Coordination of Intercalated Cu2+ Sites in Copper Doped Sodium

Sep 6, 2008 - Jadranska 19, Ljubljana, SloVenia, Faculty of CiVil and Geodetic ... SloVenia, and Faculty for Mathematics and Physics, UniVersity of Lj...
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J. Phys. Chem. C 2008, 112, 15311–15319

15311

Coordination of Intercalated Cu2+ Sites in Copper Doped Sodium Titanate Nanotubes and Nanoribbons ˇ eh, Polona Umek,* Matej Pregelj, Alexandre Gloter, Pavel Cevc, Zvonko Jaglicˇic´, Miran C Ursˇa Pirnat, and Denis Arcˇon Jozˇef Stefan Institute, JamoVa 39, SI-1000 Ljubljana, SloVenia, Laboratoire de Physique des Solides, CNRS UMR8502, UniVersite´ Paris Sud, Orsay, France, Institut of Mathematics, Physics and Mechanics, Jadranska 19, Ljubljana, SloVenia, Faculty of CiVil and Geodetic Engineering, UniVersity of Ljubljana, JamoVa 2, 1000 Ljubljana, SloVenia, UniVersity of NoVa Gorica, VipaVska 13, PO Box 301 Rozˇna dolina, 5000 NoVa Gorica, SloVenia, and Faculty for Mathematics and Physics, UniVersity of Ljubljana, Jadranska 19, 1000 Ljubljana, SloVenia ReceiVed: June 6, 2008; ReVised Manuscript ReceiVed: July 21, 2008

Copper doped sodium titanate nanotubes and nanoribbons were prepared Via two different doping methods called in situ and ex situ. In the applied in situ method, titanate nanotubes were grown from anatase TiO2 doped with Cu2+, while in the ex situ method, titanate nanotubes and nanoribbons were exposed to aqueous solution of Cu2+ species. By correlating XRD, electron microscopy, magnetic susceptibility, and electron paramagnetic resonance (EPR) measurements, we found that in the samples prepared Via the ex situ doping method, sub-10 nm CuO nanoparticles grow on the inner/outer surface of nanotubes/nanoribbons where the Neel transition temperature is strongly suppressed. In fact, they behave as superparamagnets with a blocking temperature of around 50 K. Strong evidence that Cu2+ species that form complexes between titanate layers coordinate with Na+ ions comes from the pulsed EPR data. 1. Introduction Alkali titanates have been in the past decade intensively investigated mostly because of their intriguing functional properties. Namely, they can be exploited in a wide range of applications, for instance, in catalysis, sensorial applications, nanocomposites, and energy storage.1-5 Importantly, they are chemically stable, biocompatible, and nontoxic. At the same time, we have witnessed rapid progress in preparation of alkali titanates in different forms of nanoparticles, where size and shape can improve their functional performance. In stable alkali titanates with layered or channel-like structures, alkali ions can be easily exchanged with different cations,6-8 opening an alternative route for preparation of new materials. It is expected that size reduction of nanoparticles would typically lead to shorter diffusion paths for cations, which would consequently improve the yield of ion exchange. Titanate nanotubes (TiNTs) are synthesized under hydrothermal conditions as was first demonstrated by Kasuga et al.9 It was soon reported that small modifications of reaction conditions dramatically change morphology of synthesized materials.10-15 For instance, sodium titanate nanotubes (NaTiNTs) are typically synthesized at temperatures bellow 150 °C, while crystalline sodium titanate nanoribbons (NaTiNRs) are formed at temperatures above 170 °C.10,12,16 Crystallographic description of titanate nanotube and nanoribbon structures is difficult, and different models were proposed, ranging from anatase TiO29 to layered trititanates (Na,H)2Ti3O717-20 or (Na,H)2Ti2O5 H2O.21,22 Although the detailed structure is still under debate, it is currently accepted that NaTiNTs and NaTiNRs are of layered trititanate structure characterized by a typical interlayer distance of ∼0.9 nm. Applications of NaTiNTs and NaTiNRs are still * E-mail: [email protected], tel: +386-1-4773500.

in early development despite numerous proposals appearing in the literature. It was soon realized that NaTiNTs and NaTiNRs can be, due to their relatively high elastic modulus of 260(55) GPa,23 exploited in nanocomposites as reinforcing agents.24-26 Next, it was also demonstrated that NaTiNTs show high ion exchange reactivity towards alkali metal cations13,18,27 explored for the preparation of novel electrodes for lithium ion batteries.28-30 General interest for transition metal doping of titanate nanotubes and nanoribbons is enormous. On one hand, the observation of room temperature ferromagnetism in Co2+ doped anatase TiO2 films31 stimulated studies of Co2+ doped NaTiNTs. Furthermore, ferromagnetic behavior at room temperature was reported in Co2+ doped NaTiNTs32 as well as in Co2+ and Fe3+ doped HTiNTs (protonated form of NaTiNTs).33,34 On the other hand, the changed surface chemistry of transition metal doped titanate nanostructures is a key factor for enhanced catalytic, photocatalytic, or gas-sensing properties, to mention only few possibilities. Both pristine NaTiNRs14 and Cu2+ impregnated NaTiNTs12,21 were shown to catalyze decomposition of NO2(g) and NO(g). However, the structural details, related magnetic properties, and active adsorption sites in Co2+ and Cu2+ doped NaTiNTs/HTiNTs are not yet known and call for additional studies. There are at least three different methods for doping titanate nanostructures (TiNSs) with transition elements. Epitaxial growth of metal or metal oxide phases on a surface is a standard procedure for thin films35 but may not be very useful for synthesis of nanoparticles in general. Alternatively, composition of pristine alkali TiNSs can be changed by the ion exchange process, which can be considered ex situ doping.18,21,36 Finally, the growth of NaTiNTs from already doped TiO2 may be called in situ doping. In this case, hydrothermal reaction typically starts

10.1021/jp805005k CCC: $40.75  2008 American Chemical Society Published on Web 09/06/2008

15312 J. Phys. Chem. C, Vol. 112, No. 39, 2008 from TiO2 doped with a small wt % of transition metal ions in a solution of NaOH(aq).32,33 With this in mind, we decided to explore different wet chemistry doping strategies in order to intercalate NaTiNTs/ NaTiNRs with Cu2+ species and then study their structural and magnetic properties. By using TEM (transmission electron microscopy), STEM (scanning transmission electron microscopy) in combination with EELS (electron energy loss spectroscopy) and EPR (electron paramagnetic resonance), we first determined the Cu2+ accumulation sites. During the ex situ doping, Cu2+ species preferentially adsorb on the surface of the nanotubes in the form of CuO nanoparticles, and to a lesser degree, they intercalate between the titanate layers. Intercalation of Cu2+ species is in the case of the in situ doping the dominant mechanism. In addition, magnetic properties of prepared materials were investigated with SQUID magnetometer and temperature-dependent EPR measurements and they correlate well with the proposed Cu2+ sites. 2. Experimental Methods 2.1. Ex Situ Cu2+ Doping of Sodium Titanate Nanotubes and Nanoribbons. Sodium titanate nanotubes (NaTiNTs)/ nanoribbons (NaTiNRs) were synthesized from anatase TiO2 (2 g) (Aldrich) and 25 mL of 10 M NaOH (Aldrich) under hydrothermal conditions, as described in detail elswhere.16 Ex situ Cu2+ doped NaTiNTs (Cu-TiNTs-1) and NaTiNRs (CuTiNRs-1) were prepared by dispersing 400 mg of NaTiNTs/ NaTiNRs in 100 mL of 0.016 M Cu2+ solution (CuSO4 · 5H2O, Riedel de Haen) with the help of an ultrasonic bath. Both dispersions were stirred overnight at room temperature and then centrifuged. Obtained sediments were washed at the end with 25 mL of EtOH and dried at 100 and 300 °C, respectively. In the case of NaTiNTs, an additional sample (Cu-TiNTs-2) was prepared using a 0.016 mM solution of Cu2+. 2.2. In Situ Cu2+ Doping of Sodium Titanate Nanotubes and Nanoribbons. In situ doping of NaTiNTs proceeded in two stages. In the first stage, anatase TiO2 doped with 3 wt % of Cu2+ was prepared by the sol-gel method using tetraisopropyl titanate [Ti(OCH(CH3)2] (Aldrich) as a source of titanium and cupric acetate monohydrate [Cu(CH3COO)2 · H2O] (Fluka) as a source of Cu2+.37 In a typical reaction, 38 mL of Ti(OCH(CH3)2 was dissolved in a mixture of 150 mL iPrOH and 50 mL EtOH, and then 1.542 g of Cu(CH3COO)2 · H2O dissolved in 100 mL EtOH was slowly added. The reaction mixture was aged under constant stirring overnight. Starting sol was then slowly heated to remove iPrOH and EtOH. Obtained powder was dried for 12 h at 100 °C to remove residual traces of EtOH and iPrOH and finally annealed at 400 °C for 3 × 16 h. In the second stage, in situ Cu2+ doped sodium titanate nanotubes (Cu@TiNTs) were synthesized from 2 g of anatase TiO2 doped with 3 wt % of Cu2+ and 25 mL of 10 M NaOH (Aldrich) using an analogous procedure as for the synthesis of undoped NaTiNTs.16 A Teflon-lined autoclave was filled 23 mL (degree of filling was 88%) and held at 135 °C for 72 h. The resulting pale blue product was dispersed into 100 mL of deionized water and filtered. The material caught on the filter was subsequently washed with 50 mL of EtOH and dried overnight at 100 and 300 °C. 2.3. Characterization Techniques. Typically, for elemental analysis 20-30 mg of sample was first dissolved in a 100 mL flask in the mixture of conc. HCl and 20-25 wt % H2O2. When the sample was completely dissolved, the solution was diluted with deionized water to 100 mL. An elemental analysis was

Umek et al. TABLE 1: Elemental Analysis for as Prepared NaTiNTs, NaTiNRs, ex Situ Doped NaTiNTs (Cu-TiNTs-1 and Cu-TiNTs-2), in Situ Doped NaTiNTs (Cu@TiNTs), and ex Situ Doped NaTiNRs (Cu-TiNRs-1) sample

W (Na) %

W (Ti) %

W (Cu) %

doping method

NaTiNTs NaTiNRs Cu-TiNTs-1 Cu-TiNTs-2 Cu@TiNTs Cu-TiNRs-1

8.6 14.1 2.0 6.5 8.2 5.5

49.3 40.0 43.0 46.8 49.0 40.4

11.8 0.02 1.2 11.2

ex situ ex situ in situ ex situ

performed on inductively coupled plasma atomic emission spectrometer (ICP-AES). Powder X-ray diffraction (XRD) patterns were measured on a PANalytical X’Pert PRO high-resolution diffractometer with Alpha1 configuration using Cu KR1 radiation (1.5406 Å) in the 2θ range from 10 to 65° 2θ using a step of 0.017° per 100 s. Morphology of synthesized materials was investigated with a scanning electron microscope (FE-SEM; Carl Zeiss, Supra 35LV) and transmission electron microscopes (Jeol 2000FX, 200 keV; Jeol 2010F, 200 keV). The STEM-HAADF (high angle annular dark field) and STEM-EELS measurements have been performed with a dedicated scanning-transmission electron microscope (Vacuum Generators HB501) equipped with a home-modified Gatan spectrometer. All spectra are recorded in STEM mode with 100 keV incident electrons focused on the specimen. The probe size is approximately 0.8 nm and the current intensity varies from 100 to 300 pA depending on the surface condition of the emitting tip. Energy resolution of the cold field emitter is around 0.3 eV. Such a system enables the simultaneous acquisition of EELS spectra and HAADF image with subnanometric resolution. Temperature dependence of magnetic susceptibility between 2 and 300 K as well as magnetization curves up to 5 T were measured with a Quantum Design MPMS-XL-5 SQUID magnetometer. Pulse X-band electron paramagnetic resonance (EPR) measurements were preformed on a commercial Bruker E580 spectrometer. Typically, π/2 pulse length used in these experiments was set to 16 ns. Phase cycling was used in two-pulse and three-pulse electron spin-echo envelope modulation (ESEEM) experiments. Continuous wave (CW) X-band EPR experiments were conducted on a home-built spectrometer equipped with an Oxford Cryogenics ESR900 cryostat. 3. Results and Discussion 3.1. Elemental Analysis. Several samples with different morphology, copper content, and method of doping were prepared as described in the Experimental Methods. By ion exchange, that is, by ex situ doping of NaTiNTs with Cu2+ species, two samples were prepared and named Cu-TiNTs-1 and Cu-TiNTs-2, respectively. The only difference between these two samples was in the concentration of Cu2+(aq) species to which pristine NaTiNTs were exposed. In the case of CuTiNTs-1, the concentration of Cu2+(aq) was 1000× higher than that for Cu-TiNTs-2, i.e., 0.16 M and 0.16 mM. The sample prepared Via the in situ method was labelled Cu@TiNTs. Pristine NaTiNRs were exposed to 0.016 M solution of Cu2+(aq) species. The obtained sample was labelled Cu-TiNRs-1. In order to determine a Cu2+(aq) uptake in the samples prepared by different methods, we decided to perform an elemental analysis. The results for Cu-TiNTs-1, Cu-TiNTs-2, Cu-TiNRs-1, and Cu@TiNTs were compared with those of pristine NaTiNTs and NaTiNRs in Table 1. Surprisingly, copper

Intercalated Cu2+ Sites in Copper Doped Sodium

Figure 1. XRD patterns of anatase TiO2 doped with 3 wt % of Cu2+ (Cu-TiO2), as-synthesized sodium titanate nanotubes (NaTiNTs) and nanoribbons (NaTiNRs), in situ (Cu@TiNTs) and ex situ (Cu-TiNTs1) sodium titanate nanotubes doped with Cu2+, and ex situ Cu2+ doped sodium titanate nanoribbons (Cu-TiNRs-1). Symbol A represents anatase peaks. The dotted vertical line indicates the shift of the 2θ ) 28.2° peak for Cu-TiNTs-1 and Cu@TiNTs with respect to as-prepared NaTiNTs.

content for Cu-TiNTs-1 and Cu-TiNRs-1 is almost the same, although pristine NaTiNTs and NaTiNRs strongly differ in specific surface area14 as well in a sodium content (Table 1). The lowest sodium content is seen for Cu-TiNTs-1, that is, the sample that was exposed to the solution with the highest concentration of copper. For Cu@TiNTs, the copper content is lower then expected. The reason for this is the highly alkaline environment during synthesis in which Cu2+ ion forms a soluble complex [Cu(OH)4]2-. 3.2. X-ray Diffraction. The above elemental analyses unambiguously prove the presence of copper in all doped samples. In order to check the structure of the doped nanotube and nanoribbon samples and to compare them with the structure of as-prepared NaTiNTs and NaTiNRs, we performed powder XRD measurements that are shown in Figure 1. The XRD profile of as-prepared NaTiNTs is in good agreement with the published data.36,38-40 The broad peaks in the NaTiNTs pattern can be, according to ref 38, indexed to layered monoclinic sodium trititanate structure (Na,H)2Ti3O7.41 Ex situ doped NaTiNTs samples (both Cu-TiNTs-1 and CuTiNTs-2 (not shown here)) exhibit almost similar XRD profiles as prepared NaTiNTs. Nevertheless, there are some differences in the spectra that may reveal a few structural changes. First, one may notice that the relative intensities of individual peaks of these two samples slightly differ. This effect is most prominent for the peak at 2θ ) 24.3° (assigned as (110) and (202) peaks). In addition, vaguely expressed peaks in the 2θ range between 30° and 45° become more pronounced in CuTiNTs-1. Finally, we notice a slight shift of the peak at 2θ ) 28.2° to higher values in the doped sample. The above intensity variations, as well as the peak features in the 2θ range between

J. Phys. Chem. C, Vol. 112, No. 39, 2008 15313 30° and 45°, depend on the Na+ content (Table 1) and pH value of nanotubes,22,40,42 as well as on the lattice distortions which may, in our case, be induced by the intercalated Cu2+ species.36,40 Our XRD results thus suggest that during ex situ doping Cu2+ species may indeed intercalate and exchange Na+ between the titanate layers. Since these two samples strongly differ in Na+ content (Table 1), further systematic XRD studies are necessary to distinguish between pH and Cu2+ intercalation effect on the XRD spectra. In situ doped NaTiNTs (Cu@TiNT) was prepared from CuTiO2 powder obtained by the sol-gel method. The XRD diffractogram of Cu-TiO2 (Figure 1) can be indexed as anatase TiO2.37 The XRD pattern of Cu@TiNTs is similar to those measured for pristine and ex situ doped titanate nanotubes. We stress that the intensity variations now became more pronounced and the intensity of the 2θ ) 24.3° peak even exceeded the intensity of the 2θ ) 28.6° peak. This may suggest that the amount of intercalated Cu2+ is higher for the in situ doped sample than for the ex situ and the intercalated Cu2+ is more homogenously distributed between titanate layers. Ex situ Cu2+ doped NaTiNRs (Cu-TiNRs-1) have similar diffractograms as pristine NaTiNRs. Both of them correspond to the layered trititanate structure.16 The only difference between these diffractograms is in the relative intensity of a peak that appears at 2θ ) 38.7°. In comparison to NaTiNRs, the peak for Cu-TiNRs-1 is more intense. Apart from that for Cu-TiNRs1, no peak shift was observed, which would indicate the change in the unit cell. The amount of exchanged Na+ is in the case of Cu-TiNRs-1 morphology smaller than for CuTiNTs-1, which is most probably connected to different morphology and dimensions, especially length. Nanotubes reach up to 500 nm in length, while nanoribbons can easily reach several micrometers.14,23 3.3. Electron Microscopy. Elemental analysis showssas expectedsthat the ex situ doped samples when compared to the in situ possess a higher amount of copper. On the contrary, the conclusion from the XRDs is at first glance just the opposite, since the shifts are more pronounced for Cu@TiNTs. For the Cu@TiNTs, this might be a consequence of more homogenous distribution of Cu2+, while for the ex situ doped samples, this implies that additional accumulation sites for copper might exist. To resolve this problem and to determine the exact copper accumulation sites in the doped NaTiNTs and NaTiNRs, we therefore decided to perform an electron microscopy study. Figure 2 summarizes the TEM, STEM, HREM, and EELS study of the Cu-TiNRs-1 sample. The sample is composed of elongated ribbons (length up to 5 µm with a width of around 40 nm). The nanoribbons have good crystallinity, and fringes due to the titanate layered structure are easily seen in HREM (see Figure 2a). Small nanosized particles can be observed on the surface of this nanoribbon. The diameter of these particles ranged from a few nanometers up to 40 nm. Typical size of these particles is estimated at around 5-15 nm. Fourier transformation of the HREM images of these small particles (see the marked region in Figure 2a) reveal distances which we identified as CuO phase.43 This is also confirmed by the EELS investigation of these particles. The Cu L2,3 lines corresponding to the excitations from the Cu 2p core-state to the unoccupied states show two very narrow lines at around 931 and 952 eV, indicating that the Cu 3d orbitals are partially unoccupied. Comparisons with previously reported spectra indicate that this EELS spectrum matches well with the 3d9 ground state of a Cu2+ bearing CuO phase.44 The STEM HAADF image of ex situ Cu2+ doped NaTiNR also showsa bright contrasted line at the end of a typical nanoribbon (Figure 2b). This bright line

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Figure 2. Transmission electron microscopy characterization of Cu-TiNRs-1. (A) HREM of a nanoribbon and nanosized particles; diffractogram of a particle. Values 0.23 and 0.17 nm correspond to the [111] and the [020] distances of the CuO with space group 15 (C2/c) and lattice distance (a ) 0.4653 nm; b ) 0.3410 nm; c ) 0.5108 nm). (B) STEM-HAADF of a ribbon tip, the broken lines squares indicate the area of the EELS measurement. (C) Cu L2,3 EELS spectra from the particle and the white contrasted line within the ribbon.

indicates regions with copper. Bright stripes run along the nanoribbon axis and this corroborates with the presence of intercalated copper previously suggested from the XRDs on nanotubes. These line shapes start at the end of the nanoribbon and typically disappear at around 100 nm from the end of the nanoribbon. This implies that the ion exchange is more successful at the ends of the nanoribbons than in the middle parts. These stripes never extend deeper into the nanoribbon, which implies that the ion exchange process proceeds through the nanoribbon edges, but rapidly stops because of the low copper diffusion constant and the saturation of the interlayer space. Surprisingly, the EELS spectra obtained for lines located at the tip of the nanoribbon indicated that the copper ion has a more d10 character. Indeed, the spectra shown in Figure 2c has no more narrow lines, and only excitations from Cu 2p to the broad continuum made of mixing Cu sp orbitals is seen. The spectrum is even more characteristic of a metallic-like Cu character (d10s1) than that of Cu(I) (d10s0).44 The intensity of the bright line in the STEM-HAADF images is also a kind of a qualitative indication that the copper at the tip of the rod is reduced. Indeed, the STEM-HAADF is primarily sensitive to the projected density, and a brighter line is expected more for a metallic-like density than an oxidized or weakly dense metal distribution. At this point, we should mention that the density of incoming electrons required to obtain the EELS Cu spectra for a single line of typically 40 nm × 1 nm in size is quite high and may damage the initial valence state of the inserted ion. We have also observed with a more delocalized EELS that the Cu L signal arising from the rod may also show a narrow L2,3 line demonstrating that some Cu2+ are also present in the rod. In conclusion, in the ex situ doped NaTiNRs (Cu-TiNRs-1) sample, the copper is located as CuO nanoparticles at the surface

of the rod and is also inserted within the rod as Cu2+ (d9) and Cu (d10) forms. The reduction of the copper might be due to the interaction with the titanate slab, perhaps amplified under the electron irradiation, and seems to be stronger at the tip of the rod where the quantity of inserted materials is high. Figure 3 shows electron microscopy characterization of CuTiNTs-1. This sample shows well-formed titanate nanotubes (in fact, nanoscrolls), with inner and outer diameter of typically 4-6 nm and 8-10 nm. Once again, the crystallinity of the sample is easy to observe by TEM (Figure 3b). Bright contrast on STEM-HAADF images (Figure 3a,c,d) indicates regions rich in copper. One can clearly see that NaTiNTs are decorated with small particles (often elongated) which are found (i) on the outer surface and (ii) in the inner side of NaTiNTs. The width of these particles is often less than 5 nm, while the length can be twice as long, rarely more. EELS spectra of this system show similar features as those reported previously for the ex situ doped nanoribbons. Indeed, metallic-like and divalent-like copper can be observed for particles. The particle inserted within the nanotube shows more metallic character. In the example shown in Figure 3d,e, only metallic Cu is observed within this 5 nm size particle. We were not able to detect copper inserted within the titanate structure, but it is more difficult for a tube than for a slab, since the projected contrast of intercalated materials in a tube is very weak due to the continuous curvature of the materials. In addition, the titanate nanotubes are more easily degraded under microscopic observation compared to the nanoribbon case, since they have higher surface area exposed to the electron beam. In conclusion, the ex situ doped titanate nanotubes show primary copper in oxidized nanoparticles, with some of them being reduced. Overall copper reduction may be higher for this system compared to the nanoribbon case because of the higher

Intercalated Cu2+ Sites in Copper Doped Sodium

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Figure 3. Transmission electron microscopy characterization of Cu-TiNTs-1. (A,B,C,D) STEM-HAADF and HREM images of nanotubes with nanoparticles, and (E) chemical profile obtained from the EELS analysis of the Cu L, O K, and Ti L edges along the line shown at the images D. Along the line in image E, 128 EELS spectra have been collected with an incremental step of 0.23 nm and a probe size of 0.7 nm.

surface to volume ratio of the tube. Despite the direct evidence of Cu2+ intercalation not being possible, we do not exclude it. SEM image of Cu@TiNTs shown in Figure 4a reveals that synthesized material is homogenous, but from this image, the morphology of the material cannot be clearly distinguished in detail. In contrast, TEM and STEM images (Figure 4b,c) show that the sample does not consist of well-formed nanotubes. Morphology can be described as partly rolled titanate slabs. It was observed that on average one such particle consists of 3-6 titanate slabs. Usually, half-rolled single titanate sheets are typical for NaTiNTs synthesized at lower temperatures.8 Probably, the presence of Cu2+ in a reaction mixture has a similar effect as lower reaction temperature for the synthesis of NaTiNTs. Despite the fact that STEM-HAADF may easily reveal 1- or 2-nm-sized nanoparticles, almost no contrasted particles can be seen on this sample. For example, no particles can be seen in the 380 nm × 380 nm area showed in Figure 4c. An EELS spectrum taken over this entire region is presented in Figure 4d. A small Cu L2,3 edge can be seen. A Cu/O ratio of around 9.5 × 10-3 is obtained by EELS that is on the same order of the elemental analysis obtained for the bulk Cu@TiNTs (Table 1). The presence of narrow peaks at the Cu L edges indicates that some electrons have been removed from the d band and thus that part of the copper is certainly present as Cu2+ in that sample. To conclude, in the Cu@TiNTs sample, the copper is either inserted in the slab or complexed at the surface, primarily in an oxidized form. 3.4. Magnetization Measurements. Transition metal doped titanate nanostructures are good candidates also for intriguing magnetic properties. Namely, room temperature ferromagnetism has been found in different semiconducting metal oxide films (ZnO, TiO2, SnO2, etc.) doped with a wide selection of transition

metal ions.45 The search for room temperature ferromagnetism in doped nanotubes/nanoribbons motivated us to investigate the magnetic properties of ex situ and in situ doped titanate nanotubes/nanoribbons with SQUID magnetometry. Temperature dependence of the magnetic susceptibility measured in ex situ doped sodium titanate nanotubes (CuTiNTs-1) is shown in Figure 5. Magnetic susceptibility monotonically increases with decreasing temperature for zero-field cooled (ZFC) and field cooled (FC) experiments. At high temperatures, χ(T) follows a Curie-Weiss law with a negative Curie temperature θ ) -15(1) K, and the extracted Curie constant is consistent with about 10 wt % of Cu, i.e., in agreement with the elemental analysis (Table 1). However, below ∼140 K, deviations from the Curie-Weiss behavior can be noticed and accompanied by the hysteretic behavior in ZFCFC measurements. In fact, χT (inset in Figure 5) shows a broad peak at Tb ) 53(1) K in field cooling and Tb ) 57(1) K in zero-field cooling experiments. Below Tb, χT is suddenly strongly suppressed. Another anomaly is noticed at T ) 3.9 (2) K. The observed dependence of magnetic susceptibility is suggestive of antiferromagnetic interactions between Cu2+ moments, which however never develop into a long-range antiferromagnetic order. The behavior in fact is reminiscent of superparamagnetism of magnetic nanoparticles. TEM measurements described above suggested the presence of CuO nanoparticles attached to NaTiNT inner and outer surfaces, making these nanoparticles candidates for the observed magnetic properties. However, measured magnetic susceptibility significantly deviates from the reported temperature dependence measured in bulk CuO,46 i.e., bulk CuO is an antiferromagnet with two magnetic transitions: at TN1 ) 229 K from paramagnetic to incommensurate phase takes place, followed by another transition at TN2 ) 213 K to low-temperature commensurate

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Figure 4. Electron microscopy characterization of in situ Cu2+ doped NaTiNTs (Cu@TiNTs). (A) SEM image, (B) HREM image, (C) STEMHAADF image, and (D) EELS spectrum obtained from the area imaged at C.

Figure 5. Susceptibilities of ex situ Cu2+doped NaTiNTs (Cu-TiNTs1, triangles) and in situ Cu2+ doped NaTiNTs (Cu@TiNTs, circles). Both samples were dried at 300 °C. Inset: Temperature dependence of χT parameter for both samples. In the brackets are estimated values for copper content in both samples obtained from susceptibility measurements.

phase.47-49 On the other hand, it is well-known that finite-size effects can have large influence on the Neel temperature in antiferromagnetic nanoparticles.50 We refer here to a study of 10 nm CuO nanoparticles prepared by a ball-milled technique51 where a dramatic reduction in TN from 229 K to 50 K has been reported. It is therefore not surprising for CuO nanoparticles with typical diameters not exceeding 10 nm that were grown during the ex situ doping that strong reduction in TN has been noticed. Magnetic properties measured in ex situ doped titanate nanotubes (Cu-TiNTs-1) are therefore largely determined by the CuO nanoparticles. In situ doped titanate nanotubes (Cu@TiNTs) show dramatically different magnetic behavior. Its magnetic susceptibility can be well described with a sum of diamagnetic temperature

independent part χD (which may involve vanVleck term as well) and the Curie-Weiss contribution χCW:χ(T) ) χD + χCW ) χD + [C/(T - θ)]. Unconstrained fit resulted in χD ) -4.2 × 10-7 emu/g, Curie-Weiss temperature θ ) -0.7 K, and Curie constant C ) 8.8 × 10-5 emu K/g. The extracted C corresponds to about 1.3 wt % of Cu, in excellent agreement with the elemental analysis. Nearly zero Curie-Weiss temperature signals the presence of only noninteracting Cu2+ moments in the in situ doped sample. To understand the difference between the ex situ and in situ doped samples, we need to return to our microscopy study again. For the in situ doped sample, only intercalated copper complexes were detected (Figure 4), while CuO nanoparticles were not observed. Therefore, the magnetic response is entirely due to intercalated Cu2+ moments. The above magnetic susceptibility measurements on Cu@TiNTs further suggest that the arrangement of intercalated Cu2+ between the titanate layers is so diluted that magnetic exchange interactions cannot be promoted, and hence we find very small Curie temperature. Apparently the Cu2+ content for this sample is still very low, and for this reason, we find a diamagnetic χD response of a titanate nanotube host structure. Let us now describe the magnetic susceptibility measurements for the ex situ doped titanate nanoribbons (Cu-TiNRs-1, figure 6). Again, magnetic susceptibility can be well described as a sum of two terms: temperature independent and Curie-Weiss term: χ(T) ) χ0 + χCW ) χ0 + [C/(T - θ)]. Unconstrained fit resulted in χ0 ) 2.0 × 10-6 emu/g, C ) 2.0 × 10-4 emu K/g, and θ ) -0.4 K. The value of the Curie constant is consistent with 2.8 wt % Cu2+. Additional low-temperature magnetization curve (inset to Figure 6) confirms the extracted Cu2+ concentration. Magnetization curve M(H) can be simulated well at T ) 5 K with a Brillouin function for S ) 1/2, g ) 2.1, and M0 ) 2.8 emu/g. Recalculating M0 per Cu2+, we estimate that M0 is

Intercalated Cu2+ Sites in Copper Doped Sodium

Figure 6. SQUID measurements of the magnetic susceptibility (open circles) of the ex situ Cu2+ doped NaTiNRs (Cu-TiNRs-1). The solid line represents the fit to a model involving a temperature-independent term and Curie-Weiss term with χ0 ) 2.0 × 10-6 emu/g, C ) 2.04 × 10-4 emu K/g, and θ ) -0.4 K (see text for details). Inset: Magnetization curve measured for the same sample at T ) 5 K (open circles). The solid line is a fit to a Brillouin function for S ) 1/2 and M0 ) 2.8 emu/g.

consistent with about 3.2 wt % in excellent agreement with the concentration obtained from the Curie constant. We note that this concentration is, however, much lower than was determined from the elemental analysis (Table 1) and suggest that the missing fraction of Cu2+ must contribute to some other nonmagnetic signal. The answer might be hidden in a surprising value of χ0 ) 2.0 × 10-6 emu/g, which is positive and not diamagnetically negative as in the case of the doped nanotubes (Cu-TiNTs-1 and Cu@TiNTs). The positive temperatureindependent susceptibility reflects either metallic Pauli-like susceptibility or alternatively van Vleck orbital susceptibility. 3.5. X-band Electron Paramagnetic Resonance. Summarizing the above microscopy and magnetization measurements, one may conclude that the magnetic response of the ex situ doped nanotubes is mainly because the CuO nanoparticles were grown on the titanate nanotube surface, while for the in situ doped sample, it is primarily due to uncoupled Cu2+ complexes largely accumulated between the layers of titanate nanostructures. To throw some additional light on the local coordination of the Cu2+ moments, we decided to complement the above measurements with X-band electron paramagnetic resonance (EPR) experiments that proved to be a powerful technique for the investigation of the transition metal oxide properties. A comparison of typical X-band EPR low-temperature spectra measured in the ex situ and in situ doped titanate nanotube samples is shown in Figure 7. In the ex situ doped sample (CuTiNTs-1), cw X-band EPR spectra are typically composed of two components: a very broad component with a line width of about ∆Hpp ≈ 1100 G and g ≈ 2.17 and a component displaying a hyperfine structure typical for Cu2+ ions. We have discussed already that during the ex situ doping copper prefers to form CuO nanoparticles with a typical size of few nanometers on the surface of NaTiNTs. A minor fraction of copper is incorporated into the structure Via ion exchange, or alternatively, it forms various complexes on the surface of the nanotube. On the basis of these results, we therefore assign the dominant broad component to CuO nanoparticles, while the structured component is assigned to magnetically diluted intercalated Cu2+ ions. Our assignment of the broad component is further reinforced by a recent EPR study of CuO nanopowder where signals with similar line widths were reported.52 We also notice that the broad CuO component starts to broaden below ∼50 K, i.e., below

J. Phys. Chem. C, Vol. 112, No. 39, 2008 15317

Figure 7. Comparison of CW X-band EPR spectra measured in the in situ and ex situ Cu2+ doped NaTiNTs at T ) 60 K (Cu-TiNTs-2 and Cu@TiNTs). The solid and dotted arrows indicate hyperfine structure for two coexisting signals in Cu-TiNTs-2.

TABLE 2: gi (i ) x, y, z) and Ai (i ) x, y, z) Obtained from the cw EPR Line Shape Simulations (See Text for Details)a sample

gx

Cu-TiNTs-2, 2.1065 T ) 30 K 2.0915 Cu@TiNTs, 2.1065 T ) 30 K 2.152 a

gy

gz

Axx Ayy Azz relative [MHz] [MHz] [MHz] intensity

2.0665 2.3275

30

90

490

70%

2.068 2.28 2.0665 2.3275

20 30

35 90

420 490

30% 14%

120

10

230

86%

2.06

2.335

Analyzed spectra were measured at 30 K.

the temperature where maximum in χT suggests the blocking temperature (inset to Figure 5). In striking contrast to the ex situ doped samples, we find in the in situ copper doped titanate nanotubes (Cu@TiNTs) only the central line with weakly pronounced hyperfine structure (Figure 7). The broad component that is dominant in the ex situ doped sample and attributed to CuO nanoparticles is in this case almost completely absent. This result seems to be in a full agreement with the SQUID magnetic susceptibility data (Figure 5) and microscopy study (Figure 4b). The unpaired Cu2+ component is on the other hand now dominant, and one can also notice the hyperfine structure on the low-field side of the spectrum (see arrows in Figure 5). We also find it intriguing that the g-factor anisotropy is evidently different in the case of the in situ and ex situ doped samples, suggesting that Cu2+ ions occupy different sites in both cases. We therefore speculate that the ex situ doping leads more to the formation of copper complexes on surface sites and that the in situ doping favors either substitutional doping or, more likely, intercalation. In order to further investigate the copper local coordination in NaTiNTs, we focus now on the line shape simulations (CuO signal is not taken into account in this part of the work). Simulation of the X-band spectrum taken on Cu-TiNTs-2 dried only under dynamic vacuum at 100 °C required two overlapping signals with g-factor and hyperfine coupling tensors A consistent with the orthorhombic D2h environment. Parameters for both components are summarized in Table 2. The main component (roughly 70% of the total EPR intensity) for the ex situ doped sample (Cu-TiNTs-2) has parameters close to those expected for intercalated Cu2+ while the minor component (30% of the intensity) may be also due to the surface Cu2+ sites.53 This

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Umek et al. reliable analysis of the ESEEM signals. In addition, the modulation seems to be much more complex and very difficult to fit. It is impossible to say at the moment what is the origin of drastically reduced relaxation times in the in situ doped sample. Obvious candidates are stronger coupling between neighboring Cu2+ moments due to locally increased Cu2+ concentration. Alternatively, if Cu2+ ions are really built into the titanate layer (framework Cu2+ moments), then the spin-lattice coupling would provide a very effective relaxation channel. Temperature-dependent spin-lattice relaxation time measurements are needed to solve this issue. 4. Conclusions

Figure 8. (A) Fourier transform of the modulation in the three-pulse ESEEM signal (inset) on Cu-TiNTs-1 sample. (B) Two-pulse ESEEM signal (open circles) and the fit (solid line). Both measurements were performed at T ) 30 K.

conclusion is in agreement with the above TEM results. For the in situ doped sample (Cu@TiNTs), we find the line shape simulation even more difficult because of the larger homogeneous broadening. Nevertheless, the anisotropy in the g-factor and especially the Azz component seem to suggest that some of the Cu2+ ions were probably substitutionally doped into the titanate host structure. This last assignment unfortunately cannot be directly supported with the microscopy due to instability of these structures under high magnifications. Let us now employ pulse X-band EPR techniques to throw some additional light on the details of local coordination of Cu2+ ions. A suitable technique, which has been frequently used for instance in the studies of transition metal doping of various zeolite structures, is the electron spin-echo envelope modulation (ESEEM) technique. In Figure 8a, we show a Fourier transform of a three-pulse ESEEM signal (inset to Figure 8a) measured in ex situ doped Cu-TiNTs-1. Two main peaks can be recognized in the spectrum with resonance frequencies centered at ν1 ) 3.8(1) MHz and ν2 ) 14.1(1) MHz. ν2 is exactly at the 1H Larmor frequency, so we assign this modulation to nearby protons coupled to Cu2+ center only Via dipolar interactions. On the other hand, ν1 is precisely at the 23Na Larmor frequency. Employing the simulation strategy, one can next try to estimate the mean distance between the Cu2+ centre and 1H as well as from 23Na nuclei from the two-pulse ESEEM experiment (Figure 8b). From the fitted modulation depths52 kH ) 0.025(1) and kNa ) 0.093(2) for 1H and 23Na coupling, respectively, we calculate the distance average distances 〈r(Cu2+-H)〉 ) 3.5(1) Å and 〈r(Cu2+-Na+)〉 ) 2.8(1) Å. This brings us to the conclusion: the Cu2+ complex, that intercalates between the titanate layers, prefers to occupy the site close to the co-intercalated Na+ ions. The Cu2+ complex is also coordinated with the interlayer water or alternatively with OH- groups. Similar analysis of the in situ doped sample reveals much shorter relaxation times, which prevent us from performing a

In conclusion, titanate nanotubes and nanoribbons doped with copper Via the ex and in situ methods were investigated with XRD, electron microscopy, magnetic susceptibility, and EPR measurements. Results indicate that in both cases Cu2+ species intercalate between titanate layers, but the distribution is found to be more homogenous for the in situ doping method. The homogeneity of Cu2+ distribution depends on a doping method as well on a level of Cu2+ doping. In addition, CuO sub-10-nm nanoparticles are formed on the inner/outer surface of the nanotubes/nanoribbons during the ex situ doping method. Magnetic susceptibility measurements on the ex situ doped samples confirm the presence of CuO nanoparticles, which show superparamagnetic-like behavior with Neel blocking temperature of around 50 K. Careful cw and pulsed EPR measurements helped us to distinguished between CuO nanoparticles and intercalated Cu2+ spectral components. Intercalated Cu2+ species form a complex in close vicinity to Na+ ions. Present results may appear relevant for future catalytic studies of copper doped NaTiNTs and NaTiNRs, as we identified different possible active sites in the ex and in situ doped samples. Acknowledgment. The financial support from the Slovenian Research Agency (J2-9217 and J1-9357) and ESTEEM is gratefully acknowledged. The authors also acknowledge financial support from the European Union under Framework 6 program under a contract for an Integrated Infrastructure Initiative. Reference 026019 ESTEEM. References and Notes (1) Hangfeldt, A.; Gratzel, M. Chem. ReV. 1995, 95, 49. (2) Ramı´rez-Salgado, J.; Djurado, E.; Fabry, P. J. Eur. Ceram. Soc. 2004, 24, 2477. (3) Yanagisawa, M.; Uchida, S.; Sato, T. Int. J. Inorg. Mat. 2000, 2, 339. (a) Anderson, M. W.; Klinowski, J. Inorg. Chem. 1990, 29, 3260. (4) El-Naggar, I. M.; Mowofay, E. A.; Ali, I. M.; Aly, H. F. Adsorption 2002, 8, 297. (5) Choy, J. H.; Han, Y. S.; Park, N. G.; Kim, H.; Kim, S. W. Synth. Met. 1995, 71, 2053. (6) Fiest, T. P.; Mocarski, S. J.; Davies, P. K.; Jacobson, A. J.; Lewandovski, J. T. Solid State Ionics 1988, 28-30, 1338. (7) Abe, R.; Ikeda, S.; Kondo, J. N.; Hara, M.; Domen, K. Thin Solid Films 1999, 343-344, 156–159. (8) Izawa, H.; Kikkawa, S.; Koizumi, M. J. Phys. Chem. 1982, 86, 5023. (9) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K. AdV. Mater. 1999, 11, 1307. (10) Yuan, Z.-Y.; Colomer, J.-F.; Su, B.-L. Chem. Phys. Lett. 2002, 363, 362. (11) Sun, X.; Chen, X.; Li, Y. Inorg. Chem. 2002, 41, 4996. (12) Tsai, C.-C.; Teng, H. Chem. Mater. 2004, 16, 4352. (13) Bavykin, D. V.; Friedrich, J. M.; Walsh, F. C. AdV. Mater. 2006, 18, 2807. (14) Umek, P.; Cevc, P.; Jesih, A.; Gloter, A.; Ewels, C. P.; Arcˇon, D. Chem. Mater. 2005, 17, 5945. (15) Mao, Y.; Wong, S. S. J. Am. Chem. Soc. 2006, 128, 8217. (16) Umek, P.; Cerc Korosˇec, R.; Jancˇar, B.; Dominko, R.; Arcˇon, D. J. Nanosci. Nanotechnol. 2007, 7, 3502.

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