Liang Xu, Hai-Wei Liang, Hui-Hui Li, Kai Wang, Yuan Yang, Lu-Ting Song, Xu Wang, Shu-Hong Yu ( )

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1 Nano Research DOI /s Nano Res 1 Understanding the Stability and Reactivity of Ultrathin Tellurium Nanowires in Solution: An Emerging Platform for Chemical Transformation and Material Design Liang Xu, Hai-Wei Liang, Hui-Hui Li, Kai Wang, Yuan Yang, Lu-Ting Song, Xu Wang, Shu-Hong Yu ( ), Just Accepted Manuscript DOI: /s on September 18, 2014 Tsinghua University Press 2014 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.

2 TABLE OF CONTENTS (TOC) Understanding the Stability and Reactivity of Ultrathin Tellurium Nanowires in Solution: An Emerging Platform for Chemical Transformation and Material Design Liang Xu, Hai-Wei Liang, Hui-Hui Li, Kai Wang, Yuan Yang, Lu-Ting Song, Xu Wang, Shu-Hong Yu * Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Collaborative Innovation Center of Suzhou Nano Science and Technology, Department of Chemistry, University of Science and Technology of China, Hefei, Anhui , P. R. China. Ultrathin tellurium nanowires (TeNWs), as a model material, were used to study the stability of nanomaterials in aqueous solution through an accelerated oxidation process from the viewpoint of reaction kinetics, and thus proposed a new approach for the design and synthesis of other unique one-dimensional nanostructures by chemical transformation process using the intermediate nanostructures captured during the dynamic oxidation process under different conditions. Provide the authors webside if possible. Shu-Hong Yu,

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4 Nano Research DOI (automatically inserted by the publisher) Research Article Please choose one Understanding the Stability and Reactivity of Ultrathin Tellurium Nanowires in Solution: An Emerging Platform for Chemical Transformation and Material Design Liang Xu, Hai-Wei Liang, Hui-Hui Li, Kai Wang, Yuan Yang, Lu-Ting Song, Xu Wang, Shu-Hong Yu ( ) 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 2014 KEYWORDS Stability, Oxidation, kinetics, transformation, Ultrathin nanowires Reactivity, Reaction Chemical Storage, ABSTRACT The stability and reactivity of nanomaterials are of crucial importance for their application, but the long-term effects of stability and reactivity for nanomaterials under practical conditions are still not well understood. In this study, we firstly established a comprehensive strategy to investigate the stability of a highly reactive nanomaterial from the viewpoint of reaction kinetics with ultrathin tellurium nanowires (TeNWs) as a model material in aqueous solution through an accelerated oxidation process, and thus proposed a new approach for the design and synthesis of other unique one-dimensional nanostructures by chemical transformation process using the intermediate nanostructures captured during the dynamic oxidation process under different conditions. In essence, the oxidation of ultrathin TeNWs was a gas-solid reaction which involved liquid, gas and solid phases. The results have been demonstrated that the oxidation process of ultrathin TeNWs in aqueous solution can be divided into three stages, i.e., oxygen limiting, ultrathin TeNWs limiting and mass transfer resistance limiting stages. The apparent oxidation kinetics for ultrathin TeNWs was approximatively accorded with the first order reaction kinetic model and had an apparent activation energy as low as kj mol -1, indicating that ultrathin TeNWs were unstable in thermodynamics. However, such unstable nature of ultrathin TeNWs did not seem like repugnant and useless, it actually can act as an excellent platform to help us synthesize and design unique one-dimensional functional nanomaterials with special structure and distinctive property, which are hardly achieved by a direct synthesis method. 1. Introduction As a bridge between microcosmic atoms and macroscopic materials, nanomaterials have received

5 considerable attention in recent years.[1-11] However, the long-term effects of stability and reactivity for nanomaterials under practical conditions are still not well understood.[12] As we all know, almost all nanomaterials are far away from equilibrium state.[13] Generally, nanomaterials need a stabilizing environment (such as coating with surfactants or loading on a support) to prevent aggregation between each other and, at the same time, maintain high reactivity during storage and practical applications. Many nanomaterials tend to a dramatic change in practical applications due to poor stabilizing environment, such as CdTe nanoparticles,[14, 15] CdSe nanoparticles,[16] PbSe nanocrystals,[17] Ag nanocubes,[18] Cu nanoparticles[19] and so on. Hence, the understanding of stability and reactivity for nanomaterials, especially in solution, has great values and significance for us to achieve good application performance of nanomaterials and also will be helpful for the rational design and synthesis of new nanomaterials. Unfortunately, the relevant research is extremely limited and usually just obtained some rough and un-quantified views only by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) observations. Accordingly, it is very urgent that how to establish a reasonable and reliable method to obtain in-depth and comprehensive information about the long-term effects of stability and reactivity for nanomaterials from the viewpoint of reaction kinetics. The goal of a kinetics experiment is to determine the concentration of one of the reactants or products as a function of time.[20] Comparing with other traditional quantitative methods, the absorption spectrum method is regarded as a most convenient and practical way to determine the concentration of colloidal nanomaterials, which don t require many tedious, precise, and difficult purification processes.[21] Due to their strongly size-dependent optical properties, the concentration and size of nanomaterials at a certain reaction time have a close relationship with the absorbance and the peak position of the absorption spectrum, which is unlike traditional polyatomic organic molecules and metal complexes with constant absorption peaks. Actually, many efforts have already been devoted to explore the growth processes and the growth kinetics of nanomaterials by using the size-dependent optical properties of nanomaterials.[22-25] Ultrathin TeNWs with diameters of 4-9 nm were firstly reported in 2006 by our group.[26] Owing to high reactivity, easy processability, and large-scale synthesis,[27] ultrathin TeNWs, as an efficient physical or chemical template, have evolved into an extended functional nanomaterial family, including Pt, Pd, Au, Bi2Te3, CdTe, PbTe, Ag2Te, carbonaceous nanofibers and composites, exhibiting extensive applications in many fields, such as catalysis,[28-30] supercapacitors,[31] thermoelectricity,[32] photoconductivity[33] and environmental restoration.[34, 35] Although the high reactivity of ultrathin TeNWs can provide fruitful grounds for exploring new nanoscale structural transformations as well as functionalities of the corresponding templated products, it also gives ultrathin TeNWs the metastable nature, which can be easily degraded by oxidation during use. Our group have analyzed structure and morphology evolution of ultrathin TeNWs reposited in different solvents with TEM and SEM observations,[36, 37] but the quantitative descriptions about these are strongly pursued. Herein, we choose ultrathin TeNWs as a model material to study the stability of nanomaterials in aqueous solution from the viewpoint of reaction kinetics and then develop a new approach material design for its oxidation process into a good platform for chemical transformation. An accelerated oxidation experiment for ultrathin TeNWs was performed in a self-made apparatus (Scheme 1) under different reaction conditions. The chemical kinetics information about this oxidation

6 process was easily obtained by converting ultraviolet-visible (UV-Vis) absorption spectrum data. The results indicated that the oxidation kinetics for ultrathin TeNWs was accorded with the first order reaction kinetic model, and the corresponding apparent activation energy was about kj mol -1. Interestingly, after incomplete oxidation, ultrathin TeNWs evolved into a chain-dotted nanostructure, which still maintained high reactivity and could act as an excellent platform for the templating synthesis of other functional nanomaterials. In addition, we found that oxygen-free condition and adding suitable reductants can actually and effectively preserve ultrathin TeNWs with uniform morphology and high reactivity during storage. further purification. 2.2 Synthesis of ultrathin TeNWs Ultrathin TeNWs with diameters of 4-9 nm were prepared according to the method developed by our group previously.[26] In a typical procedure, g Na2TeO3, g PVP, 1.65 ml of hydrazine hydrate (85 wt%) and 3.35 ml of aqueous ammonia solution (25-28 wt%) were added into 32 ml of deionized water with suitable magnetic stirring. The clear mixture was transferred into a 50 ml Teflon-lined stainless steel autoclave. The autoclave was maintained at 180 C for 3 h and then cooled to room temperature rapidly with cold tap water. The final products were precipitated by adding 150 ml of acetone into the final solution. The precipitates were centrifuged and washed with absolute ethanol and distilled water several times. 2.3 Oxidation of ultrathin TeNWs under different experimental conditions Scheme 1. (a) Photograph of experimental apparatus for oxidation kinetics study of ultrathin TeNWs. (b) Schematic diagram of the jacketed pilot plant reactor. (c) Top view of the jacketed pilot plant reactor. (d) The scheme of accelerated oxidation experiment for ultrathin TeNWs. 2 Experimental 2.1 Materials Na2TeO3, poly (vinyl pyrrolidone) (PVP, Mw 4000), hydrazine hydrate (85 wt%), aqueous ammonia solution (25-28 wt%), NaOH, HCl (36-38%), ethylene glycol (EG), ethanol and H2PtCl6 were purchased from Shanghai Chemical regents Co.Ltd, and Nafina ( 5 wt%) from Sigma-Aldrich. All the chemical reagents were used as received without Firstly, 3 ml of ultrathin TeNWs solution was precipitated by adding 15 ml of acetone. Then the precipitates collected by centrifuging at 5000 rpm for 3 minutes and washed with absolute ethanol and deionized water several times. After that, the precipitates were rapidly dispersed in 35 ml N2-staturated deionized H2O by using a vortex mixer, and added the suspension into 185 ml of N2-staturated deionized H2O to form a homogeneous suspension with moderate magnetic stirring under the protection of nitrogen. The final concentration of ultrathin TeNWs was about mmol/l. The ph of the suspension was carefully adjusted to the required ph with 1 M NaOH and 1 M HCl. Then oxygen was bubbled into the reaction system with different flow rate, and the suspension was examined by different characterization equipment after different time intervals. The temperature of the reaction system was controlled by a precise low temperature thermostat during the Nano Research

7 experiments. 2.4 Synthesis of nanocables The nanocables were prepared by a template-directed hydrothermal carbonization procedure developed by our group.[38, 39] Briefly, 30 ml of acetone was added into 10 ml of the prepared Te nanowires solution to precipitate the product before centrifuging at 6000 rpm, which was then dispersed into 80 ml of glucose solution (5 g glucose) with vigorous magnetic stirring for 15 min. Hydrothermal treatment of the mixed solution at 160 C for 24 h could result in highly uniform Te@C nanocables. The Te@C nanocables were centrifuged and washed with absolute ethanol and distilled water several times. 2.5 Synthesis of Ag2Te nanowires After centrifuging and washing, a certain amount of ultrathin TeNWs were redispersed in EG. Then g silver nitrate was added into the EG suspension of ultrathin TeNWs under vigorous stirring at room temperature, and the color of the suspension changed from blue to brown, suggesting the formation of Ag2Te nanowires.[40] The Ag2Te nanowires were centrifuged and washed with absolute ethanol and distilled water several times. 2.6 Synthesis of Pt nanowires In a typical synthesis, 5 ml of reaction solution was taken out from the reaction system after different oxidation intervals and was degassed by bubbling nitrogen for 15 min. Then, 20 ml of EG and 1.29 ml of 77 mm H2PtCl6 solution were added into the previous solution. The mix solution was shaken at a rotation rate of 260 rpm using an Innova 40 Benchtop Incubator Shaker for 20 h at 50 C. The products were collected by centrifugation (10,000 rpm, 15 min) and then washed with double-distilled water and absolute ethanol. 2.7 Storage of ultrathin TeNWs A quantity of TeNWs precipitate was redispersed in 10 ml deionized H2O to form a homogeneous suspension with vigorous magnetic stirring. Then the suspension was examined at different time intervals during storage. Other experimental samples were prepared by the same procedure with the addition of g PVP, hydrazine hydrate (1 ml) and aqueous ammonia solution (1 ml), respectively. 2.8 Materials characterization X-ray power diffraction (XRD) patterns were obtained from a Philips X Pert PRO SUPER X-ray diffractometer equipped with graphite-monochromatized Cu Kα radiation (λ = Å ). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) observations were performed on JEOL-2010 operated at an acceleration voltage of 200 kv. UV-Vis specta were recorded on a UV-2501PC/2550 at room temperature (Shimadzu Corp., Japan). The real-time measurements of ph, dissolved oxygen during the experiment were carried out by a digital precision meter Multi 9430(WTW GmbH., Germany). Inductively coupled plasma atomic emission spectrometry (ICP-AES) measurements were conducted using an Atomscan Advantage spectrometer (Thermo Asn Jarrell Corporation, USA). 3 Results and discussion 3.1 Morphology evolution of ultrathin TeNWs during oxidation An accelerated oxidation experiment for ultrathin TeNWs was performed in a self-made apparatus (Scheme 1) under different reaction conditions. In our previous study, we found that ultrathin TeNWs

8 Figure 1. TEM images of morphology evolution of ultrathin TeNWs in water at ph 9 at 298 K with moderate stirring after bubbling oxygen with flow rate 10 sccm for: (a) 0 hour; (b) 1 hour; (c) 2 hours; (d) 3 hours; (e) 4 hours; (f) 5 hours. Sccm stands for standard cubic centimeters per minute. were easily oxidized into tellurium dioxide (TeO2) during storage.[36] Figure 1 shows the morphology evolution of ultrathin TeNWs during an accelerated oxidation process with oxygen flow rate 10 sccm. In the initial stage, due to low dissolved oxygen concentration, the oxidation rate was remarkably slow and the oxidation etching pits were hardly found on the surfaces of ultrathin TeNWs, which still kept good one-dimensional nanostructures with smooth surfaces (Figure 1a, 1b). With adding oxygen into the reaction system continuously, ultrathin TeNWs were gradually eroded or fractured because of oxidation, and formed a special chain-dotted line structure finally (Figure 1c-1f). After long-time oxidation, the surfaces of ultrathin TeNWs became fairly rough, but they still kept one-dimensional nanostructure, which looked as if it was broken (Figure 1e, 1f) and was attached by some oxidation products and formed an incomplete oxides shell (Figure S1 in the Electronic Supplementary Material (ESM)), which blocked the contact between ultrathin TeNWs and oxygen. In addition, only part of oxidation products TeO2 fell into the solution from the surfaces of ultrathin TeNWs and formed uniform nanoparticles with a diameter about 40 nm (Figure 1d-f and Figure S2, ESM). The XRD patterns and HRTEM images and EDS spectra of ultrathin TeNWs during oxidation were shown in Figure S UV-Visible spectrum of ultrathin TeNWs and the modified Beer-Lambert law Figure 2. Evolution of UV-Vis absorption spectra for ultrathin TeNWs at ph 9 at 298 K: (a) during oxidation with oxygen flow rate 10 sccm; during stirring under the protection of nitrogen: (b) at nm, (c) at nm. As a p-type semiconductor, the UV-Vis spectrum of ultrathin TeNWs possesses two broad absorption bands located at about 278 nm (peak I) and nm (peak II),[26, 41] which are due to an allowed

9 direct transition (3-6 ev) and a forbidden transition (0-3 ev), respectively.[37, 42] Unlike traditional polyatomic organic molecules and metal complexes, the position of UV-Vis absorption peak I for ultrathin TeNWs displayed a diameter dependence, whereas the position of peak II depended on the length of ultrathin TeNWs.[41, 43] In Figure 2a, we presented the evolution of UV-Vis absorption spectrum of ultrathin TeNWs during oxidation with oxygen flow rate 10 sccm. The other UV-Vis absorption spectra of ultrathin TeNWs under different experimental conditions were given in Figures S4-S6. The absorbance intensity of UV-Vis spectrum declined with oxidation proceeding, suggesting that the concentration of ultrathin TeNWs decreased in solution. The absorption peak I (at 278 nm) weakened gradually during oxidation without apparent red-shift or blue-shift. So the changes of ultrathin TeNWs in diameter during oxidation were hardly reflected by UV-Vis absorption spectrum due to their ultrathin diameter. On the contrary, the absorption peak II (at nm) decreased rapidly with prolonging reaction time and had a significant blue-shift from 650 to 550 nm. The blue-shift suggested that the average length of ultrathin TeNWs was decreased,[41] which demonstrated that some ultrathin TeNWs were etched and fractured by oxidation (Figure 1c-1f). Compared with the absorption peak I (at 278 nm), the absorption peak II (at nm) is better able to reflect the changes of ultrathin TeNWs both in concentration and size during oxidation. Owing to tiny solid nature and flexible features of ultrathin TeNWs, the bundling and intertwining of ultrathin TeNWs in solution were inevitable, which can interfere with the absorbance measurement of ultrathin TeNWs. The effects of these factors on the absorbance mainly reflected in two aspects, i.e., i) scattering of the incident light caused by the aggregation and ii) shielding some ultrathin TeNWs from irradiation caused by the cladding. In order to diminish the influence of these effects, it is essential to disperse ultrathin TeNWs into solution completely. In this study, a long time moderate magnetic stirring was used to form a homogeneous ultrathin TeNWs solution for reducing bundling and intertwining. The absorbance gradually became stable after moderate magnetic stirring for two hours under the protection of nitrogen (Figure 2b, 2c), which demonstrated that the proper stirring was favorable for improving the dispersity of ultrathin TeNWs and the accuracy of absorbance measurement. It is noteworthy that small nanoparticles of the oxidized products can also cause light scattering. The scattering intensity of small solids in solution is generally proportional to 1/λ 4 for the solid which are sufficiently much smaller than the incident wavelength,[44] such as TeO2 with a diameter about 40 nm; but it is weakly dependent on wavelength for larger particles, such as ultrathin TeNWs with a length more than tens of micrometers. So the larger incident wavelength, such as peak II (located at nm), is conductive to diminishing the effect of light scattering as much as possible. Based on the reasons above mentioned, the typical absorption peak II (located at nm) was chosen as a basis for quantitative analysis of UV-Vis absorption of ultrathin TeNWs during oxidation. It is obvious that the absorbance of ultrathin TeNWs was closely related to several factors: concentration, size, dispersity of ultrathin TeNWs and light scattering caused by ultrathin TeNWs and oxidation products. The impact of dispersity and light scattering on absorbance can be reduced as much as possible through the methods above discussed. In view of the strongly size-dependent optical properties of ultrathin TeNWs, we introduced a correction factor into the Beer-Lambert law to discuss the absorbance of ultrathin TeNWs. Assuming ultrathin TeNWs with an average length of Li at the time of i, the modified Beer-Lambert law can be expressed as follows:

10 Figure 3. (a) The plot (black) and the differential curve (blue) of reaction time t versus MRR η during oxidation with oxygen flow rate 5 sccm. (b) The plot of natural logarithm of MRR s reciprocal ln (1/η) versus reaction time t with oxygen flow rate of 5 sccm. MRR stands for modified remaining ratio. A log i I I 0 lc i i (1) In equation 1, Ai is the absorbance of reaction solution at reaction time i, and ci is the concentration of ultrathin TeNWs with an average length of Li, and ε is the extinction coefficient of normal ultrathin TeNWs. The correction factor φi was used to adjust the size effect of ultrathin TeNWs on absorbance, because the extinction coefficient was size dependence for nanomaterials.[45, 46] Therefore, the traditional method for polyatomic organic molecules and metal complexes, which establishes working equation of the samples by linear regression method, hardly applied to the case of ultrathin TeNWs. For better comparing the variation of concentration and morphology of ultrathin TeNWs under different experiment conditions and decreasing the impact of systematic error in the same condition, the absorbance data were normalized by the formula as follows: A lc c A i i i i i 0 lc0 c0 (2) In equation 2, A0 is the absorbance of reaction solution before bubbling oxygen, and c0 is the initial concentration of normal ultrathin TeNWs with an average length of L0. It s clearly that the normalized variables η consisted of two parts: concentration term ci/c0, which represented the ratio of remaining ultrathin TeNWs in solution, and modified term φi, which adjusted the influence of size evolution. Consequently, the normalized variables η objectively reflected the oxidation process of ultrathin TeNWs from two aspects: concentration variation and size evolution, and it was a modified remaining ratio (MRR) for ultrathin TeNWs during oxidation. 3.3 Analysis of the normalized absorption data of ultrathin TeNWs during oxidation In order to gain in-depth and comprehensive information of the oxidation process, the plot and the differential curve of reaction time versus MRR η with different oxygen flow rate was drawn in Figure 3a and Figure S7. Due to its higher smoothness and better linearity of fit, the curve for oxygen flow rate 5 sccm was chosen to discuss the oxidation of ultrathin TeNWs. As the oxidation progressed, the MRR η gradually reduced from 1.0 to about 0.0, which relatively reflected the concentration and length decrease of ultrathin TeNWs during oxidation. The corresponding differential curve revealed the rate of change of MRR η with reaction time. After bubbling oxygen, the differential curve had a significant decline in peak at about 1.5 hours, which indicated that the concentration decrease rate of ultrathin TeNWs reached the maximum at this moment.

11 1 c0 ln ln c i 1 ln i (3) Figure 3b the plot of the natural logarithm of MRR s reciprocal ln (1/η) versus reaction time (t) was drawn to obtain better insight into the reaction mechanism of this oxidation process. Equation 3 derived from equation 2 was the corresponding theoretical derivation format for the plot in Figure 3b. According to the different uptrend of the plot, ultrathin TeNWs oxidation process can be divided into three stages: oxygen limiting, ultrathin TeNWs limiting and mass transfer resistance limiting. i) Oxygen limiting: in the initial stage, because of low dissolved oxygen concentration in solution, the reaction rate was mainly limited by the concentration of dissolved oxygen. ii) Ultrathin TeNWs limiting: with prolonging the reaction time, the concentration of dissolved oxygen generally increased (The inset in Figure 4a), and it s almost 4.6 times than the initial concentration of ultrathin TeNWs ( M) at about 0.5 hour with a concentration of 200% ( M). In this case, the limiting of concentration of ultrathin TeNWs was gradually loomed. When oxygen was generally saturated in solution, the effect of oxygen limiting would be further weakened, and the reaction rate became mainly limited by the concentration of ultrathin TeNWs. iii) Mass transfer resistance limiting: due to solid nature of ultrathin TeNWs, the oxidation products were easily coated on the surfaces of ultrathin TeNWs, which restrained the mass transfer between ultrathin TeNWs and oxygen. When the quantity of oxidation products accumulated to a certain degree, the mass transfer resistance could heavily limit ultrathin TeNWs oxidation process. 3.4 The reaction kinetics analysis of ultrathin TeNWs oxidation process The oxidation process was closely associated with experiment condition and the downward trend of MRR η became quicken with high reaction temperature, oxygen flow rate and ph value (Figure 4a, 4c and 4e). Based on the above, the linear region from 1.5 hours to 4.0 hours of the plot of the natural logarithm of MRR s reciprocal ln (1/η) versus reaction time (t) (i.e. ultrathin TeNWs limiting) was chosen as a basis to discuss the reaction kinetics of ultrathin TeNWs during oxidation. Figures 4b, 4d and 4f show the linear fitting plots of the oxidation process in the range of 1.5 hours to 4.0 hours under different experimental conditions. Equation 4 is a format of the linear fitting equation of these plots. Comparing with equation 3, both two equations are equivalent in theory and are consisting of two terms: concentration term ln(c0/ci) and modified term ln(1/φi) for equation 3; variables term (kt) and constant term (q) for equation 4. Because the oxidation process of ultrathin TeNWs was mainly affected by ultrathin TeNWs concentration, for simplifying the derivation process, we assumed that the concentration term ln(c0/ci) in equation 3 was approximately corresponded to the variables term kt in equation 4, and the modified item ln(1/φi) can be roughly equaled with the intercept (q) of the linear fitting equations. Equation 5 was easily obtained, and it s obviously a typical first order kinetics equation. Without ruling out the impact of mass transfer resistance during the derivation process, equation 5 was an apparent kinetic equation, and the apparent rate constant, the slope (k) of the linear fitting equations, was limited by two aspects: reaction itself and mass transfer resistance. 1 ln kt q (4) c ci ln ln kt ci c0 e c c 0 kt i o Oxygen, as a familiar oxidant, played a very crucial role in the oxidation process of ultrathin TeNWs. After bubbling oxygen into the reaction (5) solution, oxygen was generally saturated in solution

12 and the MRR η was dropped from 1.0 to about 0.0. With increasing the flow rate of oxygen from 5 sccm to 20 sccm, the downtrend of the MRR η enhanced (Figure 4a), while the apparent rate constants under different flow rates were approximate and it changed from h -1 to h -1 because of the same reaction temperature 298 K (Figure 4b). However, owing to solid nature of ultrathin TeNWs, the oxygen flow rate wasn t the only factor that affected the oxidation rate, the connecting area between ultrathin TeNWs and oxygen also heavily influenced the reaction rate. The connecting area between ultrathin TeNWs and oxygen was closely linked with the dispersity of ultrathin TeNWs. In order to obtain a good dispersity of ultrathin TeNWs in solution, a long time magnetic stirring and a suitable ph value were required. As an important index in a solution, ph could influence the surface charge state of nanomaterials, which is closely relative with the dispersity of nanomaterials. Meanwhile, hydroxyl ions or hydrogen ions could also act as a reactant to take part in the actual reaction. It s easy to find that the increase of the MRR s reciprocal ln (1/η) was much faster (Figure 4c), when the solution was more alkaline, so high ph value promoted the oxidation process. It s amazing that when the ph value reached 13, ultrathin TeNWs would be completely oxidized within 1 hour (Figure S5g, ESM). On the contrary, it was extended to more than 10 hours at ph 1.0 (Figure S5a, ESM). There are two main reasons for explaining such phenomenon: i) ultrathin Te nanowire has a negative surface potential about mv. High ph values were beneficial to improve the dispersity of ultrathin TeNWs, which increased the connecting area between ultrathin TeNWs and oxygen by reducing the bundling and intertwining; ii) hydroxyl ion can accelerate the oxidation process in a certain way: hydroxyl ions directly reacted with ultrathin TeNWs (Eq. 6) or reacted with the oxidation products TeO2 to promote the oxidation process (Eq. 7). A contrast experiment was occupied to find out the truth: ultrathin TeNWs were kept in solution at ph 9 with moderate magnetic stirring. The solution absorbance was nearly stable with prolonging stirring time under the protection of nitrogen (Figure 2b, 2c), so the reaction between ultrathin TeNWs and hydroxyl ions was impossible. But the absorbance dramatically decreased after bubbling oxygen (Figure 2a and Figures S4-S6, ESM) and the solution ph value quickly declined from 9.0 to about 7.0 within two hours (The inset in Figure 4c). Hence, hydroxyl ions actually involved in the oxidation process and reacted with oxidation products. The reaction between hydroxyl ions and oxidation products the precipitate reaction of silver tellurite (IV) (Ag2TeO3) (The inset in Figure 4e). Because process and reacted with oxidation products. The reaction between hydroxyl ions and oxidation products the precipitate reaction of silver tellurite (IV) (Ag2TeO3) (The inset in Figure 4e). Because the reaction between TeO2 and hydroxyl ion reduced mass transfer resistance by consuming the oxidation products coated on the surfaces, the apparent rate constant was increased to 2-3 times at higher ph values (Figure 4d), even although the oxidation experiments were performed at same reaction temperature (298 K) Te 6OH 2Te TeO 3 3H 2O 2 TeO 2 2OH TeO 3 3H 2O (6) (7) It s clear that the oxidation process of ultrathin TeNWs was a two steps process. Firstly, ultrathin TeNWs reacted with oxygen to form TeO2, and the TeO2 partly fell into the solution to form small particles or partly attached on the surfaces of ultrathin TeNWs to block the contact between ultrathin TeNWs and oxygen, which is the main source of mass transfer resistance. Secondly, the oxidation products TeO2 reacted with hydroxyl ions to form TeO3 2-, which can promote the oxidation of ultrathin TeNWs. With oxidation processing, the Nano Research

13 Figure 4. The plots for the reaction time t versus the MRR η under different oxygen flow rate (a); different ph value (c); reaction temperature (e). The linear fitting plots of the natural logarithm of the MRR s reciprocal ln (1/η) versus reaction time t: (b) under different oxygen flow rate; (d) under different ph value with oxygen flow rate 10sccm, (f) under different reaction temperature with oxygen flow rate 10sccm. Inset in panel a, polt of the reaction time versus the dissolved oxygen under different bubbling rate of oxygen. Inset in panel c, polt of the reaction time versus the ph value at ph 9 at 298 K. Inset in panel e, photographs of initial ultrathin TeNWs solution (I), completely oxidized ultrathin TeNWs solution (II), adding AgNO3 into the completely oxidized ultrathin TeNWs solution (III). Insets in panel b, d and f, the information tables of the corresponding linear fitting curves. surfaces of ultrathin TeNWs were coated with oxidation products. When the quantity of the oxidation products accumulated to a certain degree, the mass transfer resistance can limit the oxidation reaction heavily. The increase of conductivity of the reaction solution proved that the oxidation products TeO2 was partly dissolved by water during oxidation (Figure S8, ESM).

14 Cheng et al reported that the solubility of TeO2 zoomed exponentially with increasing solution temperature.[47] Solution temperature also play an important role in the reaction kinetics, so high reaction temperature is both good for mass transfer and reaction kinetics. For a high temperature at 338 K, ultrathin TeNWs were completely oxidized within 3 hours, but the reaction time was extended to 6 hours at 318 K, to 8 hours at 298 K and to 10 hours at 288 K due to lower oxidation rate and less dissolution rate of TeO2 (Figure 4e). At the same time, the apparent rate constant declined sharply from h -1 to h -1 (Figure. 4f). Generally, the reaction rate constant is at least twice when reaction temperature increases by 10 K, but the increase in this process is much smaller because of mass transfer resistance. The apparent activation energy of the reaction is defined as Ea in equation 8 based on the Arrhenius relation. By assuming the apparent activation energy to be indpendent of temperature from 288 K to 338 K and fitting the data under different reaction temperature, the apparent activation energy of this process was calculated about kj mol -1, indicating that the oxidation is easy to occur (Figure 5). E a ln k B RT (8) Figure 5. Linear fitting polt of the reaction temperature s reciprocal (1/T) versus the natural logarithm of the fitting apparent rate constants lnk. 3.5 Chemical transformation of ultrathin TeNWs during oxidation Figure 6. TEM images of the products templating from pristine TeNWs and TeNWs after oxidation. (a) Te@C nanocables synthesized from pristine ultrathin TeNWs, (b) TeNWs after oxidation for 2 hour, (c) Ag2Te nanowires synthesized from pristine TeNWs, (d) Ag2Te nanowires synthesized from the TeNWs after oxidation for 1 hour. The very low apparent activation energy (13.53 kj mol -1 ) demonstrated that ultrathin TeNWs have a high reactivity. In our previous report, the high reactivity of ultrathin TeNWs can provide fruitful grounds for exploring of new nanoscale structural transformations as well as functionalities of the corresponding templated products. We investigate the reactivity of ultrathin TeNWs during oxidation from two perspectives. Firstly, as a physical templating, oxidized ultrathin TeNWs were used to prepare Te@C nanocables through a template-directed hydrothermal carbonization process.[38, 48] Compared with highly uniform and long Te@C nanocables derived from normal ultrathin TeNWs (Figure 6a), the obtained Te@C nanocables from oxidized TeNWs were short and bending (Figure 6b), which was a robust evidence for the fracture of ultrathin TeNWs during oxidation. Secondly, as a chemical templating, oxidized ultrathin TeNWs were converted into Ag2Te nanowires by reacting with silver nitrate in ethylene glycol. The products inherited one-dimensional chain-dotted structure of ultrathin oxidized Te nanowires, which is hardly

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16 Figure 7. TEM images of the Pt nanostructures prepared by chemical transformation process using the ultrathin TeNWs after oxidation for (a) 0 hour, (b) 1 hour, (c) 2 hours, and (d) 3 hours, as precursors respectively. (e) HRTEM image of an individual solid wire-like Pt nanostructure obtained by chemical transformation using the ultrathin TeNWs after oxidation for 1 hours. (f) XRD pattern of the sample prepared by chemical transformation process using the ultrathin TeNWs after oxidation for 1 hours as precursor. achieved by a direct synthesis method, and had fairly rough surfaces than the normal Ag2Te nanowires (Figure 6c-6d). The Ag2Te nanowires can transform into other telluride nanowire (CdTe, ZnTe and PbTe) using cation-exchange reactions, these reactions have been demonstrated by Jeong group[40] and our group.[49] Here, we focus our study on the galvanic replacement reaction between TeNWs, as a chemical templating, and H2PtCl6. It s interesting to note that the chemical transformation products showed a dramatic morphological-evolution from hollow nanotubular nanostructures to solid wire-like nanostructures (Figure 7a, Figure S9a in ESM and Figure 7b-7d), and even large particles (Figures S9b-S9d, ESM). A high-resolution TEM (HRTEM) image taken from an individual chemical transformation nanowires was shown in Figure 7e. The observed lattice spacings are 2.3 and 1.9 Å, which are consistent with the (111) plane and (200) plane of Pt, respectively. Figure 7f shows the X-ray diffraction (XRD) pattern of these chemical transformation products. All the diffraction peaks can be indexed to face-centered-cubic (fcc) platinum, with a lattice constant of a = 3.9 Å (JCPDS No ). It is worthwhile to clearly understand why normal ultrathin TeNWs transformed into a tubular nanostructure whereas oxidized ultrathin TeNWs do not. The Kirkendall effect, a classical phenomenon in metallurgy, was used to illustrate the formation of hollow nanostructures.[50-52] On the basis of this mechanism, the different diffusion rates of two diffusion components result in a net outward flow, which leads to the formation of hollow nanostructures. In the case of Pt nanotubes, the smooth surfaces of normal ultrathin TeNWs caused the galvanic replacement reaction to limit in landscape orientation, Nano Research

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18 Figure 8. (a) Photographs of ultrathin TeNWs during Storage with (left) and without (right) mineral oil for the sample of H2O, PVP, NH3 H 2O, N2H4 H 2O at room temperature. TEM images of ultrathin TeNWs during storage with N2H4 H 2O after 4 weeks without (b) and with (c) mineral oil. therefore the diffusion components of Te and Pt could continuously diffused in reverse directions to form hollow tubular nanostructures (Figure 7a). But this reverse diffusion was heavy going at a low temperature, ultrathin TeNWs would be hard to be completely consumed. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) analyses indicated that the mass content of Te could be maintained at about 20% in the chemical transformation products, which derived from normal ultrathin TeNWs, but sharply dropped to 8%, when ultrathin TeNWs was oxidized for only 0.5 hour, and finally kept about 3% (Figure S10, ESM). In the case of oxidation, some ultrathin TeNWs were partially eroded or fractured, and the surface of all ultrathin TeNWs became fairly rough. The galvanic replacement reaction was not limited in landscape orientation, but also was carried out in the oxidation etching pits in other orientation. The galvanic replacement reaction proceeded more completely, and the less Te was maintained in these chemical transformation products. What s more, it was difficult to obtain enough the net directional flow of matter to form a hollow nanostructure after ultrathin TeNWs were oxidized. The morphology of the chemical transformation products evolved from hollow nanotubes (Figure 7a) to nanowires with little pores (Figure S9a, ESM), solid nanowires (Figure 7b-7d), and even large particles (Figure S9b-S9d, ESM), which formed by an Ostwald ripening process. 3.6 Storage of ultrathin tellurium nanowires The high reactivity of ultrathin TeNWs gives ultrathin TeNWs the metastable nature, which Nano Research

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20 Figure 9. TEM images of Te nanowires after storage for 10 days (a) stored under ambient condition, (b) stored in a vacuum dryer, which was filled up with nitrogen, (c) with the presence of citric acid, (d) with the presence of ascorbic acid, (e) with the presence of N2H4 H2O, (f) photograph of Te nanowires solution just dispersed (up) and after storage for 10 days (down) under different conditions. can be easily degraded by oxidation during use. Astonishingly, ultrathin TeNWs can maintain uniform morphology and high reactivity for more than 6 months in the original reaction solution, which include PVP, NH3 H 2O, N2H4 H 2O and other by-product.[36, 37] It is very important to investigate the influence of PVP, NH3 H 2O, N2H4 H 2O on the stability of ultrathin TeNWs. Figure 8a shows the digital photographs of ultrathin Tellurium nanowires which stored under different experiment conditions after different time intervals. with prolonging the storage time, all the samples showed a poor dispersity and settled to the bottom of sample bottle, and fade to colorless except the sample of N2H4 H 2O. The samples with mineral oil showed a lower oxidation rate, because mineral oil can, to some extend, block oxygen from dissolving into the solution. In addition, the low storage temperature was also a brilliant way to prevent ultrathin TeNWs from oxidation in thermodynamics (Figure S11, ESM). Ultrathin TeNWs were generally oxidized in solution by the dissolved oxygen form air during storage. With the abscission of oxidation products, some PVP molecules were shed from the surfaces of ultrathin TeNWs. The less PVP molecules that coated on the surfaces of ultrathin TeNWs resulted in the poor dispersity of ultrathin TeNWs in solution (Figure 8a). Comparing the sample with PVP in the system or not, we found that adding PVP capping agent didn t give ultrathin TeNWs a good dispersity and protect ultrathin TeNWs from oxidation. Adding NH3 H 2O to the storage solution could produce an alkaline reaction medium, which promoted the oxidation process (Figure 8a). Interestingly, when we added enough NH3 H 2O and a certain amount of PVP and N2H4 H 2O to the completely oxidized storage solution, uniform ultrathin TeNWs with a length of 5-6 µm can be reproduced by hydrothermal reaction at 180 C Nano Research

21 for 3 h (Figure S12, ESM). It s not only a good way to reproduce ultrathin TeNWs after degradation, but also inspired us to synthesize ultrathin TeNWs with cheaper and low toxicity TeO2. When a certain amount of N2H4 H 2O added to the storage solution, ultrathin TeNWs maintained a good stability even for more than four weeks (Figure 8a). Figures 8b, 8c show the TEM images of the sample which added N2H4 H 2O after storage for 4 weeks without or with mineral oil. Obviously, ultrathin TeNWs exhibited a uniform 1D nanowire morphology without any oxidation under the protection of N2H4 H 2O and mineral oil after storing for 4 weeks. The standard electrode potentials of TeO2/Te, O2/H2O and N2/N2H4 are 0.593, and V. The difference between the redox potentials of the two half-cell reactions, ΔE, was V for the reaction between O2/H2O and TeO2/Te (Eq. 9) and V for the reaction between O2/H2O and N2/N2H4 (Eq. 10). Due to the reaction between O2 and N2H4 H 2O with a larger ΔE, oxygen can react with N2H4 H 2O preferentially. So N2H4 H 2O can protect ultrathin TeNWs from oxidation by consuming oxygen. The reaction is thermodynamically possible only if ΔE is positive. The reaction between O2 and Tellurium has a negative potential, indicating the oxidation for tellurium is difficult. This is true for the bulk tellurium, but ultrathin TeNWs are easily oxidized in water at room temperature. The main reason for this is that the ultrathin TeNWs have a large number of surface atoms which leave chemical bonds "dangling" outside the solid and give they a higher reactivity than the bulk.[53] N Te O 2 TeO 2 2 H 4 O 2 N 2 2H 2 O (9) (10) We also investigated the storage of ultrathin TeNWs under other conditions at room temperature (Figure 9). Comparing with the sample stored under ambient condition, the sample under the protection of nitrogen still maintained 1D nanowire morphology expect that some oxides particles attached on the surfaces (Figure 9a-9b). The slight oxidation for ultrathin TeNWs could be attributed to the dissolved oxygen in water. Thus, the oxygen-free environment is beneficial to the storage of TeNWs by isolating oxygen. Obviously, ultrathin TeNWs exhibited a uniform 1D nanowire morphology without any oxidation under the protection of citric acid, ascorbic acid and N2H4 H 2O (Figure 9c-9e)., In addition, all the samples showed a good dispersity after storage for 10 days except the sample stored in ambient condition and the sample with the presence of citric acid (Figure 9f). Therefore, adding suitable reductants was actual and effective method to maintain uniform morphology and good dispersity for ultrathin TeNWs during storage. In essence, the degradation of ultrathin TeNWs during storage is a gas-solid oxidation reaction which involved liquid, gas and solid phases, so employing effective methods to prevent oxidation is the key to the storage of ultrathin TeNWs. We think that oxygen-free condition and adding suitable reductants are actual and effective ways to maintain the uniform morphology and high reactivity of ultrathin TeNWs during storage. 4. CONCLUSION In summary, the stability of ultrathin TeNWs in aqueous solution has been first investigated under different temperature, ph value and various oxygen flow rate from the viewpoint of reaction kinetics. Unlike traditional polyatomic organic molecules and metal complexes, due to the flexible features and size dependent optical

22 properties of ultrathin TeNWs, the absorbance of ultrathin TeNWs was related to several factors, i.e., concentration, dispersity, morphology and light scattering caused by ultrathin TeNWs and oxidation products. In essence, the oxidation of ultrathin TeNWs was a gas-solid reaction which involved liquid, gas and solid phases. The oxidation process can be divided into three stages, i.e., oxygen limiting, ultrathin TeNWs limiting and mass transfer resistance limiting stages. The apparent kinetics of this process accorded with first order reaction kinetic model, and the corresponding apparent activation energy of this process was just only kj mol -1, indicating that ultrathin TeNWs was unstable in thermodynamics. However, such unstable nature of ultrathin TeNWs did not seem like repugnant and useless, it actually can act as an excellent platform to help us synthesize and design unique one-dimensional functional nanomaterials with special structure and distinctive properties by using the intermediate nanostructures captured during the dynamic oxidation process under different conditions, which are hardly achieved by all existing direct synthesis methods. In addition, we found that oxygen-free condition and adding suitable reductants are actual and effective ways to prevent the oxidation of the freshly prepared ultrathin TeNWs and thus keep it stable during storage. The present work implies that knowing more about how happen on the stability and reactivity of nanomaterial is of tremendous momentousness both for their applications and reasonably designing functional nanomaterials based on chemical transformation process. Acknowledgements This work is supported by the Ministry of Science and Technology of China (Grants 2010CB934700, 2013CB933900, 2014CB931800), the National Natural Science Foundation of China (Grants , , , J ), the Chinese Academy of Sciences (Grant KJZD-EW-M01-1), and Hainan Province Science and Technology Department (CXY ) for financial support. Electronic Supplementary Material: Supplementary material (Details of characterization results and electrochemical measurements) is available in the online version of this article at (automatically inserted by the publisher). References [1] Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Shape-controlled synthesis of metal nanocrystals: simple chemistry meets complex physics? Angew. Chem. Int. Ed. 2009, 48, [2] Yin, Y.; Alivisatos, A. P. Colloidal nanocrystal synthesis and the organic-inorganic interface. Nature 2005, 437, [3] Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. One-dimensional nanostructures: synthesis, characterization, and applications. Adv. Mater. 2003, 15, [4] Lim, B.; Jiang, M. j.; Yu, T.; Camargo, P. H. C.; Xia, Y. N. Nucleation and growth mechanisms for Pd-Pt bimetallic nanodendrites and their electrocatalytic properties. 2010, 3, [5] Mackenzie, J. D.; Bescher, E. P. Chemical routes in the synthesis of nanomaterials using the sol-gel process. Acc. Chem. Res. 2007, 40, [6] Ghosh Chaudhuri, R.; Paria, S. Core/shell Nano Research