RESEARCH PAPER

Synthesis of octahedral TiO2 single crystals with {101} facetsfrom solid precursor with N2H4 as capping agent

Hui Jin • Jian Pan • Lianzhou Wang

Received: 27 September 2013 / Accepted: 5 March 2014 / Published online: 22 March 2014

� Springer Science+Business Media Dordrecht 2014

Abstract In this work, N2H4 was used as surface-

capping agent for the first time to synthesize octahe-

dral anatase TiO2 single crystals with dominant {101}

facets from H0.68Ti1.83O4 solid precursor. The resul-

tant particle size of {101} facet enriched TiO2 was

around 100 nm. The function of N2H4 in the hydro-

thermal process is not only to provide a mild basic

environment, but also to promote the growth of {101}

surface because of its strong reducibility. The amount

of N2H4 and precursor added in the solution is also

investigated, and the result reveals that nearly 100 %

{101} facet can be obtained only if N2H4 were added

to a certain amount, while the concentration of

precursor has no influence on the final result which

means that this reaction can be largely scaled up. In

oxygen evolution from photocatalytic water splitting,

the {101} facets exhibited different performance using

AgNO3 or (NH4)2Ce(NO3)6 as sacrificial agent.

Keywords TiO2�{101} facet � Octahedral

anatase � Photocatalytic water splitting �Nanocrystals

Introduction

In the recent decades, crystal facet engineering of

semiconductors has been widely studied, and signif-

icant advancements have been achieved (Peng et al.

2000; Xiang et al. 2004; Yang and Zeng 2004; Burda

et al. 2005; Han et al. 2009; Xie et al. 2009; Wang et al.

2010; Jiang et al. 2012). TiO2, as the most popular

semiconductor photocatalyst, plays a crucial role in

this research area (Chen and Mao 2007; Chen et al.

2010). Since Lu’s breakthrough on the facet engineer-

ing of anatase TiO2 single crystal with enriched {001}

facets (Yang et al. 2008), enormous efforts have been

devoted into the relevant experimental and theoretical

works(Yang et al. 2009; Li and Xu 2010; Liu et al.

2010a, 2011; Wu et al. 2010), which involved not only

anatase, but also rutile (Murakami et al. 2010; Zhang

et al. 2010; Zuo et al. 2012) and brookite (Buonsanti

et al. 2008; Lin et al. 2012). In terms of anatase single

crystal, it is commonly constructed with {001}, {101},

and {010} facets. Although the order of the photoac-

tivity and/or selectivity of these three facets varied

under different reaction conditions(Murakami et al.

2009; Tachikawa et al. 2011), it cannot be denied that

the TiO2 single crystals with dominant well-defined

and reactive facets exhibit superior photoactivity due

Electronic supplementary material The online version ofthis article (doi:10.1007/s11051-014-2352-z) contains supple-mentary material, which is available to authorized users.

H. Jin

School of Chemical Engineering, University of

Queensland, St Lucia, QLD 4072, Australia

J. Pan � L. Wang (&)

ARC Centre of Excellence for Functional Nanomaterials,

School of Chemical Engineering and AIBN, University of

Queensland, St Lucia, QLD 4072, Australia

e-mail: l.wang@uq.edu.au

123

J Nanopart Res (2014) 16:2352

DOI 10.1007/s11051-014-2352-z

to their uniform surface atomic structure and unique

properties (Liu et al. 2010b). For instance, in spite of

the relatively lower surface energy of {101} facet

(c{101} (0.44 J m-2) \ c{010} (0.53 J m-2) \ c{001}

(1.09 J m-2)), it exhibits enhanced photoactivity in

hydrogen evolution and photodegradation (Wu et al.

2010; Gordon et al. 2012; Xiong and Zhao 2012). And

there is also a strong evidence that {101} facets are

more reductive than {001} facets (D’Arienzo et al.

2011).

To obtain well-faceted single crystals of TiO2, the

hydrothermal treatment is a prevalent method. How-

ever, it is different to synthesize high-quality faceted

samples in large-scale as the hydrolysis of most

soluble titanium precursors, such as titanium halide,

titanium alkoxide, etc., is too fast to control precisely.

Recently, solid precursor such as the alkali-titanate

nanowires were used in hydrothermal treatment to

synthesize faceted anatase TiO2 single crystals (Ama-

no et al. 2009; Li and Xu 2010). In fact, alkali-titanate

and its protonated form, which have the similar TiO6

octahedral unites in TiO2, have been well studied. And

the phase transition between titanate and TiO2 has also

been illustrated via simple wet-chemical reactions

(Zhu et al. 2005). Therefore, solid titanate has been

considered as a favorable precursor to prepare tailored

anatase or rutile TiO2 single crystal with suitable

capping agent. In our previous work (Pan et al. 2011b),

we obtained tetragonal rod-like anatase TiO2 single

crystals with dominant {010} facets using lepidocro-

cite-type bulk titanate Cs0.68Ti1.83O4 as solid precur-

sor. Its protonated form, H0.68Ti1.83O4, makes itself

more suitable as precursor due to its hydrophilicity in

the presence of H? instead of Cs? for interacting with

other pH value agent or capping agent.

On the other hand, enormous efforts have been

devoted into finding suitable capping agents, which

play a crucial role in morphology control. For

instance, the substantial role of fluorine in stabilizing

{001} facets has been extensively illustrated in many

works. However, there is no works reporting the

capping agents for {101} facets. Here, N2H4 was used

as surface-capping agent for the first time in hydro-

thermal method to synthesize octahedral anatase TiO2

single crystals with dominant {101} facets from

H0.68Ti1.83O4 solid precursor. The function of N2H4

in the hydrothermal process is not only provide a mild

basic environment, but also promote the synthesis of

{101} surface due to its strong reducibility.

Experiment section

Layered titanate of Cs0.68Ti1.83O4 was prepared accord-

ing to a procedure previously reported (Sasaki et al. 1996;

Liu et al. 2009). Its protonated form of H0.68Ti1.83O4 was

prepared by ion-exchange of Cs0.68Ti1.83O4 with H? in a

1 mol L-1 HCl solution for 3 days.

50 mg of the H0.68Ti1.83O4 powder was dispersed in

20 mL MilliQ-water. And then, 1 mL hydrazine

monohydrate (98 %) was added into the solution.

The mixture was hydrothermally treated at 180 �C for

8 h in a Teflon-lined stainless autoclave with a volume

of 80 mL. The resultant product was washed for

several times with deionized water and then dried in

air at 80 �C. Then, the powder was heated in static air

atmosphere in a furnace at 600 �C for 2 h and then

cooled naturally down to room temperature.

X-ray diffraction (XRD) patterns of the samples

were recorded on a Rigaku diffractometer using Cu kairradiation. Their morphology was determined using

transmission electron microscopy (TEM,JEOL 1010

and 2100) and scanning electron microscopy (SEM,-

SUPRA 35). The Brunauer–Emmett–Teller (BET)

surface area was determined by nitrogen adsorption–

desorption isotherm measurements at 77 K (ASAP

2010). Chemical compositions and valence band

spectra of TiO2 were analyzed using X-ray photoelec-

tron spectroscopy (XPS) (Thermo Escalab 250, a

monochromatic Al Ka X-ray source). All binding

energies were referenced to the C 1 s peak (284.6 eV)

arising from adventitious carbon. The optical absor-

bance spectra of the samples were recorded in a UV–

Vis spectrophotometer (JACSCO-V650).

Photocatalytic hydrogen evolution and oxygen

evolution reactions were carried out in a top-irradia-

tion vessel connected to a glass-enclosed gas circula-

tion system. For hydrogen evolution, 100 mg the

photocatalyst with 1 % Pt loaded was dispersed in

300 mL aqueous solution containing 10 % methanol

by volume. For oxygen evolution, 50 mg powder was

dispersed in 300 mL 16.7 mM AgNO3 solution or

(NH4)2Ce(NO3)6 solution. The reaction temperature

was maintained around 10 8C. The amount of product

gas was determined using a gas chromatograph

(SHIMADZU GC-2014C). The detailed schematic of

water splitting measurement equipment can be found

in Wang’s work(Wang et al. 2009). The light source in

the above photoactivity experiments was a 300 W Xe

lamp (Beijing Trusttech Co. Ltd., PLS-SXE-300UV).

2352 Page 2 of 7 J Nanopart Res (2014) 16:2352

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Result and discussion

The precursor, protonated-titanate H0.68Ti1.83O4, have

a typical lepidocrocite-type layered structure (inset in

Fig. 1). The layered titanate consists of TiO6 octahe-

dra, which is also the basic unit of anatase titania. As

we can see from the scanning electron microscopy

(SEM) image of precursor H0.68Ti1.83O4 in Fig. 1, the

sheet-like particles were irregular with a size of

200–800 nm. For observing the nanocrystals conver-

sion process during the hydrothermal treatment, XRD

tests were conducted to characterize all solid products

obtained with various reaction times. In Fig. 2, the

XRD peak at 9.48 ({200}) is the typical characteristic

of the precursor; and the peak located at 25.38 is the

typical peaks of anatase TiO2 ({101} facet). In the first

6 h, the {200} peak of precursor gradually decreased

and disappeared finally; whereas the typical peaks of

anatase TiO2 increased simultaneously. After 8 h’

reaction, the titanate was transformed into anatase

TiO2 completely, as no precursor peaks were detected.

After hydrothermal treatment, all irregular particles of

precursor were transformed into octahedral particles

with a size of 30–50 nm (Fig. 3a). According the

previous experimental and calculation results (Die-

bold 2003; Barnard and Curtiss 2005; Amano et al.

2009), it is easily to identify the eight triangle facets of

the octahedron as {101}. To confirm the exposed

facets of the single crystal, the high-resolution trans-

mission electron microscopy (HRTEM) image in

Fig 3b reveals more detail information of its atomic

structure. The three sets of lattice fringes with spacing

of 4.8, 3.5, and 3.5 A are associated with {002},

{101}, and {10-1} facets, respectively.

In order to investigate the effect of hydrazine on the

formation of octahedral TiO2 single crystals, we

conduct a series of tests with various concentrations

of hydrazine and precursor. First, we tried to synthe-

size without any capping agents, and the particle shape

of as-prepared sample was irregular (Fig.SI-1). As the

amount of N2H4 increased to 0.1 mL, {101} facets can

be identified from a few octahedral particles. But the

sizes of particles were not uniform, as there were some

small particles with diameter around 5 nm attaching

on the surface of larger ones (Fig. 4a). Gradually, the

amount of the small particles decreased and finally

disappeared as the amount of N2H4 increased from

0.5 mL to 1 mL (Fig. 4b and c). Meanwhile, the large

particles, step by step, turned into perfect octahedral

particles. However, no obvious distinct morphological

and structural difference in the samples was found,

when the amount of precursor was increased from

50 mg to 500 mg while the amount of N2H4 was kept

at 1 mL, which means that, unlike hydrazine, the

concentration of precursor has no influence on the

shape control of the final product (Fig. 4d–f). These

results indicate that this reaction can be readily scale

up.

Conventionally, there are two main factors domi-

nating the shape of final products, pH value, and

organic/inorganic ions in solution. Hydrazine has

basic chemical properties and strong reduction power.

For investigating its effect on the morphology, several

Fig. 1 SEM images of the precursor H0.68Ti1.83O4. The inset is

crystal structure model of typical lepidocrocite-type titanate Fig. 2 XRD patterns of the solid precursor H0.68Ti1.83O4 and

solid products obtained by hydrothermal treatment for different

reaction times

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123

experiments were conducted. First, we investigated

the effect of pH value of the environment. Hydrazine

aqueous solution can provide basic environment

during the whole hydrothermal reaction. As a result,

the pH value was stable at around 11.1 during the

whole reaction. Weak alkaline environment is bene-

ficial to the formation of {010} facets of anatase TiO2.

For instance, when Na2CO3, K2CO3, and Cs2CO3

were used as pH value adjusting agent, tetragonal rod

particles with {010} facets were obtained (Pan et al.

2011b). However, in our case, anatase {101} facets

emerged instead under weak alkaline environment.

So, we substituted sodium hydroxide for hydrazine

monohydrate as pH value adjusting agent to redupli-

cate the reaction. Secondly, considering that hydrazine

tends to decompose into nitrogen, ammonia, and

hydrogen during the reaction process, aqueous ammo-

nia was also utilized to replace hydrazine in the

reaction to determine the effect of ammonia on the

shape control of TiO2. Nevertheless, from the SEM

and TEM images of the final products (Fig.SI-2), the

particles of both samples are irregular, and hard to

Fig. 3 SEM images of a octahedral nanoparticle product. b HRTEM image of octahedral nanoparticle product

Fig. 4 TEM images of the different samples by changing the

amounts of capping agent N2H4 and precursor H0.68Ti1.83O4:

a 0.1 mL, 50 mg; b 0.5 mL, 50 mg; c 1 mL, 50 mg; d 1 mL,

100 mg; e 1 mL, 250 mg; f 1 mL, 500 mg. All marked angles

are 43.4�, and the scale bar in each image is 50 nm

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identify any facets. Therefore, it is clear that pH value

and ammonia are not the only conditions to facilitate

the formation of {101} facets. As a consequence, we

suppose that the strong reducibility of hydrazine plays

an important role in the synthesis of anatase with

{101} facets. Note that the TiO2 samples synthesized

in hydrazine or aqueous ammonia solution were not

doped by nitrogen based on the following two

experimental facts we observed. The XPS results

indicate that there was no signal of N 1 s on the surface

of octahedral TiO2 nanoparticles (Fig SI-8). Normally,

N-doping will result in visible light response and

apparent color change in the samples. In our case, the

samples in white color and there is no visible light

absorption in the UV–Vis spectra (Fig. SI-7).

For better investigating the photoactivity perfor-

mance of octahedral particles (named OP) with {101}

facets, one sample of tetragonal rod-like TiO2 nano-

particles (named TP) with dominant {010} facets,

which have similar surface area with OP (25 m2 g-1),

was introduced in all tests as reference.

In the photocatalytic hydrogen evolution test with

methanol as sacrificial agents, OP showed slightly

lower output than TP as expected (Fig.SI-3, 150 vs.

200 lmol h-1), as TiO2 with {101} facets has lower

conduction band minimum (CBM) than that with

{010} facets (Pan et al. 2011a). However, it is

noteworthy that OP exhibited higher performance in

oxygen evolution test using AgNO3 as sacrificial agent

(Fig. 5a). This unexpected phenomenon was never

observed before. It has been widely accepted that

{001}, {101}, and {010} have the similar valence

band maximum (VBM)(Pan et al. 2011a; Tachikawa

and Majima 2012). And in this case, OP and TP also

showed the same VBM (Fig.SI-5). It is reasonable to

consider that they should have the same performance

in oxygen evolution from water splitting since they

have the same oxidizing ability. Nevertheless, the

photoactivity of semiconductors may possibly be

related to several factors, not only surface electronic

structure, but also adsorption energies of substrates or

the synergy of exposed facets and co-catalysts.

In order to understand this phenomenon, the

sacrificial agent was changed from AgNO3 to

(NH4)2Ce(NO3)6, which is another typical sacrificial

agent used in oxygen evolution (Gomes Silva et al.

2010). It is different from the situation in AgNO3

system that the Ag? ions were reduced to nanoparti-

cles attached on the TiO2 surface (Fig SI-5), Ce4? ions

were reduced to Ce3? ions, which was judged by

observing the colour of the solution changed from

orange to pale yellow (Fig SI-6). As a result, the

oxygen evolution of both samples became the same

(Fig 5b). So we conclude that Ag nanoparticle reduced

on the surface is the key factor to affect the behaviors

of {101} and {010} facets in AgNO3 system. On one

hand, Ag? ions tend to be adsorbed and photoreduced

on {101} facet during the whole oxygen evolution

reaction. Therefore, the XPS results show that the

amount of Ag nanoparticles reduced on the {101}

facet is higher than that on {010} facet (5.21 vs.

4.37 %). On the other hand, these adsorbed Ag

nanoparticles on the surface may also act as co-

catalyst like other noble metals, and further affect the

photocatalytic performance of each sample. There-

fore, {101} facet showed better performance than that

of {010} facet in oxygen evolution test, when AgNO3

was used as sacrificial agent.

Fig. 5 The photocatalytic performances of samples OP and TP

in oxygen evolution with: a AgNO3; b (NH4)2Ce (NO3)6 as

sacrificial agents

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Conclusion

In conclusion, octahedral TiO2 single crystals were

synthesized from solid precursor H0.68Ti1.83O4 with

N2H4 as capping agent. The results showed that the

basic and reductive N2H4 solution is beneficial to the

formation of anatase {101} facets. This method is

suitable for large-scale production. Comparing with

{010} facets, {101} facets have the same VBM.

However, {101} facet showed higher oxygen evolu-

tion with AgNO3 as sacrificial agent, due to its

selective adsorption and photoreduction of Ag ? ions.

This indicates that for evaluating the photoactivity

performance of TiO2 facets, in addition to the

photocatalyst itself, it is equally important to consider

the testing system for better understanding of the

photocatalysis nature.

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