Parts per Million Water in Gaseous Vapor Streams Dramatically

Article pubs.acs.org/JPCC

Parts per Million Water in Gaseous Vapor Streams Dramatically Accelerates Porous Silicon Oxidation Randi E. Deuro, Joseph P. Richardson, Justin M. Reynard, Caley A. Caras, and Frank V. Bright* Department of Chemistry and Materials Science and Engineering Program, Natural Sciences Complex, University at Buffalo, The State University of New York, Buffalo, New York 14260-3000, United States S Supporting Information *

ABSTRACT: Substantial research has focused on exploiting and understanding porous silicon (pSi) photoluminescence (PL) for applications in areas ranging from chemical sensing to solid-state lighting. At ambient temperature, pure H2O is well-known to slowly (over a time scale of hours to days) and irreversibly oxidize as-prepared pSi (ap-pSi) to form oxidized pSi (ox-pSi). In this paper, we report that the apparent ap-pSi to ox-pSi oxidation rates can be orders of magnitude faster in the presence of nonaqueous vapor streams that contain just ppm H2O levels. When H2O is removed from the nonaqueous vapor stream, ap-pSi oxidation ceases. The nonaqueous analyte vapors serve as a vehicle to transport H2O directly into the hydrophobic, ap-pSi matrix where the H2O then oxidizes the ap-pSi leading to ox-pSi, permanently changing the pSi PL and surface chemistry. The ap-pSi oxidation rate is much faster in the presence of nonaqueous vapors because H2O transport into the pSi matrix is no longer limited by H2O slowly percolating−oxidizing−percolating through the ap-pSi matrix.

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INTRODUCTION Porous silicon (pSi)1−5 has been used to develop electroluminescent displays,6 lasing materials,7 optoelectronics,8 photodetectors,9 and chemical sensors.1,9−15 The hope of integrating pSi platforms within other Si-based technologies remains a major driving force for research activity in these areas. Previously, researchers have shown that the pSi photoluminescence (PL) emission maxima, emission quantum yield, and excited-state luminescence lifetimes depend on the Si nanocrystallite feature size and its surface chemistry.1−5,16−21 In gaseous analyte sensing, researchers have reported that the PL from hydrogen-passivated, as-prepared pSi (ap-pSi) can be analyte dependent, and in some cases, it exhibits a reversible PL response.22−25 However, other researchers have reported irreversible PL responses in similar circumstances following analyte introduction.25−27,29,30 The PL instability in ap-pSi is well-known due to nonradiative defects at the hydrogen-terminated surface.31−33 Further, under ambient conditions ap-pSi can become oxidized by air and H2O exposure to form oxidized pSi (ox-pSi). The oxidation of ap-pSi surfaces by H2O has been studied extensively.15,22,28,31−33 Near-ambient conditions H2O slowly oxidizes the ap-pSi surface species (i.e., SiHx (x = 1−3)) to form new species (i.e., Si−O−Si, OySiH (y = 1−2), and SiOH) over a time period of several hours to days.22,28,31,33 Further, ox-pSi exhibits substantially different PL characteristics in comparison to ap-pSi.34−37 In this paper, we use infrared spectroscopy to follow the appSi to ox-pSi oxidation process when ap-pSi is subjected to nonaqueous vapor streams in the presence of small quantities of H2O. We demonstrate that nonaqueous vapors act as a vehicle to transport H2O into the ap-pSi matrix and effectively © 2012 American Chemical Society

lead to a dramatic increase in the apparent ap-pSi to ox-pSi oxidation rate. Nonaqueous vapor mediated, H2O-induced appSi oxidation occurs at detectable levels after 99.7%) (Sigma-Aldrich); ethanol (ACS grade 99.9%) (EtOH) (Pharmco); and toluene (T) (99.9%, 650 nm) when ap-pSi is challenged by small plugs of T, ACN, MeOH, and ACT vapor under ambient introduction conditions. (Note: in these particular experiments we use a temperature above each liquid boiling point to minimize the chances of species in the vapor phase permanently adsorbing to the pSi matrix. Similar profiles are seen at room temperature; the full width at half-maximum is broader.) Inspection of these data illustrates several key points. First, some PL responses are reversible (e.g., T) while others are not (e.g., ACN, MeOH, and ACT). Second, prior to vapor introduction the pSi PL response is stable. This suggests that photooxidation is not significant under the illumination conditions used in these experiments. Additional control experiments with intermittent excitation and/or lower excitation beam fluence were largely indistinguishable from the results shown in Figure 1. Third, in those cases where reversibility is poor, the time scale for onset is on the order of the peak width (90%) associated with sample transfer to and from the FT-IR spectrometer. As such, there is essentially no “vapor” induced ap-pSi oxidation from T vapors under ambient vapor introduction conditions or from T, ACN or MeOH under inert vapor introduction conditions.) Figure 4 presents typical FT-IR spectra across the SiHx (x = 1−3) and OySiH (y = 1−2) stretching and bending regions for ap-pSi before and after a 2 h continuous challenge by T, ACN, MeOH, and ACT under ambient (top row) and inert (bottom row) vapor introduction conditions. Figure 3B presents a typical FT-IR spectrum (black solid trace) and curve fitted spectra (colored traces) for an ap-pSi sample after being 23171

dx.doi.org/10.1021/jp308097d | J. Phys. Chem. C 2012, 116, 23168−23174

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though “dry” liquids were used for these experiments, ACN, MeOH, and ACT in the ambient vapor introduction experiments will all absorb additional H2O from the environment, creating dilute solutions of H2O dissolved in the vapor stream. We speculate that this small quantity of dissolved H2O in the vapor stream is then rapidly carried by the nonaqueous vapors into the majority of the ap-pSi network, and the latent H2O then causes rapid oxidation of the ap-pSi network following the known H2O oxidation mechanism.28,47 The most dramatic differences between ambient and inert vapor introduction conditions are seen for ACT (cf. Figures 2 and 4). As such, we explored the effects of ACT, H2O loading, and vapor introduction conditions on the ap-pSi surface species (dis)appearance rates (Figure 5). Si−O−Si and OySiH (y = 1−

relative humidity, and ACT and 97% relative humidity. Small values are indicative of slow rates (growth or disappearance); larger values indicate faster rates. Figure 5B presents the rate ratio (i.e., ki/kdry air) which scales the species apparent rates to the corresponding baseline rate value measured in dry air. A rate ratio greater than unity reflects an increase in rate (growth or disappearance) for a given species under a particular set of conditions in comparison to the corresponding rate measured in dry air. The data in Figure 5 show that the apparent oxidation rates (slowest to fastest) exhibit the following general trend (p < 0.05): dry air < ambient air < dry ACT < humidity chamber < ambient ACT < ACT + humidity chamber. The apparent oxidation rate in dry air having the slowest rate is not particularly surprising because this vapor has the least amount of latent H2O. Surprisingly, ap-pSi in a humidity chamber (97% RH) oxidizes more slowly in comparison to ap-pSi in the presence of ambient ACT vapors or ACT vapors in a humidity chamber. This result was initially unexpected given the substantial differences in latent water levels in these systems (ambient ACT, 0.086% H2O by Karl Fischer titration; humidity chamber, 97% RH). However, the result is consistent with our model and the ACT vapor acting as a H2O reservoir and vehicle to deliver H2O into the ap-pSi matrix where this H2O then swiftly oxidizes the ap-pSi. In contrast, neat H2O does not wet28,32 the ap-pSi well so it oxidizes the out layer of ap-pSi to ox-pSi and then slowly percolates through the ap-pSi/ox-pSi matrix oxidizing along the way as it moves through the ap-pSi matrix. Additional support for this scenario are the T results where there is no detectable analyte vapor-induced ap-pSi oxidation (Figure S1A,B); the H 2 O solubility in T under our experimental conditions is ≤0.033%.48 In contrast, H2O is fully miscible with ACN, MeOH, and ACT in all proportions.48 Significant enhancements in the ap-pSi to ox-pSi oxidation rates are observed for ACN, MeOH, and ACT vapors under ambient introduction conditions (Figure S1A,B); however, the extent of ambient oxidation sans nonaqueous vapor is comparatively much smaller (cf., the T ambient results) because the H2O in the atmosphere has no facilitating vehicle to help deliver the H2O into the ap-pSi network to effect more rapid oxidation. Figure 6 presents a typical time-dependent ap-pSi PL trace when ap-pSi is exposed to ACT analyte vapors under inert introduction conditions. In comparison to the results in Figure 1, the PL response in Figure 6 is fully reversible. Thus, under

Figure 5. Recovered oxidation rate data for the Si−O−Si and OySiH (y = 1−2) growth and SiHx (x = 1−3) disappearance in various vapors. (A) Absolute value of rate changes (0−2 h). (B) Rate ratio (i.e., ki/kdry air).

2) species rates reflect species growth over time. The corresponding SiHx (x = 1−3) species rates reflect timedependent species disappearance over time. Results for the other vapors were similar to the ACT results but slightly less dramatic (see Supporting Information). Figure 5A presents the recovered disappearance/growth rates (note the y-axis here is logarithmic) for the Si−O−Si, SiHx (x = 1−3), and OySiH (y = 1−2) species when ap-pSi has been challenged with dry air, ambient air, dry ACT vapors, ambient ACT vapors, 97%

Figure 6. Typical time-dependent PL trace for ap-pSi when exposed to ACT vapor under inert analyte introduction conditions (λex = 350 nm, λem > 650 nm, T = 385 K). 23172

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inert analyte vapor introduction conditions, one lacks significant latent H2O in the nonaqueous vapor phase vehicle for delivery into the ap-pSi matrix, there is no ap-pSi oxidation, and the ap-pSi PL response is fully reversible.

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CONCLUSIONS The IUPAC definition of a chemical sensor is a device that transforms chemical information into an analytically signal through a reversible chemical reaction or from a physical property of the system.49 Previous research using pSi photoluminescence as a vapor “sensing” platform have exhibited inconsistent reversibility.22−27,29,30 The current experiments demonstrate that the root cause of this irreversible response arises from the presence of minute quantities of H2O in the vapor stream leading to rapid, irreversible ap-pSi oxidation. Nonaqueous vapors that possess appreciable H2O solubility can uptake H2O from the ambient environment and then serve as a vehicle to deliver the H2O into the ap-pSi matrix where the H2O oxidizes the ap-pSi. The apparent ap-pSi to ox-pSi oxidation rate is much faster in the presence of the nonaqueous vapor vehicle because H2O transport into the pSi network is no longer limited by H2O slowly percolating through the hydrophobic ap-pSi. In the presence of the nonaqueous vapor vehicle, detectable levels of ap-pSi oxidation can be seen in a few seconds at room temperature if there are just ppm H2O levels in the nonaqueous liquid. In addition to demonstrating the subtle interplay between nonaqueous vapors and H2O on ap-pSi to ox-pSi oxidation, these results illustrate a strategy for rapidly and effectively oxidizing ap-pSi in a tunable manner at room temperature in a few minutes without the need for any sophisticated instrumentation (e.g., O3 generator).

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel 716-645-4180; fax 716-645-6963; e-mail chefvb@buffalo. edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is based in part upon work supported by the National Science Foundation under Grant CHE-0848171. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.

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