Nano Res

1

Growth and in situ transformation of TiO2 and HTiOF3 crystals on chitosan-polyvinyl alcohol co-polymer substrate under vapour phase hydrothermal conditions

Tianxing Wu1, Guozhong Wang1 (*), Xiaoguang Zhu1, Porun Liu2, Xian Zhang1, Haimin Zhang1, Yunxia Zhang1, and Huijun Zhao1,2 (*) Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-015-0953-1

http://www.thenanoresearch.com on November 23, 2015

© Tsinghua University Press 2015

Just Accepted

This is a “Just Accepted” manuscript, which has been examined by the peer-review process and has been accepted for publication. A “Just Accepted” manuscript is published online shortly after its acceptance, which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP) provides “Just Accepted” as an optional and free service which allows authors to make their results available to the research community as soon as possible after acceptance. After a manuscript has been technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Please note that technical editing may introduce minor changes to the manuscript text and/or graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event shall TUP be held responsible for errors or consequences arising from the use of any information contained in these “Just Accepted” manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI®), which is identical for all formats of publication.

Nano Research DOI 10.1007/s12274-015-0953-1

Nano Res

2

2

TABLE OF CONTENTS (TOC)

Growth and in situ transformation of TiO2 and

HTiOF3 crystals on chitosan-polyvinyl alcohol

co-polymer substrate under vapour phase

hydrothermal conditions

Tianxing Wu1, Guozhong Wang*1, Xiaoguang Zhu1,

Porun Liu2, Xian Zhang1, Haimin Zhang1, Yunxia Zhang1

and Huijun Zhao*1,2

1 Key Laboratory of Materials Physics, Centre for

Environmental and Energy Nanomaterials, Anhui Key

Laboratory of Nanomaterials and Nanostructures,

Institute of Solid State Physics, Chinese Academy of

Sciences, Hefei 230031, P. R. China. 2 Centre for Clean Environment and Energy, Gold Coast

Campus, Griffith University, Queensland 4222, Australia.

Page Numbers. The font is

ArialMT 16 (automatically

inserted by the publisher)

Co-polymer SubstrateInduced Crystal Growth

A chitosan-polyvinyl alcohol co-polymer with rich amino and

hydroxyl groups are used as the substrate to induce direct growth and

in situ sequential transformation of titanate crystals under HF vapour

phase hydrothermal conditions. The demonstrated substrate organic

functional groups induced structural transformation could be

applicable to other material systems.

Nano Res

3

3

Growth and in situ transformation of TiO2 and HTiOF3 crystals on chitosan-polyvinyl alcohol co-polymer substrate under vapour phase hydrothermal conditions

Tianxing Wu1, Guozhong Wang1(*), Xiaoguang Zhu1, Porun Liu2, Xian Zhang1, Haimin Zhang1, Yunxia Zhang1 and Huijun Zhao1,2(*)

1 Key Laboratory of Materials Physics, Centre for Environmental and Energy Nanomaterials, Anhui Key Laboratory of

Nanomaterials and Nanostructures, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, P. R. China. 2 Centre for Clean Environment and Energy, Gold Coast Campus, Griffith University, Queensland 4222, Australia.

Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher) © Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2011

ABSTRACT A chitosan-polyvinyl alcohol (CS/PVA) co-polymer substrate possessing rich amino and hydroxyl groups are used as the substrate to induce the direct growth and in situ sequential transformation of titanate crystals under HF vapour phase hydrothermal conditions. The process involves four distinctive formation/transformation stages. The HTiOF3 crystals with well-defined hexagonal-shape are formed during Stage I, which are subsequently transformed into {001} faceted anatase TiO2 crystal nanosheets during Stage II. Interestingly, the formed anatase TiO2 crystals are further transformed into the cross-shaped and hollow square-shaped HTiOF3 crystals during Stages III and IV, respectively. Although TiO2 crystal phase and facet transformation under hydrothermal conditions have been previously reported, the in situ crystal transformation between different titanate compounds is rarely reported. Such crystal formation/transformations are likely due to the presence of

Nano Res DOI (automatically inserted by the publisher) Research Article

Nano Res

4

4

rich amino groups in CS/PVA substrate. When celluloses possessing only hydroxyl groups are used as the substrate, rather than the sequential crystal transformation, the direct formation of {001} faceted TiO2 nanocrystal sheets are observed. The demonstrated substrate organic functional group induced crystal formation/transformation approach could be applicable to other material systems. KEYWORDS vapour phase hydrothermal synthesis, crystal transformation, titanate compounds

1 Introduction

Ability to in situ transform inorganic crystal shape and composition has been rigorously pursued by material scientists as an effective synthetic means to obtain variety of crystalline materials [1-5]. The majority of the reported crystal transformation approaches to date are based on either the chemical substitution or sacrificial mechanisms [6-11]. The chemical substitution approaches require the use of a precursor crystal as the structural template to achieve transformation via ion exchange or galvanic replacement [12-14]. The sacrificial transformation approaches are realized by removing certain structural components of the precursor material structures [7, 8, 15]. Although the crystal transformation via in situ crystal reformation has been demonstrated under high temperature thermal treatment process, such crystal transformation under hydrothermal conditions has rarely been reported. Titanium-based crystals have been extensively investigated for few decades due to their importance in environmental and energy-related applications [16, 17]. The crystal phase transformation of TiO2 from anatase to rutile can be realized in a simple high temperature thermal

treatment process [18, 19]. While the crystal facet transformation of TiO2 from {101} to {001} can be readily achieved under hydrothermal conditions in presence of hydrofluoric acid (HF) [20, 21]. Recently,

we have

demonstrated that HTiOF3 crystals can be formed on the titanium substrate and directly transformed into {001} faceted ultrathin single crystal anatase TiO2 nanosheets via an in situ crystal reformation mechanism under hydrofluoric acid vapour phase hydrothermal conditions [22]. However, an attempt for reversed transformation of the {001} faceted anatase TiO2 crystal to HTiOF3 crystal was failed.

It is known that the crystal growth can be strongly influenced by the surrounding chemical environment, especially the chemical environment at the growing crystal surface and interface [23-26]. As such, capping reagents have been widely employed to manipulating the growth of crystals [27-30]. This is because capping reagents can preferentially adsorb onto specific crystallographic planes to change the surface energy or reactant transport conditions, hence, influencing the crystal evolution process [31, 32]. Dissociative adsorption of HF on {001} facets of anatase TiO2 is a typical example of utilizing capping reagent to affect crystal growth process, enabling the synthesis of such high energy faceted crystals [29, 33, 34]. Under normal synthetic conditions, the anatase TiO2 with high energy {001} facets are diminished during the crystal growth due to the formation of lower surface energy {101} facets. However, in presence of

———————————— Address correspondence to Guozhong Wang, gzhwang@issp.ac.cn; Huijun Zhao, h.zhao@griffith.edu.au

Nano Res

5

5

HF as the capping reagent, the preferential adsorption of HF on {001} facets dramatically reduces the surface energy of {001} facets to a level lower than that of {101} facets, leading to the formation of {001} facet dominated products [35-37]. Recently, we have demonstrated that the co-capping reagent systems made of HF and organic hydroxyl acids such as citric acid, glycolic acid, lactic acid, malic acid and tartaric acid have even more dramatic effect of crystal growth, capable of producing unconventional continuously curved TiO2 crystal facets [28]. This inspires us to investigate if such co-capping reagent concept could be extended to solid organic substrates containing different functional groups for the crystal formation and transformation.

Herein, we report the growth and in situ transformation of HTiOF3 and TiO2 crystals on chitosan-polyvinyl alcohol (CS/PVA) co-polymer substrate with rich amine and hydroxyl groups under HF vapour phase hydrothermal (HF-VPH) conditions. The substrate formed hexagonal-shaped HTiOF3 crystals on the substrate surface can be in situ transformed into {001} faceted anatase TiO2 nanosheets with curved edges and then reversely transformed back to cross-shaped and hollow square-shaped HTiOF3 crystals, resulting from the synergetic effect of CS/PVA under HF-VPH conditions. The findings of this work suggest crystal formation process can be influenced by the substrate functional groups, which could be applicable for growth and transformation of other crystals.

2 Experimental 2.1 Chemicals and reagents Chitosan (CS, deacetylation: ≥ 95%, viscosity: 100-200 mPa.s), poly (vinyl alcohol) (PVA, alcoholysis degree: 87.0~89.0 % (mol/mol)) were purchased from Aladdin Reagent Company, acetic

acid and hydrochloric acid were purchased from Sinopharm Chemical Reagent Co., Ltd, titanium tetrafluoride (TiF4) was purchased from Beijing J＆K Scientific co., LTD. Hydrofluoric acid (HF) was purchased from Shanghai Su Yi chemical reagent co., LTD. All the chemicals were used without further purification. 2.2 Preparation of CS/PVA and cellulose substrates For fabrication of CS/PVA substrate, 4.00 g chitosan powders (from Aladdin) were hydrolyzed by 120 mL 3 mol/L HCl at 105 oC for 2 h under vigorous stirring. After the acid hydrolysis, the suspension was centrifuged at 14000 rpm for 5 min, washed with deionized water three times before freeze-drying. Aqueous chitosan solution (1 wt%) was prepared by dissolving 1.00 g chitosan after acid hydrolysis into 100 mL deionized water containing 0.6 mL acetic acid. 49 mL deionized water was added into 1 mL 1 wt% chitosan solution to obtain 0.02 wt% chitosan solution, then mixed with 50 mL 0.02 wt% PVA solution. After magnetic stirring, the CS/PVA mixed solution was rapidly frozen in a freezer (-50 oC), then freeze-dried in a freeze-dryer (Scientz-12N) at the shelf temperature of -40 oC for 72 h [38]. The obtained CS/PVA fibres were calcined in an oven at 250 oC for 1 h in air at a heating rate of 1 oC/min and used as the substrate. Cellulose was extracted from cornstalk [39]. A suitable amount of celluloses were compressed into sheet and calcined in air at 250 oC for 1 h and the resultant cellulose sheet was used as the substrate. 2.3 Vapour-phase hydrothermal synthesis on CS/PVA and cellulose substrates In a typical process, 1 g TiF4 was dissolved in 50 mL deionized water. Appropriate amount of CS/PVA or cellulose substrate materials was immersed into the TiF4 solution for 1 h. The CS/PVA or cellulose

Nano Res

6

6

substrate materials with sufficient amount of adsorbed TiF4 were taken up and placed on a Teflon plate (0.2×1.5×2 cm) and freeze-drying in a freeze-dryer. The Teflon plate with freeze-dried CS/PVA or cellulose substrate containing TiF4 was pleased on a Teflon holder 2 cm above the liquid level in a Teflon-lined stainless steel autoclave (100 mL) containing 20 mL of 2% HF solution. The HF-VPH reactions were carried out in sealed autoclaves subjected to 200 oC in a furnace for different reaction time (0.5-3.5 h). After the reaction, the autoclaves were removed from the furnace immediately (Hot, be careful!) and allowed to cool at room temperature. The resultant products were separated from Teflon plate for subsequent characterizations. 2.4 Characterizations Morphological properties of the as-prepared samples were investigated by a field emission scanning electron microscope (FESEM, Quanta 200FEG) operated at an accelerating voltage of 10.0 kV and an atomic force microscope (AFM, DI Innova). The microstructure was examined by a high resolution transmission electron microscopy (HRTEM, JEOL 2010) with an acceleration voltage of 200 kV. Thermal stability of samples was assessed using a thermogravimetric analyzer (Pyris 1 TGA, Perkin-Elmer), the samples were heated at 2 oC min-1 from 50 to 700 oC. Powder X-ray diffraction (XRD) patterns were recorded on a Philips X-Pert Pro X-ray diffractometer with Cu Kα radiation (λKα1=1.5418 Å). 3 Results and discussion 3.1 Crystal Growth and Transformation Chitosan (CS) and polyvinyl alcohol (PVA) are amine and hydroxyl rich compounds. In this work, tubular CS/PVA co-polymer fibres are synthesized

and used as the substrate. The as-purchased chitosan powders were hydrolyzed in hydrochloric acid solution to obtain low molecular weight chitosan (Figure S1(a) in the ESM). The CS/PVA hollow fibres were fabricated by freeze-drying of a CS and PVA mixture solution (Figure S1(b and c) in the ESM). A thermal treatment process was employed to improve the mechanical strength of the as-synthesized tubular CS/PVA. The thermogravimetry analysis (TGA) data indicate that the decomposition of tubular CS/PVA begins at 220

oC (Figure S1(d) in the ESM). Therefore, the tubular CS/PVA can be partially oxidized at 250 oC to improve inter-fibres connection strength (Figure S1(e and f) in the ESM). The thermally treated tubular CS/PVA fibres are used as the substrate for the subsequent HF-VPH reactions.

Figure 1 SEM images of the as-synthesized samples using the

thermally treated CS/PVA fibres as substrate subjected to

HF-VPH treatments at 200 oC for (a) 0.5 h, (b) 1.0 h, (c) 1.5 h,

(d) 2.5 h, (e) 3.0 h, (f) 3.5 h.

In a typical HF-VPH synthesis process, the thermally treated tubular CS/PVA with adsorbed

Nano Res

7

7

TiF4 is placed on a Teflon holder above the HF solution in a Teflon-lined autoclave (Figure S2 in the ESM). The sealed autoclave is heated to 200 oC for 0.5-3.5 h. Figure 1 shows the field emission scanning electron microscope (FESEM) images of products at different stages of HF-VPH processes. Irregular-shaped particles can be found after 0.5 h of HF-VPH treatment (Figure 1(a)). After 1 h of HF-VPH treatment, these irregular-shaped particles are developed into well-defined hexagonal-shaped crystals with a uniform size of ~2 µm (Figure 1(b)). These well-defined hexagonal crystals can be identified as HTiOF3 crystals (see later for details) [22]. When the treatment time is increased to 1.5 h, both the size and population of the formed hexagonal-shaped HTiOF3 crystals are rapidly decreased while transforming themselves into square-shaped nanosheets with a uniform size of ~3.2 µm in widths and ~200 nm in thickness (Figure 1(c)). The formed square-shaped nanosheets can be identified as {001} facets dominated anatase TiO2 (see later for details) [29]. Figure 1(d) shows a dramatically increased population of {001} facets dominated anatase TiO2 nanosheets when the sample is subjected to 2.5 h of HF-VPH treatment. At this stage, the HTiOF3 crystals are found to be completely transformed into the anatase TiO2 nanosheets. With a prolonged reaction time to 3 h, the formed anatase TiO2 nanosheets are completely transformed into cross-shaped structures with unchanged profile sizes but increased thickness (Figure 1(e)). When the reaction is proceed further, the four branches of the cross-shaped structures disappeared, leading to the formation of square-shaped hollow structures having widths approximately half of the cross-shaped structures (Figure 1(f)). To our surprise, both the cross-shaped structure and the square-shaped hollow structure are identified as HTiOF3 crystals (see later for details). This means that the anatase TiO2 transformed from HTiOF3 crystals can be reversely transformed back to HTiOF3 crystals, which has not

been previously reported. The above observed structural changes confirmed by SEM analysis suggest a four-stage crystal formation/transformation process: (i) growth of hexagonal-shaped HTiOF3 crystals (HF-VPH treatment ≤ 1 h); (ii) transformation of HTiOF3 into anatase TiO2 nanosheets (1 h < HF-VPH treatment ≤ 2.5 h); (iii) revered transformation of anatase TiO2 into cross-shaped HTiOF3 crystals (2.5 h < HF-VPH treatment ≤ 3 h); (iv) converting cross-shaped HTiOF3 crystals into square-shaped hollow HTiOF3 crystals (HF-VPH treatment ≥ 3.5 h). 3.2 Crystal Structure Confirmation

Figure 2 (a), (b) TEM images, (c) HRTEM image and (d)

SAED pattern of {001} faceted anatase TiO2 nanosheets grown

inside and across the wall of CS/PVA tubular fibres.

The X-ray diffraction (XRD) patterns of the as-synthesized products at different stages of HF-VHP process were obtained to provide the needed crystal structural information (Figure S3 in the ESM). The characteristics of XRD patterns obtained from samples treated for 0.5 and 1 h (Stage I) are the same as those obtained in our previous work for the hexagonal-shaped HTiOF3 crystals [22]. This confirms that the hexagonal-shaped crystals shown in Figure 1(b) are of HTiOF3 crystals. The precise structural and compositional information of

Nano Res

8

8

the hexagonal-shaped HTiOF3 crystals have been reported in our previous work [22]. The SEM image from the sample treated for 1.5 h (Figure 1(c)) indicating the presence of both nanosheets and small numbers of irregular-shaped particles. The corresponding XRD pattern composes the diffraction peaks of anatase TiO2 and HTiOF3 crystals, confirming that the nanosheets and the irregular-shaped particles are anatase TiO2 and the remaining HTiOF3 crystals, respectively. The XRD pattern obtained from the sample treated for 2.5 h containing pure nanosheets (Figure 1(d)) can be indexed to pure anatase TiO2 crystals, indicating the completed transformation of HTiOF3 crystals at the end of Stage II. Transmission electron microscopy (TEM) images of the nanosheets sample (treated for 2.5 h) shown in Figures 2(a and b) indicate that the nanosheets are not only grown on the surface but also the interior space and across the wall of the tubular CS/PVA co-polymer substrate. The high-resolution TEM image (HRTEM) displays well-resolved (020) and (200) atomic planes with an identical lattice spacing of 1.89 Å (Figure 2(c)). The corresponding selected area electron diffraction (SAED) pattern can be indexed as the diffraction spots of [001] zone of anatase TiO2 (Figure 2(d)). These results confirmed that the top and bottom surface of the nanosheets are enclosed by {001} facets.

Figure 3 TEM, HRTEM images and SAED patterns of the

cross-shaped HTiOF3 crystals (3.0 h HF-VPH treated sample)

with the incident beam (a)-(d) along the <001> and (e)-(g)

along <010> directions.

The XRD patterns obtained from the samples

treated for 3 and 3.5 h reveal the same diffraction peaks as that obtained from the sample treated for 1 h, confirming that the obtained cross-shaped structures (Figure 1(e)) and the square-shaped hollow structures (Figure 1(f)) are of HTiOF3 crystals. These XRD data are well corresponded to the observed structural changes by SEM for Stages III and IV. Figure 3(a) shows the TEM image of a well-defined cross-shaped HTiOF3 crystal obtained from a sample being HF-treated for 3 h. It reveals an approximately 4 µm profile size with four symmetric isometric branches. Figure 3(b, c and d) are the HRTEM images and corresponding SAED patterns of the right, bottom branches and the intersectional region, respectively. As shown in HRTEM images (Figure 3(b, c and d)), the lattice spacing of the fringes parallel to the edges of the branches were determined to be 3.90 Å, which can

Nano Res

9

9

be indexed as the {100} planes of HTiOF3 crystal. The slightly larger value compared to the published one (3.75 Å)[22] indicates a small expansion of the crystal structure induced by the presence of CS/PVA. The equivalent SAED patterns (the inset images in the Figure 3(b, c and d)) obtained in different regions of the cross-shaped structure confirm the single crystalline characteristics and they are all indexed to the (hk0) reflections with [001] zone axis. For further confirmation, TEM and SAED patterns with a different orientation (i.e., <010>, Figure 3(e, f and g)) were acquired with an erected crystal (~600 nm thick). The lattice spacing and interfacial angle (Figure 3(f)) were measured as 6.50 Å, 3.90 Å, and 90o, which were in good agreement of those for (002) and (100) planes of the HTiOF3 crystal (6.30 Å, 3.75 Å, and 90o) [22]. Again, the larger lattice spacing in c axis signifies the slightly expanded crystal unit cell. In good consistence with the XRD (Figure S3 in the ESM) data, these results prove that the obtained cross-shaped structure composed of an identical orthorhombic HTiOF3 crystal structure as the hexagon-shaped crystals. Figure 4(a) shows a TEM image of a typical square-shaped hollow crystal with a width of ~2 µm. The HRTEM image (Figure 4(b)) obtained with electron beam parallel to the normal of the planar crystal displays mutually perpendicular fringes with identical lattice spacing of 3.90 Å, consistent with the value of (100) and (010) planes of the cross-shaped structures (HF-VPH treated for 3.0 h). The corresponding SAED pattern of the single crystal (Figure 4(c)) can be indexed to be the one with [001] zone axis. Combing with the TEM and XRD (Figure S3 in the ESM) results, it can be concluded that the obtained square-shaped hollow crystals and hexagonal-shaped crystals share identical HTiOF3 crystal structures [22].

Figure 4 (a) TEM, (b) HRTEM images and (c) SAED pattern

of a typical square-shaped hollow crystal (3.5 h HF-VPH

treated sample) with the incident beam along the <001>

direction.

3.3 Mechanistic Aspects The hexagonal-shaped HTiOF3 crystals are formed on CS/PVA substrate during Stage I of the HF-VPH treatment via the reaction of the adsorbed TiF4 with the condensed water on the CS/PVA substrate surface (Equation 1).

HFHTiOFOHTiF +¾®¾+ 324 (1)

The produced HF reacts with the glucosamines of chitosan [40], facilitating the formation of HTiOF3 crystals (Equation 2).

(2)

Nano Res

10

10

The growth of HTiOF3 crystals ends when all adsorbed TiF4 is consumed.

The transformation of HTiOF3 crystals into {001} faceted anatase TiO2 during Stage I occurs via the reaction of HTiOF3 with the condensed water on the CS/PVA substrate surface (Equation 3).

HFTiOOHHTiOF 3223 +¾®¾+ (3)

A large amount of HF is produced during this stage of the reaction. When the produced HF from the dissociation of exceeded the adsorption capacity of glucosamines in the CS/PVA substrate, the built-up HF concentration in the thin-layer liquid reaction zone on the surface of the CS/PVA substrate triggers a reversed reaction to transform TiO2 back to HTiOF3, signifying the start of Stage III

process (Equation 4).

OHHTiOFHFTiO 232 3 +¾®¾+ (4)

A detailed structural evolution process during Stages III and IV, transforming the {001} faceted anatase TiO2 crystals to the HTiOF3 crystals are revealed by SEM images of the products in 10 min interval between 150 and 210 min (Figure 5).

Figure 5(a) shows a matured {001} faceted anatase TiO2 crystal obtained at the end of Stage II (150 min). A partially transformed product can be obtained at 160 min of HF-VPH treatment (Figure 5(b)), where a layer structured immature HTiOF3 crystal is formed on the {001} surface of the anatase TiO2 crystal while the dissolution of anatase TiO2 crystal is clearly visible from the inset image. This

Figure 5 The crystal transformation process on the thermally treated CS/PVA fibres under 200 oC HF-VPH conditions for (a) 150

min, (b) 160 min, (c) 170 min, (d) 180 min, (e) 190 min, (f) 200 min, (g) 210 min.

indicates that the dissolution of TiO2 crystal occurs concurrently with the formation of HTiOF3 crystal,

and the entire process is highly localized. Such localized crystal transformation processes can be

Nano Res

11

11

readily achieved because of the unique reaction environment introduced under VPH conditions, where the mass transport is limited along the surface direction of the substrate within the thin-layer liquid reaction zone [3, 22, 41, 42]. It should be noted that it is very difficult, if not impossible, to achieve similar localized crystal transformation processes under conventional liquid phase hydrothermal conditions because the dissolution product will disuse away from the precursor crystal to the bulk solution and new crystal formation could occur only after the bulk solution is oversaturated by the dissolution product. It takes a further 10 min to complete the transformation of the anatase TiO2 crystals to HTiOF3 crystals (Figure 5(c)). At this reaction point, the formed HTiOF3 crystal exhibits a square shape with a layer-like surface morphology. Importantly, the structural defects observed at the four corner positions indicate the selective etching has occurred. The well-defined cross-shaped HTiOF3 crystal is obtained when the selective etching process proceeds for another 10 min, signifying the end of Stage III (Figure 5(d)). Unexpectedly, the SEM image obtained from the sample after an additional 10 min treatment reveals that the well-defined cross-shaped HTiOF3 crystal is converted into almost identical sized near square shape HTiOF3 crystal with only small spaces in four corners left unfilled (Figure 5(e)), indicating a selective HTiOF3 crystal regrowth process has taken place at the corners of the cross. The SEM image of the sample after another 10 min treatment shows a square-shaped crystal with smaller size compared to its precursor crystal (Figure 5(f)). Additionally, the structural defect on the central square is obviously visible. This suggests that the observed size reduction and the structural defect could be the result of a selective etching process, which is totally unanticipated. Such a selective etching process is confirmed by the SEM image obtained from the sample being treated for further 10 min (210 min),

where the square-shaped precursor crystal is converted into a square frame with a size reduced to half of its precursor’s size (Figure 5(g)), resulting from the selective etching from edges and the centre parts of the precursor crystal. No noticeable structural change can be observed for a prolonged treatment time beyond 210 min, indicating the end of Stage IV. Currently, we cannot give a reasonable explanation on the selective etching mechanism of square-shaped HTiOF3 crystals. In our previous report, the theoretical calculations have revealed that there is no Ti-O-Ti bond in the HTiOF3 crystal structure composed of TiOF5 octahedra and oxygen atoms locate at the terminal corners of TiOF5 octahedra. The absence of Ti-O-Ti bonds may result in the unstable HTiOF3 crystal structure with reaction time to readily form square-shaped hollow structure with decreased sizes. This deserves a further investigation in future work [22].

The CS/PVA substrate used in this work contains rich amino and hydroxyl groups. In order to explore whether amino or hydroxyl group is responsible for the inductive effect on the crystal growth/transformation, the thermally treated cellulose (Figure S4 in the ESM) possesses only hydroxyl groups was used to replace CS/PVA as substrate for synthesis under identical HF-VPH conditions. Figure S5 in the ESM shows SEM images of products under different reaction times of 0.5 to 3.0 h. The obtained XRD patterns indicate that except the random-shaped particles products obtained from 0.5 h treated sample (Figure S5(a) in the ESM) are amorphous, all other samples are anatase TiO2 crystals (Figure S6 in the ESM). The random-shaped particles are evolved to sheet-like crystals with an average size of ~1 µm after 1 h treatment (Figure S5(b) in the ESM). The well-defined square-shaped crystal sheets with a uniform size of ~2.0 µm are obtained when the sample was treated for 1.5 h (Figure S5(c) in the ESM). The HRTEM image (Inset in Figure S5(c) in the ESM) confirms these anatase TiO2 crystals are

Nano Res

12

12

dominated by the exposed {001} facets [29]. The selective etching on {001} faceted surface start to occur with 2.5 h reaction time (Figure S5(d) in the ESM) and the entire (001) surfaces are etched away when the reaction time was prolonged to 3 h (Figures S5(e, f and g) in the ESM), similar to previously observed etch phenomena from {001} faceted anatase TiO2 crystals [43]. Although the growth of {001} faceted anatase TiO2 crystals on the cellulose substrate is observed, the formation of HTiOF3 crystals and transformation to {001} faceted anatase TiO2 crystals, and revised transformation to HTiOF3 crystals did not occur under the same HF-VPH reaction conditions. This may imply that the presence of amino groups is a necessity to achieve the reversible transformation between {001} faceted anatase TiO2 and HTiOF3 crystals.

4 Conclusions In summary, we have demonstrated the utilisation of CS/PVA substrate possessing rich amino groups to induce the direct growth of hexagonal-shaped HTiOF3 crystals and their in situ sequential transformation into {001} faceted anatase TiO2 crystal nanosheets, cross- and hollow square-shaped HTiOF3 crystals under HF vapour phase hydrothermal conditions. Such crystal formation and transformation occurred is likely due to the presence of rich amino groups as when the thermally treated cellulose only contains hydroxyl groups without amino groups is used as the substrate, rather than the sequential crystal transformation, only direct formation of {001} faceted TiO2 nanocrystal sheets is observed. The obtained results confirm that the substrate organic functional groups can be used to induce crystal growth and structural transformation. Such a substrate functional group induced crystal formation/transformation approach could be applicable to other material systems.

Acknowledgements

This work was financially supported by the Natural Science Foundation of China (Grant No. 51372248 and 51432009), the CAS/SAFEA International Partnership Program for Creative Research Teams of Chinese Academy of Sciences, China, and the CAS Pioneer Hundred Talents Program.

Electronic Supplementary Material: Supplementary material (AFM image, TGA data，XRD patterns, additional SEM and TEM images) is available in the online version of this article at http://dx.doi.org/10.1007/s12274-***-****-*

References

[1] Lee, I.; Delbecq, F.; Morales, R.; Albiter, M. A.; Zaera, F.

Tuning selectivity in catalysis by controlling particle

shape. Nature Mater. 2009, 8, 132-138.

[2] Tsung, C.-K.; Kuhn, J. N.; Huang, W.; Aliaga, C.; Hung,

L.-I.; Somorjai, G. A.; Yang, P. Sub-10 nm platinum

nanocrystals with size and shape control: catalytic study

for ethylene and pyrrole hydrogenation. J. Am. Chem. Soc.

2009, 131, 5816-5822.

[3] Liu, P.; Zhang, H.; Liu, H.; Wang, Y.; An, T.; Cai, W.;

Yang, H.; Yao, X.; Zhu, G.; Webb, R.; Zhao, H.

Vapor-phase hydrothermal growth of novel segmentally

configured nanotubular crystal structure. Small 2013, 9,

3043-3050.

[4] Yang, S.; Hou, Y.; Zhang, B.; Yang, X. H.; Zhang, H.;

Zhao, H. J.; Yang, H. G. Precisely controlled

heterogeneous nucleation sites for TiO2 crystal growth.

CrystEngComm 2014, 16, 7502-7506.

[5] Yang, S.; Huang, N.; Jin, Y. M.; Zhang, H. Q.; Su, Y. H.;

Yang, H. G. Crystal shape engineering of anatase TiO2

and its biomedical applications. CrystEngComm 2015, 17,

6617-6631.

Nano Res

13

13

[6] Chen, J.; Wang, Y.; Deng, Y. Combined effects of Bi

deficiency and Mn substitution on the structural

transformation and functionality of BiFeO3 films. J. Appl.

Phys. 2014, 116, 174102.

[7] Han, J.; Chen, R.; Wang, M.; Lu, S.; Guo, R. Core–shell

to yolk–shell nanostructure transformation by a novel

sacrificial template-free strategy. Chem. Commun. 2013,

49, 11566-11568.

[8] Liu, J.; Li, Y.; Fan, H.; Zhu, Z.; Jiang, J.; Ding, R.; Hu, Y.;

Huang, X. Iron oxide-based nanotube arrays derived from

sacrificial template-accelerated hydrolysis: large-area

design and reversible lithium storage. Chem. Mater. 2009,

22, 212-217.

[9] Liu, B.; Zeng, H. C. Direct growth of enclosed ZnO

nanotubes. Nano Res. 2009, 2, 201-209.

[10] Qiu, Y.; Xu, G.-L.; Kuang, Q.; Sun, S.-G.; Yang, S.

Hierarchical WO3 flowers comprising porous

single-crystalline nanoplates show enhanced lithium

storage and photocatalysis. Nano Res. 2012, 5, 826-832.

[11] Goubet, N.; Pileni, M.-P. Negative supracrystals inducing

a FCC-BCC transition in gold nanocrystal superlattices.

Nano Res. 2013, 7, 171-179.

[12] Chaudhari, A. K.; Nagarkar, S. S.; Joarder, B.; Mukherjee,

S.; Ghosh, S. K. Structural Dynamism and Controlled

Chemical Blocking/Unblocking of Active Coordination

Space of a Soft Porous Crystal. Inorg. Chem. 2013, 52,

12784-12789.

[13] Yuan, S.; Li, X.; Zhang, X.; Jia, Y. Fabrication of Au–Ag

Bimetallic Nanostructures Through the Galvanic

Replacement Reaction of Block Copolymer-Stabilized Ag

Nanoparticles with HAuCl4. Sci. Adv. Mater. 2015, 7,

918-923.

[14] Yao, W.; Li, F.-L.; Li, H.-X.; Lang, J.-P. Fabrication of

hollow Cu2O@ CuO-supported Au-Pd alloy nanoparticles

with high catalytic activity through the galvanic

replacement reaction. J. Mater. Chem. A 2015, 3,

4578-4585.

[15] Ai, L.; Yue, H.; Jiang, J. Sacrificial template-directed

synthesis of mesoporous manganese oxide architectures

with superior performance for organic dye adsorption.

Nanoscale 2012, 4, 5401-5408.

[16] Fujishima, A. Electrochemical photolysis of water at a

semiconductor electrode. Nature 1972, 238, 37-38.

[17] Chen, X.; Mao, S. S. Titanium dioxide nanomaterials:

synthesis, properties, modifications, and applications.

Chem. Rev. 2007, 107, 2891-2959.

[18] Yoshida, R.; Suzuki, Y.; Yoshikawa, S. Effects of

synthetic conditions and heat-treatment on the structure of

partially ion-exchanged titanate nanotubes. Mater. Chem.

Phys. 2005, 91, 409-416.

[19] Wang, C.-C.; Ying, J. Y. Sol-gel synthesis and

hydrothermal processing of anatase and rutile titania

nanocrystals. Chem. Mater. 1999, 11, 3113-3120.

[20] Zhang, H.; Han, Y.; Liu, X.; Liu, P.; Yu, H.; Zhang, S.;

Yao, X.; Zhao, H. Anatase TiO2 microspheres with

exposed mirror-like plane {001} facets for high

performance dye-sensitized solar cells (DSSCs). Chem.

Commun. 2010, 46, 8395-8397.

[21] Zhang, H.; Liu, P.; Li, F.; Liu, H.; Wang, Y.; Zhang, S.;

Guo, M.; Cheng, H.; Zhao, H. Facile Fabrication of

Anatase TiO2 Microspheres on Solid Substrates and

Surface Crystal Facet Transformation from {001} to

{101}. Chem. Eur. J. 2011, 17, 5949-5957.

[22] Liu, P.; Wang, Y.; Zhang, H.; An, T.; Yang, H.; Tang, Z.;

Cai, W.; Zhao, H. Vapor ‐ Phase Hydrothermal

Transformation of HTiOF3 Intermediates into {001}

Faceted Anatase Single‐Crystalline Nanosheets. Small

2012, 8, 3664-3673.

[23] Wulff, G. On the question of speed of growth and

dissolution of crystal surfaces. Z. Krystallogr. Mineral.

1901, 34, 449-530.

[24] Neagu, D.; Tsekouras, G.; Miller, D. N.; Menard, H.;

Irvine, J. T. S. In situ growth of nanoparticles through

control of non-stoichiometry. Nat. Chem. 2013, 5,

916-923.

[25] Borgström, M. T.; Wallentin, J.; Trägårdh, J.; Ramvall, P.;

Ek, M.; Wallenberg, L. R.; Samuelson, L.; Deppert, K. In

situ etching for total control over axial and radial

nanowire growth. Nano Res. 2010, 3, 264-270.

[26] Dong, L.; Shi, H.; Cheng, K.; Wang, Q.; Weng, W.; Han,

Nano Res

14

14

W. Shape-controlled growth of SrTiO3 polyhedral

submicro/nanocrystals. Nano Res. 2014, 7, 1311-1318.

[27] Yan, Q.; S. Raghuveer, M.; Li, H.; Singh, B.; Kim, T.;

Shima, M.; Bose, A.; Ramanath, G. Rod-Shaped

Assemblies of FePt-PtTe2 through Dynamic Templating.

Adv. Mater. 2007, 19, 4358-4363.

[28] Yang, S.; Yang, B. X.; Wu, L.; Li, Y. H.; Liu, P.; Zhao, H.;

Yu, Y. Y.; Gong, X. Q.; Yang, H. G. Titania single crystals

with a curved surface. Nat. Commun. 2014, 5.

[29] Yang, H. G.; Sun, C. H.; Qiao, S. Z.; Zou, J.; Liu, G.;

Smith, S. C.; Cheng, H. M.; Lu, G. Q. Anatase TiO2 single

crystals with a large percentage of reactive facets. Nature

2008, 453, 638-641.

[30] Mettela, G.; Kulkarni, G. U. Facet selective etching of Au

microcrystallites. Nano Res. 2015, 8, 2925-2934.

[31] Jun, Y.-w.; Choi, J.-s.; Cheon, J. Shape Control of

Semiconductor and Metal Oxide Nanocrystals through

Nonhydrolytic Colloidal Routes. Angew. Chem., Int. Ed.

2006, 45, 3414-3439.

[32] Sounart, T. L.; Liu, J.; Voigt, J. A.; Hsu, J. W. P.; Spoerke,

E. D.; Tian, Z.; Jiang, Y. B. Sequential Nucleation and

Growth of Complex Nanostructured Films. Adv. Funct.

Mater. 2006, 16, 335-344.

[33] Wen, C. Z.; Jiang, H. B.; Qiao, S. Z.; Yang, H. G.; Lu, G.

Q. M. Synthesis of high-reactive facets dominated anatase

TiO2. J. Mater. Chem. 2011, 21, 7052-7061.

[34] Barnard, A.; Curtiss, L. Prediction of TiO2 nanoparticle

phase and shape transitions controlled by surface

chemistry. Nano lett. 2005, 5, 1261-1266.

[35] Liu, S.; Yu, J.; Jaroniec, M. Anatase TiO2 with Dominant

High-Energy {001} Facets: Synthesis, Properties, and

Applications. Chem. Mater. 2011, 23, 4085-4093.

[36] Xiang, G.; Li, T.; Wang, X. Reactive facets covered

mosaic spheres of anatase TiO2 and related

pseudo-isotropic effect. Inorg. Chem. 2011, 50,

6237-6242.

[37] Han, X.; Kuang, Q.; Jin, M.; Xie, Z.; Zheng, L. Synthesis

of titania nanosheets with a high percentage of exposed

(001) facets and related photocatalytic properties. J. Am.

Chem. Soc. 2009, 131, 3152-3153.

[38] Qian, L.; Zhang, H. Green synthesis of chitosan-based

nanofibers and their applications. Green Chem. 2010, 12,

1207-1214.

[39] Abe, K.; Iwamoto, S.; Yano, H. Obtaining cellulose

nanofibers with a uniform width of 15 nm from wood.

Biomacromolecules 2007, 8, 3276-3278.

[40] Simkovitch, R.; Huppert, D. Excited-state proton transfer

of weak photoacids adsorbed on biomaterials: proton

transfer to glucosamine of chitosan. J. Phys. Chem. A

2015, 119, 641-651.

[41] Liu, P.; Zhang, H.; Liu, H.; Wang, Y.; Yao, X.; Zhu, G.;

Zhang, S.; Zhao, H. A facile vapor-phase hydrothermal

method for direct growth of titanate nanotubes on a

titanium substrate via a distinctive nanosheet roll-up

mechanism. J. Am. Chem. Soc. 2011, 133, 19032-19035.

[42] Zhang, H.; Li, Y.; Liu, P.; Li, Y.; Yang, D.; Yang, H.; Zhao,

H. A new vapor-phase hydrothermal method to

concurrently grow ZnO nanotube and nanorod array films

on different sides of a zinc foil substrate. Chem. Eur. J.

2012, 18, 5165-5169.

[43] Wang, Y.; Zhang, H.; Han, Y.; Liu, P.; Yao, X.; Zhao, H. A

selective etching phenomenon on {001} faceted anatase

titanium dioxide single crystal surfaces by hydrofluoric

acid. Chem. Commun. 2011, 47, 2829-2831.