Supporting Information Steam Etched Porous Graphene Oxide Network for Chemical Sensing Tae Hee Han,† Yi-Kai Huang,† Alvin T. L. Tan, Vinayak P. Dravid* and Jiaxing Huang* Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208 USA * Email:
[email protected] (V.P.D);
[email protected] (J.H)
† These two authors contributed equally
Experimental procedures Synthesis and processing of GO. GO was prepared using a modified Hummers method [Ref 2 in the main text] from graphite powder (Bay carbon, SP-1) as reported elsewhere. [Ref 9 in the main text] The dispersion was extensively washed and filtered first with 1M HCl and then acetone to remove the salt byproduct and excess acid. Then the dried filter cake was dispersed in deionized (DI) water by mechanical agitation. The prepared GO was deposited on SiO2/Si substrates using a Langmuir-Blodgett (LB) technique. [Ref 19 in the main text] Steaming of GO. GO deposited SiO2/Si substrates were suspended in a glass pipette and placed in a 23 mL Teflon vessel (Parr Instrument) pre-filled with 2 mL of DI water. The Teflon vessel was sealed in an autoclave and heated to 200 °C. To test the effects of steaming vapor, control experiments were carried out under the same conditions without water, or by replacing water with the same volume of toluene or ethanol. Steaming experiments were also carried out with rGO sheets, which were prepared by exposing GO coated substrates to hot hydrazine vapor (Sigma Aldrich, anhydrous, 98%) in a sealed petri dish at 100 °C for 30 min. The resulting r-GO coated substrates were soft-baked at 100 °C for 5 min to remove any residual of hydrazine. Characterization. GO sheets before and after etching were characterized by scanning electron microscopy (SEM; Hitachi S-4800-II), atomic force microscopy (AFM; Digital Instrument, MultiMode scanning probe), and transmission electron microscopy (TEM; Hitachi H-8100 microscope). AFM images were taken with tapping mode. TEM samples of free-standing GO S1
sheets were w prepareed on a laccey-carbon TEM grid (Ted Pella,, Inc.). X-raay photoeleectron spectrosccopy (XPS) measuremeents were carried c out using an O Omicron ES SCA Probe with monochrromatic Al Kα K radiation (hν = 1486.6 6 eV) at 20 m meV steps. Electrica al measurem ments and gas g sensing.. To measuree the electriccal propertyy and charactterize gas sensiing behaviorr, platinum electrodes e with w dimensi ons of 50 µ µm × 50 µm × 50 nm annd 25 µm gap were deposited on top of GO sheeets through a shadow m mask to avoid contaminnation introduceed by resist-b based lithog graphy. The gas g sensing eexperimentss were conduucted in a 1 L test chamber at room tem mperature fo or all times. During thee gas-sensingg experimennts, NO2 witth N2 carrying gas and pu ure N2 weree sequentiallly introduceed to the chhamber and the source--drain current of o the devicee was monittored. CO gas g was alsoo tested in coontrol experriments. Keiithley 4200-SCS characteriization system was used to measure the electricaal propertiess and monitoor the gas sensiing behaviorrs.
S SEM im mages showin ng typical frracturing pattterns of GO O after (a-c) heating in air at Figure S1. 400 oC for f 80 min, and at 600 0 oC for (d)) 2 min andd (e) 4 minn. (f) GO shheets complletely disappearred after heaating for 5 min m at 600 ºC C.
S2
Figure S2. S Low magnification TEM T imagees of GO sheeets suspendded on laceyy carbon griid (a) before an nd after being steamed fo or (b) 5, (c) 10, and (d) 220 hours, resspectively.
S3
Figure S3. S Effect off solvent vapor during steeaming, AFM M images shhowing GO ssheets after bbeing (a) dry heeated in the autoclave bo omb, and steeamed by (b)) toluene and (c) ethanol vapor at 2000 °C for 20 ho ours, respecttively. Etchiing was only y observed w when using w water as thee steaming vvapor. All scale bars = 2 µm m.
S AFM imaages showin ng the evoluttion of a che mically reduuced r-GO shheet with a llarge, Figure S4. pre-form med hole (a)) before and d after bein ng steamed for (b) 10, (c) 20 annd (d) 30 hhours, respectiv vely. Compaared to GO, the r-GO sh heet shows higher resisstance against steam etcching. The pre-fformed defect does not appear a to ind duce prefereential etchingg, suggestingg that r-GO sheet still conttains large number n of molecular m lev vel reactive defects thatt cannot be vvisualized bby the AFM. Alll scale bars = 400 nm. S4
