" ^^^^ du cua lop b^ mat ngoai ban dan Trang 5 Rsc - Dien tra than viing dien tich kh6ng gian ciia hi mat ban din T/,T^^ - Hang so toe do rate constant trao doi dien tich ciia dien tu,
MUC LUC 'i I Trang Loi cam doan Muc luc Danh muc cac ky hieu, cac chu viet tit M6DAU PHANI: TONG QUAN QUA TRINH DIEN HOA, PHUONG PHAP CHE TAG VA NGHIEN CtJlI DBEN ClTC BAN D A N Chirong : Qua trinh dien hoa dien cue ban Akn cau true nano-xop U Qua trinh dien hod cua dien cue ban dan 1.1.1 Tiep xiic chat ban din v6i dung dich dien giai 1.1.2 Qua trinh trao doi dien tich ban din-dien giai 1.1.3 Hieu ling quang dien hoa dien cue ban din L2 Dien cue ban ddn cdu true Nano-xop 13 16 1.2.1 Hieu ling kich thuoc hat luong tu 17 1.2.2 He hat ban din kich thu6c nho 18 7.2.2.7 18 12.2.2 Muc nang luang vo cache tdch hat tai 20 122.3 Phdn bo'dien the'trong dien cue nano-xop 22 12.2.4 Chuang : Che tao vat lieu Cc^ che ddn hat tai dien cue nano-xop 24 Cong nghe che tao va phuong phap dien hoa nghien cuu dien cue TiO, cau true Nano-xop 28 2.1 Phuong phdp soUgel che tao ban ddn TiO, nano-xop 28 22 Phuong phdp dien cue nghien cuu dien hod ehdt bdn ddn 31 2.2.1 Dae tinh Dong - The phan cue (dac tinh 1-V) 31 2.2.2 Phep dien dung-ihe phan cue (dac tinh C-V) 32 2.22.1 Dien cue kim loqi 33 2.2.2.2 Dien cue bdn dan 35 PHAN n : NHUNG KET QUA NGHIEN ClTU THUC NGHIEM VA LY THUYET TREN DIEN CUC BAN DAN CAU TRUC NANO-XOP TI02 Chuong : Xay dung mo hinh chuyen tiep ban din-dien giai (SEI) va dac trung dien dung dien hoa he dien cue ban dan nano-xop 40 3d Mo hinh thiet lap can bang bdn ddn - dien giai (SEI) 41 3.1.1 Mo hinh tiep xiic ban din dien giai he nano-xop 41 3.1.2 Phuong trinh mo ta qua trinh thiet lap can bing 46 3.1.3 Trao doi dien tich iren tiep xiic ban din-dien giai 49 3.1.4 Diiu kien trung hoa dien tich ciia be mat tiep xiic 52 3.1.5 Dang loan hoc mo hinh tiep xiic ban din-dien giai 54 3.1.5.1 He phuong trinh mo td qua trinh can bang 54 3.1.5.2 Dieu kien bien tai tiep xiic bdn ddn-dien giai 56 3.1.5.3 Dieu kien bien tai goc tog dp x=0 (R=0) 57 3.1.5.4 Dieu kien tai thoi diem ban ddu t=0 58 3.1.6 Phuong phap so giai he phuong trinh phi tuyen 58 3.1.7 Luu quy trinh mo phong thiet lap can bing SEI 62 32 Phuong phdp so tinh dien dung dien hod dien cue nano-xop 63 3.2.1 Cac lop dien tich cua tiep xuc ban din-dien giai (SEI) 63 3.2.2 Cong thuc ciia dien dung cac lop dien tich tren SEI 64 3.2.3 Quy trinh mo phong dac irung dien dung dien hoa 67 Kef luan Chuotig : 73 Dien cue TIO cau true nano-xop 4.1 Cdu true vd hinh thdi hoc cua dien cue TiO^ 74 74 4.1.1 Dien cue Ti02 che tao bing phuong phap sol-gel 4.i.2DiencircTi02chelaotirbolP25 (DEGUSSA) 4.1.3 Dien cue TiO^ dac che tao bing phuong phap dot Ti 42 Dae tinh quang dien hod cua dien cue Ti02 4.2.1 Cau true tinh the' va boat tinh quang dien hoa 4.2.2 Hinh thai hoe va boat tinh quang dien hoa 4.2.3 Kfch thude hat va boat tinh quang dien hoa 4.2.4 Thay doi dong hoc phan ling Redox dien cue ciu true nano-xop bi kich thich anh sang Ketbian Chuong : Dien dnng dien hoa dien cue Ti02 nano-xop Co che tang toe phan umg Redox 5.1 Qua trinh thiet lap can bang bdn ddn-dien giai 5.1.1 Cac thong so su dung mo phong he Ti02:171'^ 5.1.2 Ki6m chiing doi v6i tiep xiic kim loai-ban din 5.1.3 Mo phong qua trinh thiet lap can bing SEI 52 Dien dung dien hod cua dien cue Ti02 5.2.1 Dien dung dien hoa cua dien cue Ti02 dac 5.2-2 Dien dung dien hoa dien cue Ti02 nano-xop 5.2.3 Hien tuong dich chuyen bien vung cua he dien cue ban din cau tnic nano-xop bi kich thich quang Kit luan KET LUAN CHUNG VA KEEN NGHl DANH MUC CAC CONG TRINH CUA TAC GIA TAI LIEU THAM KHAO PHU LUC DANH ML C CAC KY HIEU VA CHlT VTET TAT Cac ky hieu C(o - Dien dung Warburg C(,oni - Dien dung diem cue doi Cgc - Dien dung xoing dien tich khong gian ciia b^ mat ban dkn Q s ' R-ss " Dien dung va trd thuSn ciia cac trang thai h6 mat dien cue ban din CH - Dien dung lop dien tich kep be mat Helmholtz CQX - Nong trang thai oxy hoa (oxydized species) cua cap Redox Cred" N6ng trang thai khtr (Reduced species) cua cap Redox dung dich De, Df, - He so khuecii tan cua dien tu va 16 trong ban dkn Ecs - Miic bien vung cua viing dSn ban din Evs - Miic bien vung ciia viing hoa tri ban dSn Ep - Miic Fermi ban din ERedox" Muc Redox ciia cap oxy hoa khtr EQX - Miic oxy hoa coa cap oxy hoa khu - ERed • Muc khu ciia cap oxy hoa khii ER - Nang luong cau tmic lai (Reorganized Energy) cua cap Redox G^, Gp - Toe kich thich dien tu, 16 trong ban din K^, K^ - Nhan to' chu\'en doi (Transfer factor) doi voi vxing din \'a hoa tri Loebyc • ^9 dai Debye cua chat ban din Loebye " Do d^i Dcby^ ciia dung dich dien giai rn*, nip - Khoi luong hieu dung ciia dien tu va 16 n^) - Nong pha tap irong ban din no, Po - Nong dien lu \'a 16 trong trang thai vung ph^ng Q^ - Dien lich ducua lop dien li'ch khong gian Qsuri • ^ i ? " ^^^^ du cua lop b^ mat ngoai ban dan rjj - Ban kinh Bor Rj.on, - Dien trd thuSn ciia dien cue d6i R^, Rp - Toe tai hop cua dien tir, 16 trong ban din Rsc - Dien tra than viing dien tich kh6ng gian ciia hi mat ban din T/,T^^ - Hang so toe (rate constant) trao doi dien tich ciia dien tu, 16 Vg - Do cao rao the be mat ban din ^Bias - Th^ p h ^ cue Vft, - The vung phing ciia ban din (flatband potential) Eo - Hing so dien m6i ciia chan khong - Hing so dien m6i cua ciia vat heu H^, |ih * E^9 1"^ dong cua dien tir va 16 trong ban din T]„, Tip - Do cao rao the trao doi dien tich cap Redox vdi ban din p - Nong d6 di6n tieh du vung dien tich kh6ng gian cua ban din ID - Thdi gian song nho nhat cua hat tai kh6ng co ban ban din Cac chu viet tat AFM - Hien vi luc nguyen tir Cap Redox : Cap oxy hoa- khu DTA - Phan tich nhi6t vi sai (Differential Thermo-Analysis) GCS - lop Gouy - Chapman - Stem : lop khuech tan Ion a gin bi mat dien cue C dien truong du tren hi mat dien cue (con goi la L6p Helmholtz ngoai O outer Helmholtz layer) NHE - Normal Hydrogen Electrode = SHE : The dien cue chu^n NHE PBP - Gia the phan cue (Pseudo Bias Potential) PEC - Quang dien hoa (PhotoElectroChemistry) SCE - The dien cue chuin SCE (Saturated Calomel Electrode) SEI - Chuyen tiep ban din - dien giai (Semiconductor-Electrolyte Interface) SEM - Hien vi dien tu quet MO DAU Nhung hien tuong dien hoa quang dien hoa (PhotoElectroChemistry PEC) da duoc biet den va umg dung tir nhung nam diu the ky Nhimg phai den nam 1954 Brattain va Garren [8] moi chi ban chat ciia hieu ling quang dien tren tiep xiic chat rin-dien giai dac biet la tiep xiic Ban din-Dien giai dal niu mong cho linh vuc quang dien hoa (PEC) hien dai Nam 1970 Honda \'a cac cong su [38] da phat hien hien tuong phan ly nude Oxy va Hydro chieu sang vao dien cue anode bing ban din oxit Titan loai n Tir ti^m nang ling dung ciia he PEC moi thuc su duoc quan tarn Nhung ling dung ciia he PEC de chuydn hoa nang luong mat trdi dien nang ciing nhu nhung dang nang luong khac da thu hiit n6 lac nghien cuu ciia nhieu phong thi nghiem cung nhu cac hang c6ng nghiep mot thoi gian dai Tuy nhien he PEC dien cue ban din phai doi mat v6i nhung \'an di vi ky thuat nhu hieu suit, bin \^ cua dien cue m6i truong nude Buoc dot pha thuc su chi xay vao nhung nam cu6'i thap ky 80 mot linh \arc m6i ciia khoa hoc vai lieu xuat hien : Vat heu c6 kich thudc Nanomet (vat lieu cau triic Nano) He PEC vai cac dien cue ban din c6 cau tnic Nano da lam thay doi each nhin ve tinh chat Vat ly cung nhu hieu qua ciia cac ung dung hieu ling PEC Chi khoang 10 nam nhiing ung dung ciia cac he PEC dien cue ban din c6 cau iriic Nano (hay cau triic Meso) tir nhiing Ilnh \aic ky thuat cao nhu Pin mat troi PEC, ciia so d6i mau, Pin Li, \^ den nhimg ung dung lam sach m6i truong bang quang xiic tac son chong moc va diet vi khuin, dieu tri ung thu my pham \^^ da trd nen rat quen thuoc Tuy dugc ung dung cang rong rai, nhung ban chat cac qua trinh val ly cua he tiep xiic chat ban dan (dac biet la he c6 kich ihudc hat c6 Nanomet) voi moi truong dien giai (SEI) da va dang la muc tieu nghien cuu ciia nhidu phong thi nghiem tren the gioi Su hap din ciia van de dav la iinh cap bach ve ling dung va tinh hoc thuat ciia cac qua trmh hoa ly tren bien giira Vat ly ban dSn tinh chit dien-hoa cac qua trinh hi mat va vat ly cac vat lieu C kich thuoc nho (he c6 cau triic nano) O Muc tieu luan dn tap trung vao nghien cuai mot so qua trinh dong hoc cua4ie^en-euc-ban din c6 cau tnic nano-xop m6i truong dien giai nhim lam sang to nhiing van di sau : kich thuoc hat bdn ddn co anh huong den tinh chat vT mo hay khong ? Neu co thi lam the ndo de dinh luon2 dugc tic kit qua thuc nghiem vi du C-V? Tinh chat vi mo co bi anh huong bdi tinh nano-xop (morphology) cua dien cue khong ? Moi quan he giixa sU thay doi tinh chat vi mo vd nhiing hieu ung vTmo ? Lam the ndo detdc dong len he vat ly nhdm tao nhiing dac tinh co loi cho ling dung Doi tuong nghien cim ciia luan an la v^t lieu Ti02 co cau tnic dang Anatase va Rutile Cac dien cue TiOj chii yeu la dang mang, cau tnic NanoXop (NanoPorous), che tao bing phuong phap sol-gel va tir bpt thuong phim P25 (DEGUSSA) co hat trung binh -15nxr\ Mot so nghien cuu tren mang TiOj dang dac khit (Compact), che tao bdi phuong phap dot (Pyrolysis) Ti nhiet cao duoc tieh hanh nhim so sanh tinh chat ciia hai loai dien cue Phqm vi nghien ciiu ciia luan an g6m nhimg nghien cixu thuc nghiem va ly thuyet m6 phong thuc nghiem ap dung cho doi tuong vat lieu Ti02: */ Cac mang dien cue dugc che tao va khao sat cau tnic pha bing nhi^u xa tia X ; hinh thai hoc (morphology) bing hi6n vi dien tii quet (SEM) va hi6n \i luc nguyen ttr (AFM) Dac trung quang dien hoa dugc khao sat thong qua ket qua dac tnmg dong-the phan cue (I-V) va dien dung dien hoa-the phan c\ic iC-W) di6u kien khong va co kich thich bing anh sang tii ngoai NhiJng nghien cuu nhAm he thong tfnh chat nhay hoa, nhay quang co lien quan den cau tnic, hinh thai hoc ciia mang dien cue TiOn nhiing moi truong hoa hoc khac Kel qua thu dugc la co so khoa hoc cho viec trien khai ling dung cac dien CUQ loai */ Luan an xay dung ly thuyet mo phong qua trinh thiei lap can bang va dac trung dien dung dien hoa cua he ban din cau tnic nano-xop tiep xiic voi dung dich dien giai Qua trmh khop (Fitting) ly thuyet vdi so thuc nghiem C-V ciia he dien cue TiO^ dang nano-xop va dac cac moi truong dien —giai, che dp kich thich khac da dugc tien hanh Y nghia khoa hoc vd thuc tiSn ciia di tai : luan an da xay dung dugc phuong phap ly thuyet di six ly phep thuc nghiem C-V cho he dien cue ban din cau tnic nano-xop, la huong nghien cuu dang dugc quan tam tren the gioi Su dung phuong phap de nghien cuu vat lieu Ti02 (la val lieu rat dugc quan tarn vi kha nang ung dung rgng rai) cho chiing ta nhiing hieiu biet ve hoc thuat doi vdi tinh chat vi mo cua he hat nho, nh^m ly giai ca che mot so tinh chat vat ly, d6ng thoi ggi y cho nhiing dinh hudng ling dung he dien cue ban dSn nano-xop Ngoai phan mo dau, ket luan, kien nghi huong nghien cuu liep theo, danh muc tai lieu tham khao va phu luc, noi dung luan an dugc trinh bay chuong : Chuong 1: Tong quan ve cac qua trinh dien hoa ciia dien cue ban din cau tnic nano-xop Chuong 2: Trinh bay phuong phap sol-gel che tao dien cue T1O2 nanoxop va phuong phap dien hoa dien cue dd nghien cuu tinh chat ciia dien cue ban dan Chuong 3: Xay dung m6 hinh ly thuyet mo ta qua trinh thiet lap can bang ciia tiep xiic ban dan-dien giai (SEI) cho he hat kich thuoc nho Xac lap quy trinh tinh dien dung dien hoa bang phuong phap so de' mo phong dac trung C-V cLia dien cue ban dan nano-xop Chuong 4: Trinh bay kei qua nghien cuu thuc nghiem \i cau tnic, hinh thai hoc (morphology), hoai tfnh quang hoa ciia dien cue TiO^ nano-xop cac moi truong hoa hoc va anh huong ciia che kfch thich khac Chuong 5: Trinh bay ket qua mo phong qua trinh thiet lap can bang cii< tiep xiic SEI d6'i voi he hat ban dan kich thuoc nho ; Ket qua nghien cuu v^ dien dung dien hoa dien cue Ti02 cau tnic Nano-Xop ; Khop (Fitting) I3 thuyet voi ket qua thuc nghiem C-V cua chuong 4, nit nhiing th6ng tir dinh lugng ve tinh chat ciia dien cue nano-xop Luan an dugc thuc hien tai Vien Vai ly ung dung & Thiet bi Khoa hoc Trung tarn KHTN & CNQG va bg m6n Vat ly Chat ran, khoa Vat ly, trudng Dai hoc Khoa hoc Tu nhien , Dai hoc Quoc gia Ha noi Noi dung chinh ciia luan an dugc trinh bay tai cac Seminar khoa hoc ciia khoa Vat ly, truong Dai hoc Khoa hoc Tu nhifen, Dai hoc quo'c gia Ha n6i, tap chi chuyen nganh , cac hoi nghi khoa hoc va ngoai nude CHUONG QUA TRINH DIEN HOA DIEN CUC BAN DAN CAU TRUC NANO-XOP 1.1 Qua trinh dien hoa ciia dien cue ban d^n I.IJ Tiep xuc chat bdn ddn vdi chat dien giai Xet chat ban din loai n va chat dien giai chi chiia cap oxy hoa khii (cap Redox) AVA^"" Khi chit ban dan va dien giai each ly chan kh6ng (hinh 1.1 a/), miic Redox nam dudi miic Fermi cua ban dan Muc chan khong ciia ban din tuong ling vdi muc -4,5V so voi the dien hoa chuan NHE Khi cho hai pha tiep xiic v6i nhau, trang thai can bang dugc thiet lap Khi miic \T»H X • Hdt chin khong -4.5 i ^'B CB CB ^fb Ec A*/A'* •Er A7A' VB VB Bvidanloiin CMnkh6ng U Oiini U ,T Ban dan loai n Dien giai — Ivs NHE) Hinh LI: al Trang thdi chan khong hi Tiep xuc bdn ddn-dien gidi Fermi cua ban din se bang miic Redox cua dung dich (Hinh L i b / ) , Trang thai can bang dugc thiet lap la qua trinh chuydn doi dien tich tir ban dan vao dung djch Su chuyen doi dien tu (electron transfer) thong qua phan img hoa (phan ling Redox) A ' + e > A^ , Qua trinh lay bot dien lu tu do, tao lop ngheo hat tai co ban vung hi mat tiep xiic cua ban dSn (con goi la vung dien tich khong gian) Cac dien tfch khong gian ban dan va cac ion irai dau dung dich tao lop dien tich kep cua chuyen tiep SEI la dac tnmg quang dien ciia dien cue TiO^ pha Anatase dung dich dien 'to " " " e giai nhu thi nghiem voi pha Rutile 0.20 O.lmol.dm' NaOH/O.lmol.dm CH.OH < 0.15^ E gO.lOH 1-1 D O.lmol.dm NaOH S 0.05 o 0.00 1.0 -0.5 0.0 Potential (V V5 Ag/AgCl) 0.5 Hinh 4.7: Ddc trung I-V cua dien cue Ti02 Anantase sol-gel dudi dnh sdng tic ngoai Ddng quang dien cua dien cue Anatase dung dich NaOH-^CH30H Idn hon nhieu so vdi dung dich NaOH Nguyen nhan cua su khac biet ve boat tinh cua pha Rutile va pha Anatase ddi vdi cac cap Redox khac vin dang la vam de tranh cai [13,25] De nhan thay la cau tnic tinh the cua pha Rutile va Anatase khac (hinh 4.8) Dieu dd din tdi cau tnic be mat cua cac hat Ti02 cd pha khac la khac [13] Tuy nhien bing chumg cho lap luan cdn chua dii thuyet phue vi anh hudng cua cau tnic be mat Ti02 den boat linh cho den vin dang la van de dugc cac nhdm nghien ciru tren the gidi tiep tuc lam sang to Neu nhin nhan theo gdc mo hinh muc nang lugng cua qua trinh trao ddi dien tfch (The Energy Level Model of Charge Transfer) [63] thi rat khd ly giai hien lugng tren.Theo ly thuyet nay, ddng trao ddi dien lich lien quan den muc nang lucmg cua ban din moi irudng dien giai Nhung nghien cuu [63.27] cho thay mirc nang lugng qua pha Rutile va Anatase la nhu Tuy 82 Rutile Anotose ^ AP d;;'.o=i.98oi Ti-0' = 4.593A C=2.959^ Eg = 3.iev p = 4.250 g/cm= AC^f =-2l2.6iicol/moie Eg = 3.3eV AGf =-211.4 kcol/moie //m/z ^.S.- Cd'w rriic tinh the ciia pha Rutile vd Anatase nhien cin nhd la miic nang lugng cua TiOj dugc de cap d day la trang thai bi kich thich bdi anh sang Neu trang thai kich thich, muc nang lugng cua Ti02 cd the thay doi dugc bing co che nao dd, thi sir khac ve boat tinh giira pha Rutile va Anatase hoan loan cd the ly giii dugc theo mo hinh muc nang lugng Chuong cua luan an se de cap ky hon ve hien tugng dich chuyen bien viing (Bandedge Movement) cua Ti02 pha Anatase dudi anh sang tir ngoai Hoat tinh cao cua pha Anatase so vdi Rutile cung da dugc nhieu nhdm nghien cuu khang dinh va sir dung de chgn lira vat lieu nhirng ung dung 4.2.2 Hinh thdi hoc (Morphology) vd hoqt tinh quang dien hod Trong phan se trinh bay ve anh hudng ciia hinh thai hgc mang dien cue Ti02 de'n boat tinh quang dien hoa nhu the nao Dac linh quang dien hoa cua miu dien cue nano-xop Ti02 - P25 dung dich NaOH va NaOH cho them 0,1 M Methanol (CH^OH) dugc khao sat ( Hinh 4.9 ), cho thay linh nhay cam cua dien cue P25 ddi vdi cac cap Redox khac Dac linh cua dien cue P25 la mang ban din chua pha Anatase la chu yeu 83 0.8- I , (m A/cm^) ph • • • 0.4- ,' 0.2- m ^ , / / -is**' -0.4 J * -0.2- ] 0.4 V v.r.Ag/AgCI Hinh 4.9 : I-V sdng cua dien cue Ti02-P25 Na0H/H20(I) vd'NaOH/CH,OH/H20(2) Dac tinh I-V chieu sang cua dien cue P25 cho he sd laip diy "Fill Factor" rit Idn, chiing to hieu suit chuyen boa nang lugng anh sang cua nd rit tot Ket qua cd y nghia quan trgng cho cac iing dung cua bgt P25 Do cac phan iing Redox xiy tren be mat tiep xiic SEI, nen ngudi ta cd ging tao ca'u hinh ma dien tich liep xiic Idn, nhim tang tdng hieu suit chuyen hoa nang lugng Dd chinh la mang xop bao gdm cac hat nho xep chong len (hinh 4.10) Cau hinh nano-xop cd the (ve nguyen tic) lam tang dien tich be mat noi len hang nghin lin so vdi dien tich be mat dien cue Nhd cd nhimg hoc xop (porous) Sia> kich thude nh6,trong mang dien cue xop tao cac kenh din nho Theo cac kenh din De dan dkn (TCO) dan dung dich tham sau vao mang dien cue, tao nhung liep xiic ban din-dien giai tren be mat ciia timg hat ban din, ma d dd xay cac phan iing Redox Hinh 4.10: Mo hinh mdng dien cue bdn ddn nano-xop 84 Nhu vay, neu tang day cua mang dien cue xop, l6ng dien tfch be mat noi se tang, cd nghia la tang tdng hieu suat Nhung thuc le day mang dien cue tang len den miic nao dd, xop cua nd se giim va khdng dam bao tao dugc cac kenh din chat dien giai den be mat noi cua cac hat ben trong, nen hieu suit khong the tang len nira Ngoai phin nhieu cac phan irng Redox hiru ich thudng xay tie dong ciia anh sang Khi mang dien cue day len, anh sanh bi hap thu va tan xa tren cac bien hat, nen chi cd the xuyen sau vao mang dien cue d miic nhat dinh Do vay, vin de day tdi uu cua mang dien cue xop cin tinh den cac iing dung Hinh 4.11 la ket qua Ip^,(mA/cm) nghien ciiu anh hudng cua day mang TiOj P25 den tdng hieu suit chuyen hoa quang0.4 dien Hieu suit chuyen boa tang 0.2 theo do day mang P25 cho thay cac miu ma luan an che tao -fl.2/ ^-0.8 -0.4 0.4 V vs Ag/AgCl dugc cd xop tot den day J_ IS^im M6t sd nhdm nghien ciiu Hinh 4.11: dnh hudng ddy mdng dien cue P25 :1- l/jm ; 2- lOfsm ; 3-15fm [86,44,33] da tao dugc nhung :02 dien cue xop P25 day den 40(im 4.2.3 Kich thude hqt vd hoqt tinh quang dien hod cua Ti02 Nhu da trinh bay d phin tren, mang dien cue nano-xop cho hieu suit sir dung cao la t6ng dien tieh be mat noi cua tiep xiic ban din-dien giai Idn Nhung hieu suit khong chi phu thugc vao l6ng dien tich be mat noi, ma cdn phu thugc rit manh vao khich thuoc hat ciia mang dien cue Tren the gidi [25,27] , da de cap tdi vai trd kich thude hat ban din (hat Ti02 trudng hgp ciia luan an) qua irinh chuyen hoa nang lugng anh sang 85 Khi hat ban din cd kich thuoc nho d miic nao dd (thong thudng la thang nanomet), mot sd qua trinh vat ly se xay hoan loan khac so vdi cac hat cd kich thude Idn Su inh hudng ciia kich thude hat ban din co nanomet den tinh chit val ly the hien thong qua nhieu linh chat nhu : Su thay doi rgng vung cam (hieu iing kfch thude lugng tir [27]) , kha nang chgn Igc cac phin ling quang xiic lac ciia cac oxit ban din [25], su giim mat mat lai hgp tren bien cac hat nhd w Ip,, (mA/cm ) 'T)'g" 0.4 0.2 11.2 -0.8 0.4 -0.2 0.4 V v^ Ag/AgCl Hinh 4.12 : Ddc trung W sdng cua dien cue TiO2 P25 (1) vd MCB (2) NaOH/CHjOH/H20 De khao sat anh hudng cua kich thude hat den boat tinh cua mang dien cue Ti02 , luan an da lien hanh dac linh quang dien tren miu P25 (kich thude hat trung binh - 25nm) va dien cue che tao nhu ddi vdi P25 nhung tir nguyen lieu ban diu la bgt Ti02 pha Anatase cd thuong hieu MCB (kich thude hat trung binh 200 nm) moi trudng dien giai 0,1M NaOH/CH30H/H20 Kel qua tren hinh 4.12 cho thay : tren miu dien cue P25, ddng quang dien tang manh va bao hoa nhanh phan cue ve phia dien the duong Trong dd miu dien cue MCB cho ddng quang dien nhd hon ban va the '^onset" dich ve phia dien the phan cue duong so vdi miu P25 Ket qua chiing to sir mat mat tai hgp dien cac MCB Idn hon nhieu so vdi P25 Dieu 86 trai ngugc vdi nhirng ly thuyei trudc day ve qua trinh chuyen hoa nang lugng quang dien : cac ban din da linh the ma d dd kich ihudc hat cang nho thi sir mat mat tai hgp tren bien hat cang Idn Tir nhung ket qua nghien ciiu tren cho thay, co che tang hieu suat chuyen hoa nang lugng he dien cue ban din nano-xop khong the ly giai don thuin bing sir tang tdng sd be mat noi cua tiep xiic ban din - dien giai, ma nd lien quan den co che tinh te hon, lam thay doi ve chat qua trinh val ly 4.2.4 Thay doi dong hoc (kinetic) phdn ung Redox dien cue cdu true Nano-Xdp tiep xuc vdi moi trudng dien gidi bi chieu sdng Dong hgc cua phan iing Redox tren be mat tie'p xiic ban din - dien giai cd the danh gia qua dac trung I-V Nhung kel qua nghien ciru kinh dien [63,22,54] ve qua trinh dien boa tren tiep xiic SEI, danh gia ve dong hgc phin iing Redox va lai hgp tren be mat tiep xiic dugc trinh bay tren hinh 4.13 Trudng hgp (1) dac trung dong-the phan cue cd he sd lap diy (Fill factor) Idn, tuong iing vdi tdc phan iing Redox nhanh va tai hgp be mat it Nhung trudng hgp cdn lai he sd lap diy ciia dac trung I-V nho va the "onset" dich chuyen ve phia the phan cue duong the hien sir suy giam tdc cua phan iing Redox hoac gia '' * 2/ 3/ / Doaj! bao I / i ^''.V" (11 Phan linj; Redox nhanh khonj* oi tii hop be mat (2) hop be mit nhanh (3) Phan ling Redox cham \L\ Dich chuven bien vunn xuoni: di/oi lich dien dirung Iren be niAl Hinh 4.13: Dofio hoc phdn img Redox vd tdi hop ciia tiep xiic bdn ddn-dien gidi 87 tang tai hgp be mat Luan cir dugc ap dung de danh gia hien tugng giam su mat mat tai hgp be mat ciia he dien cue kich thude hat nho 25 nm (trong P25 ) va kich thuoc Idn -200 nm (MCB) (phin 4.2.3) De nghien ciiu sir thay doi dong hgc (kinetic) cua phan iing Redox dien cue cau tnic nano-xop, ta so sanh nhung dac trung the hien dong hgc phan iing cua miu dien cue Ti02 dac vdi dien cue nano-xop Neu so sanh dac trung I-V chie'u sang dien cue TiOs dac va nano-xop (hinh 4.14) ta cd nhan xet: Dien cue Ti02 dac cd hieu suit chuyen hoa quang dien (Fill factor) thap I hon nhieu cua dien cue (10 mA/cm^) •^•••••••"3 nano-xop Ma't mat tai hgp be mat cua dien ^'-^ cue dac Idn, nhung kich thude hat ciia nd Idn gap nhieu lin dien cue nano-xop Tdc phan ling Redox cua dien cue nano-xop cao hon nhieu so vdd dien cue dac 0.8'^^V 20 -0.4 0.4 T*- V vs.Ag/AgCl •20 Hinh 4.14: I-V sdng cua dien cue Ti02 nano-xop (3) vd ddc dung dich NaOH (1 );NaOH/CH^OH/H20 (2) Hien tugng tang tdc phan iing Redox cua dien cue nano-xop so vdi dien cue dac se rat khd ly giai tren quan diem mo hinh miic nang lugng qua trinh trao ddi dien tich Theo mo hinh [63], tdc phan iing u-ao ddi dien tich giira cap Redox va cac vimg cua ban din ty le vdi ham e-mu cua binh phuong chenh lech miic nang lugng giua vi iri bien vomg ciia ban din va miic Redox (phan 1.2.2) Cac nghien curu ve dung dich cho ring miic Redox khong the thay doi dugc kich thich bing anh sang [63] Nhu vay su thay doi tdc phan ung Redox trudng hgp dien cue nano-xop cd the lien quan den hien luone dich chuven bien vung ciia ban din dudi tac dong ciia anh sang 88 De xac dinh vi tri bien viing ciia ban din, nguoi ta diing phuong phap kinh diein la dac trung dien dung ciia chuyen tiep SEI phu thuoc vao the phan cue Cac thong sd cua ban dkn va vi tri the viing phing (flatband potential) thu duoc tir dac trung 1/C- - V ( dudng Mott-Schottky) 1.2x10 I.OxlO' 8.0x10' ^ 6.0x10*4.0x10' O 2.0x1O'' Kel qua thuc nghiem ^ ^ Kel qua ly thuyet 0.0 —\ •1.2 > -0.9 > -0.6 ' -0.3 p 1—1 0.0 1 — I — — I 0.3 0.6 0.9 p 1.2 The phan cue so v6\ SCE Hinh 4.15: Ddc trung Mott-Schottky dien cue TiO2 ddc Tren hinh 4.15 la dac tnmg Mott-Schottky cua dien ctre Ti02 dac Dien cue la mau ly tudng de kiem nghiem md hinh rao the' Schottky c6 dien Trong miu dien cue nay, cac tiep xiic dien giai / ban dan khdi / kim loai cd pham cha't lot Cac khoanh iron la sd lieu thuc nghiem, cdn dudng lien net la kei qua tinh loan theo ly thuyei bang phuong phap sd trinh bay chuong cua luan an Dac tfnh tuyen tfnh cua dudng Mott-Schottky ddi vdi dien cue Ti02 dac cho iha'y mo hinh rao the' Shottky e6 dien phii hop dien cue loai Ddi vdi dien cue Ti02 nano-xop se sinh nhirng khd khan viec xac dinh cac thong sd vat lieu tur dac tnmg C-V Khi hat ban dan cd kich thude cung C vdi kfch thude viing dien tfch khdng gian (vai dai Debye) va ca'u O hinh xop tao tiep xiic SEI tren limg hat nho, hieu ung che chan tinh dien tai \aing tiep xiic se khong man iheo ly ihuyel c6 dien 89 l.OOE + 010 - Oo o 8.00E + 009 6.00E+009- 'b 4.00E + 009 O 2.00E + 009 K^t qua thuc nghiem • ^ ^ K e l q u a ly t h u y e t O.OOE-t-000•1.2 -0.9 —1"-0.6 —I— •0.3 0.0 —r— 0.3 0.9 0.6 1.2 T h e p h d n c u e so v d i S C E Hinh 4.16: Ddc trung Mott-Schottky cua dien cue Ti02 Sol-gel NaOH + Methanol (nano-xop) Dac tnmg Mott-Shottky ciia dien cue Ti02 sol-gel dung dich NaOH+Methanol tren hinh 4.16 Ro rang la dudng Mott-Schottky cua dien cue nano-xop khong tuye'n tfnh nhu dien cue dac (Hinh 4.15) De kiem chung cho luan diem tren, luan an tien hanh phan tfch ket qua ciia nhdm nghien ciiu khac cua Germany [44] (Hinh 4.17) : Dang dieu thay doi dudng Mott-Schottky cung gidng nhu miu sol-gel luan an che tao 1.0x10'" 8.0x10' O o 6.0x10'- ^*' 4.0x10 - O Ket qua thuc nghiem - ^ Kei qua ly thuyet 2.0x10' 0.0 I ' • 1.5 l l -1.2 r -0.8 • — I -0.6 ' -0.3 ' I 0.0 ' I 0.3 ' I 0.6 The phan cue so vdi SCE Hinh 4.17: Ddc trUfig Mott-Shottky cua dien cue TiO nano-xdptheoj44 90 De xac dinh cac thong so val lieu cung nhu vi trf bien viing nang lugng cua ban dan tir phep C-V cua he dien cue nano-xop ddi hoi phai cd phuong phap sir ly dac trung C-V hoan loan khac Phuong phap dd (chuong 3) ap dung cho dien cue TiOj se dugc de cap ky hon d chuong ciia luan an _ Trong luan an lien hanh khao sal dac trung C-V (trong tdi va kich thfch anh sang UV) cua he dien hoa dien cue T1O2 ed hinh thai hgc (Morphology) khac Dieu dac biet la phat hien th^y hien tugng dich chuyen ve phia the phan cue duong cua dac trung C-V d mSu Ti02 nano-xop dung dich dien giai NaOH + Methanol kfch thfch anh sang UV (hinh4.18) Mat khac, he cho hi6u SUSLI chuyen hoa quang-dien cao nh^Ll cac he dugc khao sat 1000- • \ •J u • Sing UV TOI 100•0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 The phan cue [V vs Ag/AgCI] Hinh 4.18: Hien tuang dich chuyen ddc trung C-V bi kich thich bdng dnh sdng tu ngoai cm dien cue T/Oj ^^^ ^^^^ Nano-xop Doi v6i cac he : dien cue TiOj dac hay TiOj nano-xop (sol-gel va P25) cac dung dich ma hifeu sni\ chuyen hoa quang difen tha'p nhu NaOH + HjO su khac biet dac U'nh C-V t6'i va kich thfch UV khdng dang ke TIT lap luan tren, ta dua gia thuyet : su dich chuyfen dac trung C-V ciia dien cue ban din TiOj co cau tnic nano-xop ve phfa dien the phan cue duong bi kfch thfch bang anh sang tiJr ngoai co lien quan den co che tao 91 hieu suat chuyen hoa quang-dien cao he Co che se dugc trinh bay chi liei hon d chuong cua luan an ve hien tugng dich chuyen bien viing (bandedge movement) he dien cue ban dan TiO nano-xdp/dien giai ketluan Trong chuang da trinh bay mdi sd nghien cuu thuc nghiem tren cac mau dien cue ban dan Ti02 Mot sd kei qua cd the tdm tat nhu sau : * Da che tao dugc miu dien cue Ti02 nano-xop bang phuong phap solgel va tir bgt thuong pham P25 (DEGUSSA) va MBC Mau dien cue dac che tao bang phuong phap ddt la kim loai Ti * Nghien curu mdi quan he giua ca'u tnic pha tinh the cua TiOj vdi boat tfnh quang dien hoa cua cac miu nghien curu Ke't qua cho tha'y pha Anatase cd boat tfnh cao hon ban pha Rutile * Do xop cua mau ta che tao tdt, nen cd the tang dd day mang dien cue (de tang tong hieu sudt chuyen hoa quang- dien hoa) cac iing dung * Nghien cull hieu suSil chuyen hoa quang-dien ddi vdi cac mang dien cue cd kfch thude hat khac da dugc tie'n hanh Hieu suat chuyen boa quang - dien tang kfch thude hat nhd dudi miic nguong * Ddi vdi mang dien cue Ti02 nano-xop, bi kfch thfch bdi anh sang ttr ngoai, mdi trudng dien giai thfch hgp (NaOH + Methanol), tdc dd trao doi dien tfch giira ban dan va cap Redox tang dot bien, \&a at qua trinh tai hgp tren be mat * Ke't qua dac trung C-V ciia dien cue Ti02 NaOH+Methanol, bi chieu sang bang tir ngoai, bi dich ve phfa the phan cue duong (theo thang dien hoa) so vdi C-V tdi * Dua gia thuyet ve moi quan he giua hieu sua'l chuyen hoa quang dien cao cua dien cue Ti02 nano-xop vdi hien tugng dich chuyen dac tnmg C-V ban din bi kfch thfch bang anh sang thfch hgp 92 CHUONG DIEN DUNG DIEN HOA DIEN CUC TIO, NANO-XOP CO CHE TANG TOC DO PHAN UNG REDOX Ddi vdi trudng hgp hat ban dan nho hon kfch thude vung dien tfch khong gian (vai dai-Debye), cong vung nang lugng (Band bending) khdng ddng nha'l vdi chenh lech giua muc Fermi va Redox Phan bd ndng hat tai cung nhu dien the viing dien tich khdng gian a trang thai can bang khdng cdn la don tri nhu trudng hgp kfch thude hat Idn, ma bi anh hudng bdi thdng sd kfch thude hat De giai quyet va'n de neu tren, chuong cua luan an da xay dung mo hinh ly thuyet mo phdng tinh chait he ban dan kich thude nho tiep xiic vdi moi trudng dien giai chiia cap Redox Trong chuong se trinh bay mgl sd ke't qua md phdng ddi vdi he ban din Ti02 cd kich thude hat nhd, tiep xiic vdi dung dich dien giai Noi dung gom phan chinh : * Mo phdng qua trinh thiet lap can bang cua chuyen tiep SEI ddi vdi he Ti02 tiep xiic vdi cap Redox I'fP' Ket qua mo phong thu dugc phan bd ndng hat tai va dien the he hat nhd la ham phu thugc vao kfch thude hat Chung tao co sd du lieu de tinh dac trung C-V dien cue Ti02 nano-xop * Qua trinh mo phong dac trung C-V bang phuong phap PBP(chuong 3) dugc thuc hien ddi vdi dien cue TiO^ Ket qua ly thuyet dugc khdp (Fitting) vdi thuc nghiem, da cho mdt sd thong sd ve tfnh chat cua viing chuyen tiep SEI ddi vdi he ban dan nano-xop Tren co sd ket qua thu dugc, se ban luan ve anh hudng cua cac kenh dan dien giai cd kfch thude nano (nano channel) den dien dung Helmholtz (tfnh chat be mat cua ban dan) ciing nhu hien lugng dich chuyen bien viing cua ban dan d trang thai bi kfch thfch Gia thuyet ve su gia tang tdc trao doi dien tfch (phan irng Redox) la he qua cua hien lugng dich chuyen bien \aing se dugc ly giai tren mo hinh miic nang lugng (The Energy Level Model of Charge Transfer) cua qua irinh trao d6i dien tfch 93 5.1 Qua trinh thiet lap can b^ng ban dan - dien giai (SEI) , 5.1,1 Cdc thong sdsudung mo phong he TiO2 : TII'^ Trong mo phong qua trinh thiet lap can bang ciia chuyen tiep SEI la sir dung cac thong sd vat lieu cho Ti02 nhu sau : */ Nong pha tap loai n (ban din loai n) n^^ ng = 10'^ cm"^ la ndng thudng dugc sir dung cho Ti02 pha Anatase [27] */ Mat trang thai vung din va viing hoa tri cua Ti02 dugc gia thiet [27, 28] nhu nhau: Nc = Nv=2.10*^ cm-\ */ Do r6ng vung ca'm cua Ti02 thdng thudng [63,27] E^ = 3,2 eV */ Tai nhiet phdng kT = 0,03 eV , dd ndng 16 po cua Ti02 se dugc tfnh qua cac thong sd Nc, Nv , Eg, n^ va kT */ Vi tri bien vung din E^s va viing hoa tri Eys theo thang the dien hoa nhu sau [4,63] : Ecs = - O^^V ; Evs = + 2,5V so vdi chuan SCE */ Thong sd val lieu K va b (Trong cdng thiic 3.33, 3.36): K=10; b=l */ Ddi vdi Ti02 hing sd dien moi £ = 100 Cac dai lugng vat ly : dien tfch e=l,6.10"^^ C ; hang sd dien moi chan khong £0 = 8,85.10"^^ Rem"^ ==> dai DebyeTi02: LDebye=3,5.10-^cm(35nm) Khi dd Idn cua don vi dien trudng E* (cong thiic 3.41) se cd gia tri : kT ^ , _, = 7,5.10'VolLem 1^ Debye Ddi vdi cap Redox I" /1"^ theo giao trinh ve dien boa [4,63] : */ Nang lugng tai tao lai (Reorganization energy) ER = 0,4 eV */ Miic oxy hoa E^^ = "O'^V ; khir E^,, = +0,5V so vdi chuin SCE */ Miic Redox chuin E'^^^ox = -^O^'^ so vdi chuin SCE */ Nong cua trang thai oxy hoa C,,, khir C^ cua cap Redox gia thiet la: C =Cd=10'^cm'^ (tuong duong 0.01 Moi ndng dung dich) 94 Vi tri miic Fermi ban dau ciia TiO-, tfnh nhu sau : Ep = E^j^ -kT.ln Ncy Khi hieu chenh lech giua muc Fermi va miic Redox se la : ELdox - Ep = l,33.kT (kT =0,03 eV tucmg duong T=300K) Nhan to chuydn doi {Transfer factor) K,, K^ (theo 3.52 , 3.53) dugc tinh vai ^Tr'.t n„^ gia thiet: L Diffusion / (lT\x^.n ^ D*"0 ^Diffusion = 10 (5.1) J Khi dd gia tri ciia K^ va K^ chenh lech ~ 33 bac Dieu cho thay tdc trao doi dien tich cua cap Redox vdi cac viing cua ban din rat khac nhau, nha'l la ddi vdi ban din viing cam rgng nhu TiOj (Oxide kim loai ndi chung) Ddi vdi tiep xiic kim loai - ban dan, nhan td chuyen ddi (theo cdng thiic 3.54 , 3.55) Kg va K^ cd Idn so sanh dugc vdi Su khac biet ciia nhan td chuyen ddi K^, K^ ddi vdi loai chuyen tiep ^ SEI va Kim loai - Ban din se din tdi sir khac biet Idn ve tfnh chat dong hgc (Kinetic) cua chiing 5.1.2 Kiem chitng phuong phdp ddi vdi tiep xiic Kim loai - Bdn ddn De kiem chiing su diing din cua phuong phap, luan an tien hanh mo phong qua trinh thiei lap can bing cua liep xiic Ban din - Kim loai (theo quy trinh da trinh bay a chuong 3) , gia thiet miie Fermi kim loai trung vdi miic Redox cua cap 171'^, cd gia tri tuyet ddi : Ep = +0,1 V so vdi chuin SCE Gia tri cua thong sd vat lieu (cong thiic 3.54 va 3.55) gia thiei : l +b Diffusion ( N: M Tr^T^.n, J V "o J K Ml + 1/b ^Tr'.Tij.n,, Difiusion =1 / V "o J (5.2) 95 Quy trinh dugc thuc hien theo luu dd (3.1) Ket qua mo phong trang thai chuin dirng (steady slate) ciia phan bo hat tai tai chuyen tiep ban din-kim loai nhiet phdng tren hinh 5.1 25-, g •to 20 c a/ Mo phong rao the Shottky cua chuyen tiep K i m loai-Ban d^n b/Cau hinh ban clan 10Q 10 15 Kich thinrc (Do dai Debve) Hinh 5,1: Phdn bo hat tdi tai chuyen tiep Kim loai-Bdn ddn trgng thdi chuan ditng Day cung chfnh la phan bd cua dien the viing dien tfch khdng gian Ket qua phan anh chfnh xac tat ca cac tfnh chat va thong sd ca ve dinh tinh cung nhu dinh lugng cua ly thuyet Schottky c6 dien : Dang cua cong viing nang lugng (band bending) la nhu ddi vdi ca vung din va vung hoa tri; Do cao cue dai cua rao the tuong iing chfnh xac vdi hieu khoang each giua miic Fermi cua kim loai va ciia ban din (irudng hgp 21,33 kT) Dac tfnh dong hgc (kinelic) cua qua trinh thiet lap can bing the hien qua thdi gian thiei lap can bing Ddi vdi tiep xiic Ban din-Kim loai, thdi gian thiet lap can bing cua hat tai khong co ban (16 ddi vdi ban din loai n) tuong duong vdi hat lai ca ban Ly la tdc irao doi dien lich cua kim loai vdi hai \aing (viing din va hoa iri) cua ban din (the hien qua nhan id chuyen ddi - Transfer factor) cd Idn so sanh dugc vdi nhu da trinh bay d tren ... bdn ddn - dien gidi (Charge Transfer Processes) Trong tiep xiic ban din-ban din hay ban din-kim loai qua trinh trao doi dien lich xay dien tir (16 trong) chuye''n dai giua hai pha va di chuyen 10... co the ly giai tren m6 hinh miic nang lugng (hmh 1.6) nhu sau : Trong toi, tiep xiic voi chat dien giai, - E : RwJo* Chieu sang Trong toi Hinh 1.6: Co che hieu iing quang dien hod dien cue Bdn... C3H7OH phong ; San phim thu dugc la sol (hinh 2.1) cd nhdt tuong duong Khuay lai ''-80°C Trong 8^ vdi nong ISOgA- Trong qua uinh c6 dac sol chan kh6ng mot phin H2O , C3H7OH va HNO3 du thua bi bay hoi