Download Investigation into High Efficiency Visible Light Photocatalysts for Water Reduction and Oxidation PDF

TitleInvestigation into High Efficiency Visible Light Photocatalysts for Water Reduction and Oxidation
Author
LanguageEnglish
File Size5.8 MB
Total Pages170
Table of Contents
                            Supervisor’s Foreword
Acknowledgments
Contents
List of Figures
List of Tables
Nomenclature
1 Introduction: Fundamentals of Water Splitting and Literature Survey
	1.1 Fundamentals of Semiconductor Photoelectrochemistry
		1.1.1 Semiconductor-Electrolyte Interface
		1.1.2 Charge Carrier Generation
		1.1.3 Photoelectrochemistry
		1.1.4 Photocatalytic Water Splitting
		1.1.5 Efficiency Calculations
		1.1.6 Thermodynamic Limits
	1.2 Characterisation Methods for Photocatalysts
		1.2.1 UV-Visible Spectroscopy
		1.2.2 Gas Chromatography
		1.2.3 Powder X-ray Diffraction (PXRD)
		1.2.4 Scanning and Transmission Electron Microscopy
		1.2.5 TGA-DSC-MS
		1.2.6 BET Method for Specific Surface Area Measurements [50]
		1.2.7 Zeta Potential (ZP) Using Electrophoretic Light Scattering (ELS) [51]
		1.2.8 Attenuated Total Reflectance—Fourier Transform InfraRed (ATR-FTIR) Spectroscopy [52]
		1.2.9 Raman Spectroscopy
		1.2.10 X-ray Photoelectron Spectroscopy (XPS)
		1.2.11 Elemental Analysis (EA)
	1.3 Literature Survey: Overview of Current Photocatalysts [66]
		1.3.1 UV-Active Semiconductors
		1.3.2 Semiconductors Activated by Visible Light
		1.3.3 Semiconductors Activated by Visible Light
		1.3.4 Z-Scheme Systems
		1.3.5 Effect of Morphology, Crystallinity and Size on the Activity of Photocatalysts
		1.3.6 Conclusions
	References
2 Experimental Development
	2.1 Reaction System
		2.1.1 Reactor
		2.1.2 Light Source
	2.2 Gas Chromatography: Selection and Calibration
		2.2.1 Gas Chromatography Setup
		2.2.2 Standard Gas and Calibration
	2.3 General Characterisation
		2.3.1 UV-Vis Spectrophotometry
		2.3.2 PXRD
		2.3.3 FE-SEM
		2.3.4 TEM
		2.3.5 BET Specific Surface Area
		2.3.6 ATR-FTIR Spectroscopy
		2.3.7 Raman Spectroscopy
		2.3.8 TGA-DSC-MS
		2.3.9 Zeta Potential (ZP) Measurements
		2.3.10 XPS
		2.3.11 Elemental Analysis
	References
3 Oxygen Evolving Photocatalyst Development
	3.1 Introduction
	3.2 Methodology
		3.2.1 Photocatalytic Analysis
		3.2.2 Synthesis Techniques
	3.3 Results and Discussion
		3.3.1 Initial Ag3PO4 Studies
			3.3.1.1 Method ‘A’
			3.3.1.2 Method ‘B’
		3.3.2 Facet Control of Ag3PO4 (Method ‘D’) [2]
	3.4 Conclusions
	References
4 Hydrogen Evolving Photocatalyst Development
	4.1 Introduction
	4.2 Methodology
		4.2.1 Photocatalytic Analysis
		4.2.2 Synthesis Techniques
	4.3 Results and Discussion
	4.4 Conclusions
	References
5 Novel Z-Scheme Overall Water Splitting Systems
	5.1 Introduction
	5.2 Methodology
		5.2.1 Photocatalytic Analysis
		5.2.2 Synthesis Techniques
	5.3 Results and Discussion
		5.3.1 Ag3PO4 Based Z-Scheme Water Splitting Systems
		5.3.2 Graphitic Carbon Nitride Based Z-Scheme Water Splitting Systems
	5.4 Conclusions
	References
6 Overall Conclusions and Future Work
	6.1 Overall Conclusions
	6.2 Future Work
	References
                        
Document Text Contents
Page 1

Springer Theses
Recognizing Outstanding Ph.D. Research

Investigation into
High Efficiency
Visible Light
Photocatalysts for
Water Reduction
and Oxidation

David James Martin

Page 2

Springer Theses

Recognizing Outstanding Ph.D. Research

Page 85

61

increasing error—which is nearly double that of the standard error on a hydro-
gen sample, since no hydrogen exists freely in air. The air contaminant also comes
from a small amount which is inside the injection port in the GC. All of which is
taken into account for the calibration. The successful calibration of the GC means
that gas amounts as low as 10−9 mol can be detected accurately.

0

2000

4000

6000

8000

10000

0.00 0.05 0.10 0.15 0.20
0

2000

4000

6000

8000

10000

0.00 0.05 0.10 0.15 0.20

Moles of H
2
(µmol)

A
re

a
(
µV

.m
in
)

Moles of H
2
(µmol)

A
re

a
(
µV

.m
in
)

Linear Fit

(a)

y = 55633 x + 216.54

R 2 = 0.9994

0

200

400

600

800

0.00 0.02 0.04 0.06 0.08

Linear Fit

Moles of O
2
(µmol)

A
re

a
(
µ V

.m
in
)

(b)

y = 6975x + 96.21

R 2 = 0.9947

Fig. 2.5 GC area versus molar amount calibration curves for hydrogen (a), and oxygen (b).
Equations of linear fits and R2 values are noted in the upper left hand corner of the figure

2.2 Gas Chromatography: Selection and Calibration

Page 86

62 2 Experimental Development

2.3 General Characterisation

The following section details standard characterisation methods that are used for
all necessary results sections, to avoid repetition.

2.3.1 UV-Vis Spectrophotometry

Absorption, reflection and transmission spectra were collected from a Shimadzu
UV-2550 spectrophotometer fitted with an integrating sphere. The software pro-
vided (UV-Probe 2.33) enabled reflection to be directly converted to absorption by
the Kubelka-Munk transformation. Typically, data would be collected from 250 to
800 nm, with an optimum slit with of 2 nm. This reduces noise whilst not compro-
mising the accuracy of the data. The frequency of the data was 0.5 nm, as this was
more than adequate enough to determine precise spectra.

2.3.2 PXRD

PXRD was performed on either a Rigaku RINT 2100 (40 kV, 40 mA, using a Cu
source with Ka1 = 1.540562 and Ka2 = 1.544398) or a Bruker D4 (40 kV, 30 mA,
using a Cu source with Ka1 = 1.54056 and Ka2 = 1.54439). A maximum 0.05°
step size was used, at 5 s per step, covering a maximum range of 0–90° (2θ).
Phase match and baseline corrections were performed on either MDI Jade, or

Table 2.2 Area sampling
data for H2 and O2 (0.5 cm

3)

SD was taken for entire population

Area (μV min)

H2 O2
4896.1 390.5

4834.4 394.5

4831.0 389.2

4885.1 388.2

4876.7 399.5

4867.5 397.6

4845.9 391.1

4849.8 393.3

4888.6 393.9

4820.3 399.0

Mean 4859.5 393.7

σ 25.4 3.8

% Error 0.52 % 0.97 %

Page 169

148 6 Overall Conclusions and Future Work

upon, and therefore there are many possible follow up investigations that could be
pursued in order to meet the 10 % solar to H2 conversion efficiency using a robust
system.

Chapter 3 detailed a facile method to control the exposing facet of Ag3PO4
and the influence on the activity. For any industrial application, Ag+ cannot be
used as an electron scavenger as metallic silver eventually poisons the surface, it
is used here as a method to demonstrate a proof-of-concept in terms of potential
maximum efficiency. Therefore, a sizable project would be to attempt to grow
Ag3PO4 tetrahedrons onto a thin film, for use in a PEC cell. The contact between
conducting substrate and photocatalyst would have to be exceptionally good to
shuttle electrons around a circuit, otherwise as shown by Yi et al., in the absence
of a suitable voltage, Ag3PO4 undergoes photocorrosion. Similarly, photocorrosion
occurs over Ag3PO4 in the absence of soluble electron scavengers. It might also
be possible to cover Ag3PO4 in a stable protection layer which is transparent to
visible light, in order to act as an electron scavenger and prevent photocorrosion.
Possibilities for cutting edge protection layers include; iron ferrihydrite (very high
electronegativity), TiO2 (robust over-layer), ZnO:Al or silica [4–6].

In Chap. 4, it was shown that by controlling the protonation status of g-C3N4,
it is possible to increase the quantum efficiency up to 26 % under visible light.
Previous studies have shown surface area to have an impact of sorts on the HER,
however in comparison to altering the proton content, the increase in IQY is mini-
mal. Investigations by Transient Absorption Spectroscopy (TAS) have previously
provided understanding as to rate limiting steps in other photocatalysts such as
TiO2 and Fe2O [7–9]. This same technique could be applied to carbon nitride to
examine the kinetics of photogenerated charge carriers as a function of incident
wavelength, which would illustrate a way to further improve the efficiency for the
photocatalyst, e.g. by tailoring porosity and surface acidity.

Z-Scheme water splitting systems were studied in Chap. 5, and g-C3N4 was
shown to be a functional photocatalyst for both oxidation of Fe2+ or I− [10]. Despite
the high quantum efficiency demonstrated in Chap. 4, in sacrificial systems, the
STH% of the best Z-scheme system was low, at 0.1 %. Therefore, a further study
would be required to probe this novel system. One route would be to experiment
with alternative redox mediators, yet this would prove difficult as there are only
a handful of mediators which are suitable for Z-Scheme water splitting. As men-
tioned, it is unknown whether the absorption of iodide or ferrous ions is surface-
selective. Hence, another path to investigate would be an ion study on g-C3N4
synthesised using various precursors, possibly monitoring ion absorption using
UV-Vis spectroscopy. Alternatively, it would also be possible to replace WO3 with
a photocatalyst which was more active in terms of oxygen evolution from water.
The downside to this approach though, is searching for a photocatalyst which not
only has visible light absorption and demonstrates a large OER, but also has a
band gap which straddles the reduction potential of IO3

−, and the redox poten-
tial of water, is indeed very difficult, but not impossible. It would also be worthy
exploring the incorporation of other redox mediators into the overall water split-
ting systems, such as NaNO3 or chromate complexes.

http://dx.doi.org/10.1007/978-3-319-18488-3_3
http://dx.doi.org/10.1007/978-3-319-18488-3_4
http://dx.doi.org/10.1007/978-3-319-18488-3_5
http://dx.doi.org/10.1007/978-3-319-18488-3_4

Page 170

149

References

1. Yi, Z., et al. (2010). An orthophosphate semiconductor with photooxidation properties under
visible-light irradiation. Nature Materials, 9, 559–564.

2. Wang, X., et al. (2008). A metal-free polymeric photocatalyst for hydrogen production from
water under visible light. Nature Materials, 8, 76–80.

3. Martin, D. J., et al. (2014). Highly efficient Photocatalytic H2 evolution from water using vis-
ible light and structure-controlled graphitic carbon nitride. Angewandte Chemie International
Edition, 53(35), 9240–9245. doi:10.1002/anie.201403375.

4. Qu, Y., & Duan, X. (2013). Progress, challenge and perspective of heterogeneous photocata-
lysts. Chemical Society Reviews, 42, 2568–2580.

5. Liu, G., et al. (2014). A tantalum nitride photoanode modified with a hole-storage layer for
highly stable solar water splitting. Angewandte Chemie International Edition,. doi:10.1002/a
nie.201404697.

6. Awazu, K., et al. (2008). A plasmonic photocatalyst consisting of silver nanoparticles embed-
ded in titanium dioxide. Journal of the American Chemical Society, 130, 1676–1680.

7. Pendlebury, S. R., et al. (2011). Dynamics of photogenerated holes in nanocrystal-
line α-Fe2O3 electrodes for water oxidation probed by transient absorption spectroscopy.
Chemical Communications 47, 716–718. doi:10.1039/C0CC03627G.

8. Tang, J., Cowan, A. J., Durrant, J. R., & Klug, D. R. (2011). Mechanism of O2 production
from water splitting: nature of charge carriers in nitrogen doped nanocrystalline TiO2 films
and factors limiting O2 production. The Journal of Physical Chemistry C, 115, 3143–3150.

9. Tang, J., Durrant, J. R., & Klug, D. R. (2008). Mechanism of photocatalytic water splitting in
tio2. reaction of water with photoholes, importance of charge carrier dynamics, and evidence
for four-hole chemistry. Journal of the American Chemical Society, 130, 13885–13891.

10. Martin, D. J., Reardon, P. J. T., Moniz, S. J. A., & Tang, J. (2014). Visible Light-Driven Pure
Water Splitting by a Nature-Inspired Organic Semiconductor-Based System. Journal of the
American Chemical Society, 136(36), 12568–12571. doi:10.1021/ja506386e.

References

http://dx.doi.org/10.1002/anie.201403375
http://dx.doi.org/10.1002/anie.201404697
http://dx.doi.org/10.1002/anie.201404697
http://dx.doi.org/10.1039/C0CC03627G
http://dx.doi.org/10.1021/ja506386e

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