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Sn:InO and Sn:InO/NiS Core-Shell Nanowires on Ni , Mo Foils and C Fibers for H and O Generation 2

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Matthew Zervos, Epameinondas Leontidis, Eugenia Tanas#, Eugeniu Vasile, and Andreas Othonos J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09587 • Publication Date (Web): 28 Nov 2017 Downloaded from http://pubs.acs.org on December 4, 2017

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The Journal of Physical Chemistry

Sn:In O and Sn:In O /NiS Core-Shell Nanowires on Ni , Mo Foils and C Fibers for H2 and O2 Generation

Matthew Zervos Othonos.

1

1,*

3

, Epameinondas Leontidis 2, Eugenia Tanasă 3, Eugeniu Vasile and Andreas

4

Nanostructured Materials and Devices Laboratory, School Of Engineering University of

Cyprus, P.O.Box 20537, Nicosia, 1678, Cyprus. 2

Laboratory Of Physical Chemistry of Colloids and Interfaces, Department Of Chemistry,

University of Cyprus, P.O.Box 20537, Nicosia, 1678, Cyprus. 3

4

Politehnica University of Bucharest, 313 Splaiul Independentei, Bucharest, 060042, Romania Laboratory Of Ultrafast Science, Department Of Physics, University of Cyprus, P.O.Box

20537, Nicosia, 1678, Cyprus.

Sn:In2O3 nanowires have been grown by the vapor liquid solid mechanism on Si, Ni Mo as well as on C fibers. These were used to obtain Sn:In2O3/NiS2 core-shell nanowires by the

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deposition of 10 nm Ni over the Sn:In2O3 nanowires followed by post growth processing under H2S between 100°C - 200°C. The Sn:In2O3/NiS2 nanowires have diameters of ≈ 100 nm, lengths up to ≈ 100 µm and consist of cubic bixbyite Sn:In2O3 surrounded by 3 nm NiS2 crystalline quantum dots with a cubic crystal structure. Higher temperatures of 300°C - 500°C result in the formation of NiS2 quantum dots and cubic In3S4 branches around the Sn:In2O3. We find that the p-type NiS2 in contact with n-type Sn:In2O3 NWs gives rectifying current-voltage (IV) characteristics due to the formation of a p-n heterojunction with a straddling type band alignment where electrons are confined to the n-type Sn:In2O3 core and holes in the p-type NiS2 as shown by self-consistent Poisson-Schrödinger calculations in the effective mass approximation. The gas evolution of O2 and H2 was measured using the Sn:In2O3/NiS2 nanowires as anode and Pt as cathode in a two compartment photoelectrochemical cell containing 1M KOH (aq) and 0.5M H2SO4 (aq) respectively under light of one sun. We obtain 7.8 µl/min of O2 and 15.0 µl/min of H2 at an over potential of 0.2 V and 25°C from the Sn:In2O3/NiS2 nanowires on C. These are ≈ 35 % larger than those obtained from plain Sn:In2O3 nanowires attributed to the existence of the p-n junction.


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Metal oxide semiconductor nanowires (NWs) such as SnO2 1, In2O3 investigated in detail for solar cells 4, super capacitors (PEC) generation of hydrogen

6,7

5

2

and TiO2

3

have been

but also for the photoelectrochemical

. Most of these metal oxides are n-type wide band gap

semiconductors with carrier densities of the order of ≈ 1016 to 1017 cm-3 as in the case of SnO2 NWs grown by the vapor liquid solid (VLS) mechanism at 800°C 8. Consequently they require doping to increase their conductivity and enable their use in devices. For instance Sn doped In2O3 i.e. Sn:In2O3 NWs have metallic like conductivities due to the very high carrier density of electrons i.e. 1019 - 1020 cm-3

9, 10

and have been used in devices such as quantum dot sensitized

solar cells (QDSSCs). Recently Meng et al. 11 showed that Sn:In2O3 NWs are also promising for hydrogen generation (HG) through PEC water splitting while p-type transition metal oxides like α-Fe2O3 on n-type Sn:In2O3 NWs 12 or p-type Cu2O on n-type In2O3 nanoparticles (NPs) 13 result into the formation of p-n junctions thereby promoting HG. One of the key reasons in using a p-n junction for HG is to suppress the recombination of photo generated e- and h+ pairs and allow their reaction with H2O 14, 15. However it should be mentioned that Meng et al. 11 measured only the generation of O2 from Sn:In2O3 NWs on Si(001) in a single compartment cell containing 1 M NaOH similar to Yang et al

12

who prepared α-Fe2O3 /Sn:In2O3 core-shell NWs on quartz. In

view of the fact that Sn:In2O3 NWs have a very high density of electrons i.e. 1019 - 1020 cm-3 it is desirable to grow them on highly conductive substrates such Ni , Mo foils and C graphite fibers and monitor the generation of both O2 and H2. Furthermore the p-type metal oxides like α-Fe2O3 employed by Yang et al

12

require doping in order to tailor their energy band gap and improve

their conductivity. It has been shown theoretically that the energy band gap of α-Fe2O3 which is ≈ 2.1 eV may be tailored by doping with S in which case α-Fe2O3−xSx with

= 0.17 has an

energy band gap of 1.45 eV 16. Similarly S doped NiO has a higher conductivity than p-type NiO



Page 4 of 42

and has been used as an alternative to Pt in QDSSCs 17. This naturally then leads us to consider using p-type transition metal chalcogenides such as NiS or NiS2 in conjunction with n-type Sn:In2O3 NWs since NiS and NiS2 are intrinsically p-type with metallic like conductivities that do not require doping. It has been shown that NiS and NiS2 exhibit good performance in QDSSCs

18

, super capacitors (SCs)

19

and are still being actively investigated for HG

20, 21

.

Moreover it has been shown that p-type NiS deposited on n-type CdS rods result in higher HG rates under visible-light due to the formation of a p-n junction

22

. Such NiS/CdS p-n junctions

have also been used in an aqueous solution of Na2S and Na2SO3 23. In this case photo generated holes in the p-type NiS NPs result into the oxidation of the sacrificial S2- and SO32- while electrons flow into the n-type semiconductor resulting into the reduction of hydrogen evolving from H2O. A similar mechanism has been put forward in the case of p-type Cu2S NPs on n-type CdS 24 while Zhu et al. 25 has also observed enhanced photo catalytic efficiency in TiO2/NiS2. Here we focus on the growth and properties of Sn:In2O3/NiS2 core-shell p-n junction NWs for HG. To the best of our knowledge Sn:In2O3/NiS2 core-shell NWs have not been considered previously for HG although a flexible and high-performance all-solid-state super capacitor device based on Ni3S2 deposited on Sn:In2O3 NWs on C has been obtained by Yang et al. 5. It should be noted at this point that in the past we obtained Sn:In2O3/CuS core-shell p-n junction NWs via the deposition of Cu over Sn:In2O3 NWs and processing under H2S between 100°C 200°C which were subsequently used as counter electrodes (CEs) in a QDSSC

26,27

. More

recently we obtained branched PbIn2S4/Sn:In2O3 NWs by depositing Pb over Sn:In2O3 NWs and processing these under H2S between 100°C - 500°C but we also observed the formation of a βIn2S3 shell between the PbIn2S4 branches and Sn:In2O3 NWs

28

. This formation of β-In2S3 is

interesting since it exhibits good PEC activity which has been exploited in QDSSCs 29 but also in


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conjunction with TiO2 for HG

30

while Tien et al.

31

has found evidence for enhanced photo

induced charge separation in α-In2S3/In2O3 NWs. Consequently we have grown Sn:In2O3 NWs on Si, Ni and Mo metal foils as well as on C fibers and carried out a detailed investigation of the structural, electrical and optical properties of Sn:In2O3/NiS2 core-shell NWs obtained by the deposition of Ni over the Sn:In2O3 NWs followed by post growth processing under H2S between 100°C to 500°C. We obtained a gas generation rate of 7.8 µl/min for O2 and 15.0 µl/min H2 at an over potential of 0.2 V and 25°C from Sn:In2O3/NiS2 NWs and Pt respectively, under light of one sun, which is ≈ 35 % larger than that of plain Sn:In2O3 NWs due to the existence of the p-n junction. These exceed the gas generation rates from the Sn:In2O3 NWs of Meng et al 11 and Yang et al 12 that were grown on Si and quartz respectively

12

. We discuss the contribution of the underlying foil and fibers towards the

generation of O2 and H2 which is significant only in the case of plain Sn:In2O3 NWs on Mo foils.

Initially Sn:In2O3 NWs were grown via the VLS mechanism on 1 nm Au/Si(001) using a 1″ horizontal hot wall, low pressure chemical vapour deposition (LPCVD) reactor, capable of reaching 1100°C, which was fed via a micro flow leak valve positioned on the upstream side just after the gas manifold that consists of four mass flow controllers. A chemically resistant, rotary pump capable of reaching 10-4 mbar was connected downstream. For the growth of the Sn:In2O3 NWs approximately 0.95 g of In and 0.05 g Sn (Aldrich, 2-14 Mesh, 99.9%) were weighed with an accuracy of ± 1 mg. Square samples of 10 mm

10 mm intrinsic i-Si(001) were cleaned

sequentially in trichloroethylene, methanol, acetone, isopropanol, rinsed with de-ionised H2O,



dried with N2 after which

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1 nm Au was deposited over the i-Si(001) by sputtering. The

elemental In, Sn and 1 nm Au/Si(001) substrates were loaded in a boat, which was positioned inside the 1″ quartz tube, directly over the thermocouple used to measure the temperature, after which it was pumped down to 10-4 mbar and purged with 100 standard cubic centimetres per second (sccm) of Ar for 10 min at 10-1 mbar. Following this the temperature was ramped up to 800°C at 30°C/min using the same flow of Ar. Upon reaching 800°C a flow of 10 sccm O2 was added to the flow of Ar in order to grow the Sn:In2O3 NWs for 30 min, followed by cooling down without O2. Care was taken to maintain a clean high temperature zone for the growth of high purity Sn:In2O3 NWs by changing regularly the 1″ quartz tube before commencing any growth. Note that in addition to the Sn:In2O3 NWs on i-Si(001) we have also grown Sn:In2O3 NWs on (a) 10 mm

20 mm foils of Ni and Mo and (b) an ordered 10 mm

20 mm network of

orthogonally oriented C fibres. Subsequently we deposited 10 nm Ni over the Sn:In2O3 NWs on i-Si(001) and processed these under 50 sccm H2S between 100°C to 500°C for 60 min using a ramp rate of 10°C/min. The morphology and composition of the resultant Sn:In2O3/NiS2 QD NWs on i-Si(001) was determined by scanning electron microscopy (SEM), Energy Dispersive X-Ray analysis (EDX) while their crystal structure was determined by X-ray diffraction (XRD). High resolution transmission electron microscopy (HRTEM) was carried out using a TECNAI F30 G2 S-TWIN operated at 300 kV equipped with EDX while the photoluminescence (PL) spectra of the Sn:In2O3 /NiS2 NWs on i-Si(001) was measured at room temperature with λ = 260 nm. The current-voltage (IV) characteristics of the p-n junction formed between p-type NiS2 and the ntype Sn:In2O3 NWs were measured on SiO2/Si(001) with a Keithley 2635 A at 300K. This was carried out by a dry transfer of 100 – 200 µm long Sn:In2O3 NWs over the SiO2/Si(001) receiver


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substrate after which 10 nm Ni was deposited over the Sn:In2O3 NWs and processed under H2S at 100°C to form p-type NiS2 over the Sn:In2O3 NWs. Following this 100 nm Au was deposited on top of the free ends of the Sn:In2O3 NWs. The PEC properties of the Sn:In2O3 NWs as well as the Sn:In2O3 /NiS2 NWs on Ni, Mo and C were measured in a two compartment PEC cell. The left compartment has a 35 mm ∅ diameter circular quartz window which is aligned with a Xe : Hg Newport lamp through 90° via a partially reflecting mirror in a housing with a heat sink. The Xe : Hg lamp was connected to a power supply and the current adjusted to ≈ 4.5 A in order to deliver 1 kW/m2 corresponding to one sun AM 1.5. The Sn:In2O3 or Sn:In2O3 /NiS2 NWs constituting the working electrode (WE) was positioned in the left compartment facing the quartz window under an inverted burette (0.02 ml/div) with a collecting funnel. The Pt mesh counter electrode (CE) was located under an inverted burette (0.05 ml/div) with a collecting funnel in the right compartment while the Ag/AgCl reference electrode (RE) was located as close as possible to the CE. The two compartments sit on separate stirring hotplates and are connected to N2 at the top of the cell for the purpose of purging. Both compartments are joint via a Nafion conductive membrane by Ion Power. After fixing the WE, CE and RE into position the left compartment was filled with a 1M KOH (aq) solution prepared at 25°C. The right compartment was filled with a 0.5 M H2SO4 (aq) solution. Both compartments were purged with bubbling N2 under stirring for 10 min to eliminate gasses such as CO2 in the 1 M KOH (aq). Then the inverted burettes were filled by connecting their top to a vacuum line via a trap. The open circuit potential (OCP), linear sweep voltammetry (LSV) and cyclic voltammetry (CV) were measured in the dark and under one sun (AM 1.5) using a Digi Ivy 2300 Potentiostat connected to the WE, RE and CE of the PEC cell. Care was taken to monitor the temperature and pH of the 1M KOH (aq) and 0.5 M H2SO4 (aq)



Page 8 of 42

solutions. The gas evolution and volume of the H2 and O2 generated was measured at intervals of 10 min at an over potential of + 0.2 V.

!

"

# #

$ The Sn: In2O3 NWs

obtained at 800°C and 10-1 mbar on 1 nm Au/i-Si(001) have average diameters of ≈ 100 nm , lengths up to 100 µm’s and grow by the VLS mechanism as one may observe Au nanoparticles (NPs) on their ends in Figure 1 while no growth took place on plain Si. We have shown in the past that these Sn:In2O3 NWs have the cubic bixbyite crystal structure of In2O3 and metallic like conductivities due to the very high carrier density of electrons i.e. 1019 - 1020 cm-3 10. Here we have also grown Sn: In2O3 NWs at 800°C and 10-1 mbar on 10 mm

20 mm flexible Ni foils

which have a thickness of ≈ 0.1 mm. Ni has a melting point of 1455°C so it is a robust substrate for the growth of metal oxide (MO) NWs; typical SEM images of the Sn:In2O3 NWs obtained on the Ni foils is shown in Figure 2(a). We obtain a high yield and uniform distribution of Sn:In2O3 NWs all over the 10 mm

20 mm surface of the Ni foils that do not bend or distort at elevated

temperatures. More importantly we find that the resistances between the Sn:In2O3 NWs and the back side of the 10 mm

20 mm Ni foils are less than 10 Ω which means that the Ni foil is not

significantly oxidized into NiO as this would result into higher resistances. In contrast to the case of Sn:In2O3 NWs on Ni and Si we did not obtain any Sn:In2O3 NWs on the 10 mm 20 mm Mo foils at 800°C. Mo has a very high m.p of 2623°C but the m.p of MoO2 is 1100°C and that of MoO3 is 795°C. A high yield and uniform distribution of Sn:In2O3 NWs was readily obtained on


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the Mo foils as shown in Figure 2(a) by reducing the growth temperature to 770°C so it is reasonable to suggest that one dimensional growth of Sn:In2O3 NWs via the VLS mechanism cannot occur above 795°C on Mo due to the formation and melting of MoO3. Besides Ni and Mo we have also grown Sn:In2O3 NWs on a 10 mm 20 mm network of C fibers. A typical SEM image of the C fibers is also shown in Figure 2(a) from which one may observe that they have diameters of ≈ 10 µm. The C fibers form bundles that are orthogonally oriented to each other leaving square like windows of ≈ 200 µm

µm. More importantly the C fibers have a very

high, metallic like, conductivity and a typical image of the Sn:In2O3 NWs on the C fibers grown by the VLS mechanism at 800°C and 10-1 mbar is shown in Figure 2(a) from which one may also observe the occurrence of branched Sn: In2O3 NWs. In all of the above cases Ni, Mo as well as C behave as ideal ohmic contacts to the highly conductive n-type Sn:In2O3 NWs.

!

%

"

$ In the past we showed

that Sn:In2O3/CuS core-shell p-n junction NWs may be obtained via the deposition of Cu over Sn : In2O3 NWs and processing under H2S at relatively low temperatures between 100°C and 200°C which were used as CEs in a QDSSC 26. Higher temperatures between 200°C and 500°C resulted in the reaction of Cu with the Sn:In2O3 NWs and the formation of CuInS2/Sn:In2O3 NWs. We used the same method to make Sn:In2O3/NiS2 core-shell p-n junction NWs. To do this we initially deposited 10 nm Ni over the Sn: In2O3 NWs grown on i-Si(001) and treated them with H2S in order to study the structural, electrical and optical properties of the resultant Sn:In2O3/NiS2 NWs. Therefore we consider next the structural properties of the Sn:In2O3/NiS2 core-shell p-n junction NWs on Si(001). Typical SEM images of the Sn:In2O3/NiS2 NWs on Si obtained at 100°C and 200°C are shown in Figure 2(b). These are very similar to the as-grown Sn:In2O3 NWs while we find that the Sn:In2O3/In3S4/NiS2 NWs obtained at 300°C and 400°C



Page 10 of 42

have short branches but nevertheless remain one dimensional up to 500°C as shown by the inset of Figure 2(b). Both Sn:In2O3/NiS2 and Sn:In2O3/In3S4/NiS2 NWs exhibited clear peaks in the XRD as shown in Figure 3. At low temperatures between 100°C and 200°C we observe peaks belonging to cubic bixbyite crystal structure of In2O3 and a few weak but nevertheless well resolved peaks belonging to NiS2. Note that the peaks of cubic bixbyite In2O3 are reduced in intensity but are still distinct at 300°C and 400°C while we observe the emergence of a multitude of peaks belonging to cubic In3S4 in the Sn:In2O3/In3S4/NiS2 NWs A TEM image of a Sn:In2O3/NiS2 NW is shown in Figure 4(a) from which it can be seen that it is straight and has a diameter of ≈ 200 nm while one may clearly observe the Au NP on its end. A HRTEM image of the NiS2/Sn:In2O3 NW obtained at 100°C is also depicted in Figure 4(b) showing the (111) crystallographic planes of cubic In2O3 and the (210) planes of NiS2 quantum dots (QDs) which have diameters of 2.7 nm and exist on the surface of the Sn:In2O3 NWs. Furthermore a TEM image of the Sn:In2O3/In3S4/NiS2 NWs obtained at 500°C shows that they consist of a Sn:In2O3 core surrounded by short branches of In3S4 as shown in Figure 4(b) which also depicts the (331) planes of cubic In3S4 that contains NiS2 QDs. For completeness the corresponding EDX spectrum is included showing the peaks belonging to Ni, In and S. The formation of the Sn:In2O3/NiS2 NWs described above is attributed to the reaction of H2S with Ni on the surface of the Sn:In2O3 NWs which in turn leads to the formation of NiS2 between 100°C and 200°C. It is known that H2S dissociates on metal surfaces and oxides even at 25°C. This occurs leads to the formation of NiS2 as well as In3S4 around the Sn:In2O3. We do not observe In2S3 which melts incongruently at 418°C to form In3S4 and S

32

. So far we have

considered the structural properties of the Sn:In2O3/NiS2 NWs obtained on i-Si(001) since the latter is not altered in a detrimental way by its reaction with H2S. However this is not true of the


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Ni foils which react strongly with H2S leading to the formation of NiS2 on top of Ni. A typical SEM image of the Ni foil before and after exposure to H2S between 400°C and 600°C is shown in Figure 2(b) from which one may observe the formation of spherical like protrusions which starts at 300°C as well as a propensity towards one dimensional growth which is intensified at 600°C via one dimensional stacking of ≈ 100 µm to 200 µm NiS2 crystals extending all the way through the Ni foil. This deterioration of the Ni foil via its reaction with H2S is significant only above 200°C. In contrast to Ni the Mo foil is far more robust as shown in Figure 2(b) but still reacts with H2S leading to the formation of MoS2 between 100°C and 500°C. Consequently we deposited 10 nm Ni over the Sn:In2O3 NWs on Ni, Mo and C and converted these into Sn:In2O3/NiS2 core-shell NWs only between 100°C and 200°C in order to measure their PEC properties and the generation of O2 and H2 versus Pt. However for completeness we consider next the optical and electrical properties of the Sn:In2O3/NiS2 core-shell p-n junction NWs on iSi(001).

!

%

"

$ The room temperature PL spectra of the

Sn:In2O3/NiS2 NWs obtained on i-Si(001) at 100°C and 200°C are shown in Figure 5. One may see that they are broad with a maximum at 550 nm (≡ 2.5 eV) very similar to those of Sn:In2O3 NWs. The energy band gap of Sn:In2O3 is 3.4 eV so the PL at 2.5 eV is attributed to radiative recombination between donor like states, related to oxygen vacancies, lying energetically in the upper half of the energy band gap of Sn:In2O3 and acceptor like states close to the valence band as we have shown previously by time resolved absorption-transmission spectroscopy 33. The PL at 2.5 eV is not related to NiS2 which has a smaller energy band gap but we observed a blue-shift in the PL from 550 nm to 450 nm (≡ 2.8 eV) after depositing Ni on the Sn:In2O3 NWs and post



Page 12 of 42

growth processing under H2S at 300°C and 400°C. We did not detect any PL in the red or infrared (IR) which is different to the case of other transition metals like W deposited on Sn:In2O3 NWs that result into PL between 700 nm and 900 nm after their reaction with H2S

34,35

. At first

sight one might suggest that the PL at ≈ 2.8 eV is related to the formation of In3S4 as it is close to ≈ 2.6 eV which is the optical band gap of In2S4 prepared from solution by Wang et al.

36

.

However it is also plausible that the blue shift in the PL is related to the diffusion of S into the Sn:In2O3 where it will fill O vacancies and eliminate donor like states located energetically in the energy band gap of Sn:In2O3 which in turn will promote radiative recombination between shallower states closer to the conduction and valence bands. This is further corroborated by the fact that the PL of plain Sn:In2O3 NWs changes from 2.5 to 2.8 eV after post growth processing under H2S 37. For comparison it is useful to mention that the PL spectra of the NiS2/SnO2 NWs obtained on i-Si(001) in a similar way which are shown as an inset in Figure 5 are nearly identical to those of plain SnO2 NWs. Evidently the deposition of Ni on Sn:In2O3 or SnO2 NWs and the formation of p-type NiS2 does not prevent the excitation of electrons (e-) and holes(h+) in the n-type core of Sn:In2O3 after which electrons will move downhill in energy towards the core and holes towards the p-type NiS2 shell. Consequently the band line up and potential profile of the Sn:In2O3/NiS2 NWs is important in understanding the process of e-h pair generation and separation which is considered next.

!

&

'

%

(

( )

" Strictly speaking NiS2 is a Mott insulator but its surface is metal-like as revealed by Hall effect measurements ; this has been explained by the lattice relaxation of the surface. Similarly early Hall effect measurements of NiS established a carrier concentration of ≈ 1020–1021 cm-3 but


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it is known that Ni1−xS with different values of x, i.e. Ni vacancies, has carrier densities which are nearly a linear function of x and the extrapolated density at x = 0 is very close to zero suggesting that NiS is an insulator. However recently it has been shown that NiS is an antiferromagnetic metal

38

. Both NiS2 and NiS exhibit p-type conductivity 39 so we will treat

NiS2 as a degenerate p-type semiconductor with an energy band gap of ≈ 0.3 eV which is expected to form a p-n junction with the n-type Sn:In2O3. We have calculated the conduction and valence band potential profile of the Sn:In2O3/NiS2 coreshell NWs along the radial direction but also the one dimensional electron gas (1DEG) and one dimensional hole gas (1DHG) distributions via the self-consistent solution of the Poisson Schrödinger (SCPS) equations in the effective mass approximation 40. In such a calculation one begins with the solution of Schrödinger’s equation taking a trial potential subject to specific boundary conditions, which gives the sub-band energies and wave functions. These are normalized in order to obtain the 1DEG and 1DHG charge distributions, using the one dimensional density of states (1D-DOS) in conjunction with Fermi Dirac statistics. Finally Poisson’s equation is solved, taking into account the donor and acceptor doping levels but also the electron and hole charge distributions, to get a correction potential that is added onto the initial trial potential. The process is repeated until convergence is reached, after which charge neutrality is evaluated for completeness. The SCPS calculations of the Sn:In2O3/NiS2 NWs were carried out by taking into account the effective mass and dielectric constants of (a) me* = 0.35 mo and εr = 9.3 for Sn:In2O3

41,42

(b) me* = 6 mo

43,44

and εr = 15

45

of NiS2 . In addition we have

taken into account (a) the work function φ = 5.5 eV 46 electron affinity χ ≈ 5.0 eV 47 and energy band gap EG = 0.3 eV of NiS2 (b) the work function φ = 4.5 eV, electron affinity χ = 3.3 eV and energy band gap EG = 3.4 eV of Sn:In2O3. Hence a p-n junction with a type I straddling band



Page 14 of 42

alignment occurs at the hetero junction between the p-type NiS2 and n-type Sn: In2O3. Note that we have not calculated the SCPS potential profile and charge distribution for the Sn:In2O3/In3S4/NiS2 QD NWs since the effective mass of electrons and holes as well as the dielectric constants, electron affinity, work functions and energy band gap of In3S4 are not known at all. The SCPS CB potential profile of a Sn:In2O3/NiS2 core-shell NW is shown in Figure 6 taking a core radius of 60 nm and shell of 10 nm. A flat band condition exists at the core by virtue of Gauss’s law and the conduction and valence band discontinuities at the heterojunction interface are ∆E = 1.7 eV and ∆E = 1.4 eV respectively. Given that the Sn:In2O3 core has a radius which is much larger than the thickness of NiS2 shell we find that the 1DEG distribution has a maximum at the core as shown in Figure 6 and maximum density of ≈ 10 19 cm-3. On the other hand a 1DHG exists in the NiS2 shell with a smaller density of ≈ 10 16 cm-3. Upon the application of an external electric field between the n-type Sn:In2O3 and p-type NiS2 the built-in p-n junction barrier and e- , h+ densities will increase but no current will flow when the externally applied electric field points from the core towards the surface. When the external electric field is applied in the opposite direction the built-in barrier will be reduced and a current will flow through the pn junction.

*

!

%

( )

$ We obtained a

rectifying current voltage (IV) characteristic between the n-type Sn:In2O3 NWs in contact with the p-type NiS2 as shown in Figure 7 attributed to the formation of a p-n junction with a type I straddling band alignment at the heterojunction between the p-type NiS2 and n-type Sn: In2O3 which is schematically depicted as an inset in Figure 7. However it is also important to consider


Page 15 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


here the case of NiS and specific band alignment in a Sn:In2O3/NiS core-shell NW. If we take into account (a) the electron affinity of NiS which has been suggested to be χ = 2.6 eV 48 and (b) the energy band gap of NiS i.e. EG = 0.3 eV then we expect to have a broken gap, Type III band alignment between p-type NiS and n-type Sn: In2O3 which in turn leads to the formation of an inter-band p-n tunnel junction. In this case one should observe negative differential resistance in the IV characteristics as shown in Figure 7 which is not the case. Besides we ought to mention that the Sn:In2O3/NiS2 core-shell NWs described here are very similar to the TiO2/NiS2 assembly of Zhu et al.

25

who observed enhanced photocatalytic efficiency assuming a staggered type II

band alignment between NiS2 and TiO2. The electron affinity of TiO2 with a rutile crystal structure is χ ≈ 3.0 eV while that of anatase is χ = 3.2 eV 49 both of which are close to χ = 3.3 eV of In2O3. However in this work we took χ = 5.0 eV for NiS2 from recent electronic structure calculations of transition-metal dichalcogenides and oxides

47

. Considering that the electron

affinity of In2O3 i.e. χ = 3.3 eV is smaller than χ = 5.0 eV of NiS2 and by taking into account the difference in their energy band gaps we conclude that a straddling Type I and not a staggered type II band alignment occurs at the heterojunction between the p-type NiS2 and n-type Sn:In2O3. The straddling band line-up shown in Figure 6 with ∆E = 1.7 eV and ∆E = 1.4 eV is quite pronounced and it is not expected to act as efficiently as a staggered band line-up for the separation of e-h pairs and the generation of O2 and H2 which is considered next.

+*

!

%

"

#

$ We begin by describing the

PEC properties of plain Sn:In2O3 NWs on Mo, Ni and C which were measured in a two compartment PEC cell depicted in Figure 8. The Sn:In2O3 NWs were immersed in 1 M KOH (aq) in the left compartment of the PEC cell for the generation of O2 while a Pt mesh CE in 0.5 M H2SO4 (aq) was used for the generation of H2 in the right compartment. The Sn:In2O3 NWs in



Page 16 of 42

contact with the 1M KOH (aq) result in the formation of a Schottky barrier. Upon exposure to light photo excited electron-hole pairs will be generated and separated in the depletion region of the Sn:In2O3 NWs. The photo generated e- will move towards the core of the Sn:In2O3 NWs through the Ni, Mo or C into the Pt resulting into the generation of H2 while the h+ will drift to the surface of the Sn:In2O3 NWs resulting into the generation of O2. More specifically upon illumination e- will flow from the Sn: In2O3 NWs into the Pt which is immersed in H2SO4 and the +

-

H+ will be reduced by receiving an e- from the Pt CE according to 2H + 2e → H2 ↑. The o

potential of this half reaction is E

H+/H

= 0 V. On the other hand KOH (aq) → K+ + OH- and the

OH- gives up an e- to the photo excited h+ thereby generating O2 according to the half reaction -

-

o

4OH → O2 ↑ + 2H2O +4e . The half reaction potential for a pH > 7 is E o

overall potential is then E

O2/OH-

= -0.4 V. The

o

+E H+/H

. These two half reactions leading to the generation of

O2/OH-

H2 and O2 are depicted in Figure 9. We find that the Sn:In2O3 NWs on the C fibers result into the generation of O2 and H2 as shown in Figure 10(a) with an applied over potential of + 0.2 V. The O2 and H2 generation rates of 5.8 µl/min and 10.6 µl/min are slightly larger than those of Meng et al. 11 who measured the PEC properties and generation rate of O2 from Sn:In2O3 NWs on Si at an overpotential of + 0.2 V and obtained 25µM of O2 or 0.56 ml after about 150 min, in other words 3.7 µl/min

11

. It is also larger than the gas evolution measured by Yang et al.

12

who

obtained 10µM of O2 or ≈ 0.22 ml in 100 min, in other words 2.2 µl/min from Sn:In2O3/α-Fe2O3 core-shell p-n junction NWs on quartz also in a single compartment PEC cell containing 1 M KOH (

). One of the reasons for the larger H2 and O2 generation rates might be due to the fact

that we observed the occurrence of branched Sn:In2O3 NWs on the C fibers as shown in Figure


Page 17 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2(a) . However the differences in the H2 and O2 generation rates can only be discussed in a meaningful way if one could estimate the total surface area of the Sn:In2O3 NWs as well as the contribution of the underlying substrate and it’s reaction with the 1 M KOH ( is well known that KOH (

). For instance it

) reacts strongly with Si(001) surfaces but not with Si(111) and is

used for etching inverted pyramidal structures in bulk Si. In our case the C fibers do not react at all with the 1M KOH (

) and we did not observe any H2 or O2 generation from the C and Pt. It

is reasonable then to suggest that the Sn:In2O3 NWs on C are solely responsible for the generation of H2 and O2. Note that we did not observe any changes in the morphology of the Sn:In2O3 NWs after immersion in the 1 M KOH (

) and the generation of H2 and O2. In other

words the Sn:In2O3 NWs are quite robust and remain fixed on the C fiber network while the addition of the p-type NiS2 on top of the Sn:In2O3 NWs resulted into even higher generation rates of O2 and H2 equal to 7.8 µl/min and 15.0 µl/min respectively as shown in Figure 10(b). This is attributed to the formation of the Sn:In2O3/NiS2 core-shell p-n junction. Moreover we find that the Sn:In2O3 and Sn:In2O3/NiS2 NWs on Ni result into similar generation rates of H2 and O2 as those obtained from Sn:In2O3 and Sn:In2O3/NiS2 NWs on C as can be seen in Figure 10(a) and Figure 10(b) respectively. In contrast to the case of C and Ni we find that the Sn:In2O3 NWs on Mo result into a significantly higher generation rate of 54 µl/min H2 that is not twice that of O2 as shown in Figure 10(a). This is a direct consequence of the reaction of Mo with the 1M KOH(aq) which is oxidized according to Mo + 8OH- → MoO4 2- + 4H2O + 6e- thereby releasing 6e- to the Pt and generating excess H2. In essence the Mo in the 1M KOH (aq) versus the Pt in 0.5M H2SO4 (aq) is a galvanic cell and a large current flows through the cell at + 0.2 V as shown by the cyclic voltammetry curves in Figure 10(b). However the reaction of Mo with the 1M KOH (aq) was detrimental for the Sn:In2O3 NWs that lifted-off from the surface of Mo. This



Page 18 of 42

was suppressed to a large extent by the deposition of Ni over the Sn:In2O3 NWs on Mo and post growth processing under H2S which leads to the formation of Sn:In2O3/NiS2 NWs but also MoS2 on the bare parts of the Mo foil with no Sn:In2O3 NWs or Ni. This is corroborated by Tai et al. 50 who showed that it is possible to obtain MoS2 monolayers over large areas of Mo foils. Their Mo foil had a thickness of 20 µm and MoS2 monolayers were prepared at 600°C at 1 atm under Ar using S upstream from the Mo. MoS2 is intrinsically an n-type semiconductor with an energy band gap of 1.29 eV

51

and has been shown to be suitable for HG

52

. Moreover Yang et al.

53

showed that MoS2/Ni3S2 rods may act as efficient and stable electrocatalysts for PEC HG. We find that the generation rate of H2 and O2 from the Sn:In2O3/NiS2 core-shell NWs on Mo is close to that obtained from the Sn:In2O3/NiS2 NWs on C as shown in Figure 10(b). From the above it appears that one may grow Sn:In2O3 NWs on C , Mo or Ni for the generation of O2 and H2 after the deposition of a transition metal such as Ni and its reaction with H2S but C is unequivocally the best choice. The gas generation rates of O2 and H2, obtained from the Sn:In2O3/NiS2 NWs on C, Mo and Ni shown in Figure 10(b), are equivalent to 0.35µM/min and 0.67 µM/min respectively or 21 µM/h of O2 and 40.2µM/h of H2 . We estimated the mass of the Sn:In2O3/NiS2 NWs on the C, Ni and Mo foils to be ≈ 10 mg/cm2 which translates into 4000 µM/gh of H2 and 2100 µM/gh ; this compares favorably with the highest performance photo catalysts 54,55. Further improvements may be achieved by growing hyper branched Sn:In2O3 NWs on C in order to increase the overall surface area and tailor the band line-up via the deposition of suitable oxides on the Sn:In2O3 NWs that will promote the separation of photo generated e-h pairs.


Page 19 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


We have obtained Sn:In2O3/NiS2 core-shell p-n junction NWs via the deposition of Ni over Sn:In2O3 NWs grown on flexible metal foils such as Ni, Mo as well as on C fibers and post growth processing between 100°C and 200°C under H2S. The Sn:In2O3/NiS2 NWs consist of ntype cubic Sn:In2O3 and p-type NiS2 QDs with a cubic crystal structure and diameters of a few nanometers. Higher temperatures between 300°C and 500°C lead to the formation of cubic In3S4 branches and NiS2 QDs around the Sn:In2O3 NWs. We find that the p-type NiS2 in contact with n-type Sn:In2O3 NWs gives rectifying current-voltage (IV) characteristics due to the formation of a p-n junction with a straddling type I band alignment and show that electrons are confined in the n-type Sn:In2O3 core and holes in the p-type NiS2 via the self-consistent solution of the Poisson-Schrödinger calculations in the effective mass approximation. The gas evolution of both O2 and H2 from the Sn:In2O3/NiS2 NWs and Pt were measured in a two compartment PEC cell containing 1 M KOH (aq) and 0.5 M H2SO4 (aq) respectively under light of one sun AM 1.5. We obtained 7.8 µl/min of O2 and 15.0 µl/min of H2 at an over potential of + 0.2 V and 25°C from the Sn:In2O3/NiS2 NWs on C versus Pt respectively. These are ca. ≈ 35 % larger than those obtained from plain Sn:In2O3 NWs on C attributed to the formation of the p-n junction. The C fibers are unequivocally the best choice for the generation of O2 and H2 using metal oxide and transition metal dichalcogenides since they have a metallic like conductivity and do not contribute at all towards the gas evolution of O2 and H2 although Ni and Mo may also be used when treated under H2S leading to the formation of their respective transition metal dichalcogenides.



! , & -

(, The authors declare no competing interests

.

&,

The SEM of the nanowires was possible due to EU-funding grant POSCCE-A2-O2.2.1-20131/Axa Prioritara 2, Project No. 638/12.03.2014, Code SMIS-CSNR 48652.

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Fig.1 SEM images of the Sn:In2O3 NWs obtained on i-Si(001) and schematic diagram of the VLS growth mechanism. 144x108mm (220 x 220 DPI)



Fig.2(a) SEM images of the Mo, Ni metal foils and C fibers as well as the Sn:In2O3 NWs obtained on Mo, Ni and C at 800°C . 214x170mm (150 x 150 DPI)


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Fig. 2(b) SEM images of the Sn:In2O3/NiS2 NWs obtained by the deposition of 10 nm Ni over Sn:In2O3 NWs and post growth processing under H2S at (a) 100°C (b) 200°C (c) 300°C and (d) 400°C ; inset in (d) shows those obtained at 500°C. 218x188mm (150 x 150 DPI)



Fig. 2(c) SEM images of the Mo and Ni foils after processing under H2S between 400°C to 600°C 237x130mm (150 x 150 DPI)


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Fig. 3 XRD of the Sn:In2O3/NiS2 NWs and obtained at 100°C, 200°C, 300°C, 400°C and 500°C where the peaks belonging to the cubic bixbyite crystal structure of Sn:In2O3 are shown in blue, those of cubic In3S4 in red and NiS2 in grey. 254x191mm (150 x 150 DPI)



Fig. 4(a) TEM images of the Sn:In2O3/NiS2 NWs obtained at 100°C showing the Au nanoparticles on the end of a Sn:In2O3 NW. Also shown higher magnification HRTEM images showing the (111) crystallographic planes of Sn:In2O3 but also the (210) of cubic NiS2 with lattice spacing’s of 2.38 Å and 2.5 Å respectively. 209x192mm (150 x 150 DPI)


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Fig. 4(b) TEM and HRTEM images of the Sn:In2O3/NiS2 /In3S4 NWs obtained at 500°C depicting the (210) and (311) crystallographic planes of NiS2 and In3S4 which has a lattice spacing of 3.23 Å. An EDX spectrum showing the peaks belonging to Ni, In and S is included for completeness. 220x192mm (150 x 150 DPI)



Fig.(5) Room temperature PL of the Sn:In2O3/NiS2 NWs NWs that were obtained at 100°C and 200°C, where the maximum occurs at 528 nm ( ≡ 2.5 eV ) and shifts to 438 nm ( ≡ 2.8 eV ) at 300°C to 400°C in the Sn:In2O3/NiS2/In3S4 NWs ; inset shows the PL of SnO2/NiS2 NWs obtained in the same way as the Sn:In2O3/NiS2 NWs 254x191mm (150 x 150 DPI)


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Fig. 6 SCPS CB potential profile relative to the Fermi level versus distance along the radius in a Sn:In2O3/NiS2 core-shell NW taking a core radius of 60 nm and shell thickness of 10 nm. Also shown the 1DEG and 1DHG charge distributions with maximum densities of 5 x 1019 cm-3 and 2 x 1016 cm-3 respectively ; inset shows both SCPS CB and VB potential profiles of the same Sn:In2O3/NiS2 NW core-shell NW with a straddling Type I band alignment and ∆EC = 1.7 eV, ∆EV = 1.4 eV. 254x190mm (150 x 150 DPI)



Fig. 7 Rectifying IV characteristics obtained from the n-type Sn:In2O3 in contact with p-type NiS2 at 300 K shown on the left ; top-right inset shows schematic of type III, band line-up between the n-type Sn:In2O3 and p-type NiS which gives rise to IV characteristics with negative differential resistance (NDR) ; bottomright inset shows straddling, type I, band line-up between the n-type Sn:In2O3 and p-type NiS2 which gives rise to the rectifying IV characteristic. 254x191mm (150 x 150 DPI)


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Fig. 8 Schematic diagram of two compartment PEC cell used to measure the gas evolution of H2 and O2. 255x193mm (150 x 150 DPI)



Fig. 9 Schematic of the two half reactions and gas evolution of H2 and O2 occurring at the Sn:In2O3/NiS2 NWs and Pt. 254x190mm (150 x 150 DPI)


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Fig. 10 (a) Gas evolution of H2 from Pt mesh in 0.5M H2SO4 (aq) and O2 from Sn:In2O3 NWs on C ( ) , Mo ( ● ) and Ni ( o ) in 1 M KOH (aq) under light AM1.5 and an applied over potential of + 0.2 V. 254x192mm (150 x 150 DPI)



Fig. 10(b) Gas evolution of H2 (red) from Pt mesh in 0.5M H2SO4 (aq) and O2 (blue) from Sn:In2O3/NiS2 NWs on C ( ) , Mo ( o ) and Ni ( ● ) in 1 M KOH (aq) under light AM1.5 and an applied over potential of 0.2 V ; insets show the cyclic voltammetry (CV) of Sn:In2O3 NWs on C, Ni and Mo in 1 M KOH (aq) versus Pt in 1 M H2SO4 (aq) in the dark taken at 50 mV/s over three cycles (x3). 254x191mm (150 x 150 DPI)


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NiS

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