Low-Temperature Adsorption and Diffusion of Methanol in ZIF‑8Nanoparticle FilmsAmber M. Mosier,† Hanna L. W. Larson,† Elizabeth R. Webster,† Mia Ivos,† Fangyuan Tian,‡

and Lauren Benz*,†

†Department of Chemistry & Biochemistry, University of San Diego, San Diego, California 92110, United States‡Department of Chemistry & Biochemistry, California State University, Long Beach, California 90840, United States

*S Supporting Information

ABSTRACT: The adsorption of methanol by a zeoliticimidazolate framework-8 (ZIF-8) nanoparticle thin film wasstudied in situ using temperature-programmed desorption andX-ray photoelectron spectroscopy under low-temperature, low-pressure conditions. Partial pore penetration was observed at 90 K,but upon increasing the exposure temperature of the film to 130 Kpore penetration was significantly enhanced. Although manystudies exist involving bulk powders, this is the first work to ourknowledge that demonstrates the ability to control and monitorthe entry of a molecule into a metal organic framework (MOF)film in situ using temperature. In this case, nanoparticle films ofZIF-8 were prepared and studied in ultrahigh vacuum. The ability to control and monitor surface adsorption versus poreadsorption in situ is key to future fundamental study of MOFs, for example, in the identification of active sites in reactionmechanisms.

1. INTRODUCTION

Metal−organic frameworks (MOFs) have emerged as a newtype of nanoporous material composed of metal ions or clusters(also known as secondary building units (SBUs)) connectedwith organic linkers.1,2 Compared with other porous materialssuch as primarily inorganic mesoporous silica3 and zeolites,4

and primarily organic nanostructured carbon5 and polymerssuch as polydimethysiloxane (PDMS),6 MOFs present highlycontrollable hybrid materials with nanopores formed by self-assembly and high porosities ranging from approximately 2000to 7000 m2/g.1 The high porosities, relatively large surfaceareas, tunable molecular structures, and selective adsorptionhave brought considerable attention to MOFs for potential usein the fields of gas capture,7,8 sensing,9−11 separations,12−15 andcatalysis.16,17 As such, MOFs stand to play an important role inthe solution to numerous current challenges including theenergy challenge. For example, the effective capture andconversion of CO2 into a usable energy source such asmethanol or other commercially viable chemical would be aholy grail of sorts for any material,18 and, in particular, MOFshave recently demonstrated promise to this end.19−23

Zeolitic imidazolate frameworks (ZIFs) are a particularlyinteresting subcategory of MOFs as they form with familiarzeolitic topologies because the angle of connectivity betweenthe metal and organic components matches that of the Si−O−Si bond (145°) in aluminosilicate zeolites.24 ZIF-8 is a highlypromising ZIF composed of tetrahedrally coordinated Zn ionsconnected by methylimidazolate linkers that assemble to form azeolitic sodalite topology with pore diameters of 11.6 Å that can

be accessed through 3.4 Å apertures.25 ZIF-8 is remarkablystable relative to other MOFs25 and sufficiently stable innanoparticle film form to be studied using temperature-programmed desorption (TPD) and X-ray photoelectronspectrometry (XPS) in ultrahigh vacuum, which is one reasonit is the focus of this work.26 In addition, ZIF-8 hasdemonstrated promising performance in separations14,15,27 aswell as reactions.28−31

In previous work, we investigated the interaction of CO2 andwater with supported ZIF-8 films,32 and herein we turn ourattention to methanol. High CO2 uptake has been demon-strated by several studies of porous ZIF-8, while water is knownto resist entering the hydrophobic pore structure unless highpressures are employed.33 The adsorption properties ofmethanol are more complex, falling between that of CO2 andwater, as will be shown in this in situ study. An understandingof the interaction of methanol with MOFs is of broad interestfor a variety of potential applications, including the separationof methanol from molecular mixtures and reactions involvingmethanol. We now highlight an example of each of theseapplications involving ZIF-8.A “methanol economy” is one possible intriguing alternative

to our petroleum-based economy. Methanol is currentlyproduced from petroleum-derived syngas, but it can also beproduced by methane oxidation or CO2 reduction, the latter of

Received: December 30, 2015Revised: March 3, 2016Published: March 7, 2016

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which could simultaneously mitigate CO2 levels and globalwarming.34 In the former process, biomass, an alternative topetroleum, can be degraded to produce syngas,35 which canthen lead to methanol production by reacting syngas over Cu/ZnO/Al2O3-based catalysts.36 The use of biomass to producefuels is a carbon-neutral process, as an equivalent amount ofCO2 is taken in to grow the biomass, as is produced in thecombustion of the resulting alcohols.35 Once methanol isproduced it can be used in fuel cells to generate electricalpower34 and can also be combined with various oils in atransesterification reaction to produce biodiesel.35

In many of these processes, including syngas production,water is a byproduct35 and must therefore be separated frommethanol (and other alcohols produced from biomass) prior toutilization. To separate alcohols from water, a selective materialis necessary. To this end, MOFs have been recently exploredfor use in the separations of water and alcohol. Two mainapproaches can be employed: the use of a hydrophilic sorbentlike MOF JUC-11037 to preferentially adsorb water or the useof a hydrophobic sorbent with strong alcohol adsorptionaffinity such as ZIF-8.38 In other in-between cases such as MIL-100(Fe),39 the adsorption equilibria depend on the relativeamounts of water and alcohol in the mixture. Even in the caseof hydrophobic ZIF-8, care must be taken to fully understandthe adsorption and diffusion dynamics, which can besignificantly influenced by the presence of external surfaceterminations, as was shown in a systematic study of the effect ofZIF-8 nanoparticle size on the adsorption of ethanol andwater.40 These effects can become significant in cases of dilutealcohol/water mixtures.Because hydrophobic ZIF-8 exhibits selective adsorption for

alcohols at high pressures, it has been studied extensively bothcomputationally and experimentally for the separation ofalcohols from alcohol/water mixtures.38,40,41 Previous studiesshow both simulated and experimental S-shaped adsorptionisotherms of C1−C5 primary alcohols with increasing pressurefor microcrystalline ZIF-8.27,42 The adsorption of these alcoholscan be explained by a “cluster-formation and cage filling”mechanism.43 It was found that at low pressures (1 kPa) andambient temperatures, both methanol and ethanol form clustersat the CC bonds of the methylimidazolate linkers due toweak van der Waals interactions, resulting in little adsorption inpores. With increasing pressures (∼3−5 kPa, with lower uptakepressures for longer alcohols), however, the clusters grow andcage-filling occurs in the sodalite structure.38

In addition to its ability to participate in separations involvingalcohols, ZIF-8 has recently demonstrated activity in thetransesterification of rapeseed oil to alkyl esters via reactionwith alcohols.44 What is particularly noteworthy is that thisreaction involves the framework itself rather than metalnanoparticles or other external catalyst loaded into theframework. This reactivity is surprising because the Zn2+

nodes are presumable fully coordinated in the bulk of theframework and thus not expected to be particularly active.Nevertheless, ZIF-8 exhibits considerable activity, which iscomparable to that of zinc aluminate, a competitor to zincoxide.45 In the transesterification reaction, alcohols are typicallyactivated via deprotonation by basic sites, thus rendering themnucleophilic, while esters become electrophilic when activatedby acidic sites. The reaction can therefore proceed in thepresence of acidic or basic sites. The authors of this workconclude that external surface groups that include under-coordinated Zn acidic sites and possibly basic surface sites such

as protonated imidazole groups are responsible for the activity,and indirect yet convincing evidence of the existence of thesesites is provided.Clearly, an understanding of terminating surface groups and

how they influence adsorption, separations, and reactivity isimportant. In this fundamental work, we examined methanoladsorption by a film composed of nanoparticles of ZIF-8, thusmaximizing the presence of external surface groups in an effortto uncover the interplay between external surface and internalpore adsorption and diffusion. Methanol was selected as arepresentative model alcohol with the ultimate goal of beingable to monitor alcohols in situ along with other smallmolecules such as water and carbon dioxide to distinguishsurface and bulk adsorption and reaction sites.The two main techniques used in this study, XPS and TPD,

are well established and have both been employed extensivelyin single-crystal studies, and to a lesser extent to zeolites andother porous materials. XPS is a well-known technique thatoffers chemical-state information specific to the first fewnanometers of a material and can therefore be used to identifykey surface species that often differ from that of the bulkmaterial.46 TPD helps identify the types and relativepopulations of desorption states and can also provide kineticparameters such as desorption order and activation energy.47 Inthe event that a reaction occurs, active sites can be identifiedusing TPD or XPS. For example, in the McMurry couplingreaction of benzaldehyde to form stilbene over single-crystallinereduced titanium dioxide, reactive sites were identified to bemobile interstitial bulk Ti3+ sites, which was unexpectedbecause much of the reactivity over this surface is attributedto surface oxygen vacancies.48 This reaction proceeds throughan interstitial-stabilized diol intermediate that was later imagedon the surface with scanning tunneling microscopy.49 TPD andXPS have also been employed in the interrogation of porousmaterials such as nanotubes and zeolites. For example, a varietyof binding sites for n-nonane and carbon tetrachloride oncarbon nanotubes could be identified using TPD, includingsites inside the tubes, outside on the surface of the tubes, andsites in between the nanotubes.50 In addition, TPD is routinelyused to probe the number and strength of acidic sites in zeolitesvia the adsorption and desorption of ammonia.51 Despite theproven utility of TPD and XPS in other systems, very fewreports exist to date in the literature on the application of thesemethods to metal organic frameworks. This is likely due in partto the complexity of MOFs and also the fact that MOFs arerelatively new in comparison with zeolites and single crystals.Herein and in our previous work26,32 we use XPS and TPD toprobe molecular adsorption over ZIFs, in this case focusing onmethanol adsorption and the ability to distinguish and controladsorption on outer surface sites from bulk sites. Such adistinction is key to understanding the unique behavior of thesematerials.

2. EXPERIMENTAL SECTION2.1. Ultrahigh Vacuum Chamber. All methanol adsorption/

desorption experiments were carried out in an ultrahigh vacuum(UHV) chamber with a base pressure of 1 × 10−10 Torr. The chamberhas a custom directed doser composed of a leak valve welded to a 1/4in. stainless-steel tube. The tube was positioned 0.5 cm in front of thesample during exposure to anhydrous methanol (dispensed by a PureSolv dry system, EMD OmniSolv grade), carbon dioxide (Airgas,99.999% purity), and water (Millipore filtered). Each gas was dosed ata specific pressure (range of 10−10 to 10−6 Torr, depending on the gas)and time. Longer exposure times led to greater uptake, with

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approximately linear correlation between exposure time and uptake.An XPS with a cylindrical mirror analyzer for surface analysis (PHI 15-255G) and a quadrupole mass spectrometer for temperature-programmed desorption studies (300 amu range, Hiden Analytical,HAL 301/3F) were also employed as part of this system.2.2. Sample Preparation. A ZIF-8 solution was prepared using a

previously established method52 by mixing two 100 mL methanolicsolutions and stirring for 1 h. One solution contained 1.50 g (50 mM)of zinc nitrate (hexahydrate, Baker Analyzed, 99.0−101.0%) and theother contained 3.25 g (400 mM) of 2-methylimidazole (Acros, 99%).The resulting colloidal white precipitate was separated usingcentrifugation (14 000 rpm for 15 min), then resuspended bysonication in absolute ethanol (KOPTEC, anhydrous, 200 proof) toremove the excess unreacted species. This rinsing process wasperformed a total of three times. The resulting ZIF-8 nanoparticles,as reported in our previous work, were 34.0 ± 2.9 nm in diameter.26

Gold-coated silicon wafers (50 ± 5 nm Au on 500 ± 30 μm p-typeSi with <111> orientation, Ted Pella, cut into 1 × 1 cm2 squares) werecleaned with acetone (BDH, ACS grade) and dried under nitrogen(dry house N2). After cleaning, the wafer pieces were dip-coated usinga commercial dip-coater (Chemat Technology, 180 mm/min with-drawal speed, 10 s still time) in the purified solution of ZIF-8 inethanol, heated to 130 °C for 5 min on a hot plate, dipped in ethanol,dipped a second time in the ZIF-8 solution, dried with nitrogen gas,and heated again to 130 °C for 5 min to produce “two-cycle” films.The parameters for dip-coating were determined from previous work,demonstrating the successful preparation of ZIF-8 films using thismethod.26,53 The ZIF-8-coated Au film was then mounted onto anequally sized tantalum back plate using thin tantalum wires (0.25 mm,99.9+% purity). The sample mount was in contact with a liquidnitrogen reservoir, and a type-K thermocouple was glued into a smallslit cut on the side of the Ta plate with precured UHV-compatibleceramic glue (Ceramabond 503) such that it also contacted the wafer.The sample was cooled to ∼90 K and controllably heated to 365 Kusing a tungsten wire positioned ∼2 mm behind the sample.2.3. Characterization. X-ray photoemission data were collected

using a Mg Kα X-ray source (hν= 1253.6 eV, with a maximumresolution of 1.1 eV) at 90 K. Prior to the experiments, bindingenergies were calibrated using a half-copper (2p3/2, 932.4 eV) half-gold (4f7/2, 83.8 eV) substrate. Data analysis was performed usingCasaXPS software (CasaXPS Version 2.3.14), and peaks were fittedwith a range of 2.5 to 3.5 full width at half-maximum (fwhm).Photoelectrons were collected at a 45° takeoff angle and an estimatedescape depth of ∼4 nm based on these parameters and the density ofZIF-8.54

During the TPD studies, the sample was heated at a rate of 2 K/swhile positioned ∼1 cm in front of the mass spectrometer’s ionizationsource. A small aperture (4 mm) to the mass spectrometer was used toensure that all ions collected were indeed from the ZIF-8 film ratherthan sample holder parts, which were of minimal area.

3. RESULTS AND DISCUSSION3.1. Methanol Adsorption at 90 K. Methanol (MeOH)

desorption from a ZIF-8 nanoporous film was first monitoredusing TPD following adsorption at 90 K. By comparing withprevious studies of methanol desorption from a pristine single-crystalline TiO2 (110) surface (3.2 × 1014 methanol moleculescorresponding to a monolayer on a 1 × 1 cm2 TiO2 (110)surface;55 data and additional details in Supporting Information,Figure S1), the desorption of methanol from ZIF-8 nano-particle films can be quantified. As presented in Figure 1, thedesorption spectra of methanol from a two-cycle ZIF-8 filmreveal a feature at ∼170 K at the lowest exposure, whichgradually shifts to 165 K with increasing exposure. Above thisexposure, a peak emerges at 160 K with overlapping leadingedges, suggesting zeroth-order, multilayer-like desorption.56

This multilayer desorption temperature is consistent with acontrol bare Au wafer using the same sample holder, although

slightly broader (Figure S2), and similar to that reported in theliterature (∼20 K higher, likely due to the fact that we use anas-installed Au-coated Si wafer with different thermal propertiesas compared with a sputter-cleaned Au(111) single crystal, butboth features do not saturate with increasing exposure);57

however, we will demonstrate later that this feature is not solelydue to the accumulation of a traditional surface multilayer. Atlarger exposures (Figure S3) this feature grows and continuesto exhibit zeroth-order behavior, shifting gradually to highertemperature, but no additional features were observed.To further study the interaction of methanol with the ZIF-8

film, we also employed XPS. Because XPS is a surface-sensitivetechnique that can detect the elemental composition andchemical environment of the outer ∼4 nm of the surface (seeabove), while TPD collects signal from the entire film upondesorption, the two can be used together to monitor theadsorption of methanol both in and on the film.32 For example,in a previous study, we were able to use XPS and TPD todetermine the uptake of CO2 by 2- and 4-cycle ZIF-8nanoparticle films.32 In that case, CO2 entered the pores ofZIF-8 initially rather than adsorbing to the outer particlesurfaces and was detected at the surface of ZIF-8 by XPS onlyonce the pores were filled to near capacity. Methanol differsfrom CO2 in that it has the potential ability to hydrogen bondto outer surface groups, and it also has a larger kinetic diameter(3.7 vs 3.3 Å, respectively).58,59 XPS can also be used todetermine any changes to the surface groups upon adsorptionor reaction.In our previous studies, using XPS we found that not only are

clean ZIF-8 films terminated by methylimidazolates butterminations also include carbonates, hydroxide, or watergroups and undercoordinated Zn ions as well as a small amountof protonated imidazole nitrogen.26 In the O 1s region of the“clean” sample, which shows the non-native surface termi-nations of ZIF-8 (bottom traces of Figure 2), the dominatingsignal fit with a red trace is due to the presence of carbonateoxygen atoms due to Zn-coordinated carbonate groups. Oxygencoordinated directly to Zn is a smaller component of the totalO 1s signal shown in green, while the magenta trace isattributable to Zn-coordinated hydroxyls or water molecules.After adsorption of 1.1 × 1015 molecules of methanol at 90 K

Figure 1. TPD spectra of methanol desorption from a 2-cycle ZIF-8film.

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(corresponding to 3.4 ML on a single-crystal TiO2 surface55),

the peak in the O 1s region shown in Figure 2a broadensconsiderably due to the presence of methanol in the surfaceregion. The broadening is caused by the appearance of a newcomponent shown in blue at ∼533.6 eV, attributable to the−OH moiety of methanol.60 With increasing exposure tomethanol, the intensity of the −OH feature in the O 1s regionincreases. By comparison, in previous work on H2O on a ZIF-8film an exposure of only 1.4 × 1014 molecules of water at 90 K(corresponding to 0.4 ML on a single-crystal TiO2 surface

55)produced a corresponding O 1s signal roughly 4 times greaterthan that observed here.32 In that case it was clear that waterwas building up nearly exclusively on the surface. Theunderlying substrate peaks in the region remained unchangedin relative intensity and position upon and following exposureto MeOH.In the C 1s region, the main peak on the “clean” surface can

be attributed to 2-methylimidazolate groups and is shown inyellow. Flanking smaller brown and red traces have beenpreviously attributed to adventitious carbon and carbonatecarbon, respectively.26 Upon exposure to methanol, a peakrelated to the methanol carbon appears at 286.3 eV.60 Thispeak also grows with increasing exposure, and the oxygen-to-carbon atomic ratio for the methanol peaks was calculated to be1.1:1, close to the expected 1:1 C:O stoichiometry. Theunderlying substrate peaks in the carbon region remainednearly unchanged following methanol adsorption/desorption;however, a small amount of additional adventitious carbon wasobserved following the larger exposure. Our results suggestmethanol’s adsorption behavior is between that of water andcarbon dioxide, which were studied previously. Carbon dioxide,which enters the pore structure under these conditions,required exposures roughly 2 to 3 orders of magnitude larger,and the corresponding desorption (∼1017 molecules) wassimilarly larger compared with that of methanol and water,respectively, before corresponding peaks were clearly visible inXPS.32 For further comparative purposes we did a selectnumber of TPD and XPS experiments involving butanol, whichexhibited stronger interactions, yet overall behaved similarly tomethanol. The details of these experiments can be found in theSupporting Information.

3.2. Methanol Adsorption on ZIF-8 at 130 K. To studythe adsorption behavior of methanol further, we explored thepossibility of using temperature to control methanol’s access tothe cage space in our system. Not surprisingly, it has beenpreviously shown in a comparative study of ZIF-90 and ZIF-8using a combination of molecular dynamics (MD) simulationsand nuclear magnetic resonance (NMR) that increasingtemperature will increase the diffusion of methanol in ZIF-8;27 therefore, it may be possible to enhance diffusion into thepores by increasing the methanol exposure temperature. In fact,there are a number of reports regarding the high diffusivity ofseveral gases at various temperatures in ZIF-8.61−64 Addition-ally, in a model study of a different MOF, HKUST-1, it wasshown that non-native surface groups present can actually blockaccess to the pores, significantly slowing the uptake ofcyclohexane in that case into the pore structure as comparedwith pristine films, which were never exposed to air and thusnever had the chance to build a “surface barrier” layer.65

Another recent paper on methane adsorption notes a similarbarrier acting in ZIF-8 particles.66 We therefore investigated theadsorption of methanol on ZIF-8 nanoparticle films at 130 K,the maximum temperature before which desorption rates areappreciable (as determined by TPD, Figure 1). The TPDspectra of methanol desorption from ZIF-8 films followingadsorption at 130 K are shown in Figure 3 and reveal three

main features, labeled β1, α1, and α2. The first feature topopulate, α1, appears at 170 K, similar to exposure at 90 K. Theinset in the Figure shows this in more detail, and a very broadshoulder is observed at ∼220 K as well. Upon increasing theexposure amount, this peak grows and shifts to 180 K, and asmall sharp peak appears at ∼160 K (desorption of 4.1 × 1015

molecules), labeled β1. Finally, at the highest exposures

Figure 2. XPS spectra of a 2-cycle ZIF-8 film before and after theadsorption of 1.1 × 1015 (XPS threshold amount) and 1.5 × 1015

molecules of methanol in (a) C 1s and (b) O 1s regions. Molecularvalues were calculated following subsequent desorption by TPD, afterwhich the XPS spectra reverted back to the clean state (bottom).

Figure 3. TPD spectra of methanol desorption from ZIF-8 filmsfollowing adsorption at 130 K. The TPD spectra corresponding tosmaller methanol exposure amounts are shown in the inset.

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investigated, a broad but well-defined peak, α2, appears at 260K, shifting significantly to 275 K. The peak at 160 K, the lowestdesorption temperature observed here, is similar to thatobserved both on Au and at higher exposures at 90 K;therefore, we assign this to multilayer methanol, which in thiscase is likely a true multilayer at the external nanoparticlesurfaces. Interestingly, this peak remains relatively smallfollowing exposures at 130 K, contrary to that observed at 90K. We attribute α1 and α2 to desorption states associated withthe nanoparticle film: α1 likely arising from desorption fromwithin the ZIF-8 nanoparticle pores and α2 arising fromdesorption from between particles. The α1 peak shows anoverlapping leading edge and increasing peak temperature withincreasing exposure and thus zeroth order behavior,56

consistent with the formation of clusters of MeOH withinthe pores because the main interactions within these clustersare between the MeOH molecules themselves, which break asmethanol diffuses out of the pores and desorbs. The broader α2peak with smaller area and greater shift in desorptiontemperature with increasing exposure likely correlates todesorption from between particle space and is consistent witha similar peak observed in previous experiments of carbondioxide on ZIF-8 films.32 The interparticle space is expected tohave higher density, lower overall surface area, and likely morecomplex diffusion dynamics due to the presence of non-nativesurface groups such as carbonates and hydroxyl/water groups65

and thus the broader peak and higher desorption temperature.3.3. TPD/XPS of Methanol, Water, and CO2 Adsorp-

tion on ZIF-8 at 90 and 130 K. Because of the complexity ofthe TPD spectra at 130 K, we further explored the increasedaccessibility of methanol to the pore space of ZIF-8 by carefullycomparing TPD and XPS data following exposure to MeOH at90 and 130 K. Because XPS probes the first few nanometers ofthe surface region, while TPD monitors total desorption fromboth the surface region and inner pores, the combined use ofXPS and TPD over a broader range of exposures gives insightinto the tendency of a species to enter the film porosity orremain on the surface. Figure 4 compares the integrated TPDpeak area to the corresponding integrated O 1s peak area inXPS with varying exposure amount and exposure temperature.If molecules accumulate primarily at the surface, both XPS andTPD signals will be collected, with the surface signal in XPSgrowing quickly, as shown by the bottom arrow in the Figure,eventually saturating as the photoelectron escape depth isreached. The data shown is before this saturation point isreached. On the contrary, if molecules tend to enter the porestructure, the XPS signal will remain small while the TPD signalgrows rapidly, as shown by the top arrow. If a moleculesimultaneously populates both the external surface and internalporosity, the XPS signal will grow more slowly, with the slopebetween the two extreme cases and indicative of the relativeamounts of surface accumulation and pore penetration. As a setof comparison points, because we previously found that waterdoes not enter the pore structure of ZIF-8 under theseconditions, while carbon dioxide readily enters the pores for afour-cycle film, we collected and correlated XPS and TPD areasof these two molecules as well on the two-cycle films preparedhere (open black squares and open blue circles, shown inFigure 4). Because ZIF-8 is hydrophobic and water is expectedto reside mainly at the surface, the XPS signal increases rapidlyeven at very low exposures and correspondingly smalldesorption amounts; therefore, the slope is relatively small inthe TPD/XPS correlation. On the contrary, for CO2, the

opposite is observed: Following a short initially flat region likelydue to some CO2 accumulation at bare Au regions (someunderlying Au is exposed on the two-cycle ZIF-8 sample),because CO2 can access the pore structure, the XPS signalsaturates, while the TPD signal continues to build as most ofthis desorption comes from CO2 beneath the outermost surfaceresiding in the bulk pores. (Note that the data have beencorrected for the fact that CO2 will produce a related XPS O 1ssignal roughly twice as large as that of H2O due to theadditional oxygen atom, and the TPD data are scaled down by afactor of 4 for ease of viewing on the scale shown.). Wecompared adsorption of MeOH at 90 K (open red triangles,Figure 4) to these two molecules (also at 90 K) and noticedthat the slope of the plot is flat initially and then increases,falling between that of water and CO2. This behavior is likelyindicative of initial nucleation at the surface, followed bysimultaneous nucleation at the external surface and in thepores, which was not immediately clear from the observedsingle TPD peak. In TPD, following adsorption at 90 K (Figure1), the initial peak at 170 K can be attributed to adsorption atexternal ZIF-8 surface groups due to hydrogen bonding withthese groups. At higher exposures the peak shifts to 160 K,which is likely due to nucleation in the pores (governed byweaker van der Waals forces) as well as multilayers on thesurface. It is likely that the energies of desorption are similarand therefore difficult to distinguish with TPD alone, althoughas previously mentioned the width of the 160 K peak is slightlybroader than that observed on pure Au (S2) and also broaderthan the previously observed pure surface multilayer peaks ofother small molecules on dense surfaces using this experimentalsetup and identical heating ramp parameters.67 Finally, wedosed MeOH at 130 K, then collected XPS at 90 K. We cooledthe sample to 90 K following exposure at 130 K to minimizeany potential differences in the XPS data caused by differencesin diffusion. The slope of the correlation plot at 130 K (filledred triangles, Figure 4) is greater than that at 90 K, suggesting agreater degree of pore penetration at higher exposure

Figure 4. Correlation of XPS O 1s peak area and number of moleculesdesorbed for CO2 (open black squares), methanol (open redtriangles), and H2O (open blue circles) adsorbed at 90 K on a two-cycle ZIF-8 thin film and methanol (filled red triangles) and water(filled blue circles) adsorbed at 130 K. The number of molecules iscalculated from corresponding TPD spectra. The measured O 1s XPSsignal for CO2 was divided by a factor of 2 as a stoichiometriccorrection, and the TPD signal was divided by a factor of 4 for visualscaling purposes.

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temperature, likely caused by enhanced diffusion into the poresupon exposure. Note that this phenomenon is distinct from“gate-opening” which has been shown to occur with othermolecules under higher pressure conditions. Recent calcu-lations show that “gate-opening” does not occur in the case ofmethanol adsorption.43 We performed this same experimentwith water and noted that the two lines were much closertogether, so in the case of water the higher temperature was notsufficient to cause water to enter the porosity of ZIF-8.

4. CONCLUSIONSIn this work, methanol adsorption by ZIF-8 nanoparticle filmswas followed in situ under low-pressure, low-temperatureconditions using a combination of XPS and TPD. At 90 K,methanol adsorption occurred both at the surface and in theporosity of the ZIF-8 nanoparticles. This was determined bycomparison with CO2, which was found to penetrate the poresnearly exclusively, and water, which resides primarily at surfacesites. Methanol’s behavior was between that of CO2 and waterin that it is capable of interacting with outer surface groupsthrough hydrogen bonding while also interacting with theporosity of ZIF-8 through weaker van der Waals forces.Additionally, we found that this pore penetration process ofmethanol can be kinetically enhanced by increasing theexposure temperature. This work lays a foundation for futurefundamental study of molecular adsorption and reaction inmetal organic frameworks.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.lang-muir.5b04455.

Calibration and control TPD studies of methanol on aTiO2 (110) and methanol on the supporting gold surfaceas well as additional TPD and XPS spectra related toFigures 1 and 2. (PDF)

■ AUTHOR INFORMATIONCorresponding Author*Tel: 1-(619)-260-4117. Fax: 1-(619)-260-2211. E-mail:laurenbenz@sandiego.edu.

Author ContributionsThe manuscript was written through contributions of allauthors. All authors have given approval to the final version ofthe manuscript.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis material is based upon work supported by the NationalScience Foundation under grant number DMR 1255326.Additional financial support was provided by the Henry LuceFoundation’s Clare Boothe Luce Program and the Arnold andMabel Beckman Foundation.

■ REFERENCES(1) Furukawa, H.; Cordova, K. E.; O’Keeffe, M.; Yaghi, O. M. TheChemistry and Applications of Metal-Organic Frameworks. Science2013, 341, 1230444.

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