Studies with P-incorporated Nanocrystalline ZnO thin filmsshodhganga.inflibnet.ac.in/bitstream/10603/17898/11/11_chapter 5.p… · Studies with P-incorporated Nanocrystalline ZnO

CHAPTER-V Studies with P-incorporated Nanocrystalline ZnO thin films

5.1: Background

Low-dimensional phosphorous (P) doped ZnO nanostructures have aroused great

interest in recent years due to their unique physical and chemical properties and wide

spread applications [Sun and Kwok, 1999; Gao et al. 2011; Wenckstern et al. 2007].

Most studies were performed to generate technologically more valuable p-type ZnO;

however, associated difficulty in its generation and stability has hindered the progress

so far. Further, the issues of alteration in optical and microstructural characteristics in

ZnO on P doping have also been investigated. Allenic et al. studied the microscopic

defects and their effects on the electrical properties of P-doped ZnO films grown

epitaxially on (0001) sapphire and ZnO substrates by pulsed laser deposition [Allenic

et al. 2009]. Zhang et al. reported that shallow-donor and deep-acceptor impurity

bands exist in P-doped ZnO nanostructure [Zang et al. 2009]. Investigating the

structural properties of P-doped ZnO, Huong et al. observed that when P

concentration is above 9%, peaks of P appear clearly in the X-ray spectra, and

simultaneously the compounds are of n-type. However, at P concentration below 6%,

p-type ZnO is formed temporarily, which reverts to n-type with aging [Huong et al

2011]. In contrast, Li et al. observed that phosphorus-doped ZnO nanowires can be

changed from n-type to p-type with increasing P concentration [Li et al. 2009].

Besides above observations, ZnO has rich but complex defect chemistry, as nominally

undoped ZnO exhibits n-type conductivity due to native defects, viz. O vacancies and

Zn interstitials [Look et al. 2005].

With intriguing defect status and marked effects on microstructure and optical

properties, P doping in ZnO may impinge positive implications on its usage in PEC

splitting of water. The present piece of work aims to explore this, so far lesser

understood, aspect. The study involves phosphorus incorporation in ZnO lattice to

generate Zn1−xPxOy thin films by sol-gel (sample set I) and electrodeposition (sample

[Chapter V] P-incorporated Nanocrystalline ZnO

80

set II); characterization of the samples by XRD, EDX, SEM, AFM, UV-visible

spectroscopy, Mott-Schottky analysis; and PEC measurements. In the films grown

over ITO substrate phosphorus content was varied as: 0.5, 1, 2, 3, 5 and 7 % at.

5.2 Experimental

5.2.1 Sample synthesis by Sol-Gel

The ITO glass sheets, used as substrate were cleaned by soaking for ~ 2 min. in

detergent solution followed by washing with double-distilled-deionized water.

Subsequently, these were washed for ~ 10 s under dripping HCl (0.1 M), rinsed with

water and acetone, and air-dried. Orthrophosphoric acid (OPA) was used as source

compound for P. To the solution of zinc acetate di-hydrate (ZAD), prepared in di-

methyl formamide (DMF), calculated quantity of OPA was slowly dissolved. Kept at

33 ± 2 °C the content was stirred for 3 h. The resulting solution was spin coated over

ITO sheets (1 cm x 1 cm) at 2500 rpm using a photoresist spinner. For five

successive layers, deposited one over other, each layer was dried in air at 50 °C for 10

min. before the deposition of next layer. After the final deposition, the film was

sintered in air at 250 °C for 30 min. to remove organic impurities. Subsequently, these

were sintered in air at 600 °C for 60 min. and cooled slowly for 24 h to complete the

crystallization cycle. As a secondary step to induce surface homogeneity, the sintered

films were dip-coated (at a rate of 10 s per dip) for two successive coats, with a

solution of zinc nitrate and hexa-methylene-tetrammine, prepared in DMF with added

amount of OPA. The films were dried at 50 °C on a hot plate and sintered in air at 600

°C for 60 min.

5.2.2 Sample synthesis by Electrodeposition

Pre-cleaned (as described above) ITO sheets (1 cm x 1 cm) were used as working

electrode, in conjunction with saturated calomel reference electrode and platinum

auxiliary electrode. Aqueous solution of zinc nitrate (0.1M), mixed with potassium

chloride (KCl, 0.1M), ethylene-di-amine (EDA, 0.01M) and OPA (source compound

for P), was used as electrolyte. Electrochemical work station (ECDA-001, Con-Serv


81

Enterprises) was employed to obtain films under CV mode with applied voltage

varied from -1 to +1 V/SCE at a sweep rate of 20 mV/s for 30 cycles [Kumar P et al.

2011]. During film growth, electrolyte solution was stirred continuously with

temperature maintained at 80-85 °C. The deposited films were smooth and strongly

adherent to the substrate. These were washed in a gentle flow of water, dried at 50 °C

over hot plate, and sintered at 250 °C for 30 min and at 600

°C for 60 min., which

ensured proper crystallization. Sintering led to the evolution of nanocrystalline ZnO

films under conditions similar to as employed in sol-gel method.

Films were grown at varying concentrations (0.5, 1.0, 2.0, 3.0, 5.0 and 7.0 % at.) of P,

besides the pristine samples (Table 5.1). Nearly 3/4th

area of ITO substrate was used

in film deposition. The residual portion was employed to establish Ohmic electrical

contact and convert films into working electrodes for PEC splitting of water.

5.2.3 Characterization

Film thickness (t) was measured using Alpha-step surface profilometer (Tencor Alpha

Step D-120). Density (d) was evaluated by gravimetric determination of

weight/volume ratio, employing thickness and geometrical surface area of films

[Sharma et al. 2011]. XRD measurements, recorded with X-ray diffractometer

(Bruker AXS D8 Advance, Germany) equipped with CuKα as the radiation source,

revealed crystal phase and microstructural details. With angular accuracy ~ 0.00° and

the angular resolution > 0.01°, measurements were made at 2 varying from 31 to 50

°

with a step size of 0.02 degrees/min. Scherrer’s computations based on equation 5.1

and the observed broadening of XRD peaks, yielded average crystallite size (p)

[Scherrer 1918; Sharma et al. 2012].

p = k λ / B Cos θ (5.1)

Here, λ is the wavelength of X-ray (1.542 Å for CuKα) and θ the half diffraction

angle of the centroid. The XRD data was further utilized to estimate the dislocation

density (δ) and microstrain (ε), using equations 5.2 and 5.3 [Kathirvel et al. 2009].

δ = 1/t2

(5.2)

ε = B Cos θ/4 (5.3)


82

Optical absorption data, recorded from 200 to 800 nm using UV-Visible

spectrophotometer (Shimadzu UV-2450) was utilized to evaluate optical band gap

energy, based on equation 5.4,

αhυ = C (hυ – Eg)1/2

, (5.4)

where, hυ is the photon energy, C the constant, and α the optical absorption

coefficient [Ray 2001]. From αhυ2 vs. hυ plots, the direct band gap energy (Eg) was

determined from the intersection of extrapolated linear absorption edge to the energy

axis.

Surface topography of films was explored by AFM analysis employing AFM/Surface

Profilometer (Nanosurf easy Scan, Version 1.8, Switzerland). Images were recorded

at set point force 20 µN for 256 × 256 data points for each scan size of 5 µm × 5 µm.

Root mean square (RMS) surface roughness (SR) was estimated from AFM data.

SEM images recorded through Scanning Electron Microscope [Carl Zeiss SUPRA

40VP and INCA Penta FET x3, TESCAN] combined with energy dispersive X-ray

analysis (EDX) at accelerating voltage of 10 kV and 15 kV respectively with the

working distance 4 mm (Sample Set I) and 10.27 mm (Sample Set II) revealed

surface morphology. A tentative pattern of particle size distribution was drawn from

the dimensions of 80-100 randomly chosen particles in the SEM images and using J-

image software.

Measuring variation in space charge capacitance (C) with applied voltage, through

impedance data recorded by employing LCR meter (Agilent Technolgy, Model

4263B), helped evaluate flat band potential (Vfb) and charge carrier density (Nd). In

the Mott-Schottky (MS) computations, based on equations (5.5) and (5.6), any

additional capacitance introduced by surface states was ignored [Singh et al. 2009].

1/C2

= [2 (ε0εsqNd) ] [V-Vfb - (kB T/q)] (5.5)

S = 2/ (ε0εsqNd) (5.6)

Here, ε0 and εs are, respectively, permittivity of free space and semiconductor

electrode, q the electronic charge, T the temperature in Kelvin, kB the Boltzmann’s


83

constant, and S the slope of MS plot. Using NaOH (0.01 M, pH 12) as electrolyte, the

capacitance at ZnO - solution interface was measured, with smallest possible AC

voltage amplitude (~ 0.36 V, a value comparable to the open circuit potential of the

anode) at 1 kHz signal frequency. The intersection of the linear portion of the MS plot

on the potential axis and the slope yielded, respectively, the Vfb and Nd.

5.2.4 PEC studies

Electrochemical cell filled with aqueous solution of NaOH (0.01 M, pH 12) and

having quartz glass window for illumination was used. Water jacket surrounded the

PEC cell and prevented from IR heating. Thin films were used as working electrode

(WE) in conjunction with platinum mesh counter electrode (CE) and saturated

calomel reference electrode (SCE). Films were converted to WE by creating Ohmic

electrical contact from the uncoated portion of the substrate using silver paint and

copper wire. Subsequently, the electrical contact and all side-edges of the film were

thoroughly sealed by a non-transparent, non-conducting epoxy resin, Hysol (Dexter,

Singapore). Potentiostat (Model ECDA-001, Con-Serv Enterprises) and a 150 W

Xenon Arc light source (Oriel, USA) were employed to record current–voltage (J–V)

characteristics of the cell, both under darkness and illumination. Electrochemical

measurements were also used to estimate the operational (in-situ) electrical resistivity

(ρ) of ZnO thin film photoanodes by a method described elsewhere [Kumari et al.

2007].

Triplicate-quadruplicate measurements, using chemicals with purity > 99.9%, and

double distilled deionized water (specific conductance < 10–6

mho cm–1

) yielded

reproducible results (± 15.2 % deviations).

5.3 Results and Discussion

5.3.1 Chemistry of film synthesis

The formation of ZnO thin films by electrodeposition proceeded via the reduction of

nitrate ions (NO3-

), generated through the decomposition of the precursor salt ZnNO3,


84

to nitrite ions (NO2-

) in mild acid solution of Zn2+

. This resulted in an increase in the

pH near the anode. The electrochemically generated hydroxide ions then reacted with

Zn2+

ions in the solution to form Zn(OH)2 which was deposited at the cathode. The

deposited Zn(OH)2 was subsequently dehydrated into ZnO at a temperature of 80-85

°C. [Li et al. 2007].

Zn NO3 2 → Zn2+ + 2NO3− (I)

NO3− + H2O + 2e− → NO2

− + 2OH− (II)

Zn2+ + 2OH− → Zn OH 2 (III)

Zn OH 2 → ZnO + H2O (IV)

A tentative set of reactions occurring in the formation of films by sol-gel method has

been written below [Sharma et al. 2010]. The use of DMF, a highly polar solvent, was

preferred over alcohols in this study, as it is able to dissolve zinc acetate precursor

even at room temperature (<35 °C), avoiding drastic reaction conditions. Further, the

relatively low vapour pressure of DMF prevented premature and uneven drying and

helped in crack-free and ordered crystallisation in the films. No additive was required

to improve sol stability and homogeneity.

Zn CH3COO 2 . 2H2O → Zn CH3COO 2 + 2H2O (V)

Zn CH3COO 2 + 2HCON CH3 2 → Zn CON CH3 2 + 2CH3COOH (VI)

Zn CON CH3 2 + 2H2O → Zn OH 2 + 2HCON CH3 2 (VII)

Zn OH 2 → ZnO + H2O (VIII)

5.3.2 General physical characteristics

Thickness and density of films ranged, respectively, as 520-545 and 3.96-4.29 (Set I)

and 740-765 nm and 3.81-4.09 g cm-3

(Set II) (Table 5.1, Fig. 5.1). Films appear

porous as observed density is significantly less compared to the theoretical bulk

density of pure zinc oxide (5.60 g cm-3

). Density fell almost linearly with increase in

P concentration (up to 3% at. P), suggesting partial opening-up of the lattice during

crystallization. The effect, however, seems to saturate in samples with 5 and 7% P

with possible coalescing of crystallites. Considerable size differences among Zn2+

(0.72 pm), O2-

(140 pm) and P (44 pm – trivalent, 38 pm – pentavalent) support above


85

contention. Samples exhibited n-type conductivity indicating non-stoichiometric

growth and existence of O vacancies and/or Zn interstitials. P substituting the O

lattice site (PO) is known to generate deep acceptor level due to the increase of p-

orbital energy. However, PO has high ionization energy of about 0.62 eV that makes

difficult to achieve p-type ZnO. Further, the formation energy of Zn vacancy (VZn) is

also greatly reduced under O-rich growth conditions and this acceptor defect seems

more favorable under the conditions employed for film growth [Vlasenflin and

Tanaka 2007; Ding et al. 2008]. The in-situ electrical resistivity varied from 0.15 to

0.22 (Set I) and from 0.31 to 0.41 kΩ cm (Set II). Denser films are more resistive as

density correlated positively with electrical resistivity, especially at lower

concentrations of P (Fig. 5.1). However, despite being denser compared to Set II

samples, Set I samples were less resistive. The effect may be attributed to prevailing

influence of probably increased recombination centers in earlier case.

Table 5.1: Thickness (t), average crystallite size (p), dislocation density

(δ), microstrain (ε) and RMS surface roughness (SR) of film samples.

Sample

index

[P]

(% at.)

(t)

(nm)

(p)

(nm)

(δ) ×10-14

(line2/m

2 )

(ε) 10-3

SR

(nm)

SG0.0

SG0.5

SG1.0

SG2.0

SG3.0

SG5.0

SG 7.0

-

0.5

1.0

2.0

3.0

5.0

7.0

520

526

530

545

528

532

525

30

29

27

25

22

29

30

1.32

1.41

1.43

1.65

1.52

1.12

1.11

0.96

0.99

1.06

1.33

1.19

1.02

1.00

9

12

15

16

16

11

10

ED0.0

ED0.5

ED1.0

ED2.0

ED3.0

ED5.0

ED7.0

-

0.5

1.0

2.0

3.0

5.0

7.0

740

759

760

762

760

765

765

36

26

25

24

24

28

29

0.77

1.45

1.63

1.74

2.10

1.25

1.40

0.90

1.13

1.14

1.14

1.15

1.12

1.13

12

20

13

15

18

20

23

Sam

ple

Set

I

Sam

ple

Set

II


86

0 2 4 6 8

3.95

4.00

4.05

4.10

4.15

4.20

4.25

4.30 Density

Electrical Resistivity

[P]/ % at.

Den

sit

y/

g c

m-

3

0.14

0.16

0.18

0.20

0.22

Ele

ctric

al R

esis

tivity

/ k o

hm

cm

0 2 4 6 8

3.78

3.84

3.90

3.96

4.02

4.08

Den

sit

y /

gm

cm

-3

[P] / % at.

Density

Electrical Resistivity

0.30

0.33

0.36

0.39

0.42

Ele

ctric

al R

esis

tivity

/ k o

hm

cm

Sample Set I

Sample Set II

Fig. 5.1: Variation in density and electrical resistivity in P

incorporated sol-gel (Set I) and electrodeposition derived

(Set II) ZnO films.


87

5.3.3 Crystal phase and microstructure: XRD Analysis

Polycrystalline structure with no secondary phase formation is evident from X-ray

diffraction pattern of films (Fig. 5.2). The peaks observed at 2θ angles 35.4, 37.6,

38.1, 41.8, 45.6 and 49.2° can be ascribed to the underlined ITO substrate. These

peaks were recorded with even bare substrate. The additional peaks in sample films at

2θ angles 31.8, 34.4, 36.2 and 47.5° correspond to diffraction from planes (100),

(002), (101) and (102) of hexagonal wurtzite ZnO (JCPDS-89-1397) [Schulz and

Thiemann 1979], indicating its dominant occurrence. Hexagonal wurtzite is the most

stable phase of ZnO with its ionicity lying at the border of ionic and covalent

compounds [Sharma et al. 2010]. The dislocation density (δ) and microstrain (ε) in

samples increased up to 3% at. P and decreased thereafter (Table 5.1). Set I films

exhibited slightly higher values. The Zn–P bond length is predicted to be 2.18 Å;

considerably larger than Zn–O bond length of 1.93Å [Wenckstern et al. 2007;

Seetawan et al. 2011]. Hence, the substitution of O by P may lead to strapping

deformations in ZnO host lattice. The variations in average crystallite size (Set I: 22-

30 nm; Set II: 24-36 nm) with change in film preparation method and/or the P

concentration seem to be guided by (in negative correlation with) the changes

incurred in δ and ε (Fig. 5.3) [Seetawan et al., 2011].


88

32 34 36 38 40 42 44 46 48 50

Inte

nsit

y (

a.u

)

(102)

(101)

(002)

(100)

*

**

*

*

*

2

ED0.0

ED0.5

ED1.0

ED2.0

ED3.0

ED5.0

ED7.0

35 40 45 50

**

Inte

nsit

y (

a.u

)

** *

*(102)

(101)(002)(100)

2

SG0.0

SG0.5

SG1.0

SG2.0

SG3.0

SG5.0

SG7.0

Sample Set I

Sample Set II

Fig. 5.2: XRD pattern of films.


89

0 2 4 6 8

24

26

28

30A

vera

ge C

ryata

llit

e s

ize (

p)

/nm

p

[P]/ % at.

0.96

1.04

1.12

1.20

1.28

1.36

Dis

locatio

n d

en

sity

() / lin

e2/m

2

0 2 4 6 8

24

26

28

30

32

34

36

Avera

ge C

ryata

llit

e s

ize (

p)

/nm

[P]/ % at.

p

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Dis

locatio

n d

en

sity

() / lin

e2/m

2

Sample Set I

Sample Set II

Fig. 5.3: Average crystallite size (p) and dislocation

density (δ) varied with P incorporation incorporation.


90

5.3.4 Film surface characteristics: AFM and SEM analysis

AFM images of films (Fig. 5.4-5.5) indicate preferential vertical (c-axis oriented)

growth of nanocrystallites. The observation is in accordance to XRD pattern and

earlier reports [Umar et al. 2009]. Root mean square (RMS) surface roughness

values obtained from AFM data (Table 1) indicate Set II films being rougher than Set

I. SEM images (Fig. 5.6-5.7) reveal continuous and homogenous growth of

nanocrystallites. However, when seen under high magnification, Set II films

(electrodeposition derived) revealed square/rectangular shaped cavity like formations

which appear to be grain agglomerates possibly evolved through unequal growth of

nanocrytallites in different regions (Fig. 5.8). The observation signifies the critical

role of processes pertaining to the transport/diffusion of chemical species from

solution to solid phase during film growth by electrodeposition. Further work is

needed on this aspect. Estimated from SEM images, tentative distribution of particle

size (Fig. 5.9-5.10) indicates that most particles fall to a size range 40-80 (Set I) and

50-150 nm (Set II). Values being higher than the size estimated through Scherrer’s

computations suggest that particles seen in SEM images are indeed grain

agglomerates. The particle size decreased on P incorporation up to a concentration of

3% at. Above this concentration grains seem to agglomerate with increase in particle

size.

5.3.5 EDX analysis

EDX analysis of representative samples revealed O/Zn atomic ratio lying in the range

2-3 (± 5.6 % error) (Fig. 5.11, Table 5.2). The higher number of O atoms seems

attributable to the part contribution from underlying substrate [Sharma et al. 2012].

The observed concentration of P in Set I samples matched with the concentration

added during synthesis (Table 5.2). However, a non-uniform distribution of P

prevailed in samples prepared through electrodeposition (Set II); with much greater

concentration observed inside the cavity like structures (seen in SEM images; Fig.

5.9) while the concentration being undetectable at other locations. This observation

illustrates again the crucial role of kinetics of material transport across electrode-

http://pubs.acs.org/action/doSearch?action=search&author=Umar%2C+A&qsSearchArea=author


91

electrolyte interface during film evolution. Part segregation and preferential

localization of added P indicates the limitation of the electrodeposition method in

generating films with homogenous distribution of P.

Fig. 5.4: AFM images of sol-gel derived ZnO

films.


92

Fig. 5.5: AFM images of electrodeposition derived ZnO

films.


93

Fig. 5.6: SEM images of sol-gel derived films.


94

Fig. 5.7: SEM images of electrodeposition derived films.


95

Fig. 5.8: A high magnification SEM image of sample

ED5.0 exhibiting cavity like formations.


96

20 40 60 80 100 120

05

101520253035

Diameter/nm

Pe

rce

nta

ge

Nu

mb

er

of

pa

rtic

les

SG0.5

20 40 60 80 100

05

101520253035

SG1.0

20 40 60 80

05

101520253035

SG2.0

20 40 60 80 100

05

101520253035

SG3.0

Fig. 5.9: Particle size distribution in sol-gel derived films.


97

30 60 90 120 150 180 210

0

10

20

30

40

Diameter/nm

Pe

rce

nta

ge

Nu

mb

er

of

pa

rtic

les

ED0.5

30 60 90 120 150 180

0

10

20

30

40

ED1.0

50 100 150 200 250 300

0

10

20

30

40

ED2.0

50 100 150 200 250 300

0

10

20

30

40

ED3.0

Fig. 5.10: Particle size distribution in electrodeposition derived films.


98

Fig. 5.11: EDX pattern of representative samples.


99

Table 5.2: EDX-derived elemental composition in representative samples.

Element Sample: SG5.0

Weight% Atomic%

Sample: ED5.0

Weight% Atomic%

Sample: ED7.0

Weight% Atomic%

O K

Zn K

P K

In L

Total

35.71 68.42

51.10 23.96

5.67 5.61

7.52 2.01

100.00

34.24 62.09

48.06 21.33

17.70 16.58

100.00

27.14 54.25

54.25 26.54

18.61 19.21

100.00

5.3.6 Optical characteristics

In the absorption spectra of samples recorded between 200-800 nm, a prominent

absorption edge at ~ 400 nm was attributable to the onset of fundamental O:2p

Zn:4s charge-transfer absorption band of hexagonal wurtzite ZnO. The band gap

energy (Eg) values, corresponding to absorption threshold and determined from (αhυ)2

vs. (hυ) Tauc plots (Fig. 5.12), ranged 3.21-3.23 (Set I) and 3.22 to 3.34 eV (Set II)

with minor variations apparently guided by changes in crystallinity (Table 5.2).

However, no direct correlation could be established between the two. The band gap

values are in the expected range for wurtzite ZnO and P incorporation did not yield

any significant shift in the band gap.


100

3.20 3.28 3.360

2

4

6

(h)2 /e

V2

m-2

Energy (h)/eV

SG0.0

SG0.5

SG1.0

SG2.0

SG3.0

SG5.0

SG7.0

3.2 3.3 3.40.00

0.03

0.06

(h)2

/eV

2m

-2

ED0.0

ED0.5

ED1.0

ED2.0

ED3.0

ED5.0

ED7.0

Energy (h)/eV

Sample Set I

Sample Set II

Fig. 5.12: Tauc plots between (αhυ)

2 and hυ.


101

5.3.7 Flat Band potential (Vfb) and charge carrier density (Nd): Mott-Schottky

Analysis

Table 3 depicts the values of Vfb and Nd, estimated by observing variation in space

charge capacitance with applied voltage. Conforming to the n-type nature of samples,

Vfb values derived from Mott-Schottky plots are negative (Fig. 5.13). Vfb increased

with P incorporation and was most negative with 3% at. P incorporated films. Further

increase in P concentration led to reverse shift in the value; the pattern probably

tracing its genesis again from the changes incurred in microstructural properties. As

expected the PEC response of the films also followed the same sequence. Further, Vfb

values deviated from onset potentials, (obtained from I2 vs. V relations), which

indicates the existence of surface states at the electrode-electrolyte interface where

carriers may recombine easily [Gupta et al. 2009]. Following the pattern of changes

seen in Vfb, the Nd values also increased on P incorporation and reached maximum in

films with 3% P. The observation is in tune to previous results where P-doping led to

increase in the carrier concentration [Li et al. 2006]. The effect of P incorporation on

raising Nd are more pronounced in Set I samples, so much so that 3% at P films

obtained by sol-gel (Set I) possesses charge carrier density almost double to the value

recorded with films obtained by electrodeposition (Set II).


102

-0.8 -0.6 -0.4 -0.2 0.0 0.20.00

0.05

0.10

0.15

0.20

Voltage/V

C-2/(

cm

4F

- 2)

ED0.0

ED3.0

-1.2 -0.9 -0.6 -0.3 0.0 0.3 0.6 0.90.0

0.1

0.2

0.3

0.4

0.5

Voltage/V

C-2/(

cm

4F

- 2)

SG0.0

SG3.0

Sample Set I

Sample Set II

Fig. 5.13: Mott-schottky plots for sol-gel (Set I) and

electrodeposition derived (Set II) films.


103

5.3.8 PEC splitting of water

J-V plots of PEC cells (Fig. 5.14), obtained by employing the sample films as

working electrode for photosplitting of water, are in accordance to their n-type

character. The open circuit potential (Voc) and short circuit current (Jsc) (Fig. 5.15,

Table 5.3) increased on P incorporation and recorded maximum with 3% P

incorporated films. The photocurrent (Jph = Jillumination – J darkness) values, estimated at

fixed bias of 500 mV (vs. SCE), also varied significantly with P incorporation, in

direct correlation with changes in Jsc (Fig. 5.15). Sol-gel derived films (Set I) yielded

higher Jsc and Jph compared to Set II samples and also recorded highest gain in the

value (250-400%) compared to pristine samples, at 3% P incorporation. Both Jsc and

Jph decreased with further increase in P concentration.

Applied Bias Photon-to-Current Efficiency (ABPE) of PEC cell as defined by

equation 5.8, was computed from J-V data (Table 5.3).

total

Ph

P

VJABPE

23.1

(5.8)

Here Jph is the photocurrent density obtained under an applied bias (V) and Ptotal is

incident illumination power density in mW cm-2

. Compared to Set II samples, sol-gel

derived films (Set I) yielded higher PEC response; with films prepared at 3 and 5% P

incorporation exhibiting the maximum ABPE of 3.86 and 2.83%, respectively. A

careful perusal of results presented here suggests that, despite no significant drop in

the band gap energy, P incorporation has a significant influence on the efficacy of

nanocrystalline ZnO films for PEC splitting of water. The effect seems to largely

originate via a close linkage of semiconductor microstructure with its PEC

performance. Much improved PEC response offered by films with 3 and 5% P

incorporation, as exhibited by high values of Jsc, Jph and Voc, is attributable to increase

in charge carrier density and reduced electrical resistance. The shifts in Vfb are in

agreement to the changes in Voc and the maximally negative Vfb in 3% P incorporated

samples seems facilitating charge transport across electrode-electrolyte interface


104

resulting in increased PEC response. At P concentrations >3%, photocurrent values

dropped. The study highlights the fact that the PEC response of high band gap

semiconductors viz. ZnO, even after impurity incorporation, is largely regulated by

carrier concentration and mobility; minor enhancement in optical absorption being

just the secondary.

Finally it may be added that the incorporation of impurities like P in ZnO, principally

aimed to lower the band gap energy, may or may not lead to the intended effect

depending upon to what extent added P is able to enter in the lattice and create

optimally placed defect states. Nevertheless, microstructural properties of ZnO do

change significantly with P incorporation. The magnitude and direction of such

changes are critically reliant on material preparation/processing conditions and, being

decisive to the use of material as photoelectrode in PEC cell, can also be optimized

for efficient photosplitting of water.

Table 5.3: Band gap energy (Eg), flat band potential (Vfb), charge carrier density

(Nd), open circuit potential (Voc) and % ABPE values.

Sample

index

Eg

(eV)

Vfb

(V)

Nd × 10-21

(cm-3

)

Voc

(V)

ABPE

(%)

SG0.0

SG0.5

SG1.0

SG2.0

SG3.0

SG5.0

SG 7.0

3.21

3.21

3.21

3.22

3.22

3.23

3.23

-0.57

-

-0.60

-

-1.17

-

-0.68

0.10

-

0.19

-

0.60

-

0.24

0.62

0.63

0.68

0.71

0.77

0.74

0.73

1.12

1.15

1.24

1.25

3.86

2.83

1.85

ED0.0

ED0.5

ED1.0

ED2.0

ED3.0

ED5.0

ED7.0

3.26

3.22

3.26

3.32

3.30

3.34

3.33

-0.60

-

-0.66

-

-0.71

-

-0.64

0.18

-

0.26

-

0.31

-

0.21

0.63

0.63

0.64

0.74

0.75

0.73

0.72

0.65

0.75

0.74

0.65

0.79

0.70

0.73

Sam

ple

Set

I

Sam

ple

Set

II


105

-1.0 -0.5 0.0 0.51.5

1.0

0.5

0.0

Cu

rren

t d

en

sit

y/

mA

cm

-2

Applied Potential/ V

Under Darkness

ED0.0

ED0.5

ED1.0

ED2.0

ED3.0

ED5.0

ED7.0

-1.0 -0.5 0.0 0.5 1.06

5

4

3

2

1

0

Under Darkness

SG0.0

SG0.5

SG1.0

SG2.0

SG3.0

SG5.0

SG7.0

Cu

rre

nt

de

ns

ity/

mA

cm

-2

Applied Potential/ V

Sample Set I

Sample Set II

Sample Set I

Fig. 5.14: PEC current density varied with applied potential

(vs. SCE).


106

0 2 4 6 80

1

2

3

4

JS

C/m

A c

m-2

[P] / % at.

Jsc

Jph

2

4

6

Jp

h /mA

cm

-2

0 2 4 6 80.45

0.50

0.55

0.60

0.65

0.70

0.75

J

SC/m

A c

m-2

[P] / % at.

0.80

0.85

0.90

0.95

1.00

Jp

h /mA

cm

-2

Jsc

Jph

Sample Set II

Fig. 5.15: Variation in short circuit current (Jsc) and photocurrent

(Jph, measured at 500 mV bias vs.SCE).


107

5.4. Conclusions

Following conclusions may be drawn: (i) Films prepared in the study, with dominant

evolution of wurtzite ZnO phase, proved to be prospective candidate for sustainable

hydrogen energy generation via PEC splitting of water. Compared to elctrodeposition,

sol-gel process yielded denser and smoother films. (ii) Films are efficient UV

absorber and moderate-weak absorber of visible light. P incorporation alters

microstructural properties, viz. film thickness, density, particle size and distribution,

and film surface characteristics; most of these are crucial to the PEC splitting of

water. However, the band gap energy was only marginally varied. (iii) Films prepared

by sol-gel (Set I) exhibited higher Jsc and Jph. The reduced response of Set II samples

is possibly due to rise in recombination centers and hindered carrier mobility. (iv)

Films obtained by sol-gel at 3% at. P incorporation yielded the most significant gain

in photocurrent and ABPE. The effect is largely attributable to increase in charge

carrier density fall in electrical resistivity.

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