I 0.6 hien cua qua trinh hoa ly
O Ket qua thuc nghiem ^ Kei qua ly thuyet
- ^ Kei qua ly thuyet
I ' l l r • — I ' 1 ' I ' I ' I
• 1.5 -1.2 -0.8 -0.6 -0.3 0.0 0.3 0.6
The phan cue so vdi SCE
De xac dinh cac thong so val lieu cung nhu vi trf bien viing nang lugng cua ban dan tir phep do C-V cua he dien cue nano-xop đi hoi phai cd phuong phap sir ly dac trung C-V hoan loan khac. Phuong phap đ (chuong 3) ap dung cho dien cue TiOj se dugc de cap ky hon d chuong 5 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 nhaụ 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 trong dung dich dien giai NaOH + Methanol khi kfch thfch anh sang UV (hinh4.18). Mat khac, he nay cho hi6u SUSLI chuyen hoa quang-dien cao nh^Ll trong cac he dugc khao sat.
1000-0 1 0 1 •J u 100- • • T O I \ Sing UV •0.6 -0.4 -0.2 0.0 0.2 0.4
The phan cue [V vs Ag/AgCI]
0.6
Hinh 4.18: Hien tuang dich chuyen đc trung C-V khi 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)
trong cac dung dich ma hifeu sni\ chuyen hoa quang difen tháp nhu NaOH +
HjO su khac biet dac Únh C-V trong t6'i va kich thfch UV khdng dang kẹ TIT lap luan tren, ta dua ra 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 khi bi kfch thfch bang anh sang tiJr ngoai co lien quan den co che tao ra
hieu suat chuyen hoa quang-dien cao trong he naỵ Co che nay se dugc trinh bay chi liei hon d chuong 5 cua luan an ve hien tugng dich chuyen bien viing (bandedge movement) trong he dien cue ban dan TiỌ nano-xdp/dien giaị
ketluan
Trong chuang 4 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 sol- gel va tir bgt thuong pham P25 (DEGUSSA) va MBC . Mau dien cue dac che tao bang phuong phap đt la kim loai Tị
* Nghien curu mdi quan he giua cáu tnic pha tinh the cua TiOj vdi boat tfnh quang dien hoa cua cac miu nghien curụ Két qua cho tháy pha Anatase cd boat tfnh cao hon ban pha Rutilẹ
* Do xop cua mau do ta che tao tdt, nen cd the tang đ day mang dien
cue (de tang tong hieu sudt chuyen hoa quang- dien hoa) trong cac iing dung.
* Nghien cull hieu suSil chuyen hoa quang-dien đi vdi cac mang dien cue cd kfch thude hat khac nhau da dugc tién hanh. Hieu suat chuyen boa quang - dien tang khi kfch thude hat nhd dudi miic nguong.
* Ddi vdi mang dien cue Ti02 nano-xop, khi bi kfch thfch bdi anh sang ttr ngoai, trong mdi trudng dien giai thfch hgp (NaOH + Methanol), tdc đ
trao doi dien tfch giira ban dan va cap Redox tang dot bien, \&a at qua trinh tai
hgp tren be mat.
* Két qua dac trung C-V ciia dien cue Ti02 trong NaOH+Methanol, khi bi chieu sang bang tir ngoai, bi dich ve phfa the phan cue duong (theo thang dien hoa) so vdi C-V trong tdị
* Dua ra gia thuyet ve moi quan he giua hieu suál chuyen hoa quang - dien cao cua dien cue Ti02 nano-xop vdi hien tugng dich chuyen dac tnmg C-V khi ban din bi kfch thfch bang anh sang thfch hgp.
CHUONG 5. 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 do dai-Debye), do cong vung nang lugng (Band bending) khdng đng nhál vdi chenh lech giua muc Fermi va Redox. Phan bd ndng do
hat tai cung nhu dien the trong 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 ván de neu tren, chuong 3 cua luan an da xay dung mo hinh ly thuyet mo phdng tinh chait he ban dan kich thude nho khi tiep xiic vdi moi trudng dien giai chiia cap Redox.
Trong chuong nay se trinh bay mgl sd két qua md phdng đi vdi he ban
din Ti02 cd kich thude hat nhd, khi tiep xiic vdi dung dich dien giaị Noi dung
gom 2 phan chinh :
* Mo phdng qua trinh thiet lap can bang cua chuyen tiep SEI đi vdi he
Ti02 tiep xiic vdi cap Redox ÍfP'. Ket qua mo phong thu dugc phan bd ndng
do hat tai va dien the trong he hat nhd la ham phu thugc vao kfch thude hat. Chung tao thanh 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 đi vdi dien cue TiỘ Ket qua ly thuyet dugc khdp (Fitting) vdi thuc nghiem, da cho mdt sd thong sd ve tfnh chat cua viing chuyen tiep SEI đi 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 do 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.
5.1 Qua trinh thiet lap can b^ng ban dan - dien giai (SEI)
, 5.1,1 Cdc thong sdsudung trong mo phong he TiO2 : TIÍ^
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 do pha tap loai n (ban din loai n) n^^ ng = 10'^ cm"^ la ndng do thudng dugc sir dung cho Ti02 pha Anatase [27].
*/ Mat do 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 cám cua Ti02 thdng thudng [63,27] Ê = 3,2 eV . */ Tai nhiet do phdng kT = 0,03 eV , khi đ ndng do 16 trong po cua
Ti02 se dugc tfnh qua cac thong sd Nc, Nv , Eg, n^ va kT.
*/ Vi tri bien vung din Ês va viing hoa tri Eys theo thang the dien hoa nhu sau [4,63] : Ecs = - Ô^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"^ ==> do dai DebyeTi02: LDebye=3,5.10-^cm(35nm). Khi đ do Idn cua don vi dien trudng E* (cong thiic 3.41) se cd gia tri :
kT 1 ,.. . _,
^ 1^ Debye
= 7,5.10'VolLem
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 Ê^ = "Ó^V ; khir Ê,, = +0,5V so vdi chuin SCE . */ Miic Redox chuin É^^^ox = -^Ố^ so vdi chuin SCE .
*/ Nong do cua trang thai oxy hoa C,,, khir C ^ cua cap Redox gia thiet la: C = C d = 1 0 ' ^ c m ' ^ (tuong duong 0.01 Moi ndng do dung dich)
Vi tri miic Fermi ban dau ciia TiO-, tfnh nhu sau : Ep = Êj^ -kT.ln
.Ncy
Khi do hieu chenh lech giua muc Fermi va miic Redox se la :
ELdox - Ep = 2 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
gia thiet: ^Tr'.t..n„^ (lT\x^.n ^ L Diffusion /
D*"0
^ D i f f u s i o n J
= 10 (5.1) Khi đ gia tri ciia K^ va K^ chenh lech nhau ~ 33 bac. Dieu nay cho thay tdc Khi đ gia tri ciia K^ va K^ chenh lech nhau ~ 33 bac. Dieu nay cho thay tdc do trao doi dien tich cua cap Redox vdi cac viing cua ban din rat khac nhau, nhál la đi vdi ban din viing cam rgng nhu TiOj (Oxide kim loai ndi chung). Ddi vdi tiep xiic kim loai - ban dan, nhan td chuyen đi (theo cdng thiic 3.54 , 3.55) Kg va K^ cd do Idn so sanh dugc vdi nhau .
Su khac biet ciia nhan td chuyen đi K^, K^^ đi vdi 2 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 đi vdi tiep xiic Kim loai - Bdn đn
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 đi : Ep = +0,1 V so vdi chuin SCẸ Gia tri cua thong sd vat lieu (cong thiic 3.54 va 3.55) gia thiei :
l + b Tr^T^.n, Diffusion J N: M V " o J ( K Ml ^ T r ' . T i j . n , , 1 + 1/b Difiusion / = 1 V " o J (5.2)
Quy trinh dugc thuc hien theo luu đ (3.1) . Ket qua mo phong trang thai chuin dirng (steady slate) ciia phan bo hat tai tai chuyen tiep ban din-kim loai 0 nhiet do phdng tren hinh 5.1.
25-, g 20 •to c
1 0 Q
a/ Mo phong rao the Shottky cua chuyen tiep K i m loai-Ban d^n