inverse opal architectures have been produced 
 in a wide range of materials due to potential applications as photonic 
 band gap materials , battery electrodes , gas sensors , catalysts , membranes , and biomaterials . these materials are usually synthesized using 
 close - packed arrays of polymer or silica spheres as sacrificial templates . 
 the void space between the spheres is typically infiltrated with precursors 
 of the target material and the template spheres subsequently removed 
 by thermal processing , solvent extraction , or chemical etching . the 
 most intensively studied inverse opal materials are metals , binary 
 oxides , such as sio2 , tio2 , al2o3 , and zro2 , ternary transition / main 
 group metal oxides , semiconductors , and carbon . for example , sicn has been obtained using a polysilazane 
 preceramic precursor templated around silica spheres and was proposed 
 as a catalyst support for high - temperature fuel reforming owing to 
 its surface geometry and good stability up to 1200 c . very high quality inverse opals of wn and ta3n5 have been produced by atomic layer deposition also using 
 silica sphere templates , though this method is not suited to large - scale 
 applications , and an aggressive hf etching process is required to 
 remove the template . there are a small number of examples of conversion 
 of inverse opal powders to metal nitrides . graphitic carbon nitride 
 obtained by templating cyanamide around silica spheres has been converted 
 to a tin / c composite powder with inverse opal structure by high - temperature 
 reaction with ticl4 . these 
 were suggested as catalyst supports , and ti0.7w0.3n has similarly been produced from the oxide for the same application . ta3n5 inverse opal powders 
 have been obtained by a templated sol - gel synthesis of the porous 
 oxide powder followed by nitridation , and these materials were studied 
 as photocatalysts for water splitting . conversion of oxides to nitrides is often an aggressive high - temperature 
 process , and we previously developed a nonoxide sol - gel route to 
 tin inverse opals using cross - linked 
 polystyrene spheres and infiltrating using an amide - derived sol precursor , 
 chemistry that previously we also used to directly access tin films . the relatively gentle conditions used in this 
 method have the potential to produce inverse opal nitride films on 
 a range of substrates , e.g. , for use as fuel cell or battery electrodes , 
 optical materials , or photocatalysts . nonoxide sol - gel routes could 
 also produce a variety of structured and porous metal nitride materials , as demonstrated in the more extensively studied 
 silicon nitride system where films , membranes , aerogels , inverse opals , and molecule - templated microporous catalysts have been reported . hence , expansion of our 
 tin processing route to other metals is an important target of our 
 research program . ta3n5 has been the focus 
 of many recent investigations due to its high photocatalytic activity 
 with visible light . the most intense 
 region of the solar spectrum is centered at 2.6 ev , and ta3n5 has a band gap of 2.1 ev ; hence , it could potentially utilize more than 45% of the 
 incident solar energy . materials investigated 
 range from films and powders to nanoparticles and nanotube arrays . recently , a study 
 on nanoporous ta3n5 films showed high activity 
 with high surface area porous structures as photoanodes to oxidize 
 water , raising the possibility of separated 
 oxygen and hydrogen generation . here we report the synthesis 
 of ta3n5 inverse opal films using a nonoxide 
 sol - gel process based on ta(nme2)5 cross - linked 
 by propylamine . this sol is infiltrated into a template formed of 
 hexagonal close - packed 500 nm cross - linked polystyrene spheres before 
 firing in ammonia to crystallize the ta3n5 and 
 calcine the template . the template - forming method has been improved 
 relative to our tin and sinx inverse opal 
 preparations by employing methods previously 
 developed for photonic crystals to produce 
 large ordered domains of inverse opal . all sol preparations and infiltrations were carried 
 out under dry n2 using glovebox or schlenk line conditions . ta(nme2)5 was obtained from safc hitech and 
 used as supplied . gaseous ammonia ( air products anhydrous grade ) these were cleaned by sonication in saturated koh(aq ) , 
 h2o , propan-2-ol , and acetone before drying with a stream 
 of dry n2 . divinylbenzene cross - linked , amidine - capped 
 polystyrene spheres ( invitrogen , 41 cm of 0.1% suspension 
 in deionized water ) were sonicated for 10 min to ensure an even dispersion 
 and used to produce a batch of 7 templates . a cleaned silica tile 
 was placed into a 25 mm diameter specimen tube such that it formed 
 an angle of 70 to the base of the tube . the polystyrene 
 suspension ( 6 cm ) was then carefully syringed into the 
 base of the tube . seven of these were equally spaced on an insulating 
 tray with an eighth tube containing water ( 6 cm ) to moderate 
 the humidity . the tray was then carefully placed into an oven at 65 
  1 c . as the water evaporated to dryness a striated these templates were then dried , removed 
 from the vials , stored in a desiccator , and then further dried under 
 vacuum before use . sols were produced in hexane due to a previous 
 finding that the cross - linked ps spheres were stable in it . ta(nme2)5 ( 4.013 g , 10 
 mmol ) was dissolved in dry hexane ( 7.5 cm ) , and prnh2 ( 1.64 cm , 20 mmol ) was injected slowly . 
 gas evolution was observed . stirring continued at room temperature 
 for 15 h , during which the sol darkened slightly but remained 
 a clear , yellow solution . the sol was infiltrated into the ps arrays 
 by submerging 12 mm of the edge of the tiles into 
 the sol in the glovebox so that the liquid was drawn in by capillary 
 action and the solvent could then evaporate from the surface . the 
 tiles were left to infiltrate for 1 h , over which time period the 
 volume reduced to around 20% of its original value , and then removed 
 and allowed to dry for 2 h. the infiltrated arrays were heated 
 under a flow of dry ammonia to 600 , 700 , or 800 c at 1 c 
 min and held for 10 h before cooling to room temperature 
 at 5 c min . for comparison , continuous solid 
 ta3n5 films ( i.e. , with no template ) were produced 
 by dip coating the same silica tiles into the sol and firing under 
 the same conditions as the arrays . similarly , yellow xerogel samples 
 were also obtained by solvent removal from the infiltration sols in 
 vacuo and then heating under the same conditions to provide bulk samples . x - ray diffraction ( xrd ) data were collected using a bruker c2 discover 
 with gadds diffractometer using cu k1 radiation . scanning electron microscopy ( sem ) used a jeol jsm6500f feg - sem or 
 a philips xl30-esem with an edax energy - dispersive x - ray ( edx ) detector . h nmr spectra were collected with a bruker av-300 spectrometer ; 
 samples were produced by adding an appropriate molar quantity of n - propylamine to a 0.0012 moldm solution 
 of ta(nme2)5 in c6d6 , 
 and chemical shifts were corrected by referencing to the benzene h peak (  = 7.26 ppm ) . thermogravimetric analyses ( tga ) 
 were obtained with a mettler toledo tga851e with samples in an alumina 
 crucible heated to 800 c at a rate of 10 c min under flowing high - purity nitrogen ( 80 cmmin air products bip grade ) . combustion microanalyses ( c , h , n ) were outsourced 
 to medac and collected using aluminum capsules and a wo3 combustion aid , conditions that we found previously to provide 
 good combustion of metal nitride samples . we previously reported the use of a nonoxide sol - gel route to produce 
 tin inverse opal films using the same divinylbenzene cross - linked , 
 amidine - capped 500 nm polystyrene spheres as the template as are used 
 in this work . previously the regions 
 of ordered macroporous film were limited to a few micrometers , and 
 one of our key interests in expanding this work to another system 
 was to increase the size of the ordered regions . large area ordered 
 close - packed arrays of polymer spheres can be produced by self - assembly 
 on a surface at the meniscus of an evaporating suspension of the spheres , 
 and this process can be controlled by changing the concentration of 
 the suspension , the temperature and the humidity of the environment 
 in which the evaporation is being carried out . at 65 c with a 0.1% suspension of the cross - linked 
 spheres it was possible to obtain arrays of 1  1.5 cm 
 as shown in figure 1 . these exhibited long - range 
 alignment of the orientation of the hexagonal close - packed sphere 
 array , resulting in green opalescence across the entire sample when 
 the template was held at an appropriate viewing angle . they contain 
 cracks along ( 110 ) planes in the array due to shrinkage during drying 
 and larger horizontal striations between the macroscopic domains , 
 the size of which ( 0.5 mm ) is competitive with other templates 
 produced in this way . sem image of a polystyrene 
 sphere template showing overall ordering of the domain orientation , 
 the network of small cracks , and the larger gap between domains ( bottom 
 left ) ; ( inset ) photograph of the whole template on a silica tile with 
 a coated region of 1  1.5 cm showing the bulk striations . primary amines can act as the 
 cross - linking agent in nonoxide sol - gel processing to form metal nitrides 
 from metal dialkylamides by transamination of the metal center followed 
 by condensation reactions resulting in bridging alkylamide groups 
 and formation of polymeric species in solution . previously 
 we demonstrated this chemistry to be effective for processing of tin 
 as the sols could be used to deposit films by dip coating or inverse 
 opals by template infiltration , and then the resultant xerogels could 
 be decomposed to tin by heating in ammonia . the work reported herein is the first example with another metal . 
 with the larger ta centers there is a higher likelihood that the polymers 
 will also contain some coordinated neutral amine and hence increase 
 the ta coordination number above the 5-coordinate level assumed in 
 the equations above , but this need not change the behavior for sol - gel 
 processing . reactions of ta(nme2)5 with 
 ammonia result in precipitation of a polymer with bridging amide groups 
 that decomposes to ta3n5 or tan on heating in 
 ammonia or inert gases , respectively . these facile 
 reactions are unsuitable for sol - gel processing as precipitation occurs 
 due to formation of large aggregates . transamination reactions of 
 ti(nme2)4 with 2 equiv of primary n - alkyl amines also appeared to be facile as the propylamine was not 
 observed in the h nmr spectra after a short period of 
 aging , but the condensation process was well controlled , with a strong 
 color change observed over the first few minutes of reaction but then 
 no precipitation even after the sols had been kept for several days . condensation was promoted by solvent evaporation 
 as the resulting xerogels were then found to be insoluble . slow addition 
 of 2 mol equiv of propylamine to a thf or hexane solution of ta(nme2)5 resulted in some gas evolution from the solution , 
 presumably loss of the volatile dimethylamine byproduct , but only 
 minor color changes were observed . larger quantities of propylamine 
 up to 5 mol equiv also did not produce any color changes , and in contrast 
 to the titanium reactions , these larger quantities also did not lead 
 to formation of any precipitate . in order to better understand 
 the speciation in the tantalum amide - derived sols h nmr 
 spectra were collected on solutions of ta(nme2)5 with various quantities of added propylamine ; a selection of these 
 are in figure 2 . propylamine is only observed 
 very weakly in these spectra until sufficient quantities are added 
 to fully displace the dimethylamide groups , as previously observed 
 in the analogous reactions with ti(nme2)4 . the 
 dimethylamine signal , a doublet at 2.2 ppm , is strong once only 0.7 
 mol equiv of propylamine has been added to the sol and then remains 
 fairly constant due to vaporization of the excess of this volatile 
 compound and formation of a saturated solution . the ta(nme2)5 signal is split by addition of small amounts of amine , 
 presumably due to ta(nme2)x(nhpr)y species , and becomes very weak 
 beyond 2 equiv of amine . broad features at slightly higher 
  than those of propylamine could be due to coordinated terminal 
 or bridging propylamide groups . coordinated groups were not observed 
 with ti(nme2)4 , and in combination with the 
 lack of precipitate with excess amine and the less obvious changes 
 to the sol on amine addition , this suggests that condensation occurs 
 less readily in ta(nme2)5-derived sols and that 
 smaller species will be present in solution . h nmr spectra 
 of ta(nme2)5 ( bottom ) , prnh2 ( top ) , and the products of reactions between ta(nme2)5 and the number of molar equivalents of propylamine 
 shown in the labels . in vacuo removal of solvent from a sol produced from ta(nme2)5 and 2 mol equiv of propylamine in thf resulted 
 in a foamy yellow xerogel containing gas bubbles produced by solvent 
 evaporation . combustion microanalysis of this material gave 22.48% 
 c , 4.82% h , and 9.25% n. the high carbon content is consistent with 
 the expected presence of large numbers of propylamide groups in the 
 gel . after solvent evaporation the gels were found to be insoluble , 
 showing that although condensation was found to be slower in solution 
 than in ti(nme2)4-derived sols it still occurs 
 sufficiently on solvent evaporation to produce a covalently bound 
 network . tga of the xerogel in nitrogen showed a gradual mass loss 
 of 31% between 100 and 375 c and then no further mass loss up 
 to 800 c ( supporting information ) . ammonia cross - linked polymers produced from metal dialkylamides have 
 been shown by mass spectrometry to lose mass in this temperature range 
 associated with continuing condensation reactions accompanied with 
 ammonia and amine loss and then to lose nitrogen with reduction of 
 the metal at higher temperatures . by 
 analogy , it is likely that no reduction of ta occurs up to 800 c . heating the xerogel in flowing ammonia at 600 , 700 , or 800 c 
 resulted in collapse of the foam structure and formation of reddish - black 
 residues ; the diffraction patterns obtained from these samples are 
 shown in figure 3 . the material heated to 600 
 c was found to be amorphous , while at either 700 or 800 c 
 the observed diffraction peaks correspond to the normal structure 
 of ta3n5 . use of the scherrer 
 formula indicates the observed peak widths correspond to a crystallite 
 size of 20 nm . the material heated at 800 c contained 
 2.34% c , 1.1% h , and 12.86% n ( 11.4% n calculated for ta3n5 ) , suggesting incomplete reaction and possibly some 
 carbon incorporation ( also consistent with the color as ta3n5 is bright orange - red ) . it is possible that the foam 
 structure contains small or poorly interconnected pores and hence 
 restricts access of the ammonia flow to the bulk of the xerogel . xrd patterns 
 of a bulk ta(nme2)5/propylamine - derived xerogel 
 heated to 600 ( bottom ) , 700 ( middle ) , and 800 c ( top ) with the 
 major reflections of the usual orthorhombic ta3n5 structure labeled with miller indices . sols in thf and hexane were also 
 used to produce thin films by dip coating a silica slide to a depth 
 of 20 mm four times in the glovebox and allowing it to dry in between 
 each dipping . the xerogel film coated the substrate smoothly , but 
 on heating the films were observed to crack and become flaky , especially 
 with hexane - based sols . no good quality films after firing were obtained 
 from these sols . in forming tin films it was found that a longer chain 
 amine , n - octylamine , produced better adhered and 
 less cracked films , and this change could 
 be pursued if ta3n5 films were desired . xrd 
 patterns of films fired at 600 c contained broad peaks at 38and 
 44 consistent with cubic tan . interestingly , cvd at this temperature 
 from ta(nme2)5 and ammonia also yields tan . increasing the temperature to 700 or 800 c 
 yielded red films and xrd patterns consistent with ta3n5 . formation of nanocrystalline tan at 600 c and the 
 red color of the films produced at higher temperatures are significant 
 changes in behavior relative to the bulk xerogels , and both could 
 be due to better access to the gel by ammonia in the planar morphology 
 and hence more effective removal of organic ligands during the firing 
 process . infiltration of the polystyrene opal templates was 
 achieved by capillary action and evaporation over a 1 h period when 
 the edge of the film was placed into a hexane - based ta(nme2)5/propylamine sol in the glovebox . thf - based sols are 
 unsuitable for this purpose as the template dissolves in thf . the 
 striations shown in figure 1 prevented effective 
 infiltration from the bottom of the slide , but after infiltration 
 from the side of the slide the yellow color of the xerogel was clearly 
 visible throughout the polystyrene template . the same procedure has 
 been used to produce tio2 inverse opals with long - range 
 order . the infiltration time was important 
 to achieving a good quality inverse opal as leaving the material in 
 the sol for too long results in a solid layer of material coating 
 the porous structure ; similarly , use of dip coating to fill these 
 templates resulted in a porous structure with a solid coating over 
 the surface . the sem images in figure 4 show 
 three of these films after heating in ammonia . note that the template 
 has been removed cleanly by the heating process and that at 600 , 700 , 
 and 800 c the films have retained the pore structure originally 
 introduced by the template . this has a high degree of order at 600 
 c and only a small loss of order at 700 c , but heating 
 at 800 c resulted in loss of order in the pore structure as 
 seen in the image and the fast fourier transform ( fft ) and a thickening 
 of the pore walls . we previously found in ta3n5 samples produced by ammonolysis of ta2o5 that 
 the crystallite size only increased at temperatures of 800 c 
 and above and attributed this to the start of interparticle sintering ; the same effect is probably operating here . sem images , 
 photographs ( inset ) , and calculated ffts ( inset ) of macroporous films 
 produced by infiltrating a ta(nme2)5/propylamine 
 sol in hexane into a close - packed polystyrene sphere template and 
 heating in ammonia at 600 ( top ) , 700 ( middle ) , and 800 c ( bottom ) . the porous films produced by heating 
 to 600 c were black and metallic but displayed green opalescence 
 across the entire film . the highly ordered structure ( figure 4 ) results in bragg - type optical diffraction from 
 the periodic spacing of the macroscopic lattice , the length scale 
 and dielectric properties of which are consistent with the color observed . films heated to 700 c were orange in color , suggesting ta3n5 , and also exhibited green opalescent regions 
 when viewed from certain directions . the uv  vis transmission 
 spectrum of one of these films ( figure 5 ) shows 
 a dip in the transmission at 510520 nm that is consistent 
 with the expected photonic stop band , superimposed on the general 
 trend of steeply decreasing transmission below 650 nm due 
 to the band edge of the red ta3n5 . films heated to 800 c had a deep orange 
 color but no opalescence due to disruption of the long - range ordering 
 of the film shown in figure 4 . the film colors 
 suggested that , similarly to the thin films , ta3n5 was only crystallized at 700 and 800 c , and the diffraction 
 patterns ( figure 6 ) confirmed this to be the 
 case . the observed peak widths are consistent with crystallite sizes 
 of around 20 nm ( scherrer formula ) . films heated to 600 c contained 
 broad peaks due to a nanocrystalline material with a rocksalt - type 
 structure similar to cubic tan ( 7 nm crystallites ) and a broad 
 background suggesting some amorphous content . transmission spectrum 
 of an inverse opal film heated at 700 c , and ( inset ) photograph 
 showing one of the regions of the film displaying green opalescence 
 due to the ordered porous structure . patterns from inverse opal films heated to 600 ( bottom ) , 700 ( middle ) , 
 and 800 c ( top ) showing a broad amorphous background and broad 
 peaks characteristic of nanocrystalline tan at 600 c and sharper 
 ta3n5 peaks at 700 and 800 c . edx analysis of an inverse opal film heated to 700 c 
 confirmed tantalum , nitrogen and oxygen , and a strong silicon peak 
 indicating that the beam was penetrating through to the substrate 
 owing to the film thickness and the low volume fraction of solid in 
 the porous films . consequently , it is not possible to determine whether 
 the detected oxygen originated entirely from the substrate or also 
 from the film . the observed ta : n ratio was 1 in each analysis , 
 which is significantly less than the 1.67 expected for ta3n5 ; however due to the poor sensitivity to the probe to 
 light elements the xrd characterization of ta3n5 was considered more reliable . film thicknesses were estimated by 
 imaging a crack and measuring the close - packed pores ( figure 7 ) ; typical measured thicknesses were 510 
 m , consistent with previous work . sem 
 image of a crack in a tan inverse opal film heated at 600 c , 
 showing an approximate depth of 5 m . silica substrate is seen 
 at the base of the edge , and note that the edge of the crack follows 
 ( 110 ) planes of the hexagonal close - packed pores . ta(nme2)5 undergoes rapid 
 transamination reactions with n - propylamine , but 
 self - condensation reactions occur less readily than shown in previous 
 work using ti(nme2)4 . nonetheless , this is an 
 effective system for the non - oxide sol - gel processing of ta3n5 . sols produced using 2 or more molar equivalents of n - propylamine were shown to condense irreversibly on solvent 
 evaporation . amidine - capped , divinylbenzene cross - linked polymer spheres 
 that we previously demonstrated to be effective templates for such 
 nonoxide sol - gel chemistry have now been assembled into close - packed 
 templates with long - range order by temperature- and humidity - controlled 
 evaporation of a suspension in water onto silica tiles . infiltration 
 of the nitride precursor sols was achieved by capillary action and 
 evaporation , and the template was then cleanly removed by heating 
 in ammonia . ammonolysis at 600 c removed the template but failed 
 to crystallize ta3n5 . at 700 c well - ordered 
 crystalline ta3n5 inverse opal films were obtained , 
 but at 800 c ordering was lost as the samples started to anneal 
 strongly .
reactions 
 of ta(nme2)5 and n - propylamine 
 are shown to be an effective system for sol - gel processing of ta3n5 . ordered macroporous films of ta3n5 on silica substrates 
 have been prepared by infiltration 
 of such a sol into close - packed sacrificial templates of cross - linked 
 500 nm polystyrene spheres followed by pyrolysis under ammonia to 
 remove the template and crystallize the ta3n5 . 
 templates with long - range order were produced by controlled humidity 
 evaporation . 
 pyrolysis of a sol - infiltrated template at 600 c 
 removes the polystyrene but does not crystallize ta3n5 , and x - ray diffraction shows nanocrystalline tan plus amorphous 
 material . 
 heating at 700 c crystallizes ta3n5 while retaining a high degree of pore ordering , whereas at 
 800 c porous films with a complete loss of order are obtained .