Nanowires Versus Low Temperature Growth of Pb

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C: Physical Processes in Nanomaterials and Nanostructures

High Temperature Pb Doping of SnO2 and Growth Limitations of PbxSn1-xO2 Nanowires Versus Low Temperature Growth of PbxSn1-xO for Energy Storage and Conversion Matthew Zervos, Andreas Othonos, Eugenia Tanasa, and Eugeniu Vasile J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02865 • Publication Date (Web): 10 Jun 2019 Downloaded from http://pubs.acs.org on June 10, 2019

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The Journal of Physical Chemistry

High Temperature Pb Doping of SnO2 and Growth Limitations of PbxSn1-xO2 Nanowires Versus Low Temperature Growth of PbxSn1-xO for Energy Storage and Conversion

Matthew Zervos1,*, Andreas Othonos 2, Eugenia Tanasă 3, Eugeniu Vasile 3

1 Nanostructured

Materials and Devices Laboratory, School of Engineering, University of Cyprus,

PO Box 20537, Nicosia, 1678, Cyprus 2

Laboratory of Ultrafast Science, Department of Physics, University of Cyprus, PO Box 20537,

Nicosia, 1678, Cyprus 3

Department of Science and Engineering Of Oxides Materials and Nanomaterials, Politehnica

University Of Bucharest, 313 Splaiul Independentei, Bucharest, 060042, Romania

*E-mail : [email protected]

1 ACS Paragon Plus Environment

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Abstract Pb doping of SnO2 nanowires grown by the vapor-liquid-solid mechanism on 1 nm Au/Si has been investigated between 500°C and 1000°C via the reaction of Sn containing Pb with O2 at 10-1 mbar. The SnO2 nanowires have diameters of 50 nm, lengths up to 100m and a tetragonal rutile crystal structure but they do not contain Pb due to its significant depletion during the temperature ramp and re-evaporation from the surface of the SnO2 nanowires. Consequently we do not observe a semiconductor to semimetal transition and band gap narrowing. Instead the Pb reacts with O2 leading to the deposition of PbO directly on Si but not on the SnO2 nanowires which have carrier densities of ≈ 1016 cm-3. Furthermore, one dimensional growth was completely suppressed by increasing the amount of Pb in Sn. As such Pb doping of SnO2 and the growth of PbxSn1-xO2 nanowires is difficult, if not impossible, due to the fact that PbO2 nanowires can’t be grown by the vapor-liquid-solid mechanism irrespective of the growth temperature. In contrast we find that the composition of PbxSn1-xO nanostructures may be tuned over a broad range by low temperature growth at 400°C. We discuss the properties and prospects of this ternary oxide for energy conversion and storage.


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1. Introduction Tin oxide SnO2 is an n-type metal oxide semiconductor with an energy band gap of 3.7 eV that has been used extensively for energy conversion, storage and sensors 1-4. In the past we have grown n-type SnO2 NWs via the vapor-liquid-solid (VLS) mechanism at 800°C and 10-1 mbar which had a carrier density of the order of 1016 cm-3 5. Higher conductivity SnO2 NWs have been obtained previously via the incorporation of substitutional donors like Sb 6 and other elements like Mo 7 both of which have atomic radii nearly equal to that of Sn i.e. 145 pm. Recently we explored the doping and conductivity limitations in Sb: SnO2 NWs and showed that the maximum attainable carrier density in Sb: SnO2 NWs may not reach that of Sn: In2O3 NWs 8. In contrast to Mo and Sb, Pb has an atomic radius of 180 pm i.e.  24% larger than that of Sn but Ma et al. 9 showed theoretically that substantial band-gap tuning and a strain-controlled, semiconductor to gapless semimetal transition, is possible via the incorporation of Pb in SnO2 which could result into higher carrier densities than those obtained with Sb and Mo. To the best of our knowledge no one has previously investigated Pb doping of SnO2 NWs via the VLS mechanism which is interesting from a fundamental and technological point of view. However it is worthwhile pointing out that Zhou et al. 10 investigated Pb doping of ZnO NWs and observed a red-shift in the photoluminescence (PL) emission attributed to band-gap narrowing while more recently we also considered Pb doping of In2O3 NWs 11,12. In addition to the strain-controlled, semiconductor to semimetal transition that was predicted by Ma et al. 9 to occur via the incorporation of Pb into SnO2, Ganose et al.13 showed theoretically that the energy band gap and work function of SnO2 may be tailored by alloying it with PbO2 so that it may be used as a transparent conducting oxide (TCO). PbO2 is an n-type semiconductor with an 3 ACS Paragon Plus Environment


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energy band gap of ~ 0.5 eV and a high carrier density so no doping is required. The origin of the high carrier density and metallic conductivity in PbO2 is attributed to oxygen vacancies, which form a donor state, resonant in the conduction band, while the dipole-forbidden gap combined with a large carrier induced Moss-Burstein shift results in large optical band gaps 14. PbO2 has been used for a very long time in lead acid batteries (LABs) after its discovery in 1859 by Planté while the growth of PbxSn1-xO2 was originally investigated by F. Lappe 15 in the early 1960’s who found a linear dependence of the lattice constant of PbxSn1-xO2 layers with x. The incorporation of Pb into SnO2 shifted the absorption curve of pure SnO2 to lower energies and at the same time the layers became more conductive. Other early efforts on PbxSn1-xO2 layers were carried out by Czapla 16,17 who found that the optical band gap changed from 3.7 eV for x = 0.1, to 2.2 eV for x = 0.6, while the electrical conductivity increased by three orders of magnitude. More recently Senguttuvan et al. 18 investigated the properties of Pb doped SnO2 thick films as a gas sensor but in fact alloyed PbO and SnO2 at 900°C for 120 min and obtained PbSnO3 which belongs to the family of lead stannates that include Pb2SnO4, Pb3SnO4, Pb2SnO3, PbSnO2 and PbSn6O7. Many of these have also been known for long and are still an active topic of investigation. It has been shown for instance that Pb2Sn2O6 nanoparticles with average sizes of  9 nm exhibited enhanced photo catalytic activity compared to Zn2SnO4, SrSnO3 and Y2Sn2O7

19.

Similarly the photo-catalytic

degradation of isopropanol over PbSnO3 nanoparticles under visible light has also been demonstrated by Chen et al. 20. However, to the best of our knowledge, no one has previously tried to obtain PbxSn1-xO2 NWs via the VLS mechanism over a broad range of x. Besides alloying SnO2 with PbO2 it has been shown theoretically by Butler et al. 21 that the use of an ultrathin epitaxial layer of PbO2 on SnO2 can be used to tune its work function and achieve energy levels commensurate with important technological materials. Recently we deposited PbO2 4 ACS Paragon Plus Environment

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over SnO2 NWs and the SnO2/PbO2 core-shell NWs were used to make an advanced LAB

22.

Similarly Dan et al. 23 combined SnO2 and PbO2 particles in a super capacitor for energy storage. Considering the above it is evident that Pb doping of SnO2 and the combination of PbO2 and SnO2 are interesting, not just from fundamental, but also from a technological point of view. As such we have carried out a detailed investigation into (a) Pb doping of SnO2 NWs and (b) the growth of PbxSn1-xO2 NWs via the VLS mechanism and show that these are difficult to achieve at elevated temperatures. In contrast low temperature growth leads to the preferential formation of PbxSn1-xO nanostructures with a composition that can be tuned over a broad range due to the fact that both PbO and SnO have similar crystal structures and may be obtained using the same growth conditions. We discuss the properties and prospects of this ternary oxide for energy storage and conversion.

2. Experimental Methods Initially we tried to grow Pb doped SnO2 and PbxSn1-xO2 NWs via the VLS mechanism at 800°C using a low pressure chemical vapour deposition (LPCVD), hot wall reactor, capable of reaching 1100°C. This was fed with Ar and O2 via a micro flow leak valve positioned on the upstream side, just after the gas manifold which consists of four mass flow controllers. A chemically resistant, rotary pump was connected downstream. In all cases we used 0.1 g of Pb and Sn (Aldrich, 2-14 Mesh, 99.9%) which were weighed with an accuracy of ± 1 mg. Note that although Pb has a melting point of 327° C, which is higher than that of Sn, i.e. 232°C, it has a higher vapor pressure of 10-1 mbar at 800°C and both readily form the well-known Pb: Sn alloy.



Square substrates of Si (001) and fused silica (f-SiO2)  10 mm x 10 mm were cleaned sequentially in trichloroethylene, methanol, and acetone, isopropanol, rinsed with de-ionised water, dried with nitrogen and then  1 nm of Au was deposited by sputtering on the clean surfaces. The elemental Sn and Pb as well as the 1 nm Au/Si and f-SiO2 substrates were loaded in the same quartz boat which was positioned at the centre of the 1˝ hot wall reactor after which it was pumped down to 10-4 mbar and purged with 100 sccms of Ar for 10 min at 10-1 mbar. Subsequently the temperature was ramped up to 800°C at 30°C/min under the same flow of Ar. Upon reaching 800°C a flow of 10 sccm’s O2 was added to the 100 sccm’s of Ar for 30 min, followed by cool down without O2. Care was taken to maintain a clean, high temperature zone, by changing regularly the 1 reactor tube before commencing any growth. These are exactly the same conditions used previously for the growth of SnO2 NWs 24. We varied the mass of Pb from 10 % up to 100% of the combined mass of Sn and Pb in increments of 10%. In addition we varied the growth temperature between 500°C and 1000°C at 10-1 mbar using 100% Pb and keeping all else equal. Finally we tried to grow Pb doped SnO2 and PbxSn1-xO2 NWs at lower temperatures. In this case both Sn and Pb were maintained at 800°C in order to provide an adequately high flux to the Au particles on the Si (001) and f-SiO2 that were positioned on the downstream side, outside the single heated zone, where the temperature was 400°C at 10-1 mbar. The morphology, composition and crystal structure of the resultant nanostructures were determined by scanning electron microscopy (SEM), Energy Dispersive X-Ray analysis (EDX) and X-ray diffraction (XRD) in conjunction with high resolution transmission electron microscopy (HRTEM) using a TECNAI F30 G2 S-TWIN operated at 300 kV. The electrical properties of the SnO2 NWs were measured by the Hall effect at B = 0.2 Tesla in the Van der Pauw geometry. This


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was carried out by a dry transfer of the SnO2 NWs on the clean 10 mm x 10 mm surface of fused SiO2 by applying pressure which resulted into the formation of a dense, planar network of interconnected SnO2 NWs. Subsequently In contacts with  1 mm diameter were deposited by thermal evaporation through a shadow mask at the four corners. The optical properties of the SnO2 NWs were determined by measuring the photoluminescence (PL) spectra at room temperature using an excitation of =230 nm.

3. Results and Discussion In the past we have shown that the reaction of Sn with O2 at 800°C and 10-1 mbar results into a high yield and uniform growth of SnO2 NWs with diameters of 10 to 100 nm and lengths up to  100 m. The SnO2 NWs grow by the VLS mechanism whereby Sn vapor enters the Au particles on the surface of Si or fused SiO2 thereby forming liquid Au: Sn particles at elevated temperatures. Upon saturation solid SnO2 forms beneath the liquid Au - Sn particles via the reaction with O2 at the triple phase junction leading to one dimensional growth 25. A schematic representation of the VLS growth mechanism is shown in Figure 1(a) which permits the selective area location growth of SnO2 NWs on Si and f-SiO2 as we always observed Au: Sn particles on the ends of the SnO2 NWs and no growth took place on plain Si or f-SiO2. The n-type SnO2 NWs grown under these conditions have a tetragonal rutile crystal structure and carrier densities of the order of  1016 cm3

due to donor like states related to oxygen vacancies as we have shown previously using THz

conductivity spectroscopy.

24

More recently we investigated the doping and conductivity

limitations in Sb: SnO2 NWs and showed that the maximum attainable carrier densities due to the incorporation of Sb are a few 1019 cm-3 which is at least an order of magnitude lower than that in Sn: In2O3 NWs 8. However Ma et al. 9 showed theoretically that a substantial band-gap tuning and 7 ACS Paragon Plus Environment


a strain-controlled, semiconductor to gapless semimetal transition, is possible via the incorporation of Pb in SnO2 which could result into higher carrier densities than those obtained with Sb and Mo so we investigated Pb doping of SnO2 NWs grown by the VLS mechanism.

3.1 High Temperature Growth of Pb doped SnO2 and PbxSn1-xO2 Initially we tried to obtain Pb doped SnO2 and PbxSn1-xO2 NWs via the VLS mechanism by using the same growth conditions used for SnO2 NWs and by mixing Pb with Sn. The Pb: Sn alloy is liquid above 300°C for all Pb: Sn compositions and reacts readily with Au. In this case Pb, Sn and AuSn4 are the equilibrium phases 26, 27. Consequently one expects that Pb and Sn from the vapor phase to enter the Au particles and form liquid Au: Sn: Pb particles leading to one dimensional growth of Pb doped SnO2 NWs as shown in Figure 1(b) similar to the Zn2SnO4 NWs obtained by Jie et al. 28 who proposed that Zn and Sn form a Zn: Sn alloy with the Au particles. The resultant SnO2 NWs were white and their morphology did not change upon increasing the amount of Pb used in conjunction with Sn. We observed Au particles on the ends of the SnO2 NWs suggesting that they grow via the VLS mechanism as shown by the SEM images in Figure 2. All of the SnO2 NWs exhibited clear and well resolved peaks in the XRD shown in Figures 3 (a) and (b) that belong to the tetragonal rutile crystal structure of SnO2. We did not find any peaks related to PbO2. However we find only one weak, but nevertheless well resolved peak, corresponding to PbO for 10% to 60 % Pb. This is analogous to the findings of Zhou et al. 10 who also observed one peak belonging to PbO in ZnO NWs. One more peak belonging to PbO was observed by increasing further the amount of Pb used in conjunction with Sn while the XRD trace corresponding to PbO obtained at 800°C and 10-1 mbar using just Pb is also included for comparison in Figure 3(b). The formation of PbO and not PbO2 at 800°C is not surprising since PbO2 has a low melting point of 8 ACS Paragon Plus Environment

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290°C which in turn means that it can’t be grown at 800C in contrast to PbO which has a melting point of 888°C. Despite the formation of PbO along with SnO2 we did not obtain any Pb2SnO4 via the reaction of PbO with SnO2 that is commonly known as lead-tin-yellow. At first, it appears then that the Pb is incorporated into the SnO2 NWs and that Pb also reacts with O2 giving PbO that may surround the SnO2 NWs. A TEM and HRTEM image of a SnO2 NW is shown in Figure 2 from which the (101) crystallographic planes of tetragonal rutile SnO2 are clearly observed but we find no evidence of PbO around the SnO2 NWs. The latter consisted mainly of Sn, O and less than 1at. % Pb. Consequently we did not observe any strain-induced shift of the strongest, principal (101) peak of SnO2, due to the incorporation of Pb. Instead we found that the Pb reacts with O2 and leads to the formation of circular domains of PbO on the surface of Si between the SnO2 NWs as depicted in Figure 4(a). A typical EDX spectrum of the SnO2 NWs is shown in Figure 4(b) from which one may clearly observe peaks belonging to Sn but no significant levels of Pb. In contrast Pb and Sn are clearly detected in the circular domain between the SnO2 NWs as shown in Figure 4(c). Note that we did not find any Pb at the interface between the SnO2 NWs and Si upon inspection from the side as shown in Figure 4(d). The incorporation of Pb in the SnO2 NWs was not promoted by using different catalysts like Ni, Cu and Mo keeping all else equal. Before providing an explanation for the low level of incorporation of Pb it should be noted that the one dimensional growth of SnO2 NWs was actually suppressed under Pb rich conditions, i.e. 90 % Pb, and the reaction of pure Pb with O2 at 800°C and 10-1 mbar did not result into PbO NWs. Instead we always obtained layers of PbO on Si (001). One dimensional growth of PbO NWs was not promoted by carrying out the reaction of pure Pb with O2 at 700C, 600C, and 500C. In particular no deposition occurred at 500°C due to the complete suppression of the vapor and flux of Pb that was maintained at the center of the reactor close to the Si. For completeness we must also mention that one dimensional



growth of PbO NWs was not promoted by increasing the temperature to 1000C as this anyway exceeds the melting points of PbO, PbO2 as well as many other oxides of Pb like Pb3O4 etc. This suppression of one dimensional growth is attributed to the small surface tension of the liquid particles of Pb

29, 30, 31

which prevents the formation of well-defined particles with sufficiently

large contact angles that are required for the VLS mechanism as shown in Figure 1(c). In addition Pb-rich conditions also lead to a suppression of the one dimensional growth of SnO2 NWs due to the elimination of Sn by the excess of Pb during the temperature ramp and before the growth step at elevated temperatures. Now the main reason prohibiting the incorporation of Pb into the SnO2 NWs is the depletion of Pb during the temperature ramp and thermal re-evaporation of the Pb arriving on the surface of the Au particles or sides of the SnO2 NWs during the growth step as shown in Figure 1(d). This is similar to the doping limitations that have been identified previously in connection with Sb. We have shown that the incorporation of Sb into the SnO2 NWs did not occur at 10-1 mbar but only at 1 mbar due to a suppression of re-evaporation. We tried to promote the effective incorporation of Pb into the SnO2 NWs by growing at 800°C and 1 mbar but we did not observe any significant changes in the carrier density of the SnO2 NWs. Our findings then suggest that Pb doping of SnO2 via the VLS mechanism at elevated temperatures is not as straightforward as in the case of Sb. On the other hand the growth of PbxSn1-xO2 NWs over a broad range of compositions at elevated temperatures is also difficult, if not impossible, due to the fact that its binary constituents i.e. SnO2 and PbO2 can’t be obtained under similar growth conditions and that PbO2 has a melting point of 290°C but SnO2 NWs, which have a higher melting point of 1630°C, can only be grown above 400°C. In actual fact PbO2 can’t be obtained at any temperature via the reaction of Pb and O2 which leads to the preferential formation of PbO that is a more stable oxide than PbO2. 10 ACS Paragon Plus Environment

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The Pb doped SnO2 NWs we tried to grow at 800C and 10-1 mbar with 10 and 20% Pb exhibited PL at  550 nm or 2.25 eV, as shown in Figure 5, which is very similar to the PL we obtained previously from un-doped SnO2 NWs 32. SnO2 has a direct energy band gap of 3.7 eV but the evenparity symmetry of the conduction-band minimum and valence-band maximum states prohibits band edge radiative transitions. The PL at  550 nm or 2.25 eV is attributed to radiative recombination via donor-like states, related to oxygen vacancies that are located energetically in the upper half of the energy band gap as we have shown in detail previously for SnO2 NWs 33. We did not observe a reduction in the energy band gap i.e. red shift in the PL in accordance with theoretical predictions. This is also in contrast to the observations of Zhou et al. 10 who observed a redshift of the PL emission from 378 nm to 478 nm upon increasing the content up to 7% in Pb: ZnO NWs. Instead we observed a small blue shift to  400 nm or 3.1 eV as shown in Figure 5. This is not related to the incorporation of Pb but most likely due to a change in the ratio of Sn to O2. In all cases the amount of Sn and Pb was kept fixed and equal to 100 mg and the flow of O2 that was added during the growth step was also fixed. Consequently the amount of Sn was reduced upon increasing that of Pb so the SnO2 NWs were grown under O-rich conditions. It is reasonable then to suggest that the blue shift in the PL is related to the elimination of deep donor levels leaving shallower states closer to the conduction band edge of SnO2 through which radiative recombination occurs via transitions down to the valence band. More importantly we did not observe any significant change in the carrier density or resistivity due to a strain-controlled, semiconductor to semimetal transition as predicted by Ma et al. 9 simply due to the fact that the Pb was not effectively incorporated in the SnO2 NWs. The carrier density was of the order of 1016 to 1017 cm-3 close to that obtained previously in un-doped SnO2 NWs on fused SiO2. In order to prevent the depletion of Pb during the temperature ramp and thermal re11 ACS Paragon Plus Environment


evaporation of the Pb arriving on the sides of the SnO2 NWs at elevated temperatures we deposited the Pb directly on the surface of the SnO2 NWs and annealed them to promote thermal diffusion and surface doping.

3.2 Surface doping of Pb into SnO2 NWs by thermal diffusion When a metal is deposited on the surface of an oxide and annealed under vacuum conditions, oxygen will move into the metal giving a metal oxide while the metal will move into the oxide leading to the formation of a ternary oxide.

The thermal diffusion of Zn into SnO2 has been

investigated by Ni et al. 34 who deposited Zn on SnO2 and annealed from 300C to 600C in air. They observed a change in the lattice parameter of SnO2 from a = b = 4.725Å, c =3.134Å to a = b = 4.782Å, c = 3.173Å i.e. a 1% change in a, 4 % change in c upon the introduction of Zn. However the resistivity increased from 10-2 up to 102 Ω cm which was accompanied by change from n- to p-type. Conductive Al: SnO2 was obtained in a similar fashion by Zhao et al. 35 who annealed an Al/SnO2/Al multilayer film between 450C and 600C. To the best of our knowledge no one has considered the diffusion of Pb in SnO2. Interestingly the diffusion of one monolayer of Pb deposited on 0.8 nm Al2O3 grown on Ag (111) was observed to occur even at room temperature by Vizzini et al.

36.

This suggests that Pb doping of SnO2 may not be impossible by thermal

diffusion considering that the lattice constant of Al2O3 i.e. a = 4.785Å is close to that of SnO2 i.e. 4.737Å. We deposited 100 nm Pb directly over the un-doped SnO2 NWs that were grown on fSiO2 resulting into SnO2/Pb core-shell NWs. The Pb was deposited by sputtering of a high purity Pb foil (Aldrich 99.99%) under Ar at 10-2 mbar. The SnO2 NWs changed from being white to greyblack. For comparison we also deposited 100 nm Pb on fused SiO2 without any SnO2 NWs. Following this we annealed the SnO2/Pb core-shell NWs under Ar at 200°C, 400°C, 600°C and 800°C at 10-1 mbar. A typical SEM image of the Pb as-deposited on the SnO2 NWs is shown in 12 ACS Paragon Plus Environment

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Figure 6(a) and (b) .Upon annealing above the melting point of Pb i.e. 327°C we observed that the SnO2 NWs turned white and no Pb was left on the fused SiO2 or the SnO2 NWs as shown in Figure 6(c). The XRD spectra of the SnO2 NWs after thermal annealing are shown in Figure 7 from which we observed peaks belonging to the tetragonal rutile crystal structure of SnO2 as well as a few, very low intensity peaks belonging to PbO only at 200°C due to the reaction of the solid Pb on SnO2 NWs with residual O2. Interestingly we observed a shift in the positions of the principal peaks of SnO2 by increasing the annealing temperature as shown by the inset in Figure 7 suggesting that the Pb is incorporated into the SnO2 NWs resulting into a strain of 2 %. We have annealed SnO2 NWs without any Pb for the same times at elevated temperatures but we do not observe any shifts in the position of the XRD peaks consistent with Attar et al.

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According to Ma et al. 9 a

moderate 4% tensile strain is able to reduce the energy band gap to approximately 1.5 eV, which is an ideal gap for solar-cell materials, indicating their promising applications as light absorbers. A strain of up to about 11%, gradually closes the band gap and SnO2 becomes a gapless semimetal. We find that the PL spectra of the SnO2 NWs after depositing Pb and thermal annealing are similar to un-doped SnO2 NWs as shown in Figure 6(d). In other words we do not observe a red shift due to band gap narrowing. In addition the carrier density was very close to 1016 cm-3 as in the undoped SnO2 NWs. Nevertheless it is evident that the deposition of Pb over the SnO2 NWs and thermal annealing results into a strain that may be increased further by rapid thermal annealing which could reduce the loss of Pb deposited over the SnO2 NWs thereby inducing a semiconductor to semi-metal transition.



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3.3 Low Temperature Growth of Pb doped SnO2 and PbxSn1-xO2 NWs The growth and compositional tuning of a ternary alloy such as PbxSn1-xO2 NWs requires that its binary constituent components i.e. PbO2 and SnO2 have similar crystal structures and can be obtained under similar growth conditions. The -form of PbO2 has a tetragonal rutile crystal structure with a = 0.491 nm and c = 0.3385 nm which are close to the lattice constants of tetragonal rutile SnO2 i.e. a = 0.4737 nm and c = 0.318 nm, so they differ only by  3%. In the past Kumar et al.

38

has grown SnO2 NWs via the VLS mechanism as low as 450°C and Zhu et al.

39

has

described a rational concept for the low temperature growth of metal oxide (MO) NWs. On the other hand Pan et al.

40

claimed to have obtained PbO2 NWs by physical vapor deposition using

PbO at 950°C and 400 mbar under an inert gas flow of Ar. The PbO2 NWs were collected on 10 mm x 60 mm Al2O3 at lower temperatures but these were not explicitly stated. We find that the reaction of Pb with O2 at 400°C and 10-1 mbar actually leads to the formation of PbO not PbO2. In fact we did not obtain any PbO2 via the reaction of Pb and O2 at low, intermediate or higher temperatures, which is different to the findings of Pan et al.

40.

It should be noted that all PbO2

NWs obtained in the past, have been prepared from solution, mostly by templated electrodeposition (ELD) as shown for instance by Moncada et al.41, 42 who used them to make a high performance LAB. It appears therefore that it is not possible to obtain both PbO2 and SnO2 using the same growth conditions primarily due to the fact that PbO2 has a melting point of 290°C and SnO2 NWs can only be obtained above 400°C. Consequently the growth of PbxSn1-xO2 NWs via the VLS mechanism is difficult if not impossible but PbxSn1-xO2 NWs may be obtained by other methods such as templated ELD using Pb+4 and Sn+4 solutions 22.


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Similarly, Pb doping of SnO2 NWs grown at low temperatures via the VLS mechanism is not an option since the re-evaporation of Pb arriving on the sides of the SnO2 NWs might be suppressed but low temperatures also lead to a suppression of the thermal diffusion. In other words the effective incorporation of Pb into SnO2 NWs might only be achieved at high temperatures by providing ex-situ a steady supply of Pb or by surface doping and rapid thermal annealing. Despite these difficulties we find that low temperature growth actually favors the formation of PbxSn1-xO which to the best of our knowledge has not been investigated in any detail previously.

3.4 Low temperature growth of SnO, PbO and the ternary oxide PbxSn1-xO In the past we have grown SnO2 NWs via the VLS mechanism at temperatures as low as 500°C even on soda lime glass (SLG) but even lower temperatures actually favor the formation of SnO nanoplates (NPLs). A typical SEM image of the SnO NPLs obtained at 400°C and 10-1 mbar is shown in Figure 8(a). These are very similar to those obtained by Li et al. 43 who used SnO and C at 1050°C but positioned the Si at 400°C. They are also similar in morphology to the ZnO NPLs obtained by Liu et al.44 On the other hand we find that the reaction of Pb with O2 at 400°C leads to the formation of PbO particles as shown in Figure 8(b). The growth and compositional tuning of the ternary PbxSn1-xO is possible primarily due to the fact that its binary constituent components i.e. PbO and SnO have similar crystal structures and can be obtained under the same growth conditions. It is well known that SnO has a tetragonal crystal structure with a = 3.80 Å that is comparable to the tetragonal form of PbO which has lattice constants of a = 3.97Å. Considering that PbO and SnO can be obtained at 400°C and 10-1 mbar as shown in Figure 8(a) and (b) we attempted to grow the ternary oxide PbxSn1-xO at 400°C but maintained the Pb and Sn at 800C and 10-1 mbar in which case we obtained PbxSn1-xO nanoplates 15 ACS Paragon Plus Environment


which grow on particles. Typical SEM images of the PbxSn1-xO are shown in Figure 8(b) and (c) from which it appears that they are a combination of the SnO and PbO shown in Figure 8 (a) and (b). The PbxSn1-xO exhibited clear and well resolved peaks in the XRD as shown in Figure 9 that are different compared to those in Figure 3(a) and (b). We observe peaks corresponding to SnO and PbO and a few related to the tetragonal rutile crystal structure of SnO2. However the peaks belonging to SnO are considerably stronger in intensity than those of SnO2. One may also observe that the addition of Pb leads to a suppression of the peaks related to both SnO2 and SnO. A typical EDX spectrum of the PbxSn1-xO2 is shown in Figure 8(e) from which it is evident that they contain both Sn and Pb. More specifically the Pb content of the PbxSn1-xO versus the % Pb used in conjunction with Sn is shown in Figure 8(f) demonstrating that the composition of PbxSn1-xO nanostructures may be tuned over a broad range at low temperatures due to a suppression of the re-evaporation of Pb

45.

All of the PbxSn1-xO nanostructures with different Pb and Sn contents

contained ≈ 50 at. % of O. However it should be noted that the PbxSn1-xO nanostructures do not constitute all together a single crystalline alloy but instead a polycrystalline structure containing separate phases of SnO and PbO as well as SnO2. The PbxSn1-xO NPLs exhibited broad PL with a maximum at 2.3 eV, shown as an inset in Figure 5, but we do not observe any significant changes with increasing content of Pb. SnO is a p-type semiconductor with an indirect energy band gap of  0.7 eV but the optical band gap determined from absorption-transmission spectra is as large as 2.7 eV. Similarly PbO has an indirect energy band gap of  0.2 eV and large optical band gap of 2.5 eV. The PL at 2.3 eV is attributed to the occurrence of SnO2 and is similar to that shown in Figure 5 but had a considerably smaller intensity due to the low content of SnO2.


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Interestingly Liao et al. 46 has investigated the effect of Pb doping on the properties of SnO films prepared by pulsed laser deposition on glass at 500°C. They found that the Pb was incorporated as a substitutional donor which changed slightly the optical band gap from 2.68 eV to 2.75 eV. In addition they observed a reduction in the hole carrier density from 9 x 1016 to 1 x 1016 cm-3 while the mobility changed from 1.95 to 0.68 cm2/Vs which was attributed in a semi quantitative way to the formation of traps. Consequently we do not expect that the energy band gap will change significantly via the incorporation of Pb into SnO and the growth of ternary PbxSn1-xO nanostructures compared to the case of ZnxSn1-xO

47.

However the ability to change the

composition of the PbxSn1-xO nanostructures over a broad range is important as the electron affinity may be tuned all the way from  = 0.7 eV in PbO to  = 3.7 eV in SnO 47, 48. Pb rich PbO contains O vacancies and each vacancy is occupied by two electrons so it is n-type but O-rich PbO contains O interstitials that form localized states occupied by two holes so it is p-type 49. On the other hand SnO is a p-type semiconductor so the formation of p-type SnO over n-type PbO would lead to a p-n heterojunction with a broken gap band line up as depicted in Figure 10 which is interesting for photo catalysis. In the past Lim et al.

50

has investigated the properties of layered SnO and PbO

separately from one another for energy applications. They found that SnO is better for electron transfer and hydrogen evolution than PbO but to the best of our knowledge no one has investigated previously the photocatalytic properties of the ternary oxide PbxSn1-xO or PbO/SnO heterojunction. In addition to the above the PbxSn1-xO nanostructures can be used in advanced batteries. PbO and SnO have been used as anodes in Li batteries 51, 52 so their combination and the high surface area of the ternary PbxSn1-xO nanostructures is indeed an attractive feature for energy storage. Finally the ability to tune the electronic properties and electron affinity of PbxSn1-xO appears to be also important for the realization of photo detectors. 53 17 ACS Paragon Plus Environment


Conclusion We have investigated Pb doping of SnO2 and the growth of PbxSn1-xO2 NWs on Si and fused SiO2 via the VLS mechanism at 800°C and 10-1 mbar. We find that the Pb is not incorporated into the SnO2 NWs at elevated temperatures due to a significant depletion of Pb during the temperature ramp and the thermal re-evaporation of Pb arriving at the surface of the SnO2 NWs. Instead the Pb reacts with O2 leading to the formation of separate domains of PbO between the SnO2 NWs. The latter had carrier densities of the order of n=1016 to 1017 cm-3 and exhibited PL at =500 nm ( 2.5 eV) similar to un-doped SnO2 NWs. One dimensional growth was completely suppressed by increasing the amount of Pb used in conjunction with Sn due to the small surface tension of the liquid Pb and Pb: Sn at elevated temperatures which prohibits the formation of well-defined particles that are necessary for VLS growth. This in turn imposes a restriction on Pb doping of SnO2 NWs but we show that surface doping via the deposition of Pb over SnO2 NWs followed by annealing and thermal diffusion gives rise to a 2% strain that is most likely related to the incorporation of Pb and could be improved further by rapid thermal annealing. The growth of PbxSn1-xO2 NWs is also difficult, if not impossible, due to the fact that its binary constituents i.e. SnO2 and PbO2 can’t be obtained under similar growth conditions. In contrast to the above lower growth temperatures favored the formation of polycrystalline PbxSn1-xO NPLs at 400°C consisting mainly of tetragonal SnO and PbO. The composition of the PbxSn1-xO NPLs was tuned over a broad range making it attractive for energy conversion and storage due to the broken gap, band line up of its binary constituent components and high surface to volume ratio.


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 AUTHOR INFORMATION Corresponding Author *e-mail:

[email protected]

Notes: The authors declare no competing interests.

 ACKNOWLEDGMENTS There is no funding to be reported.



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Figure 1(a). VLS growth mechanism of a free standing SnO2 NW; top-left inset shows the triple phase junction between the Sn and O2 vapor, liquid Au-Sn and solid SnO2.



Figure 1(b). Pb doping of SnO2 NWs via the VLS growth mechanism.


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Figure 1(c). Pb reaction with O2 leading to the formation of separate domains of PbO between the SnO2 NWs.



Figure 1(d). Pb doping of SnO2 NWs via the thermal diffusion; also shown the thermal reevaporation of Pb from the surface of the SnO2 NWs.


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Figure 2. SEM images of Pb: SnO2 NWs grown on Si(001) at 800°C under a flow of Ar and O2 at 10-1 mbar using Sn and Pb with 10, 20, 30, 40, 50, 60 and 80 % Pb ; also shown a TEM and HRTEM image of a SnO2 NW obtained using 70% Pb.



Figure 3(a). XRD traces of Pb: SnO2 NWs grown on 1 nm Au/Si (001) at 800°C and 10-1 mbar with 10, 20, 30, 40, 50 and 60% Pb.


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Figure 3(b). XRD traces of Pb: SnO2 NWs grown on 1 nm Au/Si (001) at 800°C and 10-1 mbar with 70, 80, 90 and 100 % Pb.



Figure 4. (a) SEM image of SnO2 NWs grown on Si (001) at 800°C using Pb and Sn under a flow of Ar and O2 and 10-1 mbar; inset shows the SnO2 NWs in area (i) and a circular domain (ii) which contains Pb as verified from the corresponding EDX spectra shown in (b) and (c); (d) a section of SnO2 NWs grown on Si. We did not detect any Pb along the interface between the SnO2 NWs and Si.


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Figure 5. Room temperature PL spectra of SnO2 NWs grown at 800C and 10-1 mbar using Pb and Sn; inset shows the PL of the PbxSn1-xO nanostructures grown at 400C.



Figure 6. (a), (b) SEM images of Pb/SnO2 core-shell NWs after the deposition of 100 nm Pb over SnO2 NWs that were grown on fused SiO2 (c) SEM image of the SnO2 NWs after annealing between 200°C and 800°C (d) corresponding room temperature PL.


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Figure 7. XRD traces of SnO2 NWs after the deposition of 100 nm Pb and subsequent thermal annealing under 100 sccm Ar at 200°C, 400°C, 600°C, 800°C and 10-1 mbar; inset shows the shift of the (211) peak of SnO2.



Figure 8. (a), (b) SEM images of SnO and PbO grown on Si(001) at 400°C by maintaining Pb and Sn at 800°C under a flow of Ar and O2 at 10-1 mbar (c),(d) SEM images of PbxSn1-xO obtained on Si under the same conditions (e) typical EDX spectrum confirming the existence of Pb and Sn (f) variation of the at.% content of Pb and Sn in the PbxSn1-xO versus wt.% of metallic Pb used with Sn.


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Figure. 9 XRD traces of PbxSn1-xO nanostructures grown on 1 nm Au/Si (001) at 400°C and 10-1 mbar using Sn and Pb with different Pb metal contents of 20, 40, 50, 60, 70 and 90 % ; also shown for comparison the trace of SnO2.



Figure. 10 Band line up between p-type SnO and n-type PbO showing the conduction and valence band edge, electron affinity  as well as the energetic position of the Fermi level.


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 TOC Graphic



VLS growth mechanism of a free standing SnO2 NW; top-left inset shows the triple phase junction between the Sn and O2 vapor, liquid Au-Sn and solid SnO2 . 164x125mm (220 x 220 DPI)

ACS Paragon Plus Environment

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Pb doping of SnO22 NWs via the VLS growth mechanism. 164x125mm (220 x 220 DPI)



Pb reaction with O2 leading to the formation of separate domains of PbO between the SnO2 NWs. 164x125mm (220 x 220 DPI)


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Pb doping of SnO2 NWs via the thermal diffusion; also shown the thermal re-evaporation of Pb from the surface of the SnO2 NWs. 164x125mm (220 x 220 DPI)



SEM images of Pb: SnO2 NWs grown on Si(001) at 800°C under a flow of Ar and O2 at 10-1 mbar using Sn and Pb with 10, 20, 30, 40, 50, 60 and 80 % Pb ; also shown a TEM and HRTEM image of a SnO2 NW obtained using 70% Pb. 164x141mm (220 x 220 DPI)


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XRD traces of Pb: SnO2 NWs grown on 1 nm Au/Si (001) at 800°C and 10 -1 mbar with 10, 20, 30, 40, 50 and 60% Pb. 262x198mm (150 x 150 DPI)



XRD traces of Pb: SnO2 NWs grown on 1 nm Au/Si (001) at 800°C and 10 -1 mbar with 70, 80, 90 and 100% Pb. 260x196mm (150 x 150 DPI)


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(a) SEM image of SnO2 NWs grown on Si (001) at 800°C using Pb and Sn under a flow of Ar and O2 and 101

mbar; inset shows the SnO2 NWs in area (i) and a circular domain (ii) which contains Pb as verified from

the corresponding EDX spectra shown in (b) and (c); (d) a section of SnO2 NWs grown on Si. We did not detect any Pb along the interface between the SnO2 NWs and Si. 164x123mm (220 x 220 DPI)



Room temperature PL spectra of SnO2 NWs grown at 800°C and 10-1 mbar using Pb and Sn; inset shows the PL of the Pbx Sn1-xO nanostructures grown at 400°C. 254x190mm (150 x 150 DPI)


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(a), (b) SEM images of Pb/SnO2 core-shell NWs after the deposition of 100 nm Pb over SnO2 NWs that were grown on fused SiO2 (c) SEM image of the SnO2 NWs after annealing between 200°C and 800°C (d) corresponding room temperature PL. 164x120mm (220 x 220 DPI)



XRD traces of SnO2 NWs after the deposition of 100 nm Pb and subsequent thermal annealing under 100

sccm Ar at 200°C, 400°C, 600°C, 800°C and 10-1 mbar; inset shows the shift of the (211) peak of SnO2. 258x190mm (150 x 150 DPI)


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(a), (b) SEM images of SnO and PbO grown on Si(001) at 400°C by maintaining Pb and Sn at 800°C under a flow of Ar and O2 at 10-1 mbar (c),(d) SEM images of Pbx Sn1-xO obtained on Si under the same conditions (e) typical EDX spectrum confirming the existence of Pb and Sn (f) variation of the at.% content of Pb and Sn in the Pbx Sn1-xO versus wt.% of metallic Pb used with Sn. 165x119mm (220 x 220 DPI)



XRD traces of Pbx Sn1-xO nanostructures grown on 1 nm Au/Si (001) at 400°C and 10-1 mbar using Sn and Pb with different Pb metal contents of 20, 40, 50, 60, 70 and 90 % ; also shown for comparison the trace of SnO2. 259x190mm (150 x 150 DPI)


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Band line up between p-type SnO and n-type PbO showing the conduction and valence band edge, electron affinity χ as well as the energetic position of the Fermi level. 259x195mm (150 x 150 DPI)


Nanowires Versus Low Temperature Growth of Pb

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