Traditionally, photosynthesis has been seen as the domain of biology, with some input from chemistry when dealing with chromatography, while, apart from a passing reference to the colour of leaves, physics has tended to steer clear of the process that provides the lifeblood of human existence. This article outlines how a recent technological advance can be used as a teaching resource in all three branches of science

Ireson, G

Mortimer, R

Twidle, J

Worrall, DR

English

John Twidle (1250097)

Gren Ireson (7152671)

Roger J. Mortimer (1942423)

David Worrall (1249377)

Loughborough University Institutional Repository

   This item was submitted to Loughborough’s Institutional Repository (https://dspace.lboro.ac.uk/) by the author and is made available under the following Creative Commons Licence conditions.      For the full text of this licence, please go to: http://creativecommons.org/licenses/by-nc-nd/2.5/  	 SSR  September	2009,	91(334)	 71Why don’t zombies like hibiscus tea? A multi-subject approach to photosynthesis through the use of Grätzel cellsJohn Twidle, Gren Ireson, Roger J. Mortimer and David R. WorrallABSTRACT  Traditionally, photosynthesis has been seen as the domain of biology, with some input from chemistry when dealing with chromatography, while, apart from a passing reference to the colour of leaves, physics has tended to steer clear of the process that provides the lifeblood of human existence. This article outlines how a recent technological advance can be used as a teaching resource in all three branches of science.The	development	of	new	sources	of	electrical	energy	is	always	at	the	forefront	of	scientific	research	and	the	use	of	batteries	and/or	cells	in	school	is	often	cited	in	published	work	to	promote	interest	in	science	(see,	for	example,	Lee,	2005	and	Bullivant	and	Hare,	2006).	Indeed,	what	follows	in	this	article	is	not	new	(see,	for	example,	Siemsen	et al.,	1998),	but	it	provides	an	improved	fabrication	process	that	can	be	carried	out	more	easily	in	schools,	together	with	guidance	on	investigative	work	using	the	cells.	Although	the	underpinning	science	may	appear	complex,	the	assembly	and	use	of	Grätzel	cells	is	relatively	simple;	working	units	have	been	successfully	constructed	by	mixed-ability	groups	of	pupils	in	year	7	(age	11/12)	and	above.Grätzel cellsThe	first	published	reference	to	Grätzel	cells	was	in	1991	(O’Regan	and	Grätzel,	1991).	These	cells	work	by	effectively	mimicking	photosynthesis	but	without	the	production	of	glucose.	The	cells	transfer	energy	from	incident	light	to	electrical	energy	and	require	the	use	of	an	iodide	electrolyte	and	a	dye;	as	such,	the	cells	may	be	referred	to	as	dye-sensitive	cells.	A	typical	structure	of	a	Grätzel	cell	is	illustrated	in	Figure	1.The	negative	side	of	the	cell	consists	of	a	thin	layer	of	titanium	dioxide	onto	which	dye	molecules	are	adsorbed.	The	titanium	dioxide	particles	are	in	turn	attached	to	a	commercially	available	conducting	glass	plate	(rendered			Figure 1	 The	basic	structure	of	a	Grätzel	cell72	 SSR		September	2009,	91(334)conducting	by	a	coating	of	tin-doped	indium	oxide,	so	thin	that	light	can	pass	through	it	almost	without	loss).	Incident	light	excites	the	electrons	in	the	dye.	These	excited	electrons	become	free	electrons	and	pass	into	the	conducting	band	of	the	titanium	dioxide.	The	titanium	dioxide	layer	thus	develops	a	net	negative	charge	and	electrons	flow,	via	the	top	conducting	glass	and	a	low-current	device,	such	as	a	calculator,	acting	as	an	external	load,	to	the	iodide	electrolyte	via	the	lower	conducting	glass	plate	and	graphite	layer.To	complete	the	circuit,	the	dye	ions	(D+)	regain	their	lost	electrons	from	the	iodide	electrolyte.	Iodide	ions	(I–)	are	oxidised	(loss	of	electrons)	to	tri-iodide	ions	(I3–)	as	in	the	equation:3I–	→	I3–	+	2e–D+	+	e–	→	DElectrons	from	the	graphite	layer	reverse	this	reaction	and	reduce	(gain	of	electrons)	the	tri-iodide	ions	back	to	iodide	ions.I3–	+	2e–		3I–The	dye	molecules	(D)	are	now	ready	for	the	next	excitation/oxidation/reduction	cycle.Constructing a Grätzel cellThere	are	three	steps	in	constructing	a	Grätzel	cell:1	 preparation	of	graphite	and	titanium	dioxide	plates;2	 adsorption	of	dye	onto	titanium	dioxide;3	 assembly	of	the	Grätzel	cell.These	steps	are	described	below.1. Preparation of graphite and titanium dioxide platesBoth	titanium	dioxide	and	graphite	plates	are	prepared	from	commercially	obtained	transparent	glass	plates	that	have	been	treated	on	one	side	with	a	transparent	thin	layer	of	conducting	substrate	(tin-doped	indium	oxide).	Both	plates	are	prepared	onto	the	conducting	sides.	(Currently,	six	pairs	of	pre-coated	plates	are	available	for	the	sum	of	15	euros	–	see	Sources.)The	graphite	plate	is	prepared	by	colouring	in	the	plate	with	graphite	using	a	pencil	(Figure	2a).	The	titanium	dioxide	plate	can	be	re-coated	with	a	thin	layer	of	titanium	dioxide	paste,	which	is	then	sintered	(baked)	over	a	blue	Bunsen	flame	for	around	3	minutes	(Figure	2b).	In	order	to	avoid	cracking	the	glass	plates,	it	is	best	to	raise	their	temperature	gently	by	holding	them	in	tongs	and	slowly	introducing	them	to	the	Bunsen	flame.	If	available,	an	oven	(set	at	500	°C),	such	as	those	commonly	used	for	ceramics	work	in	a	school	art	department,	would	give	a	more	controllable	source	of	heat.	Removing	the	plates	from	the	oven	and	cooling	them	rapidly	in	air	tends	to	produce	some	discolouration	of	the	titanium	dioxide	layer;	this	can	be	avoided	by	turning	the	oven	off	and	allowing	the	plates	to	cool	more	slowly.	Although	this	sintering	process	is	relatively	straightforward,	it	can	be	time-consuming	and	we	would	recommend	that	this	be	carried	out	in	advance	by	either	the	teacher	or	laboratory	technician.2. Adsorption of dye onto titanium dioxideFor	those	wishing	to	make	explicit	links	to	photosynthesis,	chlorophyll	extracted	from	green	plants	or	purchased	as	a	water-soluble	reagent	from	a	chemical	supplier	such	as	Philip	Harris	(see	Sources)	would	work.	However,	hibiscus	tea	is	found	to	work	exceptionally	well.	1	g	of	hibiscus	tea	leaves	is	added	to	50	cm3	of	water	in	a	100	cm3	beaker	and	allowed	to	boil	for	2	minutes.	The	titanium	dioxide	plate	is	then	immersed	in	the	hibiscus	tea	solution	for	5	minutes	to	allow	the	dye	to	adsorb	onto	the	titanium	dioxide	layer.	The	excess	dye	is	then	washed	off	using	water	and	the	plate	(Figure	3)	is	left	to	dry	in	air,	or	the	process	can	be	speeded	up	by	use	of	an	oven	or	hairdryer.A multi-subject approach to photosynthesis through the use of Grätzel cells Twidle et al.Figure 2	 Prepared	graphite	(a)	and	titanium	dioxide	(b)	platesa b	 SSR  September	2009,	91(334)	 73Other	vegetable	dyes	can	be	used	for	adsorbing	onto	the	titanium	dioxide	plate	in	a	similar	manner.	These	include:	blackcurrants,	black	tea,	cranberries,	onion	skin,	raspberries,	Ribena,	strawberries	and	even	turmeric	powder.	Suggested	quantities	to	use	are	listed	in	Table	1.3. Assembly of the Grätzel cellIn	our	trials,	we	used	a	commercially	available	(from	Man	Solar	–	see	Sources)	iodide	electrolyte.	However,	should	the	reader	wish	to	make	up	his/her	own	version,	an	effective	(although	not	cost-effective)	electrolyte	solution	can	be	made	from	0.1	mol	dm–3	lithium	iodide	and	0.05	mol	dm–3	iodine	in	a	solvent	of	80%	ethylene	carbonate	and	20%	propylene	carbonate.To	assemble	the	Grätzel	cell,	the	dyed	titanium	dioxide	plate	is	placed	on	top	of	the	graphite	plate.	It	is	important	that	the	plates	are	placed	slightly	offset	to	allow	for	electrical	contact.	To	keep	the	plates	in	position,	a	paperclip	is	used	to	firmly	clamp	the	two	plates	together.	Finally,	a	drop	of	iodide	electrolyte	(from	a	small	dropping	pipette)	is	placed	on	the	exposed	part	of	the	lower	plate,	next	to	the	edge	of	the	upper	plate,	and	left	for	2	minutes	to	allow	the	electrolyte	to	flow	through	the	whole	of	the	cell.	The	Grätzel	cell	is	complete	and	ready	for	generating	electricity	(see	Figure	4).Examples of curriculum materialsIf	natural	light	is	not	available	or	is	not	sufficiently	strong,	the	light	from	an	overhead	projector	or	desk	lamp	will	power	the	cells.	Using	dye	from	hibiscus	tea,	between	three	and	five	cells	connected	in	series	will	generate	a	total	voltage	of	1.2	V,	sufficient	to	power	a	small	electronic	calculator.	Some	commercially	available	kits	include	low-powered	fans	and	melody	modules.This	work	was	piloted	with	a	group	of	40	trainee	teachers,	a	mixed	group	of	biology,	Figure 3	 Prepared	titanium	dioxide	plate	dyed	with	hibiscus	teaFigure 4	 Prepared	Grätzel	cell,	ready	for	generating	electricityTable 1	 Suggested	quantities	for	various	vegetable	dyesVegetable material Suggested quantities to useblackcurrants crush	5	blackcurrants	and	simmer	in	10	cm3	waterblack	tea simmer	a	tea	bag	in	50	cm3	watercranberries crush	4	cranberries	and	simmer	in	10	cm3	water	onion	skin simmer	5	pieces	of	skin	in	50	cm3	waterraspberries crush	2–4	raspberries	and	simmer	in	10	cm3	waterRibena simmer	10%	solution	for	2	minutesstrawberries crush	2–4	strawberries	and	simmer	in	10	cm3	waterturmeric	powder simmer	0.3	g	in	50	cm3	waterTwidle et al. A multi-subject approach to photosynthesis through the use of Grätzel cells74	 SSR		September	2009,	91(334)A multi-subject approach to photosynthesis through the use of Grätzel cells Twidle et al.BOX 1 Zombie attackYou	are	walking	through	a	graveyard	on	your	way	back	from	a	friend’s	house	just	as	the	Sun	is	about	to	rise.	You	hear	something	rustling	in	a	bush.	You	begin	to	walk	faster.	Your	heart	is	pounding.	Just	as	you	get	close	to	the	gates	to	leave	the	graveyard	you	feel	cold	breath	on	the	back	of	your	neck.	You	gasp	as	you	fall	to	the	ground.	 A	few	moments	later	you	wake	up	and	feel	a	stabbing	pain	in	your	leg.	You	look	down	and	see	a	bloody	wound.	You	notice	something	familiar	about	the	marks	on	your	leg.	You	have	seen	them	before	in	a	research	paper	your	uncle	wrote.	You	put	all	the	links	together	and	realise	you	have	been	bitten	by	a	ZOMBIE!	 You	remember	your	uncle	investigated	suspicious	attacks	in	the	village	a	few	months	ago	but	kept	everything	top	secret	in	his	lab.	You	hobble	your	way	towards	your	uncle’s	lab	and	break	in	through	the	window.	You	manage	to	find	the	files	about	the	zombie	attacks	in	a	cabinet.	You	realise	you	have	one	hour	to	take	the	antidote	or	you	will	become	a	zombie.	The	antidote	is	in	a	locked	cabinet	with	an	electronic	keycode	pad	...	but	there	is	no	power.Your taskConstruct	a	Grätzel	cell,	which	can	be	used	to	generate	a	voltage	using	sunlight,	from	the	equipment	found	in	the	lab	kitchen.	Each	group	will	investigate	the	effects	of	a	different	dye	on	the	voltage	produced.	We	will	share	all	our	results	at	the	end	of	the	lesson	to	decide	which	dye	would	be	best	to	use	to	power	the	electronic	keycode	pad	and	get	to	the	antidote	in	time!Items available in the lab kitchenl	 conductive	glass	from	LCD	screen	on	microwave*l	 titanium	dioxide	(TiO2)	from	the	chemical	cupboardl	 iodide	solution	from	chemical	cupboardl	 pencill	 paperclipsl	 wiresl	 voltmeterl	 Ribenal	 tea	bagsl	 cranberriesl	 keycode	pad	(calculator	used	in	class	to	represent	this)*[NB	While	we	are	not	advocating	that	pupils	dismantle	a	microwave	oven	to	source	conducting	glass	plates,	this	is	an	example	of	a	commercial	application	of	conducting	glass.	If	a	teacher	were	to	resort	to	such	a	source,	it	would	be	essential	that	any	mains	lead	were	removed	and	all	precautions	taken	to	avoid	pupils	cutting	themselves	on	sharp/broken	glass.]Table of resultsType of dye Voltage generated (volts) Does calculator (keypad) turn on?RibenaTea	bagsCranberriesIn	our	experience,	we	have	found	that	as	few	as	three	hibiscus	tea	cells	connected	in	series	are	sufficient	to	power	a	typical	button-battery	operated	calculator.	 SSR  September	2009,	91(334)	 75chemistry	and	physics	graduates,	all	following	a	one-year	Postgraduate	Certificate	in	Education	course	in	secondary	science.	Working	in	mixed-subject	groups,	the	students	suggested	and	piloted	a	series	of	investigations,	two	of	which	are	outlined	in	Boxes	1	and	2.	Whilst	both	scenarios	may	appear	rather	contrived,	the	‘zombie	attack’	in	particular	is	sufficiently	macabre	to	have	a	certain	‘gross	appeal’	to	many	teenagers.	Other approachesAs	asserted	in	the	early	sections	of	this	article,	photosynthesis	has	traditionally	been	seen	as	the	domain	of	biology.	However,	Grätzel	cell	technology	can	be	adopted	for	use	as	a	vehicle	through	which	all	branches	of	science	in	the	secondary	curriculum	can	be	addressed.	In	particular,	the	action	of	light-sensitive-dye	molecules,	as	part	of	an	energy-transfer	process,	parallel	the	reaction	in	photosynthesis.	Other	approaches	that	have	been	suggested	include:l	 Investigating	the	effect	of	connecting	Grätzel	cells	in	series	circuits.l	 Investigating	the	effect	of	connecting	Grätzel	cells	in	parallel	circuits.l	 Investigating	the	effect	of	different	organic	dyes	on	the	power	output	of	a	Grätzel	cell.l	 As	a	prompt	for	discussion	on	advantages	and	disadvantages	of	different	renewable	energy	sources.l	 For	a	comparison	of	conventional	solar	cells	with	Grätzel	cells.l	 Investigating	the	effect	of	using	different-coloured	lights	on	the	current	output	of	a	Grätzel	cell.l	 Investigating	the	relationship	between	the	distance	between	the	source	of	light	and	the	power	output	of	a	Grätzel	cell.l	 Use	of	the	chemistry	underpinning	Grätzel	cell	technology	as	a	practical	application	of	oxidation/reduction	and	reversible	reactions.l	 Using	dyes	obtained	from	green	plants,	investigating	the	output	of	a	Grätzel	cell	with	light	of	different	wavelengths.Risk analysisRisk Dangers PrecautionsIodide	solution	splashed	into	eye	(low	hazard)Harmful Wear	eye	protectionSintering	the	titanium	dioxide	plates Burns	to	fingers Present	pupils	with	pre-prepared	platesRemoving	LCD	display	on	microwaveCut	fingers	on	broken/sharp	glass.ElectrocutionDo	not	allow	pupils	to	dismantle	microwave	ovensRemove	mains	lead	before	dismantlingSimmering	titanium	dioxide	plates	in	solution	of	vegetable	dyeBurns	from	hot	apparatus	/	spillage	of	boiling	waterStandard	laboratory	precautionsTitanium	dioxide	(low	hazard) Standard	laboratory	precautionsEating	any	of	the	vegetable	samples All	natural	products,	regularly	consumed	by	the	public	–	negligible	riskStandard	laboratory	precautionsBOX 2 ShipwreckOn	a	school	trip	around	the	Caribbean	you	become	shipwrecked	and	stranded	on	a	deserted	island!	 The	supplies	you	have	been	able	to	salvage	include	a	pack	of	Grätzel	cell	kits.	You	also	have	a	mobile	phone,	which	you	can	use	to	contact	an	oil	tanker	you	can	see	on	the	horizon	...	but	the	battery	is	damaged	beyond	repair.	 To	get	the	Grätzel	cells	to	work	you	need	to	go	out	and	find	a	suitable	dye	to	act	as	the	electron	donor.	 Things	to	consider:l	 What	colour	dye	will	work	best?l	 What	voltage/current	do	you	need	to	power	the	mobile	phone?l	 How	many	Grätzel	cells	will	you	need	to	achieve	this?l	 How	will	you	configure	them?Twidle et al. A multi-subject approach to photosynthesis through the use of Grätzel cells76	 SSR		September	2009,	91(334)SourcesIndividual	components	or	complete	Grätzel	cell	kits	(from	49	Euros	upwards)	are	available	from	Man	Solar	at:	www.mansolar.com/As	detailed	earlier,	an	alternative	electrolyte	could	be	made	up	from	its	constituents	although	the	organic	solvents	required	are	not	likely	to	be	readily	available	in	a	secondary	school	chemical	store	and	the	small	volumes	required	for	the	experiment	may	not	make	it	a	cost-effective	option.Conducting	glass	plates:	For	an	alternative	supplier	of	conducting	glass	which	could	be	used	for	Grätzel	cell	work	see:	www.delta-technologies.com/The	LCD	on	a	microwave	oven	is	made	of	several	layers,	one	of	which	is	tin-doped	indium	oxide	conducting	glass	(see	Howstuffworks	on:	www.howstuffworks.com/lcd2.htm)	and	therefore	non-functioning	microwaves	are	a	potential	source	of	conducting	glass	electrodes.	A	cheaper,	and	more	readily	available,	source	would	be	the	LCD	from	a	calculator,	although	the	non-display	areas	have	been	etched	away	which	results	in	only	a	small	area	remaining.	Larger	surface	areas	of	conducting	glass	can	also	be	found	in	self-dipping	rear	view	car	mirrors	and	some	high-tech	conservatory	windows.Vegetable	dyes:	Water-soluble	chlorophyll	is	available	from	suppliers,	such	as	Philip	Harris.	At	the	time	of	writing,	10	g	is	available	(reference	number:	C7A6	7003)	at	a	cost	of	£15.	Alternatively,	a	green	vegetable,	such	as	spinach,	could	be	used	in	the	same	way	as	any	of	the	other	sources.Most	supermarkets	would	carry	a	range	of	suitable	vegetable	materials	for	experimentation;	hibiscus	tea	bags	are	less	commonly	stocked	and	we	sourced	these	from	health	food	stores	(fresh	hibiscus	tea	bags	were	found	to	be	far	more	effective	than	older	ones).Further	information	on	experiments	and	resources	can	be	obtained	from	the	Department	of	Chemistry,	Loughborough	University:	www.lboro.ac.uk/departments/cm/business/Gratzel.htmlReferencesBullivant,	M.	and	Hare,	J.	(2006)	Rough	science	and	homemade	batteries.	Education in Chemistry,	43,	12–14.Lee,	K.	B.	(2005)	Urine-activated	paper	batteries	for	biosystems.	Journal of Micromechanics and Microengineering,	15,	210–214.O’Regan,	B.	and	Grätzel,	M.	A.	(1991)	A	low-cost,	high-efficiency	solar	cell	based	on	dye-sensitized	colloidal	TiO2	films.	Nature,	353,	737–740.Siemsen,	F.,	Bunk,	A.,	Fischer,	K.,	Korneck,	F.,	Engel,	H.	and	Roux,	D.	(1998)	Solar	energy	from	spinach	and	toothpaste:	fabrication	of	a	solar	cell	in	schools.	European Journal of Physics,	19,	51–58.John Twidle is	PGCE/MSc	with	QTS	secondary	science	course	leader	at	Loughborough	University.	Email:	J.Twidle@lboro.ac.uk	Gren Ireson is	professor	of	science	education	at	Nottingham	Trent	University.	Email:	gren.ireson@ntu.ac.uk	Roger J. Mortimer	is	professor	of	physical	chemistry	at	Loughborough	University.	Email:	R.J.Mortimer@lboro.ac.uk	David R. Worrall	is	a	lecturer	in	physical	chemistry	at	Loughborough	University.	Email:	D.R.Worrall@lboro.ac.ukA multi-subject approach to photosynthesis through the use of Grätzel cells Twidle et al.

Why don't zombies like hibiscus tea? A multi-subject approach to photosynthesis through the use of Gratzel cells

Nottingham Trent Institutional Repository (IRep)

	 SSR  September	2009,	91(334)	 71Why don’t zombies like hibiscus tea? A multi-subject approach to photosynthesis through the use of Grätzel cellsJohn Twidle, Gren Ireson, Roger J. Mortimer and David R. WorrallABSTRACT  Traditionally, photosynthesis has been seen as the domain of biology, with some input from chemistry when dealing with chromatography, while, apart from a passing reference to the colour of leaves, physics has tended to steer clear of the process that provides the lifeblood of human existence. This article outlines how a recent technological advance can be used as a teaching resource in all three branches of science.The	development	of	new	sources	of	electrical	energy	is	always	at	the	forefront	of	scientific	research	and	the	use	of	batteries	and/or	cells	in	school	is	often	cited	in	published	work	to	promote	interest	in	science	(see,	for	example,	Lee,	2005	and	Bullivant	and	Hare,	2006).	Indeed,	what	follows	in	this	article	is	not	new	(see,	for	example,	Siemsen	et al.,	1998),	but	it	provides	an	improved	fabrication	process	that	can	be	carried	out	more	easily	in	schools,	together	with	guidance	on	investigative	work	using	the	cells.	Although	the	underpinning	science	may	appear	complex,	the	assembly	and	use	of	Grätzel	cells	is	relatively	simple;	working	units	have	been	successfully	constructed	by	mixed-ability	groups	of	pupils	in	year	7	(age	11/12)	and	above.Grätzel cellsThe	first	published	reference	to	Grätzel	cells	was	in	1991	(O’Regan	and	Grätzel,	1991).	These	cells	work	by	effectively	mimicking	photosynthesis	but	without	the	production	of	glucose.	The	cells	transfer	energy	from	incident	light	to	electrical	energy	and	require	the	use	of	an	iodide	electrolyte	and	a	dye;	as	such,	the	cells	may	be	referred	to	as	dye-sensitive	cells.	A	typical	structure	of	a	Grätzel	cell	is	illustrated	in	Figure	1.The	negative	side	of	the	cell	consists	of	a	thin	layer	of	titanium	dioxide	onto	which	dye	molecules	are	adsorbed.	The	titanium	dioxide	particles	are	in	turn	attached	to	a	commercially	available	conducting	glass	plate	(rendered			Figure 1	 The	basic	structure	of	a	Grätzel	cell72	 SSR		September	2009,	91(334)conducting	by	a	coating	of	tin-doped	indium	oxide,	so	thin	that	light	can	pass	through	it	almost	without	loss).	Incident	light	excites	the	electrons	in	the	dye.	These	excited	electrons	become	free	electrons	and	pass	into	the	conducting	band	of	the	titanium	dioxide.	The	titanium	dioxide	layer	thus	develops	a	net	negative	charge	and	electrons	flow,	via	the	top	conducting	glass	and	a	low-current	device,	such	as	a	calculator,	acting	as	an	external	load,	to	the	iodide	electrolyte	via	the	lower	conducting	glass	plate	and	graphite	layer.To	complete	the	circuit,	the	dye	ions	(D+)	regain	their	lost	electrons	from	the	iodide	electrolyte.	Iodide	ions	(I–)	are	oxidised	(loss	of	electrons)	to	tri-iodide	ions	(I3–)	as	in	the	equation:3I–	→	I3–	+	2e–D+	+	e–	→	DElectrons	from	the	graphite	layer	reverse	this	reaction	and	reduce	(gain	of	electrons)	the	tri-iodide	ions	back	to	iodide	ions.I3–	+	2e–		3I–The	dye	molecules	(D)	are	now	ready	for	the	next	excitation/oxidation/reduction	cycle.Constructing a Grätzel cellThere	are	three	steps	in	constructing	a	Grätzel	cell:1	 preparation	of	graphite	and	titanium	dioxide	plates;2	 adsorption	of	dye	onto	titanium	dioxide;3	 assembly	of	the	Grätzel	cell.These	steps	are	described	below.1. Preparation of graphite and titanium dioxide platesBoth	titanium	dioxide	and	graphite	plates	are	prepared	from	commercially	obtained	transparent	glass	plates	that	have	been	treated	on	one	side	with	a	transparent	thin	layer	of	conducting	substrate	(tin-doped	indium	oxide).	Both	plates	are	prepared	onto	the	conducting	sides.	(Currently,	six	pairs	of	pre-coated	plates	are	available	for	the	sum	of	15	euros	–	see	Sources.)The	graphite	plate	is	prepared	by	colouring	in	the	plate	with	graphite	using	a	pencil	(Figure	2a).	The	titanium	dioxide	plate	can	be	re-coated	with	a	thin	layer	of	titanium	dioxide	paste,	which	is	then	sintered	(baked)	over	a	blue	Bunsen	flame	for	around	3	minutes	(Figure	2b).	In	order	to	avoid	cracking	the	glass	plates,	it	is	best	to	raise	their	temperature	gently	by	holding	them	in	tongs	and	slowly	introducing	them	to	the	Bunsen	flame.	If	available,	an	oven	(set	at	500	°C),	such	as	those	commonly	used	for	ceramics	work	in	a	school	art	department,	would	give	a	more	controllable	source	of	heat.	Removing	the	plates	from	the	oven	and	cooling	them	rapidly	in	air	tends	to	produce	some	discolouration	of	the	titanium	dioxide	layer;	this	can	be	avoided	by	turning	the	oven	off	and	allowing	the	plates	to	cool	more	slowly.	Although	this	sintering	process	is	relatively	straightforward,	it	can	be	time-consuming	and	we	would	recommend	that	this	be	carried	out	in	advance	by	either	the	teacher	or	laboratory	technician.2. Adsorption of dye onto titanium dioxideFor	those	wishing	to	make	explicit	links	to	photosynthesis,	chlorophyll	extracted	from	green	plants	or	purchased	as	a	water-soluble	reagent	from	a	chemical	supplier	such	as	Philip	Harris	(see	Sources)	would	work.	However,	hibiscus	tea	is	found	to	work	exceptionally	well.	1	g	of	hibiscus	tea	leaves	is	added	to	50	cm3	of	water	in	a	100	cm3	beaker	and	allowed	to	boil	for	2	minutes.	The	titanium	dioxide	plate	is	then	immersed	in	the	hibiscus	tea	solution	for	5	minutes	to	allow	the	dye	to	adsorb	onto	the	titanium	dioxide	layer.	The	excess	dye	is	then	washed	off	using	water	and	the	plate	(Figure	3)	is	left	to	dry	in	air,	or	the	process	can	be	speeded	up	by	use	of	an	oven	or	hairdryer.A multi-subject approach to photosynthesis through the use of Grätzel cells Twidle et al.Figure 2	 Prepared	graphite	(a)	and	titanium	dioxide	(b)	platesa b	 SSR  September	2009,	91(334)	 73Other	vegetable	dyes	can	be	used	for	adsorbing	onto	the	titanium	dioxide	plate	in	a	similar	manner.	These	include:	blackcurrants,	black	tea,	cranberries,	onion	skin,	raspberries,	Ribena,	strawberries	and	even	turmeric	powder.	Suggested	quantities	to	use	are	listed	in	Table	1.3. Assembly of the Grätzel cellIn	our	trials,	we	used	a	commercially	available	(from	Man	Solar	–	see	Sources)	iodide	electrolyte.	However,	should	the	reader	wish	to	make	up	his/her	own	version,	an	effective	(although	not	cost-effective)	electrolyte	solution	can	be	made	from	0.1	mol	dm–3	lithium	iodide	and	0.05	mol	dm–3	iodine	in	a	solvent	of	80%	ethylene	carbonate	and	20%	propylene	carbonate.To	assemble	the	Grätzel	cell,	the	dyed	titanium	dioxide	plate	is	placed	on	top	of	the	graphite	plate.	It	is	important	that	the	plates	are	placed	slightly	offset	to	allow	for	electrical	contact.	To	keep	the	plates	in	position,	a	paperclip	is	used	to	firmly	clamp	the	two	plates	together.	Finally,	a	drop	of	iodide	electrolyte	(from	a	small	dropping	pipette)	is	placed	on	the	exposed	part	of	the	lower	plate,	next	to	the	edge	of	the	upper	plate,	and	left	for	2	minutes	to	allow	the	electrolyte	to	flow	through	the	whole	of	the	cell.	The	Grätzel	cell	is	complete	and	ready	for	generating	electricity	(see	Figure	4).Examples of curriculum materialsIf	natural	light	is	not	available	or	is	not	sufficiently	strong,	the	light	from	an	overhead	projector	or	desk	lamp	will	power	the	cells.	Using	dye	from	hibiscus	tea,	between	three	and	five	cells	connected	in	series	will	generate	a	total	voltage	of	1.2	V,	sufficient	to	power	a	small	electronic	calculator.	Some	commercially	available	kits	include	low-powered	fans	and	melody	modules.This	work	was	piloted	with	a	group	of	40	trainee	teachers,	a	mixed	group	of	biology,	Figure 3	 Prepared	titanium	dioxide	plate	dyed	with	hibiscus	teaFigure 4	 Prepared	Grätzel	cell,	ready	for	generating	electricityTable 1	 Suggested	quantities	for	various	vegetable	dyesVegetable material Suggested quantities to useblackcurrants crush	5	blackcurrants	and	simmer	in	10	cm3	waterblack	tea simmer	a	tea	bag	in	50	cm3	watercranberries crush	4	cranberries	and	simmer	in	10	cm3	water	onion	skin simmer	5	pieces	of	skin	in	50	cm3	waterraspberries crush	2–4	raspberries	and	simmer	in	10	cm3	waterRibena simmer	10%	solution	for	2	minutesstrawberries crush	2–4	strawberries	and	simmer	in	10	cm3	waterturmeric	powder simmer	0.3	g	in	50	cm3	waterTwidle et al. A multi-subject approach to photosynthesis through the use of Grätzel cells74	 SSR		September	2009,	91(334)A multi-subject approach to photosynthesis through the use of Grätzel cells Twidle et al.BOX 1 Zombie attackYou	are	walking	through	a	graveyard	on	your	way	back	from	a	friend’s	house	just	as	the	Sun	is	about	to	rise.	You	hear	something	rustling	in	a	bush.	You	begin	to	walk	faster.	Your	heart	is	pounding.	Just	as	you	get	close	to	the	gates	to	leave	the	graveyard	you	feel	cold	breath	on	the	back	of	your	neck.	You	gasp	as	you	fall	to	the	ground.	 A	few	moments	later	you	wake	up	and	feel	a	stabbing	pain	in	your	leg.	You	look	down	and	see	a	bloody	wound.	You	notice	something	familiar	about	the	marks	on	your	leg.	You	have	seen	them	before	in	a	research	paper	your	uncle	wrote.	You	put	all	the	links	together	and	realise	you	have	been	bitten	by	a	ZOMBIE!	 You	remember	your	uncle	investigated	suspicious	attacks	in	the	village	a	few	months	ago	but	kept	everything	top	secret	in	his	lab.	You	hobble	your	way	towards	your	uncle’s	lab	and	break	in	through	the	window.	You	manage	to	find	the	files	about	the	zombie	attacks	in	a	cabinet.	You	realise	you	have	one	hour	to	take	the	antidote	or	you	will	become	a	zombie.	The	antidote	is	in	a	locked	cabinet	with	an	electronic	keycode	pad	...	but	there	is	no	power.Your taskConstruct	a	Grätzel	cell,	which	can	be	used	to	generate	a	voltage	using	sunlight,	from	the	equipment	found	in	the	lab	kitchen.	Each	group	will	investigate	the	effects	of	a	different	dye	on	the	voltage	produced.	We	will	share	all	our	results	at	the	end	of	the	lesson	to	decide	which	dye	would	be	best	to	use	to	power	the	electronic	keycode	pad	and	get	to	the	antidote	in	time!Items available in the lab kitchenl	 conductive	glass	from	LCD	screen	on	microwave*l	 titanium	dioxide	(TiO2)	from	the	chemical	cupboardl	 iodide	solution	from	chemical	cupboardl	 pencill	 paperclipsl	 wiresl	 voltmeterl	 Ribenal	 tea	bagsl	 cranberriesl	 keycode	pad	(calculator	used	in	class	to	represent	this)*[NB	While	we	are	not	advocating	that	pupils	dismantle	a	microwave	oven	to	source	conducting	glass	plates,	this	is	an	example	of	a	commercial	application	of	conducting	glass.	If	a	teacher	were	to	resort	to	such	a	source,	it	would	be	essential	that	any	mains	lead	were	removed	and	all	precautions	taken	to	avoid	pupils	cutting	themselves	on	sharp/broken	glass.]Table of resultsType of dye Voltage generated (volts) Does calculator (keypad) turn on?RibenaTea	bagsCranberriesIn	our	experience,	we	have	found	that	as	few	as	three	hibiscus	tea	cells	connected	in	series	are	sufficient	to	power	a	typical	button-battery	operated	calculator.	 SSR  September	2009,	91(334)	 75chemistry	and	physics	graduates,	all	following	a	one-year	Postgraduate	Certificate	in	Education	course	in	secondary	science.	Working	in	mixed-subject	groups,	the	students	suggested	and	piloted	a	series	of	investigations,	two	of	which	are	outlined	in	Boxes	1	and	2.	Whilst	both	scenarios	may	appear	rather	contrived,	the	‘zombie	attack’	in	particular	is	sufficiently	macabre	to	have	a	certain	‘gross	appeal’	to	many	teenagers.	Other approachesAs	asserted	in	the	early	sections	of	this	article,	photosynthesis	has	traditionally	been	seen	as	the	domain	of	biology.	However,	Grätzel	cell	technology	can	be	adopted	for	use	as	a	vehicle	through	which	all	branches	of	science	in	the	secondary	curriculum	can	be	addressed.	In	particular,	the	action	of	light-sensitive-dye	molecules,	as	part	of	an	energy-transfer	process,	parallel	the	reaction	in	photosynthesis.	Other	approaches	that	have	been	suggested	include:l	 Investigating	the	effect	of	connecting	Grätzel	cells	in	series	circuits.l	 Investigating	the	effect	of	connecting	Grätzel	cells	in	parallel	circuits.l	 Investigating	the	effect	of	different	organic	dyes	on	the	power	output	of	a	Grätzel	cell.l	 As	a	prompt	for	discussion	on	advantages	and	disadvantages	of	different	renewable	energy	sources.l	 For	a	comparison	of	conventional	solar	cells	with	Grätzel	cells.l	 Investigating	the	effect	of	using	different-coloured	lights	on	the	current	output	of	a	Grätzel	cell.l	 Investigating	the	relationship	between	the	distance	between	the	source	of	light	and	the	power	output	of	a	Grätzel	cell.l	 Use	of	the	chemistry	underpinning	Grätzel	cell	technology	as	a	practical	application	of	oxidation/reduction	and	reversible	reactions.l	 Using	dyes	obtained	from	green	plants,	investigating	the	output	of	a	Grätzel	cell	with	light	of	different	wavelengths.Risk analysisRisk Dangers PrecautionsIodide	solution	splashed	into	eye	(low	hazard)Harmful Wear	eye	protectionSintering	the	titanium	dioxide	plates Burns	to	fingers Present	pupils	with	pre-prepared	platesRemoving	LCD	display	on	microwaveCut	fingers	on	broken/sharp	glass.ElectrocutionDo	not	allow	pupils	to	dismantle	microwave	ovensRemove	mains	lead	before	dismantlingSimmering	titanium	dioxide	plates	in	solution	of	vegetable	dyeBurns	from	hot	apparatus	/	spillage	of	boiling	waterStandard	laboratory	precautionsTitanium	dioxide	(low	hazard) Standard	laboratory	precautionsEating	any	of	the	vegetable	samples All	natural	products,	regularly	consumed	by	the	public	–	negligible	riskStandard	laboratory	precautionsBOX 2 ShipwreckOn	a	school	trip	around	the	Caribbean	you	become	shipwrecked	and	stranded	on	a	deserted	island!	 The	supplies	you	have	been	able	to	salvage	include	a	pack	of	Grätzel	cell	kits.	You	also	have	a	mobile	phone,	which	you	can	use	to	contact	an	oil	tanker	you	can	see	on	the	horizon	...	but	the	battery	is	damaged	beyond	repair.	 To	get	the	Grätzel	cells	to	work	you	need	to	go	out	and	find	a	suitable	dye	to	act	as	the	electron	donor.	 Things	to	consider:l	 What	colour	dye	will	work	best?l	 What	voltage/current	do	you	need	to	power	the	mobile	phone?l	 How	many	Grätzel	cells	will	you	need	to	achieve	this?l	 How	will	you	configure	them?Twidle et al. A multi-subject approach to photosynthesis through the use of Grätzel cells76	 SSR		September	2009,	91(334)SourcesIndividual	components	or	complete	Grätzel	cell	kits	(from	49	Euros	upwards)	are	available	from	Man	Solar	at:	www.mansolar.com/As	detailed	earlier,	an	alternative	electrolyte	could	be	made	up	from	its	constituents	although	the	organic	solvents	required	are	not	likely	to	be	readily	available	in	a	secondary	school	chemical	store	and	the	small	volumes	required	for	the	experiment	may	not	make	it	a	cost-effective	option.Conducting	glass	plates:	For	an	alternative	supplier	of	conducting	glass	which	could	be	used	for	Grätzel	cell	work	see:	www.delta-technologies.com/The	LCD	on	a	microwave	oven	is	made	of	several	layers,	one	of	which	is	tin-doped	indium	oxide	conducting	glass	(see	Howstuffworks	on:	www.howstuffworks.com/lcd2.htm)	and	therefore	non-functioning	microwaves	are	a	potential	source	of	conducting	glass	electrodes.	A	cheaper,	and	more	readily	available,	source	would	be	the	LCD	from	a	calculator,	although	the	non-display	areas	have	been	etched	away	which	results	in	only	a	small	area	remaining.	Larger	surface	areas	of	conducting	glass	can	also	be	found	in	self-dipping	rear	view	car	mirrors	and	some	high-tech	conservatory	windows.Vegetable	dyes:	Water-soluble	chlorophyll	is	available	from	suppliers,	such	as	Philip	Harris.	At	the	time	of	writing,	10	g	is	available	(reference	number:	C7A6	7003)	at	a	cost	of	£15.	Alternatively,	a	green	vegetable,	such	as	spinach,	could	be	used	in	the	same	way	as	any	of	the	other	sources.Most	supermarkets	would	carry	a	range	of	suitable	vegetable	materials	for	experimentation;	hibiscus	tea	bags	are	less	commonly	stocked	and	we	sourced	these	from	health	food	stores	(fresh	hibiscus	tea	bags	were	found	to	be	far	more	effective	than	older	ones).Further	information	on	experiments	and	resources	can	be	obtained	from	the	Department	of	Chemistry,	Loughborough	University:	www.lboro.ac.uk/departments/cm/business/Gratzel.htmlReferencesBullivant,	M.	and	Hare,	J.	(2006)	Rough	science	and	homemade	batteries.	Education in Chemistry,	43,	12–14.Lee,	K.	B.	(2005)	Urine-activated	paper	batteries	for	biosystems.	Journal of Micromechanics and Microengineering,	15,	210–214.O’Regan,	B.	and	Grätzel,	M.	A.	(1991)	A	low-cost,	high-efficiency	solar	cell	based	on	dye-sensitized	colloidal	TiO2	films.	Nature,	353,	737–740.Siemsen,	F.,	Bunk,	A.,	Fischer,	K.,	Korneck,	F.,	Engel,	H.	and	Roux,	D.	(1998)	Solar	energy	from	spinach	and	toothpaste:	fabrication	of	a	solar	cell	in	schools.	European Journal of Physics,	19,	51–58.John Twidle is	PGCE/MSc	with	QTS	secondary	science	course	leader	at	Loughborough	University.	Email:	J.Twidle@lboro.ac.uk	Gren Ireson is	professor	of	science	education	at	Nottingham	Trent	University.	Email:	gren.ireson@ntu.ac.uk	Roger J. Mortimer	is	professor	of	physical	chemistry	at	Loughborough	University.	Email:	R.J.Mortimer@lboro.ac.uk	David R. Worrall	is	a	lecturer	in	physical	chemistry	at	Loughborough	University.	Email:	D.R.Worrall@lboro.ac.ukA multi-subject approach to photosynthesis through the use of Grätzel cells Twidle et al.

A low-cost, highefficiency solar cell based on dye-sensitized colloidal TiO2 films.

Rough science and homemade batteries.

Solar energy from spinach and toothpaste: fabrication of a solar cell in schools.

Urine-activated paper batteries for biosystems.

Why don't zombies like hibiscus tea? A multi-subject approach to photosynthesis through the use of Grätzel cells

https://repository.lboro.ac.uk/articles/Why_don_t_zombies_like_hibiscus_tea_A_multi-subject_approach_to_photosynthesis_through_the_use_of_Gratzel_cells/9346664/files/16955714.pdf

Why don't zombies like hibiscus tea? A multi-subject approach to photosynthesis through the use of Grätzel cells

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