Ga-Promoted Photocatalytic H2 Production over Pt ... - ACS Publications

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Ga-promoted photocatalytic H2 production over Pt/ZnO nanostructures Julio Nuñez, Fernando Fresno, Ana E. Platero-Prats, Prabhas Jana, Jose Luis Garcia Fierro, Juan M. Coronado, David P. Serrano, and Victor Antonio de la Peña O'Shea ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07599 • Publication Date (Web): 19 Aug 2016 Downloaded from http://pubs.acs.org on August 22, 2016

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ACS Applied Materials & Interfaces

Ga-Promoted Photocatalytic H2 Production over Pt/ZnO Nanostructures Julio Núñez,†,# Fernando Fresno*,† Ana E. Platero-Prats,§ Prabhas Jana,# José L. G. Fierro,‡ Juan M. Coronado,# David P. Serrano#,║ and Víctor A. de la Peña O’Shea†* †

Photoactivated Processes Unit, IMDEA Energy Institute, Avda. Ramón de la Sagra 3, 28935

Móstoles, Spain. # Thermochemical Processes Unit, IMDEA Energy Institute, Avda. Ramón de la Sagra 3, 28935 Móstoles, Spain. § X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA. ‡ Group of Sustainable Energy and Chemistry (EQS), Institute of Catayisis and Petrochemistry (ICP-CSIC), c/ Marie Curie 2, Cantoblanco, 28049 Madrid, Spain. ║Chemical and Environmental Engineering Group, ESCET, Rey Juan Carlos University, c/ Tulipán s/n, 28933 Móstoles, Madrid, Spain.

ABSTRACT. Photocatalytic H2 generation is investigated over a series of Ga-modified ZnO photocatalysts that were prepared by hydrothermal methods. It is found that the structural, textural and optoelectronic properties remarkably depend on the Ga content. The photocatalytic activity is higher in samples with Ga content equal or lower than 5.4 wt.%, which are constituted by Zn1-xGaxO phases. Structural, textural and optoelectronic characterization, combined with theoretical calculations, reveals the effect of Ga in the doped ZnO structures. Higher Ga incorporation leads to the formation of an additional ZnGa2O4 phase with spinel structure. The presence of such phase is detrimental for the textural and optoelectronic properties of the

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photocatalysts, leading a decrease in the H2 production. When Pt is used as co-catalyst, there is an increase of one order of magnitude in activity with respect to the bare photocatalysts. This is a result of Pt acting as an electron scavenger, decreasing the electron-hole recombination rate and boosting the H2 evolution reaction.

KEYWORDS. Photocatalytic hydrogen production, ZnO, Ga, doping, co-catalyst, Pt. 1. INTRODUCTION One of the most promising routes to produce H2 is through photocatalytic reactions using alcohols as electron donors. In this way, sacrificial hydrogen production can be viewed as an applicable, environmentally benign process if biomass-derived products (e.g. bioethanol) or biofuel production wastes (e.g. glycerine) are used, in a process that can be considered a more feasible alternative than water splitting.Error! Reference source not found. However, there are still difficult challenges to overcome, mainly related to the efficiency of light absorbing and managing materials and devices. Thus, in order to achieve a reliable H2 production it is important to use a semiconductor with adequate chemical stability, efficient light harvesting, suitable band edges for hydrogen formation, and limited recombination of the photogenerated charge carriers. In particular, a number of semiconductor materials prepared as nanostructured photocatalysts have demonstrated to be effective in this reaction.Error! Reference source not found.-Error! Reference source not found. Among the materials applied to this process, TiO2 constitutes the archetypical photocatalyst, although ZnO could be an interesting alternative due to its similar optoelectronic properties, its abundance and its environmentally harmless characteristics. Nevertheless, its activity in the photocatalytic hydrogen evolution reaction is limited, especially when pure water splitting is considered, as a result of photocorrosion.Error!

Reference source not found.

This process is, however,


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stongly suppressed in the presence of suitable hole scavengers like alcohols or sulfide/sulfite mixtures.Error! Reference source not found.,Error! Reference source not found. Chemical modifications of ZnO, like coupling it with other semiconductors, has displayed the capability to enhance the photocatalytic efﬁciency by promoting electron transfer.Error! Reference source not found. In addition, nanostructuring ZnO and obtaining it in the form of 1-D and 2-D ZnO nanomaterials like nanorods, nanowires, nanotips and nanoplatelets, has helped to further improve its photoactivity.Error! Reference source not found.-Error! Reference source not found.

Another approach to improve the photoactivity of ZnO is the

substitution of Zn2+ ions with higher valence cations such as In3+, Al3+ and Ga3+ (n-doping) that can act as efﬁcient shallow donors,Error!


an thus increase the electronic

conductivity. Among these, Ga is a good candidate due to its high solubility in ZnO structures as a result of the smaller ionic radius of Ga3+ (0.62 Å) compared to Zn2+ (0.74 Å). Therefore, Gadoped ZnO materials are good candidates for photocatalysis due to the influence of the dopant on the structural and electronic properties that promote the separation of photoinduced charges. Several works have reported Zn2GaO4 structures as active catalyst for CO2 photoreduction or H2 productionError!

Reference source not found.-Error! Reference source not found.

. However, Ga-doping ZnO

materials have been tested in the degradation of organic dyes in aqueous solutionsError! Reference source not found.-Error! Reference source not found.

. In view of this and of the inherent interest of this kind of

photocatalysts, we have chosen to study their application to alcohol-based sacrificial hydrogen production. For this purpose, methanol appears as an ideal model alcohol due to its simplicity and its known reactivity.Error! Reference source not found. Besides the modification of the semiconductor photocatalyst per se, one of the most common ways to increase the charge separation and, accordingly, enhance the photoefficiency, is the use of metals such as Pt, Au or Ag as co-catalysts.Error! Reference source not found.,Error! Reference source not found.-


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Error! Reference source not found.

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Noble metal nanoparticles (NPs) act as electron scavengers, which

delay the recombination processes, and as H2 evolution sites.Error! Reference source not found.,Error! Reference source not found.

Reference source not found.-Error!

Among different metal co-catalysts, Pt appears as

one of the most suitable ones for photocatalytic evolution reactions.Error! Reference source not found.Error! Reference source not found.

Considering the above-mentioned, in the present work, we examine the effect of Ga3+ doping on the structural, textural, morphological and optoelectronic properties of ZnO photocatalysts, in order to determine its influence on the photoactivity. With this aim, we have prepared a series of mixed oxide phases, with Zn/Ga ratio varying from 40 to 0.5. All these materials were assayed for photocatalytic H2 production using methanol as a model sacrificial alcohol. In order to further enhance the photoactivity, Pt nanoparticles were photodeposited as co-catalyst. The effect of metal loading was further evaluated on the semiconductor with the best performance. 2. EXPERIMENTAL 2.1. Synthetic procedures Zn1-xGaxO photocatalysts were prepared by hydrothermal treatment using different Zn:Ga molar ratios: 1:0.025 (40), 1:0.05 (20), 1:0.1 (10), 1:0.2 (5), 1:0.4 (2.5) and 1:2 (0.5) (hereafter referred to as Zn:GaI, Zn:GaII, Zn:GaIII, Zn:GaIV, Zn:GaV and ZnGa2O4, respectively). In a typical procedure, for obtaining a catalyst with Zn:Ga molar ratio of 1:0.1, an aqueous solution was prepared using 10 ml of deionized MilliQ water containing 0.023 mol of Zn(NO3)2·6H2O (Aldrich) and 0.0023 mol of Ga(NO3)3·xH2O (Alpha Aesar). CO(NH2)2 (Scharlau) was used as precipitating agent (PA) with an equimolar amount with respect to the Zn reagent (0.023 mol). For the remaining Zn:Ga molar ratios (1:0.025, 1:0.05, 1:0.2, 1:0.4 and, 1:2) and pure ZnO the procedure was the same but the corresponding amounts of reagents. The resulting solutions were


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stirred vigorously for 15 minutes, subsequently transferred to a Teflon-lined stainless steel autoclave for hydrothermal synthesis and heated in an oven at 130 °C for 24 hours. The obtained powders were filtered, washed and dried overnight in an oven at 70 °C. Finally, the samples were calcined in air at 400 °C for 3 h. Pt was loaded on the catalysts by in-situ photodeposition method, in the range of 0.5-2 wt.%. This was done just before starting the reaction, using H2PtCl6·xH2O as a Pt precursor and methanol as a sacrificial agent. The light source used in the Pt photodeposition was a 150 W medium pressure Hg UV lamp. 2.2. Characterization Methods The metal content was measured by ICP-OES analysis with a Perkin Elmer Optima 3300 DV instrument after digesting the solids in a mixture of HF and HNO3. Specific surface areas were calculated from N2 adsorption–desorption isotherms at 77 K that were measured on a QUADRASORB instrument after degassing at 250 °C under vacuum for 12 h. The samples structure was characterized by X-ray powder diffraction (XRD) with a Panalytical EMPYREAN diffractometer using CuKα radiation (λ = 1.54178 Å) at a scanning rate of 0.2 ºs-1. Diffractogram treatments and refinements were performed using the X’Pert HighScore Plus software. In-house X-ray total scattering data suitable for pair distribution function (PDF) analyses were collected on Zn:GaIII and ZnO samples within a 2-150º 2θ-range using a BRUKER D8 advance diffractometer equipped with a Ag Lα X-ray source (λ = 0.56083 Å). For that purpose, the powder samples were loaded into 1-mm-diameter borosilicate capillaries. The PDFs, G(r), were extracted from the total scattering data using PDFgetX3 to Qmax = 17 Å−1. Structural models for Zn:GaIII and ZnO systems were optimized using DFT calculations (see computational methods section). Differential PDFs (dPDFs) of ZnGaIII systems were obtained by subtracting the


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contribution of ZnO from the PDF of the sample. For comparison with the experimental data, PDFs based on these optimized models were simulated using PDFGui. A Philips Technai 20 transmission electron microscope (TEM), operating with a tungsten filament working at 200 kV, was used for morphological characterization. Optical measurements were performed using a UV/vis/NIR Perkin Elmer Lambda 1050 Spectrometer with a Harrick Praying Mantis™ diffuse reflection accessory. Photoluminescence spectra were obtained with a fluorescence spectrometer Perkin Elmer LS 55, with an excitation wavelength of 280 nm and a cut-off filter at 350 nm. Xray photoelectron spectra (XPS) were recorded with a VG 200R electron spectrometer operated in a constant pass mode and provided with a non-monochromatic Al Kα (hv = 1486.6 eV) X-ray source operated at 120 W. Prior to the analysis, the powder samples were degassed at 300 ºC in the treatment chamber of the spectrometer. The residual pressure in the ion-pumped analysis chamber was maintained below 4.2 × 10−9 mbar during data acquisition. In addition to survey spectra, the Zn 2p, Ga 2p, Pt 4f, O 1s and C 1s energy regions were recorded for each sample and the respective binding energies (BE) were calibrated using the C 1s line at 284.9 eV as internal reference. BE values within an accuracy of 0.2 eV were obtained. Data processing was performed with the XPSPEAK program. 2.3. Photocatalytic reaction system Hydrogen production experiments were conducted in a slurry photoreactor, which consists of a three-neck cylindrical flask made of borosilicate glass with an effective volume of 1 L. 0.2 g of photocatalyst was added to 750 ml of an aqueous solution containing 10 % in volume of methanol. After adding the photocatalyst, the solution was irradiated with a 150 W medium pressure Hg UV lamp for 6 hours. Argon was used as inert carrier gas with a flow rate of 30 ml/min. The reactor was tightly closed and maintained at atmospheric pressure and at 20 ºC by a


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water cooling circuit. The reaction temperature was measured with a thermocouple situated on the photoreactor wall. After 30 min of Ar purge, irradiation started and H2 evolution was monitored every 3 min by means of a Varian micro-GC system equipped with two channels with a molecular sieve and a PPQ column, respectively. 2.4. Computational methods Theoretical calculations by periodic density functional theory (DFT) were carried out using a Zn:Ga model where oxygen vacancies are located near the Ga or Zn atoms. The DF plane-wave calculations were carried out by means of the VASPError! Reference source not found. considering spinpolarization and dipole corrections explicitly. The total energies corresponding to the optimized geometries of all samples were calculated using the spin-polarized version of the Perdew−Burke−Ernzerhof functional.Error!

(PBE)


implementation

of

the

GGA

exchange

The effect of the core electrons on the valence electron

density was described by the projector augmented-wave (PAW) method.Error! found.

correlation

Reference source not

The cut-off for the kinetic energy of the plane-waves was set to 415 eV to ensure a total

energy convergence better than 10−4 eV. A Gaussian smearing technique with a 0.2 eV width was applied to enhance convergence, but all energies presented in the following were obtained by extrapolating to zero smearing (0 K). Integration in the reciprocal space was carried out using the Γ point. The VESTA package v.3 was used to represent the electron localization. Error! Reference source not found.

3. RESULTS AND DISCUSSION 3.1. Materials characterization


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ZnO photocatalysts were prepared using different concentrations of Ga and with urea as a precipitating agent (PA). The selection of this mild base was made in order to improve the solubility of the Zn(OH)2 precursor and decrease the Ga(OH)3 formation during the dissolutionrecrystallization processes. This fact is corroborated by the chemical composition determined using ICP-OES. These analyses show that, although a linear relationship between real molar content and nominal one exists, a noticeable decrease in the Ga concentration with regards to the nominal loading is detected for all samples (Table 1). Table 1. Physicochemical properties of prepared materials. Cell parameters (Å) Atomic Ga/Zn ratio bulka

surfaceb

ZnO

Ga (wt.%)a --

--

Zn:GaI

1.24

Zn:GaII

2.5

Catalyst

ZnO

D (nm)c

ZnGa2O4

a=b

c

a=b=c

ZnO

ZnGa2O4

--

SBET (m2g-1) 30

3.261

5.222

--

24.8

--

Eg (eV)d 3.19

0.017

0.058

46

3.263

5.226

--

17.9

--

3.17

0.033

0.114

48

3.263

5.223

--

16.9

--

3.21

Zn:GaIII

4.9

0.066

0.154

73

3.266

5.236

--

10.8

--

3.22

Zn:GaIV

11.2

0.142

0.205

34

3.256

5.225

--

36.8

--

3.18

Zn:GaV

24.3

0.386

0.254

49

3.256

5.208

8.395

42.9

6.7

3.14

ZnGa2O4

47.1

1.58

--

60

--

--

8.358

--

10.6

4.60

a

Obtained from ICP-OES measurements b Obtained from XPS c Determined from XRD d Estimated from DRS UVvis spectra

The structural properties of these materials were studied using X-ray diffraction. Most of the Zn:Ga samples present the characteristic diffraction peaks of the ZnO phase with wurtzite structure (ICDD PDF 01-079-0207) (Figure. 1A). However, the progressive incorporation of Ga leads to the formation of an additional mixed oxide ZnGa2O4, with spinel structure (ICDD PDF 00-038-1240), for the materials with Ga:Zn molar ratio higher than 0.1. The content of this phase is around 25 % for the sample Zn:GaV, as determined by a Pawley profile fit of the diffraction pattern to the mentioned phases. In the case of Zn:GaIV photocatalyst this is hard to calculate due to the low intensity of the diffraction peaks. Finally, in the case of the sample with Zn:Ga=


8

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1:2, the mixed oxide is detected as a pure phase. A closer inspection of the XRD profiles reveals a slight shift in the position of the most intense peaks. A magnification of the (101ത1) reflection (Figure. 1B) clearly depicts this displacement, indicating that some changes occurred in the ZnO structure due to Ga incorporation. It is also evident that these variations are different depending on the Ga content. In order to evaluate these modifications, the lattice parameters of the wurtzite structure were calculated by refining the unit cell of the corresponding phase from the XRD fitted profiles (Table 1). These values reveal small modifications in the a=c and b parameters with respect to ZnO. Snure et al.Error! Reference source not found. explained the structural variations in Ga-doped ZnO as the convergence of two competitive factors. On the one hand, substitution of Ga3+ for Zn2+ introduces electrons in the system. A high concentration of electrons results in a lattice expansion in order to minimize the Coulomb repulsion energy. This factor is dominant at low doping levels. On the other hand, with higher Ga contents, the smaller ionic radius of Ga3+ (0.62 Å) with respect to Zn2+ (0.74 Å) plays an increasingly significant role, leading to a decrease in the cell parameters. A similar trend can be observed in the materials described here, especially in the second case related to heavily doped samples. These variations in the structural properties of the materials also produce small changes in the crystallite size depending on the Ga content (see Table 1), showing a decreasing trend with increasing Ga concentration that can be expected upon introduction of a guest cation.Error! Reference source not found. When Zn-Ga spinel phase appears, this trend is reversed.


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Figure 1. (A) XRD profiles of all prepared catalysts (B) Magnification to show the changes in the most intense reflection with the increase of Ga content. To probe the local structure of Ga sites and to provide more insightful findings, we combined theoretical calculations and pair distribution function (PDF) analyses using X-ray scattering data. The first peak observed in the experimental dPDF is located at 1.87 Å and corresponds to the average Ga-O first neighbour distance (Figure 2A-B), fitting well with the distances obtained by DFT calculations. In the case of Zn-O, the bond distance scattering is broader than that of Ga-O, for which reason two average distances have been considered, at 1.98 Å (nearest to Ga) and 2.02 Å (average Zn-O distances). The dPDF peak at ~3.25 correspond to Zn…Ga bond and also to Ga…O2nd distances(Figure 2C). In addition, taking into account the possibility of the existence


10

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of oxygen vacancies (Ovac), further calculations were performed including two different options: (1) Ovac nearest to the Ga atom (Gavac), where the Ga-O, Ga…O2nd and Ga…Zn bond scattering is increased leading to a highly reactive site; and (2) Ovac associated to Zn (Znvac), where the structural changes are more evident for Zn-O bonds, while Ga only shows variations with distances associated to Zn atoms (Ga…O2nd and Ga…Zn). These results indicate that Ga is doping the ZnO structure that provoke important changes in the bond distances in nearest to Ga atoms but which are minimal in the overall structure. In addition, these studies seems indicate the creation of oxygen vacancies in the nearest to Ga.

Figure 2. (A) Experimental ZnO (black), ZnGaIII (red) and differential (blue) PDF analysis including a detail of the Zn:Ga structure. (B) and (C) Scattering of the M-O (M= Zn or Ga) and selected distances for each DFT calculated sites (average values are marked with vertical bars).


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The structural characterization was complemented by Raman spectra as displayed in Figure 3. The ZnO sample presents the characteristic Raman-active phonon frequencies of wurtzite-type ZnO: 330 (E2, high) and 437 cm-1 (E2, high). Likewise, the small band centred around 581 cm-1 is ascribed to a superposition of the A1 (LO) mode (574 cm-1) and E1 (LO) mode (590 cm-1). Error! Reference source not found.,Error! Reference source not found.

In the case of the doped samples, the increase

in Ga content produces a growth in the relative intensity of the peak at 581 cm-1. Previous works assign this Raman mode to defects associated to zinc interstitials or oxygen vacancies, the presence of the latter well fitted by PDF analysis, or to the combination of both types of defects.Error!

Reference source not found.,Error! Reference source not found.

On the other hand, this change in

intensity has been previously assigned to the inclusion of an electron donor, such as Ga3+, in the ZnO framework.Error! Reference source not found. In addition, the Raman spectra also exhibit a blue shift in the E2 (437 cm-1) for low Zn/Ga ratio, which may be related with the progressive replacement of Zn by Ga in the crystal lattice as suggested by the XRD and PDF analysis.Error! Reference source not found.

Finally, the ZnGa2O4 sample, which possesses a spinel structure, exhibits the following

modes: 1A (685 cm-1), 1E (290 cm-1), and 3F (220 cm-1). The band at 685 cm-1 is attributed to the “disorder peak” previously reported for materials with spinel structure.Error! Reference source not found.


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Figure 3. Raman spectra of the synthesized samples. XPS spectra (Supporting Information, Figures S1 and S2 and Table S1) give surface Zn 2p3/2 BEs (1021.5 eV in all cases) in good agreement with that expected for ZnO,Error! Reference source not found.,Error! Reference source not found.

regardless of the Ga concentration or the deposited Pt amount.

The Ga 2p3/2 peak is constant for all samples too, with a BE value (1117.8 eV) in the range reported for Ga3+ cations in an oxidic environment.Error! Reference source not found.,Error! Reference source not found.

Two bands contribute to the O 1s signal without significant variations, that can be attributed

to surface chemisorbed carbonate and hydroxyl groups (531.5-531.6 eV) and to lattice oxygen in ZnO (530.1-530.2 eV). Error! Reference source not found. A contribution of surface Ga2O3 could not be confirmed or discarded from this photopeak, since the O 2p signal is expected is the same range as in ZnO.Error!


In most of the samples, the surface Ga/Zn atomic ratio

resulting from the integration of XPS signals is higher than the bulk one obtained from ICP-OES (Table 1). This suggests a preferential doping of ZnO at the surface or the segregation of an


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amorphous layer of Ga2O3, ZnGa2O4 or other Ga-rich phase not detected by XRD. In any case, the absence of additional peaks in the Raman spectra, more sensitive to amorphous phases, and the changes observed both in the cell volume of the ZnO phase and in PDF analyses, point at the former of these possibilities. However, the surface Ga:Zn ratio approaches the bulk one as the Ga content increases, revealing a randomization of Zn substitution. With the appearance of a significant amount of the ZnGa2O4 phase, the situation is reversed. Textural properties were studied by N2 adsorption-desorption isotherms (Supporting Information, Figure S3). All the prepared samples, except for Zn:GaV and ZnGa2O4, depict type III isotherms according to the IUPAC classification,Error!


which indicates

that the materials are non-porous and their surface area is due to interparticle voids (Table 1). The existence of a hysteresis loop at high pressures suggests that open spaces between particles are in the meso-macropore range. In this way, the loop can be considered of type H3 for all the samples except for ZnGa2O4; this shape is characteristic of non-rigid porous structures formed by nanoparticles. The sample ZnGa2O4 depicts an isotherm close to type IV, which indicates the presence of mesopores.Error! Reference source not found. In the case of Zn:GaV sample, where a mixture of Zn1-xGaxO and ZnGa2O4 phases is detected by XRD, both types of isotherms are overlapped, giving rise to the irregular shape of the hysteresis loop. Regarding BET values, a fairly good correlation with particle size is observed, revealing the interparticle voids as the main contribution to surface area. The highest BET area is observed for Zn:GaIII (73 m2/g), whereas the lowest one corresponds to ZnO. As perceived in the TEM images (Figure 4A-F), all photocatalysts are constituted by aggregates of pseudo-spherical nanoparticles. The ZnO sample presents particles with higher size than the Zn:Ga materials. Moreover, the particle size distribution changes with increasing the


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dopant concentration. Thus, the samples with Ga content lower than 11.2 wt.% contain particles with diameters between 30-60 nm. A further increase of the Ga content leads to a decrease in the particle size, mainly due to the formation of the ZnGa2O4 phase, which has a lower crystallite mean diameter (Table 1).

Figure 4. TEM images of bare samples : (A) ZnO; (B) Zn:GaI; (C) Zn:GaII; (D) Zn:GaIII; (E) Zn:GaIV; (F) ZnGa2O4 and 2 wt.% Pt-loaded catalysts: (G) ZnO; (H) Zn:GaI; (I) Zn:GaII; (J) Zn:GaIII; (K) Zn:GaIV; (L) Zn:GaV


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Optoelectronic properties were studied by means of UV-vis diffuse reflectance spectroscopy (Figure. 5A). All the materials show an absorption spectrum similar to bare ZnO, except for the ZnGa2O4 sample that exhibits an absorption edge at lower wavelength (

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