ISSN 1998-0124 CN 11-5974/O4

2019, 12(1): 000–000 https://doi.org/10.1007/s12274-020-2711-2

Res

earc

h Ar

ticle

Phase transformation at controlled locations in nanowires byin situ electron irradiation Hongtao Zhang1, Wen Wang1, Tao Xu1, Feng Xu1 (), and Litao Sun1,2 ()

1 SEU-FEI Nano-Pico Center, Key Laboratory of MEMS of Ministry of Education, Southeast University, Nanjing 210096, China 2 Center for Advanced Materials and Manufacture, Southeast University-Monash University Joint Research Institute, Suzhou 215123, China © Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020 Received: 30 November 2019 / Revised: 9 February 2020 / Accepted: 10 February 2020

ABSTRACT Solid state phase transformations have drawn great attention because they can be effectively exploited to control the microstructure and property of materials. Understanding the physics of such phase transformation processes is critical to designing materials with controlled structure and with desired properties. However, in traditional ex situ experiments, it is hard to achieve position controlled phase transformations or obtain desirable crystal phase on nanometer scale. Meanwhile the underlying mechanisms of the reaction processes are not fully understood due to the lack of direct and real-time observation. In this paper, we observe phase transformation from body-centered tetragonal PX-PbTiO3 to monoclinic TiO2(B) on the atomic scale by in situ electron irradiation during heat treatment in transmission electron microscope, at pre-defined locations on the sample. We demonstrate that by controlling the location of the incident electron beam, a porous TiO2(B) crystal structure can be formed at the desired area on the nanowire, which is difficult to achieve by traditional synthesis methods. Upon in situ heating, the Pb atoms in the crystal migrate out of the pristine nanowire through inelastic scattering under incident electrons while high temperature(> 400 °C) provides energy for the crystallization of TiO2(B) and the volatilization of a substantial number of Pb atoms, which makes the resultingTiO2(B) nanowires to be porous. In contrast, at temperatures < 400 °C, the segregated Pb atoms form Pb particles and the TiOx nanowires remain in the amorphous state. This work not only provides in situ visualization of the phase transition from the PX-PbTiO3 to monoclinic TiO2(B), but also suggests a crystallography engineering strategy to obtain the desired crystal phase at controlled locations on the nanometer scale.

KEYWORDS phase transformation, controlled locations, in situ, transmission electron microscopy

1 Introduction Solid state phase transformations in different materials have drawn great attention because they can be effectively used to control the microstructure of materials [1–5]. Since this process is usually accompanied by the formation of a new structure, understanding the physics of such phase transformation processes is critical to designing materials with controlled structure/shape and having the desired properties [1, 2]. For amorphous alloys, phase transformation has been proposed as an effective method to improve mechanical properties [5]. Additionally, for materials or complex structures that are difficult to directly synthesize by traditional physical or chemical reactions, phase transformation processes can provide effective alternative methods of preparation [3, 6–8]. For example, it has been reported that ferroelectric PbTiO3 nanowires with different diameter and length can be obtained after phase transformation during heat treatment in air [3, 6]. However, in traditional ex situ experiments, due to the lack of direct and real-time observation of the reaction process, the underlying mechanisms are not always fully understood. In recent years, there has been great progress in the development of in situ transmission electron microscopy (TEM) techniques [9, 10]. The possibility to directly follow reactions in real time on the atomic scale during in situ TEM experiments has evidently

facilitated research on phase transformation processes [2, 5]. TiO2 is a commonly used semiconducting material with

great application potential in various areas and has therefore been extensively studied [8, 11]. Several phases of TiO2 have been identified, including anatase, rutile, brookite, and the monoclinic TiO2(B) structure. These TiO2 structures can be obtained under different conditions and the transformations of one phase into another have been investigated by several methods [8, 11–15]. Among the different TiO2 structures, monoclinic TiO2, also known as TiO2(B), has recently been applied in many fields, such as lithium-ion batteries [16–19], catalysis [8], and as a humidity sensor [20]. TiO2(B) used as a lithium-ion battery anode shows better performance when compared with other TiO2 phases due to its unique open channel structure (Fig. 1(j) and Fig. S7(b) in the Electronic Supplementary Material (ESM)) and the pseudocapacitive lithiation/delithiation process [18, 19, 21]. Various methods including solid phase reaction [23], sol-gel [11], and electro-phoretic deposition [24] have been proposed and developed for the synthesis TiO2(B), each of them resulting in TiO2(B) nanostructures with different shapes and morphologies, such as nanoparticles [21], nanotubes [25], nanowires [26], and nanosheets [19]. In addition, porosity can be introduced in these TiO2(B) nanostructures by tuning and controlling the reaction conditions in experiments [19, 21, 25, 27]. The

Address correspondence to Litao Sun, slt@seu.edu.cn; Feng Xu, fxu@seu.edu.cn

Nano Res.

| www.editorialmanager.com/nare/default.asp

2

decomposition of H2Ti3O7 to TiO2(B) has been studied [7, 28]. Nevertheless, there is still a lack of a direct and real-time observation of the phase transformation to TiO2(B) on the atomic scale and the mechanism of the formation process is still not fully understood, which means that experimental parameters including reaction temperature and time cannot be precisely controlled [8, 11, 21, 23, 25, 26, 29]. In this context, it is of great interest to perform in situ TEM to study the phase transformation and growth process of TiO2(B) at the atomic scale.

In this work, we have used a heating holder (Protochips Inc.) in an aberration-corrected transmission electron microscope to observe the phase transformation of PX-PbTiO3 into a porous and hollow TiO2(B) structure. Furthermore, by con-trolling the location of electron beam irradiation, the reaction area can be precisely defined to prepare or build complex structures with nanometer-scale precision, while simultaneously studying the phase transformation process at the atomic scale. We find that inelastic scattering of incident electrons, and high temperature are the two necessary conditions to initiate this phase transformation. The bond breakage of Pb atoms happens through an inelastic scattering process and then due to thermal effect, Pb atoms move out of the PX-PbTiO3 nanowires. At temperatures < 400 °C, the segregated Pb atoms form Pb particles leaving behind amorphous TiOx nanowires due to the lack of sufficient energy for the crystallization of TiOx at these temperatures. In contrast, at higher temperatures,

Pb atoms volatilize into the surrounding environment and the TiOx nanowires transform into monoclinic TiO2(B). The continuous volatilization of Pb atoms during the phase transformation results in TiO2(B) with a porous structure. In this study, we directly observe the phase transition from the PX-PbTiO3 to monoclinic TiO2(B) at the atomic scale, with the aim to elucidate the mechanism of phase transformation. Our results also point to a crystallography engineering strategy to obtain the desired crystalline phase in selected areas with nanometer precision.

2 Results and discussion

2.1 Results

PbTiO3 was first synthesized by a hydrothermal method as reported previously [30, 31]. The morphology and structural characterization of the as-synthesized PbTiO3 are detailed in Fig. S1 in the ESM. All the obtained samples showed a tendency to grow into PX phase PbTiO3 in the form of wires or needles with diameters ranging from tens to hundreds of nanometers, and lengths greater than several micrometers.

In situ heating experiments were then carried out on PX-PbTiO3 nanowires using a Protochips heating holder in an aberration-corrected Titan transmission electron microscope. As shown in Figs. 1(a)–1(f), the structure of the PX-PbTiO3 nanowires undergoes a complete transformation following

 Figure 1 In situ heat treatment of PX-PbTiO3 nanowires at different temperatures. (a)–(c) Morphology of one nanowire during the in situ heat treatment at different temperatures with only the part between the two blue lines exposed to electron beam. For better view, the part which is exposed to the electronbeam is colored blue. (d) and (e) HRTEM images and (f) filtering result of the HRTEM image of the middle part of (a)–(c), respectively. (g) and (h) EDS results from yellow and red rectangles in (c), respectively. The Si peak comes from the SiNx chip used for the heat treatment. (i) and (j) Atomic structures of PX-PbTiO3 and TiO2-B, respectively. (k) Scanning electron microscopy (SEM) images of (left) PX-PbTiO3 and (right) porous TiO2 nanowire. Scale bars: (a)–(c) and (k): 200 nm; (e): 5 nm; (d) and (f): 2 nm

Nano Res.

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

3

in situ heat treatment for more than 10 min at different temperatures in the presence of electron beam irradiation. As shown in Figs. 1(a)–1(c), several nanowires were gradually heated to 300 °C/400 °C with only a part of the nanowire (middle) exposed to the incident electron beam. In Fig. 1(a), it is seen that the nanowire is initially compact and dense; the corresponding high resolution TEM (HRTEM) image shows that the nanowire has a PX phase structure viewed along the [11

＿

0] zone axis, as shown in Fig. 1(d). At 300 °C (Figs. 1(b) and 1(e)), some molten particles start to grow on the nanowire at certain areas (in the middle part) and in these areas, the nanowires become amorphous, while other parts of the nanowires maintain their original structure. These molten particles were found to be liquid Pb; this aspect will be discussed in detail later in article. Moreover, when heated at 400 °C, the Pb particles are volatilized and the energy dispersive spectroscopy (EDS) result in Fig. 1(g) shows that after the heat treatment, the nanowire contains only Ti and O elements and no Pb is detected. This indicates that the heat-treated nanowire has the chemical composition of TiOx. Furthermore, it can be seen from Fig. 1(f) that the nanowires have already transformed into a monoclinic TiO2 structure, which we name TiO2(B), in accordance with previous reports [28]. In contrast, other parts of the nanowires which are unexposed to electron beam still remain in the pristine state, as shown in the HRTEM images in Fig. S2(c) in the ESM; this is also verified by the EDS result in Fig. 1(h). On the other hand, due to the difference in atomic structure between the initial PX-PbTiO3 and the final monoclinic TiO2(B), as shown in Figs. 1(i) and 1(j), following heat treatment, the nanowire grows into a porous and hollow structure, as displayed in Fig. 1(k).

To further illustrate this process, another porous nanowire after heat treatment was also imaged, as shown in Fig. S3 in the ESM. These results imply that under electron beam irradiation, PX-PbTiO3 nanowires undergo phase transformation

to form porous TiO2(B) during in situ heat treatment at 400 °C and this behavior is common to all the PX-PbTiO3 nanowires.

Figure 2 shows the details of the phase transformation from the original PX phase PbTiO3 into monoclinic TiO2(B). As shown in Figs. 2(a) and 2(b), a PX-PbTiO3 nanowire turns into porous TiO2(B) when heated at 400 °C for 9 min. Figure 2(c) shows a series of pictures taken during the experiment illustrating the process of phase transformation in real time. As shown in Fig. 2(c), the pristine PX-PbTiO3 nanowire grown along the [001] direction is viewed along the [11

＿

0] direction. During in situ heat treatment at 400 °C, the tunnel structure of PX-PbTiO3 along the c axis is gradually destroyed and as a result, the (110) planes gradually disappear. In the meantime, diffraction spots due to (110) and (220) planes begin to gradually stretch, as indicated by green arrows; after 435.6 s, these stretched spots finally merge into a new spot that can be indexed to the (200) plane of TiO2(B). Note that the spots corresponding to the (002) and (330) planes in the fast Fourier transform (FFT) images of PX-PbTiO3 at 38.1 s seem to be nearly immobile but gradually become bigger. The inter-planar spacing of these two planes in PX-PbTiO3 are 1.91 and 2.89 Å, which are quite close to 1.87 and 2.91 Å of the (020) and (400) planes in TiO2(B), respectively. In addition, the interfacial angles are 90° in both cases. Thus, it is reasonable to conclude that the spots attributed to the (002) and (330) planes of PX-PbTiO3 have evolved into those corresponding to the (020) and (400) planes in TiO2(B). Further evidence shows some new spots originating from the (110) plane of TiO2(B). Ultimately, after 435.6 s, the initial PX-PbTiO3 nanowire is completely converted into a TiO2(B) nanowire.

In addition, for the above-mentioned phase transformation from PX-PbTiO3 to TiO2(B), we have also considered the process through which Pb atoms disappear. As shown in Fig. 2(d), at the beginning of the phase transformation (1.1 s), there is a puddle of liquid matter observed in the nanowire (highlighted

 Figure 2 Evolution of a nanowire from PX-PbTiO3 to TiO2(B) structure during in situ heat treatment at 400 °C. (a) The morphology of one PX-PbTiO3

nanowire before and (b) after heating at 400 °C for 9 min. (c) Structure change of PX-PbTiO3 nanowire at 400 °C. The corresponding FFT results are given below each image. Red arrows indicate the appearance of new spots and green arrows show the movement of spots. (d) Disappearance of liquid in ananowire at 400 °C. For a better view, the liquid is colored yellow. Scale bars: (a) and (b): 10 nm; (c) and (d): 5 nm.

Nano Res.

| www.editorialmanager.com/nare/default.asp

4

by yellow dotted lines). This liquid material gradually volatilizes when kept at 400 °C and finally disappears after 287.5 s, which is also shown in Video ESM1. We also observed some molten particles that grow bigger (Fig. S5 in the ESM). These particles also eventually disappear albeit after a longer time. Since the liquid particles do not grow on the surface of nanowires in these images, their elemental composition cannot be determined by energy dispersive spectroscopy (EDS) or electron energy loss spectroscopy (EELS) during the evaporation of molten particles. However, considering the fact that the peaks in EDS due to Pb disappeared after heat treatment (Fig. 1(g)) and some liquid particles solidified into solid Pb at room temperature as seen in Figs. S7 and S11 in the ESM, it is reasonable to conclude that the liquid material seen in Fig. 2(d) is molten Pb. Since the melting point of bulk Pb is 327.5 °C, Pb should be in the liquid state at 450 °C and solidify at room temperature, which is in accordance with our experimental observations. Figures S5(h) and S5(i) in the ESM show the morphology evolution of one nanowire before and after the complete disappearance of Pb. Clearly, most of the liquid has evaporated, as noted by the many holes as shown in Fig. S5(i) in the ESM, which in turn creates pores in the nanowire. In fact, this porous morphology is more obvious in thin nanowires, which is shown in Fig. S6 in the ESM. After the complete evaporation of Pb, no further changes are observed even if the heat treatment is continued for hours, indicating complete phase transformation and the formation of the final product TiO2(B).

2.2 Discussion

All the TiO2(B) with porous structure formed in this process is grown from the starting PX-PbTiO3 nanowires following as structural transition resulting from the volatilization of a substantial number of Pb atoms. As shown in Figs. 1(i) and 1(j) and Figs. S7(a) and S7(b) in the ESM, the lattice parameters of the pristine PX-PbTiO3 are: a = b = 12.3853 Å, and c = 3.8200 Å with α = β = γ = 90°, while that of TiO2(B) are: a = 12.2077 Å, b =3.7488 Å, c = 6.5350 Å, with α = γ = 90° and β = 107.36°; both PbTiO3 and TiO2(B) contain 8 Ti atoms in one unit cell. The cell volumes of PbTiO3 and TiO2(B) are 585.9714 and 285.4463 Å3, respectively, showing a 51.29% decrease in cell volume during the phase transformation. Considering that there is no obvious decrease in the total volume of nanowires during the phase transformation (Figs. 1(a)–1(c)), we can conclude that the observed volume reduction of 51.29% is due to the formation of holes or pores in the nanowires, which account for the porous structure of the final TiO2(B) nanowires. The holes or pores are present both on the surface and in the interior of the nanowires. As shown in Fig. S5(i) in the ESM, most of the molten Pb particles are volatilized but some still remain even after several hours of heat treatment at 450 °C. Furthermore, when the heated nanowires are cooled to room temperature, the solidification of liquid particles into solid Pb is also observed, as seen in Figs. S7(c)–S7(e) in the ESM. The two series of pictures shown in Figs. S7(d) and S7(e) in the ESM, indicate that the Pb particles are trapped inside the holes and move around in a confined space under the electron beam. These results indicate that some Pb particles may be trapped in closed cavities surrounded by compact and dense TiO2(B) walls inside the nanowire; this further confirms the porous structure of TiO2(B).

We also found that the phase transformation from the original PX-PbTiO3 into monoclinic TiO2(B) did not happen at low temperatures. As shown in Fig. 3(a), in the series of pictures taken during in situ heat treatment at 350 °C, it is seen that some liquid particles are growing on the surface of the PX

phase PbTiO3 nanowires, similar to those in Fig. 1(b) and Figs. S5(f) and S5(g) in the ESM. With time, the crystal structure of the nanowire is gradually destroyed and the nanowire becomes amorphous after 210.2 s. However, the liquid particles are not volatile in this case (also shown in Fig. S8 in the ESM). Based on these images, the growth rate of the liquid-like particles can be qualitatively estimated. In Fig. 3(h), the sizes of four particles (as marked in Fig. S9 in the ESM) are plotted as a function of growth time during heat treatment at 350 °C. All the particles grow at a nearly uniform speed of about 0.02 nm/s. It should be noticed that the growth in Fig. 3(h) does not involve the merging of particles although it is frequently observed (Fig. 3(e) and Fig. S10 in the ESM). In fact, after heating for a sufficiently long time, many particles are formed on the surface, and the merging of particles can be observed as shown in Fig. 3(a) and Fig. S10 in the ESM. Both by individual growth and by merging, the liquid particles can grow much bigger with radii > 50 nm as shown in Fig. 1(b). Figure 3(a) also shows that the nanowires heated at 350 °C finally become amorphous, in contrast to the phase transformation from PX-PbTiO3 to TiO2(B) observed at higher temperatures (Figs. 1(c) and 2).

Thus, comparing the results in Figs. 1 and 2, we infer that a high temperature is required not only to vaporize Pb atoms but also to provide energy for the crystallization of TiO2(B). To further confirm this, as shown in Figs. 3(b)–3(g), a PX-PbTiO3 nanowire was first heated at 300 °C for several minutes and after the nanowires became totally amorphous and covered with liquid Pb particles, it was then heated at 500 °C. It is observed that at high temperature, all the Pb particles disappear, while the amorphous nanowire grows into a porous TiO2(B) structure. In this work, we heated the PX-PbTiO3 nanowires to different temperatures in the range from room temperature to 500 °C and we find that the phase transformation can occur only at temperatures greater than or equal to 400 °C, as shown in Fig. 2.

Furthermore, to elucidate the composition of the liquid particles, the nanowires with the liquid particles were cooled to room temperature. As shown in Fig. S11 in the ESM, the liquid particles are solidified at room temperature. HRTEM images show that most of the particles are made of pure Pb such as the one in Fig. S11(a) in the ESM, while some of them are covered with few layers of PbO on the surface, such as the one in Fig. S11(b) in the ESM. All the Pb and O atoms originate solely from the pristine PX-PbTiO3 nanowires; this aspect will be discussed later. It is observed that the surface layer of PbO has the orthorhombic phase, which is usually stable only at high temperatures (> 500 °C). However, previous works have indicated that the high tensile stress arising from the lattice mismatch between PbO and the Pb particle could stabilize orthorhombic PbO at room temperature [32, 33]. Besides, we also measured the size of the Pb particles in Fig. 1(b) and the distribution is shown in Fig. 3(i). Particles with different sizes are produced. The radius of the largest particle is > 55 nm while the smallest ones have < 7 nm radius. Based on these results, we believe that the size-selective preparation of Pb particles can also be achieved by suitably varying experimental conditions.

More importantly, besides high temperature, incident electrons are also vital for the observed phase transformation during in situ heat treatment, as recorded in Figs. 1(a)–1(c) and 4, and Figs. S12–S14 in the ESM. In the absence of electron beam exposure, the pristine PX-PbTiO3 nanowires remain unchanged after in situ heat treatment at different temperatures, as shown in Fig. S12 in the ESM. Analogously, when PX-PbTiO3 nano-wires are heated at 400 °C (Fig. 1(c)), only the middle part of the nanowire that is exposed to the electron beams transformed

Nano Res.

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

5

into TiO2(B) while the other parts still remained in the pristine state. This was further confirmed by converging the electron beam to small circles of ~ 50 nm diameter. As shown in Figs. 4(a)–4(d), on the circle which were exposed to electron beam during the in situ heat treatment at 450 °C, Pb atoms had volatilized, whereas no change was observed in the rest of the nanowire. This implies that the growth of TiO2(B) structures in selected areas can be achieved by controlling the location of the electron beam, which is difficult to achieve by traditional methods. As shown in Figs. 4(e) and 4(f), and Fig. S14 in the ESM, TiO2(B) structures with different shapes were formed on the nanowires by converging and moving the electron beam during in situ heat treatment. These results also show that the incident electrons are crucial to the process and without them, the phase transformation from PX-PbTiO3 to TiO2(B) cannot take place even when heated to high temperatures for extended periods of time, which is also in accordance with previous work [3]. Our results have shown furthermore, that the selective growth of Pb particles or TiO2(B) can be realized by combining heat treatment with controlled exposure to electron beam.

The effect of the incident electrons is due to the inelastic scattering of electrons by the pristine PX-PbTiO3 nanowires. The results in Fig. 3 demonstrate that high temperature provides energy for the crystallization of TiO2(B) whereas, as inferred from Fig. S12 in the ESM, in the absence of electron beams, nanowires would maintain their pristine structure irrespective of the temperature. This means that just high temperature is not sufficient to move the Pb atoms out of the PX-PbTiO3 nanowire. Since the temperature and the electron beam are the two parameters that have been varied, we can conclude

that the outward migration of Pb atoms from the nanowires is caused by the incident electrons. According to previous reports, in oxides containing metallic elements in their highest valence state, the O2− ions are more easily oxidized to O0 atoms or O+ ions through the Knotek–Feibelman mechanism [34, 35] and thus lose chemical binding to the metal ions. The free metal ions can be reduced by the incident electrons to zero valence atoms and diffuse to the surface of the sample or volatilize to the environment [35–42]. For instance, a similar process induced by the inelastic scattering of incident electrons can usually be observed in perovskite structures [38]. In addition, similar to our results, the growth Pb particles has also been observed when perovskite CsPbBr3 particles were exposed to electron beams with a low accelerating voltage inside a TEM instrument [38]. Inelastic scattering of incident electrons was proven to be the key to triggering this process [35, 38].

In this work, the pristine PX phase PbTiO3 has a similar structure to the traditional perovskite materials [30, 31], while the Pb atoms are all in their high valence state, which means they are all unstable when exposed to electron beam irradiation and Pb2+ ions can be reduced to Pb0 atoms by incident electrons. At temperatures less than 400 °C, Pb atoms diffuse to the surface of the nanowires and form Pb particles, but at tem-peratures > 400 °C, they are volatilized. This phenomenon is much more obvious when the nanowires are exposed to incident electrons at a low accelerating voltage. To further prove this and for comparison, the pristine PX-PbTiO3 nanowires were exposed to electron beam with different accelerating voltages without in situ heat treatment. As shown in Fig. S16 in the ESM, the Pb particles grew much faster at the accelerating

 Figure 3 Evolution of PX-PbTiO3 nanowires under in situ heat treatment at different temperatures. (a) Growth of hemispherical liquid Pb particles at the surface of a PX-PbTiO3 nanowire. Green and yellow circles show the growth and merging of liquid Pb particles. (b)–(g) TEM images of a PX-PbTiO3

nanowire during in situ heat treatment at different temperatures. (c), (e), and (g) are the HR-TEM images of (b), (d), and (f), respectively. (h) Sizes of four liquid particles as a function of growth time during heat treatment at 350 °C. (i) Histogram showing the size distribution of liquid particles in (b). Scale bars: (a), (b), (d) and (f): 10 nm; (c), (e) and (g): 2 nm.

Nano Res.

| www.editorialmanager.com/nare/default.asp

6

voltage of 80 kV than at 300 kV for the same beam rate of 4.03 × 104 A/m2, which means that the inelastic scattering of incident electrons in TEM is much more intense at low accelerating voltages. Hence, as shown in Fig. 4(g), we propose the reaction mechanism for the observed phase transformation to be as follows: When PX phase PbTiO3 nanowires are exposed to incident electron beams, some O2− would be oxidized to O0 or O+ by the electrons and these out-diffuse into the environment in the form of O2 or O+. The free Pb2+ would be reduced to Pb atoms and can also migrate to the surface of the nanowires. At temperatures > 400 °C, the Pb atoms will evaporate into the environment to leave behind TiOx, which crystallizes into TiO2(B). However, at low temperatures, there is not enough energy for the volatilization of Pb and the crystallization of TiO2(B). Thus, the Pb atoms form Pb particles and the nanowires remain in the amorphous TiOx state. Based on all these results, we conclude that the inelastic scattering of the incident electrons with the nanowires destroys the pristine PX-PbTiO3 structure and the high temperature (more than 400 °C) provides energy for the crystallization of TiO2(B) structure.

3 Conclusions In this work, monoclinic TiO2(B) with porous structure and Pb particles with different sizes was prepared by controlling the heating temperature and time, respectively, during the in situ heat treatment of the PX-PbTiO3 nanowires. During these processes, incident electrons from the TEM caused Pb atoms

to migrate out of the pristine PbTiO3 through inelastic scattering. For heating temperatures > 400 °C, the Pb atoms volatilize into the environment leaving TiOx nanowires, that subsequently crystallize into TiO2(B). And at temperatures < 400 °C, the Pb atoms diffuse to the surface of the nanowires and form Pb particles while the TiOx nanowires remain in their amorphous state. Besides, the obtained TiO2(B) have porous structures due to the volatilization of a substantial number of Pb atoms and a 51.29% decrease in cell volume during the phase transition from PX-PbTiO3 to TiO2(B). Based on these results, porous TiO2(B) structures or Pb particles can be selectively prepared at the desired location on the nanowires by controlling the incident electron beams, which is difficult to achieve by other traditional methods. Finally, this study not only directly records the formation process of monoclinic TiO2(B) at atomic scale, but also provides a crystallography engineering strategy to obtain the desired crystal structures with nanometer precision.

4 Methods

4.1 Synthesis of PbTiO3 nanowires

PX phase PbTiO3 nanowires were prepared by a hydrothermal method reported previously [31]. First, 4 mmol Ti(OC4H9)4 was dissolved in 8 mL of ethanol and the transparent solution was hydrolyzed in 8 mL of deionized H2O. Next, 20 mmol KOH, 5.2 mmol Pb(CH3COO)2·3H2O and 0.050 g polyvinyl alcohol (PVA) were added into the solution and mixed under

 Figure 4 Selective growth of TiO2(B) and schematic illustration of the decomposition of PX-PbTiO3 nanowires controlled by electron beams. (a) ScanningTEM (STEM) image of PX-PbTiO3 nanowire after in situ heat treatment at 450 °C with the circular area was exposed to the electron beam. (b)–(d) EDS mapping of (a). The three images correspond to the distribution of Ti, O, and Pb, respectively. (e) Schematic illustration of the position-controlled phase transformation by in situ electron irradiation. (f) STEM images of different nanowires after in situ heat treatment at 450 °C with only the yellow areas exposed to the electron beam. (g) Schematic illustration of the decomposition of PX-PbTiO3 nanowires at different temperatures when exposed to electron beams. Scale bars: 50 nm.

Nano Res.

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

7

stirring. After adjusting the volume to 40 mL with deionized H2O, the feedstock was transferred to a 50 mL Teflon-lined autoclave and held at 200 °C for 3.5 h. The obtained products were repeatedly washed with deionized H2O and 10 wt.% CH3COOH solution to completely remove excess PbO. Finally, the solid product was dried at 60 °C in in air to obtain pure PX-phase PbTiO3 nanowires.

4.2 Characterization of phase and microstructures

Structural information on the samples was obtained from scanning electron microscope (SEM, Zeiss Ultra Plus) and aberration-corrected transmission electron microscope (TEM, FEI Titan 80-300). X-ray diffraction (XRD) patterns of the samples were recorded with a Rigaku Smartlab(3) diffract to meter with Cu Kα radiation as the X-ray source.

4.3 In situ TEM experiments

All the reported TEM results were recorded inside the TEM, FEI Titan 80-300. In situ heating experiments were conducted using a Protochips TEM heating holder (Aduro) inside the FEI Titan 80-300 TEM; heat was generated by electrical-heating chips equipped with a supporting silicon nitride film. Arrays of holes on the film served as observation windows. In the experiments, the nominal ramp rate of the heating chips was 1,000 °C/ms.

To control the phase transition location and form different patterns (such as the ones shown in Fig. 4) on the PX-PbTiO3 nanowires, the nanowires were kept at constant temperature (at least 400 °C) and the electron beam was converged on them. Beam intensity only affected the phase transition time and thus in principle, it can be controlled optionally. In this paper, the beam rate was kept at about 2 × 104 A/m2. Then after the phase transition process had totally finished on the electron irradiation area, the electron beam was moved along the special ways to next area. The movement of the beam was continuous to ensure the completeness and continuity of the final patterns. Both the size and moving way of electron beam were determined by the target patterns. The whole process was carried out in TEM mode.

Acknowledgements This research was supported by the National Natural Science Foundation of China (Nos. 11327901, 11525415, 51420105003, 61974021, 51972058, and 11774051) and the Fundamental Research Funds for the Central Universities (No. 2242018K41020).

Electronic Supplementary Material: Supplementary material (additional details of the experiments conditions, surface treatment, simulations and complete growth process) is available in the online version of this article at https://doi.org/10.1007/ s12274-020-2711-2.

References [1] Huang, X.; Liu, Z. Q.; Millet, M. M.; Dong, J. C.; Plodine, M.; Ding,

F.; Schlögl, R.; Willinger, M. G. In situ atomic-scale observation of surface-tension-induced structural transformation of Ag-NiPx core-shell nanocrystals. ACS Nano 2018, 12, 7197–7205.

[2] Zhang, Z.; Liu, N. S.; Li, L. Y.; Su, J.; Chen, P. P.; Lu, W.; Gao, Y. H.; Zou, J. In situ TEM observation of crystal structure transformation in InAs nanowires on atomic scale. Nano Lett. 2018, 18, 6597–6603.

[3] Wang, J.; Durussel, A.; Sandu, C. S.; Sahini, M. G.; He, Z. B.; Setter, N. Mechanism of hydrothermal growth of ferroelectric PZT nanowires. J. Cryst. Growth 2012, 347, 1–6.

[4] Xiao, Z.; Ren, Z. H.; Xia, Y.; Liu, Z. Y.; Xu, G.; Li, X.; Shen, G.; Han, G. R. Doping and phase transformation of single-crystal pre- perovskite PbTiO3 fibers with TiO6 edge-shared octahedra. CrystEngComm 2012, 14, 4520–4524.

[5] Liu, Y.; Wang, H.; Zhang, X. In situ TEM nanoindentation studies on stress-induced phase transformations in metallic materials. JOM 2016, 68, 226–234.

[6] Wang, J.; Wylie-van Eerd, B.; Sluka, T.; Sandu, C.; Cantoni, M.; Wei, X. K.; Kvasov, A.; McGilly, L. J.; Gemeiner, P.; Dkhil, B. et al. Negative-pressure-induced enhancement in a freestanding ferroelectric. Nat. Mater. 2015, 14, 985–990.

[7] Zhu, G. N.; Wang, C. X.; Xia, Y. Y. Structural transformation of layered hydrogen trititanate (H2Ti3O7) to TiO2(B) and its elec-trochemical profile for lithium-ion intercalation. J. Power Sources 2011, 196, 2848–2853.

[8] Li, W.; Bai, Y.; Liu, C.; Yang, Z. H.; Feng, X.; Lu, X. H.; van der Laak, N. K.; Chan, K. Y. Highly thermal stable and highly crystalline anatase TiO2 for photocatalysis. Environ. Sci. Technol. 2009, 43, 5423–5428.

[9] Liu, X. F.; Xu, T.; Wu, X.; Zhang, Z. H.; Yu, J.; Qiu, H.; Hong, J. H.; Jin, C. H.; Li, J. X.; Wang, X. R. et al. Top-down fabrication of sub-nanometre semiconducting nanoribbons derived from molybdenum disulfide sheets. Nat. Commun. 2013, 4, 1776.

[10] Xu, T.; Xie, X.; Yin, K. B.; Sun, J.; He, L. B.; Sun, L. T. Controllable atomic-scale sculpting and deposition of carbon nanostructures on graphene. Small 2014, 10, 1724–1728.

[11] Zhu, J. F.; Zhang, J. L.; Chen, F.; Anpo, M. Preparation of high photocatalytic activity TiO2 with a bicrystalline phase containing anatase and TiO2(B). Mater. Lett. 2005, 59, 3378–3381.

[12] Durandurdu, M. Two successive amorphous-to-amorphous phase transformations in TiO2. J. Am. Ceram. Soc. 2017, 100, 3903–3911.

[13] Lei, Y. M.; Li, J.; Wang, Z.; Sun, J.; Chen, F. Y.; Liu, H. W.; Ma, X. H.; Liu, Z. W. Atomic-scale investigation of a new phase transformation process in TiO2 nanofibers. Nanoscale 2017, 9, 4601–4609.

[14] Pan, X. Y.; Ma, X. M. Study on the milling-induced transformation in TiO2 powder with different grain sizes. Mater. Lett. 2004, 58, 513–515.

[15] Yang, J.; Gao, M. Z.; Jiang, S. B.; Huo, X. J.; Xia, R. Hysteretic phase transformation of two-dimensional TiO2. Mater. Lett. 2018, 232, 171–174.

[16] Gao, M.; Bao, Y. B.; Qian, Y. X.; Deng, Y. F.; Li, Y. W.; Chen, G. H. Porous anatase-TiO2(B) dual-phase nanorods prepared from in situ pyrolysis of a single molecule precursor offer high performance lithium-ion storage. Inorg. Chem. 2018, 57, 12245–12254.

[17] Giannuzzi, R.; Manca, M.; De Marco, L.; Belviso, M. R.; Cannavale, A.; Sibillano, T.; Giannini, C.; Cozzoli, P. D.; Gigli, G. Ultrathin TiO2(B) nanorods with superior lithium-ion storage performance. ACS Appl. Mater. Interfaces 2014, 6, 1933–1943.

[18] Zukalová, M.; Kalbáč, M.; Kavan, L.; Exnar, I.; Graetzel, M. Pseudocapacitive lithium storage in TiO2(B). Chem. Mater. 2005, 17, 1248–1255.

[19] Liu, S. H.; Jia, H. P.; Han, L.; Wang, J. L.; Gao, P. F.; Xu, D. D.; Yang, J.; Che, S. N. Nanosheet-constructed porous TiO2-B for advanced lithium ion batteries. Adv. Mater. 2012, 24, 3201–3204.

[20] Wang, G.; Wang, Q.; Lu, W.; Li, J. H. Photoelectrochemical study on charge transfer properties of TiO2-B nanowires with an application as humidity sensors. J. Phys. Chem. B 2006, 110, 22029–22034.

[21] Liu, H. S.; Bi, Z. H.; Sun, X. G.; Unocic, R. R.; Paranthaman, M. P.; Dai, S.; Brown, G. M. Mesoporous TiO2-B microspheres with superior rate performance for lithium ion batteries. Adv. Mater. 2011, 23, 3450–3454.

[22] Marchand, R.; Brohan, L.; Tournoux, M. TiO2(B) a new form of titanium dioxide and the potassium octatitanate K2Ti8O17. Mater. Res. Bull. 1980, 15, 1129–1133.

[23] Kobayashi, M.; Petrykin, V. V.; Kakihana, M.; Tomita, K.; Yoshimura, M. One-step synthesis of TiO2(B) nanoparticles from a water-soluble titanium complex. Chem. Mater. 2007, 19, 5373–5376.

[24] Sugimoto, W.; Terabayashi, O.; Murakami, Y.; Takasu, Y. Elec-trophoretic deposition of negatively charged tetratitanate nanosheets and transformation into preferentially oriented TiO2(B) film. J. Mater. Chem. 2002, 12, 3814–3818.

Nano Res.

| www.editorialmanager.com/nare/default.asp

8

[25] Cai, Y.; Wang, H. E.; Huang, S. Z.; Jin, J.; Wang, C.; Yu, Y.; Li, Y.; Su, B. L. Hierarchical nanotube-constructed porous TiO2-B spheres for high performance lithium ion batteries. Sci. Rep. 2015, 5, 11557.

[26] Zhao, B.; Chen, F.; Liu, H. Q.; Zhang, J. L. Mesoporous TiO2-B nanowires synthesized from tetrabutyl titanate. J. Phys. Chem. Solids 2011, 72, 201–206.

[27] Li, X. D.; Wu, G. X.; Liu, X.; Li, W.; Li, M. C. Orderly integration of porous TiO2(B) nanosheets into bunchy hierarchical structure for high-rate and ultralong-lifespan lithium-ion batteries. Nano Energy 2017, 31, 1–8.

[28] Lei, Y. M.; Sun, J.; Liu, H. W.; Cheng, X.; Chen, F. Y.; Liu, Z. W. Atomic mechanism of predictable phase transition in dual-phase H2Ti3O7/TiO2 (B) nanofiber: An in situ heating TEM investigation. Chem.—Eur. J. 2014, 20, 11313–11317.

[29] Ben Yahia, M.; Lemoigno, F.; Beuvier, T.; Filhol, J. S.; Richard- Plouet, M.; Brohan, L.; Doublet, M. L. Updated references for the structural, electronic, and vibrational properties of TiO2(B) bulk using first-principles density functional theory calculations. J. Chem. Phys. 2009, 130, 204501.

[30] Ren, Z. H.; Xu, G.; Liu, Y.; Wei, X.; Zhu, Y. H.; Zhang, X. B.; Lv, G. L.; Wang, Y. W.; Zeng, Y. W.; Du, P. Y. et al. PbTiO3 nanofibers with edge-shared TiO6 octahedra. J. Am. Chem. Soc. 2010, 132, 5572–5573.

[31] Wang, J.; Schenk, K.; Carvalho, A.; Eerd, B. W. V.; Trodahl, J.; Sandu, C. S.; Bonin, M.; Gregora, I.; He, Z. B.; Yamada, T. et al. Structure determination and compositional modification of body- centered tetragonal PX-phase lead titanate. Chem. Mater. 2011, 23, 2529–2535.

[32] Light, T. B.; Eldridge, J. M.; Matthews, J. W.; Greiner, J. H. Structure of thin lead oxide layers as determined by X-ray diffraction. J. Appl. Phys. 1975, 46, 1489–1492.

[33] Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Lead oxide nanobelts and phase transformation induced by electron beam irradiation. Appl. Phys. Lett. 2002, 80, 309–311.

[34] Knotek, M. L.; Feibelman, P. J. Stability of ionically bonded surfaces in ionizing environments. Surf. Sci. 1979, 90, 78–90.

[35] Gonzalez-Martinez, I. G.; Bachmatiuk, A.; Bezugly, V.; Kunstmann, J.; Gemming, T.; Liu, Z.; Cuniberti, G.; Rümmeli, M. H. Electron- beam induced synthesis of nanostructures: A review. Nanoscale 2016, 8, 11340–11362.

[36] Bachmatiuk, A.; Dianat, A.; Ortmann, F.; Quang, H. T.; Cichocka, M. O.; Gonzalez-Martinez, I.; Fu, L.; Rellinghaus, B.; Eckert, J.; Cuniberti, G. et al. Graphene coatings for the mitigation of electron stimulated desorption and fullerene cap formation. Chem. Mater. 2014, 26, 4998–5003.

[37] Citrin, P. H. Interatomic auger processes: Effects on lifetimes of core hole states. Phys. Rev. Lett. 1973, 31, 1164–1167.

[38] Dang, Z. Y.; Shamsi, J.; Palazon, F.; Imran, M.; Akkerman, Q. A.; Park, S.; Bertoni, G.; Prato, M.; Brescia, R.; Manna, L. In situ transmission electron microscopy study of electron beam-induced transformations in colloidal cesium lead halide perovskite nanocrystals. ACS Nano 2017, 11, 2124–2132.

[39] El Mel, A. A.; Molina-Luna, L.; Buffière, M.; Tessier, P. Y.; Du, K.; Choi, C. H.; Kleebe, H. J.; Konstantinidis, S.; Bittencourt, C.; Snyders, R. Electron beam nanosculpting of kirkendall oxide nanochannels. ACS Nano 2014, 8, 1854–1861.

[40] Li, Y. X.; Bunes, B. R.; Zang, L.; Zhao, J.; Li, Y.; Zhu, Y. Q.; Wang, C. Y. Atomic scale imaging of nucleation and growth trajectories of an interfacial bismuth nanodroplet. ACS Nano 2016, 10, 2386–2391.

[41] da Silva Pereira, W.; Andrés, J.; Gracia, L.; San-Miguel, M. A.; da Silva, E. Z.; Longo, E.; Longo, V. M. Elucidating the real-time Ag nanoparticle growth on α-Ag2WO4 during electron beam irradiation: Experimental evidence and theoretical insights. Phys. Chem. Chem. Phys. 2015, 17, 5352–5359.

[42] Sepulveda-Guzman, S.; Elizondo-Villarreal, N.; Ferrer, D.; Torres- Castro, A.; Gao, X.; Zhou, J. P.; Jose-Yacaman, M. In situ formation of bismuth nanoparticles through electron-beam irradiation in a transmission electron microscope. Nanotechnology 2007, 18, 335604.