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Black anatase formation by annealing of amorphous nanoparticles and the role of the TiO shell in self-organized crystallization by particle attachment 2
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Mengkun Tian, Masoud Mahjouri-Samani, Kai Wang, Alexander A. Puretzky, David B. Geohegan, Wesley Daniel Tennyson, Nicholas Cross, Christopher M. Rouleau, Thomas A. Zawodzinski, Gerd Duscher, and Gyula Eres ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 06 Jun 2017 Downloaded from http://pubs.acs.org on June 7, 2017
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Black anatase formation by annealing of amorphous nanoparticles and the role of the Ti2O3 shell in self-organized crystallization by particle attachment Mengkun Tian1, Masoud Mahjouri-Samani2, Kai Wang2, Alexander A. Puretzky2, David B. Geohegan2, Wesley D. Tennyson2, Nicholas Cross3, Christopher M. Rouleau2, Thomas A. Zawodzinski, Jr.1,4, Gerd Duscher3,4* & Gyula Eres2,4* 1
Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, TN37996, USA
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Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN37831, USA
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Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN37996, USA
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Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN37831, USA
ABSTRACT We use amorphous titania nanoparticle networks produced by pulsed laser vaporization at room temperature as a model system for understanding the mechanism of formation of black titania. Here, we characterize the transformation of amorphous nanoparticles by annealing in pure Ar at 400°C, the lowest temperature at which black titania was observed. Atomic resolution electron microscopy methods and electron energy loss spectroscopy show that the onset of crystallization occurs by nucleation of an anatase core that is surrounded by an amorphous Ti2O3 shell. The formation of the metastable anatase core before the thermodynamically stable rutile phase occurs according to the Ostwald phase rule. In the second stage the particle size increases by coalescence of already crystallized particles by a self-organized mechanism of crystallization by particle attachment. We show that the Ti2O3 shell plays a critical role in both black titania transformation and functionality. At 400°C Ti2O3 hinders the agglomeration of neighboring particles to maintain a high surface-to-volume ratio that is beneficial for enhanced photocatalytic activity. In agreement with previous results, the thin Ti2O3 surface layer acts as a narrow bandgap semiconductor in concert with surface defects to enhance the photocatalytic activity. Our results demonstrate that crystallization by particle attachment can be a highly effective mechanism for optimizing photocatalytic efficiency by controlling the phase, composition and particle size distribution in a wide range of self-doped defective TiO2 architectures simply by varying the annealing conditions of amorphous nanoparticles. KEYWORDS：Black anatase, amorphous nanoparticle network, Ostwald rule of stages, crystallization by particle attachment. transmission electron microscopy, pulsed laser vaporization. Corresponding Authors * Email:
[email protected], * Email:
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INTRODUCTION The mechanism of self-doping of TiO2 resulting from lattice distortions and defects has been attracting great attention recently.1-5 The primary effect of self-doping is the formation of extra electronic states that reduce the large intrinsic bandgap6-8 of TiO2. The narrowing of the TiO2 band gap from 3.2 eV to below 2 eV is enough to shift the maximum of the absorption spectrum from the ultra violet into the visible spectral range producing a dramatic improvement in the photocatalytic performance of TiO2.9 Recently, black titania characterized by significant bandgap narrowing coupled with a large increase in photocatalytic activity became the most intensely studied form of self-doped TiO2.10 Black titania nanoparticles (NPs) have a unique structure consisting of a defective outer shell surrounding a crystalline core. The remarkable enhancement in photocatalytic performance of this core-shell structure is generally attributed to lattice distortions caused by surface structure amorphization,10,
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and the presence of self-doping defects such as Ti3+ and
oxygen vacancies.3, 11-13 The differences in synthesis methods were found to affect critically the properties and performance of such core-shell structures.14 More specifically, it is still not well understood what role the relationship between the crystalline structure of the core and the outer shell play in the formation and performance of black titania.15 A variety of techniques including electron paramagnetic or spin resonance (EPR or ESR),3, 16 Raman spectroscopy11, 17 and X-ray photoelectron spectroscopy (XPS)18, 19 have been used to determine the presence of Ti3+ and oxygen vacancies. However, these bulk characterization methods cannot provide information on the atomic structure of black titania and the location and spatial distribution of Ti3+ and oxygen vacancies. In our previous report20 we describe characterization of the structure of core-shell black titania nanoparticles (NPs) using a combination of atomic resolution transmission electron microscopy (TEM) techniques. We used annealing of amorphous titania precursors produced by pulsed laser vaporization (PLV) as a model system for understanding how the formation kinetics affects the structure and properties of black titania NPs. We showed that annealing in oxygen free atmosphere at 700 ºC converts the amorphous precursor into core-shell NPs with a perfectly crystalline rutile core and a 1-2 nm thick shell consisting of Ti2O3. By probing the crystalline structure of the near surface region as a function of depth by scanning transmission electron microscopy (STEM) imaging, electron energy loss spectroscopy (EELS), and nano-beam 2
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electron diffraction (NBED) we demonstrated that the relationship between the crystalline structure of the rutile core and the Ti2O3 shell is not arbitrary20. The TEM images show already formed amorphous precursor NPs around 3±1 nm size loosely bound together into a percolation network. The structure and the increase in size suggest that the formation of 40±10 nm black rutile NPs during annealing, in addition to crystallization, must also involve some mechanism for transformation of smaller NPs into larger NPs. In addition, the perfect rutile core and the accumulation of oxygen deficient forms of titania in the near surface region of the NPs suggest that the out diffusion of oxygen vacancies is a key driving force in the formation mechanism of core-shell structures. However, at the annealing temperature of 700 ºC we observe no indication for the presence of intermediate structures or phases, suggesting that both the transformation of already crystalized NPs and the oxygen vacancy diffusion are very fast and carried to completion or near completion. To examine how the relationship between the core and the shell evolves starting from amorphous precursors in this work we study the products of the annealing process in the temperature range from room temperature to 700 ºC. The phase of the crystalline core forming by annealing of amorphous NPs is dictated by the kinetics of the crystallization process and does not necessarily follow the thermodynamic stability of the bulk TiO2 polymorphs. To understand the variety of structures forming by crystallization at different temperatures we are guided by the Ostwald rule of stages, which states that an unstable system does not directly transform into the thermodynamically most stable state.21-23 Instead, an unstable state could transform into a succession of transient states that are structurally similar and energetically close to the unstable state minimizing the decrease of free energy before the final state is reached. Therefore, the free energy landscape plays a critical role in governing what metastable states are encountered along the way to the final state. The Ostwald rule of stages explains why amorphous NPs do not directly crystallize into rutile without formation of the intermediate metastable anatase phase. In parallel with crystallization a self-organization process known as crystallization by particle attachment (CPA) occurs that accounts for the observed increase in the black titania NPs size compared to the original amorphous NPs. CPA is a unique non-classical crystallization mechanism in which larger crystals form by attachment of already existing 3
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smaller crystalline particles.24-26 A version of CPA referred to as oriented attachment (OA) was first observed involving 5-6 nm size anatase NPs in a solution environment, and the major elements in current understanding of the CPA mechanism derive from solution studies.24 However, as CPA gradually becomes recognized as a general crystal growth mechanism it has been observed in vapor-phase and other environments. The key to OA is a spontaneous alignment of neighboring particles in search of perfect lattice match, which corresponds to a minimum free energy configuration that facilitates their contact and fusing together to form a larger crystalline particle. The finer details of this mechanism including continuous interaction and rotation upon approach, followed by atomic rearrangements after contact was established, depend on the environment in which OA occurs. A particularly intriguing feature of OA is the formation of highly anisotropic 2D and 3D crystalline architectures facilitated by the presence of a dominant high surface energy facet, such as for example the (002) facet enabling anatase nanorod growth.27 In this work we use the same atomic resolution electron microscopy techniques that previously revealed the crystalline structure of black rutile to investigate the early stages of amorphous TiO2 NP crystallization by annealing in Ar at 400°C. We used monochromated EELS to characterize the electronic structure, and TEM techniques including selected area electron diffraction (SAED), HRTEM and NBED to determine the atomic structure and phase of NPs. The results show that black anatase, an intermediate metastable phase between black rutile and amorphous precursors, is the dominant self-doped TiO2 that forms at 400°C. We show that the Ostwald phase rule describes the transformation of amorphous titania into crystalline NPs and CPA and OA are the key kinetic mechanisms that govern the formation of larger crystalline structures by self-organized agglomeration of partially crystallized NPs. RESULTS AND DISCUSSION The data in Figure 1 illustrate that amorphous titania synthesized by PLV is the perfect model system for understanding the formation of black titania because it captures both the particle size evolution and the changes of the TiO2 phases. The image in Figure S1 shows a black titania sample formed at 400°C, which represents the lowest temperature at which the formation of black titania was observed to occur by annealing of PLV amorphous titania NPs in pure Ar. The SAED patterns in the left column and the corresponding intensity profiles in Figure 1 show that the TiO2 phase starts from anatase at 400 °C, turning to a mixture of 4
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anatase and rutile at 500-600 °C, and finally to pure rutile at 700°C. The histograms of particle size distributions in the right column obtained from TEM images in Figure S2 show that the particle size increases from 3±1 nm for amorphous NPs to 40±10 nm for black rutile NPs. The red lines represent log-normal fits of the particle size distribution (PSD). The tail extending toward large particle sizes is characteristic of ripening by dynamic particle attachment mechanisms such as Smoluchowski ripening, instead of Ostwald ripening, which is in contrast skewed toward small particle sizes.28 In addition to the two main phases of anatase and rutile, the SAED line profiles indicate traces of minor phases that include brookite and TiO2(B) in the temperature range between 500°C and 600°C. The HRTEM image in Figure S2a shows 3 nm amorphous NPs already linked together into a percolation network at RT, and in Figures S2b-S2d show the formation of much larger black anatase and black rutile NPs with clearly resolved lattice fringes observed after annealing of the amorphous NPs in Ar. The new result from the SAED data is that the amorphous NPs do not tranform directly into the thermodynamically stable rutile phase. It is known that the formation of a particular phase is governed by its nucleation rate Jn given by: Jn = A exp(-Bγ3/S2)
(1)
where A and B are constants, γ is the surface free energy, and S is the supersaturation.29 In general, the cubic γ dependence is the dominant factor and this equation gives the highest nucleation rate for the amorphous phase because it has the smallest surface free energy
γamorphous
