- Email: [email protected]
Surface property control of semiconducting metal oxide nanoparticles
Pergamon
NanoSlruduredMaraials. Vol. 10. No. 5, pp. 699-713.1998 Elsevia Science Ltd 8 1998 Ati MetallurgicaInc. Printed in the USA. All lights rea.zwd 09659773/98 $19.00 + .oO
PI1 SO965-9773(98)00108-l
SURFACE PROPERTY CONTROL OF SEMICONDUCTING METAL OXIDE NANOPARTICLES M.-I. Baratont and L. Merhari2 ‘LMCTS-ESA 6015 CNRS, Faculty of Sciences, 87060 Limoges, France, [email protected] *CERAMEC, 64 Avenue de la Liberation, 87000 Limoges, France (Accepted June 15,1998) Abstract-The comprehension of the underlying mechanism attributing either a reducing or oxidizing ef$ect to humidity on the metal oxides used in gas sensors is still a challenge as the literature on this subject shows contradictory results. The high surface-to-bulk ratio of nanosized powders conferring them a much higher surface reactivity compared to their micronsized counterpart, clearly helps in a better understanding of the gas-surface interactions. A thorough FTIR sulfirce study of a nanosized titanium dioxide powder subjected to various sequences of carbon monoxide and mixtures of carbon monoxide and water vapor is reported. The effect of acetic akid and hexamethyldisilazane grafting on TiO2 is studied under the same conditions. It is found that water molecules have a reducing or oxidizing effect, depending on the acido-basic@ of the metal oxide. Our study clearly shows that a thorough knowledge of the surface reactivity of metal oxides is aprerequisiteforfurther majorgas sensing optimization. Ol998ActaMetallurgica Inc. INTRODUCTION
When properly synthesized, handled and processed, nanocrystalline materials show superior properties in a wide range of applications including sit&ring, catalysis, microelectronics, optoelectronics, magnetism, medicine, etc. It has been realized lately that for these materials which exhibit a high surface-to-b&ratio, the chemical composition of the first atomic layer is of utmost importance in the macroscopic properties of the final product (see for example ref. 1). Control and tailoring of the surface of nanocrystalline materials are the key to solving problems currently hindering the development of nanoparticle-based materials. Their electrical properties also critically depend on the chemical composition of the surface. It is indeed well-known that surface defects as well as impurities adsorbed on the surface sites can greatly modify the work-function of semiconductors by changing the affinity of their surface for electrons (2). In the rapidly growing field of gas sensing for indoor or outdoor air quality monitoring, humidity is known to have adverse effects on the response of metal oxide-based sensors (3-5). In these chemical gas sensors, the role played by the first atomic layer of the sensing material is preponderant(3,6). Asaconsequence,thesensitivityandtheelectricalcharacteristicsof thesensor depend on the chemical composition of this first atomic layer. The aim of this work is to show how
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the sensor response can be tailored by impurities purposely adsorbed on the surface. The sensor was simulated by a pellet made of pressed titanium oxide nanosized powder. However, we must underline here that we are not considering the TiO;! pellet as a gas sensor operating under its standard conditions since oxygen-free environments will be used. Indeed, we intend to highlight the surface reactions and to understand their mechanism so as to extend this knowledge to the very complex case of a real gas sensor in an atmosphere whose exact composition is unknown. The challenge was to obtain simultaneous results on the surface chemical composition and the electrical variations in presence of various adsorbates. Most of the experimental techniques providing chemical surface analyses of materials actually probe a depth ranging from a few nanometers to ten nanometers. Under these conditions, it is obvious that the so-determined chemical composition is only an average over several atomic layers and cannot resolve the very first atomic layer. Under specific experimental conditions, Fourier transform infrared (FAIR) spectrometry is a powerful tool to characterize the very first atomic layer because the surface chemical species generated during the nanostructured material synthesis process and/or generated by surrounding contaminants can be identified as well as reactive sites in anon destructive manner. These surface groupsand the reactive sites as well can be modified in situ by controlled adsorption of molecules (referred to as grufting in the literature) while the resulting modifications of the surface properties can be continuously followed in the same cell by FTIR spectroscopy. On the other hand, independently from fundamental vibrational studies, some semiconducting properties can be deduced from the infrared spectra without requiring electrical contacts (7) since the variations of the infrared energy transmitted by a sample are related to the variations of the electrical conductivity of this sample according to the Drude-Zener theory. Therefore, the FTIR spectrometry allows the simultaneous study of the electrical conductivity variations and the surface chemical species perturbations under various gaseous environments. EXPERIMENTAL The titanium dioxide (P25) was supplied by the Degussa company (Germany). The BET specific surface area given by the supplier is 50 m2gm1corresponding to an average particle size of 21 nm. The anatase phase was preponderant although mixed with about 30% of rutile phase. To perform the experiments, the powder (80 mg per sample) was slightly pressed on a stainless grid in order to ensure a homogeneous temperature distribution. The resulting titanium dioxide pellets were placed into the furnace of a homemade vacuum cell. This cell enables in situ studies at different temperatures (from room temperature to SOO’C) under vacuum or controlled pressures of gases and liquid vapors through a valve system. The cell was placed in the sample compartment of a Perkin-Elmer Spectrum 2000 spectrometer equipped with a MCI cryodetector. The analyzed range varied from 7800 to 450 cm-l with a 4 cm-’ resolution. The infrared energy transmitted by the titanium oxide pellet was measured over the whole infrared range, even though the infrared spectra presented in the following are limited to the wavenumbers ranges of interest. To emphasize the modifications undergone by the sample, some difference spectra are presented. In these spectra, the positive bands correspond to appearing or increasing species during the experimental step under consideration, whereas the negative bands correspond to disappearing or decreasing species. In the difference spectra, attention must be called to frequency shifts which appear as negative and positive double bands at very close
SURFACE PROPERTY CONTROL OFSEMKXNDVCTING METAL 0x1~ NANOPART~CLES
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frequencies. Therefore, when analyzing a difference spectrum, the original spectra must also be checked to confii the bands assignments to decreasing or increasing species or frequency shifts. Prior to measurements, the titanium dioxide pellet was activated, that is heated up to 450°C under dynamic vacuum and cooled down to room temperature while still under dynamic vacuum. This thermal treatment cleans the surface from all physisorbed and weakly chemisorbed species. The species removed during the activation are mainly water and scarce carbonate groups originating from the atmospheric and environmental contaminations. After the activation treatment, the sample was heated to the temperature of analysis under dynamic vacuum to keep the surface as clean as possible. It is worth noting that, after the activation treatment, the surface is no longer in equilibrium. Therefore, as soon as molecules are in contact with the activated surface, they adsorb on and/or possibly react with the surface sites, leading to new chemical species which may be irreversibly bound. Taking advantage of this irreversible adsorption, we modified theTi& surface by adding a few mbars of various liquid vapors to the activated sample at room temperature. The vapor in excess was eliminated by an evacuation at room temperature followed by a desorption at 450°C to remove all the reversible species. The infrared spectrum recorded after cooling at room temperature allowed to check whether the surface is irreversibly modified and to possibly determine the kind of surface sites that are poisoned by the chemisorption. Carbon Imonoxide was supplied by Air Products (Hz0 content less than 20 ppm). Water was bi-distilled and de-ionized. Hexamethyldisilazane (HMDS) from Fluka Chemie (99% pure) and acetic acid from Prolabo (Normapur) were used with no further purification.
RESULTS AND DISCUSSION Surface Characterization of the Titanium Oxide Nanopowder
The FflR spectrometry is particularly relevant to the study of surface species on nanosized powders (8). Moreover, the adsorption of probe-molecules at room temperature on an activated surface brings evidence of its acidic and basic sites. The characterization of the titanium oxide surface will not be presented here in details, but results can be found in references 9-12. As an example, Figure 1 shows the spectra of the Ti& pellet under vacuum before (Figure la) and after activation (Figure lb). The disappearing of the very intense band centered around 3320 cm-’ obviously indicates the elimination of adsorbed water. At the end of the activation treatment, several bands in the 3780-3600 cm-’ range are due to the v(0I-l) stretching mode of different types of hydroxyl groups bonded to titanium atoms of the first atomic layer. The number and the frequencies of these hydroxyl groups depend on the extent of the dehydroxylation and on the presence of impurities (9,lO). Therefore, it is very difficult to correlate all the results found in the literature about Tia surface characterization. It is generally admitted that titanium oxide strongly retains adsorbed undissociated water due to the strong Lewis acidity of the coordinatively unsaturated Ti4+ surface sites (10). In addition, a charge separation can occur in these coordinated water molecules because of the surface ionicity (13). This charge separation strengthens the formation of hydrogen bonds between the coordinated water molecules, thus stabilizing their link onto the surface. The remaining bound water molecules are indicated by the broad band at 3480 cm-’ assigned to the v(OH) stretching mode in perturbed hydroxyl groups and by the band at 1620 cm-‘assigned to the &OH) bending mode in water molecules. The 37153670-3676 and 3645
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M-l BARATON ANDL MERHARI
I
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Wavenumber (cm-l) Figure 1. Infrared spectra of Ti@: (a) before activation; (b) after activation; (c) after addition of the CO and [CO+H20] doses.
cm-’ absorption bands have been assigned by several authors (10,14) to thev(OH) modes of TiOH groups on different faces of anatase and in different coordinative unsaturation states. These TiOH groups are either acidic or basic.
Simulation of the Chemical Sensor and Experimental Procedure
According to the Drude-Zener theory, the infrared energy absorbed by a semiconductor is essentially due to surface states and to free carriers and, therefore, it is related to the sample electrical conductivity (7). By measuring the infrared energy transmitted (Em) by a titanium oxide pellet, it is then possible to follow the variations of the electrical conductivity versus different gaseous environments. These variations are precisely the response of the Ti& sensor under the considered gases. In addition, the infrared spectra give access to the in situ analysis of the surface chemical modifications during the gas exposures and possibly to information on the redox mechanism. It is worth noting that the infrared energy variations give a picture of the “true” conductivity evolution of the material, that is without perturbation due to electrodes whose metal can diffuse in the sensitive layer. In the case of n-type semiconductors, oxidizing adsorbates lower the IR absorption (En increases) whereas reducing adsorbates increase it (Em decreases). To make the Ti@ pellet simulate an operating sensor, the following procedure was conducted. Adose of gas and/or liquid vapor (CO or mixture of CO and H20) was introduced inside the cell for 10 minutes followed by an evacuation for 5 minutes. The “addition-evacuation”
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sequence was repeated as often as necessary. Infrared spectra were recorded at the end of the 10 minute-addition and at the end of the 5 minute-evacuation. ER was noted before and after the spectrum recording (2 minutes interval) to minimize the errors. The admitted gas pressures were set to 6 mbar of pure CO, 7 mbar of mixture of CO and H20 (6 mbar CO and 1 mbar H20). Usually, the commercial chemical gas sensors based on titanium oxide powder have a working temperature’close to 400°C. Therefore, we performed our simulation experiments at this temperature. The tremendous problem with these sensors is the sensitivity toward humidity which leads to adverse effects in the CO sensing mechanism. The phenomena are extremely complex because water can act as a reducing or an oxidizing agent with an apparent lack of reproducibility (4). Since the surface hydroxyl groups seem tobe somehow involved in the humidity effects (4,5), we modified these OH groups in order to study the influence of these surface modifications on the humidity response. For this purpose, two molecules were chosen with different reactivities: acetic acid (CHYXOH) and hexamethyldisilazane ([(CH$$i]2NH). Acetic acid strongly reacts with oxide surfaces by forming acetate species through the basic OH groups (13,15-17). The acetate groups can bridge on the surface Lewis acid sites, such as Ti4+. As for hexamethyldisilazane (HMDS), it reacts with OH groups yielding ammonia (18). The schematic reactions can be written as follows, keeping in mind that the grafting may be complicated by interactions with other surface sites such as biwis acids or bases. Ti- .0H+CH3C-
OH+Ti-0-CCH3+H20
s>
d
2 Ti-OH + [(CH&Si]zNH + 2 Ti-OSi(CH3)3
+NH3
To analyze the electrical conductivity variations of the TioZ pellets in dry or humid CO environments and to possibly detect memory effects, the main sequence of experiments consisted in the introduction of 4 doses of pure CO (6 mbar), followed by 4 doses of a CO and Hz0 mixture (7 mbar, referred to as [CO+H20] mixture in the following) and again followed by 4 doses of pure CO (6 mbar). As a remark, it is worth mentioning that the addition of any gas in the cell should theoretically contribute to a ER decrease. However, due to the small number of CO molecules (weak pressure), the energy variations caused by the CO addition was checked to be below the spectrometer detection limits that is less than 0.1% in absence of Ti@ pellets. For the same reasons, no energy variations are noted when 1 to 5 mbar of Hz0 are admitted inside the cell. Response of the “Clean “ TiOzSurjace The first series of experiments was performed at 400°C on an activated TiO2 surface (n-type semiconductor). The addition of the fust CO dose caused aEm (Figure 2) decrease corresponding to the CO redulcing effect, thus implying an increase of the electrical conductivity. The formation of gaseous CO2, according to the following well-known possible reactions:
co+o2-+co2+2e-
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Time (mm) Figure 2. ER variations versus CO “addition-evacuation” sequences for the “clean” Ti@ sample.
a
I
3000 Wavenumber (cm- 1) Figure 3. Evolution of the TiO2 IR spectrum: (a) after addition of the first CO dose; (b) after addition of the first [CO+H20] dose; (c) after addition of the fifth CO dose (the spectrum of Ti& at 400°C has been subs&acted).
IO
is proved by the absorption bands centeredat 2348 cm-‘, clearly visible on the difference spectrum (Figure 3a). Concomitantly, negative bands are noted in the 3720-3650 cm“ region showing either a decrease of the surface hydroxyl groups or their perturbation by the surrounding CO and Cti
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Time (min) Figure 4. EIR variations versus CO and [CO+HzO] “addition-evacuation” sequences for the “clean” Ti@ sample.
molecules. After evacuation, Em does not get back to its originalvalue (Figure 2). On the spectrum, only slight changes are noted in the v(0I-I) bands (3750-3600 cm-’ region) (not shown). This CO “addition-evacuation”sequence was repeated 12 times. The ER curve (Figure 2) indicates a steady overall reduction of the sample simultaneously with a weakening of the amplitude of the Em variation during the CO-addition period (in the following, the amplitude of the ER variation will be referred to as the response, by analogy with the sensor response). This can be explained by the steady removal of the surface oxygen atoms available for the Co2 formation. It must be noted that under our oxygen-free experimental conditions, the Tio2 sample is already reduced at the beginning of the experiment and that, in addition, each CO dose contributes to a further reduction of the surface with no possibility for surface regeneration. However, if the formation of COZ was the only mechanism involved in the electrical conductivity changes, the Em increase under evacuation would not be observed. Indeed, by removing the newly formed COz,,the evacuation irreversibly removes oxygen ionosorbed on the surface and, therefore, irreversibly adds electrons in the conduction band. The partial reversibility, that is, the weak Em increase under evacuation shows that another mechanism may be involved implying a reversible and weaker interaction between CO and the Ti% surface. On the infrared spectra, a reproducible perturbation of the surface hydroxyl groups each time CO is added, may be related, at least partially, to [this reversible part of the electrical changes. The second series of experiments on Ti@ consisted in the introduction of four CO doses followed by four doses of the [CO+H20] mixture and then by four new CO doses (Figure 4). The response toward the first four CO doses is obviously similar to that just described above. The addition of the [CO+H20] mixture causes a strong decrease of the electrical conductivity although without reproducibility. Only the fourth dose leads to a weak conductivity increase. In parallel, the infrared spectra show the intensity increase of thev(OH) absorption range (Figure 3b). The overall effect of the four [CO+H20] doses is an oxidation.
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When pure CO is addedagain, the E~evolution (Figures4) is similar to that observed during the addition of the first CO doses. Moreover, the baseline drift shows the same downward trend, thus indicating an overall reducing effect. At the end of the experiment, the sample was cooled under dynamic vacuum and compared with the activated sample (Figure 1c). The major change in the IR surface spectrum of Ti@ is the increase of the v(OH) band at 3720 cm‘*. These new hydroxyl groups have been created by the addition of water vapor. They reflect modifications of the surface structure and particularly distribution changes of the titanium atoms in the surface sites. Response of theAcetic Acid-ModifiedTiO2Surface Acetic and formic acids are known to strongly react with oxide surfaces yielding acetate or formate ions irreversibly bound. It is also possible that decomposition of acetate or formate species at high temperature may oxidize the Tio2 surface through CO production (16). To check the effects of the acetate groups considered as impurities, 4 mbar of acetic acid (AA) were added to the activatedTie pellet. Figure 5 compares the infrared spectra of the sample
I
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I
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Figure 5. Infrared spectra of the Ti@ sample: (a) after activation: (b) after acetic acid addition and subsequent desorption at 45O’C; (c) after addition of the CO and [CO+H20] doses; (d) difference spectrum b-a.
SURFACEPFXYERTYCOMR~L OF SEMIWN~~~TINGMETALOxm NANOPA~CES
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before addition (Figure 5a) and after evacuation at 450°C (Figure 5b) along with their difference (Figure 5d). These spectra show that AA is irreversibly bound as acetate species and that the hydroxyl grolups on the TioZ surface are involved in the reaction. On the difference spectrum, it is easier to see the bands corresponding to the v&X0) vibration at 1538 cm-l and thev&OO) vibration at 1.446 cm-‘. The 1352 cm-l band is assigned to the g(CH$ bending mode. The Av splitting of the v&00) andv,(COO) frequencies indicates that these acetate species are bound in the bidentate geometry which is in agreement with their thermal stability (17). A close look at the 1538 and 1446 cm-l bands reveals that they are complex because several kinds of bidentate acetate species can simultaneously exist. Indeed, the link of the acetate ion on the surface in the bidentate geolmetryinvolves both hydroxyl groups and Lewis acid sites. At least three types of OH groups are involved since three main negative bands can be discriminated at 3720.3674 and 3643 cm-l. Moreover, several types of Lewis acid sites exist on the Tio2 surface including Ti3’ possibly produced by the reducing effect of the activation treatment. Therefore, it is easy to understand that the multiple possibilities combining both OH groups and Lewis acid sites imply slightly different coordination strengths for the acetate groups, thus resulting in slightly different v,,(COO) and v,(COO) frequencies. The first dose of CO added at 400°C to this AA-modified surface causes a slight shift of the v&DO) band toward the lower wavenumbers (not shown). This shift is not reversible after the
a
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Figure 6. Rvolution of the AA-modified TioZ IR spectrum: (a) after addition of the first CO dose; (b) after addition of the first [CO+H20] dose; (c) after addition of the fifth CO dose (the spectrum of AA-modified Tiq at 4OO’Chas been subs&acted).
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CO evacuation. On the other hand, one notes a slight intensity decrease of the overall acetate groups and a reversible intensity decrease of the v(0I-I) absorption bands (Figure 6a). like in the case of the “clean”Ti@. Like the shift of thev,(COO) band, the modification of the acetate groups is not reversible after CO evacuation. The formation of a small amount of C@ is observed at 2348 cm-‘, proving that surface oxygen atoms are still available. As in the case of the non-modified surface, the surface is reduced by this first CO addition and Ti3+ ions may be formed to the detriment of theTi4+ ions. Therefore, the linkof the bidentateacetate groups onto the surface would be weakened in the new electronic distribution, thus causing the vS(COO) slight shift. The same effects are observed after the addition of the subsequent three CO doses, although weaker and weaker. Concomitantly, Em strongly decreases after the first dose addition due to the increase of the electricalconductivity of the sample (Figure 7). The subsequent three COdoses cause the same reduction. The overall evolution of Em under the four CO doses proves that, on the one hand, the response amplitude is roughly constant and, on the other hand, the sample is steadily reduced. A correlation with the decrease of the acetate groups, visible on the IR spectra, may be attempted considering that the acetate groups act as oxygen providers. When the [CO+H20] mixture is in contact with the surface, new hydroxyl groups are created around 3658 cm-’ (Figure 6b). No changes are noted for the two v(CO0) bands except a slight apparent band intensity increase due to the surface reduction. The formation of C@ is observed. Under evacuation, part of the newly formed OH groups are eliminated. The same modification of the OH groups is observed over the following three [CO+H20] doses but notably weaker. As already noted, the first dose of either CO or [CO+H20] has a more or less different effect than the following three doses. This is particularly obvious in the present case of [CO+H20] additions. The first dose leads to an oxidation of the sample (Figure 7) which can be related to new OH formation like in the case of a “clean”Ti@ surface. But, the effect of the subsequent three [CO+H20] doses
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Time (mm) Figure 7. Em variations versus CO and [CO+H20] “addition-evacuation” sequences for the AA-modified TiOz sample.
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SURFACEPROPERTY GDNTIWLOF SEMIC~NDUCTING METALOXIDE NANOPARTICIES
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is different since they lead to a reduction, A possible reason for these different behaviors may lie in the acidobasicity of the surface when water molecules strike it. Indeed, it has just been noted that acetic acid adsorption involves acidic hydroxyl groups thus changing the acido-basicity of the surface. As a consequence, the surface reactivity toward water is modified and the electron transfer may be reversed. The effect of the subsequent four CO doses is quite comparable to that of the first four ones, either on the infrared spectra (Figure 6c) or on the ER curve (Figure 7). At the end of the experiment (Figure 5c), the acetate groups are still grafted on the surface although part of them have been eliminated. Modifications of the surface hydroxyl groups brought by water are similar to those observed in the case of a “clean” Ti% surface. It can also be noted that the intensities of thev(OH) bands are weaker at the end of the experiment than after the sample activation. This is due to the reaction leading to acetate formation as explained above. Response of rhe HMDS-modifiedTiOzSurface The modification of the surface caused by the hexamethyldisilazane (HMDS) grafting has been discussed earlier (18-20). To summarize, HMDS reacts with the surface hydroxyl groups to
d
3000 2500 Wavenumber (cm-l) Figure 8. Infrared spectra of the Ti@ sample: (a) after activation; (b) after HMDS addition and subsequent desorption at 450°C; (c) after addition of the CO and [CO+HzO] doses; (d) difference spectrum b-a.
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form ammonia which is eliminated during the thermal desorption. The Ti-OH groups transform into Ti-0-Si(CH3)s groups, but the grafting is selective as not all the OH groups are transformed. When water is added at high temperature to this HMDS-modified surface, Si-OH groups are generated on the titania surface. Their v(OH) stretching modes absorb at the characteristic 3736 cm-’ frequency (shifted to 3730 cm-’ at 4OO’C). The spectrum of the HMDS-grafted titanium oxide desorbedat 450°C is presented in Figure 8b and compared with that of the activated TQ surface (Figure 8a,d). The perturbation of the O&I absorption range is obvious and the v(CI-I) stretching bands are noted at 2973 and 2914 cm-l in addition to the very intense &(CHs) band at 1267 cm-l. Due to its strong intensity, this latter band can be used as a marker of the persisting presence of the grafted HMDS. It is worth noting that the HMDS grafting followed by thermal desorption at 450°C increases the reduction of the titanium oxide surface, probably because of the presence of ammonia formed during the grafting process. The Em evolution during the CO additions (Figure 9) shows the strong reduction caused by the first dose. During the introduction of the four CO doses, no perturbation is noted on the spectra and the C@ absorption bands are hardly visible (Figure 1Oa).The overall variation is a very slight increase of the OH band intensities which might be due to the small amount of water contained in gaseous CO (less than 20 ppm) (Figure lOa). The original oxidation state of the surface is not restored by evacuation (Figure 9). But, unlike the previous two cases (“clean” and AA-modified surfaces), the “addition-evacuation” sequence of the three subsequent CO doses leads to a noticeable oxidation of the surface possibly due to the original strong reduction state caused by the HMDS grafting, tlms implying an electronic rearrangement which may look like an oxidizing effect. The addition of the first two [CO+H20] doses amplifies the oxidation effect (Figure 9). At this step, the increase of the 3730 cm-l band on the IR spectrum is noted and corresponds to the creation of new S&OH groups as explained above (Figure lob). Then, for the subsequent two
” 2300 Z 2100
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Time (min) Figure 9. Em variations versus CO and [CO+H20] “addition-evacuation” sequences for the HINDS-modified TioZ sample.
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Figure 10. Evolution of the H&IDS-modified Ti@ IR spectrum: (a) after addition of the first CO dose; (b) after addition of the first [CO+H20] dose; (c) after addition of the fifth CO dose (the spectrum of HMDS-modified Ti@ at 4OO’Chas been subs&acted).
[CO+H20] doses, the reducing action becomes preponderant. This time, the only effect observed on the S&OH groups is a very weak but irreversible increase of the v(OH) band intensity along with a reversible frequency shift (2 cm-‘)of the 3730 cm-l band (not shown). This shift can be explained by a perturbabtion of the Si-OH environment caused by the presence of surrounding molecules. When :pure CO is again added, the reversible shift of the 3730 cm“ band persists and the presence of C@ is detected at 2348 cm-’ (Figure 1Oc). Surprisingly, the Em evolution shows a perfectly reproducible response and no drift of the baseline, that is no change in the oxidation state of the surface after the “CO addition-evacuation” sequence (Figure 9). The formation of COr is much more visible on the IR spectra than after the first four CO doses (Figures lOa,c). At the end of the experiment, it is checked that HMDS is still grafted on the surface as proven by the v(CH) and &(CH3) bands.(Figure 8~). These bands are slightly weaker and slightly shifted toward higher wavenumbers proving that changes have occurred in the grafted species. Indeed, the hydroxyl group absorption range is modified by the formation of Si-OH groups whose v(0I-I) stretching band is clearly seen at 3736 cm-l as noted during the course of the experiment. Discussion
The different behaviors of the non-grafted and grafted Ti@ samples, particularly toward humidity, reflect the difference in the chemical nature of the surface hydroxyl groups. To some
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extent, the non-graftedand AA-grafted surfacesare comparable since the chemical nature of these surface hydroxyl groups is not modified by acetic acid. Even though acetic acid reacts with the OH groups, thus decreasing their relative concentration on the surface, the OH groups left behind are basically of the same nature as that of the original OH groups existing before the grafting. We have indeed notedthat the OH groups modifications brought by the [CO+H20] doses on the AA-modified surface are similar to those observed on the “clean” surface. However, since acetic acid is expected to preferentially react with OH basic groups, the relative amounts of acidic and basic hydroxyls are changed with respect to the original surface. Therefore, when water is adsorbed on the AA-modified surface, there should be a higher probability that water molecules react with H+ ion (acidic hydroxyl groups) compared to the case of the “clean” surface. This is an explanation of the different response of the AA-modified Ti@ surface toward humidity. The HMDS grafting on the Ti@ surface causes both a strong decrease of the surface hydroxyl amount and a reduction of the sample. The oxidizing effect of the first two [CO+H20] doses is expected because of this strong reduction state. Moreover, after these [CO+H20] additions, the chemical nature of the hydroxyl groups is drastically changed since new SiOH groups are formed whose acido-basicity should be close to that of the OH groups on a silica surface, that is quite different from that of the TiOH groups. The reproducibility of the response toward the second sequence of CO additions indicates a stabilizing surface effect of HMDS but its complete mechanism is still unknown. The presence of SiOH silanol groups formed during the addition of the [CO+H20] doses may play a key role. We have noted that these SiOH groups are slightly and reversibly perturbed by the CO additions. Although the elimination by evacuation of the formed gaseous (2% should irreversibly bring electrons in the conduction band, the reproducibility of the response gives rise to the question of surface regeneration which at this point is still unanswered. CONCLUSION This work has demonstrated that Fourier transform infrared surface spectrometry is an investigation tool whose performances go far beyond the basic chemical analysis of regular materials. Indeed, it can be successfully applied to the investigation of nanosized particles used for various high-added-value applications. In addition, this technique simultaneously allows the analysis of the surface species perturbations and the study of the electrical conductivity variations under the same conditions. In the field of gas sensing, our reported series of experiments on a nanosized TiO2 powder have proven that the chemical composition of the first atomic layer plays a critical part in the sensing mechanism. In a dry and oxygen-free atmosphere, whatever the surface acido-basicity of a “clean”TiO;?, an acetic acid-grafted Tie and an HMDS-grafted Tiq, it has been demonstrated that pure CO always has a reducing effect. On the contrary, when CO is in presence of humidity, we have observed an oxidizing or reducing effect depending on the surface chemical species adsorbed on Tie, thus on the surface acido-basicity. These experiments also prove that the lack of control in the surface chemical composition will imply a lack of reproducibility of the sensor response. Since the electrical behavior of semiconductors is very dependent on the surrounding atmosphere, forthcoming studies on the oxygen effects will bring more information. In the light of the above results, the humidity effects will be particularly analyzed because the presence of a third gas may have drastic consequences on the kinetics of the electron transfer.
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REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
MRS Pnweedings Symposium Series, Vol. 501, eds. K.E. Gonsalves, M.-I. Baraton et al., MRS Publisher, Warrendale, 1998. Prutton, M., Introducticbt to Sutfbce Physics, Oxford Science Publications, Oxford,1994. Gopel, W.,Hesse, J. and Zen&, J.N. (eds), Sensors -A Comprehensive Survey, Chemical Sensors, Vol. 7, Verlag Chemie, Weinheim, 1990. Clifford, PK., Ph. D. Thesis, Carnegie-Mellon University, 1981. Heir&h, V.E. and Cox, P.A., TheSugace Science of Metal Oxides, Cambridge University Press, 1994. Baraton., M.-I., Sensors &Actuators, 1996, B 31,33. Harrick, N.J., Internal Reflection Spectroscopy, Interscience, Wiley, New York, 1967. Second printing by Harrick Scientific Corporation, Ossining, N.Y. 1979. Baraton, M.-I., in Nanostructured Materials: Science and Technology, NATO Advanced Study Institute, Kluwer Academic Publishers, Dordrecht, 1998, p. 303. Morterra, C., Journal Chemical Society, Faraday Transactions 1, 1988,84,1617. Morrow, B.A., in Spectroscopic Characterization of Heterogeneous Catalysis, ed. J.L.G. Fierro, Elsevier Science Publishers, Amsterdam, 1990, p. A161. Busca,G., Saussey, H., Saur, O., Lavalley, J.C. and Lorenzelli, V.,Applied Catalysis, 198514,245. Tsyganenko, A.A., Pozdnyakov, D.V. and Fihmonov, V.N., Journal ofMolecular Structure, 1975, 29,299. Xu, C. and Koel, B.E., Journal of Chemical Physics, 1995,102,8158. Prime& :M., Pichat, P and Mathieu, M.V., Journal of Physical Chemistry, 1971.75, 1216. Cocks, I.D., Guo, Q., Patel, R., Williams, E.M., Roman, E. and de Segovia, J.L., Surface Science, 1997,377, 135. Wang, L.Q., Ferris, K.F., Shultz, A.N., Baer, D.R. and Engelhard, M.H., Surface Science, 1997, 380, 35:!. Pei, Z.E and Ponec, V., Applied Surface Science, 1996, 103, 171. Baraton, M.-I., Chancel, F. and Merhari, L., Nanostructured Materials, 1997,9,319. Baraton, M.-I, Merhari, L., Chancel, F. and Tribout, J., MRS Proceedings Symposium Series, Vol. 448, eds. S.M. Prokes, S. Brierley et al., MRS Publisher, 1997.81. Chancel,F.,Tribout, J. andBaraton,M.-I.,MRSProceedingsSymposiumSeries,Vol.50l,eds.K.E. Gonsalves, M.-l. Baraton et al., MRS Publisher, Warrendale, 1998, p. 89.
in
NanoSlruduredMaraials. Vol. 10. No. 5, pp. 699-713.1998 Elsevia Science Ltd 8 1998 Ati MetallurgicaInc. Printed in the USA. All lights rea.zwd 09659773/98 $19.00 + .oO
PI1 SO965-9773(98)00108-l
SURFACE PROPERTY CONTROL OF SEMICONDUCTING METAL OXIDE NANOPARTICLES M.-I. Baratont and L. Merhari2 ‘LMCTS-ESA 6015 CNRS, Faculty of Sciences, 87060 Limoges, France, [email protected] *CERAMEC, 64 Avenue de la Liberation, 87000 Limoges, France (Accepted June 15,1998) Abstract-The comprehension of the underlying mechanism attributing either a reducing or oxidizing ef$ect to humidity on the metal oxides used in gas sensors is still a challenge as the literature on this subject shows contradictory results. The high surface-to-bulk ratio of nanosized powders conferring them a much higher surface reactivity compared to their micronsized counterpart, clearly helps in a better understanding of the gas-surface interactions. A thorough FTIR sulfirce study of a nanosized titanium dioxide powder subjected to various sequences of carbon monoxide and mixtures of carbon monoxide and water vapor is reported. The effect of acetic akid and hexamethyldisilazane grafting on TiO2 is studied under the same conditions. It is found that water molecules have a reducing or oxidizing effect, depending on the acido-basic@ of the metal oxide. Our study clearly shows that a thorough knowledge of the surface reactivity of metal oxides is aprerequisiteforfurther majorgas sensing optimization. Ol998ActaMetallurgica Inc. INTRODUCTION
When properly synthesized, handled and processed, nanocrystalline materials show superior properties in a wide range of applications including sit&ring, catalysis, microelectronics, optoelectronics, magnetism, medicine, etc. It has been realized lately that for these materials which exhibit a high surface-to-b&ratio, the chemical composition of the first atomic layer is of utmost importance in the macroscopic properties of the final product (see for example ref. 1). Control and tailoring of the surface of nanocrystalline materials are the key to solving problems currently hindering the development of nanoparticle-based materials. Their electrical properties also critically depend on the chemical composition of the surface. It is indeed well-known that surface defects as well as impurities adsorbed on the surface sites can greatly modify the work-function of semiconductors by changing the affinity of their surface for electrons (2). In the rapidly growing field of gas sensing for indoor or outdoor air quality monitoring, humidity is known to have adverse effects on the response of metal oxide-based sensors (3-5). In these chemical gas sensors, the role played by the first atomic layer of the sensing material is preponderant(3,6). Asaconsequence,thesensitivityandtheelectricalcharacteristicsof thesensor depend on the chemical composition of this first atomic layer. The aim of this work is to show how
700
M-l BARATON ANDL MER~RI
the sensor response can be tailored by impurities purposely adsorbed on the surface. The sensor was simulated by a pellet made of pressed titanium oxide nanosized powder. However, we must underline here that we are not considering the TiO;! pellet as a gas sensor operating under its standard conditions since oxygen-free environments will be used. Indeed, we intend to highlight the surface reactions and to understand their mechanism so as to extend this knowledge to the very complex case of a real gas sensor in an atmosphere whose exact composition is unknown. The challenge was to obtain simultaneous results on the surface chemical composition and the electrical variations in presence of various adsorbates. Most of the experimental techniques providing chemical surface analyses of materials actually probe a depth ranging from a few nanometers to ten nanometers. Under these conditions, it is obvious that the so-determined chemical composition is only an average over several atomic layers and cannot resolve the very first atomic layer. Under specific experimental conditions, Fourier transform infrared (FAIR) spectrometry is a powerful tool to characterize the very first atomic layer because the surface chemical species generated during the nanostructured material synthesis process and/or generated by surrounding contaminants can be identified as well as reactive sites in anon destructive manner. These surface groupsand the reactive sites as well can be modified in situ by controlled adsorption of molecules (referred to as grufting in the literature) while the resulting modifications of the surface properties can be continuously followed in the same cell by FTIR spectroscopy. On the other hand, independently from fundamental vibrational studies, some semiconducting properties can be deduced from the infrared spectra without requiring electrical contacts (7) since the variations of the infrared energy transmitted by a sample are related to the variations of the electrical conductivity of this sample according to the Drude-Zener theory. Therefore, the FTIR spectrometry allows the simultaneous study of the electrical conductivity variations and the surface chemical species perturbations under various gaseous environments. EXPERIMENTAL The titanium dioxide (P25) was supplied by the Degussa company (Germany). The BET specific surface area given by the supplier is 50 m2gm1corresponding to an average particle size of 21 nm. The anatase phase was preponderant although mixed with about 30% of rutile phase. To perform the experiments, the powder (80 mg per sample) was slightly pressed on a stainless grid in order to ensure a homogeneous temperature distribution. The resulting titanium dioxide pellets were placed into the furnace of a homemade vacuum cell. This cell enables in situ studies at different temperatures (from room temperature to SOO’C) under vacuum or controlled pressures of gases and liquid vapors through a valve system. The cell was placed in the sample compartment of a Perkin-Elmer Spectrum 2000 spectrometer equipped with a MCI cryodetector. The analyzed range varied from 7800 to 450 cm-l with a 4 cm-’ resolution. The infrared energy transmitted by the titanium oxide pellet was measured over the whole infrared range, even though the infrared spectra presented in the following are limited to the wavenumbers ranges of interest. To emphasize the modifications undergone by the sample, some difference spectra are presented. In these spectra, the positive bands correspond to appearing or increasing species during the experimental step under consideration, whereas the negative bands correspond to disappearing or decreasing species. In the difference spectra, attention must be called to frequency shifts which appear as negative and positive double bands at very close
SURFACE PROPERTY CONTROL OFSEMKXNDVCTING METAL 0x1~ NANOPART~CLES
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frequencies. Therefore, when analyzing a difference spectrum, the original spectra must also be checked to confii the bands assignments to decreasing or increasing species or frequency shifts. Prior to measurements, the titanium dioxide pellet was activated, that is heated up to 450°C under dynamic vacuum and cooled down to room temperature while still under dynamic vacuum. This thermal treatment cleans the surface from all physisorbed and weakly chemisorbed species. The species removed during the activation are mainly water and scarce carbonate groups originating from the atmospheric and environmental contaminations. After the activation treatment, the sample was heated to the temperature of analysis under dynamic vacuum to keep the surface as clean as possible. It is worth noting that, after the activation treatment, the surface is no longer in equilibrium. Therefore, as soon as molecules are in contact with the activated surface, they adsorb on and/or possibly react with the surface sites, leading to new chemical species which may be irreversibly bound. Taking advantage of this irreversible adsorption, we modified theTi& surface by adding a few mbars of various liquid vapors to the activated sample at room temperature. The vapor in excess was eliminated by an evacuation at room temperature followed by a desorption at 450°C to remove all the reversible species. The infrared spectrum recorded after cooling at room temperature allowed to check whether the surface is irreversibly modified and to possibly determine the kind of surface sites that are poisoned by the chemisorption. Carbon Imonoxide was supplied by Air Products (Hz0 content less than 20 ppm). Water was bi-distilled and de-ionized. Hexamethyldisilazane (HMDS) from Fluka Chemie (99% pure) and acetic acid from Prolabo (Normapur) were used with no further purification.
RESULTS AND DISCUSSION Surface Characterization of the Titanium Oxide Nanopowder
The FflR spectrometry is particularly relevant to the study of surface species on nanosized powders (8). Moreover, the adsorption of probe-molecules at room temperature on an activated surface brings evidence of its acidic and basic sites. The characterization of the titanium oxide surface will not be presented here in details, but results can be found in references 9-12. As an example, Figure 1 shows the spectra of the Ti& pellet under vacuum before (Figure la) and after activation (Figure lb). The disappearing of the very intense band centered around 3320 cm-’ obviously indicates the elimination of adsorbed water. At the end of the activation treatment, several bands in the 3780-3600 cm-’ range are due to the v(0I-l) stretching mode of different types of hydroxyl groups bonded to titanium atoms of the first atomic layer. The number and the frequencies of these hydroxyl groups depend on the extent of the dehydroxylation and on the presence of impurities (9,lO). Therefore, it is very difficult to correlate all the results found in the literature about Tia surface characterization. It is generally admitted that titanium oxide strongly retains adsorbed undissociated water due to the strong Lewis acidity of the coordinatively unsaturated Ti4+ surface sites (10). In addition, a charge separation can occur in these coordinated water molecules because of the surface ionicity (13). This charge separation strengthens the formation of hydrogen bonds between the coordinated water molecules, thus stabilizing their link onto the surface. The remaining bound water molecules are indicated by the broad band at 3480 cm-’ assigned to the v(OH) stretching mode in perturbed hydroxyl groups and by the band at 1620 cm-‘assigned to the &OH) bending mode in water molecules. The 37153670-3676 and 3645
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M-l BARATON ANDL MERHARI
I
3500
I
3000
I
2500
I
2000
I
1500
Wavenumber (cm-l) Figure 1. Infrared spectra of Ti@: (a) before activation; (b) after activation; (c) after addition of the CO and [CO+H20] doses.
cm-’ absorption bands have been assigned by several authors (10,14) to thev(OH) modes of TiOH groups on different faces of anatase and in different coordinative unsaturation states. These TiOH groups are either acidic or basic.
Simulation of the Chemical Sensor and Experimental Procedure
According to the Drude-Zener theory, the infrared energy absorbed by a semiconductor is essentially due to surface states and to free carriers and, therefore, it is related to the sample electrical conductivity (7). By measuring the infrared energy transmitted (Em) by a titanium oxide pellet, it is then possible to follow the variations of the electrical conductivity versus different gaseous environments. These variations are precisely the response of the Ti& sensor under the considered gases. In addition, the infrared spectra give access to the in situ analysis of the surface chemical modifications during the gas exposures and possibly to information on the redox mechanism. It is worth noting that the infrared energy variations give a picture of the “true” conductivity evolution of the material, that is without perturbation due to electrodes whose metal can diffuse in the sensitive layer. In the case of n-type semiconductors, oxidizing adsorbates lower the IR absorption (En increases) whereas reducing adsorbates increase it (Em decreases). To make the Ti@ pellet simulate an operating sensor, the following procedure was conducted. Adose of gas and/or liquid vapor (CO or mixture of CO and H20) was introduced inside the cell for 10 minutes followed by an evacuation for 5 minutes. The “addition-evacuation”
%JRFACE
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sequence was repeated as often as necessary. Infrared spectra were recorded at the end of the 10 minute-addition and at the end of the 5 minute-evacuation. ER was noted before and after the spectrum recording (2 minutes interval) to minimize the errors. The admitted gas pressures were set to 6 mbar of pure CO, 7 mbar of mixture of CO and H20 (6 mbar CO and 1 mbar H20). Usually, the commercial chemical gas sensors based on titanium oxide powder have a working temperature’close to 400°C. Therefore, we performed our simulation experiments at this temperature. The tremendous problem with these sensors is the sensitivity toward humidity which leads to adverse effects in the CO sensing mechanism. The phenomena are extremely complex because water can act as a reducing or an oxidizing agent with an apparent lack of reproducibility (4). Since the surface hydroxyl groups seem tobe somehow involved in the humidity effects (4,5), we modified these OH groups in order to study the influence of these surface modifications on the humidity response. For this purpose, two molecules were chosen with different reactivities: acetic acid (CHYXOH) and hexamethyldisilazane ([(CH$$i]2NH). Acetic acid strongly reacts with oxide surfaces by forming acetate species through the basic OH groups (13,15-17). The acetate groups can bridge on the surface Lewis acid sites, such as Ti4+. As for hexamethyldisilazane (HMDS), it reacts with OH groups yielding ammonia (18). The schematic reactions can be written as follows, keeping in mind that the grafting may be complicated by interactions with other surface sites such as biwis acids or bases. Ti- .0H+CH3C-
OH+Ti-0-CCH3+H20
s>
d
2 Ti-OH + [(CH&Si]zNH + 2 Ti-OSi(CH3)3
+NH3
To analyze the electrical conductivity variations of the TioZ pellets in dry or humid CO environments and to possibly detect memory effects, the main sequence of experiments consisted in the introduction of 4 doses of pure CO (6 mbar), followed by 4 doses of a CO and Hz0 mixture (7 mbar, referred to as [CO+H20] mixture in the following) and again followed by 4 doses of pure CO (6 mbar). As a remark, it is worth mentioning that the addition of any gas in the cell should theoretically contribute to a ER decrease. However, due to the small number of CO molecules (weak pressure), the energy variations caused by the CO addition was checked to be below the spectrometer detection limits that is less than 0.1% in absence of Ti@ pellets. For the same reasons, no energy variations are noted when 1 to 5 mbar of Hz0 are admitted inside the cell. Response of the “Clean “ TiOzSurjace The first series of experiments was performed at 400°C on an activated TiO2 surface (n-type semiconductor). The addition of the fust CO dose caused aEm (Figure 2) decrease corresponding to the CO redulcing effect, thus implying an increase of the electrical conductivity. The formation of gaseous CO2, according to the following well-known possible reactions:
co+o2-+co2+2e-
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.+
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l +\
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80
100
160
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Time (mm) Figure 2. ER variations versus CO “addition-evacuation” sequences for the “clean” Ti@ sample.
a
I
3000 Wavenumber (cm- 1) Figure 3. Evolution of the TiO2 IR spectrum: (a) after addition of the first CO dose; (b) after addition of the first [CO+H20] dose; (c) after addition of the fifth CO dose (the spectrum of Ti& at 400°C has been subs&acted).
IO
is proved by the absorption bands centeredat 2348 cm-‘, clearly visible on the difference spectrum (Figure 3a). Concomitantly, negative bands are noted in the 3720-3650 cm“ region showing either a decrease of the surface hydroxyl groups or their perturbation by the surrounding CO and Cti
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Time (min) Figure 4. EIR variations versus CO and [CO+HzO] “addition-evacuation” sequences for the “clean” Ti@ sample.
molecules. After evacuation, Em does not get back to its originalvalue (Figure 2). On the spectrum, only slight changes are noted in the v(0I-I) bands (3750-3600 cm-’ region) (not shown). This CO “addition-evacuation”sequence was repeated 12 times. The ER curve (Figure 2) indicates a steady overall reduction of the sample simultaneously with a weakening of the amplitude of the Em variation during the CO-addition period (in the following, the amplitude of the ER variation will be referred to as the response, by analogy with the sensor response). This can be explained by the steady removal of the surface oxygen atoms available for the Co2 formation. It must be noted that under our oxygen-free experimental conditions, the Tio2 sample is already reduced at the beginning of the experiment and that, in addition, each CO dose contributes to a further reduction of the surface with no possibility for surface regeneration. However, if the formation of COZ was the only mechanism involved in the electrical conductivity changes, the Em increase under evacuation would not be observed. Indeed, by removing the newly formed COz,,the evacuation irreversibly removes oxygen ionosorbed on the surface and, therefore, irreversibly adds electrons in the conduction band. The partial reversibility, that is, the weak Em increase under evacuation shows that another mechanism may be involved implying a reversible and weaker interaction between CO and the Ti% surface. On the infrared spectra, a reproducible perturbation of the surface hydroxyl groups each time CO is added, may be related, at least partially, to [this reversible part of the electrical changes. The second series of experiments on Ti@ consisted in the introduction of four CO doses followed by four doses of the [CO+H20] mixture and then by four new CO doses (Figure 4). The response toward the first four CO doses is obviously similar to that just described above. The addition of the [CO+H20] mixture causes a strong decrease of the electrical conductivity although without reproducibility. Only the fourth dose leads to a weak conductivity increase. In parallel, the infrared spectra show the intensity increase of thev(OH) absorption range (Figure 3b). The overall effect of the four [CO+H20] doses is an oxidation.
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blATON
AND L kRHARl
When pure CO is addedagain, the E~evolution (Figures4) is similar to that observed during the addition of the first CO doses. Moreover, the baseline drift shows the same downward trend, thus indicating an overall reducing effect. At the end of the experiment, the sample was cooled under dynamic vacuum and compared with the activated sample (Figure 1c). The major change in the IR surface spectrum of Ti@ is the increase of the v(OH) band at 3720 cm‘*. These new hydroxyl groups have been created by the addition of water vapor. They reflect modifications of the surface structure and particularly distribution changes of the titanium atoms in the surface sites. Response of theAcetic Acid-ModifiedTiO2Surface Acetic and formic acids are known to strongly react with oxide surfaces yielding acetate or formate ions irreversibly bound. It is also possible that decomposition of acetate or formate species at high temperature may oxidize the Tio2 surface through CO production (16). To check the effects of the acetate groups considered as impurities, 4 mbar of acetic acid (AA) were added to the activatedTie pellet. Figure 5 compares the infrared spectra of the sample
I
I
3000 2500 Wavenumber (cm- 1)
I
I
2000
1500
Figure 5. Infrared spectra of the Ti@ sample: (a) after activation: (b) after acetic acid addition and subsequent desorption at 45O’C; (c) after addition of the CO and [CO+H20] doses; (d) difference spectrum b-a.
SURFACEPFXYERTYCOMR~L OF SEMIWN~~~TINGMETALOxm NANOPA~CES
707
before addition (Figure 5a) and after evacuation at 450°C (Figure 5b) along with their difference (Figure 5d). These spectra show that AA is irreversibly bound as acetate species and that the hydroxyl grolups on the TioZ surface are involved in the reaction. On the difference spectrum, it is easier to see the bands corresponding to the v&X0) vibration at 1538 cm-l and thev&OO) vibration at 1.446 cm-‘. The 1352 cm-l band is assigned to the g(CH$ bending mode. The Av splitting of the v&00) andv,(COO) frequencies indicates that these acetate species are bound in the bidentate geometry which is in agreement with their thermal stability (17). A close look at the 1538 and 1446 cm-l bands reveals that they are complex because several kinds of bidentate acetate species can simultaneously exist. Indeed, the link of the acetate ion on the surface in the bidentate geolmetryinvolves both hydroxyl groups and Lewis acid sites. At least three types of OH groups are involved since three main negative bands can be discriminated at 3720.3674 and 3643 cm-l. Moreover, several types of Lewis acid sites exist on the Tio2 surface including Ti3’ possibly produced by the reducing effect of the activation treatment. Therefore, it is easy to understand that the multiple possibilities combining both OH groups and Lewis acid sites imply slightly different coordination strengths for the acetate groups, thus resulting in slightly different v,,(COO) and v,(COO) frequencies. The first dose of CO added at 400°C to this AA-modified surface causes a slight shift of the v&DO) band toward the lower wavenumbers (not shown). This shift is not reversible after the
a
C
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4000
c-4
I
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3000 Wavenumber (cm- 1)
2500
20(IO
Figure 6. Rvolution of the AA-modified TioZ IR spectrum: (a) after addition of the first CO dose; (b) after addition of the first [CO+H20] dose; (c) after addition of the fifth CO dose (the spectrum of AA-modified Tiq at 4OO’Chas been subs&acted).
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CO evacuation. On the other hand, one notes a slight intensity decrease of the overall acetate groups and a reversible intensity decrease of the v(0I-I) absorption bands (Figure 6a). like in the case of the “clean”Ti@. Like the shift of thev,(COO) band, the modification of the acetate groups is not reversible after CO evacuation. The formation of a small amount of C@ is observed at 2348 cm-‘, proving that surface oxygen atoms are still available. As in the case of the non-modified surface, the surface is reduced by this first CO addition and Ti3+ ions may be formed to the detriment of theTi4+ ions. Therefore, the linkof the bidentateacetate groups onto the surface would be weakened in the new electronic distribution, thus causing the vS(COO) slight shift. The same effects are observed after the addition of the subsequent three CO doses, although weaker and weaker. Concomitantly, Em strongly decreases after the first dose addition due to the increase of the electricalconductivity of the sample (Figure 7). The subsequent three COdoses cause the same reduction. The overall evolution of Em under the four CO doses proves that, on the one hand, the response amplitude is roughly constant and, on the other hand, the sample is steadily reduced. A correlation with the decrease of the acetate groups, visible on the IR spectra, may be attempted considering that the acetate groups act as oxygen providers. When the [CO+H20] mixture is in contact with the surface, new hydroxyl groups are created around 3658 cm-’ (Figure 6b). No changes are noted for the two v(CO0) bands except a slight apparent band intensity increase due to the surface reduction. The formation of C@ is observed. Under evacuation, part of the newly formed OH groups are eliminated. The same modification of the OH groups is observed over the following three [CO+H20] doses but notably weaker. As already noted, the first dose of either CO or [CO+H20] has a more or less different effect than the following three doses. This is particularly obvious in the present case of [CO+H20] additions. The first dose leads to an oxidation of the sample (Figure 7) which can be related to new OH formation like in the case of a “clean”Ti@ surface. But, the effect of the subsequent three [CO+H20] doses
1200 , -
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900
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800
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700
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500 0
20
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60
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120
140
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Time (mm) Figure 7. Em variations versus CO and [CO+H20] “addition-evacuation” sequences for the AA-modified TiOz sample.
180
SURFACEPROPERTY GDNTIWLOF SEMIC~NDUCTING METALOXIDE NANOPARTICIES
709
is different since they lead to a reduction, A possible reason for these different behaviors may lie in the acidobasicity of the surface when water molecules strike it. Indeed, it has just been noted that acetic acid adsorption involves acidic hydroxyl groups thus changing the acido-basicity of the surface. As a consequence, the surface reactivity toward water is modified and the electron transfer may be reversed. The effect of the subsequent four CO doses is quite comparable to that of the first four ones, either on the infrared spectra (Figure 6c) or on the ER curve (Figure 7). At the end of the experiment (Figure 5c), the acetate groups are still grafted on the surface although part of them have been eliminated. Modifications of the surface hydroxyl groups brought by water are similar to those observed in the case of a “clean” Ti% surface. It can also be noted that the intensities of thev(OH) bands are weaker at the end of the experiment than after the sample activation. This is due to the reaction leading to acetate formation as explained above. Response of rhe HMDS-modifiedTiOzSurface The modification of the surface caused by the hexamethyldisilazane (HMDS) grafting has been discussed earlier (18-20). To summarize, HMDS reacts with the surface hydroxyl groups to
d
3000 2500 Wavenumber (cm-l) Figure 8. Infrared spectra of the Ti@ sample: (a) after activation; (b) after HMDS addition and subsequent desorption at 450°C; (c) after addition of the CO and [CO+HzO] doses; (d) difference spectrum b-a.
710
M-1 ~FIATON
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form ammonia which is eliminated during the thermal desorption. The Ti-OH groups transform into Ti-0-Si(CH3)s groups, but the grafting is selective as not all the OH groups are transformed. When water is added at high temperature to this HMDS-modified surface, Si-OH groups are generated on the titania surface. Their v(OH) stretching modes absorb at the characteristic 3736 cm-’ frequency (shifted to 3730 cm-’ at 4OO’C). The spectrum of the HMDS-grafted titanium oxide desorbedat 450°C is presented in Figure 8b and compared with that of the activated TQ surface (Figure 8a,d). The perturbation of the O&I absorption range is obvious and the v(CI-I) stretching bands are noted at 2973 and 2914 cm-l in addition to the very intense &(CHs) band at 1267 cm-l. Due to its strong intensity, this latter band can be used as a marker of the persisting presence of the grafted HMDS. It is worth noting that the HMDS grafting followed by thermal desorption at 450°C increases the reduction of the titanium oxide surface, probably because of the presence of ammonia formed during the grafting process. The Em evolution during the CO additions (Figure 9) shows the strong reduction caused by the first dose. During the introduction of the four CO doses, no perturbation is noted on the spectra and the C@ absorption bands are hardly visible (Figure 1Oa).The overall variation is a very slight increase of the OH band intensities which might be due to the small amount of water contained in gaseous CO (less than 20 ppm) (Figure lOa). The original oxidation state of the surface is not restored by evacuation (Figure 9). But, unlike the previous two cases (“clean” and AA-modified surfaces), the “addition-evacuation” sequence of the three subsequent CO doses leads to a noticeable oxidation of the surface possibly due to the original strong reduction state caused by the HMDS grafting, tlms implying an electronic rearrangement which may look like an oxidizing effect. The addition of the first two [CO+H20] doses amplifies the oxidation effect (Figure 9). At this step, the increase of the 3730 cm-l band on the IR spectrum is noted and corresponds to the creation of new S&OH groups as explained above (Figure lob). Then, for the subsequent two
” 2300 Z 2100
60
80
100
160
Time (min) Figure 9. Em variations versus CO and [CO+H20] “addition-evacuation” sequences for the HINDS-modified TioZ sample.
180
SURFACE
&XJt’EFiTY
&NT-L
CF SEMICCNOUCTING
bkTAL
&IDE
f’hNCPARTlCLES
711
C
3
s t*
4600
I
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1
3doo Wavenumber (cm-l)
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21 30
Figure 10. Evolution of the H&IDS-modified Ti@ IR spectrum: (a) after addition of the first CO dose; (b) after addition of the first [CO+H20] dose; (c) after addition of the fifth CO dose (the spectrum of HMDS-modified Ti@ at 4OO’Chas been subs&acted).
[CO+H20] doses, the reducing action becomes preponderant. This time, the only effect observed on the S&OH groups is a very weak but irreversible increase of the v(OH) band intensity along with a reversible frequency shift (2 cm-‘)of the 3730 cm-l band (not shown). This shift can be explained by a perturbabtion of the Si-OH environment caused by the presence of surrounding molecules. When :pure CO is again added, the reversible shift of the 3730 cm“ band persists and the presence of C@ is detected at 2348 cm-’ (Figure 1Oc). Surprisingly, the Em evolution shows a perfectly reproducible response and no drift of the baseline, that is no change in the oxidation state of the surface after the “CO addition-evacuation” sequence (Figure 9). The formation of COr is much more visible on the IR spectra than after the first four CO doses (Figures lOa,c). At the end of the experiment, it is checked that HMDS is still grafted on the surface as proven by the v(CH) and &(CH3) bands.(Figure 8~). These bands are slightly weaker and slightly shifted toward higher wavenumbers proving that changes have occurred in the grafted species. Indeed, the hydroxyl group absorption range is modified by the formation of Si-OH groups whose v(0I-I) stretching band is clearly seen at 3736 cm-l as noted during the course of the experiment. Discussion
The different behaviors of the non-grafted and grafted Ti@ samples, particularly toward humidity, reflect the difference in the chemical nature of the surface hydroxyl groups. To some
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extent, the non-graftedand AA-grafted surfacesare comparable since the chemical nature of these surface hydroxyl groups is not modified by acetic acid. Even though acetic acid reacts with the OH groups, thus decreasing their relative concentration on the surface, the OH groups left behind are basically of the same nature as that of the original OH groups existing before the grafting. We have indeed notedthat the OH groups modifications brought by the [CO+H20] doses on the AA-modified surface are similar to those observed on the “clean” surface. However, since acetic acid is expected to preferentially react with OH basic groups, the relative amounts of acidic and basic hydroxyls are changed with respect to the original surface. Therefore, when water is adsorbed on the AA-modified surface, there should be a higher probability that water molecules react with H+ ion (acidic hydroxyl groups) compared to the case of the “clean” surface. This is an explanation of the different response of the AA-modified Ti@ surface toward humidity. The HMDS grafting on the Ti@ surface causes both a strong decrease of the surface hydroxyl amount and a reduction of the sample. The oxidizing effect of the first two [CO+H20] doses is expected because of this strong reduction state. Moreover, after these [CO+H20] additions, the chemical nature of the hydroxyl groups is drastically changed since new SiOH groups are formed whose acido-basicity should be close to that of the OH groups on a silica surface, that is quite different from that of the TiOH groups. The reproducibility of the response toward the second sequence of CO additions indicates a stabilizing surface effect of HMDS but its complete mechanism is still unknown. The presence of SiOH silanol groups formed during the addition of the [CO+H20] doses may play a key role. We have noted that these SiOH groups are slightly and reversibly perturbed by the CO additions. Although the elimination by evacuation of the formed gaseous (2% should irreversibly bring electrons in the conduction band, the reproducibility of the response gives rise to the question of surface regeneration which at this point is still unanswered. CONCLUSION This work has demonstrated that Fourier transform infrared surface spectrometry is an investigation tool whose performances go far beyond the basic chemical analysis of regular materials. Indeed, it can be successfully applied to the investigation of nanosized particles used for various high-added-value applications. In addition, this technique simultaneously allows the analysis of the surface species perturbations and the study of the electrical conductivity variations under the same conditions. In the field of gas sensing, our reported series of experiments on a nanosized TiO2 powder have proven that the chemical composition of the first atomic layer plays a critical part in the sensing mechanism. In a dry and oxygen-free atmosphere, whatever the surface acido-basicity of a “clean”TiO;?, an acetic acid-grafted Tie and an HMDS-grafted Tiq, it has been demonstrated that pure CO always has a reducing effect. On the contrary, when CO is in presence of humidity, we have observed an oxidizing or reducing effect depending on the surface chemical species adsorbed on Tie, thus on the surface acido-basicity. These experiments also prove that the lack of control in the surface chemical composition will imply a lack of reproducibility of the sensor response. Since the electrical behavior of semiconductors is very dependent on the surrounding atmosphere, forthcoming studies on the oxygen effects will bring more information. In the light of the above results, the humidity effects will be particularly analyzed because the presence of a third gas may have drastic consequences on the kinetics of the electron transfer.
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REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
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