Graphene Phase

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Interactions of NO2 with Zinc (Hydr)oxide/Graphene Phase Composites: Visible Light Enhanced Surface Reactivity Mykola Seredych, Oluwaniyi Mabayoje, and Teresa J. Bandosz* Department of Chemistry and CUNY Energy Institute, The City College of New York, 160 Convent Avenue, New York, New York 10031 ABSTRACT: Composites of zinc hydr(oxide) with either graphite oxide or graphene (ZnGO/ZnGr) were obtained using an in situ precipitation method. The surface properties were studied using Fourier transform infrared spectroscopy, X-ray diffraction, thermal analysis, potentiometric titration, and adsorption of nitrogen. The materials were exposed to NO2 to evaluate their suitability as adsorbents of toxic industrial compounds at ambient conditions. The highest NO2 adsorption capacities were found on Zn(OH)2 and its composite with graphite oxide either in an as-received form or heated at 600 °C. In the majority of cases zinc nitrates were found as the main reaction products. In the case of zinc hydroxide the high efficiency for NO2 retention was linked to: (1) developed porosity, (2) presence of OH groups in the zinc hydroxide phase, (3) photocatalytic activity of Zn(OH)2. That activity was enhanced when graphite oxide, being able to activate oxygen and spatially separate the electron hole pairs, was added as a composite component. Graphite oxide also contributes to the high dispersion of active terminal OH groups. After dehydroxylation, the resulting ZnO loses its activity to retain NO2 on the surface. On the other hand, the activity of the composite increases owing to the combined effects of zinc oxide photoactivity, high dispersion of an inorganic phase, separation of electron hole pairs, increase in electronic conductivity, reactive carbonaceous component, and the presence of OH groups on partially reduced graphene phase.

’ INTRODUCTION An increase in the energy consumption due to a growing trend in urbanization has led to a surge in fossil fuel burning. Consequently, there is a rise in the emission of toxic and environmentally detrimental gases to the atmosphere.1 An example of a pollutant, which is also classified as a toxic industrial compound (TIC) is NO2. This species is involved in the formation of photochemical smog and acid rain, and it contributes to the process of ozone layer depletion and eutrophication in coastal waters.2 Nitrogen dioxide has also adverse effects on human health, especially on the respiratory system. Studies link longterm exposure to NO2 to hair loss, throat inflammation, impaired vision, aggravation of existing respiratory illnesses, and lower resistance to respiratory infections.3,4 These harmful effects of NO2 on human health and environment have led to strict regulations governing the emission of this pollutant.5 Consequently, there is a search for effective ways of minimizing the emission of NO2 and its presence in the atmosphere.6 13 An important problem to be addressed related to mentioned above properties of NO2 is also a potential release of this species in confine/open spaces as a result of terrorist actions. One of the ways to remove TICs from ambient air is the use of reactive adsorbents as filtration media. Various carbonaceous materials such as carbon nanotubes,10 activated carbon,11,12 activated carbon fibers,13 and graphite oxide (GO) composites6 9 have been tested as NO2 adsorbents. Graphite oxide (GO) is especially interesting because of its emergence as a potential precursor in the synthesis of single-sheet graphene, and the specific electronic properties of these materials.14 The presence of oxygen groups r 2012 American Chemical Society

make GO hydrophilic.15 It is thus able to take part in hydrogen bonding with water molecules that are present between the basal planes helping in maintaining its stacked structure. When dispersed or sonicated in a suitable solvent GO loses its stacked structure and becomes exfoliated into singles layers.16 Its dispersibility in polar solvents (especially in water) and its oxygen functional groups open new ways of using its chemistry and surface structure in the synthesis of composites materials.17 GO and its composites with metal organic frameworks (MOF) or manganese oxides have been tested in the removal of ammonia (NH3).18 20 Zirconium hydroxide/GO composites were found as excellent adsorbents of sulfur dioxide (SO2)21 and hydrogen sulfide (H2S).22 MOF/GO, iron oxide/GO, and polyaniline/GO composites were also tested for nitrogen oxides’ removal (NO and NO2).6 9 Studies of NO2 adsorption using graphite oxide-based composites present interesting results relevant to the removal of TICs at ambient conditions. Metals and metal oxides have also been extensively studied in the processes of environmental remediation. The group includes ZnO,23 MgO,24 TiO2,25 CeO2,26 and Fe2O3,27 among others. Underwood and co-workers provided evidence for the formation of surface-bound nitrite and nitrate species after the adsorption of NO2 on TiO2, Al2O3, and Fe2O3.27 They proposed a mechanism where NO2 initially reacts with a mineral oxide surface to form nitrite which then reacts with either another surface nitrite or gas Received: November 18, 2011 Revised: December 22, 2011 Published: January 12, 2012 2527

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The Journal of Physical Chemistry C phase NO2 to form nitrates.27 Another study on the adsorption of NO2 on TiO2 also gives evidence for the formation of nitrate although with a different mechanism involving disproportionation of NO2 on the metal’s surface and the interaction of NO2 with oxygen vacancies.25 Regardless of the kind of metal oxide used, the nitrite and nitrate are expected as the products of surface reactions and such surface features as oxygen vacancies23,25 or OH groups23 27 were indicated as the most important for the retention process. Interesting results were obtained when composites of GO with metals and metal oxides were used as adsorbents of NO2. Morishige and Hamada reported the high activity of iron oxide pillared graphite oxides in the adsorption of NOx at low pressures.9 It was attributed to the chemisorption of NO2 on the surface of FeO on Fe3O4 layers. Even though the importance of porosity in such composites was also indicated,6 the lack of porosity can be compensated by the presence of active species like lepidocrocite (α-FeOOH) and carbonate, which participate in the removal of NO2.6 The objective of this paper is to investigate the performance of zinc (hydr)oxide and its composite with GO or graphene as adsorbents of NO2 in dynamic conditions at room temperature. Previous studies have shown that NO2 is a radical molecule that is able to strongly interact with metal centers, O centers, and cations of ZnO.23 Since zinc oxide is known of its photoactivity28,29 and the graphene phase can help with the separation of charges, these unique composites have a potential to combine surface chemical reactivity with the photocatalytic properties at visible light. This can lead to NO2 removal media working efficiently at ambient conditions.

’ EXPERIMENTAL SECTION Materials. Graphite oxide was synthesized by oxidation of graphite (Sigma-Aldrich) using the Hummers method.30 Ten grams of graphite powder with particle size 10 mg/g).

adsorbed water, dehydration, and dehydroxylation of zinc hydroxide, respectively.34 After exposure to NO2 the peak representing dehydroxylation disappears, which confirms that those hydroxides were engaged in formation of zinc hydroxide nitrate detected on the surface and therefore they thermal stability decreases. The XRD pattern (Figure 3) indicates that a significant quantity of unreacted hexagonal zinc oxide is still present on the surface of this material. A complex weight loss pattern revealed between 100 and 300 °C for ZnO must be related to the removal/decomposition of the products of surface reactions. The peaks at about 140 and 250 °C are assigned to the two steps decomposition of zinc hydroxide nitrate to zinc oxide. This process was reported to take place at 133 and 247 °C.39 The first peaks represents dehydration and the second - decomposition of formed zinc nitrates into ZnO, NO2, NO, and O2. The peak at about 100 °C is linked to the removal of water and that at 200 °C might represent decomposition of zinc nitrate.25 As discussed elsewhere,31 DTG curves for ZnGO reveal overlapping peaks at about 200 and 225 °C representing dehydration of zinc hydroxide phase and the removal of water associated with a Zn(OH)1.58(CH3COO)0.42 3 0.31H2O.36 Between 250 and 400 °C decomposition of zinc acetates and new species formed on the interface zinc hydroxide and graphite oxide takes place. At temperature over 800 °C reduction of zinc oxide by carbon is visible. After heating at 600 °C the DTG curves do not show any removal of water or hydroxyls indicating that only zinc oxide is present in ZnGO-T and ZnO. These weight loss patterns are significantly altered after exposure to NO2. For both ZnGO and ZnGO-T sharp peaks are visible at

about 110 and 200 °C. In the case of ZnGO an overlapping sharp peak at 180 °C is also revealed. We assign the peak at 110 °C to the removal of water and dehydroxylation of zinc hydroxy nitrate.25,39 It is more intense for ZnGO than for ZnGO-T, which is consistent with the much higher contents of hydroxy groups in the former sample. In fact in the case of ZnGO-T the only source of hydroxyl groups are partially reduced graphene layers,40 and there must be a complex mechanism that can lead to their disattachment from the basal planes. The peak at 200 °C is assigned to decomposition of nitrates. The overlapping sharp peak at 180 °C is at the same position as that observed in the case of the decomposition of epoxy groups.16 18 It is observed only for the unreduced sample and may represent the reappearance of epoxy groups on the graphene sheets after the reaction of zinc, previously bound to epoxy oxygen, with NO2. The new carboxylic groups formed as a result of graphene phase oxidation may also contribute to it. The comparison of FTIR spectra for the selected initial and exhausted samples of high NO2 removal capacity is presented in Figure 5. In the case of ZnH a strong OH band separated into two bands at 3500 and 3455 cm 1 is assigned to OH groups coordinated to Zn(II) ion and OH groups of water molecules, respectively.41 OH bending and OH twisting vibrations in Zn OH are also in the region between 1040 and 860 cm 1.41,42 The two bands observed at 1360 and 1560 cm 1 are assigned to vibrations of OH bound to zinc hydroxide and water.41,42 After NO2 adsorption new bands are observed at 1310 and 1410 cm 1. They are assigned to nitrate ions with D3h symmetry and NO2 asymmetric stretching of the nitrate ion with a lower C2v symmetry, 2531

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Figure 5. FTIR spectra for the initial and exhausted samples of high NO2 removal capacity (>10 mg/g).

respectively.43 Apparently they are due to interactions of nitrates with zinc cation.43 The band on 1557 cm 1 is assigned to the bending mode of H2O, and the weak shoulder at 1050 cm 1 is assigned to the NO stretch v1 band of a free nitrate group.43,44 The observed broadening and splitting of the band around 3500 cm 1 is caused by stretching of OH group linked with the inorganic layer, OH water group, and OH group linked with the nitrate.41,42 Finally, the peak at 1652 cm 1 is attributed to the H2O bending mode.43,44 The same patterns but much more pronounced are seen for the composites with GO either initial or heat-treated. Here the presence of nitrates is very strongly pronounced at 1310 and 1410 cm 1. This confirms our assignment of the DTG peak at 200 °C to the decomposition of these species. Other samples with low adsorption capacities showed no significant change in their spectra after exposure to NO2 as shown on the examples of the spectra for ZnGr and ZnGr-E. An interesting feature, supporting the interpretation of DTG results, is a very intense peak at 1050 cm 1 seen on the spectra for ZnGO-E. Besides the aforementioned nitrate groups this band can be also linked to C O

bond vibrations in C OH and C O groups.45 Moreover, the band at 1750 cm 1 seen on the spectra for this sample and ZnGO-T-E can be also associated with CdO vibrations, and this suggests that new carboxylic groups are formed on the surface of graphene layers as a result of the exposure to NO2. The release of NO as a product of surface reaction confirms that the oxidation of the graphene phase takes place.6,11,12 In fact that reaction seems to be the most efficient for ZnGO taking into account the high amount of NO2 adsorbed. That release of NO is observed also on Zn(OH)2. The distributions of the pKa for the species present on the surface of our adsorbents before and after adsorption of NO2 are presented in Figure 6. The titration was performed only with NaOH from the initial pH of the suspension (∼7). As discussed elsewhere46,47 we assigned the species at about pKa 7 to the bridging hydroxyls of Zn(OH)2 and that between 10 and 11 to the terminal hydroxyls. For both, ZnH and ZnGr which have the similarities in zinc (hydr)oxide phase31 exposure to NO2 decreases the amount of terminal groups. The involvement of such hydroxyls was also found for adsorption of H2S on these 2532

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Figure 6. pKa distribution for the initial materials studied and those after NO2 adsorption.

materials46 and for adsorption of NO2 on ceria-zirconia mixed oxides.48 After NO2 adsorption on ZnH the peak at pKa about 7 significantly increases which suggests the precipitation reaction of NaOH with the nitrates present on the surface of ZnH-E. Interestingly, for ZnGO this reaction is much more pronounced, and the terminal OH seems to be more involved in interactions with NO2 than in the case of ZnH. This is likely linked to the more amorphous character of zinc (hydr)oxide phase in this composite. These results are consistent with the FTIR spectra (Figure 5). In the case of ZnGO also a new peak at pKa of about 9 is visible, which likely represents the new species formed on the interface of both phases of the composite components. The pKa distribution for ZnGO-T shows a very small quantity of detected surface species, which might be linked to residual hydroxides and OH groups of the partially reduced graphite oxide phase. After exposure to NO2 the reaction of NaOH with nitrates is seen as a peak at pKa about 7.5. The new species visible at pKa about 11 are likely the weakly acidic groups formed as a result of the oxidation of the graphene phase by NO2. Those new carbon oxygen bonds were also detected on FTIR spectrum for this sample. The results presented above clearly indicate that nitrates are the predominate products of surface reactions. Therefore on the surface of the well-performing adsorbents, ZnH, ZnGO, and ZnGO-T, the oxidation of nitrogen dioxide must take place. In the case of Zn(OH)2 we link this process to photoactivity of zinc hydroxide, which was also found as a crucial process in the case of H2S adsorption on ZnH and ZnGO.46 Interestingly, zinc oxide,

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which is known of its photoactivity in visible light,28,29 does not show any enhanced activity for NO2 retention. It is likely that even though NO2 is oxidized on this sample, the absence of hydroxyls enabling acid base reaction prevents its reactions with zinc. The involvement of hydroxides in surface reactions is clearly seen on the DTG curves for ZnH-E (Figure 4) and also on the pKa distributions (Figure 5). It is likely that NO2 is oxidized by activated oxygen to N2O5, and owing to the water of hydration associated with hydroxyl, HNO3 is formed. It then reacts with the hydroxyls on the surface. When the graphene-based phase is present, especially that containing oxygen as graphite oxide, it also participates in oxygen activation.49,50 Owing to its chemical bonds with the zinc (hydr)oxide and to the conductivity of distorted graphene layers,31,51 it helps in electron transfer. Positive effects of the carbonaceous phase on photocatalytic activity in visible light were also found on its composites with titania.52,53 On the other hand, composite with the graphene phase does not exhibit high reactivity owing to the fact no bonds with the zinc (hydr)oxide phase are present and that phase is much more crystalline and densely packed between the large graphene flakes which limits the active sites accessibility.46 Potentiometric titration results analyzed in details elsewhere46 also showed the higher contribution of reactive terminal OH on the surface of ZnGO than those on ZnH or ZnGr. The water of hydration is present in ZnGO and thus the acid base reactions between the nitric acid and zinc hydroxide take place. Since the inorganic phase in ZnGO is more amorphous than that in ZnH more zinc nitrates are formed on the surface as shown by our surface analysis. Moreover, and this seems to be an important issue, in the ZnGO and ZnGO-T composites the electron hole pair (e —H+) formed upon exposure to light is more spatially separated as redox active sites than on pure ZnH.54 It leads to the more efficient generation of reactive oxygen species as superoxide ion (O2 ), atomic oxygen, O , OH*, and HO2*. In the formation the two latter species terminal OH groups and water of hydration are involved, and this can take place only on ZnGO or ZnH. The zinc acetate present on the surface of the initial ZnGO is converted to zinc hydroxy nitrate. In the case of ZnGO-T, zinc oxide becomes a photoactive phase,28,29 and thus oxygen can be more effectively activated. Moreover, the electron transfer is enhanced by even more conductive partially reduced graphene phase. Even though the OH species associated with zinc hydroxide are not present, the OH groups attached to partially reduced graphene layers are available for photo activation, and these groups must contribute to the formation of zinc hydroxy nitrate detected on the surface. Since in the case of these materials zinc oxide is an amorphous phase dispersed between the nanosheets of graphene, its reactivity with NO2 toward formation of nitrates25 is expected to be much higher than that in ZnH-T. Moreover, XPS results obtained on ZnGO-T suggested that highly dispersed metallic zinc is also present on the surface of this composite.31 Those zinc centers were also found as very active toward NO retention.25 Even though Rodriguez and co-workers indicated that nitrates are formed on zinc at temperatures less than 23 °C and at room temperature NO was found as predominant species,25 the presence of graphene sheets likely changes the mechanism toward formation of zinc nitrates . This is supported by about 50% lower NO2 to NO conversion ratio on ZnGO-T than that on ZnO. Therefore even though the apparent reactivity of ZnGO and ZnGO-T are similar, the mechanisms of surface reactions differ in their complexity in the case of these two composites. They are presented in the schematic way in Figure 7. 2533

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Figure 7. The schematic representation of surface activity on ZnH (A), ZnGO (B), and ZnGO-T (C).

In both cases the graphene phase, besides participation in oxygen activation also enhances the electron transfer via separating the electron hole pair. Formation of NO as mentioned above is either the result of the reduction of graphene phase or disproportionation reaction, which is known to take place on the oxide surface.25 The effects of complex chemistry of the NO2 interactions with our materials are visible in Figure 8 where the adsorption bed of ZnGO-T is shown after 45 min of exposure to NO2. As seen, the color apparently changed from dark gray for the initial composites to white and then to dark brown. That white color suggests not only formation of different inorganic phase but also changes/ oxidation of the graphene layers. Highly oxidized graphite oxide is known of its yellow/brown color.55

Figure 8. Color changes for the ZnGO-T material exposed to NO2.

In the case of ZnGO-T it is zinc oxide photoactivity, OH groups from partially reduced graphene layers, the highly amorphous inorganic oxide phase, and small metallic zinc particles that govern the reactivity. For ZnGO the presence of hydroxyl groups and water is important along with photoactivity of Zn(OH)2.

’ CONCLUSIONS The results presented in this paper show that porous zinc hydroxide and its composite with graphite oxide can be used as efficient adsorbents NO2 at room temperature. Their activity is linked to the photocatalytic properties of Zn(OH)2 in visible light and the presence of terminal hydroxyl groups/water of hydration reacting with N2O5 formed in oxidation reactions. In the case of the composite, the presence of graphite oxide layers is important since it provides the spatial separation of electron hole pairs, contributes to the high dispersion of active terminal hydroxides, affects the crystallinity degree, and further enhances the oxygen activation process. When the composite is 2534

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The Journal of Physical Chemistry C heated at 600 °C, highly dispersed and photoactive in visible light zinc oxide contributes to the oxidation of NO2. OH groups of graphite oxide also take part in surface reactions and increased DC conductivity enhances the electron transfer processes. Such effects are not noticed on zinc oxide, in spite of its known photoactivity.

’ AUTHOR INFORMATION Corresponding Author

*Phone: (212) 650-6017. Fax: (212) 650-6107. E-mail: tbandosz@ ccny.cuny.edu.

’ ACKNOWLEDGMENT This work was supported by ARO Grant No. W911NF-10-10039, NSF collaborative Grant No. 0754945/0754979, and PSCCUNY Grant No. 63098-0041. ’ REFERENCES (1) Fenger, J. Atmos. Environ. 2009, 43, 13. (2) Jin, Y.; Veiga, C. M.; Kennes, C. J. Chem. Technol. Biotechnol. 2005, 80, 483. (3) Penard-Morand, C.; Charpinw, D.; Raherisonz, C.; Kopferschmittz, C.; Caillaudk, D.; Lavaud, F.; Annesi-Maesano, I. Clin. Exp. Allergy 2005, 35, 1279. (4) Lee, Y.-W.; Kim, H.-J.; Park, J.-W.; Choi, B.-U.; Choi, D.-K.; Park, J.-W. Carbon 2003, 41, 1881. (5) Menil, F.; Coillard, V.; Lucat, C. Sensors Actuators B 2000, 67, 1. (6) Bashkova, S.; Bandosz, T. J. Ind. Eng. Chem. Res. 2009, 48, 10884. (7) Levasseur, B.; Petit, C.; Bandosz, T. J. ACS Appl. Mater. Interfaces 2010, 2, 3606. (8) Seredych, M.; Pietrzak, R.; Bandosz, T. J. Ind. Eng. Chem. Res. 2007, 46, 6925. (9) Morishige, K.; Hamada, T. Langmuir 2005, 21, 6277. (10) Ellison, M. D.; Crotty, M. J.; Koh, D.; Spray, R. L.; Tate, K. E. J. Phys. Chem. B 2004, 108, 7938. (11) Pietrzak, R.; Bandosz, T. J. Carbon 2007, 13, 2537–2546. (12) Kante, K.; Deliyanni, E.; Bandosz, T. J. J. Hazard. Mater. 2009, 165, 704–713. (13) Shirahama, N.; Moon, S. H.; Choi, K. H.; Enjoji, T.; Kawano, S.; Korai, Y.; Tanoura, M.; Mochida, I. Carbon 2002, 40, 2605. (14) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Nature 2006, 442, 282. (15) Lerf, A.; He, H.; Forster, M.; Klinowski, J. J. Phys. Chem. B 1998, 102, 4477. (16) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. Chem. Soc. Rev. 2010, 39, 228. (17) Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. M.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Carbon 2007, 45, 1558. (18) Seredych, M.; Petit, C.; Tamashausky, A. V.; Bandosz, T. J. Carbon 2009, 47, 445. (19) Petit, C.; Bandosz, T. J. Adv. Funct. Mater. 20010, 20, 111. (20) Seredych, M.; Bandosz, T. J. Microporous Mesoporous Mater. 2012, 150, 55. (21) Seredych, M.; Bandosz, T. J. J. Phys. Chem. C 2010, 114, 14552. (22) Seredych, M.; Bandosz, T. J. Chem. Eng. J. 2011, 166, 1032. (23) Rodriguez, J. A.; Jirsak, T.; Dvorak, J.; Sambasivan, S.; Fischer, D. J. Phys. Chem. B 2000, 104, 319. (24) Miletic, M.; Gland, J. L.; Hass, K. C.; Schneider, W. F. Surf. Sci. 2003, 546, 75. (25) Rodriguez, J. A.; Jirsak, T.; Liu, G.; Hrbek, J.; Dvorak, J.; Maiti, A. J. Am. Chem. Soc. 2001, 123, 9597. (26) Nolan, M.; Parker, S. C.; Watson, G. W. J. Phys. Chem. 2006, 110, 2256.

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