Thermal Sensitive Quantum and Phonon Confinements for

Mar 8, 2014 - interior of high tempeature solid oxide fuel cells and opaque fireballs, where real-time readout of temperature is not feasible. Our stu...
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Thermal Sensitive Quantum and Phonon Confinements for Temperature Mapping in Extreme Environments Ashish Kumar Mishra, Junwei Wang, and Liping Huang* Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180, United States ABSTRACT: Spatial and/or temporal temperature mapping in extreme environments that cannot be physically accessible remains a grand challenge, such as in the interior of high tempeature solid oxide fuel cells and opaque fireballs, where real-time readout of temperature is not feasible. Our study showed that photoluminescence (PL) spectra of ZnO nanoparticles (NPs) and Raman spectra of anatase TiO2 NPs exhibit thermal sensitive quantum confinement and phonon confinement, respectively. Here, we explored the highly temperature sensitive irreversible growth of NPs together with the strong quantum and phonon confinements in these oxide NPs as unique signatures for ex situ temperature sensing, especially for applications in extreme environments. By distributing these NPs, a spatially and temporally nonuniform thermal mapping can be determined by a direct read-off of their PL and Raman spectra at various locations.

1. INTRODUCTION Temperature sensors have been developed for a wide range of energy, environmental, and biological applications. Increasing demands for precise temperature mapping with high spatial resolution have led to the development of nanothermometers, which are rapidly substantiated with the advancement of material synthesis and characterization tools enabled by nanotechnology and nanoengineering.1 Different temperature-dependent properties and principles have been identified to enable thermometry at the nanoscale. Miniaturizing the geometrical size of conventional thermometers has been commonly employed to develop nanothermometers.2 Luminescent nanothermometers based on emission intensity, IR nanothermometer based on blackbody radiation, nanoscale thermocouples fabricated from point contact junction based on the Seebeck effect, and liquid- and solid-intube nanothermometers and switches based on thermal expansion of liquids are a few among others that have been investigated recently for temperature sensing with submicrometric resolution.3−19 Coulomb blockade of tunneling, temperature-dependent Fermi level shifts, or resonator quality factor for MEMS devices have also been identified as potential means to measure temperature at the nanoscale.20−23 In all of the above nanothermometers, basic temperaturedependent properties, like luminescence spectrum, thermal expansion, etc., are restored to their original state at room temperature after the temperature drops from high temperature. These thermometers are usually employed for real-time and in situ temperature detection. However, they are of limited use for temperature sensing in areas that cannot be physically accessible, where real-time readout of temperature is not feasible, such as in the interior of high temperature solid oxide fuel cells, opaque fireballs, or high-vacuum chamber of electron microscopes, etc. In order to overcome such limitations, recent efforts have been devoted to develop ex situ thermometers, © 2014 American Chemical Society

whose physical properties modified by a thermal event are retained and measured at a later time.24−34 In other words, these thermometers can forensically record the temperature of the thermal event that they experienced. Furthermore, due to the small sizes, they can respond very fast to the rapid changes in the temperature profile. A guided approach has been proposed for the development of thermoluminescent and optically stimulated luminescent materials by using yttrium aluminum garnet, LiF:Mg,Ti, or Al2O3:C for temperature sensing.24,25 Twodimensional ex situ nanothermometers have been developed using single crystal silver nanospheres, while thermometry studies have been demonstrated using the phonon confinement effect in anatase and rutile TiO2 nanoparticles (NPs) in the temperature range of 400−700 °C.26,27 Eu-doped Y2O3 and ZrO2 have been studied for thermal sensing.28−31Luminescent microparticles have been used to measure the rapid temperature profiles.32 Temperature sensing has been performed using ultrathin gold island films.33 Thermal imaging was performed in temperature range 20−80 °C using multiphoton excited CdTe quantum dots (QDs) as nanothermometers.34 However, the relationship between thermal sensitivity and size of CdTe QDs has been debated.35,36 In this study, we explored the thermal sensitive quantum confinement effect in ZnO and phonon confinement effect in TiO2 NPs as distinct signatures for temperature mapping in extreme environments.37−40 By virtue of the irreversible fast grain growth upon heating, these NPs have the potential to forensically retain not only the temperature but also the time of a thermal event.41,42 Knowledge of the temperature profile of a thermal event is crucial for various applications, such as to ensure the complete destruction of Received: January 16, 2014 Revised: March 6, 2014 Published: March 8, 2014 7222

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Figure 1. SEM images of (a) ZnO and (b) TiO2 NPs clusters. TEM images of (c) ZnO and (d) TiO2 NPs.

biological hazards using fireballs. The easy synthesis and high stability of ZnO and TiO2 NPs make them especially attractive to be used as robust temperature sensors in extreme environments. Furthermore, by distributing these NPs, a spatially and temporally nonuniform thermal mapping can be determined by a direct read-off their photoluminescence (PL) and Raman spectra at various locations. NPs can be mixed with test agents and fly with them during a thermal event. Sizes of ZnO and TiO2 NPs before and after the thermal event can be determined from their PL and Raman spectra based on the quantum and phonon confinement effect, respectively, and used in grain growth equations to extract the temperature and time of their journey.

and aging time were varied to obtain different particle sizes for ZnO. 2.2. Characterization. TiO 2 and ZnO NPs were characterized by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), photoluminescence (PL), and Raman spectroscopy techniques. SEM, TEM, and XRD were used to examine the morphology, structure, and average size of NPs, while PL and Raman spectroscopy were performed to analyze the quantum and phonon confinement in ZnO and TiO2 NPs, respectively. SEM study was done with a JSM 6335 field emission scanning electron microscope operated at 30 kV. SEM specimens were prepared by depositing a small amount of sample on a carbon tape. TEM study was performed with a JEM 2010 transmission electron microscope operated at 200 kV. TEM specimens were prepared by placing ultrasonically dispersed suspension of NPs onto a carbon-coated copper TEM grid, followed by drying in air. XRD study was performed with a Panalytical X-ray diffractometer. Diffraction patterns were collected using the Cu Kα radiation (40 mA, 45 kV) in the step-scanning mode. The Scherrer equation was used to determine the average particle size of NPs:

2. EXPERIMENTAL METHODS 2.1. Synthesis of NPs. Anatase TiO2 NPs were prepared by the modified two-phase hydrothermal method.27,42 In a typical synthesis of 5 nm TiO2 NPs, 0.1 mL of tert-butylamine (TBA) was dissolved in 10 mL of water, and the solution was transferred to a 45 mL stainless-steel autoclave. Subsequently, 0.15 g of titanium(IV) propoxide (TPO, 0.5 mmol) and 1.0 mL of oleic acid (OA) were dissolved in 20 mL of toluene in air, and the solution was transferred to the autoclave without any stirring. The sealed autoclave was maintained at 180 °C for 12 h. The precipitated NPs with methanol were further isolated by centrifugation and decantation. These NPs were redispersed in toluene, which could be precipitated again with methanol. Sizes of TiO2 NPs were controlled by the concentration of tertbutylamine and the ratio of Ti/oleic acid. ZnO NPs were synthesized by the ethanolic distillation method.43 In this method, different molar concentrations (0.02−0.1 M) of Zn(II) acetate−ethanolic solution was prepared by stirring the appropriate amount of Zn(II) acetate dyhydrate in ethanol for 30 min at room temperature. Then the ethanolic solution was transferred to an autoclave for aging at 80 °C for 3−12 h. Samples were collected and washed further with ethanol and deionized water and dried. Molar concentrations

D=

0.9λ β cos θ

(1)

where λ is the wavelength of the X-ray, β the full-width at halfmaximum height, and θ the diffraction angle. Raman study of TiO2 NPs was performed with a LabRAM HR800 Raman microscope using a 532.18 nm green laser as the probing light source. A resolution of 0.5 cm−1 for the Raman peak position was achieved by using an 1800 grooves mm−1 grating. The scattered light was collected in the backscattering geometry using a CCD detector. The PL spectrum of ZnO NPs was performed using a Spex fluorolog tau-3 spectrofluorometer with an excitation wavelength of 325 nm and a spectral resolution of 0.1 nm. ZnO and TiO2 NPs of different initial sizes were 7223

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Figures 1a and 1b show the clusters of ZnO and TiO2 NPs. TEM images in Figures 1c and 1d show the NPs at higher resolution. Figures 1c and 1d show the almost uniform particles size distribution for spherical NPs of ZnO and TiO2, respectively. Particles sizes were observed in the range of 3−4 nm for ZnO and 4−5 nm for TiO2. Figure 2 shows the XRD spectra for one set of ZnO and TiO2 NPs with average size around 10 nm. It exhibits the hexagonal wurtzite structure for ZnO and the tetragonal structure for anatase phase of TiO2. 3.2. Optical Spectroscopy Studies. Figures 3a and 3b show the PL and Raman spectra for ZnO, TiO2 NPs, and their mixture with 1:1 ratio. Figure 3a clearly suggests that ZnO NPs exhibit a strong PL spectrum, while TiO2 NPs in the mixture have a negligible influence on it. Figure 3b indicates that the dominant Raman signal in the mixture comes from TiO2 NPs. These results show that the PL and Raman spectrum of the mixture can be used as the “fingerprint” of ZnO and TiO2 NPs, respectively, without the need of spectral deconvolution. The quantum confinement effect in ZnO NPs and the phonon confinement effect in anatase TiO2 NPs were investigated by measuring their respective size-dependent PL and Raman spectra and showed in Figures 4a and 4b, which are consistent with previous studies.39,40,44 These master curves will be used to determine the particle sizes after thermal treatments without the need of XRD/TEM measurements. This is particularly convenient for field applications using portable Raman and PL spectrometers. Figure 5a shows the PL spectra of ZnO NPs after heat treatment in the temperature range of 25−700 °C for 15 s, while Figure 5b shows the PL spectra of ZnO NPs upon isothermal heating at 550 °C for 5−60 s. The irreversible grain growth of NPs size upon heating is responsible for the red-shift in the PL spectra of ZnO NPs as seen in Figures 5a and 5b.37,42 A similar red-shift was observed in the Eg band Raman peak of TiO2 NPs with increasing temperature and time, as shown in Figures 5c and 5d.27,45 Figure 6 shows the TEM images of TiO2 and ZnO NPs with initial size of 5 nm after heated at 600 °C for 60 s. These images clearly show the increased sizes of these NPs in response to the fast heat treatment and confirm the shift in Raman and PL spectra observed in Figure 5. 3.3. Grain Growth Studies. The growth kinetics of ZnO and TiO2 NPs was studied using a pyroprobe thermal heater. Heating of NPs assembly results in the growth of large particles

used to study the quantum confinement in the former and the phonon confinement in the latter based on their PL and Raman spectra, respectively. 2.3. Heat Treatment. NPs were heated on a platinum ribbon using a CDS Analytical Pyroprobe 5000 heater, with which the temperatures can go up to 1400 °C and the rates may range from 0.01 °C min−1 to 20 000 °C s−1. In the present study, the fastest heating rate was used to reach the target temperature. NPs were kept at certain temperature for certain amount of time before cooling down naturally in air when the power to the heater was turned off. ZnO NPs with initial average particle size of 3.5 nm and TiO2 NPs with initial size of 4.5 nm were used for heat treatment experiments. Then the thermally treated ZnO NPs were analyzed by PL and TiO2 NPs by Raman spectroscopy to study the grain growth in each case.

3. RESULTS AND DISCUSSION 3.1. Structural and Morphology Studies. Structural and morphology characteristics of ZnO and TiO2 NPs were analyzed by SEM, TEM, and XRD techniques. SEM and TEM images of these NPs are shown in Figure 1. SEM images in

Figure 2. XRD patterns of ZnO and TiO2 NPs.

Figure 3. (a) PL spectra of ZnO, TiO2, and 1:1 mixture of ZnO and TiO2 NPs. (b) Raman spectra of TiO2, ZnO, and 1:1 mixture of ZnO and TiO2 NPs. 7224

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Figure 4. Size dependent photon energy in (a) ZnO and Raman shift in (b) TiO2 NPs.

Figure 5. PL spectra of ZnO NPs with (a) constant heat treatment time of 15 s and (b) isothermal temperature of 550 °C. Raman spectra of TiO2 NPs with (c) constant heat treatment time of 15 s and (d) isothermal temperature of 500 °C.

Dn = D0 n + k 0t me−Ea / RT

at the expense of smaller ones. Therefore, the average particle size increases as a function of both temperature (T) and time (t). The irreversible grain growth of these NPs helps in forensically retaining the complete thermal history (T and t) of a thermal event.27 Assuming normal grain growth, the following equation can be used to describe the growth kinetics for spherical particles:

(2)

where D is the average particle size at a time t and temperature T, D0 is the initial particle size, n is the growth exponent, m is the time exponent, k0 denotes a pre-exponential constant, Ea is the activation energy for grain growth, and R is the gas constant.46 These parameters can be determined from 7225

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Figure 6. TEM images of (a) TiO2 and (b) ZnO NPs after heated at 600 °C for 60 s.

Figure 7. Grain growth kinetics of ZnO NPs with (a) constant time and (b) constant temperature. Grain growth kinetics of TiO2 NPs with (c) constant time and (d) constant temperature.

calculated by taking different combinations of curve fittings in Figure 7 for ZnO and TiO2 NPs. The activation energy for the grain growth in ZnO and TiO2 NPs were found to be 71.3 ± 0.6 and 89.3 ± 0.1 kJ mol−1, respectively. The fast growth of these NPs within few seconds and their distinct identification determined by optical techniques make them very attractive as ex situ nanothermometers for short-time thermal events. To demonstrate the use of these nanothermometers to extract the T and t of a thermal event simultaneously, ZnO

temperature-dependent and isothermal time-dependent heat treatments of NPs as shown in Figure 7. The grain growth equation for ZnO and TiO2 NPs were found as follows: D2 = D0 2 + 7.4(6) × 104t 0.88(1)e−71.3(6)/ RT

(3)

D2 = D0 2 + 1.4(2) × 106t 1.11(1)e−89.3(1)/ RT

(4)

Numbers in parentheses in eqs 3 and 4 are errors in the last digit of the corresponding parameters. These errors were 7226

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and TiO2 NPs were thermally treated with different sets of temperature and time. Optical characteristics (PL specrtra of ZnO and Raman spectra of TiO2) of these NPs after heat treatments were collected to analyze the particles sizes using the master curves in Figure 4. The particle sizes before and after the heat treatments were used in eqs 3 and 4 to solve for the temperature and time (assuming a constant temperature profile) and compared with those set in the thermal treatments. The calculated and actual values of temperature and time are shown in Table 1. It clearly shows the high degree of accuracy

set T/t (K/s)

calculated T/t (K/s)

1 2 3 4 5

823/15 853/10 773/60 873/30 973/5

812/16 867/9 795/53 855/34 948/6

in revealing the thermal history of the event. Accuracy of 97% and above is found in the measurement of temperature, while 90% and above is found for time measurement. The minimum temperature studied in our current work is 400 °C, while our earlier study41 suggests that above 800 °C anatase TiO2 transforms into the thermodyamically stable rutile phase. Therefore, our method can detect temperatures in the range of 400−800 °C. Detectable change in temperature may vary with time as growth of NPs is a function of both temperature and time. For longer heat treatment smaller change in temperature can be detected. In our study, we were able to detect the change of 30 °C with 10 s of heat treatment time (Table 1).

4. CONCLUSION Our study demonstrated the quantitative measurement of temperature and time with high accuracy in the range of 400− 700 °C and 5−60 s by using ZnO and TiO2 NPs as ex situ nanothermometers based on their quantum and phonon confinement effect and the fast growth kinetics. The distinct signatures of the thermal response of these oxide NPs have great potential in forensically recording and storing the complete thermal history (temperature and time) of explosionlike events with high degree of accuracy, compared with existing techniques. It is a natural extension to use these nanothermometers for a spatially and temporally nonuniform thermal mapping by a direct read-off of their PL and Raman spectra at various locations, either by seeding them into explosive fireballs or by physically distributing them in static applications.

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Table 1. Set and Calculated Temperature and Time from Grain Growth Kinetics of ZnO and TiO2 NPs number

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (L.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Defense Threat Reduction Agency (DTRA Grant HDTRA1-09-1-0046). 7227

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