IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 23 (2012) 075705 (6pp) doi:10.1088/0957-4484/23/7/075705

In situ transmission electron microscopy

of light-induced photocatalytic reactions

F Cavalca1, A B Laursen

2, B E Kardynal

3, R E Dunin-Borkowski

1,4,

S Dahl2, J B Wagner

1and T W Hansen

1

1 Center for Electron Nanoscopy, Technical University of Denmark, DK-2800 Kongens Lyngby,Denmark2 Center for Individual Nanoparticle Functionality, Technical University of Denmark, DK-2800 KongensLyngby, Denmark3 Department of Photonics Engineering, Technical University of Denmark, DK-2800 Kongens Lyngby,Denmark4 Ernst Ruska—Centre for Microscopy and Spectroscopy with Electrons (ER-C) and Peter GrunbergInstitute (PGI), Forschungszentrum Julich, D-52425 Julich, Germany

E-mail: twh@cen.dtu.dk

Received 3 November 2011, in final form 19 December 2011Published 20 January 2012Online at stacks.iop.org/Nano/23/075705

Abstract

Transmission electron microscopy (TEM) makes it possible to obtain insight into the structure,composition and reactivity of photocatalysts, which are of fundamental interest for sustainableenergy research. Such insight can be used for further material optimization. Here, we combineconventional TEM analysis of photocatalysts with environmental TEM (ETEM) andphotoactivation using light. Two novel types of TEM specimen holder that enable in situ

illumination are developed to study light-induced phenomena in photoactive materials,systems and photocatalysts at the nanoscale under working conditions. The technologicaldevelopment of the holders is described and two representative photo-induced phenomena arestudied: the photodegradation of Cu2O and the photodeposition of Pt onto a GaN:ZnOphotocatalyst.

(Some figures may appear in colour only in the online journal)

1. Introduction

As a result of diminishing fossil fuel reserves, there is anincreasing need to switch energy dependence to renewableresources such as sunlight. Photocatalysts provide a viableroute for converting solar energy into chemical bonds. In orderto optimize the performance of such materials, it is necessaryto understand the fundamentals of their reaction mechanisms,chemical behavior, structure and morphology before, duringand after reaction using in situ investigations. Here, we focuson the in situ characterization of photocatalysts [1] in anenvironmental transmission electron microscope (ETEM) [2].Such fundamental insight can subsequently be used forfurther material optimization with respect to performance andstability [3].

Our experiments are aimed at exposing a specimen tolight and detecting the resulting microstructural and chemical

changes using in situ TEM techniques. It is important toinvestigate photoactive materials under light illumination inorder to remove the effects associated with handling of thespecimen between ex situ reactions and TEM experiments.

In addition to the extensively addressed influence of theelectron beam on ETEM experiments [4, 5], the challengesunderlying in situ studies of photocatalysts in the TEMinvolve both technical and scientific issues. Taking intoconsideration the spatial limitation due to the confinedgeometry in the sample chamber of the electron microscope,which is limited by microscope components such as energydispersive x-ray (EDX) detectors, cold traps, gas inletsand outlets, the present work focuses on the design of amicroscope-independent system that allows a sample to beexposed to light stimuli through the specimen holder.

The interaction of the specimen with both light andaccelerated electrons, each of which has a specific intensity,

10957-4484/12/075705+06$33.00 c� 2012 IOP Publishing Ltd Printed in the UK & the USA

Nanotechnology 23 (2012) 075705 F Cavalca et al

Figure 1. Schematic cross-sectional view of the lens-based specimen holder. (a) The feedthrough on the left hosts a laser diode, which isconnected through a mini-DIN connector installed in the lateral port. Two lenses (shown in blue), selected for the chosen laser wavelength,collimate and focus the light onto the sample. (b) A close-up cross-sectional side view of the tip. The sample (shown in yellow) is bent inorder to allow it to be exposed to both light and electron beams. (c), (d). Photographs of the tip with the illumination off and on, respectively.

wavelength and direction, must be considered. Althoughlight and electron beams interact with specimens in similarways, the energy of the electrons is higher and can thereforeinduce effects which are absent under light illumination. Thechallenge lies in how to distinguish the effect of the twotypes of radiation, thus being able to interpret light-drivenphenomena.

Only a limited number of previous studies haveinvestigated light-induced effects on materials in theTEM [6–8]. However, these experiments were very system-specific. Here, we present a flexible and adaptable specimenholder system that permits straightforward light sourcereplacement, simultaneous irradiation using different lightsources, several sample geometries, electrical biasing andin situ heating. Flexibility is a key feature of the system.

2. Experimental details

Two specimen holder designs are presented below, each ofwhich is compatible with modern FEI TEMs and contains alight source illuminating the sample along the holder axis thatpermits exposure to light and electrons at the same time. Ineach design, the sample is tilted by 30◦–45◦ with respect tothe holder tip plane, as shown in figure 1.

2.1. Lens-based holder design

The lens-based holder design, which is shown in figure 1,comprises a standard TEM specimen holder with acustomized tip. All of the custom parts can be adaptedeasily to other holder shafts if needed. The holder shaftwas machined along its axis to make it hollow. The holdertip was drilled axially up to the specimen recess location.The specimen is fixed by a single clamp (see figure 1(c)).A set of custom tips and feedthroughs was designed usingSolidWorksTM software for easy plug and play operationand in situ interchangeability. The feedthrough contains anaxial lead cylinder, which blocks x-rays emitted from thesample during electron irradiation. When the holder is inserted

Figure 2. Photograph of the fiber-based specimen holder. Theholder is shown without a tip for ease of visualization.

into the microscope, the evacuated volume extends to thefeedthrough. The holder contains a laser diode casing locatedon-axis in the feedthrough, in close proximity to the first lensof a two-lens optical system that guides light onto the samplewhile minimizing transmission power losses. Power to thelaser diode is provided using three of the contacts on a fivepin mini-DIN connector leaving two contacts for specimenbiasing or heating.

2.2. Fiber-based holder design

Figure 2 shows the fiber-based holder design, constructedusing a feedthrough equipped with single or multiplemulti-mode (MM) optical fibers. X-ray shielding is achievedin the same way as for the lens-based design. The fibers areintroduced via an additional four-fiber Teflon-sealed ‘3/4’national pipe thread (NPT) vacuum feedthrough. The fibersthat have been implemented in the holder are summarized intable 1.

All the fibers are buffer jacketed, terminated with SMA905 connectors on the air side, stripped (3–5 mm) and flat

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Nanotechnology 23 (2012) 075705 F Cavalca et al

Table 1. List of optical fibers implemented in the fiber-based holder.

Wavelengthrange

Numberof fibers

Outerdiameter (µm)

Corediameter (µm)

Claddingdiameter (µm)

Numericalaperture (NA)

UV–vis(180–1150 nm)

2 750 550 600 0.22

UV–vis(180–1150 nm)

1 1000 600 630 0.39

IR–vis(400–2400 nm)

1 900 600 660 0.22

cleaved on the vacuum side. The illumination cone escapingfrom each fiber is limited by its numerical aperture (NA),which is defined as [9]

NA = �ncore − ncladding = n sin θ, (1)

where θ is the half-angle of acceptance/radiation and n, ncoreand ncladding are the refractive indices of the medium wherethe radiation propagates out of the fiber, the fiber core andthe fiber cladding, respectively. θ is approximately 5.6◦ forthe chosen fibers, corresponding to a total divergence angle of∼11◦. Collimating and focusing optics can be implementedas needed. A key advantage of this design is the possibilityof using any fiber-coupled external light source, independentof monochromaticity and coherence, interchangeably duringmicroscope operation. All five of the contacts on the mini-DINconnector can be used simultaneously to perform electricalbiasing measurements, electrical heating, in situ operation ofMEMS devices, etc (see figure 2).

As the fibers are made from silica and are in closeproximity to the sample, they need to be coated with a suitableconducting layer (e.g. silver paint) and to be connected to theholder in order to avoid the effects of charging on the electronbeam. The illumination cone is defined by the fibers. Theilluminated region, ∼1.5 mm in diameter, can be observed inthe light optical microscope prior to TEM examination.

The lens-based design offers the advantages of greaterpower transmission, no need for auxiliary equipment, lightfocusing capability and the possibility of producing a smalllaser probe of very high intensity, while the versatility ofthe fiber-based design is preferable for some applications.As the laser casing in the lens-based model, once evacuated,does not dissipate the heat produced by the laser diodeduring operation, the laser efficiency is not constant overtime. Furthermore, the choice of wavelength is limited tothat of commercially available laser diodes and there is alimited possibility of using broadband sources. Finally, asthe entire cavity is connected directly to the microscopevacuum system, light sources cannot be exchanged while theholder is in operation. All of these aspects are relevant forthe experimental design but do not significantly limit theapplicability of the lens-based model, which remains effectiveand easy to operate. The two models in combination offercomplementary options for in situ experiments.

3. Results and discussion

Two experiments were performed using both TEM holderdesigns for in situ illumination. Each experiment was

designed with an emphasis on observing optically drivenphenomena. The experiments were carried out using amonochromated FEI Titan 80-300 ETEM with imagespherical aberration correction.

3.1. Photo-induced degradation of cuprous oxide

Cuprous oxide (Cu2O) is reported to be active for watersplitting and hydrogen evolution from an ethanol solution [10,11]. However, it has a tendency to photo-corrode. Thisdegradation results in a loss of photocatalytic activity, asreported independently by Nagasubramanian [12] and Fleischand Mains [13]. Here, the photodegradation of Cu2O wasinvestigated by direct observation of the transformationprocess using imaging, diffraction and spectroscopy.

Photodegradation in a water vapor atmosphere proceedsaccording to the following reactions:

Cu2O + H2O + hν → 2CuO + 2H+ + 2e− (2)

Cu2O + 2H+ + 2e− + hν → 2Cu + H2O. (3)

In order to maintain the cubic shape and electrontransparency, size-selected cuprous oxide nanocubes in therange 100–200 nm were prepared using solution-phasesynthesis [14]. TEM samples were prepared by dispersingdrops of Cu2O nanocubes in aqueous suspension onto laceycarbon-coated gold TEM grids. The experiments were carriedout using both the lens-based and the fiber-based holders,yielding the same results. Since the electron beam, in thepresence of water vapor, can induce reaction (3) very quickly,in order to ensure that the observed phenomena were opticallydriven and not induced by the electron beam, the experimentalconditions were designed carefully before performing in situ

TEM experiments. Water is known to be a strong oxidizingagent. However, here it is shown that the reduction inequation (3) can occur in an aqueous environment. This canonly be true if the reaction pathway is the one proposedin equations (2) and (3) and light is the reaction drivingforce. Moreover, the light intensity used for the experiments(≤12 W cm−2) does not contribute significantly to particleheating [15], ruling out thermal contributions to the reaction.Most importantly, blank experiments were carried out toconfirm that the reaction occurs exclusively in the presenceof light and water.

Three control experiments were performed to investigatethe effects of water, light and electrons independently (seetable 2). The sample did not undergo a noticeable change in

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Figure 3. (a), (b) Bright-field TEM images of Cu2O nanocubes before and after reaction, respectively, showing changes in particle shapeand morphology. No significant damage to the supporting carbon film was observed. The growth of large single crystals of Cu is observed.(c), (d) Selected area diffraction patterns in the [011] zone axis, indexed for Cu2O (c) and for both Cu2O and Cu (d). The area sampled bythe selected area aperture is 400 nm in diameter and encompasses at least two of the particles shown in (a), (b). After the reaction,(d) contains spots representative of metallic copper, labeled in red. Spots from Cu2O are still present as some portions of the particles arenot completely reduced. (e) Electron energy-loss spectra of the same nanocubes. The upper curve is offset vertically for ease of comparison.The positions of the peaks of the L2 and L3 EELS edges are indicated by the vertical black lines.

Table 2. List of control experiments performed. After initial imaging, the particles were exposed in the electron microscope to theconditions given in the table. At the end of each experiment, the column was evacuated to 10−6 mbar and the electron beam was restoredbefore a second set of images was acquired. In experiment 1, if the system was not evacuated to a pressure lower than 10−6 mbar before newimages were acquired, the sample degraded rapidly in the electron beam due to the presence of residual water vapor. In experiment 3,images were recorded during exposure.

Experiment GasPressure(mbar) Electron beam

Light intensityand wavelength Duration

Observedchanges

1 H2O vapor 3 Absent Absent 5 h None2 Vacuum 10−6 Absent λ = 405 nm, 6 W cm−2 5 h None3 Vacuum 10−6 ∼100 MW cm−2 Absent 20 min None

any of the control experiments, whereas it underwent rapiddegradation if exposed to the electron beam in the presenceof 1 mbar of water vapor. In the latter case the nanocubeschanged in shape and were reduced to metallic copper asobserved by electron energy-loss spectroscopy (EELS).

In order to study the combined effect of light and water,the electron beam was blanked by closing the column valvesduring the reaction. Water vapor was leaked into the specimenchamber to a steady state pressure of ∼3 mbar and the samplewas exposed to light with λ = 405 nm and a power density

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Figure 4. Schematic sketch of the Pt photodeposition process.Light generates an electron–hole pair in the GaN:ZnOphotocatalyst. Electrons and holes migrate to the surface andactivate reactions with the surrounding chemical species. Theelectrons are responsible for the Pt precursor decomposition, whilethe holes drive the water oxidation reaction.

of 6 W cm−2 for various time periods. The column was thenevacuated to 10−6 mbar for 5 h in order to reduce the watervapor in the column before the specimen was exposed to theelectron beam. This procedure allowed sample analysis afterthe reaction without electron beam-induced sample changes.

The particle morphology was observed to changeconsiderably during the light-induced reaction in the presenceof water vapor (see figures 3(a) and (b)).

Electron diffraction patterns indicated a change in latticespacing during the reaction (figures 3(c) and (d) fromthe lattice spacing characteristic of Cu2O (4.25 A) to aspacing which is consistent with that expected for metallicCu (3.61 A). The oxidation state of copper particles wasstudied before and after reaction using EELS. The ratio ofthe Cu L2 and Cu L3 edges (also known as white lines)reflects the chemical state of Cu [16]. Figure 3(e) showsthe energy-loss spectrum of the sample before and after

reaction, indicating a change to metallic copper. The absenceof CuO, whose presence is predicted from equation (2)and would be noted by a characteristic fingerprint in theelectron energy-loss spectrum, is not readily explained fromthe presented diffraction and EELS data and a more completeanalysis of the reduction reaction is ongoing. The experimentdemonstrates the advantage of using in situ characterization tostudy a photoactive material under illumination.

3.2. Platinum photodeposition

GaN:ZnO is a highly active photocatalyst for water splittingunder visible light exposure [11, 17]. Its activity can beenhanced by adding Pt as a co-catalyst for the hydrogenevolution reaction (HER) on its surface [18]. Pt is a reversiblecatalyst for the HER, meaning that unless precautions aretaken it will also efficiently catalyze the reverse reaction,resulting in a loss of net activity. Domen and co-workerscovered Pt with a shell of hydrated Cr2O3 to reduce thereverse reaction significantly [19]. Here, Pt was reduced ontoGaN:ZnO in situ by modifying a commonly used procedureto load Pt onto e.g. TiO2, which is described in detailelsewhere [20]. Reproducing the encapsulation procedure isbeyond the scope of this work. The deposition is usuallyperformed by stirring the semiconductor powder in aqueousethanol or methanol solution containing H2PtCl6 (the Ptprecursor) and illuminating the suspension for several hourswith a UV lamp. The electron–hole pairs that are generatedin the semiconductor material migrate to the surface anddecompose the salt and ethanol, respectively, leaving Pt onthe particle surface. The reaction pathway without ethanol isdepicted in figure 4.

As this procedure is not feasible in TEM, it was modifiedso that the H2PtCl6 precursor was evaporated gently ontothe GaN:ZnO surface (to avoid thermal decomposition). Theas-prepared powder was deposited onto a gold TEM gridcovered with an amorphous carbon film and inserted into

Figure 5. Bright-field TEM images of a GaN:ZnO particle (a) before and (b) after reaction in 5 mbar H2O and with 6 W cm−2 light at405 nm wavelength. The two images were acquired in vacuum and the reaction was carried out in the absence of the electron beam. Thearrows in (b) show some of the deposited Pt nanoparticles.

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the microscope. TEM investigations were carried out usingthe fiber-based holder. Preliminary control experiments werecarried out to distinguish effects other than that of light (seetable 2). None of the experiments resulted in changes to thesample, while the Pt deposited rapidly in the electron beamin the presence of water vapor. These tests allow designingexperiments in which the role of electrons can be neglected.

After a preliminary inspection of the sample, the electronbeam was blanked (column valves closed), water vapor wasleaked into the specimen chamber at a pressure of 5 mbarand the sample was exposed to light at λ = 405 nm with anintensity of 6 W cm−2 over the entire grid for 4 h. The lightwas then turned off and the specimen chamber was evacuatedfor 5 h before analysis. The results are shown in figure 5.

Pt nanoparticles were not observed on the surface of theGaN:ZnO substrate prior to the reaction (see figure 5(a)),with the salt forming a thin amorphous layer that wasdistributed homogeneously over the surface. After reaction,nanoparticles ∼1 nm in diameter appeared on the surface ofthe GaN:ZnO, while the amorphous layer disappeared almostcompletely. The particles were distributed homogeneouslyover the surface, with no evidence for particle incorporationinto the support.

4. Conclusion

Two types of TEM specimen holder for in situ lightillumination have been designed, manufactured and tested.Each holder configuration has different advantages and allowsa variety of experiments to be performed. Two photo-inducedreactions were used as proof-of-concept experiments todemonstrate potential applications of the holders. An ETEMwas used in order to provide the environment necessaryto perform the experiments. The holders are microscope-independent, i.e. they offer the advantage of being usedin different microscopes. The opportunity to use in situ

illumination in different setups is a unique feature offered bythis system and enables any TEM characterization techniqueto be used in combination with light. Such investigationscan provide insight into the working state of photoactivematerials and guide future developments. The holders offeropportunities for further development which are presentlybeing pursued.

Acknowledgments

This work was supported by the ‘Catalysis for SustainableEnergy’ (CASE) research initiative, which is funded by theDanish Ministry of Science, Technology and Innovation. TheA P Møller and Chastine Mc-Kinney Møller foundationis gratefully acknowledged for the contribution toward theestablishment of the Center for Electron Nanoscopy at theTechnical University of Denmark.

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