ASIAN JOURNAL OF CHEMISTRY

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19001 REVIEW Progress in Design nd Friction of Novel Grphene-Bsed Semiconductor Photoctlysts KEFAYAT ULLAH, ASGHAR ALI, SHU YE, LEI ZHU nd WON-CHUN OH * Deprtment of Advnced Mterils Science nd

1 Asin Journl of Chemistry; Vol. 26, Supplementry Issue (2014), S1-S5 ASIAN JOURNAL OF CHEMISTRY REVIEW Progress in Design nd Friction of Novel Grphene-Bsed Semiconductor Photoctlysts KEFAYAT ULLAH, ASGHAR ALI, SHU YE, LEI ZHU nd WON-CHUN OH * Deprtment of Advnced Mterils Science nd Engineering, Hnseo University, Seosn-si, Chungnm-do , Repulic of Kore *Corresponding uthor: Fx: ; Tel: ; E-mil: wc_oh@hnseo.c.kr Pulished online: 24 Decemer 2014; AJC Grphene, 2D mteril with extrordinry mechnicl, opticl nd electricl properties hs recently ttrcted enormous interest in photoctlysis science. It hs een considered one of the most useful entities for the friction of vrious composite mterils in environmentl nd energy pplictions. This review summrizes the recent progress in the design nd friction of grphene-sed semiconductor photoctlysts through vrious strtegies including hydrotherml, microwve techniques, ultrsoniction nd sol gel methods. In ddition, the photoctlytic properties of the prepred grphene-sed composite systems nd their ppliction in photoctlytic degrdtion of orgnic pollutnts re lso discussed. Keywords: Grphene, Photoctlyst, Microwve, TEM, Nnocomposites. INTRODUCTION The improvement of photoctlysis science hs een the focl point of considerle ttention in recent yers, ecuse of the wide rnge of reserch res especilly in environmentl nd energy relted fields. So fr the ctlytic properties of lrge numer of mterils hve een used to trnsform solr energy into chemicl energy to perform oxidtion/reduction rections for otining useful products like hydrogen energy, hydrocrons nd rdicl species to degrde pollutnts. Among them Ti hs een used extensively in mny ppliction ecuse of its chemicl stility, oxidizing ilities, long durility, nontoxicity nd low cost. The interction of light energy comprle with the nd gp of semiconductors genertes electrons nd holes in the conduction nd vlence nds, respectively. These chrge crriers either recomine or trnsfer to the surfce of the photoctlyst to contriute to series of photoctlytic rections. For proficient semiconductor photoctlyst, slow recomintion rte, fewer chrge trpping centers, proper energy level offsets nd stility ginst light re highly desirle for improving the photoctlytic ctivity. In the previous studies, vrious strtegies, including crystl nd texturl modifiction, nd gp djustment, heterostructuring, surfce improvement of semiconductor photoctlysts, hve een developed for the dvncement of the photoctlytic performnce. Recently grphene sed semiconductor mterils ttrct incresing ttention in photoctlyst science. Grphene hs ttrcted strong interest due to its higher therml, mechnicl nd opticl properties. It is promising mteril for energy-storge, electronic nd opticl devices, interfcing to iologicl mterils nd other numerous pplictions. In ctlysis in sensing ppliction the hyrid grphene mterils is used ecuse of promising cpilities. Severl pproches hve een reported for the production of grphene sheets supporting semiconductor nnocrystls. Friction nd designing of grphene sed mterils: There hve een severl reports highlighting the rtionl design of grphene sed photoctlysts with high photoctlytic ctivity. Grphene hs een considered to provide excellent support mterils ecuse of two dimensionl structures y providing lrge surfce re nd ttrctive potentil to efficiently control their redox properties. In generl when semiconductor nnoprticles re ttched on the grphene sheet very smll mount of nnoprticles is in direct contct with grphene surfce. Such smll interction etween nnopr-ticles nd grphene ffect the ctlytic properties of the desire grphene sed ctlyst. Therefore rtionl design nd control synthesis process re highly desirle to overcome these discrepncies. Recently modified microwve techniques hve een developed to synthesize Pt/grphene nd Pt/grphene-Ti s n enhnced photoctlyst mteril. The synthesis process ws

S2 Ullh et l. considered to e fst nd fcile for the friction of these mterils. The visile light photoctlytic ctivities of the s prepred smples were tested y methylene lue nd rhodmine- B decomposition.

The DRS lso confirms the lrge sorption in the visile rnge which further confirms tht visile light cn e used s incident rdition to study the photoctlytic properties of the Pt/grphene nnocomposites 1.

To further study the enhnced photoctlytic properties Ti is considered to ttch on Pt/grphene composites vi microwve techniques. The propose mechnism nd synthesis process re given in Fig. 2.

2 S2 Ullh et l. considered to e fst nd fcile for the friction of these mterils. The visile light photoctlytic ctivities of the s prepred smples were tested y methylene lue nd rhodmine- B decomposition. High photoctlytic ctivity ws oserved for Pt/G3 nnocomposite. Enhnced ctivity is ttriuted to homogeneous distriution of Pt nnoprticles on grphene sheet. The DRS lso confirms the lrge sorption in the visile rnge which further confirms tht visile light cn e used s incident rdition to study the photoctlytic properties of the Pt/grphene nnocomposites 1. The TEM nd XRD imges of the Pt/grphene nnocomposites re depicted in Fig. 1(-). To further study the enhnced photoctlytic properties Ti is considered to ttch on Pt/grphene composites vi microwve techniques. The propose mechnism nd synthesis process re given in Fig. 2. It is clerly oserve tht the Ptgrphene/TiO 2 nnocomposite cn e used s efficient photoctlyst under UV-visile light irrdition. This high ctivity is ttriuted to the synergetic effect of high chrge moility nd the oserved red shift in the sorption edge of the Ptgrphene/TiO 2 nnocomposites. Asornce (.u.) Fig. 3. [F(R)h ν ] 1/ PGT. 3.5PGT () h ν (ev) wvelength(nm) Asin J. Chem. () 3.5PGT 4.5PGT (c) Ti UV-visile diffuses sornce spectr () Corresponding nd energy clcultion using KM function [Ref. 2] c e = Pt Intensity (.u.) d c Fig. 1. Fig. 2. OH θ ( ) XRD pttern of Pt/grphene nnocomposites () TEM imges of Pt/grphene composite presenting grphene sheet decorted with Pt prticles [Ref. 1] COOH Grephen oxide O H2PTCl 6 + TNB M.W. rdition 300 s () Ethylene glycol Pt-grephene/Ti Schemtic digrm of grphene nchoring Pt nd Ti nnoprticles [Ref. 2] The sorption spectr nd the corresponding oserved nd gp re expressed 2 in Fig. 3(-). Similrly PtSe 2 - grphene nnocomposites were synthesized through microwve techniques. Attchment of PtSe 2 on grphene sheet increses the sorption towrds visile rnge. The visile light ctlytic response ws studied nd enhnced photoctlytic performnce ws oserved. Fig. 4 depicts the TEM imges nd the distriution of PtSe 2 nnoprticles on grphene sheet. PtSe 2 -grphene nnocomposite ws lso synthesized vi ultrsonic techniques. In comprison to microwve techniques the distriution of the prticles were found to e prtilly gglomerted 3,4 s shown in Fig. 4 (). PS/grphen-Ti nnocomposites were synthesized vi sol gel method. Fig. 4. TEM imges of PtSe 2/grphene nnocomposite Microwve ssisted () Ultrsonic ssisted [Ref. 3-4] The photoctlytic ctivities of the smples were evluted. As prepred PS-grephene/Ti composite exhiit much higher photoctlytic ctivity towrds the degrdtion of methylene lue thn PS-grephene. This higher ctivity is ttriuted to the coupled semiconductor with Ti which provides mximum interfcil contct with grephene surfce without ggregtion which is necessry fctor for incresing the photoctlytic performnce. The couplings of PS with Ti extend the photoresponse to visile region. And enhnced photoctlytic response ws oserved 5 s shown in Fig. 5. CdSe/grphene-Ti heterogeneous ctlysts were prepred y clcintions of CdSe-grphene composites with titnium(iv) n-utoxide s the source of Ti t 873 K. The UV spectrophotometric study indicted the photo derivtive ility of the CdSe/grphene-Ti composites in visile light is significnt nd the smple dyes methyl ornge degrded 71 % wheres methyl ornge degrded round 85 % y the end of just 180 min of exposure to visile light. CdSe/grphene-Ti composites hve excellent photoctlytic ctivity in cyclic experiment which emphsizes the rillint stility of the ctlyst nd the photochemicl stility. The proposed mechnism for the CdSe/grphene-Ti ctlyst 6 ws given in Fig. 6. In this context severl heterogeneous photoctlyst mterils were synthesized nd the influence of grphene hs een oserved y performing numerous experiments for the degrdtion of orgnic pollutnts Ag 2Se-grphene/Ti nnocomposites were synthesized vi sonochemicl methods nd oserved the dye dsorption experiments to show the ctlytic ctivity using

3 Vol. 26, Suppl. Issue (2014) Progress in Design nd Friction of Novel Grphene-Bsed Semiconductor Photoctlysts: A Review S3 Asornce (.u.) 0 min 30 min 60 min 90 min 120 min 150 min λ mx = 665 nm PS-grphene Asorption Degrdtion Asornce (.u.) 0 min 30 min 60 min 90 min 120 min 150 min λ mx = 665 nm 0.1 PS-grphene/Ti Asorption () Degrdtion Wvelength (nm) Wvelength (nm) (c) 0.9 Asornce (.u.) 0.7 PS-grphene PS-grphene/Ti Fig Wvelength (nm) Photoctlytic degrdtion efficiency of PS-grphene () PS-grphene/Ti nnocomposites (c) DRS sorption spectr of PS-grphene nd PS-grphene/Ti [Ref. 5] MO or RhB CO + H O +... Fig. 6. Visile light Ti VB h + + H 3.0 ev CdSe VB ev H + OH h + Grphene nnosheet OH OH MO or RhB CO + H O +... Proposed mechnism for CdSe-grphene/Ti nnocomposites [Ref. 6] orgnic dyes 11. Grphene/Ti photoctlyst hs een synthesized vi sonochemicl method nd the ctlytic ctivities of the mterils were investigted using rhodmine-b s n orgnic dye 12. Hydrothermlly synthesized CdSe-grphene nnocom-posite ws used s ctlyst for the degrdtion of methyl ornge nd rhodmine-b s orgnic dyes. Interesting results were found y oserving the ctlytic effect under drk mience. At the end of 150 min of ultrsoniction in drk stte, rhodmine-b went through the highest degrdtion efficiency of 98 nd 90 % of methyl ornge ws degrded. The degrdtion rection kinetics of rhodmine-b with sonoctlyst ws found to e greter thn methyl ornge following first order rection. Reused CdSe-grphene sonoctlyst showed very minute chnge in degrdtion efficiency which indictes dmirle stility of the ctlysts. As ll the sonoctlytic degrdtion occurs in drk conditions, tht proves the degrdtion of zo dyes (methyl ornge nd rhodmine- B) requires no specil conditions such s dditionl irrdition with visile light or UV light. The proposed mechnism nd the corresponding degrdtion efficiency 13 is given in Fig. 7(-). Heterogeneous CdS-grphene nd CdS-grphene/Ti composites were synthesized y simple sol-gel method. The photoctlytic studies were crried out y using methylene lue s n orgnic test dye. The corresponding TEM imge of the nno-composites ws given in Fig. 8. TEM imges of CdSgrphene nd CdS-grphene/Ti with scles of 0 2 µm-100 nm re shown in Fig. 9. TEM imge of CdS-grphene shows 2D structure of the grphene sheets nd indictes tht the surfce is very smooth s depicted in Fig. 8 (-). The morphology of grphene oxide is thin stcked flkes with well-defined

S4 Ullh et l. Asin J. Chem. Ultrsound irrdition Ctlytic reduction CdSe E Recomintion 4 + 2H O + O 2OH + 2OH Asornce 1.1 0.9 0.

fter 150 min of sonoctlysis () VB h + OH + RhB/Mo C + H 2 O 0.1 OH H2O/OH Ctlytic oxidtion 0 350 400 450 500 550 600 Wvelength (nm) Fig. 7.

4 S4 Ullh et l. Asin J. Chem. Ultrsound irrdition Ctlytic reduction CdSe E Recomintion 4 + 2H O + O 2OH + 2OH Asornce [5] (1) CdSe-grphene + MO As 30 min (2) CdSe-grphene + MO As 60 min (3) CdSe-grphene + MO As 90 min (4) CdSe-grphene + MO As 120 min (5) CdSe-grphene + MO As 150 min [1] [2] [3] [4] Degrdtion of MO fter 150 min of sonoctlysis () VB h + OH + RhB/Mo C + H 2 O 0.1 OH H2O/OH Ctlytic oxidtion Wvelength (nm) Fig. 7. Trnsfer of chrges in CdSe/grphene nnocomposite under ultrsonic irrdition; () Sonoctlytic degrdtion efficiency over methyl ornge y CdSe/grphene [Ref. 13] Fig. 8. (-) TEM imges of CdS/grphene (c-d) CdS-grphene/Ti [Ref. 14] multilyered structures t the edge. The TEM imges of CdSgrphene/TiO 2 show the grphene sheets nd CdS nnoprticles. A uniform dispersion of CdS prticles on the grphene sheet cn e oserved in Fig. 8 (c-d) 14. It is cler from the ove discussion tht mny fctors cn exert considerle influence on the ctlytic properties of the grphene sed mterils. These properties include size nd shpe, surfce re, pore structure nd homogenous distriution of nnoprticles on the surfce of grphene sheet. Thus for the development nd enhncement of photoctlytic properties of these mterils the ove mentioned prolems should ddress crefully in synthesizing nd fricting process. A generlized photoctlytic mechnism is discussed under the light of ove studies on grphene sed mterils. The photoctlytic performnce of the grphene sed nnocomposites in terms of photo degrdtion of orgnic dyes molecules under UV/ visile light irrdition is investigted. Three steps re minly required for photoctlytic mechnism. The dsorption of the pollutnts, light sorption y the ctlytic mteril used nd chrges trnsport. This trnsfer of chrges will responsile for the cretion of rdicl species which further decompose the pollutnts. Cron mterils hve stonishing sorption properties therefore minly used in vrious environmentl pplictions. Most of the industril dyes re romtic in nture nd they crete π-π stcking interction with the grphene romtic domins The concentrtion of the orgnic molecule increses t the surfce of the photoctlytic mterils due to dsorption process. By the irrdition of light electron re excited from the vlnce nd of photoctlyst to conduction nd creting hole in the vlence nd. The photo excited electron nd holes will rect with wter molecule to crete rdicl oxygen species which further decompose the pollutnts. The intrinsic electron hole pir recomintion is normlly 10-9 second which results in emission nd low photoctlytic ctivity very less electron-hole pir re trpped nd prticipte in the ctlytic rection. In order to increse the photoctlytic ctivity coctlysts re ttched on the grphene sheet. Therefore the trpped electron will trnsfer to coctlysts nd seprte the excite molecules nd gretly retined the recomintion process. Light irrdition (UV-visile) produces electrons (e - ) in the conduction nd () nd holes (h + ) in the vlence nd (VB) of the photctlyst in the nnocomposite. Thus numer of electrons (e - ) nd holes (h + ) were generted. Menwhile grphene nnosheets trnsfer electrons (e - ) to the conduction nd of ctlyst, therey incresing the numer of electrons s well s the rte of electron-induced redox rections. The grphene coupled with semiconductor photoctlyst system shows enhnced ctlytic ctivity due to high chrge sepertion induced y the synergetic effects of grphene nd semiconductor photoctlyst. It is known tht the frctionl reduction of grphene oxide only prtilly restores the sp 2 networks therefore the remining oxygen sites still le to to ccept electron nd undergo reductoion 18. The generted electrons (e - ) rect with dissolved oxygen molecules nd produce oxygen peroxide rdicls -. The positive chrge hole (h + ) cn rect with OH - derived from H 2 O to form hydroxyl rdicls OH. The orgnic dyes my degrded y oxygen peroxide rdicls - nd hydroxyl rdicls OH to C, H 2 O nd other minerliztion products 19,20. In future some more complex systrem re needed i.e. ternry or multicomponent for the

5 Vol. 26, Suppl. Issue (2014) devlopment nd enhncement of grphene sed semiconductor mterils. Recently multicomponenet grphene sed mterils hs een fricted nd enhnced ctlytic properties were reported 21. The tuning of nd gp of the one component in ternry system cn fcilitte the chrge seprtion ility t different levels. Also, through the proper choice of size, shpe nd composition of the multi component grphene-sed photoctlysts, momentous improvements cn e ttined in the photoctlysis. In ddition, the step up of more efficient synthesis methods or strtegies is highly desirle to improve the interfcil contct etween grphene nd photoctlyst components in grphene-sed photoctlysts system in order to improve the ctlytic efficiency. Conclusion Progress in Design nd Friction of Novel Grphene-Bsed Semiconductor Photoctlysts: A Review S5 In summry, well-designed grphene cn e introduced into vrious semiconductor photoctlysts to form grphenesed semiconductor composites. The introduction of grphene in these semiconductor mterils cn improve their properties enormously. These properties include high dye dsorption ilities, extension of light sorption to visile region, enhnced chrge seprtion, which increses the photoctlytic efficiencies of these systems. A vriety of methods hs een developed for the synthesis of grphene sed semiconductor mterils. These composite photoctlysts hve een widely used for the degrdtion of pollutnts nd other environmentl pplictions. REFERENCES 1. K. Ullh, S. Ye, L. Zhu, Z.D. Meng, S. Srkr nd W.C. Oh, Mter. Sci. Eng. B, 180, 20 (2014). 2. K. Ullh, L. Zhu, Z.-D. Meng, S. Ye, Q. Sun nd W.-C. Oh, Chem. Eng. J., 231, 76 (2013). 3. W.-C. Oh, K. Ullh, L. Zhu, Z.-D. Meng, S. Ye nd S. Srkr, Mter. Sci. Semicond. Process., 25, 34 (2014). 4. K. Ullh, L. Zhu, S. Ye, J. S. Bok nd W.C. Oh, J. Mter. Sci., 49, 4139 (2014). 5. K. Ullh, Z.D. Meng, S. Ye, L. Zhu nd W.C. Oh, J. Ind. Eng. Chem., 20, 1035 (2014). 6. T. Ghosh, K.-Y. Cho, K. Ullh, V. Nikm, C.-Y. Prk, Z.-D. Meng nd W.-C. Oh, J. Ind. Eng. Chem., 19, 797 (2013). 7. K. Ullh, S. Ye, S. Srkr, L. Zhu, Z.-D. Meng nd W.C. Oh, Asin J. Chem., 26, 145 (2014). 8. L. Zhu, S.B. Jo, S. Ye, K. Ullh, Z.D. Meng nd W.C. Oh, Asin J. Chem., 26, 1264 (2014). 9. S. Ye, L. Zhu, S.B. Jo, K. Ullh, Z.D. Meng, K.Y. Cho nd W.C. Oh, Asin J. Chem., 26, 1833 (2014). 10. K. Ullh, S. Ye, L. Zhu, S.B. Jo nd W.C. Oh, Asin J. Chem., 26, 1575 (2014). 11. Z.D. Meng, L. Zhu, T. Ghosh, C.-Y. Prk, K. Ullh, V. Nikm nd W.-C. Oh, Bull. Koren Chem. Soc., 33, 3761 (2012). 12. L. Zhu, T. Ghosh, C.-Y. Prk, Z.-D. Meng nd W.-C. Oh, Chin. J. Ctl., 33, 1276 (2012). 13. T. Ghosh, K. Ullh, V. Nikm, C.-Y. Prk, Z.-D. Meng nd W.-C. Oh, Ultrson. Sonochem., 20, 768 (2013). 14. C.Y. Prk, U. Kefyt, N. Vikrm, T. Ghosh, W.C. Oh nd K.Y. Cho, Bull. Mter. Sci., 36, 869 (2013). 15. S. Ye, K. Ullh, L. Zhu nd W.-C. Oh, J. Multifunct. Mter. Photosci., 5, 69 (2014). 16. S. Min nd G. Lu, Int. J. Hydrogen Energy, 37, (2012). 17. S. Stnkovich, D.A. Dikin, G.H.B. Dommett, K.M. Kohlhs, E.J. Zimney, E.A. Stch, R.D. Piner, S.B.T. Nguyen nd R.S. Ruoff, Nture, 442, 282 (2006). 18. P. Kmt, J. Phys. Chem. Lett., 2, 242 (2011). 19. Y. Yu, L.-L. M, W.-Y. Hung, F.-P. Du, J.C. Yu, J.-G. Yu, J.-B. Wng nd P.-K. Wong, Cron, 43, 670 (2005). 20. C. Deng, H. Hu, X. Ge, C. Hn, D. Zho nd G. Sho, Ultrson. Sonochem., 18, 932 (2011). 21. Q.J. Xing, J. Yu nd M. Jroniec, J. Am. Chem. Soc., 134, 6575 (2012).

Silicon Nanowire Based Single-Molecule SERS Sensor

Silicon Nanowire Based Single-Molecule SERS Sensor Supporting informtion Silicon Nnowire Bsed Single-Molecule SERS Sensor Hui Wng, Xuemei Hn, Xuemei Ou, Chun-Sing Lee, Xiohong Zhng* nd Shuit-Tong Lee S1, A series of Si nnowires coted with compct ggregtes