I 0.6 hien cua qua trinh hoa ly
4.2.2 Hinh thdi hoc (Morphology) vd hoqt tinh quang dien ho d.
Trong phan nay se trinh bay ve anh hudng ciia hinh thai hgc mang dien cue Ti02 dén boat tinh quang dien hoa nhu the naọ
Dac linh quang dien hoa cua miu dien cue nano-xop Ti02 - P25 trong 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 đi vdi cac cap Redox khac nhaụ Dac linh nay cua dien cue P25 la do mang ban din chua pha Anatase la chu yeụ
0.8- • • • 1 ,' m ^ / / -is**' J * -0.4 0.4- 0.2- -0.2- I , ph (m A/cm^) , ] V v.r.Ag/AgCI 0.4
Hinh 4.9 : I-V sdng cua dien cue Ti02-P25 trong Na0H/H20(I) vd'NaOH/CH,OH/H20(2)
Dac tinh I-V khi 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 nay cd y nghia quan trgng cho cac iing dung cua bgt P25.
Do cac phan iing Redox xiy ra tren be mat tiep xiic SEI, nen ngudi ta cd ging tao ra cáu 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 nhau (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 cuẹ Nhd cd nhimg hoc xop (porous)
kich thude nh6,trong mang dien cue xop tao ra cac kenh din nhọ Theo cac kenh din nay dung dich tham sau vao mang dien cue, tao ra nhung liep xiic ban din-dien giai tren be mat ciia timg hat ban din, ma d đ xay ra cac phan iing Redox.
Sia> De dan dkn
(TCO)
dan
Hinh 4.10: Mo hinh mdng dien cue bdn đn nano-xop
Nhu vay, neu tang do day cua mang dien cue xop, l6ng dien tfch be mat noi se tang, cd nghia la tang tdng hieu suat. Nhung thuc le khi do day mang dien cue tang len den miic nao đ, do xop cua nd se giim va khdng dam bao tao ra dugc cac kenh din chat dien giai den be mat noi cua cac hat ben trong, nen hieu suit khong the tang len nirạ Ngoai ra phin nhieu cac phan irng Redox hiru ich thudng xay ra do 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 do nhat dinh. Do vay, vin de do day tdi uu cua mang dien cue xop cin tinh den trong cac iing dung.
Hinh 4.11 la ket qua nghien ciiu anh hudng cua do day mang TiOj P25 den tdng hieu suit chuyen hoa quang- dien. Hieu suit chuyen boa tang theo do do day mang P25 cho thay cac miu ma luan an che tao dugc cd do xop tot den do day IS^im. M6t sd nhdm nghien ciiu [86,44,33] da tao dugc nhung dien cue xop P25 day den 40(im
.Ip^,(mA/cm) 0.4 0.2 -fl.2/ ^-0.8 -0.4 J_ :02 0.4 V vs. Ag/AgCl
Hinh 4.11: dnh hudng do đy mdng dien cue P25 :1- l/jm ; 2- lOfsm ; 3-15fm
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 do 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 cuẹ Tren the gidi [25,27] , da de cap tdi vai trd kich thude hat ban din (hat Ti02 trong trudng hgp ciia luan an) trong qua irinh chuyen hoa nang lugng anh sang.
Khi hat ban din cd kich thuoc nho d miic nao đ (thong thudng la thang nanomet), mot sd qua trinh vat ly se xay ra 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 do rgng vung cam (hieu iing kfch thude lugng tir [27]) , kha nang chgn Igc trong cac phin ling quang xiic lac ciia cac oxit ban din [25], su giim mat mat do lai hgp tren bien cac hat nhd ....w .
'T)'g" 0.4 0.4 0.2 11.2 -0.8 0.4 -0.2 Ip,, (mA/cm ) 0.4 V v^. Ag/AgCl
Hinh 4.12 : Ddc trung W sdng cua dien cue TiO2 P25 (1) vd MCB (2) trong 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 do dac linh quang dien tren miu P25 (kich
thude hat trung binh - 25nm) va dien cue che tao nhu đi vdi P25 nhung tir
nguyen lieu ban diu la bgt Ti02 pha Anatase cd thuong hieu MCB (kich thude hat trung binh 200 nm) trong moi trudng dien giai 0,1M NaOH/CH30H/H20 .
Kel qua tren hinh 4.12 cho thay : tren miu dien cue P25, đng quang dien tang manh va bao hoa nhanh khi phan cue ve phia dien the duong . Trong
khi đ miu dien cue MCB cho đng quang dien nhd hon ban va the '^onset"
dich ve phia dien the phan cue duong so vdi miu P25. Ket qua nay chiing to
trai ngugc vdi nhirng ly thuyei trudc day ve qua trinh chuyen hoa nang lugng quang dien : trong cac ban din da linh the ma d đ kich ihudc hat cang nho thi sir mat mat do 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 trong 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 lỵ
4.2.4 Thay doi dong hoc (kinetic) phdn ung Redox trong dien cue cdu true Nano-Xdp tiep xuc vdi moi trudng dien gidi khi bi chieu sdng
Dong hgc cua phan iing Redox tren be mat tiép 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) khi dac trung dong-the phan cue cd he sd lap diy (Fill factor) Idn, tuong iing vdi tdc do phan iing Redox nhanh va tai hgp be mat it. Nhung trudng hgp cdn lai khi 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 do cua phan iing Redox hoac gia
' ' / Doaj! bao I * /
2/ 3 / i
^''.V"
(11 Phan linj; Redox nhanh. khonj* oi tii hop be mat (2) 1 ai hop be mit nhanh
(3) Phan ling Redox cham
\L\ Dich chuven bien vunn xuoni: di/oi do lich dien dirung Iren be niAl
tang tai hgp be mat. Luan cir nay dugc ap dung de danh gia hien tugng giam su mat mat do tai hgp be mat ciia he dien cue kich thude hat nho --25 nm
(trong P25 ) va kich thuoc Idn -200 nm (MCB) trong (phin 4.2.3).
De nghien ciiu sir thay doi dong hgc (kinetic) cua phan iing Redox trong 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 chiéu 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
hon nhieu cua dien cue nano-xop. Mát mat do tai hgp be mat cua dien cue dac Idn, nhung kich thude hat ciia nd Idn gap nhieu lin dien cue nano-xop. Tdc do phan ling Redox cua dien cue nano-xop cao hon nhieu so vđ dien cue dac.
^ ' - ^ 20 0.8'^^V -0.4 0.8'^^V -0.4 T*- •20 2. I (10 mA/cm^) • ^ • • • • • • • " 3 0.4 V vs.Ag/AgCl
Hinh 4.14: I-V sdng cua dien cue Ti02 nano-xop (3) vd đc trong dung dich NaOH (1 );NaOH/CH^OH/H20 (2)
Hien tugng tang tdc do 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 đi dien tich. Theo mo hinh nay [63], tdc do phan iing u-ao đi 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 khi kich thich bing anh sang [63] . Nhu vay su thay doi tdc do phan ung Redox trong 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.
De xac dinh vi tri bien viing ciia ban din, nguoi ta diing phuong phap kinh diein la do dac trung dien dung ciia chuyen tiep SEI phu thuoc vao the
phan cuẹ 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 ỊOxlÓ 8.0x10' ^ 6.0x10*- 4.0x10' 2.0x1Ó' 0.0.