ZnO-InN nanostructures with tailored photocatalytic

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 6 7 2 7 e1 6 7 3 2

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ZnOeInN nanostructures with tailored photocatalytic properties for overall water-splitting Maofeng Dou a,*, Gustavo Baldissera a, Clas Persson a,b a b

Department of Materials Science and Engineering, Royal Institute of Technology, SEe100 44 Stockholm, Sweden Department of Physics, University of Oslo, P.O. Box 1048, Blindern, NOe0316 Oslo, Norway

article info

abstract

Article history:

ZnO-based electrodes for one-step photocatalytic water splitting are designed by incorpo-

Received 1 November 2012

rating InN. The electronic and optical properties of (ZnO)1x(InN)x alloys and ZnO with InN-

Received in revised form

like cluster formations ZnO:(InN)x are analyzed by means of first-principles approaches. We

12 February 2013

calculate the energy gaps Eg, the band-edge energies relative to the vacuum level, and the

Accepted 13 February 2013

optical absorption, employing the GW0 method to describe single-particle excitations and

Available online 19 March 2013

the BetheeSalpeter equation to model the two-particle exciton interactions. For ZnO and InN, the valence-band maximum (VBM) is EVBM z 7.3 and 5.7 eV, and the energy gap is

Keywords:

Eg z 3.3 and 0.7 eV, respectively. Incorporating InN into ZnO, the random (ZnO)1x(InN)x

ZnO

alloys up-shifts the VBM and down-shifts the conduction-band minimum (CBM). In addition,

InN

the presence of InN-like clusters enhances this effect and significantly narrows the band gap.

Hydrogen production

For instance, the VBM and the energy gap for 12.5% InN are EVBM z 6.5 and 6.1 eV, and

Alloys

Eg z 2.2 and 1.9 eV for the alloy and the cluster structure, respectively. This impact on the

Band gap

electronic structure favors thus visible light absorption. With proper nanoclusters, the band

Dielectric function

edges straddle the redox potential levels of Hþ/H2 and O2/H2O, suggesting that ZnOeInN nanostructures can enhance the photocatalytic activity for overall solar-driven water splitting. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

With the progress in nano-scale semiconductor technology, ZnO is a good example of a functional material suitable for numerous and different types of nano-technical applications [1,2]. Bulk ZnO and cation alloying of ZnO are today extensively investigated. Surprisingly, however, only little attention has been paid to understand more advanced alloy structures. One may expect that ZnO nanostructured alloys can be suitable for tailoring the material properties for a variety of novel integrated nano-systems ranging from catalysis, solid-state lighting, photonics, bio-sensing, to

nano-piezoelectricity applications, and even more. For instance, achieving p-type bulk ZnO is a major goal for further development of ZnO-based devices. It has been demonstrated that the ZnOeZnS alloy exhibits strong bandgap bowing that can be utilized to enhance dopability and activate p-type acceptors [3,4]. This is a conceptual new type of doping method. Moreover, experimental works demonstrate that random alloys of ZnOeGaN h (ZnO)1x(GaN)x with x < w15% or x > w85% can be synthesized [5e9]. The results are supported by phase stability modeling [10,11]. It has been reported that spatially random GaN alloying in ZnO host (with 15% GaN) reduces the band-gap energy Eg from 3.4 eV

* Corresponding author. E-mail address: [email protected] (M. Dou). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.02.071

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to w2.5 eV, and random ZnO alloying in GaN host (with 22% ZnO) reduces the gap energy to w2.6 eV [5,6]. This kind of compounds exhibit visible luminescence, and they have band-edge levels in a suitable energy region for an efficient splitting of water under visible light [8,9,12]. Theoretical modeling by Huda et al. [11] has shown that impact on the gap narrowing is not symmetric with respect to alloying. That is, incorporating w2% GaN in ZnO narrows the gap more than a corresponding ZnO incorporation in GaN. Although there are only few reports about co-doping of InN in ZnO [13], one can expect that InN could be incorporated in ZnO due to the fact that ZnO and InN crystallize in the same wurtzite structure with similar bond characters, Recently, ZnOeInN compound with 10% IneN in ZnO matrix was synthesized by Mapa and co-workers [14] using solution combustion method. The authors observed that the synthesized material displays a homogenous distribution of doped elements and InN formations. The band-gap energy was significantly reduced to 2.3 eV. Very recent theoretical studies of the optical properties of these ZnOeGaN and ZnOeInN isovalent alloys reveal intriguing material properties [15,16]. In this work, we discuss the somewhat unconventional type of ZnO-based materials, namely ZnOeInN structures with x 0.25. Depending on the synthesis method and growth condition nanocluster structures can be formed, and we therefore investigate the impact of cluster formations. The structural, electronic, and optical properties of the ZnOeInN alloys and the presence of clusters are studied by firstprinciples means, and the impact due to the excitonic coupling is analyzed. We employ the partial self-consistent Green’s function GW0 single-particle excitation approach to obtain accurate band-gap energies Eg, density-of-states (DOS), dielectric functions 3 (u), and optical absorption coefficient a(u). Thereafter, we employ the BetheeSalpeter equation (BSE) to study the two-particle exciton effects on the dielectric response, and we analyze the influence on the free excitons due to alloying and clustering configurations. We find that random alloys of ZnOeInN exhibit a strong band-gap bowing and, as a consequence, the gap energy is reduced by more than 1.45 eV to a value of about 1.89 eV in ZnOeInN with a 25% alloying content (Fig. 1). Intriguingly, we find that cluster formations in ZnOeInN have a strong impact on the band-gap narrowing. Both the conduction-band minimum (CBM) and the valence-band maximum (VBM) are affected, and the band-edge levels EVBM and ECBM suit well for an efficient H2 production by water splitting. Furthermore, although alloying implies broken crystalline symmetry and semi-localized states of the band edges, the strong exciton peak Eb(A) z 60 meV in ZnO is not diminished by the incorporation of InN randomly. That is, the strong exciton absorption remains in the alloy structures. These results together suggest that ZnOeInN with nanostructures can be a suitable material for hydrogen production by photocatalytic water splitting.

2.

Computational details

The electronic structures calculations were performed by means of the projector augmented wave method with a 390 eV

cutoff energy for the plane-wave basis set [16]. The quasiparticle energies were obtained from the GW0 calculation [16,17]. The Brillouin-zone integrations were performed with a G-centered symmetry reduced 8 8 8 k-mesh for the ZnO and InN binaries. The ZnOeInN alloys and clustering were scaled to 32-atoms 2 2 2 supercells with wurtzite-like structures. The electrostatic potentials relative to the vacuum potential were determined by utilizing a slab model approach [18] using the local density functional approximation. The slabs were built perpendicular to the non-polar (1010) surface, and the convergence tests ensured that the potential in both the crystal and the vacuum region were converged. The electronic structure and the band-edge positions (i.e., EVBM and ECBM) relative to the electrostatic potential were calculated for the ZnOeInN bulk materials with the GW0 approach. It should be noted that the semiconductor/water interfaces interaction were excluded in the calculations, and thus surface band bending are not considered. The average complex dielectric function 3 (u) ¼ 3 1(u) þ i3 2(u) was determined by employing an all-electron and fullpotential augmented plane wave method [19]. The imaginary part of the macroscopic dielectric function was calculated with both the GW0 and BSE method, involving single-particle excitations and two-particle excitonic interactions, respectively [20]. The optical absorption coefficient a(u) was obtained directly from the dielectric response 3 (u). Since the absorption near the band-gap energy is of special interest in this work, the ten topmost valence bands and the nine bottommost conduction bands were considered for ZnO and InN, whereas seventeen valence bands and fifteen conduction bands for the ZnOeInN compounds. The k-mesh was 6 6 6 for the binaries, whereas 4 4 4 for the ZnOeInN compounds. The cut-off energy was determined from Rmt$Kmax ¼ 6.8 with A as the smallest muffin-tin radius. Rmt ¼ 0.83

3.

Results

3.1.

Crystalline structure and electronic properties

The relaxation of the (ZnO)1x(InN)x alloys and the ZnO:(InN)x cluster compounds shows that the unit cell volume follows roughly Vegard’s law for alloys. The calculated lattice constants for ZnO are a ¼ 3.197 A and c ¼ 5.157 A are known to be slightly underestimated; the experimental data are a ¼ 3.249 A and c ¼ 5.206 A. For the alloy with x ¼ 0.25, a ¼ 3.304 A and c ¼ 5.286 A. It is remarkable that although ZnO and InN have rather different cell volumes and cationeanion bonds, the bond lengths of d(ZneO) ¼ 1.95 0.04 A and d(IneN) ¼ 2.07 0.03 A are very similar in the ZnOeInN structures compared with it binary constituents. Nevertheless, the ZnOeInN compounds maintain a wurtzite-like structure, and instead the relaxation twists the structure locally to keep the bond lengths d(ZneO) and d(IneN) close to the covalent radii in the compounds. The additional bonds between the non-isovalent pairs of cations and anions are also fairly constant for all structures: d(ZneN) ¼ 1.94 0.04 A and d(IneO) ¼ 2.11 0.05 A. Also here, the bond lengths follow the expected values considering the covalent radii of the sole atoms.


16729

Fig. 1 e The band-edge levels of ZnO, (ZnO)0.75(InN)0.25 alloy, and InN (gray areas) relative to the vacuum level. The presence of InN-like clusters (colored bars with marks) significantly shifts the VBM up and the CBM down even at low InN content; the black dashed lines represent the average effect from three cluster types (left panel). The red dashed lines indicate the standard hydrogen electrode potential (SHE) and the oxidation potential of O2/H2O relative to SHE at pH [ 0. Right panel shows the DOS for the ZnO:(InN)0.188 cluster, demonstrating that the clusters imply a broad VB dispersion and not localized defect states. The clusters favor absorption in the visible light spectrum, and the results suggest that ZnOeInN nanostructures can enhance a photocatalytic activity for overall water splitting. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

For the (ZnO)1x(InN)x alloys, we calculate the relative formation enthalpy DEt(x) ¼ Et(ZnOeInN) {(1 x)$ Et(ZnO) þ x$Et(InN)} using the LDA method (Table 1). The relative enthalpy for x ¼ 0.25 is DEt(0.25) ¼ 52 meV per atom, and this indicates that it might be difficult to grow ZnOeInN with InN content larger than w20%, but that it should be possible to incorporate InN with lower concentrations. This is in agreement with experimental founding by Mapa et al. [14] who have reported In2O3 formations in (ZnO)1x(InN)x alloys for x > 0.10. It is not surprising if ZnOeInN alloys are less stable than the corresponding ZnOeGaN alloys, because the bond length of InN is considerable longer than of ZnO, whereas GaN and ZnO have very comparable bond lengths. This implies a larger relaxation effect in the ZnOeInN alloys. Since it might be difficult to grow (ZnO)1x(InN)x alloys with high InN concentrations, one can instead try to tailor the electronic and optical properties of ZnOeInN with low InN content by utilizing cluster formations. We model this by analyzing different configurations of the IneN dimers. First, we find that a pair of In and N atoms will form a spatially close InN configuration. The relative enthalpy to have an In-on-Zn site far away from the N-on-O site cost w16 meV more compared to having them closely together. This is a direct consequence of Lewis’ octet rule applied to defects [22]. Second, the IneN dimer will primarily form a c-axis-oriented (jj) bond in the ZnO host. However, the energy to form a dimer at

one of the three-fold (t) bonds is only 2 meV higher, and thus both these types of dimers will be present in the ZnOeInN compounds. For higher InN contents, there is a possibility to form larger clusters of InN. Here, we present three types of nano-clusters (Fig. 1): cluster type C1 forms a chain of IneN bonds, cluster type C2 is more curled up, however each In has at most two IneN bonds, and cluster type C3 where the In atoms in the more compact clusters can have three IneN bonds. We find that although the energies to form the three types of clusters are rather similar, the energy to form cluster C1 is in general lowest (Table 1). The band-gap energies of the (ZnO)1x(InN)x alloys and of the ZnO with InN-like cluster formations ZnO:(InN)x are presented in Fig. 2. The calculated gap energy for ZnO is Eg ¼ 3.34 eV, which is in good agreement with the experimental data of 3.37 eV [1]. Both our theoretical analysis and the absorption measurements by Mapa et al. [14] demonstrate that InN incorporation has a strong impact on the fundamental band-gap energy. Clearly, the gap is smaller for compounds with cluster formations compared to the alloy with same InN content. Thus, a stronger narrowing of the gap can be achieved by growing ZnOeInN compounds with nanoclusters. For instance, the band-gap energy can be reduced to Eg z 1.03 eV for x ¼ 0.25, which is w0.86 eV smaller than that of the corresponding alloy. To describe this change in the

16730


Table 1 e The band-gap energy Eg for the (ZnO)1Lx(InN)x alloys and the ZnO:(InN)x clusters. The band-edge levels EVBM and ECBM are relative to the vacuum level. DEt is the relative formation enthalpy of the compound with respect to its binary constituents, indicating that it is possible to synthesize the ZnOeInN nanostructures. The numbers in brackets are the experimental values from Refs. [1,21]. Structure x ¼ 0.000; ZnO x ¼ 0.063 Dimer t Dimer jj x ¼ 0.125 Alloy Cluster C1 Cluster C2 x ¼ 0.188 Alloy Cluster C1 Cluster C2 Cluster C3 x ¼ 0.250 Alloy Cluster C1 Cluster C2 Cluster C3 x ¼ 1.000; InN

DEt (meV/atom)

EVBM (eV)

ECBM (eV)

Eg (eV)

0

7.32

3.98

3.34 (3.37)

19 17

6.45 6.32

4.12 4.11

2.33 2.21

30 27 29

6.51 6.20 6.04

4.31 4.30 4.12

2.20 1.90 1.92

44 34 36 41

6.31 5.88 5.75 5.59

4.35 4.42 4.43 4.47

1.96 1.46 1.32 1.12

band gap, we parameterize the composition-dependent bandgap energy as Eg(x) ¼ (1 x)$Eg(ZnO) þ x$Eg(InN) b$x(1 x). The estimated value for the alloy compounds is b z 2.5e5.5 eV; this is indicated by the gray area in Fig. 2. The corresponding value for the cluster formations ranges between b z 7.0e11.0 eV depending of cluster types; yellow area in the figure. This demonstrates a strong gap bowing. Moreover, by calculating the VBM energy with respect to the vacuum level, the activation energy for photocatalytic activity can be estimated. Importantly from a technological point of view, we find that with a control of the cluster formation, one can tune in the band-edge levels EVBM and ECBM to better suit the redox potential for efficient water splitting (Fig. 1). The energy states associated with the InN incorporation form a broad N-p like valence-band DOS; shown for ZnO:(InN)0.188 in the right panel of Fig. 1. Thus, high concentrations of InN clusters generate energetically delocalized

7 InN ZnO:(InN) (ZnO) (InN) ZnO

6 6.27 5.77 5.71 5.41 5.69

4.38 4.34 4.70 4.73 4.98

1.89 1.43 1.01 0.67 0.71 (0.69)

5

ε1 = Re [ε]

52 47 49 60 0

4

3

2

Expt. Alloy Cluster

50

1.5 1 0.5

2

30

1

25

0

3.34

2

3

35

1.89

4

40

0.71

2.5

0

5

45

1.43

3

1

Absorption [104/cm]

Band-gap energy [eV]

3.5

(a)

0

2

4

20 15 10

(b)

5

0

0.1 0.2 InN content x

0.3

Fig. 2 e The band-gap energy Eg versus the InN content x for different ZnOeInN compounds. The band gap is strongly narrowed by random alloying (black triangles) of (ZnO)1Lx(InN)x, and the gap is narrowed even more by the presence of InN-like clusters (black crosses). The gray and yellow areas represent the estimated energy ranges for a distribution of different alloys and cluster structures. The filled red triangles are the experimental results by Mapa et al. [13]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

0

0

2

4

6

8

Energy [eV] Fig. 3 e (a) The real part 31(u) of the dielectric function for ZnO (black thin line), (ZnO)0.75(InN)0.25 alloy (red thick line), and ZnO:(InN)0.25 C1 cluster structure (blue thick line), and InN (brown thin line). (b) The corresponding optical absorption coefficient a(u), where the band-gap energies are marked with arrows. The data are obtained from GW0 calculations, and the spectra include a 0.01 eV Lorentzian broadening. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


energy bands, and not localized defect states. In addition however, having cluster formation in ZnOeInN also allows for benefitting the spatially semi-local properties of the nanostructures, and further investigation of this effect is therefore requested.

3.2.

Optical properties and free exciton coupling

Fig. 3(a) presents the real part of the dielectric function for the ZnOeInN structures, calculated within the GW0 method. Compared with the binary ZnO, the dielectric responses of both the (ZnO)0.75(InN)0.25 alloy and the ZnO:(InN)0.25 cluster C1 structure are larger in the low-energy region of 1e4 eV. This is consistent with the smaller gap energy of the ZnOeInN compounds. The average high-frequency dielectric constant 3 N ¼ 3 1(u z Eg/2Z) is 3.6, 3.5, 3.6, and 5.0 for ZnO, (ZnO)0.75(InN)0.25, ZnO:(InN)0.25, and InN, respectively; the value for ZnO and InN is close to the experimental data 3.7 and 5.8, respectively [23]. Overall, the dielectric response of ZnOeInN is comparable to that of ZnO. The optical absorption coefficient a(u) of the ZnOeInN compounds is calculated directly from the dielectric response [Fig. 3(b)]. The onset to the optical absorption is affected by the strong compositiondependent band-gap reduction. The absorption coefficient of the ZnOeInN compounds is however comparable to ZnO and InN. This high absorption in ZnOeInN is due to that InN incorporations form delocalized energy band with a broad valence-band DOS (e.g. Fig. 1). By calculating the imaginary dielectric response with the BSE method, the exciton absorption can be analyzed. This excitonic coupling generates a red-shift of the optical response edge. The strong peak Eex from the free excitons in ZnO is observed for photons with energy just below the band-

InN ZnO:(InN) (ZnO) (InN) ZnO

Eex(ZnO)

8

4.

Conclusion

To conclude, theoretical first-principles analyses of ZnOeInN nanostructures reveal that (i) InN incorporation in ZnO narrows the band gap even for relatively low alloy concentrations. This is described by a strong band-gap bowing b z 2.5e5.5 eV of the alloy. The gap energy is Eg(0.25) ¼ 1.89 eV in (ZnO)0.75(InN)0.25. (ii) Formation of nanoclusters decreases the energy gap further. This impact from the clustering can be as much as 0.86 eV. The band-gap bowing is b z 7.0e11.0 eV for the clustered structures. (iii) With nanoclusters in ZnOeInN, the VBM with respect to the vacuum level is strongly upshifted, and EVBM comes closer to the O2/H2O oxidation potential. (iv) A relatively strong exciton peak remains in the ZnOeInN alloys for x 0.25. This is of advantage for solar-light absorption as well as for laser technologies. Hence, with a control of the cluster formation, the band-edge levels of the ZnOeInN nanostructures can be tailored for an efficient photocatalytic H2 production by an overall water-splitting.

This work is supported by the China Scholarship Council, the European EM-ECW scholarship program Eubranex, the Swedish Energy Agency, and the Swedish Research Council. We acknowledge access to high-performance computer resources at NSC and HPC2N through SNIC/SNAC and Matter network.

4

Eex(Alloy)

6 Eex(InN)

ε2 = Im [ε]

gap energy of 3.34 eV. This is in good agreement with low temperature spectroscopy measurements [24]. Intriguingly, this strong exciton peak does not diminish upon InN incorporation randomly. Instead, the exciton coupling in the alloy is relatively large compared to binary InN. However, for the ZnO:(InN)0.25 cluster structure, where the VBM states are more spatially localized, the exciton peak disappears. This indicates that the optical absorption of the free exciton is dependent of the disorder of the ZnOeInN compounds. Nevertheless, the advantage of a strong exciton coupling in ZnO can be benefited also in the ZnOeInN alloys for x < 0.25 (Fig. 4).

Acknowledgments

10

reference

2

0

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0

2

4

6

8

Energy [eV] Fig. 4 e The imaginary part 32(u) of the dielectric function from the BSE calculation; see Fig. 3 for the definition of the lines. The result demonstrates that a strong free exciton peak is present in the ZnOeInN alloy but not in the cluster structure.

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