Article pubs.acs.org/cm
Nb-Doped Colloidal TiO2 Nanocrystals with Tunable Infrared Absorption Luca De Trizio,† Raffaella Buonsanti,‡ Alina M. Schimpf,§ Anna Llordes,‡ Daniel R. Gamelin,§ Roberto Simonutti,† and Delia J. Milliron*,‡ †
Department of Materials Science, University of Milano Bicocca, Via Cozzi 53, 20125 Milano, Italy The Molecular Foundry, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States § Department of Chemistry, University of Washington, Seattle, Washington 98195, United States ‡
S Supporting Information *
ABSTRACT: We report a new colloidal synthesis of niobiumdoped TiO2 anatase nanocrystals (NCs) that allows for the preparation of ∼10 nm NCs with control over the amount of Nb doping up to ∼14%. The incorporation of niobium ions leads to the appearance of a tunable, broad absorption peak that ranges from the visible range to the mid-infrared. This optical behavior is attributed to the substitution of Nb5+ on Ti4+ sites generating free carriers inside the conduction band of the TiO2 NCs as supported by optical and electron paramagnetic resonance spectroscopic investigations. At the same time, the incorporation of progressively more niobium ions drives an evolution of the shape of the NCs from tetragonal platelets to “peanutlike” rods. KEYWORDS: doping, titanium oxide, plasmonic nanocrystals, transparent conducting oxide, metal oxide
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INTRODUCTION In the past few years, the use of transparent conducting oxides (TCOs) has led to the rapid development of many interesting devices such as flat panel displays, smart screens (i.e., touch panels), light-emitting devices, electrochromic devices, and thin-film photovoltaics.1−5 Among the TCOs, Sn-doped In2O3 (ITO) and F-doped SnO2 (FTO) have been most widely used because of their excellent visible transparency and conducting properties.1,6 Because of the increasing industrial demand for these materials, present research is mainly focused on the development of new compounds based on earth abundant elements as sustainable alternatives. Aluminum-doped ZnO (AZO) has been shown to be a suitable low-cost and highly available substitute for ITO in some applications.2,7,8 At the same time, the industrial need for high-efficiency devices requires a broader range of TCOs with specific optical properties. For example, high refractive index TCOs are desirable to increase the external quantum efficiency of GaNbased LEDs,9−13 whereas compounds with enhanced transparency across the visible and near-infrared (IR) can be used in transparent electrodes to improve the efficiency of solar cells.1,3,13,14 In addition to the required electrical and optical characteristics, the processing steps for some applications demand TCO materials that are stable in hostile environments containing acidic or alkali solutions, oxidizing or reducing atmospheres, and elevated temperature.15 Recently, Nb-doped TiO2 (NTO) with its low cost, nontoxicity, earth abundance, and both thermal and chemical stability (especially in reducing atmosphere)15 has received © XXXX American Chemical Society
significant attention as a new promising alternative to ITO. It has been shown, in fact, that anatase NTO can exhibit an electronic conductivity comparable to that of ITO while retaining high optical transmittance in both visible and IR regions.12−14,16−23 Moreover, NTO possesses a high refractive index that is particularly attractive for use in transparent electrodes for solar cells and GaN-based LEDs.11,12,15,22 So far, Nb-doped TiO2 has been obtained in the form of films mainly through pulsed laser deposition,13,14,16,18,24−27 sputtering,28−30 and sol−gel17,23,31,32 techniques. Much less effort has been spent on the synthesis and characterization of NTO nanocrystals (NCs).21,33,34 Colloidal NCs offer a potential strategy for cost-effective deposition of NTO anatase films over large areas or on specialized substrates. For this purpose colloidal TCO NCs, that could be obtained through low-cost solution processes, would be extremely attractive for “inkjet” microfabrication of display, LED, or solar cell components.35 Moreover, electrochromic devices36 or dyesensitized solar cells34,37 prepared from nanoscale materials, rather than dense thin films, can show improved performance.38−41 Hence, the synthesis of NTO NCs with control over size, shape, crystal structure, and both optical and electrical properties, is a worthy challenge.42−46 Here, we present a new high-yield colloidal synthesis of Nbdoped TiO2 NCs with diameters around 10 nm and controlled Nb content up to ∼14%. The incorporation of Nb as a dopant Received: July 17, 2013
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Optical Measurements. Visible/NIR absorption spectra were obtained with an ASD QualitySpec Pro Spectrometer of dispersions of NTO NCs in tetrachloroethylene and of NTO NC films deposited on glass. Absorption spectra of the NCs dispersed in toluene, used for the band gap extrapolation, were recorded using a Shimadzu UV-3600 UV−vis−NIR Spectrophotometer (in the 250−800 nm spectral range). Infrared spectra were recorded on a Perkin-Elmer FTIR Spectrum One spectrophotometer. NTO NCs dispersions were analyzed by drying a drop on a ZnSe ATR (attenuated total reflectance) plate. Transmission Electron Microscopy Analysis. Low- and highresolution TEM were carried out on JEOL 2100 and a JEOL 2100F microscopes, respectively, both operating at 200 kV. The latter was also used for high-angle-annular-dark-field scanning transmission electron microscopy (HAADF-STEM) images and electron diffraction (ED). Energy-dispersive X-ray (EDX) analysis was performed on the JEOL 2100F with a liquid-nitrogen-cooled Oxford Instruments INCAEDS detector. X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy (XPS) surface analysis was performed using a PHI 5400 XPS analyzer. The XPS analysis was performed in ultra high vacuum (UHV) conditions, with the electron analyzer at 45° to the sample surface normal, and the Al Kα X-ray source (1486.6 eV) at 54.4° relative to the electron analyzer. Raman Spectroscopy. The bonding arrangement of the amorphous NbOx matrix was studied by Raman spectroscopy, using a Horiba LabRAM Aramis instrument with confocal aperture. All Raman spectra were obtained with a 100x microscope objective (numerical aperture = 0.90), 17.7 mW of laser power at excitation wavelength of 532 nm and an acquisition time of 300s. The spectral resolution was 1.5 cm−1 and the laser spot size ∼720 nm. Several spots were examined on each sample and showed similar spectra. Elemental Analysis. Elemental analysis was performed by ICPAES with a Varian 720/730 Series spectrometer. The NTO samples were digested in concentrated HCl/HNO3 (3/1 v/v) at 80 °C for 48 h. The relative error on the extracted Nb content was within 3%, as evaluated on the basis of 9 replicates per each measurement. Electron Paramagnetic Resonance Spectroscopy. EPR spectra were collected using a Bruker E580 X-band spectrometer with a SHQE resonator operating at 9.4 GHz. The sample and probe were mounted inside an Oxford Instruments dynamic continuous flow cryostat. The temperature was controlled and monitored with an Oxford Instruments ITC5035 temperature controller and a Cernox Resistor CX-1050-AA-1.4L temperature sensor (LakeShore). Film Preparation and Characterization. NTO NC films were deposited by spin coating on glass and silicon substrates. A Zeiss Gemini Ultra-55 Analytical Scanning Electron Microscope was used for in-plane and cross-sectional imaging. Film thickness was measured using a Vecco Dektak 150+ Profiler. Four-probe resistivity measurements were performed on NC films deposited from ligand-stripped NC solutions in dimethylformamide and annealed at 400 °C under Argon for 2 h.47 Ellipsometric measurements were carried out using a Horiba JY UVSEL large band spectroscopic ellipsometer.
leads to the appearance of a tunable broad absorption peak that spans from the red edge of the visible to the mid-IR region. By combining X-ray diffraction, electron microscopy, and spectroscopic analysis, we interpret this absorption as an effect of the “extra” electrons accompanying the substitution of Nb5+ in Ti4+ sites within the NCs. The uniquely high crystalline quality and excellent dispersibility of our NCs allow us to clearly observe the signature of ionized dopants without any other charged defects convoluting the spectroscopic signatures. Greater Nb5+ incorporation is accomplished by increasing the relative amount of Nb precursor in the reaction mixture, which also has the effect of evolving the shape of the NTO crystals. They transform progressively from tetragonal platelets to “peanutlike” rods (Scheme 1). Scheme 1. Shape Evolution of Nb-Doped TiO2 NCs with Increasing Nb Precursora
a
The colors shown are indicative of the coloration of the NCs.
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EXPERIMENTAL SECTION
Chemicals. All the solvents and reagents were purchased by SigmaAldrich, 1-Octadecene (ODE) 90%, titanium ethoxide (TEO) technical grade, titanium isopropoxide 99.999%, oleic acid (OAc) 90%, 1-octadecanol (ODAL) 99%, 1-dodecanol 98%, and niobium(V) chloride 99%. Synthesis of Nb-Doped TiO2 NCs. All reactions were carried out under nitrogen using standard air-free techniques. To produce Nbdoped or pure TiO2 anatase NCs, a mixture of octadecanol (ODAL) (13 mmol), oleic acid (OAc) (1 mmol), and octadecene (ODE) (4 mL) was degassed under vacuum at 120 °C for 1 h in a three-neck flask. In a second flask, a solution of titanium ethoxide (TEO) (1 mmol), ODE (1 mL) and the desired amount of NbCl5 was prepared under inert atmosphere and then heated at 80 °C for 1h to get a clear solution, after which it was cooled to room temperature and its contents were rapidly injected via a syringe into the first reaction flask. The temperature was then raised to 290 °C for 60 min to let the NCs grow. NTO NCs were obtained under these synthetic conditions (i.e., at 290 °C for 60 min) by keeping the amount of all reactants fixed and adding the desired amount of NbCl5 to the solution of TEO and ODE that was injected into the flask. Different samples of NTO were prepared varying systematically the relative Nb:Ti precursor ratio from 0.025:1 to 0.2:1 (corresponding to a NbCl5 variation from 2.5% to 20%). In the rest of this report, the amount of the Nb precursor used will be expressed as a percentage ratio NbCl5/TEO. After cooling to room temperature, the NCs were isolated from the crude reaction mixture by precipitation with acetone and redispersion in hexane, repeated four times. At each washing step, 100 μL of OAc were added to the NC solution to prevent aggregation. Finally, the NCs were dispersed in 4 mL of hexane and 25 μL of OAc to yield a stable colloidal solution. The yield of the described synthesis was as high as 75% (∼60 mg of total inorganic product was deduced by ICP against the theoretical yield of 80 mg). X-ray Powder Diffraction (XRD). XRD patterns were acquired using a Bruker D8 Discover X-ray diffractometer operating with CuKα radiation (λ = 1.5406 Å) and equipped with a General Area Detector Diffraction System (GADDS). Samples were prepared by depositing NCs from solution on a silicon substrate.
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RESULTS AND DISCUSSION All NCs exhibited the body-centered tetragonal crystal structure of anatase TiO2, as evident from X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) (Figures 1 and 2a, and Figure S1 in the Supporting Information). The anatase phase of the solid solutions NbxTiyO2 has been shown to be metastable at low temperature and evolves to TiNb2O7 + TiO2, as predicted by the Nb2O5− TiO2 phase diagram,48 if annealed above 800 °C (see Figure S2 in the Supporting Information) as already observed also by M. Hirano and Y. Ichihashi.44 In the absence of niobium, 9.6 ± 1.6 nm NCs with a tetragonal platelet shape were the only product of the synthesis (Figure 1a,b). The TEM micrographs in Figure 1 (and Figure S1 in the Supporting Information) show that B
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Figure 2. (A) XRD patterns of the as-prepared NTO NCs with different Nb precursor loadings. The reference bulk reflections of TiO2 anatase are shown at the bottom (JCPDS card 00−021−1272). (B) Trend of the fwhm of the (004) and (200) TiO2 anatase peaks in function of the percentage of Nb precursor used is shown.
Figure 1. Low- (left) and high-resolution TEM (HRTEM) images of NTO NCs prepared with relative amounts of NbCl5 precursor of (A, B) 0, (C, D) 10, and (E, F) 20%. The scale bar of the images A, C, and E is 20 nm and that in B, D, and F is 5 nm. (B) HRTEM image of a tetragonal shaped crystal and its fast Fourier transform (FFT) exhibiting the (101) and (004) lattice planes compatible with the body-centered tetragonal structure of TiO2 anatase and showing measured d-spacing of 3.51 Å and 2.42 Å, respectively; D) HRTEM image of a slightly elongated crystal and its FFT exhibiting (101) and (004) lattice planes with measured d-spacing of 3.52 Å and 2.40 Å, respectively; (F) HRTEM image of a “peanutlike” shaped crystal and its FFT exhibiting (101) and (004) lattice planes with measured dspacing of 3.54 and 2.39 Å, respectively.
Table 1. Relevant Parameters for Syntheses of NTO NCs Prepared at Different Nb Precursor Concentrations % NbCl5 precursor
%Nb in NTO NCs (ICP)a
%Nb in NTO NCs (XPS)b
0 2.5 5 10 20
0 3.5 5.0 9.6 15.9
0 3.4 5.3 10.4 24.7
size (nm)c 9.6 8.6 9 11.3 15.4
± ± ± ± ±
1.6 1.5 1.4 1.6 2.4
a The amount of Nb in the products was measured by ICP-AES. bThe amount of Nb and Ti was calculated from XPS high-resolution measurements considering the Ti 2p and the Nb 3d peaks. In both footnotes a and b, the percentage of niobium is expressed as Nb/(Nb +Ti). cThe mean size (longest dimension) of the NCs was estimated from TEM images.
increasing the amount of Nb precursor changes the aspect ratio of the NTO crystals by systematic elongation along the c axis such that they eventually reach a stretched “peanutlike” shape. For small amounts of Nb precursor (2.5% and 5% of NbCl5), the mean size and shape of the NTO NCs are only slightly changed compared to the undoped TiO2 NCs (see Table 1 and Figure S1 in the Supporting Information). At higher Nb precursor concentrations NC longitudinal dimension increased up to ∼15.4 nm and took on an anisotropic shape (see Table 1). Analysis of the XRD patterns collected for the NTO samples confirmed this trend: the full width at half-maximum (fwhm) of the peak deriving from the lattice plane (004) of TiO2 anatase decreased progressively with increasing Nb as a consequence of the NC elongation along the c direction (Figure 2b). However, the overlap between this reflection and nearby (103) and (112) peaks prevents quantitative analysis of
crystallite dimensions. At the same time, the fwhm of the (200) reflection gradually increased indicating a reduced dimension of the NTO NCs in the ab plane (Figure 2b) and confirming what is observed by TEM. Control experiments were conducted to gain insight into the reaction mechanism. When ODAL was omitted from the reaction mixture, no product was collected, as already observed by others synthesizing undoped TiO2 under similar conditions.49 In the absence of OAc, TiO2 NCs were produced anyway, but they lacked a well-defined shape (see Figure S3a-b in the Supporting Information). Inverting the relative amount C
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of OAc and ODAL (i.e., using 1 or 2 mmol of ODAL and 13 mmol of OAc), a poor yield of very small dots and rods was the only product of the synthesis (see Figure S3c-d in the Supporting Information). In addition, when substituting the alcohol with one containing a shorter alkyl chain (dodecanol instead of octadecanol), no obvious changes to the reported synthesis results were observed. Although a detailed comprehension of the reaction mechanism would require further investigation, these control experiments provide a general framework for understanding the reaction. Although the OAc acts mainly to promote faceting and uniformity, the results point to a more central role played by the alcohol in the crystallization reaction. This role could be either in an alcoholysis reaction or in a dehydration reaction, releasing water into the reaction environment that then drives the hydrolysis. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) confirmed the presence of niobium in all of the NTO samples (see Table 1). It is well known that substitutional Nb5+ ions in the TiO2 anatase structure can release electrons to the conduction band resulting in generation of free carriers and, consequently, in a semiconductor-metal transition of the material.13,16−18,21 A first indication of the generation of free carriers inside the Nb-doped TiO2 NCs was found looking at the UV−vis−IR absorption spectra of the NTO samples (Figure 3). A broad absorption peak, tailing from the red edge of the visible region and having a maximum in the IR region, appeared in the doped samples. Moreover, the absorption peak increased in intensity (Figure 3a) and shifted to higher energies as the dopant concentration was raised (Figure 3b). These spectral features are consistent with NTO NCs exhibiting localized surface plasmon resonance (LSPR) absorption.12,16,18,29 It is also true that anatase TiO2 could incorporate oxygen vacancies that introduce n-type carriers in the Ti-3d conduction band.13,50 It has been shown by Murray et al. that TiO2−x NCs show a blue coloration and exhibit a broad absorption peak ranging from the visibile to the NIR region.50 By contrast, no IR absorption peak and no coloration were observed in our undoped samples highlighting that our synthetic approach does not lead to oxygen vacancy-doped TiO2. The optical properties of our NTO NCs are then most likely due to extra carriers injected by Nb5+ doping ions. Moreover the systematic blue-shift in the LSPR wavelengths is consistent with an increase of free carriers induced by substitutional Nb5+. This shift runs counter to the trend expected based on the elongating shape of our NCs, which would tend to red shift the plasmon absorption peak as the longitudinal mode becomes pronounced. The fact that our absorption peak blue shifts with increasing Nb content, despite the elongated shape of our NCs, indicates that variations in carrier concentration due to doping are dominant over any possible shape effects in this case.51 The optical bandgap (EgOPT), derived from (αhν)1/2 versus hν plots of the UV-absorption edge, considering that TiO2 anatase is an indirect bandgap semiconductor, systematically increased with increased Nb content (Figure 3a, inset and Supporting Information). This shift is consistent with the Burstein−Moss effect in which conduction band filling by “extra” electrons suppresses absorption at the band edge. Thus, the trend observed in our NTO samples suggests that substitutional Nb5+ ions act as electronic donors, as previously observed in dense NTO films prepared by sol−gel, sputtering, vapor, and pulsed laser deposition methods.16,17,19,27,29,32
Figure 3. Optical properties of NTO NCs. (A) UV−vis−NIR absorption curves of solutions of NTO NCs equimolar in titanium dispersed in tetrachloroethylene (TCE). The weak peaks appearing around 2300 nm are due to the oleic acid necessary to stabilize the colloidal solutions. The inset shows (αhν)1/2−hν plots for different NTO samples. The value of the optical band gap (EgOPT) can be derived from the extrapolation to α = 0. (B) Fourier Transform Infrared-Attenuated Total Reflectance (FTIR-ATR) spectra of the corresponding films obtained by dropcasting the NTO solutions.
Further investigation of the nature of the extrinsic electrons in our NTO NCs was made using high-resolution X-ray photoemission spectroscopy (XPS), which is sensitive to differences in metal oxidation states. The Ti 2p and Nb 3d spectra were both simple spin−orbit doublets with a Ti 2p3/2 binding energy of 459.6 ± 0.2 eV and a Nb 3d5/2 binding energy of 207.9 ± 0.2 eV. These results are consistent with introduction of niobium in its Nb5+ oxidation state, i.e., as an aliovalent substitution for Ti4+.23,29,31 No shoulders at lower energies, normally related to Ti3+ and Nb4+ species, were found for the Nb and Ti peaks for any of the samples, highlighting the uniform nature of the incorporated ionic defects and in contrast to literature reports of variously prepared NTO, which can exhibit a mixture of oxidation states23,29,52 (see Figure S4 in the Supporting Information). The goal of doping with Nb is the generation of extra electrons in the TiO2, which compensate the additional positive charge on Nb5+ substituting a Ti4+ site. Frequently, aliovalent doping of NCs does not successfully introduce extra electrons because of effective charge compensation via surface nonstoichiometries or the presence of other defects within the crystals.53 Electron paramagnetic resonance (EPR) spectroscoD
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Figure 4. (A) 4.5 K EPR spectra of Nb-doped TiO2 NCs. (B) Variable-temperature EPR spectra of NTO NCs obtained using 2.5% of NbCl5.
cryogenic temperatures but thermally detrapped at room temperature. To explore the limit of adding free carriers through niobium doping, further NbCl5 was used in the synthesis of NTO NCs. No further evolution was observed in the optical properties when varying the amount of NbCl5 from 20 to 30%, whereas the quantity of Nb detected by ICP-AES was found to increase (see Figure S5a and Table S1 in the Supporting Informatiion). Looking carefully at the XRD peak relative to the lattice plane (200), we observed a systematic shift to lower angles when increasing the amount of NbCl5 from 0% to 20%, but no further shift occurred at higher Nb concentrations (Figure 2a and Figure S5b in the Supporting Information). Because this shift was directly correlated with a lattice distortion (i.e., the expanded d-spacing of the planes perpendicular to the adirection) induced by Nb5+ ion substitution, XRD analysis also suggested that no further incorporation of dopant was achieved when increasing the concentration of Nb precursor above 20%. To resolve these apparent discrepancies, the Nb content determined by ICP-AES was compared to that revealed by XPS, which is sensitive primarily to the 1−2 nm surface region. Up to NbCl5 amounts as high as 10%, the compositions of the products measured by ICP-AES and XPS matched. Using more Nb precursor (20% and 30% of NbCl5), discrepancies between ICP-AES and XPS data were observed (see Table 1 and Table S1 in the Supporting Information). More specifically, higher amounts of Nb were detected by XPS, suggesting that additional Nb was outside or on the surface of the NTO NCs. Ultimately, this extra Nb was traced to amorphous NbOx around the exterior of the NCs. Being amorphous, the NbOx was not detectable by XRD, but evidence for its presence was found by high-angle annular dark field-scanning transmission electron microscopy (HAADFSTEM, Figure 5a-d) and Raman spectroscopy (Figure 5e). Bright contrast regions detected outside the NCs in the samples obtained using ≥20% of NbCl5 (Figure 5a) suggested the presence of an inorganic network material. This amorphous phase was not observed in samples made with less Nb precursor (see Figure S6a in the Supporting Information). EDS mapping in STEM mode verified the absence of Ti and the presence of Nb in this inorganic network. Moreover the EDS
py is well suited to probe the nature of the extra electrons introduced by aliovalent doping in colloidal NTO NCs.53,54 Figure 4 summarizes the EPR data collected for these NTO NCs at various temperatures and Nb concentrations. The low temperature (4.5 K) EPR spectrum of undoped TiO2 (Figure 4a) shows just a weak cavity background signal (g ≈ 2.05) and a weak, sharp signal at g ≈ 2.0, possibly due to a deep trap.55 More specifically, this signal has been observed before and shown to be consistent with the presence of acceptor levels above the valence band edge, possibly related to the surfacecoordinating ligands. Upon addition of niobium, an intense new axial signal appears with g|| ≈ 1.94, g⊥ ≈ 1.96 that is consistent with the formation of substitutional Ti3+ (Figure 4a).44,56 There is no evidence in the EPR spectra of Nb4+, which would be expected at g ∼ 1.97.57 Additionally, there is no evidence of the interstitial Ti3+ species frequently observed in reduced TiO2.44,56 With increasing niobium concentration, the Ti3+ signal increases and broadens (Figure 4a), as expected for increasing numbers of extra electrons in the NCs. At very high niobium concentrations the Ti3+ signal is broad and poorly resolved as a consequence of increased spin−spin relaxation rates. For a fixed niobium concentration, increasing the temperature causes a small change in relative peak intensities and a rapid decrease in total intensity (Figure 4b). The Ti3+ signal is not observed above ∼100 K (depending on niobium concentration), consistent with carrier detrapping at elevated temperatures. The EPR data thus confirm that aliovalent doping with niobium generates extra electrons in colloidal TiO2 NCs. The absence of a distinct EPR signal from these extra electrons at room temperature likely reflects rapid relaxation upon carrier delocalization. The EPR data are thus consistent with the room-temperature IR absorption spectra, which also suggest delocalized extra electrons in either the ground or excited states of the IR transition. The increased bleach in the UV bandgap with added Nb5+, and the absence of any Ti3+ in XPS measurements, are also consistent with electron delocalization at room temperature. Collectively, the EPR, electronic absorption, and XPS data all point to the conclusion that incorporation of Nb5+ into colloidal TiO2 NCs introduces extra electrons for charge compensation that are localized in Ti3+ shallow traps at E
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precursors.8,60,61 Other reaction conditions and precursors were less effective in striking this balance. For example, the infrared absorption (a signature of effective introduction of free carriers) was less intense when the reaction temperature was reduced from 290 to 260 °C (see Figure S7 in the Supporting Information). Likewise, reducing the reactivity of the titanium precursor by using titanium isopropoxide instead of TEO gave similar results to lowering the reaction temperature (see Figure S7 in the Supporting Information). In both cases the variation of the synthetic parameters led to a lower incorporation of Nb inside the crystals. These experiments suggested that the best reactivity matching in our synthetic conditions is achieved using NbCl5 and TEO precursors at 290 °C. Under these optimized conditions, we observed that working with percentages of Nb precursor ≤10% the composition of the nanocrystals reflected that of the feedstock almost perfectly, similar to the case of ITO NCs.62 In both the ITO and NTO systems, the similar Lewis acidity of the metal precursors (In3+, Sn4+, Nb5+, and Ti4+ are all hard Lewis acids) together with a comparable ionic radii of host and dopant ions are likely responsible for the high doping yield compared to other syntheses of doped NCs, such as Mg-doped ZnO,63 AZO,8 Mn-doped CdSe,64 and Mn-doped ZnSe.65 Because of the less favorable matching between dopant and host ions, each of these syntheses required an excess of the dopant precursor. The doping yield in our system is higher also compared to the only other two examples of heterovalent metal ion doping in TiO2 colloidal NCs reported in literature: Cr3+doped (30% yield) and Co2+-doped TiO2 (70−80%).66,67 These differences can be understood by considering both the ionic radii of these dopants and the reactivity of the respective precursors used. Cr3+ is a hard Lewis acid and its ionic radius is substantially bigger than Ti4+. Co2+ is a relatively soft Lewis acid, therefore its reactivity compared to chromium is higher for the same hard coordinating ligands (acetate). This likely explains why the Co2+-doping yield is higher in TiO2 NCs compared to Cr3+. Nonetheless, Co2+ is also a larger cation, which may contribute to the lower yield compare to that observed for Nb5+ in our work. To demonstrate the potential applicability of our Nb-doped TiO2 NCs, we studied the optical properties of NTO films obtained through direct deposition on quartz from NTO colloidal solutions (Figure 6a). Thin, homogeneous films of a thickness up to ∼1 μm were deposited by spin coating concentrated (60 mg/mL) 10% doped NTO NC solutions from a 50:50 hexane/octane solvent mixture (Figure 6b). The films exhibited high transparency in the visible region and a strong LSPR absorption in the NIR region indicating that the optical properties of the starting NCs were retained and could be applied for spectrally selective optical coatings. Complex refractive index in the visible region was extracted from ellipsometric measuraments (see Figure S8 in the Supporting Information). NTO NC films showed a lower refractive index value compared to sputtered films (n = 1.95 versus n = 2.63 at λ = 450 nm for 10% doped NTO), consistent with a certain fraction occupied by air and ligands in NC films.68 NTO NC films had a lower sheet resistance than undoped TiO2 NC film. The sheet resistance of 10% doped NTO NC films was lower compared to undoped TiO2 NC film prepared from ligand stripped NCs47 (3 kOhm/cm2 versus 20 kOhm/cm2). Although the conductivity is likely limited by hopping of electrons between individual nanocrystals, these results indicate that free carriers introduced by Nb doping have the expected effect of increasing conductivity. Such films could be further
Figure 5. (A) HAADF-STEM images of NTO NCs obtained using 20% of NbCl5. (B−D) Different areas of the imaging region in A were analyzed through STEM-EDS mapping: EDS spectrum of (B) the whole area, (C) the amorphous inorganic network outside of the particles, and (D) a single NC. The mean percentage of Nb inside the NCs was 14% and no Ti was found in the inorganic network around the NCs. (E) Raman spectra of NTO NCs deposited on quartz substrates under 532 nm excitation.
map did not show any preferential segregation of Nb near the surface of NTO NCs. Instead, these data suggest that the extra Nb detected by XPS was due to the presence of amorphous NbOx outside of the NTO NCs. Further evidence for the formation of amorphous niobium oxide when excess NbCl5 was present in the reaction mixture was found by Raman spectroscopy. A broad, weak scattering peak emerged at ∼850 cm−1 in the samples obtained with concentration of NbCl5 higher than 10% (Figure 5E). This peak is commonly assigned to the vibration mode of the terminal NbO bonds typically present as network termination in amorphous NbOx.58,59 That said, the insertion of Nb ions inside the TiO2 anatase structure was also reinforced by Raman spectroscopy. The strong anatase Raman peak at 144 cm−1 broadened and shifted to higher energies most likely as a consequence of the formation of Nb−O−Ti bonds. At the same time, the anatase peak at 639 cm−1 broadened and got more intense due to the formation of Nb−O bonds (Figure 5e). Finally, no peaks at 235 or 449 cm−1, which would be characteristic of the rutile phase, were observed. These data confirmed the findings by XRD and HRTEM analysis that no rutile phase was formed even at high Nb doping content, which is distinct from the observations by Arbiol et al., who studied NTO films.42 The successful incorporation of Nb into TiO2 during colloidal crystal growth implies our reaction conditions provide a balance between the kinetic reactivity of the Ti and Nb F
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ASSOCIATED CONTENT
S Supporting Information *
TEM and HRTEM images of NTO NCs obtained using a concentration of NbCl5 of 5%; TEM images and XRD pattern of TiO2 NCs resulting from control experiments; low and high resolution XPS spectra of NTO NCs; comparison between NTO NCs synthesized using 20% and 30% of Nb precursor; HAADF-STEM images and EDS mapping of NTO NCs; optical absorption of NTO NCs obtained varying synthetic conditions. This information is available free of charge via the Internet at http://pubs.acs.org/.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was carried out primary at the Molecular Foundry, Lawrence Berkeley National Laboratory, a user facility supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy (DOE) under contract No. DE-AC02-05CH11231. D. J. M. was supported by a DOE Early Career Research Program Award under the same contract. Additional funding was provided by the U.S. National Science Foundation (CHE 1151726 to D. R. G. and Graduate Research Fellowship DGE-0718124 to A. M. S.). The authors thank M. V. Altoe for helpful advice regarding XPS and STEM-EDS measurements.
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Figure 6. Optical and morphological characterization of thin films prepared from NTO NCs with a 9.6% Nb5+ content. (A) UV−vis− NIR transmittance of a 300 nm NTO film deposited on glass (that was background subtracted). The inset shows a photograph of the sample. (B) Cross-sectional-view SEM image of a thick (∼1 μm) NTO film deposited on silicon.
REFERENCES
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processed to pursue additional applications, such as electrochromic devices or transparent conductive coatings.
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CONCLUSIONS We developed a new colloidal synthesis of Nb-doped TiO2 anatase NCs. The synthesis was optimized for the incorporation of Nb5+ ions up to ∼14%. The effect of the doping was the introduction of free electrons into the TiO2 anatase host matrix to compensate the Nb5+. These electrons were evidenced by a broad, tunable absorption with its maximum in the IR region and characteristics consistent with LSPR. EPR spectroscopy confirmed that Nb5+ charge compensation is achieved by added electrons, showing signatures of Ti3+ at low temperatures but evidence of carrier detrapping well below room temperature. At the same time, the conduction band filling was evidenced by the blue shift of the absorption edge in the UV region as a consequence of the Burstein−Moss effect. For [NbCl5] ≥ 20%, the formation of phase segregated amorphous NbOx was detected using HAADF-STEM and Raman spectroscopy. At such high Nb concentrations, formation of this species competes with doping, hence fixing the upper doping limit. G
dx.doi.org/10.1021/cm402396c | Chem. Mater. XXXX, XXX, XXX−XXX
Chemistry of Materials
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