Molibden katkılı bivo4 katalizörlerinin geliştirilmesi,karakterizasyonu ve fotokatalitik etkinliklerinin incelenmesi
Development and characterization of molibden metal doped bivo4 catalysts and investition of their photocatalytic activities
Yazar: TUĞBA YALÇIN
Danışman: PROF. DR. MUSTAFA KAMİL ÜRGEN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı / Malzeme Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: Enerji = Energy

Özet:
Güneş enerjisini sayesinde suyun fotokatalitik (FK) olarak parçalanması sonucunda
hidrojen gazı ve oksijen gazı oluşmaktadır. Fotokatalitik sistemde oksijen evrim
reaksiyonu (OER) fotoanot malzeme yüzeyinde ve hidrojen evrim reaksiyonu (HER)
fotoanot malzeme yüzeyinde gerçekleşmektedir. Ancak, fotoanot malzemelerin hem
OER aktivitesi zayıf olması hem de yük taşıyıcıların verimsiz kullanılması nedeniyle
pratik kullanımlar için elverişli değildir. Bu yüzden, gün ışığında yüksek verimde
çalışacak bir foto elektrot malzemesi gerekmektedir.
FEK sistemlerinde kullanılacak olan ışığa duyarlı elektrot görünür ışık spektrumun
aralığında yüksek soğurma özelliğine, yüksek fotoakım yoğunluğuna, yüksek elektron
ve boşluk hareketliliğine, yüksek yük taşıyıcı ömrüne ve yüksek kimyasal kararlılığa
sahip olmalıdır. Diğer metal oksit göre bizmut vanadat (BiVO4) malzemesi nispeten
dar bant boşluğu (Eg = 2.4 eV) ve suyun parçalanması için uygun bant sınırlarına sahip
olan n-tipi bir yarı iletkendir. Bu yarı iletkenin sahip olduğu üç faz yapısından
monoklinik kristal yapısı tetragonal kristal yapısına göre çok güçlü fotokatalitik
performans göstermektedir. Bu özelliklerine rağmen BiVO4 görünür ışık
spektrumunda, zayıf elektron hareketliliği göstermesi, yüklü parçacıkların ayırımının
yetersiz kalması ve elektron-boşluk (e-/h+) çiftlerinin yeniden birleşme olasılığının
olması nedeniyle endüstriyel uygulamak için geliştirilmesi gerekmektedir.
Bu tez kapsamında, sol jel yöntemi ile oluşturulan BiVO4 çözeltisi daldırarak kaplama
metodu ile flor katkılı kalay oksit (FTO) cam üzerine kaplanmıştır. Sentezlenen BiVO4
esaslı kaplamaların fotokatalitik özelliklerinin, optiksel özelliklerinin ve morfolojik
özelliklerinin incelenmesini kapsamaktadır. Katkısız BiVO4 filmin fotoakım
yoğunluğunu iyileştirmek ve e-
/h+
yeniden birleşme ihtimalini azaltmak için molibden
katkısı yapılmıştır.
BiVO4 ince filmlerin zamana karşı verdikleri fotoakım değerlerini karşılaştırmak için
KA yöntem ile suyun teorik olarak parçalandığı sabit potansiyel (1,23VRHE )
uygulanmıştır. Bu kronoamporemetri deneyi ile BiV0,99O4Mo0,01 numunesi (0,32
mAcm-2
), saf BiVO4 göre 5 kat daha fazla akım yoğunluğu malzeme yüzeyinden
geçtiği ispatlanmıştır. Mo katkılı diğer fotokatalizörler artan Mo oranına ( %0,05,
%0,1, %0,2 mol) göre sırasıyla KA deneyinde ölçülen akım yoğunlukları 0,24, 0,19,
0,13 mAcm-2
'dir.
Katkısız ve Mo katkılı BiVO4 filmlerin arayüzünde gerçekleşen yük transfer kinetiği
elektrokimyasal empedans spektroskopi (EIS) yapılarak ve düz bant potensiyeleri
Mott-Schottky (M-S) analizi yapılarak belirlenmiştir. EIS elde edilen Nyquist
grafiğine göre %0,01 mol Mo katkılı numunenin OER reaksiyonuna karşı gösterdiği
direnç, saf BiVO4 numunesine göre daha küçük olması daha iyi polarize edildiği
anlamına gelmektedir. Bu numuneni kolay polarize olması OER reaksiyonun gerçekleşmesi için gereken enerjiyi azalttığı için FEK performasını arttırdığı
düşünülmektedir.
BiVO4 esaslı sentezlenen tüm malzemelerin Mott-Schottky grafiğindeki eğimin pozitif
olmasından dolayı n-tipi bir yarı iletken davranışı gösterdiği görülmektedir. Ayrıca,
bu eğimin ne kadar küçük ise yük taşıyıcı konsantrasyonu o derece yüksek olduğunu
belirtmektedir.
Sentezlenen ince film kaplamaların morfolojik özellikleri belirlemek için yapılan
FESEM analizi ile yapıdaki Mo konsantarsyonun artması morfoloji olumsuz yönde
etkilediği görülmektedir. Yapısal karakterizasyonu belirlemek için yapılan XRD
analizi sonucunda sentezlenen numuneler monoklinik fazda olduğu ispat edilmiştir.
Numunelerde çok azda olsa dönüşmemiş tetragonal faz olduğu görülmüştür. DRS
spektrumundan Kubelka-Munk modeli kullanarak yarı iletkenin enerji bant aralığını
hesaplanmıştır. Mo katkısı artıkça saf BiVO4'ün enerji bant aralığını artırdığı
görülmüştür.

Abstract:
The photoelectrochemical is a process that uses sun energy to convert to dissociate
water molecules into hydrogen and oxygen. In this system, the oxygen evolution
reaction (OER) takes place on the photoanode material surface and the hydrogen
evolution reaction (HER) takes place on the photoanode material surface. However,
both poor OER activity of photoelectrochemical materials and the inefficient use of
charge carriers on the surface of materials create a barrier for practical use. Thus, photo
absorber materials are required which will operate at high efficiency in daylight.
The light sensitive electrode to be used in photoelectrocatalytic systems must have
high absorption property, high photocurrent density, high electron / gap mobility, high
charge life and high chemical stability in the range of visible light spectrum. Compared
to other metal oxide semiconductors, the bismuth vanadate (BiVO4) material is a ntype semiconductor having a relatively narrow band gap (Eg = 2.4 eV) and band edge
positions suitable for water splitting. BiVO4 has three reported phases including
monoclinic scheelite, tetragonal scheelite and tetragonal zircon. Energy band gap of
tetragonal BiVO4 was as reported 2.9 eV possessing a UV absorption band, while
monoclinic BiVO4 with a 2.4 eV band gap has a characteristic visible light absorption
band besides the UV band. the monoclinic crystal structure of this semiconductor
shows very strong photocatalytic performance compared to tetragonal crystal
structure. Therefore, photocatalytic properties of BiVO4 are a very strong relationship
with way of preparation and its crystal structure. Only the monoclinic scheelite BiVO4
shows very strong photocatalytic performance compared to tetragonal crystal
structure. To achieve monoclinic-scheelite BiVO4 in this thesis, heat treatment was
kept to 450°C for 5 hours and then cooled to room conditions for the phase transition
between monoclinic scheelite structure and tetragonal scheelite structure of BiVO4.
Despite these properties, BiVO4 needs to be developed for industrial application in the
visible light spectrum, due to poor electron mobility, insufficient separation of charged
particles, and the possibility of electron-hole (e-
/h+
) recombination.
In this thesis, BiVO4 solution formed by sol gel method was coated with fluorine doped
tin oxide (FTO) glass by dip coating method. Additionally, Mo doped BiVO4
photoanodes were synthesized by same coating method and the property of Mo doped
BiVO4 on the FEK water splitting was studied. The reason for using the dip coating
method is that the method of dip coating for the preparation of industrial surface
photoanodes is quite simple which concludes deposition a uniform thin film on a
substrate and solidification on the coating subsequently.
The characterization of the synthesized samples was made as follows: XRD for
structure properties, FESEM morphological properties, Raman spectroscopy for
optical properties and UV-VIS for calculation of energy band gap the BiVO4
based samples. Also, photocatalytic properties were investigated by chronoamperometry, electrochemical impedance spectroscopy and Mott Schottky
analysis.
Molybdenum was added to improve the photocurrent density of the non-doped BiVO4
film and to reduce the possibility of e-
/ h+
recombination. Molybdenum doped BiVO4
has five times more photo-current density than pure BiVO4 material and the highest
photo activity value was observed in 0.01% Mo-doped BiVO4 sample. To compare
their photocurrent responses against time, the constant potential (1,23VRHE), in which
the water is theoretically splitting, was applied to BiVO4 film. The highest
photocurrent density obtained from BiV0,99O4Mo0,01, BiV0,95O4Mo0,05,
BiV0,90O4Mo0,10 and BiV0,80O4Mo0,2 was measured 0,32, 0,24, 0,19 and 0,13 mAcm-2
at 1.23VRHE, respectively. The photocurrent values of this film materials were
increased by the molybdenum doped BiVO4 because Mo acts as shallow donors for
excess electrons form small polarons around reduced V4+ centres. A multitude of
polarons help charge migrating to an adjacent V5+ site and render potent overlap at
large scale.
According to the result, it was observed that our best photocurrent result was obtained
from 0.01 mol% Mo-doped BiVO4 samples. In addition, the charge transfer kinetics
performed at the interface of both pristine BiVO4 and Mo doped BiVO4 films were
determined by electrochemical impedance spectroscopy (EIS) method and flat band
potential calculation by Mott-Schottky (M-S) analysis.
According to EIS analysis, pristine BiVO4 exhibited less charge transfer resistance so
its water splitting properties is low because of poor whole transfer at the
semiconductor/electrolyte interface. When Mo was doped in pristine BiVO4, the
charge transfer resistance value was much lower, so this increased
photoelectrocatalytic water splitting performance. However, after a critical addition
amount of Mo was added to monoclinic BiVO4 structure, the trend of a decrease in its
photoelectrochemical water splitting performance properties were observed because
of increasing diameter of semicircle decreasing in the sequence of BiV0,99O4Mo0,01,
BiV0,95O4Mo0,05, BiV0,90O4Mo0,10 and BiV0,80O4Mo0,2 under illumination.
According to Mott-Schottky analysis, adding amount of Mo doped, plot curve
becomes linear, this show that Mo doped BiVO4 increase in donor concentration.
Moreover, flat band potential of pristine and Mo doped BiVO4 was calculated intercept
of the tangent at the x-axis, thus flat band potential ranges from 0,1 to 0,03 V. We
observed that Mo-doped BiVO4 samples demonstrated a higher donor concentration as
compared to pristine BiVO4 film. If the slope of Mott-Schottky plot is positive, it is a
proof that BiVO4 shows n-type semiconductor behaviour. The slope of M-S plot was
positive for all synthesized samples, it show that all synthesized BiVO4 based samples
were n-type semiconductor.
When the structural properties were examined by XRD, the BiV0,95O4Mo0,05,
BiV0,90O4Mo0,10 and BiV0,80O4Mo0,2 structure was found to be a mixture of
monoclinic and tetragonal phase, and non-doped BiVO4 both a mixture of monoclinic
and tetragonal structure and including Bi2O3 phase. Tetragonal structure
BiVO4 reducing effect on photocatalytic activity is known, which was consistent with
the photocatalytic test results. However, 1% Mo-doped BiVO4 was only shown our
best photocatalytic performance due to matching monoclinic phase peaks of
monoclinic-scheelite BiVO4 sample.
When surface morphology was examined by FE-SEM, with the addition of 0,01% of
Mo element, there is apparent change in morphologies, which are composed of closely arranged particles. However, as increasing of Mo concentration, some was broken and
grown into large particles, so morphology change was a decreasing effect on
photoelectrocatalytic performance, which consistent with the chronoamperometry test
results.
Energy band gap were calculated via UV-VIS to use Kubelka-Munk function.
According to calculations, as the amount of molybdenum increased, the energy band
gap generally increased. However, the sample with the smallest additive (%0,01 Mo)
had a reducing energy band gap. Calculated energy band gaps respectively 2,55, 2,52
2,61, 2,73, 2,74 eV and for BiVO4, BiV0,99O4Mo0,01, BiV0,95O4Mo0,05, BiV0,90O4Mo0,10
and BiV0,80O4Mo0,2. The increase in energy band search has a negative effect on the
optical properties and it has been seen in the results of raman analysis.
Raman spectroscopy can provide structural morphology. Also, it is research of a
sensitive method for the investigation of the crystallization, local structure, and
electronic properties of materials. Raman bands of monoclinic scheelite structure of
BiVO4 for all samples were observed around 210, 324, 366, 640, 710, and 826 cm–1
.
These are the typical vibrational bands of BiVO4. The structural information of BiVO4
are obtained by the band centered at 210 cm-1
. The asymmetric and symmetric
formations of VO4 tetrahedron are given by the bands centered at 324 and 366 cm-1
,
respectively. The asymmetric stretching vibration of the shorter V–O bond are given
by bands centered at 640 cm–1
. The stretching modes of two vibrational modes of VO bonds are determined by the bands centered at 710 and 826 cm-1
. It was seen for all
samples that a slight blue shift in chaarcteristic peak at 826cm-1
as molybdenum is
added to monoclinic scheelite BiVO4 structure. It can be explained that an
incorporation of Mo+6 ions into the BiVO4 crystal lattice so stretching of the V-O bong
length due to Mo+6 ion substitution V+5 ion in the crystal side. Therefore, as amount of
the molybden increases, the monolinic scheelite structure of BiVO4 is undergoes
deterioration in even worse. It is observed that some characteristic peaks of
BiV0,90O4Mo0,10 and BiV0,80O4Mo0,2 samples are not included in the Raman spectrum
and the shift is stronger in BiV0,90O4Mo0,10 and BiV0,80O4Mo0,2 than pristine BiVO4
around 826 and 640 cm-1
. However, the peak position is found to be at the same point
BiV0,99O4Mo0,01 and pristine BiVO4. This result is parallel to what was observed in
UV-VIS diffuse reflectance spectrum.
In the summary, light sensitive BiVO4 semiconductor material was added
molybdenum element in different molar ratios to to examine the changing properties
of photoelectrochemical, morphological, structure and optical properties.