Edinburgh Research Explorer Polymers of intrinsic microporosity as high temperature templates for the formation of nanofibrous oxides

The highly rigid molecular structure of Polymers of Intrinsic Microporosity (PIM) – associated with a high thermolysis threshold – combined with the possibility to fill intrinsic micropores allows the direct “one-step” templated conversion of metal nitrates into nano-structured metal oxides. This is demonstrated here with PIM-EA-TB and with PIM-1 for the conversion of Pr(NO 3 ) 3 to Pr 6 O 11 . Nano-templating offers rapid access to novel nano-structured materials and interfaces 1 in particular for technologies where high surface area inorganic architectures are desirable. 2 Template hosts such as regular opaloid structures, 3 surfactant-based nano-structures, 4 or novel MOF structures 5 have been proposed. Here, polymers of intrinsic microporosity (PIM) 6 are introduced as “high temperature templates” for “one-step” metal oxide nano-structure growth as demonstrated for the case of Pr 6 O 11 . that prevents them from collapsing into a close-packed conformation even when allowing leading to a the formation of nanostructured praseodymium oxide via thermal decomposition of praseodymium nitrate with and without carbon-based templates. Here, report the formation of praseodymium oxide structures not in bulk, but directly at the surface of tin-doped indium oxide (ITO) electrodes. Praseodymium oxide nano-structures have been formed in a convenient and novel “one-step“ process using a high temperature template based on polymers of intrinsic microporosity. The resulting structures differ from those obtained through simple calcination and show a leaf-like or nano-fibrous structures. Finer structures are formed with the more thermally stable PIM-1 template. The results indicate that this methodology could be used beneficially for the rapid formation of a wider range of nanostructured metal oxide as well as mixed metal oxides with future applications in electronic, sensor, or solar cell components. cross-section of the F1s photoelectron signal using atomic sensitivity factors. 24 An Elite Thermal Systems Ltd. tube furnace was used to remove the possible organic contamination on the ITO electrodes and for calcination of metal oxides. Electrochemical testing was performed using an Ecochemie Autolab PGSTAT12 potentiostat system. TGA data were collected on a Setaram Setsys Evolution TGA instrument. The samples were heated under Ar from 20 o C until 800 o C at 10 K per minute. Highly rigid polymers of intrinsic microporosity (PIM) offer novel high temperature template materials for the formation of nanostructured metal oxides. Electroactive are prepared on tin-doped indium oxide substrates in a “one-step” process by calcination of Pr(NO 3 ) 3 embedded in a PIM template.

Nano-templating offers rapid access to novel nano-structured materials and interfaces 1 in particular for technologies where high surface area inorganic architectures are desirable. 2 Template hosts such as regular opaloid structures, 3 surfactantbased nano-structures, 4 or novel MOF structures 5 have been proposed. Here, polymers of intrinsic microporosity (PIM) 6 are introduced as "high temperature templates" for "one-step" metal oxide nano-structure growth as demonstrated for the case of Pr 6 O 11 .
Polymers of intrinsic microporosity represent a novel group of polymers with a rigid backbone (see structures in Fig. 1) that prevents them from collapsing into a close-packed conformation even when heated up. Space is created within the polymer, allowing for permanent microporosity and leading to a surface area as high as 900 m 2 g À1 for PIM-1 (ref. 7) and 1027 m 2 g À1 for PIM-EA-TB. 8 PIM materials are readily casted from solution into lms and have been investigated for applications in gas separation membranes, catalysis, and gas storage. 9 From thermogravimetric data (TGA, see Fig. 1) it is clear that these rigid polymer structures also show considerable high temperature stability (aer some initial weight loss due to water desorption below 100 C, decomposition onset occurs for PIM-EA-TB at 310 C and for PIM-1 at 480 C, both with charring). Therefore, in this study we contrast the ability of PIM-EA-TB and PIM-1 to function as template hosts for high temperature metal oxide nanostructure synthesis. A suitable model nano-structured metal oxide with promise for application in sensors 10 and in catalysis 11 is Pr 6 O 11 .
The synthesis of praseodymium oxides has been carried out previously by chemical vapour deposition, 12 calcination of praseodymium hydroxide (Pr(OH) 3 ), 13,14 electro-deposition, 15 or by thermal transformation of a praseodymium-containing precursor compound. 16 The products obtained through thermal oxidation depend on both the precursor as well as oxidation conditions such as temperature and oxygen partial pressure. The oxygen decient Pr 6 O 11 phase can be formed as dominant phase at temperatures higher than 465 C. 17 Bäumer and coworkers 11b investigated the formation of nanostructured praseodymium oxide via thermal decomposition of praseodymium nitrate with and without carbon-based templates. Here, we report the formation of praseodymium oxide structures not in bulk, but directly at the surface of tin-doped indium oxide (ITO) electrodes.
When the PIM host solution (1 mg mL À1 in chloroform) and Pr(NO 3 ) 3 solution (1 mg mL À1 in DMF) are mixed in 1 : 1 weight ratio and deposited onto ITO, calcination at 500 C in air affords a thin lm of oxide materials on ITO (see Experimental, Fig. 2).
The presence of praseodymium oxide is conrmed by EDX (see Fig. S1D †) and by XRD (Fig. 2E, with characteristic lines 15,11b ). Electron micrographs show brous deposits of Pr 6 O 11 on the ITO substrate (Fig. 2). With PIM-EA-TB as host template "leaf-like" nano-structures are seen. Doubling the amount of precursor deposit resulted in slightly courser structures, which must reect the pore geometry of the precursor at the point when solidication of the oxide precursor occurs. Changing the ratio of PIM-EA-TB to Pr(NO 3 ) 3 resulted in similar structures (see Fig. S1A-C †). When investigating the PIM-1 template (see Fig. 2C and D) it became apparent that a much ner nano-structure with laments down to 20 nm or less are formed. BET-based pore size data for PIM-EA-TB polymer (12-40Å (ref. 18)) and for PIM-1 polymer (5 to 15Å (ref. 19)) suggest that in both the parent polymers only comparably smaller pores are present. The feature size in the Pr 6 O 11 deposits appear considerably bigger for PIM-EA-TB but more similar to the original pore size for PIM-1. Therefore the feature size could be linked to the behaviour of the polymer template at elevated temperature. TGA data in Fig. 1 clearly show the higher thermal stability of PIM-1, which is likely to result in a ner oxide nanostructure that more closely reects the original PIM-1 template pores.
In order to demonstrate the absence of polymer remnants, further surface analysis has been performed with XPS (Fig. 3). Apart from the underlying ITO surface elements clear evidence for Pr, C, and O is observed in the survey scan. Carbon signals are very low and assigned to adventitious surface-adsorbed molecules (or possibly remnants of the template). Oxygen signals are assigned predominantly to Pr 6 O 11 , but with some other species present at the surface. Wolffram et al. 20 have studied thin Pr x O y lms made from Pr 6 O 11 targets and their work is the primary basis for tting the O1s spectra here. Four peaks were required to curve t the O1s spectra. The two main component at z528.5 eV and z531 eV most likely belong to Pr 2 O 3 and Pr 6 O 11 , respectively. Lutkehoff et al. 21 indicated that the signal at z532 eV can be ascribed to Pr-based hydroxides, such as Pr(OH) 3 . These would be expected from the breakdown of Pr 6 O 11 in the presence of surface water (eventually leading to PrO 2 formation). 20 The feature at z529.5 eV could be either indicative of the presence of PrO 2 (ref. 22) or be related to surface adsorbates in the form of Pr-O-R; 20 both species have O1s signal known to overlap with the Pr 6 O 11 O1s signal. As seen in Table 1, the Pr 6 O 11 and Pr-hydroxide content at surfaces seem  Wolffram et al. 20 note that unambiguous tting of the Pr3d core levels is difficult and remains controversial. Again using the above reference as a model, four chemical environments were curve tted. The line pair at higher binding energy (z931 eV and z935 eV) are ascribed to Pr 4+ species (e.g. Pr 6 O 11 and PrO 2 ). Assuming that they reect chemical environments at the surface and are not satellites. 20 The line pair at lower binding energy (z928 eV and z933 eV) originate from Pr 3+ (e.g. Pr 2 O 3 and Pr(OH) 3 ). The ratio between Pr 3+ : Pr 4+ species is approximately 2 : 1 and 4 : 3 for PIM-EA-TB and PIM-1, respectively. Notwithstanding the different chemical states, the surface O/Pr ratio is 4.6 and 2.6 for PIM-EA-TB and PIM-1, respectively. These ratios are quite highthe stoichiometric and expected O/Pr ratio for Pr 2 O 3 (Pr 3+ ) and Pr 6 O 11 (Pr 4+ ) is 1.5 and 1.8, respectively. This may indicate that the O1s signal is inuenced by other sources of surface oxygen, other than the Proxides (e.g. hydroxides, water, the underlying substrate, etc.). One expects an O/Pr ratio <2 : 1 for the Pr-based oxides. 20 It is clear from Table 2 that PIM-EA-TB has a higher Pr 3+ component than PIM-1, and vice versa in the case of the Pr 4+ species. In future, bulk elemental analytical methods have to be employed to further investigate bulk phase purity and possible impurities from the thermolysis process in the resulting products as a function of thermolysis time and temperature.
Electrochemical testing of Pr 6 O 11 nano-structures was performed in aqueous 0.1 M KNO 3 (Fig. 4). Nyquist plots (Fig. 4A) and Bode plots (not shown) suggest a high frequency switch from resistive to capacitive behaviour associated with the Complementary cyclic voltammetry data (Fig. 4B) also demonstrate the decrease in impedance as an increase in charging current. Full charging and therefore full capacitive characteristics would require more time (or a higher conductivity of the oxide). The electrochemical properties are consistent with those reported previously for Pr 6 O 11 with potential applications in charge storage and sensing. However, the methodology for oxide nano-structure formation in PIM templates will be applicable for a much wider range of oxides and mixed oxides.

Chemical reagents
Praseodymium nitrate hexa-hydrate, N,N-dimethylformamide (DMF) and chloroform were obtained from Sigma-Aldrich and used without further purication. Polymers with intrinsic microporosity PIM-EA-TB 8 and PIM-1 (ref. 23) were prepared following literature procedures. Tin-doped indium oxide glass plates (ITO) with a resistivity of 15 U per square were obtained from Image Optics Components Ltd (Basildon, UK). A KCl-saturated calomel (SCE) reference electrode was obtained from radiometer.

Instrumentation
The morphology of the samples was analysed using a JEOL FESEM6301F eld emission scanning electron microscopy (FE-  SEM). XPS experiments were conducted using a Thermo K Alpha (Thermo Scientic) spectrometer (operating at z10 À8 -10 À9 Torr) with a 180 double focusing hemispherical analyser running in constant analyser energy (CAE) mode and a 128channel detector. A mono-chromated Al Ka radiation source (1486.7 eV) was used. Peak tting was conducted using XPS Peak Fit (v. 4.1) soware using Shirley background subtraction. Peaks were referenced to the adventitious carbon C1s peak (284.6 eV) and peak areas were normalized to the photoelectron cross-section of the F1s photoelectron signal using atomic sensitivity factors. 24 An Elite Thermal Systems Ltd tube furnace was used to remove the possible organic contamination on the ITO electrodes and for calcination of metal oxides. Electrochemical testing was performed using an Ecochemie Autolab PGSTAT12 potentiostat system. TGA data were collected on a Setaram Setsys Evolution TGA instrument. The samples were heated under Ar from 20 C until 800 C at 10 K per minute.

Procedure for nano-Pr 6 O 11 lm deposition
Tin-doped indium oxide (ITO) coated glass slides were cut into 1 cm Â 3 cm strips and cleaned by rinsing with water and ethanol, followed by calcination at 500 C for one hour. A solution of 1 mg mL À1 PIM in chloroform was mixed with a solution of 1 mg mL À1 Pr(NO 3 ) 3 $6H 2 O in DMF in the desired ratio. From the resulting mixture, 25 mL was deposited onto a clean ITO plate covering approximately 1 cm 2 and dried in an oven at 100 C for 15 minutes. This deposition process was repeated for a desired number of layers and nally followed by calcination in a tube furnace at 500 C for 1 hour.

Conclusions
Praseodymium oxide nano-structures have been formed in a convenient and novel "one-step" process using a high temperature template based on polymers of intrinsic microporosity. The resulting structures differ from those obtained through simple calcination and show a leaf-like or nano-brous structures. Finer structures are formed with the more thermally stable PIM-1 template. The results indicate that this methodology could be used benecially for the rapid formation of a wider range of nano-structured metal oxide as well as mixed metal oxides with future applications in electronic, sensor, or solar cell components.