Tunable Photoluminescence across the Visible Spectrum and

Oct 9, 2017 - The spectra for the Re L1- (E0 = 12,527 eV, Figure S8) and L3- (E0 = 10,535 eV, Figure 2c) edges were taken in a transmission mode at ro...
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Article Cite This: J. Am. Chem. Soc. 2017, 139, 15088-15093

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Tunable Photoluminescence across the Visible Spectrum and Photocatalytic Activity of Mixed-Valence Rhenium Oxide Nanoparticles Yong-Kwang Jeong,† Young Min Lee,‡ Jeonghun Yun,‡ Tomasz Mazur,† Minju Kim,†,‡ Young Jae Kim,†,‡ Miroslaw Dygas,† Sun Hee Choi,§ Kwang S. Kim,‡ Oh-Hoon Kwon,†,‡ Seok Min Yoon,*,† and Bartosz A. Grzybowski*,†,‡ †

Center for Soft and Living Matter, Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea § Pohang Accelerator Laboratory (PAL), Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea ‡

S Supporting Information *

ABSTRACT: Materials exhibiting excitation-wavelength-dependent photoluminescence, PL, are useful in a range of biomedical and optoelectronic applications. This paper describes a nanoparticulate material whose PL is tunable across the entire visible range and is achieved without adjusting particle size, any postsynthetic doping, or surface modification. A straightforward thermal decomposition of rhenium (VII) oxide precursor yields nanoparticles that comprise Re atoms at different oxidation states. Studies of time-resolved emission spectra and DFT calculations both indicate that tunable PL of such mixed-valence particles originates from the presence of multiple emissive states that become “active” at different excitation wavelengths. In addition, the nanoparticles exhibit photocatalytic activity that, under visible-light irradiation, is superior to that of TiO2 nanomaterials.

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INTRODUCTION The ability to tune photoluminescence (PL) of nanoparticles (NPs) over the entire visible spectrum is highly desirable in bioimaging and in various optoelectronic devices including wavelength-tunable lasers, light emitting devices, or solar energy converters.1,2 Other than by adjusting particle size,3−5 shifting of emission wavelength can be achieved by using various dopants (although introducing appreciable amounts of such dopants remains challenging6) or by chemical posttreatments to modify surface states.1,2,7−9 In the latter context, a notable recent development has been the discovery of the socalled carbon nanodots whose emission can be tuned over the visible range by appropriate postsynthesis surface modification with dehydrating or reducing agents.1,7−12 The motivation for our current work has been to identify a nanomaterial in which tunable PL would derive from bulk properties (as opposed to surface states) and would neither depend on particle size nor require any postsynthetic modifications. As we show here, these criteria are met by rhenium (Re) oxide nanocrystals in which the Re atoms are present at IV, VI, and VII oxidation states. Both experiments and theoretical considerations indicate that tunable PL, remarkably, across the entire visible spectrum, in such mixed-valence Re oxide nanoparticles (henceforth, MVReO NPs) originates from the presence of multiple emissive states that become “active” at different excitation wavelengths. In addition, the MVReOs exhibit photocatalytic © 2017 American Chemical Society

activity in the visible regime that is more efficient than for commonly used TiO2 nanostructures. Overall, our results document a conceptually novel approach to the engineering of optical/catalytic properties of nanostructures comprising oxidatively labile elements. This approach could potentially be extended to other elements with multiple stable oxidation states (e.g., Mn, Pb, Co, etc.).13−15

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EXPERIMNTAL DETAILS

Nanoparticle Synthesis. In a typical procedure for the synthesis of MVReO NPs, 0.1 mmol of Re2O7 powder (≥99.9%, Sigma-Aldrich) was dissolved in 1-octadecene (10 mL, 90%, Sigma-Aldrich) with 0.1 mmol oleic acid (90%, Sigma-Aldrich) acting as a surfactant. The solution was heated up to 200 °C, and the temperature was maintained for 1 h. The solution was then allowed to cool down to room temperature. As the reaction proceeded and the NPs were formed, the solution’s color changed from pink to dark brown. Nanoparticle Separation. To separate the NPs according to size, the solution of as-grown NPs was centrifuged at 8000, 5000 and 3000 rpm for 20 min, and the supernatant was then decanted. The NPs collected at these rotation rates had average sizes of, respectively, ca. 20, 30, and 70 nm. Furthermore, to remove any residues present at NP surfaces, three cycles of washing with 10 mL of n-hexane (≥90%, Merck) each followed by centrifugation (at the original rpm) were performed. The supernatant was then decanted and the NP powders Received: July 31, 2017 Published: October 9, 2017 15088

DOI: 10.1021/jacs.7b07494 J. Am. Chem. Soc. 2017, 139, 15088−15093

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Journal of the American Chemical Society were dried at 60 °C for an hour and subsequently dispersed in ethanol (≥99.8%, Sigma-Aldrich). Synchrotron Studies. X-ray absorption fine structure (XAFS) experiments were performed on the 7D beamline of Pohang Accelerator Laboratory (PLS-II, 3.0 GeV). Prior to X-ray absorption measurements, the as-prepared NP material was mixed with BN (Sigma-Aldrich) to give an optimum absorbance of one unit and mounted in a sample holder. The spectra for the Re L1- (E0 = 12,527 eV, Figure S8) and L3- (E0 = 10,535 eV, Figure 2c) edges were taken in a transmission mode at room temperature. The incident beam was not detuned for the L1-edge but detuned by 10% for the L3-edge to attenuate the flux from higher order Bragg diffractions of Si (111) crystals in the monochromator. The intensities of the incident and transmitted beams were monitored using separate N2-filled IC SPEC ion chambers. AHENA in the IFEFFIT suite of programs was used to analyze the obtained data for the local structure study of Re in Re oxide nanocrystals. Fluorescence Lifetime Measurements. A time-correlated single-photon counting spectrofluorometer (Fluotime 300, PicoQuant) was used with a picosecond-pulsed laser diode emitting at 375 nm (LDH-D-C-375, PicoQuant), 450 nm (LDH-D-C-450, PicoQuant), and 510 nm (LDH-D-C-510, PicoQuant) as excitation sources. The total instrument response function (IRF) for the fluorescence decay was 120−150 ps. The deconvolution of the fluorescence decay profiles from IRF was performed by using a fitting software (FluoFit, PicoQuant) to deduce time constants from the obtained kinetic profiles. The femtosecond-resolved fluorescence transients were collected utilizing the fluorescence upconversion technique as follows. The beam source was an amplified ytterbium-based laser system (Pharos, Light Conversion), which produces ∼170 fs pulses centered at 1030 nm at the repetition rate of 200 kHz with an output power of 6 W. The output beam was split into two parts, one for a pump pulse train and the other for a gate pulse train. For the pump-beam generation, the fundamental beam was used to pump a collinear optical parametric amplifier (COPA) (Orpheus, Light Conversion). The signal beam of 750 nm from COPA was frequency-doubled to generate the 375 nm pump beam. The pump pulse of 375 nm was attenuated to ∼25 nJ and focused on the sample in a fluorescence spectrometer (Chimera, Light Conversion) equipped with a monochromator (MSA-130, Solar Laser System) and a photomultiplier tube (PMC-100, Becker & Hickl). The pump beam was focused on the cuvette (2 mm thickness) containing the sample. While the sample was under illumination of the pump beam, stirring bar continuously mixed the sample. A long-pass filter was installed in order to cut off the pump light but pass through the fluorescence light. Then, the fluorescence was driven and focused onto a BBO crystal (type II). The gate beam passed through a computercontrolled optical delay line in fluorescence spectrometer and was noncollinearly overlapped with the fluorescence in the BBO crystal. After passing through the crystal, the up-converted signal was separated from both the gate beam and the fluorescence by an array of iris apertures, UG5 filter, and a prism, and it was focused on the entrance slit of the monochromator. A UG5 filter was placed so as to cut background signals including the second harmonic (515 nm) and the third harmonic (343 nm) of the gate beam from the fluorescence. The gate beam polarization was set to be vertical and pump-beam polarization was set at the magic angle (54.7°) with respect to the upconversion crystal axis to eliminate the influence of anisotropy on the signal.

Figure 1. Structure and tunable photoluminescence of rhenium oxide nanoparticles. (a) TEM and (b−d) HR-TEM images of the NPs. Insets in (c) and (d) show selected area electron diffraction (SAED) patterns taken near the NP’s edge (red rectangle in b) and at the particle’s center (yellow rectangle in b). (e) Photograph of tunable emission of the NPs dispersed in ethanol and excited with, from left to right, 365, 405, 445, 532, and 650 nm. (f) STED images from the particles deposited on glass surfaces and excited with 405, 500, and 590 nm. All particles in each sample have the same emission wavelength (450 nm (left), 550 nm (middle), and 640 nm (right)). The colors are false-color code.

ABO3 perovskites but without the large A cation at the center (Figures 1d and S1; JCPDF number: 00-033-1096 space group: pm−3m, a = b = c = 3.75, α = β = γ = 90°).17 The d-value of the (110) reflection in the PXRD spectrum matches exactly the 2.65 Å distance between lattice fringes in HR-TEM, indicating that the particles can form by regular radial stacking of the ReO3 unit cells from seeds along the {110} growth direction. The ReO3 particles have been prepared before17 but they showed metallic behavior (with a plasmon band at ∼520 nm) and no photoluminescence (PL). The results illustrated in Figure 1e (and further quantified by the normalized fluorescence and excitation spectra in Figure 3b,c) are therefore intriguing−namely, the particles we prepared are not only photoluminescent but depending on the excitation wavelength, the PL wavelength changes continuously across the entire visible regime, from blue (λemmax = 426 nm at λex = 330 nm) to

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RESULTS AND DISCUSSION The mixed-valence rhenium oxide nanoparticles, MVReO NPs, were synthesized as described in the Experimental Details and had sizes ranging from 20 to 70 nm (Figure 1a−c). Analyses by high-resolution transmission electron microscopy (HR-TEM) and selected-area electron diffraction (SAED) (Figure 1c) and also by powder X-ray diffraction (PXRD, Figure S1) revealed that, during thermal treatment, the crystal structure transformed from Re2O7 to ReO316,17 with a unit cell similar to 15089

DOI: 10.1021/jacs.7b07494 J. Am. Chem. Soc. 2017, 139, 15088−15093

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Journal of the American Chemical Society red (λemmax = 646 nm at λex = 630 nm). Moreover, this tunable PL does not depend on the size of the particles and is therefore not due to quantum confinement effects. This was confirmed in two ways. First, stimulated emission depletion (STED) imaging of polydisperse samples shows all particles emitting at the same wavelength (cf. images in Figure 1f for different excitation wavelengths, 405, 500, and 590 nm). Second, with polydisperse particles fractioned according to size by centrifugation18 (cf. Supporting Information (SI), Figure S2), different fractions exhibit identical PL characteristics (SI, Figure S4). Integratingsphere measurements19 give the absolute PL quantum efficiency of the MVReO NPs at room temperature to be 6.6%. It is important to note that the effect of excitationwavelength-dependent PL is not due to Raman scattering since the changes in the frequencies of emission and of excitation are different (i.e., Δνem ≠ Δνex). In search of an explanation for the particles’ tunable PL, we characterized their compositions by X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) measurements. Due to its limited penetration depth, XPS probed only the first few nanometers from the particles’ surface;20 in this outer layer, Re(VII) and Re(IV) are detected in addition to Re(VI) (Figures 2a and S3). Studies of

increases (from 39.5% to 66.4%) as the particle size increases from 20 to 70 nm (SI, section S3, Table S1). Moreover, EXAFS analysis in transmission mode probing local structure around a specific element reveals the presence of two types of Re−O bonds (∼1.3 and ∼1.8 Å vs a single distance of 1.5 Å in pristine ReO3 commercially available from Sigma-Aldrich) which is consistent with the presence of different “types” of Re atoms. Considering the results of PXRD, HRTEM and SAED in conjunction with those of XPS, XANES and EXAFS points to the following conclusions: (i) the particles are composed of one crystalline, ReO3 phase but with Re atoms on different oxidation states; (ii) locally, Re(VII) and Re(IV) atoms are distributed randomly which is congruent with the fact that no significant shifts are observed in PXRD reflections (with respect to pristine ReO3;17 see Figure S1); (iii) on a longer length scale, the distribution of these species is such that Re(IV) is present mostly near the particle’s surface while the content of Re(VII) increases toward the center. Such a “gradient” architecture implies inclusion of some Re(VII) from the thermally decomposing Re2O7 into the lattice of MVReO NP but also further reduction of this species to Re(IV) at particle’s surface, likely due to increased free energy at circumferential edges.21 Naturally, any variations in the content of Re(VII) and Re(IV) must be accompanied by the corresponding changes in the oxygen content (as depletion or accumulation of the oxygen atoms) to balance net charge. The mixed-valence structure of the NPs is in accordance with their optical properties observed experimentally. To substantiate this claim, we first considered the UV/vis absorption spectra. Figure 3a shows spectra recorded for the MVReOs and for commercially available rhenium oxide powders (ReO3, Re2O7, and ReO2); as seen, the MVReO NPs exhibit broader absorption, significantly enhanced in the visible region. We modeled the band structure and absorption spectra using density functional theory (DFT) in a 2 × 2 × 2 supercell of ReO3 with added Re(IV) or Re(VII) dopants and corresponding oxygen vacancies (cf. SI, section 4). In qualitative agreement with experiments, the Re(VII)- or Re(IV)-doped ReO3 lattices exhibit enhanced absorption between ∼500 and ∼700 nm; in contrast, absorption of pristine ReO3 in this region is weak (SI, Figure S10). In the total density of states (DOS; Figure S11) resolved in terms of angular momentum, the Re(IV) and Re(VII) doped systems feature an unoccupied s-orbital-based DOS ca. 1.5−3.7 eV above the Fermi level. This unoccupied s-level DOS matches the DOS of the Re(VII) impurity, to which it can thus be ascribed (Figure S12). Deducing that the impurity s-band plays a key role in boosted absorption, we suggest that the major absorption comes from the p → s transition without spin-state change upon the excitation (since the atomic selection rule for the optical transition is |Δlz| = 1). Finally, we address the key property of tunable PL. Figure 3d shows a typical-nonexponential decay of PL (in the example shown, for 375 nm excitation and for emission with maximum at 440 nm). The nonexponential behavior of the PL decay was fitted to a sum of exponential functions, which we interpret to originate from the spectral inhomogeneity due to the distribution of multiple emissive states, I(t)/I(0) = ΣAiexp(−t/τi), where Ai represents the fractional amplitude of each decay component having the lifetime of τi.22 To obtain the average carrier recombination time, we used ⟨τ⟩ = ΣAiτi/ΣAi,23 which yielded the weight-averaged decay time ⟨τ⟩ = 1.16 ns monitored at 460 nm when excited at 375 nm. As the probed

Figure 2. Composition of mixed-valence rhenium oxide, MVReO, nanoparticles. (a) XPS spectrum of particles ca. 20 nm in diameter (separated by centrifugation). (b) Quantification of the contents of Re(IV), Re(VI), and Re(VII) based on data from panel (a). (b) X-ray absorption near edge structure (XANES) spectra of commercially available Re2O7, ReO3, and ReO2 (all from Sigma-Aldrich) and for our MVReO particles with different average sizes (see also Figure S8). (d) Extended X-ray absorption fine structure (EXAFS) spectra of differently sized MVReO NPs.

particles of different sizes reveal that the content of Re(VII) increases slightly with increasing size (from 7% in 20 nm NPs to 14% in 70 nm NPs), but is still significantly less than the ca. 35−40% content of Re(IV) (Figure 2b). Next, we quantified the overall composition, i.e., beyond the surface layer, using synchrotron source in XANES and EXAFS modalities. Both Re L1- and L3-edge XANES spectra (Figures 2c and S8, respectively) evidence the presence of Re atoms at VII and VI states, but not at the IV state. XANES performed in the transmission mode shows that the content of the VII state decreases (from 60.5% to 33.6%) while that of the VI state 15090

DOI: 10.1021/jacs.7b07494 J. Am. Chem. Soc. 2017, 139, 15088−15093

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Figure 3. Optical properties of MVReO nanoparticles. (a) UV−vis absorption spectra of commercial Re2O7 (black line), commercial ReO3 (red line), commercial ReO2 (blue line), and MVReO NPs (orange line) in hexane. (b) Normalized fluorescence spectra of ∼20 nm MVReO NPs excited at different wavelengths from 330 to 630 nm. Raw, non-normalized fluorescence spectra are shown in Figure S4a. (c) Normalized excitation spectra of 20 nm MVReO NPs monitored at different emission wavelengths in the range of 400−600 nm. (d) Fluorescence kinetic profiles of 20 nm MVReO NPs in 1-octadecene at various monitoring wavelengths. The excitation wavelength is 375 nm, solid lines are nonexponential fits to the experimental data. Monitored wavelengths are listed in the legend. (e) Normalized time-resolved PL spectra of MVReO NPs at several representative time delays with excitation at 375 nm. Log-normal peak functions are used to fit the spectra constructed at every 5 nm and a series of times. (f) Time-dependent PL maxima of MVReO NPs at different excitation wavelengths: 375 nm (black markers and fit), 450 nm (red), and 510 nm (blue). (g) Femtosecond-resolved fluorescence kinetic profiles of MVReO NPs in 1-octadecene probed at various wavelengths. Samples were excited at 375 nm and monitored at 410, 440, and 470 nm. Solid lines are the best-fitted curves to extract time constants.

PL wavelength was tuned to the red, the decay time increased to 1.33 ns at 490 and to 1.53 ns at 580 nm. The time-resolved PL spectra with excitation at 375 nm show a single-band structure with monotonous, time-dependent red shift (Figure 3e). When their PL maxima are plotted against time, the timedependent spectral shift is as much as ∼840 cm−1 in ∼5 ns and reaches an asymptotic wavelength at ∼487 nm (20 534 cm−1) (Figure 3f). This spectral trend is attributed to the Burstein− Moss band filling effect because the time scale of the spectral shift is of the same order as ⟨τ⟩.24,25 With excitation at 450 and 510 nm, the single bands in the time-resolved PL spectra continuously red-shift in time to eventually give peaks at around 510 and 550 nm, respectively (Figure S7). Excitation at these wavelengths also results in time-dependent spectral shifts on the same time scale of 5 ns but with different magnitudes (370 cm−1 for excitation at 450 nm and 530 cm−1 for excitation at 510 nm). In a femto- (fs)-to-picosecond (ps) time window, an ultrafast decay component (≤100 fs) is prevalent at the blue side of the peak in the PL spectrum (Figure 3g). The fractional amplitude of this component greatly diminishes around the peak wavelength, indicating that the intraband electron cooling is mainly ultrafast.26 Furthermore, as excitation wavelength increases so as to shift the PL maximum to the red, the decay time at the peak decreases (Figure S5). When the monitored wavelength for PL is fixed at 530 nm, the increase of the excitation wavelength causes the reduction of the lifetime (Figure S6). For example, the weight-averaged 1.54 ns decay time for the 530 nm peak with excitation at 375 nm decreases to 1.39 ns when excitation wavelength was tuned to 510 nm. Similar trends are observed when the emission is monitored at 550 and 600 nm. Taken together, these spectroscopic and dynamic behaviors evidence that the tunable PL of MVReO NPs is due to the band gap, which continuously changes with different mixed valence states from the surface to the core of individual particles (Scheme 1). This is in sharp contrast to

Scheme 1. Schematic Illustration for a Plausible Mechanism of MVReO NPs’ Excitation-Wavelength-Dependent PLa

a

Because of the varying proportion of Re(IV) and Re(VII) along the particle’s radial coordinate (cf. top-right inset), a continuous series of band gaps are generated in this particle. Upon photoexcitation, hot electrons with excess energy in the conduction band cool down on a femtosecond time scale and are piling up from the bottom of the conduction band. According to the Burstein−Moss effect, the lowestenergy electrons in the conduction band survive the longest to result in time-dependent Stokes’ shift of PL on the time scale of electronhole recombination (ns).

pristine ReO3 or ReO3 NPs synthesized by Rao and Biswas17 that were reported as metallic materials. We also note that intraparticle energy transfer from shell to core appears negligible in our system because MVReO NPs are significantly larger than typical quantum dots (radii 10−35 nm vs few nm), translating into much longer carrier diffusion times.27 Whereas the above analyses rationalize fully the PL properties of MVReO NPs, we found these particles to exhibit another useful property, namely, efficient photocatalytic activity 15091

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together can decrease the recombination of the photogenerated electron−hole pairs. In this way, the mixed oxidation state of Re in our NPs might efficiently provide electrons to oxygen to generate reduced oxygen radicals which can be a component dissociating methyl orange.

under visible light, for which we can, at present, offer only qualitative explanation. These experiments were motivated by the long-known uses of Re in catalysis28 combined with the strong visible absorption we observe for our MVReOs. As a model system, we used aqueous solution of methyl orange (MO) (5 μM in DI), a well-known organic dye pollutant in wastewater from industrial processes and commonly used to quantify the photocatalytic activity of TiO2 nanostructures.29,30 Figure 4 shows a direct comparison of the photocatalytic

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CONCLUSIONS In summary, a simple synthetic procedure yields structurally complex, mixed-valence Re oxide nanoparticles that, without any postprocessing, exhibit tunable PL across the visible spectrum and also show some promising photocatalytic activity. From a conceptual perspective, the key novelty in this work is the opening of a range of pseudoband gaps that derive from the presence of Re atoms at different oxidation states. On the other hand, the tunability of PL comes at an expense of the quantum yield being significantly lower than in carbon nanodots or traditional CdSe or CdS particles. We see improving this parameter as the main objective for future application-oriented work, possibly by changing the proportions of IV, VI, and VII species, but also by controlling the electronic structure by the addition of other transition metals.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b07494. Experimental and theoretical details (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Kwang S. Kim: 0000-0002-6929-5359 Bartosz A. Grzybowski: 0000-0001-6613-4261

Figure 4. Photodegradation of methyl orange (MO) by MVReO NPs vs. P25 TiO2 under visible-light irradiation. Photographs of 5 μM MO solutions taken after 4 h of 5 mW LED irradiation (a) without any nanoparticles present, (b) with P25 TiO2 present, and (c) with the same amount of like-sized MVReO NPs present. (d) Quantification of the MO’s photodegradation in the dark (open markers) and under 5 mW LED irradiation (solid markers) with either P25 TiO2 NPs (black curves) or MVReO NPs (red curves) present. In all cases, 10 mg of particles was used. The TiO2 (P25) particles had average diameters of 21 nm; the MVReO NPs were 20 nm in size.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge generous funding from the Institute for Basic Science Korea, Project Code IBS-R020-D1. This work was also supported by NRF Korea funded by the Ministry of Science, ICT and Future Planning (MSIP), 2017R1A2B4010271. We thank Prof. H. Lim for XPS measurements.

efficiencies, measured by the decay of MO solution’s absorbance with time, for the same amounts (10 mg) and particle sizes (20−21 nm) of MVReO NPs and TiO2 NPs (Evonik P25 from EVONIK Industries; highly photoactive mixed anatase/rutile = 80:20 phase of TiO2). The comparisons were made under ambient conditions and in the dark vs under irradiation with white light from a 5 mW LED lamp (model #F4057C620, Kumho Electric). The plot in Figure 4b demonstrates that the performance of MVReO NPs is better than that of TiO2 both in the dark and under irradiation; in the latter case, the rate of MO decomposition is ca. 5 times higher for the Re-based particles. Interestingly, the opposite is true for UV irradiation (see Figure S14), under which P25 TiO2 can efficiently generate electron−hole pairs31 and is more catalytically active than MVReOs. We hypothesize that the superior performance of MVReOs under visible light may result from the presence of both electron rich Re(IV) and electron deficient Re(VII) (compared to VI main oxidation state) which

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REFERENCES

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