þ 2NO3



! 2Cu

2þ

þ 2NO2 " þ 3H2 O

Cu þ 4HNO3 ! CuðNO3 Þ2 þ 2NO2 " þ 2H2 O

(1)

(2)

As easy to passivation, it is diffcult to prepare a thick TiO2 nanomaterial (more than 1 mm) with porous or hierarchical structure. Most of the papers about synthesis of TiO2 nanomaterials reported thin TiO2 layers with maximum thicknesses of serveral 100 nm. However, it seems that the TiO2 materials with nanostructure and thicknesses in the 10 mm range are interesting for solar cell applications. In Gra¨tzel-type solar cell, thick nanoporous TiO2 layers with thickness of approximately 10 mm are applied. In general the nanoporous layers were assembled using TiO2 particles with nano size by doctor-blading or spincoating approaches. The layers then are sensitized with a suitable dye and mounted into various solar cell configurations [30]. In the present work, the thick TiO2 nanomaterial with size of serveral 10 mm can be formed directly. In addition, TiCu binary alloy was chosen as starting alloy. The selective corrosion of Cu provides the space for the TiO2 nucleation and growth inside the alloy. And asformed TiO2 layer on the surface exhibits nanoporous structure, hence the HNO3 solution can penetrate into inside and continue the further etching. The amorphous Ti–Cu alloy with higher Ti content, such as Ti60Cu40 amorphous alloy, cannot form the TiO2 nanomaterial as immersing in HNO3 solution, indicating that the

TiO þ 2HNO3 ! H2 TiO3 þ 2NO2 "

(3)

Ti2 O3 þ 2HNO3 þ H2 O ! 2H2 TiO3 þ 2NO2 "

(4)

Ti þ 4HNO3 ! H2 TiO3 þ 4NO2 " þ H2 O

(5)

H2 TiO3 ! TiO2 þ H2 O

(6)

The oxidation of TiCu amorphous alloy in hot HNO3 aqueous solution can bring forth hydrated titania (H2TiO3) on the substrate. Then, new rutile seeds can nucleate and nanorods grow. After 48 h immersion, the Ti/O atom ratio inside the sample is lower than that of the surface and match to the Ti/O atom ratio of hydrated titania (1/3), as shown in Fig. 3(a) site (IV). It is a evidence of formation of hydrated titania during the etching. After 96 h immersion, full sample exhibits a atom ratio of 1/2, indicating that the hydrated titania transforms to TiO2 completely. The chemical state of Ti and O in the TiO2 nanomaterials has been analyzed through XPS. Fig. 6 shows the O1s and Ti2p spectra of the samples subjected to 48 and 96 h immersion, respectively. Ti2p spectra of both samples are simliar. Ti2p3/2 and Ti2p1/2 located at approximately 458.7 and 464.3 eV, respectively. The Ti2p3/2 binding energy exceeds that of Ti metal (454.0 eV), TiO (455.0 eV), and Ti2O3 (456.7 eV), but is similar to that of TiO2 (458.4–458.7 eV), which suggests that Ti is in the +4 oxidation state and directly bonded to oxygen. The O1s spectrum of the sample subjected to 96 h immersion consists of a main peak at about 530.26 eV which is attributed to the TiO2. For the sample subjected to 48 h immersion, a doublet appears at about 530.24 and 532.12 eV in the O1s spectrum. The former peak is attributed to TiO2 and the latter peak confirms the presence of hydroxyl group originating from hydrated titania. The nucleation process happens and the formed nuclei aggregate together to decrease the surface energy at early stages. The formation of many small crystalloids is kinetically favored

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464.03

470

465

460

455

537

450

531

528

530.26

(d)

Ti2p

458.51

534

525

O1s

Intensity / a.u.

Intensity / a.u.

(c)

464.26

465

O1s

530.24

Binding Energy / eV

Binding Energy / eV

470

532.12

(b)

Ti2p

458.34

Intensity / a.u.

Intensity / a.u.

(a)

1965

460

455

450

Binding Energy / eV

537

534

531

528

525

Binding Energy / eV

Fig. 6. XPS spectra of O1s and T2p of hierarchical rutile TiO2 nanomaterial formed by immersing in HNO3 solution for (a and b) 48 and (c and d) 96 h.

Fig. 7. SEM images of rutile TiO2 nanomaterial formed by immersing in HNO3 for (a) 48 and (b) 96 h (some small crystalloids and branches were marked using circles).

because they nucleate more easily. However, small crystalloid has a larger surface area to volume ratio than large crystalloid. Molecules on the surface are energetically less stable than the ones already well ordered and packed in the interior. Large crystalloids are thermodynamically favored because they represent a lower energy state. Thereby, many small crystalloids would transform into large crystalloids. This phenomenon can be attributed to ‘‘Ostwald ripening’’ [25,26]. In our case, lots of small crystalloids and branches were found on the nanorods in sample subjected to 48 h immersion (marked in Fig. 7(a) using circles), however disappeared for the sample subjected to 96 h immersion, as shown in Fig. 7(b). Compared with original ribbon, the thickness of sample subjected to 96 h immersion was decreased by about 10%, indicating the shrinkage in the shape. In addition, due to the

presence of large quantity of Cu in the substrate, the formation of hydrated titania cannot follow the nucleation and growth of TiO2 nanorods. Hence, a loosened layer is caused after certain duration. With further etching, a new TiO2 nanorods layer is formed on the substrate. This is the reason of formation of multilayer structure. The formation mechanism of multilayer structure is illustrated in Fig. 8. According to this mechanism, we can control the thickness of layers by changing the composition of the original TiCu ribbon. The further investigation will be carried out in future work. The multilayered structure provides highly ordered, precisely located and periodically distributed chemical environments, which are attractive platforms when designing smart and responsive materials with nanoscaleorganized architectures, hence would be benefit in design of catalysis materials [31–33].

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Fig. 8. Illustration of formation mechanism of multilayer hierarchical rutile TiO2 nanomaterial.

4. Conclusions We synthesized a new type of rutile TiO2 nanomaterial with a hierarchical nanostructure using a novel and simple method. The method combined a dealloying process with chemical synthesis. The as-formed TiO2 nanomaterial is thick in size (several 10 mm). The hierarchical structure consist of large quantities nanorods and nanoflower arrays. The nanoflowers consist of serveral nanopetals with diameter of 100–200 nm. The cross section of TiO2 nanomaterials present a multilayer structure with the layer thickness of about 3–5 mm. XRD and HRTEM results confirm the rutile phase structrue of the nanomaterial. The multilayer hierarchical nanostructure results in high specific surface area. The rutile TiO2 nanomaterial has potential applications in catalysis, photocatalysis and solar cells. Acknowledgements This work was supported by National Natural Science Foundation of China (50901051 and 51172159) and Key Projects in the Tianjin Science & Technology Pillar Program (09ZCKFGX29100). References [1] [2] [3] [4] [5]

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