Interactions Between Cations and Nanoparticulate Layered Mn Oxides -- A X-ray
Atomic Pair Distribution Functions (PDF) Analysis
Mengqiang Zhu* and Donald L. Sparks
Department of Plant and Soil Sciences, Delaware Environmental Institute, University of Delaware, 152 Townsend Hall, Newark, DE 19716
*Present affiliation: Lawrence Berkeley National Laboratory; address: 307 McCone Hall, University of California, Berkeley, CA 94720; Email: mzhu@lbl.gov
2. Metal-sorption effects on δ-MnO2 structure
H2O/Na+

(Zhu et al. Geochim .Cosmochim. Acta., 2012)
Figure 1. δ-MnO2 crystal structure obtained by pair distribution function analysis.
Pair distribution function (PDF) analysis is an excellent tool to study nanoparticle
structure. In this study, we used PDF analysis to determine possible structural changes of
δ-MnO2 induced by sorption of H+, Zn2+, and Ni2+. The results indicate that H+ (by
controlling pH) and Zn2+ sorption caused contraction of δ-MnO2 structure whereas Ni2+
showed negligible impacts. The effects were ascribed to the charge compensation that
reduced internal electrostatic repulsion. In addition, differential PDF analysis on the Zn2+
and Ni2+ sorbed δ-MnO2 revealed that Zn2+ and Ni2+ sorption occurred on vacant sites,
consistent with previous EXAFS studies.
Data
Fit
Rw = 18.3%
pH = 7
Rw = 18.8%
pH = 5
(a)
Mn-O
Mn-Mn
4
0
pH5
Rw = 19.9%
pH = 4
-4
1
pH = 3
Rw = 23.6%
12
pH = 1
pH1
3.0
3.5
r (Å)
4.0
4.5
2
5.0
4
6
8
10
12
14
16
18
20
r (Å)
8
9
10
2
(c)
Zn1
Zn2
Zn3
Zn4
r(Å)
3
(d)
4
0
-4
Unit Cell
a (Å)
pH 1
pH 3
4.948
pH 4
4.953
4.952
pH 5
4.953
pH 7
4.957
3. Differential PDF for metal sorption geometry
(a)
b (Å)
2.815
2.815
2.815
2.816
2.818
Ni-Mn correlation
Ni-O correlation
3.48
2
a/b
c (Å)
1.758
1.759
7.130
1.759
7.134
7.139
1.759
7.137
1.759
7.162
β
98.882
98.102
97.868
97.902
97.829
Scale
1.419
1.419
1.4566
1.440
1.416
diameter(Å)
23.6
25.6
25.51
25.88
25.69
Rw (%)
23.6%
19.9%
19.3%
18.8%
18.3%
6.01
1
increasing pH
4.40 4.74
1
2
3
5
6
7
8
9
10
(b)
2
Zn-Mn correlation
Zn-O correlation
3.50
2.11
5.34
6.0
4.77
4.36
7.78
7.41
Zn1
Zn2
Zn3
Zn4
8.75
(d)
0
expand
- H+
1
2
3
4
5
6
7
8
9
10
r (Å)
(c)
Calculated partial G(r)
50nm
δ-MnO2 was synthesized via reduction of KMnO4 by Mn(NO3)2 in solutions.
4
-1
Figure 4. The obtained δ-MnO2 unit cell dimension (a, b and c) increased as pH
increased whereas the stacking angle (β) decreased. These indicate that increasing
(decreasing) pH caused structural expansion (contraction) of δ-MnO2.
Figure 2. High-resolution TEM images of δ-MnO2 particles.
8.71
r(Å)
a
a
7.79
-1
b
β
7.37
0
d-G(r)
increasing pH
Ni4
Ni3
Ni2
Ni1
5.34
2.06
c
Zn-O + Zn-Mn
Zn-Mn
Zn-O
+ H+
contract
Total X-ray scattering analysis: Data were collected from 11-ID-B at the
Advanced Photon Source, Argonne National laboratory, IL. After background removal
and appropriate correction ,the data were converted to pair distribution functions
(PDFs) using the software PDFgetX. The PDF modeling was performed using
PDFgui.
7
Figure 3. (a) PDFs within 10 Å for a closer inspection. The arrows indicate right shifts
of the peaks, suggesting structural expansion. (b) Comparison of PDF experimental data
1
2
3
4
5
6
7
8
9
10
r(Å)
2
3
r(Å)
and modeling. The modeling was performed over 1-20 Å and the results are
summarized below.
Figure 6. PDFs of Ni2+ (a and b) and Zn2+ (c and d) sorbed δ-MnO2. In the order of
2+ sorption
Zn/Ni1,
Zn/Ni2,
Zn/Ni3
and
Zn/Ni4,
the
sorption
loadings
increased.
Zn
Table 1. Unit cell and atom-independent parameters of δ-MnO2 obtained from
2+ sorption is less
decreased
the
nearest
Mn-Mn
distance
whereas
the
impact
from
Ni
PDF modeling using spherical shape.
evident. The induced change is ascribed to decreased internal electrostatic repulsion.
MATERIALS AND METHODS
1) pH effects (pH = 1, 3, 4, 5 and 7) : NaOH or HNO3 was used to adjust pH of five
δ-MnO2 suspensions to target pH values. pHs were maintained for 24 hours.
2) Zn/Ni sorption: Sorption experiments were conducted at pH 4. Various loadings of
metal were added to δ-MnO2 suspension. The reaction last for 24 hours. Background
electrolyte used was 0.01 M NaNO3. The reacted δ-MnO2 was collected by vacuum
filtration and then air-dried.
6
blk -MnO2
1
5nm
5
-2
2.5
4
8
G(r) (Å )
2.0
3
r(Å)
pH4
pH3
2
-2
-2
Rw = 19.3%
G(r) (Å )
pH7
1.5
(b)
-4
Mn-O
G(r) (Å )
Mn-O
Mn-Mn
8
G(r) (Å)
(b)
blk -MnO2
Ni1
Ni2
Ni3
Ni4
(a)
12
d-G(r)
Surface sorption has been shown to be able to affect nanoparticle bulk structure,
such as H2O adsorption on ZnS nanoparticle surfaces (Zhang et al., Nature, 2003).
Extensive previous studies have investigated metal sorption on layered Mn oxide
nanoparticles by focusing on the metal side. It remains largely unknown whether
sorbed cations can also alter the crystal structure of MnO2 nanoparticles.
1. pH effects on δ-MnO2 structure
G(r) (Å )
Layered manganese (Mn) oxides are the major naturally-occurring Mn-oxides in
soils and sediments. They have extraordinary ability to sorb metal cations and oxidize
redox-active species, playing an important role in metal(loid) and nutrient cycling.
Layered Mn oxides usually occur as nanoparticles, such as vernadite, in the
environment. The oxides are enriched with Mn vacant sites and accordingly are
negatively charged (Figure 1). Thus, a large quantity of cations can be sorbed on vacant
sites for charge compensation.
G(r) (Å)
RESULTS
INTRODUCTION
1
2
3
4
5
6
7
8
9
10
r (Å)
Chalcophanite
H+
Figure 7. Differential PDF of Ni2+ (a) and Zn2+ (b) sorbed δ-MnO2 with respect to
pristine δ-MnO2.(c) Calculated partial atomic correlations involving Zn atoms from
chalcophanite, ZnMn3O7·
3H2O. (d) Chalcophanite crystal structure. The good
Figure 5. Protonation and deprotonation of O atoms of Mn vacant sites. Protonation
alignment of d-G(r) to chalcophanite partial G(r) indicates Zn2+ and Ni2+ sorption
decreased the negative charge of Mn layers and internal electrostatic repulsion, and
occurred on the vacant sites of δ -MnO2.
accordingly inter-atomic distances (i.e., structural contraction).
ACKNOWLEDGEMENTS
We would like to thank beamline scientists Dr. Karena Chapman and Dr. Peter
Chupas on beamline 11-ID-B at the Advanced Light Source, Argonne National
Laboratory (APS).
CONCLUSIONS AND IMPLICATIONS
pH and heavy metal sorption are able to alter the δ-MnO2 crystal structure. The
structural changes may modify band gap energy of δ-MnO2 and accordingly its
photochemical reduction properties that affect Mn cycling in euphotic zones in oceans
and lakes.